Gases normally used in the TIG welding process are:
Argon is obtained as a by product in the manufacturing of oxygen. Argon may be obtained in the gaseous state in cylinders or as a liquid in specially constructed cylinders or in bulk tanks.
When choosing a shielding gas, a fact that must be considered is the ionization potential of the gas. Ionization potential is measured in volts and is the point where the welding arc will be established between the electrode and the work piece through the shielding gas. In other words, it is the voltage necessary to electrically charge the gas so that it will conduct electricity.
The ionization potential of argon is 15.7 volts. So this is the minimum voltage that must be maintained in the welding circuit to establish the arc or to weld with argon. The ionization potential is different for every gas and has a major effect on the arc and weld bead.
The ionization potential for helium is 24.5 volts. Comparing two welding circuits, each being equal except for shielding gas, the arc voltage produced with argon would be lower than that produced by helium.
Argon has low thermal conductivity which means it is not a good conductor of heat. This results in a more compact, higher density arc. Arc density refers to the concentration of energy in the arc. With argon this energy is confined to a narrow or more ‘pinpointed’ area. Argon provides excellent arc stability and cleaning action even at low amperages.
Unlike argon, helium has high thermal conductivity. Due to this higher thermal conductivity, the arc column expands, reducing current density in the arc. The arc column will become wider and more flared out than the arc column with argon shielding gas. The more flared out the arc column, the more work surface area is being heated.
The heat at the centre of the arc can move more readily downward towards the colder metal at the bottom of the work piece. This results in a deeper penetrating arc. It was mentioned previously that with an equivalent arc length, helium will produce a higher arc voltage than will argon. Since the total power is a product of voltage and amperage, it is apparent that more heat energy is available with helium.
Helium or argon-helium mixtures are desirable on thick material and where high travel speeds are desired. The use of 2:1 helium to argon gas mixture has also been shown to yield lower porosity welds in production situations by allowing wider variation in welding parameters.
With helium shielding any slight variation of arc length can have quite an affect on arc voltage and consequently total arc power. For this reason, helium is not as desirable as argon for manual welding applications.
Because of its higher ionization potential, it is more difficult to start an arc with helium shielding gas, especially at lower amperages.
Argon is used almost exclusively when welding at 150 amps and lower. As helium is a light gas, flow rates are usually two or three times higher than argon for equivalent shielding. The cost of helium is considerably more than argon and with the increased flow rate, total cost of shielding goes up sharply. The cost must be weighed against increased penetration on thick material and the increased travel speed attainable.
Just as helium is mixed with argon to take advantage of the best features of both gases, hydrogen is mixed with argon to further constrict the arc and produce a cleaner weld with a greater depth to width ratio (penetration).
This mix is used primarily for welding austenitic stainless steel and some nickel alloys. The addition of hydrogen to argon also increases travel speed. It should be noted that an argon hydrogen mix will introduce the risk of hydrogen cracking and metal porosity particularly in multi pass welds.
Nitrogen when mixed with argon provides the capability of producing more energy to the work than with argon alone.
This can be particularly beneficial when welding materials of high conductivity such as copper. However, a nitrogen mix cannot be used on ferrous metals such as steel and stainless steel because nitrogen pick up in the weld pool causes a significant reduction in strength and a weaker, more porous bead.
Gas Flow Rates
The correct flow rate is an adequate amount to shield the molten weld pool and protect the tungsten electrode. More than this amount is wasted. The correct flow rate in litres per min (or cubic feet per hour) is influenced by many variables that must be considered on each application. Generally speaking, when the welding current, cup diameter or electrode stick out is increased, the flow rate should be increased.
When welding in the AC mode the current reversals have a disturbing affect on the shielding gas and flow should be increased by 25% and of course, when welding in a draft situation, flow rate should be doubled.
When welding in difficult areas excessive flow rates can cause turbulence and air entrapment. In this situation, the effectiveness of the shielding gas can be improved by reducing the gas flow by about 25%.
As a guide the flow rate is normally around 8-12 l/m for argon but can be double that for helium.