The Eastern Committee of Cofrend (French Confederation for Non-Destructive Testing) organized a regional technical day on NDT and additive manufacturing. This conference day was held at the Lycée Henri Loritz in Nancy, on December 9, 2021. Read the report.
At this technical day, Daniel Chauveau, former Director of Innovation at the Institut de Soudure and former Director of the Expert sector, presented the progress made by the working group he leads within Cofrend. This "Additive Manufacturing" working group is currently drafting a guide for manufacturers. Its work focuses on DED (Directed Energy Deposition) ) - which integrate the arc-wire - and PBF (Powder Bed Fusion). This document will be published in several volumes as they are written.
The Institut de Soudure group focused its presentation on the impact of surface finish on part controllability. Parts produced using arc and laser-wire additive manufacturing have an irregular surface finish inherent to the deposition process. Inclusions, porosities and lack of fusion (between beads) are the three major defects resulting from arc-wire additive manufacturing. Several techniques (dye penetrant inspection, radiography, ultrasound, etc.) can be used to "see" defects. It is therefore important to know how the part was manufactured, in order to adapt the inspection process to the typology and orientation of the defects sought. Results from experiments carried out on parts inspected by X-ray and immersion ultrasonics were presented. The presentation also highlighted the possibilities of tomography inspection for certain parts of suitable thickness. Techniques are evolving to improve the controllability of parts, but it is also possible to intervene during the manufacturing process. Two areas were highlighted during the presentation: microstructure modification and surface finish control. The Surfab project (surface in wire additive manufacturing. Characterization, control and optimization) will characterize surface finish using 3D scans and provide an optimization tool. The Icwam (International Congress on Welding, Additive Manufaturing), which will take place next June, has been announced.
Other speakers included representatives from SNCF and Safran. SNCF has been interested in additive manufacturing since 2016. SNCF's equipment engineering focused on different additive manufacturing technologies and materials to meet a variety of parts to be produced, maintenance constraints and normative requirements specific to the rail environment. Powder melting, DED-wire, FDM and rapid foundry are the technologies SNCF turns to today to respond quickly to a need or shortage, and also to set up an efficient production process. supply chain more agile and digital. Parts exposed to high mechanical stress and manufactured using these processes are subjected to non-destructive (NDT) and sometimes destructive testing. The magnetic particle, ultrasonic and dye penetrant testing methods recognized by the French Railway Sector Committee (CFCM) are sometimes combined with more specific testing methods, such as tomography, to detect defects after manufacture and at the end of fatigue tests. These tests can also be used to identify any unusual discontinuities, mainly related to these processes. They can also be used to assess the ability of parts to meet commercial service requirements. Initial investigations have been encouraging, prompting SNCF to continue its work on adapting NDT methods applied to railway components to these new manufacturing methods.
For Safran, additive manufacturing provides answers to major issues: making high value-added parts accessible, enabling geometries inconceivable with conventional processes, allowing the functionalization of parts, all while reducing costs and environmental impact. The Group recently set up a specific entity dedicated to additive manufacturing, a "Campus Factory" bringing together in a single location the latest generation machines, as well as experts in additive manufacturing and post-additive manufacturing processes. Some 200 people will eventually work here. Non-destructive testing will play a key role within the structure, with new issues such as varied surface conditions, inaccessible zones, and geometric complexities not previously encountered. The detection performance of "mature" methods such as dye penetrant testing, radiology and ultrasound will be reassessed to take account of these new issues, and to ensure that they are incorporated as accurately as possible into the technical definition of parts. Emerging technologies will also be studied.
A wide range of NDT techniques
LNE, for its part, pointed out that checking the integrity of critical parts, such as lattices or parts with internal cavities or channels, produced by additive manufacturing, is problematic due to the complexity of their geometry. At present, to fully inspect such geometries and internal structures without damaging the parts, it is necessary to use non-destructive volume inspection techniques. One of these, X-ray tomography (XCT), is proving to be the most effective. However, this is a costly method, with a lengthy image analysis process and large files that are difficult to handle. Furthermore, it is not suitable for large, high-density parts, nor for routine inspection. Alternative methods are therefore required. Linear resonance ultrasonic spectroscopy (RUS for Resonant Ultrasound Spectroscopy) have shown great potential and are currently the most promising methods for inspecting complex parts produced by additive manufacturing after XCT.
Novitom's presentation focused on synchrotron X-ray microtomography (S-µCT): a powerful non-destructive testing and 3D characterization tool that is ideally suited to the field of additive manufacturing. Synchrotron X-ray microtomography enables us to achieve high resolutions and image the inner structures of objects without destroying them, giving access to internal areas inaccessible with conventional surface techniques. S-µCT data can be used for image analysis to obtain the 3D roughness of surfaces inaccessible with standard profilometry techniques. Surface meshes of the internal zones of a part can also be generated and compared directly with the CAD of the parts studied. The use of the synchrotron beam, with a higher energy than laboratory X-ray sources, enables the inspection of large 3D-printed metal parts. What's more, the parallel nature of the synchrotron beam paves the way for multi-scale analysis.
