Automotive casting technology chapter one-The development direction of automotive castings

At present, the developed countries in the European and American automobile industry have advanced technology for automobile casting production, good product quality, high production efficiency, and low environmental pollution. Series and standardization of casting raw and auxiliary materials have been formed, and the entire production process has been mechanized, automated and intelligent. These countries generally adopt digital technology to improve the design level of casting process, the casting reject rate is about 2% ~ 5%, and multinational service systems have been established and network technical support has been implemented. In contrast, although China's automotive castings have a large output, most of them are black castings with relatively low added value and technical content and relatively simple structure, which is far from the foreign level. This article mainly discusses the development direction of automobile castings and automobile technology from the development requirements of environmental protection and other aspects of automobile festival.

Casting is one of the oldest metal forming methods. About 15% to 20% of automobile parts are used for castings produced by different casting methods. These castings mainly need to focus on the key components of the power system and the structural components needed.

1. Development direction of automobile castings

1.1 Integrated Design of Automotive Castings

With the increasing demand for environmental protection and reducing production costs of the car festival, the advantages of casting and forming are fully utilized, and the original stamping, welding, forging and casting parts are integrated through reasonable design and structural optimization to achieve integrated parts. The casting molding can effectively reduce the weight of the part and reduce unnecessary processing processes, so that the lightweight and high-quality parts can be achieved.

The integrated axle housing is used to replace the new products of the welded axle housing and the cast axle housing with half-shaft casing, to realize the integrated casting of the casting, and to make full use of the advantages of casting. At present, the main required form of the cast monolithic axle housing is to press the seamless steel pipe at both ends of the axle housing as a casing for the axle shaft, and fix it with pins to form the axle housing assembly. To further improve the strength, rigidity and simplified process of the axle housing, FAW Group has developed an integrated axle housing that casts half-shaft sleeves (parts on both sides of the axle housing) directly on the axle housing, which is characterized by reduced It is difficult to process, the cost is reduced, the structure of the axle housing tends to be simple, and the axle housing is relatively rigid. It can be made into a complex and ideal shape, the wall thickness can be changed enough, and the ideal stress distribution can be obtained. Its strength and stiffness are very high. Large and reliable. Due to the integration of the axle sleeve, the casting size has increased significantly, with a casting length of 2 258 mm and a single piece weight of more than 200 kg. In response to the characteristics of this integrated casting, the company has established a dedicated production line to ensure production.

The development trend of automobile casting integration is more obvious in the development of non-ferrous alloy castings. Given the characteristics that the full use of the casting process can be used to achieve the production of complex structural castings, integrated design of door interior panels, seat skeletons, instrument panel skeletons, front frame and firewall integrated design of high-pressure castings, whose size is significantly larger than the current The production of castings requires 4 to 5 000 t or more tonnage die-casting machines for production.

The development trend of automobile casting integration is more obvious in the development of non-ferrous alloy castings. Given the characteristics that the full use of the casting process can be used to achieve the production of complex structural castings, integrated design of door interior panels, seat skeletons, instrument panel skeletons, front frame and firewall integrated design of high-pressure castings, whose size is significantly larger than the current The production of castings requires 4 to 5 000 t or more tonnage die-casting machines for production.

1.2 Lightening of automotive castings

On the premise of ensuring the strength and safety of the car, it is possible to reduce the quality of the car as much as possible to achieve weight reduction, thereby improving the power of the car, reducing fuel consumption, and reducing exhaust pollution. For every 100 kg of reduced vehicle mass, the fuel consumption per 100 kilometers can be reduced by 0.3 to 0.6 L. If the total vehicle weight is reduced by 10%, fuel efficiency can be increased by 6% to 8%. With the need of environmental protection and energy saving, the lightweighting of automobiles has become a trend for the development of automobiles in the world, and the lightweighting of automotive castings has also become one of the development directions for automotive castings.

1.2.1 Lightweight Design of Automotive Castings

Due to the need for the overall safety factor of castings, equal thickness design is one of the main design methods for automotive castings. However, the main disadvantage of the equal-thickness design is that it can not give full play to the structure, and it leads to an increase in the weight of the casting. CAE description, topology optimization and other methods are used to optimize the design of parts so that the stress values ​​of parts of the parts are close, that is, the wall thickness of each part is inconsistent, the thickness of the part with less force is reduced or the material is not needed, thereby reducing The weight of the part. Considering that casting can realize the formation of complex structure castings, various irregular shaped sections can be realized. When designing, CAE or topology optimization is used to explain the stress of components. According to the force distribution, the shape of the component and the material thickness of the specific part are determined. By stiffening the castings, thickening the holes and drilling, the weight of the parts can be greatly reduced.

