Vaporization cutting
Under the heating of high power density laser beam, the speed at which the surface temperature of the material rises to the boiling point is so fast that it is enough to avoid melting caused by heat conduction, so part of the material vaporizes into steam and disappears, and part of the material is blown away from the bottom of the slit as ejecta by the auxiliary gas flow. Some materials that cannot be melted, such as wood, carbon materials and some plastics, are cut and formed by this vaporization cutting method.
During the vaporization cutting process, the steam takes away the molten particles and washes away the debris to form holes. During the vaporization process, about 40% of the material turns into steam and disappears, while 60% of the material is driven away by the air flow in the form of molten droplets.
Melting cutting
When the power density of the incident laser beam exceeds a certain value, the material inside the beam irradiation point begins to evaporate and form a hole. Once this small hole is formed, it will absorb all the energy of the incident beam as a black body. The small hole is surrounded by the molten metal wall, and then the auxiliary airflow coaxial with the beam takes away the molten material around the hole. As the workpiece moves, the small hole moves synchronously in the cutting direction to form a slit. The laser beam continues to irradiate along the front edge of the slit, and the molten material is blown away from the slit continuously or pulsatingly.
Oxidation melting
Melting cutting generally uses inert gas. If it is replaced by oxygen or other active gases, the material is ignited under the irradiation of the laser beam, and a violent chemical reaction occurs with oxygen to produce another heat source, which is called oxidation melting cutting. The specific description is as follows:
⑴ The surface of the material is quickly heated to the ignition temperature under the irradiation of the laser beam, and then a violent combustion reaction occurs with oxygen, releasing a large amount of heat. Under the action of this heat, small holes filled with steam are formed inside the material, and the small holes are surrounded by molten metal walls.
⑵ The transfer of the burning material into slag controls the combustion rate of oxygen and metal. At the same time, the speed at which oxygen diffuses through the slag to the ignition front also has a great influence on the combustion rate. The higher the oxygen flow rate, the faster the combustion chemical reaction and the removal of slag. Of course, the higher the oxygen flow rate, the better, because too fast a flow rate will lead to rapid cooling of the reaction product, namely the metal oxide, at the outlet of the slit, which is also detrimental to the cutting quality.
⑶ Obviously, there are two heat sources in the oxidation melting cutting process, namely the laser irradiation energy and the heat energy generated by the chemical reaction of oxygen and metal. It is estimated that when cutting steel, the heat released by the oxidation reaction accounts for about 60% of the total energy required for cutting.
Obviously, compared with inert gas, using oxygen as an auxiliary gas can achieve a higher cutting speed.
⑷ In the oxidation melting cutting process with two heat sources, if the burning speed of oxygen is higher than the moving speed of the laser beam, the slit appears wide and rough. If the moving speed of the laser beam is faster than the burning speed of oxygen, the resulting slit is narrow and smooth.
Controlled fracture
For brittle materials that are easily damaged by heat, high-speed and controllable cutting by laser beam heating 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 severe mechanical deformation in the area, resulting in cracks in the material. As long as the heating gradient is balanced, the laser beam can guide the crack to form in any desired direction.
It should be noted that this controlled fracture cutting is not suitable for cutting sharp angles and corner slits. It is also not easy to succeed in cutting extra-large closed shapes. Control the fracture cutting speed quickly, and do not need too high power, otherwise it will cause the workpiece surface to melt and damage the slit edge. Its main control parameters are laser power and spot size.
