Design of process parameters for direct squeeze casting

Journal of Unlversity of Sclence and Technology BeillngMaterialsVolume 15, Number 3, June 2008, Page 339EL SEVIERDesign of process parameters for direct squeeze castingMilan Zhang ), Shuming Xing), Liming Xiao”, Peiwei Bao", Wen Liu ), and Qiao Xin 2)2) Department of Mahecmatics, Ii Normal University. Yining 835000, China,(Received 2005-24)2Abstract: On the basis of the analysis of solidification interval and temperature distribution of components manufactured by thesqueeze casting method, formulas for calculating the solidification interval and compaction pressure were deduced according to theprincipal request that the compaction pressure should be equal to or greater than the plastic deformation resistance of the formingcomponent when solidification ended. The solidification interval was proven to be associated with many factors, such as weight ofthe component, specific heat of the alloy, latent heat, pouring temperature, component temperature at the end of solidifcation andbeat-transfer cofficients. The compaction pressure was related to the strain rate, deformation temperature, and dimension of the de-forming component. The solidification interval and compaction pressure calculated by the formulas deduced in this article wereadopted in the production of 45 steel bidirectional chapiter valves, and components with excelleat performance were manufactured.◎2008 University of Science and Technology Bejing. All rights reserved.Key words: squeeze casting; solidification time; temperature distribution; compaction pressure1. Introduction2. Compaction pressure and solidifcationintervalSqueeze casting, which is described as solidifyingCompaction pressure means the smallest efctiveunder high mechanical pressure maintained till solidi-fication ends, offers high productivity and excellentpressure of special forging hydropress acting on thenear-net- shape formability. Moreover, the potentialalloy fusant in squeeze casting. The purposes ofadvantages of squeeze casting process include elimi-adopting compaction pressure are to ensure well fill-nation of shrinkage and the formation of a fine-ing, welll compelled feeding, and plastic deformationgrained structure with small dendrite arm spacing andof the solidification system, so as to prevent the for-small constituent particles [1-2]. In squeeze castingmation of shrinkage voids and gas porosities in com-process, compaction pressure and solidification inter-ponents. The compaction pressure assures more com-val are the significant parameters, which are crucialpact structure, improved mechanical properties andfor the high quality of components manufactured byimproved suface and intemal qualities for the com-this forming method. Till now, some relevant re-ponents manufactured though squeeze casting.searches about compaction pressure and solidificationTo ascertain the compaction pressure, Luo [3]sup-interval have been performed, but only few studiesposed the solidification system as a discrete non-uni-have been performed on the compaction pressure con-form system, which was composed of three regions.sidering the temperature variation of a component atThe three regions were outside solidifed closed shell,the end of its solidification and the deformation veloc-solidifying sold-liquid region, and liquid region. Theity of the solidifying system; and few studies considerdeformation region was supposed to be the circum-the solidification interval under squeeze casting con-ferential region with external radius Ro and inner ra-dition. So solidification interval and compaction pres-diusand the heioht of the solidification systemsure were analyzed and discussed in the followingwas a中国煤化工1paction stress waspassage.detenCNMH G:Correspoodlng author: Milan Zhang, E-m l: bjmian@ 126.comAlso svlable ollie at www.sdecedlrect.com。2008 University of Science and Technology Bejing. All nights reserved.340J. Unir. Sci. Technol Beijing, VoL15, No.3, Jun 2008( 2R -3R37o+r )enough, solidification shrinkage of residual fusant canσ=σ,(1)not be compensated, which leads to shrinkage voids,[+(R3-π月but if the time is too long, chances of generatingwhere o, is the cnitical deformation stress of the out-cracks will be enlarged. Accordingly, the basic princi-side-closed shell, μ the friction coefficient betweenple to ascertain the pressure holding time is to ensurethe closed shell and die. Some dissatisfactions havethe casting component being solidified completely.been found when the formula is used. First, Ro andro are variable, which lead to much difficulty to de-Ytermine their values; second, the solidifying system inthe hypothesis is viewed as a combined model of threeregions, actually the solidifying system is sequential;third, the effects of squeeze temperature and velocityof plastic deformation on the critical deformationstress σ。 