According to the statistics of the Foundry Association, the die casting industry is the fastest growing field in China's foundry industry at present, and has formed three major die casting industry centers in South China, East China and Southwest China. Among them, South China is a die casting industry cluster dominated by Zhaoqing. In this region, nearly 20 die casting enterprises led by Guangdong Hongtu Science and Technology Co., Ltd., Guangdong Hongte Precision Technology Co., Ltd., Guangdong Hongtai Science and Technology Co., Ltd. are concentrated. The annual output of aluminum alloy die castings reaches 140000 tons, with an output value of more than 5 billion yuan.

In recent years, based on the requirements of lightweight, high integration and high performance of products, the structure and shape of die-casting aluminum alloy parts are becoming more and more complex, and the quality requirements are getting higher and higher. For example, for some large aluminum alloy parts used in automobiles, in order to reduce the process and avoid the problems of poor rigidity and deformation of the combined parts, new design schemes often integrate several previous parts into a single part and require direct forming. Therefore, a large number of product parts with very large wall thickness differences, high performance requirements, complex structures, and even can not be smoothly molded and die cast with very poor processability have emerged, This brings great challenges to the forming and manufacturing of parts, especially to the die-casting forming of parts.

Although in the die casting industry, new processes and technologies such as vacuum die casting and local extrusion die casting have been adopted. However, in the process of die casting, there will still be a large number of dispersed pores, shrinkage porosity and other defects. For these defects, the products that must be leak tested and have pressure requirements must be scrapped. Even if they are used for products that do not have pressure requirements, they may also cause internal corrosion due to the detergent, acid solution, etc. entering the parts during electroplating, painting and other surface treatment, resulting in bubbles or unevenness during surface painting and electroplating, which will cause the plating layer to fall off in the later period, and shorten the service life of the products.

Gear chamber aluminum alloy die-casting infiltration parts for heavy truck engine

Infiltration technology is the most effective method to solve the leakage of die castings

During the forming process of die castings, internal porosity, shrinkage cavity, air hole and other defects are easy to occur. After machining, the surface dense layer parts of these defective castings are removed and the internal organizational defects are exposed. For automotive aluminum castings with sealing requirements, such as steam cylinder block, cylinder head, intake manifold, carburetor, brake valve body, etc., during the pressure seal test, the existence of defective micropores will lead to leakage of the sealing medium and cause a large number of waste products, and these defects are often found after machining and pressure test, resulting in serious waste of labor, raw materials and energy. In order to solve the problem of high scrap rate of die castings and save the castings that may be scrapped due to the above defects, certain treatment measures should be taken in production. At present, the most popular technology is infiltration treatment, that is, plugging. The so-called "impregnation" refers to the process technology that penetrates the impregnating agent into the micropores of the casting under certain conditions, after solidification, the filler that penetrates into the pores is connected with the inner wall of the casting pores, and the micropores are blocked so that the parts can meet the conditions of pressure bearing, anti-seepage and leakage prevention.

Application of Impregnation Technology in Large and Medium Sized Die Casting Aluminum Alloy Products Infiltration method

The infiltration treatment methods include atmospheric infiltration, vacuum infiltration, pressure infiltration and vacuum pressurized infiltration.

The attached figure shows the aluminum alloy die-casting infiltrating parts for the gear chamber of heavy-duty truck engine undertaken by our company, with the weight of 19.5kg and the maximum overall dimension of 893mm × 645mm × 106mm, the maximum wall thickness is 25mm, and the minimum wall thickness is 9mm. Several screw holes and sealing grooves need to be machined, and the leakage under the pressure of 1.4kg/cm2 shall not exceed 0.5cc. According to the nature and distribution of defects in die castings, working conditions of die castings, structural dimensions, types of impregnating agents and other specific conditions, we adopt the vacuum pressure integral impregnation method.

