Arc welding based additive manufacturing or WAAM techniques are attracting interest from the manufacturing industry because of their potential to fabricate large metal components with low cost and short production lead time. This process exists alongside other high deposition rate metal AM technologies such as powder and wire based DED. While these use either laser or an electron beam as energy source to melt a metal powder or wire, WAAM technologies melt metal wire using an electric arc.
A recently published paper introduced wire arc additive manufacturing (WAAM) techniques, reviewed mechanical properties of additively manufactured metallic components, summarised the development in process planning, sensing and control of WAAM, and finally provided recommendations for future work.
The research indicates that the mechanical properties of additively manufactured materials, such as titanium alloy, are comparable to cast or wrought material. It has also been found that twin-wire WAAM has the capability to fabricate intermetallic alloys and functional graded materials. The paper concluded that WAAM is a promising alternative to traditional subtractive manufacturing for fabricating large expensive metal components. On the basis of current trends, the future outlook will include automated process planning, monitoring, and control for WAAM process.
Like EBAM and DED, WAAM technologies produce parts at near net shape which can then be CNC’s for optimal surface finish and dimensional accuracy.
TWI, one one of the world’s foremost independent research and technology organizations, explains that 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, molds, dies or furnaces offers potential for significant cost and lead time reductions, increased material efficiency and improved component performance.
Back to the basics
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.
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.
Materials and deposit properties
TWI goes on to explain that if a material is available as a welding wire, it can generally also 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 (Al-Mg) and titanium alloy Ti-6Al-4V.
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 optimization and careful selection of wire feedstock can make added material optimization 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.