Development Trend of Automotive Casting and Foundry Technology

1 Development direction of automotive castings

Integrated Design of Automotive Castings

With the increasing demand for automotive energy-saving and environmental protection and reduction of production costs, and taking full advantage of the advantages of casting and forming, the original parts of stamping, welding, forging and casting are molded to achieve the integration of parts through reasonable design and structural optimization. Forming can effectively reduce the weight of the parts and reduce unnecessary processing processes, thereby achieving lightweight and high-performance parts.

Figure 1 shows an integrated axle housing developed by FAW Group to replace welded axle housings and new products with semi-axle casing cast axle housings to realize integrated casting of castings and fully utilize the advantages of casting and forming. At present, the common form of the integrated cast axle housing is that the seamless steel tube is pressed into the half shaft sleeve at both ends of the axle housing, and is fixed with a pin to form the axle housing assembly. In order to further improve the strength and stiffness of the axle housing and simplify the process, FAW Group has developed an integrated axle housing that directly casts the axle casing (the parts on both sides of the axle housing in Figure 1) on the axle housing. Processing difficulty, cost reduction, axle housing structure tends to be simple, and the axle housing rigidity is good, can be made into a complex and ideal shape, the wall thickness can be changed, the ideal stress distribution can be obtained, and its strength and rigidity are large Reliable work. Thanks to the integration of the semi-axle casing, the size of the casting is significantly increased. The casting is 2 258 mm long and its single piece weight exceeds 200 kg. For the characteristics of this integrated casting, the company has established a dedicated production line to ensure production.

The development trend of automotive castings integration is more obvious in the development of non-ferrous alloy castings. In order to make full use of the casting process's ability to realize the production of complex structural castings, integrated design of high-pressure castings such as interior door panels, seat frames, instrument panel skeletons, front-end frames, and firewalls has emerged, and its size is significantly larger than that of current production. Castings require 4 000 to 5 000 t or even larger tonnage die casting machines for production.

Lightweight automotive castings

Under the premise of ensuring the strength and safety performance of the vehicle, the vehicle's curb weight is reduced as much as possible to achieve lighter weight, thereby improving the vehicle's dynamic performance, reducing fuel consumption, and reducing exhaust pollution. For every 100 kg of vehicle trimming mass, the fuel consumption per 100 km can be reduced by 0.3-0.6 L. If the vehicle's overall vehicle weight is reduced by 10%, the fuel efficiency can be increased by 6%-8%. With the needs of environmental protection and energy saving, the lightweighting of automobiles has become the trend of the world's automotive development. The lightweight of automobile castings has also become one of the important development directions of automobile castings.

Lightweight design of automotive castings

Due to the need for the overall safety factor of the castings, equal thickness design is one of the main design methods for automotive castings. However, the main drawback of the equal-thickness design is that it fails to give full play to the structural performance and leads to an increase in the weight of the casting. The use of CAE analysis, topology optimization and other means to optimize the design of parts, so that parts of the parts of the stress value close, that is, all parts of the wall thickness is inconsistent, the part of the small force to reduce the thickness of the material or not to reduce the parts the weight of. Considering that casting can realize the forming of complex structure castings, various irregular profiled sections can be realized. During design, stress analysis is performed on components using CAE or topology optimization. According to the distribution of force, determine the shape of the part and the thickness of the material in the specific part. By stiffening, digging, and thickening castings, the weight of parts can be greatly reduced.

Fig. 2 is a comparison of the castings before and after the Dongfeng Precision Casting Co., Ltd. has optimized the design of commercial vehicle bearings. It can be seen that the initial weight of the castings is 6.6 kg, and its design is a typical equal thickness design. After a series of lightweight design methods such as stiffening, hole digging and variable cross-section of the casting, the casting weight becomes 3.0 kg, and the weight reduction effect can reach more than 50%.

Light alloy automotive castings

The use of light alloy materials such as aluminum and magnesium is a major weight loss measure for automakers in various countries. The density of aluminum is only 1/3 of that of steel, and it has excellent corrosion resistance and ductility. Magnesium has a lower density, only 2/3 of that of aluminum, and has excellent flowability under high pressure casting conditions. The specific strength of aluminum and magnesium (the ratio of strength to mass) is quite high, which plays a decisive role in reducing self-weight and improving fuel efficiency. The competitiveness of the US auto industry in the last two years has been closely related to its significant adoption of aluminum-magnesium structural castings and integrated castings.

