The study investigates the use of friction stir processing (FSP) for forming mechanical joints, specifically focusing on joining steel and aluminum alloy plates. The authors previously explored mechanical joining by FSP using a pre-pierced hole on the steel plate by stamping. In this study, they used a chamfering punch instead of a straight punch to improve joint strength. The process involves piercing a hole in the steel plate by stamping, placing the aluminum alloy plate on top, and using a rotary tool to cause the aluminum to flow into the hole, forming a mechanical interlock between the steel and aluminum alloy plates. Experiments were conducted using 0.8-mm-thick JIS SPCE steel and 3-mm-thick JIS A5083 aluminum alloy plates, with pre-pierced stamping process in various punch-and-die clearances. The strength of the joints was evaluated using cross-tensile strength (CTS) and tensile shear strength (TSS) tests. Results showed that the use of the chamfering punch in pre-piercing improved CTS of the joint by FSP compared to a straight punch, but excessive punch-and-die clearance in pre-piercing reduced CTS. The study found that the mechanical joint strength depends on the hole’s narrowest diameter for TSS and the roll-over depth for CTS. The proposed method showed high strength among spot-joining methods without weight increment but was inferior to the joining methods with weight increment using fasteners, rivets, and dowels. The study suggests that adding metallurgical joining through friction stir spot welding (FSSW) could further enhance joint strength.