Shell Die-Casting and Its process Based on

Shell Die-Casting and Its process Based onNumerical SimulationYao Huang 1, a , Yue Dong 2, b , Yong Huang 2,c1School of Machanical and Electronic Enginerring, Beijing University of Chemical Technology,Beijing 100029, China2 School of Machanical Science and Engineering, Shenyang ligong University,Shenyang 110159, Chinaa hy06@,b 410454744@,c hy195953@Keywords: Shell, Numerical Simulation, Die Casting ProcessAbstract. In this paper, the distribution and variation trend of temperature and velocity fields were obtained by numerical simulation of the casting process of shell die casting. According to the results of the numerical simulation, the die casting technology was optimized. The sound castings were produced. It was verified that shrinkage cavity and porosity could be reduced by increasing injected pressure and decreasing pouring and mould temperatures. The reliability of numerical simulation of die casting process was proved. In addition, the qualified products were obtained on the basis of the optimized die-casting parameters from the simulation results.IntroductionWith the rapid development of industrial manufacte and equipment in China, both the quality and quantity of die casting machine are also improved obviously. The type of components produced is enlarged contiously, and the precision of die castings and the complexity of parts get the larger promotion[1].Numerical simulation of the mold filling and solidification in casting is a method used for optimizing the design and process of casting. And it can also objectively reflect the change of temperature field at different stages. It is benefit to control casting process effectively. Therefore, the sound castings were produced finally. The shell is a major part on gas meter. It needs good seal of vacuum without shrinkage cavity and porosity. The process of casting is optimized to solve the practical problems according to the die casting defects [2].Mathematical modelThe flow of liquid metal follows the hydromechanical rule in the mold filling. Mold filling is the process in which high temperature liquid metal flows in the compliex cavity and transfers heat to the environment. Tensor form of control equations used in the mold filling is as follows:ΦΦ+∂Φ∂∂=∂Φ∂+∂Φ∂S x D x u T jj j )()()(ρρ (1) Where ρ is liquid metal density ; for Φ=1, u j , T, F/ρ in Eq.1 represents continuity, momentum, energy and volume function equations, respectivel y; u j is velocity component , x j is coordinate component . F represents volume function and its physical meaning is the ration of the unit cell to the volume of all the cell , 0≤F≤1. D Φ is generalized self-diffusion coefficient and S Φ is source term[3]. Simulation detailsThe shell is a compliex thin-walled part. and Aluminum alloy ZL104 is selected in this study. Its three-dimension size of the shell is 138mm×140.5mm×76mm. The average wall thickness is about3mm. The side runner is selected as gating system in simulation.The mold filling is very rapid, so pouring temperature is assumed at 660°C. The preheated temperature of die is 215°C. The injecting pressure is 35MPa. And injecting velocity is 3m/s. Because the temperature on outside of die is not too high, the convection transfer heat is only considered. Environmental temperature is 25°C. The heat transfer coefficient is25W/(m2∙k), 1000W/(m2∙k) and 1500W/(m2∙k) for the mold and environmental, casting and mold and mold and mold, respectively[4,5].Results and DiscussionThe melting point of ZL104 is 598°C. The freezing point is 546°C. And critical solid fraction is 70%. The process of the mold filling and solidification in simulation is shown in fig.1. As seen from Fig.1, the velocity of ingate is 50.765 m/s (Fig.1 (a)); From Fig.1(b), the solid fraction in casting at 20s is shown ; the filling time of casting gradually increases (Fig.1(c)); the complete solidification time is 34.07s (Fig.1(d)); and the colored part is shrinkage cavity and porosity of casting (Fig.1(e)). From it, the porosity of the casting is 0.0214. Generally, the porosity of the casting for the sound casting is less than 0.01. This indicates that the casting is not sound under this technical parameters[6].(a) ingate velocity (b) solid fraction at 20s (c) filling time of casting(d) solidification time (e) shrinkage cavity and porosity in castingFig. 