The application and introduction of laser cutting machine part.1

Laser cutting machine is the laser emitted from the laser, through the optical path system, focused into a high power density laser beam.The laser beam is irradiated to the surface of the workpiece, so that the workpiece reaches the melting point or boiling point. At the same time, the 

high-pressure gas coaxial with the beam will blow away the molten or gasified metal.With the light beam and the workpiece relative position of the movement, the material eventually formed a slit, so as to achieve the purpose of cutting.Laser cutting processing is to use invisible light
beam instead of the traditional mechanical knife, with high precision, fast cutting, not limited to the cutting pattern, automatic typesetting to save material, smooth cut, low processing cost, will gradually improve or replace the traditional metal cutting process equipment.The mechanical
part of the laser tool head has no contact with the workpiece, so it will not scratch the surface of the workpiece.Laser cutting speed is fast, the incision is smooth and flat, generally without subsequent processing;Small heat-affected area of cutting, small plate deformation, narrow slit
(0.1mm~0.3mm);No mechanical stress, no shear burr;High machining precision, good repeatability, no damage to the material surface;Numerical control programming, can process any plan, can be a large format of the whole board cutting, no need to open a mold, economical and time saving.

 

1) introduction

 And compared to traditional oxygen acetylene, such as plasma cutting technology, laser cutting speed, narrow kerf width, small heat affected zone, vertical degree good, trimming smooth kerf edge, at the same time can be variety of laser cutting materials, including carbon steel, stainless steel, alloy steel, wood, plastic, rubber, cloth, quartz, ceramic, glass, composite materials, etc.With the rapid development of market economy and the rapid development of science and technology, laser cutting technology has been widely used in automobile, machinery, electric power, hardware and electrical appliances and other fields.In recent years, laser cutting technology is developing at an unprecedented speed, with an annual growth rate of 15%~20%.Since 1985, China has been growing at a rate of nearly 25 percent per year.

  At present, the overall level of laser cutting technology in China is still a big gap with the advanced countries, so the laser cutting technology in the domestic market has a broad development prospect and huge application space.

During the cutting process of laser cutting machine, the beam is focused into a small focus through the lens of the cutting head, so that the focus can reach a high power density, in which the cutting head is fixed on the z axis.At this point, the heat input by the beam far exceeds the part of the heat reflected, transmitted or diffused by the material.
The material is soon heated to the melting and vaporization temperature. At the same time, a high-speed airflow blows the melted and vaporized material out of the coaxial or non-coaxial side, forming the holes cut by the material.As the focus moves relative to the material, the hole is formed into a continuous slit of narrow width to complete the cutting of the material.

 At present, the flight optical path system is mainly used in the outer optical path of laser cutting machine.
The beam from the laser generator passes through the reflector 1, 2 and 3 to the focusing lens on the cutting head, and after focusing, it forms a light spot on the surface of the material to be processed.The reflective lens 1 is fixed on the fuselage and does not move.The mirror 2 on the beam moves in x direction with the motion of the beam.The reflection lens 3 on the z-axis moves in the y-direction with the motion of the z-axis.It is not difficult to see from the figure that, during the cutting process, the length of the optical path changes at any time as the beam moves in the x direction and the z-axis moves in the y direction.

 At present, the laser beam emitted by civil laser generator has a certain divergence Angle and is "conical" due to the manufacturing cost and other reasons.When the height of the "cone" changes (equivalent to the length of the path of the laser cutter), the cross-sectional area of the 

beam on the surface of the focusing lens changes.In addition, light also has the property of waves, so it is inevitable that there will be diffraction, which will cause the beam to expand laterally in the process of propagation. This phenomenon exists in all optical systems and can determine the theoretical limits of the performance of these systems.

 Because the gaussian beam is "tapered" and diffracted by light waves, when the length of the optical path changes, the diameter of the beam acting on the surface of the lens changes at any time, which will cause changes in the size and depth of the focus, but has little influence on the focus position.If the focus size and depth of the focus change in the continuous machining, it will inevitably have a great impact on the machining, for example, will cause the width of the cutting seam is not consistent, at the same cutting power will not cut through or ablative plate.

 

2) principle

 A laser is a type of light that, like other natural light, is produced by the transition of atoms (molecules, ions, etc.).However, it is different from ordinary light in that the laser only relies on spontaneous radiation for a very short period of time at the beginning, and the subsequent process is completely determined by excitatory radiation, so the laser has a very pure color, almost no divergence of direction, extremely high luminous intensity and high coherence.