The new 5 series introduced by BMW Germany has been equipped with a newer generation of Mg-Al compound in-line six-cylinder engine block, which has reduced the weight by 10 kg compared to the previous generation, significantly improving the performance and fuel economy. However, it should be noted that the price of raw materials for light alloys such as aluminum and magnesium is much higher than that of steel materials, which limits its wider application in the automotive industry. However, despite the high raw material prices, the current consumption of magnesium and aluminum castings has increased year after year. This aspect is to make up for the increase in costs through technological advances. On the other hand, market competition forces automakers to reduce profits and adopt more light alloys. However, to significantly increase the amount of light alloys and reduce the purchase price of magnesium aluminum ingots, the development of advanced forming technology is one of the keys.

High-performance automotive casting materials

Improve the performance of the material, so that the unit weight can withstand higher loads, is one of the ways to effectively reduce the weight of the casting. The bracket-type structural castings account for a considerable proportion of automotive castings, and thus the development of their castings has also become one of the focuses of attention. Through heat treatment and other measures, the microstructure of the material is changed, thereby increasing the strength, rigidity or toughness of the part, and the weight of the part can be effectively reduced.

Austempered ductile cast iron not only has higher strength than ordinary cast steel materials, but also has lower density than steel, its density is 7.1 g/cm3, and the density of cast steel is 7.8 g/cm3, which is widely recommended in recent years. . Austempered ductile iron is used, which is 10% lighter than cast steel under the same casting size. Dongfeng Motor Co., Ltd. carried out lightweight verification of a commercial vehicle using isothermally quenched ductile iron instead of cast steel parts, and redesigned 14 suspension parts into expert forums for the high-strength characteristics of isothermally quenched ductile iron. Table 1 shows the lightweight effect of replacing the austempered ductile iron material with a total weight reduction of nearly 40%. The effect is significant. It should be noted that the lightweighting effect in Table 1 is not only due to material substitution, but also includes the contribution of lightweight design. In general, the material replacement of automotive castings is often accompanied by the lightweight design of the parts.

High-strength, high-toughness materials are also used in aluminum alloy and magnesium alloy castings. Based on the light weight reduction of the original light alloys, high-performance materials are used for further weight reduction. US General Motors Corporation replaces high-performance AE44 alloys. The original aluminum alloy was produced by high-pressure casting method to produce a sub-frame, and the weight loss of aluminum alloy was further reduced by 6 kg.

1.3 Digitalization of Automotive Castings Development

The comprehensive combination of automotive casting development and digital technology can significantly increase the level of casting technology and shorten the product design and trial period. At present, digital manufacturing technology has been widely used in the development of automotive castings. At the casting structure design and casting process design stage, three-dimensional design software such as Pro/E, CATIA, and UG has been widely used, and some advanced foundry companies have realized paperless design. Softwares such as MAGMA, ProCAST, and Huacao CAE have been widely used in the simulation of solidification process, microstructure, composition segregation, and material properties of automotive castings, as well as the velocity field, concentration field, temperature field, and phase in the casting process. Field, stress field and other aspects of the simulation can ensure that the process plan is optimized before mass production.

To meet the demand for rapid development of automotive castings, RP (Rapid Prototyping Technology) has been widely used in the rapid trial production of automotive castings based on the design and development of CAD/CAE. After obtaining the CAD/CAE raw data, the prototype of the casting or the prototype of the mold needed to form the casting is obtained by means of layer-by-layer stacking by bonding, sintering or sintering. The former can be used for investment casting, gypsum casting and other methods to make casting sample parts, the latter can be used directly as a mold manufacturing sand core, casting castings through core molding. In addition, it is also possible to use the powder laser sintering method (SLS) to directly manufacture sand cores and sand molds, thereby obtaining sand molds required for casting trial production. For a relatively simple structure of the outer mold, you can also use CNC machine tools, CAM processing with plastic processing, so as to obtain the core box and pattern required for casting trial, or directly to the sand block processing, directly to obtain the outer mold sand type.

