New method for the preparation of semi-solid AISi7Mg alloy slurry

Journal of University of Science and Technology BeijingMaterialsVolume 14, Number I, February 2007, Page 56New method for the preparation of semi-solid AISi7Mg alloy slurryWeimin Mao, Hai Lin, Yuelong Bai, and Songfu GaoMaterials Science and Engineering School, University of Science and Technology Bejing, Bejing 100083, China(Received 2006-01-18)Abstracts: The effects of pouring temperature and short electromagnetic strring with low strength on the microstructures of AISi7Mgalloy were investigated. The results show that if liquid AISi7Mg alloy is poured at 630 or 650°C, many primary a-Al grains in thesuperheat and meanwhile is stired by an electromagnetic field at a low power for a short time, then most primary a-Al grains in thesolidifed melt become spherical, and only a few are rosette-like. The theoretical analysis indicates that the strengthened melt flowmotion induced by the short electromagnetic strring makes the temperature field more homogeneous in the melt, which is poured at alower superheat, and makes the primary a-Al grains deposit in a larger region at the same time, so this new solidification kineticcondition leads to the microstructure of spherical primary a-Al grains. The experiments also demonstrate that pouring at an appropriatesuperheat and stirring by an electromagnetic field at a low power for a short time is a good new method for preparing the semi- solidslury or bllt of AISi7Mg alloy.Key words: AISi7Mg alloy; semi-solid; pouring temperature; electromagnetic stiring[This work was financially supported by the National Nature Science Foundation of China (No. 50374012), the National High TechResearch and Development Program of China (863 Program) (No.2006AA03Z115), and the National key Basic Research Foundationof China (No.2006CB605203).]1. Introductionduction, the later five methods are yet in developmentnow [4-11]. As a result, searching for new and low-costThe casting technology with a semi-solid aluminummethods for preparation of the semi-solid aluminumalloy is a new technology. It has many advantages, suchalloy slurry or billet is an object of many experts andas less porosity and segregation in the casting, immanufacturers both domestic and overseas. The effectsproved mechanical properties; near net-shape castingof pouring temperature and short electromagnetic stir-and a lower machining cost; filling at a lower temring with low strength on the microstructures ofperature and a longer die life. Therefore, the castingAlSi7Mg alloy were investigated in this study, and atechnology of a semi- solid aluminum alloy has suc-new method for preparing a semi-solid aluminum alloycessfully been applied to manufacturing many kinds ofslurry was put forward.car parts [1-3]. During the semi solid casting process,the preparation of a specific slurry or billet with spheri-2. Experimentalcal primary grains has played an important role, be-The experimental material was a commercialcause the specific slurry or billet is closely related tothe production economy and the process stability. AtAlSi7Mg alloy. Its composition is Si, 7.1wt%; Mg,present, there are many methods for the preparation of0.31wt%; Fe, 0.20wt%; Mn, 0. 10wt%; Zn, 0. l0wt%;semi-solid aluminum alloy slurry or billet, such asAl, balanced.mechanical stirring, electromagnetic stirring, strain-The AISi7Mg alloy was first melted in an electricinduced melt activation process (SIMA)，new rheo-furnace and then fined. The liquid melt was eventuallycasting，single screw process, twin screw process,poured at 630 or 650°C into a preparing crucible,pouring at a low superheat and mechanical stirringwhich was made of stainlece steel and. was 80 mm inweakly, near liquidus casting process, and so on. How-diameter and中国煤化工poured melt :ever, only the electromagnetic stirring and SIMA proc-was about 120MHCNMHGthemeltwasesses have been successfully used for commercial pro-stirred by an electromagnetic field at a low power for 2Corresponding author: Weimin Mao, E-mail: weiminmao@ 263.netAlso available online at w ww .sciencedirect.com.W.M. Mao et al, New method for the preparation of semi-solid AISi7Mg alloy slurry57or 8 S respectively. If the pouring temperature waif the pouring temperature was 615°C, it was more dif-6309C, the slurry with the crucible was quenched intoficult to control the pouring process because the liquidwater after cooling continuously for 15 s. If the pouringalloy would cool too quickly, and some of the alloytemperature was 650C, the slurry with the cruciblewould solidify before and during the pouring and ad-was quenched into water after cooling continuously forhere to the inner surface of the melting crucible. If thepouring