Electric heating – Metal heating – By arc
Patent
1997-06-05
1999-02-09
Evans, Geoffrey S.
Electric heating
Metal heating
By arc
21912163, 21912164, B23K 2604
Patent
active
058698051
DESCRIPTION:
BRIEF SUMMARY
CROSS REFERENCE TO RELATED APPLICATION
This is a national stage of PCT/DE95/01017 filed 4 Aug. 1995 based, in turn, upon German national application P4434409.0 of 26 Sep. 1994 under the International Convention.
FIELD OF THE INVENTION
Our present invention relates to a method of working materials with plasma-inducing high energy radiation, especially laser radiation, in which radiation from the workpiece and originating in the region of the workpiece is monitored as a function of time along the axis of the laser radiation focused on the workpiece.
BACKGROUND OF THE INVENTION
Processes with the aforedescribed process features are generally known as laser welding processes. The thus observed optical and acoustical signals are used for monitoring and control. Such signals arise from the workpiece surface or from above the workpiece surface. It is thus possible, for example, to detect a weld breakthrough only in isolated parameter regions. It is also generally known to observe the underside of a workpiece to detect a weld breakthrough. The workpiece is, however, during industrial finishing processes generally only accessible with difficulty at the underside. It is therefore necessary to detect the penetration depth in the workpiece from the upper side, i.e. from the side to which the laser radiation is fed.
OBJECT OF THE INVENTION
The invention so improving a process with the aforedescribed features that the penetration depth can be continuously detected from above the workpiece during the working of the material.
SUMMARY OF THE INVENTION
This object is achieved in that the observation is confined exclusively to the cross section of the vapor capillary with a depth of focus encompassing the total workpiece thickness and in that the mean value of the intensity of the plasma beam determined by this observation is used as a measure of the penetration depth.
For the invention it is significant that an observation is effected over the entire depth of the zone of interaction of the laser beam with the workpiece. It is also required to acquire signals from the interior of the vapor capillary reliably and to evaluate them. This is achieved in that the observation is confined to the cross section of the vapor capillary is exclusively and thus detrimental lateral signals are excluded. The requisite depth of focus required for this is so determined that the entire workpiece thickness is encompassed in the observation.
The light intensity of the plasma which arises in the plasma working is observed. Since this intensity fluctuates strongly, however, the mean value of the intensity of the plasma beam is used as a measure for the penetration depth. If the laser beam penetrates so deeply into the workpiece that the depth of the interaction zone is equal to the workpiece thickness, the breakthrough of welding is imminent. There is a characteristic change in the mean value of the intensity as a function of time during the progress of the material working. If necessary, the working process can be interrupted or so carried out that a breakthrough of welding is avoided thereby avoiding an undesired result. With the aforedescribed process, for simplification it is accepted that the density and the temperature of the radiation-emitted particles in the vapor capillary are constant. With this presumption, the measured radiation intensity is, to a first approximation proportional to the penetration depth of the vapor capillary in the workpiece. There are however a number of influence factors upon the density and the temperature within the vapor capillary so that the presumption is not accurate. Influence factors include, for example, dynamic processes within the vapor capillary as a consequence of the movement of the absorption front and the melt front. The working geometry also plays a measurable role. As a consequence, significant intensity fluctuations can disturb the measured result. These fluctuations can however be excluded in that the mean value of the radiation intensity can be determined while excluding intensity
REFERENCES:
patent: 5155329 (1992-10-01), Terada et al.
patent: 5304774 (1994-04-01), Durheim
patent: 5486677 (1996-01-01), Maischner et al.
Abels Peter
Beersiek Jorg
Beyer Eckhard
Nitsch Holger
Schulz Wolfgang
Dubno Herbert
Evans Geoffrey S.
Fraunhofer-Gesellschaft Zur Forderung Der Angewandten Forschung
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