Note: Descriptions are shown in the official language in which they were submitted.
:~'. ~'U\' : FYA S1UE:.NLHE:.V U 1 , i _ , . (~ : 7 4 : 51 : 4 LEi 9E~ 1 ~Ufi
1-~ +.~cJ. ~~ _~~ J~.~.3.~1-Eib t,'
CA 02335546 2000-12-19
METHOD OF LASER WELDING TAILORIED BLANKS
SCOPE OF THE 1NVENTTON
The scope of the invention relates to an imtgroved method of laser welding
together
two or more sheet blanks along a seamline, and more preferably, to an improved
method of
using either a single or multiple beams from a yttrium alun'iinwn garnet (YAG)
laser to butt
weld together t:ailorcd blacks.
BACKGROUND OF THE I1WENTION
In current day manufacturing processes, it is known to form finished.
vvorkpiece
components by welding together two or more sheet metal blanks of different
thickness and/or
shapes to produce a tailortd blank. Tailored blanks are ma,ie by joining
various sheet
material which may have different gauges, surface coatings andlor properties
to achieve a
finished workplace having maximum strength with maximum mil>xrial costs and
weight. The
automobile industry is an emerging area where tailored blanks arc achieving
more and more
prominence and where such blanks arc formed into various automotive parts and
vehicle
panels. For example, it is known to maruzfacture automotive doors which
incorporate a
munber of small strategically placed strengthening components b;r spot
welding.
Patent Co-Opara~tion Treaty Application No. PCTIUS9f~/O51Z2 to The Twentyfirst
Century Corporation, published on October 17, 1996 ss WO 96/a2219 discloses a
method of
butt welding sheets whereby a laser is used to butt weld the sheets together
along a seatnline.
Laser welding is accomplished by orienting the laser beam ir. an acute angular
position
tangential to the direction of welding.
The conventional manufacture of tailored blanks has suffered the disadvantage
in that
the use of lasers has necessitated that the edges of the compo><:erit blanks
to be joined be
prefinished to high tolerances with edges polished to a mirror smooth finish.
SUMMARY OF THE IN'VENT1ON
In International application No. PCTICA98I00153 fled on February 24, 1998,
the applicant has disclosed an improved apparatus which may be used to butt
weld
together sheet metal blanks, and which incorporates a multiple beam laser
welding
apparatus. In this regard, International application No. PCT~CA98f00153
relaxes to a
welding apparatus used in industrial processing as, for exauEple, would
include the
~4M~t~~ED SHEET
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manufacture of tailored blanks used to form automobile components. The
apparatus
used a multiple beam of two or more coherent light sources to weld together
the
proximal edge portions of sheet blanks. In addition, a mechanism is provided
to
selectively position the orientation of the coherent light sources relative to
the seam line
so that welding may be achieved where a gap between the sheet blanks exists.
It is an object of the present invention to provide a method of optimizing the
selective positioning or orientation of the multiple beams relative to the
seam line, to
ensure a complete welding of the blanks having regard to the gap spacing
between the
blanks, the relative thicknesses of the abutting portions of the blanks and/or
the
materials which are to be joined together.
The present invention envisions the use of a YAG laser and more particularly
an Nd: YAG laser used to weld the tailored blanks as a most preferred coherent
light
source. It is to be appreciated, however, that other lasers including CO,
lasers are also
envisioned as being potentially useful with the present method. A comparison
of the
relevant criteria between Nd: YAG lasers and CO, lasers is as shown in Table
1.
Table 1 - Characteristics of Nd: YAG-laser and CO,-laser
CO,-LASER ND: YAG-LASER
Wave length (~cm) 10.6 1.06
Beam quality (mm mrad)4-15 20-60
I
Beam delivery metallic mirrors glass fiber
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- Beam focusing metallic mirrors glass tens
Intensity profile Gaussian flat top
fifficiency about 10% 2-5
Polarization plane or circular no
Operation cw or pulsed cw or pulsed
Operation consume Gas: He, CO,, N, Kr-lamp
Power (W) 5000-6000* 3000-4000
m one manuracture or tanored blanks Tnstalled lasers
The Nd: YAG laser is capable of producing butt welds on various steel Sheets
with satisfactory properties at welding speeds meeting the automotive industry
demands. In comparison with the CO,-laser welding the Nd: YAG appears to be
preferable as it is more tolerable to joint gap variations, seam edge
straightness and
offset of the sheared sheets.
Although the present method may be used with single laser beam techniques,
the use of a dual-beam or multiple beam technique for laser material
processing has the
advantage of using increased laser power for faster welding speeds and the
possibility
of achieving better quality, improved efficiency and flexibility with the
system. The
two principal purposes by which a dual-beam or other multiple three or more
beam
technique is introduced to weld different tailored blanks are to increase the
processing
speed and to extend the processing quality, by welding joints with greater
edge and gap
tolerances. --
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Accordingly, in one aspect the present invention resides in a method of using
,
a composite laser beam to weld together adjacent edge portions of two work
piece
blanks along a seam line, said composite beam including a first laser beam and
a
second Laser beam, each of said first and second laser beams being focused
towards a
portion of said blanks to be welded at respective focal areas having an optic
centre, the
optic centers of said first and second laser beams being spaced from each
other and
defining one end of a focal line of said composite beam, and wherein the
effective
diameter c~~yj~of the composite beam is defined by the maximum spread of the
first and
second laser beams in a direction transverse to said weld direction and said
seam line,
said blanks joined by the steps of
(a) determining the gap spacing between the abutting edge portions of the
blanks
to be welded;
(b) adjusting the effective diameter of the composite laser beam to infill the
gap
substantially in accordance with the formulas:
(rI+ Vie) - 2R' and where ryw/_= ?.r~
(h~'h~-I )
wherein ~ is the gap spacing, c~,,~ is the transverse distance the laser beam
center is offset trom the seam line, h, is the thickness of a first thinner
blank
and h~ is the thickness of the second other thicker blank;
(c) altering the rotational angle ~ of the focal line of the composite beam
relative
to the seam line substantially in accordance with the formula
ry= c~~ + h-sin~
2
wherein d, is the focus diameter of said first laser beam and h is the
distance
separating the optic centers; and
moving the laser beam along the adjacent portions of said blanks to weld the
workpiece blanks together.
In another aspect, the present invention resides in a method of using an
apparatus
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to butt join an edge portion of a first workpiece blank to an edge portion of
a second
workpiece blank along a seam line, said first workpiece having a thickness h,
selected less
than the thickness h~ of the second workpiece blank, the apparatus including,
a laser for emitting a coherent light source to weld said blanks along said
seam Line
and substantially infill any gap between the edge portions, and a controller
for controlling
said coherent light source, wherein during welding said controller maintains
said coherent
light source under effective power substantially in accordance with the
equation:
wherein I',: represents the effective laser power, n the welding speed, p is
the
f ~=,y . p . p . (L~.,W ~ I ni + h,n t L'r,4 ~ ~~)
density blank material, c,.", and c~", are the specific heat of solid and
liquid blank material,
Tm the melting temperature, h," the melting enthalpy of the blank, and DT the
medium
overheating temperature of the melt above the melting point, and wherein S
equals the
area of weld cross section, and .S is determined substantially in accordance
with the
formula:
.f=h= ~ (r~. + cl,~. ) + h~ . (,~ _ ~l«~_ _ ~,~)
wherein r~ is the radius of the coherent light source spot at the seam line in
a direction
transverse to the seam line, c~,~~ is the transverse offset of the center of
coherent light
source spot from the seam line and ~~ is the gap width between the edge
portions.
In a further aspect, the present invention resides in a method of using an
apparatus
to butt join an edge portion of first workpiece blank to an edge portion of a
second
workpiece blank along a seam line, the first workpiece blank having a
thickness h,, and
the second workpiece blank having a thickness h~ selected greater than or
equal to h,, the
apparatus including,
a laser for emitting a coherent fight source as a laser to butt weld said
blanks
together along said seam line,
said blanks being joined by,
(a) positioning said edge portion ~f said first blank proximate said edge
portion of said second blank,
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(b) activating said laser to weld said edge portions while maintaining a gap
spacing (,~) between said proximate edge portions in accordance with the
formula:
__! h,
'~ 2 h I ~r~. + c~"n. )
wherein rf is the radius of the coherent light source in a direction
transverse to the seam
line, and c~!,lj is the distance the center of the coherent light source is
transversely offset
from the seamline.
