| Since the friction stir welding (FSW) was invented, it has gone through the successful transition from experimental research to industry application and large-scale production, and has now becoming one of the most widespread technology in aluminum alloy manufacturing industry. Although there are successful application instances of this technology in aerospace, ships, high speed trains and other industry contexts, up to now, no public documents on the standard of nondestructive testing (NDT) of FSW have ever been reported due to its military application background and technological protecting restriction. In order to evaluate the welding quality of FSW welds, a discussion was made in this present thesis on the basis of ultrasonic inspection, concerning the nondestructive testing of inner defects in FSW welds as well as the influencing factors with respect to the inspection accuracy. At the same time a novel method was recommented to nondestructively evaluate the micro structural changes inside welds and to predict the tensile strength of the entire welding.FSW joints with and without inner discontinuities were manufactured by different welding parameters. The two kinds tipical kinds welding defects: tunnel hole and tightly appressed lack of penetration were considered in the NDT process. Choice of both requency and beam angle were discussed aiming at the optimized inspection parameters. Detectability of traditional C-scan and ultrasonic phased array technology for tiny flaws were compared. Moreover, diffraction effect of the special periodic rough surface of FSW on ultrasonic wave was studied both theoretically and experimentally via traditional ultrasonic C-scan. It was proofed that the periodic rough surface was an important factor for the NDT of tiny flaws in FSW joint.Microstructure evolution of FSW joint including grain size, second phase particles and hardness distribution were discussed, besides, ultrasonic velocity and attenuation distribution were measured. The ultrasonic parameters were coincident with microstructure evolution well. Comparing to velocity variation, ultrasonic attenuation has better relativity with microstructure evolution. Base on the difference between attenuation values on different positions, the entire joint can be segmented to subareas: base metal, heat-affected zone and nugget zone. Take the nugget zone for instance, while the scattering effect of both grain and second phase paticles were taken into account, the name hardness deduced by corrected attenuation value via microstrucutre size has good coincidence with the testing hardness distribution. Results show the good relativity between ultrasonic attenuation and materials hardness, which has positive effect on quantity analysis of micro mismatch, and it's also an alternative method for microstructure change evaluation of structures in server.In chapter 5, feasibility of prediction mechanical properties of FSW joint by nondestructive methods was discussed. NDE methods for tensile strength prediction under two situations: with or without inner flaws were proposed according C-scan images of inner flaws and ultrasonic attenuation distribution, respectively. For the previous situation, C-scan images with abundant masseges of inner flaws were segmented by constant threshold, flaw size was obtained, and then, tensile strength can be deduced according to the theory that the reverse influence of the inner flaws on tensile strength is equal to the decrease of the area whitch bear the weight. For the joints without inner flaws, ultrasonic attenuation distribution was analysised by the methods combining both clustering analysis and morphological image processing. Features of subareas of FSW joint were extracted, and used to evaluate the joints'micro mismatch. Moreover, relativity between the subareas features and corresponding joints'tensile strength were discussed, and pluralistic regression model was proposed for prediction of FSW joints'tensile strength.
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