Effect of vacuum on solidification process and microstructure of LFC magnesium alloy

Available online at www.sciencedirect.comsCIENCE(s @oInEoT.Transactions of.Nonferrous MetalsSociety of ChinaScienceTrans. Noferrous Met. Soc. China 16(2006) s1685- -s1689Presswww.csu.edu.cn/ysxb/Effect of vacuum on solidification process and microstructure ofLFC magnesium alloyLIU Zi-li(刘子利)2，LIU Xi-qin(刘希琴)', XU Jiang(徐江)，GUO Hua-ming(郭华明)',PAN Qing-lin(潘青林)尸, ZHOU Hai-tao(周海涛)1. College of Materials Science and Engineering , Nanjing University of Aeronautics and Astronautics, Nanjing 210016; .2. School of Materials Science and Engineering, Central South University, Changsha 410083, ChinaReceived 28 July 2006; accepted 15 September 2006Abstract: Lost foam casting (LFC) is regarded as a cost-effective, environment-friendly vital option to the conventional castingprocess for production of near-net shape castings with high quality. Effect of vacuum on the solidification process and microstructureof LFC magnesium alloy were explored. The results indicate that vacuum plays a very important role in the heat transfer duringmould flling and solidification periods, it increases the cooling rate of the flling melt, but greatly decreases the cooling rate of thecasting during solidification period, and the solidification time of the casting is greater than that without vacuum. The microstructureof LFC magnesium alloy is rather coarse. Compared with that without vacuum, the microstructure of the LFC magnesium alloyunder vacuum is more refined and has less precipitated B-phase, which is formed at the grain boundry and around the Al-Mncompound particle.Key words: magnesium alloy; lost foam casting; vacuum; solidification process; microstructuresolidification characteristics in LFC process without1 Introductionvacum have been investigated by many researchers,especially for aluminum alloys and cast iron[12- -17], butMagnesium alloys exhibit great potential in masslitte is known about effect of vacuum on thereduction for aerospace and automotive industries.solidification and microstructure of LFC casting, thoughCurrently, casting is the main industrial forming methodit has been reported that vacuum can increase the fluidityfor magnesium alloys， the lag of research andand metal fill velocity for aluminum and ferrous alloys,development on casting technology has become and lessen the carbon defects in ferrous alloy[4- -5].bottleneck for their further application[1- -3]. Lost foamPrevious works on mold filling suggested that vacuumcasting (LFC) has been regarded as a cost- effctive,plays a vital role in the magnesium alloy LFC process[3,environment-friendly vital option to the conventional6, 7]. In the paper the effects of vacum on thecasting process for production of near -net shapesolidification process and microstructure of LFCcastings[3- -5]. As the great advantages over themagnesium alloy were investigated.traditional casting process and the bright applicationfuture, magnesium alloyLFC process has been paid2 Experimentalincreasing attentions in recent years[3, 6-11].The great difference between LFC and conventional2.1 Foam, molding, melting and pouringempty cavity casting is that there is foam pattern in the30/15 mm(height) X 30 mm(width) X 250 mmmold which will be gasified and removed out during the(length) patterns and a gate(30 mmX 30 mmX 200 mm)mold flling process. The endothermic degradation of thewere hot-wire cut from EPS plates of nominal densityfoam pattern produces a chilling effect in the liquid metal, 0.017 g/cm'， and assembled using commerciallyand this will influence the solidification of the fllingavailable glue. The ceramic coating was a lowmetal and microstructure of castings. Currently, thepermeability c中国煤化工nd ChemicalFoundation item: Project (2005037697) supported by China Postdoctoral Science Foundation; proyTYHCNMHG-ative Program ofNanjing University of Aeronautics and AstronauticsCorresponding author: LIU Zi-li; Tel: +86 25-52112626; E-mail: zililiu@ sohu.com.s1686LIU Zi-li, et a/Trans. Nonferrous Met. Soc. China 16(2006)Company. The refractory slurry was brushed onto theFigs.2 and 3 show the cooling curves at differentpatterns, and dried to a final thickness about 0.3 mm. Thelocations in the castings of the magnesium alloy LFCcoated pattern was placed in the 