Laser surface modification of the wrought and LPBF-printed biomedical Co-28Cr-6Mo alloys: Effects on nanoindentation and tribological behaviors

The object of this work is the effect of a laser beam surface treatment on the micro-mechanical and tribological properties of biomedical Co-28Cr-6Mo alloy manufactured by different methods (casting-forging or Laser Powder Bed Fusion (LPBF)). The wrought and LPBF alloys were superficially melted by the laser beam of 400 W power (fiber laser «TruFiber 400» (TRUMPF) of 1064 nm wavelength) under a scanning velocity of 5 mm s – 1. The research was fulfilled using an optical (GX71 OLYMPUS) and electron scanning microscopy (JSM-7000F JEOL), X-ray diffraction (X'Pert PRO, PANalytical, Cu-K( radiation), nanoindentation (“G200 Nano Indenter”, Agilent Technologies) and tribological testing (a “Ball (Al2O3)-on-Plate” sliding in a simulated body fluid). It was found that laser treatment resulted in a modified (remelted) layer with a depth of 500-550 (m having mainly the (Co phase (HCP) structure formed by dispersed dendrites. The dendrite cross-section size varied from 5-12 (m (in the primary arms) to 1.5-6.0 (m in the secondary arms; these values are much lower than the average grain size in the wrought unmodified alloy (44 µm). Laser treatment increased the hardness and yield strength of the wrought alloy by 23 % (to 5.21 GPa), and decreased the volume wear by 25 %. In contrast, laser surface melting hardly changed the micromechanical and tribological properties of the LPBF alloy since its structure was not refined relatively the as-printed (unmodified) state. However, the melting-induced densification (porosity elimination) of the modified layer was observed in the LPBF alloy thus indicating the positive effect of laser modification on the structure of the LPBD-manufactured components.