At high laser beam intensity values (in the range 1x105 W/cm2 for metals), the material being heated will change state. Most materials will melt due to the different physical mechanisms in play (see ' What is laser vaporisation?'), and in the molten state, the absorption of laser light increases. The absorption coefficient rises to about 90% for common metals subject to infra-red laser light.
Melting progresses by conduction on an approximately hemi-spherical front through the material. Convective heat transfer, as well as conduction, becomes important and this can be driven by changes in surface tension of the molten pool arising from temperature gradients. These latter factors will distort the melt shape away from being spherical. Under these circumstances, a 'conduction limited weld' can be produced by moving the heat source, usually a focused spot, relative to the material along the joint path. Diode, solid state and CO2 laser systems are all used to produce such welds in metallic materials.
In plastics materials such as polypropylene, the 10.6µm wavelength of the CO2 laser is completely absorbed in a less than 1mm of material. These lasers are capable of melting such plastics at very high speeds to produce lap welds. For lap welding of thicker plastics, the diode and other solid state near infra-red laser wavelengths can be used effectively in 'transmission laser welding'. In this process the laser beam passes through the top, translucent (to the laser) plastic, and is absorbed in the lower plastic sheet, which has been loaded with an absorbing medium such as carbon black. Sufficient heat is generated for melting both the lower plastic and, by conduction, the lower surface of the upper plastic, thus, forming a joint.
Laser cutting is achieved by rapid removal of molten material from the beam/material interaction zone. Laser cutting is usually achieved using the continuous wave (CW) CO2 gas laser or the pulsed solid state laser; however, the cutting mechanism is different for the two. With the power densities available from modest CW CO2 lasers, the molten material can be removed by a high pressure gas jet (up to 20 bars nitrogen can be used). The effect of this, when combined with the fact that the laser melts only a small section of the workpiece due to its small spot, is to produce a narrow kerf width. Figure 1 shows schematically how the laser beam is absorbed on an inclined melt front within the kerf. The angle of this front varies with cutting speed and as the beam interacts at a high angle of incidence, the degree of polarisation of the laser beam becomes important. The molten material generated is blown out of the bottom of the cut with the high pressure N2 jet, (which also blows away any laser induced plasma). The melt is ejected as droplets, leaving a clean edge.
FAQ: What is laser vaporisation?