Monitoring method and system for laser beam welding

Abstract: In a method for monitoring laser beam welding, a sequential signal representing a laser oscillating wave form is received, a light of predetermined wave length contained in the light emitted from the weld zone is received, and the two signals thus received are compared and processed to determine a minimum intensity of the light of the predetermined wave length for accurately deduce welding beam input and penetration of welding. An apparatus used in practicing the monitoring method comprises a monitoring optical fiber for receiving and transmitting the light emitted from a weld on a work, an interference filter for selectively passing the light of predetermined wave length, and a processing device for determining an intensity of the light beam of the predetermined wave length so as to monitor a welding condition.

Description: This invention relates to a method and apparatus for monitoring the behavior of laser beam welding according to the preamble of claims 1 and 3 (see: "Laser und Optoelektronik", vol. 21, no. 3, June 1989, STUTTGART, pages 69-72). Description of the Prior Art: Laser beam welding is widely practiced nowadays because welding beam input (energy) is applied to only a desired weld zone and little thermal influence is imposed on the vicinity thereof. In this regard, to finely complete welding, the welding beam input and resultant depth of penetration of welding must be controlled properly. It is generally difficult in laser beam welding to directly detect the welding beam input being actually applied to a work. Therefore, as shown in Fig. 11(a), a part of a laser beam 15 generated by a laser oscillator 1 is guided by a quartz plate (semi-transparent mirror) 26 to a photodiode 6 such that the detection output of the photodiode 6 indicates the (laser beam) output of the laser oscillator 1. Conventionally, the behavior of welding is deduced from monitoring the laser beam output of the oscillator. In the foregoing conventional system wherein only the output of the laser oscillator is detected, however, if the transmission loss of an optical system for transmitting the laser beam from the laser oscillator to a weld zone increases suddenly during welding due to the degradation in property of optical parts, for example, a large discrepancy arises between the laser beam output and the welding beam input being actually applied to a work; therefore, accurate monitoring cannot be attained as illustrated in Fig. 11(b). Further, even if welding becomes unstable to decrease the depth of penetration due to a change in welding condition, this causes no change in the output of the laser oscillator; therefore, the behavior of welding cannot be grasped by an operator properly. A different system is known in which the behavior of laser beam welding is deduced from monitoring infrared radiation at a weld zone. Specifically, U.S. Patent No. 4,443,684 (corresponding to Japanese Patent Laid-Open No. 57-124586) discloses a system applicable in the field of laser beam processing inclusive of welding which uses a carbon dioxide gas laser as a laser beam source and sends the infrared radiation at a spot under laser beam processing to a detector to obtain the temperature of the spot. In this U.S. patent, an infrared ray of 10.6 µm in wave length is used and the temperature of a weld zone is measured. However, since the infrared ray reaching the detector includes a reflected part of the laser beam and laser plumes appear in the vicinity of a molten pool to cause much infrared radiation, it is difficult to accurately detect the temperature of the weld zone. Further, since the temperature of the weld zone generally does not indicate the welding beam input and the depth of penetration, it is difficult in laser beam welding to attain the monitoring of the welding beam input and the depth of penetration on the basis of the detected value of temperature. Another apparatus for providing a real time determination of temperatures and cooling rates experienced on a weld zone of a workpiece being processed with a laser is disclosed in US-A-4,817,020. For this purpose, a temperature in the weld zone is detected from the intensity of infrared radiation emitted from the weld zone at two different wavelengths by means of a ratio pyrometer. Preliminary results of research on methods to control laser beam welding by monitoring optical signals emitted or reflected from a weld zone are disclosed in an article in "Laser und Optoelektronik", 1989. In the described method, like in the invention, the laser oscillating waveform and the intensity of a ray of light of a predetermined wavelength reflected from the weld zone are monitored. The evaluation or analysis following the measuring process relies on a correlation between