The asymmetric material flow, severe plastic deformation and thermal cycle imposed on the base material during friction stir welding (FSW) result in unique microstructural development, which causes a gradient in local mechanical properties in the weld region. Micro-tensile and indentation testing were applied to determine the local mechanical properties in a friction stir welded joint. The local stress-strain curves exhibited a drastic change at the advancing side (AS) due to a steep gradient of mechanical properties. Finite Element Model (FEM) predictions of the tensile performance of the welded joints, based on the local mechanical properties measured by micro-tensile testing, were in very good agreement with the macro-tensile test data.

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This article presents an improved nonlinear ultrasonic technique for fatigue damage detection utilizing a kind of carefully designed aluminum-lead composite metamaterial. It focuses on developing a bandgap metamaterial to improve the accuracy and identifiability of the superharmonic features from fatigue cracks by eliminating the inherent nonlinear components in the nonlinear ultrasonic technique. The study starts with the unit cell design through modal analysis by applying the Bloch-Floquet boundary condition to obtain the band structure. Based on the local resonance mechanism, by adjusting the height ratio between the aluminum and lead cylinders, the bandgaps covering the required frequency ranges can be opened up. Then, a chain of regularly arranged unit cells is modeled to analyze the spectral response and verify the bandgap effect through harmonic analysis. The targeted ultrasonic frequency component of guided waves within the bandgap can be mechanically filtered out. Subsequently, a finite element model of the pitch-catch active sensing procedure for fatigue crack detection is constructed via the coupled-field transient dynamic analysis. Nonlinear ultrasonic experiments with the designed metamaterial are carried out to verify the theoretical and numerical investigations. This paper demonstrates that the metamaterial, with its outstanding wave manipulation capability, shows great potential on structural health monitoring and nondestructive evaluation applications. The paper finishes with summary, concluding remarks, and suggestions for future work.

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Heat input of welding causes temperature gradient in the steel structure. After welding and along with temperature cooling down, residual stresses appear around the welding zone, cause the welding deformation. Residual stresses are a result of the temperature gradient, restraint intensity and the dependency of material properties on the temperature, such as yield strength, elasticity modulus, and thermal expansion coefficient. Vibratory weld conditioning is a new technology which vibrates the workpiece while welding by vibrator. Its main purpose is to refine grain, relieve residual stress, minish welding deformation and improve the properties of welded joint. Firstly, electro-slag weld was done in the two kinds of vibration condition: 0.3g and 0.6g. Then, the distribution of welding residual stress was measured with blind hole-drilling method. Results show vibratory weld conditioning may relief welding residual stress greatly and σ max is less than 1/2σ s . From metallograph, vibratory weld conditioning can refine the grain effectively. The grain size of 0.6g was smaller than that of 0.3g. While from the measuring values of residual stress, 0.6g's are higher than that of 0.3g's.And also conclude that after finishing welding continuing vibrating a period to increase the inputting energy is benefit to the relief of welding residual stress. 0.IntroductionElectroslag welding (ESW) firstly developed in the early 1950's by Paton, was developing forward at the steady speed all the time and is especially widely used in heavy manufacturing. ESW has great superiority in thick plate structure, composite structure and thick wall vessel etc [1,2] . However, while welding, the large heat inputting associated with the process results in a coarse-grained cast structure with a wide coarse grained region in the heat affected zone (HAZ) and at the same time it results in the high residual stress [3] .Vibratory weld conditioning (VWC) is also called the process that vibrating the workpiece while welding, which is developed based on the vibratory stress relief (VSR). VWC puts the periodical force on the weld and the whole workpiece in the process of welding to improve the quality of the weld [4] . Serial researches founded that VWC can refine the grain effectively and reduce the magnitude of residual stress and the weld deformation. Comparing to the heat stress relief, VWC has the good qualities of less investment, more convenient operation, no pollution and shorter manufacturing period. Relative to VSR, VWC vibrates the workpiece from the beginning to the end of the welding