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Arc-based Additive Manufacturing

   

Job Knowledge 137

Introduction

The development of arc-based additive manufacturing (AM) is being driven by the need for increased manufacturing efficiency of engineering structures. Its ability to produce very near net shape preforms without the need for complex tooling, moulds, dies or furnaces offers potential for significant cost and lead time reductions, increased material efficiency and improved component performance.

First patented in 1920, electric arc-based AM is probably the oldest, outwardly simplest, but least talked about of the range of AM processes. Using welding wire as feedstock, the process has been used to manufacture round components and pressure vessels for decades, but not until quite recently has interest in AM in general, and arc-based AM in particular, increased. With a resolution of approximately 1mm and deposition rate between 1 and 10kg/hour (depending on arc source), the operating window of arc-based AM is between, and complementary to, accurate but slower laser-based systems and less accurate high-deposition-rate plasma and electron beam systems.

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Figure 1. Robotic AM system at TWI
Figure 1. Robotic AM system at TWI

Arc-based AM equipment

There is not currently a specific commercial arc-based AM system available, but all that is required is a three-axis manipulator and an arc welding power source. The potential range of manipulators is vast, but most fall into one of two types: robotic or machine tool-based. Similarly, there are different types of power source available and to some extent the material in use will drive the arc deposition process selected. For example, titanium alloys are usually deposited with more stable TIG or plasma transferred arc, whilst most other materials are deposited with MIG/MAG equipment. The emerging range of low-heat-input MIG/MAG systems are proving particularly suitable for AM. Figure 1 shows one of the robotic systems used for arc-based AM at TWI; this is an industry standard robotic welding setup which is also used for AM projects. The adaptions for AM on this system include modification of the turntable for endless rotation, modified control software, increased thermal management and robust wear parts in the power source to cope with long arc-on durations.

Machine tool-based systems into which the deposition equipment has been integrated have additional potential to allow the combination of AM and subtractive (cutting) (SM) processes in a layer-by-layer manner, allowing features to be created and finish machined that would not otherwise be possible. There are laser/powder-based combination AM/SM machines available; development of arc-based systems is underway and it is only a matter of time before a system is brought to market.

Arc-based process variables and control

Although described above as ‘outwardly simple’, arc-based AM is not a simple process to use. Whilst the main controllable variables are the same as robotic welding, AM is a different process. All the process variables combine to produce deposit bead geometry, and it is manipulation of this bead that results in the desired component shape. Unfortunately and unlike welding, bead geometry is affected by more than just the deposition parameters; the residual heat build-up as the part is built results in a continuously changing thermal field that must be accounted for if a deposited layer is to be accurate and free from defects.

As parts become more complex, the programme path of each layer becomes significantly more so. It is rarely possible to strike an arc at the beginning of a layer and extinguish it at the end. Most layers consist of several ‘sub-shapes’ which are programmed and deposited separately but joined together. Figure 2 shows an example of a relatively simple part, which is made up from ten sub-shapes in four different configurations (ie L, T, angled T and five-legged X).

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Figure 2. Relatively simple AM part made from ten separate sub-shapes.
Figure 2. Relatively simple AM part made from ten separate sub-shapes

Until a fully capable AM offline programming software product becomes available, the success of the process is reliant upon the skill of the operator and their ability to break a component down into sub-shapes, decide the order in which they are to be built, predict thermal field, residual stress and distortion, assign appropriate deposition parameters and compile a part programme. Despite the difficulties, examples of some very complex parts have been presented in the public domain.

Materials and deposit properties

As a generalisation, if a material is available as a welding wire, it can be used to manufacture parts by arc-based AM. TWI has deposited materials including carbon and low alloy steels, stainless steel, nickel-based alloys, titanium alloys and aluminium alloys. For many of the materials, the deposit properties are similar to those expected from weld metal in a joint. The notable exceptions to this are precipitation strengthening aluminium alloys and αβ titanium alloy Ti-6Al-4V.

Al-Mg alloys can be strengthened significantly by work hardening after deposition. The strength of heat-treatable alloys will be increased by solution treatment and ageing, but due to the absence of stretching to create nucleation points for precipitates, they are unlikely to be fully equivalent to the peak strength of a T6 tempered wrought material. Aluminium alloy deposits can suffer from porosity, but it can be minimised by the use of special deposition arc waveforms, high-quality consumables and welding best practice for preparation and handling of all materials.

Deposition of Ti-6Al-4V will result in a strongly columnar β grain structure which has isotropic tensile properties. In the horizontal (parallel to the layers) direction, the material will exhibit a 0.2% proof and ultimate strength of 850 and 950MPa respectively, but in the vertical direction these are reduced to 800 and 900MPa. However, it has been shown that introduction of high-pressure rolling as each layer is deposited leads to recrystallisation of the grain structure, resulting in anisotropic tensile properties of 990MPa 0.2% proof and 1070MPa ultimate strength.

Figure 3 shows macro sections of Ti-6Al-4V, AA 4043 and IN718 arc-based AM walls.

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Figure 3. Macro sections of (a) Ti-6Al-4V, (b) AA4043, and (c) IN718 Arc Based AM walls
Figure 3. Macro sections of (a) Ti-6Al-4V, (b) AA4043, and (c) IN718 arc-based AM walls

Summary

Arc-based AM has significant potential for cost and lead time reduction for medium-to-large engineering components of medium complexity. Careful design for arc-based AM can enable partial topological optimisation and careful selection of wire feedstock can make added material optimisation and multi-material components possible. If AM is combined with a machining platform, it becomes possible to create some otherwise impossible shapes.

Arc-based AM is not a net-shape or automated process at this time; the surface finish (waviness) usually means the part must be finish-machined, but the envelope of material to be removed can be as little as 1mm. Some operator skill is required for successful part build; until commercial AM software becomes available, the part model must be interpreted and the manufacturing process manually prepared.

TWI has developed extensive knowledge of arc-based AM over several years' work in generic and contract research. If you would like to discuss this topic, or for more information, please contact us.