58 巻 (2017) 682 号 p. 1029-1034
A mechanical clinching process with preforming of a lower sheet was developed to join two ultra-high strength steel sheets having low ductility. The thickness reduction of the upper sheet around the punch corner becomes small because of the decrease in the pressure from the preformed lower sheet, and then the interlock is increased by the expansion in the radial direction in the final compression. The shapes of the die and preformed lower sheet were optimized to increase the interlock. Furthermore, the strength of the sheets joined by mechanical clinching with the preformed lower sheet was compared with that of the sheets joined by resistance spot welding. The fatigue limit of the sheets joined by clinching was greater than that of the welded sheets, while the static load of the mechanically clinched joint was smaller. It was found that the mechanical clinching process with preforming of the lower sheet was effective for joining ultra-high strength steel sheets having low ductility and high flow stress.