Laser welding device and method

Abstract: In a laser welding device, a photographing device (31) is provided to photograph the plasma (7) caused by exposing an object to laser beams (6). An image processing device (33) is provided to measure at least one of an area and height of a plasma image photographed by the photographing device (31). A distinction device (34) is provided to compare a value measured by the image processing device (33) with an allowable value so as to determine whether the laser welding condition is good or not.

Description: This invention relates to a laser welding device and a method of checking laser welding condition which is provided at the time of welding to monitor plasma caused by concentrically irradiating laser beams to an object so as to determine whether the laser welding condition is good or not. In laser welding, a laser output measurement device has been used to control energy level of laser beams generated at the time of the laser welding. Fig. 12 shows a schematic view of an energy level measuring system of laser beams with the laser output measurement device employed. In Fig. 12, laser beams 6 generated by a laser oscillator 1 are applied on a measurement portion 11 of the laser output device through an optical fiber 2, light-converging lenses 3, 4 and a protective glass 5 so as to display an energy level of the laser beams 6 on a display device 12. An energy level value on the display device 12 makes it possible to recognize an energy intensity of the laser beams 6. In YAG laser welding device, there is provided a laser energy monitoring member referred to as a laser output monitoring device 25 which has often been used to control an energy level of laser beams. Fig. 13 shows a schematic view of an energy level measuring system of laser beams with the laser output monitoring device 25 employed. In Fig. 13, a part of laser beams 6 radiated from a laser source 21 of the laser oscillator 1 is passed through a semi-reflective mirror 22 to measure an energy level of the laser beams 6 by a detector 23 so as to display it on a display device 24. The detector 23 has a photodiode and a solar cell so as to recognize an energy intensity of the laser beams 6 radiated from the laser oscillator 1. However, in the laser welding technique, even when the energy level of the laser beams 6 radiated from the laser oscillator 1 is constant, the energy level of the laser beams 6 is subject to change so as to attenuate the laser energy due to damage and stain on optic-related components such as the optical fiber 2 and the light-converging lenses 3,4 located between the laser oscillator 1 and the object. It also holds true when a distance between the laser oscillator 1 and the object is subject to change so as to vary the laser energy density at the time of replacing the objects or the like. Since the laser output measurement device is normally used before and after the laser welding operation so as to confirm whether or not the energy level of the laser beams 6 is subjected to change by comparing the energy level before the laser welding operation and the one after the laser welding operation, it is difficult to use the laser output measurement device concurrently during the laser welding operation. In the laser output monitoring device 25 which serves as an output value measuring device of the laser oscillator 1, the device remains immune to positional changes of the optical fiber and the light-converging lenses 3,4, and the device has no direct relation to measuring the laser energy level of the welding portion on which the laser beams 6 are applied. In both the laser output measurement device and the laser output monitoring device 25, therefore, it is impossible to detect the welding condition of the object on which the laser beams 6 are applied. This necessitates to frequently carry out a sampling inspection for breaking test, X-rays photographing and chemical analysis so as to increase the number of manufacturing steps. When the product is inspected as defect during the sampling operation, all the rest of the product lot have to be rejected, thus adding an unnecessary step during the manufacturing working. In "LASER UND OPTOELEKTRONIK, vol 22, No. 6, 1 October 1990, pages 56 - 62, "OPTOELEKTRONISCHER SENSOR FUER DIE EINZELBEOBACHTUNG BEIM LASERSCHWEISSEN ZUR NAHTFUEHRUNG UND ADAPTATIVEN PROZESSBEEINFLUESSUNG" it is disclosed "a videooptical sensor supported by an image processing unit leading to the evaluation of the characteristics of the melting pool, e.g. fluctuations and geometry allowing continuous quality control and regulation of the [welding] process aditional to the parametres of the laser plant." Therefore, it is one of the objects of the invention to provide another laser welding device and method of checking a welding condition which is capable of determining whether or not the welding condition is good while concurrently carrying out the welding operation. According to one aspect of the invention, there is provided a method of monitoring the condition of a laser welding operation including the steps of: photographing the plasma caused by irradiating, with laser radiation, an object being welded; and measuring the value of at least one of the area and height of the plasma image to assess the laser welding condition. Preferably the measured value of at least one of the area and height of the plasma image is compared with an allowable value to determine whether the condition of the welding operation is satisfactory or not. The method may further comprise the step of adjusting the energy level of the laser radiation generated by a laser oscillator used in irradiating the object depending on the difference between the measured value of at least one of the area and height of the plasma image and a predetermined value. According to another aspect of the invention, there is provided a laser welding device including: photographing means provided to photograph the plasma caused by irradiating, with laser radiation, an object being welded; image processing means provided to measure the value of at least one of the area and height of