Structure and mechanical properties of Ti2AlNb-based alloy welded joints using keyhole plasma arc welding with subsequent heat treatment

Abstract

Using keyhole plasma arc welding, welded joints of a Ti2AlNb-based alloy, VTI-4, were obtained, and their structure and mechanical properties were studied. It has been established that the dynamic effect of a keyhole arc had a positive effect on the quality of the welded joint; namely, lack of penetration, porosity, and microcracks were eliminated. The welded joint consisted of a fusion zone (FZ), a heat-affected zone (HAZ), and a base metal (BM). Depending on the phase composition and morphology of the obtained phases, the HAZ can be divided into four zones: HAZ1 with large β-phase grains near the melting line, HAZ2 with large β-phase grains + α2, HAZ3 with more fragmented β-phase grains retaining more α2-phase, and HAZ4 with the phase composition β + α2 + O. Subsequent heat treatment (HT: quenching at 920 °C for 2 h, cooling in air, followed by aging at 800 °C for 6 h, cooling in air) preserved the zone structure of the weld but led to the formation of the O-phase within β-grains. The microhardness of the weld in the zone corresponds to 360±15 HV0.2, but after HT, it increased to 382±20 HV0.2. The strength properties of the welded joint after HT were above 90 % of the base metal (σucs = 1120 MPa, σ0.2 = 1090 MPa), while elongation to failure is close to the initial condition (δ = 2.1 %).