Micro-Cracking in Medium-Carbon Steel Layers Additively Deposited on Gray Cast Iron Using Directed Energy Deposition

Single track and multi-layers of medium-carbon steel (AISI H13) powders were additively deposited on gray cast iron using a directed energy deposition (DED) technique. Multi-scale microstructure characterization using a scanning electron microscopy, an electron probe micro-analyzer and a scanning transmission electron microscopy, and the thermal analysis using finite element method were performed to understand the mechanism for the micro-crack formation, which was found at the single, double and triple layers of medium-carbon steel powders, except for the single track, deposited on a cast iron substrate. The crack formation mechanism was thoroughly investigated by combining the layer-by-layer chemistry variations and the resulting microstructural evolution upon exposure to thermal cycles during the DED process. It was concluded that micro-cracking was accompanied by the initiation of the local graphitization and the formation of plate martensite in the vicinity, which seemed to be kinetically facilitated by an indirect exposure to multiple heating-cooling thermal cycles, not by a direct exposure to a melting-cooling (solidification) cycle.