3D Characterization of Additively Manufactured Materials

High cooling rates mean that rapidly solidified metals are now available in bulk form. It is important for part qualification, however, to understand the microstructure and, in particular, porosity in additively manufactured metallic parts. Absent manufacturing defects, pores are the primary origin of fatigue failures under cyclic loading, for example.  The morphology and location of these pores can help indicate their cause; lack of fusion pores with irregular shapes can usually be linked to incorrect processing parameters, while spherical pores suggest trapped gas. Synchrotron-based 3D X-ray computed microtomography was performed at the Advanced Photon Source on additively manufactured samples of Ti-6Al-4V using electron beam powder bed and Al-10Si-1Mg using laser powder bed.  Outside of incomplete melting and keyholing, porosity appears to be inherited from pores or bubbles in the powder. This explanation is reinforced by evidence from high-speed radiography, also conducted at the APS.

Beyond measurements of porosity, 3-D printed parts are known to have residual stress as a consequence of the shrinkage that occurs on solidification as well thermal contraction.  Thanks to recent advances in high-energy (synchrotron) x-ray methods, a combination of near-field and far-field high energy diffraction microscopy (HEDM) enables the mapping of both 3-D grain structure and the lattice strains.  Preliminary measurement results are presented for printed Ti-6Al-4V. Remarkably enough, both the majority hexagonal phase and the minority bcc phase can be reconstructed. Moreover, parent bcc orientations inferred from the product hcp material agree well with the HEDM reconstructions of the bcc grains.  Once such data are available, the impact of microstructure on properties can then be evaluated.  The application of image-based spectral methods for calculating the micro-mechanical response is described, where the measured image is used as direct input.