Welding is a fabrication process that joins separate workpieces into a single component, by allowing liquefied material along the contacting segments to combine and solidify. This is often achieved, as with MIG, TIG and Arc welding, through adding a filler material that combines with the surrounding liquefied workpiece material to form a pool of molten material known as the weld pool. The workpiece cools to become a robust molecular level joint. Certain types of welding such as friction welding for example, in contrast to MIG, TIG and Arc welding, creates a weld pool from only the liquefied workpiece material at the mating surfaces, hence no filler material is required.
In contrast to welding, soldering and brazing bonds workpieces through liquefying and then re-solidifying a joining material at the intended joint. This joining material has a lower melting point than the parent material. Thus the join is made without actually melting the workpieces themselves as within a molecular level welded joint, hence acting more like glue.
A link to a book on soldering and brazing is included here:
Various materials can be welded including glass, thermoplastics and even ceramics under certain conditions.However this guide will deal with the most prominent form of welding that of metallic materials, specifically the welding of steel and to a lesser extent aluminium.
Links to a couple of suitable practise materials (untreated, reasonably thick mild steel) are included here:
However buying in bulk from a local supplier is likely to be more economical.
Although outside the scope of this website, below is an interesting example of ceramic welding.
The strength of a weld can be comparable to the strength of the parent material. Various calculations can be used to assess weld strength, the specifics of which is outside the scope of this insight, however the following factors all contribute to the strength of a weld:
When calculating designs work to safety factors of between two and five for all welded joints under static loads, depending upon the situation. An additional margin is incorporated if the work piece is under fluctuating loads to account for fatigue. When possible the design should be such that the load path does not pass directly though a weld. It is advisable that critical joints (in that a failure could result in injury) should be bolted rather than welded, to enable dependable stress calculations.
Weld penetration is the distance that the weld (the atomic union or fusion of the two mating workpieces) extends below the surface of the workpiece. Inadequate weld penetration is likely to result in a poor strength weld.
Material thickness, power settings, weld speed and technique all effect weld penetration, the details of which are discussed within the relevant MIG and TIG sections. In summary increasing power while reducing weld speed and material thickness should result in increased penetration. Aesthetically the weld on the weld face can be deceptive, looking pleasant due to the filler whilst still having little penetration. The reverse side of the weld is more revealing visually in respect to weld penetration.
The images below show a cross section of two welds, produced through slicing though the weld with a band saw. The left image shows adequate penetration was as the right image has inadequate penetration.
Unless correctly managed, welding can be a dangerous process. In summary you can be incapacitated, maimed or killed through a lack of precaution in the following areas;
Heavy metal poisoning occurs through inhaling toxic metal vapours. Unfortunately the welding process can generate some of these vapours and as such precautions should be made to minimize the risk.
Galvanizing is used to protect steel from corrosion. This process applies a zinc coating to act as a sacrificial anode; it is very effective at resisting corrosion, extremely common and completely safe if not heated to high temperatures. However welding achieves the temperatures required to release the zinc as a vapour. Therefore it must be ensured that the zinc layer is removed from galvanized steel before welding to limit zinc oxide release. To remove a galvanized layer rather than using an acid, I would suggest grinding the layer away whilst using a respirator. Ensure the area is re-treated after welding with a galvafroid paint to ensure corrosion protection.
Although not recommended due to the environmental impact, cadmium plating can also be used to protect steel from corrosion. As with galvanised steel, if cadmium plated steel is welded the cadmium oxide can be released.
Lead oxide can also be released from some paint and primers when welding a material that has already been treated, thus must be removed with a wire bursh prior to process. Manganese oxide can also be released during the welding process from both the welding rod and base material.
Exposure to the metal oxides listed can result in metal poisoning. Symptoms of metal poisoning may include:
Nitrous oxide (NOx), carbon dioxide (CO2), carbon monoxide (CO) and ozone gas (O3) fumes are all produced from welding.
Ozone gas (O3) exposure, while not as risky as heavy metal poisoning can result in emphysema, can agitate existing conditions such as asthma and can also cause irritation to the eyes and nose.
When welding mild steel chromium and nickel oxides are released, stainless steel will release these vapours in higher concentrations. Nickel oxide exposure can agitate asthma while chromium oxide exposure can lead to sinus issues. There is now evidence that inhaling nickel and chromium oxides may be carcinogenic.
When welding aluminium, magnesium and aluminium oxides are released, that can cause irritation to the lungs.
