JP2018153851A - Laser welding method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a laser welding method capable of improving the strength of a welding part.SOLUTION: When executing welding by laser screw welding on a plurality of steel plates W1, W2 and W3 stacked on top of each other, a welding process for welding the plurality of steel plates W1, W2 and W3, after which a re-heat input process is executed for re-heat-inputting an outermost periphery N1 of a welding nugget N of a welding part. A softened part is expanded by executing the re-heat-input process, so that a hardness of the peripheral part of the welding nugget N can be enhanced. Thus, a strength of the welding part can be improved.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a laser welding method. In particular, the present invention relates to a measure for improving the strength of a welded portion.

  Conventionally, in the manufacturing process of an automobile body, for example, steel plates such as high-tensile steel plates are generally overlapped and the overlapped portions are joined by resistance spot welding.

  By the way, in this resistance spot welding, a heat-affected zone (hereinafter referred to as HAZ) having a hardness lower than that of the base material is formed due to tempering of the structure around the weld nugget by heat generated during welding. Is done. This softening of the HAZ is particularly remarkable in resistance spot welding of a high-tensile steel sheet having a tensile strength of a predetermined value or more.

  FIG. 4 shows experimental results showing the relationship between the tensile strength of the base metal and the peel strength when resistance spot welding is performed. In FIG. 4, the weld nugget diameter is larger in the solid line than in the broken line. As is apparent from FIG. 4, when a steel plate having a base material tensile strength of a predetermined value or more is used, the higher the base material tensile strength, the lower the peel strength. FIG. 5 shows experimental results showing the relationship between the base material tensile strength and the cross tensile strength when resistance spot welding is performed. Also in FIG. 5, the welding nugget diameter is larger in the solid line than in the broken line. As is apparent from FIG. 5, when a steel plate having a base material tensile strength of a predetermined value or more is used, the cross base tensile strength decreases as the base material tensile strength increases.

  Patent Document 1 discloses that resistance spot welding is performed by superposing heat-treated steel sheets, and a welding nugget is tempered by applying a high-frequency current to the weld. Thereby, the ductility and toughness of the welded portion are recovered, and the welded portion is made difficult to break.

Japanese Patent Laid-Open No. 2006-55337

  However, even if the tempering process disclosed in Patent Document 1 is performed, there is a limit to sufficiently increasing the peel strength and the cross tensile strength. That is, as described above, when a high-tensile steel plate having a tensile strength of a predetermined value or higher (for example, a high-tensile steel plate having a tensile strength of 1.5 GPa or higher) is used, the higher the base material tensile strength, the higher the peel strength and the cross tensile strength. It becomes low, and there is a concern about fracture (brittle fracture). In other words, there is a limit to the tensile strength of the high-tensile steel sheet that can be used to sufficiently secure these strengths. Moreover, when performing the tempering process as disclosed in Patent Document 1, the time required for the welding operation becomes long, so that it is not practical in terms of productivity.

  Inventors of the present invention have recently focused on a welding method using laser welding performed in place of or in addition to resistance spot welding, and improving the laser welding method increases the hardness of the HAZ. Considered what to do.

  This invention is made | formed in view of this point, The place made into the objective is to provide the laser welding method which can aim at the improvement of the intensity | strength of a welding part.

  The solution of the present invention for achieving the above object is premised on a laser welding method in which a plurality of steel plates stacked on each other are welded by laser screw welding. The laser welding method includes a welding step of welding the plurality of steel plates by the laser screw welding, and re-entry heat to the outermost peripheral portion of the weld nugget of the welded portion generated in the welding step. And a heat input step.

  By this specific matter, the softened part of the welded part expands to the outer peripheral side by performing the re-entry process after the welding process by laser screw welding, and the hardness of the peripheral part of the weld nugget can be increased. For this reason, the strength of the welded portion can be improved.

  In the present invention, after the welding process by laser screw welding, a re-entry process is performed in which re-entry heat is applied to the outermost peripheral portion of the weld nugget of the welded portion. Thereby, a softening part will expand to an outer peripheral side and the hardness of a welding nugget peripheral part can be raised. As a result, the strength of the welded portion can be improved.

It is a schematic block diagram which shows the laser welding apparatus used for the laser welding which concerns on embodiment. It is sectional drawing of a welding part. It is a figure showing the Vickers hardness of each location in each of the welding part which concerns on embodiment, and the welding part which concerns on a prior art. It is a figure which shows the relationship between base material tensile strength at the time of performing resistance spot welding, and peeling strength. It is a figure which shows the relationship between base material tensile strength at the time of performing resistance spot welding, and cross tensile strength.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the present embodiment, a case where the present invention is applied as a laser welding method by a laser welding apparatus used in a manufacturing process of a car body of an automobile will be described.

