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j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 7 ( 2 0 0 8 ) 1–12

journa l homepage: www.e lsev ier .com/ locate / jmatprotec

eview

eview of warm forming of aluminum–magnesium alloys

erkan Toros, Fahrettin Ozturk ∗, Ilyas Kacarepartment of Mechanical Engineering, Nigde University, 51245 Nigde, Turkey

r t i c l e i n f o

rticle history:

eceived 31 October 2007

eceived in revised form

1 March 2008

ccepted 31 March 2008

eywords:

arm forming

luminum–magnesium (Al–Mg)

lloys

XXX series

a b s t r a c t

Aluminum–magnesium (Al–Mg) alloys (5000 series) are desirable for the automotive industry

due to their excellent high-strength to weight ratio, corrosion resistance, and weldability.

However, the formability and the surface quality of the final product of these alloys are

not good if processing is performed at room temperature. Numerous studies have been

conducted on these alloys to make their use possible as automotive body materials. Recent

results show that the formability of these alloys is increased at temperature range from 200

to 300 ◦C and better surface quality of the final product has been achieved. The purpose of

this paper is to review and discuss recent developments on warm forming of Al–Mg alloys.

© 2008 Elsevier B.V. All rights reserved.

ontents

1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12. Aluminum for passenger vehicles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23. Formability of aluminum–magnesium sheets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3.1. The effects of blankholder force and drawbead geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53.2. The effects of temperatures and strain rates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63.3. The effects of lubrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

4. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . .

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. Introduction

luminum alloys are produced and used in many forms suchs casting, sheet, plate, bar, rod, channels and forgings inarious areas of industry and especially in the aerospace

∗ Corresponding author. Tel.: +90 388 225 2254.E-mail address: fahrettin@nigde.edu.tr (F. Ozturk).

924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.jmatprotec.2008.03.057

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

industry. The advantages of these alloys are lightweight,

corrosion resistance, and very good thermal and electricalconductivity. The aforementioned factors plus the fact thatsome of these alloys can be formed in a soft condition andheat treated to a temper comparable to structural steel make

s i n g t e c h n o l o g y 2 0 7 ( 2 0 0 8 ) 1–12

Fig. 1 – Average use of aluminum (International Aluminum

which replaces 2 kg of steel, can lead to a net reduction of10 kg of CO2 equivalents over the average lifetime of a vehi-cle (Ungureanu et al., 2007). In Fig. 3, the effects of the carcomponents on CO2 emissions are shown. CO2 emission is

2 j o u r n a l o f m a t e r i a l s p r o c e s

it very attractive for fabricating various aircraft and missileparts. The present system utilized to identify aluminumalloys is the four digit designation system. The major alloyelement for each type is indicated by the first digit, i.e., 1XXXindicates aluminum of 99.00% minimum; 2XXX indicatesthat copper is the main alloying element. Manganese for3XXX, silicon for 4XXX, magnesium for 5XXX, magnesiumand silicon for 6XXX, zinc for 7XXX, lithium for 8XXX, andunused series for 9XXX are main alloying elements.

In industry, low carbon steels have been commonly usedfor a long time due to their excellent formability at roomtemperature, strength, good surface finish, and low cost. How-ever application of the aluminum and its alloys in this fieldwere ranked far behind steels because of cost and formabilityissues, despite their high-strength-to-weight ratio and excel-lent corrosion resistance. For expanding use of aluminumalloys or replacing steels in many areas, however, there havebeen challenging formability problems for aluminum alloysto overcome. The formability of the aluminum alloys at roomtemperatures is generally lower than at both cryogenic andelevated temperatures. At cryogenic temperatures, the ten-sile elongation is significantly increased for many aluminumalloys especially 5XXX series alloys and is related to theenhancement of work hardening, while at elevated temper-atures it is mainly due to the increased strain rate hardening.Forming at cryogenic temperatures is technologically morechallenging than at high temperatures. At hot forming tem-peratures, other issues should also be taken into considerationsuch as creep mechanisms which may affect the formingdeformation and cavitations at grain boundaries which mayinduce premature failure at low strain rates.

2. Aluminum for passenger vehicles

Lightweight vehicles have become a key target for car man-ufacturers due to increasing concerns about minimizingenvironmental impact and maximizing fuel economy withoutsacrificing the vehicle performance, comfort, and marketabil-ity (Cole and Sherman, 1995). Aluminum will probably playan important role in the future car generations. Its materialproperties give some advantages and open the way for newapplications in the automotive industry (Carle and Blount,1999). As a result of the developments in the aluminum indus-try, improving the mechanical properties of the aluminumalloys by adding various alloying elements increased theapplication area of these alloys in automotive and aerospaceindustries (Richards, 1900). Design of aluminum structurescan also have a big influence on the sustainability of a car.Some of the important design aspects of a car which influencethe environment are weight, aerodynamic and roll-resistance.DHV Environment and Transportation Final Report indicatesthat the material has a big influence on the car weight.(DHV Environment and Transportation Final Report, 2005).Lightweight car consumes less material resources in the longrun (300,000 km), although it would cost about 30% more

than the conventional car. Therefore, its production woulddecrease employment in the car industry by about 4% overa decade while increasing the employment in the short term(Fuhrmann, 1979).