are not considered; finally, the compac-ion stress σ obtained through Eq. (1) is the pres-sure during the process of squeeze casting, which isnot the compaction stress for determining the smallest日effective pressure of a special forging hydropress.文To avoid these dissatisfactions associated withD_Eq. (1), an empirical equation for direct squeeze cast-Fig.1. Simplifned model of squeeze casting, dr- diametering [4] is expressed as p= K/K2[1+0.001()，of the die cavity; dr diameter of the punch; h- -height ofwhere K is the cofficient of different alloys, Kzthe component; hr height of plastic deformation;D- outer dimension of the die.the coefficient of different forging methods, H theheight causing great loss of effective pressure, a theThe square-root law is used to evaluate the solidi-average thickness of the solidification system in thefication interval chronically. However, it is not fit forheight H. However, the empirical equation is not sat-the squeeze casting because of the high-mechanicalisfied to determine compaction pressure accurately.pressure used in the process. In the following para-At the end of solidification, the temperature of thegraph, a formula for calculating the solidification in-terval was deduced based on the solidification theorysolidification system varies from its center to edge,and heat transfer theory. Suppose W is the weight ofand there is much lttle separated local fusant in thethe component being formed, c is the specific heat ofmiddle shown in Fig. 1. So the compaction pressurethe alloy, L is the latent heat, Tc is the pouring tem-should be great enough to make the solidification sys-perature, and T。is the component temperature at endtem as Bingham body to get plastic deformation, othof the solidification. According to the heat transfererwise compelled feeding can not be realized andtheory, in the interval from the time when pouringcasting defects can not be conquered, ie. the compac-ends to the time when solidification is completed, theion pressure should be equal to or greater than theplastic deformation resistance of the solidificationtotal heat quantity Q1 required to be transferred out-side through die/fusant interface is:system at the end of the solidification.2.1. Solidification intervalQn=W[c(T-T,)+4]Solidification interval is the interval from the timeAs the thermal transfer conditions of die/fusant in-when pouring ends to the time when the alloy fusantterface are different before and after the application ofsolidifes completely, it includes the interval from thepressure, the heat flux that go through the interfacetime when pouring ends to the time when mechanicalbecome different. Supposing the heat fux densities ofpressure is applied, which is named as delay time, anddie/fusant interface that before and after the applica-the interval from the time when punch contacts withtion of pressure are q1 and q2, respetively, the intervalthe fusant to the time when it detaches away from thefrom中国煤化工。to the time whencomponent manufactured, which is named as pressurethe ape interface area ofholding time. The main aim of the pressure holdingdie/fuMHCNMHGrtofeidssipatedtime is to realize the volume shrinkage being com-during interval ts, the other part of Q1 is dissipatedpensated. If the pressure holding time is not longentirely during pressure holding time tp, which isM.L Zhang et ah, Design of process parameters for direct squeeze castingexpressedas e =qnSts +q2Sip, and lp=9-giSts.Associating with Eq. (4), the temperature distribu-qzStion of the component at the end of squeeze casting is:Associating with Bq. (2), pressure holding time fp isexpressed asT =To+(T。-To)erf;2Jat, /" /erfn(7)，_w[c(T。 -T,)+L]-qn$ts(3)where as is the thermal diffusivity of the component,qz$exp(-η2)√πLηobtained bySo olidification interval t。in a squeeze castingerfnc,(.-T0)，andprocess is:erf ;is a Gaussian distribution function that cant,=fs+tp=Is+w[c(T-T,)+Z]-qnSts4)be obtained from the table or chart of Gaussian distri-2Va.tbution functions.where q1 =--Ao， q2=kzq1, h is the2.3. Design of compaction pressurehk h2Plastic deformation of metal at high temperature isheat-transfer coefficient from fusant to the coat usedgenerally considered as a thermal acting process, thein squeeze casting, h2 the heat-transfer coefficienteffect of temperature and deformation rate on thefrom the coat to the die, d。