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Die Design and Process Optimization of Low Pressure Casting of Aluminum Alloy Wheel

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China’s aluminum alloy wheel industry started relatively late. In the late 1980s, the first modern-scale wheel manufacturing enterprise appeared. In the early 1990s, aluminum alloy wheel factories appeared in Guangdong. Subsequently, many of them have considerable strength and Large-scale private capital enters this industry.


As of 2010, the loading rate of aluminum alloy wheels for Chinese cars has been close to 70%, and with the growth of personalized demand, wheels not only have practical functions as a part of the vehicle, but also follow the innovation of its surface treatment technology. And the richness of the shape becomes a kind of decoration, and then becomes a platform for showing one's personality.


Aluminum alloy wheels (and aluminum-magnesium alloy wheels) are increasingly becoming the future development trend due to their light weight, beautiful appearance, good shock absorption and heat dissipation.


1. Overview of the manufacturing method of aluminum alloy wheels


There are two main production processes for aluminum alloy wheels, casting and forging.


Compared with cast wheels, the metallographic structure of forged wheels is broken grains and forged structures, while the latter is dendritic grains and as-cast structures. In comparison, the mold for forged wheels is much more expensive than casting, and it is also more difficult to open the mold, but the mechanical properties of forged wheels are 30%-50% higher, and the corresponding price is much higher; from the perspective of production technology, the use of The casting process is easier to mass produce, and the price is low, and the market demand is greater.


In addition, there are also individual manufacturers that use spinning and welding assembly methods to form. The spinning method is to cast a part of the wheel hub with a corresponding margin, and then use the extrusion method to form it. This requires special equipment and production lines, which are less used in China; the welding assembly method generally only casts or Forged spokes and rims are rolled with shaped coils, and then the spokes and rims are welded into the hub, which can greatly reduce the weight and reduce the production cost, but the process is more complicated, and Japan is quite mature in this technology.


2. Casting process of aluminum alloy wheel hub


Casting forming wheels is the main production method used by most aluminum alloy wheel manufacturers. The commonly used casting methods mainly include gravity casting, low pressure casting, squeeze casting and oxygenated casting.


(1) Gravity casting relies on the gravity of the molten aluminum to fill the mold. This method is relatively simple and the equipment conditions are not high, but the production stability is poor. The temperature field during the solidification of the molten aluminum is easily affected by external conditions and shrinkage occurs. Casting defects such as, shrinkage cavity, high labor intensity of workers, poor mechanical properties of the product, but low cost, so some enterprises still use it when producing smaller-sized wheels.


(2) At present, as the main method of aluminum alloy wheel hub production, low pressure casting is to fill the mold at a lower pressure (usually 20-60kPa). After the mold is filled, the pressure is maintained for a period of time, and then the pressure is relieved and opened. Figure 1 shows the specific process of taking parts from a mold. This production method can produce high-quality wheels in large quantities. The liquid aluminum solidification process is carried out under pressure, so the product qualification rate and the utilization rate of liquid aluminum are high. The low-pressure casting machine has a high degree of automation and relatively low labor intensity. , The production efficiency is high, and the cost is slightly higher than that of gravity casting.


(3) Squeeze casting, also called liquid forging, is a process that integrates the characteristics of casting and forging, which is divided into multiple forms such as compound squeeze casting, positive squeeze casting, and double squeeze casting. The common feature is: in addition to the metal mold, there are also a die punch and a die ejector rod. Generally, a pressure of about 7000N is applied to the punch for casting. The surface of the casting is smooth, the metallographic structure, and various mechanical properties are close to Forgings. The process method is to use a hydraulic press to press the aluminum melt with a melting point of 5-l0℃ higher than the alloy into the casting mold, and apply high pressure (p≥100MPa) to the solidified melt at a speed of about 5mm/s. Compression crystallization makes the eutectic and dendrites fully broken, and can produce high-density, non-porous aluminum wheels. The mechanical properties can be improved by more than 15%, but the technology is difficult and the efficiency is low.