BRIEF DESCRIPTION OF THE DRAWINGS
Further objects and advantages of the invention will appear from the following
description, taken together with the accompanying drawings in which:
Figure 1 shows the schematic top view of a production assembly line for
forming composite work pieces in accordance with the present invention;
Figure 2 shows the schematic side view of a laser welding head used in the
production assembly line of Figure 1;
Figure 3 shows the laser welding apparatus shown in the production assembly
of Figure 1, taken along lines 3-3' showing the use of a laser to weld sheet
blanks;
Figure 4 shows schematically a test production facility for performing dual
beam laser welding using Nd: YAG lasers;
Figure Sa shows graphically the change in focus by radii relative to lens
distance;
Figure Sb shows graphically the focus spot radii relative to the change in
laser
power;
Figure 6 shows schematically the processing and welding parameters used in the
test installation shown in Figure 4;
Figure 7 shows graphically an energy intensity profile of a test of a dual
composite beam used in the method of the present invention;
Figures 8a and 8b illustrate graphically the influence of offset and gap in
laser
welding;
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Figure 9 shows schematically the weld cavity used in the evaluation of weld
acceptance;
Figure 10 illustrates schematically the theoretical principal of gap filling
by
laser welding;
Figure 11 shows graphically the maximum allowable gap in joints as a relation
to sheet blank thickness;
Figure 12 shows cross-sectional views of sample welds illustrating the
influence
of gap on weld concavity;
Figure 13 illustrates graphically the effect of gap and laser beam size as
related
to weld concavity;
Figure 14 shows schematically the energy distribution of laser welding
processes;
Figure IS illustrates laser energy absorption versus incident angle on a
workpiece;
Figure 16 illustrates graphically the calculated coupling rate percentage by
workpiece thickness and spot diameter ratio;
Figure 17 illustrates graphically the relationship between welding speed and
workpiece thickness;
Figure 18 shows schematically a model used to calculate surface absorption of
laser power;
Figure 19 shows graphically the effect of gap and offset on surface
absorption;
Figure 20 shows graphically the effect of welding speeds in relation to gap
and
offset;
Figure 21 shows graphically the welding speed differences between single beam
and dual beam welding techniques;
Figure 22 shows cross-sectional views of welds illustrating the effect of head
angle on laser weld concavity;
Figure 23 shows graphically the relationship between changing offset and weld
concavity;
Figure 24 shows graphically the relationship between gap and concavity;
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Figure 25 illustrates the maximum allowable gap in relation to the head angle;
-
Figure 26 shows the relationship between welding speed and head angle in
relationship to 2 to I.5 mm galv. to x 300 W offset 0.3 mm;
Figure 27 shows schematically a model illustrating surface absorption and head
angle;
Figure 28 shows graphically the calculated surface absorption versus head
angle;
Figure 29 shows graphically the intluence of gap width on welding speed;
Figures 30a to 30c are photographs of weld cross-sections showing the effect
of offset on weld concavity;
Figure 31 shows graphically the effect of offset on weld concavity;
Figure 32 shows graphically the effect of a gap on weld concavity using a dual
beam technique with an offset of 0.3 mm and a head angle of 6° to weld
2 to 1.5 mm
galvanized sheets;
Figures 33a through 33d show photographs of failure locations of weld
specimens produced by an Olsen test;
Figure 34 shows the intluence of offset and gap on the cracking behaviour of
welds;
Figure 35 illustrates schematically the use of dual laser beams to increase
the
effective beam size;
Figure 36 shows graphically the effect of defocusing the laser beam on welding
speed;
Figure 37 shows graphically the effect of rotating a dual beam coherent light
source on welding speed;
Figures 38a and 38b illustrate the effect of melting efficiency and welding
speed
relation to beam diameter;
Figure 39 shows sectional views of sample weld protiles in relation to the
rotation angle of the laser beam focal line;
Figure 40 (shown together with Figure 38) illustrates graphically the
influence
of beam rotation on concavity welding 2.0-1.~ mm sheets;
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Figure 41 (shown together with Figure 38) illustrates graphically the
relationship of weld concavity and beam rotation angle with a 0.3 mm offset;
Figure 42 illustrates graphically the comparison of positive and negative beam
rotation angle versus concavity in welding 2.0-I .5 mm galvanized sheets using
a dual
beam coherent light source with 0.3 mm offset and a head angle of minus
6°;
Figure 43 illustrates the effect of beam rotation angle on maximum allowable
gap;
Figure 44 shows graphically the effect of beam rotation angle, welding speed
and gap in automatic welding processes using a dual beam technique;
Figure 45 shows graphically the offset window which exists having regard to
gap size in using a dual laser beam welding technique;
Figure 46 shows graphically the effect of head angle on the offset window in
which a qualified weld may be achieved;
Figure 47 shows graphically the relationship between the offset window and the
thickness ratio of the sheet blanks to be joined;
Figure 48 shows graphically the effect of f)uctuating cap size on the offset
window;
Figure 49 shows graphically the effect of the rotation angle of a dual beam
coherent light source on the offset window;
Figure 50 shows the effect of rotation angle of a dual beam coherent light
source on the offset window joining 2.0 to 0.75 mm sheets;
Figure 51 illustrates schematically a prototype tailored blank produced in
accordance with a method of the present invention;
Figures 52a and 52b show cross-sectional views of the sample single beam and
dual beam weld seams for the prototype shown in Figure 51;
Figure 53 shows a photograph of the Olsen test of the welds conducted on the
prototype in accordance with the present invention;
Figure 54 shows a prototype tailored blank used to form a Cadillac rear door
and the resulting weld cross-section formed in accordance with the present
invention;
Figure 55 shows schematically a prototype tailored blank for a Jeep Cherokee:
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Figures 56 to 58 show cross-sectional views of weld joints achieved in forming
-
a prototype Jeep Cherokee tailored blank in accordance with the present
invention;
Figure 59 illustrates the results of Olsen testing on weld joints produced in
the
production of the Jeep Cherokee prototype; and
Figure 60(I) and (II) illustrate various non-linear welds formed in accordance
with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Reference is made to Figure I which shows a production assembly line 10 used
in the simultaneous manufacture of two composite tailored blank workpieces
12a,12b.
With the assembly line 10 shown, robot vacuum lifts l 8a, l8b are used to move
pairs
of sheet metal blanks 14a,16a, 14b, l6b from respective supply stacks. Each
robot
18a,18b is adapted to move the paired blanks 14a,16a, 14b,16b, respectively
onto a
conveyor array 20 used to convey the blanks 14a,16a, 14b,16b and finished
workpieces
12a,12b along the assembly line 10. The conveyor array 20 consists of three
sets of
elongated magnet stepping conveyors 22,24,26 which are operable to move the
pairs
of blanks 14a, l6a and 14b, l6b and workpieces 12a, I2b in the longitudinal
direction
of arrow 28. The magnetic stepping conveyors which comprise each conveyor set
22,24,26 are shown in Figure l arranged in a parallel orientation to both each
other
and the conveyors in the remaining sets. It is to be appreciated that other
conveyor
configurations are also possible.
The first set of conveyors 22 are used in the initial positioning of the
blanks
14a,16a and 14b,16b in the production fine 10, and the conveyance of the
positioned
blanks 14a,16a and 14b, ! 6b on to the second set of conveyors 24.
Conveyors 24 are provided as part of a laser welding station 32 in which the
proximal edge portions of the blanks 14a, l6a and 14b,16b are welded together
along
a seamline by a yttrium aluminum garnet (YAG) laser 36. The conveyors 24 thus
are
used to move the unwelded blanks 14a, l6a and 14b, l6b to a welding position,
and then
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after welding convey the completed workpiece I2a,12b onto the third set of
conveyors
26. Conveyors 26 are used to convey the completed composite workpieces 12a,12b
to robotic vacuum lifts 38a,38b which lift the workpieces 12a,12b therefrom
and onto
output stacks.
The production line 10 shown in Figure 1 is configured for the concurrent
manufacture of two completed workpieces 12a,12b by a single laser 36. As shown
best
in Figures 1 to 3, the YAG laser 36 includes a coherent light source generator
40 used
to generate two coherent light sources or laser beams, a movable laser head
assembly
42 (Figure 2) and a fibre optic coupling 44 (Figures 1 and 3) optically
connecting the
generator 40 and laser head assembly 42. The fibre optic coupling 44 consists
of a
bundle of two fibre optic cables (not shown). The energy of the two coherent
light
sources generated in the generator 40 thus travels via a respective fibre
optic cable to
the laser head assembly 42.
Figure 2 shows the laser head assembly 42 as including a light emitting laser
head 46 from which laser energy is emitted. As disclosed, the laser energy
comprises
the composite beam which consists of the two coherent light sources. The
assembly
42 further includes a support 48 which rotatabiy mounts the laser head 46, and
a drive
motor 52 used to rotate the laser head 46 on the support 48. The laser head
assembly
42 is provided with a microprocessor controlled seam-tracking sensor 49
(Figure 2)
which senses the spacing between the proximal edge portions of each pair of
sheet
blanks 14a,16a, 14b,16b to be joined. The sensor 49 may, for example, be of
the type
disclosed in Canadian Patent Application Serial No. 2,199,355 filed March 6,
1997.
The sensor 49 includes a separate coherent light source which directs a beam
of
coherent Iight downwardly onto the proximal portions of the sheet blanks and a
vision
or optic sensor for sensing light reelected therefrom. The sensor 49 provides
control
signals to the drive motors 52 and 64 and the gantry robot 54 to automatically
position
the laser head 42 so that the composite beam 30 is directed at the weld seam.
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Figure 1 shows best the laser 36 as being entirely housed within an enclosure -
50. The enclosure 50 is provided with mailbox type entry and exit doors 51,53.
Clamping units 60 are also provided within the enclosure 50 for maintaining
the sheet
blanks in position during welding operations. While numerous types of clamping
arrangements are possible, the clamping units 60 preferably each consist of a
magnetic
clamping unit of the type disclosed in Canadian patent application serial No.
2,167,1 I 1, which was laid open to the public on 12 July 1997.
The entire laser head assembly 42 is configured for two axis movement
horizontally. The assembly 42 is movable in a first horizontal direction over
the
conveyors 24 and blanks 14a,16a, 14,16b via a gantry robot 54, along a paired
overhead support and slave support 56a,56b. The laser head assembly 42 moves
in the
first direction via the gantry robot 54, along a track 58 (Figure 3) provided
on the
overhead support 56a. Each of the pairs of supports 56a,56b are further
slidable in a
second horizontal direction which is perpendicular to the first on parallel
spaced end
supports 62a,62b.
The end supports 62a,62b in turn movably support the ends of the parallel
supports 56a,56b. A servo drive motor 64 (Figure l ) at the end of support 56a
engages
a track 66 which extends along one end of support 62a. The movement of the
laser
head assembly 42 along the supports 56a,56b, and the movement of the supports
56a,56b on the end supports 62a,62b permits the laser head 46 to move over the
blanks 14a,16a, 14b,16b in any horizontal direction. The laser head 42 is also
vertically movable, and may be inclined relative to a vertical orientation, as
for
example to the position shown in phantom in Figure 2, by means of a pneumatic
slide
68.
During welding operations, two coherent light sources are produced in the
coherent light source generator 40. The coherent light sources travel via a
respective
fibre optic cable in the coupling 44 to the laser head 42 and are emitted
therefrom
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towards the portion of the seamline 34 to be laser welded. Two laser beams are
thus
emitted from the laser head 42 to weld proximal edges of the blanks 14a,16a
and
14b,16b as a composite laser beam 30 having an elongated focal tine which
intersects
the optical center of each beam.
To achieve optimum welding, experiments were carried out using two 3 kW
Nd: YAG lasers and a dual fiber optical cable to explore characteristics of
the dual
beam welding method and build a series of experimental data, with which to
base the
development of suitable welding procedures and construct advanced laser
welding
systems.
a) Test Installation
A research installation, shown in Figure 4, consists of two Haas HL3006D
Nd:YAG lasers and a I .2 m x l.2 m lab gantry robot and a welding station
equipped
with a tracking system described with reference to Figures ! to 3 The laser
beams are
led into the workstation with a dual step index Mass fiber which consisted of
two
single glass fibers whose ends are jointed together The beams were focused
through a
standard Haas 1:1 optic head with two 200 mm lenses. A compressed cross air
stream
was provided as protective air flow to prevent the optical head tfom smoke,
spray and
weld spatter from the welding.
