Ultrasonic phased array imaging with nonlinear enhanced total focusing and phase coherence for microscale cracks in SLM 316L stainless steel

Ultrasonic detection of micron-scale cracks in 316 L stainless steel fabricated by Selective Laser Melting (SLM) is hindered by velocity anisotropy and strong interlayer reflections. Conventional Total Focusing Method (TFM) often suffers from geometric distortion, while Nonlinear Synthetic Focusing (NSF), though effective in isotropic metals, leads to blurred crack edges in complex SLM microstructures. To address these issues, a Nonlinear Enhanced Total Focusing Method (NETFM) is proposed, integrating zonal velocity inversion and delay compensation with bidirectional autocorrelation synthesis to suppress reflection and microstructural noise. Experiments demonstrate that NETFM improves signal-to-noise ratio (SNR) by ~348% compared to TFM and by over 35% relative to NSF, achieving reliable detection of cracks as narrow as 20 μm. In addition, a systematic evaluation of coherence factors further shows that the Circular Coherence Factor (CCF) provides optimal imaging performance for horizontally oriented cracks, while the Vector Coherence Factor (VCF) enhances lateral resolution and SNR when crack spacing is below the acoustic wavelength.