Tig Welder and Arc Welder
In addition to the information set out below, more information is available in Graham's blog
Tig Welding (Tungsten Inert Gas), also know as Gas Tungsten Arc Welding (GTAW) is a long established process at the professional end of welding. Tig Welding has always been the preserve of the pro because of the high cost of the Tig Welder. Times have changed and it is now possible to buy a professional quality Tig Welder for under £500. There are two basic types of Tig Welder, DC output only Tig Welders and AC/DC Output Tig Welders. A Tig Welder that is DC Only will Tig Weld most metals except Aluminium. To Tig Weld Aluminium you need an AC Output and therefore a Tig Welder that can be used with an AC or DC Output.
The key advantages of a Tig Welder are highly controllable weld quality, the ability to Tig Weld almost any metal and the ability to Tig Weld very thin metal. Key disadvantages are the relatively high cost of AC/DC Tig Welders and greater skill needed to operate the Tig Welder, primarily, hand eye co-ordination, though anyone who has Gas Welded should have no problem mastering a Tig Welder.
If you only need a DC Tig Welder because you won't be Tig Welding Aluminium, then there are 3 types of Arc Initiation systems to consider, Scratch Start Tig Welders, Lift Tig Welders and High Frequency (HF) Tig Welders. Scratch Start Tig Welders are essentially an Arc/Stick Welder with a Scratch Start Tig Torch. When used, the Tig Torch is permanently live and a valve on the Tig Torch has to be manually opened to start the gas flow. Once the Gas has been started, the Tungsten Electrode has to be "Scratched" on the surface of the job to start the arc. When welding is complete the Tig Torch is lifted away to break the arc. Lift Tig Welders are similar to Scratch Start except they have some clever electronics to help prevent the Tungsten Electrode sticking to the job when it is touched down on the job. Some Lift Tigs still have a manual gas valve on the torch, some have an electronic gas solenoid and non live torch, so details on Lift Tig Welders need to be examined carefully. High Frequency Start Tig Welders are by far the best choice as the Tungsten electrode does not need to be touched down at all, additionally, when the Tig Torch Trigger is pressed, the Tig Welder will turn on the power, turn on the gas and then use a burst of High Frequency to jump the air gap between the Tungsten Electrode and the job. HF Tig Welders usually also have a number of other useful features.