a study on the hot-deformation bahavior and dynamic recrystallization of AL-5 wt% mg alloy

A study on the hot-deformation behavior and dynamic recrystallizationof Al±5wt.%Mg alloyJ.R.Cho a,*,W.B.Bae b ,W.J .Hwang b ,P.Hartley caSchool of Mechanical Engineering,Korea Maritime University,1Dongsam-Dong,Yeongdo-Gu,Pusan 606-791,South KoreabERC for Net Shape and Die Manufacturing,Pusan National University,Pusan,South Korea cSchool of Manufacturing and Mechanical Engineering,University of Birmingham,Birmingham,UKAbstractA numerical analysis was performed to predict ¯ow curves and the dynamic recrystallization behaviors of an Al±5wt.%Mg alloy on the basis of results of hot-compression tests.These tests were carried out in the ranges 350±5008C and 0.005±3s À1to obtain the Zener±Hollomon parameter Z .The modeling equation for ¯ow stress was a function of strain,strain rate and temperature,and the in¯uence of these variables was quanti®ed using the Zener±Hollomon parameter.In the modeling equation,the effects of strain hardening and dynamic recrystallization were taken into consideration.A model of the evolution of microstructure in the Al±5wt.%Mg alloy was developed to predict grain size and recrystallized volume fraction during hot working.The microstructure model was combined with a thermo-viscoplastic ®nite element method (FEM).Model prediction shows a good agreement with microstructures obtained in compression tests.#2001Elsevier Science B.V .All rights reserved.Keywords:Al±Mg alloy;Recrystallization;Finite element method1.IntroductionRecent developments in the basic metallurgical theories for microstructural evolution and in the technology for computer simulations have contributed to promoting studies on the predictions of recrystallization behavior in hot rolling [1±4].The microstructural evolution of aluminum alloys in hot working has been the subject of a number of studies [5±10].This technology for the prediction of mechanical prop-erties using mathematical models can be applied in the selection of optimal manufacturing conditions.The Al±5wt.%Mg alloy is widely used for components of automobiles and ships that require high strength and anti-corrosion.Hot deformation is commonly applied for for-mation of the large-sized structures and high strength mate-rials,because the materials are easily softened by annealing.The interior structure of the material during hot deformation does not change homogeneously.Thus,important para-meters,such as temperature,strain rate and stress must be controlled properly to keep the structure uniform [8].The ®nite element method (FEM)is one of the most effective methods to investigate the effects of these parameters [11,12].The objective of this study is to describe the plastic behavior of Al±5wt.%Mg alloy in constitutive equations and to predict the evolution of dynamic recrystallization.The grain size and fraction of recrystallization can be determined using a ®nite element model to predict deforma-tion,strain,strain rate and temperature,and an appropriate mathematical model for the recrystallization.The mathe-matical model for dynamic recrystallization in this alumi-num alloy was developed for compression test data.Analyses of deformation and heat transfer were undertaken using a thermo-viscoplastic FEM.2.Experimental procedure 2.1.Basic experimentThe chemical compositions of the alloy are shown in Table 1.This aluminum alloy was normalized for 3h at 4408C and 5h at 5258C after casting,and then machined into the cylindrical specimens,8mm diameter and 12mm high.Fig.1shows a schematic diagram of the equipment used in the experiments.The specimen was heated at the rate of 58C/s by induction heater under vacuum,and compressed to 50%at the strain rate of 0.005,0.01,0.1,1and 3s À1in 350,400,450and 5008C,respectively.Each specimenwasJournal of Materials Processing Technology 118(2001)356±361*Corresponding author.Tel.: 82-51-410-4298;fax: 82-51-403-3856.E-mail address :cjr@hanara.kmaritime.ac.kr (J.R.Cho).0924-0136/01/$±see front matter #2001Elsevier Science B.V .All rights reserved.PII:S 0924-0136(01)00978-5rapidly cooled by nitrogen gas to keep the dynamically recrystallized microstructure.2.2.Non-isothermal upsetting