Abstract : This paper summarizes the advantages of aluminum alloy squeeze casting, and three aspects that need to be paid attention to in order to realize ' casting instead of forging ' : first, select high-strength aluminum alloy that can be strengthened by heat treatment ; secondly, it is necessary to communicate closely with the parts design engineer and continuously optimize the casting structure to facilitate the squeeze casting process. Third, there must be strict casting process control, from different aspects to reduce the air content of extruded castings.
Keywords : squeeze casting ; aluminum alloy ; a345 ; casting structure ; process control
Squeeze casting, also known as liquid die forging, is a special casting process that integrates casting and forging process characteristics. In this process, a certain amount of metal liquid is directly poured into the metal mold, and a certain pressure is applied to the liquid or semi-solid metal through the mold or punch to make it fill, form and crystallize and solidify. The metal crystallizes and solidifies under pressure and produces a certain amount of plastic deformation, thus becoming a metal forming method for obtaining high-quality castings [ 1 ]. Squeeze casting can be divided into two categories : direct squeeze casting and indirect squeeze casting. The direct extrusion process is similar to metal die forging, and the pressure is directly applied to the entire surface of the liquid or semi-solid metal. The indirect extrusion process is close to the high pressure die casting, and the pressure acts indirectly on the liquid or semi-solid metal through the runner. At present, most of the squeeze casting equipment used in the market that can be continuously and mass-produced belongs to indirect squeeze casting. In the following, squeeze casting refers to indirect squeeze casting. Squeeze casting has the following advantages.
( 1 ) The barrel of the squeeze casting machine is generally vertical. The metal liquid fills the cavity from bottom to top, and the filling speed is slow. This not only ensures that the metal liquid is laminar flow filling in the cavity and will not be involved in the gas, but also helps to eliminate the air in the mold cavity. Therefore, the gas content of the squeeze casting is very low, and it can be heat treated or welded.
( 2 ) The large ingate must be used in the design of squeeze casting mold, and the metal liquid solidifies under a large pressurization pressure. The feeding effect of the casting is very good, and there are basically no shrinkage cavity, porosity and other casting defects in the casting, thus improving the mechanical properties of the casting.
( 3 ) During the filling and solidification process of the casting, the molten metal is subjected to a large high pressure from beginning to end, so the casting grains are fine and the structure is uniform. At the same time, the pressure makes the outer side of the casting close to the inner wall of the mold, and the dimensional accuracy of the casting is high.
( 4 ) Squeeze casting process is more suitable for some products with complex shape and high performance requirements than forging.
( 5 ) Because the squeeze casting parts can be heat treated, if the alloy which can be strengthened by heat treatment is selected, the mechanical properties of the castings can be greatly improved by heat treatment, and the quality of the castings can be close to or equivalent to that of the die forgings.
In recent years, the author has been fortunate to participate in the development projects of two engine mounting brackets in our company, and has achieved success. The two brackets are similar in shape, one is the left part and the other is the right part. The bracket was originally designed with aluminum alloy die forgings ( Figure 1 ). During the development process, the project team continuously optimized the structure of the product through close cooperation with the same level of suppliers, and finally produced two engine brackets suitable for squeeze casting ( as shown in Figure 2, only the right bracket is listed ). By carefully combing the development process of the project, it is believed that the success of the project is mainly due to the following three aspects.
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1 Aluminum alloy with high strength and heat treatment strengthening is selected.
In this project, the bracket uses our company's self-developed patented high-strength cast aluminum alloy.
Patent high-strength cast aluminum alloy. The alloy is registered with the Aluminum Association of Aluminum Association (AA) as A345. The chemical composition is similar to A356.2, which appropriately reduces the star content of silicon, improves the ductility of the casting and ensures the fluidity of liquid aluminum alloy during the production of parts. In addition, the alloy also adds some chemical elements that are conducive to heat treatment reinforcement, which have no precious metals and rare earth elements. Therefore, the design idea of the new alloy is to effectively improve the strength of the cast parts while realizing the parts cost control requirements required by the automobile industry. The chemical composition of A345 alloy is shown in Table 1, and the mechanical properties of the alloy are shown in Table 2. It is suitable for the casting process of thick wall parts such as low pressure casting, differential pressure casting or extrusion casting. In the T6 heat treatment state, both the yield strength and tensile strength far exceed the traditional A356.2 aluminum alloy, which is close to the properties of deformed aluminum alloy 6061.