To develop a full understanding of the characteristics of the Nd: YAG laser
beam, the optics (focal length) and the glass fiber delivery system used, the
focused
laser beams were measured. The following documents the results of a complete
series
of experiments with the laser beam guided by: a) a single fiber and b) a dual
fiber,
using a PRO1~TECT"' laser scope. The size, intensity profile and relative
positions)
of the focus point for optics with focus lengths of 100, I S0. 200 mm lens are
accurately determined.
Caustics and Radius Laser Beams
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The focused laser beam for the three optics were measured. The minimum '
focus spot radii were 0.3mm, 0.43mm and O.s6mm respectively, for f = 100, 150
and
200mm optics, as illustrated in Figure Sa. The smaller the optic the steeper
the curve
rises as it deviates from the true point of focus. The radius reaches a
minimum value
near the focus point and increases as an exponential condition while the
distance is
moved away from the focal point. The results of measuring the beam radius at
distinct
power levels is also shown in Figure Sb, whereby the radius of the focused
beams)
remained almost constant, while the power was changed from 300W to 3000W. That
is another advantage of fiber conducted Nd: YAG-laser. A comparison of the
beam
characteristics for different optics shows the optic with loner focal length
ie: 200, has
a loner rally length. This is to be expected knowing the basics of solid state
laser
beams, but having the enact data allows for a more accurate setup of the weld
parameters. The greater the distance over which the beam radius remains
constant the
more it increases the stability of the process. Therefore the 200 mm focal
lens is
selected in the research and the production.
The position of the focus spot for each particular optic is very important
because the welds are normally produced while the focus spot is set at the
surface of
the sheet. The focal position for each of 200 mm optic is 179 mm, measured
from the
materials surface to the cover of the protective ;lass. This dimension will
remain
constant if the lens and the lens's keeper are identical.
The major processing parameters for laser welding of tailored blanks are shown
schematically in Figure 6. These parameters can be divided into two groups:
(a) the
welding parameters; and (b) the properties of the sheets used for the tailored
blanks.
The first group includes the laser power at the surface of workpiece P, of
laser l, P
of laser 2, travel speed >>, focus position ~, angle of the head 8, beam
rotating angle ~
of the laser beams to the joint and the offset ci!,~ from the joint.
Figure 7 reveals, through a three dimensional display, the intensity of the
dual
beam and the relationsh~:- of ~' ~ spots at '_' x 3000 W The profile indicates
that the
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power distribution is almost constant over the full diameter of the beam when
it is in
focus. Each beam emanates from a 3000 W laser. The diameter of each spot is
about
0.6 mm, the same as each single beam. The distance between two focus spot is
l.2mm
and there is a space of 0.6mm between two spots. The maximum width of coverage
by
rotating the dual laser beam to 90° (ie. so that the focal line
connecting the optical
centers of the beams is transverse to the seam) is I .8mm. In addition, the
power of
each spot can be changed individually according to the requirements. It gives
a useful
method to process some particular joints.
The second group includes materials, coatings. thickness of two sheets,
shearing edge
condition and gap between the sheets. As will be described, the gap is one of
the most
important factors affecting the selection of weld parameters, the weld
concavity, and the
results of the Olsen tests. The set up of welding process is generally
described as follows:
1 ) the laser power is normally selected at the maximal output power of two
lasers to
achieve the maximal welding speed;
2) the focus position is an important process parameter of laser welding, so
that a
correct and accurate setup of the focus position is the condition to get a
stable and
effective welding process. The focus spot of laser beams for welding tailored
blanks is preferably located on the surface of the thinner sheet;
3) normally, by welding tailored blanks from 0.8 to 2.0 mm, a head ankle of ~6
degrees is proposed. The selection of the head angle is basically dependent on
the
thickness ratio of a joint. For welding a joint with a large thickness ratio,
a
positive head angle is proposed, and for joint with small one, a negative
angle is
preferred;
4) the offset is also an important welding process parameter. It may be
determined
experimentally to minimize the weld concavity and achieve optimal weld cross-
sect~onmg;
S) the necessity of the beam rotating is based on the maximal gap in joints.
It is
applied only in case the maximal gap is beyond the gap filling capability of
the
single beam technique;
6) the welding speed is determined by increasing step by step until the joint
is not
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completely penetrated. So a maximal weldin~V speed can be found out. The
welding speed can be selected at about 90% of the maximal value for a
optimally
reliable welding process.
Two testing methods were developed to concentrate the investigation on the
influence of the offset and gap on welding processing. One was the welding by
varying
offset (Figure 8a}, in which the offset is being changing continuously along
the whole
joint. At the start of each welding, the offset is zero, at the end of the
specimen the
offset reaches a designated value, for instance 0.3, 0.6 or 0.9 mm.
Occasionally, a
certain gap can be added to welding tests. After welding the specimen was
checked to
find the minimal and maximal offset, at which the sheet is not fully
penetrated or a
proper weld is not achieved. The specimen was then cut at those positions with
special
offset values, for example at 0, 0.1 mm. . . etc. to check the weld cross
section as well
measure the weld concavity. An offset range, in which the weld concavity is
below a
certain value (typically l0%), can be decided based on Figure 8. In many cases
there
exists an optimal offset value from these results.
Another was the welding by varying gap, as illustrated in Figure Sb. The two
sheets are so clamped that at start of welding there is no gap between the
sheets, at the
end of the joint a designated gap is set up. Then the width of the gap was
measured
with thickness gauge and the positions are marked. The welding was carried out
at a
constant offset (normally near the optimal offset). After welding the
specimens were
accurately sheared at those marked positions to check weld appearance. A
typical
result drawn from the testing can also be seen in Figure 8b. Normally the weld
concavity increases with larger gapping. The maximal allowable gap can be
determined
from such a kind of diagrams according to the maxima! allowable weld concavity
(for
example 10% or I S%).
To reduce testin<, errors caused by the variation of the straightness of
sheared
edges, short sheets (600 mm long) were used as welding specimens in the
research
work. Two characteristics of the welded joints have been selected to evaluate
the
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acceptance of the welds, weld concavity and the Olsen test.
As shown in Figure 9, cross sections of the welded joints are ground (600
grit) and
etched (12 % vital) to examine the weld fusion area and measure the minimum
through
thickness dimension of the weld, under the microscope. The ratio of the
measured minimal
section to the original thickness of the thinner sheet is the concavity
expressed as a percent
of the thickness of the thinner sheet. The concavity is an important weld
property. To
ensure the weld quality and formability, there is a upper limit of 15% for
concavity in
welding specifications.
The Olsen test is a qualitative formability test. The welded coupon is
stressed to
fracture. The fracture location is noted. A weld sample is accepted if the
crack starts
and expands in the base metal and does not have problem in the form process.
The
Olsen test is much stricter than the form in dies, so that a weld passed Olsen
test
formability of welds ought not to fail in the die process.
b) Gap Filling by Laser Welding
Using a simple model the relationship amon~,_ the offset, gap, laser focus
spot
and thickness of both sheets can be described for a welding processing without
additive filler material, as illustrated in Figure 10. Assuming the metal on
the edge of
the thicker sheet would be melted to fill the dap, the shape of this edge
would
approximately be triangular. The range of the melted metal would be decided by
the
laser beam dimension, i.e. the material just under the radiation of the laser
beam is
melted. To completely fill the dap ( S~ ) the area Sm of the melted thicker
sheet must
be equal to that of the gap Sg, thus the following relation exists:
,S~ -~,~.f?~ (3.1)
Therefore, the allowable width of gap is:
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~,T=~~~- -IJ(y +cl~~.)=~(TR-I)(~l ~cf~,~.) {3.?)
In the equation, the offset (ci,~. ), dap width (~,r) the focal spot radii (r~
) the
thickness h, and h~ of thicker and thinner sheets are shown in Figure 3. I .
TR is the
thickness ratio of welds(h~:h,). According to this model the gap will be
filled. The
following factors must be considered when examining the outcome of the process
variables: (a) Increasing the offset {c~~,~. ); (b) Changing the shape of the
molten zone
through altering of the head angle will effectively melt more or less of the
thicker
sheet; and {c) Increasing the focus spot size (r~ } of laser beam through
using dual
beam or defocusing the beam
But the offset is limited by laser spot size and gap, i.e. the maximal offset
is
equal to rr-g. If the offset is larger than this value, the edge of the
thinner sheet cannot
be touched and heated by the laser beam. It results in an unstable weld
process.
Therefore, the maximal dap is:
rf(TR-1)
'~'",'" - 1 + 0.5( TR - 1 ) ( 3 . 3 )
Figure I I shows the maximum allowable gap in joints as a function of
thickness
ratio by two laser beam spot sizes. The ~; = 0.3mm corresponds to a single
beam
welding, while y =0.6mm corresponds to dual beam spots with a rotating angle
of 30°.
It states, in one side, that the maximum allowed dap has to be considered with
the joint
configuration. The larger the thickness ratio, the easier to get weld without
concavity.
On the other hand, by welding a certain joint, to get better dap filling is to
use a Laser
beam with a larger focus spot. In Table 2, the maximum allowable gap by laser
welding
several typical tailored blanks is listed.
Table 2 The calculated maximal gap by laser welding tailored blanks
II Thick I Thin I TR I Max. Gap ~ Max. Gap
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(mm) (mm) (single beam c~, (dual beam clf
0.6mm) 1.2mm)
1.8 0.8 2.25 0.23 mm 0.4b mm
2.0 1.0 2.0 0.20 mm 0.40 mm
1.3 0.8 1.6 0.14 mm 0.28 mm
1.3 1.0 1.3 0.08 mm 0. l6 mm
1.0 0.8 I.25 0.07 mm 0. I3 mm
Figure l 2 shows the influence of gap on weld cross sections by welding of 2.0
to 0.75 mm galvanized tailored blanks. From this picture, it is evident to
observe how
the gap is to be filled in the welding process. The laser beam melts the edge
of the
thick material, which flows down to the joint. In the case of zero or small
gap the
volume of the melted material on the thick side is larger than the amount
which gap
needs. Therefore it overflows the thin sheet, a near triangle shaped weld
section is
formed. If the gap becomes larger, this part of the material will get into the
gap, the
weld gets flat. Also another useful result is worth notice, namely the lamest
area of the
melted transversal section in the weld is achieved under the zero gap. It
means that the
smaller the dap, the more amount of material is melted so that the higher
effective
melting power is needed.
