Plasma Cutting and Laser Cutting Explained

Plasma cutting

Plasma cutting is the use of high-energy density equidistant arc and high-speed plasma flow to blow molten metal away from the cutting edge to form a continuous cutting edge.

The plasma arc cutting speed is fast, and there is no combustion of the workpiece during oxygen acetylene cutting.

Therefore, the heat of the workpiece is relatively small, and the deformation of the workpiece is also small.

It is suitable for cutting various metal materials.

However, due to the high flow rate of the plasma arc, the noise, smoke and dust are serious, and the working hygiene conditions are poor.

Plasma arc can be used for welding, spraying, surfacing and cutting.

When cutting carbon steel plates with a thickness of less than 25mm, plasma arc cutting is about 5 times faster than oxyacetylene cutting, while when cutting plates with a thickness of more than 25mm, oxyacetylene cutting is faster.

Characteristics and classification of plasma arc cutting

1.1 Characteristics of plasma arc cutting

Plasma arc is an arc that compresses the free arc between the cathode (such as tungsten electrode) and the anode into an arc with high temperature, high ionization degree, high energy density and high flame flow rate by using a plasma gun.

Plasma arc cutting is carried out with a very hot high-speed jet, and the arc and inert gas are forced through the small-diameter hole to produce this high-speed jet.

The arc energy is concentrated in a small area, melting the plate, and the high-temperature expanded gas jet forces the molten metal through the notch.

When cutting carbon steel or cast iron, adding oxygen to the gas stream can also provide additional cutting energy.

The plasma arc cutting method has the advantages of large cutting thickness, flexible movement, simple workpiece clamping and cutting curves.

Compared with oxygen acetylene flame cutting, plasma arc has concentrated energy, small cutting deformation, no preheating at the beginning of cutting, can cut almost all metals, and the speed of cutting carbon steel is faster than oxygen cutting.

However, due to the wide cutting edge, more metal is melted. When the plate is thicker, the cut is not as smooth and flat as that of oxygen acetylene cutting.

In order to ensure that the sides of the incision are parallel, a special cutting nozzle is required.

In order to obtain certain groove forming, special cutting technology is also required.

The characteristics of plasma arc cutting mainly include:

① It can cut various metal materials that are difficult to be cut by oxygen (some non-metallic materials can also be cut by plasma arc);

② When cutting metal with small thickness, the cutting speed is fast, especially when cutting carbon steel thin plate, the speed can reach 5-6 times of gas cutting method;

③ The cutting surface is bright and clean, and the thermal deformation is small, especially suitable for processing various shaped parts;

④ The width of the cut and the bevel angle of the cutting surface are large, but when cutting thin plates, a special cutting torch or process can be used to obtain a nearly vertical cutting surface;

⑤ The ability to cut thick plates is not as good as gas cutting.

The disadvantages of plasma arc cutting are: large cutting tolerance, arc radiation, smoke and noise during cutting.

Compared with oxygen acetylene flame, plasma arc cutting equipment is expensive and the no-load voltage of the cutting power supply is high, which not only consumes large power, but also easily causes electric shock to operators in the case of poor insulation of cutting gun.

The plasma arc for cutting is formed by compressing the arc with a special cutting torch.

Plasma arc cutting requires a high arc voltage, so it needs a special power supply with a high no-load voltage.

According to the material and thickness to be cut, the required power is between 25-200kw.

The range of cutting current is 30-1000 A.

Argon or a mixed gas of nitrogen and hydrogen is usually used, and the cutting torch must be cooled with water.

There are plasma arc cutting torches for manual cutting.

The technical requirements for manual ISO arc cutting are similar to those for manual oxygen acetylene cutting.

However, in order to adjust more parameters, more training is required.

When cutting thin plates, the moving speed does not need to be carefully controlled, so the cutting quality is better.

Plasma arc cutting uses more mechanical automation equipment.

The cutting torch and other accessories are the same as those used in manual plasma arc cutting. The walking system is automated.

The moving mechanism of the cutting torch is similar to that used in oxygen acetylene cutting, but requires a higher moving speed.

Multi torch equipment requires additional power supply and control box for each torch.

In addition, in order to absorb noise and smoke, a water jacket or a water tank can be used.

1.2 Working principle of plasma arc cutting

The plasma arc temperature for cutting is generally between 10000-14000 ℃, which is far higher than the melting point of all metals and nonmetals.

