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National Institute of Technology Warangal

PLASMA ARC MACHININGPRESENTED BYL SURESH KUMARMTECHMANUFACTURING ENGINEERING

CONTENTS Introduction About plasma Generation of plasma Process Equipment Parameters influencing PAM performance Process capability Applications Advantages Disadvantages Referrences

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.

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.

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 column2. By exposing gas column to radiation

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.

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 H2,N2, O2 . plasma arc can be used for cutting,

welding, spraying, etc.., fig. Schematic diagram of PAM

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

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.

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 N2 and H2 i the

compressed air. SS,Al, and other non ferrous alloys are cut with Ar, N2 and H2 . gas flow rates are 2 to 11 m3/hr and are controlled by using mass flow

controllers(MFCs)

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

IMAGES OF THE NOZZLE

Air plasma torch Dual gas plasma torch Water injected plasma torch

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.

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.

PARAMETER ASSOCIATED WITH DESIGN AND OPERATION OF THE TORCH

i. OPERATION ASPECTS there are two modes of operation of dc plasma torches: i. Non-transfered mode ii. Transfered mode Turbulent and Laminar mode

DESIGN ASPECT the plasma torch is designed for the maximum thermal output. the following design parameters affect the performance of the torch. 1.cathode size 2.convergence of nozzle 3.nozzle orifice diameter 4.orifice length 5.electrode gap 6.cooling of electrode

GENERAL DESIGN CONSIDERATIONS

large current require larger dia cathode,orifice length and electrode gap. while operating at larger arc voltages, it is preferable to increase the angle of

taper at the cathode tip. while operating at transferred arc anode,flat face cathode is used due to high

arc voltages. edges of constrictions are rounded off to avoid turbulence. non transferred arc modes use long throat lengths, while for transferred arc

mode, the min length is used. no water leakage into the plasma chamber from the cooling chamber is

allowed. oxygen or compounds of oxygen in the plasma gas are detremental to the

cathode.

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

Power supply Shielding Nozzle (a) type: DC (a) type : CO2, H2O Ar,H2 (a) SOD: 6-30mm (b) Voltage: 200V (b) pressure: 11bar (b) diameter:0.80-6.30mm (c) Current: 50-1000A (c) flow rate: 50m3/hr (c) life: 2-5 hrs (d) Power: 2-200KW (d) Material: Zr, HfPrimary gas Cutting parameter (a) Composition:N2:H2=90:10 (a) tool : plasma jet (b) Pressure: 10bar (b) max velocity of plasma jet:500m/s (c) Flow rate: 50 m3/min (c) max temperature: 16000-20000℃ (d) MRR: 150 cm3/min (e) max plate thickness: 200mm (f) cutting speed: 0.1 - 7.5 m/min.

PARAMETERS ASSOCAITED WITH TE PHYSICAL CONFIGURATION OF THE SYSTEM

In PAM, important vaiables are, stand-off distance,angle, depth of cut, speed and feed of the work towards the torch. the feed and depth is determined by material removed. Stand - off distance is the distance between the nozzle and work piece. the effect of SOD are minntain min SOD for better quality cuts since arc divergence is less increased power input is necessary when SOD distance is large. SOD can range from 6.5-76.2mm

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

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.

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 150cm3/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..,

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.

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.

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.