Vol.13 №.3 T rans.Nonferrous Met.Soc.China J un.2003 Article ID:10036326(2003)03047311Solidif ication of Al alloys under electrom agnetic f ield①CU I Jian2zhong(崔建忠)(K ey Laboratory of Ministry of Education for Electromagnetic Processing of Materials,Northeastern University,Shenyang110004,China)Abstract:New theories and technology in the electromagnetic field were put forward about DC casting of Al alloys,in2 cluding the fundamental research works,i.e,effects of the electromagnetic field on solidus and liquidus,macrosegregation of the main alloying elements,microstructures,content of alloying elements in grains and grain size after solidification un2 der electromagnetic field,and also including a new process2DC casting under low frequency electromagnetic field (L FEMC),which can refine microstructure,eliminate macrosegregation,increase the content of alloying elements within grains,decrease the residual stress,avoid cracks and improve surface quality,and another new process2DC casting under low frequency electromagnetic vibration(L FEVC),which is a high effective method for grain refining.K ey w ords:Al alloy;electromagnetic field;frequency;intensity;solidus;liquidus;content of alloying elements;macrosegregation;residual stress;surface qualityC LC number:TG146.2Document code:A1 INTR OD UCTIONCasting is a key step in Al alloy processing. From production we know that almost all quality problems are related to the defects in casting,most of which can not be eliminated in following forming and heat treatments,so casting is not only a forming pro2 cess but also a process for structural control.To apply outfield is an efficient and economical method to con2 trol structures.The electromagnetic field is widely used in industry,because of its high efficiency,econ2 omy,cleanness and no contact[15],for example, electromagnetic stirring,electromagnetic pumping, electromagnetic brake,and so on.A series of advan2 tages are brought out in the electromagnetic casing without mold,especially in the electromagnetic soft contact[611].What do changes occur in fundamental rules during solidification under electromagnetic field, in the flow field and temperature field during solidifi2 cation,and in the microstructures and distribution of elements after solidification?How to use these changes to control the microstructure and to improve the properties of the alloys and develop new technolo2 gies are the task in this study.2 FUN DAMENTAL RESEARCH WORK IN SO2 L IDIFICATION OF Al ALLOY UN DER E L EC2 TR OMAGNETIC FIE LDThe phase diagram is the basic rules in alloy phase transition.The grain size and the distribution of the alloying elements are the key factors to the properties of the alloys.2.1 E ffects of electromagnetic f ield on solidus andliquidus of Al alloys[12]2.1.1 Effects of DC electromagnetic field on solidusand liquidus of Al alloySolidification is a nonequilibrium process under electromagnetic field,which will result in some changes in solidus and liquidus.The effects of DC electromagnetic field on the melting point of Al and solidus and liquidus of Al2Cu alloys are shown in Fig.1.The melting point of Al and both solidus and liq2 uidus temperatures decrease with the increase of the intensity of the DC electromagnetic field.However, both the interval and the slope of solidus and liquidus increase with the increase of the intensity of the DC electromagnetic field.2.1.2 Effects of AC electromagnetic field on solidusand liquidus of Al2Cu alloyThe effects of AC electromagnetic field on the melting point of Al and solidus and liquidus of Al2Cu alloys are shown in Fig.2.The melting point of Al and both solidus and liquidus temperatures of Al2Cu alloys increase with the increase of the intensity of the DC electromagnetic field.However,both the interval and the slope of solidus and liquidus decrease with the increase of the intensity of the DC electromagnetic field.