Poor quality welding rods and wires can also contain unnecessary harmful substances, including asbestos coatings, barium and fluorine, these should be avoided. Good quality MIG wire can be purchased here and Stainless steel TIG filler rods can be purchased here .
As stated above, several harmful fumes can be produced during the welding process. Steps need to be taken to negate risk, arguably most importantly knowledge on what should not be welded/removal of coatings.
During my professional welding career extraction systems were installed in all workshops and on site welding was conducted in well ventilated areas. If selecting a ventilation system for professional use, standard propelling fans are not suitable as the shielding gas would be blown away in addition to the harmful gases. If home project welding, I would recommend welding outside if possible, although also out of view of others due to the risks of arc eye.
In a profession setting I would recommend a Hydrogen / Argon shielding gas mix rather than pure Argon when TIG welding, this significantly reduces the ozone gases produced.
It is important you do not place your head directly in the rising plume of the fume, this is where the harmful gases are concentrated. Smaller respirators can be worn under a weld mask, although I did not during my career. Welding masks can also be purchased with respirators incorporated or designed to work with separate compatible respirators .
Arc eye is sunburn of the eye. The cornea is very susceptible to the hazardous ultraviolet radiation generated by most welding processes. UV radiation occurs as the extreme temperatures within the welding process (Arc welding temperatures can reach over 6000 degrees) enable the higher energy wavelengths to be released.
Arc eye damages the cornea's protective cell layer. The damaged protective cells gradually die exposing the sensitive nerves below to the rear surface of the eyelid. The onset of arc eye can be several hours after the initial exposure to the damaging UV radiation, causing an intense burning pain and blurred vision which lasts between 12 to 24 hours. Although usually a temporary condition, repeated exposure can cause permanent damage.
As a note, snow blindness is a mild form of arc eye, caused by reflected UV light due to snowy conditions and increased altitude. The higher altitude means less UV light is blocked by the atmosphere.
Due to arc eye, full face welding marks are essential. Closing your eyes will not prevent arc eye, even for the smallest of welds. Welders should be aware there is also a possible chance UV radiation could be reflected around the mask via reflective surfaces and should act accordingly to reduce this risk.
Bystanders are also at risk of arc eye, warnings should be issued before welding commences and welding screens should be erected when appropriate.
The Review Page includes a breakdown of the best welding masks for your price range (that i have used), I have also included a few links to welding masks below:
As discussed above, the high temperatures involved in welding causes the base material to radiate large amounts of ultraviolet light. UV radiation causes the radiation burn known as Sunburn, named after the most common case. However the quantity of UV radiation released from welding is vastly higher than the quantity of UV radiation that will reach you from the sun.
It takes less than 15 minutes for Sunburn to occur when naked skin is exposed to welding arcs. As the red tinge to the skin can develop after exposure for up to six hours, the damage caused may not be immediately obvious. It can take several weeks for an intense welding burn to fully heal. Excessive UV radiation is also the leading cause of primarily non-malignant skin tumours and significantly increases the risk of skin cancers; melanoma, basal-cell carcinoma and squamous-cell carcinoma.
To prevent radiation burns all skin should be covered, including arms, legs and neck. Welding gloves will protect the hand and wrists and are required in order to handle the work pieces regardless of the risks behind radiation burns.
Full concealment of the body will also provide protection from spitting molten metal and grinding sparks. Overalls should be selected as a clothing option over thin materials for this reason, as the metal could burn through. Care should be taken to ensure no clothing is flammable.
Spitting molten metal can reach several meters from a weld site, while auxiliary processes such as grinding can cause sparks to reach even further. In conjunction with this hazard the shade on a welding visor can make flames more difficult to spot.
A thorough environmental risk assessment should be conducted on your factory/work area ensuring flammable materials (such as paper, sawdust and compressed gas containers) are placed out of harms way, fire exit routes are clear and the correct C02 fire extinguishers are in place. Ensure work areas are tidy.
The metal inert gas (MIG) welding process forms an electric arc between the workpiece and a consumable, continuously fed wire. The wire is fed through the weld gun held just off the surface of the workpiece. The extreme temperature of the arc causes the workpiece to melt almost instantly, forming the weld pool. Once the metal re-solidifies a joint is formed. The physics and details behind electric arcs can be found in the relevant section below.
