Friction stir welding (FSW) is a solidstate welding process that shows promise in the aerospace industry. A new system of experimentation has been used to quickly make and screen new tool designs. After conducting a literature review, friction stir tools were designed to optimize material flow around the tool.
Plastic prototypes of these tools were produced. FSW has been simulated by running these tools through clay with a milling machine. Surface appearance, nugget appearance, void size and location, and the effects of plunge depth and shoulders on mixing in FSW clay were all similar to metallic FSW, and show promise for using this technique as an inexpensive way to test the effectiveness of tool design.
A rotating FSW tool is slowly plunged into two adjacent pieces of metal and traverses along the seam between them. Friction and plastic deformation of material by the pin creates heat that lowers the yield strength of the material. The deformed material flows behind the pin, and is forged by the downward force on the shoulder resulting in the recrystallization of the material. The two pieces are “stirred” together and a weld is created between them.
Aluminum alloys of 2xxx and 7xxx-series previously considered “unweldable” by traditional methods can be welded with FSW. Aluminum alloys, titanium alloys, copper alloys, lead, zinc, magnesium, ferrous alloys (including stainless steel), and nickel alloys can all be welded using FSW. This process is highly repeatable, and can weld most joint configurations and structural shapes. FSW is also environmentally friendly, producing no harmful emissions, and requiring no solvents for degreasing.
This process is more cost-effective than other forms of welding because few consumables are required, and surface cleaning is minimal. FSW has lower residual stresses than other welding methods, and does not encounter solidification cracking. FSW components can result in lightweight aerospace machines that require less fuel. For example, the space shuttle external tank was originally made with 2219, but later was composed of 2195, lowering its weight, but increasing its weld complexity. Using FSW, this process becomes simpler and more cost-effective because of its repeatability and reduced weight. FSW also shows promise for use to quickly combine aluminum with carbon nanotubes to produce advanced composites.
Thirty-one new pin and shoulder prototypes were tested, and some were found to be superior to conventional friction stir tools based on preliminary testing.