Microstructural heterogeneity and strength mismatch in laser powder bed fusion maraging steel tooling repaired by laser direct energy deposition

This study demonstrates the feasibility of laser-directed energy deposition (LDED) for repairing a service-aged maraging steel high-pressure die-casting insert originally fabricated by laser powder bed fusion (LPBF). Repair of the complex geometry involved defect removal, reverse-engineered groove modelling, and optimization of LDED parameters. Dimensional restoration was achieved; however, internal defects including lack-of-fusion gaps, spherical pores, and Ti-Al-O inclusions formed due to spatter entrapment and melt pool instability. Multiscale microstructural characterization revealed pronounced differences between the repaired region and the original LPBF material despite identical nominal chemistry. The LDED repair material (LRM) consisted primarily of martensite (98.8%) with limited austenite, coarser laths, and reduced precipitate density, whereas the original LPBF material (OPM) contained higher austenite content, refined laths, and a denser precipitate population. These differences arise from the distinct thermal histories associated with LDED solidification and LPBF fabrication followed by aging. Hardness mapping and physically based strengthening analysis were used to relate microstructural features to local mechanical response. Theoretical and experimental yield strengths showed strong agreement for the OPM, while a 7.3% deviation in the LRM was attributed to distributed defects. Direct comparison of the LRM, heat-affected zone, and OPM establishes a microstructure–property map for AM repair of hierarchical maraging steel and highlights how repair-induced thermal cycles generate localized strength gradients in nominally identical alloys. • Demonstrates L-DED repair of a service-aged LPBF maraging steel tool • Links LPBF, HAZ, and L-DED processing to distinct local strength gradients • Highlights challenges of AM repair in complex, groove-confined geometries • Quantifies phase, martensite lath, and precipitate evolution in repair vs base • Identifies Ti–Al–O inclusions and porosity as potential ductility-limiting defects