In Figure l3, the influence of laser beam size and thickness ratio on the weld
concavity is shown and proves the model introduced above. Generally, for
welding
tailored blanks with large thickness ratio (TR), there is less problem with
the gap
filling. The seam even with a gap of 0.3 mm can be still welded, without the
concavity
of the weld exceeding customers specification, for instance 10%, even using
single
beam welding technique. Based on the Equation (3.3), the allowable maximal
gaps for
2.0 to l.5 mm sheets is 0.085mm, so that a normal welding technique such as
single
beam or dual beam in Line is insu~cient to ensure a weld without concavity, if
the gap
is too large. For this reason, enhanced dual beam welding techniques such as
beam
rotation are to be applied for weidin~ low thickness ratio seam with a large
gap.
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Energy Balance by Laser Welding Tailored Blanks
A stable and continuous welding process is a result of energy (or power)
balance among laser power, coupling rate and loss power and effective power.
as
schematically illustrated in Figure l4. The essential energy for welding comes
from the
laser beam. The materials absorb a part of laser energy and convert it into
heat. This
process can be described by using an important number: coupling rate A. It
indicates
how many percent of the laser energy(power) f'~ will be absorbed into the
material.
The rest (f R) is reflected on the surface of the materials. The absorbed
laser energy can
be further divided into two parts. One of them contributes to melt the
material to form
the seam and is defined as effective power I'F. Another part is power loss
into the base
metal through heat conductivity and described as f',~. For laser welding
process, the
absorbed laser power has to cover the total effective power and power loss, so
following basic equation is valid:
A~f~ = fF+f',. (3.4)
From the principle of a welding process, this equation states that the
absorbed
laser power should be equal to the sum of the effective power and power loss.
If A ~f'~
is smaller than l',: - t',. , it means not enough power in the joint and can
result in no or
poor penetration. On the contrary, if A ~f'~ 1S larger than f'F T f',.., it
indicates too much
power and can often cause overheating, pinholes, blowing out or even cutting
in welds.
The purpose of introducing the energy balance is building a mathematical
formula to explore the relationship among the material and welding parameters.
It
enables the quantification of the maximal speed, the effect of the gap, offset
on welding
process as well as the requirement on tracking system.
The absorption of laser energy into materials is dependent on their optical
properties (temperature dependent), the wavelength and polarization direction
of the
laser beam and the incidence angle of the laser related to the surface. The
relationship
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among those parameters is given by Fresnel equation. A typical absorption
factor of
iron (valid also for normal steel) on Nd: YAG-laser beam (wavelength 1.06
micron) at
the melting temperature of iron is illustrated in Figure 3.6.
However by the penetrating welding with "keyhole" mechanism, the coupling
rate is not only dependent on the surface absorption, but also a function of
the shape of
keyhole because the multi-reflection-absorption effect of laser beam. In
Figure 16, the
coupling rate is shown.
For laser welding tailored blanks, thickness of sheets is in the range of 0.75-
3.0 mm, the laser beam diameter is 0.6 mm for 0.6mm glass fiber and a 1 v 1
focusing
optic, so the Thickness/diameter ratio of welding process is in the range
about I .25-5.
Then the coupling rate of Nd:YAG-laser welding process verifies between 60-
80%.
For COZ-laser welding, it verifies between 3S-60%. Therefore the coupling rate
using
Nd:YAG-laser is expected higher than using COZ-laser even in penetration
welding
process with the keyhole mechanism. For laser welding sheets with unequal
thickness,
the thickness/diameter ratio can be calculated by:
h, + h
Thickfie~.sslc~iameterrcr~iu- _ ~ (3.5)
2d,
The effective power required to heat and fuse weld metal can be calculated
according to the following equation:
f F - ,f . >>. P. (c.".~ ~ T,. +h"~ '~' era, ~ ~~ (3.6)
In the equation, a is the welding speed, p the density of material,
c.~.°, and c,;~l the
specific heat of solid and liquid melting blank material, T", the melting
temperature, h"~
the melting enthalpy and dT the medium overheating temperature of the melt
above
the melting point. For laser welding a medium overheating temperature of dT=
0.?-
0.-1Tm is normally reasonable. .f is the area ofweld cross section and a
function of
sheets thickness, offset and dap. It can be calculated as follow:
f =h: ~(r~ +c!"n.)+h~ ~(r, -d"". -,~_) (3.7)
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under the condition -y ~ ~J~~is~-~,~. Ln the equation, /~, and hr are
thicknesses of thick
and thin sheets, y the radius of laser spot, c!"J/.the offset and ~ the gap
width. ,
The power loss can be approximately expressed as[]:
h,_ +h >>.h
f;, = 4.4 ~ K ~ r, ~ ~ ' ~ 4I) (3. 8)
In the equation K is the thermal conductivity, D the temperature conductivity
of the
material, w the weld width.
d) Theoretical Welding Velocity
From the energy balance as well as equations (3.4), (3.5), (3.6), (3.7) and
(3.8),
the theoretical welding velocity can be derived:
A~fL
_ (3.9)
.5,,". ~ p~(c,",T, +H", +,~T ~c,;y )+O.SS(h_ +h, )KwT, lD
For single beam welding, the weld width u~ is normally lamer than the laser
spot
diameter. According the experimental observation, v~ can approximately be
calculated
as l.3df . So the effective area ,S~~y of weld cross section is determined at
between about
1.1 and 1.55, and most preferably is equal to 1.3.f. The medium overheating
temperature is dT is O.l Tm. For dual beam welding, because of higher energy
input and
two spots in line, the weld width is even slightly larger than single beam
welding and a
higher overheating of melting pool is expected, so w is taken as 1.~~,1,, dT
is O.:tTm .
Using Equation (3.9), the theoretical velocities for welding several typical
steel tailored
blanks are calculated and compared with the experimental results, as shown in
Figure
17. An excellent correspondence between the calculated and experimental values
can
be observed.
Gaps and offsets influence welding speed in two points. On one side, they have
affect on the quantity of melted metal in welds. which is already involved in
Equation (3.9). On other side, they change the Absorption factor a. To
describe the
absorption behavior between laser beam and joint of sheets under different gap
and
offset, a simple mode! is here introduced, as shown in Figure 18. The
absorption of
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laser energy takes place on three places of a joint: a part of laser power is
absorbed by
the top surfaces of two sheets, whereby the incidence angle of the laser beam
is the
same as the head angle; the second one is absorbed by the edge of the thick
sheet over
the thin sheet, whereby the incidence angle of the radiation is 90 °-9,
the third part of
laser power will be absorbed in the gap by a mufti-reflection-absorption
process which
occurs between both edges of thicker and thinner sheets, whereby the incidence
angle
is equal to 90 °-B, too. The proportion of the absorbed laser power is
a function of the
head angle, diameter of the focused laser beam, width of gap, offset as well
thickness
of two sheets. In calculating absorption in joints, the incidence angle, gap
width, offset
as well as thickness of two sheets are also considered. The calculated results
for 2.0 -
I .0 mm joint are shown in Figure 19. By the results, it is obvious that the
surface
absorption is strongly dependent on gap size. For a joint combination, it
increases
firstly with the gap, then reaches a peak value at a certain gap. If the gap
becomes too
large, it drops down again. In contrast, the offset has hardly an affect on
the surface
absorption.
Using Equation (3.9), the model above as well Figure 16, the affect of gap and
offset on welding speed can be estimated. For a certain sheet combination, the
coupling rate A is calculated by using the result in Figure I 5. Then it has
to be
modified with the results in Figure 19. The calculated welding speeds by
different gap
and offset are shown in Figure 20.
By inclined laser beam radiation, from zero to a certain gap width. the
absorption
factor increases with gap, and then it reaches a maximal grad at a certain gap
width. This
is because the bigger gap, the more percent of laser power can enter into the
gap, which
will be reflected and absorbed several times between the gap. This results in
higher
absorption. Also the amount melted metal decreases with gap size. Both factors
causes a
higher welding speed. If the gap is too big, the absorbed laser energy becomes
less because
the times of absorption and retlection of laser beam in the gap decrease with
the gap size
and a part of laser beam passes through the gap without touching the sheets
edges.
Although the amount of melted metal is reduced, but the loss of laser power
through gap
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becomes a determined factor. Therefore welding speed decreases. The selection
of the
speed for a welding process has to be made based on the zero gap and the
maximal gap of
the sheared sheets to guarantee the proper welding penetration along the whole
joint. The
offset has almost no affect on coupling rate and only change the amount of
melted metal,
so that the welding speed decreases with increasing offset.
e) Welding Tailored Blanks with Dual Beam in Line
The welding of tailored blanks with two Nd:YAG-lasers guided by a double
lass fiber are detailed hereafter. The double tiber is so aligned that the
double focus
points of the laser beams and the focal line connecting such points are
parallel with the
joint (beam in line). The tests concentrate on determining the effect of head
alignment,
offset the laser beam, gap filling, welding speeds and welding parameters
appearance
(concavity) as well as properties of the welds using the Olsen formability
test.
Comparison of Welding Speeds with Dual Beams
One of the purposes using the dual-beam technique in laser welding tailored
blanks is that increased laser power results in higher production efficiency,
i.e. faster
welding speeds can be achieved. To compare the speeds of the dual beam welding
technique with the single beam welding, a series of tests on similar thick-
thin sheets
combinations and under the same test conditions were conducted. The results
are
shown in Figure 2 t . The laser power of single beam is 3000W, the dual beam
2x3000W. head angle is ~ 6 °. The gaps are set from 0 to 0.2mm. The
offset varies
between 0.15 and 0.3mm according to the thickness of thinner sheet. Studies
have
indicated that the welding speeds are normally limited by the thickness of
both sheets.
However, the thinner side plays a more important role by deciding the welding
speed.
Figure 21 reveals that the welding speeds for different sheets combinations
with dual-
beam are almost twice as fast as those with single beam. The welding speed is
greatly
increased with double the laser power. Therefor, the dual beam welding
technique can
provide possibilities for customers who need higher productivity (welding
speed), to
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get immediate results, without having to wait for newer Nd:YAG-lasers with
higher
power. The construction shown in dual beam technique displays an additional
advantage of having twin beams in reducing technical risk of welding system.
tf one
laser should be defective and require repair, the other laser can be used at a
reduced
welding speed, and production would run continuously.
2 . Influence of the Head Angle on Welding Process
The head angle is an important processing parameter. In one hand. the head
angle determines the direction of the keyhole, the penetration as well as the
shape of
welding pool. In another hand, the absorption of the laser power into
workpiece is
strongly dependent on the beam incident angle. To investigate the influence of
the head
angle on welding process, four head angles were chosen to weld the sheets.
Their
effects on melting and weld profile are schematically shown in Figure 22.
