JP7513892B2 - Lap fillet welded joint, automobile part, and manufacturing method of lap fillet welded joint - Google Patents

Description

The present invention relates to lap fillet welded joints, automotive parts, and methods for manufacturing lap fillet welded joints.

In general, lap fillet welded joints of thin plates are structurally designed so that tensile stress occurs at the weld toe. Therefore, in conventional lap fillet welded joints, the fatigue strength of the toe has been given great importance. However, in recent years, there has been growing concern about fatigue crack initiation from the root of lap fillet welded joints. Therefore, there is a demand for lap fillet welded joints that have excellent fatigue strength at the toe and can also suppress fatigue crack initiation from the root.

Examples of conventional technologies related to lap fillet welded joints include the following:

Patent Document 1 discloses a method for lap fillet welding of metal plates, characterized in that when lap fillet welding is performed between a front metal plate and a back metal plate that are overlapped on each other, a high-density energy beam is irradiated onto the end face of the front metal plate so that the joining surfaces of the front metal plate and the back metal plate are welded from the end face of the front metal plate toward the back side.

Patent Document 2 discloses a method for welding a T-joint in which the end face of a second welded member is brought into contact with the flat portion of a first welded member to form a T-shape, and the corners of the welded members are laser-welded and arc-welded in one pass from one side, and the laser output as a welding condition for the laser welding is set to 1 to 6 kW, and the laser target position is set based on the laser beam angle and the plate thickness of the second welded member.

Patent Document 3 discloses a non-filler fillet welding method using a laser, characterized in that the optical axis of a laser beam is aligned with a position above the middle part of the thickness of the upper plate at the weld end surface of the upper plate to be fillet welded to the lower plate, and the optical axis of the laser beam is held at an irradiation angle of about 30 degrees to 60 degrees with respect to the plane of the lower plate, and welding is performed in a plasma generating gas atmosphere.

Patent Document 4 discloses a lap fillet welded joint of steel plates, which is formed by overlapping two steel plates so that the end of the upper steel plate is located on the surface of the lower steel plate, and welding the upper plate and the lower plate along the end face of the upper plate, and is characterized in that the end face of the upper plate and the upper plate side surface of the lower plate are connected via the weld metal, and the end face of the lower plate and the lower plate side surface of the upper plate are also connected via the weld metal.

However, none of the techniques described in these patent documents take into consideration the occurrence of fatigue cracks at the root of lap fillet welded joints, nor do they have any special configurations for this purpose.

JP 2003-1454 A JP 2011-36883 A Japanese Patent Application Laid-Open No. 4-270084 JP 2012-183542 A

The objective of the present invention is to provide a lap fillet welded joint, an automobile part, and a method for manufacturing a lap fillet welded joint that has excellent fatigue strength at the toe and can suppress the occurrence of fatigue cracks from the root.

The gist of the present invention is as follows.
(1) A lap fillet welded joint according to one aspect of the present invention is a lap fillet welded joint comprising an upper plate having a thickness _tH , a lower plate having a thickness _tL overlapping the upper plate, and a weld metal joining an end face of the upper plate and a surface of the lower plate on the upper plate side, wherein a toe angle measured on a cross section perpendicular to a weld line is more than 0 degrees and less than 30 degrees, a penetration width measured on the cross section perpendicular to the weld line is 2.50 × _tL or greater, a lapped surface between the upper plate and the lower plate has an unwelded portion at an end of the lower plate, and a shortest distance measured on the cross section perpendicular to the weld line between a root and a surface of the weld metal is _tH or greater.
(2) In the lap fillet welded joint described in (1) above, one or both of the thickness t _H of the upper plate and the thickness t _L of the lower plate may be 0.8 to 4.0 mm.
(3) In the lap fillet welded joint according to (1) or (2) above, a width of the unwelded portion measured on the cross section perpendicular to the weld line may be 0.3×t _L or greater.
(4) An automobile component according to another aspect of the present invention includes the lap fillet welded joint according to any one of (1) to (3) above.
(5) A manufacturing method of a lap fillet welded joint according to another aspect of the present invention includes the steps of: overlapping an upper plate having a thickness _tH and a lower plate having a thickness _tL ; laser welding an end face of the upper plate to a surface of the lower plate on the upper plate side; and, after the laser welding, gas-shielded arc welding the end face of the upper plate to the surface of the lower plate on the upper plate side, wherein a laser incidence angle in the laser welding is set to 10 to 30 degrees, a torch inclination angle in the gas-shielded arc welding is set to 45 to 70 degrees, and (I×E)/(V A ×t L )+P/(V L ×t L ) is calculated from a laser welding output P (W) and a laser welding speed V _L (mm/ _s ) in the laser welding, a current value I (A), a voltage value E (V) and an arc welding speed V _A (mm/s) in the gas- _shielded arc welding, and the thickness t _L (mm) of the lower _{plate .} ) is set to 0.100 to 0.250 kJ/ ^mm2 , a ratio P/(I×E) of the heat input of the laser welding to the heat input of the gas shielded arc welding is set to 0.8 to 4.5, and an unwelded portion is provided at an end of the lower plate at the overlapping surface between the upper plate and the lower plate.
(6) In the method for manufacturing a lap fillet welded joint described in (5) above, the laser welding and the gas shielded arc welding may be laser-first laser-arc hybrid welding.
(7) In the method for manufacturing a lap fillet welded joint described above in (6), a forward thrust angle of the torch in the gas shielded arc welding may be 20 degrees or less, and a backward thrust angle of a laser welding head in the laser welding may be 30 degrees or less.
(8) In the manufacturing method of a lap fillet welded joint described in (6) or (7) above, the target position of the laser welding may be a region on the end face of the upper plate between the center of plate thickness and the end of the lower plate.
(9) A method for manufacturing a lap fillet welded joint according to another aspect of the present invention includes the steps of: overlapping an upper plate having a thickness _tH and a lower plate having a thickness _tL ; and gas-shielded arc welding an end face of the upper plate and a surface of the lower plate on the upper plate side, wherein (I×E)/(V A ×t H ) calculated from _the gas-shielded arc welding current value I (A), voltage value E (V), and arc welding speed V _A (mm/s) and the thickness t _H ( _mm ) of the upper plate is set to 0.200 to 0.300 (kJ/mm ² ), and the distance between a target position of the gas-shielded arc welding and an end of the end face of the upper plate on the lower plate side is set to -0.20×t _H to -0.70×t _H , and an unwelded portion is provided at the end of the lower plate at the overlapping surface between the upper plate and the lower plate.

The present invention provides a lap fillet welded joint, an automobile part, and a method for manufacturing a lap fillet welded joint that has excellent fatigue strength at the toe and can suppress fatigue crack initiation from the root.