The following equipment is the minimum necessary for MIG welding;
In this process the key factors include selecting the correct gas mixture and flow rate (see Arc Shielding / Shielding Gas below), wire speed and therefore current (see MIG: Wire Speed below), voltage and the correct torch position and technique (see MIG: Power Settings below).
Due to the large supply of filler wire on the spool MIG welding can be considered a continuous process in comparison to TIG welding that relies upon a rod length to supply filler. Therefore massive uninterrupted welds can be accomplished.
In summary the wire is stored on a spool, driven by rollers through a feed tube into the weld gun. The power is applied directly to the wire.
The wire spool is placed on the mounting which contains a spring tensioner, located on the top or side of the MIG welder. The tensioner should be set to ensure the spool cannot unravel under its own coil spring, ensuring the wire is kept taut when the tip is held by the operator. Excess tightening of the spool spring tensioner will place unnecessary strain on the wire feed roller mechanism limiting the life of the equipment.
The wire tip should be straightened and inserted through the open roller mechanism next to the spool mounting via the guild tube. The wire should then be threaded past the adjoining wire feed roller mechanism and into the feed tube linking the wire feed roller mechanism to the weld gun.
The wire feed roller mechanism can now be closed. Over tightening of the mechanism clamp can cause damage to the welder including roller mounting distortion and motor gear shearing if the wire where to snag. Therefore the minimum tension required to provide adequate drive friction on the rollers to feed the wire should be used. It should be ensured the wire sits snugly in the correct groove of the mechanism associated to the wire gauge being used. The height of the roller can be adjusted to align the various wire gauge grooves with the centre line between the guild and feed tubes.
With the wire feed roller mechanism closed and the feed tube to the gun held straight to avoid catching the wire, the wire feed roller mechanism trigger on the weld gun can be engaged to feed the wire through the feed tube to the weld gun tip. The gas shroud and contact tip of the weld gun can be removed prior to engaging the wire feed roller mechanism to reduce the frequency of wire catches. The gas shroud is retained via a spring and ¼ turn fastening where as the contact tip is retained via a screw thread.
It should be ensured the welding wire has no rust before installing the spool. Rust on the wire will act as a lubricant on the wire feed rollers causing slip while also catching the wire in the feed tube.
The professional equipment used by TG welding is capable of automatically setting the wire speed in relation to the setting on the power supply; however most equipment requires the wire feed speed to be set independently. Power Settings are covered below.
The correct wire speed setting is crucial in MIG welding. Set the feed speed outside the narrow optimal range and the process will be subject to both burn back and stutter if the feed speed is too slow, or blow though and excess splatter if the feed speed is excessive.
High wire feed speed results in the wire deforming as it hits the work piece resulting in excess splatter and an unsightly weld.
Burn back is the formation of a weld on the welding gun contact tip as the wire burns back having made contact with the workpiece. Burn back is caused by either a slow wire speed or the weld gun being held to close to the work piece. Burn back is likely to result in damage to the contact tip and thus require the tip to be replaced. Stuttering will also occur at insufficient wire speeds as a lack of current is provided to maintain a constant arc resulting in an inconstant weld. A constant electrical arc should always be maintained to ensure welds are continuous and constant.Electric Arcs are covered below.
In MIG welding the wire feed speed (and weld gun to workpiece offset) sets the current and the welding power supply sets the voltage. As Voltage x Current = Power, it is clear that both the wire speed (current) and voltage setting can effect the power ploughed into the weld.
High power can cause the material to blow through, especially on thin material. Blow through is a hole in the workpiece with no filler material.Voltage settings depend primarily upon the metal thickness, but also secondary variables including speed, weave pattern (see MIG: technique sections below), joint design and equipment used.
Joint design such as chamfering the edges of mating workpieces to create a 'V' shape and also/either leaving a 1mm root gap between the mating workpiece can reduce the voltage required for a particular metal thickness. The joint designs listed enable the energy from the arc to reach the rear face of the workpiece therefore granting sufficient weld penetration. Such adaptations are generally only required on larger material thickness, around 4-5mm for a single V joint. At around 20mm material thickness double sided V joints are required to limit the filler content of the joint.
Once the equipment has been correctly set, the workpieces cleaned in anticipation of the weld and the operator is in a comfortable and maintainable position the welding can commence.
There are two primary techniques for MIG welding. The first is the push, also known as the forehand technique which pushes the weld gun and puddle along the weld line, with the weld gun angled toward the previously formed weld. The second is the pull, also known as the backhand technique which drags the weld gun and puddle along the weld line, with the weld gun angled toward the joint yet to be welded.