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

1.2.2 Light Alloy Automotive Castings

Handling light alloy materials such as aluminum and magnesium is currently the main need for weight reduction measures of auto manufacturers in various countries. The density of aluminum is only 1/3 that of steel, and it has excellent corrosion resistance and ductility. Magnesium has a smaller density, only 2/3 of aluminum, and has excellent fluidity under high pressure casting conditions. The specific strength (ratio of strength to mass) of aluminum to magnesium is quite high, which plays a significant role in reducing self-weight and improving fuel efficiency. The competitiveness of the American automotive industry has increased in the past two years, and it has a close relationship with its large-scale use of aluminum-magnesium structural castings and integrated castings.

The new BMW 5 series launched by the German company BMW is equipped with a new generation of magnesium-aluminum composite inline six-cylinder engine block, which reduces the weight by 10 kg compared with the previous generation, which greatly improves performance and fuel economy. However, it should be noted that the price of raw materials for light alloys such as aluminum and magnesium needs to be much higher than steel materials, which limits their wider application in the automotive industry. However, despite the higher raw material prices, the current bicycle consumption of magnesium and aluminum castings has increased year after year. On the one hand, the cost increase is compensated by technological progress; on the other hand, market competition forces auto manufacturers to reduce profits and use more light alloys. However, it is necessary to greatly increase the amount of light alloys, reduce the purchase price of magnesium and aluminum ingots, and develop advanced forming technology is one of the keys.

1.2.3 The high performance of automotive casting materials can be achieved

It is one of the methods to effectively reduce the weight of castings that the properties of materials can be improved so that parts with a unit weight can bear higher loads. Bracket-type structural castings account for a considerable proportion of automotive castings, so the development of its castings has also become one of the focuses of attention. Through measures such as heat treatment, the microstructure of the material is changed, thereby improving the strength, stiffness or toughness of the part, which can effectively reduce the weight of the part.

Isothermally quenched ductile cast iron not only has higher strength than ordinary cast steel materials, but also has a lower density than steel. Its density is 7.1 g / cm3, and the density of cast steel is 7.8 g / cm3. It is a widely recommended material in recent years. . Using isothermally quenched ductile iron, it is 10% lighter than cast steel under the same casting size. Dongfeng Motor Co., Ltd. has carried out lightweight verification of a type of commercial vehicle using isothermally quenched ductile iron instead of steel castings, and redesigned 14 suspension parts into an expert forum for the high strength characteristics of isothermally quenched ductile iron. Table 1 shows the weight reduction effect after the replacement of the isothermally quenched ductile iron material. The total weight is reduced by nearly 40%, and the effect is significant. It should be noted that the weight reduction effect in Table 1 is not only caused by material substitution, but also includes the contribution of lightweight design. Usually, the replacement of materials for automobile castings is often accompanied by lightweight design of parts.

In terms of aluminum alloy and magnesium alloy castings, high-strength and high-toughness materials are also used as replacements. On the basis of the original light alloy weight reduction, high-performance materials can be used to further reduce weight. General Motors of the United States uses AE44, which has high-performance. The alloy replaced the original aluminum alloy, and the sub-frame was produced by high-pressure casting. The weight of the aluminum alloy was further reduced by 6 kg.

1.3 Digitalization of Automotive Casting Development

The specific combination of automotive casting development and digital technology can significantly improve the level of casting technology and shorten the product design and trial production cycle. The current digital manufacturing technology has been widely used in the development of automotive castings. During the casting structural design and casting process design stage, Pro / E, CATIA, and UG 3D design software has been widely used, and some advanced casting companies have realized paperless design. Software such as MAGMA, ProCAST, and Huazhu CAE have been widely used to simulate the solidification process, microstructure, composition segregation, and material properties of automobile castings. It can also simulate the velocity field, concentration field, temperature field, Phase field and stress field simulation can ensure that the process plan is optimized before mass production.

To meet the needs of rapid development of automotive castings, based on the design and development of CAD / CAE, RP (Rapid Prototyping Technology) has been widely used for rapid trial production of automotive castings. After obtaining the original CAD / CAE data, a layer-by-layer method is used to obtain a prototype of the casting or a prototype of the mold required to form the casting by means of bonding, sintering or sintering. The former can be used to trial-produce castings by investment casting, gypsum casting, etc. The latter can be directly used to make sand cores for molds, and the castings are poured out through core molding. In addition, the powder laser sintering method (SLS) can also be used to directly complete the production of sand cores and sand molds, so as to obtain the sand molds required for the trial production of castings. For external molds with relatively simple structures, numerical control machine tools can also be used to perform CAM processing with machinable plastics to obtain core boxes and patterns required for trial production of castings, or directly process sand blocks to directly obtain sand molds for external molds.

In general, digital technology has run through all aspects of casting design, development and trial production, effectively improving the development speed and efficiency of castings. At present, the main problem that needs to exist is that the digital technologies in design, description, and rapid manufacturing are independent. When the development process is transformed from one stage to another, it also needs to perform tedious data conversion. It is hoped that in the future, a unified data interface platform can be developed for the digital technology applied in all aspects of casting development, and standardized data conversion standards will be established to achieve seamless conversion of data between different software, thereby further improving the development speed of castings. .