Mechanisms of Improving the Thermal and Electrical Conductivity of Aluminum Alloy Cylinder Heads

Abstract

<p>One of the major causes of premature failure in automotive cylinder heads is the accumulation of thermal stresses, due to their large internal temperature gradients in-service. To ensure mechanical integrity, engine operating temperatures are typically restricted. However, this response reduces fuel efficiency, which increases costs and carbon emissions. A more sustainable solution can involve enhancing alloy conductivity, which improves temperature uniformity in the heads. In aluminum alloys, solidification rate, silicon modification, and precipitation heat treatment have each been reported to influence conductivity. Yet, the mechanisms and interactions regarding these parameters were unclear in the literature. Accordingly, the objective of this dissertation was to provide an in-depth, systematic investigation of the individual and combined effects of these processes on the microstructure and conductivity of automotive 319 aluminum alloy.</p> <p>After performing a baseline characterization of commercial cylinder heads, permanent-mould castings were produced with a range of solidification rates and strontium contents. These castings were analyzed to establish the dominant microstructural factors affecting electron mobility in the as-cast condition. Then, samples were subjected to multiple combinations of solution, natural aging, artificial aging, and direct aging heat treatments, to elucidate the roles of the dissolution, spheroidization and precipitation transformations. Furthermore, this work was complimented by several innovative, state-of-the-art experiments, which were the first to analyze the kinetics of solidification, chemical modification, thermal expansion, dissolution, and precipitation in 319. As a result, this dissertation contributed numerous, novel and meaningful insights regarding heat transfer in aluminum alloys. This included elucidation of the independent, major impacts of porosity, silicon morphology, and the size, distribution and coherency of the aluminum-copper precipitates, despite insensitivity to the dendritic size. The appropriate combination of solidification rate, strontium content, and heat treatment was demonstrated to promote superior conductivity to what was possible through their individual variation. Given low-porosity castings, this research revealed the potential for approximately 40% improvements in thermal and electrical conductivity, compared to the strontium-free, as-cast condition. Overall, the extensive conductivity data generated in this research supports strategic and practical materials processing procedures, demonstrates opportunities for balancing high conductivity with mechanical integrity, and ensures enhanced component performance and environmental sustainability.</p>