Abstract
In order to join two ultra-high strength steel sheets with low ductility for automobile parts, a joining process by hemming with a pre-bent inner sheet was developed. In this joining, the pre-bent inner sheet instead of the conventional flat inner sheet was used to relax the deformation concentration of the outer sheet. Although 780 MPa steel sheets were joined without the pre-bent inner sheet, a fracture in the outer sheet occurred in joining the 980 MPa sheets due to the low ductility of the sheets. The 980 MPa and 1180 MPa sheets were successfully joined by hemming with the pre-bent inner sheet. In this process, the deformation of the upper sheet was relaxed by contacting with the inner sheet, and then the strain on the outer surface reduced. Although softening around a weld nugget occurred by heating in the conventional welded joint, work-hardening occurred in the hemmed joint. The joint strength was investigated and then the peel strength of the hemmed sheets was about a half of the welded one. It was found that the hemming process with the pre-bent inner sheet was effective for joining ultra-high strength steel sheets with low ductility.
Highlights
The light weight and the collision safety of automobiles are important
The galvanized high strength steel sheets with zinc alloy coating layers are joined by this welding, dressing the electrodes to remove the adhered zinc alloy is frequently performed—i.e., the tooling cost is increased [2]
The liquid metal embrittlement that causes the reduction in elongation and failure and early fracture of the galvanized steel sheets is prevented by the cold hemming process, whereas the problem may occur by heating in spot welding [3]
Summary
For the reduction in weight and the increase in collision strength of automobile body parts, the application of high specific material sheets, such aluminum alloy sheets and ultra-high and high-strength steel sheets is effective [1]. These parts are usually formed by multi-stage stamping, and the stamped parts are joined to an assembled part. The deforming behavior of the sheets in the joining process of the ultra-high strength steel sheets by hemming with the pre-bent inner sheet was investigated by finite element simulation and experiment. The joining limit in the proposed hemming process was evaluated with that of the conventional hemming one, and the joint strength of the hemmed and welded sheets were compared
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