-Schematic configuration of laser welding equipment-
FIG. 1 is a schematic configuration diagram showing a laser welding apparatus 1 used for laser welding according to the present embodiment. As shown in FIG. 1, the laser welding apparatus 1 includes a laser oscillator 2, a scanner head 3, a welding robot 4, and a robot controller 5.

  The laser oscillator 2 generates a laser beam. The generated laser beam is guided to the scanner head 3 through the optical fiber cable 21. As the laser beam, for example, a carbon dioxide laser, a YAG laser, a fiber laser, or the like can be used.

  The scanner head 3 irradiates a steel plate (a steel plate on which a plurality of sheets are superposed; a workpiece to be welded) W, which is a workpiece, with a laser beam guided through the optical fiber cable 21 (see the one-dot chain line in FIG. 1). A plurality of mirrors (not shown) are accommodated in the scanner head 3. Each of these mirrors is configured to be rotatable about one rotation axis. The laser beam is scanned by these mirrors, and the laser beam is scanned in a predetermined region of the steel plate W. These mirrors can be configured using, for example, galvanometer mirrors.

  The laser welding method according to this embodiment is laser screw welding (hereinafter referred to as LSW). That is, the laser beam is scanned over the entire circumference around the center position of the welded portion (welding target location) of the steel plate W, and welding is performed by melting the welded portion (this LSW). Will be described later).

  Moreover, LSW in this embodiment is used as what welds the area | region between mutually adjacent resistance spot welding parts, after performing the resistance spot welding of the said steel plate W. FIG. That is, the vehicle body manufactured by the laser welding method according to this embodiment has a configuration in which a plurality of steel plates (vehicle body panels) W are joined by combining resistance spot welding and LSW.

  The welding robot 4 is configured to make the scanner head 3 movable. The welding robot 4 is constituted by an articulated robot. Specifically, the present embodiment includes a base base 41, a rotation mechanism (not shown) housed inside the base base 41, joints 42, 43, 44, and arms 45, 46, 47. . The scanner head 3 can be moved in an arbitrary direction by the rotation operation of the rotation mechanism and the swinging operation of the arms 45, 46, 47 in the joints 42, 43, 44.

  Information for moving the scanner head 3 toward the welding location (information such as the rotation angle amount of each joint 42, 43, 44) is stored in advance in the robot controller 5 by offline teaching. When the vehicle body is transported to the welding process on the vehicle body production line, the welding robot 4 is operated based on the information according to the control signal from the robot controller 5 so that the LSWs are sequentially applied to the respective welding locations. Will be carried out.

-Re-input heat process-
The feature of this embodiment is that the welded portion generated in the LSW after the aforementioned LSW (welding by scanning a laser beam over the entire circumference around the center position of the welded portion of the steel plate W) is performed. This is to perform a re-input heat process for performing re-input heat on the outermost peripheral portion of the welding nugget.

  FIG. 2 is a cross-sectional view of the weld. As shown in FIG. 2, in this embodiment, three steel plates W1, W2, and W3 are superposed and these steel plates W1, W2, and W3 are welded by LSW. In FIG. 2, N is a weld nugget in which a part of the steel plates W1, W2, and W3 is fused and integrated.

  In the re-entry heat process, the outermost peripheral portion of the weld nugget N is scanned by scanning the laser beam from the scanner head 3 over the entire outer periphery of the outermost peripheral portion N1 of the weld nugget N of the weld generated in the LSW. This is performed by sequentially heating N1 in the circumferential direction. The amount of heat input to the outermost peripheral portion N1 of the weld nugget N in the re-input heat process is a heat input amount that can sufficiently increase the hardness by annealing of the outermost peripheral portion N1 of the weld nugget N, and this weld nugget N The amount of heat input that can suppress the softening of the tissue existing further on the outer peripheral side of the outermost peripheral portion N1 is set based on experiments or simulations.

  In FIG. 2, the arrow indicated by a broken line indicates the scanning direction of the laser beam in the LSW (the scanning direction of the laser beam over the entire circumference around the center position of the welded portion of the steel plates W1, W2, and W3). Moreover, the arrow shown as a continuous line in FIG. 2 has shown the incident direction of the heat | fever in a re-entry heat process.

