*3.3. Effects of Heat Treatment on Joint Microhardness*

The microhardness tests were carried out on as-welded Ti3Al LFW joints and joints after different heat treatments. The variation of microhardness with temperature is shown in Figure 10. In the as-weld condition, the microhardness of the Ti3Al weld zone and the TMAZ was similar. It reached about 375 Hv, much higher than the microhardness of the base metal (300 Hv). After heat treatment at 700 ◦C, the microhardness in the weld zone increased rapidly to 450 Hv, while that in the TMAZ also increased to 425 Hv, and in the base metal did not change significantly. As the heat treatment temperature increased, the joint microhardness in the WZ and the TMAZ gradually decreased. After heat treatment at 800 ◦C, the joint microhardness in the TMAZ was close to that of the as-welded joint. When the heat treatment temperature reached 850 ◦C, the microhardness in the WZ and the TMAZ was much lower than that of the corresponding zones of the as-welded joint, which was reduced to approximately 340 Hv. When the heat treatment temperature was raised to 900 ◦C, the joint microhardness in the WZ and the TMAZ was close to that of the base metal, which was about 300 Hv. The LFW process is a non-uniform deformation process in which the temperature varies from high to low and the stress/strain also vary from high to low when the location changes from the base metal region to the weld zone. In the secondary process, accompanied with processing-hardening, phase transition, and dynamic recrystallization, there are variations in the quantity, size, and morphology of the α2, O, and β phases. For instance, the predominant action on the zone close to the base metal is processing-hardening, while the phase transition is predominant in the weld zone. As a result, the microhardness of the TMAZ and the WZ of the Ti3Al joint increases. The heat treatment on the Ti3Al LFW joint is equivalent to the aging treatment on the Ti3Al after rapid cooling and with the deformation in the O + β, α<sup>2</sup> + β + O, α<sup>2</sup> + β, and β phase regions. In the treatment, the α<sup>2</sup> phase precipitates from the metastable β matrix, which produces a strengthening effect and increases the microhardness. However, when the heat treatment temperature is too high, the α phase grows, decreasing the microhardness and reducing the strengthening effect.

**Figure 10.** Microhardness of as-welded and heat-treated joints.
