Gas Shielded Tungsten Arc Welding
Terminology
Description of the Process
Consists of an AC/DC arc burning between a non-consumable electrode and the workpiece to melt the joint area. The area of the arc is enveloped in a protective gas shield which protects the weld pool and electrode from contamination while allowing a stable arc to be maintained. The filler wire is added either by hand, or in the mechanised process, by feed rollers attached to the torch.
Application
Common applications consists of high quality welding on stainless steels; aluminium and its alloys; copper; nickel alloys and rare materials such as titanium and zirconium. Mild steels are welded when full penetration welds are required within certain tolerances, both in size and x-ray quality.
Used widely in the aerospace, chemical and nuclear industries.
Electrical Conditions
DC (direct current) - most normally used for TIG welding of stainless steels and heat resisting steels, mild and low alloy steels, copper, nickel alloys, lead, silver, titanium and other metals that do not contain aluminium, magnesium or their alloys.
The electrode must be connected to the negative terminal, where one third of the heat is generated. Two thirds of the heat of the arc is generated at the positive pole. A tungsten electrode connected to the positive pole would overheat and melt causing contamination of the weld and cracks in the ceramic shroud.
AC (Alternating current) - is used only for TIG welding of aluminium, magnesium and their alloys. Surface oxidation is automatically removed by the action of the arc each time the electrode becomes positive, (50 times per. second), thus the need for corrosive fluxes is unnecessary. Equal heat distribution is achieved by the rapid reversal of the polarity between positive and negative.
If DC supply was used and the electrode was connected to the positive pole it would overheat. If DC supply was used, and the electrode was connected to the negative pole there would be no removal of the surface oxides.
Equipment
The basic equipment for TIG welding is the same as that used for MMA, but with the additions of a torch/electrode holder which can provide a gas supply to the weld area, and additional components with specific functions are also included.
Fig. 1 Gas, water and electrical circuits for TIG welding equipment
connected to existing power sources
Requirements
Power source
high frequency unit
DC suppressor
contactor unit
cables and hoses
torch
electrodes
filler wire
Electrode Preparation
A silicon carbide grinding wheel should be reserved for tungsten grinding to prevent contamination of the electrode by other materials. As a general guide for DC welding a sharp point with a 1-3 x diameter taper should be used, the lower the current the longer the taper and vice-versa. For 100% X-ray quality, a slightly rounded point will prevent any tungsten inclusions within the weld.
Under these conditions the arc is so short that the molten globules at the electrode (wire) tip short circuit to the work piece at rapid time intervals. The rise in the current melts the electrode tip and this re-establishes the arc. This cycle occurs approximately 100 times per second.
Shielding Gas
A chemically inert gas is required for TIG welding. Argon is the most common gas used, it is present in the atmosphere to approximately .94% volume. Argon is tasteless, odourless, non-toxic and does not burn. The gas is supplied in light blue steel cylinders and generally at a pressure of 2500 lbs per square inch (14.5 lbs = 1 Bar). It is extracted through a pressure regulator at 30 lbs per square inch. Cylinders should be changed when the reading on the contents gauge falls to 30 lbs.
Argon combined with either Hydrogen, Helium, or Oxygen is used to improve the welding characteristics on various materials.
Consumables
Apart from the shielding gases and the electrical power used, the main consumable is the filler wire. The process may be used without the addition of filler especially on thin material. If filler is required then it is added to the weld pool in the form of a rod which can either be added manually or by an automatic wire feed unit. It may also be added in the form of a fusible insert to produce an accurate penetration bead through a joint.
Control of the Process
The main controls are :-
current
filler wire addition
travel speed
These are the main parameters for the TIG welding process. Current range is dependent on the type and size of electrode used, and the thickness of the material and joint position. A high level of manual dexterity is required to co-ordinate the addition of filler wire to the travel speed and also to maintain a constant arc length.
Features of the Process
In spite of the low speed of the process, its ability to produce high quality welds in a wide range of materials has made it a viable proposition for more demanding applications. The process is not considered economic for thicker materials or low integrity joints in plain carbon steels.
Safety
The main safety points using the TIG process is good insulation of all power cables, secure welding return clamps and protection against arc radiations. The high frequency spark used to initiate the arc can occur through any break in the insulation, and can cause deep seated burns which can be very painful, medical advice should be sought as soon as possible.
Precautions in Use of Inert Gas for Welding
The delivery of inert gases such as Argon, and Nitrogen into confined spaces may lead to the oxygen content of the air being reduced to a point where there is a danger of asphyxiation. For this reason adequate ventilation should be used.
| TIG | Tungsten Inert Gas |
| ARGONARC | Tungsten Inert Gas (BOC Tradename) |
| GTAW | Gas Tungsten Arc Welding |
| HELIARC | CO2 + Oxygen (US Tradename) |
Description of the Process
Consists of an AC/DC arc burning between a non-consumable electrode and the workpiece to melt the joint area. The area of the arc is enveloped in a protective gas shield which protects the weld pool and electrode from contamination while allowing a stable arc to be maintained. The filler wire is added either by hand, or in the mechanised process, by feed rollers attached to the torch.
Application
Common applications consists of high quality welding on stainless steels; aluminium and its alloys; copper; nickel alloys and rare materials such as titanium and zirconium. Mild steels are welded when full penetration welds are required within certain tolerances, both in size and x-ray quality.
Used widely in the aerospace, chemical and nuclear industries.
Electrical Conditions
DC (direct current) - most normally used for TIG welding of stainless steels and heat resisting steels, mild and low alloy steels, copper, nickel alloys, lead, silver, titanium and other metals that do not contain aluminium, magnesium or their alloys.
