Hotspot revealed in casting defect processes
Scientists working at the Diamond Light Source have shown that treating semi-solid alloys more like soils reveals key insights into how casting defects form.
I12 was well-suited to the specific ratios of sample size, grain size and displacement rate that we needed to use, requiring relatively large samples Ø5x5 mm to be scanned at 12s per tomogram at 12µm voxel size. I12 was also ideal for incorporating the bespoke rotating tension-compression rig with a furnace developed by Professor Peter Lee and colleagues. The combination of the beamline and the rig enabled us to directly test previously proposed micromechanics in semi-solid alloys and to prove the active mechanisms.
Dr. Kristina Maria Kareh, Department of Materials, Imperial College London.
When the sample was subject to uniaxial (from one direction) compression, the team of researchers from University College London, University of Manchester, and Diamond Light Source, showed that grains move and rotate independently of each other, which had not previously been predicted.
This caused previously tightly packed grains to push each other apart, forming small vacuums into which free surface liquid was drawn, forming the hotspots that lead to casting defects.
This finding showed that the soil mechanics model, also applicable to other systems such as rock and magma flows, explains how the solid packing density moves towards a constant value.
We are now exploring the role of welding between grains on the mechanical response of mush, and working to develop soil-inspired models to better understand defect formation casting processes.
Dr. Chris Gourlay, Department of Materials, Imperial College London.