From Figure 22, it can be seen that there are three head angle ranges which
can
be selected by welding tailored blanks with different thickness. The first is
that the
laser beam comes from thinner side to thick side ofjoints, which is indicated
as
positive head angle. The second is that the laser beam is set up to be
perpendicular to
sheet surface, which is zero head angle. The advantage of positive head angle
is that
the joint is more easily penetrated, because the laser beam has only to
penetrate the
thinner sheet and to melt some material on the ticker side to fill the gap. A
higher
welding speed is therefore expected. By using dual beam technique more laser
power
is available so that the laser beam can be set in the third range, namely the
laser beam
comes from the thicker side to the thinner side of joints, whereby the head
angle is
described as negative. The influences of the head angle on weld concavity are
shown in
Figure 23 (welding by changing offset) and Figure 24 (welding by changing
gap). The
dependence of the maximal allowable gap on the head angle by a constant offset
is
illustrated in Figure 25.
Generally, from Figure ?3, Figure ?4 and Figure ?5, it can be seen: weld
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concavity decreases with increased offset; there exits an optimal offset to
minimize
weld concavity; weld concavity increases with gap size, etc. This indicates
that the
weld concavity can be reduced by setting proper head angle. For welding 2 to I
.5 mm
sheets the best gap filling was achieved with a head angle of -6 degree and a
joint with
0. l8 mm gap. The bigger the negative head angle, the farther the laser beam
gets into
the thicker sheets and the more material on the thicker side can be melted to
flow
down into the melting pool. Another advantage of negative head angle in
comparison
with positive head angle lies in the penetrating direction of the keyhole. By
the positive
head angle the keyhole is towards the root edge of the thicker sheet, the
distance
between the keyhole and the bottom of the joint increases with either
increasing the
offset or the head angle. This distance peferably should not exceed a certain
value.
otherwise the bottom edge of the thinner sheet will not be fully melted and
may result
in an improper weld. By the negative head angle the keyhole penetrates joints
from the
thicker side to the thinner side and is towards the bottom edge of the thinner
sheet.
The proper increasing of the offset and the head angle at same time does not
cause the
position changing of the keyhole at the joint bottom. Accordingly, on one
side, the
more offset and head angle can be set to melt more thicker sheets, on other
side, the
root of joint can be still melted to get a sound weld. The negative head angle
is
specially useful for welding the joints with small thickness difference to get
better weld
filling. Its disadvantage is that melting more material means more laser power
and
therefor lower welding speed.
From the Figures it is apparent that the worst gap filling is obtained by zero
head angle if any gap exits. One reason for this may be caused by the
interaction
between the keyhole and gap. In this case the keyhole at the sheet bottom may
become
bigger because a part of keyhole consists of the dap surface. For a deep
penetrating
welding with keyhole mechanism, it means that more material may be lost
through the
keyhole. Another reason may lie in the different absorption and interaction
between
laser beam and joint caused by varying radiating an;le.
The relationship between the welding speed and the head an,ie is shown in
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Figure 26. According to the experiments, higher welding speed can be obtained
in the
range of positive head angle as well at zero head angle. Generally, the
welding speed
decreases with the decline of the head in negative direction.
The welding speed is decided by the enemy balance of the heating process. For
laser welding the speed is determined by: l ) absorbed laser power by
workpiece, with
other word, the absorption; 2) the amount of melted material, under the
condition that
the heat loss through the conductivity into the base metal would keep same for
a
certain joint. As discussed above the head angle influences the amount of
melting
material. The negative head angle can melt more thicker sheets to get better
gap filling,
but more energy or laser power is needed. The welding speed is naturally
Power. By
the positive head angle and zero head angle the material to be melted is less
in
comparison with negative head angle, so that a higher welding speed is
expected.
The absorption behavior between laser beam and joint of sheets under different
head angles is shown in Figure 27. To simplify calculation process, the
percent of the
second part is assumed to be equal to the ratio: .S'~l to focus spot area. In
calculating
absorption in gap, the incidence angle, gap width, offset as well as thickness
of two
sheets are also considered. The calculated results are shown in Figure 28.
From the
calculation, several interesting results can be drawn. The absorption factor
by positive
head angle is the largest in three head angle ranges. The zero head angle has
the
smallest absorptance. As well, by zero head angle the absorption of laser
power
declines with increasing gap. The maximal absorbing of laser power occurs by
zero
gap. The bigger the gap, the more laser beam goes through the gap without
taking
interaction with material because of 90° incidence angle to the sheets
edges. For
welding tailored blanks it means that the welding speed would decrease with
increasing
gap size, if the amount of melted material should be kept the same.
This conclusion is experimentally confirmed, as shown in Figure 29, in which
the
welding speed is indicated as a function of the <,ap size. The welding speed
increases
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with the dap and reaches its maximum at a gap about 0. t mm and then
decreases. The
trend exactly follows the behavior of the absorption of the laser power shown
in
Figure 28.
Accordingly, the head angle is an important parameter and can strongly
influence
the welding processing. For a better gap filling the laser welding head should
be set in
negative angle range. However this kind of head angle setting is only suitable
for
welding two sheets with less thickness difference (say less than 25%). For
welding
joint consisting of two sheets with large thickness difference, it should not
be
recommended because the laser beam has to penetrate thicker side, which means
a
large loss of welding speed and so tar the productivity of the welding
processing. For
more effective absorption of laser enemy and higher welding speed it is
meaningful to
choose a positive head angle. In this case the proper welding speed is
determined by
zero gap and the possible maximal gap. The zero head angle has not only the
least
ability to fill gap, but also the smallest absorption of laser energy, so it
should be most
possibly avoided in welding tailored blanks.
f) Affect of the Offset on Concavity
By welding typical tailored blanks, the fused weld zone combines part of
thinner
sheet and a greater part of the thicker sheet. Once there is a gap between the
two
sheets, it must be filled to form a sound weld. As previously described, to
overcome
the concavity, good results may be obtained when the beam alignment radiates
more
into the thicker sheet. The offset of the laser beam is therefor another
important
processing parameter. Typical effects of the offset on weld cross section are
shown in
Figure 30. To quantitatively determine the affect of the offset on the welds
concavity
and to search for a optimal offset for the sheets combination, a series of
welding
experiments were done using three welding velocities and joints with three
sizes of
gap. The results of the affect of offset on the concavity of the welds are
shown in
Figure 31.
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Offset plays an important roll when welding tailored blanks as it applies to
the size of
gap when welding at constant welding speeds. If there is no gap, qualified
welds can be
achieved with a wide range of offset settings. From an offset of 0-0.3 mm, the
weld's
concavity is below 10% and the weld is well filled. The weld's concavity
decreases with
increasing the offset position because of melting more of the thicker sheet
material. If a
gap exits, a certain offset should be maintained to keep the weld's concavity
below l0%.
Most preferably, the offset is maintained when the automatic tracking system
is employed.
If the gap is too large, a qualified weld cannot be achieved. The concavity
and undercut
often appear in both sides of the weld (Figure 30b).
There exists an optima( offset at which the weld's concavity is a minimum. In
tests
for a 2.0 mm to t .5 mm sheet combination the optimal offset is about 0.25-0.3
mm. Above
this value any increasing of the onset results in more concavity. However,
another
phenomenon should be noted, in that there exists an upper limit of offset for
different
welding speed and gaps. Once the offset exceeds this limit, high quality welds
cannot be
created. The laser beams) heat only the thicker sheet, which is burned out.
The thinner
sheet is not melted at the bottom corner (Figure 30c). In this case there is a
small notch
at the root and a sound weld is not available under this condition.
In the range of the optimal offset the welding speed has minimal effect.
Welding
carried out at higher speeds is, however, preferred because the higher speed
leads not only
to greater productivity, but also the weld's concavity can be kept below 10%
over a much
greater offset range, increasing processing tolerance and safety.
g) The Maximum Allowable Gap
The existence of a gap between two sheets to be jointed is considered to be
unavoidable, certainly over joint lengths exceeding one meter. Investigations
have
shown that sheets cut with a normal shear do not have straight edges. As such
for any
specific welding technique under a certain welding condition, there is a
maximum
allowable gap up to which a satisfactory weld is achievable.
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In Figure 32, the dependence of the weld's concavity on the joint gap is
shown. It is not surprising that the weld's concavity increases with
increasing gap size.
From Figure 32 it is shown that the maximal allowable gap can be read out by
setting
different maximal weld concavity. For example, the maximum allowable gap is
0.1 mm
to 0.15 mm, whereby a concavity measurement of 10 % is obtained, using the
dual
beam welding technique with 0.3 mm offset, G° head angle and 2 to 1.5
mm galvanized
sheets. It should also be noted that the welding speed can effect the maximum
gap.
Slowing the welding speed has not proven to be a satisfactory method of
filling wider
Gaps by a constant offset. That is because the slower the welding speed, the
greater the
loss of metal resulting from evaporating and spraying of the molten material
through
the keyhole. In order to obtain better joint tilling the speed has to be
slowed along with
a corresponding defocusing of the beam or increasing of the offset at the same
time.
The results of Olsen tests carried out to qualitatively investigate the
mechanical
properties, i.e. the strength and formability of welds are shown in Figure 33.
The
photographs shown in Figure 33 reveal the failure locations of the welded
specimens
produced by the Olsen test. The crack initiated in the base metal (normally in
the
thinner sheet) and extended in the base metal parallel to the joint or cross
the welds
(Figure 33a and 33b). In these cases the mechanical properties of the welded
joints are
satisfied. Figure 33c shows the crack initiated in the base metal adjacent to
weld in the
thinner sheet parallel to the welds. In this situation the joints have
satisfactory
properties and the condition is not thought to be critical where the crack is
initiated
and extended in welds {Figure 33d), the joints are not qualified.
Figure 34 show how the offset and gap influence the cracking behavior of the
welded joints under the Olsen test. When the offset is too big, the thinner
sheet in
question is not completely melted and the joint has minimal formability. This
condition
has to be carefully avoided. Joints with wider gaps and/or welded with
improper offset
may also fracture at the weld because excessive concavity and undercut reduce
the
transverse section at the weld considerabiv Proper processing,; parameters
will ensure
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that joints have no problem with the Olsen test. Cracks initiating and
extending in the
base materials ensure that the welds have suitable mechanical properties.
h) Application of Duai Beam Technique
Two purposes using dual beam technique into welding tailored blanks are
increasing welding speed and extending the processing quality, by welding
joints with
greater edge/gap tolerance. According to Figure 1 1, one of the possibilities
to get
better gap filling is to increase the size of the focus spot. As an example,
if the gap is
0.2 mm and offset is 0.3mm, on welding 2 to I ., mm sheet, a focus spot with
an
approximate diameter of I .8 mm should be necessary for proper dap filling. To
meet
this technical specification on laser beam, the way for single beam welding is
increasing
the defocus or using lens with longer focus length. However, a fundamental
property
of laser beam, i.e. the power intensity is strongly reduced in either cases.
