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Tig-welding-tips: this page reminds important points, that risk to be neglected.
These are actually fundamentals of this process.
The purpose of this page is to refresh basic knowledge and to help solve practical problems.
It is intended to be periodically updated following questions of our readers,
Even experienced welders may occasionally forget those Tig-welding-tips features they do not use often enough.
A few moments dedicated to this page will refresh basic knowledge that adds to your skill and performance.
For basic information on the physics of electric arc for welding, see our new page on Arc Welding.
Best of Tig-welding-tips
The Gas Tungsten Arc Welding (GTAW) also called Tig has the following
- High quality welds, low distortion
- Easy to mechanize and automate
- Flexibility and ease of heat control
- All metals, all positions
- Lower deposition rates
- Good skills required
- Sensitive to cleanliness and contamination
- Arc blow sensitive
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The Tungsten Electrode in Gas Tungsten Arc Welding (GTAW or Tig) can make contact with a Direct Current (DC) power supply either at the positive (+) or at the negative (-) terminal. In the first case the connection is for DCEP (electrode positive) or reverse polarity. In the second one for DCEN (electrode negative) or straight polarity.
The heat generated in the electric arc is not distributed uniformly.
Tig-welding-tips - In DCEP:
- 30% of the heat is concentrated in the workpiece
- 70% of the heat in concentrated in the tungsten electrode
- Shallow penetration
- Wide weld area
Tig-welding-tips - In DCEN:
- 70% of the heat is concentrated in the workpiece
- 30% of the heat is concentrated in the tungsten electrode
- Deep penetration
- Narrow weld area
As a consequence of heat distribution, for the same current one has to select in DCEP a larger electrode size than what is proper for DCEN.
Due to the electrode limited current carrying capacity (about 1/10 of DCEN), DCEP is used only for welding thin sheet metal where low current is sufficient. For most other applications welded with higher currents, DCEN is selected if welding other materials than aluminum or magnesium (see below).
Tig-welding-tips: one of the factors of the DC Electrode Negative staying cooler is the evaporation of the emitted electrons, actively contributing to electrode tip cooling.
Tig-welding-tips: Pulsed DC
Advances in electronic controls of power supplies permitted to introduce new wave forms, generally with electrode negative, characterized by current pulsed between a low maintenance or background value, that keeps the arc but allows the weld to cool somewhat, and a high peak value capable of melting metal in a controlled way while ensuring the required penetration.
|Browse through our recent
Mid October 2012 PWL - Bulletin 78 - on Pulsed Arc Welding, to find links to Online Resources.
Tig-welding-tips: This type is called officially GTAW-P, where P stands for Pulsed. The result is better control of the weld puddle without unduly high heat input. Adjustments include both current levels and pulse duration time for each level.
Welding of thin metals or joints of dissimilar thicknesses or dissimilar alloys for manual, mechanized or automatic setups are common applications.
A variant of this process, involving high frequency switching, is useful for precision mechanized and automatic applications where directional properties (stiffness) and stability of the arc are important. Here the current is varied between the two levels as before, but at a selectable fixed high frequency, permitting the resulting low average current.
Read on for more Tig-welding-tips.
Tig-welding-tips: Alternating Current (AC)
In Alternating Current (AC) polarity is inverted at every cycle. The frequency is 50 hertz or cycles per second in Europe and 60 Hz in the USA. In the standard transformer power supply the heat of the arc, because of alternate polarity, is evenly distributed between electrode and workpiece. The results of penetration depth and weld width are somewhat intermediate between DCEP and DCEN.
Tig-welding-tips: Using AC one profits from the cathodic cleaning action (of electrons being emitted by the workpiece towards the electrode) during the half cycle when the electrode is positive (EP). This action, that removes oxides from the surface of the metal, is essential for welding aluminum and magnesium.
Furthermore the higher setting value of Alternating Current provides, for every half cycle, deeper penetration than what would be possible with DCEP at lower current values. Practically the alternating current carrying capacity of a given tungsten electrode of a certain size is about one half of the DCEN it could carry.
Tig-welding-tips: A power supply transformer connected to the power grid would provide arc extinction at every half cycle. Reestablishment of the arc is easy when the electrode is negative, but almost impossible (except for quite high open circuit voltage) when the electrode is positive.
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Therefore it has become customary to provide AC power supplies with means to stabilize the arc independently of the instantaneous polarity. Stabilization is achieved by superposing a high frequency high voltage with very low current, either continuously or as timed bursts.
One common alternative currently used for mechanized or automatic welding involves square wave alternating current power supplies. These change the flowing direction of current in a very short time. High voltages and already hot electrode and workpiece surfaces ensure immediate arc re-ignition without additional external circuits.