temperature was increased, it was easier toThe metallographic specimens were cut off thecontrol the pouring process, but the solidified micro-solidified slurries. They were roughly ground, finelystructure became worse, and the primary a-Al grainsvere changed from spheres to dendrites [10-11]. Forpolished and etched with an aqueous solution of0.5vol% HF. The microstructures were observed andexample, when poured at 630°C and not stirred, theanalyzed with an optical microscope of Neuphoto 21solidified melt would have many rosette-like and lessspherical primary a-Al grains, so the microstructuretype.was not homogeneous, as shown in Figs. 1(a) and (b).3. Experimental results and discussionWhen compared, there are many fine dendrites or3.1. Effect of weak electromagnetic stirringrosettes in the melt edge as shown in Fig. 1(c). WhenDuring preparation of the semi-solid slurry or billetthe melt is poured at 650°C and not stirred, the innerof AISi7Mg alloy, the primary a-Al grains in themicrostructure of the melt is worse than that poured atsolidified melt appeared like spheres and were630°C, and there are more rosett-like primary a-Alstretched homogeneously, if the liquid alloy wasgrains, as shown in Fig. 1(d).poured at its liquidus temperature of 615°C. However,中200uM200 um(d)Fig. 1. Microstructures of semi-solid AISi7Mg alloy slurries poured at 630°C and not be stirred: (a) inner microstructure;(b) inner microstructure; (c) edge microstructure; (d) inner microstructure.If liquid AISi7Mg alloy is poured at 630°C andprimary 0.-Al grains in the melt edge are rosett-like,meanwhile stirred momentarily by an electromagnetictoo, as shown in Fig. 3, if the melt is stirred momentar-field at 0.54 kW, most of the primary a-Al grains insideily at 0.54 kW.the solidified melt appear spherical, and only a few are3.2. Effect of stirring strength and stirring timerosette-like, as shown in Fig. 2(a). However, there aremany fine rosettes in the melt edge region, as shown inThe electromagnetic stirring power is generally aFig. 2(b). Even when the pouring temperature is in-processing factor and mav have. some effect on thecreased to 6509C, most of the primary a-Al grains in-solidified micr中国煤化工w superheatside the solidified melt also appear spherical, and theand stirred elec|YHC N M H Gpower. Whenrosette-like grains increase somewhat, and the manythe liquid AlSi7Mg alloy is poured at 630°C and.58J. Univ. Sci. Technol. Bejing, Vol.14, No.1, Feb 2007meanwhile stirred by an electromagnetic field, with thethe above. Therefore, given the pouring temperature,stirring power increased to 1.2 kW, it is found that the630 or 650°C, it is not needed to further increase theprimary a-Al grains in the solidified melt are verystirring power, and 0.54 kW is enough to obtain thesimilar to that stirred at 0.54 kW, as shown in Fig. 4.ideal spherical primary a-Al grains in the semi-solidMoreover, the effect of stirring power on the micro-slurry or billet of AISi7Mg alloy.structure of the melt poured at 650C is also similar to2001m日XFig. 2. Microstructures of semi- solid AISi7Mg alloy slurry poured at 630°C and stirred by electromagnetic field at 0.54 kW for8s: (a) inner microstructure; (b) edge microstructure.(b)200umFig. 3. Microstructures of semisolid AISi7Mg aloy slurry poured at 6509C and stirred by electromagnetic field at 0.54 kW for8 s: (a) inner microstructure; (b) edge microstructure.Fig. 4. Microstructures of semi-solid AISi7Mg alloy slurry poured at 630°C and stirred by electromagnetic field at 1.2 kW for 8s: (a) inner microstructure; (b) edge microstructure. .The stiring time is another processing factor andinside the solidified melt appear spherical, and only amay have some effect on the microstructures solidifiedfew are rosette-like, but many primary a-Al grains inon the same conditions such as the ones mentionedthe melt edge are rosette-like. The experiments alsoabove. When the liquid AISi7Mg alloy is poured atshows that if poured at 650°C, the effect of stirring time630°C and meanwhile stirred by an electromagneticon the microstructure is the same as that of the meltfield at 0.54 kW, but the stirring time is decreased frompoured at 6309C. As a result, poured at 630 or 650°C,8 to 2 s, the primary a-Al grains in the solidified melt2-8 s of stirir中国煤化工ain the idealare similar to those stirred for 8 s, as shown in Figs. 2spherical prima二i-solid slurryand 5. That is to say, most of the primary a-Al grainsor billet of AISiMHCNMHG.W.M. Mao et al, New method for the preparation of semi-solid AISi7Mg alloy slurry593.3. Discussioncident solidification and restrains the large primary a-If liquid AISi7Mg alloy is poured at 615°C, the liq-Al dendrites or rosettes. On the other hand, the tem-uid melt only