The development direction of automobile castings

Casting is one of the oldest metal forming methods. About 15% to 20% of auto parts are castings produced by different casting methods. These castings are mainly key components of the power system and important structural components.

At present, the developed countries of the European and American automobile industries have advanced automobile casting production technology, good product quality, high production efficiency, and low environmental pollution. Casting raw and auxiliary materials have been serialized and standardized, and the entire production process has been mechanized, automated and intelligent. These countries generally use digital technology to improve the level of casting process design, the casting rejection rate is about 2% to 5%, and have established a multinational service system and realized network technical support. In contrast, although my country's auto castings have a large output, most of them are black castings with low added value and technical content and relatively simple structure, which is far behind foreign countries. The following mainly discusses the development direction of automobile casting technology from the development needs of automobile energy saving and environmental protection.

The development direction of automobile castings

1 Integrated design of automotive castings

With the increasing requirements for energy saving, environmental protection and production cost reduction of automobiles, the advantages of casting forming are fully utilized to realize the casting of integrated parts through reasonable design and structural optimization of the original stamping, welding, forging and casting forming parts. Forming can effectively reduce the weight of parts and reduce unnecessary processing processes, so as to realize the lightweight and high performance of parts.

At present, the integrated axle housing is used to replace the welded axle housing and the new products of cast axle housing with half-shaft casings, to realize the integrated casting of castings and make full use of the advantages of casting and forming. The main form of the common casting monolithic axle housing is that seamless steel pipes are pressed into the two ends of the axle housing as half shaft sleeves, and fixed with pins to form the axle housing assembly. It is characterized by less processing difficulty, more cost reduction, simpler axle housing structure, better axle housing rigidity, can be made into complex and ideal shapes, wall thickness can be changed, and ideal stress distribution can be obtained. Its strength and The rigidity is relatively large and the work is reliable.

The development trend of the integration of automotive castings is more obvious in the development of non-ferrous alloy castings. In order to make full use of the characteristics that the casting process can realize the production of complex structural castings, there have been integrated high-pressure castings with integrated designs such as door inner panels, seat frames, instrument panel frames, front-end frames, and firewalls, and their sizes are significantly larger than those currently produced. Castings require 4 000-5 000 t or even larger die-casting machines for production.

2 Lightweight of automotive castings

On the premise of ensuring the strength and safety performance of the car, the curb weight of the car should be reduced as much as possible to achieve light weight, so as to improve the power performance of the car, reduce fuel consumption and reduce exhaust pollution. For every 100 kg reduction in vehicle curb weight, the fuel consumption per 100 kilometers can be reduced by 0.3-0.6 L. If the weight of the vehicle is reduced by 10%, the fuel efficiency can be increased by 6%-8%. With the needs of environmental protection and energy saving, the lightweight of automobiles has become the trend of automobile development in the world, and the lightweight of automobile castings has also become one of the important development directions of automobile castings.

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 the structural performance cannot be fully utilized, and it leads to an increase in the weight of the casting. CAE analysis, topology optimization and other means are used to optimize the design of parts, so that the stress value of each part of the part is close, that is, the wall thickness of each part is inconsistent, and the part with small stress reduces the thickness of the material or does not need the material, thereby reducing the weight of the part. the weight of. Considering that casting forming can realize the forming of complex structural castings, various irregular shaped cross-sections can be realized. During design, CAE or topology optimization is used to analyze the stress of components. According to the force distribution, the shape of the component and the material thickness of the specific locality are determined. By reinforcing, digging and thickening the casting, the weight of the part can be greatly reduced.

2.2 Light alloy automotive castings

The use of light alloy materials such as aluminum and magnesium is currently the main weight reduction measure for automobile manufacturers in various countries. The density of aluminum is only 1/3 of that of steel, and it has excellent corrosion resistance and ductility. The density of magnesium is smaller, only 2/3 of that of aluminum, and it has excellent fluidity under high pressure casting conditions. The specific strength (ratio of strength to mass) of aluminum and magnesium is quite high, which plays a decisive role in reducing the weight and improving fuel efficiency.

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 their wider application in the automotive industry. Although the price of raw materials is high, the current consumption of magnesium and aluminum castings for single vehicles has been increasing year after year. On the one hand, the increase in cost is compensated by technological progress; on the other hand, market competition forces automakers to reduce profits and adopt more light alloys. However, in order to greatly increase the amount of light alloy and reduce the purchase price of magnesium and aluminum ingots, the development of advanced forming technology is one of the keys.

2.3 High performance of automotive casting materials

Improving the performance of materials so that parts per unit weight can withstand higher loads is one of the effective ways to reduce the weight of castings. Bracket structural castings account for a large proportion of automotive castings, so the development of its castings has also become one of the focuses. 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.

Austempered nodular cast iron not only has higher strength than ordinary cast steel, but also has a lower density than steel, with a density of 7.1 g/cm3, while cast steel has a density of 7.8 g/cm3. It is widely recommended in recent years. . Austempering ductile iron is used, which is 10% lighter than steel castings under the condition of the same casting size.

In terms of aluminum alloy and magnesium alloy castings, high-strength and high-toughness materials are also used to replace them. On the basis of the original light alloy weight reduction, high-performance materials are used to further reduce weight.

3 Digitization of automotive casting development

The comprehensive 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. At present, digital manufacturing technology has been widely used in the development of automotive castings.

In order to meet the needs of rapid development of automotive castings, on the basis of CAD/CAE design and development, RP (rapid prototyping technology) has been widely used in the rapid trial production of automotive castings. After obtaining the original CAD/CAE data, the method of layer-by-layer accumulation is adopted to obtain the prototype of the casting or the prototype of the mold required for the casting through bonding, sintering or sintering. The former can use investment casting, gypsum casting and other methods to trial-produce casting samples, while the latter can be directly used as a mold to make sand cores, and cast out castings through core molding. In addition, powder laser sintering (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 casting trial production. For the outer mold with a relatively simple structure, CNC machine tools can also be used to process CAM with machinable plastics to obtain the core box and pattern required for the trial production of castings, or directly process sand blocks to directly obtain the sand mold of the outer mold.

Generally speaking, digital technology has penetrated all aspects of casting design, development and trial production, effectively improving the development speed and efficiency of castings. The main problem at present is that the digital technologies in design, analysis and rapid manufacturing are independent, and when the development process is transformed from one stage to another, quite cumbersome data conversion work is required. It is hoped that in the future, a unified data interface platform will be developed for the digital technology applied in each link of casting development, and standardized data conversion standards will be established to realize seamless data conversion between different software, so as to further improve the development speed of castings.