Laser Cutting: Principle, Characteristics And Application

1. Principle of laser cutting

Laser cutting is to use high-power density laser beam to scan the material surface, heat the material to thousands to tens of thousands of degrees Celsius in a very short time, melt or vaporize the material, and then blow the melted or vaporized material away from the incision with high-pressure gas, so as to achieve the purpose of cutting the material, as shown in figure below.

Principle of laser cutting

Figure1 Principle of laser cutting

Laser beam can cut all kinds of metallic and non-metallic materials.

Due to the different properties of processed materials, the methods and mechanisms of laser cutting are also different.

There are mainly three common laser cutting methods.

(1) Laser thermal stress cutting

Under the irradiation of the laser beam, the workpiece will produce an obvious temperature gradient after heating.

If the upper temperature is high, it will expand, while the inner temperature is low, it will hinder the expansion.

As a result, tensile stress will be generated on the surface of the workpiece, and radial extrusion stress will be generated on the inner layer.

When these two stresses exceed the tensile strength of the workpiece, cracks will appear on the workpiece.

As a result of the development of such cracks, make the workpiece break along the crack.

Because it is high-speed and controllable cutting through laser beam heating, it is also called controlled fracture cutting.

This cutting method is suitable for cutting hard and brittle non-metallic materials such as glass, ceramics, fused quartz, gem and diamond.

(2) Laser evaporation cutting

High power density laser beam is used for heating to avoid melting caused by heat conduction, so some materials vaporize into steam and disappear, forming a notch.

When the laser beam irradiates the surface of the material to be cut, the temperature of the workpiece rises rapidly to the vaporization temperature after heating, and a large amount of material vaporizes to form high-pressure steam, which is sprayed outward at supersonic speed.

Vaporization holes appear in the laser irradiation area, and the air pressure rises sharply, rapidly vaporizing and removing a large amount of material in the incision.

In the process of high-pressure steam high-speed injection, at the same time, the molten material in the cutting seam escapes outward until the workpiece is completely cut off.

The cutting using this mechanism to realize separation is called laser evaporation cutting.

This cutting method mainly depends on a large amount of vaporization removal of materials, which requires a high laser power density, which should generally reach 108W/cm2.

Suitable for cutting some hard and brittle materials, such as wood, carbon, ceramics, glass, fused quartz, asbestos cement, etc.

When cutting metal materials, generally speaking, laser evaporation cutting is mostly used for cutting very thin metal materials.

Because this cutting method needs to be carried out in vacuum or special occasions, and the laser power density required to vaporize the material is about 10 times higher than that required for other melting, this method is rarely used.

(3) Laser melting cutting

Melting cutting is to make the power density of the incident laser beam exceed a certain value, so that the interior of the material at the beam irradiation point begins to evaporate and form holes, as shown in Fig. 1.

In this laser cutting device, an auxiliary blowing system coaxial with the laser beam is added to blow away the melted material with the compressed gas coaxial with the laser beam, and make the laser beam move relative to the material along a certain track, so as to form a notch of a certain shape, which can greatly improve the ability of laser cutting and reduce the laser power density.

The laser power density required by this cutting method is only about 1 / 10 of that of laser vaporization cutting method.

The commonly used auxiliary gases include O2, N2, Ar, He, CO2 and compressed air, and the air pressure is generally 0.15 ~ 0.3MPa (1.5 ~ 3atm).

The function of auxiliary gas is to blow away the molten material from the cutting groove;

Blow away the flying spatter to protect the focusing lens;

When oxygen is used as auxiliary gas, the oxidation reaction can also produce energy.

The types of auxiliary gas used are also different according to different cutting materials.

Generally, oxygen is used when cutting low carbon steel, and nitrogen is more used when cutting stainless steel.

Due to the different auxiliary gases used in laser melting cutting, laser melting cutting is divided into laser melting blowing cutting and laser reactive gas cutting.

1) Laser melting and blowing cutting.

When the laser beam irradiates the surface of the metal material to be cut, the metal is quickly heated to the melting point, and the molten metal is blown away from the cutting seam by spraying inert gases, such as argon, helium, nitrogen and so on.

The separation cutting is called laser melting and blowing cutting.

Laser melting and blowing cutting is mostly used for cutting paper, cloth, wood, plastic, rubber, rock, concrete and other non-metallic materials.

Because non-metallic materials are generally not easy to oxidize, and the laser absorption rate of 10.6 μm wavelength is particularly high, and the heat transfer coefficient is very low, so the energy required for melting and evaporation is small, which is conducive to CO2 laser cutting.

This cutting method can also be used to cut stainless steel, easily oxidized titanium, aluminum and aluminum alloys.

