New developments in advanced welding

The laser as a machining tool

As a machining tool the laser alone is very ineffectual. It must be used in conjunction with several different items of optical and mechanical equipment, which need to be integrated into a functional unit, in order to be able to process materials. Illustrated in Fig. 5.7 is a typical laser machining system. It comprises three key elements:

• laser source

• beam delivery and processing optics

• multi-axis, numerically controlled motion system.

Of the many laser sources that have been discovered over the years, the CO2 and the Nd:YAG lasers now dominate in industrial environments. The beam delivery systems for the two laser sources differ in detail. In the case of CO2 lasers, the system comprises optical elements located within rigid tubes which transport the beam from the laser and focus it onto the workpiece surface. The system may include a number of optical components such as telescopes for diverging the beam, beam splitters for the sharing of power between different processing optics, polarisers for producing circularly polarised beams, energy-share modules for delivering the beam simultaneously at several locations as well as mirrors for bending the beam. In the case of Nd:YAG lasers, the beam can be guided both by mirrors and by glass optical fibres making delivery of the laser beam to difficult and tight spaces relatively easy.10,11 Nd:YAG lasers with output powers of 5kW transmitted through 0.6 mm diameter fibres are now commercially available.7 At present there are no commercial fibres for high power CO2 lasers.

Beam delivery

Processing optics

5.7 Components of a laser machining system.

The fibres manually used with industrial Nd:YAG lasers are of the ‘step index’ design. This means that they have a core with a high refractive index surrounded by a cladding with a lower refractive index (Fig. 5.8). Transmission of light occurs by total internal reflection at the core/cladding interface due to the difference in refractive index between the core and the cladding. A property of these fibres is that the exiting beam has a relatively homogeneous intensity distribution over its diameter. Depending on the power of the laser, the core diameters range in size from 0.2 to 1.0 mm. The fibres are manufactured from high purity fused silica and possess minimal loss at the laser wavelength. Optical losses of the order of 8 % per fibre occur in fibres which have no coatings on the ends. As this loss can cause problems at high laser powers, companies such as Rofin-Sinar have developed special coated quartz blanks at the ends of the fibre, which reduce this loss to less than 2%.12

To launch a laser beam into a fibre, so that it experiences a minimal transmission loss as it propagates along the fibre, requires that the diameter of the focused spot on the fibre face is smaller than or equal to the fibre core diameter. The focused spot size in commercial systems is normally 80 % to 90% of the core diameter, which allows for easier adjustment of the fibre and for any variation in the spot diameter due to laser parameters.10 In addition, the divergence of the input laser beam must be less than the acceptance angle of the fibre defined by its numerical aperture, NA.

The laser beam exiting the fibre diverges, so to generate a high power density on the workpiece, an optical system is used to recollimate and focus it onto a workpiece. The diameter of the focused spot is determined by the magnification M of the optical system, where M = diameter of focus spot/ diameter of fibre core. Typical magnification ratios are 0.5, 1 and 2, which generate spot sizes of 0.3mm, 0.6mm and 1.2mm with a 0.6mm diameter fibre.

I

5.9 Schematic diagram of a fibre-optic cable construction.

Illustrated in Fig. 5.9 is a schematic diagram of a fibre-optic cable construction.11 In addition to the core and cladding layer, most optic fibres used with high power lasers now include continuity detection, which senses if an accidental burn-through has occurred and turns the laser off. For some applications, the fibre is contained within a steel-armoured cable to prevent any mechanical damage. Typical fibre lengths are from 5 to 20m with a number of reports14 indicating that lengths in excess of 100 m can be used effectively.

Raw laser beams normally do not have sufficient intensity to cause melting or vaporisation of materials. To increase the intensity in order to process materials, laser beams are focused using both lenses and mirrors. Lenses are generally used with laser powers up to several kW; beyond this catastrophic damage can occur, particularly in the case of CO2 lasers. At higher powers, mirrors are used because of their high power handling capability. The characteristics of a focused laser beam are shown in Fig. 5.10. The key parameters are the focused spot diameter, d, and the depth of focus, L, defined as the distance over which the focal spot size changes by +/-5%. The focused spot diameter affects the maximum irradiance that can be achieved while the depth of focus influences the process working range.

For circular beams the focused spot diameter, d, is proportional to f/D, where f is lens focal length and D beam diameter at the lens, whereas the depth of field, L, is proportional to f 2/D2. As can be seen, the two quantities work in opposition. To obtain the smallest spot diameter and therefore the highest power density, the focal length should be small. To obtain the greatest depth of field the focal length should be large. So a compromise must be made to ensure that the correct processing conditions are maintained. The precise spot diameter and depth of focus also depend on the mode structure of the beam as well as on the optical aberrations of lenses and mirrors such as spherical aberration, astigmatism and thermally induced distortion. All

these quantities tend to increase the spot diameter or shorten the focal length, which can dramatically affect the process.

The material commonly used for lenses and mirrors for Nd:YAG lasers is borosilicate crown glass, designated BK7.15 BK7 has excellent optical and thermal properties and is relatively cheap. Lenses are normally coated with antireflection coatings to minimise reflection losses at the laser wavelength. The optical materials used with CO2 lasers are somewhat diverse depending on the laser power and operating conditions.16 Both reflective and transmissive optics are used. Focusing lenses can be made from gallium arsenide (GaAs), potassium chloride (KCl) and zinc selenide (ZnSe). ZnSe is now the most commonly used lens material because of its very low absorption and high transmission in the visible part of the spectrum, allowing red laser diode lasers to be used to align the invisible CO2 beam on the workpiece. The lenses are coated with antireflection materials to minimise the nearly 17% loss of the incident laser power that would otherwise occur at each surface. A limitation with ZnSe lenses is that some absorption of laser radiation does occur and as the laser power increases, the absorbed laser radiation becomes significant causing the lens to heat up. This heating changes the imaging properties of the lens, most notably shortening its focal length. The focal length change can be severe enough to cause process capability loss. Because of this problem ZnSe lenses tend to be used with laser powers up to about 3kW and metal focusing optics above this level. Recently developed air cooled doublet lenses, however, show potential for overcoming the thermal lensing effect and extending the operating laser power levels up to 20kW.13 Focusing mirrors tend to be made from copper because it is highly reflective at the laser wavelength and can withstand high energy densities, above 100kW/ cm2, without sustaining thermal damage. Because copper is soft, the mirrors usually have a coating to protect the surface and are water cooled to minimise thermal distortion. Metal focusing optics is normally used for welding and surfacing applications.

In order to control the motion between the laser beam and the workpiece, a number of approaches can be used, the choice of which depend mainly upon the laser and the workpiece to be processed. For processing flat sheet, material and tubes, two-dimensional systems are employed. These systems involve moving the workpiece using rotational and translational stages while the beam remains stationary, moving the laser beam while the workpiece is stationary or a combination of the two. The systems involving stationary beams allow a high degree of repeatability and accuracy at high processing speeds (up to 20m/min). The main problem with this method is that the process area is limited because of the large overhang of the machining head required with large sheets. In addition, as one of the axes is positioned on top of the other, the weight of the workpiece becomes an issue. The system is therefore mainly used for precision processing of relatively small parts.

In robot systems, there are also a number of possibilities: moving the laser, the beam or the workpiece. Robot systems with moving optical systems are further divided into systems with interior or external beam guidance. Robotic applications involving Nd:YAG lasers are now becoming increasingly common because of the ease of manipulating the processing head as well as the ability to transmit the beam over long distances.