JP2014024069A - Bead inspection method in laser welding - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a bead inspection method capable of stably performing quality determination of the whole of a weld bead or a wide range thereof while avoiding influence of a plume.SOLUTION: In a bead inspection method, after laser welding for a prescribed section is finished, in a state where heat remains in a weld bead within a range of the prescribed section, the whole of the weld bead or a part thereof is re-irradiated with a laser beam at a low energy density, and then an image of the weld bead is obtained in a state where the weld bead is allowed to emit light by heat. From the obtained image, quality determination of the weld bead is performed.

Description

  The present invention relates to a bead inspection method in laser welding.

Laser welding is capable of high-speed processing, but in lap welding for plated steel sheets, hole defects due to vaporized metal gas or the like, and weld defects such as sink marks may occur at the end of welding due to the flow of molten metal. Therefore, conventionally, while strictly managing the welding conditions, it has been performed to monitor the welding location with an imaging means and determine in real time whether or not there is a welding defect and the quality of the weld bead through image analysis or the like (Patent Document 1). reference).

  In this case, it is difficult to determine the quality of the weld bead directly in the vicinity of the laser irradiation point due to the high-luminance plume or spatter, so the effect of the plume or the like can be reduced by optical conditions or image processing. Has been studied. However, when the amount of plume ejection is large, such as three-ply welding, the influence of the plume cannot be completely eliminated, and there is a problem that the image processing process becomes complicated.

  Therefore, in order to avoid the influence of the plume, a method has been proposed in which an image of the weld bead is acquired in a state in which heat emission from the metal remains immediately after the end of laser welding, not during laser welding, and the quality is determined. (See Patent Document 2). However, this method is limited to the quality determination of the weld bead end portion where thermoluminescence remains, and for other portions, the light emission ends with a decrease in temperature, or there is a large luminance difference between the end portion and the start end side. Therefore, it is impossible to make a judgment including the entire weld bead.

JP 2006-43741 A JP 2012-45610 A

  The present invention has been made in view of such a situation of the prior art, and the object thereof is a bead inspection capable of stably performing pass / fail judgment of the entire weld bead or a wide range while avoiding the influence of the plume. It is to provide a method.

  In order to solve the above-described problem, a welding bead inspection method according to the present invention includes a whole or part of the weld bead in a state where residual heat is present in the weld bead over the predetermined section after completion of laser welding in the predetermined section. The section is irradiated again with a laser at a low energy density, and the weld bead is imaged in a state of thermal emission, and the quality of the weld bead is determined from the obtained image.

  According to the above method, after the laser welding is completed, by re-irradiating the laser having a low energy density with the residual heat, the heat input of the re-irradiation is added to the residual heat of the weld bead, and all the sections of the inspection target section In addition, relatively uniform thermoluminescence can be obtained, and it is possible to carry out a pass / fail judgment inspection of the weld bead over a wide range without depending on complicated image processing or the like. Moreover, since the laser to be re-irradiated has a low energy density, spatter and plume do not occur, and in addition to obtaining a stable and stable image, the laser can be scanned as quickly and as quickly as possible. There is almost no effect on the overall processing time due to the addition of.

  The “energy density” mentioned here is input to the irradiation spot per unit time in consideration of the scanning speed of the laser in addition to the setting energy density determined from the conventional laser output and focal spot diameter. Energy. Thus, low energy density laser re-irradiation can be implemented by a combination of one or more of these parameters, and there are several aspects of the present invention depending on the choice or combination. Further, depending on the combination of parameters, any parameter may be adjusted in the direction of higher energy than that during laser welding.

  In the present invention, it is preferable that the laser re-irradiation is performed at a higher speed than the optical axis scanning during the laser welding. The heat input to the workpiece by laser irradiation is a function of energy density and speed (time), and is generally proportional to the laser output and inversely proportional to the focal spot diameter and speed. Therefore, in terms of shortening the processing time, it is advantageous to obtain a desired energy density by speed control rather than output control.

  In the present invention, the laser re-irradiation may be performed within the width of the welding bead and shifted from the optical axis scanning during the laser welding. Since the temperature drop of the weld bead after the laser welding has progressed from the peripheral part and the central part in the width direction is kept at a relatively high temperature, shifting the re-irradiation on the same scanning line will cause a temperature drop. This is advantageous in performing heat input for compensation. In addition, since heat input to the weld bead is suppressed by the amount of deviation, for example, even if laser re-irradiation is performed under the same conditions as during welding, the same result as re-irradiation with a laser having a substantially low energy density is obtained. As a result, the control can be simplified by substituting the output control.

  For the same reason as described above, it is preferable that the laser re-irradiation is performed with a larger defocus amount than during the laser welding.

