Microstructures and properties of Nd:YAG laser welded 800 MPa TRIP steel

Nd:YAG laser welding of cold rolled TRIP steel sheet with tensile strength of 800 MPa and thickness of 1.2 mm was carried out. The effects of welding speed on bead morphologies and cross-section shape were investigated. The microstructures of welded joint were observed. The hardness distribution,tensile strength and formability of the welded joint were tested. The results show that for the same laser power,blowholes occur on weld bead at low welding speed and spatters occur at high welding speed. In addition,welding speed also has influence on penetration and bead width. The weld metal is mainly composed of martensite due to high cooling rate of laser welding. The amount of martensite decreases rapidly approaching base metal from weld bead center. Therefore,the weld metal or heat-affected zone has maximum hardness for the whole welded joint. The tensile strength of the welded joint perpendicular to the weld line is equal to that of the base metal,but the tensile strength of welded joint parallel to the weld line is higher than that of the base metal. The plasticity and formability of the welded joint are impaired because of the formation of martensite in the weld metal.