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.

They were able to demonstrate that the grains re-arranging under compression opened internal pores and drew free liquid into the material, resulting in cracking.  This way of modelling the casting process could lead to improved manufacturing techniques.
 
The technique of taking a liquid alloy, and allowing it to form a solid is one of the oldest engineering processes developed by mankind, yet scientists and engineers still have difficulty in understanding and controlling defects that occur during casting processes.
 
Many of the most damaging defects develop during the final stages of casting, when a large solid network has formed, which means that the material has become less malleable and more difficult to manipulate. (continues below)
 

 

(a) Transversal (xz) slices at increasing compressive axial strain (scale bar, 1 mm), (b) change in volume of internal voids and surface-connected voids with increasing strain and (c–e) 3D rendering of surface-connected meniscus development at 73 and 93% solid, respectively (scale bar, 300 μm).
Fig. 2 from Kareh K. M. et al. Revealing the micromechanisms behind semi-solid metal deformation with time-resolved X-ray tomography. Nature Communications 5. 4464 (2014). DOI: 10.1038/ncomms5464
 

 

At present, several models for this process have been proposed; one similar to a saturated sponge being squeezed, one analogous to dispersed clay slurries and one comparable to saturated soil, where force is transmitted via contact between the grains of soil.
 
This work took place using a bespoke furnace and tension-compression rig on the Joint Engineering, Environmental, and Processing (JEEP) beamline (I12). Using synchrotron X-ray tomography, the researchers have shown that the solid grains formed within high percentage solid samples (73 % - 93 %) begin to become less spherical (more raisin shaped than pea shaped), which increases the surface area of the grains.
 

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.

 

 

Position and volumetric strain of each assembly for increasing axial strain at (a) 73% solid (scale bar, 1 mm) and (b) 93% solid (scale bar, 1 mm); 3D rendering of the polyhedron formed by the grain centroids at (c) 73% solid (scale bar, 300 μm) and (d) 93% solid (scale bar, 500 μm); Euler distance travelled by each grain per 2% incremental strain at (e) 73% solid (scale bar, 300 μm) and (f) 93% solid (scale bar, 500 μm); rotation of each grain per 2% incremental strain at (g) 73% solid (scale bar, 300 μm) and (h) 93% solid (scale bar, 500 μm); and internal porosity response at (i) 73% solid (scale bar, 100 μm) and (j) 93% solid (scale bar, 100 μm).
Fig. 4 from Kareh K. M. et al. Revealing the micromechanisms behind semi-solid metal deformation with time-resolved X-ray tomography. Nature Communications 5. 4464 (2014). DOI: 10.1038/ncomms5464.
 

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.

 
To find out more about using the I12 beamline, or to discuss potential applications, please contact Michael Drakopoulos: michael.drakopoulos@diamond.ac.uk
 

Related publication

Kareh K. M., Lee P. D., Atwood R. C., Connolley T. and Gourlay  C. M. Revealing the micromechanisms behind semi-solid metal deformation with time-resolved X-ray tomography. Nature Communications 5. 4464 (2014). DOI: 10.1038/ncomms5464