Heterodyne Eddy Current Testing Using Magnetoresistive Sensors for Additive Manufacturing Purposes

Henrik Ehlers,Roland Thewes,Matthias Pelkner

doi:10.1109/jsen.2020.2973547

Abstract

In recent years additive manufacturing technologies have become widely popular. For complex functional components or low volume production of workpieces, laser powder bed fusion can be used. High safety requirements, e.g. in the aerospace sector, demand extensive quality control. Therefore, offline non-destructive testing methods like computed tomography are used after manufacturing. Recently, for enhanced profitability and practicality online non-destructive testing methods, like optical tomography have been developed. This paper discusses the applicability of eddy current testing with magnetoresistive sensors for laser powder bed fusion parts. For this purpose, high spatial resolution giant magnetoresistance arrays are utilized for testing in combination with a single wire excitation coil. A heterodyne principle minimizes metrology efforts. This principle is compared to conventional signal processing in an eddy current testing setup using an aluminum test sample with artificial surface defects. To evaluate the influence of the powder used in the manufacturing process on eddy current testing and vice versa, a laser powder bed fusion mock-up made from stainless steel powder (316L) is used with artificial surface defects down to $100~\mu \text{m}$ . This laser powder bed fusion specimen was then examined using eddy current testing and the underlying principles.

Highlights

C OMPARED to conventional manufacturing techniques like milling, additive manufacturing (AM) enables production of relatively complex workpieces without wasting much material
MR sensors based on other quantum mechanical effects like tunnel magnetoresistance (TMR) offer much higher sensitivities of up to feasibility is clearly demonstrated as the powder does not have an impact on the measurement results due to its much lower conductance
ET with MR sensors is a promising technique for the characterization of Laser powder bed fusion (LPBF) manufactured parts

Summary

Introduction

C OMPARED to conventional manufacturing techniques like milling, additive manufacturing (AM) enables production of relatively complex workpieces without wasting much material. Metal parts with complex geometries are produced by melting subsequently layer after layer of powder with a laser retaining the properties of the base material [1]. This manufacturing process is not easy to control and slight differences in powder, temperature, speed, or other parameters can lead to a variety of defects in the processed material changing its mechanical properties and decreasing its structural. Date of publication February 12, 2020; date of current version May 5, 2020. The associate editor coordinating the review of this article and approving it for publication was Prof.

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