Composite squeeze casting forming of large castings

With the rapid development of economy, the domestic demand for large and complex castings in the fields of automobile, aviation, architecture and military industry is increasing day by day, and the lightweight development trend of large castings is becoming more and more obvious, such as large aluminum alloy automobile seat frame, automobile gearbox box, etc. Generally, large castings of such non-ferrous metals are formed by low-pressure casting. Because the castings have many defects such as pores, shrinkage cavities and inclusions, the performance indexes of formed parts are often difficult to meet the requirements. Although forging can form parts with high performance, it has large deformation resistance, high requirements for equipment, and poor metal flow filling performance. For parts with complex structure, it often needs multiple processes to complete, and the production cost is high. It is not suitable for the production of parts with complex shapes, especially large castings with complex structure. Squeeze casting is a process combining casting and forging characteristics. It has the advantages of high utilization of liquid metal, simplified process and stable quality. It is an energy-saving liquid forming technology. Research at home and abroad shows that squeeze casting is suitable for manufacturing hub, valve body, piston and sheet metal castings. It is an alternative forming technology for low-cost manufacturing of large and complex castings.

According to the process characteristics, squeeze casting forming methods can be divided into direct squeeze casting and indirect squeeze casting. The direct squeeze casting method is that the pressure is directly applied to the entire liquid surface of the metal melt. The mold is not equipped with a mold filling system, so the solidification speed of the metal melt is fast, and the obtained casting structure is dense. Theoretically, it can produce nearly 100% dense castings. This method is suitable for producing castings with simple shape and symmetrical structure, such as pistons, calipers, cylinders, etc. The main disadvantage of this method is that the molten metal poured into the mold cavity must be accurately quantified, otherwise the dimensional accuracy of the casting will be unstable due to the change of the amount of molten metal; The casting is required to solidify unidirectionally in the direction of the punch to ensure the effective transmission of pressure, which limits the shape of the product. It is difficult to form the castings with complex shape.

Indirect squeeze casting method is to close the mold to form a closed mold cavity. The pressure acts on the metal melt of the mold cavity filling system, and the metal melt is sent into the closed mold cavity for pressure maintaining forming. Indirect squeeze casting has the advantages of short sprue length, large cross-section, low mold filling speed and long holding time. It is easy to control the casting size and does not need to configure an accurate quantitative mold filling system. This method can make full use of the heat capacity of the liquid metal in the mold filling device, so that the casting is easy to form and feed the parts under pressure. Therefore, it is suitable for producing castings with complex shape, uneven wall thickness, high dimensional accuracy and surface roughness requirements. However, this pressing method will not form plastic deformation structure in the casting, and the mechanical properties of the casting are lower than those produced by the direct method. At the same time, because the indirect squeeze casting process adopts the mold filling system, it is difficult to maintain a high pressure during the casting solidification process, which is not conducive to the production of castings with large solidification range, and the internal quality of the casting is also poor.

Squeeze casting is used to form large and complex castings. If the direct squeeze casting method is used, the molten metal is directly filled into the mold cavity. When the amount of molten metal filled is large, the molten liquid waits for extrusion in the mold cavity for a long time, the castings are easy to produce cold shuts, and it is difficult to ensure the precise amount of molten metal; If the indirect squeeze casting method is adopted, it is difficult to transfer the squeeze force to the position far away from the sprue due to the large volume of the casting, so defects are easy to appear at the far end of the sprue.

From the above analysis, it can be seen that if the direct squeeze casting method or indirect squeeze casting method is used alone, there are shortcomings in forming large castings. In this paper, a new composite squeeze casting method is proposed according to the squeeze casting characteristics of large castings, and the composite squeeze casting forming equipment for large castings is systematically developed, To solve the key technical problems such as the design and manufacture of metal melt conveying device in forming equipment and composite squeeze casting die for large castings, and to realize the forming of large and complex castings through experiments, so as to establish a technical foundation for the research and development of larger squeeze casting equipment and the forming of various large castings.

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