FIG. 2 is a schematic diagram of a cross section perpendicular to the weld line of a lap fillet welded joint according to one embodiment of the present invention. FIG. 2 is a schematic diagram of a cross section perpendicular to the weld line of a lap fillet welded joint having no unwelded portions. FIG. 2 is a cross-sectional schematic diagram showing a state in which a convex deformation occurs on the upper plate side or the lower plate side in a lap fillet welded joint having no unwelded portion. FIG. 2 is a cross-sectional schematic diagram showing a state in which a convex deformation occurs on the upper plate side or the lower plate side in a lap fillet welded joint having an unwelded portion. FIG. 1 is a schematic diagram of the stress range at the root when cyclic stress is applied to a lap fillet welded joint. FIG. 2 is an enlarged cross-sectional view of the vicinity of the toe of a lap fillet welded joint. FIG. 2 is a schematic diagram of laser-arc hybrid welding in a cross section perpendicular to the weld line. FIG. 2 is a schematic diagram of laser-arc hybrid welding in a cross section perpendicular to the weld surface of the lower plate and along the welding direction. FIG. 2 is a cross-sectional schematic diagram showing a target position for gas-shielded arc welding when producing a lap fillet welded joint by gas-shielded arc welding alone.

First, the terms used in this embodiment are explained below.

The lap fillet welded joint in this embodiment is defined as a lap joint manufactured by fillet welding. A lap joint is a joint in which parts are placed parallel to each other at an angle of 0 degrees ≦ α ≦ 5 degrees and overlap each other (JIS Z 3001-1:2018). A fillet weld is a weld that has a triangular cross section between the members without providing a groove in a lap joint, T-joint, corner joint, etc. (JIS Z 3001-1:2018). Therefore, the weld metal 13 of the lap fillet welded joint 1 according to this embodiment is a fillet weld metal.

The weld metal is a part of the weld that melts and solidifies during welding (JIS Z 3001-1:2018). The weld metal is the metal that has migrated from the filler metal to the weld (JIS Z 3001-1:2018), and is a different concept from the weld metal. In this embodiment, the weld metal 13 includes the upper plate 11, the lower plate 12, and the weld metal that melted and solidified during welding.

The weld line is an imaginary line when a bead or a welded portion is represented as a single line (JIS Z 3001-1:2018). In this embodiment, the toe angle _θWT is defined as an acute angle formed by a straight line b formed by an intersection X of a parallel line a, which is 0.3 mm away from the surface of the lower plate 12 on the upper plate 11 side, and the surface of the weld metal 13, and a toe 131 on the lower plate 12 side, and a straight line c along the surface of the lower plate 12 on the upper plate 11 side (see FIG. 6).

The toe is the point where the surface of the base metal intersects with the surface of the weld bead (JIS Z 3001-7:2018), and here, in this embodiment, the unwelded parts of the upper plate 11 and the lower plate 12 correspond to the base metal, and the weld metal 13 corresponds to the weld bead. Therefore, the lap fillet welded joint 1 according to this embodiment has a toe on the upper plate 11 side and a toe on the lower plate 12 side. As is clear from its definition, the above-mentioned toe angle θ _WT represents the shape of the toe on the lower plate 12 side. Hereinafter, unless otherwise specified, simply referring to "toe 131" refers to the toe 131 on the lower plate 12 side.

In this embodiment, the penetration width W is defined as the distance between the toe 131 on the lower plate 12 side and the root 132, measured on a cross section perpendicular to the weld line (see FIG. 1). In this embodiment, the root 132 is defined as the intersection of the surface of the lower plate 12 on the upper plate 11 side and the surface of the weld metal 13 at the overlapping surface 14 between the upper plate 11 and the lower plate 12. However, when the weld metal 13 is formed beyond the end of the lower plate 12 and there is no boundary between the surface of the lower plate 12 on the upper plate 11 side and the weld metal 13 (see FIG. 2), for the sake of convenience, the root 132 is defined as the intersection of the weld metal 13 and an imaginary line along the surface of the lower plate 12 on the upper plate 11 side at the overlapping surface 14 between the upper plate 11 and the lower plate 12.

In this embodiment, the throat thickness is defined as the shortest distance between the root 132 and the surface of the weld metal 13, measured on a cross section perpendicular to the weld line. Note that in the definition of throat thickness, the "surface of the weld metal 13" does not include the boundary between the weld metal 13 and the base metal.

In this embodiment, the incidence angle θ _b of the laser is defined as a narrow angle formed between the optical axis of the laser light and the surface of the lower plate 12 on the upper plate 11 side in a plane perpendicular to the weld line. The inclination angle θ _a of the torch 3 is defined as a narrow angle formed between the central axis of the torch 3 and the surface of the lower plate 12 on the upper plate 11 side in a plane perpendicular to the weld line. For reference, the inclination angle θ _a of the torch 3 and the incidence angle θ _b of the laser are shown in Fig. 7. Fig. 7 is a schematic diagram of the upper plate 11 and the lower plate 12, the laser welding head 2, and the torch 3 on a plane perpendicular to the weld line.

The advance angle ψ _a of the torch 3 in this embodiment is defined as an angle between the central axis of the torch 3 and a normal line to the surface of the lower plate 12 on the upper plate 11 side in a cross section perpendicular to the welding surface of the lower plate and along the welding direction, and is positive when the angle is inclined in the opposite direction to the welding direction. The sweep back angle ψ _b of the laser welding head 2 in this embodiment is defined as an angle between the optical axis of the laser light 21 and a normal line to the surface of the lower plate 12 on the upper plate 11 side in a cross section along the welding direction, and is positive when the angle is inclined in the welding direction. For reference, FIG. 8 shows the advance angle ψ _a of the torch 3 and the sweep back angle ψ _b of the laser welding head 2. FIG. 8 is a schematic diagram of the upper plate 11 and the lower plate 12, and the laser welding head 2 and the torch 3 in a plane parallel to the plate surface of the lower plate 12.

Laser-arc hybrid welding is a type of welding in which a laser beam and an arc are irradiated simultaneously to form a single molten pool. When the laser beam precedes the welding direction, this is called laser-first (JIS Z 3001-5:2013).

The target position 32 for gas-shielded arc welding in this embodiment is defined as the intersection between the central axis of the torch 3 and the surface of the lower plate 12 on the upper plate 11 side. On the other hand, the target position for laser welding in this embodiment is defined as the intersection between the optical axis of the laser light 21 and the end face of the upper plate 11. Note that there are cases where the optical axis of the laser light 21 does not intersect with the actual end face of the upper plate 11 (i.e., when the laser light 21 is irradiated to the lower plate 12), but in such cases, the intersection between the optical axis of the laser light 21 and a virtual plane including the end face of the upper plate 11 is regarded as the target position for laser welding.

Next, the technical concept of the lap fillet welded joint according to this embodiment will be explained. The inventors have conducted extensive research into means for improving the fatigue strength of the toes of lap fillet welded joints and further suppressing fatigue crack initiation from the root. As a result, the inventors have come to the following findings.

(i) Shape of the toe 131 First, it is necessary to give a gentle shape to the toe 131 on the lower plate 12 side of the lap fillet welded joint 1. This makes it possible to alleviate stress concentration at the toe 131 on the lower plate 12 side and increase its fatigue strength.

