Method for determining welding quality using thermal radiation during laser welding

Abstract

The objective is to propose a welding quality determination method using thermal radiation during laser welding, particularly an evaluation method that accurately detects the presence and / or size of gaps between workpieces during overlap laser welding. [Solution] A method for detecting gaps between samples during overlap laser welding, comprising the steps of: detecting the intensity of thermal radiation from a processing point on the surface of a welding sample 1300 and the oscillation period of the intensity fluctuations; extracting an intensity measurement value in which the oscillation period of the detected thermal radiation intensity fluctuations is 600 to 800 Hz; and determining the presence and / or size of gaps between samples based on the extracted measurement value.

Description

Technical Field

[0001] The present invention relates to a method for determining the quality of a welded product using thermal radiation light during laser welding. In particular, it relates to an evaluation method for detecting the gap between the objects to be processed by extracting and monitoring only a specific frequency of infrared radiation light of 1300 nm from the overlapping welding points.

Background Art

[0002] Conventionally, as a method for monitoring the welding state at the melting point of an object to be processed during laser welding, a technical concept of measuring plasma light, thermal radiation light such as infrared light generated by heating the object to be processed, and reflected light of the laser light for processing is known. In such a laser welding monitoring method, the laser light emitted from the laser oscillator reaches the processing melting point through an optical system such as a condenser lens, is reflected from the processing melting point and returns to the optical system, and is converted into an electrical signal by an optical sensor including a photodetector.

[0003] In addition, the plasma light and thermal radiation light generated by heating the processing melting point are converted into electrical signals through the optical system. From the change in the signal according to the detection intensity of the light acquired as such an electrical signal, the state of the processing melting point can be observed in situ, and for example, it is known that the quality of the welded product such as a real-time quality defect of the object to be processed or the occurrence of spatter can be evaluated.

[0004] For example, Patent Document 1 below describes an invention aimed at providing a laser processing state determination method that can accurately determine the processing state while suppressing over-detection of processing defects when determining the processing state in laser welding. The laser processing state determination method includes the steps of: using an optical sensor to detect reflected laser light at the weld formed on the surface of a workpiece by irradiation of the workpiece with laser light, as well as thermal radiation light and plasma light generated at the weld; making a first determination regarding the degree of shape change of the weld due to irradiation with laser light based on the intensity of the reflected light detected by the optical sensor; making a second determination regarding the degree of temperature change of the weld based on the intensity of the plasma light and thermal radiation light detected by the optical sensor; determining whether or not a processing defect has occurred in the laser processing based on the determination results of the first and second determinations; and outputting the determination result of the processing defect.

[0005] Furthermore, in the said patent document, it states: "

[0057] In addition to determining a hole abnormality (S3), the CPU 51 determines various processing states based on changes in the intensity of each signal, etc. For example, processing states include the focus position of the laser beam 4 irradiated onto the processing point 8, the processing point output of the laser output that reaches the processing point 8, and abnormalities in the gaps between the components of the workpiece 7.

[0058] After performing the determination of a hole abnormality and the determination of various abnormalities (S3, S4), the CPU 51 performs an overall determination of whether or not an abnormality in the processing state has occurred, for example, according to the results of each determination (S5). For example, if it is determined that there is no abnormality in all the determinations in steps S3 and S4, it is determined that the processing state is normal; otherwise, it is determined that an abnormality has occurred. The CPU 51 outputs the determination result, for example, by the communication circuit 52 (S6). The determination result can be received and displayed by, for example, an external device that can communicate with the determination device 15." Furthermore, the determination device 15 may be equipped with a display device (e.g., a display) that is communicably connected to the CPU 51, and the determination result may be displayed on the display device.

[0059] According to the above process, the determination device 15 acquires the signal generated by the photodetector 22 of the spectrometer 14, etc. (S1), and determines the processing state (S5) by comprehensively combining the results of the determination of hole abnormalities (S3) and the determination of other abnormalities (S4). This makes it easier to identify abnormal processing conditions, such as hole abnormalities, that can cause joining defects in laser processing of overlap welding, and to avoid situations such as defective products flowing out to subsequent processes after welding.'

[0006] Furthermore, Patent Document 2 states, "

[0050] When performing overlap welding, in which multiple workpieces 20 are overlapped and laser light 26 is irradiated from the overlapping direction to weld the multiple workpieces 20 together, it is preferable to keep the multiple workpieces 20 in close contact with each other. This is because if there are gaps between the workpieces 20 due to deformation caused by heat during laser welding or distortion of the workpieces 20 before processing, problems such as the workpieces 20 not being joined or insufficient strength of the joint will occur. If there are gaps between the workpieces 20, the molten portion 38 will penetrate one of the workpieces 20 and pass through to the gap, resulting in a decrease in thermal radiation intensity. For this reason, in the case of overlap welding, the occurrence of gaps between the workpieces 20 can be inferred from the decrease in thermal radiation intensity." However, according to the inventor's experiments, as far as the integrated value of thermal radiation intensity over processing time is observed, no difference in thermal radiation intensity due to the presence or absence or size of gaps was observed, and there was no difference, so it could not be used to determine the presence or absence of gaps.

