The Unique Technology Of CO2-Resonator By Mitsubishi Electric: The Benefits In Modern Industry
To talk about the features and benefits of different types of sources and structures, it is necessary to mention the basic principles of their work. Let’s start with a more detailed consideration of CO2-technology and the operating principle of the emission of CO2-source (Fig.1). Initially, gas, active medium, which is a 3-component mixture of gases in the proportion determined by the manufacturer: CO2: N2: He, is pumped in a sealed tube or casing. In the case of installations by Mitsubishi Electric, the gas mixture additionally includes CO. In any arrangement of the cavity, a mixture of gases is between the electrodes. When the laser is activated, influenced by high voltage between the electrodes, the electrons collide with N2 molecules, thus increasing the amplitude of their oscillations. The collisional energy transfer mechanism between N2 and CO2 causes the excitation of the CO2 molecules, which in turn leads to the emission of photons with wavelength of 10.6 microns. The component of CO gas is used to maintain the gas properties. Furthermore, the photons multiply reflected between opaque (TR – Total Reflective) and translucent (PR – Partial Reflective) mirror, amplifying the processes of emission of photons in the active medium, create radiation generation. And as soon as the beam has gained enough power, it overpasses the partial reflective mirror, and by passing through the cavity mirror system, it enters the cutting head along the optical path [1, 2].
A typical disadvantage of this design is strong heating of the gas during its passage along the optical axis, which in turn affects the quality of radiation. The solution to this problem, in addition to using a heat exchanger, is the use of high-speed turbine, which significantly increases the rate of gas flow, not allowing its heating. Approximate gas circulation rate is 200 m/s, which reduces the stability of the radiation parameters. This raises the question of the duration of the life of this expensive turbine supercharger and the cost of its replacement. According to the manufacturers, the turbine life is no more than 20 000 hours, which corresponds to the 5-year lifetime of the equipment when working in 2 shifts of 8 hours, and the cost of its replacement is 25 000–35 000 Euro. It should be noted that this type of cavitys comprises glass tube, inside which the gas circulates. These tubes are exposed to gradual overheating and loss of cylindrical shape, which leads to the necessity of replacement and downtime. It is necessary to take into account the number of mirrors used in the cavity. This parameter influences the cost and complexity of its service. Depending on the manufacturer and the cavity power, the design comprises from 7 to 24 mirrors. Choosing the power of coaxial cavity, it is necessary to know that in some designs its increase is due to an increase in the number of mirrors. Voltage in these cavitys is usually transmitted from multiple electrodes, which should be replaced according to the service rules.
The second subtype is a 3-axis cavity with a transverse-flow gas laser – Cross-Flow (Fig.3). In this case, the gas is pumped across the optical axis passing through the heat exchanger and the four fans, which pump the gas in pairs in opposite directions, eliminating distortion of the optical axis direction. The discharge is fed perpendicularly to gas stream and the optical axis, thus the cavity structure acquires a 3-axial direction.
This construction allows for operation with gas circulating at a speed of 10 m/s without requiring high gas pumping speed. This in turn increases the radiation stability and eliminates the need to costly increase the number of turbines and mirrors; the cavity comprises only 5 mirrors. The presence of one of the ceramic electrode, which, according to the regulations does not require replacement, but only maintenance, suggests an even greater savings. Also single electrode driving system leads to a reduction of power consumption when the laser is turned off, by supplying only the desired discharge time. According to this principle of operation, the rectangular radiation pulse is created and the peak power is maintained for a long time. This minimizes the thermal influence on the metal. Other manufacturers the pulse can have triangular shape and is characterized by sharp decay of power.
In addition to the above, we should note that this design involves injecting gas into the sealed enclosure. Then, minimal consumption of laser gas is provided – 1–3 l/h. This differs from the coaxial cavity, where the gas pumping occurs at all times during operation of the cavity, and thus the gas consumption is 10–20 l/h.
When again addressing the issue of the composition of the gas mixture for Cross-Flow cavity, we should also note that there is an aspect of cost savings for the customer. The most expensive component of the laser gas mixture is helium (He). For cavitys by Mitsubishi Electric, the share content of this element is 28%, whereas the other producers of its share is from 60% to 80%, which means a lower cost of 1 balloon of laser gas for Cross-Flow cavity.
All these advantages make it possible to talk about the reliability of Cross-Flow the cavity by Mitsubishi Electric and its low cost of routine maintenance. We know that it is 2–3 times lower than the cost of servicing for the CO2-laser of the same power by any other manufacturer (Fig. 4, 5).
