Laser Welding of Medium-Manganese Steel
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Medium-Manganese (MMn) third generation advanced high strength steel (AHSS) was joined using laser welding. The effects of rapid heating and cooling thermal cycles imposed by laser welding on MMn steel was investigated. Microhardness profiles of large heat input diode laser bead-on-plate (BoP) welds found that the steel was not susceptible to heat affected zone (HAZ) softening. The peak temperature above the upper critical temperature (A3) was determined to have a significant effect on the microstructure and morphology of austenite in the HAZ. The solidification mode of the diode laser fusion zone (FZ) was primarily columnar dendritic in nature and microsegregation of Mn to the inter-dendritic spaces was observed. Higher energy density fiber laser welding was used to produce laser welded blanks (LWB) containing MMn steel, high strength low alloy (HSLA) and dual-phase (DP) steel. Columnar dendritic solidification was also observed in fiber laser welds of MMn steel. Dissimilar welds containing HSLA and DP980 were found to produce a martensitic FZ with a fraction of stable austenite. HAZ softening was also absent from microhardness profiles of fiber laser welded MMn steel due to the absence of pre-existing martensite and austenite grain growth. Sub-sized similar MMn steel laser welded tensile blanks were observed to exhibit high joint efficiency with respect to the BM. Tensile testing conducted on dissimilar blanks of HSLA and DP980 were observed to fracture in their respective BM due to a difference in yield and ultimate tensile strength. Formability of similar MMn steel LWB was limited, but welding to HSLA or DP980 improved the LWB formability. LWBs of MMn steel were investigated for potential as a new press hardened steel (PHS) chemistry. Standard size tensile geometries of MMn steel LWB showed that the FZ was sensitive to loading conditions where large strains begin to accumulate. Heat treating and quenching a LWB at an inter-critical annealing temperature showed that the austenite reverse transformation can occur in the laser weld FZ. Laser weld joint ductility was determined to significantly improve by heat treating at a lower temperature (700 °C) and a shorter time (3-4 minutes) compared to conventional boron press hardened steels.