Institute (IAI), 2002; Martchek, 2006; Mildenberger andKhare, 2000; Schwarz et al., 2001).

Aluminum alloys are effective materials for the reductionof vehicle weight and are expanding their applications. Fig. 1illustrates the usage of aluminum for European and Ameri-can vehicles over years. In addition to USA and Europe, Japanhas recently increased their aluminum alloy usage. Analystsexpect that the aluminum alloys usage in Japan AutomotiveIndustry will reach 1.5 million tons by 2010. Assuming vehi-cle production holds steady at around 10 million units, theaverage yearly growth will be around 2.5% (McCormick, 2002).As shown in Fig. 1, the amount of aluminum used in 1960is substantially low. The main reasons are forming difficul-ties of aluminum alloys at that time and the smaller rangeof alloys available. The demand for aluminum alloys as lightweight materials has increased in recent years. Fig. 2 demon-strates the amount of produced aluminum products in theworld.

In the past, the main aluminum products were producedby casting such as engines, wheels, exhaust decor; how-ever nowadays wrought aluminum products are finding moreapplications in sheets including exterior panels such as hoodsand heat insulators, in extrusions including bumper beams,and in forgings including suspension parts Fig. 2.

One of the most important benefits of using aluminumalloys in automotive industry is that every kg of aluminum,

Fig. 2 – Aluminum products for automobile over years (Coleand Sherman, 1995; Inaba et al., 2005; Patterson, 1980;Miller et al., 2000; Turkish Statistical Institute, 2004).

j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 7 ( 2 0 0 8 ) 1–12 3

Fig. 3 – Effect of technical measures on the CO2 emission (Mordike and Ebert, 2001).

Table 1 – Saving in fuel consumption (Mordike and Ebert, 2001)

Measure taken Potential saving (%) fuel Importance innovative materials

Short-medium term Long term

Light constructions 3–5 10–15 ++Cw value 2 4–6 +Motor\gear control 5 10 ±

c(tabtnteer

irtaetaga

Resistance to rolling 1–2Motor preheating 2Equipment 2

ritical in terms of environmental pollution. Schwarz et al.2001) inspected that the relationship between the usage ofhe aluminum products in new designs and the CO2 emissionnd emphasized that the CO2 emission ratio could be reducedy using lightweight materials such as aluminum in newransportation designs. Weight reduction of the car’s compo-ents influences fuel consumption considerably. In Table 1,

he effect of weight reduction on fuel savings is seen. Fuelconomy improvements of around 6–8% or as much as 2.5xtra miles per gallon can be realized for every 10% in weighteduction (Mordike and Ebert, 2001).

Recyclability of alloys has also become an important issuen view of energy and resource conservation. For example,ecycling potential of the aluminum products is much bet-er than the ferrous metals. Martchek (2006) and Mildenbergernd Khare (2000) investigated the recycling potential and nec-ssary energy to reproduce the aluminum products. According

o Martchek (2006), increasing the recycled metal usage in theluminum production consumes less energy and emits lessreenhouse gas to produce the aluminum ingots. Sillekens etl. (1997) investigated the formability of recycled aluminum

Table 2 – Comparison of several Al–Mg alloys

Strength Formability

Excellent 5454, 5652 –Highest 5052 –High 5456 –Good 5154, 5254 5005, 5050, 5083

3 +4–6 ±4 ±

alloy 5017. In their study, they focused on changes in theamounts of alloying elements (particularly iron) to see howthey affect the formability of products. It is observed that thechange in the iron content does not lead to a dramatic degen-eration in the performance of the material.

Aluminum alloy sheets are widely used in the car, ship-building and aerospace industries as substitutes for steelsheets and fiber reinforced plastic (FRP) panels, due to theirexcellent properties such as high-strength, corrosion resis-tance, and weldability (Naka et al., 2001). The features ofthe most used aluminum–magnesium alloys in automotiveapplication were summarized in Table 2. Figs. 4 and 5 illus-trates aluminum and other materials usages in automotiveand aerospace industry, respectively.