experimentSpecimens of50mm diameter and60mm in height were compressed to50%by a200t hydraulic press under non-uniform temperature.Experimental results were compared with simulated results that calculated using the model equation derived from the basic experiment.The specimens were heated with an electric furnace to initial temperatures of380and4608C.The corresponding die sets were heated with heating cartridges to given initial die temperature of 250and4208C,respectively.To measure the temperature change in the specimen during hot working,a thermocouple of1.2mm diameter was inserted into the center.Specimens were compressed to50%at0.6mm/s and water-quenched rapidly after deformation.The internal microstructure was observed by optical microscope.3.Experimental results and discussions3.1.Modeling of dynamic recrystallization behaviorAs shown in Fig.2,the shapes of the¯ow curves are quite different from those of steels that are classical in nature,with a work-hardening rate at a given strain and the strain at the peak stress increasing with decreasing temperature and increasing strain rate.In the case of steels,the strain,where the stress reaches its peak value is reported to have relatively high value[1±4].In the present experiment,the peak stress was observed at very low strain(below0.02)and the¯ow stress was sharply decreased to eventually a steady-state.The microstructure-induced dynamic recrystallization,however,was not found at low strains around the critical strain.Thus,it cannot be concluded that the dynamic recrystallization initiated around strain associated with the peak stress.The Arrhenius equation is widely used to describe the relation between the strain rate _e ,¯ow stress(s),and temperature(T)at high temperatures[4,10].It can also be shown with the Zener±Hollomon parameter as follows: _e A sinh as p n expÀQRT(1) s CZ1=n(2) It is found that the index,n,is a parameter related to the hardening or softening rate of the material[11].As shown in Fig.3,the value of n depends on the current strain.This value sharply decreases due to the hardening process and then increases due to the dynamic recrystallization.Finally, the rates of hardening and softening are balanced with each other,and the value of n reaches a steady-state.The critical strain(e c)at which dynamic recrystallization is started could be determined as the point where the value of n is a minimum,and de®ned as follows:e c 6:865Â10À3exp1951T(3)Table1Chemical compositions of Al±5wt.%Mg alloyElement Si Fe Cu Mn Mg Cr Zn Ti AlComposition(wt.%)0.080.270.30.366 5.00.030.0020.037Bal.Fig.1.Testingequipment.Fig.2.Experimental stress±strain curves at varioustemperatures.Fig.3.Strain dependence of strain rate sensitivity(n).J.R.Cho et al./Journal of Materials Processing Technology118(2001)356±361357The volume fraction of dynamic recrystallization (X dyn )and the grain size (d dyn )were determined using an opti-cal microscope and image analyzer to give the following expressions:X dyn 1Àexp Àk e Àe ce cm H "#;k 8Â10À4 2Â10À4ln Z A ;m H 3:3123À0:0792lnZA(4)d dyn m m 14:834À9:96ln ZA (5)Considering the reduction of the original grain size,D o ,by the growth of the recrystallized fraction,the resulting mean grain size was given by [13]D D o 1ÀX dyn 2 d dyn X 3=4dyn (6)3.2.SimulationThe dynamic recrystallization models were combined with a thermo-viscoplastic FEM [14]to predict microstruc-ture evolution during the hot-deformation process.The initial data [15]and boundary conditions are shown in Table 2and the initial meshes of the workpiece and dies for the FEM are shown in Fig.4.The number of isopara-metric elements in the model are 238,187and 160for the workpiece,upper and lower dies,respectively.The heat-transfer coef®cient between workpiece and die is derived by an inverse method using measured temperature with time.The upper and lower dies were assumed rigid and no lubricant was used.The ¯ow stress equation was modeled considering the dynamic softening as shown in Fig.5[2,9].s s e ÀD s(7)s e s p 1Àexp ÀC e m(8)D s s p Às s 1Àexp Àke Àae p pmH "#()(9)here,s p 17:75lnZ A 80:189;C 245:15ln ZA À0:0491;m 4:263Z A 0:2882;s s 42:906ZA 0:2898k 0:3081Z A À0:1873;m H 0:6789ZA 0:0972;e p 0:0178ZA 0:1094where s p and s s represent peak stress and steady-state stress,respectively.The effect of the work-hardening and softening processes is shown in Eqs.