JP5253777B2 - Overlapping laser welding method and laser welded product - Google Patents

Description

  The present invention relates to a method for laser welding of thin plates and a laser welded product. In particular, the present invention relates to a laser welding method and a laser welded product that can suppress weld cracking in a laser welded portion when laser welding is performed on the vicinity of end surfaces of stacked thin plates.

In recent years, there has been an increasing number of cases in which laser welding is applied in place of spot welding, which is a conventional vehicle body assembly welding method, aiming to increase the functionality and weight of a vehicle body such as an automobile. The advantages of laser welding over spot welding are:
(1) Since laser welding is possible from one side, the degree of freedom of construction is high, and welding is possible even in a structure in which the material to be welded cannot be held by the electrode of spot welding.
(2) Compared to spot welding, which is spot welding, wire welding is possible, and improvement in strength and rigidity of the welded member can be expected.
And so on.

  For example, in Non-Patent Document 1, in a thin plate member having a flange, a flange width of 14 mm or more is required for spot welding, but in the case of laser welding, if a flange width of 8 mm is provided, overlap welding is possible. Have been described. In addition to the above-mentioned advantages, further improvement in member rigidity can be achieved by forming a larger closed cross-section even if the weight is reduced by flangeless (shorter flange width) and the same member width (member cross-sectional area) is provided. It also shows what can be expected.

  As for conventional laser welding of body parts, as described in Non-Patent Document 1, application to a member having a flange width of 8 mm or more has been studied as an alternative to spot welding which is the current main welding method. However, there is no study on laser welding to a member having a flange width of less than 8 mm, and no knowledge about the cracks generated is shown.

  On the other hand, as for cracking at the time of laser welding, for example, as disclosed in Patent Documents 1 to 3, it is intended for thick plates with large generation of welding thermal distortion and steel materials that are likely to generate high temperature cracks. However, little is known about laser weld cracking countermeasures for automotive thin plates, where cracks are smaller.

  In addition, laser welding enables welding with low heat input compared to arc welding methods, etc. Furthermore, with thin steel plates, the amount of metal to be melted is small, and the amount of heat distortion that occurs is extremely low compared to thick plates. Because of the small conditions, no knowledge about the cracking of automotive thin plates is shown.

"The present and the future of laser technology in the automotive industry", European Automotive Laser Application 2006 Proceedings, Germany, Jan. 26-27. JP-A-6-190575 JP 2004-25278 A JP 2004-90091 A

  Accordingly, the present invention provides a superposition laser that prevents welding defects that occur when a laser beam is scanned along the end face and welds the vicinity of the end face in the superposition welding of a thin plate for automobiles, thereby ensuring welding quality and member quality. It is an object to provide a welding method and a laser welded product.

  Since the width of the laser welded portion is usually about 1 mm, the present inventors diligently studied to further reduce the flange, that is, reduce the distance between the laser welded portion and the end face, and the distance from the end face is less than 8 mm. The present invention relating to a laser welding technique in which the vicinity of the end face is overlapped and welded along the end face has been completed. As a result, the weight of the welded product can be reduced, or when the width of the parts is the same, it is possible to improve the member rigidity by increasing the closed cross-sectional area.

Specifically, the present inventors used various steel plates with different strengths of 0.7 mm to 2.0 mm, which are thin plates, superposed two steel plates of the same type, and laser welding positions (laser beam irradiation positions and end faces). And the following findings were obtained by carrying out a test in which the laser beam was scanned along the end face and the vicinity of the end face was welded.
(A) When a portion very close to the end face is welded, the edge is melted, and the molten metal is solidified in a suspended state, and the joint shape (appearance quality) is impaired.
(B) In order not to melt the edge, it is necessary to weld a position at a distance of 1.75 mm or more from the end face. That is, even in the case of laser welding with low welding heat input, when welding with a target position within 1.75 mm from the end face (this is an unmelted portion of 1.0 mm or more with respect to the normal laser weld width ( = 1.75 mm-0.5 mm)), the end face side has a small effect of heat removal, so that the edge becomes hot and melts as welding proceeds.
(C) No welding crack occurs when welding a position 8 mm or more away from the end face.
(D) When the vicinity of the end surface less than 8 mm from the end surface is welded, a crack may occur in the welded portion.

