Process and apparatus for welding workpieces with two or more laser beams whose spots are oscillated across welding direction

Laser welding process and apparatus wherein a plurality of laser beams are focussed by a focussing device on surfaces of workpieces such that spots of the laser beams are located in the vicinity of an interface as viewed in a direction perpendicular to a direction of extension of an interface of the workpiece, and the beam spots and the workpieces are fed relative to each other by a feeding device in the direction of extension of the interface, while at the same time the beam spots are oscillated by an oscillating device at a predetermined frequency relative to the workpieces in a direction intersecting the direction of extension of the interface such that the beam spots are moved across the interface.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to laser welding process and 
apparatus, and more particularly to such laser welding process and 
apparatus using two or more laser beams which are oscillated in a 
direction intersecting a direction of extension of an interface of 
workpieces at which a weld seam is formed. 
2. Discussion of the Related Art 
For welding together workpieces such as steel plates, there is known a 
laser welding process in which a laser beam is focussed such that a spot 
of the laser beam is located in the vicinity of an interface of the 
workpieces at which the workpieces are butted together. The spot of the 
laser beam is moved relative to the workpieces, along the interface, that 
is, along the welding line. An example of an apparatus capable of 
practicing such a known laser welding process is shown in FIG. 9, wherein 
a laser beam 10 such as carbon dioxide gas laser is generated by a laser 
source 8, and is incident upon two workpieces in the form of steel plates 
14a, 14b, through a torch head 10, such that the laser beam 10 is focussed 
in the vicinity of an interface 16 of the workpieces as viewed in a 
direction perpendicular to the direction of extension of the interface 16. 
Generally, the spot of the laser beam 10 in such a laser welding process is 
oscillated at a predetermined frequency relative to the workpieces in a 
direction intersecting the direction of extension of the interface 16 such 
that the spot is moved across the interface in opposite directions. To 
this end, for instance, the torch head 12 incorporates a parabolic mirror 
18 having a concave reflecting surface 18a for focussing the laser beam 10 
on the surfaces of the steel plates 14a, 14b, and a scanning mirror 22 
which is pivotable over a predetermined angular range, about a pivot axis 
20 thereof which is parallel to the direction of extension of the 
interface 16 and the surfaces of the workpieces 14. The laser beam 10 
reflected by the parabolic mirror 18 is incident upon the scanning mirror 
22, so that the spot of the laser beam 10 on the workpieces 14 is 
oscillated by oscillatory pivoting movements of the scanning mirror 22. 
While the workpieces 14 are irradiated by the laser beam 10 with the spot 
of the laser beam 10 being moved relative to the workpieces 14 along the 
interface 16, the scanning mirror 22 is oscillatingly pivoted at a 
predetermined speed, so that the spot of the laser beam 10 focussed on the 
surfaces of the workpieces 14 is oscillated across the interface 16 over a 
predetermined distance "W" in the direction perpendicular to the direction 
of extension of the interface 16, as shown in FIG. 10. At the same time, 
the laser beam spot and the workpieces 14 are moved relative to each other 
in the direction of extension of the interface 16. As a result, the spot 
of the laser beam 10 takes a movement path 24 in the form of a sinusoidal 
wave as shown in FIG. 11. 
The oscillatory movements of the laser beam spot 10 in the laser welding 
process using the torch head 12 provide an increase in the width of fusion 
of the materials of the steel plates 14a, 14b to be welded together. This 
laser welding process is effective to minimize the formation of a shoulder 
which would be created at the interface of the two workpieces due to a 
difference in thickness of the workpieces, as indicated in FIG. 12. Broken 
line in FIG. 12 indicates the welding condition where the spot of the 
laser beam 10 is merely moved along the interface 16, without oscillation 
in the direction perpendicular to the interface 16. 
In the known laser welding process in which the spot of the laser beam 10 
is moved relative to the steel plates 14 along the interface while the 
spot is oscillated, the distance between the adjacent peaks of the 
sinusoidal wave of the movement path of the laser beam spot in the 
direction of extension of the interface 16 is considerably large. In other 
words, the laser beam spots are not contiguous with each other or not 
continuous at positions which are relatively distant from the interface 16 
in the direction perpendicular to the direction of extension of the 
interface 16. Accordingly, the heat generated by irradiation by the laser 
beam is not evenly distributed or the heating or fusion of the materials 
of the workpieces is not uniform, in the direction of extension of the 
interface 16, that is, in the welding direction. To assure a high degree 
of laser welding reliability, the materials of the two workpieces 14a, 14b 
should be continuously fused over a sufficiently large width along the 
interface 16, even on the back side of the workpieces 14 remote from the 
surfaces upon which the laser beam 10 is incident. That is, the so-called 
"back bead" should cover a sufficiently large region. To meet this 
requirement, the distance between the adjacent peaks of the sinusoidal 
wave of the movement path of the spot of the laser beam 10, namely, the 
wavelength of the sinusoidal wave, is determined so as to obtain a 
sufficiently large "back bead" or weld zone on the back side of the 
workpieces 14. In this respect, it is noted that the materials are least 
likely to be fused at the positions corresponding to the peaks of the 
sinusoidal wave of the movement path of the laser beam spot. 
