Characterization of microstructures and mechanical properties of laser welded TWIP steel plate

In the present study, laser welding of medium-thick TWIP steel plates was studied and the microstructures and mechanical properties of the welded joints obtained at varied welding speeds were characterized. It is shown that a full penetration was achieved in the welded joint, which exhibits a keyhole-shaped configuration. The welded joint displays a single austenitic phase and typical solidification structure composed of cellular, columnar and dendrite grains. With increasing the welding speed, the grains in both the central fusion zone and adjacent heat-affected zone were refined. The latter was attributed to inadequate recrystallization because of decreased heating time at elevated welding speed. The mechanical properties of all welded joints were lower than those of base metal, in which the decline of elongation was more serious than that of strength. This is attributed to the formation of thick columnar crystals and the Al2O3 core-shell structured nanoscale precipitates in the grains of the fusion zone. Moreover, there existed a certain degree of non-uniformity in the microstructures and mechanical properties along the thickness direction of welding seam, and the samples in the upper area of welding seam showed better comprehensive properties than the lower ones. When welded at a moderate speed as 1.0 m/min, the welding seams exhibited the best mechanical properties and the least difference across the thickness direction of the welding seam. This is because the moderate welding speed resulted in the most beneficial macro- and micro-structures as well as grain boundary characteristics.