- Investigation of Arc Based Additive Manufacturing
In 2019, we undertook a core research project on behalf of our Industrial Members to investigate the use of the metal inert / active gas (MIG/MAG) process as a heat source for melting the feedstock in arc additive manufacturing. Our experts conducted experiments into the use of industry standard deposition and manipulation equipment to build simple parts with wire and arc additive manufacture.
This included the development of deposition parameters for definable bead shapes as well as determining process capability and reliability. The tests explored combining single beads into larger geometric shapes in order to produce mechanical test samples. We also generated basic mechanical properties and microstructural data for a range of different materials. The research concluded that modifications need to be made to deposition parameters in order to maintain a consistent bead geometry and thermal profile. In addition, it was found that the mechanical properties of the deposited material differed from those found using the same consumable wire for conventional welding processes, with a lower yield and ultimate strength and a greater impact and fracture toughness. Figure 1 shows a test part made with a parallel bead deposition profile and figure 2 shows a vertical cross section of the alloy wall.
- Additive Manufacture of Complex Components using CMT
Another core research project saw us investigate the use of the cold metal transfer (CMT) technique to create simple aluminium structures. This proof-of-concept project used CMT as a low heat input variant of the metal inert gas (MIG) process in which the current carrying wire is mechanically oscillated to provide controlled dip transfer. The aim of the project was to demonstrate the use of arc welding techniques to additively manufacture complex components with a high rate of deposition, thereby minimising machining costs and material wastage.
Test structures were created using the CMT process through the use of multipass welds with either a vertical or a horizontal offset. The experimental trials showed that structures could be produced with a range of orientations by varying the torch angle in line with the desired orientation. Figure 3 shows an example of the range of orientations achieved by building up on a horizontal substrate. All of these different orientations were produced with similar deposition rates.
The technique was shown to be capable of producing good quality deposits that are free of porosity, with good inter-layer fusion and an impressive regularity when performed even at an angle (Figure 4). This research showed that the technique had great potential for the production of structures made from expensive or difficult to machine materials such as nickel or titanium alloys.
The high deposition rates shown in the TWI core research trials showed that the process could be used for larger-scale challenges, as shown with the additive manufacture of a railway crossing for Network Rail…
- Additive Manufacture of Railway Crossing for Network Rail
This EU-funded project was created to establish which additive manufacturing processes would be best suited to achieve the required material properties to withstand the contact of train wheels with the rails at railway crossings, which are subjected to greater levels of wear than other areas of track. Austenitic manganese steel (AMS) is typically used for these areas of track due to the good wear and impact properties. AMS crossings are usually supplied in a soft cast state and harden into a wear resistant surface as train wheels pass over them. However, the casting process can lead to defects that can develop into fatigue cracks. Repairing or replacing these tracks is not just a costly exercise but it can also lead to track downtimes that disrupt train services. Increasing the durability of the rails would therefore lead to increased service and fewer downtimes. Our experts identified a preferred deposition / additive manufacturing process and suitable feedstock, before optimising the chosen process through a series of trials and metallurgical testing in line with railway standards requirements, producing a demonstrator part.
Our experts investigated cold metal transfer, plasma arc welding, and laser powder deposition processes for the initial deposition trials, to determine that the metal active gas (MAG) welding process was most able to deposit weld layers of acceptable quality on the carbon steel substrate using AMS flux cored filler wire as a feedstock. Network Rail supplied us with a mock-up railway crossing to be used as a deposition trial test block (figure 5). The tests determined that the chosen process could successfully produce an additive manufactured weld deposit using an AMS metal-cored filler wire as the feedstock. The deposit hardness and the fatigue performance exceeded the minimum requirements and the non-destructive examination (NDE) and mechanical tests were deemed acceptable to BS EN 15689 and ISO 5817 standards. However, the AMS weld deposit impact toughness needs to be improved for the part to reach acceptable service requirements for the rail / wheel contact area. As such, our experts recommended further work to improve the impact toughness using an AMS solid wire consumable with optimised composition or tailor the composition of the present metal-cored wires for AM application. We also recommended an investigation into the use of a lower heat input to improve the impact toughness of the AM weld deposit.
