Study On Laser Additive Manufacturing Process And Microstructure Of Nickel-Saving Stainless Steel

The high cost of nickel in stainless steels(e.g.,austenitic and duplex types)and the adverse effects of nickel ion release on the human body have driven the development of nickel-free or nickel-saving stainless steels.The highly liberal and flexible characteristics in alloy design and fabrication of laser powder bed fusion(LPBF)gives it a unique advantage in the development of nickel-savings stainless steels.In this paper,different types of stainless steels(ferritic,duplex and austenitic types)were obtained by in situ alloying through LPBF based on Fe-Cr-Mn alloy powder mixed with certain amount of Ni powder,Cr_xN and Mn powders,and the effect of LPBF process on the forming quality,phase composition,microstructure and mechanical properties of the stainless steels was studied.Fe-Cr-Mn ferritic stainless steels were manufactured by LPBF,and it was found that laser energy densities above 100 J/mm~3 resulted in the formation of coarse columnar ferrite grains with fine acicular austenite precipitation,where the austenite has a Nishiyama-Wassermann(N-W)orientation relationship with the matrix and a high density of dislocations(stacking faults).The high crack sensitivity of high-angle columnar grain boundaries(>30°)leads to an extremely narrow process window(porosity≤0.5%and crack density≤2 mm/cm~2)based on laser power and scanning speed.The laser rescanning strategy can eliminate cracks without changing the grain morphology,crystallographic texture,and phase distribution through the synergistic effect of remelting metal to repair cracks and reducing residual stresses.Duplex(ferrite+austenite)stainless steels with high relative density(>99.5%)were successfully manufactured using mechanically mixed Fe-Cr-Mn alloy and pure Ni powders,especially samples fabricated at high energy densities of more than 194 J/mm~3,which exhibited a ferrite+austenite duplex solidification mode with nearly equivalent content of ferrite and austenite that are fine equiaxed-grained,possess significantly higher strength than duplex stainless steels prepared by conventional manufacturing.The samples prepared at low energy densities exhibit a ferrite dominated solidification mode.The difference is that at energy densities less than 60 J/mm~3,the microstructure consists of bimodal ferrite of columnar and equiaxed grains and a small amount of austenite grains located at the boundary of the melt pool;while at energy densities between 80 and 100 J/mm~3,the fine acicular Widmanst?tten austenite having Nishiyama-Wassermann(N-W)and Kurdjumov-Sachs(K-S)orientation relationships with the ferrite matrix was generated within the ferrite grains.Austenitic stainless steels were obtained by in-situ alloying of Fe-Cr-Mn alloy,Cr_xN and Mn powders.At low laser input(<80 J/mm~3),compositional inhomogeneity caused by differences in viscosity and density between melts induces ferrite nucleation.When the energy density exceeds 194 J/mm~3,fine ferrite having the K-S and N-W orientation relationships with the matrix precipitates inside the columnar austenite grains.The high melt pool temperature will lead to a decrease in the solubility of N,while the decrease in liquid viscosity and the extension of solidification time also minimize the number of pores caused by N degassing.The pitting potential is highly sensitive to the level of porosity,and the sample manufactured at high energy density(194 J/mm~3)exhibits the lowest pitting tendency due to the highest density.Due to the solid solution strengthening and grain boundary strengthening induced by N addition and the process characteristics of high dislocation density of LPBF,the samples process extremely high yield strength.