Effect of post-weld heat treatment on fracture toughness and fatigue crackgrowth behaviour of electron beam welds of a titanium (alpha+beta) alloy

The effects of a post-weld heat treatment on the fracture toughness and fat igue crack growth behaviour of electron beam welds of an alpha + beta titan ium alloy, Ti-6.5Al-1.9Zr-0.25Si have been studied. Welds in the stress-rel ieved condition exhibited poor fracture toughness due to poor energy absorb ing capacity of the thin alpha and alpha' phases. Post-weld heat treatment which resulted in the decomposition of a' to alpha + beta and the coarsenin g of intragranular and intergranular alpha resulted in improved toughness. This improvement in the toughness is related to improved ductility leading to crack blunting, crack path deviation at the thick intragranular and inte rgranular a phase. Fatigue crack growth resistance of welds was superior to the base metal in the alpha + beta heat-treated condition. The superior cr ack growth resistance of the welds is due to the acicular alpha microstruct ure which results in a tortuous crack path and possible crack closure arisi ng from crack path tortuosity.