Fused Filament Fabrication of 316L Stainless Steel—Microstructures and Properties Arising from the Sintering Step

Fused filament fabrication of metals (FFFm) represents a potent alternative to the more established beam-based fabrication processes. Extrusion-based additive manufacturing (AM) techniques like FFFm are less expensive, result in more homogeneous microstructures and properties, and can be used for a wider range of materials [1].<br/>In FFFm the metal powder is mixed with a polymeric binder and extruded into filaments. These can be printed on commercial FFFm machines. Subsequently, the polymers are partially removed in a solvent debinding step, followed by complete thermal debinding and sintering. The multi-component binder is of paramount importance in the FFFm process. The binder composition needs to be tuned carefully to ensure that all processing steps can be performed successfully.<br/>In this study, we characterize the properties of sintered 316L stainless steel based on a binder system developed in an earlier work [2]. The influence of the sintering conditions (binder content, peak temperature, atmosphere, and cooling rate) on the microstructure is examined. The microstructure is investigated by mechanical microscopy, a recent technique using nanoindentation to map the hardness and modulus with micron-scale resolution. This technique allows for the characterization of the porosity and to distinguish between different phases and even grains. The findings are correlated with analytical SEM techniques and good agreement is found. Based on the microstructure study, the sintering conditions are optimized. The material obtained, exhibits a high macroscopic ultimate tensile strength of 561 MPa and a maximum elongation of 64%.<br/>We demonstrate that FFFm, using the stainless steel filaments developed, enables fabrication of complex shaped geometries with excellent mechanical properties, comparable to other, sintering based, fabrication routes.<br/>Further findings suggest, that high-speed nanoindentation mapping is a promising tool for studying metals with complex microstructures as fabricated by additive manufacturing, and for optimizing the processing parameters.<br/>[1] N. Tuncer and A. Bose. "Solid-state metal additive manufacturing: a review." Jom (2020): 1-22.<br/>[2] M. A. Wagner et al. “Fused filament fabrication of stainless steel structures - from binder development to sintered properties”, Additive Manufacturing (2021). (under revision)