PLASMA ARC WELDING PROCESS

& APPLICATION

PRINCIPLE OF PLASMA ARC WELDING

The Plasma Arc welding process is an advanced mode of welding divided in 3 modes Micro Plasma, Medium

Plasma and Keyhole Plasma. The basic principle of plasma arc welding is an arc is maintained between the

electrode and the work piece in an inert atmosphere to fuse metal in the joint area and produce a molten weld

pool. The similarities between TIG and Plasma welding are readily apparent in that the arc is formed between a

tungsten electrode and the work piece but the uniqueness in plasma arc welding is the operating characteristic of

the plasma arc torch. The electrode is positioned within the body of the torch and the plasma forming gas is

separated from the shielding gas envelope. Thus the emanating plasma is constricted by a fine copper nozzle. The

capacity of the arc is determined by the degree of constriction of the plasma. Some of the Examples for plasma

welded components and plasma weld are show below,

Schematic Plasma welding process

Example for Plasma weld:

Process: Plasma Welding system with wire feed

Material: Stainless Steel 304L

Size: 6mm thick

Examples for Plasma Welded Components

VARIANTS OF PLASMA ARC WELDING

Depending on the properties of constricted plasma the plasma gas flow rate are classified into three different

variants,

Micro Plasma welding

The current range of micro plasma arc varies from 0.1A – 15A

Medium Plasma welding

The current range of medium plasma arc varies from 15A – 50 A.

Keyhole Plasma welding

The current range of keyhole plasma arc is greater than 50A.

PLASMA WELDING EQUIPMENT 

The plasma welding equipment consists of

1. Welding torch

2. Shielding gas cylinder

3. Plasma gas cylinder

4. Power source

5. Water chiller

6. Wire feeder

7. Welding torch

8. Job holder/positioner/automation

Basic circuit for fully automated Plasma Welding equipment

TORCHESMicro plasma torches are available in both manual and machine mountable modes. Medium and Keyhole plasma are mostly automated processes as it is very difficult for a welder to maintain the keyhole as well as move on the weld seam.

The basic Plasma Welding torch consist of

1. Orifice

2. Electrode

3. Shielding gas inlet

4. Plasma gas inlet

5. Water cooling circuit ( medium plasma arc or keyhole plasma arc )

6. Powder/wire inlet

7. Electric supply cable

TYPES OF ORIFICEThe arc is transferred from the electrode to the work piece through the orifice which are of 2 types,

Cylindrical Orifice

Divergent Orifice

Cylindrical orifice:

The diameter of orifice is around 0.6 mm to 2.5mm. Minimum current in case of micro plasma as low as 0.1 A .The

upper limit of current is 180A.

Divergent orifice:

The diameter of orifice is around 3mm – 3.8mm depending on the thickness to be welded. The upper limit of currents are

o For 3mm it is 280A.

o For 3.8mm it is 460A.

GASES  A plasma welding torch is characterized by the use of two gases

Plasma Gas

Shielding Gas

Plasma Gas:

The plasma gases used in this process are, Pure Argon or Argon + 2% Hydrogen for Stainless Steel, Pure Argon on

other metals and Helium.

Shielding Gas:

The plasma gas itself cannot protect the weld pool from contact with other gases hence it is necessary to provide an

extra shielding gas which is fed through outer annulus. The shielding gases used are, Argon or a mixture of argon

and hydrogen is used for protection of stainless steels, Pure argon is used for protection of metals such as titanium,

tantalum, etc, Mixture of argon and carbon dioxide is used for mild and low alloy steel, Pure helium is used for

aluminum alloys.

CHARACTERISTICS OF PLASMA ARC WELDING 

The temperature as high as 5,000 degree celsius and it is possible up to 28,000 degree celsius.

The most significant effect of plasma constriction is that the arc becomes very directional and deep penetration

is possible.

The electrode angle is not very important for effect on penetration but is usually maintained at 30 degree.

The pilot arc system ensures reliable weld starting even under adverse conditions such as long welding cables,

well used electrodes, rust, oil and moisture on components (unclean components will result in a poor weld).

An attractive controlled weld bead reducing cost / wastage of consumables with little or no ripple reducing

time and cost of post weld machining.

ADVANTAGES

It is possible to weld up to 3mm in single pass sheared plates without edge preparation and filler wire.

It is possible to weld up to 6mm in single pass sheared plates without edge preparation and minimum quantity of

filler wire is used as compared to TIG welding.

Concentration and directionality of the arc is much more precise

Complete penetration is possible through opening jet

Quality of welding is much more high

Less distortion because of a better use of the welding energy resulting in minimum heat affected zone (HAZ). This

results in minimum stresses in the component being welded.

Reduction of number of passes which reduces the assembly cost by simplifying preparation and minimizing cleaning

needed during welding

APPLICATIONS

It can be used to weld all Ferrous and non-ferrous alloys

All kind of heat resisting steels.

This welding process can be used to weld titanium, tantalum/niobium

It can also weld nickel, silver, copper, gold alloys, etc.

Aluminum is welded using AC current.

In mass production of pipes the welding is completed by Tri plasma and TIG process.

Arcraft Plasma Equipments (I) Pvt Ltd., 124, Diamond Industrial Estate, Ketkipada, Dahisar (East),

Mumbai- 400068.

INDIA.Ph : 00-91-22-28965890, 28965745, 28963247 Fax: 00-91-22-28966418