Comparison of Three Spiral Coil EMAT Configurations for Defect Detection in High-Temperature Forgings

In engineering practice, carbon steel and aluminum alloy forgings exhibit an inherent susceptibility to defects such as porosity and inclusions during high-temperature forging. Their non-destructive inspection using electromagnetic acoustic transducers (EMATs), however, poses a significant challenge. Existing EMAT designs suffer from a poor signal-to-noise ratio (SNR) when operating under high-temperature conditions, which severely compromises the detectability of defect signals in both material types. To address these challenges, this study proposes two novel permanent magnet configurations for EMATs. Compared to conventional designs, the new configurations (Configuration 2 based on a Halbach array and Configuration 3 incorporating soft magnetic materials) aim to enhance detection sensitivity. Their parameters were optimized through finite element simulation and orthogonal experiments to maximize the vertical magnetic flux density. Experimental tests were conducted on specimens containing Φ3-mm flat-bottomed holes under both room-temperature and high-temperature conditions (up to 600°C for carbon steel and 500°C for aluminum alloy). The results demonstrate that in the high-temperature environment, the optimized Configuration 2 EMAT achieved a maximum SNR of 10.44 dB, whereas the Configuration 3 EMAT reached up to 12.04 dB. Both configurations exhibited significantly superior performance compared to the conventional structure, with test results differing by only approximately 13.3 % between carbon steel and aluminum alloy materials.