This may also
change the welding mechanism from "keyhole" (deep penetration) welding to
normal
laser fusion (heat conduction) welding, and so all the advantages of laser
welding
connected to the high power intensity will be lost.
Laser welding tailored blanks with dual beam technique provides an innovative
method to solve this problem. The key processing technique is the rotating
dual beam
and so increasing the effective beam size to meet the special demands on
welding heat
sources. From Figure 35, it is easy to understand that the effective beam
diameter can
be continuously verified by turning the two spots around their common center
without
reducing the power intensity of the laser beam. The beam rotation provides the
maximal flexibility to handle the joints which are very difficult for single
beam welding.
The increasing effective beam size, whether by defocusing or rotating laser
beam, means melting more material in the workpiece and results in a wider
weld. The
lower welding speed is therefore expected. In order to determine the influence
of the
beam size on welding speed using both technical concepts, a compare
investigating
was carried out welding 2.0 -I ._i mm galvanized sheets. The results are shown
in
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Figure 36 and Figure 37. In Figure 36, the beam diameter was determined by
Prometec Laser ScopeTM. In Figure 37, the effective beam diameter was
calculated by
following equation:
cl ,~. = c~, + h ~ si n ~p
In the equation, c~~. is the focus diameter of a single spot, h is the
distance between two
focus spots centers, ~p is the beam rotating angle. The variation of the
welding speed
with the laser spot size is clearly shown. From Figures 36 and 37, an
important
relation between welding speed and spot diameter by welding with defocusing or
rotating laser beams exists as seen in Figures 38a and 38b.
For laser welding with single beam, the melting effciency, which is
proportional to the multiplication of welding speed and beam diameter, keeps
normally
constant for welding a certain joint at a constant laser power up a certain
welding
speed. It has been proved to be correct experimentally and theoretically with
heat
transfer equation. This result can also be applied In the case of laser
welding with two
spots in line (see Figure 38a), in which the multiplication of welding speed
and beam
diameter stays almost constant or slightly decreases with increasing of the
beam
diameter. That indicates the welding speed is inversely proportional to beam
diameter.
In the example above, if a joint with 0.2 mm dap is to be optimally welded, a
laser spot
of 1.8 mm diameter would be needed and a welding speed should slow down to
about
2.7 m/min. However, the same conclusion is not valid for dual laser beam
welding by
rotating beams. The melting e~ciency grows with increasing effective beam
diameter.
It can be explained with less latent heat losses through conductivity and
higher
coupling efficiency connected to the aspect ratio depth/focus diameter.
Although
welding speed slows down still with increasing effective focus diameter
(rotating
angle), (see Figure 38b), the welding speed by rotating beams is much higher
than that
of defocusing beams. For welding tailored blank of 2 to l .S mm galvanized
sheet by
90° beam rotating angle, the welding speed is S.4 m/min and will be
twice as fast as
welding by simply defocusing, laser at same effective focus diameter.
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In Figure 39 photographs of weld cross sections at different beam rotation
angles
are shown. From these photos the ati~ect of the beam rotation angle can be
clearly
observed. The width of the weld top surface is decided by the effective beam
diameter, namely the beam rotation angle, while the width of the weld bottom
is almost
independent on the beam rotation angle. The larger the beam size, the wider
the top
weld.
In addition, the two beams play a different role by welding processing, one of
them
is mainly used to penetrate the joint to form a sound weld, while another one
is mainly used
to meii the thicker material to get better gap tilling. For positive beam
rotation angle, the
front or leading beam incidents at the thicker sheet. which heats and melts
thicker sheet,
while the behind or lagging beam makes the penetration. Because of the greater
thickness
of the thick side it cannot be fially penetrated. The front laser beam leaves
only a bead on
plate weld with a half penetration on the thicker side. It can be obviously
seen that the weld
consists of two melting spurs. The front beam makes an important contribution
on welding
processing; it melts thicker side of sheet for a better gap filling; it also
preheats the joint
material, so that the behind beam can penetrate the joint more easily. The
welding speed
of such a beam arrangement is therefore higher.
The two beams of negative rotation angle are contrary. The front beam
penetrates the
joint, while the behind beam radiates the thicker side to provide more melting
to gap filling.
In this case the front beam has to penetrate cold material, so that the
welding speed is
somehow lower than that of positive beam rotation angle. Because of the
preheating effect
of the front laser beam the amount of melted material on the thicker side is
obviously more.
This is particularly the case on smaller beam rotation angle, for example by
30°, a deep
melting pool on the thicker side is formed by the behind beam. The two melting
pools form
together. With this kind of beam rotation the laser power of the behind beam
should be
properly reduced for a optimal weld profile. This behavior provides another
perspective of
the dual beam welding technique: weldinv; with laser power combination of two
beams.
As well. up to a certain beam rotation angle, the profile of weld becomes
similar. That can
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be seen in Figure 39 from the comparing of weld profiles at 60° as well
at -60° beam
rotation angle.
The influences of beam rotation angle on weld concavity are shown in Figures
40 and
41 by welding with varying offset and gap. The comparison of weld concavity by
welding
with changing gap under positive and negative beam rotation angle is shown in
Figure 42.
The maximal allowable gap by the different beam rotation angle setting is
shown in Figure
43.
Accordingly, from the etfects of bean rotation angle and welding,; processing
and gap filling, it can be concluded that throe<,h the rotation of laser
beams, the
effective beam size is increased, so that better gap filling can be achieved.
As well, by
positive beam rotation angle the welding speed is somehow higher. However
better
gap filling can be obtained by negative beam rotation angle. Generally, with
increased
beam rotation angle sheets can be optimally welded with a bigger maximal gap.
By
positive beam rotation angle, the allowed maximal gap increase is not very
evident
with the beam rotation. in the negative beam rotation range the rotation of
laser beam
brings much better gap fillinw. This chanvin~.: rate is obvious from 0 to 30
degree. Up
30 de;ree the further rotating of laser beams has no evident affect on the
allowed
maximum gap.
Table 3 overviews the basic specifications of the dual beam technique for
laser
welding of 2 to I .S mm sheet blanks. Three important characteristics are
welding
speed, maximum allowed ~appin~ and offset tolerance.
Table 3 Basic technical specifications of dual beam welding process (2 to f
.Smm).
Gap Power Head Beam Speed Offset
Mm W Angle Rotation m/min Mm
i
I
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0-0.10* 3000* 6 - 4.S-S O.IS-0.25*
~
0 - 0.1 2x3000 6 0 9.0 0.1 S - 0.30
S
0 - 0.17 2x3000 -b 0 8.0 O.OS - 0.
! 0
0 - 0.17 2x3000 -6 0 7.0 O.OS - 0.3
S
0 - 0.18 2x3000 -6 30 7.0 0.25 - 0.35
0 - 0.22 2x3000 -6 -30 6.6 O.OS - 0.50
0 - 0.23 2x3000 -6 -60 6.0 O.OS - 0.75
0 - 0.24 2x3000 -6 90 S.4 O.OS - 0.90
~rererence valua of single beam weidmg
The economical and effective welding process is also strongly dependent on the
edge state of the sheet blanks. The straightness clung the whole Length of the
sheared
edges is an important characteristic because at the worst situation the
maximum gap is
twice the variation in the straightness of the sheets. That means, if the
variation of
straightness is O.OS mm, the maximum gap can be 0.1 mm. The following examples
of
this procedure are described for welding 2 to t.> mm blanks.
Maximum gap of sheets below 0.1 mm
Either single or dual beam welding technique can be chosen only in accordance
with demands on productivity. Where the gap is below 0.1 mm, it is not
necessary to
rotate Laser beam. The head angle should be positive and the beam rotation
angle
should set up at 0 degree to get the highest welding speeds possible. The
maximum
welding speed is dependent on the beam position without gap, so it is also not
necessary to adapt the welding speed to match the gap. The dual beam welding
technique is especially attractive on its economical aspect. namely increased
costs of
about l0-lS% but with about 100% higher welding speed. In this case the demand
on
the accuracy of the tracking system is ~ 0.05 mm to keep a optimal offset.
Maximum gap of the sheets about 0. f S mm
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The dual beam welding technique should be selected if the dap is about 0.15
mm.
If the maximum gap is less than 0.1 _5 mm, the welding process with two beams
in line
(beam rotation angle 0 degree) should be still optimal, whereby the welding
speed is as
high as twice the single beam welding. The accuracy of the tracking system
should be
~0.075mm. If the maximum gap is in the range of 0.17-0.18mm, the optimal
choice
remains a welding process with 0 degree beam rotation angle or a small beam
rotation
angle (say 30 degrees). The laser head further ought to be set in negative
angle range.
Higher welding speeds need more precise tracking system (8 m/min. ~0.025mm),
the
lower welding speed, the more tolerant the welding processing (7 m/min, ~0.
l5mm).
The welding speed with two lasers is 40-60% higher than single beam welding.
At this
negative head angle setting the welding speed does almost not change with the
gap, so
it is simple to weld at a constant speed determined by the minimal or maximum
gap.
Maximum gap exceeds 0.2 mm
If the gap changes between 0 to 0.25 mm along the whole welding joint, it is
not possible to get a proper weld without using dual beam welding technique
with
rotating beams. In this situation there exist two different technical methods
which can
be considered into the construction of the whole laser welding system. A
simple way to
do this is the welding at a fixed beam rotation angle, whereby both the beam
rotation
angle and the welding speed are decided by the maximum gap and zero gap. The
disadvantage of this welding process is the slight loss of welding speed. For
example,
to weld sheets with 0.25 mm dap the welding speed is 5.4 mlmin, this is about
a 12%
improvement in welding speed in comparison with single beam welding
techniques.
The preferred method is by welding with automatically adapted beam rotation
angle and welding speed. This method is in principle based on the basic
relationship
between gap, beam rotation angle and welding speed. The sensor integrated in
the
tracking system detects the gap. The gap width is then sent to the control
unit of the
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welding system, where an optimal beam rotation angle, as well as the related
welding
speed will be calculated by using the fimction among gap, rotation angle and
speed.