The fusion of the materials of the workpieces in a laser welding process 
takes place primarily by conduction of heat through the materials. The 
width of the "back bead" is determined or influenced by the thickness of 
the workpieces 14, the output of the laser beam 10, the period or 
frequency of oscillation of the laser beam spot (wavelength of the 
sinusoidal wave of the movement path of the laser beam spot, which 
determines the distance between the adjacent peaks of the sinusoidal wave 
in the direction of extension of the interface 16), and the speed of 
relative movement of the laser beam spot and the workpieces. The width of 
the "back bead" increases with a decrease in the thickness of the 
workpieces 14, an increase in the output of the laser beam 10, a decrease 
of the oscillation period and a decrease of the relative movement speed. 
To minimize the welding cost, however, it is desirable not to use the laser 
beam 10 having a high output and not to oscillate the laser beam 10 with a 
short period (at a high frequency). For assuring a sufficiently large 
width of the back bead of the workpieces 14, therefore, the the speed of 
the relative movement of the spot of the laser beam 10 and the workpieces 
14 should be made as low as possible. However, the lower limit of this 
relative movement speed is determined by the thickness of the workpieces, 
and the relative movement speed may not be lowered sufficiently in some 
cases. 
SUMMARY OF THE INVENTION 
It is therefore a first object of this invention to provide a laser welding 
process which permits a high speed of relative movement of the laser beam 
spot and the workpieces, without having to increase the output of the 
laser beam and the oscillation frequency of the laser beam spot. 
It is a second object of this invention to provide a laser welding 
apparatus suitable for practicing such a laser welding process. 
The first object indicated above may be achieved according to a first 
aspect of this invention, which provides a process of welding together 
workpieces butted together at an interface, comprising the steps of: (a) 
focussing a plurality of laser beams on surfaces of the workpieces such 
that spots of the laser beams are located in the vicinity of the interface 
as viewed in a direction perpendicular to a direction of extension of the 
interface; and (b) feeding the spots of the laser beams and the workpieces 
relative to each other in the direction of extension of the interface, 
while at the same time oscillating the spots of the laser beams at a 
predetermined frequency relative to the workpieces in a direction 
intersecting the direction of extension of the interface such that the 
spots are moved across the interface. 
In the laser welding process of the present invention, the plurality of 
laser beams are focussed on the surfaces of the workpieces such that the 
beam spots are located in the vicinity of the interface, and the beam 
spots are moved relative to the workpieces along the interface while at 
the same time the spots are oscillated in the direction intersecting to 
the interface such that the spots are moved across the interface, whereby 
the workpieces are welded together. Consequently, the present welding 
process makes it possible to increase the speed of the relative movements 
of the laser beam spots and the workpieces, that is, the welding speed, 
without increasing the oscillating frequency of the laser beam spots, even 
when the total energy of the laser beams is the same as the energy of a 
single beam used in the known laser welding process. 
The above advantage is provided by the use of the plurality of laser beams, 
as explained below in detail. To begin with, it is considered that the 
uniformity of heating or fusion of the materials of the workpieces is 
improved owing to movement paths of the spots of the laser beams in the 
form of sinusoidal waves which have a relatively short distance between 
the adjacent peaks in the direction of extension of the interface, whereby 
the spacing in this direction between the spots moved along the sinusoidal 
waves is significantly reduced. Secondly, the efficiency of energy 
absorption in the material in a given portion of the workpieces adjacent 
to the interface is improved owing to successive irradiations of that 
portion of the workpieces by the two or more laser beams, whereby the 
efficiency of heating or fusion of that portion adjacent to the interface 
is accordingly improved. Consequently, the uniformity of heating or fusion 
of the materials is increased, leading to a reduced variation in the width 
of the "back bead" or weld zone or region on the back side of the 
workpieces. Further, the increased heating efficiency results in a reduced 
irradiation time required to obtain a sufficiently large width of the back 
bead. Thus, the present laser welding process permits an increase in the 
speed of the relative movement of the laser beam spots and the workpieces, 
without increasing the amount of energy of the laser beams and the 
oscillating frequency of the beam spots, as compared with the conventional 
laser welding process in which a single laser beam is used. 
Where the movement paths of the two or more laser beams are completely 
coincident or aligned with each other, the distance between the adjacent 
peaks of the sinusoidal waves of the movement paths of the beam spots in 
the direction of extension of the interface is the same as the distance 
between the adjacent peaks of the sinusoidal wave of the single laser beam 
used in the prior art. However, each portion of the workpieces irradiated 
by one of the laser beams is subsequently irradiated by the other laser 
beam or beams. Namely, the workpieces are irradiated along the interface 
successively by the two or more laser beams, whereby the heating 
efficiency of the workpieces is improved. Where the movement paths of the 
laser beams are offset from each other, for example, where the phase 
difference of the two laser beams is equal to a half of the wavelength of 
the sinusoidal waves of the movement paths, or an odd number of times the 
half wavelength of the sinusoidal waves, the distance between the adjacent 
peaks of the sinusoidal waves in the direction of extension of the 
interface can be reduced or minimized (in the case of the above two laser 
beams), whereby the uniformity of heating of the workpieces is further 
improved. In this case, a significant improvement in the heating 
efficiency owing to the successive irradiations by the two or more laser 
beams is not expected. In either case, the present laser welding process 
is effective to improve the efficiency of heating or fusion of the 
workpieces. 