the thickness of the coat,critical deformation stress is expressed as [7-8]k\ the heat-transfer coefficient of the coat, kr aσ2 =二sinh~feexp[ Q2/(RT2)]'(8)correction coefficient of squeeze casting that is usu-Azally obtained through contact thermal resistance, Aohe cooling area of component manufactured by thewhere E is the strain rate, R the gas constant, T2squeeze casting method.the deformation temperature, Qr the deformation2.2. Temperature distributionactivation energy， A2， a and n are coefficientscorresponding to critical deformation stress that canSuppose the solidification system of the formingbe obtained by the nonlinear regression method withcomponent is a semi-infinite slab, and its heat conduc-experimental data. And for low microalloyed carbontion conductivity is one-dimensional and unsteady. Asteel, the values of A2, a, and n| are 5.98x10*,reference coordinate is assumed in the solidification6.057x10-, and 5.09, respectively.system as shown in Fig. 1, and the thermo-physicalAccording to Eqs. (6) and (7), when solidificationparameters of the system are supposed to be constantin squeeze-casting process. A differential equation ofends, the critical deformation stress of a squeeze cast-one-dimension and unsteady heat conduction exists ining component isthe solidifcation system [5- 6]:σ=一sinh"eexp[2(Rri)]1(9)J2T2x2Assuming the plastic deformation stress for the partand its general solution is:of the component between the punch and die is equalto that of the part below the punch, the pressure ofT=A+Berf，2√a,tsqueeze casting is:where t is a random interval from the time whenp=hσxdx{D- d)/2'pouring ends to the time when solidification is com-pleted.-sinh-[ex[2/(RT)]("xdx (10)When 1=0 and x=0, it is assumed that thetD山/2 aA2pouring temperature is T。and the temperature of thediefusant interface is To which remains constant inTor中国煤化工nmation method isthe solidifcation process. The temperature distributionadopt model shown inin the component along X axis is shown as:Fig. 2MHCNMHGthesummationofeach compression pressure Pm of each area Sm .T=To+(T。-To)ert 20 /efn6)Equidistance points are assumed on the section, andthe number of equidistance points is 2n2 chosen ac-342J. Unir. Sci TechnoL Beijing, VoL15, No.3, Jun 2008cording to the requirement of the squeeze castingperature is Tm, the critical deformation stress is omcomponent, generally the value of n2 is greater thanThe diameter at the n2 point is dm= dm/n2 and10. Supposing the n2 point is a random point amongthe areais Sm = ndm .d/n 2=umd}/n吃, and then Pthese 2n2 points, and at the point of n2, the tem-is approximately calculated as follows:p= p sinh[exp[ Q/()]("xdx==so.=imsinh-[xp[ 2(r.)]"(11){D-d)/2 aA2气anξ(nz - m)d;Tn=To+(T。-To).erf-/erfin(12)w[c(T。-T,)+L]-qnSts 14nyoilaqzStwo flling cylinders, and two lateral squeezing cylin-ders, which carries out two individual types of mo-tions: locking motions and flling motions.s,_p 942RS中4677 ZFig. 2. Simplited model of integral.Fg 3. Bidiretional chapiter valve.3. Experimental identification and discussionThere are many factors that affect the solidificationAs shown in Fig. 3, the 45 steel bidirectional chapi-interval in Eq. (4), such as component weight, pouringter valve, which is used for colliery supporting, is usedtemperature, heat-transfer cofficients from fusant toto validate the formulas deduced above. At present,coat and from coat to die, thickness， and thecasting and turming round-rolled bar methods areheat-transfer coefficients of die, coating and formingmainly used for the manufacture of this kind of bidi-material. For 45 steel bidirectional chapiter valvesrectional chapiter valve. For these casting defects,with a kind of special die coat, the solidification in-such as gas voids and shrinkage cavities, valvesterval of the chapiter valve under the experimentalmanufactured by casting process leak easily under acondition is calculated to be 6.5 s. But according toworking oil pressure of 39.2 MPa. The engineeringthe traditional method for the calculation of solidifica-requirements cannot be full by the types of valvestion interval named as the square-root law [9]:fabricated by the casting method. And the tuminground-olled bar method has the disadvantages that the(答)turming procedures are complex, the production effi-ciency and yield ratio are low, the cost is high, and thewhere M is the equivalent thickness of the formingrejected and worm parts cannot be reclaimed. Becausecomponent andthe solidification constant, the so-of all these reasons mentioned above, the squeezelidification interval tpis 11 s. The heat-transfer con-casting method is adopted because it reduces the com-dition of a die and fusant under the application ofplexity of processing procedures, improves materialpressure during squeeze casting process