 Laser cutting is realized by the high power density energy generated by laser focusing.And controlled by computer, through the pulse laser discharge, thus the output controlled repeated high frequency pulse laser, the formation of a certain frequency, pulse width of the beam, the pulse laser beam through the light path transmission and reflection and through the focusing lens group focused on the machining surface, forming a subtle, high energy density, intensity, focal spot near to processing surface, with instantaneous high temperature melting or gasification materials processed.Each high-energy laser pulse instantly sputtered out a tiny hole on the surface of the object. Under the control of the computer, the laser machining head and the processed material were continuously moving relative to each other according to the pre-drawn figure, 

thus the object would be processed into the desired shape.

The process parameters (cutting speed, laser power, gas pressure, etc.) and the movement trajectory are controlled by the numerical control system. The slag at the slit is blown away by the auxiliary gas under a certain pressure.

 

3) main processes

1. Vaporized cutting

 During the process of laser gasification and cutting, the surface temperature of the material rises to the boiling point at such a rate that it avoids the melting caused by heat transfer, so that some of the material vaporizes into steam and disappears, and some of the material is blown 

away from the bottom of the slit as an ejective by the auxiliary gas stream.This requires very high laser power.

 The thickness of the material must not greatly exceed the diameter of the laser beam in order to prevent the vapor condensing on the slit wall.The process is therefore suitable only for applications where the exclusion of molten material must be avoided.This process is actually only 

used in a very small field of iron - based alloys.

 The process cannot be used for materials, such as wood and certain ceramics, that do not have a melting state and are therefore less likely to allow the material to condense again.In addition, these materials are often used to achieve thicker incisions.In laser gasification cutting, the 

optimal beam focusing depends on the material thickness and beam quality.

 Laser power and gasification heat have only a certain influence on the optimal focus location.The maximum cutting speed is inversely proportional to the gasification temperature of the material when the thickness of the plate is certain.The required laser power density is greater than 108W/cm2 and depends on the material, cutting depth and beam focus.
 The maximum cutting speed is limited by the gas jet velocity when the plate thickness is certain, assuming enough laser power.

2. Melt and cut.

 In laser fusion cutting, the workpiece is partially melted and the molten material is sprayed out by airflow.Because the material is transferred only in its liquid state, the process is called laser fusion cutting.

 The laser beam coupled with a high purity inert cutting gas causes the melted material to leave the slit, and the gas itself is not involved in the cutting.Laser fusion cutting can achieve higher cutting speed than gasification cutting.It usually takes more energy to gasify than to melt the material.In laser fusion cutting, the laser beam is only partially absorbed.
The maximum cutting speed increases with the increase of laser power and decreases almost inversely with the increase of plate thickness and material melting temperature.In the case of a certain laser power, the limiting factors are the pressure at the slit and the thermal conductivity of the material.Laser fusion cutting for iron and titanium materials can be obtained without oxidation incision.The laser power density, which produces melting but less than gasification, is between 104W/cm2 and 105w /cm2 for steel materials.

3, oxidation melting cutting (laser flame cutting).

 Melting cutting generally USES an inert gas, if it is replaced by oxygen or other active gas, the material is ignited under the irradiation of a laser beam, and the oxygen has a fierce chemical reaction to produce another heat source, so that the material is further heated, known as oxidation melting cutting.

 Because of this effect, for structural steel of the same thickness, the cutting rate obtained by this method is higher than that obtained by melting cutting.On the other hand, this method may have a lower notch quality than melting cutting.In fact, it generates wider slits, more 

pronounced roughness, increased heat affected areas, and poorer edge quality.Laser flame cutting is not good for machining precision models and sharp corners (there is a danger of burning off sharp corners).A pulsed laser can be used to limit the thermal impact, and the power of the laser determines the cutting speed.In the case of a certain laser power, the limiting factors are the supply of oxygen and the thermal conductivity of the material.

4. Control fracture and cutting.

 For brittle materials which are easy to be damaged by heat, high-speed and controllable cutting can be done by laser beam heating, which is called controlled fracture cutting.The main content of this cutting process is: the laser beam heats a small area of brittle material, causing a large thermal gradient and serious mechanical deformation in the area, leading to the formation of cracks in the material.As long as the heating gradient is balanced, the laser beam can guide the crack in any desired direction.

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