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Plasma Arc Machining

Presented by:

KARAMJIT SINGH

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INTRODUCTION

Plasma arc machining is a material removal process.

➢ The material removal takes place by directing a high velocity jet of high temperature (11,000-30,000℃) ionized gas on work piece.

➢ Narrow plasma jet melts the workpiece material in its path.

➢ With the high temperature involved, can be used for any

material those resistant to oxy-fuel gas cutting.

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WHAT IS PLASMA...?

Mixture of free electrons, positively charged ions and neutral atoms.

➢ Obtained by heating gas at high temperature so that it is partially ionized.

➢ The central zone of plasma reaches a temperature of 15,000℃and it is completely ionized.

➢ Heating of gas takes place in restricted

region of the nozzle duct, resulting in high-velocity

gas exit.

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GENERATION OF PLASMA

Heating the gas to high temp to ionize the gas is popular method for generating plasma. this can be done by two methods....

1. By applying suitable electric field across the gas column

2. By exposing gas column to radiation

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When gases are heated the following can occur :

(i). elastic and inelstic collisns between the atoms

(ii). ionization of gases takes place resulting in production of electrons and ions

(iii). electrons collide with atoms, create high temp and

thermal KE of atoms, further collision produce ions

and electrons. thus, new matter which is called

plasma is characterized by is ability to conduct

electricity due to presence of free charges is

produced.

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PROCESS

➢ Gases are heated and charged to plasma state.

➢ Plasma state is the superheated and

electrically ionized gases at approximately 5000℃

➢ These gases are directed on the w/p in the form of high velocity stream.

➢ gases Used H 2 ,N 2 , O 2 .

➢ plasma arc can be used for cutting, welding, spraying, etc..,

fig. Schematic diagram of PAM

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EQUIPMENT

Plasma arc machining consists of

1. Power supply (1000A, 200V DC) 2. Gas supply (1.4MPa)

3. Cooling water system

4. control console and plasma torch

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POWER SUPPLY

➢ Uses DC power supply.

➢ The power supply should have saturation current, beyond which the current should not rise even for short circuit.

➢ for easy ignition and maintenanceof arc,OCV must be higher than load value.

➢ normally, DC motor-generator sets or trnsductor controlled

rectifier units with saturable core reactors are used.

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GAS SUPPLY

➢ Any gas that should not attack the tungsten electrode or the work piece.

➢ The flowing pressure can be upto 1.4MPa results in plasma velocity of 100 m/s.

➢ higher the gas flow rate higher the momentum of the plasma jet.

➢ higher flow rate also helps to remove the molten mterial from the molting zone.

➢ carbon alloy and cast steels are cut with mixture of N

2

and H

2

i the compressed air.

➢ SS,Al, and other non ferrous alloys are cut with Ar, N

2

and H

2

.

➢ gas flow rates are 2 to 11 m

3

/hr and are controlled by using mass flow

controllers(MFCs)

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PLASMA TORCH

➢ In PAM, electric arc constricts from nozzle, generates basic plasma jet.

➢ Instead of diverging into open arc, The nozzle constricts arc into a small cross section.

the plasma arc are classified as.., 1. Air plasma torch

2. Dual gas plasma torch

(plasma gas,shielding gas)

3. Oxyen injected plasma torch

4. Water injected plasma torch

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IMAGES OF THE NOZZLE

Air plasma torch Dual gas plasma torch Water injected plasma torch

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METHODS TO STABILIZE ARCS

✓ Plasma torches transform electrical

energy into thermal energy in the form of the plasma jet.

✓ it is important to stabilize the electric arc which is burns in constricted space and frcedd away by gas or liquid.

✓ If liquid is used vapour is surrounds the arc column.

✓ Acc. to stabilizing medium, therre is gas or liquid stabilized plasma torches.

i. Wall or sheath stabilization.

ii. vortex stabilization, and

iii. Liquid sabilized plasma torches.

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PARAMETERS INFLUENCING PAM PERFORMANCE

Parameters associated with,

i. Design and operation of the torch.

ii. The physical configuration of the set-up

iii. The environment in whch work is performed.

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➢ for successful utilization of PAM it is necessary to analyse the following process parameters..,

Power supply Shielding Nozzle

(a) type: DC (a) type : CO 2 , H 2 O Ar,H 2 (a) SOD: 6-30mm

(b) Voltage: 200V (b) pressure: 11bar (b) diameter:0.80-6.30mm (c) Current: 50-1000A (c) flow rate: 50m 3 /hr (c) life: 2-5 hrs

(d) Power: 2-200KW (d) Material: Zr, Hf

Primary gas Cutting parameter

(a) Composition:N 2 :H 2 =90:10 (a) tool : plasma jet

(b) Pressure: 10bar (b) max velocity of plasma jet:500m/s (c) Flow rate: 50 m 3 /min (c) max temperature: 16000-20000℃

(d) MRR: 150 cm 3 /min

(e) max plate thickness: 200mm

(f) cutting speed: 0.1 - 7.5 m/min.

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CUTTING SPEED

➢ The speed depends on the thickness of the material to be machined.

➢ thick materials require lower speed than thin materials.

➢ the typical values of cutting speed are..,

Material Thickness Arc Current Cutting speed (mm/min)

plasma arc cutting speeds

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parameters associated with environment in which work is performed

➢ the protective atmosphere has created to reduce the oxidationof exposed high temp machined surface

➢ the deflection of the arc should be kept min.

➢ the spread of the plasma column should be uniform on

impingement area.

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Process capabilities

➢ carbon steel plates thicker than 50 mm can be cut 10 times faster than oxy- fuel cutting.

➢ Accuracy of 0.8mm upto 35mm plates, 3mm upto 150mm plates.

➢ sharp radius upto 4 mm is possible but increases upto 38mm.

➢ MRR upto 150cm 3 /min is possible kerf angle 2° to 7° with the normal cutting condition, 1° to 2° with special arrangement.

➢ smoother surface can be achieved compared to oxy-acetylene flame.

➢ Mg,Ti,Cu,Ni znd alloys of copper and nickel can be cut with PAM.

➢ It is used for profile cutting of many metals like mild steel, alloy steels,

titanium etc..,

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Applications

➢ PAM can be used to make cuts in electrically conductive metals.

➢ PAM is used for cutting grooves in the hardened shaft.

➢ PAM is used for underwater cutting.

➢ plasma arc can be used for depositing filler material on the on the surface or corroded or worn out part to obtain desired

properties like corrosion resistance,wear resistance, anti friction properties.

➢ PAM Is used for profile cutting stainless steel, Al, Ti, other non-

ferrous material.

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Advantages

➢ used for cutting ferrous and non ferrous materials at high speeds.

➢ profile cutting of SST and titanium is posible.

➢ no contact between the tool and the work piece. hence simple work holding devices are enough.

➢ cutting rate of all materials are high.

➢ equally effective regardless of hardness of material and other propertiesof materials.

➢ can be automated easily.

➢ PAM can machine exotic metals at high rates.

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Disadvantages

➢ Burr is often produced.

➢ Taper on the work piece may occur.

➢ process alters the surface metullurgically. it produces hard and uneven surfaces.

➢ smoke and noise is produced duringthis process. hence eyes and nose shielding is required.

➢ Sharp corners are difficult to make using PAM.

➢ the process produces HAZ. HAZ can cause premature

failure of tHe component.

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Electro-chemical Machining

PAM PAM

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THANK YOU

References

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