Microstructure and mechanical properties of TiN particles strengthened 316L steel prepared by laser melting deposition process

Large columnar grains of 316L stainless steel prepared by laser melting deposition (LMD) additive manufacturing process lead to low yield strength and anisotropic mechanical properties, therefore limit its usage. In this study, different contents of nanoscale TiN particles (0 wt%, 2 wt%, 4 wt% and 6 wt%) have been added into the LMD process of 316L steel to promote heterogeneous nucleations of grains, and to improve the strength. The results show that the TiN particles additions can refine the grain sizes significantly, and prompt columnar to equiaxed transitions of grains. The grain sizes can be refined more than 20 times after adding 2 wt% TiN particles. The strength of the 316L-based composites increases with the increase of TiN addition, however, at the expense of ductility. With 2 wt% TiN particle addition, the 316L-based composite can have balanced mechanical properties with high yield strength of 323–355 MPa, high tensile strength of 629–640 MPa, and good elongations of 26–30%. The strength improvements are quantitatively analyzed considering the Hall-Petch strengthening, Orowan strengthening, thermal mismatch strengthening, and load transfer strengthening. The calculations agree well with the experimental values, and reveal that the grain refinement strengthening is the main strengthening mechanism. The present study provides an important basis for fabricating high strength 316L-based composites using the LMD process.