Gas tungsten arc welding (GTAW) is well known as one of the highest quality welding processes, but fewer people are as familiar with keyhole GTAW. The process was developed by Australia’s national science research agency, CSIRO, and first reports of its benefits were published in 2000.
As the name implies, keyhole GTAW or keyhole TIG (tungsten inert gas) welding, is a variant of the GTAW process. With vast speed improvements over standard TIG welding and fewer complications than laser or plasma welding, other manufacturers are bringing versions of this high-performance technology to market. Understanding the benefits and limitations of keyhole TIG is the first step in solving welding challenges.
The keyhole technique covers a variety of welding processes that use the arc to push through the workpiece. During the welding process, this “keyhole” is back filled with the molten weld pool for a solid seam. Though keyhole welding uses a tungsten electrode and inert gas like standard TIG welding, the arc is focused, with a narrower heat-affected zone (HAZ).
Keyhole TIG welding blends concepts from standard TIG and plasma welding, forging a unique mix of benefits. Anyone acquainted with TIG welding will understand the keyhole version. The same parameters that modify a standard TIG arc modify the keyhole TIG arc – it’s just faster and penetrates deeper.
In terms of the Fronius ArcTig system, which utilizes the TransTig power supply for both keyhole and standard TIG welding, ArcTig uses a specialized torch in combination with a chiller to cool the tungsten during welding. Cooling the tungsten forces the arc to the tip of the electrode, narrowing the arc considerably and increasing energy density during welding. This focused arc creates the keyhole effect and deep penetration but provides the same clean, quality weld as standard TIG welding. The similarities in the processes also help reduce training time.
The increased energy density and arc pressure produced by keyhole TIG welding require high travel speeds, making it a perfect fit for mechanized or robotic welding. On average, any mechanized or robotic system is faster than manual welding, but keyhole TIG welding can be up to 99 percent faster than manual welding.
The biggest benefit of speed comes from the ability of keyhole TIG welding to complete a weld in a single pass where standard TIG welding requires a root pass and filler passes for the same joint. By reducing arc time, the machine uses less gas and produces less wear on replaceable parts, such as the electrode. Faster welding and increased productivity equal major cost savings.
Another time saver is weld preparation. Most mechanized TIG welded butt joints need a J-prep, or U-groove, to ensure proper root penetration. This type of prep, known as beveling, is unnecessary for most keyhole TIG welding, which uses a square butt joint. Beveled edges require more material to fill the space left by the cut-away material. Keyhole TIG welding removes the need for beveling in any but the thickest joints and reduces or eliminates the need for filler material.
Though many manufacturers are developing keyhole TIG welding systems, improving the simplicity and cost of the wear parts needs to be a focus. Expensive and single-source replacement parts may have contributed to the limited use of this process in the past. Switching suppliers, adding space for new products and stocking specialty items are all costly undertakings.
Newer technology, such as the ArcTig, allows companies to use whatever brand or type of tungsten they choose and regrind when necessary. Other brands on the market utilize a cartridge set that encompasses the tungsten. This means the whole set has to be replaced when the tungsten wears down.
The cooling characteristics of the ArcTig process also extend the life of the electrode. These improvements make keyhole TIG welding a viable option for mechanized and robotic applications.
How does keyhole TIG welding compare to other processes? The gold standard of welding quality, TIG, has many benefits including those it shares with keyhole TIG: high-quality, precision welds on almost any metal with a clean, spatterless finish. Where standard TIG welding may be the better option is on thinner materials, out-of-position welds or when mechanized welding isn’t an option. For thicker materials, keyhole TIG welding provides a lot of savings, but for materials thinner than 3 mm, standard TIG or plasma welding may be a better choice.
Plasma is another interesting area to explore in terms of where keyhole TIG welding may be a better option. Plasma welding uses super-heated gas and a tungsten electrode to join metals. While plasma welders aren’t difficult to use, they rely on precise setup by the operator for every application.
A minimum of five input parameters affect the plasma arc: amperage, travel speed, plasma gas flow, tungsten recession and nozzle diameter. A simple tungsten change requires specific measurements for proper electrode recession to produce consistent welding results. In contrast, after setting the voltage, dialing in the arc for keyhole TIG welding requires just two parameters: amperage and travel speed. Fewer parameters allow keyhole TIG welds to be more consistently reproducible.
Compared to standard TIG welding, plasma welding provides a narrower HAZ and a 10 to 15 percent increase in travel speed, but keyhole TIG welding can see speed gains of an additional 10 percent increase over plasma welding. However, for applications that need precise weld bead width control or are extremely heat-input sensitive, plasma welding can provide that precision and potential for further reduction in HAZ width with the use of plasma nozzle variations.
A few limitations
One of the major benefits of keyhole TIG, single-pass welding, can also be its biggest trouble spot. With a single-pass weld, there aren’t additional layers to get the job right. Everything from the grind on the tungsten to the joint fit-up and alignment needs to be precise. Welding at such increased travel speeds doesn’t leave room for error and, combined with the narrowed arc, could lead to undercutting. This is more problematic when welding without the use of filler material (referred to as autogenous welding).
Other limitations are found in the welding positions and types of joints. Keyhole TIG welding is limited to horizontal and flat positions. Application possibilities can be expanded by using part positioners or rotational devices. The process is ideal for long seam welds, pipe joints, and fabricating tanks and pressure vessels.
High-quality, consistent and reproducible welds are the hallmark of keyhole TIG welding. Increases in weld speed and travel speed, paired with reduced weld prep and simple setup ensure that keyhole TIG welding systems like the Fronius ArcTig save money and increase productivity for most mechanized joining applications.