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Table 1. Other Properties of selected aluminum casting alloys Alloy


Density (gm/cc) 319 A356


A356-Cu C355


A356-CuZrMn 319+MnVZr 224


224 (RTA) 354 351


2.79 2.67


(2.68) 2.71


(2.68) (2.79) 2.81


(2.81) 2.71 2.70


to improve engine efficiency. Tis re- quires increased specific power output (horsepower per liter of engine dis- placement), accomplished by use of di- rect fuel injection, higher compression ratios and turbochargers. However, these improvements result in higher operating temperatures and pressures. Tis causes increased thermo-mechan- ical fatigue, particularly in the valve bridge between inlet and exhaust ports on the combustion face of cylinder heads. Engine designers have realized that further improvements will require better materials. Tis has been the main impetus for the development of new materials. Briefly, the alloys currently in use or


proposed are: • 319 alloy. • Primary A356 or 357. • A356+0.5Cu.


• A356+0.5Cu-Zr-Mn. • C355 alloy. • Optimized 319/320. • 224 alloy. • 351 alloy. It will also be useful to look at some


other properties. Te relative costs have been calculated using A356 alloy as a base point. Tese calculations used typical costs and recoveries for alloy- ing materials. (In these calculations copper was $3 per pound; and 351 alloy was assumed to contain 0.15% each of V and Zr.) Other factors enter into the cost of production, of course, but the values tabulated here are a good starting point for comparison. Te costs tabulated for 319 and 319+MnVZr are for primary versions of these alloys. Secondary versions would normally be less costly, but they would have signifi- cantly lower fatigue strength. How these values may be used are


illustrated is an example, comparing the two strongest alloys: RTA’s version


Many of the sessions at Metalcasting Congress were interactive and challenged the people in attendance.


May 2018 MODERN CASTING | 49


Thermal Conductivity (25 C, W/m-K)


112 150


(<150) 150


(<150) (<112) 117


(d117) 125


140-160


Temper F


T6 T6 T6 T6 F


T62 T6


T61 T7


of 224 alloy and Alcoa’s 351 alloy. Tey have nearly the same strength, so the design (shape and size) of a cylinder head should be nearly the same for both materials. However, the density of 224 is 2.81/2.70=1.041 times greater. Te cost per pound is also slightly higher. If the base price of A356 is $1 per pound, then the material cost of a 20-lb. cylinder head in A356 alloy will be $20. Using 351 alloy, the cost is: 20 lb. x 2.70/2.67 x $ 1.126/lb. or


$22.77 Te same cylinder head in 224 alloy


will cost: 20 lb. x 2.81/2.67 x $ 1.128/lb or $23.74 Obviously, there is no cost incentive to use the 224 alloy. In addition, the thermal conductivity of 224 is less; as expected, because of its higher copper content. Similar comparisons can be made with other alloys. Castability is not presented in Table


1, but this is an important consider- ation. Typical experience with alloys similar to 224 (especially A206) sug-


Cost Differential (cents/lb)


6.9 -0- 1.1 2.2 2.7


13.1 7.5


12.8 4.9


12.6


gests that typical production costs will be 20 to 30% greater than for A356. With experience casting 354 alloy, and considering its similarity, 351 alloy was expected to exhibit similar castability. For these reasons 351 alloy was


chosen when customers requested material in cylinder heads and blocks for racing engines.


THE DISCOVERY Te production of automotive


blocks and heads puts significant demands on the foundry to produce sound, pressure tight castings in a wide range of shapes that have not neces- sarily been optimized for the casting process, from strong alloys. Many high strength alloys, particularly those that contain copper, sometimes suffer from castability concerns and may require more sophisticated gating or the use of extensive chilling to produce. 351 alloy generally has better mechanical proper- ties than A356 or C355, with cast- ability very similar to aluminum-silicon alloys without copper.


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