Diode Lasers in Tool- and Mould-Making
Heat treatments can be realized with the help of diode lasers in a more flexible, precise, and often more economical way than with other laser beam sources or other tools like gas flames, infrared rays, and induction coils, e.g. for the selective hardening of gripping tools or mold surfaces at particularly stressed areas.
In the last few articles of our little series, we presented some key areas for the application of diode lasers. Now we focus on a further application field: heat treatment. Heat treatment is used for hardening machine components, tools, accessory and commodities, but also for the softening of high-strength materials.
The heat treatment of metal, such as the hardening of steel, is one of the oldest industrial processes. Its roots go back to antiquity. Even today's laser heat treatments still follow the classical objectives: The targeted application of heat to defined surface areas alters material properties and protects components from wear and corrosion.
Diode lasers are especially suitable for surface hardening of steelwork components. The laser beam heats up areas close to the surface of a workpiece at between 900°C and 1500°C, causing a heat-induced austenitization of the originally ferritic-perlitic steel structure. When the beam moves on, the material on the processed part cools down quickly (self-quenching) and hard martensite is created. For many reasons, this process can clearly be realized more efficiently with laser than with induction coils or gas flames. First, the material physically heats up more quickly than with other methods. Second, hardening relevant zones can be treated more selectively, meaning that by local heat input only certain parts of the workpiece can be heated. And third, the heat input can be dosed in an exact manner by special beam shaping optics with integrated pyrometers or thermal cameras, last but not least with particular focus on local different heat conductions at the same component. Due to these characteristics, the diode laser is extremely well-suited for treating geometric complex structures that require in some zones a hardening; in others, however, they have to keep their ductility. Furthermore, these process advantages allow for cost savings. That is, because of the low deformation and self-quenching of the material, usually no (or only little) actions are required for distortion compensation and cooling. Finally, this is reflected positively in terms of required time and material.
Particularly predestined for this treatment are wear or cyclically stressed components such as camshafts. In every gasoline and diesel engine, steel rubs against steel. Thus, the contact zones must be hardened, otherwise the longevity of the parts would be hard to imagine. At these complex structures, the inductive method does not help much; the construction of modern camshafts (with different shaped cams and shifting gates for the cylinder deactivation or change in the engine control) requires a precise surface hardening, which is only possible with diode lasers. Even in manufacturing of large sheet metal tools, diode lasers have established themselves for a long time now. The oldest hardening plants have here already been in use since 10 to 15 years. As the prices of laser beam sources have continued to decrease for years now, additional to these typical fields of use are new applications that are permanently being explored.
Heat treatment with diode lasers does not only allow hardening but also the exact opposite, namely, the softening of materials. Here, the martensite structure is softened in certain zones by heat exposure (“tempered”) or is turned back into a ferrite-pearlite structure by austenitization followed by slow cooling. The result is a more elastic metal sheet that can be better welded, formed or cut. Here, diode laser is the better tool compared to other methods like induction, gas flames or infrared rays. On the one hand, material processing is highly flexible and precise, even with hardening; on the other hand, the created transition zone between the processed and unprocessed raw material is smaller than that created with other technologies. But in particular, diode lasers have a very homogeneous intensity distribution even at big spots, which makes the softening results specifically even.
Its application can actually save lives: In car body construction, soft deformation zones can be created at press-hardened areas by means of lasers, which absorb in case of crashes the impact energy and thus can protect the body. By this flexible material processing in the remaining zones, the total stiffness of the steel remains. A further application field is deep drawing, at which the circuit board is softened before pressing in the intended bending zones to avoid cracks or fractures during the forming process.
By the way, diode lasers can also be used for drying printing inks. The color layers are heated with a laser beam, and because of temperature induced increased viscosity they are more quickly absorbed by the printing material. The modular and compact components of the laser can be directly integrated into the systems of offset printers.