Rotary Friction Welding for Oilfield Drill Pipe

This is very interesting to see how the body of drill pipe and tool joint together using the rotary friction welding.

The friction welding or FRW is solid-state welding process which generates the heat using the friction between the work pieces in relative motion to each other, with addition of lateral force known as “upset” in order to displace and fuse materials plastically. Technically, no melt takes place and in the traditional sense friction welding isn’t a process of welding, rather it is a forging technique. Because of similarities between traditional welding and these techniques, it has become a commonly used term. The friction welding is used with thermoplastics and metals in a variety of automotive and aviation applications.

Benefits of Friction Welding

Combination of the quick joining times (on order of just few seconds), and the direct input of heat at weld interface, results in relatively smaller zones being affected by heat. Generally the techniques of friction welding are melt-free, which avoids the grain growth in the engineered materials, like high strength heat-treated steels. There is another advantage of this that motion tends to “clean” surface between materials that are being welded. This means that less preparation is required for joining them. During the process of welding, depending on which method is used, small metal or plastic pieces are forced out of working mass (flash). It’s a common belief that flash carries away the dirt and debris.

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Friction welding has another advantage that it allows the joining of dissimilar materials. It is especially useful in the aerospace, where it’s used to join the lightweight aluminum stock in order to high-strength the steels. Usually the huge difference in the melting points of these two materials makes it impossible to weld using the traditional techniques, and some kind of mechanical connection will be required. The friction welding gives a bond of “full strength” without any additional weights. Such bi-metal joins have other uses in nuclear industry as the steel-copper joints are very common in reactor cooling systems as well as in transport of the cryogenic fluids where the friction welding is used for joining the aluminum alloys to the stainless steel and high nickel alloy materials for the cryogenic fluid piping and the containment vessels.

Another use of friction welding is with thermoplastics. These act in fashion analogous to the metals under pressure and heat. The pressure and heat used on the materials is lower than that on metals, but this technique is used for joining metals to plastics with material interface being machined. For example, this technique might be used for joining the frames of eyeglasses to pins in the hinges. As lower pressures and energies are used, it allows for more techniques to be used.

The Rotary Friction Welding

Rotary friction welding is the standard method of the industry for various processes including the welding API drill rods and drill pipes, joining of spindles, axle cases and welding of piston rods. In this process one component is held still while other component spins and the two components are brought together.

This is a robust and flexible process. It is also tolerant to various qualities of the materials. Parameters involved in this process are rotational speed, the time and the force applied. Thompson has calculated the optimum parameters for each particular weld.

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