The heat of the fiber laser mainly comes from the pump source and the gain cavity. For the pump source, its conversion efficiency is about 50%, which also means that an energy equivalent to the output optical power is generated in the form of heat. If the heat cannot be dissipated in time, the temperature of the internal chip will rise rapidly, and the center wavelength of the laser will drift as the temperature rises. For the gain cavity, after the pump light enters the active gain fiber, only a part of it is converted into laser output, and the rest of the energy is converted into heat energy. Thermal energy will increase the temperature of the gain medium, resulting in broadening of the fluorescence spectrum and a short lifetime of spontaneous emission, thereby reducing the energy conversion efficiency. Therefore, thermal management has a non-negligible significance for fiber lasers. At present, the commonly used thermal management technologies are mainly air-cooled and water-cooled. Among them, the air-cooled heat dissipation technology is mainly used in low-power pulsed lasers and low-power continuous lasers. Most of the medium and high-power fiber lasers use water-cooled heat dissipation as the main heat dissipation.
Two ways to dissipate heat
1. Water cooling
As the name implies, water cooling is the use of water to take away heat through a heat exchanger (such as a water cooling plate). Its working principle is also very simple, that is, the cold water in the chiller flows into the heat exchanger through the water pipe, and then comes out from another port of the heat exchanger, and then flows back to the chiller through the water pipe. Heat is carried away from the inside of the laser.
The water-cooled heat dissipation method has a simple structure and is easy to maintain; the heat dissipation capacity is strong and the temperature uniformity is good. The cooling performance of the laser can be improved by using a chiller with a larger cooling capacity. At present, there are more than 500 manufacturers integrating and selling handheld laser welding machines on the market, and they generally use water cooling. However, in addition to the laser itself, the hand-held laser welding machine with water cooling also requires additional chillers and water, which results in a substantial increase in the overall volume and weight of the equipment, and limited use environments.
2. Air cooling
In a broad sense, air-cooled heat dissipation refers to the use of fans to enhance air convection and complete heat exchange inside the machine. With the improvement of technology, major laser manufacturers have begun to set foot in the field of air cooling and heat dissipation. In June last year, the global fiber laser giant I company launched the air-cooled LightWELD 1500W handheld laser welding product; in August, GW launched the air-cooled A1500W intelligent laser welding machine in China; in October, Reci company also released the FCA1500 air-cooled laser welding machine. laser.
▲ Air cooled laser welder：reci、IPG、GW
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These three lasers are mainly aimed at the market segment of handheld laser welding. Air-cooled lasers can make work more flexible and portable. All three lasers use air-cooled heat dissipation without additional water-cooling equipment, which reduces costs. At the same time, the size and weight of the equipment are greatly reduced. Although they are both called air-cooled lasers, the air-cooled heat dissipation schemes used are different, including fan cooling, heat pipe radiator cooling, and compressor cooling and cooling. (1) Fan heat dissipation In the laser, the heat generated inside the pump source and gain cavity is dissipated using a substrate with good thermal conductivity (such as copper, aluminum nitride, etc.), and then the heat is dissipated by convection. This method is called convection cooling. Convective heat transfer can be divided into natural convection and forced convection heat dissipation according to the driving force of fluid flow. In the absence of external force, only the temperature difference of the fluid can make the fluid flow spontaneously to conduct heat transfer, which we call natural convection; when there is an external driving force, that is, the fluid is driven by fans, fans and other components. flow, thereby removing heat, we call it forced convection. Due to the extremely slow heat dissipation and poor effect of natural convection, it cannot fully meet the heat dissipation requirements of lasers. Therefore, it is necessary to add a fan to the entire cooling system to speed up the flow of air and turn natural convection into forced convection.
▲ Fan cooling principle
(2) Heat pipe radiator to dissipate heat
The heat dissipation of the heat pipe radiator means that the heat pipe relies on the phase change of the working liquid inside itself to achieve heat transfer. This liquid has a low boiling point and is easy to volatilize. One end of the heat pipe is the evaporation end, which is connected to the heat sink inside the laser; the other end is the condensation end, which is connected to the external heat sink and the fan. The tube wall has a liquid-absorbing wick, which is composed of capillary porous materials. When the laser is heated, the evaporating end is heated, the working liquid evaporates rapidly, the steam flows to the condensing end under the pressure difference, and the heat is released, which is discharged through the fan; at the same time, the steam condenses into liquid again, and the liquid flows back to the evaporation section through the wick. (If it is a gravity heat pipe, there is no wick, and the liquid adheres to the tube wall and flows back to the bottom evaporation section by gravity). This cycle does not stop, and the heat is transferred from the inside of the laser to the outside.
