25-Apr-16

PLASMA ARC WELDING Presented by

Khalid Rasheed2k14-me-20Shafqatullah2k14-me-07

INTRODUCTION Plasma are initially employed to cut metals that are

difficult to machine by conventional methods.

Later, plasma arc has been sucessfully used for spraying ,surfacing & welding metals like aluminium, stainless steel, titanium , brass and copper though other conductive metals may be cut as well.

Plasma cutting is a process that cuts through electrically conductive materials by means of an accelerated jet of hot plasma.

THE 4TH STATE OF MATTER• The first three states of matter are

solid, liquid and gas. For the most commonly known substance, water, these states are ice, water and steam. If you add heat energy, the ice will change from a solid to a liquid, and if more heat is added, it will change to a gas (steam). When substantial heat is added to a gas, it will change from gas to plasma, the fourth state of matter.

PLASMA

• Plasma is an electrically conductive gas. The ionization of gases causes the creation of free electrons and positive ions among the gas atoms. When this occurs, the gas becomes electrically conductive with current carrying capabilities. Thus, it becomes a plasma.

REASONS OF PLASMA STATE This change takes place when gases are heated to very

high temperature

The number of collisions between the atoms, either elastic or inelastic increases.

The gas ionises ,so that a portion of atoms are stripped off from outer electrons

The electrons thus produced, in turn colloids with atoms, so that there thermal kinetic energy increases, and a light is emitted from them ,thus producing more number of atoms and electrons.

GENERATION OF PLASMA

First of all the gases are heated to very high temperature of about 16000 degree Celsius so as to generate plasma

This can be achieved by applying a suitable electric field across the gas column.

The gases are then heated by an applied electric field. An igniter supplies the initial electrons ,which accelerated before colliding &

ionizing.

The free electrons get accelerated & causes ionisation & heating of gases.

This process continues till the steady state is obtained.

The actual heating takes place when atoms recombines into molecules.

PLASMA (CUTTING) GAS SELECTION

• Plasma gas is also called the cutting gas. This is the gas that is ionized in the plasma process and exits through the nozzle orifice. Examples of plasma gas are:

• 1.Air 2.Oxygen 3.Nitrogen 4.Argon-Hydrogen

SHIELD GAS SELECTION

• The shield is the secondary gas in the plasma process. It surrounds the arc and is used to help constrict the arc and cool torch. It creates and protects the cutting environment which among other things affects the edge quality. Examples of shielding gas are:

Air CO2 Oxygen-Nitrogen Air-Methane Nitrogen Methane

PRINCIPLE

In this case, the high velocity electrons of are collide with the gas molecules & metal to form ionisation of beam.

The plasma gas is forced through nozzle duct & is made to direct on the work piece to be machined.

Much of the heating takes place in duct at about 1600 degree celcius & metal removal is due to electron bombardment & hot plasma.

MECHANISM OF PLASMA ARC CUTTING

Accurate cuts can be made in stainless steel and non-ferrous metals such as aluminum by plasma arc cutting.

The cuts are made by a high temperature, high velocity gas jet generated by constricting an arc between a tungsten electrode and the component.

The heat from the arc melts the metal and the gas jet removes the molten metal from the cut.

CONTINUED

The arc operates in an inert inner shield, whilst an outer shield provides protection for the cut surface.

Argon, helium, nitrogen and mixtures of these gases are used for both the inner and outer shields.

Plasma arc cutting is characterized by fast cutting speeds and is mainly used in mechanized systems.

The cutting is accompanied by a high noise level which can be reduced by operating the torch under water.

MODES OF OPERATION OF DC TORCH Non Transferred Arc Torch Transferred Arc Torch

25-Apr-16

Non Transferred Arc Torch The DC power source is connected directly across

cathode & anode (nozzle), so that the cathode and nozzle carry same current.

Plasma is in the form of flame.

Useful for spraying ceramic working & chemical synthesis.

The hottest portion does not appear outside the nozzle.

The electrothermal efficiency is about 65% for sheat stabilized torches and 75% for vortex stabilized torches .

Transferred Arc Torch Cathode is connected to negative terminal & anode is connected to

positive terminal of DC power source through a suitable resistor to

limit the current through the nozzle to about 50 amp.

Here arc is struck between tool &work piece.

Once this arc is struck , the pilot flame circuit is disconnected.

Useful for welding , cutting and hard surfacing of metals.

Argon or nitrogen can be used during the operation.

Electrothermal efficiency is increased to 85-90% , the only loss

being at the cathode and the unconnected nozzle.

Advantages of PAM Process It gives faster production rate.

Very hard and brittle metals can be machined.

Small cavities can be machined with good dimensional accuracy.

Disadvantages of PAM Process

Its initial cost is very high. It is uneconomical for bigger cavities to be machined. Inert gas consumption is high.

APPLICATIONS OF PLASMA JETS

• Plasma arc process for cutting aluminum and other nonferrous materials was first introduced in 1955.

• Due to the remarkable results, the process has now been widely accepted by industries for varied applications.

• The major areas of industrial production where plasma jets have successfully been employed are:

• Welding of material like titanium, stainless steel, etc. which are other-wise difficult to weld.

• Plasma arc surfacing.• Plasma arc spraying.