Product category:
Automatic and robotic welding systems
News Release from: Orbimatic | Subject: AC TIG orbital tube welding
Edited by the Manufacturingtalk Editorial
Team on 19 August 2003
AC TIG eases orbital welding of
aluminium tubes
AC orbital welding power supplies allow aluminium and aluminium alloys to be welded much more successfully with AC TIG in a production environment when compared with DC TIG.
The Orbital TIG Welding technique has been used extensively in a range of industries since it's introduction in the 1960's The process was developed by in the aerospace industry for welding of small fittings to tubes, but these systems used large power supplies and cumbersome fixtures which were only suited to workshop applications
This article was originally published on Manufacturingtalk on 16 Apr 2004 at 8.00am (UK)
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Arc voltage control module made for orbital welds
Orbital welding power supply can now be supplied with an arc voltage control module, which allows the unit to be used with the a new tube to tubesheet weld head.
System prevents wrong weld head being fitted
Orbital weld head recognition system allows the power supply to recognise which weld head is fitted to the power supply so it will limit output and operation range to suit that weld head.
From these first systems the modern day, portable equipment has been evolved and the equipment has especially made remarkable progress since the introduction of the first computerised equipment, which was introduced around 15 years ago.
However this equipment was always used for DC application which limited to steels as aluminium is difficult to weld with the DC TIG process, and where success has been found the components have to be cleaned meticulously before welding.
The recent introduction of AC orbital welding power supplies has meant that aluminium and aluminium alloys can now be welded much more successfully in a production environment.
Further reading
Orbital welding equipment maker opens UK office
Manufacturer of automatic orbital arc welding equipment has opened a UK office in Peterborough, Cambridgeshire.
Orbital welding programs include calculation
To take the stress and strife out of calculating orbital welding procedures with complicated and long winded formulae the latest equipment features automatic calculation capabilities.
When two welding heads are better than one
When high rate production tube welding is required the Orbitwin unit allows the connection of two orbital welding heads to one orbital welding power source
Basics of the TIG welding process - the TIG (Tungsten Inert Gas) Welding Process makes use of either DCEN (direct current, electrode negative) or AC (alternating current).
DCEN has a constant heat distribution with two thirds of the head being in the work piece and the other third in the electrode.
With AC the heat is balanced with polarity switching at a pre-set rate of between 50-200 Hz.
The AC balance and frequency can then be adjusted in order to control the arc contour and in turn the weld profile.
The process creates an arc in an inert atmosphere between a non-consumable tungsten electrode and the work piece.
This arc creates enough heat to melt the work piece and create a weld pool.
The whole process has to be surrounded by and inert gas, the most commonly used gas for this argon.
Some gas mixtures which will allow faster welding speeds, increased penetration and give a cleaner weld bead.
Most common gas mixtures are argon/hydrogen and argon/helium in varying ratios.
The gas type selected will depend on the material, application and economical factors.
For AC applications the use of zirconiated tungsten electrodes is most common.
It is important to select and electrode type which will provide a stable balled end when welding.
Because of the characteristics of AC welding the electrode will not sustain a pointed profile.
The AC TIG Welding Process It has been commonly accepted that an AC (alternating current) TIG welding power supply is need to obtain successful results when welding manually.
This means it is makes sense that in order to get the best results when welding aluminium with the orbital welding process an AC power supply should be used.
Aluminium is a difficult material to weld because of the oxide layer that it forms in the presence already created a molten pool inside the oxide layer.
Then when the oxide layer finally becomes molten the molten aluminium will fall straight through the oxide layer of oxygen.
This oxide films melts at a temperature of around 1200degC where as the parent material melts at only 650degC.
In order to weld aluminium easily the oxide layer must be removed, this can be achieved by the mechanical removal of the layer prior to welding or by using and AC arc to breakdown the oxide layer and allow the parent material to then be melted by the arc.
The material can then be welded with little trouble.
The profile of the weld bead profile can be optimised with adjustment of the AC Balance and AC Frequency.
The AC Balance control allows the ac wave pattern to be biased towards either predominantly electrode positive or electrode negative.
