Alloy and Casting Process Optimization for Engine Block Application

摘要

The decision of many car manufacturers to use aluminium cast alloys not only for cylinder heads but also for highly stressed diesel engines leads to increasing requirements on the casting quality, the microstructure and the casting alloy. By means of new sand casting processes very low manufacturing tolerances can be achieved, while the application of local cooling gives the opportunity to locally improve the cast microstructure and hence the mechanical properties. Therefore in this paper, firstly, the beneficial effects of the application of local cooling with metal chills on the microstructural and mechanical properties of sand castings is described. Secondly, a newly developed alloy of the type AlSi7MgCuNiFe is presented and characterised in terms of tensile tests at room and elevated temperatures, fatigue tests as well as creep tests. The achieved properties measured in samples from prototype engine castings are compared with the mechanical characteristics of existing alloys for engine block applications. Due to moderate Cu, Ni and Mg additions the alloy, which is based on an A356 alloy with higher Fe tolerance provides a good compromise between high strength and creep resistance at elevated temperatures while maintaining an acceptable ductility, good fatigue life and a high thermal conductivity compared to commonly used secondary alloys. However, the improvement of mechanical properties by alloy optimisation remains marginal if the casting process and the cooling conditions are not optimised at the same time.