The comprehensive body of knowledge that has built up with respect to the friction stir welding (FSW) of aluminium alloys since the technique was invented in 1991 is reviewed. The basic principles of FSW are described, including thermal history and metal flow, before discussing how process parameters affect the weld microstructure and the likelihood of entraining defects. After introducing the characteristic macroscopic features, the microstructural development and related distribution of hardness are reviewed in some detail for the two classes of wrought aluminium alloy (non-heat-treatable and heat-treatable). Finally, the range of mechanical properties that can be achieved is discussed, including consideration of residual stress, fracture, fatigue and corrosion. It is demonstrated that FSW of aluminium is becoming an increasingly mature technology with numerous commercial applications. In spite of this, much remains to be learned about the process and opportunities for further research and development are identified.

Linear friction welding (LFW) is a solid state joining process in which a joint between two metals can be formed through the intimate contact of a plasticised layer at the interface of the adjoining specimens. This plasticised layer is created through a combination of frictional heating, which occurs as a result of pushing a stationary workpiece against one that is moving in a linear reciprocating manner, and applied force. The process is currently established as a niche technology for the fabrication of titanium alloy bladed disc (blisk) assemblies in aeroengines, and is being developed for nickel based superalloy assemblies. However, interest is growing in utilising the process in a wider range of applications that also employ non-aeroengine metallic materials. Therefore, it is the objective of this report to provide a broad view of the capabilities of the LFW process for joining metals. This review paper will cover relevant published work conducted to date on LFW. The basics of the process and the fundamental aspects of operating a LFW machine will first be described, followed by a description of the different materials that have been welded using the process. The review will then go on to describe the microstructural changes, including texture variations, and residual stresses that are produced as a result of the welding process.

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Electron backscattering diffraction (EBSD) was used to study a stationary shoulder friction stir weld in Ti-6Al-4V. Weld temperatures exceeded the b-transus, resulting in a supertransus zone (STZ) that encompassed all of the thermomechanically affected zone (TMAZ) and a portion of the heat-affected zone (HAZ). Standard EBSD provided limited information on the material behavior at high temperature in the b phase field, so in-house software was used to reconstruct the crystallographic orientations of the high-temperature b phase. The portion of the HAZ that lay within the STZ exhibited the same b texture at high temperature as the retained b phase in the unaffected parent material. In the TMAZ, material was deformed in the high-temperature b phase field and, on cooling, transformed to a fully lamellar microstructure. The b textures at high temperature were dominated by the D 2 " 1 " 12 À Á 111 ½ simple shear texture component. The a phase textures in the fully lamellar microstructure that formed on cooling were inherited from the shear textures of the b phase, but significant variant selection occurred.

The use of a double sided friction stir welding tool (known as a bobbin tool) has the advantage of giving a processed zone in the workpiece which is more or less rectangular in cross section, as opposed the triangular zone which is more typically found when conventional friction stir welding tool designs are used. In addition, the net axial force on the workpiece is almost zero, which has significant beneficial implications in machine design and cost. However, the response of these tools in generating fine microstructures in the nugget area has not been established. The paper presents detailed metallographic analyses of microstructures produced in 25mm AA6082-T6 aluminium wrought alloy, and examines grain size, texture and mechanical properties as a function of processing parameters and tool design, and offers comparison with data from welds made with conventional tools.