By adjusting the balance of the wave form the amount of heat in the parent material can be controlled.
The result of having the electrode predominantly positive the material will have less input and this give a material cleaning effect.
By having the electrode predominantly negative there will be more heat in the material and therefore you will achieve a deeper penetration.
A high AC frequency will cause the arc to constrict and will in turn produce a narrower weld bead and profile.
As the frequency reduces the arc becomes wider as does the weld profile.
The high frequency produces a stiffer arc which will make it more stable in precise applications.
For some applications it will be necessary to introduce an additional filler material.
The filler material required will depend on the parent materials being welded.
A selection chart is shown below for selection of the correct filler material.
Basics of Orbital Welding - orbital welding equipment - to create an orbital welding system there are two main components.
An orbital welding power supply to provide the power and control of the system and an orbital welding head which will rotate the torch or electrode and perform the weld cycle.
Power supply - modern day orbital welding power supplies have the capability of being used for all applications that may be required of them.
The most up to date equipment, such as the Orbimat 160C will control welding current, rotation control, wire feed and gas flow with feed backs from the weld head to maintain a consistent weld profile.
These power supplies also have a built in cooling unit which provides a liquid coolant to the sensitive parts of the weld head to eliminate heat build up.
Additionally the units can store hundreds of weld procedures which include details of the tungsten electrode, gas type and flow, joint preparation, parent material and filler materials used.
They have extensive integral data acquisition capabilities, this allows weld data logs to be load to a PC Card and easily transferred directly to a PC for interrogation or storage.
Modern power supplies also include an Auto-Program facility which drastically reduces the amount of time needed to develop a suitable weld procedure for application.
These systems will take tube size, material type and gas used to achieve near perfect results in seconds.
Large well laid out display panels make the equipment welder friendly and easy to use.
Other features such as integral printers, fault sensors, key switch and weld head recognition are also standard features of this type of equipment.
For AC applications a AC/DC power supply is used which offers all the standard functions of a DC machine but contains a AC generation module and the operator is able to control AC frequency and AC balance in addition to the the standard controls.
Weld head - there are three common types of weld head, these combined cover most applications you may encounter.
The three types of weld head are fully enclosed fusion weld head, open arc wire feed weld head and tube to tube sheet weld heads.
Enclosed weld head - fully enclosed weld heads perform the weld in a localised welding chamber formed by the clamping system around the tube being welded.
This chamber is filled with inert gas prior to welding which fully protects the outer surface of the tube and the tungsten electrode.
These heads also have the additional advantage that when an internal gas purge is being used and gaps in the weld joint are encountered the internal quality of the weld is not degraded as no oxygen is allowed to enter the tube.
These style of weld heads are available for tube outside diameters from 2mm to 170mm and it is possible to produce acceptable welds on materials of up to 4mm wall thickness.
Open Arc weld heads - open arc weld heads are used for thick wall applications, generally materials with a wall thickness of more than 3.5mm and for materials which require the addition of a filler material.
These heads allow the use of a additional filler material and multiple pass weld sequences.
The weld torch block can be tilted to 45 degrees to allow fillet welding and the welding of short leg fittings.
The tungsten gap is controlled by an arc length controller which ensures a consistent profile, even on oval pipes.
Tube to tube sheet weld head - tube to tube sheet weld heads are used in the manufacture and repair of heat exchangers.
Usually a heat exchanger will have a tube sheet with several hundred tubes to be welded to them.
The nature of this work is very mundane and repetitive and operator fatigue is a key factor.
With the use of a tube to tube sheet weld head this operator fatigue can be eliminated with the welder only having to ensure the tube are fitted correctly and operating the machine.
The operator then simply supervises the machine while in operation and can be preparing the next joint while the head is welding another.
Material cleaning - before carrying out any welding process on aluminium components it is important they are cleaned thoroughly.
The cleaning process should remove not just dirt, oil and grease but also the aluminium oxide which has formed on the surface of the material.
Aluminium oxide can only be removed by mechanical means and the most effective method is to scrape the surface of the material.
The more thorough the cleaning process the better the results will be and the more consistence the results.
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