the plasma image photographed by the photographing means; and means to employ the measured value in controlling the device or assessing the laser welding condition. Preferably the device includes distinction means provided to compare a value measured by the image processing means with an allowable value to determine whether the condition of a welding operation is satisfactory or not. Advantageously, the device may include control means provided to control the energy level of the laser radiation, generated by a laser oscillator included in the device, depending on the difference between a value measured by the image processing means and a predetermined value. According to still another aspect of the invention, there is provided a method of making an electrode for a spark plug by laser welding a noble metal portion to a base electrode and monitoring the condition of the laser welding operation using any of the above methods or apparatus. Thus the invention provides a method of checking a laser welding condition, a welding device and a method of making a spark plug electrode respectively, each of which is capable of determining whether the welding condition is good or not. With this invention the plasma caused from the welding surface of the object on which the laser spot beams are applied by the light-converging lens is photographed by the photographing means. With the use of the image processing means, at least one of the area and height of the plasma image photographed by the photographing means is measured to determine whether the laser welding condition is good or not. This can be done concurrently while carrying out the welding condition so that whether or not the laser welding condition is good is detected early. This also makes it possible to decrease the number of frequent sampling inspections to guarantee the quality of all the products so as to reduce the length of manufacturing steps. Alternatively or additionally, with the use of a control means provided to control an energy level of laser beams depending on a difference between a value measured by the image processing member and a predetermined value, the invention makes it possible to maintain good welding condition. By adjusting the tolerance imparted by the control means, it is possible to make the welding quality of the individual products uniform so as to reduce the quality variation which leads to stability of the products. It is to be observed that there are thought to be two reasons why the area or height of the plasma image becomes reduced. One is reduction in output of the laser oscillator, and the other is damage or stain on the optic-related components such as an optical fiber and light-converging lens. It is a fault with the optic-related components when the area or height of the plasma image is reduced, while the value on the laser output monitor device corresponds to an energy level of the laser beams inherently necessary for the welding operation. As a consequence, by comparatively controlling the area or height of the plasma image and the value on the laser output monitor device, it is possible to prevent the damage on the optic-related components due to an excessive output from the laser oscillator. With the method of checking laser welding condition comprising photographing the plasma caused by irradiating the laser beams on to the welding object, and measuring at least one of the area and height of the plasma image, it is possible to determine whether or not the laser welding condition is good concurrently while carrying out the welding operation. With the method of making a spark plug electrode by pulse laser welding a noble metal tip, comprising photographing the plasma caused by irradiating the laser beams to the object, and measuring at least one of the area and height of the plasma image so as to determine whether the laser welding condition is good or not, it is possible to assess quickly the welding condition. With the use of CW laser (Continuous-Wave Gas Laser), the thermal conduction varies a penetrating depth of the welding portion due to gradual temperature rise in the neighbourhood of the welding portion despite the uniform energy level of the laser beams applied from the start until the end of the welding operation. This is remarkable particularly when a small-sized component such as a center electrode for a spark plug is used. In this instance, it is possible to control the temperature rise so as to prevent the variation of the penetrating depth of the welding portion by alternately repeating heating and cooling operations by pulsing the laser beams. The invention will be better understood from the following description given by way of example only, with reference to the accompanying drawings in which: Fig. 1 is a schemative view of a welding device according to a first embodiment of the invention; Fig. 2 is an object on which laser beams are applied in Fig. 1; Fig. 3 is a graph showing relationship between durability time and increase of a spark gap when a durability experimental test is carried out with a spark plug mounted on a gasoline engine according to the first embodiment of the invention; Fig. 4 is a graph showing a relationship between Pt-containg rate and area of plasma image; Fig. 5 show some plasma images photographed at various laser energy levels by CCD camera; Fig. 6 is a graph showing a relationship between Pt-containg rate and area of plasma image taken when a protective glass is clouded; Fig. 7 is a graph showing a relationship between Pt-containg rate and area of plasma image taken while changing a distance from an object to a light-converging lens; Fig. 8 is a graph showing a relationship between Pt-containg rate and area of plasma image taken while changing flux of argon gas; Figs. 9a and 9b are views how welding portions of center electrodes are observed according to the first embodiment of the invention; Fig. 10 is a schematic view of a welding device according to a second embodiment of the invention; Fig. 11 is a schematic view of a welding device according to a third embodiment of the invention; Fig. 12 is a schematic view of an output