Microstructure characteristics and mechanical properties of fiber-diode hybrid laser welded 304 austenitic stainless steel

In the laser welding process using a focused Gaussian beam, high thermal gradient and molten pool instability usually lead to coarse columnar grain, poor surface quality and degradation of mechanical properties. In this paper, a beam shaping strategy that coaxially combines fiber laser and diode laser is applied in laser welding of austenitic stainless steel. Effects of the diode laser power on both the microstructures and mechanical properties are systematically studied by experiments and simulations. Results show that a fiber-diode hybrid heat source could effectively avoid unstable states of the keyhole, and has significant advantages in controlling spatter and surface morphology. Equiaxed grains tends to form on both sides of the junctions in synergistic and dominant regions due to the reduction in the thermal gradient produced by the hybrid beam. EBSD results show that the addition of diode beam weakens the formation of columnar grains along the easy slip texture <100> due to the disturbance in the liquid metal by a transverse distribution of the diode laser. Compared with the Gaussian heat source, the joint welded by the fiber-diode hybrid laser welding provides higher strength and elongation, with typical fracture morphologies of deeper and larger dimples. The optimum mechanical properties of the joint can be obtained by 2000 W fiber laser power and 1000 W diode laser power, where cracks occur in the base metal.