is differentutilization ratio, brings down manufacture cost, andfrom that without the application of pressure, which isespecially, excellent components can be manufactureddescribed as having no space between the die andby the forming method,fusant中国煤化工re-root law, exte-A special forging hydropress with the maximal ex-rior nYH_nsidered, thus thesquareC N M H Ge squeeze catingtrusion force of 1500 kN and the maximal die-lockingcondition.force of 2000 kN is used in the production process.The forging hydropress contains a locking cylinder,If squeeze casting process parameters for a compo-M.L Zhang et al, Design of process parameters for direct squeere casting343nent have been settled and the velocity of the hydro-pressure holding time. These effects are reflected inpress punch is chosen, the strain rate E is constant inEq, (4), which can be used as the designing formula ofone forming process. Then compaction pressure Psolidification interval in squeeze casting process.of a component under certain process parameters in(2) The temperature distribution of the componentthe forming process can be calculated without consid-formed by the squeeze casting method is not linear asering e. In the production of a chapiter valve, theconsidered usually when solidification ends. Theworking speed of the upper hydraulic ram is 20 mm/s.temperature distribution at the end of the solidificationUsing Eq. (11), it can be found that the major fac-process obeys Gaussian distribution, which is ex-tors that affect compaction pressure include compo-pressed as Eq. (7).nent's material, dimension, strain rate, and forming(3) To ensure that the components manufacturedtemperature ditribution etc. In the production of athrough the squeeze casting method have no defectschapiter valve, the value of n2 was chosen to be 20.and possess excellent properties, the compactionUsing Eq. (11), the value of P was calculated to bepressure should be equal to or greater than the critical160 MPa under the production condition. As the em-deformation resistance at the end of solidification.pirical equation for direct squeeze castingConsidering the strain rate, the deformation tempera-P= K1K2{1+0.001(the value of compactionture, and the dimension of the deforming component,()]the designing formula of compaction pressure is ex-pressure is about 120 MPa.pressed as Eq. (11).With the solidification interval of 6.5 s and theReferencescompression pressure of 160 MPa, components are[1] M.R. Ghomashchi and A. Vikhrov, Squceze casting: anmanufactured with inner compact microstructure andoverview, J. Mater. Process. Technol, 101(2000), No.1-3,uniform mechanical properties and without castingdefects. These components satisfy the requests of col-[2] S.M. Skolianos and G. Kiourtsidis, Effect of applied pres-liery supporting. But the components, manufacturedsure on the microstructure and mechanical properties ofwith the solidification interval of 11 s and the com-squeeze cast aluminum AA6061 alloy. Mater. Sci. Eng,pression pressure of 120 MPa, are not satisfactory forA231(199), p.17.engineering requests as there are some casting defects[3] s.Q. Luo and s.Y. He, Squeeze Casting for Sieel, HarebinInstitute of Technology Press, Harebin, 1998, p.130.and cracks. Therefore, it can be concluded that theformulas deduced in the article is suitable for ascer-[4] B.G. Chen, Die Design of Liquid Metal, Huazhong Uni-versity Press, Wuhan, 1989, p.21.taining the solidification interval and the compaction5] M. Rapqaz, M.R. Ozgu, and K.W. Mahin, Modeling ofpressure in squeeze casting process. But their applica-Casting, Welding and Advanced Soldification Processes,ions for designing the parameters of other materialsThe Minerals, Metals and Material Society, 1991, p.297.and components are to be proven in the future.[6] L. Jia and Z.H. Fang, Advanced Heat Transfer, HigherEducation Press, Beijing, 2003, p.91.4. Conclusions[7] B. Kowalski, C.M. Sellars, and M. Pietrzyk, Developmentof a computer code for the interpretation of results of plain(1) The basic principle to ascertain the pressurestrain compression tests, ISIJ Int, 40(2000), No.12,holding time in squeeze casting is to ensure the com-p.1230.plete solidification of components manufactured.[8] A. Laasraoui and J. Jonas, Prediction of steel flow stressThere are many factors such as weight of the compo-at high temperatures strain rates, Metall. Trans, 22A(1991), No.7, p.1545.nent, specific heat of the alloy, latent heat, pouring[9] H.Q. Hu, Principles of Metal Solidification, China Ma-temperature, component temperature when solidifica-chine Press, Beijing, 2000, p.6.tion ends, and heat-transfer coefficient that affect the中国煤化工MYHCNMHG