Deep drawing aluminum—not as hard as it looks
Keeping a couple key tips in mind can help you turn aluminum stamping from a source of frustration to a source of income and satisfaction in a hurry.
Aluminum unfortunately often gets a bad rap.
For those manufacturing operations that revolve around the deep drawing of steel, the sheer mention of aluminum brings out a grimace. It is universally known for its inability to stretch, its abrasive nature, and its tendency to wrinkle. Although these statements may be partially true, aluminum does have several advantages.
I usually tell the attendees at one of my metal forming conferences, that in order to form aluminum, you must think like aluminum. Aluminum reacts completely differently from steel in a drawing or forming operation. This certainly doesn't mean that aluminum is bad -- it just means that it is different.
For those people who draw or form aluminum with a great deal of success, congratulations! You will be better prepared for forming high strength steels. Why? Because high-strength steel generally has poor elasticity, as well.
Aluminum has many advantages, including:
- It is about one-third the weight of steel, which makes it an ideal candidate for aerospace products that require certain strength-to-weight ratios.
- It has good dent resistance for outer body automotive panels.
- It is fully recyclable.
- It is rust-proof (brown rust).
- Far more expensive that steel.
- Limited to certain geometric features using economical processes.
- Abrasive to tooling (aluminum oxide is very abrasive).
- Difficult to weld.
- Prone to severe springback.
If I could give one bit of advice to those forming aluminum, it would be to limit extensive stretching in small areas. Aluminum strains locally, meaning that it has poor stretch distribution as well as poor elasticity.
Most general use aluminum alloys have elongation percentages ranging from 10 percent up to perhaps 25 percent. This is poor elongation compared to commonly used forming steels.
Stretch Forming. A successful aluminum stamping begins with a good product design. Because aluminum has poor strain distribution characteristics, the part either must be designed to stretch evenly, or the part's geometry must be obtained by feeding the metal into the draw cavity.
Part shapes can be made using three basic methods: stretching, drawing, or a combination of both.
Stretch forming is the process of increasing the surface area of a part (as the name implies) by stretching the metal over the forming punch. Stretch dies force the metal to thin out and strain during tension. Most automobile hoods are made with stretch forming dies.
These products are relatively shallow. Large radii and gentle flowing geometries are used to help force the metal to stretch as evenly as possible. Aluminum inherently does not like to stretch extensively within a small area, but it can be stretched a little over large areas. The key is to design the part so that it forces even stretch distribution of the metal.
Figure 1 (at top of page) shows an ideal product designed for an aluminum stretch draw.
Material Feeding. You also can obtain severe deep draws by forcing the metal to flow into the female draw cavity. Unlike a stretch drawing operation, the metal is displaced into a geometry that contains nearly the same surface area as the starting blank.
The key to deep drawing aluminum is to avoid stretching it. How is this done? Use an acceptable draw ratio and use the proper lubricant and blank holder force.
Acceptable Draw Ratio. The draw ratio is the direct relationship between the draw punch diameter and the size of the starting blank. Metal in compression resists flow; therefore, you must limit the amount of metal in compression.
If the blank edge is too far from the edge of the punch, the metal does not flow and stretches extensively. If the blank edge is close enough to the punch, the metal is forced to compress, resulting in inward flow.
Remember: The geometry is not being obtained by stretching but by displacement of metal in compression. Compression is the key (see Figure 2). Keep in mind that metal does not fail in compression, only in tension. (Yes, even so-called compression fractures occur in tension, but that's a topic for another article).
Figure 3 shows an example of a draw reduction chart used for various types of metal and demonstrates the direct relationship between each of the draws. Keep in mind that the surface area from the first draw to the final draw is the same or less. This is critical to the drawing process.
Figure 4 shows a deep drawn aluminum part utilizing draw reductions or "redraws."
Proper Lubricant and Blank Holder Force. Keeping friction to a minimum while controlling metal flow is the key to successful deep drawing. Excessive blank holder pressure forces the aluminum to stretch. Use just enough pressure to keep the aluminum from wrinkling, or use stand-offs to control the gap between the die face and the blank holder surface.
Use a proper lubricant for aluminum—lubricants that work well for steel most likely won't work well for aluminum. See your lubricant specialist about this.
Many controlling factors contribute to the success of an aluminum stamping, and many of those factors are controlled to a great degree by the mechanical properties of the aluminum sheet.This article covers only a couple of factors. It doesn't matter if you making an automobile hood or a deep-drawn cup. Keep these two basic things in mind when you are working with aluminum -- stretch distribution and compression. You shouldn't go wrong.