Understanding the Microstructure Evolution during Laser Metal Deposition of HSS M4 Obtained from Various Building Strategies, through Thermal Modelling, and Both Microstructural and Mechanical Characterizations

In this contribution, AISI HSS M4 thick deposits of various geometries (thin walls, bulk specimens, and large plate samples) are obtained from a Laser Metal Deposition (LMD) process using a medium alloy steel as the substrate. Such geometries allow us to highlight the influence of heat accumulation during LMD processing on both the solidification structure and the subsequent solid state transformations that set the macrostructure and the microstructure respectively, those features being known to strongly influence the final mechanical properties.

A validated thermal model is used to set the thermal histories of defined positions within the deposits of various geometries. Restored thermal histories are then used to determine peak temperatures and thermal gradients that influence both the solidification structure and the solid state transformations, the latter phenomena being also highlighted under dilatometry tests that have been performed. Hardness measurements performed at various scales and microstructural characterization under SEM/EDX/EBSD have been carried out, thus leading to the identification of phases that are present in the as-built samples.

The influence of both hardness and microstructure on mechanical properties and wear behavior are also discussed.