Laser weld method and weld structure

1. A laser welding method comprising:

forming a melted section in which plural metallic workpieces within a weld region are melted by projecting a first laser beam onto said weld region with parts of the workpieces being set as the weld region when the workpieces are welded; and

projecting a second laser beam while the melted section is being solidified or after a solidified section in which the melted section is solidified is formed such that the second laser beam circles around a center of the melted section or the solidified section from an irradiation start position deviated from said center toward said center or such that the second laser beam is focused to the center of the melted section or the solidified section from an irradiation start region that includes the center and a periphery thereof,

wherein

the melted section including the center is progressed to solidify, or the solidified section is re-melted and a re-melted section is progressed to solidify while projecting the second laser beam; and

a solidification rate of at least the center of the melted section or the re-melted section is lowered by the projection of the second laser beam.

2. The laser welding method according to claim 1 , wherein

the second laser beam is projected until the melted section is solidified, and

in irradiation of the second laser beam, the second laser beam is projected onto the melted section along with progression of solidification in a direction from a peripheral edge of the melted section toward the center of the melted section, so that the progression of the solidification in the direction from the peripheral edge of the melted section toward the center of the melted section is delayed.

3. The laser welding method according to claim 2 , wherein

in the irradiation of the second laser beam, the irradiation start position is located on the peripheral edge of the melted section, or the irradiation start region is a region that is surrounded by the peripheral edge of the melted section.

4. The laser welding method according to claim 2 , wherein

in the irradiation of the second laser beam, the irradiation start position is located between the peripheral edge of the melted section and the center of the melted section, or the irradiation start region is a region, a peripheral edge of which is located between the peripheral edge of the melted section and the center of the melted section and which includes the center.

5. The laser welding method according to claim 3 , wherein

in the irradiation of the second laser beam, the second laser beam is projected such that a columnar crystal structure grows from the peripheral edge of the melted section to the center of the melted section in conjunction with the progression of the solidification.

6. The laser welding method according to claim 4 , wherein

a second laser beam irradiation is started after a columnar crystal structure grows from the peripheral edge of the melted section toward the center of the melted section so as to surround the center of the melted section in conjunction with the progression of the solidification and then growth of an equiaxed crystal structure is started after growth of said columnar crystal structure is completed in a period after irradiation of the first laser beam and before the irradiation of the second laser beam, and

in the irradiation of the second laser beam, the second laser beam is projected such that the equiaxed crystal structure remains in a manner to surround the center of the melted section and that the columnar crystal structure grows from said equiaxed crystal structure to the center of the melted section.

7. The laser welding method according to claim 1 , wherein

in the irradiation of the second laser beam, the second laser beam is projected onto the solidified section in which the melted section is solidified, so as to melt the solidified section again, and the second laser beam is projected such that a cooling rate, in which the region that includes the center of the solidified section and a vicinity thereof and that is melted again by the irradiation of the second laser beam is solidified, is slower than a cooling rate, in which the region that includes the center of the melted section and the vicinity thereof is solidified after the irradiation of the first laser beam, over a position on a peripheral edge of the region that includes the center of the solidified section and the vicinity thereof being set as the irradiation start position, or over a region that is surrounded by the peripheral edge of the region that includes the center of the solidified section and the vicinity thereof being set as the irradiation start region.

8. The laser welding method according to claim 7 , wherein

the solidified section includes: an equiaxed crystal region that is formed with an equiaxed crystal structure in a manner to include the center of the solidified section; and a columnar crystal region that is formed with a columnar crystal structure in a manner to surround the equiaxed crystal region from the peripheral edge of the solidified section toward the equiaxed crystal region, and

the second laser beam is projected such that the equiaxed crystal structure of the equiaxed crystal region becomes the columnar crystal structure.

9. The laser welding method according to claim 7 , wherein

the solidified section includes: an equiaxed crystal region that is formed with an equiaxed crystal structure in a manner to include the center of the solidified section; and a columnar crystal region that is formed with a columnar crystal structure in a manner to surround the equiaxed crystal region from the peripheral edge of the solidified section toward the equiaxed crystal region, and

the second laser beam is projected onto the equiaxed crystal region such that a part of the equiaxed crystal structure of the equiaxed crystal region surrounds the center of the solidified section and such that the remaining equiaxed crystal structure of the equiaxed crystal region becomes the columnar crystal structure.

10. A laser welding method comprising:

forming a melted section in which plural metallic workpieces within a weld region are melted by projecting a first laser beam onto said weld region with parts of the workpieces being set as the weld region when the workpieces are welded; and

projecting a second laser beam while the melted section is being solidified or after a solidified section in which the melted section is solidified is formed such that the second laser beam circles around a center of the melted section or the solidified section from an irradiation start position deviated from said center toward said center or such that the second laser beam is focused to the center of the melted section or the solidified section from an irradiation start region that includes the center and a periphery thereof,

wherein in the irradiation of the second laser beam, the irradiation start position is located between a peripheral edge of the melted section and the center of the melted section, or the irradiation start region is a region, a peripheral edge of which is located between the peripheral edge of the melted section and the center of the melted section and which includes the center.

11. A laser welding method comprising:

forming a melted section in which plural metallic workpieces within a weld region are melted by projecting a first laser beam onto said weld region with parts of the workpieces being set as the weld region when the workpieces are welded; and

projecting a second laser beam while the melted section is being solidified or after a solidified section in which the melted section is solidified is formed such that the second laser beam circles around a center of the melted section or the solidified section from an irradiation start position deviated from said center toward said center or such that the second laser beam is focused to the center of the melted section or the solidified section from an irradiation start region that includes the center and a periphery thereof,

wherein in the irradiation of the second laser beam, the second laser beam is projected such that a columnar crystal structure grows from a peripheral edge of the melted section to the center of the melted section in conjunction with progression of the solidification.