Original Date: 10/07/2002
Revision Date: 01/18/2007
Best Practice : Friction Stir Welding for Aluminum Alloys
Friction stir welding is a new joining technology that is being pursued by several industry sectors to enhance material properties, improve fabricability, and boost affordability to meet ever increasing design requirements for new products. General Tool Company has taken the initiative to embrace this new technology.
General Tool Company (GTC) obtained a job to produce a number of large units for the National Ignition Facility of the Lawrence Livermore National Laboratory (LLNL)/University of California. 194 units were to be produced during a two-year period at a value of $12 Million. This job opportunity represented a significant portion of business for GTC. The job, as originally quoted, consisted of producing a box type assembly made from 356-T6 aluminum alloy casting. This required a huge casting of inconsistent quality to be precision machined extensively to achieve the final product shape and configuration. In an attempt to reduce the cost and enhance the quality of the units, GTC suggested producing the units as a weld fabricated assembly using extruded shapes and plates of 6061 aluminum alloy. An estimated $1 Million was saved by producing the units as a welded fabrication. However, producing a weld fabricated assembly from aluminum presented a number of challenges. Items to be considered were the employed welding process, weld quality, distortion, inspection, and possible repair/rework schemes. Several conventional arc welding and high energy density weld processes were considered (e.g., plasma arc welding, gas tungsten arc welding, gas metal arc welding, electron beam). The merits of each of the weld processes were evaluated with respect to the other items mentioned.
GTC became aware of a new welding process, Friction Stir Welding (FSW) developed by The Weld Institute in Great Britain, as a method of fabrication. The process can create weldments in aluminum alloys with enhanced mechanical properties and less distortion compared to conventional welding processes. FSW is accomplished by frictional heating caused by a rotating tool that is plunged into the seam of a weld joint and traversing it along. As the tool is rotating and traversing, the base material is frictionally heated to a temperature in which the material can be plastically deformed and forged, thus producing coalescence between the materials to be joined.
GTC embraced the FSW process. Because there was no affordable or quick commercial source for FSW equipment, GTC, using its knowledge of machining equipment and resources, designed and built an FSW machine using specifications from The Welding Institute (TWI) in England. The equipment was built, tooling was made, a joining process procedure was developed in 20 weeks for this application and approximately $1 Million in savings was realized. The FSW fabrication of the units for LLNL was a technical and financial success. Because of the financial success with this project, GTC shared some of its cost savings with LLNL.
The success of this project with GTC has not gone unnoticed. As a result of its success in making FSW equipment, Lockheed Martin Manned Space Systems contracted GTC for NASA to build three machines to make longitudinal welds in the space shuttle main tank. Two of the three machines made by GTC are among the largest vertical FSW systems produced in the world. Other machines are similar in size, but do not have the retractable pin spindles. Figure 2-3 depicts the longitudinal welds to be friction stir welded for the shuttle's main tank. GTC has received several benefits using FSW technology by acquiring new markets for design and build of FSW equipment (AccuStir × ), new job shop business, a new process to take to existing customers, and increased global exposure.
Figure 2-3. Space Shuttle Main Tank and Welds
For more information see the
Point of Contact for this survey.