With the increasing requirements of performance from cast light metal components, there is a need for an improved understanding of the phenomena occurring during solidification. All solidifying metals experience deformation as they are subjected to stresses, thermal contraction, solidification shrinkage and flow. The deformation of partially solid alloys leads to a variety of casting defects in industrial processes, including porosity, segregation and hot tearing.
Much of the recent work on partially solidified alloys has focused on two areas: the first area of study is concerned the semi-solid behaviour at high solid fractions, usually fs>0.9, where hot tearing occurs. The second area of study is concentrated with low solid fractions, fs<0.5 where the semi-solid behaves as a suspension containing globular crystals, in order to create materials suitable for semi-solid metal processing.
Much less work has been done between these two solids fractions where the solid has formed a network but its strength is too small to sustain tensile stresses. Recent work has shown that the behaviour of the semi-solid metal is similar to the behaviour of a cohesionless compacted granular material, which deforms by crystal rearrangement and exhibits Reynolds' dilatancy. It has been shown that this deformation mechanism is unstable and leads to concentration of the deformation into shear bands which are responsible for defects observed in high pressure die casting. This thesis explores the rheology of solidifying alloys containing 0-60\% solid with a range of solid morphologies in order to widen the link between the behaviour of semi-solid alloys and the behaviour of compacted granular materials.
The first series of experiments on magnesium alloy AZ91 has shown two transitions in the behaviour of the semi-solid with increasing solid fraction: (i) dendrite coherency solid fraction and (ii) the cracking transition. Below dendrite coherency the material flows as a dilute suspension. Samples deformed after crystal impingement exhibit the characteristic rheological response of dilatant granular materials, including a volumetric expansion associated with the adjusting packing density of rearranging crystals (Reynolds' dilatancy). Shortly after fscoh deformation readily localizes into dilatant shear bands and liquid flow is able to accommodate the local dilatancy as the shear band contains no excess porosity in the quenched sample. During post-deformation solidification, concentrated porosity can form within these dilatant shear bands because solidification shrinkage is difficult to feed completely in AZ91 due to the wide freezing range. The transition to shear cracking was suggested to result from inadequate liquid flow in response to Reynolds' dilatancy.
Additional experimentation on the same alloy have shown that shear bands initiate close to the peak stress in samples deformed after dendrite coherency. These bands are found to be present from their formation all around the vane and measure several grain sizes wide. The bands evolve slightly during strain softening to reach an almost constant width at the end of the strain softening region.
Experiments performed on Al-10Cu with different levels of grain refiners have shown that the dendrite coherency solid fraction corresponds to the onset of dilatancy (i.e. from then on the material needs to expand in order to accommodate shear). It was also shown that the major part of the volume change occurs in the shear bands and as the size of the bands is related to the grain size, the amount of dilatancy increases with grain size. The influence of crystal morphology was difficult to observe on the volume change, but the morphology strongly influenced the dendrite coherency solid fraction. Characterisation of the shape of the shear band especially at the top and bottom surface of the sample was then performed in order to confirm that the major part of the volume change occurs within the shear bands. Investigations have also confirmed the hypothesis that the shear stress during vane rheometry acts on a cylinder defined by the extremities of the vane.
Finally, a new method to measure the dendrite coherency solid fraction is proposed. This method is based on the measurement of the volume change of a free floating ceramic board situated on top of the sample. At the dendrite coherency the feeding mechanism change from mass feeding to interdendritic feeding, leading to a dramatic decrease in the rate of contraction of the sample. It is concluded that, the rheological behaviour of semi-solid alloys at solid fraction between fscoh and fscoh +20% can be considered an assembly of largely cohesionless crystals in contact, saturated with liquid. Correspondingly, the rheology over this fs range can be interpreted within the framework of saturated granular materials consisting of deformable particles.