Application of Conductive Heat Resistance Seam Welding for Joining a 7075-T6 Alloy and a 5754 Dissimilar Thickness Combination

Conductive heat resistance seam welding (CHRSEW) is a new process developed at Edison Welding Institute for creating butt joints on aluminum sheet. The process uses conventional resistance seam welding equipment, and takes advantage of steel cover sheets on either side of the intended joint. Resulting joints are fusion in character, and can be manufactured at very high welding speeds (∼ 3 to 4 m/min). In this study, the conductive heat resistance seam welding process was extended to some new applications. These included joining a 7075-T6 alloy, and a dissimilar thickness 1- to 2-mm 5754 configuration. The former is generally considered unweldable by fusion methods, and is of considerable interest for aerospace applications. The latter is representative of a tailor welded blank for automotive applications. Resulting welds were evaluated using metallurgical examinations and mechanical testing. Microstructurally, the welds for both applications consisted of a fine cellular/dendritic structure. Cracking and porosity were also non-existent. These observations were related primarily to the high hydrostatic forces applied to the welds during solidification, but also to the solidification modes and paths in these welds themselves. Mechanically, the welds on the 7075-T6 material failed in the heat affected zone with roughly a 50% joint efficiency. This reduction in performance was related primarily to overaging of the HAZ during welding, but also stress concentrations associated with extruded metal from the fusion zone itself. Mechanical tests on the dissimilar thickness 5754 welds showed failures in the base metal, with joint efficiencies between 90 to 100%. Formability tests also suggested considerable ductility in these dissimilar thickness welds.