2) Laser reactive gas cutting.

When the laser beam irradiates the surface of the metal material to be cut, the metal material is quickly heated above the melting point, and pure oxygen or compressed air is used as the jet gas. At this time, the molten metal will produce fierce oxidation with oxygen. The reaction formula is as follows:

3Fe+2O2=Fe3O4+1116.7kJ

The large amount of heat released by the oxidation reaction heats the next layer of metal and continues to be oxidized. In this way, the steel plate is cut through.

At this time, the cutting torch is moved according to a certain track and the oxide is blown away from the incision by the pressure of oxygen, which is called laser reactive gas cutting.

Due to the combustion supporting effect of oxygen, if oxygen is used as the auxiliary gas to make the cut material burn in oxygen, the ability of laser cutting can be greatly enhanced, and liquid slag with fluidity can be formed in the incision, which is continuously sprayed and removed by high-speed oxygen flow.

Because oxygen produces exothermic reaction in the process of laser cutting, this cutting method is called laser reaction melting cutting method, which can also be called laser flame cutting method.

This cutting method is much more powerful than the ordinary laser vaporization cutting method and laser melting cutting method.

The laser power density it needs is only about 1 / 20 of that of the laser vaporization cutting method.

Therefore, this cutting method has the advantages of low cost, high efficiency and the widest application.

It is suitable for cutting all kinds of metal materials and some fusible non-metallic materials.

However, when cutting some metal materials, in order to prevent the oxidation of the incision, inert gas should be used as the auxiliary gas instead of oxygen.

In addition, for most non-metallic materials containing carbon, in order to prevent carbonization of the incision in the process of laser cutting, oxygen cannot be used as the auxiliary gas, but inert gas should be selected as the auxiliary gas.

Laser reactive gas cutting is mainly used for cutting metal materials, such as carbon steel, titanium steel and heat treated steel.

The function of oxygen is not only to support the combustion of metal, but also to improve the cutting speed and efficiency, so as to narrow the incision, reduce the heat affected zone, improve the cutting quality and accuracy, and cut thicker workpieces with the help of oxygen.

Principle of laser cutting

2. Characteristics of laser cutting

(1) Advantages of laser cutting laser cutting is an advanced cutting technology in the world.

Its biggest advantage is that due to the small laser spot and concentrated energy, the cut is small, there is no slag, there is almost no thermal deformation, and the cutting surface is smooth.

In general, compared with other cutting processes, such as flame cutting, plasma arc cutting and high-pressure water jet cutting, laser cutting process has the following advantages:

1) Non contact cutting.

Laser cutting is processed by irradiating the object surface with the focused laser, so it is non-contact processing.

The mechanical part of the laser cutting head has no contact with the cut material and will not scratch the working surface in the work;

The cutting process has low noise, low vibration, no pollution, and the workpiece has no residual mechanical stress.

2) Laser cutting has good quality and high efficiency.

Due to the good focusing of the laser beam, that is, the focal spot is small, and the heating area of laser cutting is only 1 / 10 ~ 1 / 1000 of that of oxyacetylene flame, the scope of oxidation reaction is extremely concentrated compared with oxyacetylene flame cutting, which promotes the oxidation reaction.

Therefore, the incision is small, precision cutting can be carried out, and the workpiece can be cut according to the program with high dimensional accuracy.

The width of the notch is narrow (generally 0.1 ~ 0.5mm), the accuracy is high (generally the error of hole center distance is 0.1 ~ 0.4mm, the error of contour dimension is small (0.1 ~ 0.5μm), and the roughness value of the notch surface is small (generally Ra12.5 ~ 25μm). Generally, the notch can be welded without reprocessing;

The notch shall be free of burr, good perpendicularity, deformation and heat affected zone.

3) High cutting efficiency.

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

Combined with the numerical control device, when using CAD / CAM software programming, the overall efficiency is very high.

During operation, it can be applied to the cutting of parts with different shapes by changing the control program, which can not only carry out two-dimensional cutting, but also realize three-dimensional cutting.

4) The laser cutting speed is fast, the light spot of the laser beam is small, the energy is concentrated, and the cutting speed is fast.

For example, the cutting speed of 25.4mm thick titanium plate can reach more than 5m per minute.

For 6mm thick titanium plate, the cutting speed can reach about 16m per minute.

For example, with 2kW laser power, the cutting speed of 8mm thick carbon steel is 1.6m/min;

The cutting speed of 2mm thick stainless steel is 3.5m/min.

The cutting speed is mainly determined by the laser power density, but if the injection gas is different, it will also directly affect the cutting speed.

For cutting steel plates with the same thickness, the cutting speed of argon injection is almost 50% lower than that of oxygen injection.

