Studies on microstructural and mechanical properties of Ti-15V 3Al-3Cr-3Sn electron beam and gas tungsten arc welds

Abstract

The main aim of the current investigation was to study the microstructural features and mechanical properties, such as hardness and tensile strength of Ti-15V-3Al-3Cr-3Sn (Ti-15-3) weldments created through electron beam welding (EBW) and gas tungsten arc welding (GTAW) at varying welding speeds. Additionally, investigated the effect of Nickel (Ni) and Silicon (Si) modified fillers on microstructural and mechanical characteristics of Ti-15-3 welds during GTAW. The studies made on weldments in as-welded and post-weld heat treatment (PWHT) conditions. Ti-15-3 alloy sheets of 3mm thickness received in the form of solution treated condition, were used in the present investigation. Autogenous full penetration bead-on-plate EB welds (by varying welding speeds - 500, 700, and 800 mm/min) and GTA welds (by varying welding speeds - 15, 20, and 25 cm/min) were made on coupons. In order to obtain complete penetration several initial trails were made with varying heat input in both welding processes. Also, welds prepared using Ni and Si modified fillers on Ti-15-3 coupons by GTAW. The varying compositions of prepared fillers are Ti-15-3-xNi (x = 0.15, 0.3, 0.5) and Ti-15-3-xSi (x = 0.15, 0.3, 0.5), respectively. Tensile test specimens were fabricated by electric discharge machining (EDM). Few tensile test specimens were subjected to a PWHT of pre-aging at 300°C for 4 h, followed by aging at 500°C for 7 h in vacuum atmosphere and furnace cooled to room temperature. Microstructural observations were made by light microscope, scanning electron microscope (SEM) and transmission electron microscope (TEM). Mechanical properties were made by hardness and tensile testing. Hardness was measured across the weldments. Tensile tests were carried out on transverse weld samples at room temperature. The fractured surfaces of tensile tested samples were examined in a SEM. The microstructure of the fusion zone (FZ) in the as-welded condition, revealed a coarse columnar β grains and the heat-affected zone (HAZ) displays coarse equaixed β grains in both EB and GTA welds. Conversely, the base metal (BM) retains fine equaixed β grains across all welding speeds in both welding conditions. The average width of the fusion zone (FZ) decreased with an increase in welding speed in both cases, primarily due to reduced heat input and increased cooling rates. The EB welds produced at a higher welding speed of 800 mm/min iii resulted in greater ultimate tensile strength (UTS) at 751 MPa and increased hardness at 245 HV, surpassing the values obtained from welds produced at a lower welding speed of 500 mm/min, with UTS at 670 MPa and hardness at 235 HV. A similar trend has been observed in welds made subjected to GTAW. The GTAW welds prepared at a higher welding speed (25 cm/min) exhibited superior UTS of 654 ± 5 MPa and hardness measuring 240 HV, surpassing the performance of welds produced at a lower speed (15 cm/min), which had a UTS of 593 ± 5 MPa and hardness of 230 HV. The enhanced strength observed at the higher welding speed can be attributed to the reduced width of columnar β grains and the development of equiaxed grains at both the weld root and center in both welding conditions. The welds after PWHT exhibited a significant rise in UTS and hardness but expense of ductility. However, this increase in UTS and hardness attributed to the presence of uniform α precipitates within the β matrix, as confirmed by TEM analysis. Microstructural examination of welds with Ni and Si modified fillers revealed equaixed grains and refined prior-β grains and characterized by nonlinear grain boundaries within the FZ. The welds prepared using Ti-15-3-0.5 Ni filler exhibited a yield strength (YS) of 688 ± 6 MPa, UTS of 721 ± 5 MPa, and % elongation (%El) of 9 ± 0.5% and Ti-15-3-0.5 Si filler (YS = 693 ± 6 MPa, UTS = 725 ± 5 MPa, %El = 8 ± 0.5%) showed higher strength compared to autogenous weld (YS = 575 ± 4 MPa, UTS = 597 ± 4 MPa, %El = 11 ± 0.5%). The increased strength in welds produced using Ti-15-3-0.5 Ni filler and Ti-15-3-0.5 Si filler can be ascribed to the reduced columnar width of β grains and the development of equiaxed grains within the FZ. A significant enhancement in both UTS and hardness were evident in samples that underwent PWHT of weldments, as compared to the as-welded specimens.