7 - Failure mechanisms in friction stir welds

Abstract

This chapter provides an overview of the types of defect that can occur in friction stir (FS) welds and identifies their influence on failure mechanisms under static and cyclic loading. Consideration will primarily be given to FS welding for linear joints, although some results of a new repair welding technique for pipework, friction taper stud welding (FTSW), will also be presented. FS spot welds will also be briefly considered. Whilst certain defect types are generic across a broad range of alloys, there appear to be several defect categories specific to particular strain hardening FS welded aluminium alloys, which have been termed ‘onion skin’ and planar defects by the present authors,1,2 these will also be discussed in this chapter. Finally, brief attention will be paid to the residual stress distributions arising from FS welding as relationships exist between process parameters, heat input, peak values of longitudinal residual stresses and weld performance.3