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Additive manufacture of complex components using (CMT)

TWI continues to investigate the use of arc welding techniques for additive manufacture of complex components in order to minimise machining costs and material wastage.

Current additive manufacturing techniques are often based on laser or electron beam processes with powder consumables. These can offer high precision, but are relatively slow techniques. The use of arc welding allows the deposition rates to be greatly improved, although with a loss of precision.

As part of TWI’s ongoing core research programme (CRP) trials have been performed making use of the cold metal transfer (CMT) technique to form simple aluminium structures as a proof of concept. CMT is 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 arc welding technique is particularly suited to the deposition of aluminium due to the high reflectivity making it traditionally, a difficult material to deposit with laser processes

Test structures were performed using the CMT process, as this possesses certain characteristics that make it excellent for controlled deposition. The mechanical wire control leads to low spatter, which is important for minimising the amount of post process machining and the low heat input nature of the process results in minimal melting of the substrate material or prior runs.

The relatively high deposition rate allows the production of high volume structures in a relatively short period of time. For example, generating a 5183 grade aluminium structure using a 1mm diameter wire resulted in a deposition rate of 0.94kg/hour. It is predicted that it is possible to deposit similar volumes of steel and nickel-based alloys.

The CMT process is designed for use only on a robotic system and therefore suited for additive manufacture, where a pre-determined path will be used to generate complex structures repeatedly in a production line process.

The CMT process was able to construct “walls” and “pads” by performing multipass welds with either a vertical or horizontal offset. While the production of vertical walls and pads was easily achieved with a standard vertical torch alignment, 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 1 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.

It is reasonable to extrapolate from these initial trials that it is possible to produce multi-angled structures for use in either repair of pre-existing components, or to generate complex geometries without significant machining.

Figure 1 – Range of produced orientations achievable by the CMT process
Figure 1 – Range of produced orientations achievable by the CMT process

The technique was demonstrated to be capable of producing good quality deposits free of porosity, showing good inter-layer fusion, and with an impressive regularity when performed even at an angle as shown in Figure 2.

A range of weld bead sizes was produced, allowing variation in the bead height and width between 1-3mm and 1-5mm respectively. This allows the process to be tuned based on the desired final component shape, with the ability to minimise the number of passes needed for a given geometry.

This technique also has great potential in the production of structures made from expensive or difficult to machine materials such as nickel or titanium alloys and TWI continues to investigate these as part of its ongoing core research programme.

TWI also continues to work on the development of more complex structures to prove out the technique beyond the simple structures shown here.

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Figure 2 – Example of weld beads produced with a 60 degree angle
Figure 2 – Example of weld beads produced with a 60 degree angle