METHOD OF LASER BEAM WELDING OF ZINC-COATED STEEL SHEET

METHODE DE SOUDAGE AU LASER DE TOLES D'ACIER ZINGUE

English Abstract

A pair of coated components are laser welded to one another by initially forming
protuberances on one of the components. The protuberances maintain juxtaposed surfaces
separated and the components are then laser welded to one another. The separation of the
surfaces vents vapor generated by the coating.

French Abstract

La présente invention vise une paire d'éléments enduits soudés l'un à l'autre au laser. Des protubérances sont d'abord formées sur l'un des deux éléments pour maintenir un écart entre les surfaces juxtaposées de ces derniers avant le soudage proprement dit, par où peuvent être évacués les gaz générés au moment de la fusion de l'enduit.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR
PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A method of laser welding a pair of juxtaposed components comprising the steps of forming on
one surface of one component a protuberance to project above said surface, juxtaposing said components
such that opposed surfaces are maintained separated by said protuberances, and laser welding said
components by impingement of a laser beam in the region in which said surfaces are separated.
2. A method according to claim 1 wherein said protuberances are formed by impingement by laser
beam.
3. A method according to claim 1 wherein said weld is formed by rotating said beam about one of
said protuberances.

Description

CA 02209804 1997-08-lF7
METHOD OF LASER BEAM WELDING
OF ZINC-COATED STEEL ~11~;1
The present invention relates to a method of making lap joint of zinc-coated steel sheets with a
s laser beam.
BACKGROUND OF THE INVENTION
The increased use of zinc-rich coated steel sheets in auto-body components for enhanced
corrosion resistance poses a demand for an acceptable welding method to join these kinds of sheets.
10 Laser beam welding has an advantage of low total heat input and thus causes a low distortion to the zinc-
rich coating on the sheet. Therefore laser beam welding is being evaluated as a desirable joining technique
for such sheet steel in butt and lap joint configurations. However, a problem arises with welding these
m~teri~l~ in the lapjoint configuration due to the low boiling point of zinc (906 ~C) compared with the
melting temperature of steel (~1550 ~C).
If there is no joint clearance between the sheets the zinc vapor during welding can only escape
through the molten welding pool, and this typically results in excessive weld porosity or complete
expulsion of the weld metal. In order to make a good quality weld, there are, in principle, two solutions
to get around this problem: (l)creating a zinc-vapor venting channel during welding; or (2)removing the
zinc coating in the welding pass. Both of these approaches need additional techniques to be realized.
20 Many techniques have been developed to provide a gap between sheets to perform laser welding and to
remove the zinc coat at the welding spot. These approaches typically require the use of supplementary
components or spacers and cannot be employed in the production line, since additional equipment is

CA 02209804 1997-08-1~
required to create a gap or remove the coating. This will incur a signifi~nt expense and increase
production time.
It is therefore an object of the present invention to obviate or mitig~te the above disadvantages.
5 SUMMARY OF THE INVENTION
The present invention is intended to provide a practical and flexible way of making laser beam lap
weld of zinc-coated steel sheets. In general terms, a laser beam interaction with one surface of the
m~tt-ri~l is used to create an acceptable gap between the sheets before the welding pass is performed.
More specifically, protuberances are formed on one surface of one sheet by impingement of a
10 laser beam which m~int~in~ opposed surfaces of the sheets in spaced relationship. In this manner, the
welding may be completed entirely on a single welding apparatus in an efficient cost-effective manner.
With such a technique, it is possible to make a curved welding pass and it is applicable to 3D welding
configuration. Therefore, this technique makes the production of laser beam lap welds of the zinc-coated
steel sheets possible using an exiting butt welding system.
The preferred embodiment of the invention relates to a method of creating a gap for vapor gas
venting by the laser beam before welding is performed. The principle is that a laser beam pulse with an
appropriate pulse length can melt a spot on the metal sheet when it interacts with steel and soli(lifi~;~tion
of the molten metal forms a protuberance. The protuberance height above the sheet surface could be a
few tenths millimeters. A series of protuberances in a line or curve serves as natural spacer. When the
20 laser pulse pre-processed sheet is put together with another sheet, a gap is formed. Laser welding can
thus be performed along the spotted line/curve and go over the protuberances. The appropriate distance
between two laser created protuberances depends upon the clamping force, laser beam power, sc~nning