40 cmX 40 cmX 40 cmprocess without vacuum and under vacuum, respectively.flask with plenum chamber in its base, and fllel withFrom analysis of cooling curves, times and temperatureswashed silica sand of 40/70 screen mesh until it was .at the beginning and end of solidification for the LFCcompletely embedded in it. Then, a plastic sheet wasmagnesium alloy castings are listed in Table 1.covered onto the flask, and a pouring cup of baked claysand was placed on the position of the sprue. When novacuum was applied, the pouring cup was put directly on700a)the top of the sprue.LiquidusCommercial AZ91 ingot was melted in an electric600595 Cresistance furnace. During melting and pouring of themagnesium alloy, a protective atmosphere o500| Sprue1:234 560.1%SF6+CO2 was used to prevent its oxidation andburming. The metal was poured under vacum of 0 and0.02 MPa, and with the pouring temperature of 750 C.If vacuum was applied, it was maintained throughout thewhole pouring process and for another 5 min.2002.2 Data collctionSix thermocouples were positioned at 2, 6, 10, 14,18, 22 cm from the sprue along the length of the foam10 15202pattern, and their tips lay at sectional center of theTime/slocations, and a thermocouple was also inserted into theb)top of the sprue to monitor the heat transfer and700 ,solidification behavior (Fig.1). Data from ththermocouples were collected and disposed bcomputerized data acquisition system. The process of thedata collctions was divided into 2 periods. For the firstperiod, which was about 45 s from the beginning ofpouring, the thermocouples were scanned every 0.02 s;Sprueafterwards, the thermocouples were scanned every 0.08 s二23for the second period to record the heat transferEutectic"characteristic and solidification behaviors after the mold400信temperaturefilling finished.300道0100 200300 400 500 600Fig.2 Cooling curves at each location in casting duringmagnesium alloy LFC process without vacuum(30 mm highpattern and 750 C pouring temperature were used): (a) Moldflling period; (b) Whole casting period; 1一2 cm; 2- 6 cm; 3品品- -10cm;4一14 cm;5- -18 cm; 6- 22 cmPattern250Without vacuum, a positive temperature gradientFig.1 Schematic diagram of experimental setup: Six thermo-was established from gating to the end of castingcouples were located at itevals of 40 mm along the lengthof immediately after mold fling finished, and thethe foam pattern: 1- 2cm; 2一-6cm; 3- 10 cm;4一14 cm; 5-temperature in the casting was near the liquid:18cm;6- -22 cmtemperature (Fig.2). As the melt cooled down,solidification began at nearly the same temperature of3 Results594 C from th中国煤化工(Table 1).Under vacuYHCNMHGliquidmetal3.1 Solidification process of LFC magnesium alloydecreases sharply when It enters 1nto sprue, most of thecastingliquid metal is in the state of supercooling after mold.LIU Zi-li, et a/Trans. Nonferrous Met. Soc. China 16(2006)s168700-a)| (b)700Liquidus006004001:213 4|54. Sprue. 500300?200-Eutectic大--4400 t temperature1002100200 300400 500 600Time/sFig.3 Cooling curves at each position in casting during magnesium alloy LFC process under vacuum(30 mm high pattern, 750 Cpouring temperature and 0.02 MPa vacuum were used): (a) Mold flling period; (b) Whole casting period; 1- 2 cm; 2- -6cm;3- -10cm;4- -14 cm;5--18 cm; 6- -22 cmTable 1 Time and temperature at beginning and end of solidification, and cooling rates during solidification processes for LFCmagnesium alloy castings (30 mm thick patterns)Pouring conditions andBeginning of solidificationEnd of solidifcationCooling rate/Positions of thermal couplesTemperature/C(C min')Sprue4059557242818.92840426.624374750 C,3219434530.80 MPa1959432232.851529435.761327342738.55553052813.1305947718.42951743340816.30.02 MPa2754340741420.3223537241520.555634342224.7filling (Fig.3), and solidified in a greater supercooling3.2 Microstructures of LFC magnesium alloy castingsthan that without vacuum (Table 1). A positiveAs shown in Fig.4, the microstructure of LFC AZ91temperature gradient is also formed along the sprue to magnesium alloy is much coarse, disconnected nettedthe end of casting in the late of casting solidificationB(Mg17Al2) phase distributes along grain boundary oprocess (Fig.3), and solidification times in the casting ;a-Mg matrix. There are two types of β phase in thincrease inversely with the distance