Numerical simulation of chilled cast iron camshaft in sand casting. process

Transcription

1 Applied Mechanics and Materials Online: ISSN: , Vols , pp doi:10.408/ 011 Trans Tech Publications, Switzerland Numerical simulation of chilled cast iron camshaft in sand casting process Binfeng He a, Zhuqing Zhao b Department of Mechanical & Electronic Engineering, Xi an University of Arts and Science, Xi an, China a greatman_he@yahoo.com.cn, b zhaozhuqing15@16.com Keywords: chilled cast iron; filling and solidification; numerical simulation Abstract. There are many kinds of casting defects such as insufficient pouring, cooling separation, crack, and shrinkage and soon on were formed in the mold filling and the solidification process, which affect the final casting performance. Based on the mathematical models of mold filling and solidification process, the numerical simulation of chilled cast iron camshaft in sand casting process has been done. The filling behaviors at each stage in the filling process were presented. The temperature distributions in the solidification process were obtained, and the positions of shrinkages were predicted. According to the simulation results, an improved technology is proposed, and the shrinkages were eliminated efficiently. The simulation results are in good agreement with practical. Introduction Camshaft is a key part in engine. The development of automobile industry and engine power brings up more advanced requirement for the properties of camshaft. Due to high contact stress and rapid sliding speed on the surfaces of camshaft and tappet, many defects were generated such as insufficient pouring, cooling separation, crack, shrinkage and soon on [1-3] in the mold filling and solidification process. There are many factors to affect the casting quality, but the most important affection is come from casting technology. Traditional casting process is based on trial-and-error method which could not satisfy the modern manufactures [4]. In this paper, View Cast software was introduced to optimize the casting technology of the camshaft. The simulation results could be used to judge whether the technology parameters are reasonable, which improves the technological scheme. It makes a significant meaning to short the testing period and decrease the product cost. Numerical simulations of mold filling and solidification process, which play the key function in the casting production, are the world known leading area, widening and promoting the development of casting subject by using the advanced computer technology. And it also initiates a new way to improve the casting quality [5]. Mathematical models The mold filling process that closely related to molten metal flow, heat transfer and mass transfer process is a flow process at changeable temperature accompanied with heat loss and solidification. It can be described by the continuity and momentum equations, and the heat exchange between the molten metal flow and casting chamber can be presented by heat-balance equation. So the mathematical equations are expressed as follows [6-7]: Continuity equation: u v w D= + + = 0. x y z (1) All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-1/05/16,16:07:1)

2 118 Frontiers of Manufacturing and Design Science N-S equation: u u u u P ρ( ) = + ρ x+ µ t x y z x u v w g u. () v v v v P ρ( ) = + ρ y+ µ t x y z y u v w g v. (3) w w w w P ρ( ) = + ρ z+ µ t x y z z Energy equation: u v w g w. (4) T T T T T T T ρc( + u + v + w ) = ( κ )+ ( κ )+ ( κ )+S. (5) t x y z x x y y z z Volume function equation: f f f f + u + v + w = 0. (6) t x y z Where D is the divergence; u, v, w are the speed vector components on x, y, z; P is the pressure of the unit density; m is the dynamic viscosity; g x,g y,g z are the gravitational acceleration vector on x, y, z; is the Laplancian; ρ is the density; c is the specific heat; к is the coefficient of heat conductivity; S is the endogenous pyrogen; f is the rate of the liquid volume. In equation(5), the,3,4 item on left are the temperature transformations for liquid flow, which state that heat-transfer process is composed of two parts, capacity of heat transmission and macro-displacement of liquid. Solidification is the complicated physical chemistry process, coupling quantity of heat transmission, momentum transmission, mass transmission and phase change. The core of predicting and controlling the quality of casting is that reasonably predicting the position and degree of the potential shrinkage of castings. The G gradient. T T T G= [( ) + ( ) + ( ) ] x y z Tu TL R= [ ]. t t u L 1/ R method was adopted, which is the improved temperature. (7) (8) Where G is the temperature gradient; T, the temperature; x, y, z, the coordinate; R, the cooling velocity; T u, the liquid line temperature; T L, the solidus line temperature; tu, time of reaching the liquid line temperature; t L, time of reaching the solidus line temperature. Simulation results and discussion of original Filling process. Filling is an important stage in casting process, which has the critical effect on the casting quality. Many defects are related to this stage, such as insufficient pouring, cooling separation, crack, shrinkage and soon on. The original casting technology and the dimensions of the filling system are shown in Fig.1. The diameter of pouring cup is 30mm, and the diameter of ingate

