Influence of post-processing on 17-4 PH printed via Fused Deposition Modelling of Metals
The precipitation hardening, corrosion-resistant steel 17-4 PH (1.4548) is a versatile material with high mechanical strength and good corrosion resistance. The additive manufacturing of this steel via Fused Deposition Modelling of Metals (FDMet) has only been commercially available for a few years, so there are currently few scientific sources providing information on which mechanical properties can be achieved. To fabricate parts made of 17-4 PH using FDMet, a green part is printed via FDM from a plastic filament filled with steel powder. The component is then debound and sintered in a multi-stage process similar to the one used for Metal Injection Moulding (MIM). In this way, material properties between those of components manufactured using MIM and those processed according to ASTM A564 are to be expected. Since the powder is bound by the polymer filament the process is less hazardous than Selective Laser Melting (SLM). Another benefit of this technology lies in the possibility to fabricate closed cavities. With a dual extruder it is also possible to fabricate support structures that are easily removable. In this study FDMet parts were subjected to different heat treatments like annealing and precipitation hardening. The effects of these treatments on hardness, tensile strength and porosity were compared to the as-sintered state. A further Hot Isostatic Pressing (HIP) process was carried out in an attempt to eliminate the porosity. Moreover the heat treatment was optimized using the critical temperatures calculated via CALPHAD with the as-is chemical composition of the steel. This study shows that the triangular filling of the 17-4 PH parts reduces the tensile strength significantly. The open porosity prevents a successful HIP process. However the part properties can be significantly improved by an appropriate heat treatment.