Friction stir welding, a relatively new process, is having a tremendous impact on welding dissimilar alloys. It was developed in 1991 by the Welding Institute, a British research and technology organization, and is used in automotive, aircraft, electrical, and aerospace industries. The process transforms metals from a solid state into a "plasticlike" state and then mechanically stirs the materials together under pressure to form a welded joint.
Friction stir welding succeeds where other welding technologies fail. It melds together "unweldable" alloys like 7075 aluminum, various forms of aluminum/lithium, and 2519 armor plate aluminum. Friction stir welded joints show minimal changes in structural properties of the parent materials and extremely low dimensional distortion. It is especially useful when strength and ductility are important.
Friction stir welding is unlike traditional welding methods. In the process a cylindrical shouldered tool with a profiled pin is inserted into the joint line between the two pieces being joined. The shoulder spins, creating frictional heat between the wear-resistant pin and two work pieces, which are butted together and clamped onto a backing bar. Heat softens the materials but doesn't reach the melting point, and the pin traverses the entire joint.
As the tool moves, frictional heat plasticizes the material as it travels from the front of the rotating pin to the back. Here it consolidates and cools, forming a solid-state weld. Unlike fusion welding, there is no actual melting and the weld has the same fine-grained condition as the parent metal.
Most friction stir welding systems use a retractable pin in the rotating shoulder to prevent keyholes in the weld. The pin retracts or expands within the material to allow tapered welds on thin-to-thick parts, circumferential tank welds, and other complex operations not possible with other systems.
STRONG WELDS, LOW DISTORTION
Unlike fusion welding where pieces are melted and resolidified to form a new and different material structure, friction stir welding produces solid-phase bonds between parts. The plasticized material is stirred and mixes across the joint boundary. This produces welds both strong and ductile, with up to twice the fatigue resistance of fusion welds. Often the weld has a finer microstructure than the parent material.
A significant benefit of friction stir welding is that it has few variables that must be controlled. In a fusion weld, for instance, technicians have to closely monitor the purge gas, voltage, amperage, wire feed, travel speed, shield gas, and arc gap. In stir welding, there are only three variables: rotation speed, travel speed, and pressure, which are all easily controlled.
Stir welding is also safer compared to other welding techniques. It does not create or use fumes, radiation, high voltages, liquid metals, or arcing. It also does not require welder certification.
BRINGING IT TO LIFE
The main factors standing in the way of widespread use of friction stir welding are cost and unfamiliarity. But the initial investment can yield long-term savings in terms of speed, repair, and ease of maintenance. Stir-welding equipment with data-acquisition capabilities helps bolster statistical-process control. It is feasible to produce consistently good welds by monitoring weld parameters.
Developments in production tooling from companies like MTS Systems, and MCE Technology, are rapidly bringing friction stir welding into the foreground. It has already proved itself in aerospace and aviation. For example, it's been used to weld a full-scale external tank hydrogen barrel at NASA's Marshall Center, Huntsville, Ala. NASA will also use stir welding on longitudinal barrel welds on both the liquid oxygen and hydrogen tanks.
MCE Technology, Seattle, is developing the largest fiveaxis curvilinear stir-welding machine for Concurrent Technologies Corp. of Johnstown, Pa. It will be used to weld materials up to 2.0-in. thick and on the amphibious assault-vehicle program. The unit has a retractable pin and independent pin rotation speed, and will handle workpieces 26 ft long and 13 ft high.
Friction stir welding was also used in the assembly of the Eclipse 500 jet from Eclipse Aviation Corp, Albuquerque. It is the first company to use friction stir welding in production on thin-gauge aircraft aluminum. The technology eliminates thousands of rivets, resulting in reduced assembly costs, better quality joining, and stronger and lighter joints. Because stir welding is significantly faster than other structural jointing processes production cycle times are drastically reduced. Friction stir welding also replaces rivets in most major assemblies of the Eclipse 500 including the cabin, aft fuselage, wings, and engine mounts.
MTS Systems Corp of Eden Prairie, Minn., developed Eclipse Aviation's system and assisted in developing weld patterns. MTS is also developing a patented self-reacting tool, which will further reduce tooling complexity and cost. Basic stir-welding systems use a conforming anvil to counteract the forging force of the shoulder tool. The self-reacting tool has two shoulders to counteract the load and produce a better weld. The shoulders are controlled independently and eliminate the need for an anvil.
According to the Friction Stir Welding Assoc., weld zones can be divided into distinct regions as follows:
Unaffected material or parent metal: This is material remote from the weld, and has not been deformed. It may experience a thermal cycle from the weld but is not affected by the heat in terms of microstructure or mechanical properties.
Heat-affected zone: In this region material has experienced a thermal cycle which has modified the microstructure or mechanical properties. However, there is no plastic deformation. This is often referred to as the "thermally affected zone."
Thermomechanically affected zone (TMAZ): In this region, material has been plastically deformed by the friction stir welding tool, and heat from the process has exerted its influence on the material. In the case of aluminum, it is possible to get significant plastic strain without recrystallization in this region, and there are generally distinct boundaries between recrystallized zones and deformed zones of the TMAZ.
Weld nugget: The recrystallized area in the TMAZ is traditionally called the nugget.
Beyond friction stir welding
Friction stir welding looks like a promising technology. In friction stir welding a cylindrical rotating shoulder passes along a joint, plasticizes the material, and creates a solid-state weld. Developed in Britain in 1991, it is heralded as a safer alternative to fusion welding, with better mechanical properties. It joins materials that were unweldable, such as armor plate aluminum and dissimilar metals.
But there are still several drawbacks that need to be addressed before stir welding can make a big impact. For starters, powerful clamping fixtures are needed to hold workpieces down and counteract forging forces from the probe tool. The system requires high forces to move the shoulder through the plasticized material which, in turn, wears down the shoulder.
According to the American Welding Society, several variations of friction stir welding are being developed. One patent-pending alternative uses an induction coil moving in front of the rotating tool to provide controlled heating beneath the probe. The rotating pin would control flow of the preheated material and break up any oxide skin from welded parts.
But there are problems with this method also. It is hard to focus the induction coil on a specific location, and the coil heats all conductive materials, including the shoulder and clamping tools. The coil can only heat conductive materials, which excludes nonmetallic and nonconductive materials. Also, induced currents may flow across the path of the weld and generate sparks.
Another patent-pending system, laser-assisted friction stir welding, shows more promise. It combines a conventional milling machine and a Nd:YAG laser. The laser preheats workpieces ahead of the shoulder and the probe moves through the material as in conventional stir welding. Less force is needed to push the probe through the material so it moves faster and wears less. Heat is also concentrated to avoid damaging other parts of the system.
Experiments have produced defect-free welds with laser-assisted friction stir welding. Since lasers can heat nonconductive materials like plastics or ceramics, this system has a lot of potential.