Plasma Arc Technology

Plasma technology developed taking inspiration from TIG welding: adding gas constriction – reducing the

orifice diameter to ≈ 5 mm – it was possible to reach a very high concentrations of energy through Joule

effect, obtaining T ≈ 20000 K. Keeping the same current it was possible to reach higher voltages. The effect

of constriction through the nozzle is increased by the presence of a tubular layer of not-ionized gas whose

motion maintains separated the arc from the nozzle periphery. This phenomenon is called nozzle-clogging

and allow to distinguish two regions inside the nozzle:

1) Internal hotter region, with higher current conduction capacity;

2) External colder region, responsible of mass transport.

The layers of cold gas, because of their greater intensity and weak ionization, further constrict the jet.

This constriction is responsible of mechanical action: the back-pressure generated by the upstream nozzle-

clogging phenomenon accelerates the plasma, resulting into high pressure in the

stagnation point (anodic surface) which allows material removal.

2 Stand Off Distance: distance between the workpiece surface and the nozzle. It must be low enough to transfer the

pilot arc from the torch to the surface but large enough to avoid short voltages and high damages.

3 Phenomenon for which a dielectric material becomes a conductor. Over a certain threshold voltage, the behaviour of

the material changes suddenly: current intensity increases and a light emission occurs – electronic emission is able to

auto-sustain the discharge. For atmospheric P and low electrical resistance, the pilot arc starts. 2

Dry torches with W cathode and N plasma was for years the dominant technology. Unluckily, along the

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propagation axis inside the material, the plasma lost energy density, creating top edges rounding at the top

edge and burrs at the bottom. To avoid this, the double arc solution was adopted:

an increase of absolute T and its more uniform radial profile were achieved. To

limit burrs formation and improve cutting quality SOD was decreased. By the way,

double arc can cause damages to the torch (high thermal stresses in small points).

Later, the Dual Flow Torch was introduced. A

second gas flux was introduced to protect the

cutting zone. Furthermore, it was easier to cool

down the nozzle, reaching T good for ceramic

material implementation. The adopted gas is function of the workpiece:

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oxygen or air for steels , CO for stainless steel and Ar and oxygen for aluminium.

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One of the major limit of plasma technology was the poor quality of the workpiece – rounded edges and

imperfect shapes, but it was the only process able to cut high thicknesses (up to 250 mm). Moreover, material

removal was not efficient and re-solidification was responsible of burrs formation on the bottom edge.

Water radial injection torch was the next step: water was introduced radially towards

the arc, just after the nozzle orifice. 10% of the it evaporates, increasing in volume,

further forcing the arc: double temperatures are obtained compared to every other

torch produced before. Cutting velocity increases; nozzle cooling improves avoiding

double arc; better quality surface are obtained; reduction of secondary gas – just

nitrogen, good for different materials. The variation of the vortex velocity is a way to

control the diameter of the plasma jet.

Dual plasma torch underwater was implemented to reduce the noise and to eliminate fumes: the workpiece

is immersed with the torch under a water level of 5 cm. Also light discharge was reduced and cooling was

improved. This process is exploited for high-power application, involving current higher than 100 A.

Drawbacks were the reduction in speed (up to 20%), the lack of visibility and the dissociation of water.

High Definition Plasma

Before Eighties, arc plasma technology cannot be still considered a precision process. It was a competitive

processes in terms of costs and productivity, but the physic

nature was responsible of irregular cutting, requiring post-

processing. Nowadays a more efficient control of the beam

energy is possible, assuring more precision. HDP is characterized

by:  Increased cutting speed

 Better cut quality

o More straight profile of the groove

o Absence of burrs

o Clean cut surface

 5 2

Higher energy density (6200-9300 A/cm )

 Lower components consumption

4 Higher cutting speed (25%) is allowed by the exothermic reaction of oxygen with the iron. On the other hand, rapid

erosion of tungsten electrode was a big issue. Today, electrodes in zirconium or hafnium oxide are used for increased

stability (lower affinity with oxygen) and higher electronic emissivity.

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Traditional: 1800-3100 A/cm . 3

To increase the energy density, nozzle diameter is decreased and better co-axiality of nozzles are introduced,

achieving a lower kerf taper (from 7° of traditional PAC up to 2°). The latter is achieved though the High Flow

Vortex Nozzle: around the electrode surface, a dense vortex is created through a swirl (diffusion) ring. The

generated gas rotation causes a depression in the central part of the vortex, claiming hot particles inside.

Pressure is stabilized and cold layers are separated from the hot central one directed to the workpiece. Excess

throughput of this cold layers of gas are then ejected.

Moreover, the double arc was solved through Shield Technology: a thin Cu dome containing the nozzle was

introduced.

