Laser powder bed fusion of immiscible steel and bronze: A compositional gradient approach for optimum constituent combination

The microstructures and the mechanical properties of a laser powder bed fusion (LPBF) manufactured alloy coupons that were compositionally graded with austenitic stainless steel 316 L and bronze (Cu10Sn), which are immiscible, were investigated. While the microstructure of the pure 316 L contains only columnar γ-Fe grains, the formation of the equiaxed α-Cu, with α-Fe particles embedded in them, at the grain boundaries of the γ-Fe grains was observed upon alloying with Cu10Sn, for up to 50 wt.%. In the graded alloys with > 50 wt.% Cu10Sn, the microstructure inverts and contains α-Fe particle embedded α-Cu matrix along with some scattered γ-Fe grains. The two-dimensional phase-field simulations were employed to reproduced the phase transition and microstructure evolution in different compositions, revealing that the nucleation of γ-Fe occurs synchronously with the spinodal decomposition of liquid and that with further cooling, Cu-rich liquid solidifies and nanoscale spherical γ-Fe precipitates appear. Cross-sections with 10–40 wt.% Cu10Sn were found structurally unreliable owing to the formation of liquation cracks and shrinkage pores, whereas the rest of the build contained few pores and was crack-free. The Scheil solidification simulations together with the crack susceptibility index calculations over the entire compositions range of the CGA, reveal that when with < 50 wt.% Cu10Sn, liquid feeding to interdendritic regions of γ-Fe is limited, which is otherwise necessary to mitigate shrinkage-induced stresses. The hardness, yield and tensile strengths of the compositionally graded alloy in the cross-section with 50 wt.% Cu10Sn are higher than those of the parent constituents and the one with 80 wt.% Cu10Sn has the best combination of strength and ductility. The variations in strength and ductility are attributed to the microstructural and compositional changes in α-Cu matrix that were influenced by the formation of α-Fe particles. Finally, the appropriate proportions and the influence of the LPBF method in successfully mixing Cu10Sn and SS316L was discussed.