Observations on impact toughness of electron beam welds of an α+ β titanium alloy

Abstract

Electron beam welds of an α+ β titanium alloy equivalent to Russian origin VT 9 has been studied for its impact toughness characteristics. The effect of base metal heat treatment in the α+ β region and β regions has been investigated. The width of the fusion zone grains controlled impact toughness properties. Welds of β heat-treated base metal containing wider fusion zone grains along the crack path exhibited higher toughness than α+ β heat-treated base metal welds. This trend is reflected even in high heat input α+ β base welds containing wider fusion zone grains. Post-weld heat treatment in the supertransus and subtransus regions resulted in further improvement in weld zone toughness. β heat-treated base metal welds exhibited superior toughness even after post-weld heat treatments. The superior toughness of β base welds is mainly due to crack path deviation at high angles at the grain boundaries and crack arrest at martensite plate and or colony boundaries prior to the post-weld heat treatments. In the post-weld heat treatments the improved toughness is due to predominant crack arrest and deviation at the α/ β interfaces, and crack deviation at colony/subcolony (pocket) boundaries and grain boundaries coupled with energy dissipation due to crack blunting as a consequence of improved ductility in the post-weld heat-treated conditions. The high density of α coupled with smaller colony/subcolony size (pocket size) in β base welds resulted in superior toughness in the post-weld heat treated condition. Super transus post-weld heat treatment which resulted in large aligned colony α and continuous grain boundary α exhibited marginally low toughness compared to the subtransus treatment. Trends in toughness could be related to a combination of factors namely, increased crack path length due to deflection at grain boundaries, colony boundaries and crack arrest and deviation at α/ β and α/ α′ boundaries.