EFFECT OF POSTWELD HEAT-TREATMENT ON TI-14-PERCENT-AL-21-PERCENT-NB FUSION ZONE STRUCTURE AND HARDNESS

Gas tungsten are (GTA) spot welds were made on 1 x 0.5 x 0.018-in. (2. 54 x 1.57 x 0.0457-cm) samples of Ti-14.2Al-21.3Nb (wt-%). The samples were welded for 3 s at 32 A and 17 V. The as-welded fusion zone consi sted of a omega-type, ''tweed-like'' microstructure with a microhardne ss that was higher than the base metal by as much as 100 VHN. Two post weld heat treatment (PWHT) temperatures were chosen, which represented the upper (980 degrees C) and lower (650 degrees C) limits of the alp ha(2) + beta region of the Ti3Al-Nb pseudo-binary phase diagram. Short PWHT times at 650 degrees C resulted in secondary hardening above tha t of the as-welded condition while longer times resulted in hardness v alues near that of the base metal. Only heavily faulted alpha(2) was d etected in the 650 degrees C PWHTs. The PWHT at 980 degrees C produced rapid softening of the weld metal. A 1-min PWHT at 980 degrees C 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 t hat of the base metal. There were two distinct microstructures for the 980 degrees C PWHT. One consisted of alpha(2) subgrains with a high d islocation density and beta 2 precipitates. The other consisted of cle ar alpha(2) grains with beta 2 at grain boundaries.