Because laser cutting is connected with CNC machine tools and robots, the processing is clean, safe, pollution-free and low labor intensity.

The working environment of operators is greatly improved, and the whole page arrangement, set cutting and cutting can be realized, saving labor and materials.

(2) Disadvantages of laser cutting

Although laser cutting has incomparable advantages over traditional technology, it also has the disadvantages of high manufacturing cost and high requirements for the peripheral environment.

1) High cost.

In the construction machinery industry, the traditional blanking processing of parts mainly adopts flame cutting and plasma arc cutting.

The angle of flame cutting parts is small, but the deformation is large, and the cutting speed is slow and the efficiency is low.

Plasma arc cutting is fast and efficient, but the cutting section has a large inclination.

The cutting quality of CO2 laser cutting meets the requirements of construction machinery, but its equipment maintenance is complex and the use cost is high.

The emergence of fiber laser cutting machine just meets this demand.

See table 1 for the comparison between plasma arc cutting and laser cutting processes.

Table 2 shows the comparison of cutting quality of laser cutting, oxyacetylene cutting and plasma arc cutting methods (the cutting material is 6mm thick low carbon steel plate).

Table 3 shows the comprehensive performance comparison of CO2, fiber laser cutting, oxyacetylene gas cutting and plasma arc cutting machines.

Table 1 Comparison of plasma arc cutting and laser cutting processes

ItemPlasma cuttingLaser cutting
Applicable materialsCarbon steel, stainless steel, aluminum, copper, cast iron and other metal materialsMetal materials, special metal materials, non-metallic materials, etc
Cutting plate thicknessplateMedium thin plate
Cutting speedfastfast
Cutting accuracyHigher (within 1mm)Height (within 0.2mm)
Incision sizelessVery small (0.2 ~ 0.3mm)
Perpendicularity of cutting sectionHave a certain inclination anglegood
Heat affected zonelessVery small (width 0.1mm)
Cutting plate deformationlessVery small

Table 2 Comparison of cutting quality of laser cutting, oxyacetylene cutting and plasma arc cutting

Cutting methodNotch / mmHeat affected zone / mmRa/umIncision shapeCutting speed
Laser cutting0.2-0.30.04-0.0612.5~25parallelfast
Oxyacetylene gas cutting0.9~1.20.6~1.225~50Relatively parallelslow
Plasma cutting2.0~3.00.5~1.025~50Wedge and inclinedfast

Table 3  Comparison of comprehensive performance of several cutting machines

Cutting machineOne time investmentCutting speedOperationMaintenanceCutting qualityRunning cost
Flame cutting machineSmallslowRelatively simplesimpleGeneral, deformedin
Plasma cutting machineinfastRelatively simpleRelatively simpleGeneral, with inclinationhigher
CO2 laser cutting machinelargeFaster (depending on the thickness of the steel plate)complexcomplexhighhigh
Fiber laser cutting machinelargeFaster (depending on the thickness of the steel plate)simplesimplehighlow

Taking cutting 10mm ordinary low carbon steel plate as an example, the cost analysis of plasma cutting and laser cutting is shown in table 4.

According to the data in table 4, the operation cost of plasma arc cutting machine is 97.65 yuan / h and that of laser cutting machine is 263.73 yuan / h based on the annual base of 3860h.

According to the calculation that the cutting speeds of fine plasma arc cutting and 4kw laser are 3.4m/min and 1.5m/min respectively, the cost of fine plasma arc cutting is 0.48 yuan / M and the cost of laser cutting is 2.9 yuan / m.

When laser cutting is used to cut metals that are difficult to cut by general cutting methods, its cost can be reduced by 75% compared with plasma arc cutting.

For example, when cutting quartz tube with 1kW CO2 gas laser, the cost is 40% lower than that with diamond grinding wheel.

At the same time, because the laser spot is very small and the incision is narrow, the cost is much lower than other cutting methods.

The cost of arc cutting is 0.48 yuan / M and that of laser cutting is 2.9 yuan / m.