the dependence on time of the course of the intensity of the light monitored and the pulse frequency of the laser beam in order to determine the power density in the weld zone. SUMMARY OF THE INVENTION: It is an object of the present invention to provide a method for monitoring laser beam welding by which welding beam input and depth of penetration can be deduced with high accuracy from monitoring the intensity of emission at a weld zone inclusive of a molten pool. It is another object of the present invention to provide an apparatus for monitoring laser beam welding by which welding beam input and depth of penetration can be deduced with high accuracy from monitoring the intensity of emission at a weld zone. According to the invention there is provided a method of monitoring laser beam welding as recited in claim 1. In a laser beam welding machine wherein a laser beam is transmitted from a laser oscillator through an optical system including an optical fiber and a lens to a weld zone, the present invention continuously detects the light emitted from the weld zone and the laser beam output of the laser oscillator. As well known in the art, a pulsating laser beam applied to the weld zone causes emission, the emission at the weld zone changes in response to the pulsating laser beam applied, laser plumes present in the weld zone disappear when the laser beam decreases below a base level, i.e. when the application of the laser beam to the weld zone is suspended, and consequently, there remains only the emission of a molten pool. The present invention detects the intensity of a light of predetermined wave length contained in the emitted light, from which the welding beam input and the depth of penetration can be deduced. The present inventor has found that there is a certain correlation between the intensity of a light of predetermined wave length and both the welding beam input and the depth of penetration. In this invention, the behavior of laser beam welding can be deduced from monitoring the intensity of a light of predetermined wave length immediately before the rising of the pulsating laser beam, that is minimum emission intensity. The present invention is embodied in the form of an apparatus which comprises the features of Claim 3. BRIEF DESCRIPTION OF THE DRAWINGS: Fig. 1 is a schematic diagram showing an embodiment of the present invention; Figs. 2(a) through (c) are views showing optical fibers; Fig. 3 is a diagram showing the principle of converging the light emitted from a weld zone; Fig. 4 is a graph showing an emission intensity characteristic during pulsation welding; Fig. 5 is a block diagram showing a monitoring section of the embodiment; Fig. 6 is a graph showing the relationship between the pulsating output of a laser oscillator and the intensity of emission from a weld zone; Fig. 7 is a graph showing the measured intensity of emission at a weld zone in relation to welding beam input; Fig. 8 is a graph showing the correlation between welding beam input and emission intensity; Fig. 9 is a graph showing the correlation between emission intensity and depth of penetration; Fig. 10 is a graph showing the emission intensity plotted in relation to the welding beam input where no interference filter is included; Fig. 11(a) is a diagram showing a conventional monitoring system; and Fig. 11(b) is a graph showing the property of the conventional system. DESCRIPTION OF THE PREFERRED EMBODIMENTS: Embodiments of the present invention will now be described in greater detail with reference to the drawings, which are for better understanding of the invention, not for limiting purposes. In Fig. 1, a laser beam generated by a laser oscillator 1 is transmitted through an incident optical system 2 to a laser-beam transmitting optical fiber 3. Where the laser oscillator 1 is made of a YAG laser, it generates a laser beam of 1.06 µm in wave length. The laser beam after the laser-beam transmitting optical fiber 3 is transmitted to a welding/converging optical system 11, thus is converged at a weld zone 13 of a work 14. The weld zone 13 absorbs the laser beam to melt, forming a molten pool. When the laser beam moves by scanning, the molten pool resolidifies, attaining welding. Although not shown, an assist gas such as inert gas is applied to around the weld zone 13 as is the case of the ordinary welding. During welding, emission takes place at the weld zone 13 as designated by reference numeral 12. As shown in Figs. 2(a) through (c), a transmitting/monitoring optical line 10 is a combination of the laser-beam transmitting optical fiber 3 plus monitoring optical fibers 4. Specifically, as shown in Fig. 2(b), the laser-beam transmitting optical fiber 3 is disposed at the center, the monitoring optical fibers 4 are disposed around the central fiber 3, and