  Thus, the outermost periphery of the weld nugget N of the welded portion generated in the LSW is performed by performing a re-entry heat process for performing re-entry heat on the outermost peripheral portion N1 of the weld nugget N of the welded portion generated in the LSW. The softened portion existing in the part N1 expands to the outer peripheral side, and the outermost peripheral part N1 of the weld nugget N is annealed to increase the hardness and the outermost peripheral part N1 of the weld nugget N. Further, in the structure existing on the outer peripheral side, the tempering amount is small, so that the growth and coarsening of crystal grains are suppressed, the softening of this part is suppressed, and the entire welded part has sufficient hardness. Will be secured.

  In the prior art, when a high-tensile steel plate having a tensile strength of a predetermined value or higher (for example, a high-tensile steel plate having a tensile strength of 1.5 GPa or higher) is used, the higher the base material tensile strength, the higher the peel strength and the cross tensile strength. Was low, and there was concern about fracture (brittle fracture). That is, there is a limit to the tensile strength of the high-tensile steel plate that can be used to sufficiently secure these strengths. On the other hand, according to the present embodiment, sufficient hardness is ensured over the entire welded portion, and therefore, even when a high-tensile steel plate having a tensile strength of a predetermined value or more is used, the fracture ( There is no concern about brittle fracture. That is, the range of the tensile strength of the high-tensile steel sheet that can be used to sufficiently ensure the peel strength and the cross tensile strength can be expanded.

  In addition, since both the LSW and the re-entry heat process can be performed by scanning the laser beam from the scanner head 3, the time required for the welding operation is not significantly increased, and the productivity is deteriorated. There is no. For this reason, a highly practical welding method can be realized.

-Experimental example-
Next, an experimental example performed to confirm the above effect will be described. In this experiment, the welding according to the above-described embodiment (LSW + reheating process), the resistance spot welding as the conventional technique, and the LSW not performing the reheating process as the conventional technique are used. About what was done, it performed by measuring the Vickers hardness of each location of a welding part.

  The experimental results are shown in FIG. In FIG. 3, the point S represents the center position of the welded portion, and both the left and right outer sides are the outer peripheral side of the welded portion. Moreover, in this FIG. 3, a continuous line is a measurement result of the Vickers hardness of each location of the welding part in what performed the welding (LSW + re-input heat process) which concerns on the said embodiment. Moreover, a broken line is a measurement result of the Vickers hardness of each location of the welding part in what performed resistance spot welding (prior art). Moreover, a dashed-dotted line is a measurement result of the Vickers hardness of each location of the welding part in what performed LSW which does not implement a reheat input process (prior art).

  As apparent from FIG. 3, in the prior art (broken line and alternate long and short dash line), softening occurs in the HAZ at a predetermined distance from the center position of the welded portion, and the portion where the hardness is extremely reduced is shown. It existed (see region X surrounded by a two-dot chain line circle in FIG. 3).

  On the other hand, in the present embodiment, there was no portion where the hardness was extremely reduced, and it was confirmed that substantially uniform hardness was obtained over the entire welded portion.

-Other embodiments-
In addition, embodiment disclosed this time is an illustration in all the points, Comprising: It does not become a basis of limited interpretation. Therefore, the technical scope of the present invention is not construed only by the above-described embodiments, but is defined based on the description of the scope of claims. The technical scope of the present invention includes meanings equivalent to the scope of claims and all modifications within the scope.

  For example, in the above embodiment, the case where three steel plates W1, W2, and W3 are welded by LSW has been described as an example. The present invention is not limited to this, and can also be applied to the case where two steel plates are welded by LSW or the case where four or more steel plates are welded by LSW.

  Moreover, the said embodiment demonstrated the case where the laser welding method which concerns on this invention is used in the manufacturing process of the vehicle body of a motor vehicle. The present invention is not limited to this, and can be applied to welding of other steel plates.

  Moreover, the said embodiment demonstrated the case where the steel plate W was welded combining LSW (LSW + re-entry process) based on this invention with resistance spot welding. The present invention is not limited to this, and can also be applied to the case where the steel sheet W is welded only by LSW (LSW + re-heat input process).

  INDUSTRIAL APPLICABILITY The present invention can be applied to a welding method when manufacturing a vehicle body using laser screw welding.

DESCRIPTION OF SYMBOLS 1 Laser welding apparatus W1-W3 Steel plate N Welding nugget N1 The outermost periphery part of a welding nugget

Claims (1)

A laser welding method for welding a plurality of steel plates stacked on each other by laser screw welding,
A welding step of welding the plurality of steel plates by the laser screw welding;
The laser welding method characterized by performing the re-input heat process which performs re-input heat with respect to the outermost periphery site | part of the weld nugget of the welding part produced | generated in the said welding process.

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