Material flow behavior during friction stir welding of aluminum

Friction stir welding (FSW) is a new technique for joining aluminum alloys. Invented in 1991 at The Welding Institute (Ref. 1), this technique results in low distortion and high joint strength compared with other techniques, and is capable of joining all aluminum alloys. To date, the majority of research has concentrated on developing the tools and procedures for making reliable welds in a variety of alloys, on characterizing the properties of welds and on developing design allowables (Refs. 2-7). However, very little is known about material flow behavior during welding. The purpose of the current study is to document the movement of material during friction stir welding as a means of developing a conceptual model of the deformation process. In this paper, two new techniques for visualizing material flow patterns in friction stir welds are presented. Based on measured results in welds of 6061 and 7075 aluminum, material movement within friction stir welds is by either simple extrusion or chaotic mixing, depending on where within the weld zone the material originates. These results impact the development of welding procedures and suggest ways to model the process for predicting welding tool performance.