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. 5. Macrostructure of samples 1.4–1.5 ( а , c ) and 2.4–2.5 ( b, d ) 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 layered structure of the stir zone; 3 – area with predominantly etched grains in the stir zone а c b d sample, the structure of the stir zone is more homogeneous than in the upper layers. This position may explain more signi fi cant pressure from the welding tool pressing at the bottom of the stir zone than at the top. In part, the mechanics of the welding/processing procedure can con fi rm this. The structure in the tool inlet zone has a similar structure in all areas throughout the thickness of the sample (Figure 6). As can be seen from Figure 6 a-d , the structure is represented by a mixture of etched layers and etched grains. The heterogeneities in the structure appear clearly in the form of complex shapes in the stir zone organization (Figure 6 b,e ). The size of the layers is close to the amount of feed per revolution during tool movement. The reason why etching shows layers in some areas and grains in others is not completely clear. It can be assumed that the etchability of the grain boundaries is less than the etchability of the transfer layers at the boundary of the metal fl ows that form the joint, while the etchability of the grain boundaries is higher inside the fl ows. No weld defects characteristic of friction stir welding was detected in the structure of the samples, which may indicate potentially high mechanical properties of the material in different directions. In the tool outlet zone (Figures 7, 8), the structure of the metal, on the contrary, signi fi cantly depends on the distance from the tool shoulders. In the upper part of the sample (near the shoulders), one can see the structure of inhomogeneities described earlier in the macrostructural analysis (Figure 7 a ). Different structural patterns at various depths can be identi fi ed in the shape and size of the transfer layer and the features of its state in the tool outlet zone (Figure 7 c-e , Figure 8 a-d ). The most negligible thickness of the transfer layer and the stir zone is expected to be in the lower part of the processed area, where the tool pin diameter is minimal (Figures 8 c, d ). There are also transfer layers partially detached from the stir zone at almost every level throughout the height of the sample (Figures 7 c-e , Figure 8 a ). The adhesion of aluminum alloy causes the detachment of the stir zone part at tool output to the steel tool and the fact that quite a large portion of the material during processing is between the fi llets of screw pin. Determination of the weld and near-weld material microhardness shows that in the main structural zones of the 35 mm thick AA5056 sample, there is no material hardening as compared to the base metal value for the corresponding alloy (Fig. 9). The data obtained during Vickers microhardness measurements of the