An arc welding process, plasma arc welding (PAW) is somewhat similar to TIG welding because the arc is created between a focused tungsten electrode and the object. However, the plasma arc can be isolated from the shielding gas envelope by placing the electrode inside the torch’s body.
Instead, plasma is pushed through a fine-bore copper nozzle that constrains the arc.
What is plasma welding?
The welding or central arc burns between the workpiece and tungsten electrode in plasma welding. Alternatively, something referred to as “pilot arc” burns between the tungsten electrode and a rigorously moisture-cooled nozzle with a maximum strength of 3-30 A.
Additionally, a plasma gas (usually argon) is transmitted between the nozzle and the electrode. This slightly forces the pilot arc out of the nozzle so it can be used as an electromagnetic “flash.”
The pilot arc polymerizes the looping path, and with extremely high reliability, the main arc can now be activated without high-frequency contact.
Different physical effects (cooling effect of the nozzle, electromagnetic effects) accomplish the constriction of the arc, typical of plasma welding. The melt bath is shielded by the safe gas supplied between the external protective gas nozzle and the plasma nozzles.
Mixtures of pure argon or methane with hydrogen or helium may be used as safe methane.
Similar to other traditional arc welding methods the plasma arc has a slightly higher energy density and a lower beam separation.
Although the diameter of the TIG arc expands from the tip of the electrode to the workpiece with an angle of 45 °, the diameter of the plasma arc only rises marginally and is shaped like a column.
The working principle of plasma arc welding
In a plasma gene neutral gas atmosphere, the energy input required for welding is maintained by an electric arc. This arc formed between an infusible electrode and the fabricated parts is forced by a nozzle that pneumatically and mechanically constricts it.
- Superb visual dimension
- Decrease in assembly preparation times by excluding bevelling from layers up to 10 mm
- Faible manipulation
- Joint quality: 100 percent X-ray accuracy assured with maximum and frequent penetration
- Respect to the chemical structure of the binding agent
- Decrease of the zone affected by heat due to contraction of the arc
Basics of plasma arc welding (PAW)
An arc welding process, PAW utilizes an electrode of un-consumable tungsten or tungsten alloy, somewhat like GTAW. The main distinction between these two welding processes is that the electrode in PAW is cantilevered in a nozzle which serves to inhibit the arc.
Through the restrictive nozzle, plasma gas becomes ionized and escapes the nozzle at high speed.
The plasma gas on its own is not sufficient to protect the atmosphere from the molten weld stream. Therefore, shielding gas is supplied around the plasma base, just like with GTAW.
The power output for the plasma gas is slightly lower than the shielding gas to reduce turbulence
To maintain uniform spot size and energy density, the tapered form of a gas tungsten arc needs arc voltage control (AVC) or arc length control (ALC) apparatus to be used for automatic welding. In PAW, the restricted arc leads to a higher columnar-shaped arc.
This reduces the impact of variance in arc length on energy density and reduces the requirement for AVC or ALC. The benefit of cutting away an electrode in a nozzle is that contact with the electrode is reduced.
Usually, an electrode will last for the whole shift of production without requiring to be sandblasted.
Another special characteristic of PAW is the way it initiates the arc. Usually, high frequency (HF) current is used to create a pilot arc between the copper nozzle and the electrode.
HF is switched off after the beginning of the pilot arc. The current of the pilot arc is normally fixed at one level or can be set at one of two stages, generally between 2 and 15 amps.
The arc is passed to the job for welding and is a component of the electric circuit. Since the arc is formed before a weld is made, arc welding beginnings tend to be quite accurate.
During the welding, the pilot arc stays on, and the torch is capable of making the next weld without requiring additional HF.
It can be useful for welding in robotic applications where HF electromagnetic noise can interact with automated system controls. A consequence of the pilot arc is that it requires moisture-cooled plasma torches, particularly for minimum-current applications.
The different operating modes of PAW
By varying the flow rate of plasma gas and the bore diameter, the following three modes of operations can be produced.
Microplasma welding
Microplasma is utilized for the thin sheets welding (up to 0.1 mm thickness), as well as for parts of mesh and wire. The syringe-like, straight arc minimizes wandering arches and displacement.
