Transmission Electron Microscopy and Nanoindentation Study of the Weld Zone Microstructure of Diode-Laser-Joined Automotive Transformation-Induced Plasticity Steel

We have used transmission electron microscopy (TEM) and nanoindentation to characterize the dominant phases present in the weld zone of a diode-laser-welded transformation-induced plasticity (TRIP) steel, examining the unaffected base metal as a baseline. The microstructure of the base metal consists predominantly of ferrite, retained austenite, martensite, and occasional large carbide particles. The dominant microstructure of the weld zone is of differently oriented packets having a bainitic morphology. The weld also contains allotriomorphic ferrite, idiomorphic ferrite, as well some twinned martensite that is surrounded by austenite. The TEM analysis of the bainitic-morphology packets indicates that they consist of a lath ferrite phase separated by an interlath carbon-enriched retained austenite. In most cases, the orientation relationship (OR) between the lath ferrite and the interlath retained austenite can be approximated as Nishiyama–Wasserman (N-W). We used site-specific nanoindentation to further characterize the packets and the allotriomorphic ferrite, confirming through the hardness values the conclusions reached by TEM. While martensite was regularly present in the base metal, it was only sparsely distributed within the weld zone, boding well for the weld’s mechanical properties.