CORRELATION BETWEEN MICROSTRUCTURAL AND MECHANICAL PROPERTIES OF ALUMINUM ALLOY CASTINGS PRODUCED BY DIFFERENT FOUNDRY PROCESSES

Aluminum is quite a new material and the production of aluminumalloy castings has greatly increased in the last recent years. Nowadays, there is a continuous market requirement to produce lighter vehicles and to increase fuel efficiency, therefore roughly two thirds of all aluminum castings production is within the automotive field. Even though the applications of aluminum in this scenario are considerable, some aspects affecting the quality and the soundness of cast products are still not fully understood. The aim of the research work presented in this PhD thesis was to study the correlation between mechanical and microstructural properties of Al-Si castings, in order to contribute to fill this lack of knowledge. The majority of my research work is included here as a collection of four papers submitted in the form they were published or submitted for publication. Each paper is a standalone work with separate abstract, introduction, experimental procedure, results, conclusions and reference sections. Different kinds of automotive and motorcycle structural components, realised by means of low pressure die casting and permanent mould gravity casting, and experimental components realised by sand mould casting were produced and studied. Impact strength and tensile strength tests were performed in order to understand the correlation between microstructural and mechanical properties better. The effect of different cooling rates, eutectic modification, defects, heat treatments and type of cores was studied with the aim to improve the design of aluminum alloy structural components. Moreover, the effect of the eutectic microstructure on the anodizing surface treatment in aluminum-silicon alloys was studied. The influence of silicon content, morphology and distribution of the eutectic phase on the anodizing process was investigated on both sand-cast and die-cast samples. Different techniques for the microstructural examination were employed in the research study; Optical Microscopes (OM) and Scanning Electron Microscopes (SEM) with Energy Dispersive X-ray (EDS) microprobe were used and also X-ray investigations were carried out to verify the presence or the absence of porosities and defects in the castings. In addition, numerical simulations were carried out and the results were compared with those obtained by microscopy. In terms of utility, the numerical simulations were able to predict the formation of macro-defects and the final scale of microstructure within the castings, confirming their potential as an engineering tool for predicting microstructural and mechanical properties throughout the castings.