Abstract:
A laser lap welding method for parts made of galvanized steel sheet includes steps of press-forming two parts from galvanized steel sheet such that the two parts include elongated joining regions to be welded together on mutually opposed surfaces thereof and a plurality of protrusions are formed on at least any one of the joining regions of the two parts at predetermined intervals in a longitudinal direction of the joining region; retaining the two parts in a state in which the joining regions are overlapped one on the other such that a gap according to a height of the protrusions is formed between the joining regions; and irradiating a laser onto one surface of the overlapped joining regions of the two parts such that the overlapped joining regions are fused and welded by energy of the laser, and zinc gas produced with fusing is discharged through the gap.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority from Japanese Patent Application No. 2010-238419, filed in the Japanese Patent Office on Oct. 25, 2010, the disclosure of which is hereby incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a laser lap welding method for parts made of galvanized steel sheet, or more specifically, to a laser lap welding method suitable as an alternative technique to be carried out in place of spot welding. 
     Galvanized sheets are used in many portions of vehicle bodies of automobiles in consideration of corrosion resistance. Panels made of galvanized sheets to form vehicle body parts are press-formed into three-dimensional shapes and are welded together at peripheral portions of such three-dimensional shapes, and thereby form a vehicle body structure based on a hollow cross-section which has an advantage in strength. Generally, such vehicle body panels have been integrated mainly by means of spot welding. However, laser welding is now being introduced as an alternative technique that allows processing at a higher speed. 
     It is known that vaporized zinc may cause welding failures such as blowholes when galvanized sheets are closely overlapped and welded to each other using a laser, as the vaporized zinc may blow fused metal away or may remain in the fused metal as bubbles. To avoid this problem, JP10-216974A and JP2571976B disclose a technique in which protrusions are formed on any one of galvanized sheets to form a gap for discharging the zinc vapor in a state in which the sheets are overlapped on each other. 
     Since a vehicle body panel of an automobile is formed by press forming, protrusions are formed on such pressed parts by subjecting the parts to an embossing process with punches arranged inside a press die after a press forming process. However, in the case of employing laser welding for parts originally designed to be suitable for spot-welding, the following problems occur because such parts are not designed to allow arrangement of the protrusions necessary for laser welding. For example, the protrusions cannot be disposed due to limited space and presence of inclinations on a joining surface, or welding positions need to be changed because of the protrusions added. 
     If the welding positions are changed, a strength performance and an impact resistance performance as a vehicle body structure are changed. For this reason, the structure will need to undergo performance confirmation tests again. Moreover, the parts that are newly designed cannot directly utilize design data which have been accumulated on the premise of spot welding. Such problems have been considerable obstacles to introduction of laser welding. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above-described circumstances, and an object thereof is to provide a laser lap welding method which is capable of achieving a performance similar to that of spot welding under a joining condition based on spot welding, and which is suitable for an alternative technique to be carried out in place of spot welding. 
     In order to solve the above-described problems, a laser lap welding method for parts made of galvanized steel sheet, including the steps of: press-forming two parts ( 10 ,  20 ) from galvanized steel sheet such that the two parts include elongated joining regions ( 11 ,  21 ) to be welded together on mutually opposed surfaces thereof and a plurality of protrusions ( 1 ) are formed on at least any one of the joining regions ( 11 ) of the two parts at predetermined intervals in a longitudinal direction of the joining region; retaining the two parts in a state in which the joining regions are overlapped one on the other such that a gap (g) according to a height of the protrusions is formed between the joining regions; and irradiating a laser ( 2   a ) onto one surface of the overlapped joining regions of the two parts such that the overlapped joining regions are fused and welded ( 2 ) by energy of the laser, and zinc gas produced with fusing is discharged through the gap, wherein each of the protrusions is located between unit spots ( 2   e ) each of which is equivalent to an individual welding spot when the two parts are spot-welded, and is formed into a ridge-shape ( 1   a ) extending in a substantially orthogonal direction to the longitudinal direction of the joining regions ( 11 ,  12 ), and wherein the step of irradiating the laser includes discrete unit laser scanning ( 2   c ) each of which is scanned along a curved line surrounding the unit spot ( 2   e ). 
     According to this method, the multiple protrusions provided between the unit spots used for performing unit laser scanning described above are formed as the ridge-shaped protrusions extending in the substantially orthogonal direction to the longitudinal direction of the joining regions, and thereby, an even gap can be stably formed between the joining regions toward lateral and outside of the joining regions so as to be able to discharge zinc vapor, and laser welding can be achieved favorably without causing welding failures such as blowholes. 
