Effects of filler materials on the microstructural characteristics and mechanical properties of laser-welded lap joints of TZM alloys

Abstract

The poor weldability of molybdenum (Mo) alloys has always been the main problem that impedes their application as large-sized structural components. Two types of laser-welded lap joints, namely LW-Ti and LW-Nb joints, were prepared using titanium (Ti) and niobium (Nb) as filler materials to investigate the alterations in microstructures and mechanical properties of TZM (Mo-0.5Ti-0.08Zr-0.02C) alloy joints. The results indicate that both Ti and Nb have excellent metallurgical compatibility with Mo, resulting in well-formed joints. Due to solid-solution strengthening, the microhardness of the fusion zone (FZ) significantly increases from 269.9 HV to 397.1 HV for LW-Ti joints and reaches 389.6 HV for LW-Nb joints. Furthermore, the shear strength of LW-Ti joints is approximately 2.05 times that of LW-Nb joints, measuring at 110.97 MPa. Fractures in LW-Ti joints primarily occur at the base metal (BM) and display lamellar ductile fracture characteristics, while fractures in LW-Nb joints mainly occur at the FZ and exhibit coarse brittle intergranular fractures. The mechanical properties of LW-Nb joints are compromised due to the larger maximum pore diameter (300 μm) in the FZ compared to LW-Ti joints (50 μm), which can be attributed to the higher viscosity of liquid niobium. Additionally, the average grain size in the FZ of LW-Nb joints (86.37 μm) is larger than that in LW-Ti joints (53.28 μm). The reduction in grain size observed in LW-Ti joints also increases the grain boundary area, decreases oxygen concentration along the grain boundaries, and enhances binding strength at Mo grain boundaries. Given Ti's stronger affinity for oxygen compared to Mo, the abundant formation of second-phase TiO2 particles within LW-Ti joints hinders the specific formation of MoO2 along grain boundaries. Consequently, the proposed welding technology was employed to weld a large-scale cylinder-flange component made from Mo alloy, which is expected to greatly facilitate the widespread utilization of welded structures made from Mo alloys.