Abstract: With rapid development of modern automobile industry, application of light metal materials, represented by aluminum alloy, is becoming more and more popular. In order to guarantee qualified rate of production, quality requirement of aluminum alloy parts is higher and higher. However, due to defects in casting technology, quality of aluminum alloy castings has not been effectively resolved. Taking aluminum alloy shell castings as an example, process and flow of die casting technology for aluminum alloy parts are des blamed in detail, and some suggestions are put forward to improve production quality of aluminum alloy parts.
With enhancement of people's awareness of environmental protection, automobile manufacturing industry has taken production of ordinary cars with low fuel consumption as main direction since the 1980s, economy, light weight and miniaturization have become main features of modern cars. In order to meet requirements of new trend of automobile production, various iron castings in automobile manufacturing have begun to be replaced by aluminum castings in large quantities, thus realizing lightweight requirements of modern automobiles. With extensive use of aluminum alloy castings, production quality of related castings has also received extensive attention. Due to imperfect die-casting technology of aluminum alloy parts, quality of aluminum alloy parts has not been able to reach ideal state. Therefore, research and development of key technologies for aluminum alloy parts die-casting has not stopped in recent years.

Aluminum alloy shell castings are widely used in modern automobile industry. Due to their irregular shapes, some structures are difficult to meet quality standards through machining. Therefore, some aluminum castings are only used for rough surface structure of automobile shells. From perspective of safety of automobile use, industry has very strict quality requirements for aluminum castings. All castings have strict standards for material strength. In requirements for shell castings, dual standards of surface and internal quality are put forward. In order to ensure product quality, some manufacturers have adopted optical flaw detection technology to control the entire production process.

Table 1 Performance table of aluminum alloy shell castings
Aluminum alloy shell cover introduced in this article has a relatively simple structure, and only two parts need to be machined. However, due to irregular shape of shell, heights of the two bosses are different, maximum outer diameter, height, and main wall thickness section are 105mm, 40mm, and 3.5mm, respectively. Parts that need to be machined are outer diameter surfaces of large and small shells, and machining tolerance must be controlled within range of +/-0.1mm. Since this part does not need to process inner cavity, difficulty of die casting is relatively small.

 

Figure 1 Simplified diagram of aluminum alloy shell castings
In order to ensure that performance of castings can meet requirement of withstanding static crushing force above 20kN, production process needs to be controlled in accordance with GB6414-86 CT6 level to ensure that castings are not mixed with impurities and cracks during die-casting process, and it is not allowed to repair product by welding or dipping.

Parts production process is: incoming material → melting/die-casting → trimming/cleaning → machining → cleaning → assembly.

Although aluminum alloy shell casting introduced in this article has a simple structure, following production defects often occur in production process:
(1) Mottling: Temperature difference between metal and mold, as well as filling speed during smelting and amount of spraying in shot peening process are not well controlled, which can easily cause color of casting to become dark and black.
(2) Pore defects: This type of problem can hardly be completely avoided in production process of aluminum alloy castings. Control requirement for porosity problem of casting is to ensure the overall strength of shell, and it is necessary to ensure that no porosity exceeding ASTME505 level 2 standard is generated in critical area. Acceptable diameter range of pores in castings is ≤∉1.6mm, porosity should be controlled within 6.2%, and diameter of pores on machined surface should not exceed 2.0mm.
In terms of production quality control, visual inspection is mainly responsible for checking appearance defects of castings, while X-ray flaw detection is responsible for controlling internal quality of castings. However, in mass production, from perspective of speeding up production and controlling production costs, both methods have certain defects. Therefore, CAE and other auxiliary technologies are used to analyze and refer to aluminum alloy die-casting process, aiming to control inherent quality problems of castings in blank stage.

According to structural characteristics and production requirements of casting, casting pressure is set at 350T. According to production experience of similar products in the past, if mold combination is unreasonable and process parameters are not selected properly, liquid metal filling speed will be too fast in die-casting process, resulting in obstruction of gas discharge in cavity, and finally forming quality defects such as pores or excessive oxide impurities in finished casting, thus affecting qualified rate of casting.
According to design requirements of castings, thickness of surface dense layer of thin-walled shell castings is only 0.8mm. If it is processed properly, central structure will be loose, resulting in a decrease in shell performance and pressure resistance. Therefore, when designing mold, use positioning pins for positioning, and control machining volume within range of 0.5mm, which saves machining time and is more conducive to improving internal quality of castings.

According to design requirements of aluminum alloy shell casting, AnyCast-ing software was used for virtual design, and three different water inlet design schemes were obtained. Through comparison, the smoother flow path in terms of liquid flow filling was finally determined. Design scheme C is for production of molds, it is found in actual production that this scheme has a good effect on improving internal defects of shell castings and improving yield of shells.

 

Fig. 2 Mold flow analysis of mold scheme

Combined with structural weight of casting shell, a 350T cold-pressure chamber die-casting machine is used for die-casting of castings, and temperature is set within range of 640℃+/-20℃. In order to ensure a stable pressure during filling process of metal liquid and avoid secondary oxidation slag inclusions or erosion of core caused by problems such as turbulent flow and splashing, pressure increase rate is set at 1.3kPa/s.

In order to enhance quality of aluminum alloy shell castings and reduce impact of porosity, pinholes, and slag inclusions on product qualification rate, a secondary refining step is adopted in casting process, that is, refining once before and after alloy is released from furnace. At the same time, fiber filter screens are used at riser nozzle, ceramic filter screens are used at horizontal gate, double-layer fiber filter screens are placed at the end of horizontal gate and gap mouth for three times of filtration, so as to control occurrence of slag inclusion defects in product rate.

According to above scheme and process, 6 shell samples were randomly selected in actual production for inspection, it was found that scheme C had a good effect on improving internal defects of shell castings and increasing shell yield. It was found through X-ray flaw detection that qualified rate of internal control reached 100%; it was found through finishing test that porosity index of machined surface reached level 2 of ASTM E505; through destructive crushing test, it was found that all samples could reach A standard that withstands a pressure of 25kN or more. It can be seen that design and process selection of aluminum alloy shell castings introduced in this paper are relatively reasonable, and have achieved certain results in controlling product quality.

With continuous improvement of science and technology and related processes, die-casting process of aluminum alloy parts has been continuously improved. Traditional related process is long, which is not conducive to quality control in casting process. In order to ensure improvement of production quality, it is necessary to start quality management from incoming material stage. Production process and key technologies of above aluminum alloy shell castings have reached reasonable and feasible standards through scientific design and program selection, which proves that relevant technologies and processes can meet quality requirements of mass production for related casting products, and also verify role of key technologies in die-casting of aluminum alloy parts in improving the overall quality of related products.