Furthermore, based on this knowledge, the cause of weld cracking was examined in detail, and the following knowledge was obtained.
(E) FIG. 4 shows a state during welding when laser welding (steel plate thickness: 1.4 mm, welding condition: output 4 kW, speed 2.5 m / min) at a position 2 mm from the end face along the end face is performed. And shows the appearance of the cracks that occurred. 4A is a schematic view seen from the steel plate 1 side, and FIG. 4B is a cross-sectional view schematically showing the welded portion, showing a portion where a crack has occurred. In FIG. 4A, welding progressed from right to left on the paper. In FIG. 4, reference numerals 1 and 2 are steel plates 1 and 2, a portion indicated by A is a welded portion (weld bead), and a portion indicated by C is a crack. In addition, cracking occurred in the range indicated by B.
Here, it was found that the crack B occurred about 7 mm behind the laser irradiation point (= about 0.15 seconds after laser irradiation) during the welding. That is, it is considered that cracking occurs at a position where a molten pool formed by irradiation with a laser is solidified, and this occurs in the process of solidifying and cooling the molten pool.

(F) FIG. 5 shows the results of measuring the temperature history after welding on the end face side and the base metal side with the welded part A in between. The temperature was measured by attaching a thermocouple at a position of 1.3 mm from the center of the weld A to the end face side and the base metal side. In FIG. 5, the horizontal axis indicates time (seconds) immediately after welding, and the vertical axis indicates temperature (° C.). FIG. 5 (a) shows the result when a crack is generated by welding at a position 2mm from the end face, and FIG. 5 (b) shows the result when a crack is not generated by welding at a position 4mm from the end face.
As shown in FIG. 5A, when a crack occurs, the welding heat history on the end face side and the base metal side are greatly different. This is because even when laser welding has a low welding heat input and thermal distortion during welding, when the vicinity of the end face is welded, the distance from the welding position (= heat source) to the end face is short on the end face side, and the heat is removed. Is not enough. Therefore, the end surface side is at a higher temperature than the base material side, and the temperature difference between the end surface side and the base material side is large, and cracking occurs. On the other hand, when cracking does not occur, the difference in thermal history between the end face side and the base material side is small as shown in FIG.

  (G) FIG. 6 is an explanatory view schematically showing the mechanism of crack generation. When the difference in thermal expansion between the base metal side and the end face side becomes large, as shown by E in FIG. 6 in particular, the welded portion A is distorted so as to be greatly pulled in the end face direction perpendicular to the generated weld line. The crack C is generated.

  (H) Since cracks during welding are caused by a difference in heat removal between the base metal side and the end surface side, the frequency of occurrence of cracks increases as the distance from the end surface decreases. By welding the position where the distance from the end face is 3.5 mm or more, the occurrence frequency of cracks is greatly reduced.

  (I) In order to prevent cracking, it is important to reduce the amount of strain caused by the temperature difference between the end face side and the base material side. To reduce the amount of strain, the welding speed is less than 2.5 m / min. Performing intermittent welding (stitch welding) that repeatedly forms a welded portion intermittent in the welding direction, welding while cooling the end surface, or welding while pressing the end surface and suppressing thermal deformation on the end surface side It is effective.

Based on the above findings, the inventors have completed the present invention shown below.
According to the first aspect of the present invention, a plurality of thin plates are welded by irradiating a laser beam to an area having a distance from the end surface of 1.75 mm to 3.5 mm along the end surfaces of the stacked thin plates. This is a superposition laser welding method, in which welding is repeatedly formed in an intermittent manner in the welding direction, and welding is performed while cooling only the end face side and the end face by bringing the tool into contact with the end face side and the end face from the weld part. The problem is solved by providing a superposition laser welding method characterized in that welding is performed by simultaneously using end face cooling welding and end face press welding for welding while pressing the end face.

The invention described in claim 2 is characterized in that the thin plate in the superposition laser welding method according to claim 1 has a plate thickness of 0.7 mm or more and 2.0 mm or less.

  According to the present invention, even when a plurality of steel plates are superposed on a thin steel plate and laser welding is performed in the vicinity of the end surface, weld cracking is suppressed, and a welded joint with good appearance quality can be obtained.

  FIG. 1 is a view for explaining a laser welding method and a laser welded product of the present invention according to the first embodiment. The laser welded product 10 is obtained by laser welding of a front side steel plate 11 shown in FIG. 1 and a back side steel plate 12 (not shown in FIG. 1) at a distance indicated by I from FIG. It is integrated. Here, since the laser welded product 10 is welded by so-called stitch welding, the welded portions 13, 13,... And the idle running portions 14, 14,. It has. Here, after stitch welding is performed with a welding length that does not cause cracks to form the welded portion 13, an appropriate free-running distance is provided to form the free-running portion 14, and the welded portion 13 and the free-running portion 14 The cycle for forming is repeated.