Selection of impregnating agent

The impregnating agent (glue solution) shall have good permeability and adhesion, and shall also have good medium resistance and physical properties close to the workpiece after curing. Infiltration agents can be divided into two categories: organic and inorganic. Inorganic impregnating agents (such as sodium silicate, commonly known as water glass) have a wide range of materials, high heat resistance, but low content of cured materials, large shrinkage rate of cured materials, brittleness, poor impact and water resistance, so they must be modified. Although the organic impregnating agent is a little expensive, it is relatively stable, readily available and convenient to use. Therefore, according to the technical requirements of the product, considering the structural characteristics of the product and the parts where quality defects occur, after many tests and inspections, we use the Jiatai 90C heat curing organic impregnating agent provided by "Wuxi Shenglong Impregnation Chemical Equipment Co., Ltd.". Its composition and performance indicators are shown in the following table.

Vacuum pressure infiltration process

The vacuum pressure infiltration process is mainly carried out in the vacuum pressure tank. According to the process sequence, it can be divided into three stages: pretreatment, infiltration and post-treatment.

The pretreatment is to degrease, clean and dry the die castings for standby. The main purpose is to remove the oil stain, metal chips and dust on the surface of the parts, improve the infiltration quality, and prevent the oil stain and mechanical impurities from being brought into the infiltration agent to affect the penetration and adhesion. Common degreasing methods include solvent degreasing, alkali degreasing and electrochemical degreasing. After the above degreasing cleaning, the castings are taken out, rinsed with hot water, and then dried at 80-90 ℃. In order to improve the immersion effect and prevent a large amount of water from entering the vacuum pump, the drying after cleaning is very important.

Infiltration treatment is to put the pre treated workpieces into the infiltration tank for sealing and vacuumizing for 15min, so as to fully remove the air and fine dust in the tank and the casting pores, and create differential pressure dynamic conditions for the filling and infiltration of the penetrant. Then use the negative pressure in the tank to suck in the impregnating agent, and vacuum again for 15min. The purpose of the second vacuum pumping is to discharge the gas in the impregnating agent in the tank, prevent the gas dissolved impregnating agent from infiltrating into the casting microporous defects, and prevent the generation of pores during curing from affecting the sealing performance. Finally, pressurize 0.5~0.75MPa for 15~20min, so that the infiltrating agent that has been filled and infiltrated into the pore of the die casting can further penetrate into the deepest part of the defect.

Post treatment is to drip dry, clean and put the workpiece into the curing furnace for curing, so that the penetrant entering the pores of the casting changes from liquid to solid, forming a solid curing film. The curing temperature is 80-90 ℃, which shall be kept for more than 2h, or it shall be kept at room temperature (above 25 ℃) for 24h. If the casting is used above 300 ℃, it should be cured at 80 ℃ for 2h first, and then cured at 110 ℃ for 1h. 100% pressure test shall be conducted for pressure castings after impregnation to check whether the required air tightness is achieved after impregnation treatment. If there is still leakage within the scope of infiltration, it is allowed to conduct re infiltration, and the castings that still have leakage after three times of infiltration shall be treated as waste products. Generally, the test pressure value is twice the working pressure value.

The main process flow of this process: cleaning parts → vacuumizing → pressurizing to make the penetrant enter the micropores → removing the residual glue (centrifugal drying, repeated cleaning) → curing → inspection → identification → storage, transportation or assembly.

It is required that the workpiece shall be placed correctly in the basket during infiltration

Key points of process control in infiltration process

Due to the high price of organic impregnating agent, it is necessary to add a centrifugal dryer in the impregnating tank to make the excess impregnating agent return to the liquid storage tank for further use. This requires that the workpiece be placed correctly in the basket during infiltration, and attention must be paid to the axis direction of some blind holes (threaded holes).

Pay attention to the maintenance of the penetrant, check the viscosity and stability of the penetrant once a week, and continue production only when it is within the specified index range; During operation, water and electrolyte shall not be brought into the impregnating agent; Before impregnation, the workpieces must be cleaned and the storage temperature of the impregnating agent must be strictly controlled, which must be enforced in accordance with the relevant process documents.

