Columnar-To-Equiaxed Transition in Alloys

Microgravity Experiments

Columnar-to-equiaxed transitions (CETs) greatly affect the properties of cast materials, but the dependence on processing remains poorly understood. To under CETs better, experiments are performed on the International Space Station to study density-driven phenomena such as dendrite fragmentation. We used serial sectioning and machine-learning-based image segmentation to process data obtained from the microgravity experiments once the samples are returned to Earth. The resulting alloy microstructures are quantified by computing interface curvature values, number of independent bodies, distribution of interface normals, among other properties. The results obtained from these experiments help industry improve their predictions of the CET during casting.


Participants

Measuring Dendrite Fragmentation

3D Reconstruction of Primary Dendrites (Transparent) and Fragments (Opaque) in Al-Si7

Microstructural Evolution during Additive Manufacturing

While additive manufacturing offers exceptional control over macroscopic geometry, the properties of additively manufactured components are dictated by the solidification structures that arise during processing. One example is the “columnar-to-equiaxed transition” (CET), in which columnar grains following the direction of the melt pool suddenly transform to an equiaxed microstructure. Because equiaxed microstructures increase resistance to solidification cracking and typically impart more desirable properties, a method of controlling the CET is highly desirable.

We are developing a framework to predict the processing conditions necessary to induce the CET in modern, technologically relevant alloys. Most solidification theory deals with dilute binary alloys, but modern alloys frequently contain appreciable amounts of several different elements. Further developing solidification theory and coupling the model with a CALPHAD database will enhance our ability to predict the CET and control the microstructure and properties of additively manufactured components made from complex modern alloys.


Participants