measurement system of laser beams when a laser output measurement device is used; and Fig. 13 is a schematic view of an energy measurement system of laser beams when a laser output monitoring device is used. Referring to Fig. 1 which shows a welding device according to a first embodiment of the invention, laser beams 6 emitted from a laser oscillator 1 reach light-converging lenses 3, 4 by way of an optical fiber 2. The light-converging lenses 3, 4 concentrate the laser beams 6 into spot beams which are applied on an object through a protective glass 5 as described hereinafter. When the laser beams are applied on a surface of the object to be welded, plasma 7 appears from the surface of the object. A photographing device 31 only photographs the plasma 7, while an area of a plasma image taken by the photographing device 31 is to be measured by an image processing device 33. With the use of a pulse YAG laser, it is necessary to pick up images in sync with the emission of laser beams 6. It is possible for the photographing device 31 to pick up images of the plasma 7 by using a synchronizing trigger signal which corresponds to a laser emission synchronizing signal generated from the laser oscillator 1. In this instance, a CCD camera (Charge Coupled Device) is employed with the photographing device 31. The synchronizing trigger signal may be a trigger signal source 38 by using a photodiode which detects the laser beams 6 or plasma 7. A measurement value of an area of the plasma image is fed per pulse to a distinction device 34 which calculates an average value of all the measurement values corresponding to all the pulses required to finish the welding operation per product. By comparing the average value with an allowable value, it is determined whether or not the welding condition is good. When the obejct is taken as a center electrode for a spark plug, Fig. 2 shows the center electrode prior to being exposed to the welding operation. An electrode metal (a) of the columnar center electrode is made of Ni-based alloy (containing 95% nickel) which has 2.0 mm in diameter. An upper portion of the electrode metal (a) has a circumferential groove to serve as a welding portion around which a wire (b) is wound. The wire (b) is made of pure platinum (Pt) having 0.3 mm in diameter. The electrode metal (a) is located by a chuck 35, and the laser beams 6 are applied on the wire (b) while rotating the electrode metal (a) at approximately 70 rpm as shown in Fig. 1. The operating condition of the pulse YAG laser employed herein is under 0.8 ms in pulse width, 12 PPS and 3.5 sec for duration. The welding condition under this operating condition is shown at "PULSE" in Fig. 9a. When CW laser (Continuous-Wave Gas Laser) is used, the electrode metal (a) is subjected to a considerable amount of heat too much to put it into practical use as shown at "CW" in Fig. 9b. This is because it is necessary to apply the laser beams 6 until the wire (b) begins to melt although a melting point of the wire (b) is higher than that of the electrode metal (a). The wire (b) is fused into the electrode metal (a) to circularly form a Pt-Ni alloyed layer. The welding operation is carried out in a chamber 36 into which a stream of argon gas is flowing. The welding portion is placed in inert argon atmosphere which prevents oxidation of the welding portion. An exhaust duct 37 is provided to prevent the protective glass 5 from being stained by metallic vapor. It is to be noted that Pt- containing rate of the Pt-Ni alloyed layer changes depending on an intensity of the laser energy level applied on the welding portion. It is preferable that the Pt- containing rate may be 70 ∼ 90 % at the middle of the alloyed layer so as to enhance a spark-erosion resistant property of the center electrode for the spark plug. A durability experimental test was carried out with the spark plug mounted on four-cylinder, 1600 cc engine at full load (5500 rpm). As a result of the durability experimental test, Fig. 3 is a graph showing a relationship between durability time and increase of spark gap. Fig. 4 is a graph showing a relationship between the Pt-containing rate and an area of the plasma image photographed at various laser enegry levels. From Figs. 3 and 4, it is found that the Pt-containing rate in less than 70 % tends to increase the spark erosion so as to remarkably increase the spark gap in which the Pt-Ni alloyed layer has increased its volume so that the Pt-containing rate has decreased. When the Pt-containing rate exceeds 90 % at the time of welding operation, an unfused portion often remains at an interface between the Pt-Ni alloyed layer and the electrode metal (a). This also causes the alloyed layer to fall off due to the expansional difference between the alloyed layer and the electrode metal (a) when put into practical use by mounting the spark plug on the engine. Fig. 5 are plasma images photographed by the CCD camera at various laser energy levels. It is found from Figs. 4 and 5 that the increase of the laser energy level brings a decrease of the Pt-containing rate while increasing the area of the plasma image. This means that the Pt-containing rate is controlled by monitoring the area of the plasma image photographed. Fig. 6 is a graph showing a relationship between the Pt-containing rate, the number of products and an area of the plasma image photographed when the exhaust duct 37 is not activated. When the exhaust duct 37 is not operated, metallic vapor appeared at the time of applying the laser beams deposits on the protective glass 5 so as to decrease the laser energy density applied on the welding portion. This causes the Pt-containing rate to deviate from the allowable value, but detectable due to the gradually decreased area of the plasma image. Fig. 7 is a graph showing a relationship between the Pt-containing rate and an area of the plasma image photographed when changing a distance from the object to the light-converging lens. On the assumption that the light-converging lenses 3, 4 concentrate the laser beams 6 into spot beams which are applied on the welding portion, the welding is insufficient due