Eddy Current Probes Design for Defect Detection in Metallic Parts Made by Additive Manufacturing Processes

Abstract

Non-destructive testing (NDT) is the method where an object or materials are being tested for any type of damage or features without harming the object during the process. There are different NDT techniques; in this thesis the main focus is to explore the feasibility of using eddy current (EC) non-destructive testing technique to detect different types of flaws in metallic parts. One of the contributions in this project is to develop different models to design an EC probe that is sensitive to detect small flaw sizes in parts made by additive manufacturing. Additive manufacturing (AM) is the process where an object is created by layering materials based on three dimensional (3D) Computer-Aided Design (CAD) data. In this thesis, the main focus are the products created by laser powder bed fusion process (LPBF) which is one of the metal AM technologies. Defects such as pores and cracks will be created within the parts because of, but not limited to the unmelted powder or the excessive material vaporization during laser-material interaction. The existence of these defects will affect the quality of parts therefore, NDT should be done on these parts for early detection of defects. Eddy current probes have been designed based on modelling developed to detect subsurface defects in parts made by the LPBF technology. Several samples made of stainless steel (316L) and titanium (Ti64) were considered for the detection process to explore the probe response while testing different materials with different conductivity and permeability. Artificial defects were embedded inside the parts at different depths, with different geometries and sizes as well. The probes operate at a wide frequency range for the purpose of increasing the standard depth of penetration of the eddy currents inside the material which, increase the probes, sensitivity to detect subsurface flaws. Experiments were carried out to further test the performance of the designed probes to detect flaws embedded inside parts made by AM. Some of the challenges in testing parts made by AM are the surface roughness and the effect of edges. The problem of edge effect phenomena is a major problem when it comes to testing small samples. The edge of the small sample will create a signal that is similar to the one obtained from a crack. The small diameter of the probe coil and the tip added to it helps distinguishing between the actual defect signal and the signal produced by the test piece edge. Another contribution is that the EC probe is designed in a way that can overcome the problem of surface roughness and the unwanted resultant signal produced by rough surface by adding a coating layer made of thin paper to the bottom of the probe; some of the experimental findings and contributions are listed below: Subsurface notches that simulate subsurface cracks with different sizes in the range of (0.07 mm-0.4 mm) and subsurface blind holes with different diameters in the range of (0.2 mm-0.5 mm) embedded inside plates made of stainless steel and titanium materials have been detected successfully. The plates sizes are 50 mm×50 mm. FEM models with the same parameters were created in ANSYS Maxwell to validate the experimental results. Multiple probes with different parameters such as number of turns, length, inner and outer diameter were designed to investigate the effect of each parameter on the sensitivity of the probe to detect small size defects. Core tip analysis for focusing the magnetic field were done. Different tip shapes of the core with different geometries were considered to increase the detectability of the probe to subsurface defects. Optimization for the designed probe was conducted to determine the optimal probe parameters such as length, inner and outer diameter that gives the best values for the magnetic field. Shields with different thickness were added around the coils to increase and focus the eddy current distribution under the probes coils. Amplification circuits were attached to the probe coil winding to amplify the detected defect signal since the measured detected signals are in millivolts. In conclusion, it is clear that the eddy current testing technique has a potential to be used to inspect parts made by AM.