3 Applied Mechanics and Materials Vols is 0mm which formed a closed type gating system. The molten metal is flow fast in this kind of gating system, so the molten metals easy flashed in mold and have a strong scouring force to the mold. (a) Original scheme (b) Mesh of FVM Fig.1 The original casting technology The results of simulation process are shown in Fig., and Fig. (a) is temperature field, Fig. (b) is velocity field, the color bar of temperature means the pouring temperature is 900~1400, and the color bar of velocity means the filling velocity is 0.~. m s 1. It can be seen from the results that the molten metal almost spouted into the mold at the beginning of the filling, and the pouring velocity is about.0 m s 1. When pouring at 1.814s, the molten metal flowed into the upper mold, the temperature of mold unchanged but the subjacent velocity still very fast which because mainly molten metal came to the subjacent mold. When t=1.943s, it can be seen from Fig. that subjacent mold have full filled and the upper mold filling about 50%, which because the velocity of the molten metal flowed in two mold is equal. Finally the whole mold almost full filled at t=.086s. It can be seen from the simulation results that the rate of flow is very fast from beginning to end. The filling process is instable, wrap gas happened in the mold. t = s t = 1.814s t = 1.943s t =.086s (a) Temperature field t = s t = 1.814s t = 1.943s t =.086s (b) Velocity field Fig. Simulation results of mould filling Solidification process. Numerical simulation of the solidification belongs to the nonlinear transient thermal analysis. During the solidification process, the state of the casting varied from liquid to liquid-solid and then solid, along with the lowering of temperature. Phase transformation and latent heat releasing were happened in this process which can impact the quality of the casting. And the temperature field is variational. So the potential shrinkage can be predicted. The solidification time of each parts of camshaft are shown in Fig.3. According to the technical requirements, the hardness of the cams should be 48HRC, some chills were used which solidified the cams very quickly. So that some isolated regions formed and these

4 10 Frontiers of Manufacturing and Design Science regions would be the shrinkages. The failure prediction was shown in Fig.(c). Some shrinkage was occurred in the bearing locations. Fig.4 shows the shrinkage in the bearing location. Shrinkage (a) t=7.87s (b) t=108.41s (c) Failure prediction Fig.3 The simulation result of solidification of original scheme Fig.4 shrinkage There are two reasons caused the shrinkage. For one is the low strength of the mold. The mold was broken during the graphite expansion, which led to the graphite expansion could not feed the shrinkage of the solidification. For another is the fast flow rate which may impact the mold and the wrap gas, inclusion may occurred, and finally, shrinkage formed. But in fact, the mold generated at high-temperature and pressure, and the bolt was used during the filling process, so the strength of the mold would be enough. In order to resolve the problems a new casting technology was used. Optimization of casting technique Filling process. According to the simulation results, the original technology could not filling the mold one by one but nearly coinstantaneous with the unreasonable gating system, the rate of flow is too fast. In order to filling the mold one by one and also decrease the velocity of the molten metal, the optimization of casting technique was put forward, which changed the gating system, a second downsprue was made for decrease the flow rate. It is shown in Fig.5. (a)modified scheme (b) Mesh of FVM Fig.5 The new casting technology The results of simulation process are shown in Fig.6, and Fig.6 (a) is temperature field, Fig.6 (b) is velocity field, the color bar of temperature means the pouring temperature is 1155~1400, and the color bar of velocity means the filling velocity is 0.~. m s 1. It can be seen from the results that the molten metal filling the mold very slowly. When pouring at 1.599s, the molten metal moved along with the mold and escalated. The velocity is about 1. m s 1. When pouring at.363s, the subjacent mold has full filled and the molten metal rose along with the second downsprue, and the velocity of the molten metal is about 0.7 m s 1. When pouring at 3.855s, the molten metal flowed into the upper mold and it s stable, the velocity is about 1.0 m s 1. The mold full filled at 4.841s. it can be seen from Fig.6, the second downsprue not only filling the mold one by one, but also decrease the velocity of the molten metal in the mold. Solidification process. It can be seen from Fig.7(a) and Fig.7(b), the whole time of solidification is about 74s, also because of the effect of chills some isolated regions formed. But it can be seen from failure prediction; there were no any shrinkage in the casting. After analyses, the main reason is the different flow rate. For the original technology, the highly flow rate led to the molten metal went round and round in the mold which formed shrinkages easily. For the new technology, the

5 Applied Mechanics and Materials Vols t =1.59s t =.363s t =3.855s t =4.841s (a) Temperature field t =1.59s t =.363s t =3.855s t =4.841s (b) Velocity field Fig.6 Mould filling of new technology effect of the second downsprue was very clearly, which decreased the flow rate effectively. So there was no shrinkages occurred. Fig.7(c) is the failure prediction of the new technology. Conclusion (a) t= 41.44s (b) t=83.0s (c) Failure prediction Fig.7 The simulation result of solidification of new scheme In the paper, the View Cast software is employed to simulate the casting technique of the gray iron-cast camshaft. After analyzing the temperature field and flow field during casting process, the potential defects were predicted, and the main reason of the defects were because of the highly flow rate of the molten metal in the mold. The second downsprue was used in the new technology which decreased the flow rate effectively. And the new technology was benefit to fill the mold one by one which is also benefit to the product quality and reduce the cost. Reference [1] Li Ping, Wei Bo-kang et al: Modern Cast Iron Vol.1(00), p. 33~36 [] Cai Qi-zhou, Wei Bo-kang et al: Modern Cast Iron Vol. 5(005), p. 9~1 [3] Jin Yong-xi: Modern Cast Iron Vol.5(005), p.1~8 [4] Li Dian-zhong, Kang Xiu-hong ea tl: Foundry Vol. 54(005), p. 148~15 [5] Hong Yan, Wenwei Zhuang ea tl: Journal of Materials Process Technology Vol (007), p. 349~353 [6] Liu Zhiyuan: China Foundry Machinery & Technology Vol. 6(003), p. 6~9 [7] Fan Hongxun, Lai Huaqing: Research studies on foundry equipment, Vol.4(003), p. 3

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