In one preferred form of the present laser welding process, the spots and 
the workpieces are fed relative to each other while the spots are 
oscillated such that movements paths taken by the spots of the laser beams 
on the surfaces of the workpieces intersect each other in synchronization 
of a frequency of oscillation of the spots in the direction intersecting 
the direction of extension of the interface. In this form of the 
invention, the movement paths of the laser beams are not coincident or 
aligned with each other, but intersect each other, whereby the distance 
between the adjacent peaks of the movement paths in the direction of 
extension of the interface can be reduced with a result of increased 
uniformity of heating or fusion in the direction of extension of the 
interface. In this case, the total amount of energy given by the laser 
beams at the points of intersection of the movement paths is larger than 
the amount of energy given at the other points, thereby making it possible 
to compensate for a comparatively small amount of energy given by each 
laser beam at the points near the interface at which the distance between 
the adjacent spots of the laser beams in the direction of the interface is 
the largest. 
In another preferred form of the present process, the laser beams are 
focussed such that the spots of the laser beams lie on a straight line 
parallel to the direction of extension of the interface, before the spots 
are oscillated. In this case, the spots of all the laser beams have the 
same oscillating range in the direction perpendicular to the interface. 
The above-indicated straight line represents the center line of the 
oscillating range. In this case, the oscillating range can be minimized, 
leading to improved welding efficiency. 
In a further preferred form of the present process, the plurality of laser 
beams consist of a first beam and a second beam which are focussed at 
respective two spots on the workpieces and which have different amounts of 
energy. The two spots are spaced from each other in the direction of 
extension of the interface. In this case, the amounts of energy of the 
first and second laser beams are determined as needed depending upon the 
specific welding condition, so that the welding accuracy is improved. 
In one advantageous arrangement of the above preferred form of the 
invention, the second laser beam has a larger amount of energy than the 
first laser beam, and the first laser beam precedes the second laser beam 
in a direction of relative movements of the spots of the first and second 
laser beams and the workpieces in the direction of extension of the 
interface. In this arrangement, each region of the workpieces along the 
interface is irradiated first by the first laser beam having the 
relatively small amount of energy, and then by the second laser beam 
having the relatively large amount of energy, while the beam spots and the 
workpieces are moved relative to each other in the direction of the 
interface. Thus, the first laser beam functions as a preliminary heating 
beam, while the second laser beam functions as a primary heating beam for 
heating or fusing the spot pre-heated by the preliminary heating beam, 
with higher efficiency of absorption of energy in the workpieces upon 
irradiation by the second laser, whereby a comparatively large weld zone 
or bead is obtained on the back side of the workpieces remote from the 
surfaces upon which the laser beams are incident. 
In a still further preferred form of the present laser welding process, the 
spots of the laser beams and the workpieces are fed relative to each other 
in the direction of extension of the interface while at the same time the 
spots are oscillated in a direction perpendicular to the direction of 
extension of the interface. Where the speed of the relative movement of 
the laser beam spots and the workpieces is held constant, the speed of 
movement of the laser beam spots on the workpieces in the direction of 
extension of the interface is also held constant, permitting even 
distribution of heat in the direction of extension of the interface. 
The second object indicated above may be achieved according to a second 
aspect of this invention, which provides an apparatus for welding together 
workpieces butted together at an interface, comprising: (i) a focussing 
device for focusing a plurality of laser beams on surfaces of the 
workpieces such that spots of the laser beams are spaced from each other 
in a direction of extension of the interface; (ii) an oscillating device 
for oscillating the spots of the laser beams at a predetermined frequency 
relative to the workpieces in a direction intersection the direction of 
extension of the interface such that the spots are moved across the 
interface; and (iii) a feeding device for feeding the spots of the laser 
beams and the workpieces relative to each other in the direction of 
extension of the interface. 
In the present welding apparatus, the focussing device is adapted to focus 
the laser beams on the surfaces of the workpieces such that the beam spots 
are located in the vicinity of the interface, and the feeding device and 
the oscillating device are adapted to move the spots of the laser beams 
relative to the workpieces along the interface and at the same time 
oscillate the beam spots in the direction perpendicular to the interface 
such that the spots are moved across the interface, whereby the workpieces 
are. welded together. Consequently, the present welding apparatus makes it 
possible to increase the speed of the relative movements of the spots of 
the laser beams and the workpieces, that is, the welding speed, without 
increasing the oscillating frequency of the laser beam spots, even when 
the total energy of the laser beams is the same as the energy of a single 
beam in the known apparatus. 
