Investigation on the flow pattern in the shot sleeve of the cold chamber HPDC process. Jun-Ho Hong, Young-Sim Choi, Ho-Young Hwang, Jeong-Kil Choi

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1 Investigation on the flow pattern in the shot sleeve of the cold chamber HPDC process Jun-Ho Hong, Young-Sim Choi, Ho-Young Hwang, Jeong-Kil Choi Center for e-design, KITECH (Korea Institute of Industrial Technology), , Dongchun-Dong. Yeonsu-Gu, Incheon, , South Korea. Abstract The cold chamber high pressure die casting process is one of the most general processes to make a lightweight material alloy casting. This process usually guarantees the short cycle times and good surface quality. However, the gas porosity defect appears easily in this process due to the air entrapment during the injection stage. The flow pattern of molten metal in the shot sleeve is closely related to the air entrapment. Generally, the flow patterns in the shot sleeve are concerned with the various plunger speeds and fill rate of molten metal. To investigate the mechanism of the gas porosity generations in the shot sleeve, the numerical simulation is introduced in this study. The numerical simulation results were shown good agreement with the experimental results using water model. From the comparison results, we conclude that the developed mathematical model for the flow pattern simulation in the shot sleeve of the cold chamber high pressure die casting process is useful to determine the proper condition of the plunger speed in some rage of the molten metal fill rate. Key words shot sleeve, gas porosity, various plunger speed, numerical simulation, water model 164/1

2 Introduction Many recent research works in material engineering have been carried out to find out more effective casting methods for the lightweight materials such as aluminum alloys[1]. The cold chamber high pressure die casting process is one of the most general processes to produce the lightweight material cast. It injects the liquid molten metal into a mold at high speeds. From this process, we can usually get the short cycle times and good surface quality of casting products. This process, however, has a problem that the gas porosity defect appears easily[2]. The gas porosity is occurred mainly due to the entrapment of air in the molten metal as a consequence of the high speed injection. The gas porosity usually causes the pin-hole defects that are harmful to the mechanical properties and surface quality etc. The flow pattern of molten metal in the shot sleeve is closely related to the air entrapment. The flow pattern of molten metal in the shot sleeve depends on the die casting operation conditions; filling rate and plunger speed. The main aims of this study are the development of a numerical simulation system for the flow pattern analysis in the shot sleeve and the accuracy verification by comparing to the experimental results from a water model. Numerical model A numerical model based on the SOLA-VOF technique[3][4] was used to simulate the molten metal flow in the shot sleeve. Continuity equation for the mathematical modeling is the equation (1). V = 0, (1) where V is the vector of velocity. Volume of fluid equation is written by F + VF = 0, (2) t where t and F are time and fraction of fluid volume, respectively. Navier- Stokes equation is given by V 1 + ( V ) V = G gradp + 2 V, (3) t ρ where G, ρ and P are vector of gravitational acceleration, density of molten metal and pressure. Energy equation is T T T T f s ρ c = K + K + K + ρl = 0, (4) t x x y y z z t where T, c and L are temperature, specific heat of fluid and latent heat. f s is the fraction of solid, and k is the thermal conductivity. The VOF method is using the fractional volume function F(x, y, z, t) for tracking free surface boundaries. When averaged over the cells of a computational mesh, the average value of F in a cell is equal to the fractional volume of the cell occupied by fluid. A unit value of F corresponds to a cell full of fluid, whereas a zero value indicates the cell contains no fluid. The cells with F value between zero and one contain a free surface. 164/2

3 Volume of fluid a cell F =. (5) Volume of a cell The VOF method considers the amount of F to be fluxed through the face of a cell during a time step of duration t. The amount of F in one time step is δ F times the face cross section area, where δ F = MIN[ FAD Vx + CF, FDδx D ] (6) and CF = MAX [( 1.0 FAD ) Vx ( 1.0 FD ) δxd, 0.0]. (7) To deal with the problem where the domain keeps decreasing, a very simple moving wall boundary condition is employed. It is emphasized that in the description of plunger moving the grid system in the domain remains unchanged. We chose a method of maximum preserving the calculating system before. In this method, the moving wall is treated such as the second fluid in the multi-phase flow calculations. Fig. 1 shows the flow chart of the calculating code. Experimental model The water modeling instrument for the visualization experiments was made to prove the accuracy of the developed shot sleeve flow pattern simulation module as shown in Fig. 2. It includes a transparent plastic acrylic shot sleeve, a plunger, and the 1D robot system which can control the plunger speed of shot sleeve. The space between the plunger and the sleeve end is 60mm in diameter and 500mm in length. Plunger speeds were set in 30, 50, and 70 cm/sec. Then the fill rate of shot sleeve was 30% unique. The setting of the injection conditions in the shot sleeve for the experiments and numerical simulations is shown in Table 1. Results and Discussion Fig. 3 shows the 3D shot sleeve model and the 30% fill rate state of molten metal. In the first, we have done the verification test through the comparison with the theoretical plunger displacement and the numerical simulation result. The 3D grid system (210 x 40 x 40) was used in this calculation. The molten metal is poured initially into the shot sleeve through the left side gate system in the Fig. 4. The right side one is a vent system. The molten metal pushed out by plunger is exhausted through this vent system. The plunger shape is not drawn in this figure while the molten metal flow pattern in the shot sleeve is expressed. When the plunger speed is 50 cm/sec, the plunger displacement has to be 20.5 cm after 0.41 seconds theoretically. As we can see in the Fig. 4, the numerical simulation result shows the plunger displacement exactly. Fig. 5 shows the flow patterns in the shot sleeve for the plunger speed of 30 cm/sec. The molten metal in the shot sleeve is strongly suppressed by the plunger as the plunger start to move. Then the molten metal is pulled up to the upper wall of sleeve. In this plunger speed, the pushing pressure 164/3

