The Latest Trends In Aluminum Alloy Sheets For Automotive .
The Latest Trends in Aluminum Alloy Sheets for Automotive BodyPanelsTakeo SAKURAI; Material & Process Research Section, Aluminum Sheets & Coils Department Moka Plant, Aluminum &Copper Company Automobiles are becoming heavier in order to satisfyrequirements such as safety (e.g., collision safety andpedestrian protection), drivability, comfort andlarger interior space. Meanwhile, automobiles haveto meet more stringent regulations against exhaustemissions, including CO2 and NOX emissions, toprotect the environment. The European Commission,for example, has announced a policy to reduce CO2emissions down to 140 g/km or less by 2008 and to130g/km or less by 2012. Japanese regulations toreduce CO2 emissions will also be tightened to a levelcomparable to that of the European Commission by2012. Thus, reducing fuel consumption is nowimperative for all the automobile manufacturers.Various measures are taken to reduce fuelconsumption including new power trains involvinghybrid systems or advanced diesel engines. Aboveall, automotive weight reduction is regarded as oneof the most effective means for decreasing fuelconsumption. One hundred kilograms of weightreduction improves fuel consumption by 1 km/L,according to the Japanese 10-15 mode measurements1).Automotive weight reduction has been achievedtraditionally by reducing thickness of steel-sheetbody panels by increasing the strength of the steels,however, this approach is limited because of theresultant reduction in panel stiffness. For this reason,lighter materials are being studied extensively toreplace the steels. Especially, aluminum alloys havestarted to be used for various automotive parts for itslow specific gravity (approximately 1/3 of steel).Now the use of aluminum alloy is not limited toconventional castings such as engine blocks, butextends to hoods, trunk lids, outer panels such as doorsand protection covers including heat insulators2)-5).The present paper describes the latest trends anddevelopmental status of aluminum alloy sheets forautomotive panels. In 1985, the Mazda RX-7 applied aluminum alloysheet for its body panel (hood) for the first time inJapan. Since then, aluminum panels have been usedmostly for sport cars and luxury sedans until early1990s. Honda Motor Co., Ltd. presented the allaluminum body NSX for the first time in the world.Subsequent collapse of bubble economy reduced theapplications of aluminum panels to new models,however, aluminum panels started to be used againin around 1999 when global environmental issuesbecame a major concern. lists the vehiclemodels in Japan using aluminum panels as of 2006.More recently, aluminum panels are being used fora wider variety of models, such as the Toyota Crown( ), Toyota Prius, Subaru Legacy, Nissan Fuga(Infiniti M35/M45, )6) and Nissan SkylineTable 1 Examples of aluminum closer panels for automobilesCar makerModelApplication partsCROWNHoodTOYOTACROWN MJHoodPRIUSHood, DAMITSUBISHILSHoodGSHoodSCHood, RoofISHoodCOPENHood, RoofFUGAHood, Door, Trunk-lidCIMAHood, Trunk-lidSKYLINEHoodSTAGEAHoodFairlady ZHoodLEGACYHood, Back-doorINPRESSAHoodFORESTERHoodRX-8Hood, Rear-doorROADSTERHood, Trunk-lidLEGENDHood, Trunk-lid, fr-fenderS2000HoodLANCHER Evo.Hood, Roof, Trunk-lidPAJEROHoodAUTORUNDERRoofSource：Japan Aluminum AssociationKOBELCO TECHNOLOGY REVIEW NO. 28 OCT. 200822
Source：Toyota Motor CorporationFig. 1 Example of application of aluminum hood tDoor：Al-sheetFig. 2 Example of application of aluminum body panel(NISSAN FUGA)6）(Infiniti G35). Many aluminum panels are used forhoods or bonnets. Recent models employ aluminumpanels for trunk lids, rear doors and roofs7), 8).In Europe, automotive use of aluminum raisedsharply between 2000 and 2002. Aluminum alloypanels are now used for more than 2.5 millionvehicles every year. The Renault Clio (sold as“Lutecia” in Japan) is a high volume model employingan aluminum alloy panel for its hood, and aboutforty thousand vehicles of the model are producedevery month. Other examples of high volume modelsinclude the Mercedes-Benz E-class and BMW 5series, both of which are produced in volumesgreater than twenty thousand a month. Furthermore,the uses of aluminum alloy panels in those vehiclesare expanding. The BMW 5 series, for example, hasan all-aluminum frame and panels for the structurein front of its A-pillar. Audi presented, as early as1992, an all-aluminum body car, the Audi A8, havinga space frame structure and is regarded as aforerunner in the use of aluminum panels forautomobiles. In 2006, after presenting the Audi A2and new Audi A8, the company started using ahybrid material structure for the new Audi TTmodel, in which steel sheets are partially used withan aluminum space frame and joined together. Theexpanding automotive use of aluminum in Europeseems relevant to the more stringent