MATERIAL PROCESSING BY PLASMA TECHNOLOGY

Source: MATERIAL PROCESSING BY PLASMA TECHNOLOGY. By VISHWANATH H M NATIONAL INSTITUTE OF TECHONOLOGY KARNATAKA-SURATHKAL. Seminar on Dept of Metallurgical And Materials Engineering

CONTENTS :
CONTENTS Introduction Properties of plasma Current applications Future applications Conclusions References

INTRODUCTION :
INTRODUCTION Plasma is considered to be 4th state of matter next to solid, liquid, gas. “A gas in which an appreciable number of atoms or molecules are ionized is called Plasma”. It was first identified in Crookes tube by Sir William Crookes as Radiant Matter in 1879. Later in 1923 it was named as PLASMA by an American Physicist Longmuir. For him it resembled to be as Blood Plasma. He says, “In the Crookes tube experiment, Except near the electrodes, where there are sheaths containing very few electrons, the ionized gas contains ions and electrons in about equal numbers so that the resultant space charge is very small. We shall use the name plasma to describe this region containing balanced charges of ions and electrons.

PROPERTIES OF PLASMA :
PROPERTIES OF PLASMA Plasma is the normal state of matter at temperatures of the order of 10000K or even more. Sun and stars are condensed high temperature plasmas. The ionosphere of earth also contains plasma. Plasma is mixture of electrons, ions, neutral atoms and molecules having different kinetic energies. Plasma is electrically neutral. Electrons in it have highest energy and molecules the least. The presence of a non negligible number of charge carriers makes the plasma electrically conductive so that it responds strongly to electric fields. If an magnetic field is applied to plasma, it behaves as a diamagnetic medium having a small but negative susceptibility. radio waves are reflected by plasma as if it were a mirror.

Plasma Categories :
Plasma Categories According to the temperature of plasma:

CURRENT APPLICATIONS :
CURRENT APPLICATIONS Plasma can be utilized in 2 ways; as a heat source and as a carrier medium. In material processing plasma is used as both as a heat source and carrier medium. Current applications in material processing include welding, cutting, spraying, surface heat treatment, melting, sintering etc.
Field of Applications
variable polarity plasma arc is used electrode is +ve for short time(ms) and –ve for long time(ms) during former there will be cleaning action which removes the oxides from work piece as heavy ions strike the work piece and during the later there will be penetration. PLASMA WELDING Plasma beam is used for fusion welding process. Plasma arc welding is a gas shielding process. Done with or without filler wire addition. Coalescence of metals is achieved through heat transferred by arc created between tungsten electrode and the workpiece. The arc is constricted in a copper alloy nozzle orifice to form a highly collimated arc column

Advantages: Automation is possible Low currents are used Minimum human exposure is involved Ozone emission low. Disadvantages: Complicated torch design High maintenance cost Protection from electric shocks is required Electromagnetic waves hazards. Used for In welding titanium and its alloys. Corrosion resistance will not be affected, hence used in welding stainless steel alloys.`

PLASMA CUTTING It is an erosion process. It utilizes a constricted arc in the form of a high velocity jet of ionized gas to melt the material. An inert gas is focused under pressure through a small orifice infront of the cutting torch connected to the DC power supply. In the torch a portion of inert gas is ionized by the electric discharge from the power source. The arc is created between a negative tungsten electrode electrode in the torch through which the gas flows to the work piece. Plasma arcs are extremely hot and are in the range of 15,000 degrees Celsius.

Hand-held torches can usually cut up to 2 in (48 mm) thick steel plate, and stronger computer-controlled torches can pierce and cut steel up to 12 inches (300 mm) thick. Advantages Very high quality cuts are obtained High speed cuttings can be done Automation is possible Process is applicable to all metal and alloys Disadvantages Hazardous fumes and gases, high level of noise. Proper clothing, helmets etc is very much essential.

PLASMA SPRAYING Molten metallic or non metallic material is sprayed on to a prepared substrate to form a coating. The sprayed material is in the powder form. It is propelled by a stream of ionized gas on to the substrate. The particles striking the surface gets flattened to form thin platelets. The substrate surface which can be kept below 473K eliminating metallurgical changes in base material. The temperature is as high as 1100 deg cel. The powder size is 10-44 microns for free flow. Spherical powders are best. Powders of Al. Ni, Cr, WC-CO are sprayed by this technique.

Thickness of coating is 0.0025 - 0.075mm. The hardness of coating is 150 – 350 VHN. This technique is used in shafts, liners, pistons, valves, cast metal rams. Advantages Oxidation of powder is minimal Highly dense coating are produced Mechanical and metallurgical properties are superior due to high temperature and high velocity as well. Decomposition of powder is minimised. Disadvantages Very high temperatures are required Process is noisy Radiation hazards and toxic fumes, high volume of waste products.

PLASMA HARD FACING Plasma hard facing is a form of surfacing that is applied for the purpose of reducing wear, abrasion, impact erosion, galling and cavitations. Build up is used to restore the damaged component to the required original dimensions but not to resist wear so as to further add hard and wear resistant layers. It is to achieve desired property not to intrinsic to the base metal of component. The powder is introduced into a combined plasma stream to form a molten pool on the work piece. Advantages. Deposit thickness varies from 0.64-3.2 mm in a single pass. Rate of deposition is up to 4.5 kg/hr to 18 kg/hr.
Disadvantages Costly equipments Only straight line cylindrical goemetry operations. Pre heating is required in case of hard alloys.

