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Evaluation of NDT Techniques for Assessment of Critical Process and Manufacturing Related Flaws and Defects for a Ti-alloy

Periodic Reporting for period 1 - 3TANIUM (Evaluation of NDT Techniques for Assessment of Critical Process and Manufacturing Related Flaws and Defects for a Ti-alloy)

Reporting period: 2021-01-01 to 2021-12-31

To improve aircraft resource-efficiency and to decrease fuel consumption and CO2 emissions innovative solutions with superior mechanical properties for advanced structures are in development. Ti6Al4V alloys, due to a high strength-to-weight ratio compared to steel or aluminium can keep the structural weight and size ratio low. However, lightweight construction with this alloy is currently only possible with conventional techniques as CNC machining, casting that lead to a high proportion of raw material removal. This is not cost-efficient and works with a high ecological footprint. A primary alternative for fabrication of advanced functional lightweight metallic parts is additive manufacturing (AM, also known as 3D-printing), offering benefits in terms of weight, design and functionality, lead time and cost/manufacturability and allowing for alternative geometric shapes, thereby decreasing the weight of the component without sacrificing component strength and safety. However, AM has not yet been approved for structural components with high safety requirements to date, as there are still technology gaps: material property control, correlation between process and structural properties, effect of defects, quality control. Material and mechanical properties of AM parts differ substantially from the properties of the same parts produced by conventional casting. Therefore, a damage tolerance assessment needs to be performed for AM parts in commercial aircraft applications to meet functional and safety requirements.
The main objective of the 3TANIUM is the establishment of NDT methods that are capable to provide the secure detection of process related critical flaws and defects and to understand their effects on material and mechanical properties in Ti6Al4V AM parts. 3TANIUM will quantitatively assess the applicability of NDT methods applied on appropriately and innovatively post-treated (heat-treated, Laser-shock-peened, and surface-treated with chemical / electrochemical methods) AM parts in order to realize benefits offered by AM in the aeronautical industry. Evaluation of reliable Non-Destructive Testing and Analysis (NDT/NDA) techniques for precisely and securely assessing eventual defects and their criticality in AM parts will be performed in 3TANIUM project, together with optimization of AM- and post-treatment processes to reduce the occurrence of such defects and for substantial improvement of fatigue life. In combination with NDT/NDA methods and surface characterization, a method for modeling / simulation will be established for effective lifetime prediction of AM parts. So, will pave the way for introduction of AM manufactured parts, not only in aviation, but also in other sectors such as spacecraft, automotive and vessels. This may bring a new dimension to Europe’s future aircraft technology, with a manifold return on the investment as technological innovation, economic exploitation, and societal benefits.
According to a test and characterization matrix established at the start of the project, round (Fig. 1) and flat Ti6Al4V parts have been AM-manufactured by PREMET. The specimens have been post-treated with various methods, i.e. thermal treatment (HIP and Stress Relief), and surface treatment (pickling and electrochemical polishing methods). Laser shock Peening and CNC milling post-treatments will be performed in the second year of project. Specimens have been NDT-tested and characterized by various methods before and after post-treatment. For NDT testing, microcomputed tomography (µCT) has turned out as the most effective method for assessment of micropores (down to 12 µm) and to reveal defects such as larger pores (Fig. 2) and cracks. Characterization methods include roughness measurement (0.9 µm with fluoride-free electropolishing), electron microscopy and energy dispersive X-Ray spectroscopy (SEM, EDX) of specimens and powders, and crystallography by means of X-Ray diffraction (XRD). During electropolishing (CEST), roughness and SEM/EDX characterization has been carried out before electropolishing and after each process step.
With the first specimens, NDT (µCT) revealed a porosity of 0.02 – 0.03% (thick square samples up to 0.11%) (Fig. 2). An optimization loop has been implemented to reduce porosity and other defects. With the fourth batch of specimens, porosity (before thermal treatment) has been reduced to 0.0088%, further optimization steps are currently ongoing; the optimized specimens will be treated and characterized / tested in the same way as described above. So far, more than 40 specimens have been analyzed with µCT.
In parallel to NDT testing and characterization, mechanical testing (tensile test, fatigue testing) is performed. All results are forwarded to VZLU, who have established a machine learning approach for prediction of fatigue lifetime. As of now, still more data, especially from characterization, is required for training of the AI – the according work is currently ongoing.
The 3Tanium project is presented on the webpage www.3tanium.eu it has also been presented in the FFG-event “Guten Morgen TakeOFF” on 04.03.2021 which can be found at https://www.ffg.at/veranstaltung/takeoff_Veranstaltungsreihe_Teil_3. First results have been presented in the THERMEC 2021 INTERNATIONAL CONFERENCE ON PROCESSING & MANUFACTURING OF ADVANCED MATERIALS (online), see https://www.tugraz.at/events/thermec-2020/home/
This project has received funding from the Clean Sky 2 Joint Undertaking (JU) under grant agreement No 101007830. The JU receives support from the European Union’s Horizon 2020 research and innovation programme and the Clean Sky 2 JU members other than the Union.
The currently ongoing optimization work for the AM process and post treatment aims for a reduction of defects such as pores and for stress reduction and increase of density, in order to increase ultimate tensile stress by at least 10% and to reach a fatigue life of at least 1.5 million cycles. The AM protocol to be established in the 3Tanium project aims for a cost reduction of at least 20 % in comparison to state-of-the-art subtractive manufacturing methods (such as casting and subsequent CNC machining). With post treatment (at least two methods), a homogeneous surface roughness of 1 µm or less is aimed for. This goal has already been reached for the electropolishing method. The work carried out by the 3Tanium project partners is shown schematically in Figure 3.
Establishment of NDT protocols for AM parts for detection of flaws and onsetting fatigue cracks, and fatigue life prediction based on the data generated by NDT inspection shall pave the way for reaching the safety requirements in aeronautics and give a cost estimate for EASA and FAA approval requirements.
µCT scan reveals defects (large pores) in a specimen of the first batch
AM-manufactured specimens for fatigue testing; from left to right: As built, pickled, electropol
Work carried out by the 3Tanium project partners