TIG Keyhole Welding with Forced Cooled Tungsten Electrode

Willinger, R&D Arc Technology, Fronius Iternational GmbH
Schörghuber
Wels-Thalheim

Dipl.- Ing. Manfred Schoerghuber
FRONIUS International GmbH.
‘Keyhole welding with directly cooled tungsten electrode’, a process variant of the TIG welding method, is characterized, in contrast to plasma keyhole welding, by its use of simpler system technology with additional process benefits. Although the physical effect at amperages above 300 A has been known for a long time [1, 2, 3, 4], its advantages are hardly ever used in industrial applications. This article compares different tests with the aim of helping welders to choose a welding process.
1      Keyhole Welding – the Welding Process and Process Variants
Keyhole welding mainly works with materials of low thermal conductivity and is mainly used for butt and corner welds. The buildup of heat beneath the arc superheats the molten mass which is pushed down by the arc pressure. It results in a “keyhole” which should penetrate through the entire thickness of the workpiece.
The area of the impacting arc determines the diameter of the keyhole. In the case of plasma keyhole welding, this is controlled by the diameter selected for the plasma gas nozzle. In the case of TIG welding, the welder takes advantage of the fact that electrons escape from hot cathodes more easily. This means that if only the end of the electrode tip is superheated and the rest of the electrode tip is cooled by means of good heat dissipation, then a very narrow arc is formed at the electrode. This results in a high current density in the plasma column which can create a keyhole with a lot of arc pressure. Of course, the electrode distance, grinding angle and amperage remain important welding parameters.
Figure 1 shows a simplified comparison of the arc appearance and the penetration for the plasma keyhole process and the TIG variants with “conventional electrode clamping system” and “directly cooled tungsten electrode”. Cooling directly after the electrode tip requires significantly more cooling power for the welding torch.

Figure 1 : Arc formation comparison (left: plasma, center: “conventional” TIG, right: “electrode cooled” TIG).
When carrying out keyhole welding, it is important that the plasma jet fully penetrates as otherwise there is a risk of plasma-induced pore formation. The risk of this “frozen” pore is practically eliminated through the “blowing out” of the plasma jet, thus excellent quality seams can be achieved.
The surface tension of the liquid metal ensures that the keyhole closes again as the arc moves on and no liquid metal drips. Sometimes, both joint components moving together due to the shrinkage stresses results in weld reinforcement. The desired weld reinforcement can be directly controlled through the addition of wire-shaped filler metal.
Table 1 compares the quality aspects of plasma keyhole welding with the TIG processes. It should be noted that the required current densities for keyhole welding can also be achieved with a conventional TIG welding torch, however the electrode burns out very quickly in this case and shows signs of wear, such that a practical limit can be drawn at a sheet thickness of 4 mm.

In theory, plasma welding can also be used to join considerably thicker sheets. In practice, the limit is set by the amperage. The plasma nozzle is irreparably damaged as soon as the arc touches it directly and a practical limit is drawn at 400 A.

The cooling power of the welding torch system limits the range of applications for the directly cooled TIG variant. With significantly higher welding currents, faster welding speeds can be achieved with a lower energy input per unit length, when compared directly with plasma keyhole welding. As plasma current/gas hardware or multi-layer nozzle systems are not required, significantly less investment in technical equipment is needed and it is less effort compared to the complexity of numerous PLASMA parameter settings.
Table 1. Overview of different limit values and complexities for keyhole welding.

*Parent material is austenitic CrNi steel, butt joint without processing, single-pass, recommended limits.
2 . Components and System Design
The principle of the directly cooled tungsten electrode is not new and is sold by a range of manufacturers with different designs and brand names. There are solutions with screwed-on or push-in tungsten inserts or, as shown in figure 2, cooled clamping mechanisms from FRONIUS for commercially available tungsten electrodes.

Figure 2: Example of an automated TIG welding torch with clamping mechanism and direct cooling of the tungsten electrode [5].
In practice, keyhole welding tasks can only be carried out using mechanized welding systems. As an example, the required components are explained for a “clamping bench” system with longitudinal chassis, as used in tank manufacture:
The high process currents are generated, for example, by two TIG power sources (1+2) connected in parallel. The TIG welding torch (3) is supplied by an adequately dimensioned cooling unit (9). The carriage control (13) regulates not only the reduced and process currents but also the travel speed (6) of the filler metal and, if necessary, the height by means of AVC (Automatic Voltage Control). A portable gas nozzle (4) prevents oxidation of the surface and an optional weaving unit (11) supports the process and increases the gap tolerances and welding depth.

Figure 3: System design of a TIG keyhole welding system [6].
3        TIG Keyhole Welding – Process Limits
3.1     Comparison of Parameters
The parameters are compared for butt welds without processing with 100% argon as shielding, plasma, forming and carrier gas. For brevity, “plasma” is used below to denote the plasma keyhole welding process while “ArcTig” denotes the TIG process with directly cooled electrode.
Table 2 shows that ArcTig has twice the welding speed with the same energy input per unit length on a 6 mm sheet, due to the considerably higher welding current. If welding is performed with the same welding speed, the energy input per unit length is even somewhat higher because the required current densities for the keyhole effect only arise from 300 A.
Table 2. Comparison of process parameters – austenitic steel 1.4301 – 6 mm butt joint.