In general, digital technology has penetrated all aspects of the design, development, and trial production of castings, effectively increasing the development speed and efficiency of castings. At present, the main problem is that the digital technologies in design, analysis, and rapid manufacturing are independent. When the development process is transformed from one stage to another, the data conversion work is also quite complicated. It is hoped that in the future, a unified data interface platform can be developed for the digital technologies applied in all aspects of casting development, and standardized data conversion standards can be established to seamlessly convert data between different software, thereby further improving the development speed of castings.

The development direction of automotive casting technology

2.1 Production Technology of Thin-Walled and Complex Structure Castings

With the development of the automobile industry and the demand for energy-saving and emission-reduction, automobile parts are becoming lighter and lighter. Through thin-wall design, achieving lightweight is an important development direction of the engine block. For example, FAW Casting Co., Ltd. used FAW-Volkswagen for the production of cast iron cylinders. The early production of 06A cylinders had a wall thickness of 4.5 mm ± 1.5 mm, and EA111 cylinders had a wall thickness of 4 mm ± 1 mm. The current mass production of EA888Evo2 cylinder wall thickness 3.5 mm ± 0.8 mm, the structure of the next-generation EA888 Gen.3 cylinder is even more complex. Its wall thickness is only 3 mm ± 0.5 mm. It is currently the thin gray cast iron cylinder. Although there are problems in core production, such as broken cores, floating cores, and large wall thickness fluctuations, by controlling the quality of the sand core and molding sand, the horizontal horizontal pouring process that is currently widely used can still meet the production requirements of the EA888Evo2 cylinder. However, the production requirements of the EA888 Gen.3 cylinder cannot be met and the integral core core pouring process must be adopted.

Figure 3 shows the horizontal horizontal pouring and the vertical core assembly. For the 3 mm thin wall feature of the cylinder block, the core-in-situ pouring technology puts forward stringent requirements on both the core and the core. The core making center can realize high intelligence and automation of core production. From the addition of raw sand, resin, sand mixing, core making, core repair, assembly, coating and drying to molding and group cores, the entire process can be highly automated, making sand core making quality, assembly quality or dimensional accuracy and The drying quality of the coating has been guaranteed in a stable manner, thereby avoiding the risk of quality and size due to human factors and adapting to the needs of large-volume cylinder system core production. Can effectively solve the mass production, the problem of instability and high scrap rate, while improving the size accuracy of the sand core, it also greatly reduces the cleaning workload and cost, and can fully guarantee the 3mm wall thickness requirements.

Manufacturing Technology of Large Magnesium Alloy Components

With the increasing requirements for energy saving, environmental protection and reducing the cost of components, aluminum-magnesium alloy large-structure castings have become an important development trend, and its manufacturing technology has also become a hot spot for development. At present, the main production technologies of aluminum-magnesium alloy large-scale structural parts include high-pressure casting, squeeze casting and low-pressure casting. Due to the high production efficiency of high pressure casting and good product quality, it has become the main production process at present. The development of its manufacturing technology mainly focuses on the easy gas-rolling during the high-pressure casting process, the easy formation of pores inside the casting, and the improvement of the heat treatment problem.

German Fulai company developed a vacuum vacuum suction casting process. Its working principle is shown in Figure 4, and the entire die casting process is performed under high vacuum (less than 30 mbar). The molten metal is sucked from the furnace through a suction pipe under a vacuum in a mold, a pressure chamber, and a suction pipe. The steam of the release agent is also discharged from the vacuum system. The main features of the above vacuum negative pressure suction casting process are as follows: at the beginning of the quantitative pouring, the entire system is in a high vacuum state; in the quantitative pouring process, the gas in the cavity and the metal melt can be effectively discharged; during the pouring process The metal melt is non-oxidizing; there is no heat loss during the pouring process, and the casting can be performed at a lower pouring temperature, and the laminar flow can be filled without disturbance under real-time monitoring. The above-mentioned process has been successfully applied to the mass production of steam structural expert vehicle structural castings, and provides advanced forming methods and processes for the application of high-quality light alloy castings.

