Laser cutting is one of the most widely used technologies in laser processing. Metal cutting is a common economic and practical processing method.
In order to achieve the best cutting effect, it is necessary to select and apply the air compressor properly.
This article presents the method of selecting air compressor according to the nozzle theory, and introduces the problems needing attention when using compressed air as auxiliary gas in laser cutting machine.

1. Introduction

Laser cutting is the most widely used laser processing technology in the world. As early as the 1970s, the laser was first used in cutting.
Since the beginning of this century, with the rise and popularization of the third generation laser technology, laser cutting is more widely used in sheet metal, plastic, glass, ceramics, semiconductors, textiles, wood and paper materials processing.
Since the beginning of this century, many laser enterprises have vigorously developed high-power fiber laser cutting machine.
Due to the unique processing advantages of high-power fiber laser processing, the processing cost has been greatly reduced and a variety of flexible payment methods are combined.
At present, more and more sheet metal processing enterprises, kitchen and bathroom manufacturing enterprises and automobile parts processing enterprises are using laser cutting machine more and more, especially in the sheet metal processing industry, it has replaced the traditional processing method.
Laser cutting machine can meet the cutting requirements of various materials and complex shapes.
Besides the need for high energy laser, auxiliary gas is an indispensable material to complete the cutting process.
The auxiliary gases used for laser cutting are mainly oxygen (O2), nitrogen (N2) and compressed air.
Compressed air is easier to obtain than oxygen and nitrogen, and it is very cheap compared with oxygen and nitrogen.
It is very common to use compressed air as auxiliary gas cutting.
The quality of compressed air has a very direct impact on the quality of metal laser cutting, and the size and stability of gas pressure will affect the cutting effect.
The size of the air compressor used as auxiliary gas for laser cutting machine should be selected according to the design of laser cutting head, auxiliary gas pressure and nozzle size.
Thus, the best matching of air compressor and laser cutting machine can be obtained.

2. Principle of laser cutting

Laser cutting is to use the focused high power and high-density laser beam to irradiate the workpiece, so that the material at the irradiated place will melt, vaporize, ablate or reach the ignition point.
At the same time, with the help of the high-speed airflow coaxial with the beam, the molten material can be removed to cut the workpiece.
Laser cutting is one of the thermal cutting methods.

2.1 Main methods of laser cutting

Laser cutting can be divided into laser gasification cutting, laser melting cutting, laser oxygen assisted melting cutting and controlled fracture cutting.

1) Laser gasification cutting

Using the laser beam with high power density to heat the workpiece, the temperature rises rapidly and reaches the boiling point of the material in a very short time, and the material begins to vaporize and form vapor.
These vapors are ejected at a very high speed, and at the same time, a notch is formed on the material.
The gasification heat of materials is usually very large, so the laser gasification cutting needs a lot of power and power density.
Laser gasification cutting is mostly used for cutting extremely thin metal and non-metallic materials (paper, cloth, wood, plastic and rubber, etc.).

The cutting edge of laser gasification cutting is smooth without burr.

2) Laser melting cutting

Laser melting cutting is to melt metal materials by laser heating, and then inject high-pressure inert gas (N2, Ar and He etc.) through a nozzle coaxial with the beam.
Depending on the strong pressure of the gas, the liquid metal is eliminated and a slit is formed.
It is not necessary to vaporize gold completely in laser melting cutting, and the laser energy required is only about one-tenth of that of gasification cutting.
For sheet materials, if the cutting speed is too slow, most of the laser beams will lose energy directly through the incision.
With the increase of the speed, more beams irradiate the material, and the coupling power with the material is increased.
For thick plate materials, because laser evaporation or the speed of melting product removal is not fast enough, the beam is reflected on the material section in the slit many times.
As long as the melted product can be removed before it is condensed by cold airflow, the cutting process will continue.

The edge of the cutting edge of laser melting cutting is striped.

Laser melting cutting is mainly used for cutting some materials or active metals which are not easy to oxidize, such as stainless steel, stainless steel, stainless steel, titanium alloy, aluminum and its alloy.
The laser melting cutting uses nitrogen as the auxiliary gas for cutting quality, but the cost is higher than that of oxygen cutting.

3) Laser oxygen assisted melting cutting

The principle of laser oxygen-assisted melting cutting is similar to oxyacetylene cutting, which uses the laser as preheating heat source and oxygen as auxiliary cutting gas.
On the one hand, the oxygen emitted reacts with iron which is heated to about 1500 ℃ by laser.
Thus, a large amount of oxidation heat is continuously released to make the iron element heat up and burn;
On the other hand, molten liquid oxides and melts are blown out of the reaction zone to form a notch in the metal.
Due to the oxidation reaction in the cutting process, a large amount of heat is generated, so the energy required for laser oxygen cutting is only 1/2 of that of melting cutting.
Moreover, the cutting speed is far higher than that of laser gasification cutting and melting cutting.
The edge roughness of laser oxygen assisted melting cutting is between laser vaporization cutting and laser melting cutting.
Laser oxygen assisted melting cutting is mainly used with carbon steel and other easily oxidized metal materials.
It can also be used for stainless steel and other materials processing, but the section is black and rough, and the cost is lower than inert gas cutting.