Tig-welding-tips: Modern application involve electronic manipulation of the wave forms to provide so called balanced current, to counter output decay due to overheating.
In recent sophisticated (and expensive) power supplies, the maximum current levels are kept different at both polarities, with higher current for shorter time periods during the electrode negative half cycle. Increased DCEN time periods provides for deeper weld penetration and reduces electrode heating.
Tig-welding-tips: Tungsten Electrodes
A few tungsten alloys are used as electrodes for GTAW (tig welding). They are non consumable as they do not enter, intentionally, in the composition of the weld pool. Their function is limited to carry the currents employed, to stand the heat of the arc without melting and to provide electrons by thermionic emission.
Requirements are given in:
Specification for Tungsten and Oxide Dispersed Tungsten Electrodes for Arc Welding and Cutting
(ISO 6848:2004 MOD)
American Welding Society / 17-Apr-2009 / 38 pages
The two most important types are EWP or pure tungsten and EWTh-2 or thoriated tungsten including 2% thorium oxide (or thoria). The first is used with AC, mostly for aluminum or magnesium welding. Electrodes of the second type, used for DCEN, emit electrons more easily and have a 20% higher current carrying capacity when compared to pure tungsten.
Tig-welding-tips: Tip shape may have effect on quality of welding. A truncated cone of specified included angle, obtained by grinding, is often preferred for DCEN welding with thoriated tungsten electrodes.
Grinding dust might be dangerous if inhaled or ingested, and attention should be paid to other safety issues as explained in the above specification. For AC welding with pure tungsten electrodes a ball or emispherical shape is often selected.
Tip shape is a parameter that should be kept constant once determined that the results are acceptable for a given job.
Tig-welding-tips: Filler Wire
Manual GTAW is often performed autogenously (without added filler metal) when the joint requires only fusion of the abutting ends. However when needed, filler material can be added manually from the side, right in the weld puddle, by alternatively introducing the wire tip into the arc and withdrawing it.
For mechanized or automatic welding, in order to provide a higher weld deposition rate, filler wire is fed by a mechanical feeder, drawing wire continuously from a spool. The feeder may be similar to those used for Gas Metal Arc Welding with an adjustable feed speed control and wire guide. Normally the wire is fed at ambient temperature in what is known as cold wire feeding.
However, for special requirements, when exceptionally elevated deposition rate is needed, hot wire can be provided. The process is similar to that described above, except that the wire is resistance heated between a contact tube and the workpiece by an AC supplied by an additional power supply, independent from the DCEN power supplied to the welding arc.
Find the most recent
AWS Filler Metal Specifications by Material and Welding Process
from the current downloadable AWS 2012 Summer Catalog at:
Tig-welding-tips: Shielding Gases
The shielding gas is provided continuously through the torch to protect the molten weld metal, the tungsten electrode and the filler wire tip, if used, from air contamination. Backup shielding gas may be provided if necessary on the underside of the weld to purge and displace surrounding air.
Shielding is always needed but it may be critical especially for highly reactive metals like titanium that would be promptly oxidized and ruined if heated in air. To weld titanium by GTAW use is made of glove boxes purged with shielding gas of the highest purity before weld start or of trailing shields, specially designed accessories that provide a sufficient gas trail to protect the welded surface until cool enough.
The gases uses are mostly Argon and Helium or mixtures of the two. Argon is the standard shielding gas. Helium, which is lighter than air, must be supplied with a higher flow than argon because it tends to rise. It transfers more heat than argon for the same current and voltage. Thus it may be needed to join metals of higher conductivity or heavy plates.
When extinguishing the arc at the end of the weld, it is important to let the gas flow for a certain time to shield the hot metal from air. Equipment provides a delay before shutting off the gas flow, but the welder must linger with the torch upon the weld as long as needed.
Most weldable metals can be welded by the GTAW process. It is adaptable to thin and delicate joints but also to thick and demanding applications, always of good quality although possibly not the most fast and economic process.
Aluminum alloys of the weldable kinds are among those for which GTAW is most suited. One should always remember that not all aluminum alloys can be welded successfully or with the same ease. For a reminder of the reasons making some of the strongest alloys non readily weldable by fusion processes see Aluminum Welding.
Joining is made difficult by the surface formation of tenacious refractory aluminum oxides of melting point much higher than that of aluminum metal. The oxides though are broken up by the cathodic cleaning action of the Electrode Positive part of the alternating current cycle. Once broken they may float upon the molten metal and they no longer interfere with the welding process.
Therefore high frequency stabilized alternating current power supply is the most used GTAW process for aluminum with pure tungsten electrodes. Exceptionally, for thin sections, DCEP can be used with thoriated tungsten. Argon is the most used shielding gas.
