Characterization of Porosity Defects in Selectively Laser Melted IN718 and Ti-6Al-4V via Synchrotron X-Ray Computed Tomography

Additive manufacturing (AM) is a method of fabrication involving the joining of feedstock material together to form a structure. Additive manufacturing has been developed for use with polymers, ceramics, composites, biomaterials, and metals. Of the metal additive manufacturing techniques, one of the most commonly employed for commercial and government applications is selective laser melting (SLM). SLM operates by using a high-powered laser to melt feedstock metal powder, layer by layer, until the desired near-net shape is completed. Due to the inherent function of AM and particularly SLM, it holds much promise in the ability to design parts without geometrical constraint, cost-effectively manufacture them, and reduce material waste. Because of this, SLM has gained traction in the aerospace, automotive, and medical device industries, which often use uniquely shaped parts for specific functions. These industries also have a tendency to use high performance metallic alloys that can withstand the sometimes-extreme operating conditions that the parts experience. Two alloys that are often used in these parts are Inconel 718 (IN718) and Ti-6Al-4V (Ti64). Both of these materials have been routinely used in SLM processing but have been often marked by porosity defects in the as-built state. Since large amounts of porosity is known to limit material mechanical performance, especially in fatigue life, there is a general need to inspect and quantify this material characteristic before part use in these industries. One of the most advanced porosity inspection methods is via X-ray computed tomography (CT). CT uses a detector to capture scattered X-rays after passing through the part. The detector images are then reconstructed to create a tomograph that can be analyzed using image processing techniques to visualize and quantify porosity. In this research, CT was performed on both materials at a 30 µm “low resolution” (LR) for different build orientations and processing conditions. Furthermore, a synchrotron beamline was used to conduct CT on small samples of the SLM IN718 and Ti64 specimens at a 0.65 µm “high resolution” (HR), which to the author’s knowledge is the highest resolution (for SLM IN718) and matches the highest resolution (for SLM Ti64) reported for porosity CT investigations of these materials. Tomographs were reconstructed using TomoPy 1.0.0, processed using ImageJ and Avizo 9.0.2, and quantified in Avizo and Matlab. Results showed a relatively low amount of porosity in the materials overall, but a several order of magnitude increase in quantifiable porosity volume fraction from LR to HR observations. Furthermore, quantifications and visualizations showed a propensity for more and larger pores to be present near the free surfaces of the specimens. Additionally, a plurality of pores in the HR samples were found to be in close proximity (10 µm or less) to each other