Laser Cutting Archish Bharadwaj V S 20082202 B.E, Materials Science Engineering CEG, AU

Introduction In laser cutting and drilling, the focused laser beam is directed onto the surface of the work piece to rapidly heat it up, resulting in melting and/or vaporization, depending on the beam intensity and work piece material Lasers can be used to effectively cut metal plates of thicknesses up to about 10 cm. The cut surfaces are roughly parallel and straight edged. The total heat input required for laser cutting is relatively small. This results in a small heat-affected zone size, of the order of 0.1 mm. Laser cutting is used for both straight and contour cutting of sheet and plate stock in a wide variety of materials.

Schematic of the laser cutting process

Forms of Laser Cutting The process of laser cutting can occur in one of the three forms: 1. Fusion cutting. 2. Sublimation cutting. 3. Photochemical ablation.

Fusion cutting This involves melting of the base material, which is then ejected using a high-pressure assist gas. The assist gas may be an inert gas, in which case the energy for melting is provided entirely by the laser beam. It may also be oxygen (or air), which reacts with the base metal, and the resulting exothermic reaction provides additional energy to enhance the process. A major problem of fusion cutting is the formation of striations on the cut surface and dross (molten material that clings to and solidifies on the underside of the cut edge) at the lower cut edge. However, the fusion cutting process is more efficient, requiring less energy per unit volume of material removed as compared to the other methods.

Sublimation Cutting In sublimation cutting, the work piece material is vaporized along the cutting seam. This is often achieved using a pulsed beam, and a jet of inert assist gas that is coaxial with the beam is used to blow away the vapor produced. It is limited to thin sections since more energy is required to remove a unit volume of material as compared to the latter. However, it has the advantage of a narrower kerf width and higher quality surface. Pulsed beams with high peak power may be necessary when surface quality is critical.

Photochemical Ablation Organic materials, ceramics or difficult-to-cut materials in general are normally cut by this method. Organic compounds tend to absorb ultraviolet radiation in an efficient manner. The photon energy levels of lasers based on ultraviolet radiation range between 3.5 and 6.5 eV which corresponds with the energy levels required for molecular bonding. So, an organic material is irradiated with an ultraviolet beam, absorbs the beam’s energy in a very thin layer near the surface, of the order of submicrons thereby breaking the molecular bonds, causing ablative decomposition of the irradiated area.

Contd… The process occurs almost instantaneously (about 20 ns duration), and since the thermal conductivity of organic materials is relatively low, the resulting edges are well defined, with minimal thermal damage to the surrounding area. Thus the cut region is cleaner and smoother compared to that obtained using CO2 and Nd:YAG lasers. The process is sometimes referred to as cold cutting since little heat is generated

Components of a Laser Cutting System The basic components of a cutting system are illustrated in figure (next slide), include The laser generator that produces the beam. A beam delivery system for directing the beam to the workpiece. A nozzle assembly, usually integral with the focusing assembly and coaxial with the beam, for directing the assist gas to the workpiece. A motion unit for providing relative motion between the laser beam and theworkpiece An exhaust for the waste material.

Components of a laser cutting system

Working The beam normally emerges from the generator horizontally and is deflected vertically downward by a bending mirror. The beam is then focused by the lens onto the work piece. At the same time, a gas jet is directed through a nozzle attached to the tip of the focusing assembly, onto the work piece. A typical nozzle diameter is 1–2 mm and the delivery pressures are normally maintained at about 3–4 bar (45–60 psi or 0.3–0.4 MPa) in the gas jet nozzle for cutting thin materials at high speeds. At high gas pressures, it is often necessary to use relatively thick lenses that can withstand the pressure.

Contd… It is preferable to use reflective optics (mirrors) rather than transmissive optics at high pressures (>10kW) The distance from the nozzle tip to the work piece surface is typically maintained constant at about 0.3 mm to minimize expansion of the gas flow. Directly opposite the nozzle, on the other side of the work piece, an exhaust system is provided to absorb the transmitted beam, molten debris from the cut, and exhaust vapors. This normally consists of a vacuum pump that draws the exhaust to a disposal unit.

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