- Residual Stresses in Wire Arc Additively Manufactured Parts
Our experts worked alongside those from Coventry University on a study funded by the Lloyd’s Register Foundation to investigate residual stresses in wire arc additively manufactured (WAAM) aluminium alloy samples. This work formed part of the PhD work of TWI senior arc welding engineer, Karan Derekar, who was a student at the time of the project. The research led to the creation of a paper on the subject, titled, ‘Effects of Process Variants on Residual Stresses in Wire Arc Additive Manufacturing of Aluminium Alloy 5183,’ which discussed the effects of processing conditions on residual stress formation and distribution. The research highlighted that the base plate thickness and deposit height play a vital part in residual stress distribution, whilst the temperature of deposit (interlayer temperature) has an effect on the magnitude of residual stresses. Residual stresses were found to be lower with a higher interlayer temperature (figure 6). The samples for the study were manufactured at TWI using the pulsed metal inert gas (MIG) processes applied robotically. The residual stress measurements, using the contour method and computational approach, were conducted at Coventry University while experts from TWI, Coventry University, and the University of North Texas all provided input to the study.
Another core research project combined TWI’s expertise in microstructural analysis, non-destructive testing (NDT) and arc welding engineering, to develop a constant output approach to WAAM deposition…
- Integrated AM from Feedstock to Part Quality: WAAM Process to Achieve Consistent Bead Geometry with Conventional Feedstock Composition
Despite the high deposition rates and large build envelope, arc additive manufacturing can sometimes suffer from unfavourable deposition conditions as a result of the complex thermal characteristics, the use of conventional welding wire, and fixed process inputs. This can lead to inconsistencies in the deposited bead geometry and microstructure that, in turn, create excessive scatter in mechanical properties, which hinders part qualification and limits the use of WAAM parts in safety critical applications.
This project sought to develop a fundamental understanding of process factors that influence deposition conditions, as well as devising monitoring and analysis techniques to assess surface quality and dimensional accuracy in order to improve the robustness of the WAAM process. Our experts worked to identify critical deposition parameters that control weld bead geometry and microstructure for a variety of build geometries while also adapting conventional NDT techniques to monitor WAAM deposition, from which key information can be extracted for feedback control (figure 7). The outcomes showed that WAAM process was stable for carbon steel using commercially available synergic line and control methods. Further, physical samples and measurements were generated and used to develop an understanding of process factors that influence deposition conditions. A vision camera melt pool imaging and python-based analysis and monitoring method was also developed (figure 8).
This work was developed further via another core research project that aimed to develop an in-situ monitoring technique for the detection of flaws in WAAM depositions.
- Integrated AM from Feedstock to Part Quality: Development of an in-situ monitoring technique for the detection of flaws in WAAM depositions
The complexity of the WAAM process means that deviations from optimal deposition parameters, random changes in environmental conditions, feed rate and energy input may lead to the formation of flaws. In addition, different equipment, feedstock quality variations, and working conditions can obstruct the final quality of a build. In such cases, the late detection of unacceptable flaws may lead to unnecessary rework or, even worse, render a build or a batch into scrap. To address these challenges for the benefit of industry, we launched a core research project to develop in-situ ultrasonic inspection during the build process.
The study demonstrated that in-situ ultrasonic inspection is possible and effective in detecting flaws and changes in component growth during the WAAM manufacturing process. This trial was an important milestone to pave the way for further research and advancements in in-situ monitoring and inspection of WAAM depositions.
As well as addressing concerns over deposition, our experts recently launched a joint industry project to assess the structural integrity of additive manufactured materials, including those created using arc AM.
- Structural Integrity of Additive Manufactured Materials
In order to use AM materials in safety-critical applications, it is necessary to understand the material’s behaviour under complex loading such as fatigue. In order to address this, we launched a joint industry project to perform structural integrity assessment of materials produced via AM, with a particular focus on the fracture and fatigue performance of additive manufactured steel, copper, nickel and titanium alloys. Among the processes highlighted for investigation was wire-arc direct energy deposition (DED).
This project allows sponsors to pool their resources and gain access to the outcomes of this research, including structural integrity assessments of fracture toughness, fatigue endurance and fatigue crack growth in air, hydrogen, and elevated temperature environments, along with post-test analysis and correlation between process-microstructure-properties, leading to certification and qualification of the material.
As with other additive manufacturing processes, arc additive manufacturing offers a number of benefits for industry, but still requires expert input to ensure it meets the challenges of different applications.