     Moreover, the ridge-line protrusions require small areas in a direction of arrangement of unit laser scanning, and can be formed without affecting the layout of the unit spots equivalent to welding spots for spot welding. The unit laser scanning along a curved line surrounding the unit spot can be performed within a substantially equivalent area of the unit spot, and thus, it is possible to obtain joining strength which is equivalent to the case of spot-welding two parts. 
     Consequently, it is possible to utilize existing design data accumulated on the assumption of spot welding. This is advantageous for introducing laser welding as an alternative technique to replace spot welding. Lower power consumption is another advantage because minimum laser scanning is required therein. Meanwhile, laser welding does not cause a problem such as a non-effective shunt current to a welded spot as observed in spot welding. Accordingly, it is also possible to enhance joining strength locally by adding laser scanning to spaces between the unit spots. 
     In the laser lap welding method according to the present invention, it is preferable that the plurality of protrusions ( 1 ) further include trapezoidal protrusions ( 1   b ) each having a flat top surface; an interval between the ridge-shaped protrusions ( 1   a ) adjacent to one another is equal to or less than five times as long as a diameter of the unit spots ( 2   e ); and both, an interval between the ridge-shaped protrusion ( 1   a ) and the trapezoidal protrusion ( 1   b ) adjacent to one another, and an interval between the adjacent trapezoidal protrusions ( 1   b ), are equal to or less than ten times as long as the diameter of the unit spots ( 2   e ). 
     If the gap between the mutually adjacent ridge-line protrusions is equal to or less than five times as long as the diameter of the unit spot in the step of retaining the two parts in the overlapped state, then it is possible to avoid a gap shortage due to warpage of the joining regions, and thereby to avoid inhibition of discharge of zinc vapor at the time of welding, even when the joining regions between the adjacent ridge-line protrusions are formed of flanges of thin steel plates, for example. Furthermore, when a continuous flat portion is formed between the multiple unit spots on the joining region of one of the parts, it is possible to improve supporting stability relative to a joining region of the other part in the step of retaining the two parts in the overlapped state by forming the trapezoidal protrusions instead of the ridge-shaped protrusions. Hence, the gap can be formed by omitting the ridge-shaped protrusions adjacent to the trapezoidal protrusions, thereby reducing the number of protrusions to be formed. 
     The laser lap welding method according to the present invention is particularly performed preferably when the two parts are panels forming a vehicle body of an automobile, and at least one of the joining regions is a flange provided along a periphery of corresponding one of the panels. 
     As described above, according to the laser lap welding method for parts made of galvanized steel sheet according to the present invention, an even gap for discharging zinc vapor can be formed stably between joining regions of pressed parts having three-dimensional shapes including many inclined surfaces and curved surfaces, and thereby excellent welding quality can be achieved. Moreover, laser lap welding can be performed under minimum required welding conditions while maintaining similar performances and quality to those attained by spot welding. Therefore, laser lap welding can be introduced at low cost as an alternative technique in place of spot welding. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a perspective view showing parts made of galvanized steel sheet to be subjected to laser lap welding according to the present invention before being joined to each other, and  FIG. 1B  is a cross-sectional view of the parts made of galvanized steel sheet after being joined to each other, which is taken along line A-A in  FIG. 1A . 
         FIG. 2A  is a cross-sectional view taken along line B-B in  FIG. 2B ,  FIG. 2B  is a front view showing the parts made of galvanized steel sheet to be subjected to laser lap welding according to the present invention, and  FIG. 2C  is an enlarged view of unit laser welding. 
         FIG. 3A  is a cross-sectional view showing an embossing process of a ridge-line protrusion,  FIG. 3B  is a perspective view showing the ridge-line protrusion, and  FIG. 3C  is a cross-sectional view showing a clamped state of the ridge-line protrusion. 
         FIG. 4A  is a cross-sectional view and  FIG. 4B  is a perspective view showing a trapezoidal protrusion. 
         FIG. 5A  is a plan view showing the ridge-shaped protrusions adjacent to a stepped portion and showing unit laser scanning, and  FIG. 5B  is a cross-sectional view taken along line B-B in  FIG. 5A . 
         FIG. 6A  is a plan view showing the trapezoidal protrusions adjacent to a stepped portion and showing unit laser scanning, and  FIG. 6B  is a cross-sectional view taken along a B-B line in  FIG. 6A . 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment of the present invention will be described below in detail with reference to the accompanying drawings. 
