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Advanced Forming Techniques "Stretch" the Potential for Aluminum Sheet Applications
12-13-02
An ongoing challenge for the application of aluminum sheet in certain automotive components has been its lower stretch formability compared to sheet steel. Ongoing development of three advanced forming techniques-superplastic forming, electromagnetic forming and hydroforming-offers the potential for overcoming some of the current limits. As always is the case, the probability of adoption will be a result of both technical capability and cost-effectiveness.
Superplastic Forming
While typical aluminum alloy sheet can elongate 10-30% during forming, a superplastic material can achieve ten times these levels or more. While some early observations of exceedingly high elongations at slow strain rates and elevated temperatures were observed for specialized materials in the 1930's, it wasn't until the 1960's that superplasticity started to gain interest as anything more than a laboratory curiosity. Superplastic forming of aluminum alloys has been a niche area for the last few decades, primarily for aerospace and specialty automobiles, and is now receiving increased attention as a result of new developments.
Superplastic forming requires two components: a superplastic alloy as well as a special, high temperature, relatively low strain rate forming process for making the desired shape. For aluminum alloys, the primary metallurgical mechanism for achieving high elongations under superplastic forming conditions is grain boundary sliding accommodated by dislocation slip, which points to fine grain size in the sheet material, typically on the order of 10 microns or so, as the key attribute that's needed. Efforts to produce commercial sheet products with superplastic forming capability have primarily involved specialized alloy compositions and rolling treatments designed to develop and stabilize the fine grains at the elevated temperatures of the superplastic forming process.
The superplastic forming process typically is carried out at temperatures close to the typical alloy solution heat treatment temperatures. Forming rates expressed in terms of strain rate are 10-3-10-4 per second, which translates to several minutes to form a moderately complex part shape. Nevertheless, the substantial increase in elongation capability for the aluminum alloys with suitable microstructures under these conditions allows production of substantially more complex formed shapes, which can in turn enable part consolidation and elimination of fasteners and joints. These benefits are used to offset the higher costs of the specially prepared sheet material as well as the slow forming rates. Typical alloys available for superplastic forming are 2004 (also known as Supral), 2090, Weldalite Al-Li alloy, and special-processed 7475. Applications have included specialized aircraft components such as cowlings and auto body components.
Further extension of this process into larger scale use requires reduction in the costs of superplastically formed parts as well as increased forming rates more comparable to conventional stamping processes. One very promising development in this direction is the discovery of what has been termed high strain rate superplasticity. It has been found that aluminum-based materials with grain sizes about an order of magnitude smaller than conventional superplastic sheet products, i.e. on the order of 1 micron or so, exhibit superplastic behavior at lower temperatures and significantly higher strain rates of 10-2-101 per second, which could substantially reduce the forming times into the range of conventional warm forming and possibly even stamping. The challenge remaining here is that the aluminum materials showing these high strain rate superplastic characteristics are metal matrix composites or other alloys produced by specialized processes, and so the material cost remains high. Recent work on potentially lower cost processes such as friction stir processing and spray forming to produce the very fine grained alloys should be watched as an indicator for future commercial potential.
For those interested in further information on superplasticity and superplastic
forming, a web site maintained by Prof. John Pilling at Michigan Technological
University at www.mse.mtu.edu/~drjohn
is recommended.
Electromagnetic Forming
Electromagnetic forming utilizes energy discharged from capacitors to accelerate the sheet into the die cavity at high velocities, on the order of 200 m/s. It has been found that utilizing high forming rates enables the sheet to be stretched without fracturing, a phenomenon referred to by researchers at Ohio State University as "hyperplasticity". In interesting contrast to the conditions for achieving increased ductility for superplastic forming, electromagnetic forming is done at room temperature, at high strain rates, and apparently on sheet materials that don't require any special processing to modify their microstructure for the process. The theory for improved formability in this case is a resistance to the thinning in the sheet metal that occurs prior to fracture at conventional forming rates by inertia. In addition to enhanced formability (elongations on the order of 100% for aluminum alloy sheet has been achieved), the process also reduces springback and wrinkling of the sheet during the forming process. Work is underway at Ohio State in collaboration with the Partnership for New Generation Vehicle (PNGV) program to address issues related to the capital equipment and coil design for implementing electromagnetic forming in a commercial forming operation. In some cases, electromagnetic forming is seen as an add-on to conventional stamping, with the purpose of locally increasing the formability of the sheet in critical regions.
Further information on this emerging technique can be found at a web site maintained
by the Ohio State group at www.matsceng.ohio-state.edu/~daehn/hyperplasticity.html.
Hydroforming
Hydroforming is a technique applied to tubular products in which a fluid medium such as water is used inside the tube to apply internal pressure that results in forming of the tube to the shape of a surrounding die. Initial hydroformed aluminum automotive applications utilized roll formed and seam welded tubes of the 5xxx or 6xxx alloy series, with increasing interest in the use of extruded aluminum sections as an alternative starting stock.
The hydroforming process has been applied routinely for forming of steel tubes,
driven by the ability to produce complex, non-symmetrical cross sectional shapes
and subsequently reduce the number of required joints and increase material utilization.
These same benefits are expected for aluminum hydroformed parts, and automotive
parts, including chassis and frame members, cross members and cradles, and other
closed-section parts, have been considered and in some cases prototyped. Tooling
design and development represent a key challenges for aluminum hydroforming, since
typically cold-bending steps along with forming and thermal treatments may create
a complex material history. Modeling efforts are directed toward better simulating
the process to assist tool and forming process design. Efforts underway in this
area include work sponsored by the USCAR consortium (see www.uscar.org),
as well as research activities at the Ohio State ERC/NSM (see http://nsmwww.eng.ohio-state.edu/html/tube_hydroforming.html
). A resource on the web for links to hydroforming companies and technologies
can be found at http://www.hydroforming.net.
Article provided courtesy of The Aluminum Association - www.aluminum.org
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