Characteristics of Nd:YAG Laser Welded 600MPa Grade TRIP and DP Steels

The characteristics of Nd:YAG laser welded 600MPa grade TRIP(transformation induced plasticity) and DP(dual phase) steels with respect to hardness, microstructures, mechanical properties and formability was investigated. A shielding gas was not used, and bead-on-plate welding was performed using various welding speeds at a power of 3.5kW. In the case of TRIP steel, the hardness was the highest at the fusion zone(FZ) and HAZ near the FZ and decreased as the base metal was approached. The maximum hardness increased with increasing welding speed to 3.6m/min and then remained. The microstructures of FZ and HAZ near the FZ consisted of ferrite and bainite for all welding speeds. In the case of DP steel, the maximum hardness was obtained at the HAZ near the FZ. It increased rapidly to 2.1m/min and then showed nearly the same value. The difference between the maximum hardness of HAZ and that of FZ increased with decreasing welding speed. The microstructure of FZ was composed of acicular ferrite but the HAZ near the FZ contained bainite and ferrite at a low welding speed. Both steels had similar tensile properties and formabilities. In a perpendicular tensile test to the weld line, all specimens were fractured at the base metal, and the strengths were somewhat higher than those of raw metals. In a parallel tensile test, the strengths of the joints were higher than those of raw materials but the elongations were lower. Formability was determined to be approximately 80% as compared with raw material at the optimum conditions. Differences in hardness near the welded zone was dependant on the relative contents of ferrite.