Research on fiber laser welding formation, microstructure, and mechanical properties of 7.5 mm 304 stainless steel

Abstract

304 austenitic stainless steel with a thickness of 7.5 mm was welded by high-power fiber laser welding. The effects of main welding parameters, including laser power, welding speed, and defocusing amount, on weld formation were studied in detail. The welding parameters were optimized according to the weld formation. On this basis, the microstructure and mechanical properties of the welded joints were further analyzed. The results showed that depression and hump defects easily occurred once the joints were penetrated. The defects could be effectively avoided when two specific parameters were adopted: namely, high-power laser welding with high speed at the focal position or low speed at the far defocusing position; both could reduce the impact of metal vapor in the keyhole on the molten pool, and meet the requirements of penetration and stability to obtain well-formed welds. High-speed welding obtained nail-shaped weld, and Peanut-shaped weld was obtained by low-speed welding. The microstructure of the welds was mainly composed of massive austenite and a small amount of ferrite, indicating that the solidification abided by the F-A mode according to the pseudo-binary phase diagram. Due to the smaller grain size, the hardness of the Nail-shaped weld (238.8HV0.3) was higher than that of the Peanut-shaped weld (223.4HV0.3). The tensile strength of the Nail-shaped weld joint and Peanut-shaped weld joint were 677 MPa and 689 MPa, which reached more than 95% of the base metal.