from the sprue.microstructure: the white one is the divorced eutectic βCompared with the cases without vacuum, solidificationphase, while the dark one in the lamellar structure is thetimes in the casting are much longer, and the eutecticprecipitated β phase. From the analysis of EDX (Fig. 5),temperature is much lower (Table 1), so, the cooling ratethe dark pellet scatters in a-Mg matrix is Al-Mnof the casting under vacuum is definitely smaller duringintermetallie compound containing small amount of Fe.solidification period. From the analysis above, it can beBesides grain中国煤化工β phase alsoindicated that applying vacum increases the cooling rateforms around thCNMHGof the mold flling melt, but greatly decreases the coolingThe effects..tures of LFCrate of casting during solidification period.AZ91 alloy is shown in Fig.7. Compared with that.s1688LIU Zi-li, et a/Trans. Nonferrous Met. Soc. China 16(2006)without vacuum, the microstructure of LFC magnesiumunder vacum is refined and has less precipitated β phase.|(aThis indicates that the greater cooling rate of the moldfilling melt under vacuum refines the grain and the lowereutectic temperature suppresses the formation of theprecipitated B-phase, though the cooling rate of castingduring solidification period is greatly reduced.PrecipitatedDivorced eutecticβ-Mg1zAl2B-MgrzAl2 8Al-Mn-Fe.compound((b)20 u.mFig.4 Microstructure of LFC AZ91magnesium alloyEDAX ZAF quantificationAIElement0Min39.83 51.438.80Total_ 100.00 100.00x is atom fraction100 umMg]Fig.7 Effects of vacum on microstructures of LFC AZ91alloy<(15 mm high patterns were used): (a) Without vacuum; (b)0.02 MPa0.702.103.504.906.304 DiscussionEnergykeVFig.5 EDAX analysis of spherical phase in LFC AZ91During the mold flling process of LFC， themagnesium alloyexistence of the foam makes the heat transfer behaviorsof the metal liquid more intricate than that ofconventional casting process. On one side, the foamendothermic decomposition is a complex Stefan heattransfer process, at which the heat transfer medium is theiquid and gaseous thermal decomposed productsbetween the moving mental-foam interface. The type andamount of foam thermal decomposed products determinethe size of mental-foam interface, thus the effect ofStefan heat transfer (even if the mental-foam interface isless than 0.05 cm, it can still prevent heat transfer fromthe mental liquid to the foam greatly[13])， and theremoving process of the foam decomposition productsalso carry some heat away from the liquid metal by heat10m-288kU 482E3 0019/81convection. On中国煤化Iloving liquidproducts betweCNMHGfacewill beFig.6 β phase precipitated at interface of Al-Mn compound andgasified during une muld ig piucess and LFCa-Mg compoundemployed the dry sand, the heat transfer of the liquid.LIU Zi-li, et al/Trans. Nonferrous Met. Soc. China 16(2006)s1689metal to the surrounding sand through coating layers will2) For the microstructure of LFC AZ91 magnesiumbe quite different from the traditional casting process.alloy, disconnected netted β (Mg17Al2) phase distributesThe rate of pattern degradation and the expelling rate ofalong grain boundary, and the precipitated β phase formsthe degradation products determine the contact time of at the grain boundary and around AI-Mn compoundliquid metal with the foam pattern and the mold, thus theparticle. Compared with that without vacuum， theheat transfer effects of the liquid metal.microstructure of LFC magnesium alloy under vacuum isWithout vacum, the heat loss of the flling metal isrefined and has less precipitated β-phase， which ismainly due to the mode of Stefan radiation to liquefy theformed at the grain boundary and around Al-Mnfoam and the mode of heat conduction to the surroundingcompound particle.coating and sand, while heat loss by the removinggaseous products in the mode of heat convection is veryReferencessmall[13]. Just as that in the conventional sand castingprocess, when a-Mg forms the consecutive solid skeleton1] NICHOLAS J. High performance magnesium[J]. Advancedin the casting, the solidification shrinkage of the castingMaterials and Processes, 2005, 193(9): 65- -6LUO A A. Recent magnesium alloy development for automotivewill produce a gap at the metal-coating interface, whichpowertrain applications[J]. 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