ORNL researchers (from left) Zhili Feng, Stan David, and Alan Frederic display a length of wire more than 15 feet long fabricated with the friction-stir extrusion method. They eventually ran out of the magnesium-aluminum alloy feed stock. Friction-stir extrusion is an energy-efficient method for making wire from high-temperature, recyclable materials. Photo: Oak Ridge National Laboratory |
Researchers Zhili Feng, Alan Frederic, and
Stan David in Oak Ridge National Laboratory’s Materials S&T Division have
made significant progress toward a new metal processing technique, called
friction-stir extrusion, that could represent a major advance in converting
recyclable materials—such as alloys of aluminum, magnesium, and titanium
alloys, and even high-temperature superconductors—to useful products.
The process also represents a step forward
in energy-efficient industrial processes in that it eliminates the melting step
in conventional metal recycling and processing. The friction-stir method, as
the name implies, derives its heat from spinning metal against metal, and
direct conversion of mechanical energy to thermal energy as frictional heat generated
between two surfaces.
The ORNL team produced a solid wire of a
magnesium-aluminum alloy from machined chips, eliminating the energy and labor
intensive processes of melting and casting.
“This process is very simple. You get
the product form that you want by just using the frictional heat,” says
Stan David, an ORNL retiree and consultant who once led the division’s
Materials Joining group.
The new approach provides an opportunity to
efficiently produce highly engineered structural and functional materials.
Friction extrusion can be developed into metal recycling process of steels, Al
alloys, and other recyclable metals. It is suitable to produce a variety of
bulk nanomaterials such as nano-engineered ODS alloys. It also has the
potential to produce nano grain structure bulk materials. The impact of
economically producing nano-engineered creep resistance Al conductors in large
quantity will be enormous for the power transmission industry.
Friction-stir extrusion could also represent
a new route to the fabrication of extremely specialized materials, such as
high-temperature superconducting wires and mechanical alloyed materials.
“The process of melting and casting can
destroy the properties of a highly ordered, novel material such as an oxide
dispersion strengthened materials or a high-temperature superconductors.
Because friction-stir only takes the material up to ‘plasticizing’
temperatures, the properties of the material are not affected as much,”
says Zhili Feng, who now leads the ORNL group.
The extrusion process follows the same
principle of the friction-stir welding, in which a rapidly spinning tool is
applied to the metal, heating it until it becomes soft, but not melted. Because
the material is still in its solid state when it is extruded, it suffers none
of the degradation and transformation that would occur with actual melting.
“The process of melting is very detrimental
to those properties,” says Feng.
Wayne Thomas, who pioneered the friction
stir technology at The Welding Institute in England, says ORNL has proved the
basic principle of a new technique that could be key to working with advanced
alloys, including high-temperature superconductors.
“It is very difficult to mix silicon,
titanium, magnesium, and other materials in to alloys and turn them into molten
metals. If you can mix them in the solid phase, it is much better, and there
are mixtures you can’t even consider outside a solid phase,” Thomas says.
One such application is the fabrication of
mechanically alloyed magnesium alloys into components. Friction-stir extrusion
has potential to be a low-cost way to produce product forms with this
lightweight and high-performance metal. ORNL is extensively involved in the
magnesium alloy R&D and technology transfer and commercialization.
The energy savings of this
process are significant: The process requires only 10 to 20% of the energy
required for conventional melting with potential saving of more than 80%.