Today, many manufacturers say that there is no perfect source for all laser cutting applications. Each source has its uses, and this point of view, of course, is unanimous. However, the presence in the industry if the Cross-Flow laser cavity allows us to speak about its widespread use for cutting of various metals having different thicknesses.
When choosing a solid-state radiation source, the user often justifies his choice by the declared benefits of this technology: cost of the service and its simplicity, high performance, tolerance to reflective materials, low operating costs.
But are the advantages always clearly expressed and compared with the use of CO2-technology and in particular, Cross-Flow? To really understand this, you need to get acquainted with the peculiarities of solid-state cavitys. The main solid-state sources, which received wide industrial applications, are: YAG, disk and fiber lasers, the main difference between them is the active medium. We will try to consider all aspects wherein these structures are different from each other, and compare them with the CO2 laser cavity, Cross-Flow.
Initially, YAG-laser has appeared in the market, where YAG rod is used as the active medium which is cylindrical in shape and the pumping source is a high-intensity flash lamp, least often – pump diodes. The mirrors are still used in the design. However, the major drawback limiting the use of this full industrial application of this source has always been unstable radiation and its limit power due to the occurrence of the thermal lens effect. The point is that the rod is disposed between the pump flash lamps along the optical axis and is subject to constant internal and external heat. Cooling of the rod is possible only through its outer surface. Thus, it is exposed to thermal deformation and assumes an elliptical shape when the pump flash lamp is located on one side, or the double ellipse shape if the pump energy influences on the rod from both sides. All this leads to deterioration in the quality of the laser radiation and the need for expensive maintenance. The resolution of this cooling issue was changing the geometry of the active medium of solid-state YAG-laser. Thus, two modern versions of a solid-state power have been developed: disk cavity (stem diameter increased and length reduced) and fiber cavity (diameter reduced, but the length of the active medium increased). These two versions have become the solution to the issue of active medium cooling.
The disc cavity in a disk-shaped crystal is used as an active medium, and the diode arrays are used as the elements that emit pumping energy. The disk is cooled through the surface area by coating with high reflective properties or the heat-sinking element. The wavelength of the laser is approximately 1.064 microns. One disadvantage of this type is the source of the presence of the cavity mirrors in the design, which raises the questions of considerable ease of maintenance and lower cost in comparison with the Cross-Flow cavity.
Fiber laser active medium is in the form of an optical fiber doped with rare earth elements. The single laser diodes are used as the pumping elements. This cavity structure does not have optical elements at all, and the function of reflection of extraneous radiation inside the active fiber is performed by Bragg gratings. The wavelength of the fiber source is approximately equal to 1.07 mm.
We will not focus on the comparison of the two kinds of solid-state laser radiation sources, the allocation of their advantages and disadvantages in relation to each other. In this case, we are talking only about their common participation in the applicability of solid-state technology as compared with CO2-technology by Mitsubishi Electric, and its compliance with all the requirements of the user. Let us address those aspects which are the most important for the client when choosing a laser installation, and analyze in detail each of them.
Let’s start with the least significant difference for our market – the safe operation. In this case, the main influencing aspect is the wavelength. When λ≈1 µm, the radiation is in the near infrared range of the spectrum. Such radiation has a negative impact on a person’s vision, so the laser systems with solid-state radiation sources are provided with the safety means for the staff by the equipment manufacturers, protective glasses to visualize the cutting area are equipped by all means. The cab of the machine is covered by a roof, providing full protection for the operator during operation. The CO2 sources are safer, since in this case the working wavelength is 10.6 micrometers in the IR range. The working area of such installations is usually also protected on all sides by covers, but without a roof and special protective glasses.
The truly undeniable advantages of disk and fiber lasers in laser cutting shall be emphasized. One of them is low power consumption due to a higher efficiency of the laser, resulting in lower operating costs. This advantage is undeniable. For example, disk cavity efficiency reaches 25%, that of the fiber cavity – 25–30%, whereas the CO2laser efficiency is 10–12%.
The second advantage is the stability of the disk and fiber cavitys to the processing of materials with high reflectance, such as brass and copper. However, full utilization of this advantage is possible only in the situations where there is a constant need for treatment of these materials. Otherwise, when clients need to process these materials to 3 mm thick, in small volumes, these advantages disappear, since CO2-lasers can operate in this range, but with the additional training of the treated metal surface.