In one preferred form of the present welding apparatus, a control device is 
provided for controlling the oscillating device and the feeding device 
such that movement paths taken by the spots of the laser beams on the 
surfaces of the workpieces intersect each other in synchronization with a 
frequency of oscillation of the spots in the direction intersecting the 
direction of extension of the interface. In this case, the movement paths 
of the laser beams are not coincident or aligned with each other, but 
intersect each other, so that the distance between the adjacent peaks of 
the movement paths in the direction parallel to the interface is reduced 
with a result of increased uniformity of heating or fusion in the 
direction of extension of the interface. In the present case, the total 
amount of energy given by the laser beams at the points of intersection of 
the movement paths is larger than the amount of energy given at the other 
points, thereby making it possible to compensate for a comparatively small 
amount of energy given by each laser beam at the points near the interface 
at which the distance between the adjacent spots of the laser beams in the 
direction of the interface is the largest. 
In another preferred form of this laser welding apparatus, the focussing 
device focuses the plurality of laser beams such that the spots of the 
laser beams lie on a straight line parallel to the direction of extension 
of the interface, before the spots are oscillated. In this case, the spots 
of all the laser beams have the same oscillating range in the direction 
perpendicular to the interface. The above-indicated straight line 
represents the center line of the oscillating range. In this case, the 
oscillating range can be minimized, leading to improved welding 
efficiency. 
In a further preferred form of the present apparatus, the focussing device 
focuses a first laser beam and a second laser beam as the plurality of 
laser beams on the surfaces of the workpieces such that spots of the first 
and second laser beams are spaced from each other in the direction of 
extension of the interface, and the apparatus further comprises an energy 
amount setting device for setting energy amounts of the first and second 
laser beams at the spots thereof. In this case, the amounts of energy of 
the first and second laser beams are determined as needed depending upon 
the specific welding condition, so that the welding accuracy is improved. 
In one advantageous arrangement of the above preferred form of the 
apparatus, the energy amount setting device sets the energy amounts of the 
first and second laser beams such that the energy amount of the second 
laser beam is larger than that of the first laser beam, and the focussing 
device focuses the first and second laser beams such that the first laser 
beam precedes the second laser beam in a direction of relative movements 
of the spots of the first and second laser beams and the workpieces in the 
direction of extension of the interface. In this arrangement, each region 
of the workpieces along the interface is irradiated first by the first 
laser beam having the relatively small amount of energy, and then by the 
second laser beam having the relatively large amount of energy, while the 
beam spots and the workpieces are moved relative to each other in the 
direction of the interface. Thus, the first laser beam functions as a 
preliminary heating beam, while the second laser beam functions as a 
primary heating beam for heating or fusing the spot pre-heated by the 
preliminary heating beam, with higher efficiency of absorption of energy 
in the workpieces upon irradiation by the second laser, whereby a 
comparatively large weld zone or bead is obtained on the back side of the 
workpieces remote from the surfaces upon which the laser beams are 
incident. 
In a still further preferred form of the present laser welding process, the 
oscillating device oscillates the spots of the laser beam in the direction 
perpendicular to the direction of extension of the interface. Where the 
speed of the relative movement of the laser beam spots and the workpieces 
is held constant, the speed of movement of the laser beam spots on the 
workpieces in the direction of extension of the interface is also held 
constant, permitting even distribution of heat in the direction of 
extension of the interface. 
In a yet further preferred form of the apparatus, the focussing device 
includes a beam reflecting and condensing member having a concave 
reflecting surface for reflecting and at the same time condensing a laser 
beam generated from the laser source, and further includes a beam 
reflecting and splitting member having a reflecting surface consisting of 
a plurality of portions which are inclined relative to each other. The 
beam reflecting and splitting member reflects the laser beam reflected 
from the beam reflecting and condensing member and at the same time splits 
the laser beam into a plurality of sub-beams so that the sub-beams are 
focussed on the surfaces of the workpieces. In this arrangement permits 
the two laser beams to be focussed on the workpieces, without using two 
laser sources, and assures accurate relative position of the spots of the 
laser beams in the direction of extension of the interface, since the beam 
positions are determined by the angles of inclination of the two portions 
of the reflecting surface with respect to the base plane. 
In one advantageous form of the above preferred form of the apparatus, the 
oscillating device includes a device for pivoting the beam reflecting and 
splitting member about a pivot axis thereof. The pivot axis is parallel to 
the direction of extension of the interface and parallel to the surfaces 
of the workpieces. The present oscillating device using the pivoting 
device is comparatively simple in construction, and permits synchronous 
oscillation of the spots of the laser beams on the surfaces of the 
workpieces, so that the workpieces are evenly heated or fused in the 
direction of extension of the interface.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
A laser welding apparatus constructed according one embodiment of this 
invention will be described referring to the accompanying drawings. It is 
to be understood that the drawings do not accurately represent dimensional 
relationships of individual elements of the apparatus. 