MIG welding is commonly conducted using the push technique, however both methods are used and each has advantages and disadvantages. Generally pushing provides shallower penetration with a wider, smoother weld bead considered to be more aesthetical, where as pulling provides deeper penetration with a narrower higher weld bead. The differences in resulting penetration occur as when pushing the weld gun arc is angled towards the cooler workpiece material, where as when the weld gun arc it is angled directly towards the weld puddle. This increases the temperature and weld time hence providing deeper penetration.
Pushing provides greater visibility, as the weld gun placement is over the existing weld rather than the intended weld line. The pushing technique also aids in the containment of the shielding gas (see Arc Shielding / Shielding Gas section) .
As a note, other welding methods such as Arc welding that produce slag are limited to the pull technique, as utilising the push technique would run over deposited slag causing it to become trapped in the weld puddle.
The weld travel angle is the offset angle of which the weld gun is held, in relation to a direction perpendicular to the work piece, in a plane along the direction of the weld. This is more easily demonstrated in the figure below:
The work angle is the angle of which the weld gun is held, in relation to the work piece, in a plane perpendicular to the direction of the weld. This is more easily demonstrated in the figure below:
When welding flat the weld gun should be held with a travel angle of between 5 to 20 degrees. Exceeding this travel angle can lead to lower penetration as the arc is angled at the edge of the weld pool (see section MIG: torch position and technique – push and pull) and also must pass through more material to reach the rear of the work piece. This is represented in the following trigonometry cosine formula 'work piece thickness (adjacent) / cosine of travel angle = actual material penetration required (hypotenuse).
The work angle of which the weld gun is held is dependant upon the joint type and welding plane. When welding flat;
See the joint types section for a pictorial representation of each of the joint types discussed.
We may have to manufacture products in the factory or repair and install onsite. Therefore we are often required to weld on vertical or inclined surfaces. The travel angle, weld angle and weld patterns used need to be adjusted accordingly. A quick summary of these adjustments are covered below.
Welding along a Horizontal Edge the work angle should be adjusted to between 80 and 65 degrees in an upward direction. This will aid the fusion on the upper edge of the joint, countering the effect of gravity pulling the filler towards the lower edge.
Welding Vertically can prove to be challenging. The travel angle should be adjusted to between 35 and 45 degrees in an upward direction. A travel angle pointing downwards would be detrimental, assisting gravity in pulling the molten material from the joint.
The weld gun should stay ahead of the weld puddle. Slow travel speed will result in the molten material flowing from the joint due to gravity, where as fast travel speed will result in insufficient penetration. It can prove beneficial to incorporate a weave motion in order to slow the relative linear speed and spreading the weld over a wider area (see section weld patterns). The weave motion also provides more of a foundation on which the new molten weld can sit (each weld weave creates a shelf for the following weave) when welding vertically up.
Welding vertically up requires more skill however provides greater penetration. Welding vertically down produces more aesthetical welds but should generally not be used for critical welds on thicker material, with penetration limited due to weld speeds accelerated by gravity. Welding vertically down can be utilised in the welding of thinner materials as the limited weld penetration can limit blow through.
Welding Overhead the weld speed must be sufficient to ensure the molten metal is contained within the joint. The powering setting should be adjusted in order to limit the size of the weld puddle thus increasing controllability.
A weld pattern is a pattern made with the weld gun is it moves along the weld line. Various weld patterns exist and can provide some compensation for poor equipment setup. Patterns work by expanding the width of the weld. A steady motion using no pattern can produce a perfect weld on the flat; however the equipment settings, travel speed and technique will need to be perfect.
Common patterns include:
The contact tip of the weld gun should be held between 5mm and 10mm from the workpiece. The weld bead width, controlled via the weld speed, should not surpass the workpiece thickness.
See the Electric Arc and Polarity, Arc Shielding / Shielding Gas and Joint Type sections below for details and information common to both MIG and TIG techniques. Also see the MIG v TIG for the benefits of each method.