Speaking of simplicity and cost of service, it is necessary to carefully approach to the comparison of the two technologies. Initially, we must accept the fact that the laser systems with CO2-cavitys are produced for over 30 years. They have evolved in the price and performance level. The cost of their routine maintenance and sufficiently accurate resources for the main components is known for sure. On the other hand, the technology of using solid-state sources have developed for the last 5–6 years, respectively, laser devices with such cavitys have been operated for comparatively short time. To regulate particular resource of the components, and thus the total cost of maintenance of such cavitys is quite difficult. Presumably, the high cost of maintenance of the coaxial cavity by any manufacturer on a plant with similar power will indeed be higher than the cost of service of the considered solid-state sources because of the need to clean and replace the large number of mirrors, glass tubes, electrodes, and most importantly – the replacement of the turbine at the end of a particular resource.
If observed in more detail, in comparing each solid-state source with the analogue CO2-source, the result will be different since the characteristics of disk and fiber cavitys. For example, a disk cavity contains mirrors, which still need to be serviced and replaced periodically. The fiber cavity does not comprise mirrors. Also, it should be noted that the different elements of power pumping are used in fiber and disk lasers. Disc lasers the pumping elements are the diode arrays, fiber lasers employ single diodes that also have different resource of the claimed work. However, a key advantage of these techniques with respect to CO2-lasers is still a lack of an optical beam transmission system in the cutting head. Instead a fiber cable is used. But if these technologies are compared with CO2 Cross-Flow, this comparison carries the opposite sign, since this cavity design eliminates all the above maintenance complexity, except tract maintenance and replacement costs of components. This conclusion is based on the existing primary practice received by Mitsubishi Electric, in serving both CO2 and fiber laser systems, comparable in power. Therefore, the sharp judgment that the maintenance cost of solid-state source is lower than the maintenance cost of CO2-laser Cross-Flow is debatable.
Back on topic of the optical path, we should note that the CO2 lasers when transferring radiation to the cutting head must be purged with auxiliary gas. For installations of up to 4 kW air is used as a purge gas, and nitrogen is used in the lasers with power of above 4 kW. These are additional operating costs. While the solid-state sources uses nitrogen in completely small quantities only to purge optics in the cutting head. Also, it is possible to take into account the characteristic contrast of the optics used in these two types of sources. The point is that the optics for solid-state sources is highly susceptible to contamination. It is recommended not to clean the lens, and immediately replace it with a new one. To prevent molten metal particles or dust on the lens in the cutting head, the solid-state laser source units are equipped with protective glass. But it is difficult to calculate the resource of this glass: it depends on the treated metal, and the operator skills. The cost of the original glass is a certain line costs in the operation of such units. The CO2-installations do not have protective glass installed and the lens is more resistant to dirt, moreover, there is the possibility of its periodic cleaning. But then it should be noted that despite the fact that the resource of the lens for CO2-units is higher, its cost is higher than that of the lens for the units with solid-state cavitys.
With regard to the quality of the resulting edge, it is higher when cutting on the plants with CO2-cavitys, which is especially noticeable in the medium and large thick stainless steels.
Finally, one of the most important aspects of the comparison is the performance. Indeed, the characteristics of solid-state sources enable to faster melt the metal due to the lower wavelength. This is clearly seen in the processing of stainless steel sheet, where the speed is different about 3 times. Frequently this fact is taken as an indisputable argument in choosing laser cutting for all materials, but in fact a pronounced advantage is achieved only in the processing of stainless steel up to 6 mm thick. It should be mentioned, that the processing speed of the structural steel to a thickness of 4 mm using different laser sources differs slightly. For cutting the metal with a thickness above this limit, the CO2-laser is the most acceptable. From practical experience we knew that in some cases, solid-state sources lose speed even when processing low-carbon steel sheet. All this is due to the fact that the cutting process takes place with the combustion process using oxygen as the cutting gas. In addition, the difference is due to the different level of integrated assembly of the installation, as well as its inherent software processing parameters of various materials and thicknesses. After all, the performance is influenced not only the maximum feed rate, but many more other factors, such as bypass speed and cutting hole speed, the diameter of the nozzle, cutting gas pressure.
We can draw a conclusion about the relative qualities of solid-state and Cross-Flow CO2-technology, specifying only the most important criteria for selecting the laser system. Solid-state technology has an important source of electricity and the optimum efficiency for cutting sheet applicability of stainless steel, but it does not have high reliability. The reason is the presence of elements of the energy pumping. They have their own life, after which their failure is reflected in the loss of the power plant and the complexity of its further operation on the maximum values of operating parameters, up to replacement of the expensive module. It should be noted that to date Mitsubishi Electric has never received a claim from a client to replace the cavity on any installation. This source is characterized by minimal expenses for routine maintenance, excellent quality of cutting edge and performance on a wide range of thicknesses of various materials. Cross-Flow cavity has the highest operation resource; therefore it is the embodiment of the concept of "reliability" in the production and operation of laser systems.