Referring first to the fragmentary perspective view of FIG. 1, a laser 
welding apparatus 28 includes a torch head 30, and a laser source 31 
adapted to generate a laser beam 32 such as carbon dioxide gas laser in 
the form of parallel rays. The torch head 30 has a parabolic mirror 34 for 
condensing and reflecting the incident laser beam 32, and a twin-spot 
focussing mirror 36 for splitting the laser beam 32 reflected by the 
parabolic mirror 34, into two sub-beams 32a, 32b which are focussed on two 
workpieces in the form of steel plates 14a, 14b which are butted together 
so as to form an interface 16 corresponding to a weld seam. The spots of 
the sub-beams 32a, 32b are located in the vicinity of the interface 16 in 
the direction perpendicular to the direction of extension of the interface 
16. The twin-spot focussing mirror 36 is disposed to be pivotable over a 
predetermined angular range, about an axis 38 thereof which is parallel to 
the direction of extension of the interface or weld beam 16 and parallel 
to the surfaces of the steel plates 14a, 14b. 
The laser welding apparatus 18 further includes a galvanometer 40 fixed to 
the torch head 30, a control device 42 for controlling the galvanometer 
40, and an X-Y table 43 on which the workpieces 14a, 14b are fixedly 
mounted and which is moved in X-axis and Y-axis directions. The 
galvanometer 40 is provided for rotating or pivoting the twin-spot 
focussing mirror 36 about the pivot axis 38. The control device 42 is 
adapted to control the operating frequency and amplitude of the 
galvanometer 40, for thereby controlling the pivoting speed and angle of 
the twin-spot focussing mirror 36. The X-axis and Y-axis directions are 
perpendicular to each other and to the direction in which the sub-beams 
32a, 32b are incident upon the workpieces 14a, 14b. In the present 
embodiment, the torch head 30 functions as a focussing device for 
focussing a plurality of laser beams 32 on the surfaces of the workpieces 
14a, 14b. 
The parabolic mirror 34 has a concave reflecting surface 44 in the form of 
a parabolic surface or paraboloid, which is inclined by about 45.degree., 
for example, with respect to the direction in which the laser beam 32 from 
the laser source 31 is incident. As shown in the cross sectional view of 
FIG. 2 taken in a plane perpendicular to the interface 16, the laser beam 
32 vertically incident upon the reflecting surface 44 is condensed while 
it is reflected by the reflecting surface 44 so as to travel horizontally 
toward the twin-spot focussing mirror 36. The laser beam 32 incident upon 
the twin-spot focussing mirror 46 is reflected by this mirror 46 so that 
the laser beam 32 is focussed on the surfaces of the workpieces 14. 
As indicated above, the twin-spot focussing mirror 36 has a function of 
splitting the laser beam 32 into the first and second sub-beams 32a, 32b, 
as well as the beam focussing function. To this end, the twin-spot 
focussing mirror 36 has a reflecting mirror in the form of a generally 
circular disc having a reflectance of about 98%, as shown in the plan view 
and the front, left and right side elevational views of FIGS. 4, 5, 6 and 
7. This reflecting mirror has a reflecting surface 46 on its front side 
facing the parabolic mirror 34. The reflecting surface 46 consists of a 
first portion 46a and a second portion 46b which are inclined by 
predetermined respective angles .theta.1 and .theta.2 with respect to a 
base plane perpendicular to the axis of the above-indicated circular disc 
(i.e., plane parallel to the back side surface opposite to the reflecting 
surface 46), as indicated in FIG. 5. These two portions 46a, 46b of the 
reelecting mirror 46 are contiguous with each other at a division or 
boundary line 48 parallel to the above-indicated base plane, as indicated 
in FIG. 4. In other words, the reflecting surface 46 is bent at the 
division line 48, which is offset some distance from the above-indicated 
axis of the circular disc of the mirror 36, so that the first portion 46a 
has a smaller surface area than the other portion 46b, as also indicated 
in FIGS. 4 and 5. 
With the laser beam 32 being incident upon the reflecting surface 46 of the 
twin-spot focussing mirror 36 constructed as described above, the laser 
beam 32 is split into the first and second sub-beams 32a, 32b 
corresponding to the respective first and second portions 46a, 46b of the 
reflecting surface 46, with a splitting ratio determined by the position 
of the division line 48 which is offset from the axis of the mirror 36 as 
described above. When the twin-spot focussing mirror 36 is placed in a 
neutral position about its pivot axis 38, the two sub-beams 32a, 32b are 
focussed at respective two positions which are located in the vicinity of 
the interface 16 as viewed in the direction perpendicular to the direction 
of extension of the interface 16. The above two positions are spaced from 
each other in the direction parallel to the interface 16. Described more 
specifically, the division line 48 of the twin-spot focussing mirror 36 
lies in a plane perpendicular to the pivot axis 38 parallel to the 
interface 16. Therefore, the spots of the sub-beams 32a, 32b focussed on 
the workpieces 14a, 14b lie on a straight line which is parallel to the 
interface 16, as indicated in FIG. 1. In the present embodiment, this 
straight line is substantially aligned with the interface 16. The spots of 
the sub-beams 32a, 32b are spaced from each other by a predetermined 
distance "d" in the direction parallel to the interface 16, as shown in 
FIG. 3. 