1 The simulation results of mold filling and solidification As shown in Fig.1, the shell is filled well, however, there is much shrinkage cavity and porosity in the casting. The quantity of shrinkage cavity and porosity can be decreased by decreasing pouring temperature and mold temperature, increasing injected pressure and improving the structure of casting. In solidification process of casting, evolved gas and poor compensation in the casting result in defects such as shrinkage cavity and porosity. The solidification time is too long for above process parameters. We conclude that pouring temperature of liquid alloy and mold temperature are higher, and the injected pressure is lower. Therefore, three optimization solutions are put forward to decrease defects:Solution 1: pouring temperature of 660°C, injecting velocity of 4 m/s, mold temperature of 220°C, and injecting pressure of 35MPa.Solution 2: pouring temperature of 630°C, injecting velocity of 3.5 m/s, mold temperature of 220°C, and injecting pressure of 70 MPa.Solution 3: pouring temperature of 610°C, injecting velocity of 3 m/s, mold temperature of 180°C, and injecting pressure of 45 MPa.Solution 1 Solution 2 Solution 3Fig. 2 Shrinkage cavity and porosity in optimization solutions Fig. 2 shows the size and the position of shrinkage cavity and porosity after solidification of the casting. The sashes on the right in figures represent porosity in the casting. 0.1 denotes of this part volume in which shrinkage cavity is 10%. For the porosity<0.01, the casting is sound in the simulation. For solution 1, solution 2 and solution 3, shrinkage cavity and porosity is 0.177, 0.0053 and 0.0234 respectively. It is indicated that the solution 1 and solution 3 did not agree with the requirements of shrinkage cavity and porosity and solution 2 agree with the process requirements. Experimental VerificationAccording to the simulation results, the optimal process parameters are following: pouring temperature is 630°C, injecting velocity is 3.5 m/s, mold temperature is 190°C, injecting pressure is 70 MPa, ingate thickness is 2 mm. Under the condition of the process parameters, the casting is regularly produced, as shown in Fig.3. The shell surface is smooth and there is on defect in the casting. The shell is cut at three positions as shown in Fig.3. casting defect is given in Fig. 4. Comparing with the results in Fig.1, the shrinkage cavity and porosity of the casting decrease markedly. It attains the requirements of porosity and the press test meets industrial standard. On the basis of the optimized process parameters, shells have been produced in large scope[7].Fig. 3 Qualified products(a) positon 1 in Fig.3 (b) positon 2 in Fig.3 (c) positon 3 in Fig.3Fig. 4 Metallographic diagrams of three positionsThe microstructure and mechanical properties of the shell were observed. The three metallographic diagrams were taken from position 1, 2, 3 on the shell in Fig. 3, In the Fig. 4, the white is solid solution, the gray is needle shaped eutectic silicon. The distance between the liquid metal and gating system is small (Fig. 4(a)). The cooling rate is the slowest and the grain size is relatively large. Fig. 4(b) is in the middle of the casting. The grain size is moderate. Fig. 4(c) is at the end of the flowing alloy. It is obviously cooled by the die. The grain size is relatively small and the distribution of needle shaped eutectic silicon is relatively dense. The strength and hardness of casting are improved greatly.Summary(1)The die casting process was optimized by numerical simulation as follows: pouringtemperature of 630°C, injecting velocity of 3.5 m/s, injected pressure of 70MPa, and mouldtemperature of 190°C.(2)The castings were produced according to the optimized results obtained from decreasingpouring and mould temperature, increasing injecting pressure. The porosity of the casting is0.0053. The shell is sound.(3)The quantity of shrinkage cavity and porosity are evidently decreased by increasing pouringtemperature. This indicates that the process parameter is resonable in this simulation . It canguide production.References[1] K. Kubo, R. D. Pehlke: Metallurgical and Materials Transactions B Vol. 16 (1985), P. 359.[2] Laurent, Vrigautc: AFS Transaction Vol. 100 (1992), P.647.[3] F Q Wang: Mould Filling and Temperature Fields, Technical University of Denmark, 1988, P. 7.[4] B. Zhang, D.M. Maijer and S.L Cockcroft: Materials Science and Engineering A Vol. 464 (2007),P. 295.[5] J. P. Hattel, S. Anderson and P. Henson: Transaction of 17th International Die Casting Congressand Exposition, 1993.[6] Y Huang, D.W. Pan and X.M. Du: Special Casting & Nonferrous Alloys Vol. 30 (2010), p. 59.[7] H. J. Lin, W. S. Huang: AFS Trand, Vol. (1988), P. 447.。