Effect of Grain Orientation on Microstructure and Mechanical Properties of Laser Welded Joint of Additive Manufactured 300m Steel

Abstract

Laser welding was used to join the laser additive manufactured (LAM) 300M steel. The effect of grain orientations (0°, 45°, 90°) on the microstructure and mechanical properties of the laser welded joint was studied. The results showed that microstructure on both sides of the 0°-0°joint sample present full epitaxial growth of columnar grains. With the increase of orientation, between two sides of the base metal, the columnar to equiaxed transition (CET) occurred at the 45° side of the welding line for the 0°-45° joint sample and both sides of the weld center for the 0°-90° joint sample. Microstructures of the welding line for the samples with different orientations were mainly quenched martensite. The heat-affected zone of the as-welded samples showed the highest micro-hardness, then the welding line, and the base metal was the lowest. After heat treatment, the micro-hardness of the whole joint became uniform. Grain orientation had great influence on tensile strength of the sample. With the increase of orientation, the tensile strength of the joints increased while the elongation was decreased. However, the strength of the joints after heat treatment was significantly higher than that of the as-weld samples and presented little difference. Fractures mode of all joints before and after heat treatment showed the ductile fractures.

Keywords: Laser welding, additive manufacturing, 300M steel, microstructure, mechanical properties