(8)and (9),respectively.The peak stresses are generally used in the modeling of ¯ow curves from experimental data with no consideration of softening.As shown in Fig.6,the load prediction by the model of Eq.(7)Table 2Process parameters for FE simulation Process parameterValue Friction coefficient0.6Thermal conductivity of workpiece (N/(s K))146.24Thermal conductivity of dies (N/(s K))28.4Heat capacity of workpiece (N/(mm 2K)) 2.43Heat capacity of dies (N/(mm 2K))4.0Heat-transfer coefficient (between workpiece and dies)(N/(s mm K))0.7Convective heat-transfer coefficient (N/(s mm K))0.0029Emissivity0.15Fig.4.Initial finite elementmesh.Fig.5.Stress±strain curves predicted in accordance with the model and experimental data at 4508C and 0.1s À1.358J.R.Cho et al./Journal of Materials Processing Technology 118(2001)356±361was more consistent with the experimental result than that by the strain and strain rate-hardening model.When the dynamic recrystallization proceeds partially during the hot forming,the region where it takes place and the region it does not should be treated properly.In the recrystallized region,the additional strain and strain recovery in each iteration step are considered.In the non-recrystallized region,however,the additional strain is added to the accu-mulated strain.If the accumulated strain exceeds the critical strain,the region is divided into separate substructures.TheFig.6.Load predictions by model equation and power equation using peakstress.Fig.7.Microstructures of 50%compressed specimens.J.R.Cho et al./Journal of Materials Processing Technology 118(2001)356±361359parison of measured recrystallized volume fraction and simulationresults.parison of measured grain size and simulationresults.Fig.10.Distribution of mean grain size.360J.R.Cho et al./Journal of Materials Processing Technology 118(2001)356±361mean strain considering the volume fraction and additional strain is calculated in the elements as the initial strain in the next step.The heat was transferred from the workpiece to the die rapidly in the case of a workpiece at3808C and the die at 2508C.Therefore,the temperature was not high enough to bring about recrystallization and the dynamically recrystal-lized structure was not found.The microstructure with the workpiece at4608C and the die at4208C is shown in Fig.7. It was found that recrystallization was very active in the center compared to regions further away in the radial direction.Thus,the recrystallized grain size was relatively small.The experimental and simulated results for recrys-tallization rate and grain size were compared in the radial and axial direction,respectively,in Figs.8and9.The experimental result in the center was rather larger than the simulated result.The difference can be attributed to non-homogeneous initial grain size and measuring error. Fig.10shows distribution of the calculated mean grain size.4.ConclusionThe dynamic recrystallization behavior of the Al±5wt.%Mg alloy was modeled using the hot-compression test.The dynamic recrystallization model was combined with a thermo-viscoplastic FEM to predict microstructure evolution during the hot-deformation process.1.The dynamic recrystallization was generated after the peak strain.The critical strain was numerically found based on the decrease of the softening process rate.2.The change of the dynamic recrystallization grain size was modeled based on the forming condition.3.A themo-viscoplastic finite element program was developed to predict microstructure evolution during the hot deformation.4.The hot-compression experiment was carried out to verify the usefulness of the program and the results agree with the simulated results.5.The dynamic recrystallized grain size increase with a decrease in the Zener±Hollomon parameter,i.e.,with high temperature and low strain rate.AcknowledgementsThe authors acknowledge with gratitude the®nancial support received from the overseas Post-doctoral Program of the Korea Science and Engineering Foundation(KOSEF). 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