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Table 1. Mechanical Properties of Al-Si-1% Ba Alloys Alloy


Al-10.5% Si Al-8.5% Si Al-5.5-6.0 % Si


2.0% Ba 1.5% Ba 0.5% Ba 2.0% Ba 1.5% Ba 0.5% Ba 2.0% Ba 1.5% Ba 0.5% Ba


UTS (MPa) 145


139 135


134 132 132


127 125 123


Ductility (%) 23


24 27


25 26 26


27 29 30


Table 2. Mechanical Properties of T6 Treated Al- 5.5-6% Si-1% Ba Alloys with Various Level of Added Ba


Alloy


Al-5.5-6%%Si-2.0%Ba Al-5.5-6%Si- 1.5% Ba Al-5.5 to 6%Si- 0.5%Ba


value of about 148 MPa reported for impurity modified Al-10%Si alloys. However, ductility of the Al-10.5%Si- Ba alloys is at least 3.5 times higher than the reported alloys of about 8%


UTS (MPa) 142 137 133


Ductility (%) 31 35 32


Si. In fact, the ductility of any of these alloys is at least 2.5 times higher than any current hypoeutectic casting alloys. To see the effect of heat treat- ment, the microstructure (Fig. 3)


and mechanical properties of T6 tempered Al-6%Si alloys were investigated. A comparison of this alloy with as-cast Al-6%Si- 0.5%Ba micrograph reveals two distinct features: The silicon particles have


increased in size by 10-15% due to heat treatment. The second feature relates to


the background matrix, which contains less porosity in the heat treated alloys. Te microstructural features of the heat-treated samples changed the mechanical properties. Te UTS and ductility of the heat treated samples, shown in Table 2, were noticeably improved compared to the as-cast samples. The mechanical properties of Al-6%Si-Ba alloys show that heat treatment increased strength by 10-15% and ductility by 3-5%. The feature of mechanical properties for as-cast and T6 heat treated alloys is also evidenced in the load vs. strain plot of as-cast and T6 tempered Al-6%Si-1%Ba alloys shown in Figure 5. Te microstructure of the


hypoeutectic Al-Si-Ba alloys cast in a permanent mold exhibit high UTS and ductility values. Te silicon contents of the alloys appear uniform in size and assume sub-micron flake morphology. Te primary aluminum phase in the microstructure also is very refined compared to other lightweight Al-Si alloys. Te solid barium solu- tion in silicon appears to affect the crystallization of both the primary aluminum and eutectic silicon in the hypoeutectic Al-Si-Ba alloys when cast in a permanent mold. Te effect allows the hypoeutectic melt to nucleate eutectic silicon and primary aluminum crystal, result- ing in the development of high- strength, highly ductile Al-Si alloys. Also, additional improvements in alloy performance may be realized through heat treatment.


Tis article is based on a paper (14- 023) that was presented at the 2014 AFS Metalcasting Congress.


50 | MODERN CASTING March 2016


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