Solidification cracking occasionally occurs in a spot-welded part of aluminum alloy sheet joints. The box section beam constructed by spot-welds fractured from such solidification cracking parts in the flange under cyclic bending load. It was found that the fatigue crack started from the solidification cracking and that the fatigue strength of the box section beam has the same level as that of the spot-welded double sheet specimen under cyclic tensile load. When the solidification cracking was reduced by the optimized welding condition, the fatigue strength was improved significantly. This improvement was verified for three types of aluminum alloy sheets: Al- Mg sheet, Al-Cu-Mg sheet, and Al-Mg-Si sheet.
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