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 mens 1.1–1.5) and the zone with a tool outlet hole (specimens 2.1–2.5). Changes in the microstructure of the metal were studied on polished and etched sections using a metallographic microscope ALTAMI MET 1C . Mechanical tests were carried out on a universal testing machine UTS 110M-100 . Microhardness was measured on a Duramin 5 microhardness tester. Results and discussion During friction stir welding or processing, a complex structure is formed in the stir zone along with the entire thickness of the workpiece to the depth of tool plunge, which is the result of the formation of material fl ows along the tool contour (Figure 3). A typical monolithic nugget was not formed in the stir zone of the AA5056 sample with a thickness of 35 mm. However, several nuggets were formed in swirling layers under the tool shoulders and at the root of the stir zone (Figure 3). A similar pattern was observed when analyzing the structure of Al-Mg-Sc-Zr alloy in [13]. But, due to more qualitatively selected process parameters at the stage of preliminary studies in this work, it was possible to avoid signi fi cant discontinuities in the upper part of the sample. The modes were selected so that defects are not formed in the processed area, which is often encountered during welding large-thickness workpieces. The dotted lines highlight the distinct structural zones created in the FSW joints: stir zone (1), thermomechanically affected zone (2), heat affected one (3), and base metal (4). The thermomechanically affected zone on the advancing side ( AS ) has a sharp boundary with the stir zone, but on the retreating side ( RS ), there is a smooth transition from the thermomechanically affected zone to the weld. The macrostructure of the samples in the horizontal section is shown in Figures 4 and 5. In the upper part of the stir zone, one can distinguish a signi fi cant structural heterogeneity (4 in Figure 4, a ). This pattern may be related to the plastic deformation effect on the upper part of the sample structure by the tool shoulders during processing. In the zone of the movement started, inhomogeneity of the structure is formed in all areas along with the joint thickness (1 in Figures 4 and 5). It is connected with rather complicated Fig. 3. Macrostructure of a sample cut in cross section after friction stir processing: 1 – stir zone; 2 – thermomechanically affected zone; 3 – heat affected zone; 4 – base metal; a – nugget formation in the near-shoulder area; б – nugget formation in the weld root; RS – retreating side; AS – advancing side. Lines along which the microhardness measurement was carried out are indicated with dashed lines