PLASMA MELTING 10 kw Plasma furnaces are routinely used in industries for recycling Ni, stainless steel and tool steel scrap. Primary melting can also be done. Here DC power is used. Tungsten electrode is cathode, anode is lining. High temperature and high rate of melting can be achieved. Fine powder can also be used as charge. Reaction rates are faster.
PLASMA SURFACE HEAT TREATMENT Plasma carburizing, nitriding and nitrocarburising can be done to get a hard and wear resistant surface with a tough core. These are clean and more environment friendly than conventional carburizing, nitriding, nitrocarburising. These processes require lower temperatures. PLASMA CARBURISING It is a diffusion process in which surface has a higher carbon content of 1.2% than the interior of 0.2%. The change in composition from surface to interior is gradual. it is done at a temperature high enough for steels to exists in austenite phase. Therefore diffusion rates are higher. Active nascent carbon is formed directly from CH4 by a glow discharge plasma.

It is done in oxygen free vaccum at higher temperatures compared to conventional carburizing.
PLASMA NITRIDING It is a method of surface hardening using glow discharge to introduce nascent nitrogen into surface of the work piece Nascent nitrogen further diffuses into the interior of the component. The nitrogen ions are accelerated to impinge on the surface of the component. The mechanism used to generate nascent nitrogen is different from conventional gas nitriding. A mixture of 75%H and 25%N is used to get Fe4N.
Advantages of plasma surface treatment no toxic fumes or waste products are generated. No risk of explosion exsists No heat dust or noise occurs. Short times are required for the process Consumption of energy and gas are less High efficiency and excellent quality Case thickness can be controlled Lower temperatures Disadvantages Costly and complicated equipment Maintenance is costly Specialized training for personnel Positioning of the work piece to properly expose the right area to the ion beam

FUTURISTIC APPLICATIONS :
FUTURISTIC APPLICATIONS Thermonuclear fusion, Magneto hydro dynamic generator and plasma rocket engines and many are some of the futuristic applications of plasma. In thermo nuclear fusion high temperature of several million degrees is required. In MHD, efficiency is more for convention of thermal energy to electrical energy, but temperatures slightly low compared to nuclear fusion process. In plasma engines the weight of the fuel required is reduced due to very high velocity. This enhances the maneuverability of spaceship in mid air as weight of fuel to be carried is less.

PLASMA ENGINE Injector consists of 2 co axial metal cylinders. The cylinders are fed by a capacitor bank, which serves as a reservoir of electrical energy. A definite amount of gas is let into the space between the cylinders through a fast acting inlet valve at a time. Before the gas succeeds in expanding into the available space, the high voltage from the capacitor bank is applied to the electrodes and plasma is produced as a result of electrical discharge. A current flows in the radial direction and the interaction of the current with the magnetic field due to this it gives rise to an electrodynamic pressure which accelerates the plasma along the injector. Equation governing is IH/C=ma I- current m-mass of plasma a- accleration c-velocity of light H-magnetic field.

Larger the current passing through the plasma and smaller the mass of the gas and high velocity exhibited at exit of injection. The injector will be pushed in opposite direction with equal velocity w.r.t. equation, m1v1=m2v2=thrust m1- rocket mass m2-plasma mass v1-rocket velocity v2-plasma velocity Rate of energy consumption is large in plasma engine and is given by E=0.5 m v2

TOKOMAK NUCLEAR FUSION REACTOR The chamber itself is usually of the two layer kind. Plasma is produced inside a thin corrugated stainless shell 0.2-0.3mm thick. This thin inner chamber is known as liner and is inserted into a thick outer copper envelope. The space between the liner and the outer envelope is evacuated to a very low pressure. This enables one to maintain clean vaccum condition since the liner can be baked by passing a current through it. And thereby degassing it. The biggest of such Tokomak installation developed in Russia can produce a current of 2.5 to 3*105 amps with a stabilizing longitudinal magnetic field of 40000 oe. Such an installation, plasma filament can exist for 0.005-0.02sec. A maximum temperature of 20 lakhs deg.cel. can be achieved so far.

CONCLUSIONS :
CONCLUSIONS Plasma technology is widely used in metallurgy and materials processing for welding, cutting, spraying, sintering, melting, surface treatment etc. It holds a great assurance for the future with plasma engines, electricity production, nuclear reactors economically on a large scale.

REFERENCES :
1. Plasma Technology and Its Applications in Material Processing-K.Srinivasan-IIM METAL NEWS,Vol 12 No.2,April 2009.
2. ASM Hand Book,1993 Vol 6,Welding, Brazing, ASM Materials Park, Ohio.
3. Heat Treatment of Steels-R.C.Sharma-New Age Internationals, New Delhi-1996.
4. Introduction to Electrometallurgy-Sharan and Narain, Standard Publishers and distributors, Delhi-1999.