THE INFLUENCE OF FRICTION STIR WELDING CONDITIONS ON THERMAL STABILITY OF АА6061 ALLOY

Abstract

Friction stir welding (FSW) is an innovative technology for the solid-phase joining of metal materials. It allows producing permanent joints of materials conventionally considered to be nonweldable, in particular aluminum alloys. However, an essential drawback of FSW is the relatively low stability of the stir zone microstructure. In particular, during post-weld heat treatment, seams frequently demonstrate abnormal grain growth. Such an undesirable phenomenon is often interpreted in terms of the so-called Humphrey’s cellular model, according to which the abnormal behavior is attributed to the essential microstructure refinement and the dissolution of the second-phase particles occurring during FSW. Since these two processes significantly depend on the temperature, the authors suggested that the thermal stability of the produced FSW seams should also be associated with the FSW heat conditions. To test this hypothesis, the authors obtained two welded seams at different FSW conditions and then studied their microstructural behavior during T6 mode thermal treatment (involving solution heat treatment followed by artificial aging). The authors used the advanced electron backscatter diffraction technique (EBSD) to investigate microstructure. In full accordance with the initial idea, the investigation showed that microstructural evolution in both studied microstructure states varied wildly. Specifically, the study identified that the reduction in the FSW temperature causes the suppression of abnormal grain growth. The authors suggested that the enhanced thermal stability of the material is associated with the conservation of the second-phase particles during the low-temperature FSW.