①Found ation item:Project(G1999064905)supported by the National Key Fundamental Research and Development Program of ChinaR eceived d ate:20030305;Accepted d ate:20030409Correspondence:CU I Jian2zhong,Professor;E2mail:jzcui@Fig.1 Effects of DC electromagnetic fieldon melting point of Al and solidus and liquidus of Al 2Cu alloysFig.2 Effects of AC electromagnetic fieldon melting point of Al and solidus and liquidus of Al 2Cu alloys (f =50Hz )2.1.3 Effects of the frequency of electromagneticfield on solidus and liquidus of 7075Al alloy The effects of the frequency of electromagnetic field on solidus and liquidus of 7075alloy were studied at given electromagnetic intensity of 0.06T.The re 2sults are shown in Fig.3.Both liquidus and solidus temperatures fall down under DC electromagnetic field.The solidification interval increases because the solidustemperature falls down in a large extent.However ,both liquidus and solidus temperatures raise under AC electromagnetic field.The solidification in 2terval decreases because the solidus temperature raises in a large extent.The lower the frequency is ,the more notable the effects are.For example ,when the liquidus temperature increases from 630to 640℃,the solidus tem perature increases from 520to 540℃and the solidification interval reduces by 10℃at the frequency of 15Hz comparing with those without e 2Fig.3 Effects of frequency of electromagnetic fieldon liquidus and solidus in 7075Al alloy (B =0.06T )lectromagnetic field.2.1.4 Effects of intensity of electromagnetic fieldon liquidus and solidus of 7075Al alloyThe effects of the intensity of electromagnetic field on liquidus and solidus of 7075Al alloy are shown in Fig.4.The solidus and liquidus tempera 2tures raise and the solidification interval decreases un 2der AC electromagnetic field.For example ,at a given frequency of 15Hz ,the liquidus and solidus increase by 5and 10℃,respectively ,and the solidification interval reduces by 5℃at the intensity of 0.03T comparing with those without electromagnetic field.At the same frequency ,the liquidus and solidus in 2crease by 15and 27.5℃,respectively ,and the so 2lidification interval reduces by 12.5℃at the intensi 2ty of 0.12T comparing with those without electro 2magnetic field.2.1.5 A simple explain of effects of electromagneticfield on liquidus and solidusFig.4 Effects of intensity of electromagneticfield on liquidus and solidus of 7075Al alloy (f =15Hz )・474・Trans.Nonferrous Met.Soc.China J un.2003 The force will be produced in a moving charged particle when it cuts magnetic lines according to the electromagnetic theory.The direction of the force is opposite to the moving direction of the particle,so that DC electromagnetic field will produce a brake force,which will limit the moving particles.Howev2 er,the AC magnetic field will promote the motion of the particles.Crystallization of the metals is a process in which liquid atoms transfer to solid crystals.De2 crease in solidus under DC electromagnetic field means a decrease in the temperature at which the crystalliza2 tion finishes and an increase in need for undercooling, which results from a decrease in the atom transferring speed from liquid to solid because of a brake made by DC electromagnetic filed.Decrease in liquidus under DC electromagnetic field means a decrease in the tem2 perature at which the crystal nuclei formation and an increase in need for undercooling,which results from a decrease in the atom transferring speed from liquid to solid because of a brake made by DC electromag2 netic filed,too.The higher the intensity of the elec2 tromagnetic field is,the stronger the brake force and the more the decrease in liquidus and solidus are.Un2 der AC electromagnetic field,the motion of the atoms is promoted and the speed for atoms to transfer from liquid to crystal nuclei is increased,which means that at higher temperature the nucleation starts and the crystallization finishes.The higher the increase in the intensity of AC electromagnetic,the larger the in2 crease extent of the liquidus and solidus is.2.2 E ffect of electromagnetic f ield on content ofalloying elements within grains[13]The solution content of alloying elements within grains is a key for the workability and service proper2 ties of the alloys.It is the first report on the solution content of alloying elements within grains promoted by electromagnetic field.2.2.1 Effect of frequency on content of alloying ele2ments within grainsThe content of alloying elements within grains increases under