Influence of post-manufacturing Hot Isostatic Pressing (HIP-ing) on the microstructure and mechanical properties of WAAM 316L stainless steel

Wire-Arc Additive Manufacturing (WAAM) of 316L stainless steel typically results in columnar grains, high dislocation densities, and residual porosity, which limit toughness compared to conventional material. This study evaluates Hot Isostatic Pressing (HIP) as a post-processing route to refine the microstructure and eliminate defects by investigating four HIP cycles (1000–1200 °C, 100–150 MPa) using EBSD-EDS, neutron diffraction, tomography, and mechanical testing. Increasing HIP temperature and pressure promoted dislocation recovery and recrystallisation while dissolving metastable δ-ferrite; however, while processing at 1000 °C induced brittle σ-phase formation, the 1200 °C/150 MPa cycle (HIP-4) produced a fully recrystallised, chemically homogeneous austenitic structure. HIP-4 reduced porosity by 98.8% and restored a mechanical response comparable to conventionally processed 316L, characterised by improved ductility and strain-hardening capacity despite a reduction in yield strength. Ultimately, HIP-4 establishes an optimal post-processing window for achieving concurrent densification and microstructural homogenisation, significantly enhancing the mechanical performance and reliability of WAAM 316L components. • Systematic assessment of HIP effects on WAAM 316L stainless steel. • Increasing HIP temperature and pressure promotes recrystallisation. • HIP-4 (1200 °C, 150 MPa) eliminates δ-ferrite and σ-phase formation. • Optimal HIP improves ductility and reduces porosity by ∼99%. • HIP transforms WAAM 316L into a homogeneous, isotropic microstructure.