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

为了降低燃油消耗和随后减少污染排放的目标，汽车和航空工业必须在轻量化结构方面选择创新的方法，这与新的轻质材料的开发有关。从这个角度来看，极限抗拉强度在600 Mpa以上的高强AlZnMgCu合金具有很大的发展潜力。目前，AlZnMgCu合金主要用作锻造合金，因为这些材料对热裂纹非常敏感，限制了它们在重力压铸或高压铸造中的应用能力。通过对热裂纹形成机制的准确认识，将有可能改善其性能，使其成为轻量化铸造材料的发展趋势。本研究项目由五个次级目标组成：1.通过测定枝晶粘结性和硬度温度，研究用两个热电偶的热分析方法来预测AlZnMgCu合金的枝晶间补缩是否为合适的方法。这将通过比较热分析和流变学测试的结果来实现。搭建了一套测量装置，用于测量AlZnMgCu合金的凝固收缩起始温度、收缩应变形成的起始温度以及热裂纹的温度。计算CSC和TFR标准的假设假设应该被对树枝相干性和刚体温度的实验深刻知识所取代。由此得到的CSC和TFR准则的改进将使预测(AlZnMgCu-)合金的热裂纹倾向成为可能。如果有CSC和TFR以外的其他标准来预测热裂纹倾向，如凝固结束时残余熔体的镁含量或计算出的共晶相数量，则可通过凝固路径的热力学计算进行研究。另一个重点是综合研究了铸造工艺参数(铸型和浇注温度)和晶粒细化对热裂纹形成的影响。这将导致以具有良好热裂稳定性的AlZnMgCu为基础的用于轻量化结构的新型高强铝合金。

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.