The microstructure of metal is very important, because it determines the ease with which all later stages of manufacturing (heat treatment, extrusion, machining and anodising, for example) can be carried out, as well as the final properties of the metal product. We believe that microstructural control is most effectively achieved through a command of the nucleation stage of the solidification process, which can be developed using the principles of liquid metal engineering.
In all practical cases, nucleation happens unevenly, and the structure of liquid metals is generally considered to be amorphous – but using molecular 40 dynamics simulation, we have demonstrated that there is some predictability to the process. Prior to nucleation, within the first nanometre above the substrate, the liquid tends to be layered and to exhibit a degree of order within these layers. Based on this observation, we have developed a new epitaxial nucleation model, which allows us to take greater control over this stage in the casting process.
When casters want to change the microstructure or grain of a metal product as they cast it, they often add master alloys that fine-tune the ‘recipe’ of the metal. Some of these master alloys, however – like TiB2, used in aluminium alloys – have an uncertain effect on nucleation. Using atomic-resolution imaging and chemical analysis at the national SuperSTEM facility, we have solved this problem; free titanium adsorbs onto the surface of the TiB2 particles to form a single atomic layer, and it is this layer that acts as the nucleating substrate.
We have created new master alloys for metals which have not had them in the past, like aluminium containing, magnesium-based alloys. Our studies have shown that the ductility of Mg-Al alloys is increased by our novel grain refiner, with a subsequent improvement in the deflection before failure of vehicle front-end carriers that use the material. We used a similar technology to improve the recyclability of scrap aluminium through the removal of deleterious iron impurities. Aluminium alloys used for shape casting are generally not able to match the ductility and strength of aluminium alloys for wrought processing. We have recently developed a ‘superductile’ aluminium casting alloy, however, which has sufficient ductility to allow cast vehicle body components to be more readily joined by selfpiercing riveting. We have also developed a superior high-strength casting alloy which has been licenced to a tier 1 aerospace supplier. Thoroughly mixing alloys by exposing them to high shear forces disperses harmful oxide films into benign particles, and refines the metal’s microstructure and chemistry. We have created a simple and compact rotor-stator mechanism that can be easily incorporated into conventional casting processes which accomplishes this mixing, and we have successfully combined it with both the twinroll casting process and direct-chill casting. These revolutionary approaches have allowed us to cast thin magnesium alloy sheets that can be directly stamped into component shapes, and large ingots or billets of aluminium alloys for downstream rolling or extrusion that exhibit a refined microstructure and uniform chemical composition without the need for the addition of grain refiners.