the combination of fibers is covered with a metal sheath 27 and a flexible stainless (SUS) pipe 28 as shown in Fig. 2(a). The monitoring optical fibers 4 are divided into two bundles (as shown in Fig. 1) each including several fibers, and connected to interference filters 5 described later. Fig. 2(c) shows such division in greater detail. The optical fiber 3 is partially covered with a copper sleeve 29. The light from the emission 12 at the weld zone is transmitted through the lens of the welding/converging optical system 11 to the monitoring optical fibers 4 as illustrated in Fig. 3. In Fig. 3, a welding laser beam 15 departing from the end of the central optical fiber 3 diverges slightly, enters the converging optical system 11, and by its condenser lens, is converged on the surface of the work 14. The light from the emission 12 at the weld zone 13, i.e. emitted light 19, is converged by the condenser lens on the end of the optical fibers 4. Since the optical fibers 4 are circularly arranged, they can effectively receive the emitted light 19 irrespective of the surrounding condition of the weld zone 13. It should be noted that even after the laser beam 15 disappears, the emitted light 19 enters the optical fibers 4 as long as the emission 12 exists. The interference filter 5 selects a light of predetermined wave length, for example, of 0.8 µm or 0.94 µm in wave length in the case of "Inconel" welding, and passes the thus selected light to a photo-diode 6 where photoelectric transformation is performed. The signal obtained by photoelectric transformation as above which is representative of the light is amplified by an amplifier 7, converted by an A-D conversion board 8 into digital form, and sent to a computer 9 for signal processing. Although not shown in Fig. 1, the pulsating output wave signal of the laser oscillator 1 is also sent to the computer 9. Fig. 5 shows in functional block form a monitoring section of the apparatus shown in Fig. 1. The light passed through each monitoring optical fiber 4 is transmitted through the interference filter 5 to the photodiode 6 whose output is amplified by the amplifier 7 as described above. A signal line 17 for conveying a signal representative of the pulsating laser beam generated by the laser oscillator 1 is connected to a synchroscope 24 and a data recorder 22. The signal representative of the emission intensity that is amplified by the amplifier 7 is also sent to the data recorder 22 and the synchroscope 24. A memory recorder 23 is used for storing the foregoing signals. Fig. 6 shows the waveform 18 of the pulsating laser beam and the waveform 20 of the emission intensity at the weld zone, both being recorded in the monitoring section. As will be appreciated, the waveform 20 of the emission intensity at the weld zone corresponds to the intensity of the light of predetermined wave length. Fig. 4 shows the two waveforms of Fig. 6 in expanded scale. These waveforms are pulsating together, and two intervals of time A and B along the time axis (abscissa) are about 9 msec and about 13 msec, respectively. As will be appreciated from Fig. 4, the waveform of the emission intensity keeps a certain high level while the welding beam input is present (i.e. while the laser beam is irradiating the work), decreases abruptly in a short time after the welding beam input drops, and then rises together with the rising of the laser beam. Fig. 7 shows the minimum emission intensity immediately before the rising of the laser beam in relation to the welding beam input. Specifically, Fig. 7 shows the change of emission intensity in relation to the welding beam input being 425 W, 450 W and 470 W. Figs. 8 and 9 show the correlation between the minimum emission intensity of the light of predetermined wave length and the welding beam input, and the correlation between the minimum emission intensity and the depth of penetration, respectively. As will be appreciated from these drawings, each correlation is linear; thus, to monitor the minimum emission intensity is equivalent to monitor the welding beam input as well as the depth of penetration. For reference, in Fig. 10, the emission intensity measured with no inclusion of the interference filter 5 shown in Fig. 5 is plotted in relation to the welding beam input. As will be appreciated, no correlation is recognized between these parameters where the interference filter is not included. The foregoing predetermined wave length used in filtering the light emitted at the molten pool can be determined by performing welding testing, spectrum analysis, data processing, etc. As described above, the present invention can provide the method and apparatus for monitoring the behavior of laser beam welding by which the welding beam input and the depth of penetration can be precisely deduced.

