Gas tungsten arc welding (GTAW), also known as tungsten inert gas (TIG) welding, is an arc welding process that uses a non-consumable tungsten electrode to produce the weld. The weld area and electrode are protected from oxidation or other atmospheric contamination by an inert shielding gas (argon or helium). A filler metal is normally used, though some welds, known as autogenous welds, or fusion welds do not require it. When helium is used, this is known as heliarc welding. A constant-current welding power supply produces electrical energy, which is conducted across the arc through a column of highly ionized gas and metal vapors known as a plasma. GTAW is most commonly used to weld thin sections of stainless steel and non-ferrous metals such as aluminum, magnesium, and copper alloys. The process grants the operator greater control over the weld than competing processes such as shielded metal arc welding and gas metal arc welding, allowing for stronger, higher quality welds. However, GTAW is comparatively more complex and difficult to master, and furthermore, it is significantly slower than most other welding techniques. A related process, plasma arc welding, uses a slightly different welding torch to create a more focused welding arc and as a result is often automated.
Lichfield (/ˈlɪtʃfiːld/) is a cathedral city and civil parish in Staffordshire, England. Lichfield is situated roughly 16 mi (26 km) north of Birmingham, 8.1 miles (13.0 km) from Rugeley, 9 miles (14 km) from Walsall, 7.9 miles (12.7 km) from Tamworth and 13 miles (21 km) from Burton Upon Trent. At the time of the 2011 Census the population was estimated at 32,219 and the wider Lichfield District at 100,700.
Notable for its three-spired medieval cathedral, Lichfield was the birthplace of Samuel Johnson, the writer of the first authoritative Dictionary of the English Language. The city's recorded history began when Chad of Mercia arrived to establish his Bishopric in 669 AD and the settlement grew as the ecclesiastical centre of Mercia. In 2009, the Staffordshire Hoard, the largest hoard of Anglo-Saxon gold and silver metalwork, was found 5.9 km (3.7 mi) south-west of Lichfield.
The development of the city was consolidated in the 12th century under Roger de Clinton, who fortified the Cathedral Close and also laid out the town with the ladder-shaped street pattern that survives to this day. Lichfield's heyday was in the 18th century, when it developed into a thriving coaching city. This was a period of great intellectual activity, the city being the home of many famous people including Samuel Johnson, David Garrick, Erasmus Darwin and Anna Seward, and prompted Johnson's remark that Lichfield was "a city of philosophers".
Today, the city still retains its old importance as an ecclesiastical centre, and its industrial and commercial development has been limited. The centre of the city has over 230 listed buildings (including many examples of Georgian architecture), and preserves much of its historic character.