to the reduced density of the laser energy level when the focuses of the light-converging lenses 3, 4 are too near or too far. When the Pt-containing rate to deviate from the allowable value, it is found that the area of the plasma image decreases. Fig. 8 is a graph showing a relationship between the Pt-containing rate and an area of the plasma image photographed when changing the flux of the argon gas flowing into the chamber 36. When the argon gas is not flowing normally, it is found that the Pt-containing rate decreases because the electrode metal (a) is facilitated to melt due to the generated oxidation heat. In this instance, it is possible to detect the abnormal welding due to the enlarged area of the plasma image photographed. As a result with the experimental test, it is possible to recognize the welding condition of the product by measuring the area of the plasma image so as to check whether the product has a good welding portion or not by comparing the measurement value with the allowable value. In the case in which the welding condition is determined by a height of the plasma image photographed, the same procedure is applied as used when determined by the area of the plasma image. Further, it is possible to decrease the variation of the measurement values to obtain a stable check and maintenance by determining the welding condition by both the area and the height of the plasma image photographed. In the present embodiment of the invention, the CCD camera 31 is located on which only the plasma image is photographed. This is because the CCD camera 31 is not exposed to the scattered light of the laser beams 6 applied on the welding portion so as to improve the measurement precision. A filter may be used to block infrared-rays when the CCD camera 31 is placed in a manner to be exposed to the scattered light of the laser beams 6 applied on the welding portion. It is appreciated that the laser device may be used at the time of cutting, broaching and scribing. It is also appreciated that instead of the CCD camera, SIT tube (Silicon Intensifier Target Tube) may be used as an image pick-up tube, and CO2 laser, or excimer laser (Excited Dimer) may be used instead of the YAG laser. Fig. 10 shows a second embodiment of the invention which is substantially the same as the first embodiment of the invention except for a control device 32 provided instead of the distinction device 34. The control device 32 enables to control the laser energy level generated from the laser oscillator 1 depending on the difference between the allowable value and the measurement value generated from the image processing device 33. This makes it possible to maintain the welding condition substantially uniform without inviting any welding defect. It is possible to reduce the variation of the welding so as to make the quality of the welding uniform by narrowing a control width of the control device 32. This contributes to stable performance of individual products. Fig. 11 shows a third embodiment of the invention in which the CW laser is used to bond flat plates by means of a butt welding. The third embodiment of the invention is substantially the same as the first embodiment of the invention except for the trigger signal source 38 discretely provided so that the plasma 7 is photographed by the photographing device 31. In this instance, the inert argon gas is to be replenished by an assist gas applicator 39. While the invention has been described with reference to the specific embodiments, it is understood that this description is not to be construed in a limiting sense in as much as various modifications and additions to the specific embodiments may be made by skilled artisans without departing from the scope of the invention as defined by the claims.

Claims: A method of monitoring the condition of a laser welding operation including the steps of: photographing the plasma (7) caused by irradiating, with laser radiation (6), an object being welded; and measuring the value of at least one of the area and height of the plasma image to assess the laser welding condition. A method according to claim 1, wherein the measured value of at least one of the area and height of the plasma image is compared with an allowable value to determine whether the condition of the welding operation is satisfactory or not. A method according to claim 1 or 2, further comprising the step of adjusting the energy level of the laser radiation (6) generated by a laser oscillator (1) used in irradiating the object depending on the difference between the measured value of at least one of the area and height of the plasma image and a predetermined value. A laser welding device including: photographing means (31) provided to photograph the plasma (7) caused by irradiating, with laser radiation, an object being welded; image processing means (33) provided to measure the value of at least one of the area and height of the plasma image photographed by the photographing means (31); and means (32;34) to employ the measured value in controlling the device or assessing the laser welding condition. A laser welding device according to claim 4, including distinction means (34) provided to compare a value measured by the image processing means (33) with an allowable value to determine whether the condition of a welding operation is satisfactory or not. A laser welding device according to claim 4 or 5, including control means (32) provided to control the energy level of the laser radiation, generated by a laser oscillator (1) included in the device, depending on the difference between a value measured by the image processing means (33) and a predetermined value. A method of making an electrode for a spark plug by laser welding a noble metal portion (b) to a base electrode (a) and monitoring the condition of the laser welding operation according to the method of one of claims 1 to 3 or using the apparatus of claim 4, 5, or 6.

Inventors: MASASA MAMORU, MATSUTANI WATARA, MASASA, MAMORU, MATSUTANI, WATARA
Applicants: NGK SPARK PLUG CO, NGK SPARK PLUG CO., LTD
Application year: 1995
Main classification: B23K26
IPC codes: B23K26, G06T1