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 From the panoramic image in Figure 4, b and the enlarged image in Figure 5 , g , it can be seen that the inhomogeneity of the temperature effect has a significant impact on the fragmentation of the material and the formation of transfer layers in the area in front of the tool during friction mixing welding. The result of such heterogeneity is a change in the width of the zone of the primary fragmented material. So, in the up- per part of the tool, the width of this zone is minimal and is only 0.22 mm to the surface of the tool, while in the lower part of the tool, its width increases to 1.28 mm. From Figure 4 , b , it can be concluded that the depth of the tool grooves affects the fragmentation of the material during welding, since where the depth of the grooves is maximum – the width of the zone of the primary fragmented material is minimal. At the same time, in the lower part, where the greatest thickness of the fragmentation zone is observed, the depth of the grooves is minimal. Figure 4 shows that the destruction of the tool in sample 4 occurred in the upper part, that is, in the zone where the temperature effect of the tool on the material is minimal. In this case, the fracture of the tool has a spherical shape. As discussed above, local flows of material resist the movement of the tool in the area of the grooves, thereby increasing the tangential stresses in these zones [22]. The shape of the fracture surface of the tool and the low intensity of fragmentation and plasticization of the material in the area in front of the tool led to the fact that the stresses acting tangentially to the surface of the groove under the shoulders of the tool reached a certain critical value, as a result of which the formation of a crack began, which led to the destruction of the tool. Probably, in sample 2, the destruction process proceeded according to a similar pattern. However, in this case, the destruction occurred in the middle of the weld, which may be due to the fact that the local flows of the transferred material are formed in this area with the narrowest height (as shown in Figure 4 in the cross section of the weld). Thus, both the welding speeds and the loading force on the tool have a serious impact on the stability of the friction welding process with mixing and the welding tool when obtaining permanent joints with a thickness of 35 mm. At low loading force, but high welding speeds, a defective welded joint is formed with the presence of tunnel-type defects in the zone of influence of the tool shoulder due to the deterioration of the adhesive interaction of the material being welded with the tool. An increase in the load makes it possible to eliminate the defect, but at high welding speeds it leads to tool failure due to an increase in the resistance of the material to the movement of the tool and the formation of narrow local flows of material due to the shape of the tool. In order to stabilize the influence of the parameters, the load on the tool was increased, and the process speeds were reduced, which made it possible to obtain a defect-free welded joint without destroying the tool. However, a further decrease in the rotation speed again leads to the tool destruction due to the fact that the set speed does not provide the required thermal welding conditions in the mixing zone, due to which the width of the zone of primarily fragmented and plasticized material is very small in the upper part of the weld, which also leads to an increase in the resistance of the material being welded to the tool. Conclusion As a result of the conducted studies, it became possible to choose the optimal mode for obtaining an all-in-one joint of the AMg5 aluminum alloy with a thickness of 35 mm by friction welding with mixing, as well as to establish the dependence of the formation of defects and destruction of the tool on the parameters of the welding process. It was found that the loading force of the tool is the determining factor for the formation of tunnel-type defects and voids in the area under the tool shoulders. The destruction of the tool is affected by both the loading force and the speed of movement and rotation of the tool, but the main parameter in this case is the speed of rotation. At an excessively high rotation speed, the tool is destroyed in the middle part of the weld, where local volumes of the transferred material are formed by the tool grooves. At a low speed of rotation of the tool, the destruction occurs in the area under the tool shoulders and is caused by an inhomogeneous temperature effect during the welding process. In both cases, the destruction of the tool occurs tangentially to the surface of the tool groove, since the recesses on the surface of the tool experience the greatest tangential stresses caused by the increased resistance of the material being welded to the tool.