Electric heating – Metal heating – By arc
Reexamination Certificate
1996-05-28
2001-04-10
Heinrich, Samuel M. (Department: 1725)
Electric heating
Metal heating
By arc
C219S121830
Reexamination Certificate
active
06215094
ABSTRACT:
The invention relates to a process for determining the instantaneous penetration depth and achieving a desired penetration depth of a machining laser beam into a workpiece and a device for implementing the process.
Laser beams are used for machining workpieces in the widest variety of ways, such as superficial re-melting, producing welded joints or introducing holes. In all these cases it is very important to detect the depth of penetration of the laser beam into the workpiece and ensure that it reaches the desired value and optionally keeps it constant. In welded joints this is important, for example, because on the one hand the welding melt has to reach sufficiently far into the parts to be joined together, so that the parts are reliably joined, but also because on the other hand the weld should not emerge from the undermost part, which could cause damage to visible surfaces. This applies, for example, to the welding of sheets of metal that are used as body parts in motor vehicle construction.
In the past, in laser welding the correct penetration depth was substantially verified on the basis of random samples, by producing polished sections on completed workpieces. This process is relatively costly, however, because a penetration depth that is inadequate or too deep is not ascertained until waste has already been produced.
DISCUSSION OF RELEVANT ART
A process is known from DE-OS 37 10 816 in which the quality of a laser machining process on a workpiece is monitored by observing the laser light reflected at the workpiece. No relationship is established between the penetration depth of the laser beam and the reflected portion of the laser beam, however; irregular changes in the intensity of the reflected laser light are simply interpreted as deviations from the desired operating conditions of the machining laser beam.
EP-A-0 299 702 describes a process in which the depth of a hole produced by a laser beam in a laminate structure is monitored by means of the laser irradiation reflected in the hole. In this case, however, the differing reflectivity of the different materials contained in the laminate structure is exclusively utilized; the sudden change of the reflected portion of the laser light is identified with the reaching of a new boundary layer inside the laminate. This known process is not suitable for monitoring the penetration depth of a laser beam in a uniform material.
A process and a device of the above-mentioned kind are described in DE-A-25 38 660. In this case the criterion for the depth of the vapour capillary produced by the machining laser beam is the absorption of the laser beam which it experiences when passing through the vapour present in the vapour capillary. The results obtained in this way are, however, relatively difficult to reproduce because they depend on the nature of the vapour discharge; furthermore this process and/or this device cannot be universally used in cases where there is no notable absorption of the machining laser beam in the vapour produced when machining.
WO-A-92 14 578 describes a process for monitoring the laser machining of workpieces in which the optical and/or acoustic signals resulting from the plasma or vapour created by the laser are detected and conclusions are drawn about the progress of the machining process from them.
Pages 69-71 of the June 1989 edition of “Laser und Optoelektronik” describe a process in which optical signals are detected in the near-UV range and at the wavelength of the machining laser. In this case the main purpose of detecting the IR signal is to obtain a better understanding of the principles of the mechanisms of interaction between laser beam and workpiece. This IR signal is not interpreted in the sense of using it as a measured variable for the weld depth.
SUMMARY OF THE INVENTION
The object of the invention is to develop a process of the above-mentioned kind in such a way that the penetration depth can be continuously monitored in the course of the machining process and can be brought to the desired value, including in uniform materials.
According to the invention this object is achieved by means of the following process:
a) directing a laser beam performing the function of a measuring laser beam onto a vapour capillary formed in the workpiece by a laser beam performing the function of a machining laser beam;
b) determining a portion of the laser beam performing the function of a measuring laser beam that re-emerges from the vapour capillary;
c) in step b) determining the percentage portion of the laser beam performing the function of a measuring laser beam that actually re-emerges at the workpiece and is not introduced into the workpiece; and
d) determining enery introduced into the workpiece per unit of time and area by the laser beam performing the function of a measuring laser beam that was determined in step b).
The process according to the invention is based on the knowledge that there is an unequivocal functional relationship between the percentage portion of a laser beam directed at the vapour capillary, which portion is reflected i.e., re-emergers from the vapour capillary, and the penetration depth of the laser beam, i.e. the depth of the vapour capillary. It is highly likely that this functional relationship is based on the mechanism of the multiple reflections within the vapour capillary; according to this model, curves can be calculated which reflect the relationship between the percentage reflected portion of the laser beam and the aspect, i.e. the relationship between depth and surface area of the hole. Such curves can also be determined experimentally by means of corresponding trials in which the reflected laser beam portion is determined point by point in each case at a certain aspect.
In the simplest case the same laser is used to generate the machining laser beam and the measuring laser beam. It is quite evident that in this case the construction will be the least complex in terms of apparatus and the costs at their lowest. This variant of the process according to the invention has the following disadvantage, however: for reasons of economy the highest possible percentage (over 90%) of the energy provided by the machining laser should be introduced into the workpiece. At this high input of the machining laser beam that is important to the efficiency of the process, however, the characteristic curve which produces the relationship between depth of the vapour capillary and reflected percentage portion of the laser beam is relatively flat, so that the determination of the penetration depth is relatively insensitive.
If the sensitivity of the measuring process when a single laser is used is not sufficient in some applications, therefore, use can be made of the physical phenomenon according to which the more slowly the characteristic curves pass into a flat region, the greater the wavelength of the laser beam used for the measurement. In this case a process variant is recommended in which two different lasers are used to generate the machining laser beam and the measuring laser beam, wherein the wavelength of the measuring laser beam is greater than that of the machining laser beam. In this way the measuring laser beam can be operated independently of the machining laser beam set to the highest possible efficiency in a manner in which the measuring sensitivity is high.
The process according to the invention can be used, for example, to achieve a hole with a certain depth in a workpiece. In this case its form is such that
a) a preliminary trial determines the percentage portion of the measuring laser beam that is reflected at the workpiece at the desired depth of the hole;
b) the reflected percentage portion of the measuring laser beam is monitored continuously while the machining laser beam is making the hole with the workpiece stationary and the machining laser beam is interrupted when the reflected percentage portion of the measuring laser beam has reached the value determined in the preliminary trial.
As the depth of the hole increases, the reflected portion of the measuring laser
Dausinger Friedrich
Griebsch Juergen
Hack Ruediger
Heinrich Samuel M.
Universitat Stuttgart
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