Correlation between microstructural characteristics and cavitation resistance of Stellite-6 coatings on 17-4 PH stainless steel prepared with supersonic laser deposition and laser cladding

Abstract

Stellite-6 coatings were deposited on 17-4 PH stainless steel substrate by supersonic laser deposition (SLD) and laser cladding (LC) to improve cavitation resistance of the substrate. The microstructural characteristics of the as-deposited coatings were analyzed on the basis of OM, SEM, EBSD, XRD, Vicker’s hardness and nano-indentation results. The cavitation erosion performances in 3.5 wt% NaCl solution were comparatively investigated by a vibratory apparatus for the coatings prepared by SLD and LC technologies. The underlying mechanisms for differences of cavitation behavior between these two samples were elucidated in terms of grain size, dilution level, phase composition, hardness, elastic modulus and topographical features of the worn surfaces. Results show that SLD coating has finer grain, lower dilution and higher ratio of hardness to modulus. By analyzing the eroded surfaces, it is found that the pores formed due to mechanical bonding between particles in SLD coating are the priority position of cavitation where bubbles nucleate, grow and collapse. Repeated impact force from bubble collapse produces cracks and makes cracks propagation, leading to particle detachment and finally material removal. Although the porosity of SLD coating is higher than that of LC coating, its content is only less than 0.4%. Therefore, the negative effect of porosity is weaker than the positive effect of grain refinement, low dilution ratio and high hardness on cavitation performance. Consequently, SLD coating has better cavitation resistance than LC coating.