The information will be separated and transferred to the control and drive
unit of
related servo motors to continuously change the beam rotation angle and the
welding
speed at same time. The advantage of welding with automatically adapted beam
rotation angle and welding speed is optimal utilizing of dual beam technique.
For
example, if the gap changes smoothly from zero to 0.2Smm, an average welding
speed
of 6.7 rn/min can be obtained by from 0 to 90 degree beam rotation angle. This
is a
24% higher welding speed compared to welding at a fixed beam rotation angle,
and
about 40% higher speed than single beam welding= technique without yet
accounting in
the more processing reliability and tolerance that the dual beam welding
technique will
bring. A simplified alternative technical layout using above principle is
welding with
two fixed speeds and beam rotation angles. From Figure 44, it is apparent that
the
welding speed and beam rotation ankle do change very slightly, if the gap is
smaller
than 0.1 mm. Therefore, the beam rotation ankle can be set at zero and the
welding
can be carried out at a higher welding speed. Once the jap is between 0=1 and
0.2 mm,
a higher beam rotation angle and a lower welding speed can be set, for
example, 30
degrees and 7 m/min as shown in Figure 44 in dash lines. Only two tixed
welding
speeds and beam rotation angles are needed. The advanta<,es of the layout are
the
simple construction and control of beam rotating mechanism as well as lower
technical
demand on sensor for gap width detecting.
As indicated, the offset of the laser beam in relation to the joint is also an
very
important process parameter. One factor that can result in fluctuation of the
offset
value is if the edge of the blanks are not a perfectly straight fine. Their
shape may
change by different shearing or cutting processes. Also, the focal distance
can vary
because of wavy surface of blanks. It can also cause the variation of the
offset by
declined head angles. Another factor may be if the sheets are somehow not
properly
qualified on the magnetic bed, or if the position of the seam (eaves the
central line of
the mechanical motion. As well, a slight tluctuation of the pins position may
be
unavoidable because of wearin'; or welding spatter on ;gins after a long time
operation.
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Lastly, the motion of the axis and the gantry has a limited mechanical
precision.
The offset window is defined as a range in which a stable welding process and
a
qualified weld can be achieved. Generally, there are two critical values to
determine a
window. The lower limit of the offset is decided by the profile of welds,
which means a
certain amount of metal has to be melted to fill the joins to reduce the weld
concavity.
The upper value of the offset is limited by penetration of welds, which means
a poor
or no penetrated weld has to be avoided. The larger the offset window, the
more
tolerable the welding process to the fluctuations of materials and the welding
systems.
The influence of welding speed on the onset window is shown in Figure 45. By
zero gap, it increases with decreasing welding velocity. For the welding
practice, that
means the upper limit of an offset window can be extended through reducing
welding
speed to get more penetration. However, if there exists a gap between two
sheets. it is
reduced by slowing down welding speed. It also can be observed that the offset
window moves to a higher range with a gap.
As shown in Figure 46, the head angle can cause the change of the offset
windows. Normally, the bigger offset windows can be obtained by setting a
positive
head angle. On the contrary, the negative head an~;ie results in a smaller
offset
window. So a positive head angle is generally recommended for a more stable
and
tolerate welding process. The negative head angle is applied only in case the
gap
filling becomes a determined factor.
Figure 47 shows the influence of thickness ratio ofjoints on the offset
windows. For joints with lamer thickness ratio. it is easy to get good gap
filling, as
previously discussed. However, the offset window is much smaller than that
with
smaller thickness ratio. The larger the thickness ratio of joints, the
narrower the offset
window, and for we(din~ joints with large thickness ratio, a more accurate
positioning
of laser spot is required.
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In welding, the gap typically will change along the whole joint. The possible
minimal gap is zero and the maximum gap will be determined by the tit up of
two
abutting edges to be joined. The offset window can be further reduced because
of the
fluctuation of gap size, as shown in Figure 48. For instance, by welding 2.0
to
0. l5mm tailored blanks with dual beams in line, the offset window is from 0.1
to
0.23 mm for zero gap, and from 0.13 to 0.26 mm for a maximum gap of 0.2 mm. In
this case, the lowest offset value is determined by the limitation of the
maximum gap
which is 0.13 mm, while the highest one by the zero gap, which is 0.23 mm.
That
means the offset window becomes 0. l3-0.23 mm, which is obviously smaller than
those of a constant gap. The procedure ofdetermining offset for a welding
process is
then followed by dividing the offset window into two. The optimal offset
should locate
just in the center of an offset window. For the example above, the offset
should be set
at 0.18 mm. It allows a maximal offset fluctuation of ~0.05 mm and coves a gap
variation of 0-0.2 mm.
The Nd:YAG-laser welding tailored blanks with dual beam technique shows
the capability to welding joints with larger gap. It also provides the
capability to
expand the offset windows, as illustrated in Figure 49. 2.0-1. amm blanks were
welded
at different beam rotation angles. At zero beam rotation angle (beam or focal
line in
line with the edges to be joined), the offset window is 0.21 mm. If the dual
beam is
turned to 30°, it becomes O.Smm, which is more than two times big as
that of dual
beam in line.
The offset window increases with beam rotation angle. For welding tailored
blanks with a lame thickness ratio, it normally has a very small offset window
and
requires a very accurate beam positioning. Through the beam rotating, the
offset
window can be extended. too. Figure 50 shows an example of welding 2.0 to 0.75
mm
tailored blanks. By welding with dual beam in line. the offset window is 0.13-
0.26 mm.
It will be increased to 0-0.39 mm by using a 30° beam rotation. In
welding practice, it
means a tolerance of laser spots position of ~0.2 mm comparing with ~0.065mm
of
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single beam or dual beam in line.
h) Prototype Welding
A prototype GMT 800TM body side ring shown in Figure S 1 consisting of four 4
pieces (two pieces 2mm and two 1.0 mm galvanized sheets) was welded along 3
joints
with a total weld length of S.Sm(~ I 8' ). It is a typical linear weld. The
welding
procedure is: first welding A and B; next welding ,4B and C; and lastly,
welding ABC
and D. The GMT 800 body sides were produced with both single beam and dual
beam
techniques. The welding parameters are listed in Table 4
Table 4 Welding parameters for GMT 800 parts
Welding Focal Head Beam Laser Welding Max.
Technique Lens Angle RotationPower Speed Gap
Single 200 mm 6 - 3000 W S.0 m/min0.20
beam mm
Dual beam 200 mm 6 0 2x3000 9.0 m/min0.20
W mm
Dual beam 200 mm 6 I S 2x3000 8.0 mlmip0.35
W - mm
I Dual I50 rnm 9 ~ 30 2x2800 9.0 m/min0.30
beam ( W mm
Comparing dual beam welding process with a single one shows the welding
velocity
increases, so that the welding time of a part is reduced from 66 seconds with
single
beam to 37 seconds with dual beam in line, and 4~ seconds with a beam rotation
angle
of 30°. The tolerance on gap is increased from 0.2mm to 0.3Smm with
beam rotation
with the result that the welding process is more stable and safe. Table 4 also
shows
the testing of a lens with I 50 mm focal length. The advantage of shorter
focal length
lies in higher speed under same welding conditions.
Cross sections of various weld seams are shown in Figure 52. It can be seen
that very smooth welds can be obtained. On a comparison of single and dual
beam
welding processes, the weld profile of dual beam welding process look better
than
single one. A typical Olsen stretch formability test coupon is shown in Figure
>3. The
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cracking bean in the thinner section in the base metal. The parts appear
satisfactory
and to meet the formability in the die. Except for difficulties in the
starting phase, no
cracking of the delivered parts in forming process was reported.
A prototype CadillacT"' rear door inner panel is shown in Figure 54 and as
will
be described hereafter a prototype Jeep CherokeeTM side panel was also formed.
The
Cadillac weld consisted of two linear welds which are perpendicular to each
other.
Each part was sheared with the same cutting die, which accounted for very
accurate
joint fit-up. The daps measured through all the joints were from 0. f to 0. ~5
mm,
depending on different shearing.
A typical cross-section of the weld appears in Fi<lure ~4. There is no weld
concavity shown in the photos. To overcome the small hole which appeared at
the
intersection of the two welds, the laser power was ramped down over the last
ten ( 10)
millimeters of weld 1 to allow the crater to be filled. Subsequently, the
laser power
was ramped up at the start of weld 2. The blank was welded in one path with
the head
turning at the corner. The beam was shut down at the end of the long leg and
restarted
at the corner after the head was rotated.
Welds were produced by using: a) the vision tracking system which maintains
exact positioning of the laser beam with respect to the joint, b) in some
instances not
using any tracking to check the accuracy and stability of the gantry and c) a
I m by ( m
gantry and a special procedure which involved a compound ankle. Without the
tracking system, the sheet qualification was much more critical. Satisfactory
welds
were obtained when the tracking was not engaged if the prepared edges of the
die cut
parts were within specification. Parts welded with the tracking system turned
on
revealed no significant difference in weld appearance and profile. The welding
parameters applied in welding the parts are listed in Table 5. Again, welding
with dual
beam technique allows a much larger gap validation.
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Table 5 Welding parameters for Cadillac rear door inner panel
Welding focal Head Beam LasCI' Welding Maa.
TechniqueLCm AtIgIC Rutatic,nI'mvcr y,eed (.ral
Single ~()() ()+fi - 3()()() 7.() 0. 1 ()
t7CaII1 IlUll '~ W !Il/Illlt,II1II7
Dual heanl200 < 0 2~0()+.s00(1W9.0 nI/ntin0.1~ mtn
nun
Dual beam200 C, 30 ~ 2~(H)+3()OOW7.2 nI/minU.35 Inm
nun
~' compound angle
The prototyping parts of the Jeep body sides and the welding sequence are
shown
in Figure ~s. The tailored blank of Figure s~ consists of sheets of three
gauges and
the first weld is over 2.4 meter long. The whole part is 3.6 meters long. The
sheets
were very thin, and the minimal thickness ratio of the joint was 1.25 which
makes gap
filling extremely difficult. In addition, two thickness combinations existed
in a single
weld which could not be welded using a unique speed. The sheet A (0.8 mm
thick)
and B { 1 mm) were trimmed to the widths shown on the drawing (Figure SS) and
loaded into the gantry machine. They were welded into an intermediate part AB.
The
welded blanks were then sheared to the proper length and angle prior to
resetting the
part AB back into the welding gantry. Part C' was previously sheared to the
proper size
and shape. Lastly, the Part Af3 and (' were jointed together.
