Welding Processes: Plasma Arc Welding

Different types of welding processes 

In this series, we look at the different types of welding processes used by A&G Engineering in its fabrication processes. 

Plasma Arc Welding 

What is plasma in welding? 

Plasma is often called “the fourth state of matter,” along with solid, liquid and gas. 

When energy is added to a liquid causing it to boil, it will change into a gas. Similarly, if enough energy is applied to a gas, the molecules separate into individual atoms and the atoms will lose electrons and become ions. The material now exists in a fourth state of positively charged ions and free electrons called “plasma”. 

One of the unique properties of plasma is that the presence of positively charged particles makes plasma electrically conductive. 

In plasma arc welding, a tungsten rod is used as the electrode, and the electrode is placed within the body of the welding torch. An inert gas passes through the torch and when an electric arc is formed between the electrode and the workpiece, the gas is ionised and forms a plasma arc. The ionised plasma jet acts as the conductor for the arc current, and with the electrode positioned inside the torch, the plasma arc is separated from the shielding gas envelope.  

The plasma is then forced through a fine bore copper nozzle that constricts the arc and causing the plasma to exit the nozzle as a column at high velocities and reaching temperatures up to 28,000 degrees Celsius. The nozzle ensures that the spread of the plasma arc is limited resulting in an arc with higher current density.  

Once the workpiece contacts the high velocity, high temperature and concentrated plasma jet, the joint edges melt and coalesce which requires minimal filler. The resulting high heat concentration enables high-speed welding with little elevation of the welded joint surface and less distortion.   

Applications for plasma arc welding 

Plasma arc welding is commonly used in the marine and aerospace industries, and in the manufacture of tools, dies, and moulds. Plasma arc welding is ideal for welding stainless steel tubes and pipes, and for welding of turbine blades. 

Plasma arc welding: advantages and disadvantages 

Inside the plasma torch, the electrode is protected from outside impurities that would normally attack its hot surface and, as a result, the welding torch normally requires a change only once every 8 hours for most operations. This reduction in electrode change allows for increased productivity and reduced downtime. 

In addition, with a highly concentrated and well controlled arc, that plasma jet can travel at faster speeds and the high heat concentration allows a keyhole effect which provides a complete penetration for single-pass welding of many joints and a smaller heat affected zone. 

As a result, in high production environments, plasma arc welding is very cost effective with savings gained through easy automation, increased productivity, reduced scrap, reduced downtime and fewer electrode changes. 

Despite the good advantages that plasma arc welding offers, its main limitations are that the equipment is expensive requiring a higher initial cost of investment and it requires specialised training to perform. Also, with the arc plasma and high speed of travel, the process generates high noise levels. 

If you have a technical query or want to find out how A&G can help you with your next project, talk to our expert team by calling us on (02) 6964 3422 or email sales@agengineering.com.au with your inquiry. We look forward to hearing from you.