  The length of the welded portions 13, 13,... And the length of the idle running portions 14, 14,... Vary depending on welding conditions (output and speed), etc. Set timely. The idle run length is not particularly limited as long as no cracking occurs.

  Moreover, the distance shown by I in FIG. 1 is 1.75 mm or more and less than 8.00 mm. As a result, the weight of the welded product can be reduced as compared with the conventional case, or the member rigidity can be improved by enlarging the closed cross-sectional area when the part width is the same.

  And even if it is welding close to the end in this way, the amount of heat input to the welded material is reduced by performing the above-mentioned stitch welding, and as a result, the amount of distortion generated is reduced and the occurrence of cracks is suppressed. Can be made.

  Here, the steel type of the steel plate to be welded is not particularly limited, and is not particularly limited as long as it is a thin steel plate to which lap welding by laser can be applied. Further, it may be a bare material or a plating material. Here, the thickness of the thin steel plate is preferably 0.7 mm to 2.0 mm.

Further, the type of the laser is not particularly limited as long as it is a laser usually used for laser welding of steel materials, and examples thereof include a CO ₂ laser, a YAG laser, and a fiber laser. The spot diameter (laser irradiation diameter of the steel plate) in laser welding is not particularly limited, but is preferably 0.5 mm to 1.0 mm, and the resulting weld width is usually about 1 mm.

  FIG. 2 is a view for explaining a laser welding method and a laser welded product according to the second embodiment of the present invention. The laser welded products 20 and 20 ′ are integrated by laser welding in which the steel plates 21 and 21 ′ on the front side and the steel plates 22 and 22 ′ on the back side are at the distances indicated by J and J ′ in FIG. ing. Here, the laser welded products 20 and 20 ′ are welded while cooling the end surfaces by performing welding while bringing the tools 25 and 25 ′ into contact with the end surfaces. As a result, the end face is cooled, so that the ultimate temperature on the end face side is lowered, the occurrence of thermal strain can be suppressed, and the crack of the welded portion can be suppressed. Although the range of the edge part cooled here will not be specifically limited if a crack is suppressed, The point of 10 mm going back from the laser irradiation point shown by F and F 'in FIG. It is preferable that it is the whole range of the edge part corresponding to between.

  Further, the distances indicated by J and J ′ in FIG. 2 are 1.75 mm or more and less than 8.00 mm. As a result, the weight of the welded product can be reduced as compared with the conventional case, or the member rigidity can be improved by enlarging the closed cross-sectional area when the part width is the same.

  Here, the steel type of the steel plate to be welded is not particularly limited, and is not particularly limited as long as it is a thin steel plate to which lap welding by laser can be applied. Further, it may be a bare material or a plating material. Here, the thickness of the thin steel plate is preferably 0.7 mm to 2.0 mm.

Further, the type of the laser is not particularly limited as long as it is a laser usually used for laser welding of steel materials, and examples thereof include a CO ₂ laser, a YAG laser, and a fiber laser. The spot diameter (laser irradiation diameter of the steel plate) in laser welding is not particularly limited, but is preferably 0.5 mm to 1.0 mm, and the resulting weld width is usually about 1 mm.

  Here, examples of the tool for cooling welding are shown in FIGS. 2 (a) and 2 (b). The tool shown in FIG. 2 (a) is a fixed tool 25, which has a rectangular parallelepiped block shape, and a recess 26 for receiving the end portions of the steel plates 21 and 22 is formed on one surface thereof. Thereby, the edge part of the steel plates 21 and 22 can be cooled efficiently. The fixed tool 25 is preferably made of a material having high thermal conductivity such as copper or copper alloy. In order to enhance the heat removal effect, it is further desirable to provide a flow path for supplying cooling water to the inside.

  The tool in FIG. 2B is a rotary tool 25 ′, which has a cylindrical shape having a rotation axis parallel to the stacking direction of the steel plates 21 ′ and 22 ′, and has end portions of the steel plates 21 ′ and 22 ′ on the outer peripheral surface. A recess 26 'is formed to receive the. Thereby, the edge part of steel plate 21 ', 22' can be cooled efficiently. The rotary tool 25 'is preferably made of a material having high thermal conductivity such as copper or copper alloy. In order to enhance the heat removal effect, it is further desirable to provide a flow path for supplying cooling water to the inside.