  Further, in the present invention, when the predetermined section is a loop-shaped section in which a start point and an end point of the laser scanning are close to or coincident with each other, after the laser welding of the predetermined section is completed, Re-irradiation may be performed, or the re-irradiation may be further performed from the start point side to a partial section after the re-irradiation is performed on the entire section from the start point side. By performing such re-irradiation, it is possible to obtain the same level of thermoluminescence on the start point side and the end point side without re-irradiating the entire section of the weld bead in the same manner. If the same level of thermoluminescence is obtained, it is possible to make a pass / fail judgment with high accuracy.

The “energy density” in the welding bead inspection method according to the present invention is input to the irradiation spot per unit time in consideration of the scanning speed of the laser in addition to the setting energy density determined from the laser output and the focal diameter. As mentioned above, it means energy.
Therefore, in the welding bead inspection method according to the present invention, the energy density of the laser re-irradiation is defined by the output speed ratio (P / V) based on the laser output P and the scanning speed V, and is reduced compared to the laser welding. It is also specified by performing the laser re-irradiation at the output speed ratio (P / V).

In that case, it is preferable that the output speed ratio (P / V) of the laser re-irradiation is selected from a range of 25 to 50% at the time of laser welding. Further, when the laser output P (W), the scanning speed V (mm / sec), and the focal diameter d (mm), the energy density P / dV of the laser re-irradiation is 25 to 50 (J / mm ² ). It may be selected from a range.

  As described above, according to the bead inspection method according to the present invention, it is possible to stably perform quality determination of the entire weld bead or a wide range while avoiding the influence of the plume.

It is the schematic of the laser welding apparatus which enforces the bead inspection method concerning the present invention. It is a schematic diagram which shows the image of the weld bead immediately after (a) weld bead formation and (b) laser reirradiation imaged in the bead inspection method which concerns on this invention. It is a distribution map which shows the correlation of the laser output at the time of re-irradiation, and speed.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
In FIG. 1, a laser welding apparatus 1 includes a laser irradiation head 11, a laser irradiation control unit 12, a laser irradiation unit 10 including a laser oscillator 13, an imaging unit 21 of a weld bead B, and an inspection unit including an image processing unit 22. 20.

  The laser irradiation head 11 irradiates a laser beam transmitted from the laser oscillator 13 via a fiber optical system or the like at a predetermined energy density at a predetermined position on the surface of the workpiece w1 with 2 laser optical axes on the surface. Means (for example, galvano scanner or XY table, or multi-axis head) for scanning in the axial direction or more in multiple axes, and focus control means (for example, focus lens) for controlling the focal length / defocus amount of the laser Etc.

  The laser irradiation control unit 12 controls the laser irradiation head 11 based on a preset program, and performs laser irradiation ON / OFF and laser output control. In particular, the present invention has a function as a switching means between laser irradiation for welding and laser reirradiation for inspection with low energy density (low output and / or high speed).

  As the image pickup means 21, a well-known image pickup means (for example, a CCD camera or the like) capable of picking up an area including preferably the whole of at least one unit weld bead B can be suitably used. The image acquired by the imaging means 21 is sent to the image processing unit 22 where defect inspection and quality determination of the weld bead B are performed. The imaging means may be configured to install a spectroscopic means such as a half mirror on the laser optical axis and take an image acquired through the laser output optical system.

  Next, the weld bead defect inspection and pass / fail judgment process based on the above embodiment will be described in detail with reference to the drawings.

(First embodiment)
FIG. 1 shows a case where two workpieces (steel plates) w1 and w2 are overlapped and laser welding is performed from the surface side of one of them (w1), but in the experiment, three workpieces ( Upper: 0.65 mm thick non-plated steel sheet, center: 1.4 mm thick non-plated steel sheet, bottom: 0.8 mm thick galvanized steel sheet through a 0.1 mm gap), first, FIG. ) As an alternative to conventional spot welding, an arc shape (loop shape) having a predetermined radius from the start point Ss to the end point St at a focal diameter of 0.6 mm, a laser output of 4000 W, and a scanning speed of 3.7 m / min. ) Is subjected to laser scanning Sa to form a weld bead B.

  At this time, when the laser optical axis reaches the end point St, the terminal end portion B1 emits residual heat, but the temperature of the other start end portion B0 decreases and infrared radiation remains, but the imaging means 21 The brightness in the image acquired at the end of the image is lower than the threshold value, and the weld bead shape of the start end side portion B0 cannot be confirmed in FIG.

  Next, as shown in FIG. 2B, the laser optical axis is moved again to the starting point Ss, the scanning speed is increased to 13.7 m / min with the same laser output 4000 W, and the laser is re-irradiated to the end point St ( Sb), the entire bead B (B2) is caused to emit light. At this time, even when the laser output is 4000 W, the energy input to the weld bead B per unit time, that is, the energy density becomes 1 / 3.7 by increasing the scanning speed to 3.7 times speed. This is similar to reducing the laser output to about 1081 W (about 27%).