Magnesium is one of the most effective and widely usedalloying elements for aluminum, and is the principal elementin the 5XXX series alloys. These alloys often contain small

additions of transition elements such as chromium or man-ganese, and less frequently zirconium to control the grain orsubgrain structure and iron and silicon impurities that areusually present in the form of intermetallic particles (ASM

Resistance to corrosion Weldability

– 5454, 5652– –5456 5083, 54565005, 5050, 5083, 5254, 5652 5154, 5254, 5557

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Table 3 – Properties of Al–Mg alloys in automotive structures and other materials (IMUA, 2006; Wendt and Weiß, 2004; Beer and Johnston, 1992; Talbot and Talbot, 1998;Material Property Data, 1996–2007; Shernaz, 1991; Boyd et al., 1995)

Material � (kg/m3) �a (MPa) Strenght/density(Pa/(kg m3))

E (GPa) G (GPa) CTE (20 ◦C) εb Hardness Applications inautomotive

Prices

Aluminium5005 2700 124 45925, 9 68, 9 25, 9 23, 8 25 BS: 28 Trim, nameplates,

appliques

1486

5052 2680 193 72014, 9 70, 3 25, 9 23, 8 25 BS: 47 Interior panels andcomponents, truckbumpers and body panels

5182 2650 275 103773, 6 69, 6 26 23, 9 21 BS: 74 Inner body panels, splashguards, heat shields, aircleaner trays and covers,structural and weldableparts, load floors (sheet)

5252 2670 180 67455, 7 69 26 23, 8 23 BS: 46 Trim5454 2690 248 92193, 3 70, 3 26 23, 6 22 BS: 62 Various components,

wheels, engine accessorybrackets and mounts,welded structures (i.e.dump bodies, tank trucks,trailer tanks)

5457 2690 131 48698, 9 68, 9 26 23, 8 22 BS: 32 Trim5657 2690 110 40892, 2 69 26 23, 8 25 BS: 28 Trim5754 2670 230 86142, 3 68 22.6 HV: 55 Inner body panels, splash

guards, heat shields, aircleaner trays and covers,structural and weldableparts, load floors

MagnesiumAZ80A-F 1800 340 188888, 8 45 17 26 7 BS: 67 Headlight housing, wheels

and tires

1200

AZ31B-F 1770 260 146892, 7 45 17 26 15 BS: 49 Seats, passenger restraintsinstruments and controls,case of seat belt

AZ91D-F 1810 230 127071, 8 45 17 26 3 BS: 63 Treadle of BicycleAM50A-F 1770 228 128813, 5 45 17 26 15 BS: 60 Exhaust decor, exhaust

systemAM60B-F 1800 241 133888, 9 45 17 26 13 BS: 65 transmission or transaxle,

clutch (if manual), driveline (rear-wheel drive)

WE54-T6 1850 280 151351, 4 45 17 26 4 BS: 85 Differential, transfer casesubframes, engine block

ZK60A-F 1830 340 185792, 3 45 17 26 11 BS: 75 Fuel storage system

PlasticsNylon 6/6 1140 75 65789, 5 2, 8 – 144 50 BS: 95 320

j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 7 ( 2 0 0 8 ) 1–12 5

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Fig. 4 – Al alloys and its application for automotive industry(Sherman, 2000; White, 2006).

Fig. 5 – Current material usages for Boing 757 (Moscovitch,2005).

Metal Handbook, 1988). When magnesium is used as the majoralloying element or combined with manganese, the result isa moderate to high-strength, non-heat-treatable alloy. Alloysin this series are readily weldable and have excellent resis-tance to corrosion, even in marine applications. Selection ofsuitable aluminum alloys, for several applications, requires abasic knowledge of heat treatment, corrosion resistance, andprimarily, mechanical properties. Table 3 summarizes featuresand applications of Al–Mg alloys. Three different materialgroups, their properties and applications were compared formaterial selection.

3. Formability of aluminum–magnesiumsheets

3.1. The effects of blankholder force and drawbeadgeometry

Typical sheet metal forming processes are bending, deepdrawing, and stretching. If a doubly curved product must bemade from a metal sheet, the deep drawing process or thestretching process is used. The deep drawing process canreach production cycles of less than 10 s, and is hence asuitable process for mass production. In deep drawing andstretching, the stresses normal to the sheet are usually verysmall compared to the in-plane stresses and are thereforeneglected. Two important failure modes limit the applicabil-ity of the deep drawing and stretching process: necking and

wrinkling. Both are closely related to the material properties.The ability to accurately predict the occurrence of wrinklingis critical in the design of tooling and processing parame-ters (Xi and Jian, 2000) like sheet thickness, blankholder force,

