Warm forming of aluminum alloys
Academic exercise or practical opportunity?
Federal government and U.S. automakers to develop technologies for a newgeneration of vehicles to triple fuel efficiency without sacrificing performance, affordability, or safety.
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.
The Department of Energy and the U.S. Council for Automotive Research (USCAR) established collaboration between the federal government and U.S. automakers to develop technologies for a new generation of vehicles to triple fuel efficiency without sacrificing performance, affordability, or safety.
Results of this study show that from 20 degrees C (room temperature) up to 175 degrees C, cup height increases just 3 mm(a). However, at 250 degrees C, cup height increases significantly, up to 60 mm(b).
A report by the National Research Council found, "Aluminum is the lightweight material of choice for intensive use" and is critical in reaching the goal of tripling fuel efficiency. Innovative uses of aluminum may help to achieve this higher fuel efficiency standard, reflecting its increasing value as an automotive engineering material.
Aluminum use in automobiles doubled over the past decade and tripled in light trucks. Recently it surpassed plastic in terms of percentage of vehicle content. GM announced that all of its V8 light truck engines will be made of aluminum instead of iron which, when combined with new displacement-on-demand technology, will help to boost fuel efficiency up to 25 percent.
Displacement on demand is one of several power train technologies GM is developing to address fuel economy. An engine is always started on more than one cylinder, but once the vehicle is moving, the displacement on demand is activated. This technology saves fuel by using only half of the engine's cylinders during most normal driving conditions. The system automatically and seamlessly reactivates the other cylinders when the driver needs the engine's full capabilities, brisk acceleration, or load carrying.
Ford Motor Co. also is committed to increasing fuel economy of its light trucks. And within most leading car companies, aluminum will play a greater role in the future.
Aluminum can offer manufacturers a competitive advantage in the following areas:
Fuel savings. A 6 to 8 percent fuel savings is possible for every 10 percent weight reduction.
Emissions. Each pound of aluminum that replaces 2 pounds of steel can save a net 20 pounds of carbon dioxide equivalents over the lifetime of a vehicle.
Recycling. Nearly 90 percent of automotive aluminum is recovered and recycled.
However, the formability of aluminum alloys with yield strengths similar to low-carbon steel is very low. When aluminum stampings are produced with dies designed for steel stampings, fracture usually results in part regions that are under severe stretching.
Warm Forming of Aluminum
Warm forming is a method for improving the formability of aluminum. Interest in warm forming of lightweight materials began in the 1970s, when it was discovered that an aluminum alloy with 6 percent magnesium content could give a 300 percent total elongation at about 250 degrees C.
In warm forming of aluminum, the die and the blank holder usually are heated to a temperature in the range of 200 to 300 degrees C. Many studies show a significant increase in formability of 5XXX and 6XXX series when warm forming is used.
The dies and blank holder are heated with electrical heating rods that are located in the dies. It is necessary to heat up the corners of the dies, because the corners are critical in controlling the metal flow. So, it is not necessary to heat up the entire die in most cases. Straight sides can be cooled by using water or oil. This reduces material flow, similar to the effect of a draw bead.
Many studies have been conducted in warm forming of aluminum alloys. For example, experimental analyses of rectangular conical cups from an aluminum alloy (5754-O) drawn at room temperature (20 degrees C), 100 degrees C, 175 degrees C, and 250 degrees C were conducted by Bolt. Maximum cup heights, obtained without fracture, were compared. These cup heights were 35 mm, 38 mm, 38 mm, and 60 mm for temperatures of 20 degrees C, 100 degrees C, 175 degrees C, and 250 degrees C, respectively.1
Figures 1aand 1bshow the achievable 35-mm and 60-mm cup heights at 20 degrees (room temperature) and 250 degrees, respectively. Results of this study show that from 20 degrees to 175 degrees, cup height increases just 3 mm. However, at 250 degrees, cup height increases significantly, up to 60 mm.
In rectangular conical cup warm forming, the cup wall is formed by stretching the blank over the punch and partly by drawing the flange between the die and the blank holder. Softening of the flange from heating allows more material to be drawn into the die cavity without defects such as tearing or wrinkling.
One of the main problems encountered in warm forming is the lack of a satisfactory lubricant. Various lubricants need to be evaluated for this process, and lubricant suppliers should be encouraged to develop lubricants specifically for the process. A satisfactory lubricant has to meet the following criteria:
• Good lubrication to reduce friction
• Stability at operating temperatures (no fume or smoke)
• Good adhesion
• Ease of application
• Ease of removal
• Low cost
Warm Forming of Magnesium
Magnesium also is a lightweight material that has potential in automotive manufacturing. Warm forming of magnesium alloys currently is being studied in various laboratories around the world. The studies show that deep drawing of magnesium parts for industrial applications is possible, but the forming technology for this alloy must be developed for wide acceptance by the automotive components industry.
Further Development Needed
Warm forming of aluminum alloys offers the possibility of drawing complex aluminum sheet products, which cannot be manufactured at room temperature without extra forming and joining operations. Because of the complexity of various parts and lack of knowledge base, further development work is needed to establish design guidelines for warm forming. Even though the tooling costs for warm forming are higher compared to conventional tooling, there are applications in which this technology will be cost-effective.
Taylan Altan is a professor and director of the Engineering Research Center for Net Shape Manufacturing, 339 Baker Systems, 1971 Neil Ave., Columbus, OH 43210-1271, phone 614-292-9267, fax 614-292-7219, Web site www.ercnsm.org. 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. P.J. Bolt, N.A.M.P. Lamboo, and P.J.C.M. Rozier, "Feasibility of warm drawing of aluminum products"Journal of Material Processing Technology, Vol. 115 (2001), pp.118-121.
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