Investigating the Process-Structure-Property Relationships of Additively Manufactured 17-4 Precipitation Hardening Stainless Steel

Abstract

The present work aims to investigate the process-structure-property relationships for the additively manufactured 17-4 PH stainless steel to improve its mechanical performance. To this end, a multidisciplinary (i.e., micro-/defect-structure correlation with the mechanical properties) investigation is carried out to evaluate the impact of the process (i.e., additive manufacturing techniques, process parameter maneuvering, and post-process surface and thermal treatments) on the micro-/defect-structure, and further mechanical performance of the additively manufactured 17 -4 PH stainless steel. The micro-/defect-structural characterization was performed utilizing optical and scanning electron microscopy, X-ray diffraction, electron backscatter diffraction, electron dispersive spectroscopy, and X-ray diffraction computed tomography. The micro-/defect-structure results obtained for different conditions are correlated with the mechanical properties (i.e., tensile, fatigue, fatigue crack growth) and fracture behavior of the material. Several types of heat treatment procedures were applied on the additively manufactured 17-4 PH SS specimens with the as-built and machined surface conditions to find an appropriate heat treatment that results in a good combination of the tensile and fatigue properties. The selection of the most appropriate heat treatment is based on the heat treatment with and without hot isostatic pressing, basically with the presence of defects and elimination of defects, respectively. In addition, the effect of shielding gas type on the micro-/defect-structure and consequently on the mechanical properties is studied. It is shown that using the N2 as the shielding gas for the laser powder bed fused 17-4 PH stainless steel refines the micro-/defect-structure and enhances the tensile properties and fatigue up to the very high cycle fatigue regime. Although the N2 shielding gas results in smaller and fewer defects in laser powder bed fused specimens, the CA-H900 heat treatment results in more scatter in fatigue than in the CA-H1025 heat treatment condition. Therefore, the CA-H1025 heat treatment procedure is recommended for the laser powder bed fused 17-4 PH stainless steel. The 17-4 PH stainless steel’s microstructure is found to be dependent on additive manufacturing techniques, laser powder bed fusion, laser powder directed energy deposition, and metal binder jetting. Since the goal is to propose a universal heat treatment scheme, and metal binder jetting specimens are required to be hot isostatic pressed prior to heat treatment, the laser powder bed fused and laser powder directed energy deposited specimens were also hot isostatic pressed. It is seen that in case of defect elimination, the most common heat treatment, i.e., CA-H900, can be an option for the laser powder bed fused and laser powder directed energy deposited 17-4 PH stainless steel; however, metal binder jetted counterpart exhibit a large scatter in fatigue results. The CA-H1150 heat treatment condition is found to be an appropriate scheme for the hot isostatic pressed additively manufactured 17-4 PH stainless steel regardless of the additive manufacturing technique. Nonetheless, laser powder bed fused 17-4 PH stainless steel possesses the best mechanical performance process, which is attributed to the refined microstructure and the absence of δ-ferrite due to using N2 shielding gas for fabrication.