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I. Introduction
The aluminum alloy wheel hub is a new generation of steel wheel hubs. It has the advantages of light weight, fast heat conduction, attractive appearance, energy saving and safety. At present, it has been widely used in cars and other light passenger cars at home and abroad. With the rapid development of China's auto industry and the increasing demand for foreign accessories, the market capacity is very impressive. At present, the methods for manufacturing aluminum alloy wheels at home and abroad are mainly divided into two major categories: one is the forging method, in which the more advanced foreign technology is composed of a continuous casting process and three forging processes. Although the method is of good quality, the yield is only About 50%, the price is expensive. The other is casting, gravity casting and low pressure casting. The gravitational casting method has serious defects such as shrinkage, loosening, porosity, etc., low mechanical strength, and low yield, and has been eliminated in foreign countries. Currently, low-pressure casting methods are used mostly at home and abroad. The product quality and finished product yield have been improved. However, the technology is complicated and the investment in equipment is too high. It is necessary to invest more than 100 million yuan to import 300,000 pieces of equipment annually from abroad. The liquid die forging method is used to make the aluminum alloy crystallize under high pressure and produce a certain amount of deformation during the crystallization process, eliminating defects such as shrinkage holes, loosening, and pores. The product not only has excellent mechanical properties close to the forging, but also has a fine casting A precision molding of high efficiency, high precision, and investment is much lower than the low pressure casting method.
Second, the hub of the process characteristics and process key
The hub is similar to a shallow cup-shaped member, the wall is thin, the wall thickness is substantially uniform, the rim diameter is large, and the height is moderate, which is basically suitable for the liquid forging process. The main difficulty in manufacturing is that when using the direct liquid die forging method, a deep cold septum is easily formed between the rim and the original pouring liquid surface, and measures must be taken to avoid it.
The key to affecting the internal crystalline quality and mechanical properties of the workpiece is the control of the temperature field and the stress field. There are many factors affecting the temperature field. Therefore, an ideal temperature-time curve must be found through experiments and calculations. The stress field is directly related to the elimination of defects such as shrinkage holes, porosity, and porosity in the workpiece. It is necessary to determine the appropriate stress field distribution and lay the foundation for obtaining high-quality workpieces.
Third, mold design and molding conditions to determine
Mold design
Considering that defects such as inclusions may appear on the surface of the workpiece, the thickness dimension must have a certain machining allowance. Therefore, add 0.5 mm to the thickness dimension of the part drawing, and take the draft angle of 1.5° to draw a forging drawing. The mold is designed based on this figure. According to the structural characteristics of the workpiece, a direct liquid forging method must be adopted, and the female mold adopts a vertical parting surface so that the workpiece can be ejected. The use of vertical parting must have a horizontal locking device, taking into account the limitations of the equipment, so the use of cone-shaped retaining ring locking device, see Figure 3 in 3 . During work, the metal is poured into the concave mold composed of parts 4, 6, and 8, the upper mold is downward, and the part 3 firstly presses the parts 4 and 8 to form a mold lock, and then the punch 6 is pressed and formed. After the pressure is maintained, the punch is pressed. 6 with the upper mold up, parts 3 still under the action of the spring 4,8, so that the punch release, the spring pressing force should be greater than the ejection force of the punch, the rear part of the 3 driven parts 3, 4, 8 Remove the workpiece separately. Because it is a test mold, the guide is mainly based on equipment. The closing stroke is controlled by the pressurizing force. This may cause errors in the thickness dimension of the spoke, but it does not affect the test results but greatly simplifies the quantitative casting device.
2. Determination of forming process conditions
(1) Melting and mold preparation of aluminum alloy
The hubs are subjected to large impact loads while working, and are often made of aluminum-silicon alloys. The ZL107 alloy was selected and smelted in an electric furnace in order to control the smelting temperature more accurately and to perform refined degassing treatment. Because the metal filling distance is longer, in order to increase filling, the pouring temperature is increased to 730° C., and the mold should be preheated to 310° C. before the work. The resistance wire is used for heating, and the lubricant is applied at the same time for preheating so that the mold can be demolded smoothly. Graphite oil is used as a lubricant. In order to ensure uniformity, spraying is preferred. Pouring temperature and mold temperature are too high, it will make the workpiece surface rough or even sticky welding, the temperature is too low, the metal cooling too fast, causing difficulty in filling forming.
(2) Pouring and Pressurization
There are no gates and risers in liquid forging, so it is necessary to accurately quantify the casting. With funnel casting, the hopper needs to be heated to a temperature close to the liquid phase of the metal to perform a "bottom beating" so as to avoid the metal liquid splashing on the mold causing defects.
Due to the large size of the workpiece and rapid heat dissipation, it is necessary to complete the pouring in the shortest possible time. The speed of the large hydraulic press is slow, and it takes a certain period of time for the fast downward transfer to the working pressurization, so the punch is allowed to descend as soon as possible after casting, so that the start The pressurization time is controlled at 5-8 s, and the pressurization speed is about 0.1 m/s. The speed is too fast and the molten metal will spray outwards, resulting in insufficient watering. The pressurization pressure should be greater than 100 MPa. This is due to the fact that the rim has a certain height and the pressure is too low. The pressure on the connection between the rim and the spokes will be insufficient and the mechanical properties will be poor. The pressure holding time is about 10s, the cooling time is 15~20s, the holding pressure cooling time is too long, and the workpiece temperature is too low, which will greatly increase the stripping force, and it is difficult to remove the mold and even cause the workpiece to shrink and crack.
3. Ring cold treatment
In the case of direct liquid forging, part of the molten metal moves up and fills, forming a ring of cold septum between the liquid surface of the original metal. This cold septum is sometimes difficult to avoid, increasing the pouring temperature and the preheating temperature of the mold, shortening the start of pressurization. After the time, the cold septum has been reduced, but can not be completely avoided, there is still 1 ~ 1.5mm deep cold septum, as shown in Figure 3. For this purpose, a semicircular arc of R2 is opened at the height of the cold gap formed on the mold so that the cold gap is formed on the projecting arc and is removed at the machining step, so that the influence of the cold partition is completely eliminated.
Fourth, the detection of wheel hub mechanical properties
In order to detect the mechanical properties of the hub, heat treatment was first performed on the hub. The heat treatment conditions were quenching at 515±5°C for 6 h, tempering at 175±5°C for 6 h, and processing into specimens.
V. Conclusion
(1) The liquid die forging process for automotive aluminum alloy hubs is feasible and the product performance is superior to the current manufacturing methods.
(2) The process equipment is simple, the investment is small, the material utilization rate is high, and the product cost is low.
(3) The process is easy to automate and is suitable for mass production of auto parts.