On the pre-processing temperature of fallow primary and secondary recycled stainless steel powder for powder bed fusion

Some studies have reported the effect of pre-processing temperatures (550°C and 800°C) on mixed fallow powders to assess printability with powder bed fusion (PBF). However, the underlying physics of optimal pre-processing temperature is not reported. This study examines primary (1°) and secondary (2°) recycled powder of 17-4 precipitate hardened (PH) stainless steel (SS) to understand the physics behind optimal pre-processing temperature for possible reuse in the PBF. The 1° and 2° recycled 17-4 PH SS powders were characterized post-heat treatment (at different temperatures (°C): 250, 350, 450, 550, 650 and 750) to compare their surface properties. A comprehensive microstructural analysis was performed based on scanning electron microscopy (SEM) of powders processed at various temperature ranges. Single equilibrium simulations were performed on recycled powders, with input from energy-dispersive X-ray spectroscopy (EDS). The outcomes identified three temperature ranges (350, 450and 550°C) with the maximum volume fraction of stable phases. Further, X-ray diffraction (XRD), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) of fallow powder for selected temperature ranges were also performed. The results suggest that 450°C is the most suitable pre-processing temperature for 1° and 2° recycled 17-4 PH SS powder for successfully fabricating samples using PBF in the present case study.