(ii) Penetration width W
However, when stress that opens the overlap surface 14 between the upper plate 11 and the lower plate 12 acts on the lap fillet welded joint 1, even if the shape of the toe 131 is made gentle, stress concentration occurs at the root 132, and there is a risk of fatigue cracks occurring from the root 132. Therefore, the inventors conducted further studies and found that the stress generated at the root 132 can be reduced by making the penetration width W larger than usual.

(iii) Unwelded portion 141
However, after further investigation, the inventors found that fatigue cracks can occur at the root 132 even in a lap fillet welded joint 1 having a large penetration width W. The inventors then found that fatigue cracks are likely to occur from the root 132 when the weld metal 13 is formed beyond the end of the lower plate 12 as shown in Fig. 2. The inventors then found that fatigue cracks from the root 132 can be more effectively suppressed by providing an unwelded portion 141 at the end of the lower plate 12 at the overlapping surface 14 between the upper plate 11 and the lower plate 12.

The reason why the unwelded portion 141 can more effectively suppress fatigue cracks from the root 132 is believed to be as follows.

Stress is applied to the root 132 when convex deformation toward the lower plate 12 occurs in the lap fillet welded joint 1, and when convex deformation toward the upper plate 11 occurs in the lap fillet welded joint 1. Convex deformation toward the lower plate 12 opens the overlapping surface 14, and convex deformation toward the upper plate 11 closes the overlapping surface 14. Repeated convex deformation toward the upper plate 11 and convex deformation toward the lower plate 12 may cause fatigue fracture starting from the root 132.

For deformations that are convex toward the lower plate 12, the lap fillet welded joint 1 without the unwelded portion 141 is considered to be more advantageous. This is because high stress concentration occurs at the end of the unwelded portion 141 (i.e., the root 132) (see the schematic diagrams on the left side of Figures 3 and 4). On the other hand, for deformations that are convex toward the upper plate 11, the lap fillet welded joint 1 with the unwelded portion 141 is considered to be more advantageous. The unwelded portion 141 comes into contact and distributes the compressive stress, so the stress concentration at the root 132 is alleviated (see the schematic diagrams on the right side of Figures 3 and 4).

Here, the inventors performed a simulation to estimate the stress in the root 132, and found that the stress range of the root 132 in the lap fillet welded joint 1 with the unwelded portion 141 is smaller than that in the lap fillet welded joint 1 without the unwelded portion 141. The stress range is the difference between the maximum compressive stress (negative value) that occurs in the root 132 when a convex deformation occurs on the upper plate 11 side, and the maximum tensile stress (positive value) that occurs in the root 132 when a convex deformation occurs on the lower plate 12 side. Note that stress in the direction that opens the root 132 is considered to be a positive stress, and stress in the direction that closes the root 132 is considered to be a negative stress.

Figure 5 shows a schematic diagram of the stress range at the root 132 when repeated stress is applied to the lap fillet welded joint 1. In the schematic diagram of Figure 5, the horizontal axis is elapsed time, and the vertical axis is stress (stress in the direction of opening the root 132 is considered to be positive stress, and stress in the direction of closing the root 132 is considered to be negative stress). Therefore, the schematic diagram of Figure 5 shows the change in stress over time at the root 132 when repeated stress is applied to the lap fillet welded joint 1. The curve shown by the dashed line shows the change in stress over time in a lap fillet welded joint where the lap surface remains (i.e., there is an unwelded portion), and the curve shown by the solid line shows the change in stress over time in a lap fillet welded joint where the lap surface is completely melted (i.e., there is no unwelded portion). The above-mentioned stress range is equal to the vertical width (height difference between the peak and the valley) of the graph showing the stress change.

As shown in FIG. 5, in the lap fillet welded joint 1 having the unwelded portion 141, the positive stress is larger but the negative stress is significantly smaller than in the lap fillet welded joint 1 not having the unwelded portion 141. Therefore, the stress range of the lap fillet welded joint 1 having the unwelded portion 141 is smaller than that of the lap fillet welded joint 1 not having the unwelded portion 141. As a result, in a repeated stress application environment in which positive stress and negative stress alternate, it is estimated that the lap fillet welded joint 1 having the unwelded portion 141 can more effectively suppress fatigue cracks in the root 132 than one not having the unwelded portion 141.

(iv) Throat thickness In addition to suitably controlling the shape of the toe 131, the penetration width W, and the unwelded portion 141, it is necessary to supply a sufficient amount of weld metal to the weld to ensure the throat thickness. If the throat thickness is insufficient, the stress generated in the weld metal 13 increases, and both the static strength and fatigue strength may decrease.

A lap fillet welded joint 1 according to one embodiment of the present invention, obtained based on the above findings, comprises an upper plate 11 having a thickness _tH , a lower plate 12 having a thickness _tL overlapping the upper plate 11, and a weld metal 13 joining an end face of the upper plate 11 and a surface of the lower plate 12 on the upper plate 11 side, as shown in Fig. 1, in which the toe angle _θWT measured on a cross section perpendicular to the weld line is more than 0 degrees and less than 30 degrees, the penetration width W measured on a cross section perpendicular to the weld line is 2.50 x _tL or more, the overlapping surface 14 of the upper plate 11 and the lower plate 12 has an unwelded portion 141 at an end of the lower plate 12, and the shortest distance measured on a cross section perpendicular to the weld line between a root 132 and the surface of the weld metal 13 is _tH or more. The lap fillet welded joint 1 according to this embodiment will be described in detail below.

The lap fillet welded joint 1 according to this embodiment comprises an upper plate 11, a lower plate 12 overlapping the upper plate 11, and a weld metal 13 joining the end face of the upper plate 11 and the surface of the lower plate 12 facing the upper plate 11.

The types of the upper plate 11 and the lower plate 12 are not particularly limited and can be appropriately selected according to the application of the lap fillet welded joint 1. For example, it is preferable that the upper plate 11 and the lower plate 12 are high-strength steel plates used as materials for mechanical structural parts to which repeated displacement is applied. Specifically, one or both of the upper plate 11 and the lower plate 12 may be hot-rolled steel plates with tensile strengths of 780 MPa, 980 MPa, or 1180 MPa. The types of the upper plate 11 and the lower plate 12 may be the same or may be different within the range in which welding is possible. One or both of the upper plate 11 and the lower plate 12 may have a zinc-based plating, an aluminum-based plating, or the like. In addition, after the completion of the lap fillet welding, a chemical conversion coating, a paint coating, or the like may be provided on the surfaces of the upper plate 11 and the lower plate 12. Alternatively, a blasting treatment such as shot blasting may be performed on the surfaces of the upper plate 11 and the lower plate 12.

The thickness t _H of the upper plate 11 and the thickness t _L of the lower plate 12 are also not particularly limited and can be appropriately selected depending on the application of the lap fillet welded joint 1. For example, when the lap fillet welded joint according to this embodiment is applied to an automobile part, it is preferable that one or both of the thickness t _H of the upper plate 11 and the thickness t _L of the lower plate 12 are 0.8 to 4.0 mm. The thickness t _H of the upper plate 11 and the thickness t _L of the lower plate 12 may be 1.0 mm or more, 1.2 mm or more, or 1.8 mm or more. The thickness t _H of the upper plate 11 and the thickness t _L of the lower plate 12 may be 3.5 mm or less, 3.2 mm or less, or 2.9 mm or less. The thicknesses of the upper plate 11 and the lower plate 12 may be the same or different.

