Influence of multi-pass friction stir processing on the formation of microstructure and mechanical properties of Ti6Al4V alloy

Friction stir processing (FSP) is an advanced technology for altering the surface microstructure of metals and alloys to improve mechanical and performance properties. Previous research on titanium alloy processing showed that varying the FSP process parameters (such as rotational speed, movement speed and tool contact force) significantly affects the Ti–6Al–4V microstructure evolution and mechanical properties. However, the effect of multipass FSP on the Ti–6Al–4V alloy was not studied. Therefore, this paper studies the effect of four-pass FSP of the Ti–6Al–4V titanium alloy on the microstructure evolution, mechanical properties and wear resistance of this alloy. Microstructure analysis showed that the stirring zone forms heterogeneous microstructure with dynamically recrystallized equiaxed α grains, β grains and β areas with α phase of needle and laminar type, which is associated with the stirring zone temperature gradient during FSP. It was found that an increase in the number of FSP passes up to 3 times improves the ultimate tensile strength (up to 1173 MPa) and wear resistance (by 33 %). The improved ultimate tensile strength of samples after 3 FSP passes is caused by grain size reduction in the stirring zone by 88 % compared to the initial Ti–6Al–4V alloy. It was shown that after 4 FSP passes the grain size increases and the ultimate tensile strength decreases to 686 MPa in the stirring zone, which is associated with large defects formed along the contour of metal flows. At the same time the Ti–6Al–4V wear resistance after 4 FSP passes increases by 39 % compared to the raw material.

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