August 9, 2005
Reducing weight while maintaining or improving functional requirements is one of the major goals of automotive design and manufacturing, as it decreases fuel consumption and improves structural design. As a result of these considerations, the use of aluminum alloys in car manufacturing continues to increase, not only in body panels but also in structural, power train, and suspension components.
An important component in the assembled front axis support structure is the cross member, which is made from an aluminum-alloy tube extrusion with a rectangular cross section.
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.
Reducing weight while maintaining or improving functional requirements is one of the major goals of automotive design and manufacturing, because it decreases fuel consumption and improves structural design. To reach this goal, automakers continue to increase their use of aluminum alloys in car manufacturing, not only in body panels but also in structural, power train, and suspension components.
An excellent example of this trend is a process used by a German car manufacturer, in which the weight of a front axle structure was reduced about 30 percent using air-assisted forming.
An important component in this structure is the cross member (see Figure 1), which is made from an aluminum-alloy tube extrusion with a rectangular cross section. In addition to aluminum, the 6000 series alloy contains mainly magnesium and silicon. It has good formability, a tensile strength of 200 to 350 N/mm2 and good corrosion resistance.1
The cross member starts as an extruded aluminum-alloy tube that is formed by bending on both ends before being pierced and punched out at appropriate locations.
Figures 2 and 3 show the cross member initially is an extruded tube that has to be formed by bending on both ends before being pierced and punched out at appropriate locations. The company spent considerable effort on tryout and experimentation to develop the tooling for accurate and reproducible bending and forming of this component, without defects such as wrinkling, tearing, or buckling.
The company considered tube hydroforming (THF), but cost estimates for this application were too high, and the process would be too slow. Instead, it hired Schnupp Hydraulik, a manufacturer of hydroforming presses and electrohydraulic systems, to develop an entirely new concept to form the part, as well as the necessary tooling and automation.
The manufacturing technique used in this application was air-supported forming, during which pressurized air acts as a mandrel in the tube to prevent buckling, wrinkling, or tearing during deformation.
In the production process, the aluminum-alloy tube is measured and cut from a long extrusion. The tube is lubricated by a spongelike tool and then placed in the forming die. The part then is centered, and two mechanical plungers move to seal both ends of the part.
The air-assisted forming begins as the tube is internally pressurized to about 70 bars. The pressurized air acts like a flexible mandrel and prevents the material from buckling and wrinkling during deformation. The forming operation takes only five seconds, while the entire automated forming cycle (that is, robotic positioning, forming, knockout, and robotic transport) takes less than 20 seconds.
The cross member is formed using pressurized air as a mandrel to prevent buckling and wrinkling. After forming, holes and end configurations are punched out.
The key to this operation is a hydraulic unit. The hydraulic pressure, generated by several gear pumps, can activate various tool motions under direct drive or using an accumulator. An intensifier, energized by the accumulator and controlled by proportional valves, takes one second to provide the necessary volume of air at 70 bars' pressure. The system also ensures that the pressure is
released after deformation is completed.
The air-assisted forming operation is an essential but relatively small part of the fully automated production line, which turns out about 3,000 cross members per day with a total cycle time of 20 seconds per part.
The use of pressurized air rather than water emulsion, which is used in conventional tube hydroforming, lowers investment costs and keeps the cycle time relatively short. In addition, the formed tubes do not have to be emptied or cleaned since the pressure medium (air) simply can be released to the atmosphere.
This application illustrates how thinking "outside the box" can result in technical and cost-effective success in modern production.
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, 614-292-9267, fax 614-292-7219, www.ercnsm.org.
Note: 1. D. Kuhn, "Manufacturing of Automobile Axle Components From Rectangular Aluminum Extrusions Using Air Assisted Forming" (in German), Das Industrie Magazin, Nr. 10, 2005, p. 28.