Stamping aluminum

Its not as easy as you may think

STAMPING JOURNAL® SEPTEMBER 2007

September 11, 2007

By:

The reality is that aluminum is not steel. It doesn't behave like steel, it doesn't flow like steel, and it certainly doesn't stretch like steel. So does this make aluminum hard to form? No, not if you think like aluminum.

Deep draw part

Figure 1Depending on the type, aluminum has a good strength-to-weight ratio and is well-suited to the deep-drawing process, as well as multiple draw reductions.

I remember distinctly my very first experience with building a die that was supposed to draw a sheet of aluminum into a deep, contoured shape. Not knowing much about aluminum, I assumed that it must be extremely formable—after all, they make beverage cans from it, don't they?

My first thoughts were, "This will be a cake walk. I'll bet this stuff stretches a mile. Yep, it must stretch a lot because it's really soft."

This thought process was obviously a testimony to my ignorance regarding aluminum.

I think I lost a large portion of my hair trying to make that job work. I must have spent weeks fighting splits and wrinkles. It wasn't long before I came to the conclusion that drawing and stretching aluminum were not as easy as I had thought.

Now that I am a little wiser with respect to the formability of aluminum and aluminum alloys, I realize that my problem was really not the fault of the aluminum, but rather the fact that during the die tryout stages, I was thinking like steel rather than aluminum. Up until then, all of the things that I would have done to correct the problem with a die that was forming steel, I did with the aluminum. Needless to say, I failed.

The reality is that aluminum is not steel. It doesn't behave like steel, it doesn't flow like steel, and it certainly doesn't stretch like steel. So does this make aluminum hard to form? No, not if you think like aluminum.

Aluminum is not a bad metal; it's just a different metal. Like any metal, it has advantages and disadvantages, and the key is to understand the material's behavior before designing a part or creating the process and die that are to produce it.

Stretchability

If you are comparing aluminum to deep-drawing steel, generally you will find that aluminum does not have near the elongation ability of steel. For instance, typical deep-drawing steel has elongation somewhere around 45 percent, while a 3003–O temper, meaning "dead soft," aluminum will have elongation near 30 percent.

Stretch Distribution Characteristics

Generally speaking and depending on the alloy, aluminum has poor stretch distribution characteristics compared to deep-drawing steel. It is known as a material that strains locally, meaning that most of the stretch that occurs when the metal is subjected to a stretching operation will take place in a small, localized area.

However, keep in mind that the forming punch geometry has a greater influence on how the metal stretches than the metal itself. Stamped parts to be made from aluminum must be designed so that the part shape forces the metal to distribute stretch more evenly.

Aluminum ironing process

Figure 2Generally speaking, aluminum is a great material when ironing can be used. During ironing, the metal is squeezed down a vertical wall to increase the surface area while reducing the metal's thickness. Ironing is the basic process used to make beverage cans.

Parts requiring a great deal of stretch in a small area with small male radii are doomed for failure if designed of aluminum, especially if the final geometry is to be made in a single forming operation. In contrast, large, liberal radii and flowing, gentle geometries are best-suited for aluminum.

Drawability

First, don't confuse drawability with stretchability. Drawability is the metal's ability to flow plastically when subjected to tension, while stretchability is the increase of surface area as the result of tension.

Depending on the type, aluminum can draw very well (see Figure 1). It has a good strength-to-weight ratio and is well-suited to the deep-drawing process, as well as multiple draw reductions. The reductions percentages are very comparable to those often used when drawing deep-drawing steel.

Tooling Interface

Although aluminum is soft, it can still be abrasive. While it does not rust conventionally, it forms a white powdery substance called aluminum oxide, which is used to make grinding wheels. That means the same abrasive that you have been using to grind your tool steel die sections can be present on the aluminum sheet surface.

You can prevent this poor interface by using high-pressure barrier lubricants, which keep the aluminum from touching the tool steel sections during forming and cutting.

Compressibility

Generally speaking, aluminum is a great material when ironing can be used. During ironing, the metal is squeezed down a vertical wall to increase the surface area while reducing the metal's thickness. It increases the metal sheet's surface area by squeezing the metal rather than exposing it to tension. Ironing is the basic process used to make beverage cans (seeFigure 2).

When aluminum is ironed, it almost compressively flows like a hot liquid down the wall of the die cavity and punch, and it shines to a mirrorlike surface finish.

Springback

Aluminum has more springback than soft draw-quality steel. However, the amount of springback that occurs can be controlled by designing the stamped product with respect to the springback value.

Like steel, each type of aluminum has its own physical characteristics, and each one requires careful consideration. Until next time … best of luck!



Dieology LLC

Art Hedrick

Contributing Writer
Dieology LLC
8579 River Oak Circle
Greenville, MI 48838
Phone: 616-225-2170
Author of the "Die Science" column in STAMPING Journal®, Art also has written technical articles on stamping die design and build for a number of trade publications. A recipient of many training awards, he is active in metal stamping training and consulting worldwide.

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STAMPING Journal® is the only industrial publication dedicated solely to serving the needs of the metal stamping market. In 1987 the American Metal Stamping Association broadened its horizons and renamed itself and its publication, known then as Metal Stamping. Print subscriptions are free to qualified stamping professionals in North America.

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