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Aluminum 351 Can Take the Heat, Study Says


A study on cast aluminum 351 for cylinder heads determined the alloy has improved tensile strength and creep resistance over more common alloys while


exhibiting good castability. QIGUI WANG AND DEVIN HESS, GM PROPULSION SYSTEMS (PONTIAC, MICHIGAN); XINYAN YAN AND FRANCIS CARON, ALCOA TECHNICAL CENTER (NEW KENSINGTON, PENNSYLVANIA)


T


he higher demand for lightweighting and fuel efficiency in combustion


engines has significantly increased engine power density, exhaust tem- peratures, and peak cylinder pressures. Cast aluminum alloys have been increasingly used in the automo- tive industry to replace cast iron in applications such as engine blocks and cylinder heads to reduce mass.


With increasing demand for fuel economy, high temperature proper- ties (including tensile, creep, and fatigue strength) of the cast alu- minum alloys become critical. This poses a significant challenge on existing cast aluminum alloys for high temperature performance. Over the past decade, the maxi-


mum operating temperature of components like cylinder heads


Fig. 1. Cyclic stress-strain responses are shown from tests performed at room temperature and 250C.


36 | MODERN CASTING July 2018


increased from approximately 338F (170C) to temperatures exceeding 392F (200C). Te higher operating temperatures result in more severe high cycle fatigue and more low cycle fatigue and/or thermo-mechanical fatigue damage in areas of cylinder heads exposed to high thermal gradi- ents, where the complex out-of-phase transient thermo-mechanical fatigue loading is produced. In today’s cylinder head designs, the most commonly used cast alu- minum alloys are A356, 319 and AS7GU (A356+0.5%Cu). A356 is a primary aluminum alloy with good ductility and fatigue properties at low to intermediate temperatures. How- ever, above approximately 392F, creep resistance and tensile strength of this alloy are rapidly degraded due to the rapid coarsening of Mg/Si precipitates in the alloy. The 319 alloy is a secondary aluminum alloy representing a lower cost alternative to the A356. The copper-bearing 319 alloy has the advantage of better tensile and creep strength at intermediate tempera- tures because the Al/Cu precipitates are stable to a higher temperature than the Mg/Si precipitates in A356. However, this alloy is prone to shrinkage porosity due to the high iron and copper content and low ductility at room temperature.


Image orginally published by Engler-Pinto Jr., et al at SAE 2014.


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