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Examples of lack of fusion stochastic flaws observed in Ti-6Al-4V powder bed fusion additive manufacturing builds, post-build, and post-HIP.
This project seeks to quantify the effect of defects by building >150 Ti6Al-4V powder bed fusion additive manufacturing coupons to generate statistically significant, pedigreed data.
Stochastic flaws are observed in components produced by powder bed fusion additive manufacturing (PBFAM). The naturally occurring flaws are sometimes referred to as rogue or random flaws due to their seemingly random occurrence, even under optimized processing conditions. Stochastic flaws negatively impact fatigue properties, limit the ability to define design limits, hamper qualification efforts, and thus prevent widespread use of AM in many critical applications. To date, little is known regarding the probability of stochastic flaw formation, dependencies on primary and secondary process variables, and the impact of such flaws on fatigue properties.
The objective of this project is to explore the relationships between PBFAM processes, the generation of stochastic flaws, and the subsequent impact such flaws have on fatigue properties. To achieve this objective, fatigue testing is to be performed on a high volume of AM samples which are then scrutinized via fractography, metallography, and examination of CT scans to quantify the impact of flaw characteristics on fatigue results. An additional goal is to systematically examine specific factors expected to contribute to the likelihood of formation of stochastic flaws through targeted experiments that couple comprehensive process monitoring with high-speed video.
The technical approach is to develop statistically significant, pedigreed fatigue data for more than 150 Ti-6Al-4V components. Components are to be tested using an R-ratio of 0.1, subject to recommendations from industrial partners and the DoD sustainment community. Components are being evaluated to assess the influence of (i) virgin versus heavily reused powder and (ii) PBFAM system. Since the investigation targets stochastic flaws rather than intentionally embedded flaws, all processing is utilizing OEM-recommended process parameters. Natural build variations arising from changing location on the build plate and/or build density are being explored. Given that most fatigue-limited structures are used in critical applications, all samples are undergoing HIP prior to testing. Consistency in the post-process thermal treatment is also reducing variability in microstructure, thus allowing focus to remain on the effect of flaws.
Specific factors expected to contribute to formation of stochastic flaws, (e.g. powder thickness variations, contour-hatch and hatch-hatch overlap regions, scan direction relative to gas flow, etc.) are being systematically studied through targeted experiments that couple comprehensive process monitoring with high speed video and post-build CT scans in order to gain insight into the nature of stochastic flaw generation.