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货车轮毂温度测控系统设计
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131313Fig. Ying-chun Wang . Da-yong Li . Ying-hong Peng . Xiao-qin ZengORIGINALARTICLE Numerical simulation of low pressure die casting of magnesium wheelAbstract The application of magnesium in the automotive industry contributes to reduced fuel consumption and CO2 emissions. Nowadays, most magnesium components in automobiles are manufactured by die casting. In this paper, simulation of the low pressure die casting process of a magnesium wheel that adopts FDM (finite difference method) is presented. Through calculating the temperature and velocity fields during filling and solidification stages, the evolution of temperature and liquid fraction is analyzed. Then, potential defects including gas pores in the middle of the spokes, shrinkage at the top of the rim and the rim/spokes junctions are predicted. The reasons for these defects are also analyzed and the solutions to eliminate them are presented. The air gas pores and shrinkage at the top of the rim are eliminated effectively by reducing the pouring velocity. Furthermore, the cooling capacity at the rim/spokes junctions is also investigated in the paper. Through analysis of the shrinkage defects generated in various cooling modes, it is proven that the cooling pipe system set in the side mould alone is a valid way to enhance the cooling capacity at the rim/spoke junction areas. Finally, the strength analysis is carried out for further verification of the effectiveness of the new cooling method.Keywords Magnesium wheel . Low pressure die casting .Filling and solidification . Defects . Cooling capacity .Strength1 IntroductionMagnesium is being applied more and more widely in various industries due to a number of advantages including low density, relatively high specific strength, good cast ability, good shock absorption capability and good attributes for recycling. Its application will be further expanded supported by the currently developed Mg alloys with extended application potential 1. In recent years, the consumption of magnesium is dramatically increasing by a rate of about 15% per year 2, and the application in the automotive industry is the main driving force. Therefore, the investigation of the application of magnesium in the automotive industry becomes more and more important3.Low pressure die casting is a casting technology of near net shape method. Due to the high precision and high efficiency, low pressure die casting becomes an important technology for aluminum and magnesium castings. In the processing of the casting, the metal flow during mould filling and the solidification are undoubtedly important, so many studies of the numerical method were employed to simulate the flow of molten metal in the mould cavity. These simulations help researchers to understand the patterns of filling and solidification during pressure casting, and predict potential defects. In previous studies, there are mainly two methods for casting analysis. One is FDM (finite difference method), and the other is FEM ( finite element method). The FDM is commonly used for the fluid flow and heat transfer research 46. Many software programs for CFD (computational fluid dynamics) including FLUENT, PHOENICS and PAM-CAST adopt this method. The FDM can obtain a mass of valuable information in a shorter time and save on investment, and it fits well the regular geometry model. The FEM performs well in dealing with complicated geometric boundary conditions and is also used for casting problems 79. However, the finite element method needs considerable time and large memory space, along with difficulty in convergence.In this paper, the low pressure die casting process of a magnesium wheel is simulated with the commercial soft-Fig. 1 The magnesium wheel ware PAM-CAST which adopts the finite difference method. The velocity field of the metal flow in the die cavity and the temperature field of the casting during the filling and solidification processes are obtained by calculation.In practice, there are many defects generated in castings, such as gas pore, shrinkage and cold shut. In previous works, a number of studies have been carried out to understand the influence of these defects on the castings 1012. It has been proven that the defects have detrimental effect on the products, so they must be eliminated or reduced. Conventionally, these defects are related to the process of filling and solidification. In the present work, two kinds of defects generated in the process are predicted and the effective methods are put forward to solve these defects based on simulation.2 Filling and solidification2.1 Theory backgroundIn the present study, the flow of liquid metal is assumed to be Newtonian. The governing equations for mass, momentum and energy conservation are as followsFig. 2 Mesh of the wheel where is the density; u, the velocity vector; t, the time; , the dynamic viscosity of the molten metal; g, the gravitational acceleration vector; p, the pressure; c, the specific heat; , the thermal conductivity; T, the temperature and S, the source item.2.2 Modelling setupThe magnesium wheel of interest is shown in Fig. 1. TheCAD model of the wheel is imported into PAM-CAST, and the FDM mesh consisting of 1,894,528 elements isautomatically generated (Fig. 2). At least three elementsare created at the thinnest part of the rim in the thicknessDirection to ensure the accuracy of the fluid flow simulation.In this study, the physical property data of the alloy,AZ91D, are listed in Table 1. The casting parameters are: Pouring temperature 690C Initial mould temperature 420C Velocity at the gate 0.5 m/sFig. 3 af Temperature fieldduring filling. d t=3.76 s.e t=4.88 s. f t=6.13 s3 Simulation results and discussionThe numerical simulation has been carried out with a threedimensional mould filling program and the whole process of filling takes 6.22 s. Figure 3 shows the temperature field during filling. At first, the alloy flows through