| The welded structure of electron beam welding of thick titanium alloy had broad application prospects in the aerospace industry, but now the process of electron-beam deep-penetration welding of titanium alloy easily occured the spiking defect, which will significantly reduce the performance of welded joint, so the forming mechanism and control measures of spiking defect were studied in order to ensure the quality of welding.The numerical analytic model of temperature field of electron-beam deep-penetration welding was establish in this paper, The calculated results of temperature field show that the cooling rate and heating rate are very fast in the process of electron beam welding, the distribution of temperature field is basically unchanged when the welding process entered into the quasi-steady-state, the peak temperature of center of molten pool was maintained at about3200"C.The simulated result was verified by the thermocouple and contrast of cross section of weld, the simulated result was well consistent to the experimental data, which verified the reliability of the finite element model.The flow field of titanium-alloy electron-beam deep-penetration welding was calculated on the base of established finite element model of temperature field, the formation mechanism of flow field was preliminary analyzed. The study shows that the gradient of surface tension hydrostatic pressure and recoil pressure of metal vapor are the most important driving forces of flow of liquid metal, and the flow of liquid metal in the surface of molten pool was the most violent, the maximum flow rate is0.295m/s, the flow of liquid metal gradually waned with the increase of penetration. The tungsten element was selected as the tracer, and the EDS analysis was used to measure the distribution of tungsten element in the electron-beam deep-penetration welded joints, and the flow and mass-transfer behavior of liquid metal of molten pool was further analyzed in the process of electron-beam deep-penetration welding, finally the tracer experiments and numerical simulated results were combined to summarize the flow law of fluid of molten pool of the electron-beam deep-penetration welding.Combining the flow-field model of the electron-beam deep-penetration welding, the spiking defect of thick titanium-alloy electron-beam welded joint was studied. The study shows that the pulse of electron beam is the direct cause of forming spiking, the metal vapor with high saturated vapor pressure is the important cause of forming spiking. The precise control of penetration to achieve full-penetration welding is the most effective solution of the spiking defect of the electron-beam deep-penetration welding.The quality control system of titanium-alloy electron-beam deep-penetration welding was studied in order to control the spiking defect. A set of visual sensing system of colored image of molten pool was fabricated which is suitable to electron beam welding, the system achieved the real-time sensing of the molten-pool image. The edge-detection algorithm of molten-pool image was developed, the clearly edge of molten pool was obtained by the program, which created the condition for the further extraction of molten width. A suit of extracting program of molten width was developed to extract exactly the molten width. A predictable model of BP neural network of penetration of titanium-alloy electron-beam welding was established according to the experimental data, which could predict the penetration by the welding parameters and molten width. The fuzzy controller was designed, its controlled variable was the molten width, its control variable was the electron beam current. The model of BP neural network and the fuzzy controller were combined to establish the control system model of penetration of titanium-alloy electron-beam welding, and the simulation experiment was carried out, the results show that the dynamic performance and the steady-state performance of designed control system are eminent. |
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