The composition of the weld metal 13 is not particularly limited and can be appropriately selected depending on the composition of the upper plate 11 and the lower plate 12. The surface of the weld metal 13 may be subjected to various blasting treatments such as shot blasting, or may be provided with zinc-based plating, aluminum-based plating, a chemical conversion coating, a paint coating, or the like.

(i) Shape of the toe 131 In the lap fillet welded joint 1 according to this embodiment, the toe angle θ _WT measured on a cross section perpendicular to the weld line is made to be more than 0 degrees and less than 30 degrees. If the toe angle θ _WT is 30 degrees or more, stress concentration at the toe 131 becomes excessive, and the fatigue strength of the toe 131 becomes insufficient. For this reason, the toe angle θ _{WT is made to be less than 30 degrees. The toe angle θ WT may} also be 28 degrees or less, 25 degrees or less, 22 degrees or less, or 20 degrees or less.

The more gentle the toe 131 is, the higher the fatigue strength is. Therefore, the lower limit of the toe angle θ _WT is not particularly limited as long as it is greater than 0 degrees. On the other hand, the toe angle θ _WT may be 5 degrees or more, 8 degrees or more, or 10 degrees or more.

(ii) Penetration width W
In the lap fillet welded joint 1 according to this embodiment, the penetration width W measured in a cross section perpendicular to the weld line is set to 2.50× _tL or more. When a stress that opens the lap surface 14 between the upper plate 11 and the lower plate 12 acts on the lap fillet welded joint 1, stress concentration occurs at the root 132, and there is a risk of fatigue cracks occurring from the root 132. Here, by increasing the penetration width W, specifically by setting it to 2.50 times or more the thickness _tL of the lower plate, it is possible to reduce the stress occurring at the root 132 and suppress the occurrence of fatigue cracks. The penetration width W may be set to 2.80 times or more, 3.00 times or more, or 3.50 times or more the thickness _tL of the lower plate.

On the other hand, if the penetration width W is excessive, it becomes difficult to secure the unwelded portion 141 described below. However, as long as the unwelded portion 141 is provided at the end of the lower plate 12, a larger penetration width W is preferable because it increases the joint strength. Therefore, there is no upper limit to the penetration width W. However, the penetration width W may be specified to be 6.0 times or less, 5.5 times or less, or 5.0 times or less of the thickness _tL of the lower plate.

(iii) Unwelded portion 141
In the lap fillet welded joint 1 according to this embodiment, the overlapping surfaces 14 between the upper plate 11 and the lower plate 12 have an unwelded portion 141 at the end of the lower plate 12. This makes it possible to more effectively suppress fatigue cracks from the root 132. It is presumed that the reason that the unwelded portion 141 can more effectively suppress fatigue cracks from the root 132 is that, as described above, the provision of the unwelded portion 141 reduces the stress range at the root 132 (see FIG. 5 ), thereby suppressing the load on the root 132 in a repeated stress application environment.

The width w of the unwelded portion 141 measured in a cross section perpendicular to the weld line is not particularly limited as long as it is greater than 0 mm. When a load is applied that closes the root 132, the pressure with which the unwelded portion 141 comes into contact increases the closer it is to the root 132. Therefore, even if there is only a small amount of unwelded portion 141, the effect of reducing compressive stress can be obtained. If forced to say, the width w of the unwelded portion 141 may be _0.3tL or more, _0.4tL or more, or _0.5tL or more.

On the other hand, the smaller the width w of the unwelded portion 141, the smaller the overlap, and the more the weight of the lap fillet welded joint 1 can be reduced. For example, when the lap fillet welded joint 1 according to this embodiment is applied to an automobile part, the smaller the width w of the unwelded portion 141, the less the weight of the automobile part will be, and the more fuel efficient the automobile will be, which is preferable. For the above reasons, it is preferable to set the overlap to 20 mm or less, 15 mm or less, or 10 mm or less. The upper limit value of the width w of the unwelded portion 141 can be appropriately selected within a range in which the overlap before welding is within a preferable range and the above-mentioned specified penetration width W can be secured.

(iv) Throat thickness In the lap fillet welded joint 1 according to this embodiment, the shortest distance (throat thickness) between the root 132 and the surface of the weld metal 13 measured in a cross section perpendicular to the weld line is _tH or more. As described above, _tH is the thickness of the upper plate 11. In Fig. 1, an arc of radius _tH centered on the root 132 is shown. In the lap fillet welded joint 1 in which the entire surface of the weld metal 13 is outside this arc, a throat thickness of _tH or more is ensured.

By supplying a sufficient amount of deposited metal to the weld and setting the throat thickness to _tH or more, the static strength and fatigue strength of the lap fillet welded joint 1 can be ensured. The larger the throat thickness, the greater the stress that the weld metal can bear, so the throat thickness may be 1.1× _tH or more, 1.2× _tH or more, or 1.3× _tH or more. However, if the throat thickness is excessive, there is a risk that the stress concentration at the toe on the upper plate 11 side will become significant. Therefore, the throat thickness may be 1.6× _tH or less, 1.5× _tH or less, or 1.4× _tH or less.

In addition, in the lap fillet welded joint 1 according to this embodiment, the above-mentioned requirements do not need to be met over the entire length of the weld bead. For example, in one lap fillet welded joint 1, the above-mentioned configuration may be applied to a portion where particular strength is required, and a normal configuration may be applied to other portions. Therefore, a lap fillet welded joint in which the above-mentioned requirements are met in a portion of the weld bead is considered to be a lap fillet welded joint 1 according to this embodiment.

Next, an automobile part according to another aspect of the present invention will be described. The automobile part according to another aspect of the present invention includes the lap fillet welded joint 1 according to this embodiment. As a result, the automobile part according to this embodiment has excellent fatigue strength at the toe and can further suppress the occurrence of fatigue cracks from the root. The automobile part is, for example, a front lower arm and a suspension member (subframe). However, the use of the lap fillet welded joint 1 according to this embodiment is not particularly limited, and it can be applied to various mechanical structural parts, etc.

Next, a method for manufacturing a lap fillet welded joint according to another aspect of the present invention will be described. According to this manufacturing method, the lap fillet welded joint according to this embodiment can be suitably obtained. However, the following description does not limit the scope of the lap fillet welded joint according to this embodiment. In other words, a lap fillet welded joint that satisfies the above requirements is considered to be a lap fillet welded joint according to this embodiment, regardless of its manufacturing method. The manufacturing method described below is an example of a suitable manufacturing method for the lap fillet welded joint according to this embodiment.