an electromagnetic field.However, the increase extents are different at different frequen2 cies.The increase extents of Cu and Zn reach its maximum at about15Hz as shown in Figs.5(c)and 5(a),and the increase extent of Mg is largest at0Hz as shown in Fig.5(b),i.e,under DC electromagnet2 ic field,which shows that low frequency electromag2 netic field will promote to increase the content of the alloying elements within grains,and different ele2 ments have their own optimum frequency.2.2.2 Effects of intensity of electromagnetic fieldon content of alloying elements within grains The content increases firstly,then slowly decreases with the increase of the intensity of electromagnetic field, which means that there may be a saturated s olubility at high intensity,as shown in Fig.6.2.2.3 Microstructures of solidification under elec2tromagnetic field[13,14]The size of precipitates in Al2Cu alloy and7075alloy becomes small when it s olidifies under electromagnetic field,as shown in Fig.7and Fig.8.There are only fine GP zones when electromagnetic field intensity is high and the frequency is low,as shown in Fig.8.It is very inter2 esting that the resistance rate increases and the lattice con2 stant decreases at the frequency where the s olubility reach2 es its maximum,which is the direct confirmation of the increase in s olubility.It is notable that electromagnetic field with low frequency has stronger effects and DC elec2 tromagnetic field has a little effects on Al2Cu alloy,as shown in Fig.9.2.2.4 Discussing on increase of alloying elementswithin grains under electromagnetic field The atoms will lose their electrons and become ions in liquid metals.The moving speed,v,of the ions is stochastic and free without electromagnetic field.Under an electromagnetic field B,the speed v divides into two parts,v⊥,perpendicular to the elec2 tromagnetic field,and v∥,parallel to the electro2 magnetic field.A Lorentz force results from the int2Fig.5 Dependence of content of alloying elements on frequency in7075alloy(a)—Zn;(b)—Mg;(c)—Cu・574・Vol113 №13 Solidification of Al alloys under electromagnetic fieldFig.6 Dependence of solubility ofalloying elements on intensit y of electromagnetic field.(a )—Zn ;(b )—Mg ;(c )—CuFig.7 Microstructures of Al 24.5Cu alloy solidified under electromagnetic field(a )—Without electromagnetic field ;(b )—50Hz ;(c )—25Hz ;(d )—15Hz ;(e )—10Hz ;(f )—DC electromagnetic field (B =0.1T)eraction between ions with charge q and electromag 2netic field B [15].f =q v ⊥×B(1)where f is perpendicular to v ⊥,and changes the direction of v ⊥,but dos not change its magnitude ,so the ions go around the electromagnetic lines under the action of f .v ∥forces ions to move in the direc 2tion of electromagnetic lines.The circum 2rotation ra 2dius is [16]r c =v ⊥ωc=m v ⊥|q |B(2)From the above equation we know that r c is pro 2portional to the mass of the ions ,m ,and inversely proportional to the charge ,q .The radii of Al 3+,Zn 2+,Mg 2+,Cu +are different for their differences in mass and charge ,which results in a relative motion of the alloying elements and Al atoms ,and an in 2crease in diffusion ability of the alloying elements in Al matrix ,so the solubility of the alloying elements in α(Al )increases.2.3 E ffects of electromagnetic f ield on macrosegre 2gation of alloying elements [13]Macrosegregation of alloying elements is a famil 2iar defect in casting ,which will results in a decrease in properties for uneven element distribution.Espe 2cially ,if the segregation nubbles form on the surface of ingots ,a lot of materials must be milled out ,which results in a decrease in the rate of the finished・674・Trans.Nonferrous Met.Soc.China J un.2003Fig.8 Microstructure of 7075Al alloy solidified under electromagnetic field(a )—Without electromagnetic field ;(b )—f =30Hz ,B =0.06T ;(c )—f =30Hz ;B =0.12T ;(d )—f =15Hz ,B =0.12TFig.9 Effects of frequency on lattice constantand resistance rate of Al 24.5Cu alloyB =0.12Tproducts.To eliminate segregation many methods were used ,whose the application of electromagnetic field is an effective one.2.3.1 Effects of electromagnetic field on macroseg 2regation of 7075alloyThe segregation rate ,S ,is calculated by the fol 2lowing