the running system into the cavity of the casting, and then fills the centre of the hub smoothly and steadily. Finally, it enters into the spokes. At 1.38 s, the front metal reaches the junctions between the rim and the spokes, and joins each other at 2.13 s at the bottom of the rim. At this moment, the temperature of the alloy is 640C. The liquid metal reaches the middle of the rim at 4.88 s with a temperature of about 590C, and attains the top of the rim at 6.2 s. When the cavity is filled entirely, no misrun and cold shut is detected at the top of the rim. At this time, the temperature of the front metal is about 590C, and the alloy is in the semisolid state. The alloy temperature at the bottom of the rim is about 630C, the temperature difference of the alloy between the top and the bottom of the rim is about 50C (Fig. 4). In the simulation, three test points at the middle of a spoke, at the rim/spoke junctions and at the top of the rim are selected to record the evolution of the temperature in the whole process. During the process of filling, when the front metal reaches the middle of the spokes, the flow velocity will slow due to the existence of the scoops, which makes the gas entrapped in the scoops generate the potential shrinkage defects shown in Fig. 5.The process of solidification begins at the end of filling,and is completed at 136.25 s. Figure 6 shows the liquidfraction pattern in the various stages of the process. Thedeep coloured fraction indicates that the alloy is in theliquid stage or mushy stage. On the contrary, the lightcoloured fraction indicates that the alloy is solidifiedcompletely. At the beginning, the solidification at variousareas of the rim is not synchronous due to the inconsistentFig. 5 Generation of the gas entrapmentFig. 6 af Liquid fraction patternduring solidification.d t=30.252 s. e t=35.252 s.f t=57.252 sflow of metal flow in these areas; the solidification at theparts between two spokes is faster than that at the spokes(Fig. 6a). Due to the non-symmetry of the hub geometry,three regions form liquid islands at the top of the rim andsolidify later, which leads to the generation of shrinkages(Fig. 6b). In addition, the volumetric heat content ofmagnesium is lower. The premature solidification in themiddle region of spokes occurs (Fig. 6c). It can be seenfrom Fig. 7 that the temperature of the alloy at the rim/spokes junctions is higher than that at the middle of thespokes during 2535 s. Consequently, the hot spots (liquidislands) at the rim/spoke junctions are generated (Fig. 6d),which leads to the potential shrinkage during the last stageof the process. Due to the above reasons, the whole processof solidification is not a directional solidification patternfrom the rim top towards the hub centre during thesolidification process.Fig. 7 Temperature curves during solidification. The curves13indicate:1 mid-spoke,2 junction,3 top-rim4 Defects eliminationBased on the above analysis, two kinds of casing defects,gas pore and shrinkage, are predicted. The first type ofdefect is from gas entrapped in the liquid metal which leads to the gas pores in the middle of the spokes; they have smooth and bright interior surfaces and regular shape. The other defect is shrinkage. The reason leading to the shrinkage at the top of the rim and the rim/spoke junction is that the alloy can not be compensated well during the process of solidification, so the cavities are generated. The interior surfaces of these cavities are coarse and dim, and their shapes are irregular. Solutions to eliminate these defects include decreasing the pouring velocity and improving the mould cooling.4.1 Solution 1: decreasing pouring velocityIn this study, the casting parameters are changed to eliminate the defects in the process of casting. The pouringFig. 8 Velocity field during fillingFig. 9 af Liquid fraction patternafter reduction of velocity.d t=25.835 s. e t=33.835 s.f t=40.835 svelocity at the gate is reduced to 0.3 m/s and the otherconditions are kept invariant. Figure 8 depicts that there isno difference in the velocity between the two sides andthe middle of the spokes when the front metal arrives at the spokes; thus, the gas entrapment will not take place and th gas pores in the middle of the spokes will be avoided. At the end of filling, the metal temperature at the top of the rim is about 580C, and the front metal can still flow freely.Figure 9 shows the liquid fraction pattern in the solidification process; the solidification in various areas of the rim is also not synchronous owing to the decrease of pouring velocity. The liquid islands are not found at the top of the rim and the shrinkage is eliminated in these areas.However, hot spots at the rim/spoke junctions still exist (Fig. 9e), which will result in potential shrinkage.4.2 Solution 2: cooling capacity improvementIn order to eliminate shrinkage at the rim/spoke junctions,the cooling water pipe system is installed in the mould toimprove the cooling capacity in these areas. As shown inFig. 10, the casting mould structure consists of three parts,the top mould, the side mould and the bottom mould, toFig. 10 mould of magnesium wheelallow easy removal. The cooling water pipes are equippedin the top mould and the side mould. Three cooling methods are proposed and presented in Table 2. These methods include pipe in the top mould alone, pipe in the side mould alone, and pipe in the top and side moulds together. According to the practical process, various waterflow velocities and pipe diameters are used to analyse theeffect of different cooling water systems on the cooling capacity. The standard settings are pipe with diameter of15 mm and water velocity of 10 m/s. In the case of method I, the result reveals that the time of the liquid islands generation during the solidification stage is 24.036 s, which is 4 s ahead of the mode without a cooling system. The liquid islands also disappear 4 s earlier. However, the liquid island volumes do not decrease and their positions do not change (Fig. 11b). In mode 2, the water flow velocity is reduced to 0.7 m/s. The generation and disappearance of the liquid islands are both broughtFig. 