1997-02-24

Influence of Microstructure on the Static and Thermal Fatigue Properties of 319 Alloys 970705

Modern architectures for diesel cylinder heads, especially high performance, direct injection heads for passenger cars and light trucks, require an optimized combination of design and material properties. In aluminium castings, microstructural gradients and associated fatigue and mechanical properties can result from the process selection, e. g. gravity or low pressure, and from the variable cooling rates which have to be applied to the different parts of the casting in order to get a progressive solidification and a sound part. It is thus essential to understand the relationship between the microstructure resulting from the combination of process, material choice and heat treatment, and the properties of the material.
As the most widely used material for aluminium diesel as well as gasoline cylinder heads, the 319 alloy has been selected for its superior strength. We have carried out tensile testing and thermo - mechanical fatigue testing on a range of materials. The effect of parameters such as impurity levels (further refered as grade), DAS (secondary Dendritic Arm Space), and heat treatment has been investigated. The thermo - mechanical fatigue experienced by the inter valve seats areas has been simulated on a specific rig developped by Montupet. The thermal cycle applied to the sample was chosen between 20 to 250° C. Also the material evolution in a running engine has been simulated by soaking the specimens 200 h at 200°C.
The results show that the cooling rate, which influences directly the DAS and the size of intermetallic compounds is of prime importance for the static and dynamic behavior of the material. It is observed that a very fine DAS can even compensate for a lower grade of the alloy by reducing the size and harmfulness of intermetallic compounds.
In this paper also, the effect of alloy grade and soak will be discussed in terms of microstructure changes, in correlation with the mechanical properties.

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