R&D UPDATE Aluminum sheet forming for automotive applications, Part I Material properties and design guidelines By Tingting Mao and Taylan Altan 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 elevatedtemperature applications. The 6xxx series alloys are heattreatable 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. Editors Note: This is Part I of a two-part series that discusses the forming of aluminum sheet for automotive products. Part II, to appear in the March/ April 2013 issue of STAMPING Journal®, will discuss the lubricants used in aluminum forming. A luminum is used extensively for such automotive parts as hoods, trunk lids, and doors because of its light weight, workability, and recyclability. and its market share continues to increase. Commonly Used Alloys The 5xxx and 6xxx aluminum alloys are used most commonly for automotive applications. Their mechanical properties for automotive body sheets are listed in Figure 1, and their specific properties and main differences are shown in Figure 2. The 5xxx alloys have ultimate tensile strength of 125 to 350 MPa and cannot be heat-treated. They have relatively good formability and are highly resistant to corrosion. How- Design Guidelines and Requirements The key requirements for automotive closures are panel bending stiffness and dent resistance. The elastic modulus 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. Mechanical Properties Alloy AA5022 Ultimate Tensile Strength (MPa) 275 Yield Strength (MPa) 135 Elongation (%) 30 n-value r-value 0.3 0.67 AA5023 5000 AA5182 Series AA5052 285 135 33 - - 265 125 28 0.33 0.8 190 90 26 0.26 0.66 AA5754 212 90 22 0.34 - AA6022 275 155 31 0.25 0.6 235 130 28 0.23 0.7 290 160 28 0.26 0.6 6000 AA6016 Series AA6111 Figure 1 The mechanical properties of several aluminum sheet alloys for automotive applications are shown here. Source: T. Sakurai, “The latest trends in aluminum alloy sheets for automotive body panels,” Kobelco Technology Review, No. 28 (Oct. 2008). 12 january/february 2013 WWW.STAMPINGJOURNAL.COM 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 3 taluminum Esteel = E = 1.44 tsteel aluminum where: t = thickness E = Young’s modulus The resulting weight saving is about 50%: massaluminum = 1.44 × masssteel densityalu 1.44 × densitysteel = 2.7 7.8 = 0.5 Another important design criterion for aluminum alloys is dent resistance for static and dynamic conditions. To reach a static dent resistance comparable to steel’s, aluminum sheet should meet the following thickness requirement: taluminum ≥ tsteel × YSsteel YSaluminum Where: t = thickness YS = yield strength Figure 3 shows the aluminum sheet properties compared to those of steel, based on dynamic dent resistance 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 percent. Critical Material Parameters Some critical parameters are different for aluminum than steel, and they affect formability: STAMPING JOURNAL • an fma publication g AA6016 AA6111 AA5182 AA5754 Jo in W eld ab ili ing ty De nt Re sis ep lity De a b i aw Dr tan ce Stretchability on si ce rro tan Co sis Re Life Cycle Process Chain min Quality Surface Appearance 220 180 140 100 200 250 300 350 400 Reference Steel Panel YS (MPa) Figure 3 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 aluminum – The European Perspective,” Aluminum Alloys: Their Physical and Mechanical 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). •Elastic Modulus—With an elastic 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- 260 Automobile Hood For Properties Aluminum Panel YS for 50% Weight Saving (MPa) R&D UPDATE minum requires dry, waxlike lubricants. •Formability—For the stamping of autobody parts, the lower formability of aluminum compared to steel can be offset to some extent by using technology such as advanced addendum design, local blank holder force control with multiple-point hydraulic cushions, or warm forming. Warm forming using heated dies and heated blanks has been investigated extensively, and recent studies have shown that the use of heated dies complicates the process and increases costs. Present R&D efforts are focused on heating the sheet to warm forming temperature while keeping the dies This graph compares yield strength of aluminum and steel based on dynamic dent resistance studies. Source: Design with Aluminum (European Aluminum Association, 2011). at room temperature. Researchers hope to establish a practical and robust process that increases the formability of aluminum 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 Center 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. t Mild Steel Design Criteria Specification (YS = 200 MPa) Dent Resistance YS × t2 ≥ Cte t ≥ 0.8 Local Stiffness E × t2 ≥ Cte t ≥ 0.7 Weight (kg) 14 * After Deformation and Paint Baking High-strength Steel (YS = 300 MPa) t ≥ 0.65 t ≥ 0.7 12.4 AI 6016 (YS = 150 MPa)* t ≥ 0.92 t≥1 6.1 Figure 4 Automobile hood weight decreases substantially when Al6010 is used rather than steel. Source: Design with Aluminum (European Aluminum Association, 2011). an fma publication • STAMPING JOURNAL Figure 5 Strains of at least 2 percent are necessary when stamping aluminum to reduce springback. Source: W. Thomas and T. Altan, “Saving weight with aluminum stampings – Part II,” STAMPING Journal, July/August 1999, p. 84. WWW.STAMPINGJOURNAL.COM january/february 2013 13