Tungsten inert gas (TIG) welding, also referred to as Gas tungsten arc welding (GTAW), forms an electric arc directly between the workpiece and a non-consumable tungsten electrode contained within the weld gun. The extreme temperatures of the arc causes the workpiece to melt almost instantaneously, this forms the weld pool. Once the metal re-solidifies a joint is formed. Filler material is optional, supplied externally via a stick of filling material held in the operator's second hand. This enables the operator great flexibility in terms of the filler supplied and also the weld speed. Weld current and as such power, is controlled by the foot pedal regulator and to a lesser extent the weld gun to work piece offset. This enables 'on the fly' power adjustment. Shielding gas is supplied through the weld gun. These attributes make TIG welding one of the most versatile welding processes and the most utilised in my career.
In contrast to TIG welding, in MIG welding the filler material is fed through the weld gun which acts as the electrode. The MIG current is a direct output of the MIG wire speed, which is set pre-process, removing the ability to adjust the MIG power settings 'on the fly'.
Therefore the operator has more control over filler application and weld power in TIG welding than in MIG welding.
The following equipment is the minimum necessary for TIG welding;
Various oxides have been added to tungsten electrodes in order to improve the weld characteristics. I would advise using lanthanated electrodes for general welding of stainless and mild steel, over the commonly used thoriated tungsten electrodes. Thoriated electrodes while cheaper, are radioactive and care needs to be taken when grinding. Lanthanated electrodes also have the benefits of being suitable for DC and AC welding (aluminium), have a higher current threshold and lower voltage breakdown (arc stability).
The table below includes the more commonly available electrodes;
|Pure Tungsten||Green||AC/DC||Performance AC welding|
|1.5% Lanthanated||Gold||DC||Performance DC welding|
|2% Lanthanated||Blue||AC/DC||Good all round performance / general use|
|2% Ceriated||Orange||DC||Low current performance, small delicate parts|
|Thoriated||Red||DC||Lower cost and common but radioactive|
Each tungsten electrode diameter has a maximum current rating, the greater the diameter the greater the maximum current rating. The smallest diameter tungsten should be selected for your application to keep a tight arc, without exceeding the max current limit.
When grinding a tungsten electrode tip for use, ensure sharpness. A sharper tip provides a more precise arc that in turn provides a cleaner weld. The tungsten colour code will be removed through grinding, hence ensure that the tungsten is retained within the correct packaging enabling identification on future welds.
Links for each type of tungsten electrode listed is include below;
TIG filler rods are selected to match the parent material. Cut lengths of MIG wire can be used as an economical alternative to purchasing separate filler rods as long as no additives have been added. Common filler rod materials include;
Filler rod diameters range from 1/6" (1.5mm) to ¼" (6.35mm) and selection is dependent upon the parent material thickness. However 3/32" (2.5mm) is a good diameter to start for beginners.
This section will be updated with key process factors and methods soon.
This section will provide an overview of the SMAW process, but with less detail than the MIG and TIG welding sections. The shielded metal arc welding (SMAW) is also known as manual metal arc welding (MMAW) or stick welding.
The following equipment is the minimum necessary for SMAW welding;
The SMAW process forms an electric arc between the grounded workpiece and a consumable electrode, which also acts as the weld filler and shielding gas. The electrode is held just off the surface of the workpiece and is contained in the electrode receptacle held by the operator. As the electrode is consumed the operator moves the receptacle closer to the workpiece maintaining a constant arc length (gap) between the diminishing electrode and the workpiece.
The electrical arc in SMAW is formed through striking or tapping the electrode against the grounded work piece, thus completing the circuit. Excessive contact time on the strike or tap will result in the electrode welding to the workpiece. After the circuit is formed the arc length, the distance between the electrode and workpiece, should be approximately that of the diameter of the electrode. The weld sound and light outputs will indicate adjustments required in the arc length (electrode offset). An excessive arc length will lead to an unstable arc and excess splatter. An insufficient arc length will lead to the electrode converging with the molten slag leading, to slag inclusions in the weld and a weak joint due to impurities.
Along with arc length, other key factors in the process include travel speed and current selection. Excessive travel speed and/or insufficient current can result in a weak thin weld lacking penetration. Insufficient travel speed and/or excessive current can result in a large inconstant weld and blow through. Attempting to relieve excessive current with increased travel speeds is not advisable as this will lead to rapid cooling of the weld which can lead to weld slag inclusions.
As stated, the consumable electrode contains the filler and also the shielding gas which is produced when the flux coating on the electrode melts. Solidified flux, called welding slag, drops on top of the fresh weld, adding further protection to the air, however this needs to be removed after the weld is complete. The frequency of the SMAW consumable replacement and requirement for slag removal results in a slow weld process in contrast to other options. Specific electrodes are also required for welding aluminium, copper or cast iron.
