Hardening

The high-power diode laser, with its rectangular beam, a "top-hat" intensity distribution in one beam direction ("slow-axis") and a Gaussian curvature in the other ("fast-axis"), is particularly suitable for wide-area surface-treating applications. One advantage over the CO2 Laser is the short wavelength (808 nm and/or 940 nm), which leads to higher absorption and thus dispenses with the blackening that is necessary when CO2 lasers are used. Compared with the NdYAG laser, the major advantages are to be found in the beam profile, and in the significantly lower investment and operating costs that result from the high efficiency. These advantages make the high-power diode laser an efficient, reliable, cost-effective tool for surface treatment.

In the case of skin hardening, the laser beam heats the surface above the austenitizing temperature for a short time, but does not cause melting. The heat conductivity ensures fast cooling, so that the material solidifies in the harder martensitic structure. This way, a hardened layer , defined locally by the width of the laser beam (across the direction of travel), with hardening to a depth of approx. 1 mm, can be produced. The duration for which the temperature is applied is determined primarily by the depth of the laser beam in the direction of travel, and by the rate of travel.

Laser remelting is very similar to skin hardening, but differs from it in that the material is heated to above the melting point. It is the prefered method of hardening on castings. Here, too, the material solidifies very quickly through self-cooling, crystallizing to form a fine-grained layer that is highly resistant to abrasion

Another important surface application for lasers is coating. Coating helps prevent wear and is also used for repair purposes. One method that is frequently used with success is the precipitation of layers of powdered, hard material, which is introduced by means of a special nozzle to the zone that is being heated by the laser. The high-power diode laser is ideal for this application, too. At a moderate power density in the region of some 104 W per square centimeter, layers about 0.5 mm thick can be applied at speeds of several 100 mm per minute, depending on the power of the laser. Here, the advantages of the diode laser over the CO2 lasers that have been used hitherto lie in the higher speeds coupled with lower power requirements, that is, significantly higher process efficiency.