New extrusion process of Mg alloy automobile wheels

Available online at www. sciencedirect.comTransactions of骂P心ScienceDirectNonferrous Metals孟ScienceSociety of ChinaEL SEVIER PressTrans. Noferous Met. Soc. China 20(2010) s599- s603www.tnmsc.cnNew extrusion process of Mg alloy automobile wheelsWANG Qiang(王强)', ZHANG Zhi-min(张治民) , ZHANG Xing(张), LI Guo-ju(李国俊)1. College of Materials Science and Engineering, North University of China, Taiyuan 03005 I, China2. Engineering Research Center of Magnesium-base Material Processing Technology, Ministry of Education,North University of China, Taiyuan 030051, ChinaReceived 23 September 2009; accepted 30 January 2010Abstract: The recent rescarch and development of forged magnesium road wheel were reviewed. Methods of flow-forming, spinforging of manufacturing a forged magnesium alloy wheel were introduced. A new extrusion method was investigated especially.Extrusion from hollow billet was proposed in order to enhance the strength of spoke portion and reduce the maximum forming load.By means of the developed technique, the one-piece Mg wheels were produced sucesully by extrusion ftom AZ80+ alloy. At thesame time, the existing problems on the research and development of forged magnesium road wheel were analyzed. The impacttesting, radial fatigue testing and bending fatigue testing results show that AZ80+ wheel can meet application requirement inautomobile industry.Key words: magnesium alloy; road wheel; extrusion; flow-forming; spin forgingForging is an altermative production technology for1 IntroductionMg aloy parts with high strength requirements[9]. Theproducts manufactured by this process are characterizedMagnesium alloy is promising structural light metalby fine-grained microstructure without pores andbecause of its low densities, good recyclical potential andimproved mechanical properties compared with castingabundant resources, which is expected to become aMg parts[10-12]. For magnesium road wheel forging,next- generation material[1- -4]. This material is receivedmethods of flow-forming and spin forging had beenspecial attention due to the renewed interest in energyinvestigated[13-14]. But heavy-duty press equipment isconservation. Its use is increasing in automobileused and the fatigue strength cannot meet applicationindustries as a replacement for aluminum and steel.requirement[13, 15]. To enbance the fatigue strength andMagnesium alloy road wheels have to be considered inreduce the forming load, a new extrusion process forparticular due to their beneficial effect such as safety,magnesium alloy wheels was introduced and investigatedcomfort and energy saving.As a safety-relatedespecially in this work. The one-piece AZ80+ alloycomponent, the essential factors in wheel applicationswheel was extruded successfully by means of theare the fatigue strength and the impact strength. At thedeveloped technique.same time, we should also take account of otherrequirements such as corrosion resistance and cost.2 Forging technique used for wheelIn order to have a widespread use of magnesiummanufacturingalloy wheels, casting and forging processes have beenadopted for wheel manufacturing. In the case of the2.1 Forging and flow-forming processmagnesium alloys, the manufacture of the entire wheelThe technique of forged Mg alloy wheels requiresonly by the casting (low pressure die casting, squeezethe following two process steps: forging the wheel disccasting，gravity casting)[5- -8] results in insufficientblank and flow-forming tbe rim[13]. The billets are madestrength of, for example, a rim portion which carries aby。中国煤化工are homogeizedtire, as well as casting quality(porosity, inclusions) oftenbefordthe forging anddoes not meet the requirements.flow-fHC N M H Geated up to suitableFoundation item: Project(50735005) supprted by the National Natural Science Foundation of ChinaCorresponding author: WANG Qiang; Tel: +86 351-3921398, +86 13834166948; Fax: +86 -351-3921778; E mail: ncustwangq@nuc.cdu.cns600WANG Qiang, et al/Trans. Nonferrous Met. Soc. China 20(2010) s599 -s603temperature to provide the required formability.reduction of the wheel disc section. Rotary fatigue testThe forging consists of three steps using hydraulicshows durability lifetime (84 000 cycles under 3 000press with different pressing powers. The first forgingN.m test load) is 8.5% that of the series aluminum wheel.operation with a high degree of reduction provides aHence, it is necessary to thicken the wheel sections forwheel blank. The final shape of the wheel disc is mainlyachieving the required lifetime. Thus the actual massachieved by the second forging. The forming process issaving potential is reduced to 10%- -15% in regard tocompleted by subsequent deburring operation as the lastaluminum wheel based on FEA analysis.step. The flow-forming operation itself runs in threesteps: splitting up the forged flange, flow. forming the2.2 Forging and spin forging processrim and calibrating the rim contour, as shown in Fig.1.The technique of forged Mg alloy wheels consistsThe flow-forming device consists of mandrel andof the three process steps[14]: forging wheel blank, spintailstock plate to be used for locking the wheel disc.forging rim and roll processing rim edge portion, asThree forming rollers are positioned approximately inshown in Fig2. AZ80 magnesium alloy casting was120° orientations: roller 1 splits up the flange, roller 2prepared by casting and was subsequently forged toand 3 flow-form the rim including calibration of the finalprovide a wheel blank of a shape substantially identicalrim contour.with that of the eventually manufactured wheel.The ZK30 alloy prototype wheel has beenAn automotive wheel is obtained by forging frommanufacured by using flow-forming technique. Its massthe wheel blank using forging machine with an upper dieis 6.8 kg representing 35% mass saving than forgedand a lower die. The average crystalline particle size ofaluminum wheel. High strength can be achieved in thethe material is reduced. Then the wheel is subjected to arim especially in flow direction