Use of Gas Lasers in Metal Processing

     USE OF GAS LASERS IN METAL                                                                                                        PROCESSING

1. INTRODUCTION

            Gas lasers are lasers that can produce consistent beams of light with short wavelengths when an electric current is passed through a gas (or plasma) (Bloom, 1968).Plasma is an ionised gas with relatively no charge (SPI Lasers, 2018), it is the fourth state of matter. The first gas laser was discovered in 1960 by Ali Javan.

             According to Hecht (2012), the gas laser family is large and as such can be easily studied and tested. Moreover, they can generate radiations with diversified intensity and duration and such properties are the reason they are used for metal processing. This report may encourage the search of new gas mixtures leading to more powerful gas lasers enhancing their applications in industries.

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        The next sections in this report will briefly explain the different types of existing gas lasers, demonstrate the different types of metal processing and objects manufacturing in which they may be involved and aim at showing the possible advantages and shortcomings encountered while using such lasers to process metals and manufacture products.

2. TYPES OF GAS LASERS

  Here, a brief knowledge of the different types of gas lasers will be given which are: Helium-Neon, metal vapour, carbon monoxide, carbon dioxide, nitrogen, chemical, Ion and Excimer lasers. However, it should be noted that this report will focus on lasers involved in metal processing.

1. HELIUM-NEON LASER

        This is the most common type of lasers and it is usually abbreviated He-Ne. They consist of low red coherent radiations which are visible to naked eyes and can be found in schools for laboratory experiments as well as on scanners used to process out purchased items in shops. They can also be used for hologram projections (Hecht, 2012). Figure 1 below gives a general idea on how the laser works.

Figure 1. General He-Ne laser (Physics Open Lab, 2018)

2. EXCIMER LASER.

       According to Isner et al., (1985), these are lasers with high electrical productivity making use of a mixture of different gases and group VII elements as mobile means to create ultraviolet (UV) lights with short wavelengths. UV photons contain more energy than visible and infrared photons hence can be used for a verity of purpose as compared to others (Hitz, Ewing and Hecht, 2005). They can be used in material processing for holes drilling and to produce high performing semi-conductors.

3. CARBON DIOXIDE (CO2) LASERS.

            As shown in Fig.2, It works on the principle that when electrons flow in a gas-filled tube with reflective mirrors at its ends, light is generated. Bloom (1968) states that, the light generated is visible and formed in the infrared range of the spectrum, the gas present in the tube is made of CO2, nitrogen, hydrogen and helium. The nitrogen molecules get excited when stimulate by the current which later stimulates CO2 molecules, nitrogen is used because it can hold photons for a long period of time. For light to be released, the excited nitrogen molecules must meet the inactive helium atoms. The light generated is powerful enough to cut through a wide range of materials (Hecht,2012).

Figure 2. Sealed tube design CO2 laser (D&E Notes, 2018).

4. ION LASERS.

         They consist of a power supply, resonator assembly and plasma tube as the main component with argon producing distinct lines with wavelengths ranging from UV to infrared section of the spectrum. Here, argon has lost an electron and acts as a source of optical gain (AZoOptics.com, 2013). The main types are argon, krypton and a mixture of both, electricity pumped causes the gas to be ionised and left in an excited state for a long period of time, these ions can then be stimulated to produce a variety of laser transitions (Hitz, Ewing and Hecht, 2005).

5. CARBON MONOXIDE(CO) LASERS.

             Research has shown that the CO laser has a high-efficiency potential. Like CO2 laser, it can generate strong coherent beams of light but most of the rays are later soaked up by the atmosphere and for optimum functioning, the gas should be cooled at room temperature (Hecht, 2012).

6. CHEMICAL LASERS.

              According to Ultee (1982), chemical lasers arise from the energy emitted by the rearrangement of the ionic structure of molecules in the form of electromagnetic waves. This reaction is good because it usually generates high energy that can produce the desired molecular state for higher laser level (Hitz, Ewing and Hecht, 2005).

7. METAL VAPOUR LASERS.

                They are formed from excited metal vapours. Here, the metal vapour acts as an active medium, to obtain the vapour the metal is introduced in a tube and heated by electricity. The rays produced can be found in the UV and visible region. There are commonly two types of metal vapour lasers, the helium-cadmium which is ionised and the cupper which is neutral (Hecht, 2012).

8. NITROGEN AND HYDROGEN LASERS.

                Nitrogen lasers use nitrogen as active medium and occur in UV range. They are easily produced and generally have low power. Emission occurs in form of short vibrations. Hydrogen lasers are similar but can produce shorter wavelengths (Hecht,2012).

