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Information Sheets
Materials - Gases
Gases
Advantages can be gained from the thought put into the procurement of gas to reduce its costs, optimize the process gas composition and the method of storage and delivery.
Gases used with lasers
Gas lasers may consume gas in the resonator. These gases are specific to the type and model of the gas laser being used being closely specific by the laser manufacturer. They are often premixed and it is essential that these gases be supplied with very low levels of impurity. Many types of laser system use gas to purge the beam delivery systems and in cross jets to protect the process lens. The laser process may require shield gases and cutting accelerants.
Shield gases are used to produce a controlled inert atmosphere surrounding the point of laser welding to prevent oxidization, control the plasma and maintain weld quality and consistency. The three common gases used are nitrogen, argon and helium, the most common being argon. Argon is easily stored and delivered to the point of use. Helium is expensive and more difficult to handle due to its low density but has advantages in some applications. Nitrogen is inexpensive and is easily generated but as it is not true inert, is unsuitable for many applications.
Four gases are used for cutting accelerants.
Oxygen is the best cutting accelerant but its use increases the risk of fire so steps are required to prevent the development of oxygen enriched environments. The inflammable nature of certain materials, such as titanium or magnesium alloys may not allow this gas to be used. In addition the use of oxygen may be precluded with excessive oxidization compromises weld quality.
Compressed air is the least expensive cutting gas and is safe and convenient to produce and deliver. However it is essential that it is dry and oil free.
Nitrogen is used when cutting materials where oxidization is a problem.
Argon can be used being primarily restricted to cutting titanium as the inert gas reduces the fire/explosion hazard associated with titanium dust.
Purge gases are often required establish a clean atmosphere at positive pressure in beam delivery systems. Many high power CO2 lasers use this technique to prevent contamination inside the beam delivery protective housings and prevent the ingress of other gases giving rise to thermal blooming effects. The two gases most commonly used are nitrogen and compressed air. Nitrogen is superior as it can be provided inexpensively and is effectively inert. Where compressed air is used it is essential that it is clean, dry and oil free.
Gases supply can be derived from a variety of sources.
Gas bottle storage gives flexibility and arrangements can be made to allow the exchange of bottles without affecting the process by the disruption of the supply.
MCP (Manifold Cylinder Pack) installations hold a number of bottles in a dedicated assembly allowing the storage of larger quantities of gas. The MCP is exchanged for recharge when required.
Bulk storage systems are available for the bulk storage of gas. These are usually situation outside a plant building to allow access by a bulk supply tanker. To prevent accidental contact with very cold pipes and valves around the installation it is usually sited in a secure compound.
Where larger quantities of nitrogen are used, a dedicated on-site nitrogen generator may be a viable option being ideal for producing gas for pressurizing beam tubes and CO2 laser processing applications.
Laser Steels
The laser cutting of mild steel can be improved in speed, maximum thickness and edge finish by optimizing the composition and manufacturing process of the material; in particular minimizing the levels of impurities which hamper laser-cutting.
Several steel manufacturers in co-operation with the laser industry and especially laser cutting equipment manufacturers, have developed "laser steel". The steel characteristics usually include a uniform surface finish and good flatness together with low silicon and sulphur content. Manufactures claim up to a 20% improvement in cutting speed (depending on plate thickness), a uniform and excellent cut edge quality, a higher maximum cutting thickness and minimum problems associated with stresses in the steel sheet.
Clearly the cost benefits need to be evaluated before specifying these specialised products but these materials are being used in many applications where the thickness of sheet is pushing the limits of capability and good cut quality needs to be guaranteed.