With its range of hardware and software, X-Ris provides answers to production problems when it comes to X-ray inspection of additive manufacturing parts. Additive manufacturing is driving customers to demand the highest levels of image quality, without any certainty as to the harmfulness of the defects observed or sought. Tomography (or micro-tomography) imaging must therefore be reserved for fine-tuning printing parameters to avoid harmful defects (possibly on test specimens). According to X-Ris, production can then be limited to faster, lower-resolution tomography or 2D radiography. Nevertheless, the amount of data generated is very large, and an automatic defect detection tool quickly becomes essential.
During the technical day, participants were also able to attend a guided demonstration on the premises of the Lycée Henri Loritz, and talk to three exhibitors: Eddyfi (presentation of the TFM ultrasound technique) Mistras Group (presentation of the acoustic emission and acousto-ultrasound methods) and X-Ris.
Interview with Daniel Chauveau, leader of Cofrend's Additive Manufacturing working group
You manage Cofrend's additive manufacturing working group. Are there any particular challenges in this area?
Additive manufacturing makes it possible not only to manufacture parts with new shapes, but also to replace parts previously manufactured in other ways. This implies the use of new processes, the defects of which are not fully understood. One of the first challenges is to understand the defects likely to be generated by these new processes. However, there are no fully developed documents defining what constitutes a defect generated by additive manufacturing processes. The Cofrend working group therefore set about this task. We drafted two documents proposing a classification of defects generated by additive manufacturing processes. We based them on what we know about welding. We used existing standards to produce something similar. The ISO players - who are well aware that there is a gap in this field - showed an interest in our progress.
Are we moving towards a specific standard for additive manufacturing?
Once these defects have been defined, we can begin to imagine a standard that will indicate what is "acceptable" or "not acceptable" in additive manufacturing parts, and under what conditions. This is an important first step. We have also published a document explaining how all additive manufacturing standards should be structured. A huge number of standards have been published, but it seems to us that there is no backbone and that these standards need to be "plugged in" to each other. We have proposed to structure this standardization and to formalize as a priority the general standards that are still lacking today, while at the same time launching a reflection on the priority of NDT standards to be developed. Just as there are standards dedicated to welding, casting, etc., we need standards dedicated to additive manufacturing. Even if these take existing bits and pieces from other standards written for other subjects. Additive manufacturing is in full development, and needs its own standards structure if it is to develop and really take off industrially.
There are many different additive manufacturing processes. But are the defects they generate common?
We have focused on two areas that we believe are important for manufacturers: powder processes, in particular SLM, and Waam arc-wire additive manufacturing processes (Wire Arc Additive Manufacturing). In addition, we have tried to group together processes such as electron-beam additive manufacturing in the arc-wire processes, which should not generate very different defects. Similarly, in powder processes, we think it's possible to group together different powder-related processes. We can't put everything under the same umbrella, but by addressing these two areas, we cover a broad spectrum of additive manufacturing.
How far along is the GT in drafting its guide?
The first documents to be published have little to do with NDT, but this answers one of the first questions from the NDT community, which was "tell us what you're looking for, and we'll tell you how to find it." We are fortunate to have members in the group who are from major technical centers (representatives from Cetim, the Institut de Soudure, the foundry, etc.), which means we can build something that is fairly comprehensive, addressing just about every field. Because even if additive manufacturing is different from welding or casting, there are still many similarities with these industrial fields.
What's next?
Volume 1 lists existing standards and those in preparation in the field of additive manufacturing and NDT: terminology, manufacturing processes, testing, qualification, acceptance criteria, defectology. We are currently finalizing Volume 2, which describes the main additive manufacturing processes, compares them and reviews the acceptance criteria and quality approaches deployed by manufacturers. In this part of the guide - which also lists all the commercial acronyms - we will be adding a section dedicated to composites. Initially, we had confined ourselves to metallic materials, but we agreed with Cofrend's Composites working group - which was working on additive manufacturing issues - to include these materials. For Volume 3, we launched a round robin on typical parts. We realized that it is very difficult to proceed in a general way when you want to be able to select the right techniques and give information in terms of cadence, inspection time, detectable defects, etc. We want to give examples through the use of additive manufacturing techniques. We'd like to give some examples using non-confidential parts made available by participants. Parts that have been inspected by tomography, induction thermography, ultrasound or digital radiography, or by dye penetrant inspection. This will enable us to give results on use cases. Even if these cannot be generalized, they provide examples to which we can refer. Volume 4 will provide guidance for establishing future control procedures. Volume 5 will focus on process monitoring, as we realize that NDT is not the only answer to part quality. Monitoring parts during manufacture is also of interest. This volume will therefore review all possible monitoring techniques, as well as outlining their limitations and how they can be coupled with NDT.