Wire arc additive manufacturing (WAAM) is arousing a great deal of interest in several industries. By building up layers, arc-based manufacturing allows for a high degree of flexibility in component geometry. For prototypes and small-batch production runs in particular, WAAM is a more cost-effective solution than other additive processes for metal. The key factor in determining the quality of components produced is the welding process. A recent innovation here, called cold metal transfer, effectively fulfils all the conditions for WAAM.
Additive production methods generate components by building up material layer by layer. The most well-known example of an additive method is 3D printing. WAAM, which is based on the arc welding process, also produces metal parts layer by layer, with the layers formed by the melting wire electrode. This generative method is particularly advantageous when complex component geometries have to be produced, as the design options are virtually limitless.
In addition, parts can be manufactured at low cost and extremely quickly – which makes WAAM a very attractive option for prototype construction and/or small production batches. Processing time, tool wear and material loss during machining – especially with the conventional approach of milling out the workpiece from a solid block – all mean considerable extra costs.
What is WAAM?
There are a number of generative production methods for metal. Essentially these can be divided into two fundamental types: powder-based processes and wire-based processes. In powder-based processes, the layers are built up using molten metal powder. The most common method, the powder bed process, produces extremely precise results, but is somewhat slow in production. Wire-based processes, on the other hand, build up the component by melting a wire-shaped filler metal, requiring the use of a laser, electron beam, or arc. These processes have a high deposition rate and therefore help to cut production times.
WAAM is a wire-based process, and uses the gas metal arc welding process (GMAW). WAAM itself offers a number of advantages besides its high deposition rate (up to 4kg per hour with steel materials). In the future, multi-wire solutions could give rise to even higher deposition rates. Equipment and material costs are also important criteria, which is another strength of WAAM: all you need is a suitable welding system. There is no requirement for costly special equipment, such as the vacuum chambers needed for the faster electron beam process.
In comparison to powder-based processes, WAAM benefits from the immediate availability of a range of certified wire types. There are relatively few powder-based materials to choose from, as it can take years to acquire the necessary certification and to produce data sheets, since the use of metal powder is a relatively new technology.
“Cold” welding process essential for WAAM
A stable welding process and effective heat dissipation are essential for WAAM. The welding process needs to be sufficiently low energy such that when a new layer is applied, the existing layers do not melt again. In other words, the process needs to be as “cold” as possible. Furthermore, the weld layers need to be continuous, spatter-free, and consistent. If any flaws were to occur, these would be replicated in each subsequent layer.
The cold metal transfer (CMT) GMAW process from Fronius (and its process control variants) meets these requirements. They produce a stable arc and a controlled short circuit with long short circuit times. This means that the heat input is lower and the material transfer is practically spatter-free, which helps to prevent flaws.
Two process control variants of CMT are particularly well suited to WAAM. One is the CMT additive process characteristic, which has been optimised for WAAM. It achieves high deposition rates while transferring very little heat into the component. The CMT Cycle Step variant reduces the arc power even further through the controlled deactivation of the arc during the process phase. However, this particularly “cold” process does need longer to build up the layers, as the deposition rate is lower.
Real world WAAM applications
Countless WAAM components have already been produced using welding technology from Fronius in a variety of sectors. These include fan impellers for the electronics industry, which are made from high-grade materials. Milling the workpiece is very expensive due to the high rate of material consumption, while casting is not always able to meet the critical metallurgical properties required for walls just 1.5mm thick. With WAAM based on CMT Cycle Step, these fan impeller blades can be produced from a nickel-based alloy using an additive approach. It is even possible to repair components using WAAM.
Fronius has also implemented another application with a partner in the aviation sector. Titanium is a frequently used material in aircraft construction thanks to its tensile strength, resilience, corrosion resistance, and low weight. The majority of the components are manufactured using subtractive methods, whereby up to 90% of the material is milled away. This causes high costs, long machining times, and costly tool wear. Components produced using WAAM, on the other hand, only need reworking to produce a smooth surface. The titanium components produced using the CMT additive process do not exhibit any problems with lack of fusion and have impressive metallurgical properties. Tool costs, machining times, and wear can be reduced, meaning that overall machining costs can be brought down.
WAAM is economical and flexible
This makes WAAM a cost-effective and flexible alternative for component production. The additive process can be adopted with relative ease using welding technology from Fronius and the CMT solution.