Fiber Laser Cutting Machine BasicsTriveni
Laser cutting is the most common method to cut sheet metal and metal tubes in the world. It can be used to process a variety of metals including Mild Steel, Stainless Steel, Aluminium, Copper, Titanium, Tantalum, and many other metals and alloys. A laser is a very thin beam of light that has very high energy. One more feature that this beam has is that it is monochromatic. That is, unlike white light, it has only one frequency.
The two most popular laser machines that are used to cut sheet metal are fiber laser and CO2 laser. These lasers are very different in their construction and properties but we are not going into the details of that. What we need to know is that both produce laser light of high intensity and monochromaticity (and coherence and polarization etc.). Both produce light of different wavelengths. For fiber lasers, this frequency is 1,070 nm while for CO2 lasers it’s close to 160 nm. This means that both of the emissions are in the infrared spectrum. Both lasers behave differently with different materials. Why does this happen?
To understand this, we would ideally like to talk about stable molecular and atomic configuration levels. But we will skip that for now and look at it from a simple lens. We know that different materials absorb different wavelengths of light and this is what gives them their color. The same is true for infrared radiation also: different materials absorb different materials. This would not lead to any changes in color because infrared light is invisible to us.
When a laser of high energy falls on the material at proper focus (we will talk more about focus in one of the later articles), the material can either “absorb” or “reflect” the light. If the material absorbs the light, the energy from the light is transferred to the object (because energy cannot be created nor destroyed). When an object absorbs more energy, its properties change. For example, in sunlight, steel becomes hot. Now the energy from a laser cutting machine is much higher than that from the sun. Thus, the metal becomes “very hot”. Let us note that because the laser light is very focussed (beam diameter 100 microns and even less), most of the energy is absorbed by a very thin section of the sheet on which the laser light is falling and hence one small portion of the material becomes very unstable and with suitable optics and technology discussed, later on, this can be used to break the material from that point. If we then move the laser beam along the surface, we can cut it in the desired shape. More of that later.
When a material becomes very hot, it can melt and even vaporize. Or it may react with the surrounding gases or materials to form compounds. Please note that this will happen only when the material absorbs the light. If it reflects it, it remains in its previous state and the light continues to travel till it is absorbed by another material (or to infinity if we really go technical about this. But that is beyond the scope of this article).
As discussed earlier, the amount of light absorb differs from material to material. Some materials like CR and HR mild steel absorb a large proportion of the light and some others like Aluminium and Copper not so much. This is the reason that the fiber laser is able to cut through steel more easily than Copper and even Aluminium; even though Aluminium is lighter and is much easier to cut with shearing blades and can be bent easily with bare hands, unlike steel.
As an ending note, the material, once it absorbs energy, is generally cut using two methods in a laser cutting machine. One machine involves oxidation or “burning off” of the material and the second method involves melting and quick removal of the material along the cut line.
Both these methods require different assist gases and slightly different technology to achieve good cutting. While oxidation involves, obviously, oxygen, the second method generally uses Nitrogen as an assist gas.
When Nitrogen is used, we use a single layer cutting nozzle (also known in short as a single nozzle) and when we use Oxygen as an assist gas, a double-layered nozzle (also called double nozzle) is used. The layers in the nozzle are to control two parameters called pressure and flow rate. This will make up for the bulk of our discussion in the next post.