Piet van der Horst

Piet van der Horst

In 1970 Piet made welding his trade en since then he never stopped learning about that trade. By now he is well past his retirement age, but not welding is still not an option for him. It is not just work, it is a passion.

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Gases for welding processes

Inert and active gases

Classification according to the standard In NEN-EN-ISO 14175 standard, gases are divided into main and subgroups. In this article we discuss four main groups of

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Plasma Welding – an overview of all you need to know

Plasma Welding

1. Gas plasma, 2. Nozzle protection, 3. Shield Gas, 4. Electrode, 5. Nozzle constriction, 6. Electric arc

What is Plasma Arc Welding?

In plasma welding, the arc is formed between a pointed tungsten electrode and the workpiece. The electrode is placed within the body of the torch, so the plasma arc can be separated from the shielding gas envelope. Plasma is then forced through a fine-bore copper nozzle which constricts the arc. This results in the arc plasma exiting at very high velocities and reaching temperatures up to 28.000 degrees Celsius. 

Plasma arc welding and TIG welding; Differences and similarities

In plasma arc welding (PAW), the column of a TIG arch is squeezed together by means of a water-cooled copper nozzle. This greatly increases the energy density of the arch compared to the original TIG arch. 

For plasma welding we use a tungsten electrode and the arc is ignited by means of a high-frequency voltage. This is a pilot arc that burns between the tungsten electrode (min) and the plasma nozzle (plus) with a relatively low current  (1 to 15 amperes depending on the size of the plasma torch). The pilot arc is the current conductor for the welding current. When the plasma nozzle approaches the welding spot by a few mm, the plasma arc ignites. 

About temperatures 

The temperature around the tungsten electrode in TIG welding is about 18,000 degrees Celsius. In the plasma chamber, the temperature is 28,000 degrees Celsius and the inert plasma gas is very highly ionised. The core of the plasma arc that touches the workpiece is about 4000 degrees Celsius hotter than the 11,000 degrees Celsius of the TIG arc. There is a bore in the plasma nozzle, which forms the plasma chamber. The size of this bore depends on the strength of the current used for welding. In our Real Life Solutions you will find a more detailed article about TIG temperatures.

Three main types of plasma welding

In plasma welding, we can distinguish three main types. The main difference between the three types is the amperage used.

  1. Microplasma welding, where the current is between 0.02 and 15 amperes.
  2. Plasma welding with the “melt-in technique”, where welding is carried out in the same way as in the TIG process. The amperage is between 15 and 100 amperes.
  3. Plasma welding using the “keyhole technique”. Current levels can vary from 15 to 350 amperes, depending on the thickness of the material. 

Keyhole plasma welding

In keyhole plasma welding, the plasma arc drills a hole in the very tightly closed seam. By moving the plasma torch with a very clean movement, the seam will immediately close again. Keyhole plasma welding can almost exclusively be applied automatically, with currents of up to 350 amperes or higher. It is usually carried out without additives. However, there are also applications with filler materials, such as prop welding, which can be seen in this video

Advantages and disadvantages of plasma welding

If we compare it to TIG welding, there are a number of advantages of plasma welding:

  • Thin materials are easier to weld. The TIG-arc is less stable at the same current levels and the heat input is greater due to the larger weld pool.
  • The penetration is greater. This allows closed T-seams up to about 10 mm thick to be welded. With TIG this is a maximum of 3 mm.
  • The arc is much less sensitive to length variations because the size of the plasma column hardly changes.
  • The tungsten electrode is completely built into the torch, therefore, the chance of tungsten inclusions in the molten pool is almost non-existent. In addition, the service life of the electrode is longer.
  • Due to the fact that the current level in plasma welding is much lower for an equal material thickness, the heat-affected zone (HAZ) is narrower and the deformation smaller.
  • Plasma welding is very easy to automate and robotize.

However, there are a few drawbacks:

  • The complexity of the equipment. Setting the pressure of the plasma gas is very precise. It is a question of the correct adjustment between the shielding gas and the plasma gas, for which the equipment is fitted with two meters. 
  • The accuracy of the pre-processing must be very high, especially for keyhole welding. When the weld is slightly open, the keyhole process will not function. 
  • Due to the large torch, the accessibility of small spaces is less good.
  • The complicated torch has to be maintained very carefully. 
  • In the case of manual plasma welding, the welder’s hand steadiness must be very high, therefore, because of the very narrow welding arc, every movement of the hand is immediately visible.

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