高温铝合金电磁超声检测回波特性及因素分析

针对高温铝合金在线检测条件下，温度对铝合金电磁超声检测回波特性的影响规律尚不明确、高温检测时缺陷定量/定位补偿困难这一难题，以螺旋线圈电磁超声换能器（EMAT）为例，建立了高温铝合金EMAT检测过程的场路耦合有限元模型；研究了温度对EMAT激励/接收换能效率、EMAT激励/接收电路的功率分配特性、超声传播过程中的扩散/介质衰减特性、回波幅值和超声声速等因素的影响规律；研制了耐高温EMAT探头，对20~500℃高温铝合金试样进行了检测实验，并测定了高温铝合金的超声介质衰减系数和超声声速。在仿真和实验相结合的基础上，分析了高温检测时超声回波幅值变化特性及其影响因素。结果表明：对于铝合金这类非铁磁性金属材料，导致高温时超声回波幅值下降的主要原因是超声介质衰减系数随着温度的升高而增大，其次为高温时EMAT激励/接收电路的功率分配特性的改变。在激励EMAT在试样表面形成的洛伦兹力不变的条件下，其所激励的超声波回波幅值具有随着温度的增加而增加的特点，可以有效减缓超声回波幅值下降的趋势。

Characteristics and factor analyses for electromagnetic ultrasonic detection echoes in high-temperature aluminum alloy

To solve the uncertainty problem of the effect of temperature on the amplitude of ultrasonic echoes from an Electromagnetic Acoustic Transducer (EMAT) testing with aluminum alloys, and to overcome the difficulty in compensation for defect sizing and location evaluation at an elevated temperature, a field-circuit coupling finite element model is built for the detection process of a spiral coil EMAT operated in the aluminum alloy. Based on this model, the effects of temperature on the excitation and detection efficiency of the EMAT, power allocation characteristics of the equivalent transmitting and receiving circuits for the EMAT, and the diffusion/medium attenuation, echo amplitude, and ultrasonic wave velocity are analyzed. Subsequently, a high-temperature EMAT probe is designed and applied to an aluminum block with temperature ranging from 25 ℃ to 500 ℃. The medium attenuation coefficient and velocity of ultrasonic waves at elevated temperature are also measured. Based on the simulation and experimental results, affecting factors and characteristics of ultrasonic echoes in high temperature testing are examined. Results indicate that, for those non-ferromagnetic metal materials such as aluminum alloys, the main reason leading to the decrease in the amplitude of ultrasonic echoes is attenuation of the ultrasonic medium which increases with temperature rise. The second reason is the variation of power allocation characteristics for the EMAT excitation and detection circuit at elevated temperature. When the Lorentz force on the surface of high temperature sample produced by the transmitting EMAT is constant, the amplitude of excited ultrasonic waves increases with temperature rise. The increasing amplitude of the shear wave can compensate for the downward trend of ultrasonic echo amplitude.