Mechanical Performance and Microstructure of Resistance Element Welds of Dissimilar Metals Created with a Headless Rivet

Abstract

Resistance Element Welding (REW) is a novel processing technology developed to join dissimilar materials such as aluminum and steel. It uses an auxiliary steel rivet (element) inserted into an aluminum sheet that forms a resistance spot weld with the steel sheet, and the aluminum work piece is mechanically joined by the interlocking rivet. The versatility and agility of REW is advantageous, especially in the automotive industry. However, the head of the rivet often extends out externally over the work piece, and the misalignment of the electrode with the head can be problematic since inserting the head and the weld are performed in separate stages. In the present paper, we performed REW using a headless rivet which has minimal to no overhanging part above the aluminum surface. The lap-shear strength of REW is higher than self-piercing rivets, which makes REW a promising solution for extending applications of multi-material structures. The microstructure of the joint involves a fusion zone which mainly consists of martensite, and the failure occurs near the heat affected zone. This study finds that the electrode-rivet alignment needs to be optimized to manage the optimal nugget size and to avoid current arcing through the aluminum work piece. (Received August 12, 2019; Accepted September 30, 2019)