Dynamic Tensile Behavior of Fiber Laser Welds of Medium Manganese Transformation-Induced Plasticity Steel

During a crash situation, tailor-welded blanks (TWBs) are subjected to local high strain rate (HSR) deformations. Hence, understanding the effects of fiber laser welding on the microstructure and subsequent dynamic (1 to 100s(-1)) tensile properties of the recently developed medium-Mn transformation-induced plasticity (TRIP) steel is essential. Electron backscatter diffraction (EBSD) analysis indicated that Mn segregation during rapid solidification of the FZ resulted in retention of austenite within interdendritic regions. Moreover, it was found that the lower stability of retained austenite within the intercritical heat-affected zone (HAZ) resulted in double yielding behavior of the welds due to discontinuous yielding (Luders banding). However, by increasing the strain rate, the second yield point decreased and shifted to higher strains. In the dynamic loading range, the ultimate tensile strength (UTS) and elongation of welds showed positive strain rate sensitivity (SRS). In addition, high-speed thermal camera results indicated that the average temperature decreased with increasing strain rate as a consequence of the balance between TRIP effect suppression and HSR adiabatic heating. The overall increase in elongation of the welds at higher strain rates was attributed to the higher localized softening during pre-failure nonuniform deformation.