Microstructure and mechanical properties of friction welds of an α+β titanium alloy

The mechanical properties of friction welds of an α+β titanium alloy (Ti–6.5Al–1.9Zr–3.3Mo–0.25Si) were evaluated in the stress relieved and in the post weld heat treated (PWHT) conditions to understand the effect of post weld heat treatments on the microstructure and mechanical properties. Stress relieved welds exhibited mechanical properties comparable to the base metal except impact toughness. The impact toughness was ∼65% of the base metal. This was mainly due to a mixed microstructure of martensite and thin alpha+beta. Post weld heat treatment at 700°C that led to beta precipitation together with silicides exhibited poor impact toughness and trangranular fracture with shallow and under developed fine dimples. A PWHT at 960°C for 1 h followed by air cooling (AC) that led to the decomposition of α1 to equilibrium alpha+beta and coarsening of the transgranular alpha improved the toughness. This treatment improved all other properties. 960°C/1 h/water quenching (WQ) PWHT reduced the impact toughness and exhibited quasi cleavage fracture possibly due to α1 microstructure. Prolonged soaking at 960°C marginally reduced the toughness. This is thought to be due to a lean distribution of alpha consequent to its coarsening and possible compostional differences between the α and β phases that led to smooth and flat fracture in the transgranular locations. The smooth and flat fracture features were due to poor resistance of the microstructure to crack propagation.