  Therefore, in the laser re-irradiation Sb, in addition to the fact that no plume or spatter is generated in the weld bead B, it is completed in a short time of 1 / 3.7. The entire bead B2 emits heat with a relatively uniform luminance distribution. Therefore, an image captured by the imaging unit 21 at the end of the laser re-irradiation Sb (FIG. 2B) can detect a welding defect such as a hole defect without performing complicated image processing.

(Second embodiment)
In the above embodiment, the case where the scanning speed is increased by 3.7 times while the output at the laser re-irradiation Sb is kept equal to that at the time of the laser welding Sa is shown, but the degree of the acceleration is reduced and the laser output is reduced. The same thermoluminescence can be obtained even when the laser re-irradiation Sb is performed by actually decreasing the energy density and performing the laser re-irradiation Sb.

Therefore, in order to verify the correlation between the speed and output in laser re-irradiation and the range of effective combinations, re-irradiation was performed at various combinations of scanning speed and laser output, and the results are shown in the distribution diagram of FIG. .
In FIG. 3, (1) symbol “◯” indicates a combination in which good thermoluminescence B2 was obtained and no plume or spatter occurred, and (2) symbol “Δ” indicates a plume or (3) The symbol “x” indicates that the brightness is insufficient on the high speed side, and plumes or spatters are generated on the low speed side. It shows the combinations that existed.

  The heat input to the workpiece by laser irradiation is generally proportional to the laser output and inversely proportional to the focal spot diameter and the scanning speed. That is, when the laser output P, the scanning speed V, and the focal diameter d are set, the energy density of laser irradiation is expressed by P / dV. In FIG. 3, the critical value (upper limit value) on the left side of the effective combination range of the speed V and the output P is a straight line P = 500 V from (V, P) = (7,3500) to (9,4500). Yes, the critical value (lower limit) on the right side is on the straight line P = 250 V from (V, P) = (14,3500) to (18,4500).

  Here, if the focal diameters of the welding laser (4 m / min, 4000 W) and the re-irradiation laser are the same, the energy density can be defined by the output speed ratio (P / V) based on the laser output P and the scanning speed V. For the welding laser output speed ratio (P / V) = 1000 (W · min / m), the upper limit value of the re-irradiation laser output speed ratio is (P / V) = 500 (W · min / m). ), The lower limit value is (P / V) = 250 (W · min / m). That is, it can be seen that thermoluminescence suitable for the pass / fail inspection can be obtained when the output speed ratio (P / V) of the re-irradiation laser is in the range of 25 to 50% during laser welding.

Furthermore, when a sufficiently high scanning speed V (m / min) with respect to the focal diameter d = 0.6 mm is converted into (mm / sec) and the output P is converted into energy (W · sec = J), unit time The energy P / dV input to the hit irradiation area is 25 to 50 (J / mm ² ) at the time of laser re-irradiation with respect to 100 (J / mm ² ) at the time of laser welding. Thus, it can be seen that there is a certain correlation between the combination of the scanning speed V, the laser output P, and the focal spot diameter d, and it is possible to qualitatively select the re-irradiation output value and scanning speed value.

  The laser welding method (welding bead inspection method) according to the present invention is characterized in that the inspection laser re-irradiation Sb is performed at a low energy density immediately after the end of the welding laser irradiation Sa. The explanation was centered on two operations, low output and high speed. Operations that bring about results similar to these include operations for shifting the position of the re-irradiation Sb in the width direction of the weld bead B and operations for increasing the defocus amount. Each example will be described below.

(Third embodiment)
Immediately after the end of the welding laser irradiation Sa, the inspection laser re-irradiation Sb is shifted in the width direction of the weld bead B, so that heat input to the weld bead B is suppressed, and laser re-irradiation is performed under the same conditions as during welding. Even if Sb is performed, the same result as that obtained when the laser having a substantially low energy density is re-irradiated can be obtained. In order to verify this effect, the following experiment was conducted.

  First, similarly to the first embodiment, laser scanning Sa is performed in a circular arc shape (loop shape) with a predetermined radius from the start point Ss to the end point St at a focal diameter of 0.6 mm, a laser output of 4000 W, and a scanning speed of 3.7 m / min. To form a weld bead B. Next, as in the first embodiment, the laser optical axis is moved again to the starting point Ss, and when the laser re-irradiation Sb is performed with the scanning speed increased to 13.7 m / min, the irradiation position is determined by laser scanning during welding. The Sa was re-irradiated by shifting to the radially outer peripheral side and the inner peripheral side (Sb), and the bead B was thermally emitted and imaged.

