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Technical Insight: Additive Manufacturing NDT Inspection

Non-destructive testing (NDT) is used to validate the structural integrity of additively manufactured components, detecting internal defects and ensuring product quality without damaging the parts themselves. Techniques commonly used for NDT of additive manufactured items include computed tomography, ultrasonic testing, infrared thermography, nonlinear resonance, eddy current and penetrant testing.

Problems that can be picked up with NDT include internal voids and porosity, poor fusion between layers, cracks and inclusions, distortion or dimensional inaccuracies, and surface roughness issues.

NDT is becoming an increasingly integral part of the additive manufacturing workflow, offering crucial data to improve processes and ensure final part quality.

Our experts have undertaken a number of projects related to and involving NDT of additive manufactured parts, helping to create confidence in the manufacturing process for industry.

- Research Launched in Arc-Based Additive Manufacturing

A 2020 core research project was launched, bringing together a range of TWI expertise in order to investigate various integrated additive manufacturing strategies, from feedstock customisation to online monitoring techniques, to ensure build consistency and part quality.

Focusing on wire and arc additive manufacturing (WAAM), which has gained interest across industry for its high deposition rates and large build envelope, this project aimed to deliver data sets related to process parameters, thermal history, deposited bead geometry, and microstructures for various build geometries that will feed into future process and microstructure modelling efforts. In addition, WAAM deposition monitoring techniques were also validated and the effect of alloy element additions on reheated microstructures was investigated. The project also identified consumable compositions optimised for WAAM deposition. Our technical excellence in NDT was integral to this research project, which was created to meet recognised needs among our Industrial Members.

With reductions in material wastage, and fast deposition rates using relatively simple arc welding equipment and industrial manipulators for various alloys of steels, aluminium, titanium and nickel, WAAM has lots of benefits for industry. However, the thermal characteristics of WAAM builds along with the use of conventional welding wire and fixed process inputs could sometimes result in unfavourable deposition conditions and, subsequently, inconsistencies in the deposited bead geometry and microstructure. To improve quality and allow the use of WAAM parts in safety critical applications, our technical experts adapted existing NDT techniques to monitor WAAM deposition so as to extract key information for feedback control purposes as well as identifying critical parameters for a range of build geometries and developing improved consumable composition for WAAM applications.

NDT has also proven integral to another core research project, launched in 2024 to assess the susceptibility of additive manufactured parts to hydrogen embrittlement.

- Hydrogen Embrittlement Susceptibility of Additively-Manufactured 316L and Alloy 718

TWI has decades of expertise with the effect of hydrogen on materials. This experience was used for this core research project that investigated hydrogen embrittlement in additively manufactured parts.

Metallic materials, including corrosion resistant alloys (CRAs) can be prone to hydrogen embrittlement (HE), when a susceptible microstructure is subjected to a sufficiently high combination of hydrogen and tensile stress. Commercial exploitation of additively-manufactured (AM) materials is increasingly producing complex components for both emerging technologies and for existing applications where HE is a concern, such as for subsea service in the oil and gas sector. Whilst there is a large body of work detailing the performance of wrought alloys in hydrogen, there is limited data available for AM materials and components, which present a host of new challenges including uniquely complex microstructures, inherent residual stresses, anisotropic properties and surface finishes.

This project used both destructive and non-destructive testing methods to investigate resistance of 316L stainless steel and Alloy 718, produced as AM builds, to HE (Figures 1 and 2). Our AM experts produced the parts to be tested using laser-powder bed fusion (L-PBF), with the tests showing that L-PBF 316L and Alloy 718 are potential candidates in environments where HE is a concern, provided a suitable fabrication route (including chemical composition and heat treatment) is used.

Although many of the additive manufacturing insights provided by our NDT experts are for the widespread benefit of industry, we also conduct work for specific clients and industries. Such as with a project conducted for the European Space Agency (ESA) to investigate the use and manufacture of additively manufactured components in space…

- In-line NDT for Additive Manufacturing

Additive manufacturing (AM) promises optimised designs and subsequent weight reductions for spacecraft, which lowers launch costs. It also has the potential to manufacture parts directly in space, reducing the dependency on terrestrial supply chains and enabling long-term missions. However, AM parts also pose a number of challenges; the can be prone to defects as a result of the build process, material properties and design complexity of parts. Furthermore, conventional inspection techniques struggle with complex geometries and grain structures, while current methods (e.g., X-ray Computed Tomography) may be limited by part size and density.

To save costs, material waste and machine time on making replacements, TWI investigated in-line NDT techniques as a method for identifying and quantifying defects during Direct Energy Deposition (DED). The aim is to advance manufacturing and NDT technologies from TRL3 (technology proof-of-concept) to TRL5 (validated in relevant environment), laying the groundwork for closed-loop feedback control systems that would enhance real-time quality assurance and defect correction.

This work investigated laser profiler surface scanning as an indirect monitoring technique and laser ultrasonic testing (LUT) as a direct technique (Figures 3 and 4). These allow the inspection strategy to be determined based on the selected DED technique and build parameters to minimise disruption. At the time of writing, this project is ongoing, with laboratory trials proving both 2D line profilometry for detecting weld bead shape continuity, and LUT can detect flaws in DED produced parts. The next challenge is performing these tests in-situ during manufacture…

Additive manufacturing has the potential to deliver a wide number of benefits to different industries and our NDT experts are working to ensure the final quality of parts for an increasing range of applications.

Figure 1: EBSD inverse pole figure (IPF) map of Alloy 718 L-PBF taken transverse to the building direction in the as built condition
Figure 1: EBSD inverse pole figure (IPF) map of Alloy 718 L-PBF taken transverse to the building direction in the as built condition
Figure 2: Scanning electron micrographs of the fracture surfaces of Alloy 718 L-PBF specimens printed with the longest specimen in the horizontal (Y) direction. Specimens were heat treated in accordance with API 6ACRA N07718 150K. Nominal scale bars are shown: a) Image of specimen Y9 tested in air taken near the edge of the specimen, showing evidence on microvoid coalescence indicative of a ductile failure; b) Image of specimen Y2 tested under CP taken near the edge of the specimen, showing brittle fracture surface indicative of hydrogen embrittlement
Figure 2: Scanning electron micrographs of the fracture surfaces of Alloy 718 L-PBF specimens printed with the longest specimen in the horizontal (Y) direction. Specimens were heat treated in accordance with API 6ACRA N07718 150K. Nominal scale bars are shown: a) Image of specimen Y9 tested in air taken near the edge of the specimen, showing evidence on microvoid coalescence indicative of a ductile failure; b) Image of specimen Y2 tested under CP taken near the edge of the specimen, showing brittle fracture surface indicative of hydrogen embrittlement
Figure 3: 2D line laser profiler
Figure 3: 2D line laser profiler
Figure 4: DED-arc 2D line profilometry
Figure 4: DED-arc 2D line profilometry
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