Special features of the formation of the joints in dissimilar aluminium alloys D19 and 1420 in the friction welding of linear joints

New components for aviation technology contain integral structures, including plating and stiffener components produced from dissimilar high-strength aluminium alloys. The fusion welding of these alloys is associated with considerable difficulties and does not ensure the required reliability and stability of the quality of the welded joints. 2 Recently, to produce long linear joints in production, special attention has been given to the application of the friction stir welding method. As a result of the effect of the friction forces in the welding process, the metal in the joint zone is heated, becomes ductile and in displacement of the rotating pin instrument along the joint line it is deformed together with the formation of the welded joints. According to the investigators, the set of processes taking part in the formation of the welded joint in stir friction welding results in the formation of a complicated structure of the weld. 8–11 The application of the method for dissimilar aluminium alloys has been studied insufficiently. At the same time, because of large differences in the etching capacity and phase composition, the welded joints in dissimilar aluminium alloys represent a highly suitable model for the examination of the rheology of the plastic flow of the material in welding, the processes of mass transfer, and special features of the structure of different zones of the welded joints. The understanding of the mechanism of the processes, taking place in the welding zone, makes it possible to estimate the effect of the composition and structure of the welded materials and also the parameters of technology on the formation of the joint and, in the final analysis, optimise the process of production of high-strength welded joints. The aim of the this study was the examination of the special features of grain structure, phase composition and processes of mass transfer in the welded joints in dissimilar aluminium alloys D19 and 1420, strengthened by heat treatment, and produced by the friction stir welding process. Investigations were carried out on 3 mm sheets of D19 T and 1420T1 aluminium alloys (Table 1), welded at a speed of v w = 400 mm/min, using a tool suitable for welding of these alloys in the identical combinations. The macrostructure of the welded specimen in the cross-section is shown in Fig. 1a. The core (central part) of the joint includes two zones: 3 (alloy D19) and 4 (alloy 1420). It is produced from two welded alloys with a distinctive (because of the different etching capacity) contact boundary without any mutual mixing of the alloys in the welding process. The macrosection of zone 3 has a matt colour; zone 4 is shiny. The boundaries of the centre are difficult to see on the macrograph and are indicated by the broken line in Fig. 1b. Both zones are asymmetric because of the complex nature of combine plastic deformation of the welded alloys in the welding process. Alloy D19 is transferred from zone 3 into zone 4 of 1420 alloy, and the metal of the latter is displaced in the opposite direction into zone 3 in alloy D19. Thus, there is circular motion of the metal of the alloys with appropriate distortion of the contact boundary between them. They should be accompanied by considerable shear deformation, causing, as is well-known, the formation of the joint in pressure welding. The direction of circular motion is in agreement with the vector of rotation of the tool (Fig. 1b). The microstructure of the zone of the centre of the joint in alloy D19 (zone 3) is fine-grained (Fig. 1d) and differs from the recrystallised structure of the parent metal with the grain size of approximately 20–50 μm, typical of the sheet metal (Fig. 1c). The central part of the joint in D19 alloy contains the narrow zone 2 of white matt colour on the macrosection (Fig. 1a). Because of the different etching capacity of the alloys, examination at a high magnification shows that the zone consists of alternating layers: dark layers with the fine-grain structure (alloy D19), and light layers (alloy 1420, Fig. 1d). Possibly, the Special features of the formation of the joints in dissimilar aluminium alloys D19 and 1420 in the friction welding of linear joints