The established technologies for joining sheet metals are welding, riveting and bolting. However, it is difficult to join dissimilar sheet metals and unweldable materials by welding. Riveting and bolting have a disadvantage that additive part must be provided. The mechanical joining process of a sheet metal pair has been developed in order to replace the resistance spot welding process in case that joining of dissimailar sheet metals, mechanically unweldable materials and coated sheet metals with different thicknesses are needed. Form-joining or clinching, a kind of mechanical joining process, is defined as the joining process of a sheet metal pair by geometric constraint imposed by plastic deformation of the workpieces without any additive part . There are many commercial form-joining devices. It has been reported that the joining strength by commercial form-joining apparatus is 50 - 70 percent of that by resistance spot welding. Therefore, a new form-joining process is required in order to improve the joining strength.
In this work, a 2-step form-joining process with a secondary punch is proposed. The device is designed to improve the joining strength by increasing the geometric constraint of the deformed shape by combining a primary punch, a secondary punch and a female die. A sheet metal pair is first deformed into the circular female die by a drawing process. Then, the drawn sheet metal pair is squeezed between the primary punch and the secondary punch, The squeezed sheet metal deforms along the inclined surfaces of the inner die and the secondary punch. The design parameters are systematically investigated for the proposed 2-step form-joining process. The objective functions are defined as the geometric dimensions of the joint. The parameters are then designed by Taguchi method and finite element analysis. In order to verify the improved joining strength by the designed process, the tensile-shear strength, the peel-tension strength and the asymmetric peel-tension strength are compared with those by the TOX process and those by resistance spot welding. The tensile-shear strength by the designed process amounts to 76 percent of the strength by resistance spot welding.