Effect of heat treatment on microstructure and microhardness of spot welds in Ti26Al11Nb

The gas tungsten arc welded fusion zone of Ti26Al11Nb (atomic per cent) was investigated to determined the effect that postweld heat treatment (PWHT) had on its microhardness and microstructure. The as-welded fusion zone consisted of an ω-type, "tweed-like" microstructure with a microhardness that was higher than the base metal by as much as 100 VHN. Two PWHT temperatures were chosen which represented the upper (980°C) and lower (650°C) limits of the α₂ + β region of the Ti₃AlNb pseudo-binary phase diagram. Short times at 650°C resulted in secondary hardening above that of the as-welded condition while longer times resulted in hardness values near that of the base metal. heavily faulted α₂ was detected in the 650°C PWHTs. The 980°C, 1 min PWHT had a microhardness and microstructure similar to the 650°C, 50 h PWHT. All other PWHTs at 980°C had microhardeness values near that of the base metal. There were two distinct microstructures for these PWHTs. One consisted of α₂ subgrains with a high dislocation density and B2 precipitates. The other consisted of clear α₂ grains with B2 at grain boundaries.