Influence of alloying elements iron and silicon on mechanical properties of aluminum-copper type B206 alloys

The main objective of this research study was to optimize the iron content in B206 alloys without major loss in mechanical properties, so that it can be produced from recycled materials and become cost competitive. This was done by neutralizing iron by silicon, and achieved through studies on the effects of iron and silicon additions on solidification, hot tearing, and mechanical properties of B206 alloy.;Studies on hot tearing were conducted because it is an inherent defect in aluminium-copper alloys and is deleterious to mechanical properties of the material. A Constrained Rod Casting (CRC) mould was used in this study. It is a permanent mould made of cast iron with a cavity capable of producing four 12.7mm diameter cylindrical constrained rods with nominal lengths of 50.8mm, 88.9mm, 127mm, and 165.1mm. Alloys used for these tests had chemical compositions very close to those used during solidification studies such that the differences could not significantly affects the results. Characterization techniques include physical determination of hot tear sensitivity of alloys through tear indexation and theoretical determination using the vulnerability range theory.;Results from solidification studies show that iron is precipitated mainly as either beta(CuFe) or alpha(MnFe) phases, or both, depending on the iron and silicon content, as well as the cooling rate. It was found that in alloys having up to 0.3wt% Fe, the precipitation of beta(CuFe) phase can be largely suppressed if the Fe/Si ratio is close to 1 and the cooling rate is moderately high. The low mobility of the large facets of the beta(CuFe) platelets is likely the cause limiting the amount of this phase, especially when the iron atoms have the possibility to be captured by another phase, in this case, the alpha(MnFe) phase.;Results from hot tearing studies show that the susceptibility to hot tearing is highly influenced by the iron to silicon ratio and the nominal concentration of the single elements. This influence is exerted through the determination of the amount of liquid at the eutectic temperature and the times spent in the vulnerable regime. The best resistance is obtained with both a ratio close to one and low concentrations of iron and silicon. The resistance decreases as this ratio distances itself from one. The higher this ratio, the worse the resistance to hot tearing, especially at a ratio of about two and above.;Studies on solidification were conducted because mechanical properties of a material greatly depend on its microstructure in the as-cast condition and thereby its solidification history. Different levels of Fe/Si ratios and two cooling rates (low and high) were used, with the remaining minor alloying elements kept almost constant. For each Fe/Si ratio, two levels of copper content were used in order to assess its importance. Actual casting took place in small moulds which produced samples weighing about 80 grammes.;Results from mechanical testing show that properties are highly influenced by the iron to silicon ratio and the nominal concentration of the single elements. The best properties were obtained with both a ratio close to one and low concentrations of iron and silicon, in agreement with results obtained during solidification and hot tearing studies. Two main parameters were found to determine the properties of heat treated samples, namely solubility of Al2Cu phase and dendrite coarsening. Present experimental results show that there will not be a problem with natural aging (T4) to obtain the minimum of 7% elongation required by the automotive industry by doubling or tripling the present limit of 0.1%Fe in these alloys, while increasing the strength. From calculated maximum values of strain at T4, the loss compare to B206 alloy may be narrowed to 2.5% with a good casting practice. With artificial aging (T7), it will be very difficult, if not impossible, to reach the 7% elongation at 0.2%Fe and 0.2%Si, while at 0.3%Fe and 0.3%Si it is quite impossible. Impact energy data correlates well with tensile ductility. The results shows that most of the decrease in absorbed energy of alloys containing (0.2%Fe, 0.2%Si) and (0.3%Fe, 0.3%Si) in comparison to B206 alloy is related to the crack propagation energy. (Abstract shortened by UMI.)...