It is possible to cut most metallic and non-metallic materials.

This method was born in the 1950s and was initially used to cut metal materials that could not be cut by oxygen acetylene flame, such as aluminum alloy and stainless steel.

With the development of this cutting method, its application has been extended to carbon steel and low alloy steel.

The basic design of plasma arc cutting gun is similar to that of plasma arc welding gun.

When used for welding, low-speed ion gas flow is used to melt the base metal to form a welded joint;

When used for cutting, a high-speed ion gas flow is used to melt the base metal and blow off the molten metal to form a notch.

The flow velocity and intensity of ion gas flame for cutting depend on the type of ion gas, gas pressure, current, nozzle channel ratio and distance from nozzle to workpiece.

The basic structure of the plasma arc cutting gun is shown in Fig. 4.1.

When plasma arc cutting is used, only the current polarity of DC positive connection is used, that is, the workpiece is connected to the positive electrode of the power supply.

Transfer arc is used when cutting metal. The method of igniting the transfer arc is related to the cutting gun.

The cutting gun can be divided into two types: the maintenance arc cutting gun and the non maintenance arc cutting gun.

See Fig. 4.2 for the circuit wiring of the maintenance arc cutting gun.

The circuit wiring of the non maintenance arc cutting gun has no resistance branch, and the rest is the same as the circuit wiring of the maintenance arc cutting gun.

Fig. 1 basic structure of plasma arc cutting gun

1. Electrode;

2. Compression nozzle;

3. Compress the nozzle channel length;

4. Distance from nozzle to workpiece;

5. Compress the nozzle aperture;

6. Electrode retraction distance;

7. Lonic gas.

Fig. 2 basic circuit of maintenance arc cutting gun

1. Power supply;

2. High frequency arc starter;

3. Resistance;

4. Contactor contact;

5. Compression spray;

6. Electrode;

7. Workpiece.

The function of the resistor in Fig. 2 is to limit the arc maintenance current to the lowest value that can smoothly ignite the transfer arc.

High frequency arc starter is used to ignite the maintenance arc.

When the arc is struck, the contact of the contactor is closed, and the high-frequency arc starter generates high-frequency and high-voltage to ignite the maintenance arc.

After the maintenance arc is ignited, when the cutting gun approaches the workpiece, the high-speed plasma flame from the nozzle contacts the workpiece to form a path between the electrode and the workpiece, so that the arc is transferred between the electrode and the workpiece.

Once the transfer arc is established, the maintenance arc is automatically extinguished, and the contact of the contactor is automatically disconnected after a period of time delay.

Laser cutting

Laser cutting is an advanced and widely used cutting technology in material processing.

It is a processing method that uses a high-energy density laser beam as a “cutting tool” to thermally cut materials.

Laser cutting technology can be used to cut various kinds of metal, non-metal plates, composite materials and hard materials such as tungsten carbide and titanium carbide, and has been widely used in national defense construction, aerospace, engineering machinery and other fields.

Laser cutting principle, classification and characteristics

2.1 Laser cutting principle and classification

(1) Principle of laser cutting

Laser cutting is to use the focused high-power density laser beam to irradiate the workpiece, so that the irradiated material quickly melts, vaporizes, ablates or reaches the ignition point, and at the same time, the molten material is blown away by the high-speed airflow coaxial with the beam, so as to cut the workpiece.

Laser cutting is one of the thermal cutting methods.

See Fig. 3 for the principle of laser cutting.

Fig. 3 principle of laser cutting

(2) Classification of laser cutting

Laser cutting can be divided into laser vaporization cutting, laser melting cutting, laser oxygen cutting and laser scribing and controlled fracture.

1) Laser vaporization cutting

The workpiece is heated by the laser beam with high energy density, so that the temperature rises rapidly, reaches the boiling point of the material in a very short time, and the material begins to vaporize to form vapor.

These vapors are ejected at a high speed, and notches are formed in the material at the same time as the vapors are ejected.

The vaporization heat of materials is generally very large, so laser vaporization cutting requires a lot of power and power density.

Laser vaporization cutting is mostly used for cutting extremely thin metal materials and non-metal materials (such as paper, cloth, wood, plastic and rubber).

