Plasma Arc Welding

 Conventional methods are not suitable for machining metals such as cast alloy, Waspaloy and carbides having promising applications in various industries. Also, machining these materials using conventional methods causes the increased machining cost. So, these types of materials in special welding methods are preferred.

Principle:

Plasma is high temperature ionized gas. A plasma is the gas region in which there is practically no resultant charge, i.e., where positive ions and electrons are equal in number. The region is an electrical conductor and it is affected by electric and magnetic fields. When this high temperature plasma passes through the orifice, the proportion of the ionized gas increases and plasma arc welding is formed.

Working:

When the high heat content plasma gas is forced through the torch, an orifice is surrounded by negative tungsten electrode in the form of jet. The plasma cutting force imposes a swirl on the orifice gas flow. The arc is initiated in the beginning by supplying electrical energy between nozzle and tungsten electrode. It will release high energy and heat. This heat is normally between 10,000°C to 30,000°C. This high amount of heat energy is used to weld the metal. Narrow and deep welds can be made using this process at high welding speeds.

1. Types of Plasma Arc Welding

There are two types plasma arc welding used practically. They are as follows:

1. Non-transfer type, and

2. Transferred type.

1. Non-transferred type:

In this type, power is directly connected with the electrode and torch of nozzle. The tungsten is connected to the negative pole (cathode) of a DC supply and the nozzle to the positive pole (anode) as shown in Figure 2.25.

Gas is fed into the nozzle and when an arc is struck between tungsten electrode and nozzle. The gas is ionized in its passage through the arc. Due to the restricted shape of the nozzle orifice, ionization is greatly increased. Also, the gas is issued from the nozzle orifice as a high-temperature, high-velocity plasma jet, cylindrical in shape and of very narrow diameter realizing temperatures up to 10000°C. This type is known as non-transferred plasma. The main advantage of this type is that the spot moves inside the wall and heats the incoming gas and outer layer remains cool. This type of plasma has low thermal efficiency.

The non-transferred plasma arc possesses comparatively less energy density as compared to transferred arc plasma and it is employed for welding applications involving ceramics or metal plating (spraying). High density metal coatings can be produced by this process. A non-transferred arc is initiated by using a high frequency unit in the circuit.

2. Transferred type:

In the transferred type, the restricting orifice is in an inner water-cooled nozzle within which the tungsten electrode is centrally placed. Both work and nozzle are connected to the anode and the tungsten electrode to the cathode of a DC supply as shown in Figure 2.26. Relatively low plasma gas flow (of argon, argon-helium or argon-hydrogen) is necessary to prevent turbulence and disturbance of the weld pool. So, a further supply of argon is fed to the outer shielding nozzle to protect the weld.

It is difficult to initiate the arc initially between workpiece and electrode. For this, the pilot arc is struck between nozzle and electrode. A high-frequency generator unit fed from a separate source from the main supply initiates the pilot arc. For initiating a transferred arc, current limiting resistor is put in the circuit which permits a flow of about 50 amp between nozzle and electrode. A pilot arc is established between electrode and nozzle. As the pilot arc touches the workpiece, main current starts flowing between electrode and job. Thus, it ignites the transferred arc. The arc is transferred from electrode to work via the plasma. The pilot arc initiating unit gets disconnected and pilot arc extinguishes as soon as the arc between electrode and job is started. The temperature of a constricted plasma arc may be in the order of 8000°C to 25000°C.

To shape the arc, two auxiliary gas passages on each side of the main orifice may be included in the nozzle design. The flow of cooler gas through these orifices squeezes the circular pattern of the jet into oval form to give a narrower heat affected zone and increased welding speed. Since the tungsten electrode is well inside the nozzle (about 3 mm) in plasma welding, tungsten contamination by touchdown or by filler rod is avoided to make the welding process easier.

The base metals welded by plasma arc welding are as follows:

1. Stainless steels

2. Titanium alloys

3. Carbon and low alloy steels

4. Copper alloys

5. Aluminium alloys.

The types of joint which are made by plasma arc welding are as follows:

1. Filler welds

2. T-Welds

3. Grooves [Single groove (or) 'V' groove]

4. Square groove.

2. Advantages, Limitations and Applications of Plasma Arc Welding 

Advantages:

1. Penetration is uniform.

2. It has deeper penetration capabilities and produces a narrow weld.

3. Arc stability is good.

4. Fully penetrated keyholes can be obtained.

5. The plasma arc is more stable and it is not as easily deflected to the closest point

of base metal. Hence, high accuracy weld can be produced.

6. High speed weld can be obtained.

7. The production rate is high.

8. Greater variation in joint alignment is possible with plasma arc welding.

Limitations:

1. Huge noise occurs during welding.

2. It is limited to high thickness applications. 

3. Frequent orifice replacement is necessary.

4. Cost of the equipment is expensive.

5. Ultraviolet radiations can affect human body.

6. More skilled operator is needed than GTAW process.

7. The torch is more delicate and complex than a TIG torch.

8. Gas consumption is high.

Applications:

1. It is used in aerospace applications.

2. It is used for melting high melting point metals.

3. It is used for welding titanium plates and nickel alloys.

4. It is used for tube mill applications.