A method for manufacturing a lap fillet welded joint according to another aspect of the present invention includes the steps of: overlapping an upper plate 11 having a thickness _tH and a lower plate 12 having a thickness _tL ; laser welding an end face of the upper plate 11 and a surface of the lower plate 12 facing the upper plate 11; and, after the laser welding, gas-shielded arc welding the end face of the upper plate 11 and the surface of the lower plate 12 facing the upper plate 11; wherein the laser incidence angle _θb in the laser welding is set to 10 to 30 degrees, the inclination angle _θa of the torch 3 in the gas-shielded arc welding is set to 45 to 70 degrees, and (I×E)/(V A ×tL)+P/(V _L ×tL) calculated from the laser welding output P (W) and laser welding speed V L (mm/ _{s )} in the laser welding, the current value I (A), voltage value E (V) and arc welding speed V _A (mm/s) in the gas- _shielded arc welding, and the thickness _tL of the lower plate 12 is set to 0.100 to 0.250 kJ/mm _. ² , the ratio P/(I×E) of the heat input of laser welding to the heat input of gas shielded arc welding is 0.8 to 4.5, and a non-welded portion 141 is provided at the end of the lower plate 12 at the overlapping surface 14 between the upper plate 11 and the lower plate 12.

First, the upper plate 11 having a thickness _tH and the lower plate 12 having a thickness _tL are overlapped. As described above, the upper plate 11 and the lower plate 12 are not particularly limited. When overlapping, the overlap (the width of the overlap surface 14 measured in a direction perpendicular to the weld line) may be determined. In the manufacturing method of the lap fillet welded joint according to this embodiment, a non-welded portion 141 is provided at the end of the lower plate 12 at the overlap surface 14 between the upper plate 11 and the lower plate 12. By increasing the overlap, the non-welded portion 141 can be secured.

However, the smaller the overlap, the smaller the weight of the lap fillet welded joint 1, which is preferable. For example, when the lap fillet welded joint 1 according to the present embodiment is applied to an automobile part, the smaller the overlap, the smaller the weight of the automobile part and the better the fuel efficiency of the automobile, which is preferable. Therefore, it is preferable to appropriately select the overlap necessary to secure the unwelded portion 141 according to the welding conditions of the laser welding and gas shielded arc welding described later. For example, the overlap may be set to 3 times or less the total thickness of the upper plate 11 and the lower plate 12 (i.e., 3.0 x ( _tH + _tL ) or less).

Next, the end face of the upper plate 11 and the surface of the lower plate 12 facing the upper plate 11 are laser welded. Here, by setting the laser incidence angle θ _b to 30 degrees or less, a molten pool is generated that reaches deep into the upper plate 11 and the lower plate 12, and a wide penetration width W can be ensured. However, if the laser incidence angle θ _b is less than 10 degrees, the laser irradiation point changes significantly even with a slight deviation in the target position, making it difficult to perform stable welding. Therefore, the laser incidence angle θ _b in laser welding is set to 10 to 30 degrees.

Furthermore, the end face of the upper plate 11 and the surface of the lower plate 12 on the upper plate 11 side are gas-shielded arc-welded. Note that during gas-shielded arc welding, the end face of the upper plate 11 and the surface of the lower plate 12 on the upper plate 11 side may become a molten pool or weld metal by laser welding. In this case, the gas-shielded arc welding that forms a fillet between the location corresponding to the end face of the upper plate 11 and the location corresponding to the surface of the lower plate 12 on the upper plate 11 side is considered to be gas-shielded arc welding between the end face of the upper plate 11 and the surface of the lower plate 12 on the upper plate 11 side.

By gas-shielded arc welding, the deposited metal is transferred from the filler metal 31 to a depression formed by the end face of the upper plate 11 and the surface of the lower plate 12 facing the upper plate 11. Here, by setting the inclination angle θ _a of the torch 3 to 45 degrees or more, the throat thickness of the weld metal 13 can be ensured. Also, by setting the inclination angle θ _a of the torch 3 to 70 degrees or less, the shape of the toe 131 can be made smooth. Therefore, the inclination angle θ _a of the torch 3 in gas-shielded arc welding is set to 45 to 70 degrees.

Gas-shielded arc welding must be performed after laser welding. If laser welding is performed after gas-shielded arc welding, the deposited metal transferred from the filler material 31 to the weld will prevent the formation of a molten pool that extends deep into the upper plate 11 and the lower plate 12. In addition, the type of filler material 31 used in gas-shielded arc welding is not particularly limited, and can be selected appropriately depending on the composition of the upper plate 11 and the lower plate 12.

In the manufacturing method of the lap fillet welded joint according to this embodiment, the laser welding conditions and the gas-shielded arc welding conditions satisfy the following requirements (a) and (b).
(a) (I×E)/(VA× _tL )+P/(VL×tL) calculated from the laser welding output P (W) and laser welding speed VL (mm/s) in laser welding, the current value I (A), voltage value _E ( _V ) and arc welding speed _VA (mm/s) in gas shielded arc welding, and the thickness _tL of the lower plate 12 is set to 0.100 to 0.250 _kJ / _mm2 ^.
(b) The ratio P/(I×E) of the heat input of laser welding to the heat input of gas shielded arc welding is set to 0.8 to 4.5.

(I×E)/( _VA × _tL )+P/( _VL × _tL ) is an index established to define the total heat input of laser welding and gas-shielded arc welding. P/(I×E) is an index established to define the distribution of the heat input of laser welding and gas-shielded arc welding. By setting both of these indexes within the above-mentioned ranges, it is possible to obtain a weld metal 13 that combines the wide penetration resulting from laser welding with the smooth toe 131 and large throat thickness resulting from gas-shielded arc welding.

As long as requirements (a) and (b) are satisfied, the welding conditions are not particularly limited. Examples of preferred welding conditions are as follows. The current value I in gas-shielded arc welding may be within a range of 50 to 250 A. The voltage value E in gas-shielded arc welding may be within a range of 18 to 35 V. The arc welding speed V _A in gas-shielded arc welding may be within a range of 0.5 to 3.0 m/min. The laser welding output P in laser welding may be within a range of 4 to 10 kW. The laser welding speed V _L in laser welding may be within a range of 0.5 to 3.0 m/min.

In the manufacturing method of the lap fillet welded joint according to this embodiment, the laser welding and gas-shielded arc welding are preferably performed by laser-first laser-arc hybrid welding. A schematic diagram of laser-arc hybrid welding is shown in FIG. 7.

In laser-first laser-arc hybrid welding, the portion to be gas-shielded arc-welded is previously turned into a molten pool by laser welding, so that the arc in the gas-shielded arc welding is stable. Therefore, compared with the case where gas-shielded arc welding is performed alone, in laser-first laser-arc hybrid welding, the arc welding speed can be significantly improved, and the manufacturing efficiency of the lap fillet welded joint 1 can be improved. In addition, in normal laser-arc hybrid welding, it is considered that the best welding efficiency is achieved by making the incidence angle θ _b of the laser and the inclination angle θ _a of the torch 3 the same, unless there is a special reason. However, in the manufacturing method of the lap fillet welded joint according to this embodiment, it is necessary to make the incidence angle θ _b of the laser and the inclination angle θ _a of the torch 3 different from each other, as described above.

