The Eastern Committee of cofrend (French Confederation for Non-Destructive Testing) organised a regional technical day on NDT and additive manufacturing. This day of conferences was held at the Lycée Henri Loritz in Nancy, on December 9, 2021. Account.
During this technical day, Daniel Chauveau, former Director of Innovation at the Institut de Soudure and former Director of the Expert sector, presented the progress of the working group he leads within the Cofrend. This "Additive Manufacturing" working group has undertaken the drafting of a guide for manufacturers. His work focuses on the DED (Directed Energy Deposition) processes – which integrate the wire arc – and PBF (Powder Bed Fusion). This document will include several volumes published as they are written.
The Institut de Soudure group, for its part, focused its presentation on the impact of the surface finish on the controllability of parts. Parts made in arc and laser-wire additive manufacturing have an irregular surface condition inherent in the deposition process. Inclusions, porosities and lack of fusion (between cords) are the three major defects resulting from additive manufacturing by arc-wire process. Several techniques (PT, radiography, ultrasound, etc.) make it possible to "see" defects. Thus, it is important to know how the part was manufactured to adapt the control process to the typology and orientation of the defects sought. Results from experiments conducted with X-ray controlled parts and immersion ultrasound were presented. On the occasion of this presentation, the possibilities of control by tomography for certain parts of adapted thickness were also highlighted. To improve the controllability of parts, techniques are evolving but it is also possible to intervene during the manufacturing process. Two axes were highlighted during this presentation: the modification of the microstructure and the control of the surface finish. The Surfab project (surface in additive manufacturing metal wire. Characterization, control and optimization) will make it possible to characterize the surface finish thanks to 3D scans and to have an optimization tool. The Icwam (International Congress on Welding, Additive Manufaturing), which will take place next June, has been announced.
Representatives of SNCF and Safran also spoke. Since 2016, SNCF has been interested in additive manufacturing. SNCF's hardware engineering has 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 railway environment. Powder bed melting processes, DED-wire, FDM and fast foundry are now the technologies to which SNCF is turning to respond quickly to a need, a lack but also to set up a more agile and digital supply chain . Components exposed to high mechanical stresses and made from these processes are subjected to non-destructive tests (NDT) and sometimes destructive tests. Control methods such as magnetoscopy, ultrasound and PT recognized in the Railway Sectoral Committee (CFCM) are sometimes combined with more specific control methods such as tomography to look for defects after manufacture and at the end of fatigue tests. These checks also make it possible to identify any unusual discontinuities related mainly to these processes. They offer the opportunity to judge the ability of parts to meet commercial service requirements. The first investigations are encouraging, which encourages SNCF to continue the work undertaken to adapt the END methods applied to railway components and these new manufacturing methods.
For Safran, additive manufacturing provides answers to major issues: making parts with high added value accessible, enabling geometries inconceivable with conventional processes, enabling the functionalization of parts, all while reducing costs and environmental impact. The group has recently set up a specific entity dedicated to additive manufacturing, a "Campus Factory" bringing together in one place the latest generation machines, as well as experts in additive manufacturing and post-additive manufacturing processes. Some 200 people will eventually work there. The non-destructive testing will have a preponderant place within the structure, with new problems, such as varied surface states, inaccessible areas, or geometric complexities that did not exist until then. The detection performance of so-called mature methods such as PT, radiology, ultrasound will be reassessed to take into account these new issues and to be integrated as accurately as possible into the technical definition of the parts. Emerging technologies will be studied.
Plural NDT techniques
The LNE, for its part, recalled that controlling the integrity of critical parts such as lattices or with internal cavities or channels, made in additive manufacturing, is problematic because of the complexity of their geometry. Currently, to fully inspect such geometries and internal structures without damaging parts, non-destructive testing techniques of volume investigation must be used. One of them, X-ray tomography (XCT), is proving to be the most efficient. However, it is an expensive method, the image analysis process is long and the large files are difficult to handle. In addition, it is not suitable for large and high density parts, nor for routine control. Alternative methods are therefore needed. Linear Ultrasonic Resonance Spectroscopy (RUS) methods have shown great potential and are to date the most promising methods for inspecting complex parts made in additive manufacturing after XCT.
Novitom focused its presentation on synchrotron X-ray microtomography (S-μCT): a powerful non-destructive testing and 3D characterization tool adapted to the field of additive manufacturing. Synchrotron X-ray microtomography makes it possible to achieve high resolutions and to image the interior structures of objects without destroying them, giving access to internal areas inaccessible with the usual surface techniques. S-μCT data can be used to obtain by image analysis the 3D roughness of surfaces inaccessible with standard profilometry techniques. The surface meshes of the internal areas of a part can also be generated and directly compared to the CAD of the parts studied. The use of the synchrotron beam, with a higher energy than laboratory X-ray sources, allows the control of large 3D printed metal parts. In addition, the parallel nature of the synchrotron beam paves the way for multiscale analysis.
