Note: Descriptions are shown in the official language in which they were submitted.
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TESTING A WELD SEAM
The present invention relates to a method and an apparatus for inspecting a
weld seam of
a weld joint between sheet metal parts which is produced as a butt joint by
deep welding
with a laser beam; according to the precharacterising clause of claim 1.
Sheet metal parts which are cut and welded for specific uses, and which are
termed
"tailored blanks"; are increasingly being used in the automobile industry in
particular. They
firstly enable savings in raw material to be achieved and secondly enable
process planning
1 Q and process implementation to be simplified. "Tailored blanks" such as
these are
fabricated by forming butt joints by laser welding. In the course of this
procedure, the
edges of two sheet metal parts which are cut so that they are complementary to
each other
are abutted flush against each other, are fixed by means of a clamping device
and are
passed through a welding station equipped with a laser. High-power lasers,
with which
deep welding can be effected, are used in particular for this purpose.
Apart from melting, high-power lasers also result in the evaporation of the
metal and in
the formation of a plasma. The vapour pressure of the plasma, which is
situated above and
in the region of the melt, opens up a deep, narrow capillary in the melt pool.
The plasma
2 o cloud or plasma flare which is formed above the melt pool, which is termed
the weld pool,
can become too hot and too dense, however, which can result in screening of
the laser
beam and interruption of the welding process. As a rule, a process gas, mostly
helium or
argon, is therefore supplied in order to cool the plasma and thereby to reduce
the density
thereof. During deep welding using a laser beam, liquid metal flows
continuously into the
capillary, and under some circumstances can be explosively ejected. Most of
this spitting
or spatter flies within the "welding plane" which is determined by the weld
seam and the
laser beam.
The stability and thus the weld seam quality of a deep welding process depends
on
numerous parameters, e.g. on the speed of welding, on the properties of the
laser and on
the nature of the workpieces, particularly the edge surfaces and the course of
the edges
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thereof. The relationships pertaining to the gas dynamics, i.e. the type of
process gas feed,
the removal by suction of the welding fume and the aeration of the laser beam
tube, are of
considerable importance in order to obtain a reproducible, uniform welding
result. A
complex, labile equilibrium exists between all these parameters during the
welding
process.
If the position of equilibrium is disturbed by random changes in the
influencing variables,
this can result in transient interruptions of the deep welding operation and
in the non-
uniform ejection of melt from the weld pool. If the amounts of material
ejected are small,
the metal which is lacking locally is replaced again by the liquid weld pool.
However, if
substantial amounts are ejected, no replenishment occurs, which results in
drop-like
deposits of material on the weld seam. The deposits of material can extend to
a length of
0.5 mm to 5 mm over the entire width of the weld seam, and have a height of
about 1 mm.
Since this ejected, deposited material no longer flows back into the weld
capillary, a
crater, channel or gap is formed in the weld seam in front of a drop-like
deposit such as
this. Weld defects of this type generally occur when welding is carried out
using high-
power lasers, and are generally not tolerated according to the specifications
for the weld
seam quality of weld joints produced by laser beam which are defined in ISO
13919-1.
Various methods are therefore known, by means of which the quality of a weld
seam can
be inspected. Methods of inspecting weld seams produced by deep welding to
form a butt
joint with a laser beam, or methods which are known from Patent ............
(Soudronic), are
known in particular to one skilled in the art. The method disclosed in the
latter document
relates to the detection of the weld seam profile by means of an optical
system. Evaluation
devices can then assess whether or not the weld seam profile of a weld joint
between sheet
metal parts complies with the quality requirements. In this method, weld
profile
measurements are made at about every 15 mm.
Other known methods of weld seam inspection, such as that described in US-
4,827,099
for example, monitor the welding process itself and are based on the detection
of the UV
light emitted by the plasma cloud and of the IR radiation emitted by the
glowing weld
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spatter. In this method, weld defects are detected by comparing the measured
spectral
values with stored reference values. A method of this type for monitoring a
welding
process necessitates the costly compilation of reference values and requires a
complicated
detection system. Unfortunately, false readings are always obtained in a
method such as
this, since this method essentially takes into consideration only two measured
quantities
of the complex welding process.
GB-A-2 359 269 relates to the identification of weld defects based on the
acoustic signal
emitted by the plasma.