Table 4  Cost comparison between plasma cutting and laser cutting

Category

Unit Price

Consumption

Plasma arc cutting

Laser cutting

He (volume fraction, 99.999%)

0.25 yuan / L

nothing

13L/h

Gas consumption

N2 (volume fraction, 99.999%)

0.04 yuan / L

nothing

6L/h

C03 (volume fraction, 99.999%)

0.3 yuan / L

nothing

1L/h

N2 (volume fraction, 99.99%)

0.01 yuan / L

nothing

375L/h

02 (volume fraction, 99.6%)

0.002 yuan / L

4000L/h

2000L/h

Total power of power consuming equipment

Electricity charge 1 yuan / (kW · h) (setting factor: 50%)

45kW

88kW

Total cost of equipment depreciation, purchase and installation

Depreciation period: 10 years

130000 yuan / year

650000 yuan / year

Equipment maintenance and daily equipment maintenance cost

none

50000 yuan / year

100000 yuan / year

Consumption of vulnerable and consumable parts

Focusing lens

5000 yuan / piece

none

About 4 pcs / year

Nozzle

100 yuan / piece

10h / piece 40H / piece

Electrode

100 yuan / piece

10h / piece

none

Other consumption (filter element, porcelain ring, cooling water)

About 2000 yuan / year

About 4000 yuan / year

Note: labor cost, management expense, profit and tax and other factors are not considered in the above cost analysis.

2) High requirements for the peripheral environment.

The cleanliness of optical components (especially lenses) installed during laser transmission will have a great impact on the quality of the beam and become the cause of poor processing;

When cutting metal materials, the workpiece to be cut needs to be coated with beam absorbent to prevent laser reflection, so the process of removing absorbent is added after cutting;

High price of consumable parts;

When cutting thick plates, the change of beam absorption state on the surface of the workpiece to be cut will affect the cutting quality, so it is necessary to protect the surface to be cut from rust or too dirty.

3. Application of laser cutting

In addition to cutting carbon steel, stainless steel and other materials, laser cutting can also cut various high melting point materials, heat-resistant alloys, superhard alloys and other special metal materials, as well as semiconductor materials, non-metallic materials and composite materials.

Its cutting application range is very wide.

Especially in the case of thin plate cutting, high cutting efficiency and small thermal deformation, laser cutting has outstanding advantages.

Laser cutting mainly focuses on medium and thin plates.

Taking 4kw laser as an example, it can cut ordinary carbon steel of about 25mm at most.

Laser cutting has the advantages of high speed, high processing accuracy, narrow incision, small heat affected zone, smooth incision, small deformation of cutting plate, no damage to the cutting surface, and generally no subsequent processing is required.

Laser processing has replaced the traditional plasma arc cutting and punch in sheet metal processing and become the leader in the field of metal cutting.

The requirement of laser cutting equipment is not only flexibility and processing quality, but also to provide high-speed cutting solutions to meet the needs of mass production.

Laser cutting technology appears as an alternative process of “shear punching”, which has the characteristics of flexibility and flexibility.

But its cost is high, so it is often used in the manufacture of special-shaped (or complex shape) workpieces.

Laser cutting has been widely used in machine tool manufacturing, engineering machinery, electrical switch cabinet manufacturing, elevator manufacturing, grain machinery, textile machinery, food machinery, locomotive and automobile, shipbuilding, agricultural and forestry machinery, petroleum machinery, aerospace, environmental protection equipment, electrical manufacturing, packaging and printing and other industries. It is used for sheet metal blanking and cutting and manufacturing of mechanical parts.

Three dimensional laser cutting is mainly used for the cutting and welding of panels in the automotive industry, as well as the manufacturing of aerospace and special three-dimensional parts.

In the process of automobile production, three-dimensional laser cutting machine is generally used to accurately cut the edges of panels and door panels, and then laser welding manipulator is used for high-quality automatic welding, which not only ensures the product quality, but also has high production efficiency.

In the development of sample cars and small batch production, highly flexible laser three-dimensional cutting replaces a large number of punching and trimming molds, which not only saves molds, but also greatly shortens the development cycle of new models.

In Europe, almost all automobile manufacturers use laser processing in automobile research, development and production.

In fact, the breadth and depth of laser manufacturing technology in automobile manufacturing has become an important symbol of the advanced nature of the automobile industry.

Laser cutting machine is developing in the direction of high power, large format and thick plate. Taking the shipbuilding industry as an example, high-power CO2 laser cutting technology is widely used in shipbuilding in advanced countries and regions such as the United States, the European Union, Japan and South Korea.

The concept of “precision shipbuilding” has emerged in the world.

Cutting and welding is the most important processing technology in shipbuilding industry.

Due to the requirements of high precision, especially for the deck and hull materials of special materials, many large foreign shipyards generally use large-scale thick plate laser cutting machines.

Laser cutting has become a necessary processing means for domestic shipbuilding enterprises for special-purpose ships.

With the promotion of major projects in railway construction, highway, water conservancy, hydropower, energy, mining and construction industry, large-scale thick steel plate laser cutting machine will also be popularized and applied in construction machinery industry.

High end special CNC laser cutting machines, such as high-power fiber lasers, play an irreplaceable role in the aerospace industry.

Laser cutting and welding is the best processing method for the processing of special materials such as titanium alloy and aluminum alloy.

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