A D V A N C E D M A T E R I A L S & P R O C E S S E S | J U L Y / A U G U S T 2 0 2 2 2 5 to-machine, and facility-to-facility. Because mechanical properties are highly sensitive to small microstructure changes, they are naturally impacted by the processes which develop that microstructure. Therefore, it is essential to monitor and control all process variables such as beam power and velocity as well as production parameters such as melt pool size and build temperature. While the need to maintain process control to ensure reliable, repeatable outcomes is not unique to AM, this new technology does introduce a new family of processes to be measured and documented during production. The ASTM F42 technical committee has made significant progress in addressing these new developments and their implications for quality. Process variability is not the only source of variability in AM manufacturing. Unlike conventional casting, AM components exhibit highly directional properties based on the nature of their layer-by-layer build. Most notable is an expected lower tensile strength in the build direction (Z) as compared to the XY-plane. To qualify AM processes and parts, AM components need not only more data points through testing greater sample sizes, but also require more comprehensive data from each specimen. This requires both a quantitative increase in mechanical tests and a qualitative improvement in data handling, from supporting software architecture to managing the vast data sets generated. Inherent in the complexity and customized nature of AM component builds lies the major difficulty in part qualification and certification. Well-designed feedback controls that monitor and respond to process parameters ensure dimensional accuracy and surface condition. Load-bearing and stressed components must also be shown to exhibit satisfactory strength. Two AM components with perfectly identical geometries may exhibit wildly different mechanical properties due to their thermal history. The rapid solidification that makes AM builds possible may extend the solubility of unwanted material phases to various degrees. This in turn can negatively impact strength and ductility. Even more detrimental is possible incomplete fusion of internal structures when solidification occurs too rapidly. Digital image correlation (DIC) is an optical technique that compares images of a tested specimen’s surface to generate pictures that can be used to visualize strain and displacement over the full two-dimensional surface of the test specimen. Layer-by-layer build-up of AMmetal components presents opportunities for variability in mechanical properties. Strength and toughness must be verified through mechanical testing to confirm satisfactory performance in service.
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