E-Book, Englisch, 144 Seiten
Reihe: Woodhead Publishing Series in Welding and Other Joining Technologies
Muncaster A Practical Guide to TIG (GTA) Welding
1. Auflage 1991
ISBN: 978-1-84569-872-0
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: Adobe DRM (»Systemvoraussetzungen)
E-Book, Englisch, 144 Seiten
Reihe: Woodhead Publishing Series in Welding and Other Joining Technologies
ISBN: 978-1-84569-872-0
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: Adobe DRM (»Systemvoraussetzungen)
Comprehensive advice on applications, techniques and the best available equipment is given in clear, straightforward language.
Autoren/Hrsg.
Weitere Infos & Material
The TIG welding process
Publisher Summary
This chapter provides an overview of the Tungsten Inert Gas (TIG) welding process. Although this process is known by various names, TIG is the best known name in Europe and also generally understood world-wide. TIG welding is clean and cost-effective, though it is a bit slow compared to the Metal Inert Gas (MIG) and Metal Active Gas (MAG) welding. It can be used by hand or is automated and welds a vast range of metals and thicknesses in several different modes. Whatever mode is used, the process remains the same—namely, that the metals to be joined are fused together by the heat of an electric arc within a shield of inert gas, which surrounds the arc and prevents undue oxidation of the metal. The arc is struck between the electrode and the work piece and, in all but a very few cases, the electrode is made from tungsten, often with small quantities of a rare metal alloyed into the finished electrode rod.
Most forms of electric arc welding make use of the fact that shorting and then separating conductors connected to the positive and negative poles of an electric current source creates an arc, and thus an area of concentrated heat. If this arc occurs in air the metals being welded can become oxidised, and to some extent vaporised, by the uncontrolled intense heat produced. This unwanted effect can be reduced by use of a flux or, in the case of TIG, an inert gas shield. The power source thus controls the short circuit, directs the arc and allows the molten metal to flow evenly without oxidation.
Terminology
The process is known by various names as follows:
• – Tungsten inert gas, the best known name in Europe but generally understood world-wide;
• Gas tungsten arc welding, mostly in the USA;
• – Wolfram (tungsten) inert gas, German definition.
For the purpose of this book only the first definition is used.
Modes
• – direct current, electrode negative, work positive. Sometimes known as straight, unpulsed DCEN (or DCSP in the USA), Fig. 1.1.
1.1 DCEN polarity.
• electrode positive, work negative (not used as widely), DCEP (or DCRP in the USA), Fig. 1.2.
1.2 DCEP polarity.
: The terms DCEN and DCEP are preferable as they indicate electrode polarity for both the above.
• – alternating current. In this mode arc polarity rapidly changes giving some cathodic cleaning effect, ideal for aluminium where oxides rapidly develop in the weld bead, and for some stainless steels. An AC/DC power source gives the ability to select either DC or AC as the occasion arises and would be the best purchase for a general fabrication shop where many different metals of varying thickness to weld would often be encountered, Fig. 1.3.
1.3 AC operation.
• – This mode allows the arc to be pulsed at various rates between selectable high and low current settings and gives greater control of heat in the arc area.
TIG welding is clean, cost-effective, albeit a bit slow compared with metal inert gas (MIG) and metal active gas (MAG) welding, can be used by hand or automated and will weld a vast range of metals and thicknesses in several different modes. Whatever mode is used, the process remains the same, namely that the metals to be joined are fused together by the heat of an electric arc within a shield of inert gas which surrounds the arc and prevents undue oxidation of the metal. The arc is struck between the electrode and the workpiece and, in all but a very few cases, the electrode is made from tungsten, often with small quantities of a rare metal alloyed into the finished electrode rod.
A TIG arc is very hot and localised providing a means of applying maximum heat for welding in a small area, allowing an experienced welder to produce neat, compact weld beads with excellent penetration and strength.
Other terminology
AUTOGENOUS
A common term in TIG welding which means that the weld is formed by fusion of the parent metal(s) only and that no additional filler rod or wire has been introduced into the weld bead. It is generally considered that the thickness for autogenous butt welding of mild and stainless steels by TIG is 2.5 mm (0.100 in) with other metals pro rata, depending on their heat conducting properties.
– a situation where the electrode touches the weld pool and welding ceases is known by expressions such as touch down, stub in or plough in, amongst others (often unrepeatable).
BACK PURGE
This is a condition in which additional shielding gas is piped to the underside of the weld bead, and some high quality power sources have an extra gas circuit and controls specifically for this purpose. It ensures that the underbead has a clean smooth surface with minimum or zero porosity. It is essential when fabricating vessels and welding tubing circuits for the food and drink processing industries where porosity could occur and harbour dangerous micro-organisms. It also serves to keep the underbead as small as possible, as large porous underbeads can affect the smooth flow of liquids through pipes and tubes.
CRATER
An imperfection or dimple at the end of a weld seam which can occur if the weld is suddenly terminated at full current. Craters can be eliminated, particularly in automatic welding by correct use of slope-down. With manual welding, craters are filled by use of the current control pedal.
DOWNSLOPE
Opposite of upslope, allowing the arc to die away or decay gradually. Also known as slope out and, in the USA, ramp out.
DUTY CYCLE OF A POWER SOURCE
A term used to define the period of time for which a power source can be used at a particular current level without overloading. Stated as a percentage, generally related to a ten minute period, or in the form of a graph, see Fig. 1.4.
1.4 Duty cycle of a power source.
A typical power source may be rated as follows:
Welding shop use is nearly always intermittent, so the set usually has a chance to cool to normal operating temperature. Overload trips are often fitted to avoid electrical damage. These trips are either manually or automatically reset. When purchasing a set do not be misled by maximum current, pay attention to the stated duty cycle and select a unit with power above your normal needs.
: arc voltage generally increases slightly as duty cycle decreases. AC/DC TIG sets can be marginally less efficient than AC or DC models.
EARTH OR GROUND
These are misnomers but have become normal reference terms in all welding processes and should correctly be called welding ‘current return’. The terms refer to the return circuit to the power source. Critical to the welding process, the return circuitry should carry the of the set.
HEAT AFFECTED ZONE (HAZ)
A term used to describe the region immediately around the weld bead. This area should be kept as small as possible and heat sinks are commonly used. Keeping the HAZ small reduces discoloration in the area around the weld zone and thus minimises finishing and polishing times.
HIGH FREQUENCY ARC START (HF)
In the past, HF pulses were used at the electrode tip to provide an ionised air bridge across which the welding current flowed from the tip to the work and established an arc. Once the arc was established the HF was terminated after a few milliseconds only. Now largely replaced by specialised electronic systems which eliminate, or at least minimise, RF and HF interference with peripheral equipment such as computers and DC motor control systems.
RF interference
RF stands for radio frequency and interference with electronics circuits and DC thyristor motor control systems can be caused by airborne RF. It generally results when electromagnetic waves combine either to reinforce each other or to cancel each other out, depending on their relative phases. It is rather difficult to eliminate when caused by HF arc start systems using spark gaps, but these are gradually being replaced by modern electronic devices which minimise both HF and RF interference. The effect of RF on welding system peripherals is similar to that caused by unsuppressed automobile...