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Table 1. Chemical Compositions of 315 and A356+0.5%Cu Alloys Alloy 351


Si 9.3 A356+0.5%Cu 7.15 Cu


1.87 0.49


Te recently developed AS7GU


alloy is a variant of A356, strengthened with 0.5%Cu. Like A356, the AS7GU alloy has good castability while the small copper addition improves creep resistance and tensile strength at inter- mediate temperatures. Both Mg/Si and Al/Cu precipitates


are thermally unstable, thus all three alloys have poor mechanical properties above 482F (250C) due to the rapid coarsening of these precipitates. Figure 1 shows that, at room temperature, the AS7GU-T64 alloy is superior to “W” and “E” 319-T7 alloys. However, at 482F, all the alloys evaluated show equivalent and significantly reduced fatigue properties compared to the room temperature data. Tis indicates that the beneficial effects of precipita- tion hardening on fatigue resistance completely disappear in the typical operating temperature range desired for engine efficiency. A new high-temperature cast aluminum alloy 351 has been regis- tered with the Aluminum Association by Alcoa and recently was studied for


Fe


0.12 0.13


Mn 0.1


0.02


Mg 0.36 0.33


Ti


0.12 0.12


Zr 0.06


V 0.07


Sr 0.01 0.007


Fig. 2. Microstructures of 351 alloy cylinder heads are shown for a non-grain refined deck face sample (a), non-grain refined high pressure oil line sample (b), grain-refined deck face sample (c), and grain-refined oil line sample (d).


Fig. 3. These charts depict quantitative analysis of dendritic arm spacing (a) and porosity (b) in the deck face and high pressure oil line areas of the cylinder heads cast in 351 alloy.


July 2018 MODERN CASTING | 37


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