Metals are subject to continuous research to improve their mechanical properties for various applications. This is no different for laser cutting. There are more and more alloys whose cutting surface is of very high quality by laser cutting. But, although the alloy has an impact on the performance of the laser cutting machine, it is far from the only property that is important. In addition, there are different metal types that can be laser cut.
In this article, we will outline the most important parameters you should look for in your search for the right metal for your laser piece.
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Laser cutting offers a whole host of opportunities and challenges, from mild steel to high tensile steel to stainless steel, aluminum, titanium and nickel or copper alloys. All these metals have different applications and purposes and are used for different products. However, they can all be laser cut.
Below is an overview of the different alloys and their potential for metalworking with a laser cutting machine.
Mild steel is the most commonly used metal cut with lasers because of its high volume of industrial applications. It is also the easiest metal to laser cut.
mild steel consists mainly of iron with several other alloying elements in small controllable amounts. These are mainly Carbon (C), Manganese (Mn), Silicon (Si), Phosphorus (P) and Sulfur (S).
Most mild steels have a low carbon content (0.05-0.4%). More carbon improves the tensile strength and hardness of the metal, but also makes it more brittle and significantly reduces weldability. The lower the carbon content, the higher the cutting speed. Metals with a carbon content close to 0.4% also tend to produce small micro fractures at the cutting edge. If the carbon content is below 0.15%, micro fractures will not occur. It is important to take this into account, as these fractures can later cause cracks or metal fatigue. However, there are heat treatments to counteract this.
Manganese (0.25-0.50%) is mainly used because it helps to deoxidize steel during production. Oxidation is detrimental to steel quality. In addition, it also prevents the formation of iron sulfide. It does make laser cutting more difficult. The cutting speed will be lower, as will the quality of the cutting edge. Phosphorus (below 0.035%), on the other hand, increases the weather resistance of the steel.
Sulfur (below 0.04%) is generally considered more of a contaminant than an alloying element. It makes steel more brittle and harder to weld, but in low amounts it increases machinability. Metals with a sulfur content of about 0.4% are often used in the electrical industry. This is because the sulfur provides an increase in electrical resistance and resistance to thermal oxidation. However, the cutting speed is up to 20% lower with a high sulfur presence.
Whether thin or thick sheet metal, mild steel can always be lasered with acceptable to superior cutting edges (especially compared to other thermal cutting technologies). All alloys of mild steel remain within the tolerances of the laser cutting process and do not cause major differences in machine settings. The alloy also does not affect the cutting edge quality and the cutting speed that much.
This makes a laser cutting machine very suitable for machining the most commonly used steels in industry and that in large quantities.
Stainless steels fall under the category of chromium – nickel steels. Their alloy quantities are up to 10-20 times higher than for mild steels. Chromium (Cr) is the main element and accounts for about 18% of stainless steel. Nickel (Ni) accounts for about 8% in the most common types.
The addition of nickel and chromium makes for a less brittle material and makes welding easier. Most stainless steels are not magnetic, but there are types with a low nickel value that are magnetic.
Stainless steel is highly resistant to corrosion due to a natural Cr2O3 film on its surface when exposed to the atmosphere. This film provides a protective screen against further corrosion.
Because of this reactivity with oxygen, impurities often arise during laser cutting that make the cutting surface look very impure. Therefore, stainless steel is mainly lasered with nitrogen as a non-reactive gas. This does significantly increase the cost of cutting. The cutting thickness is also significantly lower for stainless steel. For similar quality results as mild steel can you have to halve the cutting thickness.
Aluminum (Al) is the second most common steel in the industry when it comes to laser cutting. As with stainless steel, aluminum oxidizes very quickly when exposed to the elements. An Al2O3 film appears over the surface which prevents further corrosion.
As with stainless steel, this film will react strongly with oxygen during cutting, so here too, it is often switched to nitrogen or another non-reactive gas.
Aluminum is not magnetic and therefore cannot be moved by magnetic methods.
Titanium and nickel alloys
Especially in the aerospace and energy industries, titanium (Ti) and nickel (Ni) alloys are very popular. They have good mechanical properties, high wear resistance, toughness and have a great strength to weight ratio versus other metals (including aluminum). Some also have high corrosion resistance in chemical environments and can handle high temperature rise. These advantages must always be weighed against a higher cost per pound.
