TIG Welding

Tungsten inert gas (TIG) welding became an overnight success in the 1940s for joining mag-nesium and aluminium. Using an inert gas shield instead of a slag to protect the weldpool, the process was a highly attractive replace-ment for gas and manual metal are welding. TIG has played a major role in the acceptance of aluminium for high quality welding and struc-tural applications.

Process characteristics

In the TIG process the arc is formed between a pointed tungsten electrode and the workpiece in an inert atmosphere of argon or helium. The small intense arc provided by the pointed electrode is ideal for high quality and precision welding. Because the electrode is not consumed during welding, the welder does not have to balance the heat input from the arc as the metal is deposited from the melting electrode. When filler metal is required, it must be added separately to the weldpool.

Power source

TIG must be operated with a drooping, constant current power source – either DC or AC. A constant current power source is essential to avoid excessively high currents being drawn when the electrode is short-circuited on to the workpiece surface. This could happen either deliberately during arc starting or inadvertently during welding. If, as in MIG welding, a flat characteristic power source is used, any contact with the workpiece surface would damage the electrode tip or fuse the electrode to the work-piece surface. In DC, because arc heat is distributed approximately one-third at the cathode (negative) and two-thirds at the anode (positive), the electrode is always negative polarity to prevent overheating and melting. However, the alternative power source connection of DC electrode positive polarity has the advantage in that when the cathode is on the workpiece, the surface is cleaned of oxide contamination. For this reason, AC is used when welding materials with a tenacious surface oxide film, such as aluminium.

Arc starting

The welding arc can be started by scratching the surface, forming a short-circuit. It is only when the short-circuit is broken that the main welding current will flow. However, there is a risk that the electrode may stick to the surface and cause a tungsten inclu-sion in the weld. This risk can be minimised using the ‘lift arc’ technique where the short-circuit is formed at a very low current level. The most common way of starting the TIG arc is to use HF (High Frequency). HF consists of high voltage sparks of several thousand volts which last for a few microseconds. The HF sparks will cause the elec-trode – workpiece gap to break down or ionise. Once an electron/ion cloud is formed, current can flow from the power source

Note: As HF generates abnormally high electromagnetic emission (EM), welders should be aware that its use can cause interference especially in electronic equipment. As EM emission can be airborne, like radio waves, or transmitted along power cables, care must be taken to avoid interference with control systems and instruments in the vicinity of welding.

HF is also important in stabilising the AC arc; in AC, electrode polarity is reversed at a frequency of about 50 times per second, causing the arc to be extinguished at each polarity change. To ensure that the arc is reignited at each reversal of polarity, HF sparks are generated across the electrode/workpiece gap to coincide with the begin-ning of each half-cycle.

Electrodes

Electrodes for DC welding are normally pure tungsten with 1 to 4% thoria to improve arc ignition. Alternative additives are lanthanum oxide and cerium oxide which are claimed to give superior performance (arc starting and lower electrode consumption). It is important to select the correct electrode diameter and tip angle for the level of weld-ing current. As a rule, the lower the current the smaller the electrode diameter and tip angle. In AC welding, as the electrode will be operating at a much higher temperature, tungsten with a zirconia addition is used to reduce electrode erosion. It should be noted that because of the large amount of heat generated at the electrode, it is difficult to maintain a pointed tip and the end of the electrode assumes a spherical or ‘ball’ pro-file.

Shielding gas

Shielding gas is selected according to the material being welded. The following guide-lines may help:

Argon – the most commonly-used shielding gas which can be used for welding a wide range of materials including steels, stainless steel, aluminium and tita-nium.

Argon + 2 to 5% H2 – the addition of hydrogen to argon will make the gas slightly reducing, assisting the production of cleaner-looking welds without sur-face oxidation. As the arc is hotter and more constricted, it permits higher weld-ing speeds. Disadvantages include risk of hydrogen cracking in carbon steels and weld metal porosity in aluminium alloys.

Helium and helium/argon mixtures – adding helium to argon will raise the temperature of the arc. This promotes higher welding speeds and deeper weld penetration. Disadvantages of using helium or a helium/argon mixture is the high cost of gas and difficulty in starting the arc.

Applications

TIG is applied in all industrial sectors but is especially suitable for high quality welding. In manual welding, the relatively small arc is ideal for thin sheet material or controlled penetration (in the root run of pipe welds). Because deposition rate can be quite low (using a separate filler rod) MMA or MIG may be preferable for thicker material and for fill passes in thick-wall pipe welds.

TIG is also widely applied in mechanised systems either autogenously or with filler wire. However, several ‘off the shelf’ systems are available for orbital welding of pipes, used in the manufacture of chemical plant or boilers. The systems require no manipu-lative skill, but the operator must be well trained. Because the welder has less control over arc and weldpool behaviour, careful attention must be paid to edge preparation (machined rather than hand-prepared), joint fit-up and control of welding parameters.

Equipment for TIG welding

TIG welding process is using an inert gas shield, instead of a slag to protect the weld-pool. 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

Torch

Backing system

Protective equipment

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 volt-age is slightly increased or decreased, there will be very little change in welding cur-rent. 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 be-tween the electrode and the workpiece. HF generates airborne and line transmitted interference, so care must be taken to avoid interference with control systems and in-struments 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 sur-face 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

The greater stability of these power sources allows very low currents to be used par-ticularly for micro-TIG welding and largely replaced the plasma process for micro weld-ing 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.

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

Square wave AC, or switched DC, power sources are particularly attractive for welding aluminium. 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 propor-tion 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 posi-tion (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. De-signs 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. or-bital and bore welding of pipes.

Electrode

For DC current, the electrode is tungsten with between 2 and 5% thoria to aid arc ini-tiation. The electrode tip is ground to an angle of 600 to 900 for manual welding, irre-spective of the electrode diameter. For mechanised applications as the tip angle de-termines 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 be-tween 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 under-side of the weld pool and weld bead from oxidation. To reduce the amount of gas con-sumed, 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 corro-sion 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 re-quired 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

TIG  WELDING  PARAMETERS