[0007] Furthermore, Patent Document 2 states, "

[0055] As an example of a welding defect, as shown in Figure 8, when spatter 40 is generated, the spatter 40 ejected from the molten part disrupts the molten state of the molten part, causing a change in the state of vibration 49 and a change in vibration gain. Also, since the spatter 40 is ejected from the molten part at high speed, after the spatter 40 is scattered, the molten part returns to a stable state, and the detected increase in vibration gain shows a peak shape. In the embodiment of this disclosure, only the vibration gain is shown, but frequency analysis of the obtained vibration may also be performed, and when spatter 40 is generated, the state of the molten part changes instantaneously, so a different frequency from the vibration seen in a proper molten state is detected." Those skilled in the art will understand that the frequency analysis described here means an analysis of the change over time of the obtained peak intensity vibration frequency. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2024-017906 [Patent Document 2] Japanese Patent Publication No. 2020-189305 [Overview of the Initiative] [Problems that the invention aims to solve]

[0009] A technical concept that attempts to grasp a part of laser welding quality by detecting gain and intensity changes of thermal radiation, infrared radiation, and laser reflected light from the molten pool and laser processing point during overlap laser welding, in order to evaluate the presence or absence of gaps between processed samples and determine whether or not spatter is generated, is somewhat known. However, until now, the accuracy has been low, and sufficient defect detection and quality assessment have not been obtained, so there has been a need for a method that can detect the presence or absence of gaps between samples with higher accuracy and precision.

[0010] More specifically, as mentioned above, even if the integrated value of thermal radiation intensity from the laser welding point is detected and calculated, there is absolutely no difference in the integrated thermal radiation intensity due to the presence or absence or size of gaps. Therefore, this integrated value cannot be used to determine the presence or absence or size of gaps. Furthermore, even if the frequency analysis of the thermal radiation intensity is performed by analyzing the time-dependent change in the vibration frequency of the obtained peak signal, only sudden frequency changes due to sputter generation are observed, and this cannot be used to determine the presence or absence or size of gaps.

[0011] This invention has been made in view of the above-mentioned problems, and aims to propose a welding quality determination method using thermal radiation light during laser welding, and in particular an evaluation method that can detect with high accuracy the presence or absence of gaps between workpieces and / or the size of those gaps during overlap laser welding. [Means for solving the problem]

[0012] The present invention provides a method for detecting the gap between samples during overlap laser welding, comprising the steps of: detecting the intensity of thermal radiation from the processing point during overlap laser welding; and calculating the intensity of thermal radiation at which the oscillation frequency of the detected intensity fluctuations of thermal radiation is 150 Hz or higher. The method is characterized by comprising the step of determining the presence and / or size of gaps between samples based on the calculated intensity.

[0013] Furthermore, the present invention's method for detecting gaps between samples during overlap laser welding is preferably characterized in that the calculation step involves calculating the integrated intensity value of thermal radiation light with a vibration frequency of 150 Hz or higher.

[0014] Furthermore, the present invention's method for detecting gaps between samples during overlap laser welding is more preferably characterized in that, in the determination step, the gap between samples is determined to be larger the smaller the integrated intensity value.

[0015] Furthermore, the present invention's method for detecting gaps between samples during overlap laser welding is more preferably characterized in that the thermal radiation is infrared radiation in the 1300 nm band.

[0016] The present invention provides a sample gap detection device for overlap laser welding, comprising: a detection unit for detecting the intensity of thermal radiation from a processing point during overlap laser welding; a calculation unit for calculating the intensity of thermal radiation where the vibration frequency of the detected intensity fluctuations of thermal radiation is 150 Hz or higher; and a determination unit for determining the presence and / or size of a gap between samples based on the calculated intensity.

[0017] The present invention provides a sample gap detection device for overlap laser welding, preferably characterized in that the calculation unit calculates the integrated intensity value of thermal radiation light with a vibration frequency of 150 Hz or higher.

[0018] The present invention's sample gap detection device for overlap laser welding is more preferably characterized in that the determination unit determines that the gap between samples is larger as the integrated intensity value decreases.

[0019] The sample gap detection device for overlap laser welding according to the present invention is more preferably characterized in that the thermal radiation is infrared radiation in the 1300 nm band.

[0020] Also, the method for detecting the sample gap during the laser butt welding process of the present invention preferably has the calculation step calculate the integrated intensity value of the thermal radiation light only when the vibration frequency is between 250 Hz and 1200 Hz.

[0021] Also, the method for detecting the sample gap during the laser butt welding process of the present invention more preferably has the calculation step calculate the integrated intensity value of the thermal radiation light only when the vibration frequency is between 400 Hz and 1000 Hz.

[0022] Also, the method for detecting the sample gap during the laser butt welding process of the present invention more preferably has the calculation step calculate the integrated intensity value of the thermal radiation light only when the vibration frequency is between 600 Hz and 800 Hz.

