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

Electron beam welds of an alpha + beta 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 alpha + beta region and beta regions has been investigated. The width of the fusion zone grains controlled impact toughness properties. Welds of beta heat-treated base metal containing wider fusion zone grains along the crack path exhibited higher toughness than alpha + beta heat-treated base metal welds. This trend is reflected even in high heat input alpha + beta 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. beta heat-treated base metal welds exhibited superior toughness even after post-weld heat treatments. The superior toughness of beta 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 alpha/beta 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 alpha coupled with smaller colony/subcolony size (pocket size) in beta 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 alpha and continuous grain boundary alpha 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 alpha/beta and alpha/alpha' boundaries. (C) 1998 Elsevier Science S.A. All rights reserved.