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Strain evolution during hot tearing in aluminium alloys Mitchell, Jason Brian


Hot tearing refers to cracks that frequently occur within the mushy zone during cooling from the liquid to solid state during shape and ingot casting. Both ferrous and non-ferrous alloys may be affected, and there is some evidence to suggest those with long freezing ranges are more susceptible. Due to the nature of this defect the economic impact is often significant and can result in an immediate productivity loss. It is therefore important for industry to be able to better predict the susceptibility of various alloys to hot tearing. Various theories have been proposed and several different types of experimental methods have been developed to interpret the properties of alloys in the semi-solid state. However, many of these techniques do not produce good quantitative data (i.e. strain) that can be used to calibrate a thermal-mechanical computer simulation of casting. Existing experimental methods often measure strain indirectly by means of a load train frozen into the end of the casting. However, local strain at the hot tear initiation site would be more valuable for computer model calibration. Clearly, the use of traditional measurement techniques, such as strain gauges, is not a viable option and therefore an alternative was investigated. In this work the use of digital image correlation to determine the evolution of strain and strain at the onset of localisation resulting in a hot tear has been evaluated. Data has been determined for aluminium alloys AA6111, AA3104, CA32118, Al-0.5% wt pct Cu under slow cooling conditions and AA3003 under directional solidification using a water cooled copper chill. A new hot tearing experiment has been developed which localises strain to promote hot tearing to occur in only one region of the casting and is cooled by directional solidification. Images of this region were captured during solidification via a glass window embedded in the mould of the experiment. These images were correlated with each other to determine strain accumulated during hot tearing using 3rd party commercial digital image correlation software.

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