For construction of the prototype tailored blank, both single and dual beam
welding processes were applied, as listed in Table 6. For welding 1.0 to 0.8
mm sheets,
a negative head angle was selected because of the very small thickness ratio.
The dual
beam welding technique with beam rotation provided a higher welding velocity
and the
capability to bridge larger gaps in joints. This is especially important for
long thin
sheets prepared by a normal shearing. Also, a I 50 mm focal lens was tested.
Its
advantage is welding at same speed but requiring lower laser power, which
benefits the
operating duration of the lamps and reduces the operating cost. For welding
second
welds, the welding velocity was varied up to the sheets combination. For the
section of
1.3-0.8 mm, a slightly higher speed was used.
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Table 6 Welding parameters for welding Jeep Cherokee body sides
Welding Fcx:al Head Beam (.aaer Welding Mav.
Technique Len, Angle Angle I'clwar ~peecl Crap
S I.t)-O.~ 2t)() -C, - 30()() W 7.J Ill/m111O.I~
InlI1 null tI1I11
1.3-().t( 2()() +(, - 3tH)() W 7.0 Ill/n11I1().2U
n1I11 !lull IIIlII
S 1.3-I.0 200 nun +(, - 3000 W 6.5 nl/min0.15
Inm mm
D 1.0-O.li 200 I1LI1-h 30 2?t00+3000W12 nl/min0.20
nun .- mnl
D l.3-O.R 2t)0 +6 30 260()+2H00W10 nlinlin0.25
nun ~ nun mnl
D l.3-1.0 200 nun +(, 30 2600+~tt00W').5 0.22
nun nl/nun mm
D l.0-0.H I50 nun -<, 30 ?50(1+2500W12 nl/nnn0.20
nun Inn!
D 1.3-0.H 150 nun ~, 3() 25()0+?SOt)W10 nl/nlin().20
nun ~ IIlll1
l) I. 3-I.()I Jt) +(~ 3t) 2J()l)+2J()t)W').J ().2()
nllll 111111 117/n1111IIIIII
~
The typical weld cross-sections are shown in Figures 56 to 58 with Figure S6
showing a I.0-0.8mm joint and Figures S7 and 58 showing 1.3-l.Omm joints. The
welds produced have 0% to 8% concavity maximum by the welding of 1.0 to 0.8 mm
sheets, which still meet the profile specifications for sheet steel welds set
out in the
Auto Steel Partnership standard (proposed '97) for tailor welded blanks. From
a
comparison of the cross-sections welded by using 1 ~0 mm focal lens with those
of
200 mm focal lens, there is no obvious difference in the welds.
The Olsen stretch formability dome tests (Figure 59) reveal the fracture
taking
place in the thinner materials outside the welded joint. The welded joints
meet all the
specifications for acceptability according to current specifications and no
failure at
welds was reported.
Non-linear welds are also convinced as the future application in tailored
blanks.
Vehicle designers are increasingly considering the non-linear welded blanks to
optimize
the construction and improve the formability of the parts more effectively. In
Figure 60, two kinds of non-linear welds are shown. The first part consists of
two
straight line and three arc welds with separate radii of 100 and 475 mm. The
second
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one is a complete circle weld with a diameter- of 200 mm, which is typically
used in
constructing shock absorber towers. These two non-linear welds were
successfully
produced by using the applicant's AWS 3TM-axis welding machine shown in
Figures I
to 3 combined with dual beam technique. The dual beam welding technique with a
beam rotation angle of 30 degrees decreases the requirement on part
positioning and
gap tolerance along joints.
During prototyping, the experiences with welding defects and scrap parts were
also collected. The purpose of this was to review the scraps produced in the
prototyping, and furthermore, to look for the reasons of the scraps to
minimize the
scrap rate. As an example, the GMT 800 parts produced in the very beginning of
the
prototyping were chosen. By the welding of more than 600 body side rings,
there were
23 left-hand scraps and 12 right-hand scraps to be registered. The detailed
information
is given in Tables 7 and 8.
Table 7 The left hand parts scraps
fart WCId ~)CtCCI hOStIW I7 (,.UII1117Ct1t
NO. NO.
AWS014 BO ()veriall End ~heeta nut properly
qualified
AWV02~ Al no weld ~ start '('racking error
AW502C~ I BC.' ~ no penetrationltan Tracking error
AWV036 A)_3 nu wCid End - EacC.,aC dap
AWS038 AR weiding,tuppedEnd vhCeta nut properly
qualified
AWS039 C'C) wClding End vheeta nut properly
auppCd qualified
AWVU47i BC' overlap End ~hCCt, nut properly
qualitied
AWS04c) C:D weidin~ End sheets nut properly
stul7ped qualified
AWS0~2 BC' overlap End ,beets not properly
qualified
AWS058 AB nu penetrationMart tracking ClTUr
AW~()~7t)A~ I7U l7Cnetral1t111~tarf U'aCklll~ CITUr
AWSOH2 $C.' nu penetrationMiJtilv tracking error
ASWlOI i=3C' wCldit7g laser fault
aoppCJ
AW1142 AB f nu weld End ~xc:C"iw gap - _
~
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AWS143 CI) wrung sheet
AWS144 B(: cutting ,beet nrU properly
yualitied
A WS 147 $C welding ,!reefs nc,t properly
,topped qualified
AWS183 C.D blowing End wce"ive gap
cuu
A WS 1 A$ welding End ,!reefs nut properly
H4 atopped qualified
AWS l9fi AB cutting End eacesaive gap
AWS259 ~D no penetrationEnd tracking error
AWS269 AB welding End sllCCi n()l properly
,topped qualified
AW~27li AIi no penetrationEnd tracking error
Table 8 The right-hand scraps
fart Nu. Weld Defect f'ritiun C.'ununent
No.
AWS4l4 Bl' ~to wClding one la,er nut active
",-
AWS428 CD (:offing End excessive gap
AWS430 BC: ( Cutting ~ End exce,sive gap
AWS450 BC' ~ C:utting eacewive gap
AWS451 I EiC.'~'.utting Cxcc,sive gap
AW5454 BC: Welding,toppedEnd ,beets nut properly
~ualitied
i
AWW 36 LU Welding,tuppedEnd all~C(S ttOt properly
qualified
AWW~H BC Overlap vlart- ;beet, not properly
dualitied
A W~630 gy wrong welding ,~yuence
AWV633 (:D Welding ,tuhpedEnd ,beet; neU properly
~ dualitied
AWS68lt BC.' Welding ,topped ,hret, nut properly
qualified
AW~707 $C' C.'utting EnJ Exce,aive gap
~ ~ ~
From Tables 7 and 8, of 3 S scraps, half ( I 7 pieces) were caused by improper
qualification of the sheets on the magnet bed. One reason was that sheets were
not
sufficiently pushed against the pins, so that a part of the joint is outside
of the tracking
window. The welding process is stopped by the tracking system. Another reason
is the
overlap of thick sheet aver thin sheet, all of which occur by qualifying thick
sheet against
thin sheets (weld number BC). 2S% (9 pieces) of the scraps were caused by
excessive
gaps, resulting in cutting, blowing out and series of pinholes. 7 scraps (20%)
occurred as
a result of tracking error. The welding defect in this case is no penetration
in some local
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welds (normally at weld start). In most cases, an evident jumping of the laser
beam to the
thick side can be observed. As well, 2 scrap pieces resulted from laser fault.
The scrap rate is influenced to a great extend by the qualifying sheets on the
gantry. To reduce the scrap rate, proper qualification is important, even
though the
tracking system is applied. More rigid and stable pins can help improve the
quality of
the qualification. To prevent overlap of the thicker sheet over thinner one,
the thick
sheets should preferably be qualified first. The first pin should put as close
to the
staring point of the welding as possible, thereby reducin~d the "lead-in"
tracking error.
The state of sheared edges play also an important role. In order to reduce the
scrap
rate, a tight straightness tolerance of the sheets edge is helph~l. The
tracking
parameters should be optimized to reduced the frequency of tracking errors.
After
these improvements, a very low scrap rate ( less than 1 %) was achieved during
second
phase of prototyping.
Figure l shows the simultaneous production of two work pieces t 2a, t 2b, each
having a linear seamline 34. If desired, however, the present invention may
equally be
used to weld one, two or more workpieces along straight, curved or angled
seamiines.
Although Figures I to 3 show a production assembly line 10 which incorporates
a single laser 36 used to weld pairs of blanks 14x,1 Oa 14b,16b together, the
invention is
not so limited. If desired, two or more lasers could be used. each with its
own movable
laser head for simultaneously welding a respective pair of blanks 14, t 6
along a seamline.
Although the preferred embodiment of the invention discloses the apparatus as
including a sensor 49 for continuously sensing the spacing between the sheet
blanks l4,
the invention is not so limited. In a more cost effective embodiment, the
sensor 46 may
be omitted. With such a configuration, the positioning of the laser head 42
may be
programmed or continuously manually adjusted by an operator concurrently as
welding
operations are performed. Alternately, the laser head 42 may be moved to a
fixed initial
position which is maintained constant durinv; welding, as for example, when
blanks 14 of
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different thicknesses are to be joined.
While the preferred embodiment of the invention discloses the coherent light
source generator 40 as generating two separate laser beams, if desired, the
energy source
could be used to generate a single coherent light source which is separated
into two or
more laser beams in or en route to the laser head 42. Similarly while two
coherent light
sources are disclosed as being used for welding, a single beam or multiple
beams of three,
four or even more coherent light sources could also be used.
Although the detailed description describes and illustrates preferred
embodiments of the invention, the invention is not so limited. Many
modifications and
variations will now occur to persons skilled in the art. For a definition of
the invention
reference may be had to the appended claims.
i ~ . _ _.._.
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Table of Symbols
A Absorption or coupling rate
b weld width
~.tn specific heat of solid material
specific heat of liquid material
D temperature conductivity of
the material
df diameter of laser beam
dey effective beam diameter
cirn~~offset of laser beam to joints
,f focal length of lens
width of Dap
Hm melting enthalpy of material
h thickness of sheets
h, thickness of the thinner sheet
in a joint
h~ thickness of the thicker sheet
in a joint
K thermal conductivity of material
f'c laser power (output ~ workpiece)
f'F effective power (absorbed power)
f',. loss power through heat conductivity
r~ radius of laser beam
T temperature
Tm melting temperature of material
TR thickness ratio of joint
welding speed
a beam incident angle
c~ beam rotation anDle
8 optical head angle
p density of material
0T the medium overheating temperature
