Strengthening and homogenizing strategies for PH13-8 Mo (CX) stainless steel manufactured via additive manufacturing

Abstract

Precipitation-hardening stainless steels (PHSSs) offer a unique balance of high strength, corrosion resistance, and moderate toughness, making them well-suited for critical applications in aerospace, tooling, and injection molding. This thesis focuses on processing PH13-8Mo stainless steel using two AM techniques: Wire Arc Additive Manufacturing (WAAM) and Laser Powder Bed Fusion (LPBF) with the aim of enhancing densification, controlling microstructure, and improving mechanical and tribological performance through the incorporation of ceramic particles and post-processing heat treatments. In the first phase, WAAM was used to fabricate PH13-8Mo components. The as-printed parts exhibited a columnar structure along with δ-ferrite and retained austenite, leading to anisotropic wear behavior. Post-processing heat treatments including solutionizing at 1050 °C and aging at 400–600 °C were applied. Aging at 500 °C yielded a fully martensitic microstructure and nanoscale β-NiAl precipitates. These transformations significantly enhanced hardness and wear resistance while minimizing friction and eliminating anisotropic wear response. The second phase examined the addition of TiC and TiB₂ nanoparticles during WAAM to refine grains and improve wear performance without relying solely on heat treatment. Both nanoparticles refined the grain structure and increased retained austenite content, while TiC provided the best wear resistance due to its excellent balance of hardness and fracture resistance as well as its ability to promote strain-induced martensitic transformation during sliding contact The third phase of this work focused on the LPBF processing of TiC reinforced PH13-8Mo SS powder (CX SS). A central composite design approach was adopted to evaluate the influence of laser power, scan speed, hatch spacing, and layer thickness on densification behavior across samples reinforced with 0–2 wt.% TiC. The TiC-reinforced alloys showed a broader process window and improved densification, achieving up to 100% relative density compared to 99.4% in the unreinforced counterpart. In the final phase, the mechanical behavior of LPBF-processed PH13-8Mo with TiC reinforcement was investigated. The addition of TiC simultaneously enhanced strength and ductility. The 1 wt.% TiC composition exhibited great combination of ultimate tensile strength (~1232 MPa) and elongation (~35.7%), attributed to grain refinement, retained austenite stabilization, and uniform particle distribution. Strengthening mechanisms included grain boundary strengthening, dislocation hardening, transformation-induced plasticity (TRIP), Orowan looping, and load transfer. Following heat treatment, particularly solution-aging, the 2 wt.% TiC sample achieved the highest strength (~1990 MPa) due to the combined effects of nano-precipitated β-NiAl and refined grain structure. Overall, this work advances the understanding and application of WAAM and LPBF for PHSSs. It offers practical strategies for microstructure control, mechanical and wear performance enhancement using ceramic reinforcement and targeted thermal treatments.