1.Introduction The application of aluminum and aluminum alloys is significantly increasing due to thegood formability of the materials and high strength which can be obtained in the finalproduct of forging<川.Examples of applications are tubes,cans for electronic housings andelectromotors,oil filters,pump housings,hydraulic cylinders,high pressure cylinders,shockabsorbers,bicycle components,automotive transmission and power steering components,light reflectors and components for defence. At present aluminum alloys are mainly processed by cold forging.At room temperature,the forming loads and the flow stress are very high.If the reduction in height is toolarge,the shear cracks appear(Fig.1).Forging of high carbon and alloy steels at elevatedtemperatures is widely used since it offers the advantages of a reduction in flow stress.anFig.1 Compressed billets in upsetting test at room temperature(A201 1): (a)barrel shape Ah/ho=41.25%,(b)crack Ah/ho=46.2%,(C)crack Ah/ho=51.17%,(d)crack Ah/ho=60.62%,(e)crack Ah/ho=69.11%.. 111-.increase in ductility,a reduction in work hardening and an increase in toughness of theforged parts when compared with cold forging<2J. In the present study,the relations of the flow stress of aluminum alloys with the tem。perature and strain rate are analyzed,using the upsetting test experiment data.A com—prehensive formula for the flow stress of the alloys at elevated temperatures is proposedwith work softening factor in a simple form.2.Upsetting Test In order to clarify the workability of alu—minum alloys,the upsettability test<3J is car—ried out.In the test.a cylindrical billet iscompressed with concentrically grooved toolsthat restrict the end surfaces of the billet(Fig.2). Flow stress is obtained from theload—stroke data of the upsettabliity test.Aformula converting the measured load andreduction in height into the average flowstress and equivalent strain is derived fromthe rigid—plastic finite element simulation<3,引.This test is performed under practical form-ing conditions and is free from the effect offrictional variation over the interface betweenA 120~.哭◇F≮葑Ⅶ甚 b坠j叫Fig.2 Schematic illustration of upsettability testthe workpiece and the tools. The experimental conditions for upsettability test are as follows.The diameter andheight of the initial cylindrical billet are 8.5 and 10.5mm,respectively.The testing tem.perature is chosen to be between room temperature and 400。C.The up and lower groovedtools are also heated to about 200。C to prevent the heated billet from rapid cooling、on thet001.The reduction in height is 10%～70%,and the mean strain rate ranges from 5—20s—1.The test is conducted on a mechanical press with.a maximum capacity of 600kN.In thetest,the billet is heated in a muffle furnace. The material used in the test is an aluminum alloy,、A201 1.The chemical compositionsof the aluminum alloy are Si0.09,Fle0.16,Cu5.68,Pb0.54,BiO.56,Ti0.02 and Al bal.Thespecimen is heated to 4100C and kept for 2h,then cooled down with the speed 30℃/h to2000C before testing.3.Influences of Flow Stress7.1 Influence of temperature on flow stress A keY parameter in the warm forming of aluminum alloys is the forming temperature.Fig.3 display the flow curves of the aluminum billet material A2011 at difierent temper.atures,determined in the upsetting test.From Fig.3,at higher temperatures the flowstress decreases with increasing straining.because of the thermal activated work softening.We use the stress data at the temperature of 200。C and 400。C in this study,for in thistemperature region the stress decreases monotonically with increased temperature.. 112 .StrainStrainFig.3 Temperature dependent flow curves of A2011:(a)套=5s一’,(b)~=20s一’了.2 Influence o.f strain rate On flow stress For the determination of strain rate sen-sitivity of aluminum alloys,upsetting test attwo dimjrent velocities has been carried out.Fig.4 show the influence of testing velocity onflow stress for the billet material of A201 1 ata temperature of T=400
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