Page 28 - MetalForming July 2014

Stamping Aluminum
tions must be carefully selected as they all interact to affect the cost and qual- ity of aluminum stampings. Engineers must design aluminum stampings carefully, especially if the components previously were produced from steel, to account for aluminum’s different prop- erties. Because aluminum has one- third the elastic modulus of steel, the stiffness of an aluminum product pre- viously produced from steel will be sig- nificantly less unless the product design is modified. The designer has two options: Increase sheetmetal thick- ness, or increase the number of ribs and stiffeners used in the product. The final stiffness of the stamping tends to increase in proportion to the square of its thickness, while the ribs increase stiffness by increasing cross-sectional area.
Also, the engineer must carefully specify part radii for aluminum stamp- ings. Small radii can localize strains
may fail quickly during forming. Con-
versely, large radii reduce the contact area between the blankholder and the part flange, increasing the tendency to form wrinkles in unsupported regions near the die radius.
Difficult and irregular-shaped parts usually require addendum features to promote material flow and minimize localized stretching. The ultimate alu- minum-stamping design will stretch evenly and distribute strains uniform- ly when deformed.
Cup Drawing
Cup-drawing ratio relates initial blank diameter to punch diameter. If this ratio is too large, the sheet- metal will not flow easily and may stretch extensively. Successful cup draws have minimal stretching because material displacement is pri- marily in compression.
Draw-reduction ratios for cupping will vary depending on the aluminum grade and temper being formed. Some grades of aluminum can have a limiting
draw ratio (LDR) similar to that of steel, while others will have a much lower LDR. Die-design handbooks usually contain draw-reduction tables for low- carbon steel. Due to differences in workhardening behavior, surface topography and other factors, engi- neers should not use these tables for brass, aluminum or other nonferrous metals.
For aluminum alloys, engineers should aim for an LDR below 1.6, unless they have previous experience with the alloy being stamped. In gen- eral, start with a 40-percent blank reduction in the first draw, then 20 per- cent for the second reduction, and 15 percent for the third and fourth reduc- tions [1]. Four or more successive draws have been performed on 1100, 3003 and 5005 alloys without the need for interstage annealing.
The drawing reductions for higher- strength aluminum alloys (2014, 2024, 3004, 5052 and 6061, for example) will be somewhat less. The approximate
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MetalForming/July 2014
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