Introduction
In the field of laser processing, laser cutting machines have been widely used in many industries due to their advantages such as high precision, high speed, and non-contact processing. However, when dealing with highly reflective materials, the cutting effect of laser cutting machines is significantly affected. Highly reflective materials, such as copper, aluminum, gold, silver, and their alloys, pose difficult challenges in the laser cutting process due to their high reflectivity to laser and good thermal conductivity. An in-depth exploration of the influence of highly reflective materials on the cutting effect of laser cutting machines is of great significance for expanding the application range of laser cutting technology, improving processing quality and efficiency.
Characteristics of Highly Reflective Materials and Their Influence on Laser Absorption
Basic Characteristics of Highly Reflective Materials
Highly reflective materials usually have low resistivity, which results in a low absorption rate of laser. For example, copper and aluminum rank among the top in electrical conductivity among common metals. At room temperature, the absorption rate of red copper to infrared laser is about 5%. At the same time, the surface state of highly reflective materials also has an important impact on laser reflection. The smoother the surface, the stronger the reflection of laser. For instance, the reflective ability of ordinary brushed stainless steel is relatively weak, but when the surface reaches a mirror effect, it will cause obvious reflection during laser processing. In addition, the absorption rate of materials to laser varies with their states. The absorption rate of solid metals to laser is generally low, but it will increase significantly when melted into a liquid state. For example, when red copper is heated to near its melting point, the absorption rate can reach about 20%.
Absorption Characteristics of Highly Reflective Materials under Different Wavelength Lasers
Different materials have significantly different absorption rates for light of different wavelengths. Take copper (Cu) and silver (Ag) as examples. For fiber lasers with a wavelength of 1070nm, they are typical highly reflective materials, with an extremely low absorption rate, much lower than that of iron (Fe) and steel (Steel). However, their absorption rate is relatively higher when exposed to solid-state lasers. The laser emitted by a CO₂ laser is in the infrared band (usually 10.6um), which performs well in many industrial application fields, but it is not ideal for processing highly reflective metals in special bands. This indicates that choosing an appropriate laser source with a suitable wavelength is crucial for increasing the absorption of highly reflective materials to laser and improving the cutting effect.
Multiple Influences of Highly Reflective Materials on the Cutting Effect of Laser Cutting Machines
Decrease in Cutting Quality
Dross and Burr Problems
When cutting highly reflective materials, due to insufficient absorption of laser by the materials, the heat generated during the cutting process is not enough to fully melt and vaporize the materials, resulting in the formation of dross at the bottom of the cutting surface. At the same time, the incompletely melted materials, under the blowing of the auxiliary gas, may form burrs on the cutting edge. For example, when using an ordinary laser cutting machine to cut copper materials, obvious dross is often found at the bottom of the cut, and the burrs on the cutting edge are relatively rough, seriously affecting the flatness and smoothness of the cutting surface.
Inconsistent Cut Width
The reflection and heat conduction of highly reflective materials to laser make the distribution of laser energy on the material surface uneven, thus leading to inconsistent cut widths. During the cutting process, situations such as a wider upper part and a narrower lower part of the cut or significant local width changes may occur. This not only affects the cutting size accuracy but may even lead to the scrapping of products for some parts processing with extremely high requirements for dimensional accuracy.
Expansion of the Heat-Affected Zone
The good thermal conductivity of highly reflective materials causes the heat generated during the laser cutting process to spread rapidly to the surrounding areas, expanding the range of the heat-affected zone. The structure and properties of the materials within the heat-affected zone may change, such as a decrease in hardness and a deterioration in toughness. Taking the cutting of aluminum alloy as an example, the expansion of the heat-affected zone may lead to a decline in the mechanical properties of the materials, affecting the service life of the products.
Reduction in Cutting Efficiency
Low Energy Utilization Rate Caused by Low Absorption Rate
The low absorption rate of highly reflective materials to laser means that a large amount of laser energy is reflected and cannot be effectively used for material cutting, resulting in a low energy utilization rate of the laser cutting machine. In order to achieve material cutting, it is necessary to increase the laser power or extend the cutting time, which undoubtedly reduces the cutting efficiency. For example, when cutting copper and steel materials of the same thickness, the laser power required for cutting copper materials is often higher, and the cutting speed is significantly slower.
Limitation of Cutting Speed
Due to problems such as incomplete penetration and dross formation during the laser cutting of highly reflective materials, in order to ensure the cutting quality, the cutting speed has to be reduced. An excessively slow cutting speed not only prolongs the processing time of a single workpiece but also reduces the overall production efficiency. For example, when cutting thick plates of highly reflective materials, the cutting speed may be only a fraction of that when cutting ordinary metal materials.
Increased Equipment Wear
Damage to the Laser Caused by Reflected Light
When the laser irradiates the surface of highly reflective materials, most of the energy will be reflected. These reflected light rays may return to the inside of the laser and damage the optical components of the laser, such as damaging the mirrors of the laser resonator and affecting the performance of the laser diode, thus shortening the service life of the laser. In severe cases, the reflected light may even cause the laser to have unstable light output or stop emitting light.