4 of the plunger is not so high yet, so that the front of molten metal pulled up by the plunger does not touch the upper part wall of the sleeve. When the plunger moves further, the wave moves faster than the plunger and hits the end of the shot sleeve. After that, the end part of the shot sleeve is full up firstly with the molten metal remaining the empty part in front of the plunger. The void region in the shot sleeve is undesirable in the casting operation. And, as you can see in the Fig. 5, the numerical simulation results and the experimental results are showing a good agreement. Fig. 6 shows the flow patterns in the shot sleeve for the plunger speed of 70cm/sec. As the plunger starts to move, the molten metal in front of the plunger is suppressed high and the top surface of the molten metal touches the upper wall of the shot sleeve. After that, the molten metal pushes the air in front of the melt wave and exhausts the air from the shot sleeve. In the case of fill rate of 30%, it has been reported from several previous studies[5][6] that the air in the shot sleeve is exhausted desirable by the wave propagation of molten metal in critical plunger speed 65cm/sec. The critical plunger speed is very close from this case plunger speed condition. And so, in this case, the vortex shedding phenomenon from which the molten metal flow is separated from the top wall of the shot sleeve is seen. It is not too serious yet, however, compared with the vortex shedding phenomenon in more higher plunger speed[5]. From the comparison with the two cases, the results from the numerical simulations and experiments are corresponding very well. Conclusions In this study, numerical simulation system for the flow pattern analysis in the shot sleeve is developed and the accuracy of the numerical simulation is compared with the experimental observations from a water model. In the numerical simulation system, the moving wall is treated as the second fluid in the multi-phase flow calculations. From this study, we have the following conclusions; 1) Air void region in the shot sleeve is appeared when the plunger speed is 30 cm/sec with fill rate of 30%. This plunger speed is less than the critical plunger speed, 65 cm/sec. 2) In the case of plunger speed 70 cm/sec, the vortex shedding phenomenon, which means that the molten metal flow is separated from the top wall of the shot sleeve, is seen. 3) The developed numerical simulation system for the flow pattern analysis in the shot sleeve is very useful to determine the proper condition of the plunger speed in some range of the molten metal fill rate. References 1. K. Fukizawa and H. Shiina, J. Soc. Auto. Eng. Jpn. 46(5), 1992, pp A. Kaye and A. Street, Die Casting Metallurgy, Butterworths, London, 1982, pp /4

5 3. B. D. Nicholas and C. W. Hirts et. al, Tech. Report LA-8355, Los Alamos Scientific Lab., R. A. Stoehr and C. Wang, MCWASP, 1991, pp J. H. Kuo, S. M. Pan and W. S. Hwang, MCWASP, San Dego, California, 1998, pp J. R. Brevick, M. Duran and Y. Karni, Transaction of 16 th Int. Die Casting Cong. and Expo., 1991, pp Table Table 1. The setting of the plunger speed for the water model. Fill Rate of Shot Sleeve Plunger speed (cm/sec) 1 30 % % % 70 Figures Figure 1. The flow chart of calculating code. Figure 2. Water model apparatus. 164/5

6 (a) Shot sleeve model (b) 30% fill rate state Figure 3. Shot sleeve model and 30% fill rate state of molten metal. Figure 4. Displacement verification of numerical simulation result: plunger speed is 50 cm/sec. (a) experimental results (b) numerical results 164/6

7 Figure 5. Time history of flow pattern: plunger speed is 30 cm/sec and the fill rate is 30%. (a) experimental results (b) numerical results Figure 6. Time history of flow pattern: plunger speed is 70 cm/sec and the fill rate is 30%. 164/7