The pull technique (see section MIG: Technique – Push and Pull) is required as utilising the push technique would run over deposited slag causing it to become trapped in the weld puddle.
The electrode shown below is a Mild Steel 2.5mm Type 6013; we will use this as a breakdown example
Type Code, 6013 in our example, can be broken down into the separate components as shown in the image below;
Electrode Poundage is the maximum tensile strength of the filler material, 60,000 pounds per square inch (PSI) in our example. Generally 60,000 PSI electrodes offers deeper weld penetration, whilst 70,000 PSI electrodes offer more aesthetically pleasing shallower welds.
dictates the orientation in which the electrode can be utilised
Electrode Coatings are listed in the table below. The coating effects the weld penetration and also dictates the polarity that can be used (AC/DC+/-). Some coatings require special storage once unsealed to ensure dryness.
|10||High cellulose sodium||DC+|
|11||High cellulose potassium||AC/DC+/DC-|
|12||High titania sodium||AC/DC-|
|13||High cellulose potassium||AC/DC+|
|14||iron powder titania||AC/DC+/DC-|
|15||low hydrogen sodium||DC+|
|16||low hydrogen potassium||AC/DC+|
|18||Iron powder low hydrogen||AC/DC+|
|20||High iron oxide||AC/DC+/DC-|
|22||High iron oxide||AC/DC-|
|24||Iron powder titania||AC/DC+/DC-|
|27||Iron powder iron oxide||AC/DC+/DC-|
|28||Low hydrogen potassium iron powder||AC/DC+|
Electrode material is stated outside the Type Code, in this example it is mild steel. Electrode material is selected to match the base material. Common electrode materials include;
Electrode Diameter is stated outside the Type Code, in this example it is 2.5mm. The stated electrode diameter does not include the coating thickness. Electrode diameter selection depends on the joint type and thickness of the weld material. Larger diameter electrodes provide a larger diameter weld; hence a lap joint would require a smaller diameter electrode than a fillet joint. You should stock a range of diameters to suit any requirements that may arise.
|Overview||Generally large welds||Generally short, highly technical welds|
|Speed||Up to 5x faster
|Aesthetics||Lower weld splatter than SNAW||Precise clean welds, limited finishing required|
|Bond Quality||-||Higher quality bond
(TIG welding can be stipulated in quality and code requirements, especially when working with nuclear and aerospace)
|Arc / Power Control||Preset
(on power source)
|Suitability for Thinner Material Gauges||-||Lower power range limits blow through|
|Skill Level Required||Lower than TIG||Higher than MIG|
Two Hand Process
|One Handed Process
(facilitates free hand for workpiece stabilisation etc)
|Two Handed Process|
|Material Range||Wide Range||Wide Range|
(however less so than SMAW due to requirement for a shielding gas source).
(however less so than SMAW due to requirement for a shielding gas source).
|Fumes Produced||-||Produces less fumes
than both MIG and SMAW)
|UV Produced||Less than SMAW
|Less than SMAW
In all the welding processes discussed, an electrical arc is used to create the intense heat required. An electrical arc is produced when a voltage applied across an electrical insulator is sufficient enough to cause ionization. In this case the insulator is the air between the weld gun and work piece. A current (the free electrons) will then pass through this ionized air, which has now transitioned from a gas to plasma state. The direction of travel of the electrons will depend upon the polarity of the weld process, flowing from the cathode (negative) to anode (positive) terminal. Around sixty-five percent of the arc energy (heat) flows to the positive terminal.
When using an alternating current neither the workpiece nor weld gun can be regarded as the cathode or anode. However, when using a direct current the polarity of the workpiece and weld gun can be switched between cathode and anode. The most suitable polarity will depend upon various factors including the welding process and joint type. An anode workpiece could be more suitable for a filling weld type, such as fillet weld to ensure the maximum transfer of filler.
MIG welding is commonly conducted with a cathode (negative) workpiece and anode (positive) weld gun and wire; this is identified as 'reverse polarity'. Therefore the electrons flow from the negative cathode workpiece to the positive anode wire tip, in order to melt the wire in a consistent process. In contrast MIG welding using straight polarity can cause an unstable inconstant arc, leading to poor welds as the filler wire fails to melt.