regulation onCO2 emissions which becomes compulsory in 2012.23KOBELCO TECHNOLOGY REVIEW NO. 28 OCT. 2008 shows the development history of aluminumalloy sheets for automotive panels9). In Japan, Al-Mgalloys with minor addition of Zn were first usedwhen aluminum alloy sheets started to be used forautomobiles in 1985. Around the same time in EUand USA, either 2000 series (Al-Cu) alloys or 6000series (Al-Mg-Si) alloys with minor addition of Cuwere used to serve the purpose. Currently, as therequirements for automotive panels has diversified,6000 series (Al-Mg-Si) alloys are predominantly usedworldwide with a few exceptions of 5000 series (AlMg) alloys. This is because the requirements for thematerial properties now include not only strengthand formability, but also corrosion resistance,hemming formability, surface characteristics andweldability. presents the compositions and propertiesof typical aluminum alloys used for automotivepanels8). The 6000 series alloys constitute themainstream of alloys used for automotive panelsbecause they are heat-treatable and their strengthscan easily be controlled by heat treatments. Inaddition, the 6000 series alloys may also be adaptedto exhibit bake hardening which occurs during thebaking step after painting in the automotivemanufacturing process. The bake hardening featuresof the alloys improve dent resistance and enablesreduction of the sheet thickness and weight. The bakehardenability depends on the baking conditionswhich differ slightly by countries. The bakingprocess is treated at higher temperatures, around 180to 200 C, in EU and USA, while the treatments atlower temperatures for short period (e.g.,170 C x 20min) are preferred in Japan. The alloys for outerpanels are required to harden under the bakingconditions described above, however, conventional6000 series alloys were unable to harden sufficientlyunder such conditions. Extensive studies10)-12) revealedthat pre-aging steps as shown in provides bakeTable 2 Development history of aluminum sheets forautomotive panelsJAPAN1985〜19901990〜1998 Al-Mg Series alloyHigh strength &High formability Al-Mg Series alloyAA5022, AA5023 Al-Cu Series alloyAA2036, AA2008 A 0R1.0140150160Proof stress (MPa)90 Bend radius 0.590 Bend radius 1.090 Bend radius 1.590 Bend radius 2.0170180Fig. 6 Effects of proof stress and 90 bend radius on flathemming formability in 6000 sieres alloyswith a small bending radius and/or thin outer panelthickness. To prevent such cracking, much effort hasbeen spent for improving the bendability. Thecracking is reported to occur due to relatively largeprecipitate existing in the crystal grain and/or at thegrain boundaries17). Also, bendability is reported todeteriorate when shear zones and second phaseparticles are formed in a matrix, or when the amountof the shear band and the second phase particles islarge18). In other words, bendability is considered beimproved by adjusting material properties throughcontrolling of the second phase which precipitatesduring heat treatment. Another approach forimproving bendability relates to processing method.The alloy sheets of 6000 series can be bent withoutcracking by increasing the bend radius for the downflange (90 bending) step as shown in 19).Many panel products in Europe are mechanicallyjoined together after down flange step without beingbent 180 , however, flat hems as produced in Japanare being introduced to improve aesthetics. Aluminum alloy sheets are less formable than steelsheets because of their low elongations, n-values, rvalues and modulus of elasticity. A shape that couldbe formed by steel sheet may not be formed byaluminum alloys without causing cracking, wrinkling,or spring back. Thus, aluminum alloy sheets mayimpose restrictions on the design of shapes with theirlower degree of freedom in forming.When compared to steel sheets, aluminum alloysheets exhibit proof stress and tensile strengthsalmost equivalent to those of mild-steel, however,their elongations are much smaller. showsstress-strain curves of typical aluminum alloy sheets(5000 and 6000 series) used for automotive panels.Also included is the curve for a mild steel. As can beseen from the figure, the aluminum alloys havesignificantly smaller elongations after maximum25KOBELCO TECHNOLOGY REVIEW NO. 28 OCT. 2008 depicts the relation between the LDH0 (LDH：Limit Drawing Height) and tensile strength foraluminum alloys and steels. The LDH0 is defined asthe height at which a fracture occurs when arectangular blank is press-formed using a sphericalhead punch. The vicinity of the fractured portion is ina plane-strain region. The results indicate that thestretchability of aluminum alloy sheets is inferior tothat of steel sheets. shows the results of cylindrical drawing tests350300(MPa)R2.0Normal stress22502001501006000 series Alloy5000 series AlloyMild steel5000102030Normal strain (％)4050Fig. 7 Stress-strain curves of aluminum alloys and mild steel50SteelAluminum alloy40Mild steelSteel(mm)1loads are reached (the elongations called “localelongations”) compared to the steel. The difference inlocal elongation is considered to be the cause of thedifferent formabilitites between aluminum alloysand steel. The following describes the measurestaken to improve the formabilitites of aluminumalloy sheets.LDHoPoor Hem levels Good030206000 series alloy5000 series alloy3000 series alloy100200400Tensile strength (MPa)600Fig. 8 Relationship between LDH0 and tensile strength oftypical aluminum sheet and mild steel