[0023] The device for detecting the sample gap during the laser butt welding process of the present invention preferably has the calculation unit calculate the integrated intensity value of the thermal radiation light only when the vibration frequency is between 250 Hz and 1200 Hz.

[0024] The device for detecting the sample gap during the laser butt welding process of the present invention more preferably has the calculation unit calculate the integrated intensity value of the thermal radiation light only when the vibration frequency is between 400 Hz and 1000 Hz.

[0025] The device for detecting the sample gap during the laser butt welding process of the present invention more preferably has the calculation unit calculate the integrated intensity value of the thermal radiation light only when the vibration frequency is between 600 Hz and 800 Hz.

[0026] Also, the method for detecting the sample gap during the laser butt welding process of the present invention more preferably has the calculation target in the calculation step be such that in the graph of the intensity of the thermal radiation light with respect to the vibration frequency of the intensity variation, the intensity is 3% or less of the peak intensity.

[0027] The present invention's sample gap detection device for overlap laser welding is more preferably characterized in that the target calculated by the calculation unit is one in which the intensity is 3% or less of the peak intensity in the graph of the intensity of thermal radiation light against the vibration frequency of intensity fluctuations. [Effects of the Invention]

[0028] The present invention provides a welding quality determination method using thermal radiation during laser welding, and in particular, enables an evaluation method that can detect with high accuracy the presence or absence of gaps between workpieces and / or the size of those gaps during overlap laser welding. [Brief explanation of the drawing]

[0029] [Figure 1] (a) is a conceptual diagram illustrating the configuration outline of the laser welding apparatus and overlap welding sample according to this embodiment, and (b) is a conceptual block diagram illustrating the configuration of the calculation unit. [Figure 2] (a) is a diagram illustrating a photograph of the sample surface after overlap laser welding, (b) is a diagram illustrating an example of a cross-sectional photograph of the laser-welded area when a gap of about 0.5 mm occurs between the samples, and (c) is a waveform showing the detection signal intensity as a result of the elapsed time of 1300 nm when the gap t between the overlapping samples is 0 mm. [Figure 3] (a) is a waveform showing the detection signal intensity over a 1300 nm elapsed time when the gap t between the superimposed samples is 0.5 mm, (b) is a waveform showing the detection signal intensity over a 1300 nm elapsed time when the gap t between the superimposed samples is 1.0 mm, and (c) is a figure illustrating the result of integrating the waveform intensity for 0 to 0.7 seconds for each gap t of 0 mm, 0.5 mm, and 1.0 mm at 500 μs sampling periods. [Figure 4] (a) is a graph in which the horizontal axis represents the frequency of the detected change in the intensity of thermal radiation, and the vertical axis represents the spectral intensity of thermal radiation corresponding to that frequency. (b) is a graph showing the result of integrating the spectral intensity in the range of 800 to 1000 Hz on the horizontal axis of the graph shown in Figure 4(a) for plate thicknesses of 0 mm, 0.5 mm, and 1.0 mm, respectively. [Figure 5] This graph shows the change in intensity of thermal radiation in the 1300 nm band measured from the overlapping laser welding point with a sampling period of 50 μs. (a) shows the case when the gap t = 0 mm, and (b) shows the case when the gap t = 0.5 mm. [Figure 6] (a) shows the measurement results when the gap t = 1.0 mm, and (b) is a graph showing the integrated spectral intensity values ​​for the entire same welding time for each case of gap t = 0 mm, 0.5 mm, and 1.0 mm. [Figure 7] (a) shows the results of measuring the intensity change of thermal radiation in the 1300 nm band from the overlapping laser welding point with a sampling period of 50 μs, plotted with the vibration frequency of the intensity change on the horizontal axis and the spectral intensity corresponding to the vibration frequency on the vertical axis. (b) is a graph plotting the integrated spectral intensity values ​​for each gap in the vibration frequency range of 800 to 1000 Hz of the waveform shown in Figure 7(a). [Figure 8] Figure 8(a) shows the relationship between the detected spectral intensity and the laser welding time when the gap is 0 mm, Figure 8(b) shows the relationship when the gap is 0.5 mm, and Figure 8(c) shows the relationship when the gap is 1.0 mm. Figure 8(d) is an enlarged view illustrating the cross-section after good overlap laser welding with no gap, and Figure 8(e) is an enlarged view illustrating the cross-section after laser welding of an overlap sample with a gap t of approximately 0.47 mm. [Figure 9] (a) is a graph in which the vertical axis scale is aligned to match the peak values ​​of the measured spectral intensity, and (b) is a table showing the integrated spectral intensity values ​​corresponding to the oscillation frequencies of the 1300 nm thermal radiation light when laser scan welding is performed using the laser scan welding device 2000 for each of the following ranges: 400-600 Hz, 600-800 Hz, 800-1000 Hz, and 1000-1200 Hz, for gaps t of 0 mm, 0.5 mm, and 1.0 mm, respectively, for upper plate thicknesses of welded specimens of 1.2 mm, 1.6 mm, 2.0 mm, and 2.6 mm. [Figure 10](a) shows the case where the upper plate thickness of the welded sample is 1.2 mm and the gap t is 0 mm, (b) shows the case where the upper plate thickness of the welded sample is 1.2 mm and the gap t is 0.5 mm, (c) shows the case where the upper plate thickness of the welded sample is 1.2 mm and the gap t is 1.0 mm, (d) shows the case where the upper plate thickness of the welded sample is 1.6 mm and the gap t is 0 mm, (e) shows the case where the upper plate thickness of the welded sample is 1.6 mm and the gap t is 0.5 mm, and (f) shows the case where the upper plate thickness of the welded sample is 1.6 mm and the gap t is 1.0 mm. In all cases, the lower plate thickness of the welded sample was uniformly 1.6 mm. [Figure 11] (a) shows the case where the upper plate thickness of the welded sample is 2.0 mm and the gap t is 0 mm, (b) shows the case where the upper plate thickness of the welded sample is 2.0 mm and the gap t is 0.5 mm, (c) shows the case where the upper plate thickness of the welded sample is 2.0 mm and the gap t is 1.0 mm, (d) shows the case where the upper plate thickness of the welded sample is 2.6 mm and the gap t is 0 mm, (e) shows the case where the upper plate thickness of the welded sample is 2.6 mm and the gap t is 0.5 mm, and (f) shows the case where the upper plate thickness of the welded sample is 2.6 mm and the gap t is 1.0 mm. In all cases, the lower plate thickness of the welded sample was uniformly 1.6 mm. [Figure 12] (a) is a graph plotting the relative spectral intensity with respect to the gap t between welded samples for each vibration frequency range, assuming the upper plate thickness of the welded sample is 1.2 mm. (b) is a graph explaining the differences due to each gap t, with the horizontal axis representing the vibration frequency and the vertical axis representing the spectral intensity, corresponding to (a), assuming the upper plate thickness of the welded sample is 1.2 mm. (c) is a graph plotting the relative spectral intensity with respect to the gap t between welded samples for each vibration frequency range, assuming the upper plate thickness of the welded sample is 1.6 mm. (d) is a graph explaining the differences due to each gap t, with the horizontal axis representing the vibration frequency and the vertical axis representing the spectral intensity, corresponding to Figure 12(c), assuming the upper plate thickness of the welded sample is 1.6 mm. All welded samples used were uniformly 1.6 mm thick. [Figure 13](a) is a graph plotting the relative spectral intensity with respect to the gap t between welded samples for each vibration frequency range, assuming the upper plate thickness of the welded sample is 2.0 mm. (b) is a graph explaining the differences for each gap t, with the horizontal axis representing the vibration frequency and the vertical axis representing the spectral intensity, corresponding to (a), assuming the upper plate thickness of the welded sample is 2.0 mm. (c) is a graph plotting the relative spectral intensity with respect to the gap t between welded samples for each vibration frequency range, assuming the upper plate thickness of the welded sample is 2.6 mm. (d) is a graph explaining the differences for each gap t, with the horizontal axis representing the vibration frequency and the vertical axis representing the spectral intensity, corresponding to Figure 13(c), assuming the upper plate thickness of the welded sample is 2.6 mm. All welded samples used were uniformly 1.6 mm thick. [Figure 14] This diagram illustrates the configuration of a laser scanning welding apparatus and a sample for overlap welding. [Modes for carrying out the invention]