The object of the present invention is therefore to create a method and an
apparatus for the
inspection of weld seams for the production of butt joints by deep welding
with a laser
beam, which method and apparatus do not exhibit the disadvantages of known
methods
and apparatuses, and which are capable in particular of inspecting weld seams
in a simple
and reliable manner.
The object in particular is to create a method and an apparatus by means of
which weld
defects, particularly craters, channels, gaps and/or accumulations of material
along the
weld seam can be detected in a simple manner and by means of which compliance
with the
specifications defined in ISO 13919-1 can thus be checked.
According to the invention, this object is achieved by a method of inspecting
weld seams
according to claim l and by an apparatus comprising the features of claim 6.
In order to
detect said weld defects in a weld joint between sheet metal parts which is
produced as a
butt joint by deep welding with a laser beam, means are provided in particular
with which
the weld seam height can be continuously scanned along the weld seam. Craters,
channels
and gaps can be detected therewith, as can accumulations of material on the
weld seam
such as those described above.
Since weld defects during deep welding by means of a high-power laser are
always
accompanied by adjacent drop-like deposits of material, which result in a
considerable
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excessive height of the weld seam over the entire width thereof, in a
preferred embodiment
of the method according to the invention it is merely these regions of
excessive height of
the weld seam which are detected.
It should be understood that the scanned height values are transmitted to a
display and/or
evaluation device. In particular, the evaluation device is capable of
comparing the scanned
values with the various grades of quality of ISO 13919-l, and is capable of
indicating the
exact positions of weld defects and/or of storing the measured data.
The method according to the invention is therefore distinguished by an
extremely simple
principle of measurement, which permits simple apparatus construction and
which
constitutes a reliable, uncomplicated and cost-effective method of inspecting
weld seams
and of detecting weld defects.
This simple principle of measurement enables very different means to be used
for
continuously scanning the weld seam height along the weld seam. It should be
understood
that all the means for the measurement of distance which are known to one
skilled in the
art can be used. Suitable means include optical distance measuring devices
with which the
weld seam can be continuously scanned.
In a further embodiment of the method according to the invention, a stylus is
provided
which is capable of executing mechanical excursions, which can be converted
into
electrical signals by means of piezoelectric crystals, for example.
Commercially available devices for measuring distance can be used as means for
scanning
the weld seam height along the weld seam.
The invention is explained in more detail below with reference to an example
of an
embodiment and with reference to the Figures, where:
Figure 1 is a schematic view of a defect-free weld seam;
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Figure 2 is a schematic illustration of a deep welding process;
Figure 3 is a schematic view of a weld seam with the formation of craters;
Figure 4 is a schematic view of a weld seam with a gap; and
Figure 5 is a schematic illustration of a preferred embodiment of the
apparatus
according to the invention.
Figure 1 is a schematic view of a weld joint between two sheet metal parts 12
and 13 of
different thicknesses, which is produced as a butt joint by deep welding with
a laser beam.
The weld seam width 14; the height profile 15 perpendicular to the weld seam
and the
surface structure 16 along the weld seam are dependent on the weld parameters,
e.g. on the
properties of the laser and on the nature of the workpieces, particularly the
edge surfaces,
the edge progressions and the edge preparation, and are also dependent on the
thickness
of the sheet metal edges and the speed of welding. Thus, for example, the weld
seam width
14 can vary from 0.3 to 1.3 mm. In a weld joint between sheet metal parts
which complies
with the relevant standard, the weld seam 11 generally has an imbricated
surface structure
16, the variations of the weld seam height of which along the weld seam fall
within the
range of a few hundredths of a millimetre, for example 0.02 mm.
The expression "sheet metal part" which is used here refers to a flat metal
product which
is preferably fabricated from steel, e.g. metal strips or metal plates of any
size, thickness
and surface area. The weld joint between sheet metal parts which is described
here refers
to a joint between two sheet metal edges which is produced by welding, where
the sheet
metal edges can be of different thicknesses and do not necessarily have to
originate from
two separate sheets, but can also form part of one-piece metal sheet which has
previously
been rolled. The term "butt joint" is used here to denote the position of
sheet metal parts,
the sheet edges of which are abutted and fixed flush against each other. The
term "weld
seam height" comprises both depressions as well as regions of excessive height
in the weld
seam.
