Silicon mediated twin formation in laser direct energy deposited 316L stainless steel

Microstructures of two laser direct energy deposited 316L stainless steel samples printed using the same additive manufacturing parameters and primarily differing in their Si content, 2.2wt% (316L-Si) and 0.73wt% (316L), were studied. A larger length fraction of Σ3 twin boundaries (~23% of all boundaries in austenite) was observed in 316L-Si than in 316L (~2%). The twin-related domains in 316L-Si are attributed to two mechanisms: (i) icosahedral short-range order-mediated nucleation in the melt based on observation of grain clusters sharing a common fivefold symmetry axis, and (ii) massive transformation from ferrite to austenite, confirmed by the presence of refined grains, absence of solidification cells and jagged boundaries between austenite grains. For the same printing parameters, massive transformation occurs in 316L-Si due to a higher equivalent chromium to equivalent nickel ratio (1.73) than in 316L (1.46). Thus, twin boundary fractions in additively manufactured 316L can be increased via Si addition.