  FIG. 3 is a view for explaining a laser welding method and a laser welded product of the present invention according to the third embodiment. The laser welded products 30 and 30 ′ are integrated by laser welding in which the steel plates 31 and 31 ′ on the front side and the steel plates 32 and 32 ′ on the back side are performed at the distances indicated by K and K ′ in FIG. Yes. Here, the laser welded products 30 and 30 ′ have the tools 35 and 35 ′ in contact with the end faces, and press the tools 35 and 35 ′ toward the welded portions 33 and 33 ′ as indicated by the straight arrows in FIG. 3. While being welded. Thereby, distortion acting on the welded portions 33 and 33 'can be reduced, and cracking of the welded portions 33 and 33' can be suppressed. Although the range of the edge part pressed here will not be specifically limited if a crack is suppressed, The point of 10 mm going back from the laser irradiation point shown by G and G 'in FIG. It is preferable that it is the whole range of the end surface corresponding to between.

  Further, the distances indicated by K and K ′ in FIG. 3 are 1.75 mm or more and less than 8.00 mm. As a result, the weight of the welded product can be reduced as compared with the conventional case, or the member rigidity can be improved by enlarging the closed cross-sectional area when the part width is the same.

  Here, the steel type of the steel plate to be welded is not particularly limited, and is not particularly limited as long as it is a thin steel plate to which laser welding lap welding can be applied. Further, it may be a bare material or a plating material. Here, the thickness of the thin steel plate is preferably 0.7 mm to 2.0 mm.

Further, the type of the laser is not particularly limited as long as it is a laser usually used for laser welding of steel materials, and examples thereof include a CO ₂ laser, a YAG laser, and a fiber laser. The spot diameter (laser irradiation diameter of the steel plate) in laser welding is not particularly limited, but is preferably 0.5 mm to 1.0 mm, and the resulting weld width is usually about 1 mm.

  Here, examples of tools for the press welding are shown in FIGS. 3 (a) and 3 (b). The tool shown in FIG. 3 (a) is a fixed tool 35, which has a rectangular parallelepiped block shape, and a concave portion 36 for receiving the end portions of the steel plates 31 and 32 is formed on one surface thereof. Thereby, the edge part of the steel plates 31 and 32 can be pressed efficiently. The fixed tool 35 only needs to have heat resistance that does not change during laser welding. For example, hot tool steel or heat resistant resin can be used.

  The tool shown in FIG. 3B is a rotary tool 35 ′, which has a cylindrical shape having a rotation axis parallel to the stacking direction of the steel plates 31 ′ and 32 ′ and ends of the steel plates 31 ′ and 32 ′ on the outer peripheral surface. Is formed. Thereby, the edge part of steel plate 31 'and 32' can be pressed efficiently. As the rotary tool 35 ', for example, hot tool steel having high heat resistance is preferably used.

  As described above, the first to third embodiments have been described separately, but actually, these may be performed alone or in combination.

  Further, regarding the welding speed in the laser welding of each of the above-described embodiments, if the welding speed is too high, the molten pool is greatly rearward in the welding progress direction with the center of the welded portion (13, 23, 33, etc.) as an axis. It becomes a shape that pulls. In other words, since the solidification of the molten pool proceeds from the base material toward the center of the welded portion (13, 23, 33, etc.), the end of the welded portion is solidified, and the region where the molten metal remains in the central portion becomes large. Therefore, since distortion occurs here, cracks are likely to occur. In particular, when the welded portion (13, 23, 33, etc.) is at a distance close to the end surface, the generated distortion increases. From the above, the welding speed is preferably 2.5 m / min or less.

  More specifically, the solidification of the molten pool proceeds from the base material toward the center of the welded portion (13, 23, 33, etc.), and solidifies through a state in which the molten metal (liquid phase) remains slightly in the central portion. . At this time, cracking occurs when distortion occurs with a slight liquid phase remaining in the center. For this reason, the positional relationship between the region where the liquid phase slightly remains and the region where the distortion is greatest is important. In lap welding up to a plate thickness of 2.0 mm, in laser welding in which the beam spot diameter is reduced to about 0.6 mm, the area where the molten pool is small and the liquid phase slightly remains is small, but the welding speed increases. Since the molten pool becomes large, as a result, the region where the liquid phase slightly remains also becomes large, and there is a high risk of cracking. For this reason as well, the welding speed is preferably set to 2.5 m / min or less.

