RESEARCH OF DEVELOPMENT CONDITIONS OF STRUCTURAL TRANSFORMATIONS DURING FRICTION STIR WELDING OF MULTIPHASE ALUMINUM ALLOY

Abstract

Purpose. We aim to investigate the development conditions of structural transformations during friction stir welding (FSW); establish the nature of individual influence of structural components in achieving superplastic flow conditions; determine the influence nature of grain size on the groundmass microhardness during FSW. Methodology. Friction stir welding was performed on specially designed equipment. The material was 2.9 mm thick AMg6 aluminum alloy plates with the chemical content of alloying elements within the grade composition. The temperature and pressure from the tool on the edges during welding were determined on a specially designed stand. The tool pressing force to the metal was measured with a dynamometer type DC-0.1. Microhardness measured on the PMT-3 device with the indentation load of 0.05 N was taken as a characteristic of alloy microvolumes strength. Findings. Different degrees of rotation of the working tool and normal pressure to the edges determined the degree of metal heating and the quality of the seam formation. The influence degree of the technological parameters of the FSW on the metal heating temperature in the area of the working tool shoulder is estimated. The development of recrystallization processes in the conditions of two-phase alloys is considered. It has been shown that collective recrystallization is less determined by the volume fraction of the second phase, its dispersity and ability to interact with the metal matrix. The effect of the temperature gradient on the microhardness for the structures of the heat-affected zone is estimated under conditions of a practically unchanged grain morphology. Originality. The conditions for the development of structural transformations during friction stir welding and the influence mechanism of grain size on the matrix microhardness are determined. Exceeding the optimum temperature in the joint area during welding contributes to the diffusion accelerating along the boundaries between phases and grains, resulting in the formation of a concentration gradient of alloying elements and, first of all Mg, increasing the hardening effect of the solid solution state. Practical value. According to the results, the additive character of the hardening from the influence of the solid solution and grain boundaries under the conditions of superplastic flow is determined. A state close to the superplastic flow is achieved by reducing the effect of hardening the solid solution and increasing the contribution from the small grains boundaries. Achieving a state of superplastic deformation is possible by minimizing the effect of strain hardening.