1.860.676000 series alloy-1.0t1.810.65Mild steel-0.8t2.150.97as represented by limit drawing ratio (LDR) anddrawing height, measured for sheets of aluminumalloys and a mild steel. The LDRs for the aluminumalloys are smaller than the value for the mild steel.The drawing heights of the aluminum sheets areabout 70 % of that of the steel. shows the results of conical cup forming testsperformed on the sheets of aluminum alloys and amild steel. When low blank holding forces (BHFs) areapplied, the aluminum alloy sheets exhibit wrinklingbehaviors similar to that of the mild steel. However,higher BHFs, such as applied to steels, cause aluminumalloy sheets to crack. This limits the forming ofaluminum alloy sheets compared to steel sheets.Some measures against wrinkling and crackingshould be taken for the forming of aluminum sheet,because their modulus of elasticity is rather small(1/3 of steel) which makes them more prone tobuckling. In addition, as indicated by their low rvalues, aluminum alloys tend to fracture more easilyonce local deformations occur20).Because of the issues unique to aluminum sheets,the forming technologies of steel cannot be appliedas-is to the forming of aluminum sheet in most cases.Thus, it is essential to develop technologies designedto form aluminum alloy sheets. The press forming ofa 6000 series alloy, for example, involves optimizingthe die shape for the alloy, increasing the accuraciesof the die contact face and lowering the bead height(1/2 of steel). A BHF control method is reported to beeffective in improving dimensional accuracies of theshapes formed21).The temperature dependence of superplasticdeformation is also utilized in the forming ofaluminum alloy sheets. Some aluminum alloys haveunique characteristics of exhibiting significantlylarge elongations at temperatures either higher orlower than room temperature. The characteristicsmay be exploited in the forming of aluminum alloys. depicts the temperature dependence of themechanical properties of a 5000 series (Al-Mg) alloy.Hot-blow forming utilizes the large elongation of807060breakage504030wrinkle205000 series alloy6000 series alloymild steel1000100200BHF300(kN)Fig. 9 Relationship between forming limit height and BHF ofconical cup forming(punch dianeter：100mm, die inner 4030030TS20YSElongation (％)5000 series alloy-1.0t(mm)Drawing height / Forming heightDrawing ratio(LDR)(MPa)Alloyaluminum alloy sheet at elevated temperatures. Themethod is practically used in forming complexshapes which may be difficult to form even with steelsheets. However, the approach takes long processtime and its application has been limited. Recentdevelopment has shortened the process time of hotblow forming. Honda Motor Co., Ltd., adapted theprocess to the forming of front fenders and trunk lidsfor the Honda Legend22). On the other hand, a lowtemperature forming technology has also beendeveloped utilizing the large elongation at lowertemperatures, a unique characteristics of aluminumalloy.Since aluminum alloys have smaller localelongation compared to steel, aluminum sheets tendto break more easily when local deformations occurduring press forming. Solid lubricant serves toimprove stretch flangeability and drawing formabilityof aluminum alloy sheets. Various, commerciallyavailable, lubricant of wax and dry film are appliedto aluminum sheet in Europe to form automotivepanels23).TS, YSTable 4 Comparison of drawing formability of aluminumsheet and mild steel (die inner diameter：100mm)101000 200 1000100200300Forming temperature ( )4000500Fig.10 Effects of forming temperature on mechanicalproperties in 5000 series alloysKOBELCO TECHNOLOGY REVIEW NO. 28 OCT. 200826
Table 5 Chemical compositions and mechanical properties of aluminum alloys for automotive structual partsChemical compositions (wt％)Mechanical A51820.250.35TiTSYS(MPa) (MPa)El.(％)CorrosionWeldability 50.10 ��0.200.250.20240110
23 KOBELCO TECHNOLOGY REVIEW NO. 28 OCT. 2008 (Infiniti G35). Many aluminum panels are used for hoods or bonnets. Recent models employ aluminum panels for trunk lids, rear doors and roofs7), 8). In Europe, automotive use of aluminum raised sharply between 2000 and 2002. Aluminum alloy panels
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