s i n g t e c h n o l o g y 2 0 7 ( 2 0 0 8 ) 1–12

Fig. 6 – A typical warm forming set-up (Palumbo and

6 j o u r n a l o f m a t e r i a l s p r o c e s

and the local curvature of the sheet (Hutchinson and Neale,1985). Ahmetoglu et al. (1997) determined wrinkling and frac-ture limits and developed blank holding force (BHF) controlto eliminate these defects, improve part quality and increasethe formability. A computer simulation model was developedto control aforesaid parameters. Jinta et al. (2000) examinedwrinkle behavior in 5XXX and 6XXX series aluminum alloysand compared the results with wrinkle behavior of steel. Theirresults indicate that aluminum alloys generally forms morewrinkles then steel, especially 6XXX series aluminum alloyhas a tendency to more wrinkles than 5XXX series. Gavas andIzciler (2007) examined the effect of blank holder gap (BHG)on deep drawing of square cup in order to investigate wrin-kling, tearing, and thickness distribution. As a result of theirstudy, they observed that increasing of the BHG allows morematerial to be drawn into the die cavity without tearing orshape distortions. It is also noticed that it was impossible touse too large BHG because of excessive wrinkling and buck-ling at the straight sides which cause tearing. Lin et al. (2007)determined the drawing limit under constant and variableBHF. Drawbeads are directly related with wrinkling behavior ofthe materials. They are used to control the flow of sheet metalinto the die cavity during the stretch forming of large panels.Beside that they reduce the BHF and minimize the blank sizeneeded to make a part (Demeri, 1993). The shape and positionof drawbead and the amount of force on it are very impor-tant in terms of part quality. Samuel (2002) investigated theinfluence of drawbead geometry on the drawbead restrainingforce (DBRF) and BHF numerically and experimentally for alu-minum alloy. In his study, two kinds of drawbead geometrywhich are square female and round female were investigated.As a result, it is obtained that the DBRF and BHF for the squarefemale bead are higher than those for the rounded femalebead. He emphasized that this discrepancy are occurred dueto the sharp corners. It is also observed that the total equiva-lent plastic strain and von Mises stresses at upper and lowersurfaces of square female drawbead are higher than those forthe round female drawbead.

3.2. The effects of temperatures and strain rates

Although the aluminum alloys have high-strength to weightratio and good corrosion resistance, the low formability ofaluminum sheets limits their use in some products withcomplex shapes, such as automotive body parts. The warmforming process is intended to overcome this problem byusing an elevated forming temperature which is below therecrystallization temperature (Tebbe and Kridli, 2004). A typ-ical warm forming experimental set-up is shown in Fig. 6. Inthe warm forming set-up, dies and blank holders are heated to200–300 ◦C. In order to heat dies and blank holders, electricalheating rods that are located in these parts are used but thereis a risk of necking during heating and cooling. Warm form-ing was studied for many years, e.g. in the 1970s and 1980s byShehata et al. (1978) and Wilson (1988) with increasing atten-tion being dedicated to the subject in the last decade.

The warm forming method improves the formability ofthe aluminum alloys. This improvement at the elevated tem-peratures is principal for the aluminum alloys such as 5082and 5005 alloys due to the increased strain rate hardening

Tricarico, 2007).

(Shehata et al., 1978). Schmoeckel (1994) and Schmoeckel etal. (1995) investigated the drawability of 5XXX series alloysat the elevated temperatures. Temperature has a significantinfluence on the stamping process. Further investigation onforming showed that the formability with a partial heating inthe holder or matrices area was much better when comparedwith the homogeneously heated tools (Schmoeckel, 1994).Schmoeckel et al. (1995) showed that a significant increasein the limiting drawing ratio (LDR) for the aluminum alloyAlMg4.5 Mn0.4 can be achieved by a heated and lower strainrated hydromechanical stamp. Modeling of the deep draw-ing with a rotationally symmetrical tool (stamp diameter:100 mm) which was cooled from the stamp side by additionalair ensured an increase in LDR.

It was demonstrated that the formability is improved bya uniform temperature increase, but the best results areobtained by applying temperature gradients. The formabil-ity depends strongly on the composition of the aluminumalloy. Aluminum–magnesium alloys have a relatively goodformability. A disadvantage is that these alloys suffer fromstretcher lines, which gives an uneven surface after deforma-tion. Because of this reason, 5XXX series aluminum is usedfor inner panels of vehicles. These undesired surface defectscan be eliminated by the forming processes at the elevatedtemperatures (Van Den Boogaard et al., 2001). The aluminumwhich contains 6% magnesium could give a 300% total elonga-tion at about 250 ◦C, finds more application in industry (Altan,2002). Yamashita et al. (2007) numerically simulated circu-lar cups drawing process by using Maslennikov’s technique(Maslennikov, 1957) which is also called “punchless drawing”.In this production technique, a rubber ring is used insteadof the rigid punch. Browne and Battikha (1995) optimized theformability process by using a flexible die and optimized theprocess parameters to ensure a defect-free product.