(4) Recently, a new porosity-free die-casting process (oxygenated die-casting method) has appeared abroad, but only the United States, Japan and other countries have begun to use it in production.


Third, the design of low-pressure casting process mold


The low-pressure casting process is now the main production process used by major aluminum wheel manufacturers. It has many advantages and is generally accepted by many manufacturers. The core component of the process-the design and development of low-pressure casting molds, is effective for the company's products. Low-cost production has the most important impact.


1. Factors affecting the performance of castings by low-pressure casting process


In the low-pressure casting process, the factors that can have an important influence on the forming and pass rate of castings include:


(1) The reasonable low-pressure mold design of the low-pressure mold can make the feeding channel in the casting process unblocked, produce a good and rapid sequential solidification effect, and realize the solidification from the distal end to the riser direction to the greatest extent to avoid the occurrence of casting defects. Improve production efficiency and benefits.


(2) Low-pressure casting machine Low-pressure casting machine is a factor that cannot be ignored. A good low-pressure casting machine can be automatically controlled according to specified parameters at all stages of the casting process, which greatly reduces the instability of the production process. For example, the German GIMA machine is A better model.


(3) Mold surface coating The coating on the mold surface has important functions such as resistance to thermal shock, ease of mold release, and improvement of the surface quality of the casting. The type, particle size, and thickness of the spray coating on the mold surface will directly affect the performance of the casting.


(4) The initial temperature of the mold. All molds need to be pre-heated before going on-line production. Generally, the mold pre-heating temperature is suitable to reach 400-450°C. If the mold temperature is too low or too high, the reasonable temperature field of the mold will be destroyed, and the casting will be difficult to form or the qualified rate will be low.


(5) Production cycle The so-called production cycle refers to the time period required to produce a casting. This should be a basically fixed cycle process, including mold clamping, liquid lifting, pressure increase, pressure retention, pressure relief, temperature reduction and opening. Mold taking, the total length of the process and the time distribution of each stage will affect the temperature field changes of the mold to a greater extent, thereby affecting the stable production of castings.


(6) Outside temperature The outside temperature often changes with the change of seasons. Production in an open factory area is more susceptible to interference from outside temperature. The process parameters that make stable production in summer are no longer applicable in winter.


(7) Other accidental factors These factors include aluminum clamping in the mold, which makes the pressure relief cooling time longer, the machine breaks down for maintenance, and spray paint. The occurrence of these conditions will affect the mold temperature and affect the normal production of castings.


2. Design of low pressure mold


Reasonable mold design is the most important part of achieving high efficiency and high benefit. When designing low pressure molds, factors such as mold gradient, mold wall thickness, cooling system distribution, etc. should be considered, as well as the influence of the shape characteristics of the wheel hub on the casting performance. After comprehensive analysis and reasonable configuration, the effect of twice the effort can be achieved. .


(1) Determination of mold gradient Here gradient refers to the tendency of the rim cavity of the mold to increase from thin to thick from top to bottom, and this trend meets the requirements of sequential solidification. As shown in Figure 2, the pouring and risers of basically all low-pressure molds are opened in the middle of the hub, and the spokes are fed to the surroundings, and solidify sequentially from the farthest and thinnest part to the riser, and the thickness is further toward the riser. The larger it is, it can ensure that there is a better feeding channel for liquid aluminum during solidification. The size of the rim cavity gradually increases from 8.78mm, 9.14mm, 9.9mm to 10.32mm, which meets the gradient requirements.


The size of the rim cavity, on the basis of stable forming, the smaller the better, which can reduce the amount of processing, retain more dense parts of the crystal structure, and prevent air leakage due to shrinkage, shrinkage and other defects. , And at the same time increase the utilization rate of molten aluminum and reduce the weight of the blank.