[0030] [Laser welding of overlapping metals] This welding technique uses laser light to join two or more overlapping metal plates, and is widely used in industrial fields where high precision and high-speed joining are required. Its key feature is that, because it utilizes laser light with extremely high energy density, it can locally melt and join metals within a small thermal affected zone. This reduces distortion and thermal deformation of the materials.

[0031] Furthermore, because the laser beam can be controlled with extreme precision, welding is possible in narrow areas, making it suitable for welding complex shapes and intricate details. In addition, since laser welding is a non-contact method, there are fewer consumables, and chemicals and parts do not directly come into contact with the material surface, resulting in a clean and efficient process. Laser welding can also be performed at very high speeds, contributing to increased productivity.

[0032] In laser welding, first, multiple metal plates to be welded are placed on top of each other in the designated positions. Typically, the laser is shone on the upper metal plate, reaching the lower layers to initiate the joining process. Next, the laser beam is shone on the surface of the upper metal, causing localized melting. Subsequently, the two or more metals are joined either by the laser light penetrating or by the molten metal on the surface fusing with the metal in the lower layer. When the laser irradiation is stopped, the molten metal cools rapidly and solidifies, completing the joining process.

[0033] Laser welding makes it possible to join dissimilar metals, which was difficult with conventional welding techniques. It also produces smooth welds, often eliminating the need for additional polishing, resulting in a beautiful finish. Furthermore, its high-precision and high-speed welding capabilities contribute to improved efficiency in manufacturing lines and increased productivity. For these reasons, it is primarily used in industries requiring high-precision joining of thin metal sheets, such as the automotive, aerospace, and electronics manufacturing sectors.