Medium current welding
It is an alternative to traditional TIG when utilized in the melt mode. The benefits are greater infiltration (from higher plasma gas flow), better resistance to substrate contaminants like coatings (the electrode is inside the torch), and improved resistance to differences in the gap between the workpiece and the electrode, without major changes in energy input.
Keyhole welding
Through the increase of the welding current and the flow of plasma gas, a really strong plasma beam is produced that can achieve total immersion in a material, like in beam or laser welding. A keyhole is created throughout welding, which slowly passes via the metal with the molten welded pool moving behind to create the weld bead beneath surface tension factors.
This method can be used for welding thicker material in a single process (up to 10 mm of stainless steel).
The plasma arc is usually handled with a signature power source that is DC, steady current (sagging). Since the special torch system and independent plasma and shielding gas currents are where it derives its specific functional features from, a plasma control console can be connected to a traditional TIG power source.
It also provides intent-built plasma systems.
While HF is used to activate the arc, it is first generated between the plasma nozzle and the electrode. This ‘pilot’ arc is kept inside the torch’s body until it is moved to the workpiece needed for welding.
The pilot arc device guarantees a stable beginning of the arc, and as the pilot arc is retained between welds, it eliminates any need for HF re-ignition, which can trigger electrical disturbances.
The electrode used during the plasma phase is tungsten-2 percent thoria, and copper is the plasma nozzle. The bore diameter of the plasma nozzle is vital and a bore diameter that is too little for the current stage, and the flow rate of plasma gas can cause significant nozzle oxidation or even burning.
For plasma gas, usual gas ratios are argon, with argon or argon plus two to five percent hydrogen for the shielding material. For plasma gas, helium can be utilized.
However, due to its hotness, this decreases the nozzle’s current ranking. The lower mass of helium can also make it harder for keyhole mode.
The combinations of helium argon are utilized on materials such as copper as a shielding gas.
Understanding the different types of plasma arc welding (PAW)
Now that we know the basics of what is plasma welding, we can move onto the different types of PAW used today. The following are the two categories of plasma welding:
Transferred PAW
This PAW system uses DC with direct voltage. Additionally, in this form, the metal may be associated with the + ve terminal and the tungsten electrode may be associated with the – ve terminal.
The arc generates a tungsten electrode, as well as a section of the work. For this kind of process, both arc and plasma travel toward the section of the work, which will increase the technique’s heating power.
This kind of PAW can be utilized for joining strong sheets.
Non-transferred PAW
This PAW process employs DC current with direct polarity. Additionally, in this process, it is possible to link the nozzle to the + ve tip and link the tungsten electrode to the – ve.
The arc produces inside the torch among the tungsten electrode as well as the nozzle, which will boost gas ionization inside the torch. Additionally, the torch must then pass the ionized gas to go further.
This PAW style can be used for joining thin sheets.
These are the two different types of paw arc welding that can be used for welding. You can choose the right PAW method for your applications by considering the process/stage involved in each type of plasma arc welding.
With the types of plasma welding discussed, we can now move onto the advantages and disadvantages of plasma welding.
The advantages and disadvantages of plasma welding
In spite of being utilized in high-probity applications such as aerospace, electronics, automotive, and medical industries, plasma arc welding (or PAW) is frequently ignored in steel manufacturing, aluminum manufacturing, or stainless steel manufacturing, because it is much more complicated and involves a more costly range of tools as opposed to other arc processes.
The explanation for the predicament being ignored is the idea that paw is a little slower relative to other welding methods such as laser beam welding (LBW).
PAW isn’t the slowest of all, though. It is quicker than GTAW (gas tungsten arc welding), which is also known as TIG or inert gas. Additionally, PAW provides minimum-cost welding, where LBW was the first option.
PAW also has its range of pros and cons, including metal rolling, metal bending, laser cutting etc. In truth, PAW is much more sluggish than LBW.
The intensity can touch up to five times of what PAW achieves, based on the laser used in LBW. When you find the welding costs, however, PAW is much more cost-effective than LBW and some other welding methods.
This is why PAW is still utilized in metal processing as a cost-effective alternative to costly LBW, where high quality and high speed are not a requirement. However, PAW has a downside that there is greater thermal input.
