Subsurface defect characterization using laser ultrasonic technique for metal additive manufacturing

• Main Author: Cai, Xingfang
• Other Authors: Fan Zheng, David
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Aeronautical engineering
• Online Access: https://hdl.handle.net/10356/148483
• Institution: Nanyang Technological University

With a layer-by-layer approach to part fabrication, additive manufacturing holds strong potential to revolutionize design and manufacturing processes. Selective Laser Melting (SLM) is one primary metal additive manufacturing technique to build functional parts for the automotive and aerospace industries. However, the general lack of process robustness for product quality has presented key technical challenge that impedes a wider adoption of the technology for direct part production. In this work, advancements in the defect inspection capabilities of laser ultrasonic technique applied for metal additive manufacturing have been made. A novel methodology for detection and characterization of micron-sized subsurface defects with laser ultrasound is presented. The methodology was developed from classical theories of elastic wave scattering from defects to address the surface wave scattering from subsurface defects, with defect detectability limited by wave scattering principles. Subsurface defect types considered in the study are void and powder-filled defect representing the lack-of-fusion defect. The work was conducted on the SLM samples with artificially created defects, as the groundwork before the intended application for in situ process monitoring. The methodology showed good results for defect detection and characterization in the numerical simulation and experimental studies. The methodology is also applicable for defect characterization on the as-built SLM part with poor surface finish.