Effect of tool revolutionary pitch on heat transfer and material flow in Al/steel friction stir lap welding

Abstract

The motion condition of a friction stir welding tool significantly affects weld formation and quality in dissimilar joining between Al alloy and steel owing to its direct effect on thermo-mechanical condition during the process. In this study, a finite element simulation based on the coupled Eulerian–Lagrangian formulation was used to investigate the thermomechanical field evolutions in the friction stir lap welding of Al and steel, focusing on the effect of tool revolutionary pitch on thermal energy generation and material flow. Simulation results indicated that an increasing revolutionary pitch increased the proportion of heat input generated by plastic deformation, exhibiting an approximately linear relationship under the adopted welding parameters. With a fixed revolutionary pitch, a high- parameter-matching of rotational created higher welding temperatures. A lower revolutionary pitch resulted in higher welding temperature, leading to a thicker intermetallic layer at the Al/steel interface. However, a high revolutionary pitch under high-matching parameters significantly weakened the mixing of two materials behind the pin by decreasing material migration from the Al alloy and steel at the pin bottom on the restraining side, due to reduced temperature and increased deformation resistance. A parabolic mathematical correlation between interfacial strength and revolutionary pitch was observed, suggesting that maintaining a revolutionary pitch within the range of 0.4 to 1.2 mm/rev could produce a robust Al/steel lapped interface with joining efficiencies of approximately 91% or higher compared to the Al alloy base metal