Magnesium alloys present welding problems similar to those of aluminum. These are dealt with by the same procedures.
Stainless steels are often GTA welded because of the cleanliness of the process and quality of results, especially for food or pharmaceutical applications. Mechanized and Orbital Welding are widely used as good results are repeatable. Argon is used.
Likewise also heat resisting alloys, nickel and cobalt alloys are welded by this process that permits the use of similar or different filler metal as required and usually minimizes heat input, a parameter required to control distortion.
Steels can be GTA welded, but usually other processes are found more economical, unless special requirements like presence of dissimilar joints recommend its selection.
Welding of cast irons, usually for repair of small parts, can be performed by GTA, for the good heat input control with suitable filler metals. Special heat treatments may be required if cracking is a problem.
Copper and its alloys, characterized by high heat conductivity, are suited to GTA welding because of the concentrated high heat it can deliver. If necessary for thick sections helium instead of argon can be used for its improved performance.
Refractory and Reactive metals are welded by GTA except that positive shielding from air contamination must be provided at all times, preferably within purged enclosures filled with high purity inert gas.
Additional Tig-welding-tips relative to materials can be found in the material specific pages of our website by clicking on the Site Map from the NavBar.
Tig-welding-tips: How to Develop Welding Parameters
Once determined that GTAW is the selected process:
- Consider material, thickness and joint type
- Consider applicable polarity per above data
- Consider if special wave form is needed to limit heat input
- Select tungsten type, tip shape and size
- Select filler metal type and size if required
- Select shielding gas and flow rate
- Select maximum current applicable for required penetration
- Select voltage suitable to the arc length to be obtained
- Test selected parameters by welding a test piece
- Adjust parameters as needed and retest
- Examine macro sections for penetration and absence of defects
- Write down approved parameters for future reference
Watch the following Miller TIG Welding Aluminum Basics Videos
1: Keys to Puddle Control.
2: Coordinating Movement & Filler Deposition.
3: Forming and Controlling the Puddle.
4: Introducing Filler Metal to the Puddle.
An Article on tips for Welding thin Stainless Steel Sheets was published (3) in the Issue 32 of Practical Welding Letter for April 2006.
To read it click on PWL#032.
An Article on Filler Metals for Aerospace Applications was published in the issue No. 41 of Practical Welding Letter for January 2007.
To read the article click on PWL#041.
An Article on Fluxed Filler Metals for GTAW was published (4) in Issue 65 of Practical Welding Letters for January 2009. Click on PWL#065 to see it.
An Article on Progress in Orbital Welding was published (2) in Issue 107 of Practical Welding Letters for July 2012. Click on PWL#107 to see it.
An important Article on an Improved Welding Process called TIP-TIG was published (2) in Issue 72 of Practical Welding Letter for August 2009.
Click on PWL#072 to read it.
An Article on Juggling the variables of GTAW was linked to (5.1) from Issue 101 of Practical Welding Letter for January 2012.
Click on PWL#101 to get the link.
An Article on Productivity Improvements in GTAW was published (7) in Issue 112 of Practical Welding Letter for December 2012.
Click on PWL#112 to see it.
An Article on New Rules for Storing Thorium Containing Materials was published (3) in Issue 121 of Practical Welding Letter for September 2013.
Click on PWL#121 to see it.
An Article on Trailing Shields for Welding Titanium was published (11) in Issue 130 of Practical Welding Letter for June 2014.
Click on PWL#130 to see it.
An Article on Activated TIG Welding was published (11) in Issue 135 of Practical Welding Letter for November 2014.
Click on PWL#135 to see it.
An Article on Back to Basics was published (3) in Issue 142 of Practical Welding Letter for June 2015.
Click on PWL#142 to see it.
An Article on Rotating Arc Welding Technology was published (4) in issue 143 of Practical Welding Letter for July 2015.
Click on PWL#143.
An Article on A new Variant of the GTAW process was published (2) in Issue 147 of Practical Welding Letter for November 2015.
Click on PWL#147.
An Article on Remote Control Welding Technologies was published (3) in Issue 149 of Practical Welding Letter for January 2016.
Click on PWL#149.
An Article on Welding Austenitic SMO 254 Stainless Steel was published (4) in Issue 155 of Practical Welding Letter for July 2016.
Click on PWL#155
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Find some interesting links in a special Mid Month Bulletin Page of our PRACTICAL WELDING LETTER, designed offer you, our interested readers, the opportunity to search the web quickly and effectively on the subject of TIG Welding (GTAW).
We urge you to explore this rich source of essential knowledge.
Online Resources on TIG Welding, presenting Articles, Tables, Guidelines, Handbook, Tips, Downloads, Links and Information is now available by clicking on PWL#055B.
Looking for more Online Reference Links? Click on Welding Resources
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