When laser welding and gas-shielded arc welding are performed as laser-first laser-arc hybrid welding, it is preferable to appropriately set the advance angle ψ _a of the torch 3 and the retreat angle ψ _b of the laser welding head 2 in order to avoid interference between the laser welding head 2 and the torch 3 (see FIG. 8). On the other hand, if the advance angle ψ _a of the torch 3 and the retreat angle ψ _b of the laser welding head 2 are too large, there is a concern that porosity defects will occur due to poor shielding caused by a wide molten pool. Therefore, it is preferable to set the advance angle ψ _a of the torch 3 in gas-shielded arc welding to 20 degrees or less, and the retreat angle ψ _b of the laser welding head 2 in laser welding to 30 degrees or less.

In normal lap fillet welding, unless there is a special reason, it is considered best to set the recess formed by the end face of the upper plate 11 and the surface of the lower plate 12 as the target position in order to ensure joint strength. On the other hand, in the manufacturing method of the lap fillet welded joint according to this embodiment, when the laser welding and gas-shielded arc welding are laser-first laser-arc hybrid welding, it is preferable to set the target position of the laser welding in the area between the center of the plate thickness and the end of the lower plate 12 on the end face of the upper plate 11. In other words, it is preferable to set the target position of the laser welding in the lower half area of the end face of the upper plate 11. This makes it easier to ensure a predetermined penetration width W. On the other hand, the target position 32 of the gas-shielded arc welding is not particularly limited, and the conditions of normal lap fillet welding (for example, the recess formed by the end face of the upper plate 11 and the surface of the lower plate 12) can be appropriately set.

So far, the method for manufacturing a lap fillet welded joint using both laser welding and gas-shielded arc welding has been described, but the lap fillet welded joint according to this embodiment can also be obtained using only gas-shielded arc welding. A method for manufacturing a lap fillet welded joint according to another aspect of the present invention includes the steps of overlapping an upper plate 11 having a thickness _tH and a lower plate 12 having a thickness _tL , and gas-shielded arc welding the end face of the upper plate 11 and the surface of the lower plate 12 on the upper plate 11 side, wherein I×E/(V _A ×t H ), calculated from the gas-shielded arc welding current value I (A), voltage value E (V), and arc welding speed V _A (mm/s) and the thickness _{t H (} mm) of the upper plate 11, is set to 0.200 to 0.300 (kJ/mm ² ), and the distance between a target position 32 of the gas-shielded arc welding and the end face of the upper plate 11 on the lower plate 12 side is set to -0.20×t _H to -0.70×t The welding area is within the range of _H , and an unwelded portion is provided at the end of the lower plate at the overlapping surface between the upper plate and the lower plate.

First, the upper plate 11 having a thickness _tH and the lower plate 12 having a thickness _tL are overlapped. The upper plate 11 and the lower plate 12 are not particularly limited as described above. As in the above-described method for manufacturing a lap fillet welded joint by combining laser welding and gas-shielded arc welding, it is preferable to appropriately select an overlap required to secure the unwelded portion 141 during overlapping in accordance with the welding conditions of the gas-shielded arc welding described later, even in the method for manufacturing a lap fillet welded joint performed using only gas-shielded arc welding, as in the above-described method for manufacturing a lap fillet welded joint by combining laser welding and gas-shielded arc welding. For example, the overlap may be set to 3 times or less the total thickness of the upper plate 11 and the lower plate 12 (i.e., 3.0 x ( _tH + _tL ) or less).

Next, the end face of the upper plate 11 and the surface of the lower plate 12 facing the upper plate 11 are gas-shielded arc-welded. By gas-shielded arc welding, the deposited metal is transferred from the filler metal 31 to the depression formed by the end face of the upper plate 11 and the surface of the lower plate 12 facing the upper plate 11. There are no particular limitations on the type of filler metal 31 used in gas-shielded arc welding, and it can be selected appropriately depending on the components of the upper plate 11 and the lower plate 12.

In the manufacturing method for a lap fillet welded joint according to this embodiment, I×E/(V A ×t _H ), calculated from the arc welding current I (A), voltage E (V), arc welding speed V _A (mm/s), and thickness t _H (mm) of upper plate 11, is set to 0.200 to 0.300 (kJ/mm ² ). I×E/(V _A ×t _H ) is an index established to define the heat _input in gas shielded arc welding.

In a typical method for manufacturing a lap fillet welded joint by gas-shielded arc welding, I×E/(V _A ×t _H ) is set to less than 0.200 kJ/mm ² unless there is a special reason. This is because if I×E/(V _A ×t _H ) is set to 0.200 kJ/mm ² or more, the molten pool will penetrate the lower plate 12, and there is a concern that the molten metal will burn through. In addition, those skilled in the art recognize that even if I×E/(V _A ×t _H ) is set to less than 0.200 kJ/mm ² , a sufficient amount of deposited metal can be transferred. However, in the method for manufacturing a lap fillet welded joint according to this embodiment, gas-shielded arc welding is performed with a higher heat input than normal, that is, a high current value and a large wire feed rate, to increase the penetration depth and the amount of transferred deposited metal compared to normal. This allows a wide penetration width W and throat thickness to be secured in the lap fillet welded joint 1.

Furthermore, in the manufacturing method of the lap fillet welded joint according to this embodiment, the distance between the target position 32 of gas shielded arc welding and the end of the end face of the upper plate 11 on the lower plate 12 side, where the direction away from the upper plate 11 is taken as a positive value in the direction along the surface of the lower plate on the upper plate side in a cross section perpendicular to the weld line, is set to a range of -0.20 x _tH to -0.70 x _tH (see Fig. 9). The horizontal direction in Fig. 9 corresponds to the "direction along the surface of the lower plate on the upper plate side in a cross section perpendicular to the weld line".

In a typical method for manufacturing a lap fillet welded joint by gas-shielded arc welding, unless there is a special reason, the target position 32 for gas-shielded arc welding is set near the boundary between the upper plate 11 and the lower plate 12 so that the arc and the weld metal 13 are not biased toward either the upper plate 11 or the lower plate 12. However, in the method for manufacturing a lap fillet welded joint according to this embodiment, the target position 32 for gas-shielded arc welding is shifted toward the upper plate 11. This prevents the molten pool from penetrating the lower plate 12 and the molten metal from melting through. If the molten metal melts through, there is a concern that voids will form and cause weld defects, and even if no voids form, there will be a shortage of weld metal and the required throat thickness will not be secured.

In any of the methods for manufacturing lap fillet welded joints according to this embodiment, the above requirements do not need to be met over the entire length of the areas to be welded. For example, in the manufacture of one lap fillet welded joint, the above welding conditions may be applied to areas that require particular strength, and normal welding conditions may be applied to other areas. Therefore, a manufacturing method for a lap fillet welded joint in which the above requirements are met in some of the areas to be welded is considered to be a manufacturing method for a lap fillet welded joint according to this embodiment.

