A new friction processing method for welding of dissimilar metals

Abstract

A friction processing method is proposed for joining of dissimilar metals which does not involve direct mechanical processing, significant deformation or thickness reduction of parent metals. A rotating tungsten carbide tool is plunged into a sacrificial top sheet to generate heat and pressure required for dissimilar metal welding. Constant plunge rate is applied till melting initiates at the interface. Interdiffusion of dissimilar metals at the interface, results in interface melting at (near eutectic) temperatures well below the melting of either of the base metals. Once the interface starts melting, plunging is stopped and tool is maintained at constant position for a prescribed dwell time. Rapid cooling, from self quenching, occurs in the weld zone once the tool is retracted post the dwell time. In this work, the proposed method is used for lap welding of copper and aluminium sheets. While low dwell times are seen to result in insufficient bonding, excessive dwell times are seen to result in higher amounts of brittle intermetallic phases that degrade the joint strength. High bond strength is achieved for 0.05 mm/s plunge rate and 10s dwell time. Cross-sectional scanning electron micrographs, fractographs, electron dispersive spectroscopy, X-ray diffraction, mechanical strength tests, and finite element analysis are used to investigate the physics of the process and to study the effect of process conditions on resulting microstructures, phase formations and mechanical properties of the joint. The diffusion layer at the interface is predominantly composed of lamella with alternating phases of Al and Al2Cu at homogeneous eutectic composition. Very thin Cu rich transition layers are formed towards Cu side. X-ray diffraction and SEM images of fracture surfaces on Al- and Cu-side, from peel tests, show that fracture occurs in the interface layer towards the Cu side. Finite element analysis (FEA), using a fully coupled thermo-mechanical model, is used to understand the temperature evolution in the weld zone for different plunge rates. The method holds great promise for joining dissimilar metal sheets of a wide range of thicknesses.