       FIG. 1A  shows a rear skirt  10  to be located below a back door opening (or a trunk lid opening) of an automobile and a tail end member to be joined to an upper outer surface side of the rear skirt  10 , before being joined to each other, both of which represent an example of parts made of galvanized steel sheet to be subjected to laser lap welding according to the present invention. Meanwhile,  FIG. 1B  shows the parts made of galvanized steel sheet after being joined to each other. 
     The rear skirt  10  has a flange  12  on an upper edge portion thereof. The flange  12  extends in a vehicle width direction and projects toward the rear of the vehicle. An elongated joining region  11  extending in the vehicle width direction is formed on the lower side of the flange  12 . A channel structure  14  extending in the vehicle width direction is formed between the joining region  11  and the flange  12 . The channel structure  14  has U-shaped cross section opened toward the rear of the vehicle. A portion below the joining region  11  of the rear skirt  10  is practically the portion forming the rear skirt, and a central part in the vehicle width direction thereof is formed into a swelled portion  13  which is swelled toward the rear of the vehicle so as to form a rear wall portion of a spare tire housing (not shown). 
     A tail end member  20  has flanges  21  and  22 . The flange  22  is formed on an upper edge portion of the tail end member  20 . The flange  22  extends in the vehicle width direction and projects toward the rear of the vehicle. The flange  21  is formed on a lower end portion of the tail end member  20 , and extends in the vehicle width direction and projects downward thereof. Then, as shown in  FIGS. 1A and 1B , the flange  22  of the tail end member  20  is lapped over a lower surface of the flange  12  of the rear skirt  10 , and the flange  21  of the tail end member  20  is lapped over the joining region of the rear skirt  10 . By subjecting the overlapped portions to laser lap welding, as described later, a closed cross section  15  that extends in the vehicle width direction is formed between the channel structure  14  of the rear skirt and the tail end member  20 . Meanwhile, the welded and joined flanges  12  and  22  collectively form part of a back door opening flange which extends along a lower edge of the back door opening. 
     In order to introduce gaps for discharging zinc vapor generated at the time of laser lap welding between the respective overlapped portions  11 ,  21  and  12 ,  22 , a large number of protrusions  1  ( 1   a ,  1   b ) are formed on the joining region  11  and the lower surface of the flange  12  of the rear skirt  10  while providing intervals in an longitudinal direction thereof. 
     These protrusions  1  ( 1   a ,  1   b ) are formed between unit spots indicated by reference numeral  2   e  in  FIGS. 2A and 2B , that is, the unit spots  2   e  corresponding to individual welding sports used when spot-welding the rear skirt  10  and the tail end member  20  together, so as to stay away from the unit spots  2   e . When performing laser welding, unit laser scanning  2   c  is performed on each of the unit spots  2   e , the unit laser scanning  2   c  drawing a circular shape (a curved line shape or a C-shape) surrounding the unit spot  2   e , as shown in  FIG. 2C . In this way, it is possible to obtain joining strength equivalent to the case of spot-welding two parts. 
     The joining occurs in the entire part inside the unit spot  2   e  in the case of spot welding, whereas only the periphery of the unit spot  2   e  is joined in unit laser scanning  2   c  described above without causing any joining inside the unit spot  2   e . However, laser welding achieves deeper fusion of the metal compared to spot welding. Hence, it is confirmed that joining strength at least equivalent to the strength obtained by spot welding can be obtained by unit laser scanning  2   c  in a circular shape as described above. 
     Here, it is also possible to obtain joining strength at least equivalent to the strength obtained by spot welding by means of performing unit laser scanning in a curved line shape or a straight line shape to obtain a bead area equivalent to the unit spot  2   e  instead of performing unit laser scanning  2   c  as described above. However, if unit laser scanning is performed in the shape protruding significantly from the unit spot  2   e , a strength performance may change because such a configuration is practically the same as displacement of the unit spot  2   e . This problem is also related to the shapes of the protrusions  1  ( 1   a ,  1   b ). This point will be described later. 
     The rear skirt  10  is formed by press-forming a blank galvanized sheet, and then the protrusions  1  ( 1   a ,  1   b ) are formed by use of punches  3  attached to a press die for an embossing process. Although  FIG. 3A  is illustrated upside down in comparison with the actual process, the protrusion  1  ( 1   a ) is formed by setting the rear skirt  10  on a lower die including a die  32  with a hole drilled in a position corresponding to a processing region, and then sending the punch  3  out of a hole drilled in an upper die  31  (a holding block), so that the punch  3  pushes the steel plate  11  into the hole in the die  32 . 
