Innovative mass finishing processes for intrinsically porous stainless steel components produced by metal injection moulding

Components produced by metal injection moulding (MIM) often undergo surface finishing to satisfy functional and aesthetic requirements. Most challenging surface defects are related to the intrinsic porosity, which limits the effectiveness of conventional mass finishing processes, as material removal by abrasive media is not effective on surface porosity. This study presents an innovative approach for achieving high-quality surface finishing on stainless steel MIM components. Peening processes carried out in mass finishing machines with high-density non-abrasive media are sequentially combined with conventional polishing and brightening. In-mass peening is investigated and comparatively evaluated with conventional shot-peening as pre-treatment to provide compact and pore-free surfaces potentially suitable for subsequent polishing and brightening. Porosity is determined using Archimedes’ principle and optical microscopy. Surface mechanical properties upon peening are quantified by measuring compressive stresses and microhardness. Surface finishing is assessed on the basis of roughness and gloss brightness values and by optical and electronic microscopy. All peening processes induce plastic deformation and reduce surface porosity. Shot-peening also causes a marked surface roughening, compromising further finishing steps. In-mass peening affects a thinner surface layer, avoiding surface roughening and resulting in the best performing pre-treatment. Suitably tuned in-mass peening, followed by polishing and brightening, allows for roughness decrease from Ra = 0.55 ± 0.04 µm to Ra = 0.04 ± 0.01 µm and gloss brightness values up to 457.8 ± 20.5 GU.