Equipment for TIG Welding
Job Knowledge for Welders No. 6 describes the TIG welding process. Using an inert gas shield instead of a slag to protect the weldpool, this technology is a highly attractive alternative to gas and manual metal arc welding and has played a major role in the acceptance of high quality welding in critical applications.Essential equipment
In TIG, the arc is formed between the end of a small diameter tungsten electrode and the workpiece. The main equipment
components are:
Power source
The power source for TIG welding can be either DC or AC but in both the output is termed a drooping, or constant current, characteristic; the arc voltage / welding current relationship delivers a constant current for a given power source setting.If the arc voltage is slightly increased or decreased, there will be very little change in welding current. In manual welding, it can accommodate the welder's natural variations in arc length and, in the event of the electrode touching the work, an excessively high current will not be drawn which could fuse the electrode to the workpiece.
The arc is usually started by HF (High Frequency) sparks which ionise the gap between the electrode and the workpiece. HF generates airborne and line transmitted interference, so care must be taken to avoid interference with control systems and instruments near welding equipment. When welding is carried out in sensitive areas, a non-HF technique, touch starting or 'lift arc', can be used. The electrode can be short circuited to the workpiece, but the current will only flow when the electrode is lifted off the surface. There is, therefore, little risk of the electrode fusing to the workpiece surface and forming tungsten inclusions in the weld metal. For high quality applications, using HF is preferred.
DC power source
DC power produces a concentrated arc with most of the heat in the workpiece, so this power source is generally used for welding. However, the arc with its cathode roots on the electrode (DC electrode negative polarity), results in little cleaning of the workpiece surface. Care must be taken to clean the surface prior to welding and to ensure that there is an efficient gas shield.Transistor and inverter power sources are being used increasingly for TIG welding. The advantages are:
- the smaller size makes them easily transported
- arc ignition is easier
- special operating features, e.g. current pulsing, are readily included
- the output can be pre-programmed for mechanised operations
AC power source
For materials such as aluminium, which has a tenacious oxide film on the surface, AC power must be employed. By switching between positive and negative polarity, the periods of electrode positive will remove the oxide and clean the surface.
The figure shows current and voltage waveforms for (sine wave) AC TIG welding.
Disadvantages of conventional, sine wave AC compared with DC are:
- the arc is more diffuse
- HF is required to reignite the arc at each current reversal
- excessive heating of the electrode makes it impossible to maintain a tapered point and the end becomes balled
By switching between polarities, arc reignition is made easier so that the HF can be reduced or eliminated. The ability to imbalance the waveform to vary the proportion of positive to negative polarity is important by determining the relative amount of heat generated in the workpiece and the electrode.
To weld the root run, the power source is operated with the greater amount of positive polarity to put the maximum heat into the workpiece.
For filler runs a greater proportion of negative polarity should be used to minimise heating of the electrode. By using 90% negative polarity, it is possible to maintain a pointed electrode. A balanced position (50% electrode
positive and negative polarities) is preferable for welding heavily oxidised aluminium.
Torch
There is a wide range of torch designs for welding, according to the application. Designs which have the on/off switch and current control in the handle are often preferred to foot controls. Specialised torches are available for mechanised applications, e.g. orbital and bore welding of pipes.Electrode
For DC current, the electrode is tungsten with between 2 and 5% thoria to aid arc initiation. The electrode tip is ground to an angle of 600 to 900 for manual welding, irrespective of the electrode diameter. For mechanised applications as the tip angle determines the shape of the arc and influences the penetration profile of the weld pool, attention must be paid to consistency in grinding the tip and checking its condition between welds.For AC current, the electrode is either pure tungsten or tungsten with a small amount (up to 0.5%) of zirconia to aid arc reignition and to reduce electrode erosion. The tip normally assumes a spherical profile due to the heat generated in the electrode during the electrode positive half cycle.
Gas shielding
A gas lens should be fitted within the torch nozzle, to ensure laminar gas flow. This will improve gas protection for sensitive welding operations like welding vertical, corner and edge joints and on curved surfaces.Backing system
When welding high integrity components, a shielding gas is used to protect the underside of the weld pool and weld bead from oxidation. To reduce the amount of gas consumed, a localised gas shroud for sheet, dams or plugs for tubular components is used. As little as 5% air can result in a poor weld bead profile and may reduce corrosion resistance in materials like stainless steel. With gas backing systems in pipe welding, pre-weld purge time depends on the diameter and length of the pipe. The flow rate/purge time is set to ensure at least five volume changes before welding.Stick on tapes and ceramic backing bars are also used to protect and support the weld bead. In manual stainless steel welding, a flux-cored wire instead of a solid wire can be used in the root run. This protects the underbead from oxidation without the need for gas backing.
Inserts
A pre-placed insert can be used to improve the uniformity of the root penetration. Its main use is to prevent suck-back in an autogenous weld, especially in the overhead position. The use of an insert does not make welding any easier and skill is still required to avoid problems of incomplete root fusion and uneven root penetration.Protective equipment
A slightly darker glass should be used in the head or hand shield than that used for MMA welding.Recommended shade number of filter for TIG welding:
| Shade number | Welding current A |
|---|---|
| 9 | less than 20 |
| 10 | 20 to 40 |
| 11 | 40 to 100 |
| 12 | 100 to 175 |
| 13 | 175 to 250 |
| 14 | 250 to 400 |
The article was prepared by Bill Lucas.
Copyright © 2003 TWI Ltd