equation :S =(C max -C min )/C ave(3)where C max is the highest content of an element in an ingot ;C min is the lowest content of an element in an ingot ;C ave is the average content of ten measuring points of an element from the center to the border in an ingot.The application of electromagnetic field reduces the macrosegregation remarkably ,as shown in Fig.10.However ,different frequencies will have differ 2ent effects.Zn and Mg reach their lowest value at 15Hz and Cu reaches its lowest value at DC electromag 2netic field.The intensity of electromagnetic field has stronger effects on macrosegregation ,as shown in Fig.11.The rate of segregation firstly falls down fastly then raises up slowly.The lowest rates of the three elements take place in B =0.06T ,which shows that high intensity is not necessary to eliminate macrosegregation.2.3.2 Discussion on elimination of macrosegre 2gationThe reason why the electromagnetic field elimi 2nates the macrosegregation maybe come from three aspects1)the electromagnetic field makes a stirring which forces liquid alloy to flow in a circular direc 2tion ,and the distribution of elements become even ;2)a fast diffusion and an increase in elements within grains result from the electromagnetic force ;3)the electromagnetic field has much strong ef 2fect on the temperature field and solute field at inter 2face of liquid and solid ,which will make a homoge 2nous distribution of elements.3 DC CASTING OF Al ALLOY AT LOW FRE 2QUENCY E L ECTR OMAGNETIC FIE LD (LFEMC)G etselev [17]invested a process for electromagnet 2ic casting (EMC ),which was called casting without mould because the liquid alloys do not contact with the mould by a high pressure produced by high fre 2quency electromagnetic field ,only act on the surface of the liquid surface.This technique receives high r 2・774・Vol 113 №13 Solidification of Al alloys under electromagnetic fieldFig.10 Dependence of frequency ofelectromagnetic field onsegregation rates in 7075alloy(a )—Zn ;(b )—Mg ;(c )—CuFig.11 Effects of intensity of electromagnetic field on macrosegregation in 7075alloy(a )—Zn ;(b )—Mg ;(c )—Cu ecognition in Al industry for its improvements in sur 2face quality and grain fining.However ,this tech 2nique is only used in a few workshop because it is dif 2ficult to control in beginning stage and a very smooth casting machine is needed.Another EMC ,CREM was invented by Vives [18]who cast ingots of Al alloys with fine grains and good surface by the application of an electromagnetic field of common frequency ,50Hz.In this study we will open a new ,efficient tech 2nique 2L FEMC based on the understanding principle of CREM [1928].3.1 Working principle of LFEMCThe result of the numerical simulation of the dis 2tribution of magnetic flux density in pool is shown in Fig.12.The magnetic flux density decreases quickly in the mold because of a much magnetic loss with the increase in frequency ,which results a decrease in the density in the pool.The higher the frequency and the larger the vertex in the mold which is made of met 2als ,the lower the density in the pool is ,so the high penetration ability of low frequency magnetic field is the base of L FEMC.The flow of liquid alloy in the pool without and with a low frequency electromagnetic field is shown in Fig.13.It is very interesting that the impact depth of the pouring liquid alloy decreases (Fig.13(b ))inL FEMC ,which results from the liquid flows from the center to the border in surface.This characteristic can markedly change the temperature field and great 2ly decrease the depth of liquid cavity and residual stress in ingot during casting.Dependence of temperature at the central line of the ingot on frequency is shown in Fig.14.It is seen that the temperature field becomes even and the depth of liquid cavity decreases when a low frequency elec 2tromagnetic field is applied.3.2 R esults of LFEMCThe L FEMC microstructures of 7075alloy areFig.12 Distribution of magnetic flux densityin pool of diameter 200mm・874・Trans.Nonferrous Met.Soc.China J un.2003Fig.13 Liquid alloy flowswithout and with a lowfrequency electromagnetic field(a )—B =0;(b )—f =25HzFig.14 Dependence of temperature at the central line of the ingot