Cosworth method is developed by Cosworth research and development company and GKN United Company in the mid-1970s. It is a sand mold low-pressure, widely used in the automobile and aircraft industry. Its typical products are engine cylinder block, cylinder head, etc. The utilization rate of aluminum melt is as high as 80% – 90%, while that of sand mold and metal mold process is only about 55%. The scrap rate of castings is only 2% – 4.5%, while that of metal mold and other processes is as high as 5% – 10%, sometimes even higher. Process flow of Cosworth method.
The casting size produced by this process is accurate and stable. When the casting size is within 100 mm, the deviation is ± 0.15 mm, ± 0.20 mm when it is less than 300 mm, and ± 0.25 mm when it is less than 800 mm. The machining allowance of castings like tank body and tank cover is only 1.5 mm ~ 2 mm, which can eliminate rough machining. The structure of castings is compact and the mechanical properties are high, and the porosity is generally 0.01% ～ 0.001%, However, the elongation of castings produced by the traditional process can be as high as 0.1%, and the elongation of castings is at least twice as high as that of the traditional. There is no oxide inclusion in the castings, so the surface quality of castings has been greatly improved. Generally, the surface roughness Ra is (5.3 ± 0.7) μ M.
Another main feature of Cosworth process is to eliminate the turbulence of aluminum melt during transportation and pouring, and it usually does not need flux for purification treatment or grain refiner. In order to reduce the inhalation of gas and the formation of oxide slag inclusion, a protective gas inlet device is arranged above the holding furnace.
In addition, the heat treatment of the casting is carried out continuously. The casting is hung on the special hanger of the push rod type suspension chain. The suspension chain is transported from the loading point to the linear quenching tank and the aging furnace. Before unloading, the final shot blasting is carried out in the blade type shot blasting machine. The heat treatment period (solution treatment and artificial aging) of castings is 6 h ~ 8 h, which is 2 h ~ 4 h shorter than that of traditional sand castings. In addition, the castings have good hardenability, and almost no deformation occurs after quenching.
In order to improve the strength and castability of the alloy and reduce the production cost, Cosworth research and development company has developed a kind of low-cost recycled aluminum alloy with good properties, namely, an Al Si (10% – 12%) – Cu (3%) alloy. This kind of eutectic alloy has good sequential solidification performance, and it will not cause the difficult problem of dendritic crystal gap feeding due to the wide solidification range. Even if there are pores in the casting, it will not form the connecting pores that make the two sides have leakage channels. After two times of purification treatment, the alloy melt is allowed to contain high content of iron and zinc, w (Zn) = 2%, w (FE) = 0.9%, and the magnesium content can be reduced by chlorination treatment. The mechanical properties of the as cast alloy: tensile strength RM = 190 n / mm2, yield strength Rp0.2 = 120 n / mm2, elongation A5 = 1% ～ 1.5%; mechanical properties after artificial aging: RM = 250 n / mm2 ～ 320 n / mm2, Rp0.2 = 150 n / mm2 ～ 300 n / mm2, A5 = 1.0% ～ 0.8%.
Cosworth process is mainly used to produce high-strength castings with no defects and light weight, which can meet the needs of more complex cylinder head production. In order to improve efficiency and reduce fuel consumption, the engine structure of modern automobile is developing towards multi valve. Therefore, the cylinder head casting is not only more complex, but also more and more demanding for its compactness. Sand mold low pressure casting is one of the effective processes to produce this kind of castings.