[0034] [Experimental setup 1] Figure 1(a) is a conceptual diagram illustrating the configuration of the laser welding apparatus 1000 and the overlap welding sample 1300 according to this embodiment. The laser welding apparatus 1000 uses a collimating lens 1500 to make the laser light with a wavelength of 1100 nm irradiated from an optical fiber 1400 parallel, transmits it through a dichroic mirror 1600, and then focuses it with a focusing lens 1800 to focus on the surface of the overlap welding sample 1300(1), 1300(2) (referred to as overlap welding sample 1300 as appropriate) which is the material to be welded, and performs welding.

[0035] The overlapping welding sample 1300 melts in that area due to the heat of the laser beam focused to a diameter of approximately 2 mmφ, forming a weld pool at the processing point. A portion of the thermal radiation emitted from this processing point returns to the dichroic mirror 1600 via the focusing lens 1800, and only light with wavelengths of 1200 nm or longer is reflected. This light is then focused through the interference filter 1750, which transmits only wavelengths of 1300 nm, and the sensor focusing lens 1700, and detected on the surface of the light-receiving element of the optical sensor 1900. The signal photoelectrically converted by the optical sensor 1900 is analyzed and evaluated by the calculation unit 1200 by waveform conversion and numerical analysis. The collimating lens 1500, the dichroic mirror 1600, and the focusing lens 1800 are housed in the scanner processing head 1100.

[0036] Here, the thermal radiation emitted from the processing point detected by the optical sensor 1900 is within a range of approximately 45 mm in diameter centered on the processing point on the surface of the overlapping welding sample 1300. Detecting thermal radiation within this range ensures good detection even when the laser head is moved. Furthermore, while it is preferable for the overlapping welding samples 1300(1) and 1300(2) to be joined to each other by laser welding in a state where they are closely positioned without any gaps, a gap t may occur during the welding process, as shown in Figure 1(a).

[0037] In particular, in mass production processes where multiple samples are continuously and parallelly overlapped and laser-welded, it is possible that undesirable gaps may occur between samples at a certain rate during welding. In such cases, it is necessary to promptly identify them as defective products and determine them as unacceptable in terms of quality. Laser welding of overlapping samples with gaps present results in insufficient joint strength. Conventionally, inspection for the presence or absence of gaps has been generally performed by inserting a gap gauge and taking actual measurements in a post-processing step after the laser welding is completed. However, in order to reduce process costs and further improve the accuracy of judgment, it is preferable to be able to make real-time, on-the-spot judgments digitally or numerically.

[0038] In this respect, the gap detection method according to this embodiment detects thermal radiation light of a specific wavelength from the laser welding processing point, converts it into a waveform, and monitors and observes the waveform intensity at a specific frequency of the waveform, thereby enabling more accurate real-time gap detection. Figure 1(b) is a conceptual block diagram illustrating the configuration of the calculation unit 1200. The photoelectric signal detected by the optical sensor 1900 is input to the calculation unit 1200 as an input signal. First, the waveform data conversion unit 1210 records the signal intensity for each sampling time (e.g., 50 μs to 500 μs), and then the integrated signal intensity value (spectral intensity) for each vibration frequency (e.g., 0 to 1000 Hz) of the welding signal intensity is converted into waveform data or calculated.

[0039] Then, from the waveform data input from the waveform data conversion unit 1210 to the calculation unit 1200, only components with an oscillation frequency of 150 Hz or higher in thermal radiation intensity are extracted, and the integrated value of the thermal radiation intensity that meets that condition is calculated. Furthermore, the judgment unit 1230 compares the integrated value of the thermal radiation intensity obtained from the calculation unit 1200 with a standard value or other integrated values ​​to determine the presence and size of gaps between samples and outputs it as a judgment output.

[0040] As will be explained in more detail later, various verification experiments conducted by the inventors have revealed that the larger the gap between samples, the smaller the signal intensity detected at specific vibration frequencies (especially above 150 Hz) of the 1300 nm thermal radiation light. By monitoring this value, it becomes possible to estimate the presence and size of gaps almost in real time as soon as one laser welding operation is completed.

[0041] Furthermore, simply integrating the signal intensity of 1300 nm thermal radiation, monitoring peak intensity, or performing frequency analysis of the entire signal including the dominant intensity governing the overall signal intensity, or any combination thereof, has not been able to extract differences that can be used to determine the presence or size of gaps as in the present invention. Theoretically, it is expected that if the gap is large, the molten metal in the molten pool will fall into or melt into the gap, causing a larger depression on the surface of the molten pool, and a reduction in reflected or radiated light from the processing point will be observed to some extent. However, such a reduction in reflected or radiated light involves observing the time-dependent change in the overall intensity of the detected reflected or radiated light, or the peak intensity of a specific wavelength within it, which is completely different from the data extraction and processing in this experiment by the inventors.