When determining the rim thickness and gradient, the rim width is another factor that needs to be considered. The larger the rim width, the farther the distance from the riser, the more appropriate to increase the rim wall thickness and gradient.


(2) Mold wall thickness As shown in Figure 2, the mold cavity is closed by the bottom mold, the top mold and the side mold. The mold wall thickness refers to the thickness of these three parts.

 


In the initial stage of the filling process, the heat dissipation of the casting is mainly heat conduction, that is, the mold itself absorbs heat, and the amount of heat absorption depends on the quality of the mold (the heat absorption is directly related to the material quality under the premise that the mold temperature rise is fixed. Proportional), the mold heats up and absorbs heat, and the aluminum liquid cools and dissipates heat. When the two reach a thermal balance, the heat dissipation method based on conduction and heat dissipation basically stops. At this stage, due to the rapid heat conduction and heat dissipation, and the large temperature difference between the mold and the molten aluminum, the molten aluminum has a chilling effect during solidification. At this time, the solidified structure on the outer surface of the casting is dense and the mechanical properties are good. Inferring from this situation, increasing the wall thickness of the mold can obtain a relatively long-term chilling effect, thus obtaining a high-quality tissue layer with a larger thickness.


The top mold and the side mold form the rim of the wheel. Because the rim itself has a small thickness, too thick mold wall thickness may cause uneven heating and cooling throughout the rim, resulting in casting defects. Therefore, the thickness of the top and side molds is mainly to ensure the strength of the mold, while taking into account the forming factors of the rim, and according to general experience, the wall thickness of the upper mold is 25-30mm, and the side mold is about 30mm.


The bottom mold and the upper mold form the spokes of the hub. The strength of the spokes is very important to the wheel. According to general experience, if the thickness of the bottom mold is increased, a deeper chill layer should be obtained, thereby enhancing the mechanical properties of the spokes. This has been tested and verified. Choose a wheel with wider and thicker spokes, design the thickness of the bottom mold to 45mm, and produce a total of 50 test pieces, which are difficult to form during the casting process, and the X-ray inspection of the spokes and the rim has large shrinkage holes. After heat treatment of 50 pieces, the mechanical properties of the semi-finished product did not increase as expected, and it was basically the same as the similar wheel type produced with conventional wall thickness. A large number of scraps occurred during the air tightness test after machining, and there were 27 air leaks. The air leaks were distributed in various parts of the rim. There were large slag holes at the junction of the spokes and the rim.


After analysis, the excessive mold wall thickness accelerates the overall cooling speed of the ribs (the bottom mold absorbs more heat), causing the feeder channel from the riser to the rim to be blocked prematurely. Due to insufficient feeding, the spokes and the rim are in phase. There are large shrinkage holes in the joint, and the shrinkage of the rim is caused to cause serious air tightness.


After experiencing the heat dissipation mode based on conduction and heat dissipation, the cooling mode is transformed into convection and radiation. Because the bottom mold is too thick, it is unfavorable for the heat dissipation of the molten aluminum. The cooling effect of the bottom mold cooling system on the molten aluminum is also weakened. That is to say, in the subsequent cooling process, the external cooling factors affect the internal temperature field of the hub. The influence is weakened, so that the control of the reasonable shaping of the wheel hub is correspondingly weakened, and the rib part has a tendency of coarsening and segregation in the subsequent cooling process, thus weakening the strength of the rib.


Therefore, an excessively large bottom mold thickness is not advisable, and then the bottom mold wall thickness of the mold is reduced to 25mm, and the above problem is basically solved. When selecting the wall thickness of the bottom mold, the main consideration is to facilitate the control of the internal temperature field by the external cooling conditions. Therefore, the wall thickness of the bottom mold is generally 20-25mm.


(3) Cooling system The cooling system occupies a very important position in the mold design. The additional cooling affects the temperature field of the mold, which is the key factor for the subsequent control to achieve sequential solidification.


