Toru Kato’s research while affiliated with National Institute Of Technology, Ishikawa College and other places

Ultra-high-strength steel sheets having a tensile strength of more than 1 GPa and a low ductility were joined by mechanical clinching with dies for control of metal flow. The bottom angle of the die was modified to increase interlocking between the sheets under avoidance of the sheet fracture. The effect of the die shape on metal flow in the sheet combination including an ultra-high-strength steel sheet was investigated by the finite element simulation and the experiment. As the tensile strength of the steel sheets increased, the joining range was narrow due to low ductility of the sheets. The static and fatigue strengths of the mechanically clinched joints were compared with those of the welded joints. Although the static loads of the mechanically clinched joints were smaller than those of the resistance spot welded joints in both tension-shearing and cross-tension tests, the fatigue loads of the clinched joints were larger. It was found that the mechanically clinched joint has superior fatigue strength.

A mechanical clinching using counter pressure of a rubber disk was developed to join the ultra-high strength steel sheets having low ductility. In the proposed process, the interlock was increased by the increment of metal flow with the counter pressure of rubber disk in the die cavity. The two kind of ultra-high strength steel sheets having different ductility were used in the mechanical clinching. The effect of the shape of rubber disk on the deforming behaviour of the sheets was investigated. The joinability was improved under the appropriate shape of rubber disk for both sheets, and then the sheets having 56% of reduction area were successfully joined whereas the sheets were not joined without the counter pressure. Although the joinability of the sheets having 43% of reduction area was improved, the cracks occurred in the upper sheet around the punch sidewall. The maximum static load and the fatigue limit of the joined sheets were measured in the tension-shearing and cross-tension tests. It was effective for the improvement of joinability in the mechanical clinching of ultra-high strength steel sheets to use the counter pressure of the rubber disk.

抄録 The static and fatigue strengths of mechanically clinched, self pierce riveted and resistance spot welded joints in aluminium alloy A5052-H34 sheets were investigated. In the static tension-shearing test, self pierce riveted, resistance spot welded and mechanical clinched joints were tested in order of the maximum load. The endurance ratio of the sheets joined by the mechanical clinching and self pierce riveting were larger than that of the sheets joined by resistance spot welding owing to the work-hardening and slip at the interface between the sheets, i.e. the fatigue characteristic of mechanical joining processes was better than that of the welding processes involving metallurgical bonding. To increase the maximum joint load for mechanical clinching, a step punch was proposed. In the mechanical clinching using the step punch, the wall thickness in the side wall of the punch was increased by the compression of the step. The maximum static load and endurance ratio for mechanical clinching using a step punch were increased by 21% and 45%, respectively.

To attain high recyclability of automobiles, aluminium alloy sheets were joined by means of an aluminium alloy cylindrical rivet. Although steel rivets used in the conventional self-pierce riveting are removed from the aluminium alloy sheets in the recycling, the removal is not required for the aluminium alloy rivets, and thus the joined sheets with the rivets were directly melted due to the same aluminium. To join aluminium alloy sheets by the aluminium alloy cylindrical rivets, the shapes of the rivet and die were optimised. The effects of the shapes of the rivet and die on the deforming behaviour of the sheets and rivet were investigated using a finite element simulation. The effectiveness of the designed cylindrical rivet and die was evaluated. The aluminium alloy sheets were successfully joined by the optimised aluminium alloy cylindrical rivet.

The joinability of mechanical clinching for high strength steel sheets with a low ductility and an aluminum alloy A5052 sheet was improved. For the upper steel sheet, a fracture was observed around the punch corner. For the lower steel sheet, cracks appeared around the die corner. To prevent the fracture and cracks of the steel sheets, the concentration of deformation was relaxed by modifying the shape of the die. The effect of the die shape on the deforming behavior of the sheets was investigated by finite element simulation and experiment. The joinability for high strength steel and aluminum alloy sheets was improved by modifying the depth and diameter of the die. The strength of the joined sheets was measured by a cross-tension test. The fracture in the test appeared at the aluminum alloy sheet. As the tensile strength of the steel sheet increased, the strength of the joined sheets decreased because of the small interlock of the joined sheets.