[0042] [Experiment details] Using the multimode fiber laser described in Figure 1, overlap laser welding was performed on 1.6 mm thick SPCC material (cold-rolled steel) using the C-shaped welding method shown in Figure 2(a) with a laser output of 4.5 kW and a scanning speed of 2000 mm / min. Here, SPCC material is also called cold-rolled steel sheet, and is a carbon steel manufactured by cold-rolling SPHC (STEEL PLATE HOT COMMERCIAL). It has a low carbon content of less than 0.1% and is classified as SPC material. Generally, SPCC material is widely used as steel for sheet metal and pressing, and plate thicknesses of 0.5 to 3.2 mm, especially 1.6 mm to 2.0 mm, are stably distributed.

[0043] Figure 2(a) is a diagram illustrating a photograph of the sample surface after overlap laser welding. In Figure 2(a), the C-shaped laser weld typically extends to the back surface of the sample, indicating that the two samples are closely fused together. Figure 2(b) is a diagram illustrating an example of a cross-sectional photograph of the laser welded area when a gap of approximately 0.5 mm occurs between the samples. As shown in Figure 2(b), the actual gap at the measured location was 0.47 mm, and the maximum penetration width of the weld was 1.42 mm.

[0044] [Experiment details / General spectral intensity measurement] Next, using the laser welding apparatus 1000 and overlapping welding sample 1300 shown in Figure 1, the time-dependent change in the intensity of thermal radiation at a specific wavelength of 1300 nm was simply measured for each of the following cases: when the gap t between the overlapping samples was 0 mm, when the gap t between the overlapping samples was 0.5 mm, and when the gap t between the overlapping samples was 1.0 mm.

[0045] Figure 2(c) shows the waveform of the detection signal intensity over 1300 nm when the gap t between the superimposed samples is 0 mm, Figure 3(a) shows the waveform of the detection signal intensity over 1300 nm when the gap t between the superimposed samples is 0.5 mm, and Figure 3(b) shows the waveform of the detection signal intensity over 1300 nm when the gap t between the superimposed samples is 1.0 mm.

[0046] As can be seen in Figures 2(c), 3(a), and 3(b), the waveform shape, amplitude, and their changes differ depending on the three different gap configurations. However, it can be observed that in all cases, the intensity on the vertical axis changes with fine amplitudes within the range of approximately 400 to 1200. As can be seen from these figures, the welding time in this case is approximately 0.7 seconds. Figure 3(c) shows the results of accumulating the waveform intensity from 0 to 0.7 seconds for gaps t of 0 mm, 0.5 mm, and 1.0 mm, respectively, at 500 μs sampling intervals.

[0047] As shown in Figure 3(c), simply calculating and comparing the detection intensity of 1300 nm thermal radiation as an integrated value over the laser welding time of 0 to 0.7 seconds does not allow us to identify differences in the presence or size of gaps, and therefore it cannot be used to determine the presence of gaps.

[0048] [Experimental details / Analysis and evaluation by the inventor 1] Therefore, the inventors calculated the frequency spectrum of the frequency of the intensity change of the detected 1300 nm thermal radiation and performed analysis and evaluation. Figure 4(a) is a graph in which the horizontal axis is the frequency of the intensity change of the detected thermal radiation and the vertical axis is the spectral intensity of the thermal radiation corresponding to that frequency. Figure 4(b) is a graph showing the results of integrating the spectral intensity in the range of 800 to 1000 Hz on the horizontal axis of the graph shown in Figure 4(a) for gaps t of 0 mm, 0.5 mm, and 1.0 mm, respectively (however, the upper plate thickness of the sample was assumed to be 1.6 mm).

[0049] In the graph in Figure 4(a), when comparing the cases of (1) a gap of 0 mm, (2) a gap of 0.5 mm, and (3) a gap of 1.0 mm, a clear difference in spectral intensity appears in the frequency range of 150 Hz or higher. In particular, in the range of 250 to 1200 Hz, preferably the range of 400 to 1000 Hz, and even more preferably the range of 600 to 800 Hz, the difference in spectral intensity attributable to the presence or absence and size of the gap can be clearly identified.

[0050] In the graph in Figure 4(a), the vertical axis scale is adjusted to correspond to the frequency range of thermal radiation intensity changes, specifically focusing on the range of frequencies above 150 Hz for spectral intensity analysis and evaluation. However, based on actual measurement results, the spectral intensity is approximately 1200-1400 or higher near 0 Hz, and this effectively dominates the peak intensity of thermal radiation. Figure 9(a) is a graph with the vertical axis scale aligned to the measured peak values ​​of spectral intensity, and the data is the same as that shown in Figure 4(a).

[0051] Upon closer examination of Figure 9(a), it can be seen that, compared to the peak intensity (approximately 1200-1400), only a very weak spectral intensity (approximately 10-40) is detected in the range where the frequency of the intensity change of thermal radiation is 150 Hz or higher, which is almost negligible or falls within an error range of 1-3% or less relative to the peak intensity. Generally, it is known that frequency analysis allows for the observation and evaluation of wavelength and frequency changes of the peak intensity and surrounding satellite peaks over time. However, after various experiments and trial and error, the inventors discovered that differences appear in spectral intensity in a data range where the frequency of the intensity change of thermal radiation is completely different from the frequency that indicates the peak intensity of the spectrum, a range that would normally be dismissed as an error range. These differences appear in the spectral intensity and are thought to be caused by the presence or absence and size of gaps between samples during superposition laser welding. Furthermore, it was concluded that these differences can be used to evaluate gaps and, consequently, to evaluate the quality of the welding.

