Plasma welding: What is it?
In plasma metal inert gas welding, a plasma gas serves as the heat source. This is ionised in the plasma torch by high-frequency pulses, thereby igniting an auxiliary arc (pilot arc). This burns between the tungsten electrode and the nozzle and ionises the gas column between the nozzle and the workpiece. This enables contactless ignition of the main arc, which burns between the electrode and the workpiece. The water cooling of the nozzle causes the plasma to constrict into an almost cylindrical gas column. This results in a higher energy density than when welding with the TIG process. The hardness of the plasma arc can be influenced by the choice of plasma nozzle and the amount of plasma gas. A large nozzle diameter in combination with a low gas flow results in a soft arc.
The current strength of the pilot arc is usually between three and 30 amps. The plasma gas pushes it slightly out of the plasma nozzle so that it can be seen as a bright point of light. The gas supplied between the outer shielding gas nozzle and the plasma nozzle is responsible for protecting the molten pool. This gas surrounds the plasma arc and keeps atmospheric influences at bay.
The right plasma welding process for every sheet thickness
Tungsten plasma welding is used in three different process variants:
Microplasma welding
The range of applications for this technique extends from welding 0.01 millimetre thick foils to contacting and welding wire mesh and welding very thin sheets. The welding current starts at just 0.5 amps.
Punching technique
This welding process is used to weld material thicknesses of up to approximately three millimetres. The material is only melted, but not pierced.
Punching technique
This method is used for material thicknesses of three millimetres and above. The plasma jet pierces the workpiece completely and forms the so-called welding eyelet. The arc guides the metal melted in the welding direction around the welding eye, where it solidifies as a solid weld seam. The surface tension prevents the melt from falling through. At the end of the welding process, the welding current and plasma gas pressure are reduced, causing the melt to close the piercing hole.
Which gases are used in plasma welding?
Like TIG welding, plasma welding is only possible with gases. Two gases are required: the plasma gas (centre gas) and the shielding gas (outer gas).
Plasma gas
Argon (Ar) is almost always used as the plasma or centre gas because it is easily ionisable and therefore achieves a high degree of ionisation. In some cases, gases are also mixed during plasma welding, for example Ar with hydrogen (for chromium-nickel steels and nickel-based alloys) or Ar with helium (for aluminium, zirconium and titanium), in order to increase heat transfer and achieve higher welding speeds. Depending on the type of torch and welding current, the required flow rate is between 15 and 35 litres per minute.
Shielding gas
Argon or an argon-hydrogen mixture is also generally used as a shielding or outer gas. When welding unalloyed and low-alloy steels, an active mixed gas consisting of carbon dioxide or oxygen and argon can also be used. In most cases, both gases (centre gas and outer gas) are the same in plasma welding. The required shielding gas volume is between three and twelve litres per minute, depending on the burner type and welding current.
What are the advantages of plasma welding?
In direct comparison to TIG welding, plasma metal inert gas welding scores points with higher speeds, a more stable burning arc and a longer service life for the tungsten electrode. The heat is applied in a more concentrated manner, enabling deeper penetration. In addition, the pilot arc helps the welder to find the correct starting point for the weld seam more quickly and easily during manual plasma welding. The seams are of very high quality, low in pores and free of spatter. There is also less distortion. Although plasma metal inert gas welding is not quite as fast as laser welding, it offers advantages over laser welding in terms of gap bridging, positioning tolerances, and investment and operating costs.
Plasma welding with gases from the global market leader
Air Liquide is your reliable partner for standard gases and custom gas mixtures. When it comes to plasma welding of unalloyed steels, ARCAL Prime is the right choice as a plasma and shielding gas. For non-ferrous metals, we recommend ARCAL Prime as the plasma gas and ARCAL 31 N as the shielding gas. For stainless steels, it is best to use a combination of ARCAL Prime and ARCAL 15. The latter enables oxide-free seam surfaces when welding stainless steel.
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