Microstructural Investigations of 17-4 PH Stainless Steel Fabricated via Laser Powder Bed Fusion

Abstract

Additive manufacturing (AM) offers significant advantages, such as design freedom and a reduction in material waste, compared to conventional manufacturing. 17-4 precipitate hardening (PH) stainless steel is used in a variety of applications including within the aerospace, marine, and chemical processing industries, largely due to its combination of corrosion resistance and its high strength, which is achieved by the formation of nanoscale Cu rich precipitates during thermal aging treatments. Owing to its favourable combination of properties, 17-4 PH has been widely researched for its applicability for AM via laser powder bed fusion (LPBF). However, a number of microstructural aspects of LPBF 17-4 PH remain unclear. There is inconsistency in literature surrounding the phase composition of as-printed builds. Furthermore, there has never been any systematic study using atom probe tomography (APT) to investigate the nanoscale Cu rich precipitates in additively manufactured 17-4 PH, despite the integral nature of these precipitates to the strength of the alloy. Given the fine scale of these precipitates in the industrially relevant heat treated states of 17-4 PH, APT is currently the only microscopic technique capable of adequately characterising them. This thesis applies multiple materials characterisation techniques, including APT, to comprehensively analyse 17-4 PH LPBF builds across multiple length scales and correlates these changes in the microstructure to mechanical properties. An initial study of the printing process on the grain structures and texture formed within builds was performed primarily using electron backscattered diffraction analysis. The next two chapters use APT to characterise the nanoscale precipitation behaviour of this material. The first investigates the effect of changing the intrinsic heat treatment on the Cu clustering and precipitation behaviour. The second analyses the phase transformation pathways in two variants of LPBF 17-4 PH resultant from standard heat treatments post AM. Finally, an in depth investigation of the mechanical properties and residual stress of builds as well as their microstructures’ response to deformation is undertaken, correlating these effects with data from neutron scattering and tomography. This thesis provides a deeper understanding of how LPBF can affect the microstructures and properties of builds, driving towards greater industrial adoption of AM