XCT Analysis of the Defect Distribution and its Effect on the Static and Dynamic Mechanical Properties in Ti-6Al-4V Components Manufactured by Electron Beam Additive Manufacture

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Abstract

Selective electron beam melting (SEBM) is a promising powder bed Additive Manufacturing technique for near-net-shape manufacture of high-value titanium components. An extensive research program has been carried out to characterise in 3D the size, volume fraction, and spatial distribution of the pores in model samples, using X-ray computed tomography (XCT), and correlate them to the SEBM process variables. The average volume fraction of the pores (<0.2 %) was measured to be lower than that usually observed in competing processes, but a strong relationship was found with the different beam strategies used to contour, and infill by hatching, a part section. The majority of pores were found to be small spherical gas pores, concentrated in the infill hatched region; this was attributed to the lower energy density and less focused beam used in the infill strategy allowing less opportunity for gas bubbles to escape the melt pool. Overall, increasing the energy density or focus of the beam was found to correlate strongly to a reduction in the level of gas porosity. In addition, the volume fraction of pores in bulk material was found be approximately linearly related to the volume fraction of gas pores in the powder. Rarer irregular shaped pores were mostly located in the contour region and have been attributed to a lack of fusion between powder particles. When manufacturing samples with older melt strategies an extra defect type was observed: large tunnel defects that grew through the deposited layers. They develop when capillary and wetting effects overcome gravitational forces, which leads to the melted powder tracks separating by beading up, rather than filling in large voids present in the preceding layer. These samples were also used to confirm that a hot isostatic pressing cycle was able to close all internal porosity to below the resolution limit of the equipment used (~2 µm), apart from defects with surface connected ligaments. Pores were found to be crucial in determining fatigue crack initiation sites, with those at the surface found to be far more likely to initiate a crack. Finite element modelling demonstrated that pores near the sample surface generated a much higher elastic stress concentration than those in bulk material. Plotting the fatigue cycles to failure against the estimated stress intensity factor generated by the pore at the crack initiation site was found to be more informative than using the global stress, with higher stress intensities associated with shorter fatigue lives. Furthermore, by XCT analysis of machined but untested fatigue samples it was possible to predict with reasonable accuracy (>97.5 %) where fatigue cracks would initiate based on the relative stress intensity factor of all the pores. In contrast, crack growth was found to be insensitive to porosity, which was attributed to the much higher stress concentration generated by the crack in comparison to the pores. Some crack diversion was associated with the local microstructure, with prior β grain boundaries often coincident with crack diversion.

Keyword(s)

Additive manufacture; Fatigue; Mechanical properties; Pores; Selective electron beam melting; Titanium; X-ray computed tomography

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