The electrode must be connected to the negative terminal, where one third of the heat is generated. Two thirds of the heat of the arc is generated at the positive pole. A tungsten electrode connected to the positive pole would overheat and melt causing contamination of the weld and cracks in the ceramic shroud.
AC (Alternating current) - is used only for TIG welding of aluminium, magnesium and their alloys. Surface oxidation is automatically removed by the action of the arc each time the electrode becomes positive, (50 times per. second), thus the need for corrosive fluxes is unnecessary. Equal heat distribution is achieved by the rapid reversal of the polarity between positive and negative.
If DC supply was used and the electrode was connected to the positive pole it would overheat. If DC supply was used, and the electrode was connected to the negative pole there would be no removal of the surface oxides.
Equipment
The basic equipment for TIG welding is the same as that used for MMA, but with the additions of a torch/electrode holder which can provide a gas supply to the weld area, and additional components with specific functions are also included.
Fig. 1 Gas, water and electrical circuits for TIG welding equipment
connected to existing power sources
Requirements
| Power source | Requires a controllable supply of AC/DC current, any where between 1 amp and 300 amps, with an open circuit voltage of between 60 and 100 volts. AC/DC rectifiers, AC only, or DC only power sources with a drooping characteristic supply are required. |
| High frequency unit | The two main functions of this unit are :-
|
| DC Suppressor unit | To suppress the inherent DC tendency of the AC welding arc. When welding aluminium with a tungsten electrode on AC polarity there is a tendency for the current to be changed to DC. This condition (inherent rectification) increases as the oxide film increases. A DC suppressor unit allows the AC current but prevents what is known as the DC component, thus a suppressor is only necessary when welding aluminium, magnesium, and their alloys. |
| Contactor | The two main functions of this unit are:-
Contactors may be operated by a switch on the torch or a foot pedal. The foot control may also allow the current to be adjusted during welding. |
| Cables | These must be of the type recommended by the manufacturer of the equipment with suitable plugs and connections for AC and /or DC power sources. |
| Torches | The two main types of torches are:-
|
| Air cooled torches | Used for light duty work up to approximately 1.6mm thickness, they vary from pencil type and swivel head type with a capacity of up to 50 amps, to the general purpose torch used for continuous welding at 75 amps. |
| Water cooled | Used for heavy duty work up to approximately 12mm thickness with current capacities of between 100 and 500 amps. Cooling water passes through the tube carrying the welding cable. This allows a reduction in power cable size thus reducing weight and increasing flexibility. A safety switch is incorporated on the power source which prevents the arc being initiated if the water supply fails. |
| Electrodes | The non-consumable electrodes are improved by the addition of small amounts of zirconium, thorium, and ceriated tungsten. |
| Thoriated electrodes | Colour coded red, used mainly on DC negative polarity for stainless and heat resisting steels, mild steels, low alloy steels, nickel, silver, lead, copper, monel, titanium etc. These electrodes can also be used on AC polarity but the arc characteristics are poor. |
| Zirconiated electrodes | Colour coded white, designed for AC welding of aluminium and magnesium and their alloys. |
| Ceriated electrodes | Specially designed for DC welding, with negative polarity and used exactly as thoriated electrodes. The main use of these type electrodes are in areas where no radioactivity can be allowed, i.e. aircraft industry, and nuclear industries. Colour coded blue/green. |
Electrode Preparation
A silicon carbide grinding wheel should be reserved for tungsten grinding to prevent contamination of the electrode by other materials. As a general guide for DC welding a sharp point with a 1-3 x diameter taper should be used, the lower the current the longer the taper and vice-versa. For 100% X-ray quality, a slightly rounded point will prevent any tungsten inclusions within the weld.
Under these conditions the arc is so short that the molten globules at the electrode (wire) tip short circuit to the work piece at rapid time intervals. The rise in the current melts the electrode tip and this re-establishes the arc. This cycle occurs approximately 100 times per second.
Shielding Gas
A chemically inert gas is required for TIG welding. Argon is the most common gas used, it is present in the atmosphere to approximately .94% volume. Argon is tasteless, odourless, non-toxic and does not burn. The gas is supplied in light blue steel cylinders and generally at a pressure of 2500 lbs per square inch (14.5 lbs = 1 Bar). It is extracted through a pressure regulator at 30 lbs per square inch. Cylinders should be changed when the reading on the contents gauge falls to 30 lbs.
Argon combined with either Hydrogen, Helium, or Oxygen is used to improve the welding characteristics on various materials.
Consumables
Apart from the shielding gases and the electrical power used, the main consumable is the filler wire. The process may be used without the addition of filler especially on thin material. If filler is required then it is added to the weld pool in the form of a rod which can either be added manually or by an automatic wire feed unit. It may also be added in the form of a fusible insert to produce an accurate penetration bead through a joint.
Control of the Process
The main controls are :-
These are the main parameters for the TIG welding process. Current range is dependent on the type and size of electrode used, and the thickness of the material and joint position. A high level of manual dexterity is required to co-ordinate the addition of filler wire to the travel speed and also to maintain a constant arc length.
Features of the Process
In spite of the low speed of the process, its ability to produce high quality welds in a wide range of materials has made it a viable proposition for more demanding applications. The process is not considered economic for thicker materials or low integrity joints in plain carbon steels.
Safety
The main safety points using the TIG process is good insulation of all power cables, secure welding return clamps and protection against arc radiations. The high frequency spark used to initiate the arc can occur through any break in the insulation, and can cause deep seated burns which can be very painful, medical advice should be sought as soon as possible.
Precautions in Use of Inert Gas for Welding
The delivery of inert gases such as Argon, and Nitrogen into confined spaces may lead to the oxygen content of the air being reduced to a point where there is a danger of asphyxiation. For this reason adequate ventilation should be used.