Pore morphology of thermally regenerated porosity after HIPing of Ti-6Al-4V additively manufactured partsTuesday (05.11.2019) 16:55 - 17:15 Part of:
Argon filled gas porosity is almost ubiquitous within powder bed fusion additive manufacture of titanium. Components manufactured by electron beam melting of pre-alloyed Ti-6Al-4V powder are no exception, with a large fraction porosity observed within them nearly spherical, and thus thought to be caused by gas bubbles trapped during initial solidification. To solve this problem, many organisations are applying a hot isostatic press (HIP) treatment after additive manufacturing, and this has been found to shrink/heal the gas porosity to negligible levels. However, when subsequently reheated, for example during a β-anneal, the internal gas pressure can cause the porosity to regrow, albeit at much smaller sizes than in the as-built condition. While the smaller size of the pores would suggest their effect on mechanical properties would be reduced, here we show that the regrown porosity has a faceted, angular morphology. Thus, the regrown porosity may be more detrimental to mechanical properties than their size alone would suggest, due to the higher stress concentration resulting from angular voids in comparison to spherical porosity. To investigate this morphology, a range of experimental techniques have been applied correlatively. First, X-ray computed tomography (CT) was used to track the porosity within a sample from the as-built condition, through HIPing and heat treatment. A thermally induced pore was identified close to the surface of the sample, and then a plasma focused ion beam (P-FIB) instrument was used to place a grid of fiducial markers on the corresponding surface. A subsequent X-ray CT scan allowed accurate determination of the location of the pore relative to the markers. Finally, a plasma FIB scanning electron microscope (P-FIB-SEM) system equipped with an electron backscatter diffraction (EBSD) detector was used to serial section the volume containing the pore. In this way, the pore morphology and its relationship to the grains neighbouring it was obtained and is discussed in relation to its effect on mechanical properties.