Machine Tools And Devices For Special Technologies Plasma machining Slovak University of Technology Faculty of Material Science and Technology in Trnava
Processing by plasma Plasma is considering like fourth state of substance, Plasma is high ionised gas: Free electrons, Positive electrons, Neutral atoms, Plasma is electric conductive: Partly ionised (10 – °C), Completely ionised (around °C)
Properties of plasma Electric conductivity, Final electric charge = 0, Movement of elements is possible to directed by magnetic field, Plasma is possible to stop by magnetic „wall“ possible to cover in magnetic container
Becoming of plasma Nuclear reaction, Electron beam Electric charbe. In technical praxis has meaning only plasma originated by electric charge.
Plasma origin Voltage between electrodes – ionisation of gas between electrodes – electric charge, Charge can: Turn off, Glow discharge, Permanent arc discharge We need permanent arc discharge.
Permanent arc discharge Arc discharge is needed to stabilise by cooling. Stabilisation: By shape of jet of plasma burner, By flowing gas, By water, In intensity cooling is diameter of arc smaller and his temperature is higher.
Temperature of plasma arc Temperature depend on using plasma gas. nitride – round °C helium – round °C
Sing of plasma for metal processing cutting, welding, Coating of surface layers, Hard surfacing, Machining.
Cutting by plasma Cutted metal is melted by high temperature of plasma arc and is blowing out by plasma gas. Plasma is created: Water, Mixture of gases (most often Ar, H 2 ), Mixture water and gas
Plasma cutting – stability of plasma beam Stability of plasma beam is achieved by stabilisation gas. Depended arc – cut material is anode. It secure asked stability in small flash of gas too.
Plasma cutting – materials Aluminium – soft cutting of surface, stabilisation by water, thickness of cutting till 100 mm. Copper – cutting is difficult, with thickness above 25 mm the material is taking up to the bottom part. Austenitic steels – soft cutting surface, thickness till 100 mm. Beams of plates – needed is to strong constringe because they are welded.
Deposition of surface layers by plasma Material is heated fast and partially is melted. By high speed (above 200 m/sec) is shed to support. Here is used independence arc.
Deposition of surface layers by plasma These coats increased resistance of base material against: High temperatures, Oxidation and corrosion in high temperatures too, Thermal fatigue, Mechanical wear by erosion and abrasion.
Deposition of surface layers by plasma Size of the powder grain is necessary to adapt to power of burner. Big grains are not melted and small can steamed. High temperature of plasma aided these chemical reactions: oxidation – safety by inert gas, reduction – safety by inert gas, Thermal reduction – short staying in burner, high speed of plasma flow.
Deposition of surface layers by plasma Sprayed coat is porous. Porous can be till 15%, size of poruses 20 – 200 μm. Melted elements gain spherical shape. When they are more melted and when the speed of their impact is higher then the structure of coat is denser.
Deposition of surface layers by plasma Adhesion is decreased with thickness of coating. The layers are thick 0,1 till 2 mm. In thick coatings is several layers – increased inside strain, layers leafing.
Plasma welding High stabile welding process, Fine forming of weld, mainly weld root, Welding without supporting of weld root till 10 mm of thickness, It is not become blowing out of welding pool, Jet has very small section, Only 10 – 30% of plasma gas is plasma, remainder stabilised and positioning arc, Arc is burning by high frequency
Plasma welding Plasma gas is mixture Ar + H 2, Ar + He, or clean Ar. Safety gas is usually Ar. Advantages: Simply preparation of connection, Good formation of weld, Possibility of process mechanisation, High productivity.
Plasma welding Plasma welding is used for: Connecting of plates with small and medium thicknesses – one layer weld (steel plates to 6 mm, without adjusting of rims), Welding of chrome-nickel steels, Welding of Ti and his alloys, Welding of copper and her alloys, Welding of aluminium and his alloys, Welding of nickel and his alloys, Micro plasma welding – thin plates and wires
Plasma hard surfacing Powder additional (hard surfacing) material – is not possible to do the wire of these material. Temperature of plasma arc till to K. Output speed of plasma gas more than 1000 m/sec. Base material is melted and become metallurgic mixturing with liquid additional material. Is needed to use preheating of the base material.
Plasma hard surfacing
Plasma hard surfacing – MIG Additional material is a wire that is one electrode and next is second (W, Cu) cooling electrode. Arc burning between electrodes melting hard surfacing additional material.
Plasma machining Machined material is melted and evaporated. Plasma gas is N 2, H 2, Ar, air and their mixture. Thermal inluenced area under 1 mm. Surface quality and accuracy answered like roughing. It is used for hard machining steels, coppers, aluminium and etc.
Plasma machining High rate of removing (10 x more than in classical machining of heat-resisting steels). Here is originated smoke, dust, UV emission – cutting under the water.
Machining by straight plasma beam
Machining by additional plasma beam
Devices for plasma beam processing Devices have next main parts: Source of electric energy – insuring the electric energy for burning of arc, High frequency ionisator – burn on the arc, Direct box – directed all process, Plasma burner, Container of plasma gases – get to burner the plasma gas.
Plasma burner Plasma burner is cooling by cooling liquid (water), jet must by strip the insulation from other parts. Function of burner: Input of electricity to electrode, Input of plasma, focusation and safety gas, Shaping and directing of arc, Positioning of electrodes.
Plasma burner Construction of plasma burners: With gas stabilisation, With jetting of water. Part where is the plasma originated - plasmatron. Dividing of plasmatrons: transfer (with transport of arc), notransfer (without transport of arc), combinated
Burner with gas stabilisation
Burner with water jetting
No transfer and transfer plasmatron
Plasma hard surfacing automat
Plasma cutting machine tool