We developed a small and compact system of diode lasers, which can be inserted into the lower tools of a bending press. The parts of the system allow easy plug and play operation and can be installed for any bending length. The diode laser, which is based on 200 W laser bars on microchannel cooler, allows the heating of sheet metals in the forming zone shortly before and during the bending process. There is no unnecessary heating of other parts of the bending equipment, no wear of the tool, and, if properly done, no damage of the surface of the metal. The power per bending length is 16 kW/m.
The production of complex structures out of sheet metals with bending technology is widespread in industry. Alas, this is currently mainly restricted to materials that allow a certain degree of cold forming. Brittle materials, as Mg-, Ti-, or some Al- and steel-alloys are generally avoided or, in some rare cases, heated up as a whole, before the bending operations are performed. Due to fast cooling the initial temperatures must exceed the necessary forming temperatures, such that the whole process becomes very cumbersome. The goal of laser assisted bending, not to be confused with laser-bending [
Previous work [
In order to obtain a system for industrial use, we developed a diode laser system based on diode laser bars on microchannel coolers. The system can be installed inside a special tool, which replaces the lower tool of a bending press. The challenge here was that on smallest possible space, a dust-tight solution with optics, power and cooling water supply was to be integrated. Furthermore, the solution should allow any required bending length to be installed.
Diode laser bars [
Optical beam guidance around the connections screws. (a) With reflecting distance elements for lasers 1, 4, 5, 8. (b) with prisms crossing two neighbored beams.
Figure
1.6 kW diode laser: mounting of 8 micro-channel coolers on the base part.
Cu-connectors fix the lasers and provide a serial connection as shown in Figure
1.6 kW diode laser: fixing and serial connection with Cu-diagonal-connectors; FAC-lenses and prisms for beam-crossing.
To enable a compact design, the casing is made of two parts, each of which has a contact to the first and the last diode laser bar, respectively, and contains the connectors to the neighbor lasers or to the power supply (Figure
1.6 kW diode laser: casing, thin side covers and a protection glass.
The lower tool consists of two parts and contains a cavity for the diode laser, which can be inserted from the side or from the front. (Figure
1.6 kW diode laser: assembly into a lower tool.
A finished diode laser with 1.6 kW for 100 mm bending length, ready for assembly into the lower tool.
Figure
The in- and outlets for the cooling water must be sealed with O-rings. This requires that the lasers are pressed together by plates that are mounted on the ends of the array of lower tools.
Figure
400 mm laser assisted bending tool in operation. Installed laser power: 6.4 kW.
A power supply with 30 kW output power supplies the diode lasers, for a maximum bending length of 1 m. For longer bending lengths, several power supplies can be connected in series to do the supply in a master-slave configuration.
For cooling, deionized water at 21°C with a flow rate of 3850 l/h is required for 1 m bending length at a pressure of up to 9 bar. The cooling capacity can be well below the laser heat losses of ~15 kW/m due to a maximum load ratio of <0.5.
Dirt falling down from the work piece is removed by a strong flow of pressured air.
The control is based on time or on the temperature of the work piece, measured by a thermoelement, which is integrated in the upper tool.
First, a small cold bending is performed. Then the press stops, and the laser heating starts. At a predefined temperature or time, the bending process proceeds while the lasers remain on. Usually the temperature rises further during the bending process. In some cases, for example, when the bending has to be done very slowly, a control loop keeps the temperature on a predefined level to avoid damage of the work piece.
Typical heat-up times are in the range of 1–5 s depending on the thickness and material properties of the work piece. The following materials were tested with this process. Mg-alloys (AZ31, ZE10), thickness is 1–2.5 mm: temperature range is 200–300°C. Usually the absorption is sufficient to overcome the high heat conductivity high increase of ductility (Figure Al-alloys (7075, Titanal), thickness is 1–2.5 mm; temperature range is 200–300°C with high heat conductivity, very low absorption, so an absorbing layer like graphite is necessary, high increase of ductility, but ~20%-decrease of strength in the forming zone. Ti-alloys (Titan Grade 2, WL 3.7164), thickness 2–12.7 mm: temperature range 300–600°C, good absorption, low heat conductivity, appreciable increase of ductility, very well suited for laser assisted bending. Steels (M85, St52, Hardox), thickness is 1–3 mm, temperature range is 400–600°C, good absorption, problems with blue brittleness, very accurate temperature control needed, up to now good results only with the spring steel M85, other steels were successfully bent by Geiger et al. [
Bending of the Mg-alloy AZ31 without (left) and with (right) laser heating.
We presented a laser-assisted bending system that allows the fast bending of brittle sheet metals by laser heating of the forming zone. This is done with diode lasers that are integrated in the lower bending tools. The power per bending length is 16 kW/m.
The method was successfully applied to Mg-, Al-, Ti-, and steel-alloys. With the proper processing window, bending of brittle materials become nearly as simple as ordinary cold bending. Welcome side effects are the reduction of the necessary bending force, the reduction of spring back, and sharper edges in the case of materials, that can be bent cold as well.
Possible applications are countless in general engineering, car building, aero space industry, general light weight construction, electronic devices, and in consumption industry.
This project was funded by the company TRUMPF Maschinen Austria and by the Austrian funding association FFG (formerly FFF) under the project title “Laser-Assisted Bending” (Laser-Unterstütztes Gesenk-Biegen, LUGB). The authors are deeply indebted to DI F. Killian, Trumpf GmbH & Co. KG in Ditzingen/FRG, Dr. A. Hutterer, DI A. Rau, and G. Sperrer, Trumpf Maschinen Austria in Pasching/Linz for valuable scientific advice and cooperation.