Understanding microstructure formation is key in a broad range of solidification processes ranging from casting to welding. In a new study, scientists from the University of Birmingham determined how microscopic crystals grow and change shape in molten metals as they cool. Their research could help improve the tensile strength of alloys used in casting and welding.
Using high-speed synchrotron X-ray tomography, scientists photographed the changing crystal structures in molten alloys while cooling. The method shows the growth of crystals under various solidification conditions, including upwards and downwards solidification and rotational solidification under magnetic fields.
Scientists used aluminum-copper alloy for the study. They found that- as the alloy cools, the solidification process starts with faceted dendrites, formed by a layer-by-layer stacking of basic units that are just micrometers in size.
These units start as L-shaped building blocks, forming a stack, but as they cool, they change shape and convert into a U shape, then a hollowed-out cube, with some of them piled together to produce beautiful dendrites.
Dr. Biao Cai from the University of Birmingham’s School of Metallurgy and Materials said, “The findings from this new study provide a real insight into what happens at a micro level when an alloy cools and show the shape of the basic building blocks of crystals in molten alloys. Crystal shape determines the strength of the final alloy, and if we can make alloys with finer crystals, we can make stronger alloys.”
“The results are in direct contrast with the classical view of dendrite formation in cooling alloys and open the door to developing new approaches that can predict and control the formation of intermetallic crystals.”
The prototype is currently being tested using ingots provided by the Tandom Metallurgical Group, which operates an international trading operation from its base in Congleton, Cheshire, where they produce aluminum alloys, master alloys and recycle aluminum products, scraps, and drosses.
- Zihan Song, Oxana V. Magdysyuk et al. Revealing growth mechanisms of faceted Al2Cu intermetallic compounds via high-speed Synchrotron X-ray tomography. DOI: 10.1016/j.actamat.2022.117903