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

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

Additive manufacturing of steels: a review of achievements and challenges

Metal additive manufacturing (AM), also known as 3D printing, is a disruptive manufacturing technology in which complex engineering parts are produced in a layer-by-layer manner, using a high-energy heating source and powder, wire or sheet as feeding material. The current paper aims to review the achievements in AM of steels in its ability to obtain superior properties that cannot be achieved through conventional manufacturing routes, thanks to the unique microstructural evolution in AM. The challenges that AM encounters are also reviewed, and suggestions for overcoming these challenges are provided if applicable. We focus on laser powder bed fusion and directed energy deposition as these two methods are currently the most common AM methods to process steels. The main foci are on austenitic stainless steels and maraging/precipitation-hardened (PH) steels, the two so far most widely used classes of steels in AM, before summarising the state-of-the-art of AM of other classes of steels. Our comprehensive review highlights that a wide range of steels can be processed by AM. The unique microstructural features including hierarchical (sub)grains and fine precipitates induced by AM result in enhancements of strength, wear resistance and corrosion resistance of AM steels when compared to their conventional counterparts. Achieving an acceptable ductility and fatigue performance remains a challenge in AM steels. AM also acts as an intrinsic heat treatment, triggering ‘in situ’ phase transformations including tempering and other precipitation phenomena in different grades of steels such as PH steels and tool steels. A thorough discussion of the performance of AM steels as a function of these unique microstructural features is presented in this review

Introduction of an abbreviated breast MRI service in the UK as part of the BRAID trial: practicalities, challenges, and future directions.

Abstract Introduction Women with mammographically dense breasts have an increased risk of breast cancer. Dense breasts can limit detection of small breast lesions due to overlapping fibroglandular tissue. BRAID is a randomised, multi-centre UK study assessing the impact of supplementary imaging, including abbreviated breast MRI (ABB-MRI), in detection of breast cancer in women aged 50-70 years with dense breasts. ABB-MRI is a shorter protocol breast MRI, allowing much faster acquisition times. The aim of this study is to review the feasibility of introducing ABB-MRI into the NHS setting as a part of a research trial. Protocol ABB-MRI is performed on a 1.5 tesla MRI scanner. Sequences obtained include a 2 mm axial T2 weighted sequence, a dual echo (Dixon water/fat separation) 3D DCE acquisition consisting of one pre- and two post-contrast images with generation of post-contrast subtracted images and post-processed MIP images. Scheduling ABB-MRIs are performed on a dedicated breast MRI list. Participants arrive 30 minutes prior to appointment time with pre-filled safety questionnaire. ABB-MRIs are scheduled for a 20-minute appointment slot, allowing 10 women to be scanned in a 3-hour session. Specific roles are allocated to staff members. Improving Efficiency of ABB-MRI in the UK Challenges encountered are similar to those encountered with FDP: IV cannulation, BMI and claustrophobia. Dedicated MR sessions with standard contrast doses and prefilled safety questionnaires help improve efficiency of sessions and allow examination times under 15 minutes.GE Healthcare NIHR Senior Investigator Awar