OptimizatiOn Of an aluminum Cylinder Head allOy Of
tHe alSi7Cu3mnmg type reinfOrCed by additiOnS Of periteCtiC elementS M. Garat
Rio Tinto Alcan, Voreppe, France Copyright © 2011 American Foundry Society abstract
Recent trends in engine design aimed at increasing fuel efficiency, lowering emissions, and reducing size all tend to increase the thermo-mechanical loads seen by cylinder heads. Among several approaches which can be used to combat this problem, the use of new alloys with improved elevated temperature properties (namely tensile and creep strengths) is a promising one.
In a previous paper,1 an AlSi7Cu3Mn alloy reinforced by
a combination of the peritectic forming elements V, Zr, and Ti was described. This Mg-free alloy, heat treated to a T7 temper, offered an interesting combination of elevated temperature and creep strength but suffered from a relatively low room temperature yield strength and poor low cycle fatigue strength. Hence R&D work
introduction
The present article constitutes a sequel to the paper first pub- lished in the 2007 AFS Transactions and later reprinted in Hommes et Fonderie.1
In this previous work, a new Mg-free
AlSi7Cu3, reinforced by additions of Mn, V, Zr and Ti, had been described. This alloy, mainly intended for common rail diesel cylinder heads, has excellent properties in the critical 250 – 300C (482-572F) temperature range now reached in the hottest spots of the combustion chamber. At room tem- perature, it exhibits a high ultimate tensile strength (UTS) [> 330 MPa], a good ductility (6 – 8%) and an excellent high cycle fatigue strength (HCF) for a T7 treated alloy: 120 -135 MPa at 10 million cycles (R = -1).
But this alloy also shows two weak points at room tem- perature: its limited tensile yield strength (TYS) [195 – 230 MPa] had initially been considered acceptable, but further characterization showed that its low cycle fatigue strength (LCF) was also much lower than that of the now widely used AlSi7Cu0.5Mg0.3 T7 (Figure 1).
Another potential problem with the initial formulation of this alloy is the risk of precipitation and settling of coarse V containing intermetallic compounds during the holding of the melt, because of the relatively high V content of 0.25%.
International Journal of Metalcasting/Summer 2011
The chosen amount of Z, about 0.14%, does not pose a major problem as it is sufficient to hold the melt at 700C (1292F) minimum. But the original 0.25% V is quite an- other matter and requires a minimum holding temperature of 745C (1373F), which is some 25 to 45C higher than the usual modern practice. Consequently, it also felt necessary to try and reduce the amount of V, which is economically desirable since V is a costly metal.
The recent trends in engine design, such as higher fuel efficiency, lower emissions and downsizing, all tend to increase the thermo-mechanical efforts undergone by the cylinder heads, not only in diesel engines, but also in tur- bo-charged gasoline engines, which are now coming back in force. For this reason, it was decided to continue the development of this promising alloy base and correct its weak points. It was also decided to take this opportunity to measure the heat conductivity over the relevant tem- perature range, room temperature (RT) to 300C (572F), as this property plays an important role in the design of modern cylinder heads.
17 was pursued to improve these properties.
The present paper describes the final optimized alloy that has evolved out of this work. Adding a small amount of Mg has strongly improved the room temperature properties as well as the creep strength, while allowing the amount of V needed to be reduced without undo loss in creep strength. The 250 to 350C (482-572F) tensile and creep properties of the new alloy are vastly superior to those of the existing Al-Si alloys currently used for cylinder heads.
Keywords: automotive powertrain, aluminum, cylinder head, diesel, creep strength, 319, thermal fatigue, low cycle fatigue, hot isostatic pressing
The solubilities of V and Zr have been measured in the real alloy with all elements simultaneously present by use of two methods, isothermal filtration and natural settling, which yielded very similar results (Figure 2).
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