on frequency shown in Fig.15.The finer the microstructures be 2come and the lower the frequency is ,the finer the grains are.The distribution of the alloying elements and the improvement of macrosegregation after L FEMC in a d 200mm ingot are shown in Fig.16.The best improvement takes place at 30Hz.The residual stress of the ingot after L FEMC is shown in Fig.17.The residual stresses are reduced by 50%in L FEMC comparing with the DC casting without elec 2tromagnetic field.The advantages of L FEMC are summarized as followings :1)microstructures become finer ;2)segregation is eliminated ;3)solubility of alloying elements in grains in 2creases ;4)surface quality is improved ;5)residual stress decreases a lot and the cracks are avoided effectively [29]L FEMC does not change the conventional cast 2ing process and does not need rebuilding the melting furnace and casting machine ,only needs adding a cheap electrical source and coils ,so a little increase will be in cost and L FEMC will be widely used in Al industry.Recently ,L FEMC is applied to DC casting of Mg alloys and its high strength alloys ,and thesame results as 7075have been achieved.4 DC CASTING OF Al ALLOY UN DER LOWFREQUENCY E L ECTR OMAGNETIC VIBRA 2TION (LFEMVC)4.1 Working principles of LFEMVC [30]Two electromagnetic fields were applied ,of which one is a DC or invariable electromagnetic field B with a foil going through a DC current and the oth 2er is an AC electromagnetic field of low frequency b with a foil going through an AC current of 20100Hz ,which produces an inductive current J with the same frequency as AC current in the melt.A force field will be produced for the interaction of B ,b and J ,which will force the melt to be vibrated.The working principle and the schematic diagram of L FEMC are shown in Fig.18and Fig.19.From the magnetohydrodynamics the liquid motion is the superposition of three kinds of motions ,the alter 2nating vibrations with frequencies of f and 2f and the brake function against liquid flow.The interaction of B with J will produce a vibration force F 1=J ×B ,which acts in the liquid and can reach and spread into the whole liquid by an elastic function [31].The interaction of b with J will produces a force ,F 2=J ×b [3234],which includes an indepen 2dent 2time heft and a vibration heft with frequency 2f .If the permeability is μ,the electromagnetic force can be divided into a radius part , (1/2μb 2),which is perpendicular to the center line of the ingot ,and is a balance to the hydropressure of the liquid al 2loy in the pool ,and forms a meniscus free surface in the sump ;and an axis part ,1/μ(b ・ )b ,which in 2clines to the centerline of the magnetic field ,and stirs the sump.The liquid flows at a speed U from the center to the border of the sump.The flow will cut the mag 2netic lines and produce a current J 1which interacts with B to produce a force F 3=J 1×B 1.The force will reduce the flow speed of the liquid.・974・Vol 113 №13 Solidification of Al alloys under electromagnetic fieldFig.15 Effects of frequency of electromagnetic field on microstructures of 7075alloy in L FEMC(d 200mm ,Θ=20000A )(a )—B =0;(b )—50Hz ;(c )—30Hz ;(d )—15HzFig.16 Effect of low frequency electromagnetic field on distribution of alloying elements(d 200mm ,Θ=20000A )(a )—Mg ;(b )—Cu ;(c )—Zn ;(d )—Cu・084・Trans.Nonferrous Met.Soc.China J un.2003Fig.17 Effect of L FEC on residual stressin ingot(d200mm,Θ=20000A)(a)—Radial stress;(b)—Circular stressFig.18 Principle of production ofelectromagnetic vibrationFig.19 Schematic diagram of DC castingof aluminum alloys with electromagneticvibration methodAnother force F4=J0×b comes from the interaction of AC magnetic field,b,and an induced current J0, which results from the cutting magnetic lines owing to liquid flow,and it is a periodic alternating vibra2 tion force.There is little of reports on F4.4.2 E ffects of LFEMVCThe microstructures after LFEMVC are shown in Fig.20.The microstructures become very fine,s o LFEMVC is a more effective process for grain refining than LFEMC.I f the LFEMVC process can force the al2 loying elements to s olute and improve segregation and re2 duce residual stress,it will become a more valuable casting method for Al