Arrangement for machining workpieces by means of a laser beam by

Details Arrangement for machining workpieces by means of a laser beam by Arrangement for machining workpieces by means of a laser beam by

1986-04-17

1989-06-13

Albritton, C. L.

Electric heating

Metal heating

By arc

21912185, 21912162, B23K 2600

Patent

active

048394939

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

This invention relates to an method and apparatus for machining workpieces by means of a laser beam.
In the case of this type of machining of workpieces, the laser irradiation, by means of an optical system, is focussed on the workpiece and absorbed at its surface. The resulting heat is used as process heat for the various machining processes.


STATE OF THE ART

When work is carried out in the range of low intensity of laser irradiation, only the natural absorption of the workpiece is utilized. Since only a small part of the laser energy used in this case is transformed into process heat and the largest part is lost because of heat conduction and reflection (in the case of metals) or transmission (in the case of nonconductors), only low efficiency is obtained.
It is known to apply, for the purpose of increasing absorption, onto the workpiece to be machined a layer having a high degree of absorption. This alternative is used mainly in the case of transformation hardening.
It is also known that the degree of absorption, when the laser intensity I.sub.L is increased to or above a critical limiting value I.sub.C, approaches 1. This intensity dependence of the absorption that is also called an abnormal absorption is connected with the development of a laser-induced plasma on the surface of the workpiece. This plasma acts like an energy converter that converts the incident laser energy, in the case of a high efficiency, into heat energy. In the case of removing machining, the material to be removed, by means of the controlling of the intensity of the laser irradiation, can be changed into such a plasma condition in which the kinetic energy of the particles causes a sufficiently fast material transport.
The efficiency .eta. of a removing machining may be shown as a function of the relative laser intensity I.sub.L /I.sub.C in the case of a wave length .lambda.=10.6 .mu.m, for rustproof steel and for cooper. It is found that in the range of the critical intensity I.sub.C, the efficiency .eta. comes close to the value 1.
In the case of a further increase of the laser intensity, a noticeable shielding of the workpiece occurs by means of heating and ionization of the plasma expanding in the direction of the incident laser irradiation. Also the lens effect of the plasma increases interfering with the machining geometry. In the case of an exceeding of an upper limiting value of the intensity I.sub.D of the laser irradiation, the plasma almost completely shields the workpiece which results in a laser-induced detonation wave in which case the recoil of the expanding plasma may result in cavitation and destruction of the surface of the workpiece because of stress caused by pressure.
The prerequisite of the formation of a laser-induced plasma at the surface of the workpiece is the presence of a gaseous state of aggregation of the material of the workpiece. In order to meet this requirement, the laser beam must first heat the workpiece to or above the evaporation temperature. This means that more laser energy must be absorbed by the workpiece than is removed by heat conduction. On the basis of a varying heat conduction and absorption, typical evaporation critical intensities of the laser irradiation are obtained for the different materials.
One difficulty in the case of the machining of workpieces by means of laser beams in the area of abnormal absorption is the fact that the formation of a laser-induced detonation wave may start immediately after the beginning of the evaporation process. This may, among other things, also occur during the machining of aluminum by means of CO.sub.2 -laser.
Another difficulty is the result of the fact that the laser by the machining process is changed in its system characteristics. By means of the arrangement of the workpiece in the focal plane of the optical system, a fraction of the laser irradiation is reflected into the laser resonator. This resonator, together with the workpiece, forms a system of coupled resonators, the characteristics and modes of which are clearly di

REFERENCES:
patent: 3700850 (1972-10-01), Lumley et al.
patent: 4121087 (1978-10-01), Malmuth et al.
patent: 4504727 (1985-03-01), Melcher et al.
patent: 4507536 (1985-03-01), Inoue

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