| The current study was carried out with a view to investigating the influence of a number of metallurgical parameters on the tensile properties and quality indices of two high strength Al-9%Si casting alloys, namely, 354 and 359, containing 1.8%Cu-0.5%Mg and 0.5%Mg, respectively.;An increase in the solutionizing temperature improves the strength and quality of the castings compared to the as-cast condition. In accordance with these findings, maximum safe solutionizing temperatures of 520°C and 537°C are respectively recommended for heat treating 354 and 359-type castings to produce superior strength and optimum quality. Solution treatment at these temperatures is observed to bring about a number of improvements in the microstructure by which the strength and the quality are thereby enhanced. Solution treatment at temperatures above 525°C results in the incipient melting of the Al 2Cu phases in the 354 alloys and the formation of high levels of shrinkage porosity after quenching. As a consequence of the incipient melting, silicon particles in the microstructure exhibit a polygonal morphology in areas where the melting of the Al2Cu phase has occurred.;An increase in the solution heat treatment time further enhances the tensile properties and the quality index values of the 354 and 359 alloys. Twelve hours was judged as the optimum solution heat treatment time for these castings. The solution time required to obtain a specific level of tensile properties and quality index in the unmodified 354 and 359 alloys may be shortened by modifying these alloys with strontium. However, the beneficial effect of Sr-addition on the response of the 354 and 359 alloys to solution treatment diminishes as the solution heat treatment time is increased.;Aging at 155°C is observed to produce the highest strength and optimum quality in both 354 and 359-type castings compared to aging at higher temperatures. The peak-strength observed for 354 and 359 alloys may be attained after shorter aging times on condition that the aging temperature is increased. The aging times required for reaching peak-strength in 354 alloys are 72 hours, 40 hours, 8 hours, 1 hour, and 15 minutes at aging temperatures of 155°C, 170°C, 195°C, 220°C, and 245°C, respectively, while the aging times required for reaching peak-strength in 359 alloys are 32 hours, 24 hours, 1 hour, 30 minutes, and 10 minutes, respectively, at these same aging temperatures.;An analysis of the results shows that the addition of iron has deleterious effects on both the strength and the quality of these alloys. These effects are related to the size and morphology of the iron-containing phases, specifically, beta-Al 5FeSi and pi-Al8Mg3FeSi6 present in the alloys. The addition of copper to 359-type alloys plays a significant role in improving the alloy strength; this improvement occurs, however, at the expense of ductility and therefore only a slight effect on the quality index is noted. The addition of up to 0.6% magnesium to 359-type alloys considerably improves their strength without affecting the quality. Increasing the Mg-level beyond 0.6% results in a slight increase in the alloy strength with a noticeable reduction in ductility and quality index due to the formation of a large volume fraction of the pi-Al8Mg3FeSi6 phase.;Aging at higher temperatures is accompanied by a reduction in the tensile properties and quality index value. On the other hand, it also introduces the possibility of achieving significant economic gains by minimizing the time and cost of the treatment. Aging treatment at the lower temperature of 155°C produces fine and dense precipitates having smaller inter-particle spacings, while at a higher aging temperature such as 245°C, the precipitates are coarser in size, less dense, and more widely dispersed in the matrix. The 354 alloys were observed to display higher strength levels when compared to the 359 alloys for all aging treatments. This high strength, however, was obtained at the expense of ductility, resulting in slight variations in the quality index values of the 354 castings. (Abstract shortened by UMI.)... |
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