Friction-stir: Alternative to melting and casting metal

Dr. Neil Canter, Contributing Editor | TLT Tech Beat December 2011

A new technique has been developed that minimizes degradation of metal alloys.

KEY CONCEPTS
• Metal fabrication using melting and casting can lead to alloy degradation by destroying the properties of the original metals under high-temperature and process conditions.
• A technique developed in 1990 called friction-stir processes metal at temperatures below its melting point.
• Friction-stir extrusion shows promise as a process to recycle metals using less energy than current techniques.

Newer techniques for processing metals continue to be developed as a means to improve productivity and reduce the inefficient use of energy. In a previous TLT article, high-pressure torsion was used to boost the strength of aluminum to that of carbon-steel alloys (1). This process was conducted on the widely used aluminum alloy 7075, increasing its strength from 0.4 to 1.0 gigapascals. High-pressure torsion involved the treatment of the metal for 10 revolutions under a pressure of six gigapascals at room temperature.

Dr. Zhili Feng, group leader, Material Joining Group at the Oak Ridge National Laboratory in Oak Ridge, Tenn., says, “Softening metal at temperatures below its melting point can facilitate processing of the metal in a number of ways.” One relatively new approach known as friction-stir was invented in 1990 by Dr. Wayne Thomas of The Welding Institute in the U.K.

Feng adds, “Friction-stir utilizes the frictional heat generated by a rotating tool against the material to be processed at high speeds and under high-pressure conditions to process the material. Often the process facilitates the bonding of one material into another without melting.”

The mechanical energy created as the tool spins against the material to be processed is converted into frictional heat used to facilitate the bonding.

The current technique for metal fabrication is melting and casting. But there are problems with this approach because it can lead to degradation of the metal alloys by destroying the properties of the original metals at high temperatures and process conditions where metal literally is melted. In addition, a very unattractive feature is the high amount of energy that needs to be expended to melt and process the metal. Defect formation during such processing is another disadvantage.

Feng says, “The technique of friction-stir has not been well developed and used as a manufacturing process. We have taken on the challenge to demonstrate the benefits of friction-stir in manufacturing by evaluating how the process can be used to improve productivity and save energy.”

The efforts of Feng and his associates have now shown the feasibility and benefits of using friction-stir as an alternative to melting and casting metal.

EXTRUSION
One of the processes that is under evaluation is friction-stir extrusion. Feng says, “In this application, the friction and heat generated enables the metal to be plasticized so that it can be extruded under pressure. We have demonstrated that a solid metal wire can be prepared from this operation by mixing powder, chips and other feedstock metals.”

The metal feedstocks are mixed in a plunger and then extruded. Feng and co-workers Stan David and Alan Frederic demonstrated this process by preparing a wire consisting of a magnesium-aluminum alloy. Feng says, “We were able to prepare a 15-foot-long piece of wire through the technique of friction-stir extrusion.” A longer wire would have been prepared but the researchers were limited by the amount of feedstock (see Figure 1).


Figure 1. Friction-stir extrusion was used to produce wire from a magnesium-aluminum alloy that is over 15-feet-long. This technique has the potential to fabricate metal in a more efficient manner than melting and casting. (Courtesy of Oak Ridge National Laboratory)

The wire was prepared from metal chips and testing revealed that the processed metal achieved good mechanical properties. Preliminary analysis showed that friction-stir extrusion can display significant energy savings of more than 80% compared to the current melting and casting method. Energy usage dropped from 6.21 to 0.56 kilowatt-hours per kilogram of metal. The key savings occurred because there is no need to melt or homogenize the metal.

Currently, friction-stir extrusion is mainly done on aluminum alloys because of the tooling limitations. Feng says, “Aluminum and other low melting alloys are well suited for friction-stir extrusion because the processing temperatures are low—hence the low wear of the tooling. This enables aluminum to be effectively processed without melting.”

Most wrought and cast aluminum alloys can be processed in this manner. Friction-stir extrusion is also effective in preparing alloys of aluminum with other metals such as titanium and also composites.

One other positive feature is that friction-stir extrusion is well suited for recycling metals, using less energy than current techniques. Feng says, “We believe that friction-stir extrusion will be able to process metal into various forms such as tubes and rods. One of the first viable applications will be recycling aluminum.”

Friction-stir extrusion also is conducted with near-zero emissions of carbon dioxide and no emissions of metal oxide particulates. Since the process is carried out below the metal’s melting point, no fumes are produced.

A future objective is to process ferrous alloys. Feng says, “We are unable to extrude ferrous alloys because the tooling materials currently available cannot withstand the wear and tear during processing of steels. In conjunction with industry partners, we have successfully generated friction-stir welded steels using polycrystalline cubic boron nitride (PCBN).

Another objective is to scale-up the process and make it continuous so that it can be carried out commercially. The researchers are engaged in process refinement and are looking for industry partners.

For further information and to view a recent presentation (2), contact Feng at fengz@ornl.gov.

REFERENCES
1. Canter, N. (2011), “Super-Strong, Ductile Aluminum,” TLT, 67 (1), pp. 10–11.
2. David, S., Feng, Z and Thomas, W. (2011), “Friction Technology for Processing of Advanced Materials: Challenges and Opportunities,” Presented at the Annual Meeting of the International Institute of Welding, Chennai, India.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.