In conventional processes the Hf electrode, inside a water-cooled cylinder, is consumed because during

cutting the lower extremity melts and at the end this melting part deposits on the nozzle wall or is ejected

from the orifice. This loss of material creates a cavity in the electrode. HDP

encapsulate the electrode in Ag, which confines wear deep into Hf layer, increasing

its life.

Longlife Oxygen Consumable Technology allows to control gas fluxes during the

different phases of the process. During the starting, gas pressure and current are

low in order to avoid damages to Hf and its layer of oxide. Later, the two

parameters are increased

up to the switch-off phase,

when they are decreased again in order to solidify Hf.

1. During pre-flux, no current is involved

whereas gas is inserted at low pressure;

2. Arc pilot is started switching on current.

Pressure increases to sustain it.

3. Cutting: optimal condition of current and gas

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pressure are reached .

4. Switching off is gradual, to avoid explosion of

the oxide protective layer and Hf losses from

the head. N is again introduced to reduce

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further electrode wear.

The electrode with this technology can last up to 1200 cycles with respect to the traditional 200, similarly to

the nozzle.

Components

The entire system consists in five components:

 Power generator

o DC current generator (100 A, 15 kW)

o Microprocessor with regulating and control function

o High frequency console to cause dielectric breakdown, generating the pilot arc. V ≈ 5000 –

10000 V; AC with f = 2 MHz.

 Gas console. It regulates gases throughput and eventually prepares the mixing gas for cutting.

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o Primary gas for plasma generation

 Ionization potential influences voltage and transmitted thermal power;

 Thermal conductivity influences internal energy transport;

6 The passage from 2 to 3 is guaranteed by the pressure increase, forcing the pilot arc to go out.

7 Vaporization and fusion of workpiece (kerf generation); protection of electrode. 4

 It is important to choose wisely the gas according to the reactivity with the

workpiece, in order to prevent undesirable phenomena such as formation of nitrides

or oxides, or simply to pursue the best qualitative results in terms of quality.

 Available gases are

 Air: cheap and available, for conventional cutting system under 200 A. Up to

200 cycles; problems with nitrides formation in carbon steels.

 Nitrogen: best performance in cutting quality for Al and stainless steel.

 Argon: excellent for cutting, requires a low voltage to sustain the arc, very

expensive. Often used in the ignition phase of the arc.

 Oxygen: cutting of carbon steels providing excellent surface quality.

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o Shielding gas .

o Gas channels, sensors, valves, …

 Water-cooling system

 Motion system

o Antropomorphous robots (5 controlled axes)

o Combined machines (punching + PAC)

 Torch

o Electrode

 Pure copper + high electronic emissivity insert (Hf or Zr)

 Cathode: thermoionic emitter

o Nozzle

 Copper (high purity)

 Responsible of beam costriction

o Swirl ring

 Ceramic or natural lava

o Shielding parts

 Reduce double arc phenomenon

 Avoid melted material contact with the nozzle

 Guarantee correct positioning of the nozzle and throughput of secondary gas.

Cutting: process parameters

Process parameters depend on material and thickness of the workpiece. They must be chosen in the

following order:

1. Torch parameters: torch choice is based upon the workpiece in primis (choice of gases) and then

upon the thickness. The parameters are current and voltage (stand off distance).

2. Gas parameters: primary and secondary gases flow rates, which are function of material and

thickness. Generally, throughputs are suggested by manufacturers.

3. Feed rate f (cutting speed).

Same thickness of the same material can be cut with different current values: the criterion is to centre the

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interested thickness value . Increase the current means to increase the power density, cutting deeper

thicknesses but decreasing tool life; it is always kept constant (in the range 15 – 400 A) to achieve high quality

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of the cut. With 100 A it is possible to reach 20000 K and 10 - 10 W/cm .

8 Protection of workpiece (oxides, …); cooling of kerf walls; protection of ceramic parts from thermal shock.

9 To cut 8 mm, the current is 100 A because it has the range 3-13 mm. 5

Gas flow determines arc stability and eventually the double arc phenomenon. Increasing the flow rate, the

arc becomes more stable and constricted, increasing power density and cutting speed, but also decreasing

consumable life.

Voltage increase means increase of transferred energy, of removal rate and of kerf width. To keep constant

voltage, the torch must respect roughness and flatness of the

working piece, varying continuously its distance from the

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surface in order to guarantee the same resistance . Voltage

influences the SOD: lower values of both are responsible of a

lower divergence of the beam, increasing power density and

quality, but also help the creation of double arc. Too high values

of SOD (and voltage) don’t allow arc pilot switch on.

Cutting speed / feed rate is function of material, current, power and thickness. Increasing f, keeping the

thickness constant, power density increases. By the way, really high values of f decrease quality (striations,

burrs, high taper). On the other hand, low speeds c