Study of a modified AA2618 wrought aluminum alloy to enhance thermal resistance - For automotive applications
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Aluminum alloys exhibit a huge potential due to their strength-to-weight ratio and therefore play a key role as regards achieving the desired mass reduction in the automotive field. Their main limit is the loss of strength occurring at high temperatures (250-300°C), limit which has to be overcome in particular for critical components, such as pistons and engine heads. Since alloying elements and their strengthening precipitates are crucial for high temperature performance of heat treatable Al alloys, this master thesis project deals with chemical modifications and heat treatments of AA2618 wrought aluminum alloy, used in the automotive industry to produce pistons. In particular, a comparison has been made between the base material and the same alloy with a higher level of Zr, both heat treated according to the industrial practice and to an innovative heat treatment specifically studied to enhance recrystallization resistance. In order to assess the thermal stability of the alloys, specimens have been soaked at 250°C up to 50 h. The subsequent phases involved Brinell hardness measurements, Gleeble® simulations and microstructural characterizations through OM, SEM and EDS analyses.Given the starting condition as extruded bar of the base material, a considerable effort has been made to reproduce the extrusion process in a laboratory scale, but still the microstructure show differences. Moreover, chemical segregations in the modified alloy cast at NTNU laboratories menace the perfect comparability of data. Nevertheless, significant results have been pointed out: Zr additions show a valid anti-recrystallization effect and could favor alloy resistance at high temperature, but some changes in the industrial heat treatment should be taken into account to get all the benefits of chemical modifications. In fact, increasing the level of Zr means that special attentions are required to keep it in solution during the casting procedures, and further attentions are needed to enhance the formation of nanometric Al3Zr precipitates, which are the more effective in terms of thermal resistance.