Stir zone material flow patterns during friction stir welding of heavy gauge AA5056 workpieces and stability of its mechanical properties

OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 23 No. 4 2021 Fig. 4. Macrostructure of samples 1.1 – 1.3 ( а , c, e ) and 2.1 – 2.3 ( b, d, f ) cut in the horizontal section according to the scheme shown in Fig. 1: 1 – inhomogeneities of material structure in the tool inlet zone; 2 – area with a predominantly etched structure with a layered structure of the stir zone; 3 – area with predominantly etched grains in the stir zone; 4 – inhomogeneities of the stir zone structure in the near-shoulder area; 5 – change in the width of the processing zone at the beginning of the tool movement а c e b d f conditions at the beginning of tool movement due to unsteady thermal mode. Further along the weld length, a thickening of the stir zone (5 in Figure 4) can be distinguished. In such an area, after heating the material, the tool fully presses the shoulders against the workpiece and forms a stir zone of the width that should be formed at the steady welding/processing mode stage. In the lower layers, the metal structure at the steady-state stage is formed with more excellent stability over the length of the weld (Figure 4, b , c ). In the horizontal section samples after etching, two types of structure are distinguished, containing elements with different etching characters. In the fi rst (2 in Figu- res 4 and 5), the layers of transferred metal are well de fi ned, and the distance between them correlates with the value of the feed per tool revolution. In the second region, the structure shows a tendency to etch on the grain (3 in Figures 4, 5) and does not show a layered structure of the stir zone. In the lower part of the stir zone (Figure 5a,b), the metal structure is represented by a relatively stable structure. There are also zones where etching reveals layers (2 in Figure 5) and grains (3 in Figure 5) in the stir zone. It has to be noted that the distribution of these zones along the thickness of samples is rather inhomogeneous and changes from one sample to another irregularly. In general, in the lower layers of the