[0052] As can be seen from Figure 4(b), the larger the gap between samples, the more clearly the spectral integrated intensity value corresponding to the vibration frequency range of the thermal radiation intensity change in the 800-1000 Hz range decreases. Therefore, by calculating the spectral integrated intensity value in this specific vibration frequency range, it is possible to estimate the presence or relative size of gaps in near real time. Furthermore, by accumulating and learning from a large amount of mass data and utilizing AI, it is thought that it will also be possible to derive the absolute value of the gap.

[0053] [Experimental details / Analysis and evaluation by the inventor 2] Furthermore, Figures 5 to 6(a) are graphs showing the intensity change of thermal radiation in the 1300 nm band measured from the overlapping laser welding point with a sampling period of 50 μs. Figure 5(a) shows the measurement results when the gap t=0 mm, Figure 5(b) shows the results when the gap t=0.5 mm, and Figure 6(a) shows the results when the gap t=1.0 mm.

[0054] In the experiments shown in Figures 5 to 6(a), the welding time differed from that in the experiments shown in Figures 2(c) to 3(b), being approximately 0.5 seconds. However, as with the results explained earlier, when the spectral intensity for this welding time was integrated, no difference in spectral intensity was observed between gaps t of 0 mm, 0.5 mm, and 1.0 mm, as shown in Figure 6(b).

[0055] On the other hand, Figure 7(a) shows the results of measuring the intensity change of thermal radiation in the 1300 nm band from the overlapping laser welding point with a sampling period of 50 μs. The horizontal axis represents the vibration frequency of the intensity change, and the vertical axis represents the spectral intensity corresponding to that vibration frequency. In this case as well, similar to the case with a sampling period of 500 μs (Figure 4(a)), we found that differences in intensity depending on the presence or absence and size of the gap were observed between the case where the gap t is 0 mm (1), the case where the gap t is 0.5 mm (2), and the case where the gap t is 1.0 mm (3), in the weak spectral intensity (values ​​of 1-3% or less of the peak value) in the frequency band that is higher than the frequency at which the peak value appears and which might otherwise be dismissed as noise or static.

[0056] Although not explicitly shown in the graph of Figure 7(a) due to the vertical axis scale, the spectral intensity near 0 Hz shows values ​​of 1400 or higher. As explained earlier, the spectral intensity of the intensity change of 1300 nm band thermal radiation at oscillation frequencies of 150 Hz or higher has an intensity value of only 1-3% or less compared to the peak value of the spectral intensity. This is a data portion that those skilled in the art would generally exclude from observation, analysis, and evaluation as error, noise, or static. However, the inventors have discovered that data for evaluating gaps is hidden and latent in this portion, masked by the peak value.

[0057] Figure 7(b) is a graph plotting the integrated spectral intensity values ​​for each gap in the vibration frequency range of 800-1000 Hz for the waveform shown in Figure 7(a). Similar to the results shown in Figure 4(b), this also shows that the integrated spectral value decreases as the gap increases from 0 mm to 1 mm. Judging from the chart in Figure 7(a), it is likely that similar results would be obtained if calculations were performed and plotted in the vibration frequency range of 600-1000 Hz or 400-1000 Hz.

[0058] [Experimental details / Considerations by the inventors] When observing the spectral intensity of the waveform described above, where the vibration frequency is in the range of 150 Hz or higher, preferably 250 to 1200 Hz, more preferably 400 to 1000 Hz, even more preferably 600 to 800 Hz, and most preferably 800 to 1000 Hz, the component having the intensity vibration frequency of the relatively high-frequency component corresponds to the fine amplitude portion shown in Figures 8(a) to 8(c) (direct reading from these drawings is difficult due to scale / resolution issues).

[0059] Furthermore, the small amplitude portions shown in Figures 8(a) to 8(c) decrease in amplitude as the gap size increases. Figure 8(a) shows the case with a gap of 0 mm, Figure 8(b) shows the case with a gap of 0.5 mm, and Figure 8(c) shows the case with a gap of 1.0 mm. The 1300 nm thermal radiation from the laser processing point is thought to be infrared radiation from the surface of the molten pool, but in laser welding, it is thought that the temperature near the center of the molten pool rises to above the boiling point instantaneously due to the absorption of high-power energy density laser light.

[0060] Therefore, violent behavior occurs due to the ejection of metal vapor and plume from the molten pool, and the intensity fluctuations of the 1300 nm band thermal radiation are thought to reflect this behavior. When a gap t occurs in the overlap, it is presumed that the molten material in the molten pool falls into the gap between the samples, resulting in a sparse molten material on the upper sample side and an underfill condition. Figure 8(d) is an enlarged view illustrating the cross-section after good overlap laser welding with no gaps, and Figure 8(e) is an enlarged view illustrating the cross-section after laser welding of an overlap sample with a gap t of approximately 0.47 mm.