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Figure 2 is a schematic illustration of the situation at the location of the
welding process.
A high-power laser beam 21 impinges on the sheet metal parts 22 and 33 (the
latter is not
illustrated) which are to be welded. The incident energy of the laser beam 21
results in the
melting of the sheet metal parts 22, 23 and forms a weld pool 27. In addition,
the heat
generated by the laser beam 21 gives rise to a plasma 24, which forms a deep,
narrow
capillary 25 in the weld pool 27. The relative movement R of the laser beam 21
and the
sheet metal parts 22, 23 results in a zone 26 behind the weld pool 27, which
zone
comprises solidified weld material and which forms the weld seam. It can be
seen from
Figure 2 that the liquefied metal 28 situated in front of the laser beam 21
becomes
relocated at the back 29 of the laser beam 21 during the welding process due
to the relative
movement R, and is preferentially deposited on the upper surface of zone 26.
This results
in the formation of an imbricated surface structure. With the aid of this
Figure, it can also
be understood that if the equilibrium of the plasma 24 is disturbed, the
capillary 25 is filled
with liquid weld material and that therefore no weld is produced or only a
partial weld is
produced. If the laser beam 21 impinges on an already existing weld pool 27,
larger
amounts of liquid weld material can be explosively ejected from the liquid
weld material.
These ejected amounts are generally deposited on the weld seam behind the
welding beam
and can clearly be recognised as drop-like formations.
The weld seam 31 of a weld joint between two sheet metal parts 32 and 33 of
the same
thickness which is shown in Figure 3 comprises a weld defect in form of a
crater 34, with
a drop-like deposit of material 35 directly adjacent thereto. Craters of this
type, which are
also known as pinholes with a diameter of about 0.1 mm or more, can be of
different
depths, and typically do not extend over the entire weld seam width but are
situated
preferentially in the middle of the weld seam. The drop-like deposits of
material 35 result
in regions of excessive height in the weld seam, which can extend over the
entire weld
seam width and which have a length from about 0.5 mm to about 5 mm and a
height of
several tenths of a millimeter.
The schematic view of a weld seam 41 between two sheet metal parts 42, 43 of
identical
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thickness which is illustrated in Figure 4 comprises a weld defect in the form
of a weld
seam gap 44, with a drop-like deposit of material 45 situated directly behind
said gap 44.
Weld seam gaps of this type generally have a length ranging from 0.05 to 0.2
mm and
typically do not extend over the entire weld seam, but have a width of about
0.2 mm. Weld
seam gaps 44 are often present in the form of holes with a diameter of about
0.3 mm. The
drop-like deposits of material 45 result in regions of excessive height in the
weld seam,
which can have a height of several tenths of a millimeter.
Figure 5 is a schematic illustration of an apparatus for carrying out the
method according
to the invention. Optical means 54 are preferably used for scanning the weld
seam height
along the weld seam S 1 of a weld joint between two sheet metal parts 52, 53
with different
thicknesses. In this embodiment, said optical means 54 comprise three optical
distance
measuring devices, which are preferably disposed at an angle of inclination of
30° to the
normal to the surface, and which are equipped with semiconductor lasers.
In a further embodiment, a detector stylus which is guided along the weld seam
51 is used
instead of optical detectors. By means of piezoelectric crystals, the
excursions of this
detector stylus can be converted into electrical signals, which in turn can be
processed by
corresponding threshold value circuits. It should be understood that one
skilled in the art
can also employ other distance measuring devices which appear suitable to him,
for
example ultrasonic sensors. The construction of these detectors does not form
part of the
present invention and will not be explained in detail here. Regarding the
selection of
suitable detectors, it is essential that they are capable of determining
differences in height
of the order of several tenths of a millimeter in a reliable and simple
manner. In particular,
they should be capable of detecting regions of excessive height of this order
of magnitude
in the weld seam.
The advantages of the method according to the invention and of the apparatus
according
to the invention are the simplicity of detection of weld defects - in
particular, solely by the
detection of regions of excessive height in the weld seam - during deep
welding with high-
power lasers. Moreover, the continuous scanning of the weld seam results in
said weld
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defects being detected reliably.
