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Many of the existing non destructive evaluation (NDE) standard procedures applied to conventionally wrought, forged, and molded metal components are generally applicable to parts made by AM. However, specific challenges must be addressed by newer AM-specific standardized NDE procedures. In addition, there is a recognized need to develop in-process monitoring techniques to inspect parts during the build process.
A wide range of challenges for NDE of AM needs to be addressed: complex part geometry, surface finish, deeply embedded flaws, lack of defined critical defect types and sizes, lack of physical NDE reference standards, lack of written inspection procedures tailored for AM processes, and a lack of probability-of-detection data.
Critical defects
In AM, well-characterized physical reference standards are still being developed. Furthermore, there is often industry ambiguity in defining what constitutes a critical defect in a complex-geometry AM part. This information will only become available once AM processes have matured and after exhaustive effect-of-defect studies have been completed. A further complication is presented by the numerous AM processes and the complex processing parameter space for AM. These challenges can set the stage for a dangerous scenario where true critical defect types and sizes are missed in an AM part inspection because of inexperience and unfamiliarity with the true failure modes of the materials.
Yet, it is necessary to provide critical flaw size and orientation for any part made by AM regardless of the part geometry, stress, and material properties. In addition to potentially new or unknown defect types in metal parts made by AM, finished AM parts also typically have greater ranges of porosity and lack of fusion, with defects distributed throughout the part, rendering sizing of relevant flaws difficult. Therefore, instead of defining a critical crack for a part made by AM, it is probably better to validate NDE techniques that are known to not miss standard-sized flaws at a specified probability and confidence level. Complex geometry
The AM process is most suited for parts with complex geometries, and these geometries present a challenge for conventional NDE methods like ultrasonic testing, Eddy current testing and even radiography testing. Likewise, many parts made by AM have internal structure that may not be accessible for the less geometry-sensitive methods such as penetrant testing and magnetic particle testing. Hence, the most promising technique for complex geometry parts appears to be x-ray computed tomography (CT).
Physical reference standards
Physical reference standards are a basic requirement for any NDE process. However, they are just starting to be designed and fabricated for the AM processes and are a needed first step in understanding the capabilities of the various NDE methods and techniques that are being used on these materials. Again, this will take time because of the numerous AM processes and the vast processing parameter space. Furthermore, until critical defect types and sizes are properly quantified, representative defect reference standards are being designed and fabricated using engineering judgment.
Inspection procedures
Although many of the existing NDE standard procedures for conventionally fabricated metal and plastic parts are generally applicable to parts made by AM, specific requirements (geometrical complexity, porosity, surface finish and deeply embedded flaws)must be addressed by newer standardized NDE procedures.
In-process monitoring
In-process monitoring may be a game changer in 1) improving the consistency, repeatability, and uniformity across machines and 2) qualification and certification of parts made by AM. Part qualification during manufacturing is essential given the difficulty in applying NDE methods to inspect complex parts after their fabrication. Research on in-process infrared monitoring of weld pool temperature and profile is underway, and high-speed visual monitoring techniques are available. However, automatic defect recognition algorithms need to be developed, and current AM fabrication equipment will likely require modification prior to the successful implementation of these novel inspection techniques.
Qualification and certification
Once the effects of critical defects are understood, physical reference standards have been fabricated, and suitable NDE inspection procedures have been developed, qualification and certification of parts made by AM becomes achievable and practical. Currently, there are no NDE protocols currently in place to evaluate the quality, workmanship and acceptability of these parts.
References
[1] J. M. Waller, B. H. Parker, K. L. Hodges, E. R. Burke, J. L. Walker , Nondestructive Evaluation of Additive Manufacturing - State-of-the-Discipline Report – NASA - Nov 2014
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