It triggers broader welds, and some thermally-affected areas as well as LBW and EBW. This creates more instability, as well as mechanical property damages.
Nevertheless, PAW has an edge over them besides the cost efficiency. It is the resistance of joining gaps as well as the irregularities which ensure the use of PAW in metal manufacture.
Although the arc is confined, the plasma column is considerably greater in diameter as opposed to the beam. Therefore, it is easier to generate fillers with PAW than with LBW or EBW.
Concerning the contrast between PAW and GTAW, PAW has a major drawback which is its difficulty. In fact, as you learn, the value-efficiency of GTAW is higher.
Compared to the conical gas tungsten arc, the slender arc for PAW is less resistant to the joint irregularity. Nevertheless, the nuanced arc of microplasma is easily preserved at a lower current level, thereby giving it a small advantage over GTAW.
PAW has a slightly lower current cap which represents nearly one-tenth of GTAW. This along with the effective start of the arc renders Plasma Arc Welding ideal for many smaller sensitivity applications like the medical and electronics industries.
PAW provides greater-current advantages over GTAW too. This is the reason activating welds in keyhole mode with plasma arc welding could lead to thorough penetration welding in just one pass in the finer material.
PAW removes any need for costly joint preparatory work and eliminates any need for filler material. Additionally, the majority of the material can be welded via DCEN (DC electrode negative) with plasma arc welding.
In spite of being complicated and sluggish, PAW is still used and it is easy to see why.
We discussed above some reasons to use plasma welding as well as some reasons to avoid it. You can decide whether to use plasma welding or not depending on what you want to get out of the welding process.
To make the decision easier for you, we will quickly list down the main advantages and disadvantages of not just plasma welding (PAW), but also plasma arc cutting.
Advantages of plasma arc cutting
The following is a list of the main advantages of using plasma arc cutting:
- Quick automation
- It leaves tiny kerf
- One can cut any metals
- Elevated thickness of 150 mm
- The cut is quicker (approximately 5-10x) than the oxy-fuel
Disadvantages of plasma arc cutting
The following is a list of the main disadvantages of using plasma arc cutting:
- Large initial investment costs
- Larger region affected by heat
- The system also causes burr
- Surfaces are rough
- It creates smoke and noise
- Sharp corners can be difficult to build
Advantages of PAW
The following is a list of the main advantages of using PAW:
- The affected region of heat is smaller compared to GTAW
- Torch architecture allows greater arc power
- The plasma’s high temperature and increased heat intensity allow for keyhole impact and it enables complete infiltration of several joints with single-pass welding
- The increased heat and plasma jet allows for greater speeds of transfer
- This approach ensures freer observation and monitoring of the weld
Disadvantages of PAW
The following is a list of the main disadvantages of PAW:
- The torch is heavy and therefore hand welding is very difficult and needs training
- Compared to EBW and LBW, it generates broader welds and heat-affected areas
- The system generates higher noise in the range of about 100dB
- The plasma welding systems are very expensive; it will, therefore, carry higher starting costs
- It generates UV and Infrarot radiation
- Training and specialization are required to conduct plasma welding
There you have it—the advantages and disadvantages of both plasma arc cutting, and plasma arc welding (PAW). Based on this information, you can decide whether using plasma welding for your specific application is a good idea or not.
Applications of plasma welding
One of the most common questions about plasma welding after ‘what is plasma welding’ is ‘where can plasma welding be used?’ Listed below are the main applications of plasma welding:
- It is used to cover the turbine blade in a particular way
- It can be used in sectors such as marine and aerospace industries
- PAW is used mainly to repair tools, to shape and dye
- This form of welding applies mainly to the electronic industries
- It is used to fit stainless pipes and tubes together
This list is not exhaustive, and there are more applications of plasma welding.
Summary
What is plasma welding? The information provided above is all there is to know about plasma welding.
From the above details, we can infer that the plasma arc welding method is equivalently appropriate for manual, automated applications, in addition to various operations spanning high-volume sheet metal welding to welding of physical kitchen appliances, automated refurbishment of jet engine blades to precision medical equipment welding.