Search Results

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.

Modern automobile diesel engines make use of aluminium cylinder heads that experience both high pressure and thermal loads. Maximum temperatures are above 250°C in the valve bridge area, generating microstructural transformations in the material and thus local evolution of the mechanical properties. To be able to predict the life time of this component with a reasonable amount of confidence, it is therefore necessary to describe these changes in the material. This has been done on a variety of casting materials, with various amount of silicon and copper. Two of them have been taken as references, namely the A356 and 319 type of alloys, making extensive use of Transmission Electron Microscope (TEM) associated with Automatic Image Analysis for quantitative analysis of the precipitation stages during different heat treatments, from the as-received state to saturated aging state.