The Buhler company in Switzerland has developed a two-loop vacuum system for the production of structural castings. This production technology is called structural part production technology, as shown in Fig. 5. The use of structural parts production technology can increase the speed of vacuuming, so as to obtain stable production conditions and significantly improve the quality of die castings. As shown in FIG. 5 , a suction port of a circuit in the dual-circuit vacuum system is arranged at the upper end of the pressure chamber and is mainly used for air extraction in the pressure chamber. When the injection punch seals the gate, it starts and closes immediately after the punch seals the air inlet. The other circuit setup is the same as the traditional vacuum process and is mainly used for air extraction in the cavity. At present, this technology has been successfully applied to the manufacture of aluminum alloy integrated shock absorber towers for passenger cars, door inner panels and body longitudinal beams.

2.3 Casting Accurate Casting Technology

The so-called exact cast molding of automotive castings mainly refers to lost foam and investment casting techniques. With the development of automobile casting forming technology, accurate casting molding refers to a kind of casting forming method. Castings produced by this type of forming method can be used without cutting or cutting. With the improvement of the dimensional accuracy of castings, the accurate forming technology has been rapidly developed in recent years, and a series of new casting methods such as accurate sand casting, lost foam casting, controlled pressure casting and pressure casting have emerged. The Cosworth casting method is a method developed by the United Kingdom using a combination of zircon sand core and controlled by an electromagnetic pump. It has been successfully used in the mass production of aluminum alloy cylinders, and many process variations have emerged, such as the use of low pressure casting instead of electromagnetic Pump pouring and other processes. Using this type of casting method, an aluminum alloy cylinder body with a wall thickness of 3.5-4.0 mm can be produced, which is a representative process for accurate sand casting.

The lost foam casting process has been invented since 1965. The main production automobile castings are cylinder blocks, cylinder heads, intake and exhaust pipes and other products, and have formed scale production. Since the introduction of lost foam casting technology in China in the 1990s, it has begun to take shape. It has been promoted by the state and has become a widely used high technology in the transformation of the traditional foundry industry. At present, China has three kinds of investment precision casting processes, including water glass shells, composite shells and silica sol shells. The surface quality of castings used in the production of silica sol shells for automotive products can reach Ra 1.6 μm, and the dimensional accuracy can reach CT4. Grade, smaller wall thickness can be achieved 0.5 ~ 1.5 mm. Dongfeng Automobile Precision Casting Co., Ltd. uses a composite system of silica sol and water glass to produce complex structural castings, which significantly reduces production costs. The development trend of the investment casting technology is that the distance between the casting and the final product is getting closer, the complexity and quality of the product are getting higher and higher, and the application of CAD, CAM and CAE has become the main technology for product development. Collaboration begins to show.

The vacuum casting, oxygen-filled die-casting, semi-solid metal rheology or thixotropic die-casting processes developed on the basis of the high-pressure casting process aim to eliminate casting defects, improve internal quality, and expand the application range of die castings. In the process of squeeze casting, the melt is filled and solidified under pressure, which has the advantages of stable, no metal splashing, less loss of molten metal oxidation, energy saving, safe operation, and reduction of defects in casting holes, etc., in aluminum alloy subframes, etc. High-performance aluminum alloy castings have been widely used in the development and application.

The continuous increase in the output of automobiles urgently requires the development of casting production in the direction of high quality, excellent performance, near net shape, multiple varieties, low consumption, and low cost. Since about 15% to 20% of the parts of a vehicle are castings. This requires the foundry industry to constantly apply a variety of new technologies and new materials to enhance the overall level of casting. The exact casting forming technology of castings can meet the above requirements of automotive castings, and its application will also cover different casting production processes of automobile castings.

Conclusion

In order to meet the increasingly stringent requirements of environmental protection laws, automobiles are moving toward lightweighting. As the weight of cars decreases by 10%, fuel consumption can be reduced by 5.5%, fuel economy can be increased by 3% to 5%, and emissions can be reduced by about 10%. The application of aluminum and magnesium and other non-ferrous alloy castings, the development of large-scale complex structural castings, and the extensive application of accurate casting technology are the main ways to achieve lightweight automotive castings. Therefore, it is required to realize the research, development and production of automotive castings through the use of high-performance casting materials and the wide application of automation equipment on the basis of extensive adoption of digital technologies, and to meet the needs of the modern automotive industry.