OBSERVATIONS ON IMPACT TOUGHNESS OF ELECTRON-BEAM WELDS OF AN ALPHA-ALLOY(BETA TITANIUM)

Electron beam welds of an alpha + beta titanium alloy equivalent to Ru ssian origin VT 9 has been studied for its impact toughness characteri stics. The effect of base metal heat treatment in the alpha + beta reg ion and beta regions has been investigated. The width of the fusion zo ne grains controlled impact toughness properties. Welds of beta heat-t reated base metal containing wider fusion zone grains along the crack path exhibited higher toughness than alpha + beta heat-treated base me tal welds. This trend is reflected even in high heat input alpha + bet a base welds containing wider fusion zone grains. Post-weld heat treat ment in the supertransus and subtransus regions resulted in further im provement in weld zone toughness. beta heat-treated base metal welds e xhibited superior toughness even after post-weld heat treatments. The superior toughness of beta base welds is mainly due to crack path devi ation at high angles at the grain boundaries and crack arrest at marte nsite plate and or colony boundaries prior to the post-weld heat treat ments. 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 grai n boundaries coupled with energy dissipation due to crack blunting as a consequence of improved ductility in the post-weld heat-treated cond itions. The high density of alpha coupled with smaller colony/subcolon y 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 t he subtransus treatment. Trends in toughness could be related to a com bination of factors namely, increased crack path length due to deflect ion at grain boundaries, colony boundaries and crack arrest and deviat ion at alpha/beta and alpha/alpha' boundaries. (C) 1998 Elsevier Scien ce S.A. All rights reserved.