Effect of process parameters on microstructure and mechanical properties of friction stir-welded Ti–6Al–4V joints

Abstract

In this work, the microstructure and mechanical behavior of friction stir-welded 2-mm-thick sheets of Ti–6Al–4V alloy were investigated. Specifically, the influence of friction stir welding (FSW) process parameters on microstructure evolution, defect generation, tensile, and hardness properties of the joint were studied. Optical and scanning electron microscopies were used to examine the microstructure and to determine the type and nature of the defects. A progressive change in the microstructure from equiaxed and elongated α grains (base metal) to a very fine lamellar structure with an average grain size of less than 1 μm (in the upper stir zone) was observed and related to FSW parameters. Tensile tests and microhardness measurements were also carried out to assess the mechanical properties. Important variations were observed on tensile strength and hardness distribution as a function of process parameters. The weakest mechanical properties and fracture were found in the heat-affected zone (HAZ) of the as-welded joints. The results were analyzed in terms of the pseudo heat index, and it was determined that tool rotation is the most significant process parameter influencing both the microstructure and mechanical properties of FSWed Ti–6Al–4V joints.