Gas Tungsten Arc Welding best practice

Gas tungsten arc welding is a process where a non-consumable tungsten electrode is used to produce the weld. Welders can encounter various issues in the GTAW process so we have compiled our best practice advice for gas tungsten arc welding in an effort to prevent or troubleshoot these problems.

Process

The electric current is provided from a power source to the tungsten electrode in the handpiece causing an arc. The arc is struck by either scratching the electrode across the workpiece or through a high-frequency generator which allows the arc to be initiated. The electrode and the workpiece are separated, typically about 2mm apart. The weld area is protected from atmospheric contamination by an inert gas, usually argon or helium, supplied through the handpiece and directed by a gas shroud. A filler wire is normally used. This is manually fed into the weld pool by the operator. Some welds, known as fusion welds do not require a filler wire. The power source varies depending on the application:

• Alternating current is used for material such as aluminium.
• Direct current with a negative electrode is used for most common materials.
• Direct current with a positive electrode is not often used due to the excessive heating of the tungsten.

 

Tungsten

There are various types of Tungstens for different applications. The most common are:

• 2% Thoriated (red) – used for direct current welding of carbon steel, nickel alloys, titanium alloys and copper alloys.
• 0.8% Zirconiated (white) – used for alternating current welding of aluminium alloys.
• 2% Ceriated (grey) – can be used on either alternating current or direct current.

Tungstens come in different sizes with the material thickness and amp range being the variable that will determine the selection. The common sizes are 1.6mm, 2.4mm, 3.2mm and 4mm. Using a size too small will cause the tungsten electrode to melt as it is unable to carry the higher amperage.

 

Parts

The GTAW torch has various parts that need to be correctly fitted and maintained. The quality of the fittings, torch, and leads are extremely important in the quality of welds. Damaged fittings and torch leads can lead to issues with porosity and an erratic arc.

Gas lens/gas diffuser

The gas lens has a mesh insert to spread the shielding gas that is supplied from a flow meter so that it is evenly dispersed around the weld pool. If the mesh is damaged or blocked it needs to be replaced.

Gas diffusers (also known as the Collet body) have larger holes around the diffuser to spread the gas as it flows through the torch. These larger holes can lead to vortexes which draw in oxygen as the gas passes through the ceramic. Ensure these holes are free from debris otherwise the shielding gas flow will be blocked.

 

Collet

This is used as a clamp to hold and supply current to the tungsten electrode. The size of the collet needs to directly reflect the size of the tungsten electrode selected. If the collet is no longer clamping the electrode correctly, it will need to be replaced.

 

Backing cap

The backing cap is securely tightened to the back of the torch head to assist in holding the tungsten in place and has an o ring to prevent the shielding gas from escaping. If this o ring is worn it will need to be replaced.

 

Ceramic nozzle

The ceramic directs the shielding gas over the weld pool. Without this, the gas will not correctly shield the weld pool from atmospheric contamination. The size and shape of the ceramic usually depends on joint access. A blocked or broken ceramic nozzle will need to be replaced.

 

Washers

A washer is placed between the backing cap and the torch head. It is also placed between the ceramic and the torch head to create a sealed unit so that the shielding gas does not escape.

 

 

 

Gas shielding

The gas shielding system is not that complex. Gas is delivered via a regulator and flow controlling meter on the cylinder. It passes through an on/off solenoid which is turned on and off with the trigger, so it coincides with the arc starting. This trigger process delivers the gas down to the torch. Some machines bypass the on/off solenoid and trigger to supply the torch straight from the flow meter. This requires the operator to manually turn on the gas flow from a valve on the handpiece before welding and manually turn the valve off once welding has ceased. A common mistake by welders is to turn the gas up excessively. Having the gas too high can create just as many problems as having it too low. Too low means that the weld will not be adequately shielded. Too high gas flow results in turbulence whereby the gas will hit the plate and vortex and drag some of the atmosphere into the shielding gas. This can cause porosity in the weld, this also means you are consuming too much gas. Pure Argon is the most commonly used gas for GTAW.

 

 

 

Flow rate

The flow rate of argon should be run at approximately 12 to 15Lpm because it is heavier than air. Helium gas needs to have a slightly higher flow rate to account for the fact that helium is lighter than air. Helium gases should be run at approximately 15 to 18Lpm. Correct maintenance and setup is the key to ensure you don’t have a problem with your gas system through the introduction of atmospheric contamination and therefore a reduction of the purity of your shielding gas. Ensure connections are tight, check that the hoses are not damaged, and all of the O-rings are in good condition.

Connection setup

There are a number of connections that you need to ensure are in good condition; start at the cylinder, on the bottom of the regulator there is an O-ring that should be in good condition, never lubricated, and when the regulator is put into the cylinder the regulator nut should not be overtightened. If you are running a separate flowmeter the connection between the regulator and the flow meter needs to be sound. The flowmeter should always stand vertical to ensure you have an accurate reading and it should be the correct type for the gas you are using. Different types of flow meters measure at different locations. Bobbin type at the top of the bobbin, ball type to the centre of the ball. If you’re unsure read the instructions that are directly on the flowmeter.

 

The next connection is the flow meter to the hose and the nut that connects the hose to the flowmeter. This should be firm but never over tightened, the male and female component should be of the same type because it relies on the brass seat to create an effective seal. The connection between the nut and male component, what is referred to as the tail where the hose is attached to that connection should be in good condition and there should be no cracks in the hose from damage.

 

The final connection is where the hose goes into the back of the machine; often neglected as they are hidden. Where the hose connects to the back of the GTAW unit, it should be firm but not overtightened and the hose should be in good condition. All of these fittings use a hose clamp that needs to be firm. You should not be able to twist the hose easily.

Power connection

Lastly, the least common thing people have problems with his power. The first thing you need to do is ensure all of your connections are tight, some of these checks need to be done by a licensed electrician, for example, the cable connecting the three-phase plug which is known as the primary cable and connections inside the power source body. You need to ensure that those connections are tight and in the correct polarity so the machine or power source receives the right power. You need to look at the pins on the three-phase plug and ensure there is no evidence of earthing or arcing. Where the three-phase plug has not been done up correctly, any sparking can damage the power source and create issues with what is effectively known as clean power. The connections on the secondary side or output side should be checked making sure the connections are of the right type and are tight, this ensures you are getting all of the power delivered from the power source into your machine. The connections including both electrode and work return, what is often called earth lead, need to be tight. Loose connections will create heat due to the fact there is resistance. When you get an increase in resistance you get a reduction of power, you overheat the insulators, and can damage the machine. This, in turn, can cause shorting out the output terminals to the case of the machine. This not only is costly; it is dangerous because the machine body or cables can become live. You need to ensure the output leads are of an adequate rating or they will overheat, with heat comes resistance and breaking down of the insulation. G-clamp type clamps on your work return are far better to get a good connection than spring type.

The analogy I often use is ‘a formula one car driver ensure the car is maintained so it lasts the race.’ For the welder, if you do not understand how to maintain a machine you will potentially get poor welds and your arc on time will be reduced as you are constantly fighting against the equipment. Welding can be complex but it doesn’t have to be difficult. Technoweld can create Welding Procedure Specifications for your next Gas Tungsten Arc Welding project. Contact us to find out more about how to ensure you get the best GTAW results, every time.