Welding is a remarkably versatile joining technique, with various types of specialist equipment available to join different types of metal, different thicknesses and with weld characteristics to suit the application requirements. With today’s demand for

high-speed, high-quality processes that require minimal finishing, there is a growing need for fast, spatter-free welding technologies.

In addition, optimised product designs often require dissimilar materials to be joined, as well as materials of different thicknesses. These last two requirements, in particular, pose problems for traditional welding technologies such as gas metal arc (GMA) welding.

There is, however, a newly developed welding process, known as Cold Metal Transfer (CMT), that appears to offer the combination of versatility, speed and quality that is being sought. Developed by Fronius, the CMT process is characterised by workpieces – and weld zones – that remain considerably ‘colder’ than they would in conventional welding processes such as GMA. The reduced thermal input leads to advantages such as low distortion and high precision.

CMT is closely related to GMA and is suitable for automated and robot-assisted applications. Other significant benefits for users of CMT include higher quality for the welded joints (in terms of uniformity and reproducibility), an absence of spatter, the ability to weld light-gauge sheet from 0.3mm thick, and the ability to join both galvanised and bare steel to aluminium. As well as being a suitable replacement for welding, the versatile CMT technology can also be used instead of brazing.

CMT is based on dip-transfer arc welding and features a deliberate, systematic discontinuity of the arc. The result can be described as an alternating hot-cold-hot-cold sequence that greatly reduces the arc pressure. In a normal dip-transfer arc, the electrode is deformed while being dipped into the weld-pool, and melts abruptly at a high dip-transfer

arc current. In contrast, the CMT process benefits from a wide process window and, therefore, high stability. This is important when the welding torch is abruptly reorientated, as is often the case in automated or robotic welding. There are three criteria that differentiate the CMT process from the familiar dip-transfer arc-process: the wire motions are incorporated into the

process-control; the thermal input is lower, and

the metal transfer takes place without creating spatter.

How CMT works

The principal innovation in CMT is that the motion of the wire is closely controlled as an integral part of the welding process. Every time the short circuit occurs, the digital process controller interrupts the power supply and retracts the wire, which assists droplet detachment. This forward-and-back motion takes place at a frequency of up to 70Hz. A defining feature – and sometimes critical side-effect – of arc welding is the conversion of electrical energy into heat. In CMT, the controlled interruption of the short-circuit leads to a low short-circuit current and greatly reduced heat input to the joint.

Most attempts to create a spatter-free welding process have had limited success, but Fronius boldly describes CMT as 'spatter-free metal transfer'. The elimination of spatter is said to be due to a combination of the forward-and-back motion of the wire and the controlled short-circuiting.

Although the CMT process is based on conventional GMA welding elements – such as a power – there are two separate digitally-controlled wire feeders: the front one moves the wire forward and back up to seventy times per second, while the rear one pushes the wire. A buffer between the two feeders ensures they do not influence each other.

The combination of reduced thermal input and freedom from spatter means welding and brazing can be used for applications that would not have been possible before, while also delivering benefits such as higher productivity, fewer rejects and less post-weld finishing.

These advantages are complemented by a high gap-filling ability, which leads to better control of automated processes and a near-flawless appearance for the finished joint. One notable application for CMT is the

butt-welding (or butt-brazing) of sheets as thin as 0.3mm. This means that, for example, aluminium sheets can now be welded with no need for any clamping tools, and with no burn-through.

Application examples

Meanwhile, VW Sachsen, in Germany, is the first automobile manufacturer to use the CMT process in car body manufacturing for the Bentley Continental and VW Phaëton. CMT welding the C-pillar from three parts of different thicknesses produces excellent results with 20 to 30percent less heat input and no spatter (Fig.2). This reduces clamping and logistic requirements and almost eliminates the need for post-weld finishing.

The C-pillar is an important part of the construction, comprising three high-strength galvanised sheet steel parts with thicknesses of 0.8, 10 and 1.7mm. The quality of the joint is critical for reasons of both strength and appearance.

As a result of the lower heat input, there is a 50percent reduction in metal incursion, which significantly reduces the need for time-consuming hand-finishing of the C-pillar. The second advantage lies in the fact that the CMT process is spatter-free, which also helps to minimise the finishing time required (Fig.3). Finally, CMT means improved accuracy and faster welding.

For automotive supplier ELB-Form, CMT has proved highly beneficial in its automatic and robot-assisted welding processes. ELB-Form is a specialist supplier of lightweight pipes; its core skills are centred on reshaping pipes using very high internal pressure, plus the welding of complex components. The CMT process presented the company with new opportunities in terms of weld quality and workflow. Spatter-free welding, outstanding gap-filling properties, low heat input and a high degree of process flexibility convinced the company to convert all of its welding robots to the new process (Fig.1).

Tube welding

Another innovative welding process has also recently been developed; again, it produces a small heat-affected zone, dissimilar materials can be joined, and it can be used as an alternative to both welding and brazing. However, this process is suitable only where one component is tubular, and the capital equipment cost is relatively low.

In an attempt to find a better way to joining metal components for automotive assemblies, Delphi Technologies developed Deformation Resistance Welding (DRW) with the intention of reducing costs relating to welding, manufacturing and maintenance.

DRW is now being commercialised by Spaceform, a spin-out company that Delphi created to take the patented process forward. While the first applications were to replace brazing, the main area of interest is in joints that are currently welded. In comparison with welding, DRW is said to offer a number of advantages: both capital and materials costs are lower, process cycle time is up to 79percent lower and heat is applied only locally, with no need for a filler material. In addition, solid-state joining gives excellent versatility, the process is suitable for automating, and it is geometry-independent.

For end users, product performance is also enhanced, as the joints are leak-free and non-porous. Spaceform claims that the DRW joint is stronger than the parent metal and, when tubes are welded, there is no thinning of the parent metal.

As the DRW process was created by an automotive supplier, it is not surprising that the first potential applications identified are in that industry: exhaust systems, tubular spaceframes, fluid-power systems and load-bearing structures. Nevertheless, numerous other opportunities exist beyond the automotive industry as well. These include bicycles, wheelchairs, large-scale industrial heat exchangers – and NASA is evaluating the DRW process for use in stationary spaceframes for use in the Man on the Moon and Man to Mars expeditions.

The DRW process

Prior to welding, the tubular component is first formed to create a folded flanged. The welding process is then similar in concept to resistance welding, with the DRW machine compressing the folded flange against the second component while an electric current passes through the two. An important aspect of the process is that the electrodes permit relative movement between the components while they in the plastic state – very close to melting point – which enables the joint to be formed through both deformation and displacement of material at the component interface. n