Microstructural and Thermal Characterization of 316L+WC Composite Coatings 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 is a popular candidate due to its hardness and its theoretical high melting temperature of 2600°C.
Additive manufacturing, and in particular laser cladding, is a suitable technique for the manufacturing of composite coatings, allowing for a large variability in raw materials. In laser cladding, a stream of a 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 stage and the subsequent solid state transformations, thus giving rise to out-of-equilibrium phases.
This work considers a metal matrix composite composed by 316L stainless steel and reinforcements of tungsten carbides (WC) particles (15 in vol.%)1. The dissolution and interfacial reactions of WC reinforcements during casting is a well-known challenge in the production of metal matrix composites2. Indeed, 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 microstructure away from the particles is not affected by the carbide presence and is more homogeneous. The effects of the high cooling rates on the WC particles and the resulting microstructure are evaluated. A special attention was given to the dissolution of the reinforcements by reactions with the metallic matrix and to the different phases that could form depending on the local composition. Differential Thermal Analyses (DTA) have been performed on the WC powder, and on the as-built samples at three different positions inside the deposited coating. Two constant heating rates (1 and 5°C/min) were investigated up to 1500°C in order to evaluate the stability of the reinforcements and to study the solidification sequence based on the heating curves. Scanning electron microscopy (SEM) and EBSD analyses have been performed in order to completely characterize the out-of-equilibrium phases formed around the reinforcements. The stability of the reinforcement in the original conditions and after laser cladding is verified. Most importantly, the solidification which took place during laser cladding can be restored using a cross consideration of the DTA plots and SEM observations.