Generally, cooling is divided into three modes: air cooling, water cooling and air-water mixed cooling. Regardless of the cooling method, better cooling can be achieved. The water-cooling method has a fast cooling speed, which can improve the mechanical properties of the hub to a certain extent, but it is necessary to solve the problem of cracking of the cooling pipeline due to frequent thermal expansion and contraction. Therefore, the pipeline welding process requires higher requirements.


Air cooling is a common cooling method. As shown in Figure 2, the cooling air ducts are divided into upper mold air ducts, lower mold air ducts and side mold air ducts. Air duct holes can be drilled for individual parts that require strong cooling. Pass the air duct into the mold, closer to the cavity, in order to have a better cooling effect.


The key to cooling is to determine the appropriate cooling position, cooling sequence and cooling intensity. As shown in Figure 2, the layout of the cooling air pipes and the layout principle of the cooling air pipes of the bottom mold are: facing the ribs and centrally cooling the hub flange The hot spots where the discs and spokes meet the rim; the layout of the upper mold air duct is similar to that of the bottom mold, while the side mold is generally just to add air ducts to the junction of the spokes and the rim. The control of the cooling sequence and intensity is the control of the sequence of cooling on and the air flow of the cooling air pipe per unit time, which determines the cooling sequence and the degree of heat removal per unit time. This plays a vital role in ensuring sequential cooling and is also the biggest task of on-site process adjustment.


For today's more advanced wheel casting machinery, the air-cooling flow can be accurately and automatically controlled, so that the stability of the process can be guaranteed to ensure the stability of product quality.


(4) Mould design outlines for different front-shaped wheels. Different front-side shapes of the wheels will cause large differences in the number, width, thickness, etc. of the spokes. Therefore, the temperature field of the entire casting process will be quite different. There are also different effects when feeding the mouth to the rim and ears. When designing the mold, the specific situation should be analyzed in detail.

 

Fourth, prospective research on the influence of low-pressure mold design


The design of low-pressure die for cast aluminum alloy wheels is affected by many factors, and many factors are difficult to quantitatively analyze and control, such as the influence of different rim front shapes. Due to the demand for diversified products, there are thousands of types of modeling, and we can only have an empirical design plan from the macro level, but often due to subtle differences, the molding process will be very different. For this reason, the designed mold may need to undergo multiple changes and verifications to meet the mass production requirements. This will prolong the development cycle and cause greater waste.

With the development of computer technology, the secondary development based on the three-dimensional software platform can simulate the casting process of aluminum alloy wheels. After all the external conditions are determined, the filling and solidification of molten aluminum can be simulated. Show the possible defects and corresponding parts of the wheel blank produced under the corresponding conditions, provide a direct reference for the mold design, and carry out the improvement and optimization of the mold design. This "design→simulation→improvement" process can be repeated until a satisfactory result is achieved.

The application of computer software is particularly important in combination with actual production on site. The determination of many parameters needs to be connected with actual production conditions to truly reproduce the production process. We call this process of integrating with the actual factory "internalization". The success or failure of "in-house" determines the success or failure of software application, because the actual situation of each production plant is very different, and the use of software is meaningful only when it meets the actual conditions of the factory.


Conclusion

The production of aluminum alloy wheels is a technology, capital-intensive, and relatively complex process. With the rapid development of computer technology and the continuous emergence of various advanced technologies, they are gradually being integrated into the research and development and actual production of aluminum alloy wheels. , And the use of computer to simulate the solidification process of the wheel hub in the mold will play a vital guiding role in the design of the mold and the optimization of product development after sufficient influencing factors have been perfected. In terms of metal surface treatment, processes such as vacuum electroplating are gradually being applied in China. Therefore, the design of low pressure molds should be oriented to a broader scope to meet the challenges of more advanced processes, introduce more, absorb more, and create maximum benefits in continuous improvement!

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