TOPA User Meeting

09.15 – 10.00

In this session, we‘ll explore the emerging research applications, along with underlying technology and methodology, stemming from 3D X-ray microscopy (XRM). As a nondestructive characterization method, XRM allows us to uniquely evaluate the internal structures of our samples and specimens at sub-micron resolution, covering multiple contrast mechanisms and length scales. Moreover, 4D investigation of structures via in situ or ex-situ repeated imaging provides new opportunities for understanding materials evolution/degradation processes, and correlative workflows linking XRM with other modalities such as EM or FIB-SEM offer the chance to easily span across a range of length scales. Several examples will be presented, emphasising the latest developments and an outlook towards the future.

10.00 – 10.20

A major limitation of additively manufactured metal parts using laser powder bed fusion (LPBF) processes is their low fatigue resistance vis-á-vis the parts manufactured using conventional processes. Where porosity, inevitable in the parts produced by powder-based processes, is the cause of the poor performance. In this work, the effects of process parameters such as laser power, layer thickness, and scan rotation on the defect formation and their influence on the fatigue performance of LPBF Ti-6Al-4V, the most widely used aerospace grade Ti alloy, were investigated using 3D tomography. The possibility of enhancing fatigue resistance via post-processing heat treatment and subsequent shot peening is also examined. A fracture mechanics-based failure envelope was proposed as a guide for the use of LPBF Ti64 in fatigue loading applications, assuming the presence of pores is inevitable.

11.00 – 11.20

The unnotched fatigue behavior of the 17-4 PH martensitic (α’) stainless steel, additively manufactured using the binder jet printing technique (BJP), with varying porosity levels and in two different aging conditions are investigated and are compared to that of the conventionally manufactured (CM) alloy. As expected, fatigue strength is enhanced by the reduction in porosity, with hot isostatic pressing of the BJP alloy resulting in a fatigue strength that is similar to the CM alloy. Over-aging of the alloy improves its fatigue resistance further. These variations are rationalized by recourse to the analysis of the microstructure-defect interactions based on microscopy and x-ray tomography. Furthermore, it was found that the combination of plasticity induced crack closure mechanisms and transformation induced plasticity are dominant at the fatigue crack tip of the over-aged alloy. Fracture mechanics-based Kitagawa and Takahashi diagrams were utilized to illustrate the significance of pore size on the unnotched fatigue resistance of BJP 17-4 PH alloys. Lastly, pore distribution played a critical role in the propagation of fatigue cracks, which eventually affected the fatigue life. These results are discussed in terms of designing alloy fabrication using BJP specifically for cyclic loading conditions.