needs to transfer a lttle heat from itself toperature of the preparing crucible is heated to only athe preparing crucible, and then the melt goes into so-lower value, the melt cooling rate is high, and the timelidification. Because there is almost no superheat, thefor primary a-Al grains to grow is shortened [11], somelt region in which the many primary a-Al grains lar-the primary a-Al grains are obliged to grow into finegely deposit will be very wide. The coincident solidifi-rosettes. Owing to almost coincident solidification, thecation phenomenon is greatly strengthened and therelatent heat will be released in a narrow time range, andare many more primary grains depositing concurrently.the ripening effect and the remelting phenomenon onIt is certain for so many grains in such a narrow area tothe secondary arms' roots will be intensified. Thereforelimit each other's growth. It is impossible for largea large number of spherical primary a-Al grains areprimary a-Al dendrites or rosettes to appear in the melt.gradually evolved in the melt. So the above solidifica-Meanwhile, as the melt is still being poured, the melttion phenomenon establishes a new solidification ki-poured later continuously exerts an impact effect on theetic condition, that is, if a large number of grainsmelt poured earlier, so the lttle primary a-Al grains aresolidify in a given area at the same time, the grain sizestirred, and they will be stretched more evenly in thewill be decreased greatly, and spherical grains will ea-melt. This solidification action further promotes coin-sily be obtained.a)Fig. 5. Microstructures of semi-solid AISi7Mg alloy slurry poured at 630C and stirred by electromagnetic field at 0.54 kW for2 s: (a) inner microstructure; (b) edge microstructure.With the pouring temperature increasing to 630 orand stirred momentarily by an electromagnetic field at650°C, liquid AISi7Mg alloy carries more physical a low power, the melt flow motion in the preparing cru-heat and needs to transfer more heat to the preparingcible is obviously strengthened, and has a strong effectcrucible for itself to solidify. Although the impact ef-on the whole melt. This action makes the temperaturefect from the pouring flow motion still exists, the meltfield in the melt more homogenous and increases thedoes not start to solidify or there are only a few of pri-region in which the primary a-Al grains deposit at themary a-Al grains deposited, and so the flow effectsame time. So the above-mentioned new solidificationpromoting the coincident solidification is greatly dis-kinetic condition is established, and it is possible forcounted. On the other hand, the preparing crucible isthe primary a-Al grains to be fined greatly. Moreover,heated to a higher temperature, the cooling rate of thethe flow motion also makes the deposited fine rosette-melt is low, and the time for primary a-Al grains toike primary a-AI grains move everywhere, and thegrow is lengthened, so the primary a-Al grains are ob-whole crucible region will be in solidification in a veryliged to grow into large rosettes [11]. Moreover, theshort time. Because of a more homogeneous tempera-melt is even more inclined to solidify layer after layerture field, a larger solidification region and latent is re-from the outer to the inner, that is, the melt region inleased in a shorter time, the ripening action of the finewhich the primary a-Al grains deposit simultaneouslydendrites or rosettes is intensified, and it is more possi-becomes narrower, so the latent heat will be released inble for the secondary arms to be remelted on their roots,a wider time range, and the ripening effect and theand the rosette-like primary a-Al grains are eventuallyremelting phenomenon on the secondary arms’ rootschanged to spherical grains. Therefore, it is easier towill be decreased. Therefore, a large number of ro-control the pouring process at an appropriately highersette-like primary 0-Al grains remain in the solidifiedpouring temper中国煤化工tain the idealmelt.spherical primaC N M H Gostructure, ifIf liquid AISi7Mg alloy is poured at 630 or 650°Cthe melt is momentarily stirred weakly by an electro-.6(J. Univ. Sci. Technol. Bejing, Vol.14, No.1, Feb 2007magnetic field at the same time.state, Metall. Trans, 22A(1991), No.5, p.957.In a word, the experiments demonstrate that pouring2] K.P. Young and R. Fitze, Semi-solid metal cast aluminumautomotive components, [in] Proc. of the 3rd Int. Conf. ofat an appropriate superheat and stirring by an electro-Semi-Solid Processing of Alloys and Composites, Tokyo,magnetic field at a low power for a short time is a good1994, p.155.new method for preparing semi-solid slurry or billet of3] S.P. 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