Development of high-strength AlZnMgCu-castalloys for light weight constructions - studies of mechanisms of hot tearing formation during solidification

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For the objective of reducing the fuel consumption and the subsequent reduction of polluting emissions the automotive as well as the aviation industry have to choose innovative approaches in light-weight construction, which are connected with the development of new light-weight materials. In this perspective high strength AlZnMgCu-alloys with ultimate tensile strength above 600 MPa have a very high development potential.Nowadays, AlZnMgCu-alloys are mainly used as forging alloys, because these materials are very sensitive to hot cracks which limits their ability for the usage of gravity die casting or high pressure die casting. By accurately knowing the mechanisms of the formation of hot cracks in AlZnMgCu-alloys it will be possible to improve their behavior and to establish these alloys as trendsetting casting material for light weight construction.The submitted research project consists of five sub-ordinate targets:1. It will be investigated if the thermal analysis with two thermocouples is a proper method to predict the end of mass respectively interdendritic feeding in AlZnMgCu-alloys by the determination of the Dendritecoherence- and the Rigiditytemperature. This will be accomplished by the comparison of the results of thermal analysis and those of rheological testing.2. A measurement device will be constructed which allows to measure the start temperature of solidification contraction, the start temperature of the formation of contraction strains as well as the temperature of hot cracks in AlZnMgCu-alloys.3. The hypothetical assumptions for the calculation of CSC- and TFR-criteria should be replaced with experimental profound knowledge of the Dendritecoherence- and the Rigiditytemperature. The resulting improvement of CSC- and TFR-criteria will make it possible to predict the hot crack tendency of an (AlZnMgCu-) alloy.4. It will be investigated by thermodynamic calculation of solidification paths if there are other criteria than CSC and TFR for the prediction of hot crack tendency, for example the magnesium content of the residual melt at the end of solidification or the calculated amount of eutectic phases.5. Another focus is the comprehensive investigation of the influence of casting parameters (mould and casting temperature) and grain refinement for the formation of hot cracks. This will result in new high strength aluminum alloys for light weight construction on the basis of AlZnMgCu with excellent hot crack stability.