CA 02209804 1997-08-1~
speed and the thi~knPss of the steel sheets. In lap welding, the sheet on which laser pulses generate
protuberances can be either the top one or the bottom one. For both configurations, good quality welds
can be produced.
s DESCRIPTION OF THE INVENTION
Embodiments of the invention will now be described by way of example only with reference to the
accompanying drawings, in which
Figure 1 is a plan view of a pair of lap welded components;
Figure 2 is a view on the line 2-2 of the sequential steps in performing a weld on the components;
Figure 3 is an alternative embodiment showing the production of a spot weld;
Figure 4 is a photographic representation of a portion of one of the sheets shown in Figure 2;
Figure 5a is a photographic representation of a section on the weld shown in Figure l;
Figure 5b is a photographic representation of a plan view of the weld shown in Figure l; and
Figure 6 is photographic representations of welded components produced by the technique shown
in Figure 2.
Referring therefore to Figure 1, a pair of components 10, 12 are connected by seam welding along
a seam line 13 intli~tecl in chain dot line. Each of the components is a steel sheet having oppositely
directed surfaces 16, 17 and lX, 20 respectively. Each of the surfaces has a zinc coating 22 to provide
protection from corrosion.
As shown in Figure 2, one of the components to be welded, namely zinc-coated steel sheet 10 is
placed on a welding table (any kind of table used for laser welding). A series of laser beam pulses 14
tr~n.~mitted through laser head 23 are used to impinge on one of the surfaces 16 of the sheet 10 to create

CA 02209804 1997-08-1~
a series of protuberances 15 along the intended weld line 13. This process can be done by moving laser
beam 14 over the sheet 10 or by manipulating the focusing optics of the laser head.
Once the pulsed protuberances are formed, the zinc-coated steel sheet 12is placed on top of the
laser-pulsed sheet 10 so that surface 18 overlies the surface 16. Alternatively, of course, the laser-pulsed
steel sheet 10 may be placed on top of the other zinc-coated steel sheet 12. Under either condition, the
steel sheets 10, 12 are pressed together by a clamp (not shown). As a result, two sheets to be welded are
held together and are ready to be welded.
The protuberances 15 forms an air gap 24 between the opposed surfaces 16, 18 of overlapping
portions of the zinc-coated steel sheets 10, 12 along the weld line 13.
The welding operation is carried out by applying laser beam 14 to the steel sheets 10, 12 as
in-lic~ted in Figure 2 by arrow A. During the welding, the zinc vapor 26 of the zinc coating 22 can flow
into the air gap 24 surrounding the welding keyhole. Therefore, no air holes or very few air holes are
formed in the solidified metal weld 19. Thus, the resultant weld is satisfactory in quality.
In a typical welding operation using a 1.5 kW CO2 laser, the protuberances 15 were formed by
15 beam pulses of 100 - 150 millisec in duration. The thickness of the sheet 10 was ~1.0 mm and the
m~teri~l galvanneal steel. The height of the protuberances 15 was in the order of 0.4 mm with a diameter
of in the order of 2 mm. As can be seen from Figure 4, the protuberance is irregular but of ~u~rlcienl
height to m~int~in the surfaces 16, 18 separated. The spacing of the protuberances was about 50 mm for
the material selected although this spacing may vary according to the material.
The components 10, 12 were seam welded using the 1.5 kW laser in continuous mode traveling at
a welding speed of 1.5 in/sec. The resultant weld is shown in Figure 5 where it can be seen that good
homogeneity has been obtained.

CA 02209804 1997-08-1~
The above embodiment has been described with respect to seam welding but a similar technique
may be used for spot welding.
As shown in Figure 3, one of the components 10 to be welded is placed on a working table or
frame. At the locations where laser spot welds are to be made, a laser beam pulse 14 impinges on surface
s 16 to create a protuberance 15. After all the locations are pulsed by laser beam 14, zinc-coated steel sheet
12 is placed on top and clamped. As a result of this configuration, an air gap is formed between two zinc-
coated steel sheets around the laser-generated humps. The welding operation is carried out by applying a
laser beam 14 to the steel sheets adjacent to each protuberance 15. The laser beam draws a circle of small
radius around the protuberance to provide a localized circular weld. Welding will not affected if the path
lo of the beam intercepts the protuberance.
As described in the seam welding embodiment above, the zinc vapor 26 escapes into the air gap
around the hump. Therefore, a circular weld 19 is made and the resultant weld is satisfactory in quality.
This circular weld can be considered as a spot weld.
According to the above description, the creation of natural spacer by this technique is simple and
1S flexible as the laser beam can be put on anywhere on the sheet to create protuberances and to perform
welding. Moreover, implementation of lap welding can be done with present laser welding systems with
little additional costs.

Representative Drawing