With the twin-spot focussing mirror 36 being pivoted in an oscillating 
fashion by the galvanometer 40 between two angular positions cross the 
above-indicated neutral position, the spots of the sub-beams 32a, 32b are 
oscillated by a predetermined oscillating distance "W" in the direction 
perpendicular to the interface 16, with the distance "d" being maintained, 
as shown in FIG. 1. In the present embodiment, the twin-spot focussing 
mirror 36 having the pivot axis 38 and the galvanometer 40 cooperate to 
constitute an oscillating device for oscillating the spots of the 
sub-beams 32a, 32b at a predetermined frequency relative to the workpieces 
14a, 14b in a direction intersecting the interface 16 so that the beam 
spots are moved across the interface 16 in the opposite directions. The 
amounts of energy of the two sub-beams 32a, 32b are determined by the 
surface area ratio of the portions 46a, 46 of the reflecting surface 46 of 
the twin-spot focussing mirror 36. In this embodiment, the energy amount 
of the first sub-beam 32a is smaller than that of the second sub-beam 32b. 
It will be understood that the twin-spot focussing mirror 36 functions as 
a device for setting the amounts of energy of the first and second 
sub-beams 32a, 32b. 
An X-axis drive device 54 is provided for feeding the X-Y table 43 and the 
workpieces 14a, 14b mounted thereon, in the X-axis direction parallel to 
the interface 16, while a Y-axis drive device 56 is provided for feeding 
the X-Y table 43 and the workpieces 14a, 14b in the Y-axis direction 
perpendicular to the X-axis direction. The X-axis and Y-axis drive devices 
54, 56 are controlled by either the control device 42 or another control 
device, so that the spots of the sub-beams 32a, 32b are moved relative to 
the workpieces 14a, 14b, along the interface 16. It will be understood 
that the X-Y table 43 and the X-axis and Y-axis drive devices 54, 56 
constitute a feeding device for moving the spots of the sub-beams 32a, 32b 
relative to the workpieces 14a, 14b in the direction parallel to the 
interface 16. A suitable Z-axis drive device (not shown) is provided for 
moving the torch head 30 in a Z-axis direction perpendicular to the X-axis 
and Y-axis directions, for adjusting the height of the torch head 30 so as 
to focus the sub-beams 32a, 32b on the surfaces of the workpieces 14a, 
14b. This Z-axis drive device may be adapted to move the X-Y table 43, 
rather than the torch head 30, in the Z-axis direction. 
The two workpieces in the form of the steel plates 14a, 14b to be welded 
together by the present laser welding apparatus 28 are fixedly mounted on 
the X-Y table 43 such form the interface 16 which corresponds to the weld 
seam to be formed by welding. The X-Y table 43 and the steel plates 14a, 
14b are fed in a welding or feeding direction "F" parallel to the 
interface 16, as indicated in FIG. 1, while the sub-beams 32a, 32b are 
focussed on the surfaces of the steel places 14a, 14b, and adjacent to the 
interface 16 as viewed in the direction perpendicular to the direction of 
extension of the interface 16, so that the materials of the steel plates 
14a, 14b are fused and welded together along the interface 16. 
While the spots of the sub-beams 32a, 32b are moved relative to the 
workpieces 14a, 14b by the X-Y table 43 in the feeding direction F, the 
twin-spot focussing mirror 36 is oscillatingly pivoted in the opposite 
directions by the galvanometer 40, which is operated according to the 
oscillating frequency and amplitude as specified by the control device 42. 
As a result, the spots of the sub-beams 32a, 32b take respective 
sinusoidal movement paths 50, 52 on the workpieces 14a, 14b, as indicated 
in FIG. 8 by solid and broken lines, respectively. The sinusoidal or sine 
waves of these two movement paths 50, 52 have the same wavelength and 
amplitude. In the present embodiment, the spot of the first sub-beam 32a 
precedes the spot of the second sub-beam 32b so that the spot of the first 
sub-beam 32a irradiates a given portion of the workpieces 14 before the 
spot of the second sub-beam 32b irradiates that portion while the spots 
and the workpieces 14 are moved relative to each other in the direction 
parallel to the interface 16. Further, the distance "d" between the spots 
of the two sub-beams 32a, 32b on the workpieces 14, the feeding speed of 
the workpieces 14 (X-Y table 43) and the pivoting speed of the twin-spot 
focussing mirror 36 (i.e., operating frequency of the galvanometer 40) are 
determined so that the distance "d" is equal to a half of the wavelength 
of the sinusoidal waves of the movement paths 50, 52 of the spots. In this 
arrangement, the sinusoidal movement paths 50, 52 have a phase difference 
equal to the half wavelength and intersect each other on the straight line 
aligned with the interface 16. Namely, the two sinusoidal movement paths 
50, 52 taken by the spots of the two sub-beams 32a, 32b on the surfaces of 
the workpieces 14 intersect each other in synchronization with the 
oscillating movements of the spots. 
