Thermal–microstructural–mechanical relationships in Fe-based tool steel coatings fabricated by laser directed energy deposition

Understanding the factors associated with hardness in laser directed energy deposition (L-DED) coatings is essential for controlling mechanical properties and reducing reliance on empirical parameter tuning. This study aims to clarify the mechanisms influencing the hardness of Fe-based tool steel coatings by sequentially analyzing microstructure and thermal history rather than direct parameter–property correlations. Coatings were fabricated under three process speeds to induce differences in the coating, and hardness distributions were examined. Microstructural features were characterized using electron backscatter diffraction (EBSD) to quantify BCC (martensite-dominant) phase fraction, grain size, and retained austenite distribution, while elemental dilution from the workpiece was analyzed by energy-dispersive X-ray spectroscopy (EDX). Thermal simulations were used to evaluate cooling rate, melt pool temperature, and melt pool morphology. Results show that hardness was correlated with BCC fraction and grain size. Elemental dilution from the workpiece was associated with increased retained austenite and reduced hardness. Additionally, spatial clustering of retained austenite, quantified by Moran’s I , further influenced hardness. Thermal analysis suggested that variations in melt pool temperature were more consistently associated with hardness than cooling rate alone, through their influence on dilution and cooling behavior in L-DED coating. Differences in melt pool morphology and overlap ratio were also associated with localized hardness reduction. By tracing the relationships from thermal history to microstructure to mechanical properties, this study provides a structured interpretation of hardness formation in L-DED coatings. The findings contribute to systematic process design and reducing dependency on trial-and-error in parameter selection. • Hardness in tool steel DED coatings analyzed via thermal–microstructural links. • BCC fraction, grain size, and retained austenite distribution are closely associated with hardness. • Moran’s I quantified the spatial clustering of retained austenite. • Melt pool temperature affects microstructural evolution in the coating. • Integrated analysis clarified process–structure–property relationships in DED.