(Example 1: Combined use of laser welding and gas-shielded arc welding)
Various lap fillet welded joints were manufactured by a manufacturing method for lap fillet welded joints including the steps of overlapping an upper plate and a lower plate, laser welding an end face of the upper plate to the surface of the lower plate on the upper plate side, and gas-shielded arc welding an end face of the upper plate to the surface of the lower plate on the upper plate side. Here, the laser welding and gas-shielded arc welding were performed as laser-arc hybrid welding in which the laser precedes the lower plate. Therefore, the laser welding speed and the arc welding speed were the same value.

Table 1 shows the welding conditions. In Table 1, the laser welding speed and the arc welding speed are simply written as the welding speed V. In Table 1, values outside the scope of the invention are underlined (same in other tables). In addition, in calculating "(I×E)/( _VA × _tL )+P/( _VL × _tL )" and "P/(I×E)", the laser welding output (output P) in Table 1 was converted to have a unit of W, and the welding speed V in Table 1 was converted to have a unit of mm/s. Condition 1 was normal gas-shielded arc welding without laser welding. Therefore, in Table 1, the output P of the laser welding in Condition 1 was written as "0".

The welding conditions not shown in Table 1 were as follows:
-Type of upper and lower plates: 980MPa-class thin steel plate -Thickness of upper plate _tH : 2.9mm
Lower plate thickness t _L : 2.9 mm
Type of filler metal: JIS Z 3312 YGW16 equivalent Laser incidence angle θ _b : 30 degrees Torch inclination angle θ _a : 60 degrees Laser welding head retreat angle ψ _b : 30 degrees Torch advance angle ψ _a : 15 degrees Laser welding target position: center of plate thickness of upper plate Shielding gas in gas-shielded arc welding: Ar + 20% CO ₂

The toe angle, penetration width, presence or absence of unwelded parts, and throat thickness of the various lap fillet welded joints obtained in this manner were measured. To measure these values, the lap fillet welded joints were cut and polished in a cross section perpendicular to the weld line, and the polished surface was etched to reveal the weld metal. These measured values are shown in Table 2. Note that for the penetration width, the value obtained by dividing the penetration width by the thickness _tL of the lower plate is shown in Table 2. For the throat thickness, the symbol "○" was entered when it was equal to or greater than the thickness _tH of the upper plate, and the symbol "×" was entered otherwise.

Furthermore, the fatigue strength of the lap fillet welded joints was measured. The fatigue strength was measured by a fatigue test on fatigue test pieces with a weld bead length of 20 mm cut out from the lap fillet welded joints. In the fatigue test, the loading mode was plane bending loading, and the bending stress acting at the toe position was used as the standard, with the stress ratio R = -1. The fatigue limit was set at the point of 10 million cycles without fracture. The fracture condition was set at the point when the bending moment decreased by 50% from the initial value. The lap fillet welded joints under condition 1 were obtained under normal arc welding conditions. The fatigue strength ratio of each test piece was calculated using the fatigue strength of the lap fillet welded joint under condition 1 as the standard value, and is listed in Table 2. Lap fillet welded joints that showed an improvement in fatigue strength of 10% or more were considered to be examples of the invention. In this fatigue test, the two locations where fatigue cracks are likely to occur are the toe and the root on the lower plate side. Fatigue cracks will occur in the one with the lower fatigue strength of these two locations, leading to fracture. Therefore, in this fatigue test, the fatigue strength of the test piece cannot be improved unless the fatigue strength of both the bottom plate toe and the root is improved. In other words, a lap fillet welded joint that shows a high fatigue strength ratio in this fatigue test is a lap fillet welded joint that has excellent fatigue strength at the toe and can also suppress fatigue crack initiation from the root.

In condition 1, laser irradiation was not performed and the heat input of the gas-shielded arc welding was within a normal range, so the penetration width obtained in condition 1 was smaller than that of the invention example.
In condition 2, the ratio of the heat input of laser welding to the heat input of gas shielded arc welding, P/(I×E), was insufficient, i.e., the heat input of laser welding was insufficient. Therefore, in the lap fillet welded joint obtained under condition 2, the penetration width was insufficient and the fatigue strength was insufficient.
In condition 7, (I×E)/( _VA × _tL )+P/( _VA × _tL ) was insufficient, i.e., the total heat input was insufficient. Therefore, in the lap fillet welded joint obtained under condition 7, the penetration width was insufficient and the fatigue strength was insufficient.
In condition 9, no unwelded portion was provided at the end of the lower plate at the overlapping surface between the upper plate and the lower plate. Therefore, the lap fillet welded joint obtained under condition 9 had insufficient fatigue strength.
In condition 10, the ratio of the heat input of laser welding to the heat input of gas-shielded arc welding, P/(I×E), was excessive, i.e., the heat input of gas-shielded arc welding was insufficient. Therefore, the throat thickness of the lap fillet welded joint obtained under condition 10 was insufficient, and the fatigue strength was insufficient.
On the other hand, the lap fillet welded joint obtained under appropriate welding conditions had an appropriate shape of the weld metal, and therefore showed a high fatigue strength ratio. In other words, it was confirmed that the lap fillet welded joint and its manufacturing method according to the present invention have excellent fatigue strength at the toe and can further suppress the initiation of fatigue cracks from the root.

(Example 2: Use of gas shielded arc welding alone)
Various lap fillet welded joints were manufactured by a manufacturing method for lap fillet welded joints including a process of overlapping an upper plate and a lower plate and a process of gas-shielded arc welding an end face of the upper plate and an upper plate side surface of the lower plate. Table 3 shows the welding conditions. The value obtained by dividing the distance between the target position of the gas-shielded arc welding and the end face of the upper plate on the lower plate side by the thickness _tH of the upper plate is shown in the "target position/ _tH " column of Table 3. Condition 1 was normal gas-shielded arc welding.

The welding conditions not shown in Table 3 were as follows:
-Type of upper and lower plates: 980MPa-class thin steel plate -Thickness of upper plate _tH : 2.9mm
Lower plate thickness t _L : 2.9 mm
Type of filler metal: JIS Z 3312 YGW16 equivalent Torch inclination angle θ _a : 60 degrees Torch advance angle ψ _a : 0 degrees Shielding gas in gas-shielded arc welding: Ar + 20% CO ₂

The toe angle, penetration width, presence or absence of unwelded areas, and throat thickness of the various lap fillet welded joints obtained in this way were measured. In addition, the fatigue strength of the lap fillet welded joints was measured. The measurement method for each parameter and the pass/fail criteria for the fatigue strength ratio were the same as in Example 1, which concerns the combined use of laser welding and gas-shielded arc welding.