TIG welding is commonly conducted with an anode workpiece and cathode weld gun, this is identified as straight polarity. In TIG welding the filler is held at the workpiece surface rather than weld gun. Therefore the majority of arc energy (heat) can be placed on the workpiece, which is able to quickly disperse the energy due to its conductivity. This polarity limits splatter, blow away and also protects the electrode from excess heat.
Oxygen and nitrogen present in the air will react with metals at high temperature forming oxides and nitrates. Therefore the weld pool needs to be protected to stop this reaction. If unprotected the particles would form and thus contaminate the weld pool. This contamination would destroy the weld toughness, strength and consistency.
To ensure these reactions and the associated contamination are minimized, a protective gas is used to effectively separate the weld pool from the surrounding air.
Pure argon can be used as a shielding gas for most situations. Argon can be mixed with various other gases, including carbon dioxide and helium, to improve weld characteristics such as weld penetration and fluidity. Pure carbon dioxide can be used in MIG welding as an economical but inferior substitute for an argon mix, however not for TIG welding as it would cause the tungsten to oxidise.
In MIG and TIG welding the shield gas is fed through the weld gun. In SMAW (Arc) and Gasless MIG welding a flux coating is present on the consumable electrode. This flux coating melts and produces gas which forces out the air from the welding zone. Solidified flux, called welding slag, drops onto the fresh weld adding further protection to the air. Welding slag needs to be removed in an additional cleaning process after the weld is complete.
The correct gas flow rate is dependent upon a number of factors including weld process, environment, weld joint type and gun nozzle, the use of a gas lens, tungsten stick out and to an extent weld current. A fillet joint for example will help to retain the gas within the weld location where as a butt joint will not. Elevated gas flow rates will not only result in wasted gas but also led to turbulent rather than laminar gas flow. This will cause the surrounding air be drawn into the flow and mix with the shielding gas causing poor welds.
After a vast career I have always set the flow rate by feel, after my own experience of weld quality. However some approximations are included in the tables below for a beginner start point. For TIG welding 12 litres per min (25CFH) is considered a standard flow rate.
The tables demonstrate that the electrode diameters (tungsten/wire) and therefore the current / parent material thickness is proportional to the flow rate requirement. The TIG Gas Flow Tables demonstrate the effectively of a Gas Lens in reducing gas expenditure. A comparison between the Ferrous and Aluminium Tables demonstrate the requirement to increase flow rate when working with Aluminium if all other variables are fixed.
To reiterate the tables are an approximation. Gas flow needs to be at the maximum end of the range if welding outside in windy conditions.
|TIG Set-Up||Argon Flow Ferrous Metals|
|Electrode Diameter||Cup Size||Gas Flow Rate (Litre/Min)||Gas Flow Rate With Gas Lens (Litre/Min)|
|.5mm||3,4 or 5||3-4||3-4|
|1.0mm||4 or 5||3-5||3-4|
|1.5mm||4,5 or 6||4-6||3-5|
|2.5mm||6,7 or 8||5-7||4-5|
|3.0mm||7,8 or 10||5-9||4-6|
|4.0mm||8 or 10||7-12||5-7|
|4.5mm||8 or 10||10-17||6-12|
|TIG Set-Up||Argon Flow Aluminum||Electrode Diameter||Cup Size||Gas Flow Rate (Litre/Min)||Gas Flow Rate With Gas Lens (Litre/Min)|
|.5mm||3,4 or 5||3-4||3-4|
|1.0mm||4 or 5||3-6||3-5|
|1.5mm||4,5 or 6||4-7||4-6|
|2.5mm||6,7 or 8||5-10||5-7|
|3.0mm||7,8 or 10||6-12||5-10|
|4.0mm||8 or 10||7-14||6-12|
|4.5mm||8 or 10||12-19||7-14|
|MIG Nozzle Size||Gas Flow Rate (Litre/Min)|
Post flow is required in order to protect the weld and/or tungsten from contamination as it cools. If adjustable on your power supply, a good approximate calculation is 1 second of post flow per 10 amps of welding current. For example if welding at 70 amps, post flow should be 70 seconds.
Pre flow purges the gas lines and ensures the weld pool is protected from the start of the weld. This setting is often fixed on a power supply and will be approximately 0.5 seconds.
Professional welding outfits are likely to rent gas canisters that are refilled on a rolling contract. I am unsure on the best (most economical) solution for the hobbyist, as this will depend on usage. Hence I have included a link to a disposable canister and also a link to a non-disposable canister that will require filling at a local welders merchant.