When 10:30 AM - 11:30 AM Oct 20, 2017
Where 1571 G.G. Brown
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Additive Manufacturing Inspired Synchrotron Experiments and Computation: Diffraction and Dynamic X-ray Radiography

Anthony Rollet
Materials Science and Engineering, Carnegie Mellon University

High Energy (x-ray) Diffraction Microscopy (HEDM) experiments are described that provide data on 3D microstructure and local elastic strain in 3D printed materials, including Ti-6Al-4V and Ti-7Al. The reconstruction of 3D microstructure in Ti-6Al-4V is challenging because of the fine, two-phase lamellar microstructure and the residual stress in the as-built condition. Both the majority hexagonal phase and the minority bcc phase were reconstructed. Moreover, parent bcc orientations inferred from the product hexagonal material agreed well with the HEDM reconstructions of the bcc grains. To complement the measurements, the elasto-viscoplastic behavior of dual phase titanium structures was modeled using an image-based fast Fourier transform (FFT) based model, on a spectrum of varying crystal phase fractions, activated slip systems, grain sizes/morphologies and orientation distributions. Representative 3D synthetic microstructures were generated based on additively manufactured Ti-6Al-4V characteristics, and the input structures were modified for features of interest. The sensitivity to microstructure was quantified through average and local field distribution comparisons. In Ti-7Al, a small weld bead on a sample deliberately induced void formation via keyholing. HEDM measured the microstructure and residual stress, which was compared against FFT-based simulations.

Synchrotron-based 3D X-ray computed microtomography was performed at the Advanced Photon Source on additively manufactured samples of Ti-6Al-4V using both laser (SLM) and electron beam (EBM) powder bed; Al-10Si-1Mg form SLM was also characterized. Outside of incomplete melting and keyholing, porosity is inherited from pores or bubbles in the powder. This explanation is reinforced by evidence from dynamic x-ray radiography (DXR), also conducted at the APS. DXR has revealed entrapment of voids (from powder particles) in melt pools, keyholes (i.e., vapor holes) and hot cracking. Concurrent diffraction provides information on solidification and phase transformation in, e.g., Ti-6Al-4V.

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