The laser beam is an amazing thing. The energy density of a continuous power laser beam is 4 trillion times higher than the focused energy of the sun. Manufacturers have determined that using this ultra-high power density method can complete everything from cutting and welding sheet metal to drilling holes on PCB boards. All work.
The laser can cut, join and subtract materials. They can even add materials through laser metal deposition or 3D printing. Among other things, we can also manipulate the beam diameter to change the power level, pulse frequency and energy density, so that the laser beam can trigger the correct material reaction in various processes. Indeed, the use of lasers in industry is wide and varied.
Different materials interact differently with light of various wavelengths, making some laser sources more effective than others in processing certain materials. For example, one of the known benefits of cutting metals in industrial applications with 1-μm wavelength lasers is the increased speed compared to cutting with CO
laser. A large part of it comes from the high absorption of carbon steel by light of this wavelength (for example, see
). When performing melting cutting in carbon steel (that is, using a non-reactive gas, such as nitrogen), a small beam of light that is effectively absorbed into the cut steel will be directly converted into a higher speed.
When cutting with a solid-state laser (such as a disk or optical fiber), the focused beam diameter is combined with the high absorption percentage of the laser emission to achieve a very fast cutting speed. This performance exceeds CO
Mainly seen in thin to medium-thick materials, as the thickness of the material increases, the advantages will shrink. The diameter of the beam can be controlled to a certain extent by collimating the light or moving the position of the focusing lens (see
), but the size of the beam is limited.
The beam diameter range depends on the size of the beam delivery fiber (see
). The 100-micron beam delivery fiber is usually used in lasers for cutting metal plates. This core diameter provides high beam quality and high cutting speed. As the material thickness increases, the very small spot size becomes a problem, which limits performance, cut quality and process reliability.
In order to alleviate this situation, a larger core diameter can be selected. Of course, the downside is that the minimum beam diameter becomes smaller than the diameter that the magnetic core can provide is much larger. Although quality and process reliability have been significantly improved, the speed of thinner materials has been affected.
This is where dual-core fiber can help. The small diameter core is coaxially installed with the large diameter core. Programmable blinds can change which core is active. This fiber is designed to enable the laser cutting system to achieve high speed in thin materials and high quality and reliability in thicker materials.
The beam diameter plays another role when welding. Although laser welding is not new by any means, it may save costs due to reduced rework, so it has great appeal in workshops and OEMs. Greater engineering flexibility; eliminates expensive and time-consuming downstream processes, such as grinding and polishing.
In sheet metal, there are two main methods of laser welding: heat conduction welding and deep penetration welding. Thermally conductive welding uses a strongly defocused beam located above the workpiece. The focal position of the beam is usually 6 to 12 mm above the surface of the workpiece, but can be as high as 25 mm. This process heats the metal above its melting temperature without forming steam. The power density ranges from 104 to 105 W/cm2 and depends on the thermal conductivity of the metal. For example, carbon and stainless steel are easier to weld than aluminum.
Although heat conduction welding presents a highly beautiful weld (positioned perpendicular to the laser beam (90 degrees)), although there is a certain degree of angular flexibility, it will affect the penetration depth, but the efficiency of the process is somewhat poor. When the process uses a solid-state laser that produces 1-μm light, 68% of the energy will be reflected from the illuminated area of the workpiece, resulting in low coupling efficiency, which limits penetration and welding speed. With CO
Laser, the coupling is worse, 88% of the light is reflected from the illuminated area, thus using CO for thermal conduction welding
Laser is impractical.
The 1 micron wavelength energy of the disk and fiber laser helps to increase the cutting speed during the fusion cutting process.
Despite some limitations, thermal conduction welding is still very popular among manufacturers, especially in highly visible applications that require rounded edges. Think of all the stainless steel appliances in your kitchen, or peek into the kitchen of a restaurant, and look at all the stainless steel surfaces. Observe carefully, you may find that when dealing with the problems caused by conventional welding, all manual repairs will have grinding marks and inconsistencies in radius.
Take a look at the parts produced by laser thermal conduction welding, and you will find that these problems have disappeared. This has indeed driven people's growing interest in laser welding, especially in environments that require a lot of rework.
Using the same laser source and beam delivery system, the beam density and focus position can be controlled to use the second technique for welding. Deep penetration welding or keyhole welding uses a focus position of approximately 0; that is, the focus is located on or near the surface of the material, resulting in a high energy density on the workpiece. Although the coupling efficiency is relatively low in heat conduction welding, in keyhole welding, both solid and carbon monoxide are quite high
Laser source; the coupling efficiency is 10% and 15%, respectively.
This process heats the workpiece to a temperature higher than the temperature at which steam is generated, and forms a steam capillary tube by the ablation pressure of the outflowing metal vapor, thereby forming "small holes", hence the name of the process. The power density is 105 to 106 W/cm2, and the penetration depth depends on the formation of the keyhole.
This welding technology can provide a higher welding speed, a narrow heat-affected zone and a considerable weld penetration. Due to low energy transfer and large penetration depth, deep penetration welding is more suitable for thick materials or when the weld is face-to-face or face-to-face welding. The ideal seam preparation method is side-to-side butt joints, although the keyhole process is usually well suited for various joint constructions.
In order to choose between thermal conduction welding and deep penetration welding, the application pushes the choice. Although it can be said that thermal welding looks better, the simple fact is that deep penetration welding is usually cheaper, mainly due to its high welding speed. In other words, you can still benefit from the speed and low energy transfer of deep penetration welding, while still getting the aesthetic and consistent rounding that is unique to thermal welding. Simply place the beam at a higher focal point to cross the seam that has been buttonhole welded.
Lasers continue to solve more and more manufacturing issues, and process variables such as beam diameter and manipulation continue to have a meaningful impact. From cutting and welding to adding or removing material layers, advancements in laser technology will surely be a key factor in the success of the fourth industrial revolution.
The latest technology has given us a great understanding of the potential of lasers in metal manufacturing and other areas. Consider ultra-short pulse duration lasers. To give an idea of scale, light travels at a speed of 186,000 miles per second. In one second, light can travel 7.5 times around the earth. In picoseconds, light only travels 300μm! If the absorption time of the processed material is less than the interaction time between electrons and phonons, cold ablation will occur. The metal will not be heated or melted, but will completely decompose.
Cold ablation can be used for metals and various other materials, including glass. In most cases, glass is processed through a scratch and break process, in which force scratches the material on the scratch line, or uses an ultraviolet (UV) laser for surface ablation.
The beam diameter can be controlled to some extent by moving the position of the focusing lens.
Why use ultraviolet lasers? It is related to absorption. Under normal conditions, transparent materials will not absorb infrared photons (~1μm). Those of us who are trying to cut transparent materials or coatings on disks or fiber lasers know this. This is why glass processors use UV lasers, but they can also use another method: non-linear light absorption by lasers with ultra-short pulse durations.
Similarly, under linear absorption conditions, transparent materials will not absorb photons. But in nonlinear light absorption, several photons will be absorbed at the same time, thus combining their energy and allowing IR (such as disk or fiber) to complete the work of the UV laser.
This is achieved by reaching those ultra-short pulse durations. They combine energy with ablation, not through heat treatment but through the direct decomposition of the material. This kind of cold ablation allows more precise processing of the material. This process, combined with optics that produce a slender beam profile, allows the laser to achieve extremely high cutting speeds in transparent materials.
The diameter of the transmission fiber determines the range of beam diameter.
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