Since the first and second portions 46a, 46b of the reflecting surface 46 
of the twin-spot focussing mirror 36 have the different surface areas as 
described above, the first sub-beam 32a whose spot precedes the spot of 
the second sub-beam 32b has a smaller amount of energy corresponding to 
the smaller surface area of the portion 46a, than the second sub-beam 32b 
whose amount of energy corresponds to the larger surface area of the 
portion 46b. Consequently, the workpieces 14 are irradiated along the 
interface or weld seam 16 first by the first sub-beam 32a, and then by the 
second sub-beam 32b having the larger amount of energy, so that the 
materials in the irradiated portion of the workpieces 14 are sufficiently 
heated or fused with the laser beam energy being absorbed by the materials 
with relatively high efficiency. Since the workpieces 14 are irradiated 
along the interface 16 by the two oscillating sub-beams 32a, 32b, the 
distance between the adjacent peaks of the sinusoidal waves of the 
movement paths 50, 52 in the direction parallel to the interface 16 is 
made considerably smaller than the distance between the adjacent peaks of 
the sinusoidal wave of the movement path of a single oscillating beam used 
in the prior art as indicated in FIG. 11. The present arrangement results 
in improved uniformity of the welding temperature or even distribution of 
the heat in the direction of extension of the interface 16. Thus, this 
arrangement makes it possible to sufficiently and evenly fuse the 
workpieces 14 along the interface 16, even if the amount of energy 
incident upon the workpieces 14 is relatively small. Accordingly, the 
present welding apparatus 28 makes it possible to increase the feeding 
speed and therefore the welding speed of the workpieces 14, as compared 
with the known welding apparatus using a single oscillating laser beam. 
As described above, the present welding apparatus 28 includes: the 
focussing device in the form of the torch head 30 for focussing the two 
sub-beams 32a, 32b on the surfaces of the workpieces in the form of the 
steel plates 14a, 14b; the oscillating device in the form of the twin-spot 
focussing mirror 36 pivotable about the pivot axis 38 and the galvanometer 
40, for oscillating the spots of the sub-beams 32a, 32b at a predetermined 
frequency relative to the workpieces 14a, 14b, in the direction 
intersecting the interface 16 so that the spots are moved across the 
interface 16 in the opposite directions; and the feeding device in the 
form of the X-Y table 43 and the X-axis and Y-axis drive devices 54, 56, 
for moving the spots of the sub-beams 32a, 32b relative to the workpieces 
14a, 14b, in the direction of extension of the interface 16. When the 
workpieces 14a, 14b are welded together, the sub-beams 32a, 32b are 
focussed on the surfaces of the workpieces 14 such that the beam spots are 
located in the vicinity of the interface 16. The spots of the sub-beams 
32a, 32b are moved relative to the workpieces 14 along the interface 16 
while at the same time are oscillated in the direction perpendicular to 
the interface 16 such that the spots are moved across the interface 16, 
whereby the beam spots take the respective sinusoidal movement paths 50, 
52 on the workpieces. Consequently, the present welding apparatus 28 makes 
it possible to increase the speed of the relative movements of the spots 
of the sub-beams 32a, 32 and the workpieces 14a, 14b, that is, the welding 
speed, without increasing the oscillating frequency of the beam spots, 
even when the total energy of the two sub-beams 32a, 32b is the same as 
the energy of a single beam in the known apparatus. 
In the present embodiment, the welding apparatus 28 further includes the 
control device 42 for controlling the speed of oscillation of the spots of 
the two sub-beams 32a, 32b on the surfaces of the workpieces 14a, 14b so 
that the sinusoidal movement paths 50, 52 taken by the two sub-beams 32a, 
32b intersect each other in synchronization with the frequency of 
oscillation of the spots of the sub-beams 32a, 32b. Thus, the spots of the 
two sub-beams 32a, 32b are simultaneously oscillated across the interface 
16 and moved along the interface 16, relative to the workpieces 14, so 
that the spots take the respective two intersecting sinusoidal movement 
paths 50, 52. Namely, the two sinusoidal movement paths 50, 52 are not 
coincident or aligned with each other, but intersect each other as 
indicated in FIG. 8, whereby the distance between the adjacent peaks of 
the two sinusoidal movement paths 50, 52 in the direction parallel to the 
interface 16 is reduced with a result of increased uniformity of heating 
or fusion in the direction of extension of the interface 16. In the 
present embodiment, the points of intersection of the sinusoidal movement 
paths 50, 52 lie on the interface 16. The total amount of energy given by 
the two sub-beams 32a, 32b at these points of intersection is larger than 
the amount of energy given at the other points, thereby making it possible 
to compensate for a comparatively small amount of energy given by each 
sub-beam 32a, 32b at the interface 16 at which the distance between the 
adjacent spots of the sub-beam in the direction of the interface 16 is the 
largest. 
In the present embodiment, the torch head 30 functioning as the focussing 
device is adapted such that the first sub-beam 32a precedes the second 
sub-beam 32b in the direction in which the beam spots are moved relative 
to the workpieces 14 in the direction parallel to the interface 16. 
Further, the twin-spot focussing mirror 36 functioning as the energy 
amount setting device is adapted such that the second sub-beam 32b has a 
larger amount of energy than the first sub-beam 32a, so that each spot 
along the interface 16 is irradiated first by the first sub-beam 32a 
having the relatively small amount of energy, and then by the second 
sub-beam 32b having the relatively large amount of energy, while the beam 
spots and the workpieces 14 are moved relative to each other in the 
direction of the interface 16. In the present arrangement, the first 
sub-beam 32a functions as a preliminary heating beam, while the second 
sub-beam 32b functions as a primary heating beam for heating or fusing the 
spot pre-heated by the preliminary heating beam, with higher efficiency of 
absorption of energy in the workpieces 14 upon irradiation by the second 
sub-beam 32b, whereby a comparatively large weld zone or bead is obtained 
on the back side of the workpieces 14 remote from the torch head 30. 