▲ Heat dissipation principle of heat pipe radiator
IPG’s LightWELD 1500 handheld laser welding system uses a heat pipe radiator cooling solution. The design and manufacture of LightWELD is characterized by small size and light weight, which leads a new generation of changes in the current handheld laser welding machine. In addition to welding, it also realizes the functions of handheld laser welding and cleaning. LightWELD hand-held laser welding machine adopts air-cooling method, without the power consumption required by additional chiller equipment, eliminating the chiller piping, components, control and maintenance links, reducing costs while increasing portability and improving the overall reliability of the system.
▲ LightWELD 1500 Handheld Laser Welding System
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(3) Compressor cooling and cooling
Compressor refrigeration and heat dissipation principle: The compressor compresses the refrigerant, turns the refrigerant into a high temperature and high pressure gas, and flows to the external condenser. The high temperature and high pressure gas is condensed into a low temperature and high pressure liquid, and the heat generated by the liquefaction is discharged out of the machine with the fan. The low-temperature and high-pressure liquid refrigerant is depressurized through the expansion valve and becomes a low-temperature, low-pressure, easy-to-evaporate state, and flows to the internal evaporator. The evaporator absorbs heat to reduce the internal temperature of the laser to achieve the effect of cooling, and then the refrigerant vaporizes into a high temperature and low pressure gas. The gas refrigerant evaporated by the evaporator is compressed by the compressor again and circulates back and forth, which realizes the heat dissipation inside the machine.
▲ Compressor refrigeration and heat dissipation principle
The A1500W smart air-cooled handheld welding machine launched by GW Laser uses the compressor cooling and heat dissipation scheme. GW Laser focuses on the continuous exploration and innovation of 976nm technology
Combined with the high photoelectric conversion efficiency of 976nm, it creatively solved the problem of air-cooled cooling capacity, and launched the first air-cooled 976nm technology in the industry, which solved the problems of power consumption and portability, and once again led the technological development direction of fiber lasers. This model has realized the three-in-one function of welding, cutting and cleaning.
▲ GW Laser A1500W Smart Air-cooled Handheld Welder
Comparison of several cooling methods
The structure of fan cooling is relatively simple. It simply spreads the heat in the heat sink to the heat sink, and then uses the temperature difference between the heat sink and the ambient air to dissipate heat through forced convection of the fan. When the ambient temperature is too high in summer, the temperature difference between the heat sink and the air is too small, and the heat dissipation capacity will be greatly reduced. It can only passively dissipate heat, is greatly affected by the environment, and cannot accurately control the temperature. The advantage is that the overall equipment and control system are simple.
Compared with the simple fan cooling method, the heat pipe radiator has more heat pipes, so its structure is relatively complicated. It relies on the evaporation and condensation of the working material to quickly transfer heat from the heat sink to the heat sink and then dissipate the heat into the air through the fan. It also belongs to passive heat dissipation, which cannot accurately control the temperature and is greatly disturbed by the surrounding temperature.
The compressor cooling and heat dissipation scheme belongs to active heat dissipation. Due to the existence of the compressor and the expansion valve, the temperature can be precisely controlled by adjusting the flow and pressure of the refrigerant. At the same time, the temperature of the refrigerant in the condenser is higher than that of the heat sink, which is conducive to rapid heat generation. transmitted to the air. Its control system is more complicated; at the same time, because its structure is much more complicated than the above two schemes, the volume and weight of the equipment are also increased accordingly.
Most of the traditional fiber lasers use water cooling to dissipate heat. First, the water is cooled by compressor refrigeration, and then the laser is cooled by water. The air-cooled heat dissipation scheme of Guanghui Laser directly uses compressor cooling to cool the laser, abandoning the existence of water and eliminating the intermediate heat transfer link, so the heat dissipation efficiency is higher, and the volume and weight can be made smaller.
In the laboratory, we use a constant temperature and humidity test box to set 35°C to simulate the high temperature use environment in summer, and test the temperature change of the internal gain fiber of the laser with different air-cooling schemes under the condition of full 1500W power. . From the experimental data, it can be clearly seen that the fiber temperature increases exponentially in the first few minutes and stabilizes around 10 minutes. Due to the cooling effect of the compressor, the laser can be actively cooled, so the temperature can be controlled below 60 °C, and the temperature change is relatively stable; while the other two can only rely on passive heat dissipation, so the internal temperature is slightly higher than that of the compressor cooling scheme; , Due to the high heat transfer efficiency of the heat pipe, the heat can be well exported from the inside of the laser, so its internal temperature is lower than that of a pure fan, and the temperature rise is more gentle.
▲ The temperature change with time when the laser outputting 1.5kW laser with different air-cooling schemes
(laboratory data, there may be deviations from actual field use)
Epilogue
In the field of fiber lasers, GW Laser has always been aiming at the global laser giant IPG. It is the unique brand advantage of Guanghui to create products with military quality. Many years ago, GW Laser began to organize scientific research forces to conduct continuous exploration in air cooling and heat dissipation. In the future, we will continue to improve this aspect, continuously improve the stability of products, realize iterative upgrade of products and technologies, and meet the needs of more industries. processing needs
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