Filling analysis for thixomolded process of AZ91D magnesium alloy

Available online at www.sciencedirect.com●CIENCE @DIRECT'Transactions ofNonferrous Metals骂PSociety of ChinaScienceTrans. Nonferrous Met. Soc. China 16(2006)s117-s121Presswww.csu.edu.cn/ysxb/Filling analysis for thixomolded process of AZ91D magnesium alloyZHANG You-fa(张友法), LIU Yong-bing(刘勇兵), CUI Xiao-peng(崔晓鹏), CAO Zhan-yi(曹占义)Key Laboratory of Automobile Materials, Ministry of Education, Department of Materials Science and Engineering,Jilin University, Changchun 130025, ChinaReceived 20 April 2006; accepted 30 June 2006Abstract: Mold flling in lately-developed Thixomolding process is a complex process, of which numerical simulation is necessaryfor development. Governing equations and numerical models are first given, and then the experiment and its filling simulation arecarried out. The results demonstrate that the modeling temperature changes from 864 to 873 K when the barrel temperature is 873 K.The difference is primarily in the runner system, but nearly invariable in the part. Consequently, the slurry flls the cavity smoothlywith low solid fraction, and the flling process finishes in 5 ms, the flling process completes sucessfully before soldifiation of theslurry, which has good agreement with the experiment. Through a snapshot of the flling process and defects tracking, drawbacks aredisplayed and confirmed in the experiment.Key words: analysis; semisolid; thixomolded; magnesiumprocess of mobile phone housing by Thixomolding" is analyzed1 Introductionby commercial software FLOW 3D"， some comparisonsbetween the analysis and practical production is displayed.Recently, Thixomolding2" becomes one of the hottest topics asa kind of lately-developed magnesium alloy process, which2 Governing equations and analytic modelcombines the principle of typical plastics injection molding andtraditional die casting. It has been used for making near. net-shapeTo date, the fixed rheological models based on the non-components with the advantages of lower porosity, good quality,Newtonian fluid model are mosty used to model the moldinghigh size accuracy and protecting environment, etc[1 3].process for semi-solid alloys by being implicated into commerialHowever, the die design and trial periods are too long, the qualitysoftwares[2,3.8]. In this paper, FDM is employed to model;and accuracy have not been well ensured. Futhermore, theFAVOR" method is used to generate mesh; and VOF method[9]experimental work has shown phenomenologically a strongis used to track the firee surface. In order to simulate the flowcoupling between the slury structure and the flow behavior underprocess of incompressible semi- solid slurry in the die cavity, thevarious flow conditions, which has been reviewed by Fan[2] andfollowing goveming equations are necessary:Suery et al[4] etc. Along with the fast development of computer1) Continuity equation:technique, simulation method and the gradual understandabilityof the forming characteristics, such as theology and thixotropy,du dv, dw=0(1dx dthe application of computer simulation in analyzing the moldingprocess of semi-solid alloys is growing rapidly[2 5].where u, v, W are velocity components in x y, Z directions,Nevertheless, the numerical simulation ofThixomolding processrespectively.is still a new field for development[6,7]. In this study, the flligFoundation item: Project (2002AA001022) surprted by the National High-Tech Rssarch and Development Program of China and Project 985 - Auto-motive Engineering of Jilin UniversityCorresponding author: CAO Zhanyi, Prfssor, Tel: +86 431-5095874; E mail: caozy@jlu.cdu.cn中国煤化工MHCNMH G.s118ZHANG You-fa, et al/Trans. Nonferrous Met. Soc. China 16(2006)2) Momentum equation:dimensional solid model of the mobile phone housingand its runner system are constructed, presented in Fig.1.du.. du du... du__ 1 P示“示“azρ dxG, +μV2u(2)Secondly, the modeling parameters and boundaryconditions are added into the model. The physicalproperties of slurry and die material, and the molding+G, +μV2v(3)parameters are listed in Tables 1 and 2. Fig.2 shows theat ax dyP dygeneration mesh, which is divided into 4 blocks using themulti-block technique. The total number of elements is .dw.dw..dw..dw__18PG, +μV2w(4)1 218 120. The injection