CMT arc welding for robots
May 1st 2008

CMT welding is having a profound effect on the use of robotic technology for arc welding applications.

Steve Bartholomew of ABB explains how it will change the way in which robots are employed

Until now, sheet metal product manufacturers have faced a choice of TIG (Tungsten Inert Gas) or MIG (Metal Inert Gas) welding for the arc welding of sheet metal.

MIG welding has successfully covered much of the market, the limitation being material type, minimum thickness and achievable consistent quality. The higher quality alternative, TIG, has inherent characteristics that have ruled out the use of robots, depriving users of the potential cost, time and quality benefits associated with robotic installations.

TIG welding is most commonly used for welding thin-section material of all types and provides a number of advantages over MIG welding. As a higher quality process for thinner section materials, it is better suited for applications such as stainless assemblies and non ferrous materials.

While TIG welding offers high levels of accuracy, it also needs high levels of skill when performed manually, typically requiring an operator to use both hands to perform the process, one for holding the torch, the other for feeding in filler, for maximum precision. The high level of accuracy required has meant that robots have traditionally been unsuited to TIG welding.

As TIG welding uses a tungsten electrode from which the arc is generated to provide the heat for the addition of the filler material, it needs to be kept at a constant length. However, the tungsten is subject to wear, shortening in length and losing the accurately ground angle during use. This variation in length changes the distance between the tungsten and the work piece, which impacts on the characteristics and width of the weld pool.

To maintain a constant arc length requires the tungsten to be resharpened, incurring downtime – not ideal for automated volume manufacture.

This factor, plus requirements for accurate fit up and difficulty in positioning a wire filler into the weld pool, effectively prohibits the use of robots for TIG welding.

Consequently, in cases where robots are used for TIG welding, it is usually only as a heat source to melt the work-piece.

Recent advances in CMT (Cold Metal Transfer) welding technology, however, are opening up a wealth of new possibilities for robots that could usurp the dominance of manual TIG welding in sheet metal arc welding applications.

CMT welding The concept of CMT welding was introduced in 2005 by Fronius International, a European market leader in arc-welding technology.

By using the MIG welding method of applying a wire consumable, Fronius found a way of welding with high levels of accuracy on thin-section materials usually reserved for TIG welding. Most importantly for robot manufacturers, the big advantage of CMT is that it's based on the MIG welding process, a proven process for robotic automation.

Speed and control of heat input are two key benefits of Fronius' CMT welding process.

CMT welding offers a speed that is four to five times faster than TIG welding's speed of 5mm/second, with levels of consistent quality commensurate with robotic automation.

In terms of heat input, the thermal input levels of CMT welding are much lower and more controllable than with MIG welding, which cannot go below a certain heat level to create an arc. This decreases the likelihood of burn through on the materials being welded. As the process only creates a short circuit at the weld start, the creation of spatter is eliminated, which minimises the need for post-weld finishing in a number of applications.

As CMT welding provides greater control, it also makes position welding achievable. The process enables the full potential of a six-axis robot arm to be realised, with no need to reposition the product during welding. For users, this can represent a saving on CAPEX costs, as turning equipment is no longer required. As the robot reach required is reduced, smaller robots can also be utilised.

The Fronius equipment can be used in all modes of transfer, CMT, synergic and dip pulse, providing considerable flexibility for the user should they choose to switch from one mode to another.

ABB has worked closely with Fronius to develop a process interface to make programming and using the machinery together a straightforward process.

Cost Savings The improved efficiencies available through CMT opens up a host of potential new applications for robots in markets previously dependent on costly, highly skilled labour. In robotic CMT applications, users are typically seeing payback on their investment in robotic systems being achieved in less than a year.

ABB's robotics business in the UK has been testing CMT welding with robots for the past year, applying it to different applications and developing its knowledge of the process.

We expect to see far greater use of robots for arc welding, as users become aware of their ability to simplify welding processes. We also anticipate that manufacturers will start to evolve the process of manufacturing design because of the benefits afforded by the process.

Manufacturers in the automotive and metal fabrication industries will particularly benefit from being able to apply robots using CMT. They will benefit from the ease of the process, increasing their production output rates and also reducing their CAPEX costs.

CMT welding could be the biggest advance in robotic welding for years. It has knocked down a lot of the barriers that were preventing robots from being used more effectively for quality welding applications.

ABB is already working with customers who have realised a return on their investment by the use of the CMT process with robots over labour intensive manual welding processes and looks forward to continuing to prove the combined potential of the technology in even more applications.

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