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2 Communicate closely with parts engineers to optimize the structure of castings.
In this project, the structural thickness of the original parts is uneven, which is not suitable for extrusion casting. Our project team members communicated closely with the product designers of the primary supplier, and through the use of casting simulation software, constantly change the casting and piece structure, reasonably adjust the thickness of different parts of the casting, and finally optimize a part ⑵ suitable for extrusion casting process. Figure 3 is an example diagram of increasing the local thickness of the casting. The original design of the parts has a large installation space, which causes the wall thickness of the part is too thin (the left circle), which blocks the shrinkage channel of the longest leg of the casting, resulting in serious internal shrinkage Figure 2 Optimized part diagram Fig.2 Picture of the optimized parthole of the longest leg. In order to eliminate this shrinkage area " specially add about 5 g mass of aluminum alloy in this part (Figure 3 on the right circle) to form a solidified complement and contraction channel, which finally eliminates the shrinkage hole of the casting to get a compact extrusion casting. Figure 4 shows the metallographic microstructure of a typical T6 heat treated squeeze cast A345 aluminum alloy, consisting of an aluminum matrix and eutectic zone, with only a very small amount of small pores present in the microstructure.
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Fig.3Picture showing the location where the solidification path is enlarged
3 Strict casting process control
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Fig.4 Typical optical microstructure of tthe T6 tempered A345 aluminum squeeze casting part
The material cylinder coating with low gas capacity and the mold coating with low gas capacity and good heat insulation performance are selected. Third, the speed of the punch is strictly controlled. For the casting with an average wall thickness of 3-6 mm, the filling speed of the inner pouring channel must be less than 0.7 m/s, so as to avoid the casting into the gas in the mold cavity when filling. Fourth, clean the mold regularly to ensure that the exhaust tank of the mold is smooth and prevent the gas in the casting.
The two engine mounting bracket castings developed by our company, due to the use of the new high-strength aluminum alloy A345, the manufacturing process is changed from the original forging to the current extrusion casting, and the manufacturing cost is reduced by about 70%oThe yield strength of the casting body sampling is basically the same as that of the forged aluminum alloy (Figure 5), and it also has sufficient elongation, so that the quality of the final mass production casting parts is the same as the drawing quality of the original forging design, so as to realize the "casting instead of forging". At present, these two stents have been supplied in bulk since 2019 and applied to the eighth generation Corvette (Corvette Zerv, marketed in 2020, see Figure 6) coupe. The car uses a 6.2-L middle engine layout, an all-aluminum body structure, and the acceleration time from zero to 100 kilometers per hour is only 2.9s.
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Fig.5 A comparison of tensile properties between the A345 squeeze casting and lhe AA6061 forgings
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Fig.6 Picture of the fin>i mid-cnginc 8th generation Corvette speedster
1. What is high strength aluminum alloy squeeze casting?
High strength aluminum alloy squeeze casting is an innovative manufacturing process that combines casting and forging techniques to produce high-strength aluminum components with exceptional mechanical properties. The process involves injecting molten aluminum into a pre-designed mold under high pressure, resulting in a dense and fine-grained structure that enhances the material's strength and performance.
2. What are the advantages of using squeeze casting for mass production?
Squeeze casting offers several key advantages for mass production of high-strength aluminum components. Firstly, it allows for rapid production cycles, reducing manufacturing lead times. Secondly, the process ensures precise dimensional accuracy and excellent surface finish, minimizing the need for secondary machining operations. Lastly, the superior mechanical properties of the squeeze-cast parts lead to lighter and stronger end-products, contributing to enhanced overall performance.
3. How does squeeze casting contribute to sustainable manufacturing?
Squeeze casting promotes sustainability in manufacturing by optimizing material usage and energy efficiency. The process minimizes material waste by reducing the need for extensive machining and optimizing the design of the components. Additionally, the low energy requirements during squeeze casting compared to traditional forging processes lead to lower carbon emissions, making it an environmentally-friendly choice for mass production. By adopting squeeze casting for mass production of new high-strength aluminum alloys, manufacturers can achieve cost-effective, sustainable, and high-quality production while meeting the demands of modern industries.
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