Effect of copper donor material-assisted friction stir welding of AA6061-T6 alloy on downward force, microstructure, and mechanical properties

Srinivasa Naik Bhukya,Zhenhua Wu,Abdelmageed Elmustafa,Joseph Maniscalco

doi:10.1007/s00170-021-08390-8

Abstract

In this research, copper (Cu) donor material-assisted friction stir welding (FSW) of AA6061-T6 alloy was studied. Cu-assisted FSW joints of AA6061-T6 alloy were prepared at a constant tool rotational rate of 1400 rpm and various welding speeds at 1 mm/s and 3 mm/s. The Cu donor material of different thickness (i.e., 20%, 40%, and 60%) with respect to the workpiece thickness was selected to assist the FSW joining at the plunge stage. It is observed that the downward force generated in the FSW process was gradually decreased after introducing Cu donor material with incremental thicknesses with respect to workpiece at the plunge stage. Post-weld analysis was characterized in terms of microstructure and mechanical properties. The results of microstructure analysis at the stir zone (SZ) show the formation of finer grains due to dynamic recrystallization and plastic deformation. Micro-hardness tests reveal that the hardness decreased from the base metal (BM) to the SZ across the heat-affected zone (HAZ) and thermo-mechanically affected zone (TMAZ). The lowest value of hardness appeared in the TMAZ and HAZ where tensile failure occurs. With increasing welding speed, the average hardness in the SZ decreased due to lower heat input and faster cooling rate. Tensile test plots show no significant change in ultimate tensile strength with or without Cu donor material. Fractography of tensile tested samples shows both ductile and brittle like structure for given welding parameters. This proposed work of FSW with Cu donor material is promising to increase tool life due to the decrement of the downforce during plunge and throughout the welding stage. Meanwhile, the inclusion of donor material did not compromise the weld quality in terms of the mechanical properties and micro-hardness.

Highlights

Friction stir welding (FSW) is a novel solid-state joining process invented at The Welding Institute (TWI) of UK in 1991
The constant frictional energy generates high temperatures which soften the workpiece, at this point the downward force keeps stable and increased slightly until it reaches to the exit position, it immediately reduced to zero downward force in the retraction stage
It is understandable that the higher downward forces were taking place during the plunge stage which resulted in producing frictional heat that softened the surrounding material [31,32]

Summary

Introduction

Friction stir welding (FSW) is a novel solid-state joining process invented at The Welding Institute (TWI) of UK in 1991. It has been broadly used in aerospace, aircraft, shipbuilding, automotive, railways, and marine industries [1,2,3,4]. FSW has many advantages including lower heat input and avoidance evasion of phase change during the welding process in comparison to conventional fusion joining processes such as MIG or TIG welding. FSW is used for joining high strength steel for hulls and decks

Methods

Results

Conclusion