machine is JLM220MG producedby JSW. The die material is SKD61 steel.where n is fluid density, p is pressure; G is gravityacceleration, i is apparent viscosity， V2 is Laplacianoperator.3) Energy equation:dT... dT.. d7aT_ k[a2T. 2r. d2tni●dt0xdyaρCp| dx2 dy2 dz25)where T is temperature, Cp is specific heat, k is thermalconductivity.4) Volume of fluid equation:Fig.1 Solid model of part with runner system10.200dVpVF =(6d1"Axby)zwhere Vp is fraction of opening volume in elements.5) Transport equation for viscosity:「Semi-solid alloys show non-Newtonian andN4.050thixotropic behavior. Transport equation for the apparentviscosity reflects the rheology and thixotropy [2,3,7,8].Here a generic expression based on the “ Carreau”0.975model is used for the strain rate dependent viscosity:-2.100-4.20-1.281.64 4.56 7.48 10.40Ho- μowhere io, i。are the shear viscosities at zero and shearFig.2 Multi-blocks based on generated meshrate at infinity, respectively; i is a time constant andn(0 103slury, some was brought into barrel and can't released,which possibly caused air entrapment. Fig.7 displays8.73>X 10Pclearly a snapshot of the flling process during thesimulation. Free surface of the slurry front is predicted8.72X 10exactly in the simulation. A large air pocket is found toform in the indicated station of the figure, which would.....mostly increase air hole and decrease tensile strength.00 0871X10Fracture morphology in SEM of a tensile sample wasshown in Fig.8; it revealed the existence of porosity inthe modeling area, confirmed the above analysis. In Fig.9,Fig.4 Analytic temperature distribution of part with runnerthe predicted places of drawbacks are displayed in thesystemsection of warm color. A specimen of the predictedlocation was cut down from the part. Its’ microstructureTime frame: 7.6983X 104images of metallography is shown in Fig.10, where thelocations of porosity were marked. There was a few ofdrawbacks, just as predicted in the simulation.) The pressure field of mobile phone housing by)↓0.8.72>102simulation is displayed in Fig.11， which distributes. ....homogeneously except of the overflows. The goodcompactness ofltion.中国煤化工8.71x10The last fllznodeling. Theimpurity and gYHC N M H Ghe overflowswhen the mold was filled completely. These reflectedFig.5 Simulated temperature distribution of part.ZHANG You-fa, et al/Trans. Nonferrous Met. Soc. China 16(2006)Time frame: 7.6983X10-1Time frame: 1.3903X 10-1.78X 1ou8.73X 1027.13X10*.... ,8.72X 10P-35510 IAir pocketLast flld areas8.71X10 .Fig.11 Analytic pressure field of partFig.7 Flling feature during simulationthat the places of overflows were reasonable. However,an amout of alloy flowed into the overflows. The size ofthe overflows was thought too large.5 Conclusions .An experiment and a numerical simulation of theAir poreflling process were conducted; the obtained conclusionsare as follows.1) The temperature in mobile phone housing isnearly invariable, and the solid volume fraction is about3.4%, the slurry fill the cavity smoothly.Fig.8 SEM image of tensile sample2) The injection pressure and velocity are dividednto four stages during the screw pushing ahead. TheTime frame: 7.698 3X 104Predictedtotal flling process takes about 5 ms when the injectionplaces of drawbacks3.02X102velocity is 2 m/s. The flling process completes010successfully before the solidification of semi-solid slurry.3) The free surface of the slurry is tracked●●●●●preferably. Based on the simulation results， somedrawbacks are showed clearly and explained combiningwith the experiment. They have good agreement.4) Both the homogeneous distribution of pressureand the last filleld area prove the reasonable places ofFig.9 Predicted locations of drawbacksoverflows. But, the size of the overflows is too large.Simulation is thought to be a valuable assistant forThixomolding production.References[1] CZERWINSKI F. Injection molding magnesium aloys[]I.J AdvMater Process, 2002, 16011): 31-33.2] FAN z. Semisolid metal processing[J]. Int Mater Rev, 2002, 47:Porosity[3] ATKINSON H V. Modeling the semisolid processing of metlicalloys[J]. Prog in Mater Sci, 2005, 50: 341-412.4] SUERY M, MARTIN C L, SALVO L. Overview of the rtheologicalbehavior of globular and dendritic slurries[A1. 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