Fracture toughness of Ti-6Al-4V after welding and postweld heat treatment

The fracture toughness (JIC) of the fusion zone of Ti-6Al-4V alloy welds was studied in terms of microstructural changes in the as-welded condition and following postweld heat treatment. Gas tungsten arc and electron beam welds were produced in sheet material over a limited range of heat input and subsequently heat treated at 700°C (1290°F) and 900°C (1650°F). In the aswelded condition, the weld microstructure was a mixture of diffusional and martensitic alpha phases, whose proportion varied with heat input and cooling rate. The fusion zone exhibited low ductility resulting from the highly acicular microstructure and a large prior-beta grain size. Postweld heat treatment tempered the martensite and coarsened the microstructure, but a beneficial effect on ductility was realized only after treatment at 900°C. Fracture toughness in the as-welded condition was greater than for the base metal and was attributed to the lamellar microstructure of the fusion zone and absence of continuous alpha film along the grain boundaries. Postweld heat treatment at 700°C reduced the fracture toughness considerably and, as in the case of ductility, it was necessary to heat treat at 900°C to produce an improvement. These variations in fracture toughness correlated well with fractographic features and, in terms of the microstructure, were found to be related to the coarseness of the transformed/ aged beta matrix and to the thickness and morphology of the alpha phase formed at the grain boundaries.