Research on Weld Defect Detection Using Electromagnetic Ultrasonic Based on Magnetostrictive Mechanism

电磁超声换能器(EMAT)凭借其非接触式检测等优势，在工业无损检测中得到了广泛应用。然而，对于焊缝缺陷检测，由于焊缝区域中材料组织的粗大化，超声波检测过程中漫反射回波信号增多，导致检测信号强度大幅衰减，信噪比下降。本文针对常规洛伦兹力机制EMAT在焊缝缺陷检测中存在的信号微弱、换能效率低等问题，提出通过在试件表面涂覆磁致伸缩涂层，以主动构建磁致伸缩换能区域，从而实现焊缝缺陷检测的方法。通过搭建电磁超声检测实验平台，研究了磁致伸缩机制EMAT接收信号的幅值与偏置磁场强度的关系，优选了最佳磁场配置；采用磁致伸缩机制EMAT对根部未焊透、坡口未融合和夹渣三种焊缝缺陷进行实验检测，实验结果表明，与常规洛伦兹力机制EMAT相比，磁致伸缩机制EMAT对三种缺陷的检测信号幅值分别提升了4.5倍、2.4倍和6.3倍。Electromagnetic acoustic transducers (EMATs) have been widely used in industrial non-destructive testing due to their advantages such as non-contact detection. However, for weld defect detection, the coarsening of the material microstructure in the weld zone leads to increased diffuse reflection echoes during ultrasonic testing, resulting in significant attenuation of the detection signal strength and a decrease in the signal-to-noise ratio. To address the issues of weak signals and low transduction efficiency in conventional Lorentz force-based EMATs for weld defect detection, this paper proposes a method that involves coating the specimen surface with a magnetostrictive layer to actively construct a magnetostrictive transduction zone, thereby enabling effective weld defect detection. By establishing an electromagnetic ultrasonic testing experimental platform, the relationship between the amplitude of the received signals from the magnetostrictive EMAT and the bias magnetic field strength was investigated, and the optimal magnetic field configuration was determined. Experiments were conducted using the magnetostrictive EMAT to detect three types of weld defects: incomplete root penetration, lack of sidewall fusion, and slag inclusion. The results demonstrate that, compared to the conventional Lorentz force-based EMAT, the magnetostrictive EMAT improved the signal amplitudes for detecting these defects by 4.5 times, 2.4 times, and 6.3 times, respectively.