Fabrication and improvement of 316L+WC composite coatings processed by Laser Cladding

Austenitic stainless steels (e.g. SS316L) are widely used due to their corrosion resistance and good toughness. However, their applications are still limited by their relatively poor tribological properties at high temperature. Surface damage occurs in areas under contact loadings. A composite coating in those zones appears as an interesting solution, combining the matrix material with hard reinforcements such as carbides. Among the possible reinforcements, WC and SiC are promising candidate due to their hardness and theoretical high melting temperature higher than 2200°C.
Additive manufacturing, and in particular laser cladding, is a suitable technique for the manufacturing of composite coatings with new properties, impossible to achieve with the classical manufacturing processes. In laser cladding, a stream of powder, or a mixture of different powders, is fed into a focused laser beam while being scanned across a substrate, thus leaving behind a coating or object. Laser cladding process involves ultra-fast cooling rates during the solidification leading to matrix refinement and to peculiar solid-state transformations, thus giving rise to out-of-equilibrium phases.
This work considers the production and improvement of two metal matrix composite composed by 316L stainless steel and reinforcements of tungsten carbides (WC) particles or silicon carbides (SiC) particles.
The production and control of thick deposits of those two new stainless-steel alloys, with possibly several compositions, could lead to tailored reinforced microstructure. Indeed, the dissolution and interfacial reactions of WC reinforcements during casting is a well-known challenge in the production of metal matrix composites. Because of carbide dissolution during solidification and re-precipitation, several layers composed by different out-of-equilibrium phases are formed around the partially dissolving carbide.
The possible improvement and problems of those new alloys are exhibited. Moreover, the effects of the high cooling rates on the particles and the resulting microstructure are evaluated.
In order to obtain more insights into the relative contributions of the various strengthening mechanisms at play in the composites (i.e. composite effect, precipitation and solid solution strengthening), macro-hardness tests have been performed and correlated with scanning electron microscopy (SEM), and nano-indentations that have been carried out to completely characterize the out-of-equilibrium phases formed around the original reinforcements.