An investigation on the effect of powder recycling on the microstructure and mechanical properties of AISI 316L produced by Directed Energy Deposition

Directed energy deposition (DED) has been employed to produce AISI 316L samples. Microstructure and primary cellular arm spacing (PCAS) are studied analysing the relationship with the cooling rate at the different heights of DED processed 316L stainless steel sample. It is found that, by increasing the deposition distance from the substrate, the PCAS of the sample increases from 2.9 to 4.5 mu m, as a consequence of the decreased cooling rate and thermal gradient. On the other hand, in the last deposited layers, the PCAS of the sample decreases from 4.5 to 3.3 mu m, because of the changes in cooling mechanisms. The phase composition of samples after deposition is revealed and compared with the predictions based on the Schaeffler and Pseudo-binary diagrams. It is revealed that the final microstructure is characterized by austenitic dendrites together with some residual delta ferrite located at dendritic arms location. Lastly, the effect of using fresh or recycled powders, on the microstructure and mechanical properties of DED 316L stainless steel parts is investigated. It is found that the samples fabricated using recycled powders have rather similar tensile strength levels, but much lower elongation than those produced using fresh powder due to a lower inclusions content and of their average lower size. The nature of these inclusions is discussed as well as the reason for their increase both in numbers and size.