  As an example, four types of steel plates having different strengths and thicknesses (soft steel: thickness 1.2 mm, 440 steel: thickness 0.7 mm, 590 steel: thickness 1.4 mm, 780 steel: thickness 1.6 mm) The two steel plates were stacked and welded. Here, including the examples and comparative examples, the laser welding test with N number = 5 was performed by changing the conditions such as the distance from the end face to the laser irradiation position, the presence or absence of stitch welding, the end face cooling or the end face pressing, etc. Carried out.

  In stitch welding, welding was repeated at a predetermined welding speed with a welded part length of 13 mm and an idle running part length of 5 mm. In the end face cooling welding, welding was performed while removing heat using a fixed tool made of copper having a width of 30 mm (direction parallel to the welding direction) in contact with the end faces of the two stacked steel plates. Here, in order to ensure the close contact between the end face and the fixed tool, the fixed tool was pressurized so as to be pressed against the end face with a force of 9.8 N (1 kgf). Further, when the end surface cooling and the end surface pressing were performed simultaneously, the fixed tool was pressed against the end surface with a force of 98 N (10 kgf). In end face pressure welding without end face cooling, a resin plate having a low thermal conductivity, a thickness of 2.0 mm, a length of 30 mm, and a width of 10 mm is attached to the surface of the fixed tool, and the end face is applied with a pressure of 98 N (10 kgf). Was welded while being pressurized.

  In end face cooling and end face pressing, a fixed tool was arranged so as to include an end corresponding to a range of about 10 mm from the welding point in the direction opposite to the welding progress direction.

  As another condition, a YAG laser was used for laser welding, and the machining point output was fixed at 4.0 kW. As for the welding speed, appropriate through-welding conditions were set according to the thicknesses of the stacked steel plates. Here, the laser beam was condensed so as to have a spot diameter of 0.6 mm on the surface of the steel plate, and the obtained weld width was approximately 1.0 mm.

Welding results were evaluated by appearance observation, and the presence or absence of melting of the plate edge and the presence or absence of cracks in the welded portion were evaluated. Specifically, among N = 5 specimens, O: No cracking occurred Δ: 1-4 cracks occurred x: All cracks occurred. Table 1 shows test conditions and test results.

  As shown in Table 1, in any steel type, when the distance from the plate end surface was 1.5 mm (reference numerals 1, 11 and 21), the edge part melted, resulting in poor appearance quality. In the case where the distance from the plate end surface in reference numerals 12 to 16 is 1.75 mm to 3.5 mm, the greater the distance from the plate end surface to the welded portion, the lower the frequency of occurrence of cracks, but at least one specimen The occurrence of cracks was recognized, which was not always preferable.

On the other hand, stitch welding (numeral 4, 18), the end surface cooling welding (numeral 5,7,9,19,23), welding comprising at least one end face pressing welding (numeral 6,7,10,20,24) Then, even if the distance from the plate end surface to the welded portion was 1.75 mm, no crack was observed.

  While the present invention has been described in connection with the most practical and preferred embodiments at the present time, the present invention is not limited to the embodiments disclosed herein, The invention can be changed as appropriate without departing from the scope or spirit of the invention that can be read from the claims and the entire specification, and the overlapping laser welding method and laser welded product accompanying such changes are also included in the technical scope of the present invention. Must be understood as being.

It is a figure for demonstrating stitch welding. It is a figure for demonstrating edge part cooling welding. It is a figure for demonstrating edge part press welding. It is a figure for demonstrating crack generation. It is a figure which shows the result of having measured the temperature history after the welding in the end surface side and base material side on both sides of a welding part. It is explanatory drawing which shows typically the mechanism of a crack generation | occurrence | production.

Explanation of symbols

10, 20, 30 Welded article 11, 12, 21, 22, 31, 32 Steel plate 13 23, 33 Welded part 14 Free running part 25, 35 Fixed tool 25 ', 35' Rotary tool

Claims (2)

A superposition laser welding method for welding a plurality of thin plates by irradiating a laser beam to a region having a distance from the end surface of 1.75 mm to 3.5 mm along the end surfaces of the plurality of superposed thin plates. ,
A stitch weld which forms repeatedly welds intermittently in the welding direction, and the end surface cooling welding for welding while cooling only the end face side and the end face by contacting a tool to the end face and the end face than the weld superimposing the laser welding method and performing welding by using an end face pressing welding welding while pressing the end face, at the same time.   The superposition laser welding method according to claim 1, wherein the thin plate has a thickness of 0.7 mm or more and 2.0 mm or less.

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