| 摘要 |
PURPOSE:To improve a flange cracking resistance by using the Al alloy sheet having a specific elongation % as the blank and executing a local heating before or after joining the overlapping part thereof. CONSTITUTION:What has the elongation property of 5-20% range is used for the Al alloy sheet 5 of as the blank and the four corner parts 5a, 5b, 5c, 5d of the sheet 5 which are prior to the joining only are subjected to a heat softening treatment locally. They are then made in a cylindrical shape with their curving by a roll forming, etc. and the opposing side parts are joined by overlapping. Or only the piling junction parts 2a, 2b of after joining are softened by local heating. In this case, the heat softening treatment is performed so as to satisfy the equation I respectively. A flange cracking is apt to cause on the overlapping part 2, so in this way the flange cracking resistance is improved and the production cost can be reduced.
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