Lifetime Improvement of AlSi6Cu4 Cylinder Head Alloy

Abstract

Efficiency increase of combustion engines in automotive development is directly related to improved material performance at high temperatures. In particular, the interaction of complex thermal and mechanical forces under service loading is one of the major factors affecting the lifetime. In this paper the aluminium cylinder head alloy AlSi6Cu4 is characterized concerning the high temperature low cycle fatigue (LCF) and thermo-mechanical fatigue (TMF) behaviour. Experiments were performed in air at temperatures between 200 and 400°C. The influence of the initial heat treatment and microstructure, testing temperatures, strain amplitudes and strain rates is reported. Two heat treatment modifications (i.e. overaged condition, T7, and a modified combined heat treatment, hot iso-static pressing (HIP), prior to peak hardening according to T6) are compared to the T6 standard used in service. The strain amplitudes ranged between 0.2 to 0.7%. For the standard T6 heat treatment LCF results at 400°C indicate increased ductility and longer lifetimes compared to 200°C. For the overaged microstructure and the modified treatment HIP + T6 under TMF exposure clear lifetime enhancements are observed which are significantly highest for the HIP + T6 version. Thus, the elimination of almost any microporosity reduces local stress concentrations and early crack initiation promoting failure. Special attention is focussed on microstructural changes during high temperature exposure.