Abstract: Compared with conventional Ti-6Al-4V alloy, the fine grained Ti-6Al-4V has higher mechanical properties and better machinability, whereas microstructure transformation of this alloy results in reduced properties of the welded joints under welding thermal cycle. The fine structure in coarse grain region was studied by TEM and the microhardness along the welding seam was measured under three welding parameters during welding of fine grained Ti-6Al-4V with equiaxed crystal. The results show that the primary α′ martensite is greatly coarsened and the fine acicular martensite evolves into α′ plates with increasing heat input. When the heat input is higher, the coarse colony α′ composed of parallel arrangement martensite plates decreases the strength and ductility in coarse grain region. Furthermore, the substructure in martensite plates contains predominately dislocations and staking faults with a few platelets containing twins. Continued increments in heat input increase the dislocation density of α′ phase, as well as the retained β phase. A great quantity of β phase is retained when cooling to low temperature. This less-stabilized retained β phase tends to undergo transformation to coarsener second α′ phase with increasing heat input during subsequent cooling. No softened zone exists in the heat-affected zone with different welding heat inputs.

Key words: fine grained titanium alloy; coarse grain region; fine structure; martensitic phase transformation; substructure