Optimization design of laser-EMAT and its application in high-temperature forgings detection

In high-temperature continuous forging process, according to the real-time monitoring of workpiece thickness and flaws, the processing parameters can be adjusted accordingly, so we can remove defective components in time, which has essential research value for avoiding the interruption of production line and improving their yield and quality grade. We established a finite element (FE) model of the carbon steel’s laser-electromagnetic acoustic transducer (laser-EMAT) testing process. Based on the simulation model, we analyzed the effects of laser parameters, EMAT parameters, and sample thickness on the detected ultrasonic signal amplitude, and we also achieved the optimized Laser-EMAT design parameters. Subsequently, we developed a high-temperature resistant spiral coil EMAT and measured the high-temperature forging with a thickness of 100 mm and temperatures from 300 °C to 730 °C. Based on the experiments, we researched the effect of specimen temperature on the received ultrasonic wave amplitude. The results show that the excitation efficiency of laser-induced ultrasonic waves improves by decreasing pulse duration, decreasing laser spot radius, and increasing pulse laser energy. The receiving efficiency of the shear wave (SW) detected by the EMAT enhances when reducing the diameter of the EMAT wire and increasing the permanent magnet height. When the radius of the permanent magnet is equal to the radius of the EMAT coil, the receiving efficiency of SW is the highest. As the sample thickness increases, the size of the EMAT should increase accordingly to the acoustic beam divergence for obtaining a higher ultrasonic wave intensity. The amplitude of the SW signal received by the EMAT increases by 679% after the optimization design. With rising carbon steel forging temperature, the SW signal amplitude increases first and then decreases sharply, reaching its maximum at 617 °C, which is 29% higher than at room temperature, and the signal-to-noise ratio (SNR) of the SW is 20.5 dB.