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Hot tearing predictions in direct chill cast aluminum AA5182 ingots Phillion, André

Abstract

Hot tearing is a major industrial problem because it affects both the quality and productivity of the casting process. These defects form in the last stage of the solidification sequence, in a region where the grains are surrounded by a continuous film of liquid and so they cannot sustain tensile strain. The application of strain can cause the metal to fail in the region between the dendrites, creating a hot tear. Previously, many researchers have focused their attention on hot tearing in cylindrical billets. With the development of a coupled thermal - stress finite element model of the aluminum Direct Chill (DC) ingot casting process at UBC, both hot tearing susceptibility and criteria validation can now be investigated in ingots of rectangular cross-section using the temperature, stress, and strain fields predicted by this process model. Two hot tearing criteria have been implemented into the DC ingot casting model: Pellini's Total Strain criterion, in which the strain accumulated during solidification is compared to a critical value, and the RDG Hot Tearing criterion, in which the magnitude of the pressure drop in the liquid due to volumetric shrinkage and mechanical loading is an indication of hot tearing susceptibility. To investigate the hot tearing predictions, two castings have been simulated - a non typical hot cast, and a non typical cold cast. It is the cold cast which is prone to hot tearing. The strain fields in the cold cast clearly show that there is a high accumulation of tensile strain during solidification on the rolling face, just above the ingot lip. This is also the region where hot tears are observed industrially. There is good agreement between the hot tearing predictions made by the Total Strain criterion, and industry observations. Using the strain predictions from the cold cast simulation, this criterion predicts a region prone to hot tearing in the middle third of the rolling face, in the start-up phase. In this critical region, the plastic strain accumulated during solidification exceeds the ductility limit at a temperature of about 575°C. Both further up the ingot, and out towards the edge of the rolling face, the plastic strain accumulated during solidification was less than the ductility limit. The conditions in the hot cast were such that the predicted plastic strain during solidification on the rolling face was also always less than the ductility limit. In contrast, there is poor agreement between the hot tearing predictions made by the RDG criterion and industry observations. In both the hot and cold cast simulations, the region predicted to be most susceptible to hot tearing is the steady-state phase, where hot tearing was not observed to occur.

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