| The influence of iron (0.5-1 wt%), Mn (0.5-1 wt%), Cu (2.25-3.25 wt%), and Mg (0.3-0.5 wt%) as well as of the trace elements Pb, Bi, Sn, and In, on the microstructure and mechanical properties of modified and grain-refined Al-10.8%Si near-eutectic alloy was investigated in both as-cast and heat-treated conditions.;In the matter of the addition of alloying elements, the results show that the modifying effect of Sr diminishes as the amount of added Cu and Mg is increased, due to the interactions between these elements, causes severe segregation of the Al2Cu phases in areas away from the modified eutectic Si and alters the precipitation sequence of the alpha-Al15 (Fe,Mn)3Si2 iron intermetallic phase from a post-dendritic reaction to a pre-dendritic one where the intermetallic is observed to occur within the alpha-Al dendrites. Depending upon the Fe and Mn content of the alloy, a coarser variation of the alpha-phase is observed in the form of polyhedral shaped particles known as "sludge". The Al2Cu phase is seen to dissolve almost completely during solution heat treatment, while Al5Cu2Mg8Si6, sludge, and alpha-Al 12(Fe,Mn)3Si2 iron intermetallic phases are found to persist for all the alloys studied, especially those containing high levels of Mg and Fe. The beta-Al5(Fe,Mn)Si iron intermetallic phase dissolves partially in the Sr-modified alloys, and its dissolution becomes more pronounced after solution heat treatment.;For the heat-treated alloys, peak aging is achieved at 180°C, although the highest quality index corresponds to 155°C aging temperature, for all the alloys investigated. Accordingly, 155°C may be considered as the optimal aging treatment. At 0.5% Mn, the beta-Fe phase forms when the Fe content is above 0.75%, causing the mechanical properties to decrease drastically. The same result is obtained when the levels of both Fe and Mn are increased beyond 0.75%, due to the formation of sludge. On the other hand, the mechanical properties of the Cu-containing alloys are affected slightly at high levels of Mg as a result of the formation of Al5Cu2Mg 8Si6 which decreases the amount of free Mg available to form the Al2CuMg phase. In alloys containing high levels of copper, the increased copper level lowers the impact properties significantly, since the fracture behaviour is now also influenced by the Al2Cu phase in addition to the Si particles. Regardless of alloy composition, the combined impact energy-percent elongation plots display linear relationships for all alloys for the as-cast and heat-treated conditions.;Multiple regression models were developed in order to predict the influence of compositional variations on the mechanical properties (UTS, YS, %El, and ET) of T6-aged Al-10.8%Si alloy. These equations, in the form of interpolation formulae, provide information on the non-conjugated as well as conjugated effects of individually varying the alloying element additions made to the alloy. The equations show that increasing the content of Cu, Mn, and Mg results in an increase in hardness and tensile strength.;In the context of this research study, where the main focus has been the development of the Al-10.8%Si alloys with a view to optimizing their machining characteristics and, hence, productivity, it was also thought worthwhile to investigate the microstructure and mechanical properties of the B319.2 and A356.2 alloys from this point of view. For this purpose, an examination of the microstructures of these alloys was thus undertaken after minor amounts of Sn had been added. Both the ductility and the toughness of as-cast B319.2 and A356.2 alloys are sensitive to variations in Sn content, while the yield strength remains practically unaffected. The higher ductility and toughness of Sn-containing alloys in the as-cast condition may be attributed mainly to the stress-strain state in the matrix material associated with the fineness of Sn-bearing phases. It may also be observed that the hardness and the strength of as-cast and heat-treated B319.2 and A356.2 alloys is reduced slightly by Sn, a fact which is believed to be due to softening of the tin-bearing phases. (Abstract shortened by UMI.)... |