To accurately simulate warm forming of aluminum sheet,a material model is required that incorporates the temper-ature and strain-rate dependency (Van Den Boogaard andHuetink, 2004). Because of this, the effect of temperature

distribution on warm forming performance is very impor-tant. Van Den Boogaard and Huetink (2006) observed thatthe formability of the Al–Mg alloy sheets can be improved byincreasing the temperature in some parts of the sheet and

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ooling the other parts when simulated by the cylindrical cupeep drawing at the different temperature gradients of theools and the blanks. Chen et al. (2006) investigated combinedsothermal/non-isothermal finite element analysis (FEA) withesign of experiments tools to predict appropriate warm form-

ng temperature conditions for 5083-O (Al–Mg) sheet metallanks, deep drawing and two-dimensional stamping cases.o achieve increased degrees of forming, different tempera-ure levels should be assigned to the corner and body of theie and punch. 25–250% elongation ranges were seen. Theyound that the formability of Al-5083 alloy is greatly depen-ent on the temperature distribution of the die and punch.

t is also observed that the optimal temperature distributionsor warm deep drawing and warm two-dimensional stampingere not identical.

Naka et al. (2003) investigated the effects of temperaturen yield locus for 5083 aluminum alloy sheet. In their study,hey have tried to determine the optimum condition of pressorming for an aluminum alloy sheet, the effect of form-ng temperature on the yield locus. They obtained the yieldocus for a fine grain Al–Mg alloy (5083-O) sheet by performingiaxial tensile tests, using cruciform specimens, at temper-tures of 30, 100, 170, 250, and 300 ◦C at 10 s−1 strain rate.s expected, the size of the yield locus drastically decreasedith increasing temperature. This can be exploited to improveress operations. Naka and Yoshida (1999) investigated theffects of temperature and forming rates on deep drawabil-ty of 5083 Al–Mg alloy. In their study, different temperatureradients from room temperature to 180 ◦C and the form-ng speeds between 0.2 and 500 mm/min were performed.esults show that LDR increases mostly with increasing theie temperature because the deformation resistance in flangehrinkage decreases with an increase in temperature. Besidehat the LDR becomes smaller with increasing the formingpeed at all temperatures since the flow stress of the heatedlank (at the flange) increases with increasing the strain rate.oreover, the cooled blank at the punch corner becomes less

uctile. Another comprehensive study for 5XXX series alu-inum alloys was done by Bolt et al. (2001). In that study,

he formability was compared for 1050, 5754 and 6016 typeluminum alloys from 100 to 250 ◦C by using the both boxhaped and conical rectangular products. They observed thathe minimum die temperature of 6016 alloy on the die pro-ess limits was lower than that of the 5754 alloy. Smerda etl. (2005) investigated the strain rate sensitivity of 5754 and182 type aluminum alloy sheets at room temperature andlevated temperatures. In their study, the split Hopkinson barpparatus was used to identify the constitutive response andhe damage evolution in the aluminum alloys at high strainates of 600, 1100 and 1500 s−1. It was observed that the qua-istatic and dynamic stress strain responses in the range oftrain rates and temperatures were low for both alloys. AA5754xhibited a mild increase in flow stress with strain rate, whileA5182 appeared to be strain rate insensitive. The ductilityf the materials showed little differences in the tempera-ure range between 23 and 150 ◦C at a strain rate of 1500 s−1.

owever, the final elongation was decreased for both alu-inum alloys at 300 ◦C and a strain rate of 1500 s−1 when

ompared to that at lower temperatures. Picu et al. (2005)nvestigated the mechanical behavior of the commercial alu-

t e c h n o l o g y 2 0 7 ( 2 0 0 8 ) 1–12 7

minum alloy AA5182-O. The dynamic strain aging effect wasobserved at all temperature between −80 and 110 ◦C and atstrain rates lower than 10−1 s−1. In addition, the strain ratesensitivity parameter was also determined as a function oftemperature and plastic strain. Abedrabbo et al. (2007) devel-oped a temperature-dependent anisotropic material model forFEA and formability simulation for two automotive aluminumalloys, AA5182-O and AA5754-O. Multiple temperatures tosimulate the formability of more complex automotive parts,where the temperatures of the different sections will be deter-mined automatically, can be found by this model. In additionto the temperature, the forming speed controlling the strainrate, the die and stamp corner radii and other geometricparameters of the die set-up determine the forming charac-teristics of aluminum alloy sheets.

In the finite element simulations, material models are quiteimportant in order to evaluate accurately the formability ofaluminum alloy sheets. Barlat models are commonly used todefine aluminum alloy behaviors.

