Composition dependent laser-material interactions and microstructural evolution in directed energy deposited Alloy 718-YSZ gradient tracks

Functionally graded Alloy 718-YSZ systems fabricated using Laser Directed Energy Deposition with powder feedstock (DED-LB/p) process offer a promising pathway to integrate structural metals with thermally insulating ceramics for high-temperature applications. However, the composition-dependent melt pool behaviour and microstructural evolution arising from laser interactions with dissimilar metal-ceramic powders remain insufficiently understood. Therefore, this study investigates the influence of gradient compositions on solidified melt pool characteristics and the resulting microstructural changes in compositionally graded Alloy 718-YSZ single tracks (in 10 wt% steps) deposited by DED-LB/p under constant processing parameters. The results revealed a distinct phase separation of Alloy 718-rich and YSZ-rich regions in the solidified melt pool, primarily driven by differential melting behaviour and limited wettability between metal and ceramic phases. With increasing YSZ content, the ceramic microstructure transitioned from sintered or partially fused grains to fully melted and solidified columnar grains, which was also reflected in the microhardness response, showing increasing hardness in the ceramic-rich regions. Despite the low laser absorptivity of YSZ, complete melting and solidification were achieved in the 100 wt% YSZ sample, attributed to local heat retention effects. Cracking behaviour was governed by both the microstructural defects and the thermal expansion mismatch between the separated metal and ceramic regions. This study provides a systematic experimental understanding of composition-dependent melting behaviour, phase separation mechanisms, and microstructural evolution across the full composition range of the Alloy 718-YSZ system.