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The past efforts to develop aluminum alloy engine parts were focused on the formulation of chemical composition rather than the heat treatment conditions of aluminum alloys. Only a few studies have been made on optimizing heat treatment conditions. This paper deals with the effects of artificial aging on two aluminum alloys, A356 and A319, which have been often used for engine cylinder heads. The aluminum alloys were artificially aged under several different conditions after T6 heat treatment. The alloys were tested for such mechanical properties as pure tension, cyclic loading resistance and thermo-mechanical fatigue failure. The microstructure was observed by TEM to see the effects on microstructure in terms of mechanical properties.

Critical for the use of aluminum alloys for cylinder heads are strength and durability. In our study, the mechanism of the low cycle fatigue and creep was clarified based on the mechanical properties of aluminum alloys, such as tensile, compressive, thermal fatigue and creep characteristics, which were determined by conducting tests on test pieces. The behavior of a cylinder head under cyclic thermal condition was represented by using a high-precision analysis method with cyclic creep taken into consideration. Moreover, the high-precision analysis method turned out to be effective in estimating a low cycle fatigue life under the cyclic thermal condition.

Many methods for estimating the fatigue life of an aluminum alloy have been proposed in order to save development time and cost as demand for durable and light-weighted material grows strong. None of them, however, are practical enough to estimate the life of an engine component because thermal and mechanical loads on the engine component change as time elapses. Firstly, this paper deals with a method for clarifying the inelastic characteristics of an aluminum alloy, especially the effects of strain amplitude, ductile period (compression-tension cycle time) and temperature range on inelastic deformation, by making experiment where both thermal and mechanical loads were applied in the inverted phase(‘out-of phase’). Secondly the paper discusses a possibility of improving accuracy in determining the fatigue life of the material by introducing a new index of ‘plastic work density per second’, which is based on the conventional concept of plastic strain energy density.