Barlat and Lian (1989) developed a yield function thatdescribed the behavior of orthortropic sheets and metalsexhibiting a planar anisotropy and subjected to plane stressconditions. This yield function showed similar results cal-culated by the Taylor/Bishop and Hill models. Barlat et al.(1991) extended this method to triaxial loading conditionsby using a six-component yield function. Lian et al. (1989)used this yield function to study the influence of the yieldsurface shape on failure behavior of sheet metals. Yu et al.(2007) developed a ductile fracture criterion which is intro-duced by a finite element simulation. They carried out thesimulations of aluminum alloy sheet forming based on Bar-lat’s yield function (Barlat and Lian, 1989) and Hollomon’shardening equation. In their study, the critical punch strokesof the aluminum alloy sheets of X611-T4, 6111-T4 and 5754-O in a cylindrical complex forming in which deep drawingand stretching modes were calculated by the ductile fracturecriterion. The results showed good agreements with the exper-imental results. Barlat et al. (1997) measured the yield surfacesfor binary aluminum–magnesium sheet samples with differ-ent microstructures. A generalized plastic behavior of anyaluminum alloy sheet yield description was proposed to pre-dict the behavior of the solute strengthened (precipitationhardened) aluminum alloy sheets. Barlat et al. (2003) proposeda plane stress yield function that describes the anisotropicbehavior of the sheet metals, in particular, for aluminumalloy sheets. The anisotropy of the function was introducedin the formulation using two linear transformations on theCauchy stress tensor. For the Al–5 wt.% Mg and 6016-T4 alloysheet samples, yield surface shapes, yield stress and r-valuedirectionalities were compared with those of previously sug-gested yield functions by Yoon et al. (2004). Barlat et al. (2005)proposed anisotropic yield functions based on linear transfor-mations of the stress deviator in general terms. Two specificconvex formulations were given to describe the anisotropicbehavior of metals and alloys for a full stress state (3D). Choiet al. (2007) developed analytical models for hydro-mechanical

deep drawing tests to investigate the effects of process con-ditions such as temperature, hydraulic pressure, BHF andforming speed. According to their models, the experimentalresults show good agreement with the FE models. One of the

s i n g

8 j o u r n a l o f m a t e r i a l s p r o c e s

most important problems in forming simulation programs isthe method by which the defects are analyzed during thesimulation. Forming conditions of the sheet metals were alsoinvestigated in implicit and explicit finite element simulationsby Van Den Boogaard et al. (2003). The study showed that thecomputation time for implicit finite element analyses tendedto increase disproportionally with increasing problem size.

Sheet metal deformation is considered biaxial rather thantensile deformation. For this reason, biaxial data in materialmodel should be evaluated. Li and Ghosh (2003) studied uni-axial tensile deformation behavior of three aluminum sheetalloys, Al 5182 + 1%Mn, Al 5754 and Al 6111-T4 in the warmforming temperature range of 200–350 ◦C and in the strainrate range of 0.015–1.5 s−1. It is found that the total elonga-tion in uniaxial tension increased with increasing temperatureand decreased with increasing strain rate. They contributedto the enhanced ductility at elevated temperatures primarilyfrom the post-uniform elongation which becomes dominant

at elevated temperatures and/or at slow strain rates. Theenhancement of strain rate sensitivity with increasing tem-perature accounts for the ductility improvement at elevatedtemperatures. In their study, the uniaxial tensile test is used

Fig. 7 – The effect of warm temperatu

t e c h n o l o g y 2 0 7 ( 2 0 0 8 ) 1–12

as a screening test for ranking the relative formability amongdifferent sheet alloys. The strain hardened 5XXX alloys (Al5182 + Mn and Al 5754) have shown better formability thanthe precipitation hardened alloy (Al 6111-T4). Li and Ghosh(2004) also investigated biaxial warm forming behavior in thetemperature range 200–350 ◦C for three automotive aluminumsheet alloys: Al 5754, Al 5182 containing 1%Mn and Al 6111-T4. Formability was studied by forming rectangular parts at arapid rate of 1 s−1 using internally heated punch and die forboth isothermal and non-isothermal conditions. It is observedthat the formability for all the three alloys improves at ele-vated temperatures, the strain hardened alloys Al 5754 and Al5182 + Mn show considerably greater improvement than theprecipitation hardened alloy Al 6111-T4. Results show thattemperature effects on drawing of the sheet have a largeeffect on formability. Setting die temperature slightly higherthan punch temperature was favorable in promoting forma-bility. They also determined the forming limit diagram (FLD)

under warm forming conditions which showed results con-sistent with the evaluation of part depth. Fig. 7 shows thatthe effects of temperature on FLDs of type 5754, 5182 and6111-T4 aluminum alloy. As seen in the figures, the formabil-

res on FLDs (Li and Ghosh, 2004).

