The Research Of Thermoplastic Repair Of 7050 Aluminum Alloy Internal Cracks

Large forgings were scrapped because of internal unqualified cracks detection, then cause enormous waste of resources and economic losses. Especially the tiny cracks deep inside the materials, seriously affect the service lives, safety and reliability of forgings in the process of service, due to its concealment and undetectability. At present, internal cracks are still the common technology problem of the forging production which affect the material utilization seriously. This paper selected 7050 aluminum alloy as the experimental material and studied more deeply in the physical simulation method of forgings internal cracks, the repair process of internal cracks, the microstructure evolution of internal cracks healing process and repair mechanism.Internal chevron cracks were obtained successfully using at room temperature axisymmetric extrusion method and this physical simulation method is a new attempt to study the healing of internal cracks of forgings. A control variable method was used to control different levels of die semicone angles, reductions of area, shear friction coefficients between workpiece and die. The experimental results of internal cracks preparation show that shear friction coefficient is the key factor to the evolution of the crack morphology, within a certain range, the greater the shea r friction coefficient, the more easily lead to chevron cracks, and excessive shear frictions will result in the cold surface cracking extrusion specimen and eventually lead to the sample fracture; the larger the die semicone angle is larger, the more easy to cause the core crack s initiation. Furthermore, reduction of area has a significant influence on crack size. The 7050 aluminum alloy would generate internal chevron cracks without surface cracks with a hybrid lubricant condition, a 36% reduction of area and a 35° die semicone angle during the axisymmetric extrusion process at room temperature. In order to generate internal chevron cracks in 7050 aluminum alloy bar, the critical value of die semicone angle is between 30° and 35°.Chevron cracks repair experiment of 7050 aluminum alloy samples was studied by refining the controllable factors of production into repair process, including the holding time before deformation, deformation temperature, amount of deformation and deformation rate and orthogonal design the four factors to study internal chevron cracks repair of 7050 aluminum alloy samples. The experiment results show that the four factors are influential in internal cracks repair, the longer the holding time before deformation, the higher the deformation temperature, the more the deformation and a moderate deformation rate are good for internal chevron cracks repair.The microstructure evolution of 7050 aluminum alloy mainly comesfrom the matrix atomic diffusion and its healing crystallization in crack cavity and dynamic recrystallization behaviors in matrix microstructure. The healing crystallization is not the real mechanism but the material guarantee of internal cra ck healing. The real mechanism of internal crack healing is the grow up phagocytosis of matrix dynamic recrystallization grain to healing crystal grain.