Experimental Investigation of Three-Dimensional (3-D) Material Flow Pattern in Thick Dissimilar 2050 Friction-Stir Welds

The current microstructural investigation performed at various scales deals with the three-dimensional (3-D) material flow in thick dissimilar Airware™ 2050 friction-stir butt welds (Airware, Newport Beach, CA) because of the scarcity of the results obtained with thicker than 8 mm joints and the lack of detailed interpretation of features in the longitudinal direction. An additional originality consists in the study of material flow under the probe tip. In the current case of thick plates, the variation of local temperature along the weld depth is of key importance for the material flow. Indeed, it governs the slight difference of local mechanical behavior between both materials and therefore the shift of the interface, which was clearly put into evidence by means of a difference of Mn content as small as 0.3 pct between both alloys. This importance of temperature for the malleability also entails the pear shape of the nugget as well as a change of grains orientation along the depth in the thermomechanically affected zone. Due to the modification of tool-material adhesion with temperature, a new phenomenological model of material flow for thick friction-stir welds is proposed. In accordance with their difference of origin, the coexistence of onion rings and serrated interface is also highlighted.