i n g

itpTtabebictJtdtOdtcmtdTihraafiriTAtlfoiftoidggaafBtftapTe(c

j o u r n a l o f m a t e r i a l s p r o c e s s

ty of aluminum alloys increases with increasing the formingemperature. It is also determined that Al 5754 forming tem-erature sensitivity is greater than other type aluminums.akuda et al. (2002) studied the deformation behavior andhe temperature change in cylindrical deep drawing of anluminum alloy sheet at elevated temperatures by the com-ination of the rigid-plastic and the heat conduction finitelement methods. It was clarified that the appropriate distri-ution of flow stress depending on temperature must exist

n the sheet for the higher LDR. In their study, the numeri-al results as well as the experimental show that the LDR inhe warm deep drawing increases with the die profile radius.ain et al. (1998) investigated experimentally and numericallyhe limiting draw ratios (LDRs) and other axisymmetric deeprawing characteristics of AA5754-O and AA6111-T4 automo-ive aluminum sheet materials as a function of die profile radii.ther deep drawing characteristics such as punch load versusisplacement traces, flange draw-in, strain distribution alonghe cup profile, flange wrinkling, wall ironing and fractureharacteristics are experimentally assessed for the two sheetaterials as a function of the die profile radius. They observed

hat the deep drawability of AA5754-O as measured by cupepth at fracture and LDR is superior to that of AA6111-T4.hey explained the differences in the deep drawing behav-

or of the two materials in terms of the competition in workardening between the material in the flange at the die profileegion versus the material at the punch profile region, bend-bility of the two materials, and fracture characteristics. Theylso observed that a decrease in LDR and flange draw-in as aunction of the die profile radius. Namoco et al. (2007) stud-ed embossing and restoration process of A5052 and A6061 toeduce the deformation force, the drawing resistance and toncrease the drawability of the sheet and LDR. Palumbo andricarico (2007) investigated warm deep drawing process ofA5754-O aluminum alloy. In this experimental work, they

ook into account the parameters which were temperatureevel of the blank in the centre of the specimen and theorming speed; in addition they used grease lubricant. Theybserved that the temperature in the blank centre had a strong

nfluence on the process feasibility and thus on the materialormability. Spigarelli et al. (2004) investigated the deforma-ion behavior of an Al alloy between 120 and 180 ◦C by meansf uniaxial compression tests to identify possible differences

n the deformation response compared with uniaxial tensileata. They found that the strength of the alloy was slightlyreater in compression than in tension and this differenceradually disappearing as strain rate decreased. Yoshihara etl. (2004) demonstrated spin formability of Al–Mg alloy usingn NC control machine at 300 ◦C with a main shaft rotationalrequency of 300 rpm and feed per revolution of 180 mm/min.y spin forming, it is possible to form a domed shape similaro a pressure vessel at the end of a pressurized gas cylinderor passenger and aeronautical vehicles. Their study presentshe finite element simulation of the spin forming of Al–Mglloys. This model was constructed based on the materialroperties at 300 ◦C as recorded in the real forming process.

hey also developed a new deep drawing process (Yoshiharat al., 2003a,b) and localized heating and cooling techniqueYoshihara et al., 2003a,b) to improve formability. The con-lusion is deep drawing performance of the alloy would be

t e c h n o l o g y 2 0 7 ( 2 0 0 8 ) 1–12 9

enhanced using the appropriate temperature distribution forthe local heating and cooling technique and with variableblank holder pressure control. Kim et al. (2006) investigatedthermomechanically coupled FEA which was performed forforming of aluminum rectangular cups at elevated tempera-tures. They examined applicability, accuracy and repeatabilityof three different failure criteria (maximum load, minimumload, and thickness ratio) to identify the onset of failure dur-ing FEA. They selected the thickness ratio criterion since itresulted in accurate prediction of necking-type failure whencompared with experimental measurements obtained undera variety of warm forming conditions. They also comparedpredicted part depth values from FEA at various die-punchtemperature combinations and blank holder pressures con-ditions with experiments. Results indicate that they were ingood agreement. They established forming limit diagrams atthree different warm forming temperature levels (250, 300and 350 ◦C). It is concluded that limit strain increases withincreasing forming temperatures. In addition, strain distribu-tions on the formed part obtained under different die-punchtemperature combinations were also compared to furthervalidate the accuracy of FEA. A high temperature gradientbetween die and punch (Tdie > Tpunch) was found critical toincrease formability. Naka and Yoshida (1999) investigatedthe effects of forming speed and temperature on the deepdrawability for a fine grain Al–Mg alloy (5083-O) sheet by per-forming cylindrical deep drawing tests at various formingspeeds (0.2–500 mm/min) at die temperatures of 20–180 ◦C (thedie was heated, while the punch was water cooled duringthe tests). They observed that the LDR mostly increases withincreasing die temperature, because the deformation resis-tance in flange shrinkage decreases with temperature rise andthe LDR also becomes lower with increasing forming speed atall temperatures because of the flow stress of the heated blankat the flange increases with increasing strain rate. Moreover,the cooled blank at the punch corner becomes less ductile.Naka et al. (2001) investigated the effects of forming speedand temperature on the FLD experimentally for a fine grainAl–Mg alloy (5083-O) sheet by performing stretch-forming testat various forming speeds (0.2–200 mm/min) at several tem-peratures from 20 to 300 ◦C. It is found that the forminglimit strain increased drastically with decreasing speed forany strain paths at a high temperature ranging from 150 to300 ◦C. It is known that the FLD was not sensitive to speed atroom temperature. The improvement in formability at 300 ◦Cat low forming speed is specifically due to the high strainrate hardening characteristic of the material, but below 200 ◦Cthe formability is also affected strongly by strain harden-ing. The number of available 5XXX series Al-based alloys forpassenger vehicles is very limited. At the present time, 5052and 5456 are the most commonly used alloys. Although 5052offers a good combination of mechanical properties, corro-sion resistance, and formability, it is unsuitable for use attemperatures above 120 ◦C due to its poor creep resistanceand its low strength at elevated temperatures. In order toget a better overall understanding of alloys and to identify

