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Welding, cutting and adhesive bonding
News Release from: Huntingdon Fusion Techniques | Subject: Tungsten arc welding
Edited by the Processingtalk Editorial
Team on 06 September 2004
Gas tungsten arc welding electrodes
Gas-shielded arc welding with non-consumable electrodes has been adopted in fabrication worldwide and is firmly established as one of the most versatile techniques for producing quality fusion welds
The tungsten arc welding process has been making a contribution to the development of key industries for 60 years Gas-shielded arc welding with non-consumable electrodes has been rapidly adopted in fabrication worldwide and is now firmly established as one of the most versatile techniques for producing fusion welds of the highest quality
This article was originally published on Processingtalk on 4 May 2004 at 8.00am (UK)
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Since a temperature of around 4000C is generated in the arc during welding, the role played by the electrode is crucial.
Clearly it must have a high melting point and it must be non-consumable: Tungsten quickly established itself as the most suitable material for electrodes in arc welding.
As the knowledge of arc characteristics increased it became clear that the use of pure tungsten presented some limitations on process development, particularly arc initiation, stability and electrode wear.
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Studies illustrated the effect of minor additions of refractory oxides, notably thoria, to the tungsten and that significant overall improvements in performance could be realised.
Consequently a range of tungsten electrodes containing oxide additions or 'dopants' were introduced progressively.
Notwithstanding the potential significance of electrode characteristics, some of the few scientific papers covering the performance of tungsten electrodes advocate the widespread use of dopants on the basis of improved welding performance and some highlight the hazards associated with them.
Much of the work is however contradictory and contentious and evidence supporting results of field trials is flawed and cannot be used as a basis on which to make generalised conclusions.
Darren Sewell, Managing Director of Huntingdon Fusion Techniques presents an objective review of what has become an emotive issue-the use of dopants in tungsten electrodes.
The issues under examination fall into two categories; those associated with technical and commercial advantages, and those relating to health hazards.
Electrodes are made using a metallurgical process in which powder is compressed and sintered at high temperature to produce rods.
The oxide additives are generally dispersed finely in the tungsten powder prior to forming but some products - composite electrodes - are made up of a tungsten core with an oxide coating.
The rods are subsequently heat treated and mechanically reduced in diameter to meet the standards, which are set down by the market place.
The low work function - the energy needed to remove an electron, is a major advantage of tungsten as a welding electrode material.
This is an essential requirement for efficient arc generation.
Dopants reduce the work function and thus enhance electron emission.
This also increases the usable life of the electrode and may promote arc initiation and stability.
(MST stands for Multi Strike Tungsten, the Huntingdon Fusion Techniques Limited trademark for its tungsten electrodes).
The most commonly used additives are thoria (ThO2), zirconia (ZrO2), lanthana (LaO2), yttria (Y2O3) and ceria (CeO2).
The commercial manufacturers also produce a range of electrodes which incorporate a combination of dopants.
Although the electrode material has a considerable influence on the ease with which an arc can be initiated there exist several other controlling factors.
Among these are arc gap, tip geometry, open circuit voltage, welding current, shielding gas and the characteristics of the welding power source.
There is little to choose between the effect of various doped electrodes on arc starting but none of them is any worse overall than pure tungsten.
Under certain defined conditions lanthana and ceria offer significant advantages.
Multi Strike Tungsten Electrodes are not only free of any radioactive thoria, but also offer considerable striking advantages.
For example, tests by Huntingdon Fusion Techniques Limited showed that a Multi Strike Tungsten Electrode will strike 10 times more than a thoriated tungsten under the same conditions.
A stable arc is a prerequisite for producing welds of consistently high quality.
Stability is inversely related to the electrode material work function so that although tungsten is good, additions of dopants with their lower work functions increases arc stability.
There is evidence that some dopant materials are lost from the electrode surface however and may or may not be adequately replenished by diffusion from the body of the electrode.
This would have the net effect of progressively reducing arc stability during use.
The Multi doped tungsten does provide a higher percentage of dopant to counter this effect.
Addition of any refractory oxide dopant improves arc stability but Multi Strike Tungsten is the most effective.
As with arc starting, erosion is influenced by many related variables; even sophisticated research must therefore lead to conclusions based on limited combinations of circumstances.
A major contributory factor is contamination through touch starting and contact with the weld pool.
Other factors are electrode tip geometry, welding current, open circuit voltage, shielding gas, parent material and welding power source.
Studies have shown that Dopant additions of all types reduce electrode erosion under some circumstances but no single dopant is superior to the others under all welding conditions.
Based on limited work, Anderson showed that dopant additions can reduce erosion at high current levels and Huntingdon Fusion Techniques has shown that multi-doped electrodes perform better than those containing only zirconia under most circumstances on aluminium.
Winson and Turk reported that Thoria doped electrodes exhibited longer life than pure tungsten in most applications.
Matsuda found that dopant additions offered significant advantages at higher welding currents.
Erosion by evaporation is an important consideration and the fact that refractory oxides reduce the temperature of the electrode during welding suggests that oxide additions would reduce evaporative losses: the effect of diffusion referred to earlier must be considered however.
Weld quality can be influenced by electrode quality and consistency of operation is determined by the welding electrode as it influences stability of the arc.
Whilst low cost tungsten and oxide doped tungsten electrodes are available commercially they are usually of low quality: in particular the operational consistency is poor.
A traceable tungsten electrode will go some way to ensuring the critical user of consistency and reliability.
Although the use of thoria doped electrodes constitutes a health risk, the alternative oxide doped electrodes are comparatively safe.
Health warnings are issued by many legislative and advisory bodies in connection with the use of welding electrodes, which contain refractory oxides.
The warnings refer to the potential toxicity and in particular to the risk of inhalation of dust particles from thoria-containing electrodes generated during grinding.
The general hazard may be put in subjective context by referring to the specifications for doped electrodes.
Whilst all these electrodes may contain radioactive substances, the level of risk is substantially less with those containing zirconia, lanthana, yttria and ceria than with those containing thoria.
Whilst the use of thoria doped electrodes has not been banned there is a duty of care responsibility on the suppliers and users of these products to take adequate precautions during storage and use.
Storage In all cases a dedicated storage arrangement is advised.
With low-volume users this could be a simple metal cabinet but for large-scale users a separate room may be more suitable.
In either case the cabinet/room walls will probably be adequate to provide shielding against external gamma radiation.
Use Local exhaust ventilation should be provided where occasional electrode tip grinding is carried out and any surface dust removed regularly by vacuum cleaning.
Volume production welding with thoria doped electrodes may necessitate the use of a dedicated tip grinding facility.
In these circumstances a system of work based on exhaust and respiratory aids, with suitable arrangements made for subsequent disposal of the dust should be implemented.
With respect to arc starting, all oxide doped electrodes perform better than pure tungsten electrodes.
Under certain defined conditions lanthana and ceria offer significant advantages, while Multi Strike Tungstens offer the most advantages.
Addition of any refractory oxide dopant to tungsten electrodes improves arc stability but ceria or ceria containing multi doped tungsten is the most effective.
Dopant additions of all types reduce electrode erosion under some circumstances but no single dopant is superior to the others under all welding conditions.
Electrode quality and traceability can be an important consideration.
Traceable tungstens should be used by companies with critical welds and operating to ISO standards or similar.
The use of thoria doped electrodes constitutes a health risk and alternative oxide doped electrodes should be employed. Request a free brochure from Huntingdon Fusion Techniques ...
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