According to the distribution of shrinkage porosity and porosity inside the product, and in combination with the wall thickness of the product, select the appropriate viscosity of the impregnating agent, which is very important to improve and stabilize the qualification rate of the impregnating product. The smaller the viscosity of the impregnating agent is, the stronger the permeability is; The smaller the pore, the greater the capillarity, and the easier the penetration. Generally, the viscosity of die-casting products with large wall thickness difference is higher than that of products with uniform wall thickness when selecting impregnating agent.

The vacuum degree and pressure holding time also have a great impact on the qualification rate of infiltration products. The higher the vacuum degree is, the more thoroughly the gas in the micropore is discharged, and the more easily the penetrant penetrates into the micropore; The higher the pressure is and the longer the holding time is, the deeper the microporous defects of the castings are penetrated by the penetrant.

The geometric shape of the part has a certain influence on the infiltration effect. The thicker the part wall is, the easier it is to infiltrate, otherwise, the worse the effect is. Because the thinner the part wall is, the impregnant will flow out of the part before curing, or will be lost during cleaning.

In order to prevent discoloration of the surface of solidified aluminum alloy die castings, ion exchange water shall be used as curing water, and its pH shall be adjusted to 5~7 with glacial acetic acid; The curing water shall be replaced every 3-5 days according to the production volume.

For products with cleanliness requirements, no residual impregnating agent is allowed on the outer surface, inner cavity surface and bolt hole of the parts after patching, otherwise the appearance and assembly of the parts will be affected. In order to prevent the influence of the infiltration, cleaning, packaging and logistics process of product parts on the cleanliness of product parts, the parts that have been infiltrated and patched need to be cleaned again on the cleaning equipment of the processing line according to the process to ensure that the cleanliness requirements are met.

The leakage amount of the parts after penetration repair shall meet the product technical specifications and the sealing requirements. Therefore, 100% sealing test must be carried out for the parts after infiltration treatment.

Application and Development of Impregnation Treatment Technology for Aluminum Die Castings

In recent years, the demand for vacuum infiltration in aluminum alloy processing industry has grown rapidly, mainly used in aircraft, ships, locomotives, automobiles, motorcycles and other machinery manufacturing industries. Typical infiltration parts include engine block, cylinder head, intake manifold, oil pump, nozzle, water heating equipment, water pump, valve cover, casting oil pan, carburetor housing, gearbox housing, crankcase, compressor, ABS controller, power steering gear, automobile hub, fuel pump, natural gas control valve, hydraulic valve, cooling pump, sealed aircraft instruments, electronic instrument shield, pneumatic parts and various gas instruments.

The aluminum alloy die castings of Guangzhou Honda and Chery, such as gearboxes and clutches, were fully infiltrated by our company many years ago. At present, advanced industrial countries all advocate full infiltration treatment of pressure bearing castings, and the infiltration technology has been recognized by the automobile manufacturing industry and other major industrial departments. For example, several major automobile companies in Japan regard casting infiltration as an indispensable process in engine production. In recent years, with the development of impregnation equipment and impregnating agent materials, impregnation has become a satisfactory process to save the leakage defects of automotive aluminum castings. People's understanding of it is no longer just as a method of waste product regeneration, but as a recognized production process in the subsequent processing of castings.

Good logistics management, strict control of the infiltration process, and strict control of the cleanliness and sealing of the parts are the key to ensure that the product parts after infiltration meet the product technical specifications.

In the past, because of the small production scale of aluminum die castings in Chinese Mainland, the impregnation technology could not bring benefits to the die casting industry. In recent years, with the rapid expansion of the application of automotive aluminum die castings, the application prospect of infiltration technology in production has become increasingly apparent. The practice has proved that the infiltration technology is an effective method to improve the qualified rate of aluminum die castings and ensure the product performance. Especially for some die-casting aluminum alloy products weighing more than 3 tons, the economic benefits brought by improving the product qualification rate through infiltration have been recognized and valued by leaders and managers at all levels.