Titanium has been used since the early 1950s, but is an expensive metal used mainly in aerospace as a structural steel. In the energy sector, its high temperature and high pressure resistance are useful, for example, in gas turbines. It is also biologically compatible with human tissue and bone, making it very suitable for the medical industry.
During laser cutting, titanium reacts with oxygen in a highly exothermic reaction. It also reacts with nitrogen when the temperature gets too high. This can lead to very violent reactions even when laser cutting at room temperature.
Oxidation during the cutting process creates an ink blue appearance and is going to cause a sharp increase in hardness in the oxidized zone. This is to the point where cracks appear. It makes the material very brittle.
It is therefore recommended to use an inert gas like argon (Ar) when laser cutting.
Copper can be lasered in its pure form as well as in alloyed forms. Brass is copper with 1/3rd zinc (Zn) and bronze contains 10% tin. These three forms are the main groups for commercial use when it comes to copper alloys.
Bronze is usually cast in a mold and is not often used in place form. It is therefore not often lasered, but rather mechanically treated (milling e.g.). Copper and brass, however, are very often treated with a laser cutting machine.
Copper is a good conductor and is very suitable for working with electric current. That is why you will find it especially in electronics and industrial electrical systems. It also has very good thermal conductivity making it perfect for heat exchangers. It is generally non-magnetic.
Copper alloys have a strong resistance to corrosion. Oxidation of copper by oxygen is a very moderate process compared to steel and titanium. Oxygen can therefore be used as a laser gas and provides a higher cutting speed than a neutral or inert gas.
Mechanical and physical properties
The mechanical properties of metals such as maximum tensile stress and maximum tensile strength, hardness, specific gravity, coefficient of expansion do not have a great influence on the laser cutting process. Laser cutting is a thermal process with rapid heating and cooling. However, the effects are limited to a small cutting zone. In that cutting zone, material properties can be noticeably different from the rest of the material though.
For example, for metals with a high iron content the hardness in the cutting zone can increase by up to 50-100% depending on the input variables of the laser (wavelength, power, cycle and gas type). Conductivity is lowered and resistance to metal fatigue is reduced.
In most cases, however, the effects are very localized. About half of the material thickness is affected. Thus, this is usually not significant for the whole product.
Material properties such as thermal conductivity do have a noticeable impact on the quality, productivity, cutting speed and safety of the laser cutting process. The efficiency with which the laser energy is dissipated through the workpiece via heat can make the laser process a lot less efficient.
This becomes even more important with thicker plates at a slower cutting speed. And, of course, if the entire workpiece heats up it can lead to burns. Oils or lubrication can also create a fire hazard in this case.
The viscosity of a metal also plays an important role. In general, metals have a low viscosity at their melting temperature. Nevertheless, this can cause a not nice cutting edge after cutting because the cutting surface becomes fluid. Viscosity also increases further as the temperature increases. Aluminum, for example, already has a very low melting temperature, so there it is a very important factor.
Most metal surfaces reflect up to 80% infrared light at room temperature. The remaining 20% is converted to heat and that creates a melt as the basic part of the laser cutting process.
However, the reflectivity of metals can be drastically changed by natural or artificial coatings on the surface. The surface of the metal also plays a role, what is the roughness? A polished surface has a higher reflectivity. Even the angle at which the cut is made has an influence.
How this affects is relatively complex since not only do material properties have an effect, but the wavelength of your laser is also important.
Often metal sheets are given a thin film of oil to protect the sheet metal from corrosion. It also reduces corrosion due to friction during storage and transportation. However, the laser cutting process is rarely affected since the lubrication evaporates very quickly.
It can, however, slow down productivity since handling oiled material is less easy. It is slippery, sheets stick together and it has fire safety implications. This affects the loading and unloading of the laser cutting machine.
Coating based on zinc are frequently used to protect steel from corrosion. Many sheet metal products are galvanized on the basis of zinc. It also helps to prevent scratches and stains and promotes paintability.
In most cases coatings have a melting point far below that of the base metal. Therefore, in theory, zinc coating should have little impact on the laser cutting process. In practice, however, the efficiency is significantly reduced.
This is because a deposit forms on the cutting surface, which reduces quality. This can only be counteracted by drastically reducing the cutting speed (by up to 20%).
There are no significant differences between the different electroplating processes (electrolytic, thermal or galvanic). Although the surface texture can be different and this affects the reflectivity. The vaporization of it can be toxic though and so must be properly filtered.