12 january/february 2013 WWW.STaMPInGjOurnaL.COM STAMPING JOURNAL • an fma publication

R&D UPDATE

Editors Note: This is Part I of a two-part series that

discusses the forming of aluminum sheet for auto-

motive products. Part II, to appear in the March/

April 2013 issue of STAMPING Journal®, will

discuss the lubricants used in aluminum forming.

Aluminum is used extensively for such automotive parts as hoods, trunk lids, and doors

because of its light weight, workabil-ity, and recyclability. and its market share continues to increase.

Commonly Used AlloysThe 5xxx and 6xxx aluminum alloys are used most commonly for auto-motive applications. Their mechani-cal properties for automotive body sheets are listed in Figure 1, and their specific properties and main dif-ferences are shown in Figure 2.

The 5xxx alloys have ultimate ten-sile strength of 125 to 350 MPa and cannot be heat-treated. They have relatively good formability and are highly resistant to corrosion. How-

ever, 5xxx alloys are prone to the formation of Lüders bands during forming, so they are used mostly for inner-panel applications. 5182 and 5754 are the principal 5xxx series alloys used in autobody panels. 5754 also is recommended for elevated-temperature applications.

The 6xxx series alloys are heat-treatable to reach ultimate tensile strength of 125 to 400 MPa. The alloys, especially 6022 and 6111, often are used for outer panels since they are precipitation-hardened and free of Lüders bands.

Design Guidelines and RequirementsThe key requirements for automotive closures are panel bending stiffness and dent resistance. The elastic mod-ulus of aluminum (70 GPa) is about one-third that of steel (210 GPa). As a result, parts previously designed for steel need to be redesigned to achieve the same stiffness.

One way to improve stiffness in aluminum is to increase the ribbing used in the product or increase the part thickness. For closures and body-structure sheets, the thickness should be increased by a factor of about 1.45:1

where: t = thickness E = Young’s modulusThe resulting weight saving is

about 50%:

Another important design criterion for aluminum alloys is dent resistance for static and dynamic conditions. To reach a static dent resistance com-parable to steel’s, aluminum sheet should meet the following thickness requirement:

Where: t = thickness YS = yield strength Figure 3 shows the aluminum

sheet properties compared to those of steel, based on dynamic dent resis-tance studies.

Figure 4 shows the weights of three different hoods made of mild steel, high-strength steel, and Al6016, respectively. For mild steel, the reduction in weight is limited by the dent resistance; for high-strength steel, it is limited by local stiffness. When the hood is made of Al6016, its weight can be reduced by 50 per-cent.

Critical Material Parameters Some critical parameters are different for aluminum than steel, and they affect formability:

Aluminum sheet forming for automotive applications, Part IMaterial properties and design guidelinesBy TingTing Mao and Taylan alTan

Alloy

Mechanical Properties

Ultimate Tensile Strength (MPa)

Yield Strength (MPa)

Elongation (%) n-value r-value

5000 Series

AA5022 275 135 30 0.3 0.67AA5023 285 135 33 - -AA5182 265 125 28 0.33 0.8AA5052 190 90 26 0.26 0.66AA5754 212 90 22 0.34 -

6000 Series

AA6022 275 155 31 0.25 0.6AA6016 235 130 28 0.23 0.7AA6111 290 160 28 0.26 0.6

Figure 1

The mechanical properties of several aluminum sheet alloys for automotive applica-tions are shown here. Source: T. Sakurai, “The latest trends in aluminum alloy sheets for automotive body panels,” Kobelco Technology Review, No. 28 (Oct. 2008).

taluminumtsteel = =

EsteelEaluminum

31.44

massaluminummasssteel

densityaludensitysteel

= 1.44 ×

= 1.44 × = 0.52.77.8

taluminum ≥ tsteel × YSsteelYSaluminum

13an fma publication • STAMPING JOURNAL WWW.STAMPINGJOURNAL.COM JANUARy/febRUARy 2013

R&D UPDATE

•Elastic Modulus—With an elas-tic modulus one-third that of steel, aluminum parts experience more springback. This can be reduced by increasing the blank holder force, the amount of stretching, and sheet thickness. The forming operation must be optimized to ensure at least 2 percent stretch throughout the part (see Figure 5).

•Friction—Friction between the tool and the aluminum sheet is expected to be higher compared to steel sheet because aluminum has a surface roughness (Ra) from 0.25 to 0.38 micron. In comparison, the Ra of steel sheet is about 0.63 to 0.88 micron. The smoother texture of alu-

minum requires dry, waxlike lubri-cants.

•Formability—For the stamping of autobody parts, the lower form-ability of aluminum compared to steel can be offset to some extent by using technology such as advanced adden-dum design, local blank holder force control with multiple-point hydrau-lic cushions, or warm forming. Warm forming using heated dies and heated blanks has been investigated exten-sively, and recent studies have shown that the use of heated dies compli-cates the process and increases costs. Present R&D efforts are focused on heating the sheet to warm forming temperature while keeping the dies

at room temperature. Researchers hope to establish a

practical and robust process that increases the formability of alumi-num sheet for forming more complex parts with difficult geometries.S Tingting Mao is a graduate research associate and

Taylan Altan is professor and director of the Cen-

ter for Precision Forming (CPF) at The Ohio State

University, 339 Baker Systems, 1971 Neil Ave.,

Columbus, OH 43210, 614-292-5063, www.

cpforming.org and www.ercnsm.org.

Notes:

1. “The Aluminum Automotive Manual,” version

2011, European Aluminum Association.

Figure 2

Properties and forming characteristics vary among the 5xxx and 6xxx alloys used for manufacturing car body components. Source: I.J. Hirsch, “Automotive trends in alu-minum – The European Perspective,” Aluminum Alloys: Their Physical and Mechani-cal Properties: proceedings of the 9th International Conference on Aluminum Alloys (ICAA9), eds. B. Muddle, A. Morton, and J. Nie (Institute of Materials Engineering Australasia, 2004).

Figure 5

Strains of at least 2 percent are neces-sary when stamping aluminum to reduce springback. Source: W. Thomas and T. Altan, “Saving weight with alumi-num stampings – Part II,” STAMPING Journal, July/August 1999, p. 84.

Figure 4

Automobile hood weight decreases substantially when Al6010 is used rather than steel. Source: Design with Aluminum (European Aluminum Association, 2011).

Figure 3

This graph compares yield strength of aluminum and steel based on dynamic dent resistance studies. Source: Design with Aluminum (European Aluminum Association, 2011).

Li

fe C

ycle

Forming

Joini

ng

Surface Appearance

Dent

Res

istan

ce

Stretchability

Corrosion

Resistance

Deep

Drawability

Quality

Weldab

ility

Properties Process Chain

AA6016AA6111AA5182AA5754

Reference Steel Panel YS (MPa)200 250 300 350 400

260

220

180

140

100Alum

inum

Pan

el Y

S fo

r 50%

Wei

ght S

avin

g (M

Pa)

Automobile

Hood

t

Design Criteria SpecificationMild Steel

(YS = 200 MPa)High-strength Steel

(YS = 300 MPa)AI 6016

(YS = 150 MPa)*

Dent Resistance YS × t2 ≥ Cte t ≥ 0.8 t ≥ 0.65 t ≥ 0.92

Local Stiffness E × t2 ≥ Cte t ≥ 0.7 t ≥ 0.7 t ≥ 1

Weight (kg) 14 12.4 6.1

* After Deformation and Paint Baking