**Plasma gas nozzle with 3.2 mm dia., plasma gas 1.8 l/min   *TIG electrode distance 1.5 mm, 6.4 mm dia. Electrode.
Table 3 shows a comparison for an 8 mm sheet, whereby here higher plasma currents and voltages reverse the energy input per unit length relationships at just 20 cm/min and only half the energy input per unit length or welding time is needed at a possible ArcTig welding speed of 40 cm/min.
Table 3. Comparison of process parameters – austenitic steel 1.4301 – 8 mm butt joint.

**Plasma gas nozzle with 3.2 mm dia., plasma gas 1.8 l/min   *TIG electrode distance 1.5 mm, 6.4 mm dia. Electrode.
The advantages of the welding speed can primarily be achieved on materials with low thermal conductivity. The comparison with 8 mm steel shows only a small difference in welding speed and energy input per unit length.
Table 4. Comparison of process parameters – ferritic steel S355 – 8 mm butt joint.

**Plasma gas nozzle with 4.0 mm dia., plasma gas 2.8 l/min  *TIG electrode distance 1.5 mm, 6.4 mm dia. Electrode.
3.2     Tolerance Limits
TIG keyhole welding is a high performance welding process and therefore requires precision clamping technology and tight tolerance limits. A standard value of max. 10% of the sheet thickness applies to edge misalignment and air gap. The bevel processing should ideally be butt and single-pass. If this process is used for the root pass of thick components, then Y or U bevel processing is recommended.

Figure 4: Recommended tolerance limits and bevel processing for TIG keyhole welding.
3.2     Weaving Offers Greater Process Reliability
Weaving at low amplitude provides a level of production reliability with tolerances that is not to be underestimated. Figure 5 shows the results of different offsets. The root pass is clearly wider when weaving is used on this sheet thickness and the “keyhole lance” is able to compensate for the offset of 1 mm (Figure 5 “center”). If the offset is larger than the weaving amplitude, the process is no longer able to compensate for the offset position (Figure 5 “right”).

Figure 5: Weaving comparison with and without offset on a 6 mm butt joint.
*ArcTig without filler metal, without forming gas, TIG electrode distance of 2 mm, 4.8 mm dia. Electrode.
Figure 6 shows the deflection of the “plasma flame” due to the geometry of the plasma channel as it exits the sheet from below. At a high welding speed, the plasma channel is counter to the welding direction. During weaving, a clear deflective movement against the welding direction is detected at weaving frequency. With a blunt tungsten electrode the plasma does not extend as far on the underside.

Figure 6: Photo of the deflection of the plasma channel at high welding speed (left) – comparison of the appearance of the plasma channel with a sharp (center) and blunt electrode (right) on the underside of a 6 mm sheet.
Weaving is also advantageous when welding thick sheets in terms of weld-seam formation and production reliability. Figure 5 shows a comparison of the TIG keyhole weld on an 11 mm sheet, which already exceeds the upper limit of recommended sheet thicknesses. The keyhole channel in the lower third of the seam is broader and root fusion is easier.

Figure 7: Comparison of the fusion reliability with and without weaving on an 11 mm butt joint.
*11 mm sheet 1.4301, TIG electrode with a dia. of 6.4 mm, filler metal: 316L, shielding and forming gas: I1-Ar.
4        TIG Keyhole Applications
A distinction is made between longitudinal and circumferential weld applications. Austenitic chromium-nickel steels are mainly butt welded, but duplex and nickel-based steels and titanium are also possible. Welding is normally carried out with clamping benches and gantry jibs in the PA position, and also in PC.
4.1     Applications in Tank Construction
As an example, a customer in the food industry was able to triple the welding speed by replacing TIG keyhole welding with ArcTig keyhole welding on a 4 mm thick tank. Thicker wall thicknesses, which were previously carried out in multiple passes, can now be completed in a single pass [7].

Figure 8: Austenitic tank with ArcTig keyhole longitudinal and circumferential welds in the food industry.
4.1.1  Advantages in Low Temperature Tank Construction
The lower energy input per unit length with faster welding speed delivers a distinctively finer structure compared to the plasma keyhole welding process and higher impact energy. This advantage is most evident with thicker wall thicknesses and comparatively narrow heat-affected zones.
Table 5. Comparison of the impact energy at -196 °C

**Double pass, *single pass.
4.2     A Variety of Possible Applications for TIG Keyhole Welding
On longitudinal seams, run-in and run-off plates are used, for circumferential welds appropriate parameter runs for motion, current and filler metal must be selected when welding over the start of the weld.

5        Summary and Outlook

TIG keyhole welding achieves a seam quality comparable to that of the plasma keyhole welding process but with a faster welding speed, less additional hardware and fewer parameter settings. Weaving at low amplitude or control by means of filler metal offers additional process reliability.

This process variant offers potential which is often not used. Some companies still lack the level of automation needed for possible applications, others often use multi-pass or a variety of welding processes or doubt the process reliability.
As a high performance welding process, special attention must be paid to the clamping technology and tight tolerance windows. The reward is a very cost-effective joining process which can provide a competitive advantage in the face of ever increasing wage and price pressure.
Literature
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