Effect of welding speed and postweld heat treatment on microstructural characterization and mechanical properties of gas tungsten arc welded Ti-15V-3Al-3Cr-3Sn joints

Abstract

Titanium alloys are extensively used in aerospace applications due to their high strength-to-weight ratio, corrosion resistance, and outstanding mechanical performance. However, welding these alloys is difficult as they are highly reactive to environmental gases (O, N, and H) above 500°C. Aerospace structures require joints of high integrity to meet the design requirements. To this concern, gas tungsten arc welding (GTAW) offers the potential to achieve welds of equal quality to electron beam welding or laser beam welding at much lower capital costs. The present study reports the influence of heat input on the evolution of microstructure and mechanical properties of Ti-15V-3Al-3Cr-3Sn (Ti-1533), a metastable beta titanium alloy welded by GTAW. The heat input can be controlled by different welding parameters like current, voltage, and welding speed. However, welding speed (15, 20, and 25 cm/min) is a crucial welding parameter that influences the cooling rate (product of thermal gradient and growth rate) and heat input. The microstructure of the fusion zone (FZ) consists of coarse columnar β grains, and coarse equiaxed β grains in the heat-affected zone, while the base metal comprises fine equiaxed β grains in all welding speeds. The average width of the FZ was found to decrease with an increase in welding speed due to lower heat input and higher cooling rate. The welds at 25 cm/min welding speed showed higher ultimate tensile strength (UTS) (654 ± 5 MPa) and hardness (240 HV) compared to 15 cm/min welding speed (UTS 593 ± 5 MPa; hardness 230 HV). The higher strength in the as-welded sample at 25 cm/min welding speed can be attributed to the lower columnar width of the β grains and the formation of equiaxed grains at the bottom portion of the weld zone. A similar trend was observed in samples subjected to the postweld heat treatment for all the weld speeds. Postweld aging of the welds prepared at 25 cm/min speed showed uniform α precipitates in the β matrix, as evidenced by transmission electron microscope results.