due to the highsolution treatment and an artificial treatment. Upondeformation induced by flow forming process. However,completion of T6 treatment, spin forging is carried out.the properties and influence of flow direction on theWhile the wheel is rotated together with the mandrel andmaterial properties are reduced due to lower degree of the press member, a roll is pressed against a rim of the(2)a)(b(b)中国煤化工c)MYHCNMHGFig,1 Schematic diagram of flow-forming process: (a) ProcessFig.2 Schematic diagram of spin forging proces: (a) Blankstarting position; (b) Slitting up flange; (c) Flow-forming rimforging; (b) Spin forging; (C) Roll processingWANG Qiang, et a/Trans. Nonferrous Met. Soc. China 20(2010) s599- -8603s601wheel, so that the rim may be finished by a spinningtreatment. Finally, the wheel is subjected to a rollprocessing. A roll is pressed against an edge portion ofthe rim while the rotary platform together with the wheelis rotated, micronizing crystalline particle to improve theresistance to corrosion of the edge portion.r=0.5The physical properties of the wheel using spinMwIwtwr=0.6 |forging technique are considerably improved by thesynergistic effect of the crystalline particles and the T6r=d/d,=0.6treatment, especially the strength of a rim portion isr=afdenhanced. The technical parameters such as temperaturesswaging rate and average crystalline particle size are02080optimized. But, the related reports on fatigue strength,Reduction in height/%mass and application instance have not been seen yet.Fig.4 Calculational force- -displacement curves3 Extrusion technique used for wheelmanufacturingaverage positive stress on interface. At the same time, thehole is formed directly and cutting process is reduced for3.1 Extrusion from hollow billetthe parts with center hole such as wheel by extrusionThe maximum load was chosen as the extrusionfrom hollow billet.load during extrusion process. A new concept of hollowbillet was proposed and a method of extrusion from3.2 Extrusion process of wheelshollow billet was developed[16]. The schematic diagramThe extrusion technique of wheels[17] has beenof extrusion from hollow billet is shown in Fig.3.developed for magnesium alloy wheel manufacturing, asshown in Fig.5. A can is backward extruded from hollowr PunchDie777r BilletWorpiece-(aMandrelFig.3 Schematic diagram of extrusion from hollow billetTo investigate the press force during extrusion fromhollow billet, the finite element simulation was(b)conducted using an implicit FE code MSC/Autoforge.The data on AZ80 alloy flow stress as function of strain,strain rate and temperature established based oncompression experiment have to be introduced into theFE package. Fig.4 shows the calculational force一displacement curves during extrusion from hollow billet10777十777.at different n=d/d, where d is outside diameter of中国煤化工hollow billet; d is inner diameter of hollow billet; and dsis diameter of mandrel. The results show that formingYHCNMHGforce is decreased enormously during extrusion fromFig.5 Schematic diagram of extnusion process: (a) Extrudinghollow billet, because of the decrease of contact area andcan; (b) Forging front lip; (C) Expanding rim and back lips602WANG Qiang, et al/Trans. Nonferrous Met. Soc. China 20(2010) s599- s603billet made by upsetting and punching from as-cast alloy.Upon completion of extrusion, the front lip is forgeda)from can extruded. The rim contour and back lip arefinally achieved by expanding. The metal may flowinwards and outwards simultaneously during forging thefront lip, which may make the billet deformation atrelatively low forging force and the die cavity flldsufficiently. At the same time, the degree of reduction ofthe wheel disc section is enhanced. High strength can beachieved in the wheel disc section due to the highdeformation.3.3 Experimental resultsThe AZ80+ alloy was produced by continuous6casting method for the wheel. The billets were machinedin order to remove the efect of the surface layer of theingot and homogenized (385 C， 12 h) beforedeformation. Extrusion was conducted under isothermalconditions in which the die temperatures were varieddepending on the billet temperature. The billet waheated to a temperature within 320 to 380 C duringforming process. The oil-hydraulic press of capacity 12.5MN was employed and the average forming speed of aram was 10 mm/s. A graphite coating was used tolubricate billet and punch surface.c)The one-piece magnesium alloy wheels areproduced according to the present technique, as shown inFig.6. The forming force was decreased by extrusionfrom hollow billet. The (13x10) J and (14x6) J wheelswere extruded under 12.5 MN oil-hydraulic press. Thetechnology and dies designed have many otheradvantages such as better flling for flange, highproduction precision, low surface roughness anconvenient mould unloading for near-net shape forming.The manufacture costs are reduced due to using limitedcapacity of press equipment and less forming process.The mechanical properties are remarkably improvedfor AZ80+ alloy wheel extruded in the both rim and discFig.6 AZ80+ Mg alloy wheels extuded: (a) (14x6) J; (b)section. The ultimate tensile strength is 300 -320 MPa(13x10)]; (c) (13x8)Jand the elongation is above 10%. For example, the massof (14x6) J wheel is 5.2 kg representing 28% mass4 Conclusionssaving of aluminum wheel. The impact testing, radialfatigue testing and bending fatigue testing results in1) Methods of flow-forming and spin forging ofTable 1 show that the magnesium wheels can meetmanufacturing a forged magnesium alloy wheel areapplication requirement in automobile industry.introduced and the existing problems are analyzed. TheTable 1 Testing results of wheelsImpact testingBending fatigue testingRadial fatigue testingStrikerTireDropStriking TestingBenTestingRotation中国煤化工BScrewweight/ pressure/ beightmoment/cyclespeed/torsion/ pressure/kNkPamN-m)(rmin-'MYHCNMHGi (Nm)6.174196301185/65R14 3 5100°700121.95 18.3751050121.95 450.8WANG Qiang, et a/Trans. Nonferous Met. Soc. 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