On the problem of tool destruction when obtaining fixed joints of thick-walled aluminum alloy blanks by friction welding with mixing

OBRABOTKAMETALLOV Vol. 23 No. 3 2021 MATERIAL SCIENCE EQUIPMENT. INSTRUMENTS force and speed of movement, the destruction occurred at the initial stage of welding. Figure 3 shows the tool fragment in sample 2 and the longitudinal section of the mixing zone formed by the remaining part of the tool. As can be seen from Figure 3, an increase in the load allowed to eliminate the defect in the area under the tool shoulders, but probably increased the resistance of the base metal to the tool when moving dur- ing welding. Figure 3, b , made in the longitudinal section of the sample, confirms the influence of the tool grooves on the formation of separate flows of material around the tool. Consequently, local metal flows affect the area of the grooves, creating stresses there that are tangential to its surface. In sample 4, made with a high loading force, but lower speeds of movement and rotation of the tool, a similar picture is observed. Figure 4 shows that before the destruction of the tool, the mixing zone was formed by separate flows of material formed by the shoulders ( 1 in Fig. 4, a ) and the tool grooves, while the height of the transfer layers decreases towards the middle of the weld (3) and again increases slightly towards the lower part (4). Thus, the action of local material flows in the area of the tool grooves proceeds independently of the selected mode, but its intensity is determined by both the loading force and the weld- ing speeds. At the same time, sample 3, obtained with a loading force and a tool movement speed like that of sample 4 (4100 kg and 60 mm/min, respectively), but at a higher rotation speed (280 rpm vs. 260 rpm), demon- strates both the absence of any weld defects and a fully completed welding process without the welding tool destruction. After the tool destruction, a strongly deformed zone is observed on the stuck part, represented by a lay- ered structure (Fig. 5, b ), and the closer to the contact area between the stuck and the remaining parts of the tool, the material delamination becomes more intense and is accompanied by the formation of cracks paral- lel to the plane of delamination. At the same time, in the area below the delamination zone, cracks spread deep into the tool, the length of which can reach 20 mm or more (Fig. 5, b, c ). As a result of mutual wear during friction contact, inclusions of tool fragments of two types are mixed into the mixing zone: inclusions having the structure of a strongly deformed layered region, and inclusions with the structure of an initial tool material structure (Fig. 5, a ). It is obvious that such inclusions are products of tool wear, while during the wear process the remaining (movable) part of the tool does not undergo significant plastic deformation necessary to change the structure of the material. Fig. 3. Cross-section of sample No. 2 with the stuck piece of tool ( а ) and longitudinal cross–section of sample No. 2 in the tool outlet zone ( b ): 1 – stir zone formed by tool shoulders; 2 – stir zones formed by tool grooves а b