EFFECT OF HEAT-TREATMENT ON MICROSTRUCTURE AND MICROHARDNESS OF SPOT WELDS IN TI-26AL-11NB

The gas tungsten arc welded fusion zone of Ti-26Al-11Nb (atomic per cent) was investigated to determine the effect that postweld heat treatment (PWHT) had on its microhardness and microstructure. The as-welded fusion zone consisted of an omega-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-degrees-C) and lower (650-degrees-C) limits of the alpha-2 + beta-region of the Ti3Al-Nb pseudo-binary phase diagram. Short times at 650-degrees-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. Only heavily faulted alpha-2 was detected in the 650-degrees-C PWHTs. The 980-degrees-C, 1 min PWHT had a microhardness and microstructure similar to the 650-degrees-C, 50 h PWHT. All other PWHTs at 980-degrees-C had microhardness values near that of the base metal. There were two distinct microstructures for these PWHTs. One consisted of alpha-2 subgrains with a high dislocation density and B2 precipitates. The other consisted of clear alpha-2 grains with B2 at grain boundaries.