Requirements to precision laser cutting processing of refractory metals.

Uebel, Martin ; Buerger, Wolfgang ; Schoele, Holger 等

1. INTRODUCTION

The work examination serves the method development of the precision laser cutting of refractory metallic materials; in particular of tungsten composite materials.

In previous use of these methods the cut area qualities, structure size and production costs did not suffice for many applications. Due to bad cut edge quality, rip danger as well as large warmth influence zones caused by high process energies, tolerances and other requirements of the user are not or only conditionally met.

The market of refractory metals is constantly growing. However, the potential of this market cannot be exhausted due to the insufficiently developed technologies available today.

Essential improvements of the project are the attainable cutting quality and the rise of the process safety compared to the level of technology and to the competitors.

The task in this work examination consists of planning cutting attempts for the parameter optimization, of conducting laser cutting experiments and evaluating the cut tests. Furthermore material tests are processed by outside companies. These have to be evaluated in the context of the work examination and to be compared to each other.

2. EXPERIMENTAL SETUP

The materials to be examined in the project are metal based alloys with a very large amount of tungsten. A very high temperature is characteristic for these alloys. The materials are processed with a thickness of 0.1 mm to 2.3 mm as metal sheets. The products manufactured of the semi-finished products find new possibilities for the application in different industrial areas, e.g. in medical engineering, the beam technology, the automobile industry and the aerospace.

In this work examination pulsed Nd:YAG laser plants were used. The laser cutting systems are optimized especially for fine cutting use. High-precision work pieces are mainly manufactured of metal based alloys like high-grade steel, aluminium and copper-based alloys; beside those, some non-metals are used as well. In addition to the available lasers at the work place, material tests were processed by outside companies at other laser plants.

The features of the cut opening to be determined were carried out with the VDI guideline 2906 (cut opening quality when cutting, cut and perforate work pieces made of metal). Of special significance for the work examination are conicity, roughness, the warmth influence zone, cut opening breadth and burr formation. For measuring the roughness a stylus instrument is at disposal. Interesting characteristic quantities are Ra, Rz and Rt. Also a variety of further surface identification values can be determined next to this one for the work examination according to DIN 4287.

For the visual evaluation stereo microscopes are used. It is of great importance for the regulation of the cut breadth both on the beam admission side and on the beam leaving side and the burr height. Material ejection and warmth influence zones can be swiftly recognized on the top side of the test. The micro examination views the cross section of the cut opening. The cross section test of the cuts will be poured in curable plastics and then polished. The test can be examined now with regard to the geometric qualities via microscope.

3. EXPERIMENTS AND RESULTS

Using special cutting gases different surface results can be achieved. The cutting gases which are of special interest are air, nitrogen and oxygen.

The quantitative evaluation of the cutting area of the cut tests is carried out with the stylus instrument. When compared, oxygen cut shows the lowest roughness (figure 1). Air and nitrogen produce a larger roughness. Under the reaction with oxygen the surface profile is burned off or oxidized and therefore it is smoothed. At the air gas flow the effect, however, is considerably lower due to the smaller amount of oxygen, and at the nitrogen cut it does not occur. While the oxidation of the metal also brings in an additional thermal energy which contributes to melting the tungsten alloy with the oxygen, the melt is made stiffen and cools down fast using the nitrogen gas current.

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While the nitrogen gas current has developed an oxide free surface and a low discoloration, the oxygen gas current surface is strongly oxidized and discolored by the warmth entry into the surrounding structure. The tempering tarnishes are obvious there considerably stronger and penetrate into the material further than at the nitrogen cut.

The feed rate is varied for this examination from 100 mm/min up to 500 mm/min. Apperently, the roughness increases with an increasing feed rate. As seen in figure 1, this statement applies to the three different cutting gases similarly. To be able to determine the cut surface geometry, micro examination specimen were made. While the melt is blown clean with oxygen at the slow cut tests below and the burr accumulates with a narrow, long form at the underside, the melt of the fast cut tests is deposited mainly at the top side and in the cut opening. The burr matured at the top side of the test is distributed by the cutting gas flow conditionally very broadly and can be removed only with difficulty. Therefore it is not favourable under the given conditions to cut with high speed, although this leads to higher production costs. Rather a melt which produces a narrow, well removable burr has to be striven for.

