Utilizing local orientation image analysis for microstructure quantification in additive manufacturing

One of the biggest challenges facing the field of additive manufacturing (AM) is to understand the structure/ processing relationships, particularly as a function of position within the build. This requires reliable quantification techniques to extract subtle differences in, for example, dendrite growth direction and scale. Differences in thermal gradients and cooling rates within different melt pools can influence dendrite growth, resulting in several different orientations and morphologies. Here, an automated microstructure quantification method within individual melt pools is introduced. The orientations of dendrite/cellular/grain boundaries are mapped on a local (submicron) level, and the frequency of the specific orientations is tracked relative to the build direction. This technique was used to quantify the microstructure of an additively manufactured 300 maraging steel. The orientation mapping revealed subtle differences as a function of the build height, presumably related to the different cooling rates. For example, dendrite growth at the bottom and the top of the build was characterized by a wider variation of growth directions, while in the middle part of the build, dendrite/cell growth was in more distinct directions due to more steady-state heat transfer. Local orientation image analysis provided a comprehensive approach to detect and quantify these subtle differences in microstructure and is thus an important tool for quantifying the structure/processing relationships in additively manufactured parts.