Increased Burden on the Auxiliary System
In order to overcome the adverse effects of highly reflective materials on laser cutting, it is usually necessary to increase the flow rate and pressure of the auxiliary gas to help blow away the dross and improve the cutting quality. This will increase the burden on the auxiliary gas system, increasing the operating cost and maintenance frequency of the equipment. At the same time, due to the large amount of heat generated during the cutting of highly reflective materials, higher requirements are also put forward for the cooling system of the equipment. The long-term high-load operation of the cooling system is also prone to failures.
Strategies for Coping with the Challenges of Cutting Highly Reflective Materials
Optimization of Laser Equipment
Adoption of New-Type Lasers
The development of new-type lasers for highly reflective materials is one of the key solutions to the problem. For example, the wavelength of blue lasers usually ranges from 400 - 500nm. Compared with traditional infrared lasers, highly reflective materials have a higher absorption rate for blue light. The 500w series and 1000w series semiconductor blue lasers launched by the Guangdong Hard Technology Innovation Research Institute of Guangdong-Hong Kong-Macao Greater Bay Area perform well in the welding, cladding, and 3D printing of highly reflective materials. In scenarios such as lithium-ion battery welding and electronic component processing that require the processing of copper materials, the power required for using a blue laser is only one-tenth to one-fifth of that of a conventional infrared laser, and "spatter-free welding" can be achieved. The welding speed is at least 8 times faster than that of infrared lasers.
Optimization of the Fiber Output Head
In fiber laser machines, the reflected laser caused by highly reflective materials is likely to damage the output optical cable head. Raycus Laser has adopted a new QBH fiber output head, adding a stage of reflected light stripping device on the original basis, which can strip most of the reflected laser in the first place. Combined with the water-cooling heat dissipation design, it reduces the thermal impact on the optical cable output head. After strict copper surface welding tests, the core components of the RFL-A1500D laser equipped with this output head can still maintain a normal operating temperature in a high-reflected light environment, effectively verifying its performance.
Adjustment of Process Parameters
Control of Laser Power and Energy
According to the characteristics of highly reflective materials, appropriately increasing the laser power can increase the absorption of materials to laser and improve the cutting efficiency. However, too high a laser power may cause excessive melting and vaporization of the materials, resulting in more splashes and dross. Therefore, it is necessary to precisely control the laser power and energy to achieve the best cutting effect while ensuring the cutting quality. For example, when cutting highly reflective materials of different thicknesses, determine the appropriate laser power and pulse energy through experiments to achieve good cutting quality and efficiency.
Optimization of Cutting Speed and Frequency
Adjusting the cutting speed and frequency is also an important means to improve the cutting effect of highly reflective materials. If the cutting speed is too fast, the materials cannot fully absorb the laser energy, and problems such as incomplete penetration and dross formation are likely to occur. If the cutting speed is too slow, it will lead to the expansion of the heat-affected zone and a decline in the cut quality. At the same time, reasonably adjusting the pulse frequency of the laser can control the melting and vaporization process of the materials and reduce the generation of burrs and dross. In actual processing, it is necessary to optimize the combination of cutting speed and frequency through experiments according to the thickness, material properties of the materials, and the performance of the laser equipment.
Application of Auxiliary Measures
Selection and Control of Auxiliary Gas
When cutting highly reflective materials, adding appropriate auxiliary gas can significantly improve the cutting effect. The auxiliary gas reacts chemically with the materials at high temperatures. For example, when oxygen is used as the auxiliary gas, it can undergo an exothermic chemical reaction with metal materials, playing a role in promoting combustion and increasing the cutting speed. Nitrogen can, to a certain extent, improve the quality of the cutting surface and prevent oxidation. In addition, the auxiliary gas can also help blow away the dross in the cutting area, clean the cutting seam, and cool the area near the slit to protect the focusing lens. Selecting the appropriate gas pressure and flow rate is crucial for the cutting process. Different highly reflective materials and cutting thicknesses require matching different auxiliary gas parameters.
Surface Pretreatment of Materials
Performing surface pretreatment on highly reflective materials, such as sandblasting and roughening, can reduce the surface reflectivity of the materials and increase their absorption rate of laser. Through surface pretreatment, tiny concave-convex structures are formed on the material surface. When the laser irradiates, the light is reflected and scattered multiple times in these concave-convex structures, increasing the opportunity for the materials to absorb the laser. For example, after sandblasting the aluminum plate, its absorption rate of laser is significantly increased, and the cutting effect is significantly improved.
Conclusion
Due to their unique physical properties, highly reflective materials have multiple negative impacts on the cutting effect of laser cutting machines, including a decrease in cutting quality, a reduction in cutting efficiency, and increased equipment wear. However, through the optimization of laser equipment, such as the adoption of new-type lasers and the optimization of fiber output heads; the reasonable adjustment of process parameters, precisely controlling the laser power, energy, cutting speed, and frequency; and the application of effective auxiliary measures, such as selecting appropriate auxiliary gases and performing surface pretreatment on materials, it is possible to overcome the cutting problems caused by highly reflective materials to a certain extent and achieve a relatively ideal cutting effect.
With the continuous development and innovation of technology, it is believed that more breakthroughs will be achieved in the field of laser cutting of highly reflective materials in the future, further expanding the application range of laser cutting technology and promoting the development of related industries.
--Rayther Laser Jack Sun--