2) Laser melting cutting

During laser melting cutting, the metal material is melted by laser heating, and then non oxidizing gas (Ar, Hc, N, etc.) is blown through the nozzle coaxial with the beam, and the liquid metal is discharged by the strong pressure of the gas to form a notch.

Laser melting cutting does not need to completely vaporize the metal, and the energy required is only 1 / 10 of that of vaporizing cutting.

Laser melting cutting is mainly used for cutting some materials or active metals that are not easy to oxidize, such as stainless steel, titanium, aluminum and their alloys.

3) Laser oxygen cutting

The principle of laser oxygen cutting is similar to oxyacetylene cutting. It uses laser as the preheating heat source and active gas such as oxygen as the cutting gas.

On the one hand, the blown gas interacts with the cutting metal to produce oxidation reaction and release a large amount of oxidation heat;

On the other hand, the molten oxide and melt are blown out from the reaction zone to form a notch in the metal.

Because the oxidation reaction in the cutting process generates a large amount of heat, the energy required for laser oxygen cutting is only 1 / 2 of that for melting cutting, and the cutting speed is far greater than that of laser vaporization cutting and melting cutting.

Laser oxygen cutting is mainly used for metal materials that are easy to oxidize, such as carbon steel, titanium steel and heat-treated steel.dize, such as stainless steel, titanium, aluminum and their alloys.

4) Laser scribing and controlled fracture

Laser scribing is to use high-energy density laser to scan the surface of brittle materials, so that the materials are heated and evaporated out of a small groove, and then apply a certain pressure, and the brittle materials will crack along the small groove.

Lasers for laser scribing are generally Q-switched lasers and CO₂ lasers.

Controlled fracture is to use the steep temperature distribution generated by Laser Grooving to generate local thermal stress in brittle materials and break materials along small grooves.

2.2 Characteristics of laser cutting

Compared with other thermal cutting methods, laser cutting is characterized by fast cutting speed and high quality.

It can be summarized as follows.

(1) Good cutting quality

Because of the small laser spot, high energy density and fast cutting speed, laser cutting can obtain better cutting quality.

① The laser cutting incision is thin and narrow, the two sides of the slit are parallel and perpendicular to the surface, and the dimensional accuracy of the cut parts can reach ± 0.05mm.

② The cutting surface is clean and beautiful, and the surface roughness is only a few tens of microns.

Even laser cutting can be used as the last process, without mechanical processing, and parts can be used directly.

③ After the material is cut by laser, the width of the heat affected zone is very small, the performance of the material near the cutting seam is hardly affected, and the deformation of the workpiece is small, the cutting accuracy is high, the geometry of the cutting seam is good, and the cross-sectional shape of the cutting seam presents a regular rectangle.

See table 1 for the comparison of laser cutting, oxyacetylene cutting and plasma cutting methods.

The cutting material is 6.2mm thick low carbon steel plate.

Table 1 comparison of laser cutting, oxyacetylene cutting and plasma cutting

Cutting method	Slit width / mm	Width of heat affected zone / mm	Slit shape	Cutting speed	quipment cost
Laser cut	0.2~0.3	0.04~0.06	parallel	Fast	High
Oxyacetylene cutting	0.9~1.2	0.6~1.2	Relatively parallel	slow	Low
Plasma cutting	3.0~4.0	0.5~1.0	Molded and inclined	Fast	Middle

(2) High cutting efficiency

Due to the transmission characteristics of the laser, the laser cutting machine is generally equipped with multiple numerical control worktables, and the whole cutting process can be fully numerical controlled.

In operation, it is only necessary to change the numerical control program, and it can be applied to the cutting of parts with different shapes, which can be both two-dimensional cutting and three-dimensional cutting.

(3) Fast cutting speed

A laser with a power of 1200 W is used to cut a 2 mm thick low carbon steel plate, and the cutting speed can reach 15000 px/ rain;

The cutting speed can reach 15000 px / min when cutting 5mm thick polypropylene resin sheet.

The materials do not need to be clamped and fixed during laser cutting, which can save the tooling and fixtures, as well as the auxiliary time for filling and blanking.

(4) Non contact cutting

There is no contact between the cutting torch and the workpiece during laser cutting, and there is no tool wear.

To process parts with different shapes, it is not necessary to change the “tool”, but only to change the output parameters of the laser.

The laser cutting process has low noise, low vibration and no pollution.