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 Introduction Friction Stir Welding (FSW) and Friction Stir Processing (FSP) are virtually identical processes of severe plastic deformation at elevated temperatures [1-3]. These technologies differ mainly in the purpose of their use: formation of a hardened surface layer or production of a welded joint. When a rotating tool is plunged into the workpiece and further moves, the material is heated and plastically deformed. During this process, the grain structure is fragmented, and then the transferred material cools down and simultaneously recrystallizes, resulting in signi fi cant changes in the material structure and its mechanical properties. These techniques are mainly used for welding or processing various aluminum alloys [4–16]. The most applicable are aluminum-magnesium alloys AA5056 and AA5083 [5, 7, 9, 12], alloys of the following systems: Al-Mg-Sc , Al-Mg-Sc-Zr [13, 15], Al-Cu-Li [11], Al-Zn-Mg-Cu [14, 16], Al-Cu-Mg [16], etc. It is also possible to produce joints and coatings based on dissimilar metals and alloys of Al-Cu [17], Cu-Fe [18], and other systems. It is also possible to form various reinforced metal matrix composite materials [18-20]. The technology of friction stir processing for hardening materials produced by additive technologies has a rather high degree of applicability [12, 18]. Friction stir welding can be performed both over the entire thickness of the workpiece and to adjust the depth of the processed area due to the size of the tool. Friction stir welding is possible with both butt and overlap welding and different types of bevelling before welding. Depending on the thickness of the workpiece, the structure of the stir zone differs signi fi cantly: in large- thickness workpieces, a monolithic nugget is not formed, which is characteristic for FSW ’ed or FSP ’ed samples of 2–10 mm thickness. It is shown in detail on the example of FSW joint made of an alloy of the Al-Mg-Sc-Zr system with a thickness of 35 mm in [13]. In this case, due to the large workpiece thickness, temperature gradients, and the conditions of adhesion interaction, the plastic fl ow of the material and the formation of the stir zone change compared to the thin-sheet workpieces with fundamentally different heat dissipation characteristics. In this regard, one of the most relevant tasks for research is to determine the pattern of material fl ow along the tool contour and the formation of mechanical properties of the processed or welded material of large-thickness workpieces in different directions. This work aims to study the stability of the large-thickness sample formation and the homogeneity of its mechanical properties. Methods Friction stir welding and processing were carried out on a particular unit at CJSC Cheboksary Enterprise “Sespel” . For this purpose, 200 mm wide and 35 mm thick AA5056 alloy sheets were used. Processing/ welding was carried out according to the scheme shown in Figure 1. Workpiece (1) was processed by tool (2), by penetration a pin (3) with rotation (4) of the tool, followed by movement along the joint line (5). This technique was used not to produce a weld joint but to determine the boundary values of the process parameters of intelligent adaptive friction stir welding for 35 mm thick plates. The rotational speed of the tool ( ω ) was 250 rpm, the longitudinal velocity ( v ) was 250 mm/min, the tool loading force to the workpiece ( P ) was 3,600 kg, and the tool inclination angle was 3.0 deg. The process parameters were selected experimentally. The processed area was 250 mm long. Specimens for mechanical tests were cut in area (6). Specimens from the processed area (7) were cut in the vertical direction (8), transverse direction (9), and longitudinal direction (10) in the number of eight samples for each direction. The size of the investigated area does not comply with the Interstate standards, so the samples were cut in a reduced shape, preserving the standard proportions. The size of the test specimens was 2.7×2.2×12 mm. A thin section (11) was cut out to investigate the cross-sectional structure of the processed material. In this case, the stir zone (12) and the heat and thermomechanically affected zones (13) are clearly distinguished in the section. Sections for the study of changes in the structure by thickness were cut from the workpiece in the hori- zontal plane after friction stir processing on an electric discharge machine in a planar section. Then the sections were divided into two parts (Figure 2) to study the area where the tool enters the workpiece (speci-