Laser beam cutting
A laser (light amplification by the stimulated emission of radiation) generates a coherent beam of light at an essentially constant wavelength. When this beam is focused on a surface there is sufficient energy concentrated in this focused spot that the material may be melted or vaporised (Fig. 4.6). This enables the laser to be used for either welding or cutting. The laser light is produced by exciting a lasing medium, this being either a suitable gas or solid. The excitation is provided by the passage of an electric current or by means of high-intensity light. There are two commonly used lasers to be found in industrial applications: the gas CO2 laser and the solid state crystal laser, the neodymium-doped yttrium-aluminium-garnet (Nd-YAG) laser. Of the two, the CO2 laser is the most powerful with average power outputs of up to 50 kilowatts. Both types of laser can be designed to provide
4.6 Laser cutting principles. Courtesy of TWI Ltd.
a steady output, continuous wave (CW) laser light or in a pulsed output mode. In this latter case the power output on the peak pulse may be as much as 20 times the average power.
The wavelength of light from the CO2 laser is 10.6 microns (micrometres) and at this wavelength is easily absorbed by most solids, enabling the CO2 laser to be used on a wide variety of materials. This long wavelength has a disadvantage, however, in that it cannot be transmitted by glass or fibre optics but requires reflecting metal mirrors for manipulating the beam and materials such as zinc selenide or gallium arsenide for focusing lenses. The Nd-YAG laser light is an order of magnitude less at 1.06 microns, allowing the use of glass lenses for focusing and fibre optic cable for beam transmission. This offers a clear advantage over the CO2 laser, since it permits the marriage of commercially available manipulating equipment such as NC (numerically controlled) gantries and robots with the laser. The power output of currently available Nd-YAG lasers is limited to around 6 kilowatts, however, restricting the thickness of materials that can be cut.
The laser cutting process consists of focusing the beam through a cutting nozzle onto the surface to be cut, the concentration of energy being sufficient to vaporise the material, creating a ‘keyhole’. With continuous wave lasers there is generally more melting than vaporisation and an assist gas is used to blow away the vapour and any molten metal, creating a narrow clean cut as the beam is traversed along the item. The pulsed lasers generally provide enough energy that the laser beam imparts sufficient force to the vapour that the vapour itself removes any molten metal. The assist gas, introduced either through the cutting nozzle or co-axially with it, is used not only to blow away any molten metal but also to protect the lens from spatter or debris ejected from the cut.
The assist gas for cutting aluminium may be oxygen, nitrogen or air. Oxygen is a reactive gas with aluminium and will give higher cutting speeds than nitrogen. Nitrogen, however, will give a better quality cut in terms of squareness and roughness than will oxygen. Air is a compromise but is the cheapest of the gases. Gas pressure is an important variable that needs to be controlled to give the best quality of cut - high gas pressures give the most effective metal removal but too high a pressure may damage the focusing lens, since this forms part of the pressure system. As the assist gas pressure is increased the lens also needs to be thickened in order to carry the increased pressure. The pressure of gas in the cut is also influenced by the distance between the nozzle and the workpiece. For example, high-pressure cutting may require a stand-off distance of only some 2.5 mm. The relationship between stand-off and pressure in the kerf is not simple, however, as most laser cutting is done with supersonic gas velocities. It is essential that the nozzle stand-off distance and nozzle condition are closely controlled to provide consistent and high-quality cuts. Typical laser cutting
parameters are given in Table 4.3.
A number of advantages accrue from using a laser for the cutting of weld
• Low heat input, resulting in minimal distortion and narrow heat affected zones.
• Edges that are smooth and perpendicular to the surface and often require no further cleaning before welding.
• Narrow kerfs and heat affected zones, meaning that more efficient
nesting can be achieved, resulting in material savings.
• Very thin materials can be cut without distortion.
• Very accurate cuts can be made, resulting in more easy assembling for
welding, this giving reduced fit-up time, more accurate fit-up and fewer weld defects.
• The process is easily automated and can be readily interfaced with other NC equipment (Fig. 4.7).
The main drawbacks to the use of lasers for the cutting of aluminium are
• The capital cost of equipment, which may be in the order of several hundreds of thousands of pounds for a laser interfaced with suitable manipulating equipment. A 1.5 kW CW Nd-YAG laser interfaced with a robotic system, together with its appropriate safety equipment will cost in the region of £250k to £300k at today’s (2002) prices.
• The coupling of the beam with the work surface is not very good since aluminium can be highly reflective. This means that higher power is needed to cut an aluminium component than a similar item in steel. Aluminium may also reflect the beam back into the lens, resulting in damage, although this problem has lessened with the development of more accurate lenses and focusing systems.
• Laser cut aluminium may have a heavy dross on the underside of the cut. Removal of this can make the process non-competitive with other processes. Higher gas pressures will assist in reducing or eliminating the problem.
• The cut edges of the age-hardening alloys may contain microfissures that will need to be removed.
4.4.1 Health and safety
The laser cutting process is a thermal process and therefore metal fume mixed with the assist gas will be generated. This fume will need to be removed, preferably by local fume extraction at source. As laser cutting is
Table 4.3 Parameters for laser cutting
4.7 CNC CO2 laser cutting machine. Courtesy of Messer Griesheim.
performed with mechanised or automated systems using remote control there is only a limited risk of fume exposure for the operator. However, a laser cutting system generally has a very high usage and fume extraction will be required to control the general fume level within the shop.
The voltages used in laser equipment are sufficiently high to present a serious risk of electric shock. Access panels should be secured and suitably marked to highlight the risks. Only authorised and trained personnel should be permitted access to the equipment for repair and maintenance purposes. A typical laser work cell is illustrated in Fig. 4.8.
There are two hazards associated with laser radiation which, depending upon the wavelength, can damage either the eye or the skin. The radiation can damage the retina and/or the cornea, particularly the shorter wavelength radiation which can be focused by the lens of the eye on to the retina. Exposure of the skin can result in burns. With high-power lasers these burns may be deep and can cause permanent damage. To prevent such damage it is generally necessary to position the laser inside a suitable enclosure with interlocks to prevent access when the laser is operating. Screening of the CO2 laser beam can be provided by clear glass or acrylic screens. Tinted welding screens are required for the solid state lasers since the radiation is closer to the wavelength of visible light than that of the gas laser. Personal eye protection for the operator is also recommended, selected to filter out the appropriate wavelength of laser light.
4.8 Laser welding and cutting work cell. Courtesy of TWI Ltd.
Visible radiation is also emitted during laser cutting, this light being similar to that produced from a welding arc containing both ultra-violet and infrared light. To filter this out requires tinted filter glasses, the density of the shade being sufficient that no discomfort is felt when viewing the bright plume associated with the beam. This radiation may also cause skin reddening. It goes without saying that all personnel involved in laser processing operations should be fully trained in the use of eye and skin protection equipment.