Effect of surface roughness on laser surface alloying of additively manufactured 17-4PH stainless steel

In the present work, the evolution of the final microstructure in 17-4PH stainless steel additively manufactured and subjected to the laser surface alloying with boron and nitrogen is described with special emphasis on the influence of surface topography and roughness. It was shown that character of the surface topography, and hence the surface roughness of the additively manufactured samples plays a major role in the development of microstructure during laser surface alloying. Dendritic microstructure of a solid solution with a small amount of eutectic in the interdendritic space was observed in a laser-melted zone (LMZ) of so-called smooth samples. In contrast, fully eutectic microstructure was revealed in the LMZ of the rough samples. This resulted in significantly different microhardness of the LMZ of both samples, i.e. 317.0 +/- 12.7 and 636.7 +/- 18.5 HV0.1 for the smooth and rough samples. The microstructural features and varying microhardness were found to be attributed to the different degree of the steel alloying primarily with boron in the LMZ, significantly affected by the initial roughness of the sample surface. This mechanism can be used to enhance laser surface alloying of the additively manufactured products.