The examinations to the gas pressure were carried out at the metal sheets with a thickness of 1 mm. Air served as cutting gas. The pressure varied from 2 bar to 25 bar in steps of five. During the tests it became obvious that it is difficult for a low pressure to separate the tests from the metal sheet. While at a low pressure a lot of melt is blown up, at a high pressure no melt is taken up on the top side or in the cut opening. The rough value Ra is compared in illustration 2 with the cutting gas pressure for the measuring of the metal with a thickness of 1.0 mm. The most favourable pressure is between 15 to 20 bar. The possibility of increasing the amount of air continues to exist by the use of a 1.5 mm nozzle. For later use in the batch production such a high cutting gas consumption is not economical

Another aim of this work was to reduce the energy per way as much as possible to achieve the minimal amount of energy necessary for cutting 0.1 mm thick material. To do this the parameters pulse frequency, charging voltage and pulse duration were adapted. It was observable that the cut opening was narrower with relieving energy per way. The pulse frequency had to be increased simultaneously, selected so that the pulses overlap sufficiently to produce a continuous opening. The dependence of the cut opening breadth of the way energy is represented in figure 3. The experiment shows that it is important to reduce the energy per way at a high pulse frequency as far as possible. On the one hand, the cut opening gets narrower thereby; on the other hand, the interaction zone can be reduced.

In contrast to experiments before, a fiber laser system of the company IPG Laser GmbH and a water jet conducted laser system of the company Synova S.A. are used here now. The two essential advantages of the fiber laser are the high feed rate and the almost inexistent melt deposit in the cut opening.

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A very big warmth influence zone is to be mentioned as an essential disadvantage. The water jet conducted method of laser cutting is based on the principle of the total reflection of the laser beam in a jet of water. When comparing the method of the company Synova with other methods, it achieves the highest cut edge qualities (Saint-Ghislain, 2007). Melt deposits are not available at all, the cut surfaces are parallel and the opening is very narrow. Deficits can be seen in the low processing speed and the complicated handling.

4. CONCLUSION

The knowledge of the process of cutting tungsten alloys won by the experiments and their evaluation is suitable to considerably improve the cut area quality in comparison to the qualities obtained before the work examination.

A rise of the cutting gas pressure up to 20 bar is of special significance for the improvements mentioned. Economic considerations must be included since such a rise of the pressure is achieved only with larger technical and financial effort. Broader possibilities for improvement of the cutting area quality for the thicker metals (= 0.5 mm) lie in the combination of high pulse frequency and simultaneous maximization of the application of energy per way. By contrast, it is favourable for thin metals (0.1 mm and 0.2 mm) to choose the energy as small as possible to ensure a narrow cut opening. Regarding the feed rate it can be noticed that this depends substantially on the material strength. The focus situation must be selected in order to produce low tapering at the cut opening. This ensures an optimal removal of the melt from the opening and provides a straight cutting edge simultaneously. The evaluation of the cut samples of the two outside companies has shown that other laser systems are in principle suitable as well to cut tungsten alloys with considerably higher feed speeds or with substantially better cutting area qualities. A decision on the actual efficiency of these plants can only be taken after further examinations which take the economic parameters into account.

5. REFERENCES

Forschungsprojekt der Thuringer Aufbaubank (2007): Rationalisierungsmoglichkeiten bei der Laserprazisions-bearbeitung von refraktaren Metallen. Vorhabens-kurzbeschreibung PraMet. University of Applied Sciences Jena

Guddei, Yvonne (2007): Zwischenbericht PraMet. University of Applied Sciences Jena

Norm DIN EN ISO 4287 (1998). Oberflachenbeschaffenheit: Tastschnittverfahren, Deutsches Institut fur Normung e. V., Berlin

Richtlinie VDI 2906 (1994). Schnittflachenqualitat beim Schneiden von Metallen, Verein Deutscher Ingenieure, Dusseldorf

Saint-Ghislain, Michael (2007): Thungsten plate cutting by Laser-MicroJet[R], Synova SA, Ecublens