Friction Stir Weld (FSW)

Friction Stir Welding (FSW) is a solid state joining process that produces improved joint properties as compared to conventional fusion welding techniques. Fusion welding is characterized by melting of the material, adding a filler rod, and natural cooling to form a weld. A shielding gas is required to protect the weld from contamination. FSW is accomplished at temperatures below the melting point of the material, and does not introduce filler rod nor require shielding gas to form the joint. By its nature, FSW eliminates problems associated with cooling from the melted condition such as porosity, solidification cracking and shrinkage. In comparison, FSW has been found to produce nearly defect free joints.

Friction Stir Welding (FSW) is a solid state joining process that produces improved joint properties to produce nearly defect free joints

How does FSW work

Friction stir welding is basically a metal working process that uses principals that have been understood for a long time.  It is sometimes seen as a special subset of cold forging technique.  The two elements to be joined are firmly clamped in their final joined position. A specially shaped rotating pin of a hard alloy is precisely traversed along the joint line. The rotation of the pin and tool shoulder plasticize the material, move it across the joint boundary, and then allow it to cool and consolidate.  To anyone used to the drama of arc welding, the process is pure magic in its silent elegance.  No blinding plasma arc, no dangerous sparks, no arc noise, no dirt, no noxious gasses and no contamination of the joint.  And you get a faster joining process with almost no post-processing of the surface.  Fusion welding and riveting are inferior techniques in almost every aspect. How does it work?  Very well!

Plunging

In order to perform a solid state bond of the native materials, a FSW tool consisting of a pin and shoulder are used to provide frictional heating and “Stirring” of the material to be welded. The tool is typically mounted in a spindle similar to that used on milling machines. Initially, the tool is rotated at a controlled speed and gradually plunged into the seam representing the butt line of the materials to be welded. Frictional heat developed between the rotating pin and work piece cause the material to soften thereby allowing the tool to continue plunging into the seam. The tool will continue plunging until the shoulder comes in contact with the upper surface of the plates. In order to generate additional heat, it is common practice to “dwell” or maintain position of the tool at the entry point. Dwell time can very as a function of material thickness and alloy as required to plasticize the material before traversing starts.

Traversing

The FSW tool is now traversed along a programmed path to follow the desired butt line. Frictional heat from the shoulder, and mixing heat generated by the pin, keeps the material in a plastic, workable state. The pin will “stir” or “extrude” the material from the leading edge and deposit it on the retreating side. The depositing of material can be characterized as forging process as the extruded material is consolidated or “compressed’ on the retreating side. The resulting bond creates a fine grained mixture of native material with nearly defect free properties.

Termination

There are several methods of terminating a FSW weld. Generally, forward motion is stopped, and then the tool is retracted from the material. If the pin and shoulder are retracted concurrently, a retraction hole will be left in the material as the absence of heat generated by the shoulder allows the material to cool around the pin. Alternately, the pin can be retracted over the last several seconds of a weld while maintaining shoulder contact. This allows heating from the shoulder to continue as the pin is retracted, and the void of material left by the retracting pin is able to fill and solidify. The end result is a termination point that is free of any exit holes. This method is commonly referred to a Retractable Pin, and was originally invented by NASA in the late 1990’s by a team of welding engineers, and consequently patented and licensed.

FSW Advantages

There are many advantages to FSW as compared to fusion welding. Key advantages of the process are related to the ability of welding materials that are often difficult to weld conventionally. This includes 2000 and 7000 series aluminum which are commonly used in the aerospace industries. Equally important is improved weld strength, fatigue, and bending properties that are achieved by FSW. In some cases an FSW weld can approach 90% of the base material strength.The FSW process can offer a significant economical savings as compared to fusion welding. Single pass FSW welds can be accommodated up to 50mm thick with current technology. Fusion welding may require up to 8 passes on a 50mm part. The cost of filler rod, gas, and skilled labor is reduced by using the FSW process.

FSW Tool Types

The friction stir welding pin and shoulder profiles (FSW Tool) are key elements in determining joining performance and joint quality. The FSW Tool affects the joint’s static strength, fatigue strength, corrosion resistance, translation force requirements, processing speeds and control of the metal flow. A wide range of FSW Tool geometries are available, each of which is used for a specific joint and/or material application. Three pin configurations of varying geometries are typically used; fixed, retractable, and self-reacting. Fixed pins do not move relative to the FSW shoulder. Retractable pins can change position relative to the FSW shoulder to accommodate varying material thickness and elimination of the “pin hole” at the completion of a weld. “Self-Reacting”, or commonly referred to “Bobbin” is often used to produce FSW joints. Unlike a conventional tool consisting of a single shoulder, a Self-Reacting tool contains two shoulders. One shoulder is located on the top surface of the work pieces, and the other shoulder is located on the bottom of the work pieces. This eliminates the need for a backing mandrel and reduces the process forces imposed on the welding head. FSW Tool materials play an important role as they require low wear, high strength at elevated temperatures, and good fatigue properties. Tool steels such as MP159, AISI H13, and Densimet are commonly used in aluminum welds. Special alloy material, like Polycrystalline Cubic Boron Nitride (PCBN), are being developed

Shapes and Joints

Production manufacturing is always looking for better, stronger, faster and more cost effective joint design technology. FSW is ideal for a number of common and highly used joint designs:

  • Full and partial penetration butt joints
  • Lap penetration joints
  • Tee joints
  • Corner joints
  • Lap fillet joints
  • Double sided butt joints
  • Different thickness butt joints
  • Operation and Maintenance manuals
  • Dissimilar material joints
  • Spot weld and skip weld joints

FSW is being used to join aluminum/foam sandwich panels without distorting the thin skins or melting the sandwich materials. FSW can also be used to weld aluminum castings without having problems with entrapped high-pressure gas pockets. The low thermal input, precision and repeatability of FSW give the designer a new range of joining possibilities that were previously technically or economically impossible.