  As a result, good images were obtained up to 0.7 mm on both the outer peripheral side and the inner peripheral side. Rather, as compared with the case of laser scanning Sa at the time of welding, there was a tendency that the peripheral portion where the temperature decrease first was reheated and the thermoluminescence of the weld bead B was made uniform. However, when the deviation is 0.8 mm or more, a dark portion is generated on the opposite side, and the defect detection accuracy in that region is lowered.

  Considering this point, the width of the weld bead B formed with respect to the laser irradiation focal diameter of 0.6 mm is about 1 mm. Therefore, until the deviation is 0.7 mm, the reirradiation Sb has a focal diameter of 0.6 mm and welding. There is an overlap with bead B. However, when the deviation is 0.8 mm or more, there is no overlap with the weld bead B, and the heat input is solely due to solid heat conduction, and it is considered that the energy of the re-irradiated laser is hardly transmitted to the weld bead B. In any case, the ability to allow deviation in re-irradiation is advantageous in securing practical stability.

(Fourth embodiment)
Immediately after the end of the welding laser irradiation Sa, by carrying out the inspection laser re-irradiation Sb with a large defocus amount, the energy of the laser at the time of re-irradiation is dispersed over a wide area of the welding bead B. It is considered that the same result as that obtained by reirradiation with a laser having a low energy density can be obtained. In order to verify this effect, re-irradiation was performed while changing the defocus amount, and a good image was obtained up to a focal diameter of 1.0 mm where the defocus amount is equal to the width of the weld bead B under the same conditions. However, it has been determined that if it is more than that, a low-luminance part is observed in the intermediate part or the like because of insufficient heating, and the defect detection accuracy is lowered.

  This is a result when the scanning speed is increased to 13.7 m / min. If the degree of the acceleration is reduced, it is predicted that heating that generates the necessary thermoluminescence is possible even with the above defocus amount. However, since the processing time increases accordingly, it can be said that there is no point in selecting such a defocus amount. The fourth embodiment can be an advantageous parameter in terms of suppressing plume and spatter during re-irradiation and ensuring practical stability.

  Although several embodiments of the bead inspection method according to the present invention have been described above, in addition to the laser output and the scanning speed, the setting of shifting the irradiation position of the third embodiment and the setting of the defocus amount of the fourth embodiment are described. By performing the above, it is added that the quality of the weld bead can be determined while achieving both processing time and stability.

DESCRIPTION OF SYMBOLS 1 Laser welding apparatus 10 Laser irradiation part 11 Laser irradiation head 12 Laser irradiation control part 13 Laser oscillator 20 Inspection part 21 Imaging means 22 Image processing part B Welding bead B0 Beginning end side part B1 Termination part B2 Welding bead whole Sa laser irradiation (laser welding )
Sb Laser re-irradiation Ss Start point St End point w1, w2 Workpiece

Claims (8)

  After completion of laser welding in a predetermined section, with a residual heat in the weld bead over the predetermined section, the whole or part of the weld bead is irradiated with laser at a low energy density, and the weld bead is thermally radiated. A weld bead inspection method, wherein the weld bead is picked up and picked up from the obtained image.   The welding bead inspection method according to claim 1, wherein the laser re-irradiation is performed at a higher speed than optical axis scanning during the laser welding.   The welding bead inspection method according to claim 1, wherein the laser re-irradiation is performed within a width of the welding bead and shifted from the optical axis scanning during the laser welding.   The welding bead inspection method according to claim 1, wherein the laser re-irradiation is performed with a larger defocus amount than that during the laser welding.   When the predetermined section is a loop-shaped section in which the start point and the end point of the laser scanning are close or coincident, after the laser welding of the predetermined section is completed, the re-irradiation is performed on a part of the section from the start point side, The laser according to any one of claims 1 to 4, wherein the re-irradiation is further performed from the start point side to a partial section after the re-irradiation is performed on the entire section from the start point side. Welding method.   The energy density of the laser re-irradiation is defined by an output speed ratio (P / V) based on the laser output P and the scanning speed V, and the laser is output at a reduced output speed ratio (P / V) as compared with laser welding. Re-irradiation is performed, The weld bead inspection method according to any one of claims 1 to 4.   The welding bead inspection method according to claim 6, wherein an output speed ratio (P / V) of the laser re-irradiation is selected from a range of 25 to 50% at the time of laser welding. When the laser output P (W), the scanning speed V (mm / sec), and the focal diameter d (mm), the energy density P / dV of the laser re-irradiation is within a range of 25 to 50 (J / mm ² ). The weld bead inspection method according to claim 6, wherein the weld bead inspection method is selected.

Images