11 The initial generationof liquid islands in variousModesforward by about 2 s. Volume and position of the liquid islands also do not change. In mould 3, the diameter of the pipe increases to 20 mm, and the generation and disappearance of the liquid islands are brought forward by 7 s. Similarly, the volumes and positions do not change. From the above analysis, the cooling water pipe installed in the top mould alone cannot conduct an effective enough cooling job to limit the shrinkage at the rim/spoke junctions due to the fact that the rim/spoke junctions and the spokes of mould are cooled at the same time. In the case of method II, the results present the generation time of the liquid islands to be 28.93 s, and there is no difference from the mode without a cooling system. But the disappearance of them is brought forwardFig. 12 Temperature curve in method II. The curves13 indicate:1mid-spoke,2 junction,3 top-rimby 4 s. The liquid island volumes decrease considerably, and their positions move about 11 mm to the center of thewheel (Fig. 11c). From the temperature curve in the solidification stage shown in Fig. 12, the temperature difference of the alloy between the rim/spoke junction and the middle of the spoke becomes obviously small during 2535 s compared to that without a cooling system shown in Fig. 7. The improvement in water flow velocity and diameter of the pipe shows remarkable effect. In the case of method III, generation and disappearanceof the liquid islands are brought forward by about 8 s.Liquid island volumes decrease, but not obviously. The moving distances of the liquid islands are small. Figure 11d shows generation of the liquid islands in modes 8 and 9. Through the above analysis, it can be found that all kinds of cooling modes cannot eliminate shrinkage at the rim/spoke junctions absolutely, which is coincident with the actual fact 13. The liquid islands can only be reduced to some extent. Among the three modes, the pipe in the side mould alone is the most effective method. Not only are the liquid island volumes obviously reduced, but also their positions are moved toward the centre of the wheel. On the contrary, the pipe installed in the top alone or in the side mould together can lead only to less time for the generationFig. 13 Stress distribution ofthe wheeland disappearance of the liquid islands; and these two modes play little role in the change of the volumes and positions of the liquid islands.5 Strength analysisCasting defects can bring considerable damage to the mechanical property of the casting products and reduce the service life greatly 11. Casting defects are the main reasons leading to the cracks; the size and position of the casting defects are important factors to influence the strength endurance of the casting products 12. Structure analysis is carried out to prove the validity ofcooling method II by comparing the patterns of stress andstrain of the wheel in two models. One model is that the wheel has considerable shrinkage at the rim/spoke junctions generated without a cooling system, and the other model is that the shrinkage in these areas is generated with the cooling pipe equipped in the side mould alone, and their volumes are small and the positions move about 11 mm toward the centre of the wheel. In the investigation, the solid models are imported from the result of solidification simulation. The braking status that is dangerous to the wheels is selected for the analysis. The material of the wheel is casting magnesium alloy, AZ91D, and a kind of soft material property is assigned to the shrinkage defects for the analysis. Table 3 shows the material properties.MSC.Marc software (MSC Corp, Santa Ana, CA) is used for the analysis. The wheel is subject to a moment of 2,306 Nm. Meanwhile, there is 300 kPa pressure on the inner surface of the wheel. From the result of the calculation, it is found that the maximum stress and strain appears at the spoke near the interface of the wheel and land in two models. The stress concentration is located at the part of the spoke closest to the centre of the wheel. Adopting cooling pipe in the sidemould, the value of stress here reduces from 16.42 MPa to 14.06 MPa, decreasing by 14.4% (as shown in Fig. 13).The strain reduces from 3.23810-4 to 2.87910-4,decreasing by 11% (as shown in Fig. 14). This indicatesthe stress and strain of the wheel has an obvious decreasebecause of the reduction and transfer of the shrinkage when the cooling pipe is installed in the side mould. Consequently,the strength and service life of the wheel are enhanced enormously.6 ConclusionsThe overall process from filling stage to solidification stage for the low pressure die casting of an AZ91D magnesium alloy wheel has been numerically analysed. In addition,various solutions to eliminate or reduce the casting defectsFig. 14 Strain distribution ofthe wheel. a Without coolingwater pipe. b With pipe in theside mould aloneare numerically implemented. The following conclusionscan be drawn from the present study.1. The temperature field, flow field and liquid fraction in the two stages are obtained, and two kinds of potential defects including the gas pores in the middle of the spokes, shrinkage at the top of the rim and the rim/spoke junctions are predicted.2. Through reduction of the pouring velocity, the gas pores and shrinkage in the rim are eliminated effectively.3. The cooling system involving pipe installed in the side mould alone is the most effective method to reduce the hot spots at rim/spoke junctions. This mode can diminish the liquid island volumes and make their positions move toward the centre of the wheel.4. The strength analysis shows that the reduction and transfer of the liquid islands can decrease the stress and strain of the wheel during the work condition.Acknowledgement The authors would like to acknowledge financial supp
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