Friction Stud Welding

The technique of friction stud welding is a rather expensive process compared to that of arc stud welding. The high cost makes this technique more suitable for special applications where arc welding may present problems. These areas are underwater welding, welding on live subsea pipelines in order to attach anodes, joining materials that are commonly difficult to join by standard fusion welding processes, friction plug welding and welding pipe fittings onto flat or curved surfaces.
Friction stud welding can be done virtually anywhere because of the portable equipment that has been developed. This makes it easier for welders who need to use this technique while on a construction site, offshore, underwater and in workshops. The portable units are much lighter and smaller than the larger static friction welding machines that are commonly used in factories to weld engine components.

A portable friction stud welding tool features a motor which is used to rotate the stud at high speed and a piston which is used to apply the necessary force to the stud. The unit may be either powered by air or by a hydraulic system. The welder must also use a clamping system which holds the tool onto the work piece in order to provide reaction to the force on the stud. The most common style of clamps used is either magnetic or vacuum for flat surfaces. Chain or claw clamps are required when working with pipes and other various mechanical clamps are used when welding onto I beams or other odd shaped materials.

A friction stud weld is created by rotating the stud at high speed and then forcing it onto the substrate which causes friction which then heats the stud tip and substrate surface. Metal which is located at the interface between the stud and the substrate flows plastically when under this pressure. The high speed at which the metal is spinning causes any impurities to be forced from the metal, at which point a solid phase weld is created. While this technique creates high rates of speed and heat, the temperatures are much lower than the melting points of the metals that are being used.
While this process creates sound welds there are also some disadvantages as well as other advantages of using this method. Some of the advantages are that because of the low temperatures that are used, this method of welding can be adapted for use in other applications, because of the absence of an electric arc and a liquid phase the metal does not experience some of the problems that are common with arc welding, due to the rapid weld cycle, which is usually five to ten seconds, and the strength of the weld formed results in a fine grain structure. Some of the disadvantages associated with friction stud welding include the limitation of the process only being able to be used to weld small components onto a work piece and that the system requires a rigid clamp to hold the welding tool onto the work piece, using these clamps can become troublesome because the welder is required to continually move the clamps from location to another.

Welding underwater is a common area where friction stud welding is commonly used. When friction stud welding is used underwater, a shroud must be fitted over the stud to prevent the weld from cooling too fast because of the temperature of the surrounding water. An air powered system can be used underwater up to a depth of twenty meters and are quite easy for divers to use underwater. A hydraulically powered system can also be used by divers and can be used at far greater depths. Hydraulic units can typically be used in depths over 300 meters deep from a remotely operated vehicle. Friction stud welding units that are in use today were designed to be able to function in depths of up to one thousand meters.
Friction stud welding is also commonly used to retrofit sacrificial anodes onto subsea pipelines while the pipeline is in operation. In many cases the anodes are placed on the sea bed next to the pipeline, and then a lug placed on a cable coming from the anode is then connected to the stud and then welded onto the pipeline. Another method of friction stud welding onto a pipeline is using a tripartite weld. This type of weld consists of a lug on the anode cable that is primarily made from steel and features a tapered hole. The tapered end of the stud then welds through the hole and onto the pipeline welding to both the lug and the pipe and creating a fully welded connection between the anode cable and the pipeline.
Another use of friction stud welding is pipe fittings onto flat or curved surfaces. Using a portable unit the stud can be replaced by a small pipe nipple which can then be friction welded directly onto a flat or curved surface. This is a favorite method among welders to weld onto storage tanks or pressure vessels.
There are many other fields that can benefit from the use of friction stud welding, as its uses continue to be discovered the sky is the limit. As further advances are discovered, welders will benefit from the ability to use such a strong technique in a multitude of uses.

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