The principles of the TIG welding torch
Tungsten inert gas welding (TIG) is an arc welding process, which uses a non-consumable tungsten electrode, shrouded by a protective atmosphere of inert gas, such as argon or helium. The extra weld metal can be provided by a separate filler rod if necessary. A direct current is used, with the electrode negative, to avoid overheating and undue erosion of the tungsten electrode.
The principles of a MIG welding “gun”
The metal inert gas (MIG) process uses a consumable electrode, which is usually in the form of a copper-coated coiled wire. Argon is used to shield the weld, and direct current with the electrode being positive to generate more heat for melting.
- Uses a non-consumable electrode of tungsten with addition of 1% thoria (ThO2).
- Process is particularly useful in welding thin sheet, without the need to use filler metal.
- High purity argon gas produces welds free of oxidation, which enables reactive metals such as titanium and zirconium to be successfully welded.
- No slag formation eliminates post-cleaning operations.
- Minimal weld splatter.
- Slower than MMA or MIG processes.
- Good control of welding current, arc length and filler metal additions.
- Lends itself to mechanisation.
- Power supply up to 300A a.c. or d.c.
- Process can be semi-automatic or automatic.
- Feed wire diameters range from 0.75 to 2.25 mm.
- Wires are usually copper-coated to improve conductivity.
- Argon/helium mixtures can be used for shielding.
- Produces high quality welds at high speeds, and no flux to remove (deposition rates 1.25–7.5 kg h-1).
- Power supply 60–500 A, 16–40 V d.c.
Metal active gas (MAG) and CO2 welding
- Argon or helium is replaced in the MIG process by carbon dioxide (with or without additions) at lower cost.
- CO2 is used mainly for the welding of steel.
- Addition of up to 10% oxygen to the CO2 base gives the following advantages: provides smoother transfer of weld metal, increases the fluidity of the weld pool and increases the wettability of the weld metal.
- d.c. welding used for carbon and alloy steels, heat resistant and stainless steels, copper and its alloys, nickel and its alloys.
- a.c. welding is required to TIG weld aluminium, magnesium and aluminium-bronze alloys to break down the tenacious surface oxides on the metal surface.
- Use of high purity argon enables reactive metals such as titanium and zirconium to be welded, with argon shrouds and d.c. current.
- Thin walled (1.6 mm and less) stainless steel tubing can be TIG welded by rotating the welding head and fixing the tube. This is called orbital pipe welding.
- MIG process is suitable for welding aluminium, magnesium alloys, plain and low-alloy steels, stainless and heat-resistant steels, and copper and bronze.
- Variations are in the filler wire composition, current and voltage and shielding gas.
MAG and CO2 welding
- CO2 welding is mainly used for welding mild steel and low-alloy steels (cheaper than argon).
- CO2 is really effective as a shield gas if the electrode wire contains up to 1.8% manganese, 0.5% silicon, 0.15% titanium and 0.15% zirconium, which act as deoxidising agents.
- Stainless steels are MAG welded with argon +1% oxygen.
- Can successfully weld thin-gauge materials with minimal distortion (< 0.5 mm thick).
- Aluminium alloys with plate thicknesses of 2–6.4 mm can be welded as flat butt joints. Plate with thicknesses of 5–9.5 mm welded with single V-butt joints.
- Thin section stainless steel tube can be TIG welded by orbital pipe welding.
- Plate thicknesses of 6–25 mm can be flat-butt welded in aluminium with root faces of 1.6–4.8 mm.
- Productivity is higher than TIG welding.
- Used in general engineering construction.
MAG and CO2 welding
- Automatic welding by MAG or CO2 processes produces consistent high quality welds in mild steel and low-alloy steels.
- Higher speeds of welding than by TIG or MIG.
This article is a part of Manupedia, a collection of information about some of the processes used to convert materials into useful objects.