Microstructural Characterization and Mechanical Properties of Metastable Beta and α+β Titanium Alloy Electron Beam Weldments

Abstract

In the present study, the microstructural and mechanical properties of Ti-15V-3Cr-3Al-3Sn (Ti-1533) and Ti-6Al-4V (Ti-64) electron beam welds have been studied. Optical microscopy investigations revealed the presence of three different zones, namely, the fusion zone (FZ), the heat-affected zone (HAZ), and the base metal (BM). In Ti-1533 weld, the BM comprises equiaxed β grains while the FZ consists of large columnar β grains. Further, the HAZ constitutes coarse equiaxed β grains near the FZ. However, in the case of Ti-64 weld, the BM comprises a slightly elongated α phase and transformed β phase while the FZ consists of an acicular martensitic phase. Welds prepared with Ti-1533 exhibit a lower ultimate tensile strength (UTS) of 726 ± 5 MPa, yield strength (YS) of 702 ± 5 MPa, and % elongation (%El) of 12 compared to its BM (YS: 738 ± 5 MPa; UTS: 778 ± 5 MPa; %El: 15). The lower strength in Ti-1533 weld is due to the presence of coarse columnar β grains in the FZ while Ti-64 weld exhibits superior tensile properties (UTS: 993 ± 5 MPa; YS: 959 ± 4 MPa; %El: 9) compared to its BM (UTS: 910 ± 5 MPa; YS: 856 ± 5 MPa; %El: 14). The higher strength for Ti-64 weld could be attributed to the formation of acicular martensitic α′ in the FZ. However, Ti-64 welds subjected to postweld heat treatment (PWHT) showed a decrease in strength (UTS: 922 ± 4 MPa; YS: 858 ± 4; %El: 12) compared to as-welded Ti-64 welds. This is attributed to the formation of the diffusional product α+β phase in the FZ. In contrast, Ti-1533 welds subjected to PWHT showed a rapid increase in tensile property (UTS: 1224 ± 6MPa; YS: 1205 ± 8; %El: 9) values and hardness (380 HV) values compared to as-welded Ti-1533 welds. This increase in strength after PWHT is due to uniform precipitation of alpha particles in the β matrix, which was evidenced by transmission electron microscope results.