alloy.4.2.1 Discussion on principle of grain refining ofL FEMVC[34]The reason why the grains can be refined by L FEMVC is a complex problem.We consider that the main reasons are followings:1)the number of grain nuclei increases by the nuclei deviating from the wall of the mold owing to the vibration;2)the nuclei number increases by an increase in nuclei forming at some solid particles,such as com2 pounds because the vibration can make a repeat fric2 tion to improve the wetting of liquid on the solid par2 ticles and a decrease in wetting angle[35];3)the vibration force makes a convection,which reduces the composition overcooling and temperature grads at the interface of nuclei and liquid to eliminate the condition for dendrite growth;4)the parts of the surfaces of grains are remelted and grow again and again during electromagnetic vi2 bration.Especially,it is easy for the bulges of the grains to be remelted because bulges are of high ener2 gy,so the arms of the dendrites are remelted.The solute field will become homogenous and the buds of dendrites will be broken for the shearing function be2 tween the nuclei and liquid during the vibration be2 cause the nuclei are of higher conductivity than the liquid alloy and they will receive a higher vibration force owing to the high density of current induced by electromagnetic field.When the solid fraction of the alloy is over0.55,the motion speed of liquid will re2 duce greatly for an increase in the friction.A forced vibration still takes place in the two phases region to overcome the free convection and to reduce the tem2 perature grads and the composition overcooling before・184・Vol113 №13 Solidification of Al alloys under electromagnetic fieldthe solidification ,which will restrain the growth ofdendrite ,fine grains and increase the proportion scale of theequiaxed grains.The electromagnetic lines will go deep into the center of the sump and the temperature field and the distribution of the alloying elements will become more homogenous with the decrease in the frequency.The vibration force increases greatly ,especially ,in the liquid cavity with the decrease in the frequency.When the solid fraction is over 0.55and the vibration frequency between the dendrites is closed to the iner 2tia respond frequency ,the excellent function to ho 2mogenize the temperature field and to decrease the composition overcooling takes place ,which will re 2strain the dendrite πs growth ,improve the distribution of the alloying elements and eliminate the segrega 2tion.5 CONC L USIONS1)Under AC electromagnetic field both liquidus and solidus temperatures increase and the solidifica 2tion intervals decrease.However ,under DC electro 2magnetic field both liquidus and solidus temperatures decrease and the solidification intervals increase.2)The content of the alloying elements within grains increases when an electromagnetic field is ap 2plied.Each element has its own optimum combina 2tion of frequency with the intensity of electromagnetic field.3)Under low frequency electromagnetic field ,the resistance rate of Al 2Cu alloy increases ,lattice constant decreases and the precipitates after solidifica 2tion become finer.And the precipitates in 7075alloy after solidification become finer ,too.4)The application of low frequency electromag 2netic field is an effective method to eliminate the macrosegregation.However ,different elements have different optimum combinations of frequency with in 2tense.5)L FEMC is an effective and economical pro 2cess for DC casting of Al alloys ,which can refine mi 2crostructures ,increase the solubility of alloying ele 2ments within grains ,eliminate macrosegregation ,im 2prove surface quality ,decrease residual stress and avoid cracks in ingots.6)L FEMVC is a more effective method to refine microstructures.7)Both L FEMC and L FEMVC are important techniques in the Al industry.REFERENCES[1] Boettinger W J ,Coriell S R ,Greer A L ,et al.S olidifi 2cation microstructures :recent developments ,future di 2rections [J ].Acta Mater ,2000,48:4370.[2] ZHOU Rao 2he ,HU Zhuang 2qi ,J IE Wan 2qi.S olidifica 2tion Technology [M ].Beijing :Mechanical Industry Press 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