     Each of the protrusions  1  ( 1   a ,  1   b ) is formed either into a ridge-shaped protrusion  1   a  as shown in  FIGS. 3A to 3C  and  FIGS. 5A and 5B  or into a trapezoidal protrusion  1   b  as shown in  FIGS. 4A and 4B , depending on the shapes of the joining region  11  and the flange  12  as well as positional relationships with the unit spots  2   e.    
     The ridge-shaped protrusion  1   a  includes a ridge line  1   c  extending in an orthogonal direction to the longitudinal direction of the elongated joining region  11  and the flange  12  as shown in  FIG. 3B . The ridge line  1   c  is obtained by forming the ridge-shaped protrusion  1   a  by the embossing process using the punch  3  with a tip having a V-shaped cross section. A minimum necessary area for this ridge-shaped protrusion  1   a  is the area required, in the longitudinal direction of the joining region  11  and the flange  12 , and therefore the ridge-shaped protrusion  1   a  is suitable for a region where a sufficient space cannot be ensured due to the shape of the joining region  11  or the flange  12 . Moreover, there is also an advantage that an even height g is obtained in the direction of the ridge line  1   c  irrespective of inclination angles of inclined surfaces  11   d ,  11   e.    
     Meanwhile, the trapezoidal protrusion  1   b  includes a circular top surface which is flat and broad as shown in  FIG. 4B , The circular top surface is obtained by forming the trapezoidal protrusion  1   b  by the embossing process using a punch (not shown) with a flat circular top end surface. This trapezoidal protrusion  1   b  has an advantage that a mating part  20  can be supported stably by use of the flat top surface. For this reason, there is an advantage that an even gap g can be maintained at the overlapped portion even by extending the intervals of the trapezoidal protrusions  1   b  and thereby to reduce the number of the protrusions  1  to be formed compared to locating the ridge-shaped protrusions  1   a  adjacent to one another. Here, the trapezoidal protrusion  1   b  may also be formed into shapes other than the circular shape. However, the circular shape has an advantage in light of workability. 
     The joining region  11  and the flange  12  of the rear skirt  10  provided with the respective protrusions  1   a ,  1   b  described above are lapped over the flanges  21  and  22  of the tail end member  20  to form the predetermined gap g in the overlapped portions. Moreover, the respective unit spots  2   e  shown in  FIGS. 2A and 2B  are subjected sequentially to unit laser scanning  2   c  while clamping the rear skirt  10  and the tail member  20  in multiple positions along the overlapped portions. Hence the rear skirt  10  and the tail end member  20  are welded and joined to each other. Although it is not particularly limited, remote scanner welding with an optical scanning laser welding machine utilizing a galvano scanner is preferable due to a configuration to perform unit laser scanning  2   c  in a constant shape repeatedly many times. 
     Next, the layout of the respective protrusions  1   a  and  1   b  will be described specifically. In the rear skirt  10  shown in  FIGS. 2A and 2B , the trapezoidal protrusions  1   b  are formed on flat surfaces  11   a  and  11   c  of the joining region  11  and on flat surfaces  12   a  and  12   c  of the flange  12  because of comparatively large intervals of the unit spots  2   e . Naturally, the ridge-shaped protrusions  1   a  may be formed on these respective surfaces instead of the trapezoidal protrusions  1   b . Nevertheless, in that case, it is preferable to add any of the ridge-shaped protrusions  1   a  or the trapezoidal protrusions  1   b  to a section on the flat surface  12   c  and the like of the flange  12 , for example, where there are large intervals between the trapezoidal protrusions  1   b . In other words, some of the protrusions are omitted in this section by continuously forming the trapezoidal protrusions  1   b.    
     On the other hand, the ridge-shaped protrusions  1   a  are formed on the inclined surfaces  11   d  and  11   e  of the joining region  11  irrespective of the intervals of the unit spots  2   e . No trapezoidal protrusions  1   b  can be formed in this section. If the trapezoidal protrusions  1   b  are formed in this section instead of the ridge-shaped protrusions  1   a , actual heights of the protrusions are changed by the inclinations of the inclined surfaces  11   d  and  11   e . Hence, the desired gap cannot be obtained. 