In condition 1, the heat input of the gas-shielded arc welding was within the normal range, and the target position of the gas-shielded arc welding was also within the normal range (i.e., the target position was too close to the lower plate side), so the penetration width obtained under condition 1 was smaller than that of the invention example.
In conditions 2 to 5, the target position of the gas shielded arc welding was the same as in condition 1, and the heat input was insufficient. The lap fillet welded joints obtained under these conditions had insufficient penetration width and insufficient fatigue strength.
The heat input was insufficient in conditions 6 and 7. The lap fillet welded joints obtained under these conditions had insufficient penetration width and insufficient fatigue strength.
In condition 11, the heat input was sufficient, but the target position of the gas shielded arc welding was the same as in condition 1. In the welding under condition 11, the molten metal melted down from the lower plate, the throat thickness was insufficient, and the fatigue strength was insufficient. Furthermore, the lap fillet welded joint obtained under condition 11 also had insufficient penetration width.
In condition 12, the target position of the gas shielded arc welding was too close to the upper plate. In the welding under condition 12, the weld metal bulged, making it impossible to make a smooth toe shape, and the fatigue strength was insufficient.
On the other hand, the lap fillet welded joint obtained under appropriate welding conditions had an appropriate shape of the weld metal, and therefore showed a high fatigue strength ratio. In other words, it was confirmed that the lap fillet welded joint and its manufacturing method according to the present invention have excellent fatigue strength at the toe and can further suppress the initiation of fatigue cracks from the root.

(Example 3: Effect of welding angle)
Various lap fillet welded joints were manufactured by a manufacturing method for lap fillet welded joints including a process of overlapping an upper plate and a lower plate, a process of laser welding an end face of the upper plate to a surface of the lower plate on the upper plate side, and a process of gas-shielded arc welding an end face of the upper plate to a surface of the lower plate on the upper plate side. Here, the laser welding and the gas-shielded arc welding were laser-arc hybrid welding in which the laser was first. Here, the torch tilt angle θ _a and the laser tilt angle θ _b were appropriately changed. Meanwhile, the other welding conditions were the same as those of Condition 8 described in Tables 1 and 2.
Table 5 shows the torch tilt angle θ _a , the laser tilt angle θ _b , and the evaluation results under each welding condition.

In condition 8-1, the laser incident angle was too large, and therefore the lap fillet welded joint obtained under condition 8-1 had insufficient penetration width and insufficient fatigue strength.
In condition 8-5, the torch inclination angle was too small, and as a result, the toe angle of the lap fillet welded joint obtained under condition 8-5 was excessive, resulting in insufficient fatigue strength.
On the other hand, the lap fillet welded joints obtained with an appropriate welding angle had an appropriate shape of the weld metal and therefore showed a high fatigue strength ratio.

(Example 4: Relationship between overlap and unwelded portion)
Various lap fillet welded joints were manufactured under the same welding conditions as Condition 4 in Table 1 except for the overlap, and the effect of the overlap on the unwelded width was investigated. The results are shown in Table 6.

The required fatigue strength ratio was achieved for all lap fillet welded joints. It was also confirmed that the width of the unwelded area increases as the overlap increases.

The present invention provides a lap fillet welded joint, an automobile part, and a method for manufacturing a lap fillet welded joint that have excellent fatigue strength at the toe and can suppress fatigue crack initiation from the root. Therefore, the present invention has high industrial applicability.

1 lap fillet welded joint 11 upper plate 12 lower plate 13 weld metal 131 toe 132 root 14 overlap surface 141 unwelded portion 2 laser welding head 21 laser light 3 torch 31 filler metal 32 target position of gas shielded arc welding t _H thickness of upper plate t _L thickness of lower plate W penetration width w width of unwelded portion θ _WT toe angle θ _a torch inclination angle ψ _a torch advance angle θ _b laser incidence angle ψ _b laser welding head retreat angle

Claims (8)

an upper plate of thickness _tH ;
A lower plate having a thickness t _L overlapped on the upper plate;
a weld metal joining an end surface of the upper plate and a surface of the lower plate facing the upper plate;
A lap fillet weld joint comprising:
The toe angle measured on a cross section perpendicular to the weld line is greater than 0 degrees and less than 30 degrees;
The penetration width measured on the cross section perpendicular to the weld line is 2.50× _tL or more;
The overlapping surface between the upper plate and the lower plate has an unwelded portion at an end portion of the lower plate,
The shortest distance between the root and the surface of the weld metal measured in the cross section perpendicular to the weld line is equal to or greater than _tH ;
The width of the unwelded portion measured on the cross section perpendicular to the weld line is 0.3×tL _or more and 1.9×tL _or less.
Lap fillet welded joint. 2. The lap fillet welded joint according to claim 1, wherein one or both of the thickness t _H of the upper plate and the thickness t _L of the lower plate are 0.8 to 4.0 mm. An automobile part comprising the lap fillet welded joint according to claim 1 or 2 . A step of overlapping an upper plate having a thickness of _tH and a lower plate having a thickness of _tL ;
a step of laser welding an end surface of the upper plate and a surface of the lower plate facing the upper plate;
a step of gas-shielded arc welding the end surface of the upper plate and the surface of the lower plate on the upper plate side after the laser welding;
A method for manufacturing a lap fillet welded joint comprising:
The laser incident angle in the laser welding is set to 10 to 30 degrees,
The inclination angle of the torch in the gas shielded arc welding is set to 45 to 70 degrees,
(I×E)/(VA× _tL) +P/(VL×tL) calculated from the laser welding output P (W) and laser welding speed VL (mm/s) in the laser welding, the current value I (A), voltage value E (V) and arc welding speed _VA (mm/s) in the gas _shielded arc welding, and the thickness _tL ( _mm ) of the lower plate is set to 0.100 to ^0.250 _kJ / _mm2 ;
The ratio P/(I×E) of the heat input of the laser welding to the heat input of the gas shielded arc welding is set to 0.8 to 4.5;
A method for manufacturing a lap fillet welded joint, comprising providing an unwelded portion at the end of the lower plate at the overlapping surface between the upper plate and the lower plate. The method for manufacturing a lap fillet welded joint according to claim 4 , wherein the laser welding and the gas shielded arc welding are laser-arc hybrid welding in which the laser precedes the welding. The forward angle of the torch in the gas shielded arc welding is set to 20 degrees or less,
6. The method for manufacturing a lap fillet welded joint according to claim 5 , wherein a sweep angle of a laser welding head in the laser welding is 30 degrees or less. 7. The method for manufacturing a lap fillet welded joint according to claim 5, wherein a target position for the laser welding is a region between a center of plate thickness and an end of the lower plate on the end face of the upper plate. A step of overlapping an upper plate having a thickness of _tH and a lower plate having a thickness of _tL ;
a step of gas-shielded arc welding an end surface of the upper plate and a surface of the lower plate facing the upper plate;
A method for manufacturing a lap fillet welded joint comprising:
(I×E)/(V _A ×t H ), calculated from the gas-shielded arc welding current value I (A), voltage value E (V), and arc welding speed V _A (mm/s), and the thickness t _H ( _mm ) of the upper plate, is set to 0.200 to 0.300 (kJ/mm ² );
a distance between a target position of the gas shielded arc welding and an end of the end face of the upper plate on the lower plate side, the distance being taken as a positive value in a direction along the surface of the lower plate on the upper plate side in a cross section perpendicular to the weld line, within a range of −0.20×t _H to −0.70×t _H ;
A method for manufacturing a lap fillet welded joint, comprising providing an unwelded portion at the end of the lower plate at the overlapping surface between the upper plate and the lower plate.

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