For some materials and material combinations the use of fusion welding is not practicable. Intermetallics may be formed at the joint interface or cracking may occur at or near the interface, which restricts the use of the joint in any service application where significant loading has to be withstood. However, while resistance welding, friction welding and their derivatives have been around for many years, the range of solid state joining techniques that are available may not be widely known in the fabrication and construction industry. The advantages of these technologies, apart from the avoidance of problems from molten and solidified material, are that the processing times can be very short and with minimal heat input, so they can also be attractive techniques for joining materials that are weldable by standard processes.
The Process

The key aspect of a solid state joining process is that, after the application of heat to soften the materials, a force is applied to squeeze out the softened material, thereby dispelling any existing surface oxide films that may be present. Friction welding achieves the heating usually by rotary or linear oscillation to promote relative motion of one component against the other, while pressure is applied normal to the faces. When the motion ceases and the final forging pressure is applied, there will be some additional flash extruded from the joint region, which can usually be easily machined away.
Flash Removal from Internal Surfaces

In the case of rock drills, however, the internal flash created in the bore of the tube is not easily accessible for removal, but Thompson Friction Welding has devised a gas pulsing system with oxygen and nitrogen being blown alternately down the bore during processing, which removes the flash as it is formed and can leave a concave or convex inside profile if required, figure 1. Because the softened material is being extruded out of the joint line, allowing clean metal to adhere to clean metal, there are no contamination problems, despite using oxygen.