Microstructure evolution, phase formation, corrosion, and mechanical properties of stainless steel fabricated by extrusion-based sintering-assisted additive manufacturing

In the domain of metal additive manufacturing (AM), a Fused Filament Fabrication (FFF), Debinding, and Sintering process that adopts polymer-based filaments with highly filled metal particles can be considered an economical solution to create large-size and refractory dense metal parts. Sintering, as a critical step, plays a critical role in determining the resultant microstructure uniformity of final parts. During sintering, the distribution and content of elements and phases in metal parts dictate the achieved material properties. In this work, the microstructure evolution during the entire sintering of FFF-printed 316L stainless steel (SS) parts was characterized and understood. The unique interlayer microstructures including interlayer pores and dislocation density were uncovered by characterizing grain morphologies of 316L SS at different sintering temperatures. Subsequently, the relationship between the chromium element segregation at the grain boundary and the formation of δ-ferrite phases was established through phase equilibrium simulation, phase characterization, and element analysis. Finally, the sintering temperature curve was optimized to achieve the direct preparation of single-phase austenitic SS with excellent pitting corrosion resistance and mechanical properties. The findings of this work will provide great insights into the mesoscale sintering behavior of FFF-built austenitic SS, paving new avenues for tailoring element/phase distribution to control the microstructure of metal alloys built by AM.