Cast Aluminum Alloys and Al-Based Nanocomposites with Enhanced Mechanical Properties at Room and High Temperature: Production and Characterization

The present PhD thesis summarizes the results of experimental activities carried out on the production and characterization of cast aluminum alloys and Al-based nanocomposites for room and high temperature applications. Two quaternary Al-Si-Cu-Mg alloys (A354 and C355) were studied, aiming to investigate the effect of chemical composition, solidification rate and heat treatment condition on the tensile and fatigue behavior at room and high temperature. Heat treatment optimization of A354 alloy was carried out. The overaging behavior of A354 and C355 alloys was compared to that of A356 (Al-Si-Mg) alloy, in order to evaluate the thermal stability of the alloys. As a result, the concurrent presence of Cu and Mg confers, by precipitation hardening, enhanced mechanical properties and higher thermal stability in comparison to the traditional Al-Si-Mg alloy. A preliminary study aimed to evaluate the effect of Molybdenum addition on A354 overaging response was also carried out. Enhanced mechanical properties after long-term overaging were registered in A354-0.3wt.%Mo alloy, in comparison to the base A354. Casting techniques for the production of Al-matrix composites were implemented at the laboratory scale. The stir-casting method assisted with ultrasonic treatment and in situ reactive casting were applied to produce Al2O3-A356 micro/nanocomposites and ZrB2-A356 composites, respectively. Friction Stir Process (FSP) was evaluated as possible solid state processing route to: (i) enhance Al2O3 nanoparticles distribution in a semisolid processed AA2024-based nanocomposite, and (ii) directly distribute Al2O3 nanoparticles into AA7075 alloy at the solid state. Experimental results highlighted difficulties in obtaining an even distribution of nanoparticles, by both liquid and semi-soli state routes, due to the low wettability of nano-sized ceramic reinforcement. The application of FSP led to enhanced nanoparticles distribution, mitigation of casting defects associated to nanoparticles addition (porosity, nanoparticles clusters) and microstructural homogenization, thus allowing to better exploit nanoparticles strengthening effect.