The spots of the two sub-beams 32a, 32b are oscillated in the direction 
perpendicular to the interface 16, by the oscillating device which 
includes the twin-spot focussing mirror 36 pivotable about the pivot axis 
38 and the galvanometer 40. Where the speed of the relative movement of 
the beam spots and the workpieces 14 is held constant, the speed of 
movement of the beam spots on the workpieces 14a, 14b in the direction of 
the interface 16 is also held constant, permitting even distribution of 
heat in the direction of the interface 16. 
It is also noted that the torch head 30 which functions as the focusing 
device as described above includes the parabolic mirror 34 having the 
concave reflecting surface 44, and the twin-spot focussing mirror 36 
having the reflecting surface 46 consisting of the first and second 
portions 46a, 46b which have respective angles of inclination with respect 
to the base plane. The parabolic mirror 34 function as a beam reflecting 
and condensing member adapted to reflect and at the same time condense the 
parallel rays of the laser beam 32 generated from the laser source 31. The 
twin-spot focussing mirror 36 functions as a beam reflecting and splitting 
member adapted to reflect the incident laser beam 32 and at the same time 
split the laser beam 32 into the first and second sub-beams 32a, 32b so 
that the sub-beams 32a, 32b are focussed on the surfaces of the two 
workpieces 14a, 14b and near the interface 16. This arrangement permits 
the two sub-beams 32a, 32b to be focussed on the workpieces 14a, 14b, 
without using two laser sources, and assures accurate relative position of 
the spots of the two sub-beams 32a, 32b in the direction of the interface 
16, since the beam positions are determined by the angles of inclination 
of the two portions 46a, 46b of the reflecting surface 46 with respect to 
the base plane. 
In the present embodiment, the oscillating device using the twin-spot 
focussing mirror 36 pivotable about the pivot axis 38 is relatively simple 
in construction, and permits synchronous oscillation of the spots of the 
two sub-beams 32a, 32b on the surfaces of the workpieces 14, so that the 
workpieces 14 are evenly heated or fused along the interface 16. 
While the presently preferred embodiment of the present invention has been 
described above by reference to the accompanying drawings, it is to be 
understood that the invention may be otherwise embodied. 
The illustrated embodiment wherein the reflecting surface 46 consists of 
the two portions 46a, 46b may be modified such that the reflecting surface 
46 consists of three or more portions for splitting the incident laser 
beam 32 into three or more sub-beams, which permit improved uniformity of 
heating or fusion of the workpieces. 
While the portions 46a, 46b of the reflecting surface 46 have straight 
surfaces, these portions 46a, 46b may have concave surfaces. 
Although the portions 46a, 46b have different surface areas, they may have 
the same surface area. 
The twin-spot focussing mirror 36 in the illustrated embodiment is adapted 
such the spots of the two sub-beams 32a, 32b lie on a straight line 
aligned with the interface 16 and are spaced from each other in the 
direction of extension of the interface 16. However, the mirror 36 may be 
adapted such that the spots of the two sub-beam 32a, 32b lie on a straight 
line which intersects the interface 16 at a given angle. The spots of the 
two sub-beams 32a, 32b may lie on a straight line perpendicular to the 
interface 16. In this case in which the two spots are not spaced from each 
other in the direction of extension of the interface 16, the spots are 
oscillated in a direction intersecting the interface at an angle other 
than 90.degree.. 
In the illustrated embodiment, the pivot axis 38 is parallel to the 
interface 16 so that the spots of the sub-beams 32a, 32b are oscillated in 
the direction perpendicular to the interface 16. However, the oscillating 
direction of the beam spots need not be perpendicular to the interface 16 
and may be inclined with respect to the interface 16, unless the 
oscillating direction is parallel to the interface 16. 
While the pivoting speed of the twin-spot focussing mirror 36 and the other 
parameters are determined so that the sinusoidal movement paths 50, 52 of 
the spots of the two sub-beams 32a, 32 intersect each other on the 
interface 16 as shown in FIG. 8, these movement paths 50, 52 need not 
intersect each other on the interface 16, or need not intersect each 
other. For instance, the two movement paths 50, 52 may be coincident or 
aligned with each other. 
The welding apparatus 38 according to the illustrated embodiment uses the 
twin-spot focussing mirror 36 for splitting the laser beam 32 generated by 
the laser source 31, into the plurality of sub-beams 32a, 32b, and 
focussing the sub-beams 32a, 32b on the workpieces 14. However, a half 
mirror may be used for splitting the laser beam 32. Alternatively, two 
laser sources 31 may be used to generate two laser beams which are 
focussed by suitable means. 
It is to be understood that the present invention may be embodied with 
various other changes and modifications, without departing from the spirit 
of the invention.