February 2, 2002
This brief overview discusses rules for aluminum stamping involving: die clearance, die radius, punch radius, minimum bend radii, round cups, rectangular box parts, and overhang limit.
In deep drawing, if the die clearance is too large, the component forms a cone instead of a cylinder.
Editor's Note: This column was prepared by the staff of the Engineering Research Center for Net Shape Manufacturing (ERC/ NSM), The Ohio State University, Professor Taylan Altan, Director. It is the first of a two-part article related to stamping of aluminum alloys. This column covers part design considerations and practical part and tooling design rules. The second column, to appear in the July/August issue, will cover addendum design methodology to improve the formability of aluminum stampings.
Customers and government regulators continue to demand environmentally friendly, fuel-efficient cars. Of all the ways to increase efficiency, weight reduction is by far the most effective. Automotive weight can be reduced by optimizing the design and using a lightweight material.
Aluminum is a good candidate for weight reduction. Its properties make it a versatile material for engineering and construction. It is light in mass, yet some of its alloys have strengths greater than that of structural steel.
Aluminum's specific gravity is about 2.7. Its mass (weight) is roughly 35 percent that of iron and 30 percent that of copper. Commercially pure aluminum has a tensile strength of about 13,000 pounds per square inch (PSI), so its usefulness as a structural material in this form is somewhat limited.
Different aluminum alloys have different minimum bend radii based on their sheet thickness.
By alloying and cold working, the strength of aluminum alloys can be approximately doubled. Much larger strength increases can be obtained by alloying aluminum with small percentages of one or more other elements, such as manganese, silicon, copper, magnesium, or zinc. Some alloys are further strengthened and hardened by heat treatment, giving them tensile strengths approaching 100,000 PSI.1
Aluminum has one-third the Young's modulus of steel. Thus, the final product stiffness will be reduced unless the product design is modified to account for this factor. Two possibilities are to increase the ribbing used in the product or to increase the part thickness.
The reduction of Young's modulus will result in an increased tendency for wrinkling, oil canning, and surface distortion. Flange wrinkling typically can be controlled by using a blank holder (binder), but wrinkling, oil canning, and distortion in the product body or addendum features (gainers, drawbars, and ribs) can be difficult to control. Increasing the material thickness can improve these problems, but the best method is to introduce more appropriate geometric features in the problem addendum areas.
The yield strength of aluminum alloys is less than that of steel. Consequently, the overall strength, dent resistance, energy absorption, and crash resistance of an aluminum-alloy stamped panel also are lower than in a steel stamped panel. Certain alloys such as 2XXX and 6XXX series can be baked after forming to increase yield strength. Otherwise, good design must be employed to make up for this strength loss.To achieve good part quality and accuracy in aluminum stampings, many die and part design rules must be observed.
Die clearance, uD(see Figure 1), is the distance between the punch surface and the die surface. In deep drawing, if the die clearance is too large, the component forms a cone instead of a cylinder. If the die clearance is too small, ironing can take place, which increases the drawing load and the danger of cracking.
Draw depth is limited to 7 times any corner radius or 12 times corner radii of 0.25 inch or less.
Die radius, rD, depends on the thickness of the workpiece. To lower the drawing load, large radii are desired. Large radii, however, reduce the contact area between the blank holder and the flange and increase the tendency to form wrinkles in the region of the die radius. The possibility of wrinkle formation is reduced if the die radius selected is small. Recommended values for die radii for aluminum stamping are 5 to 10 times the sheet thickness.2
Selecting the proper punch radius, rP, is essential to obtain a good-quality part. For small components of large sheet thickness, it is advisable to use a gentle transition (for example, parabolic) from the punch radius to the cylindrical portion of the punch to avoid wall thickness reductions in the transition zone from the bottom of the cup to the wall. The punch radius must never be smaller than the die radius, or the punch might pierce the material.
The recommended punch radii values for aluminum stampings are in the range of 8 to 10 times the sheet thickness. Punch radii greater than 10 times the sheet thickness will cause wrinkling because of the high compressive hoop stresses that occur while the sheet wraps around the punch radius.
An overhang of 50 times the sheet thickness or less is desirable.
Figure 2presents the minimum bend radii to sheet thickness ratios, r/s0, for a variety of aluminum alloys.
In drawing round cups from higher-strength alloys of the same gauge, the maximum single depth of draw must be reduced. The diameter and height of a part should be such that the ratio of punch diameter to blank diameter, dP/dB, is at least 0.5 for 2024-O or 5052-O aluminum alloys and 0.3 for 6061-T4/ T6 alloy. The die radius should be within the range of 5 to 10 times sheet thickness, with 8 sheet thickness optimum.
In drawing rectangular or box-shaped parts, the draw depth is limited to 7 times any corner radius, rC, from 0.5 to 1 inch (12.70 to 25.4 millimeters) or 12 times corner radii of 0.25 inch (6.35 millimeters) or less (see Figure 3). Both the draw and the corner radii must be a minimum of 5 times the sheet thickness; bottom radii, rB, can range from 3 to 8 times the sheet thickness.
For 3003-O, 2024-O, and 5052-O alloys, the box height-to-width ratio is about 0.6 for corner radii up to 0.375 inch (9.52 millimeters) and 0.75 for radii exceeding 0.5 inch (12.7 millimeters). Included corner angles of less than 60 degrees reduce the feasible depth of draw.
The overhang limit is the limit of a certain amount of material that is not supported in the die. The maximum overhang for aluminum should not exceed 75 times the sheet thickness. As shown inFigure 4, an overhang of 50 times the sheet thickness or less is desirable. When an overhang is too large, wrinkling caused by compression stresses will occur in shrink flanging areas.
Taylan Altan is a Professor and Director of the Engineering Research Center for Net Shape Manufacturing, 339 Baker Systems, 1971 Neil Avenue, Columbus, Ohio 43201-1271, phone 614-292-9267, fax 614-292-7219, Web site nsmwww.eng.ohio-state.edu. The ERC/NSM conducts research and development; educates students; and organizes workshops, tutorials, and conferences for the industry in stamping, tube hydroforming, forging, and machining.
1. Aluminum Standards and Data (Washington, D.C.: The Aluminum Association, 2000), p. 1-1.
2. K. Lange, Handbook of Metal Forming (New York: McGraw-Hill Company, 1985), p. 20.1