     No protrusions can be formed on a stepped portion  11   b  of the joining region  11  and a stepped portion  12   b  of the flange  12 . In addition, since these shape transformed portions  11   b ,  12   b  are important in terms of strength, welding spots (the unit spots  2   e ) are located comparatively close to these portions. If the trapezoidal protrusions  1   b  are formed adjacent to any of the shape transformed portions, or specifically, the stepped portion  12   b  shown in  FIGS. 6A and 6B , for example, then unit laser scanning  2   c  surrounding the unit spots  2   e  cannot be carried out. Since the top surface of the trapezoidal protrusion  1   b  is pressed against a mating part (the flange  22 ) without providing any gap, zinc gas to be generated by laser irradiation cannot be discharged. 
     If unit laser scanning is performed away from the trapezoidal protrusion  1   b  as indicated with reference numeral  2   c ′ in  FIGS. 6A and 6B  to avoid the above situation, then the welding spot (unit laser scanning  2   c ′) is away from the stepped portion  12   b . A region defined by a diameter dw=5 mm as shown in  FIG. 4A  is required for forming the trapezoidal protrusion  1   b  having a diameter d=4 mm. Furthermore, the trapezoidal protrusion  1   b  needs to be approximately 1 mm away from an upper end of the stepped portion  12   b  for stably embossing the trapezoidal protrusion  1   b . As a consequence, the unit laser scanning  2   c ′ needs to be performed at least 6 mm away from the upper end of the stepped portion  12   b.    
     Accordingly, at the stepped portion  11   b  of the joining region  11  and the stepped portion  12   b  of the flange  12 , the ridge-shaped protrusions  1   a  are respectively formed on portions of the flat surfaces  11   a ,  11   c  and the  12   a ,  12   c  close to both sides of the stepped portions  11   b  and  12   b . As shown in  FIGS. 5A and 5B , in the case of the ridge-shaped protrusion  1   a , unit laser scanning  2   c  can be performed at a position 2 to 3 mm away from the upper end of the stepped portion  12   b  even when embossing the protrusion  1   a  at a position 1 mm away from the upper end of the stepped portion  12   b . A processing margin is provided in the case of typical spot welding, and the ridge-shaped protrusion  1   a  having a width W equal to about 1 mm can be formed within this processing margin. Therefore, it is not necessary to change the position of unit laser scanning  2   c  from the original position of the unit spot  2   e.    
     In addition to the shape transformed portions such as the stepped portions  11   b  and  12   b  described above, the ridge-shaped protrusions  1   a  can be provided in the vicinity of the welding spots (the unit spots  2   e ) adjacent to ends of the flanges without changing the locations of unit laser scanning  2   c  or changing the strength performance. 
     In this embodiment, it is necessary to maintain the gap between the joining regions in a range of g=0.15±0.05 mm (0.1 to 0.2 mm) in order to obtain favorable welding quality when the length of each ridge-shaped protrusion is equal to 8 mm, a width w thereof is equal to 1 mm, the diameter d of the top surface of each trapezoidal protrusion  1   b  is equal to 4 mm, and the height of each of the protrusions  1   a ,  1   b  is equal to 0.15 mm. In the case of using galvanized sheets having plate thickness in a range from 0.6 to 1.2 mm, the gap g between the joining regions can be maintained in the range of 0.1 to 0.2 mm by setting the interval D 1  equal to 30 mm or below when the ridge-shaped protrusions  1   a  are located adjacent to one another. 
     On the other hand, when the ridge-shaped protrusions  1   a  and the trapezoidal protrusions  1   b  are adjacent to one another and when the trapezoidal protrusions  1   b  are adjacent to one another, the gap g between the joining regions can be maintained in the range of 0.1 to 0.2 mm just by setting the interval D 2  equal to or less than 60 mm, which is twice as long as the interval D 1 . These figures are respectively equivalent to five times or less (regarding D 1 ) and ten times or less (regarding D 2 ) in comparison with a diameter of 6 mm applicable to unit spots  2   e  in the case of typical spot welding. Accordingly, when the flat surface is ensured across the section equivalent to the above-described interval D 2 , the number of the protrusions to be formed can be reduced by providing the trapezoidal protrusions  1   b , and multiple sessions of unit laser scanning  2   c  can be performed between the protrusions adjacent to each other. 
     The embodiment has described the example of welding and joining the rear skirt  10  and the tail end member  20  of the automobile. However, the laser lap welding method according to the present invention is also preferably applicable to welding and joining of other vehicle body panels, constituent parts, reinforcing members, and brackets of automobiles, and moreover, to various other parts made of galvanized steel sheet press-formed in a three-dimensional shape. 
     Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.