MICRO PLASMA ARC WELDING SYSTEM 
The plasma welding process was first introduced to the welding industry about 45 years back. In micro plasma welding the current range is from 0.1Amps to 50Amps which makes it most suitable welding process for very thin sheet welding application such as bellows manufacturing, jewellery manufacturing, thermo couple junction welding, bulk filament welding & surgical instruments. 

HOW PLASMA WELDING MACHINE WORKS 
A plasma gas (normally argon) is heated to extremely high temp. between the copper nozzle & tungsten electrode & is ionized so that it becomes conductive media. This plasma arc is constricted by the construction of the nozzle & due to the pressure of inert argon gas it comes out of the nozzle orifice. This is known as a pilot arc & it remains in the torch until the machine is in ON mode.

When the torch is taken to the work piece & the pilot arc is touched to the job this arc is transferred to the metal to be welded & the main arc is established between the tungsten electrode & work piece. During the entire welding process the pilot arc also remains at the background which gives the arc stability during the plasma welding process. The torch also uses the secondary gas i.e. argon, argon + hydrogen or helium which assist in shielding the molten weld puddle, thus minimizing the oxidation of weld joint.  

PLASMA WELDING SYSTEM / KEY HOLE PLASMA WELDING 
Normally the welding current range for plasma welding or key hole plasma welding starts at 50Amps or above. At higher currents because of the constructed arc the plasma arc is more stiffer. Plasma gas flow rate can be increased to improve the weld pull penetration. Normally the Tig welding arc flares at the job but plasma welding arc remains columnar which gives more welding speed, low distortion & better penetration as compared to Tig welding process. In key hole plasma welding process the plasma blows a hole through the joint or plate. Behind the hole the molten metal flows together to fill the hole. Gravity, surface tension & gas pressure from the shielding gas causes this to take place. By the plasma key hole process we can weld I-butt joints in one single parts up to the plate thickness of 8mm.

This process has a minimum distortion due to the higher welding speed. 
 

ADVANTAGES OF PLASMA WELDING MACHINES 

• The plasma welding process is more precise than the Tig welding process as the plasma arc is columnar & stable.
• The plasma arc is stable even at 0.1Amps.
• The arc transferred takes place with without high frequency.
• Protected electrode offers longer electrode life as compared to Tig welding process.
• Presence of continuous pilot arc gives the main arc consistence & a gentle arc striking.
• Arc energy density is almost three times that of Tig welding which leads to the higher welding speeds, better penetration & less distortion

 

PLASMA WELDING PROCESS FOR TUBE MILL APPLICATION 
In tube mill manufacturing by the process of continuous roll forming & welding, the cost of the product is directly related to the production speed. To achieve higher welding speeds & at the same time maintaining the weld quality is possible by using Tig Plasma Tig i.e. TPT system.

IN this system the first torch is of Ti welding & it is used for pre heating. The middle torch is of plasma welding & it is used for welding, while third torch is of Tig welding which used for finishing weld joints.

By using the plasma welding process, he tube mill speed is substantially increased as plasma arc density is more than Tig welding arc. This also gives a smaller weld width.

The tube mill welding speed depends upon the following factors

• Strip material composition & its weld ability
• Material surface contamination
• Shielding gas
• Strip edge quality
• Pulse frequency. By increasing the pulse frequency to about 10KHZ the arc pressure can be increased to 4 times that of the steady DC ARC