The growing understanding of partially solid alloy rheology has already improved the prediction of casting defects including hot tears, and allowed engineers to create higher integrity castings through semisolid processing (SSP) techniques. These two examples have evolved from a dedicated research focus on two specific areas: the rheology of globular metallic suspensions, and the deformation of alloys with high solid fraction to relatively small strains. However, rheology across the whole spectrum of solid fractions, solid morphologies and deformation conditions is not well understood and the role of rheology in a range of industrial casting processes remains under-explored.
One group of casting processes where rheology is expected to play a significant role includes squeeze casting and high pressure die casting (HPDC) where elevated pressures are applied to fill narrow die cavities and to increase productivity. Even when these processes start with liquid alloy, solidification during filling and pressurisation results in large stresses and pressures acting on partially solid microstructures. This thesis investigates how rheology influences filling, feeding and microstructure formation in HPDC and, in so doing, more generally explores the rheology of solidifying alloys containing 0-50% solid with a range of solid morphologies.
Previous research has related mush rheology to the narrow bands of positive macrosegregation and porosity that commonly follow the surface contour of HPDC components. This thesis therefore uses defect bands as a feature with which to study the rheology relevant to HPDC. The approach has been to design reductive experiments which contain aspects of HPDC but which can be controlled and monitored sufficiently to study rheological mechanisms and phenomena. Throughout the research, AM and AZ series Mg-Al alloys and Al alloy A356 (Al-7Si-0.3Mg) have been used.
In the first series of experiments, a gravity-flow-through technique was developed to simulate the concurrent flow and solidification that occurs during HPDC. Alloys containing 0-30% solid were poured through an open ended, relatively cold steel die. The externally solidified crystals (ESCs) were found to migrate towards the centre of the cross-section during flow resulting in an ESC distribution similar to those in HPDC components. Additionally, bands of positive macrosegregation formed in A356 and bands of concentrated porosity formed in AM/AZ series Mg-alloys similar to those in HPDC. During gravity-flow-through experiments, it is proposed that defect bands form in the stagnant, partially solid layer emanating from the die wall due to solute-enriched liquid being drawn to a locally collapsing dendrite network. Localised collapse is suggested to be caused by the shear stress acting on the immobile solidifying mushy layer due to bulk flow through the die.
The formation of a localised band of dendrite network collapse is next investigated using vane rheometry in which a four-bladed vane is rotated for one revolution during equiaxed solidification at solid fractions in the range 0 <fs ≤ 0.5. It is found that, once the growing crystals have impinged, deformation becomes localised in a band at the vane path. Additionally, concentrated porosity forms in the localised band in Mg-Al alloys but not in Al-Si alloys, similar to defect bands in HPDC.
The findings from gravity flow-through experiments and shear rheometry experiments are then drawn together to show that, shortly after crystal impingement, solidifying alloys behave as cohesionless compacted granular materials and exhibit Reynolds dilatancy. Similar to compacted granular materials, it is suggested that after crystal impingement, crystals must push each other apart and increase the space between themselves in order to rearrange, causing the material to expand in response to shear. It is shown that defect bands in all laboratory experiments and HPDC are dilatant shear bands similar to those that form in a wide range of compacted granular materials including dense sand and glass beads. Moreover, dilatant shear bands are found to form in microstructures with a range of solid morphologies from equiaxed dendritic to globular.
It is concluded that, when defect bands form in casting processes, the material in that region consists of a geometrically crowded assembly of crystals where deformation occurs by dilatancy-enabled crystal rearrangement. After examination of HPDC microstructures, it is suggested that dilatant shear bands could form during die filling in the stagnant partially solid material at the die wall, and/or during the pressurisation stage when a large pressure differential exists between the plunger and the shrinking and contracting solidifying alloy.