Claims: A method of monitoring laser beam welding, said method comprising the steps of: performing welding by irradiating a workpiece (14) with a pulsating laser beam (15); generating signals (18) representative of the pulsating waveform of the laser (15); during said welding, continuously monitoring the intensity of light of a predetermined wavelength emitted from a zone (13) of the workpiece (14) at which said welding is being performed; generating signals (20) representative of the intensity of said light of a predetermined wavelength; characterized by processing said signals (18, 20) representative of both the pulsating wave form of the laser beam (15) and of the intensity of said monitored light of a predetermined wavelenght to determine the minimum intensity of said light of a predetermined wavelength that is being emitted from the weld zone (13) during said welding; and deducing an energy of the laser beam (15) at the weld zone (13) and/or a depth of penetration from the minimum intensity of said light of a predetermined wavelength on the basis of a previously obtained correlation between the welding beam input and the emission intensity of the monitored light and/or a correlation between the depth of penetration and the emission intensity of the monitored light. A method of monitoring laser beam welding as claimed in claim 1, wherein said step of monitoring comprises filtering the light (19) emitted from the weld zone (13) except for said light of a predetermined wavelenght. Apparatus for monitoring laser beam welding, said apparatus comprising: a monitoring optical fiber (4) capable of receiving and transmitting light therealong; an interference filter (5) optically coupled to said fiber (4), said filter (5) filtering light (19) transmitted by said fiber (4) except for light of a predetermined wavelength; photodiode means (6) operatively associated with said interference filter (5) for transforming the light of a predetermined wavelength passing through the filter (5) into signals (20) representative of the intensity thereof; and a processor (9) operatively connected to said photodiode means (6) so as to receive the signals produced thereby, characterized in that said processor (9) includes processing means for processing said signals to discriminate the minimum intensity of the light passing through said filter (5) during a given time period and for deducing an energy of the laser beam (15) at the weld zone (13) and/or a depth of penetration from the minimum intensity of said light of a predetermined wavelength on the basis of a previously obtained correlation between the welding beam input and the emission intensity of the monitored light and/or a correlation between the depth of penetration and the emission intensity of the monitored light. An apparatus for monitoring laser beam welding as claimed in claim 3, and further comprising a laser beam transmitting optical fiber (3) assembled with said monitoring optical fiber (4) into a single bundle of fibers, and an optical system (11) optically coupled to said single bundle of fibers, said optical system (11) having focal points located proximate the end of said monitoring optical fiber (4) and spaced from said single bundle of fibers, respectively. A laser welding apparatus including an apparatus for monitoring laser beam welding as claimed in claim 3, and further comprising laser oscillator means (1) for emitting a pulsating laser beam (15) and for generating signals (18) representative of the waveform of the laser beam (15) emitted thereby, and wherein said processor (9) is operatively connected to said laser oscillator means (1) so as to receive the signals (18) representative of the waveform of the laser beam (15) emitted by the laser oscillator means (1) and said processing means (9) processes said signals (18) representative of the waveform of the laser beam (15) emitted by said laser oscillator means (1) along with said signals (20) produced by the photodiode means (6) to determine the minimum intensity of the light passing through said filter during a given period of time.

Inventors: HASHIMOTO SHIGEO C O KOBE SHIP, ISHIDE TAKASHI C O TAKASAGO TE, MATSUMOTO OSA C O TAKASAGO TEC, NAGASHIMA TADASHI C O KOBE SHI, NAKURA YASUMI C O TAKASAGO TEC, TERADA KUNIO C O TAKASAGO TECH, HASHIMOTO, SHIGEO C/O KOBE SHIPYARD & ENGINE W., ISHIDE, TAKASHI C/O TAKASAGO TECHN. INSTITUTE OF, MATSUMOTO, OSA C/O TAKASAGO TECHN. INSTITUTE OF, NAGASHIMA, TADASHI C/O KOBE SHIPYARD & ENGINE W., NAKURA, YASUMI C/O TAKASAGO TECHN. INSTITUTE OF, TERADA, KUNIO C/O TAKASAGO TECHNICAL INSTITUTE, Hashimoto, Shigeo c/o Kobe Shipyard &amp; Engine W., Nagashima, Tadashi c/o Kobe Shipyard &amp; Engine W.
Applicants: MITSUBISHI HEAVY IND LTD, MITSUBISHI JUKOGYO KABUSHIKI KAISHA
Application year: 1991
Main classification: B23K26
IPC codes: B23K26