3. APPLICATIONS OF GAS LASERS IN METAL PROCESSING AND OBJECT FABRICATION

           CO2 or nitrogen mixture lasers are usually used for laser cutting. This process involves the use of sharp and highly concentrated rays of light to pierce through a material. It can be done by either pulsed laser beams or continuous depending on the thickness of the material (SPI Lasers, 2018). Carbon dioxide Lasers are also used in metal welding, this is a process whereby metal is been melted and joined. It usually involves metal, ionised gas and reciprocal actions between laser beams (Li, Cai and Wu, 2009). Laser marking is another type of metal processing which involves the use of a laser (excimer, carbon dioxide) to engrave, remove, stain, carbonise, foam and anneal metals (Sobotova and Badida, 2017). Hecht (2012) chapter 11, states that CO2 lasers can be used for heat treating, it is the process of changing metal coats from one form to another and is done by a continuous light ray. Gas lasers such as the CO2 laser are commonly used in drilling. Drilling is the process of making holes through a material (Majumdar and Manna, 2013). As stated by Majumdar and Manna (2011), carbon dioxide lasers are also used for metal bending, this process involves changing the shape of a metal without changing its crystalline structure.

4. ADVANTAGES AND DISADVANTAGES OF THE USE OF GAS LASERS IN METAL PROCESSING

   In this section, the focus will be given on the carbon dioxide laser because it is the most commonly used one.

1. LASER CUTTING.

1. Advantages: Using lasers to cut metal causes less metal destruction since it acts from a point of distance and there is complete control of the process (intensity, heat output, duration). Also, it produces less waste and one arrangement of the laser can be used for many purposes (SPI Lasers, 2018).
2. Disadvantages: If carelessly used, the light beam can cause burns if it interacts with the body. It requires high power usage and cannot be used on all types of metals. If not setup conveniently, it can destroy the material (Powell and Kaplan, 2012)

2. LASER WELDING.

1. Advantages: Lasers provide clean welds, they are usually fast in action and cause minor deformation to the material, provides high precision. Also, can be used on a wide variety of metals (Moskvitin, Polyakov and Birger, 2013)
2. Disadvantages: It is a costly process and requires well-trained individuals. It also cools down fast and may cause metal fracture (Moskvitin, Polyakov and Birger, 2013)

3. LASER DRILLING

1. Advantages: This process produces low damage to material, passes through metal fast and easily. It is relatively cheap to run and is a non-contact process (Dhar, Saini and Purohit, 2006).
2. Disadvantages: Laser drilling provides low material penetration with small diameters and changes the form of the metal coat (Dhar, Saini and Purohit, 2006).

4. LASER MARKING

1. Advantages: This process provides high precision marking on the metal surface and there is little or no loss of material. It is relatively cheap to run (Shannon, 2018)
2. Disadvantages: carbon dioxide marking takes a long time and should be set up precisely or else the marking will not be permanent (Shannon, 2018)

5. LASER BENDING

1. Advantages: The laser is the only required equipment, there is total control over the process, it provides greater applications than mechanical process such as forming small angles and complicated shapes (Gautam et.al., 2015).
2. Disadvantages: If laser not set up properly, it can cause major damage to metal, the process can be slow (Gautam et.al., 2015).

6. Heat treating

1. Advantages: It increases the strength of the material. This process does not alter the composition of the metal and material penetration can be adjusted using carbon dioxide laser (Ion, 2002)
2. Disadvantages: The beam produced by carbon dioxide laser is small and hence needs to be widened before use. This process requires high energy consumption, some metals are not capable of absorbing the CO2 laser wavelength (Ion, 2002)

5. CONCLUSION

            To conclude, this report demonstrated the use of gas lasers in metal processing and their respective advantages and disadvantages, but the great focus was given to the carbon dioxide laser because it is the oldest and the most commercially used gas laser. However, further research on the other types of gas lasers should be done because they show high potential.

6. REFERENCE LIST

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• Dhar, S., Saini, N. and Purohit, R. (2006). A review on laser drilling and its Techniques. In: International Conference on Advances in Mechanical Engineering-2006 (AME 2006),. p.2.
• Gautam S.S., Singh S.K., Dixit U.S. (2015) Laser Forming of Mild Steel Sheets Using Different Surface Coatings. In: Joshi S., Dixit U. (eds) Lasers Based Manufacturing. Topics in Mining, Metallurgy and Materials Engineering. Springer, New Delhi
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