Steinbeis experts develop a plasma welding burner using a mechanical rotating light arc for bonding and weld overlaying
Welding has become an extremely important task, not just for joining traditional materials – such as steel or aluminum – but also for joining different kinds of materials and plastics used in multi-material components. The most common method is arc welding using non-melting electrodes, not only because this technique is highly cost-effective and can be automated, but also because of the high quality of the seams. But welding processes are also subject to increasingly stringent expectations when it comes to productivity and product quality. As a result, Intelligent Functional Materials, Welding and Joining Techniques, Implementation, the Steinbeis Innovation Center in Dresden, joined forces with Autogen-Ritter to design and develop a plasma welding torch that allows arc rotation to be controlled mechanically.
Plasma arc welding is a form of tungsten inert gas (TIG) welding. As such, it offers an interesting alternative to laser welding, especially when used on metal sheets with a thickness of up to 8 millimeters. With plasma welding, the welding arc is constricted by a water-cooled copper nozzle. Not only does this deliver a much more intense pulse of energy, it also makes it possible to alleviate beam divergence. Whereas the diameter of TIG arcs increases rapidly between the electrode and the workpiece, with plasma arcs the diameter only widens marginally. This eliminates the need for time-consuming seam preparation work in order to “bundle” the arc.
In addition, the higher welding speed saves time and money and makes it possible to achieve a greater depth of fusion. Tungsten electrodes also have a much longer service life because they are surrounded by plasma gas, which promotes cooling. The method is used on thin and thick sheets, as well as semi-finished products with metallic coatings. A further potential field of application is repairing and maintaining components. The high power density also supports the welding of metals with good thermal conductivity, such as copper and copper alloys.
Welding processes are subject to continuously intensifying demands: High-volume production, for example, typically requires highly automated welding processes, especially compared to prototyping and small-scale production; but this also applies to the reworking or repairing of parts in serial production, which are not subject to such high demands in mechanical terms. With such applications, there are a number of ways to derive benefit from the processing advantages of plasma welding.
A plasma arc burner with mechanically controlled light arc rotation – safe and economical
Experts at Intelligent Functional Materials, Welding and Joining Techniques, Implementation, the Steinbeis Innovation Center based in Dresden, decided to join forces with Autogen-Ritter and take on this challenge as part of an R&D project. Together, they not only developed a plasma welding torch offering mechanically controlled arc rotation, they also established the required process and hardware for seam and buildup welding. The new process technology, including torch technology, also had to be economical in terms of production costs and cycle times. Similarly, it was important that any mutual dependences between the rotation and feed speed on the one hand, and the intensity of the arc energy on the other, delivered the required seam quality during the welding process.
For the first stage of the project, the team drew on an overall process engineering concept to design an intensely cooled plasma arc welding torch with a functionally reliable prototype of a mechanical arc rotation unit. To capture processing data and the process parameters that would be required for the defined burner output of I = 200 A, the experts used a high-speed camera. In order to ensure the mechanical rotation of the plasma welding torch functioned reliably within the process, the researchers also designed an efficient torch cooling and gas supply system. To do this, a water cooling system was designed along with a pump and heat exchanger such that heat generated in the areas around the functional areas of the torch could be safely transferred away during the welding process. The resulting combination of materials and technology was then applied to the design of the welding torch. The plasma and protective gases in the torch head are able to flow without issue, also taking mechanical arc rotation into account to achieve a reliable welding process. The geometries of the torch parts were coupled with the developed arc rotation unit and synchronized in terms of process technology.
This makes it possible to cool the plasma nozzle intensely such that the mechanical arc rotation is achieved without error. As a result, the functional parts of the torch head are subjected to less thermal stress during the welding process and arc rotation. To achieve this, materials that can withstand high temperatures and deliver electrical insulation were fitted between the individual parts, the cooling system, and the power connection. The experts working on the project also developed and designed the documentation required to construct the entire plasma torch head and the mechanical rotation unit. Other elements developed for the project, such as a beveled tungsten electrode, a DC motor, drive elements, plain bearings, a timing belt, and a clamping device for the plasma welding torch, were also coupled in terms of control technology and mechanical interplay. The project also resulted in the development of a feed unit for wire or powder filler materials. In the prototype torch system, this is integrated through the mechanical arc rotation unit.
The prototype meets requirements
Testing of the torch prototype not only confirmed that the mechanical arc rotation functioned without error. The torch cooling process worked reliably, as did the in-feed of process gas and filler material. The results also showed that the mechanical rotation of the plasma arc and the high arc temperature gradients result in a more intense mixing of molten pool and corresponding material particles.
This makes it easier to apply heat more evenly and achieve uniform component cooling. The result: fine-grained microstructure and component properties, in line with quality standards. This was the case with thin and thick sheets, as well as sheets with metallic coatings, depending on the applied process parameters.
When used to weld thin sheets with a maximum thickness of 6.0mm, depending on the type of joint, the mechanically rotating plasma welding torch succeeds in maintaining clearance tolerances, such that uniform seams can be achieved on component surfaces in all kinds of welding positions. The solution also makes it possible to vary component tolerances during the welding process and thus maintain required component tolerances and dimensions. The project team also proved that the newly developed plasma arc, the process technology, and the hardware make it possible to achieve welding speeds of up to 3.0m/min with the required seam quality. The quality of the produced seam show that, equipped with the mechanical rotation unit and based on a torch power of I = 200A, the developed plasma welding torch prototype is suitable for practical application.
The prototype plasma arc welding torch – technical information:
- Welding current up to 200A DC
- Working voltage up to 35V
- Tungsten electrode (diam. 2.4mm) beveled on one side at a 30 – 45° angle.
- Rotation of the tungsten electrode/cathode around its own axis (for arc rotation): plasma nozzle diameter 2.3/3.2mm
- Driven by a toothed belt (non-conductive), plain bearing (for example made of plastic)
- Total torch head size: 150 x 100mm (compact design)
- Interchangeable plasma nozzle with an indirect strong water cooling system
- Specially adapted water cooling system with a cooling unit for the rotary unit