Micromechanical, corrosion and wet sliding wear behaviours of Co-28Cr-6Mo alloy: Wrought vs. LPBF

Abstract

The work is dedicated to the comparative study of a Co-28Cr-6Mo alloy fabricated by two methods (forging and laser bed fusion (LPBF)), in terms of micro-mechanical, corrosion, and tribological properties. The wrought alloy had an ε-HCP (60.4 vol%)+ γ-FCC structure with a grain size of 43.9 ± 7.1 µm and a cast-based dendritic pattern. The LPBF alloy presented a γ-FCC (72.8 vol%)+ ε-HCP structure featuring three main microstructural patterns: (1) melt pools and columnar grains, (2) a solidification cellular network, and (3) dislocations walls and strain-aged elemental segregations. The solidification of the alloys was accompanied by preferential segregations of Mo and Cr in the interdendritic areas (wrought specimen) and along the cell boundaries (LPBF specimen) with an increased (by 1.7–1.8 wt%) content of each element. The hierarchical cellular structure of the LPBF alloy with a cell size of 0.60 ± 0.05 µm ensured increased properties as compared to the wrought alloy, specifically: elastic modulus - by 6 GPa, hardness - by 0.93 GPa, wear resistance under sliding in simulated body fluid (SBF) - by 25%. Both alloys exhibited similar electrochemical behaviour, featuring true passivation, low corrosion current densities (of the order of 10–4 mA/cm2) and very high resistance to localized corrosion. Despite a more complicated and finer microstructure, the LPBF alloy presented (moderately) lower corrosion current density and passive current density as compared to the wrought alloy attributed to a lower level of residual stresses and a lower amount of martensite.