4) Controlled fracture cutting

Through laser beam heating, brittle materials which are easy to be damaged can be cut off in a high speed and controllable way, which is called controlled fracture cutting.
The cutting principle is as follows: the laser beam heats the small area of brittle material, which leads to thermal gradient and serious mechanical deformation, which makes the material form cracks.
The cutting speed of fracture control is fast, only a small laser power is needed;
If the power is too high, the workpiece surface will melt and the cutting edge will be damaged.
The main controllable parameters are laser power and spot size.

2.2 Characteristics of laser cutting

Compared with other cutting methods, laser cutting is characterized by high cutting speed and high quality. It can be summarized as follows:
a. The cutting quality is good. Due to the small laser spot, high energy density and fast cutting speed, laser cutting can obtain better cutting quality.
b. The laser gas cutting incision is thin and narrow, the two sides of the cutting seam are parallel and perpendicular to the surface.
c. The cutting surface is smooth and beautiful, which can even be used as the last processing procedure.The parts can be used directly without machining.
d. After laser cutting, the width of the heat-affected zone is very small, the properties of the material near the slit are almost unaffected. The shape of the workpiece is small and the cutting accuracy is high.
e. The cutting speed comparison between laser cutting and other cutting methods is shown in Table 1, and the cutting material is low carbon steel plate.

Cutting method	Laser cutting	Plasma cutting	Oxyacetylene cutting	Hydraulic cutting
Cutting speed	Very fast	Fast	Slow	Very slow

f. The cutting speed is fast, for example, when 2500 W laser cutting 1 mm thick cold-rolled carbon steel plate, the cutting speed can reach 16-19 m/min.
g. It is a non-contact cutting. There is no contact between the nozzle and the workpiece during laser cutting, and there is no tool wear.

3. Comparison of laser cutting with different auxiliary gases

When laser cutting, different cutting gas is selected according to the different material of cutting plate.
The choice of cutting gas and pressure has a great influence on the quality of laser cutting.

3.1. Types of auxiliary gas

The commonly used auxiliary gases for laser cutting are oxygen (O2), nitrogen (N2) and compressed air (compressed air), and sometimes argon (Ar) is used.
According to the gas pressure, it can be divided into high-pressure gas and low-pressure gas.

3.2. Selection of auxiliary gas types

The main functions of auxiliary gas in laser cutting are:

• combustion supporting and heat dissipation
• timely blowing off the molten stains produced by cutting
• preventing the cutting molten stains from rebounding upward into the nozzle, protecting the focus lens, etc.

According to the different cutting materials and combined with the power of laser cutting machine, when choosing different laser cutting technology, auxiliary gas selection is not the same.
The characteristics, uses and application scope of different auxiliary gases are as follows:

1) Oxygen

O2 is mainly used for cutting carbon steel materials.
The chemical reaction heat of oxygen and iron promotes the endothermic melting of metal, which can greatly improve the cutting efficiency.
It can also realize the cutting of thicker materials, and obviously improve the processing ability of the laser cutting machine.
However, due to the presence of oxygen, an obvious oxide film will be formed on the end face of the incision.
Moreover, it will produce a quenching effect on the materials around the cutting surface, improve the hardness of this part of materials, and have a certain impact on the subsequent processing.
The color of the oxygen cutting material incision end face is black or dark yellow.
General carbon steel plate uses oxygen cutting, low-pressure drilling and low-pressure cutting.

2) Nitrogen

When N2 is used as auxiliary gas for cutting, nitrogen will form a protective atmosphere around the molten metal to prevent the material from being oxidized and the formation of oxide film, realizing non-oxidative cutting.
But at the same time, because nitrogen does not react with metals, the cutting ability is not as good as oxygen without reaction heat.
Moreover, the nitrogen consumption of nitrogen cutting is several times larger than that of oxygen cutting, and the cutting cost is higher than that of oxygen cutting.
The non-oxidation cutting surface has the characteristics of direct welding, coating and strong corrosion-resistance etc., and the end face of the incision is white.
Generally, nitrogen is used to cut stainless steel, galvanized sheet, aluminum and aluminum alloy plate, brass and other materials, with low-pressure perforation and high-pressure cutting.
When cutting with nitrogen, the change of gas flow rate has a great influence on cutting。
In the case of ensuring the cutting gas pressure, it is necessary to ensure sufficient gas flow.

3) Compressed air

Compressed air can be directly provided by air compressor, which is easy to obtain compared with oxygen and nitrogen, and the price is very cheap.
Although the air contains only about 20% oxygen, the cutting efficiency is far less than that of oxygen cutting.
However, the cutting capacity is close to that of nitrogen, and the air cutting efficiency is slightly higher than that of nitrogen cutting.
However, the cutting capacity is close to that of nitrogen, and the air cutting efficiency is slightly higher than that of nitrogen cutting.
The end face of air cutting is yellow.
It is the most economical and practical choice to use compressed air instead of nitrogen when there is no strict requirement on the surface color of the material incision.

4) Argon

Ar is an inert gas, which can prevent oxidation and nitriding in laser cutting, and can also be used in solution welding.
However, the price of argon is higher than that of nitrogen, so it is not cost-effective to use argon electrode in general laser cutting.
Argon cutting is mainly used for titanium and titanium alloy etc.
The end face of the incision is white after argon cutting.