the most promising compositions, most researchers examineand evaluate the micro structural features, tensile propertiesand creep resistance. Zhang et al. (1998) presented some newAl–Mg alloys with good creep resistance, acceptable formabil-

s i n g

r

10 j o u r n a l o f m a t e r i a l s p r o c e s

ity, and low cost. They also investigated the influence of smalladditions of Ca and Sr on the tensile and creep properties.Another room temperature formability testing was performedon an Al–Mg6.8 type alloy sheet with a fully recrystallizedstructure (average grain diameter ∼18 �m) and after partialannealing with a retained deformed structure by Romhanjiet al. (1998). The yield strengths attained after full recrystal-lization and after partial annealing, were 175 and 283 MPa,respectively. Such an increase in strength is followed by forma-bility degradation, maximized around the plain strain state toeither 42%, as obtained using the limiting dome height test(LDH), or 35% after using forming limit curves (FLC). A com-parison with known high-strength formable alloys has shownthat the tested alloy in the recrystallized condition has a bet-ter stretch formability (at the same or even higher yield stresslevel), while in the unrecrystallized-partially annealed con-dition it has a lower formability, limiting its application tomoderate forming requirements for very high-strength parts.

3.3. The effects of lubrication

One of the important parameter for the forming of aluminumsheets is lubrication. It is used during the forming processto get better surface quality and to decrease the frictionof die surfaces. This contributes to increasing the die life-time by reducing wear. Meiler et al. (2003) investigated theeffects of dry film lubricants on aluminum sheet metal form-ing and compared the results with other type lubricants. Theyobserved that dry film lubricants showed advantages over con-ventional oil lubricants because of their high deep drawingperformance, especially on complex shaped body panels. Theyalso emphasized in their study the formability is increased asa consequence of reduced friction and it is possible to get morehomogeneous sheet thickness distributions. Wu et al. (2006)studied a super plastic 5083 Al alloy under biaxial deformationby deforming the sheet into a rectangular die cavity with andwithout lubrication. Results indicate that reducing the inter-facial friction by use of a lubricant altered the metal flow afterthe deformed sheet had made contact with the die surface.Besides, they observed that changes of the metal flow dur-ing forming not only developed a better thickness distributionof the formed part, but also improved cavitations distribution(Kelly and Cotterell, 2002).

4. Conclusion

In this paper, formability of Al–Mg alloys at warm temperatureis presented. In general, at temperatures above 225 ◦C the flowstress becomes strain rate dependent.

The warm forming process is beneficial in terms of forma-bility. Researchers have been conducted their studies at labenvironment for years. No well-know procedure have beendeveloped for press shop. It is very important to transfer warmforming from lab to press shop. As known sheet metal partmanufacturing is a mass production process. It is necessary to

develop the procedure for successful warm forming operationin press shop.

The properties of aluminum alloys at elevated tempera-tures have been determined by various researchers around the

t e c h n o l o g y 2 0 7 ( 2 0 0 8 ) 1–12

world. Many of these researchers have used material prop-erties which are obtained from tensile test results in theirinvestigations. However, information on properties obtainedat elevated temperatures under a biaxial state of stress islimited. Mostly, they are not available for finite element sim-ulation. This area needs to be studied extensively.

In terms of numerical simulations, there are no welldefined material models including temperature and strain rateeffects for aluminum alloy. Further investigations on materialmodels are required. In future study, material models shouldbe developed and the effect of process parameters should beinvestigated for process optimization.

Acknowledgements

This work is supported by The Scientific and Technologi-cal Research Council of Turkey (TUBITAK). Project Number:106M058, Title: “Experimental and Theoretical Investigationsof The Effects of Temperature and Deformation Speed onFormability”. TUBITAK support is profoundly acknowledged.

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