Optimization of beam oscillation on macro/micro formation and mechanical property in low-expansion superalloy laser-welded joint

Abstract

Low-expansion superalloys have been widely used in aerospace, ships, nuclear power fields due to the high strength and low expansion coefficient. Generally, laser beam oscillation was adopted to solve the defects including poor bead formation, inner pores, and solidification cracks in the weldment of GH909 low-expansion superalloys medium-thick plates. Therefore, the molten pool flow, joint formation, microstructure, and mechanical properties were systematically studied in this paper. Results showed that beam oscillation effectively stabilized the molten pool and keyhole inside, thus forming a well-formed joint without obvious pores. In tensile tests, the strength and elongation of the joint at 300 Hz were measured as 859.5 MPa and 38.6 %, significantly higher than those of laser joint (731.1 MPa and 17.9 %) with plenty of pores and cracks. The average microhardness of the fusion zone (FZ) using laser oscillating welding (LOW) was about 16 HV larger than that of single laser welding (SLW). In FZ, γ/Laves eutectic precipitated phase was distributed discretely without obvious solidification cracks. There existed coherent and incoherent interfaces between eutectic γ and Laves phases. Moreover, coherent interfaces were detected to account for 73.9 %, presenting a major fraction. The improvement of the mechanical properties of LOW joint was mainly caused by solid solution strengthening and precipitation strengthening. This research has a guiding reference for well-formed and high-quality low-expansion superalloy welding and relevant industrial application.