6 Questions about stamping aluminum

How to take advantage of aluminum’s properties

STAMPING JOURNAL® NOVEMBER 2008

November 25, 2008

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Aluminum's unique metallurgical properties make it suitable for multiple applications. Aluminum has very high corrosion resistance, Aluminum's specific weight is 2.7 kilogram/dm3 compared to 7.8 kg/dm3 for steel and 8.8 kg/dm3 for copper, and is thermally and electrically conductive.

Note: References to aluminum may also include its alloys.

Aluminum and its alloys are vital to the world economy. Globally, aluminum usage is second only to iron and steel. The latest data shows that the world primary aluminum production is more than 33 million tons. China, Russia, the U.S., and Canada are the major producers.

A snapshot of world aluminum production from 1950 to 2006 shows its exponential growth rate. Production figures are (in million tons):

1950 – 1.5
1970 – 9.0
1990 – 19.3
2000 – 24.3
2006 – 33.7

These figures reflect the increasing role of aluminum in the world. Growth is expected to continue because alumina, the raw material used to produce aluminum, is abundant and available globally. In addition, environmental concerns and fuel efficiency requirements are likely to keep aluminum in the forefront.

1. Why are aluminum's metallurgical properties significant for metalworking?

Aluminum's unique metallurgical properties make it suitable for multiple applications.

Corrosion Resistance. Aluminum has very high corrosion resistance mainly because of a thin protective layer of oxide formed almost instantaneously when the metal is exposed to air or any oxidizing medium.

Light Weight (low density). Aluminum's specific gravity is 2.7 compared to 7.8 for steel and 8.8 for copper. Alloying or cold-working the metal can achieve high strengths. The combination of low density and high strength gives aluminum a unique, high strength-to-weight ratio.

Thermal and Electrical Conductance. Aluminum is a suitable replacement for copper in thermal and electrical applications because of its high conductivity and low density. By weight, its electrical conductivity is nearly twice that of copper's, yet aluminum is currently priced at about half that of copper.

Aluminum is highly reflective of radiant energy. It is nonmagnetic, making it suitable for electronic and electric industries.

2. Why is aluminum being used more in vehicle production?

Aluminum usage in the automotive industry has almost doubled during the last decade. It is aluminum's superior corrosion resistance and low density that make it highly suitable for the automotive and aircraft industries. In the automotive industry, both financial issues and environmental priorities have prompted a push for lightweight cars and trucks. According to the International Aluminium Institute (IAI), every pound of aluminum that replaces two pounds of steel in a vehicle saves 20 pounds of carbon dioxide over the vehicle's lifetime.

Substituting steel components with aluminum alloys greatly reduces a car's weight without reducing its size. With every 10 percent reduction in vehicle weight, a 6 percent to 8 percent decrease in fuel consumption results. Gradually iron-based autobody panels are being replaced with aluminum-based alloys, such as aluminum-magnesium-silicon (AlMgSi) 6000 series.

Because aluminum is highly recyclable, it is good for the environment, and producing it from scrap uses less energy than from raw materials.

3. What are the alloy and temper designation systems for wrought components?

Alloy Designation. The Aluminum Association uses a standard four-digit numerical system. The table below shows the main alloying elements used in each (for wrought aluminum products only).

1xxx Aluminum (Al) 99.0 percent minimum, no alloying element
2xxx Copper (Cu)
3xxx Manganese (Mn)
4xxx Silicon (Si)
5xxx Magnesium (Mg)
6xxx Mg and Si
7xxx Zinc (Zn)
8xxx Other elements
9xxx Unused series

Temper Designation. The temper designation system indicates the cold- working and heat-treating histories:

F As fabricated
O Annealed
H Strain hardened (wrought product only)
W Solution heat-treated
T Heat-treated to produce stable tempers other than F, O, or H

4. What factors should be considered before replacing an existing steel part with an aluminum part?

The mechanical and physical properties of carbon steel and aluminum vary, necessitating the following general precautions for stamping operations.

  • Formability for deep drawing and biaxial stretching of aluminum is about 65 percent of carbon steel. Therefore, part design must limit forming to an area two thirds of that which would be permitted when forming steel.
  • The bend radii of the tools should be about three times those of steel for similar formability requirements.
  • The strain rate sensitivity is low for aluminum, which generates high stresses in the initial phase of drawing, and the part might fracture. To compensate for this, the hold-down pressure should be lowered, and the radii for the draw-ring and punch nose must be increased.
  • Aluminum is more notch-sensitive than carbon steel, so any blanking tool should be designed with close tolerances and must be sharp. Burrs must be minimized to reduce edge splitting in subsequent bending operations.
  • Because of its lower elastic modulus, springback is higher for aluminum. The machines must be designed to compensate for the increased springback compared to steel parts.

5. What can prevent galling when punching aluminum?

Galling is caused by friction between the sheet and the tools. Any action to reduce friction will help reduce galling.

Immediately after exposure to air, aluminum forms an oxide on the surface, which is quite abrasive to punches, so it is imperative that proper lubrication be applied. The lubricant should be stable enough so that a very thin layer is always present between the punch and the part being drawn to reduce or eliminate the abrasive action.

Currently many punches are coated by the manufacturer. These coatings greatly reduce friction and reduce galling.

The design of the punches and dies is also critical. Because aluminum has low yield and tensile strengths, grains can be torn off the tool's surface readily if the radii are not adjusted properly. The grain size of the alloy should be small. Large microstructural grains have a tendency to pull out of the surface easily and stick to the punch, causing galling. The sheet's surface also needs to be as smooth as possible. It is important for the surface to be porous enough to retain lubrication but smooth enough to minimize friction.

6. Do T tempers age-harden over time?

Yes. Most T tempers of the age-hardenable series actually age- or precipitation-harden over time. How long it takes to harden and by how much depends on which alloying elements are present in the aluminum alloy. Age-hardening is a process of hardening of the alloy through time, thereby increasing hardness along with yield and tensile strengths.

The metallurgical principle behind this phenomenon is slight oversaturation, with the alloying element at a higher temperature than the solution, followed by fast cooling to retain the alloys in the solid solution. As time progresses, the supersaturated elements tend to precipitate (or simply distort the lattice), and eventually some actually develop an extremely fine microstructure. Depending on the volume, hardness increases slowly during this process of precipitation. For example, 2xxx alloys attain full hardening in a few days, but the 6xxx and 7xxx alloys may take years to complete full hardening naturally.



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