With its hardware and software offer, X-Ris provides answers in terms of production issues when it comes to controlling additive manufacturing parts by X-rays. Additive manufacturing pushes contractors to demand advanced image quality performance without certainty about the harmfulness of the defects observed or sought. Therefore, imaging by tomography (or micro tomography) should be reserved for the development of printing parameters to avoid harmful defects (possibly on test tubes). According to X-Ris, it is then possible to limit oneself, in production, to a faster tomography of lower resolution or to 2D radiography. Nevertheless, the amount of data generated is very important and an automatic defect detection tool quickly becomes essential.
During this technical day, participants were also able to attend a commented demonstration carried out in the premises of the Lycée Henri Loritz, and discuss with three exhibitors: the companies Eddyfi (presentation of the TFM ultrasonic technique), Mistras Group (presentation of the methods by acoustic emission and acousto-ultrasound) and X-Ris.
Interview with Daniel Chauveau, leader of the Additive Manufacturing working group at Cofrend
You manage the additive manufacturing working group within Cofrend. Are there any particular issues concerning this theme?
Additive manufacturing makes it possible to manufacture parts with new shapes, but also to replace parts previously manufactured differently. This implies the use of new processes whose defectology is not known. One of the first challenges is to know the defects likely to be generated by these new processes. However, there are no completely constructed documents to define what a defect generated by additive manufacturing processes is. The Cofrend working group has thus set about this task. We have drafted two proposal documents for the classification of defects generated by additive manufacturing processes. We based ourselves on what we have in welding. We built on existing standards to produce something similar. ISO stakeholders – who are well aware that there is a gap in this area – have shown interest in our progress.
Are we moving towards a specific standard for additive manufacturing?
Once these defects are defined, it is possible to start imagining a standard that will indicate what is "acceptable" or "not acceptable" in parts from additive manufacturing, and under what conditions. This is an important first step. We have also published a document that explains how all additive manufacturing standards should be structured. A lot of standards have come out, but it seems to us that there is a missing backbone and that we must "connect" these standards with each other. We have proposed to structure this standardization and to formalize as a priority the general standards that are still lacking today, while launching a reflection on the priority of standards NDT to be developed. In the same way that there are standards dedicated to welding, molding, etc., we need standards dedicated to additive manufacturing. Even if these take up existing pieces of other standards written for other subjects. Additive manufacturing is in full development and needs its normative structure to be able to develop and really take its industrial boom.
There are a large number of additive manufacturing processes. Still, are the defects generated common?
We focused on two areas that we believe are important for manufacturers: powder processes, including SLM, and Waam (Wire Arc Additive Manufacturing) additive manufacturing processes. In addition, we have tried to group in arc-wire processes processes such as additive manufacturing by electron beams that should not generate very different defects. Similarly, in powder processes, we believe that it is possible to group different processes related to powders. We can't put everything under one umbrella, but already, by addressing these two areas, we cover a broad spectrum of additive manufacturing.
Where is the WG in writing its guide?
The first documents that will appear have little to do with the NDT but it answers one of the first questions of the community of NDT which was: "tell us what you are looking for and we will tell you how to find it." We are fortunate to have, in the group, members who are large technical centers (representatives of Cetim, the Institute of Welding, the foundry, etc.), which makes it possible to build something quite complete, which addresses a little all the fields. Because even though additive manufacturing is different from welding or foundry, there are still a lot of similarities with these industrial fields.
What are the next steps?
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 in the process of 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 add a section dedicated to composites. Initially we had confined ourselves to metallic materials, but we agreed with the Composites working group of Cofrend – which was working on additive manufacturing issues – to integrate these materials. For volume 3, we launched a round robin on typical pieces. We have indeed 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, control times, detectable defects, etc. We want to give examples through non-confidential documents that have been made available by the participants. Parts that have been controlled by tomography, induction thermography, ultrasound or digital radiography, or PT. This will allow us to give results on use cases. Even if this is not generalizable, it offers examples that can be referred to. Volume 4 will give advice on how to establish future control procedures. Volume 5 will focus on process monitoring, since we realize that the NDT is not the only answer to the quality of parts. Monitoring parts during manufacturing also brings interest. This volume will review all the possible monitoring techniques and will also give the limits and indicate how to couple this to the NDT.