Research On Process And Mechanism Of Aluminum Alloy Friction Stir Crack Repairing

Fracture of mechanical components and metal structures caused by cracks is the most dangerous and common failure mode. Friction Stir Crack Repairing (FSCR) has unique advantages compared with existing methods, but the crack healing mechanism is still indefinited. In this paper, surface cracks of 2A12 aluminium alloy were friction stir repaired, during which the process range of FSCR and the physical model and healing mechanism were studied through experimental observation and theory analysis.Firstly, repairing process tests and mechanical properity tests of repaired specimens which were made into tensile and fatigue specimens according to the National Standard were carried out; the healing zone microstructure and micro-hardness were analyzed using optical microscope and scanning electric microscope and micro-hardness tester including sizes and shape of grains, fracture morphology analysis and hardness distribution curve of surface and different cross-section. The result showed that the repaired specimens can get good mechanical properties during the repairing process range of v=70-110mm/min, ?=700-1100rpm. The hardness curves on the cross-section appeared to be the type of "W" shape, the more heat which upper section got than the bottom made the precipitated phase strengthen during the repairing process, and the dislocation density decreased greatly near the precipitated phase during the process of plastic deformation, which made the high angle crystal boundary formed easily and improved the mechanical properties. So the hardness of upper microstructure of healing core center is higher than the lower.Secondly, the precipitations distributions of healing zone, the dislocation density and the healing zone organization evolution were analyzed by TEM (Transmission Electron Microscope, TEM) and EBSD (Electron Back-Scattered Diffraction, EBSD). The result showed that the healing core zone contained dynamic recrystallization microstructures, part of the grain size achieved nanoscale, there were some precipitated phase dispersedly distributing near the grains and some dislocation lines in the grains, which enhanced the healing zone strength. The grain size of the BM (Basic Metal, BM) and the AZ (Affect Zone, AZ) were big, there were a lot of rod and ellipsoidal precipitated phase in the grain interior and boundary. In AZ, the small precipitated phase occurred to cluster and contained high density of defects such as dislocations and vacancies which decreased the strength of the healing zone.Thirdly, through the analysis of microstructure evolution, dynamic recrystallization and the metal plastic flow, the physical model of friction stir cracks repairing was established. The physical mechanism was put forward that the plastic metal flowed to the original crack center with the effect of the pin. The healing metal underwent the cyclical process of broken, deformation, grew up, recrystallization, metal atoms interaction and the metallic bond was repaired well and finally the crack get repaired. The driving force is the mechanical stirring under thermal cycle.