[0061] In Figure 8(e), the outermost surface of the molten area is concave, forming an underfill, where the molten metal in that area has fallen into the gap, essentially filling it. Due to this phenomenon, when detecting thermal radiation from above, the molten components of the molten pool are concave, weakening their behavior, and the amplitude of the waveform of the intensity fluctuation becomes smaller, a phenomenon that becomes more pronounced as the gap increases. However, the extraction range of the vibration frequency in the intensity fluctuation of the 1300 nm thermal radiation observed by the inventors in this study corresponds to a portion of the high-frequency component, which is only 1-3% or less of the main peak intensity of the thermal radiation. In this respect, this is considered to be an entirely different technical approach from the frequency analysis or time-dependent analysis of peak intensity and peak intensity frequency that is typically performed by those skilled in the art.

[0062] [Sample metal used in overlay laser welding] The following are some of the metals that are well-suited to and can be welded using overlap laser welding: Stainless steel is suitable for laser welding because of its high corrosion resistance, its use in parts requiring strength, and its relatively low thermal conductivity. By using a high-energy-density laser, stainless steel can be welded with relatively little thermal effect, resulting in a beautiful finish with minimal deformation and oxidation. For this reason, it can be used in automotive parts, home appliances, and medical equipment. Aluminum alloys are lightweight and possess strength and corrosion resistance, but their high thermal conductivity makes them relatively difficult to laser weld. However, by using a high-power laser, the molten pool can be stabilized, enabling efficient joining. Furthermore, because lightweight materials are important in the aerospace and automotive fields, they are used in aircraft, automobiles, and electronic equipment casings.

[0063] Carbon steel is widely used in various applications and offers excellent cost performance, but it is a material that is prone to thermal expansion and distortion during cooling. However, laser welding allows for rapid localized heating and cooling, minimizing distortion and enabling high-precision joining, resulting in a strong joint. Therefore, it is used in structures, pipes, steel frames, and other applications.

[0064] Titanium and titanium alloys are lightweight yet possess extremely high strength, and also have excellent corrosion and heat resistance. However, they react easily with oxygen and nitrogen, so care must be taken during welding. The high precision of laser welding minimizes the reaction of titanium, enabling extremely clean and high-strength joints. In particular, performing the welding in a vacuum or inert gas environment to block oxygen is effective. For this reason, it is used in the aerospace industry, medical equipment, and sporting goods.

[0065] Furthermore, nickel and nickel alloys have excellent heat resistance and corrosion resistance, making them suitable for use in harsh environments. Laser welding produces highly durable welds and allows for reliable joint formation while minimizing metal surface degradation. For this reason, it is applied to high-temperature and high-pressure environments such as gas turbines, nuclear power plants, and chemical plants.

[0066] Furthermore, while copper and copper alloys possess high thermal and electrical conductivity, their high reflectivity during laser welding can make welding difficult. To suppress this laser light reflection, using short-wavelength or high-power lasers enables efficient joining. Additionally, because welding can be performed while maintaining conductivity, it is suitable for electrical connection components and electronic equipment. Known applications include electronic equipment, electrical wiring, and heat sinks.

[0067] Furthermore, joining dissimilar metals, such as aluminum and copper, or stainless steel and titanium, was often difficult with conventional welding techniques. However, laser welding allows for precise and localized heating, resulting in superior joining capabilities for dissimilar metals. Even metals with different physical properties can be joined to high quality by setting appropriate laser parameters, and this technology is used in electrical and electronic equipment, lightweight automotive parts, and medical devices.

[0068] As mentioned above, overlap laser welding enables joining a wide range of metal materials, including stainless steel, aluminum, titanium, and nickel, with excellent precision. It is particularly effective in situations requiring precise control of thin metal sheets or joining dissimilar metals. Furthermore, it excels in achieving high-quality finishes by minimizing thermal effects.

[0069] [Problems to be aware of and countermeasures for the overlap laser welding process] While laser welding of metals offers advantages such as efficient and high-precision joining, it also presents several points to consider and challenges. However, understanding these points in advance can ensure welding quality and prevent problems.

[0070] • Problems with controlling welding heat: Laser welding uses high energy concentrated locally, but applying excessive heat can cause metal distortion and cracking. In particular, with stacked thin metal sheets, there is a concern that one metal may melt too much due to uneven heat distribution. Therefore, it is important to optimally set welding parameters (laser power, irradiation time, irradiation speed, etc.) to control the melting range and welding depth. Appropriate cooling and heat treatment processes are also worth considering.

[0071] • Problems with metal reflectivity: Metals with high surface reflectivity, such as aluminum and copper, easily reflect laser light, which can reduce welding efficiency and raise concerns about the possibility of reflected laser light damaging equipment. Therefore, it is effective to suppress reflection by using short-wavelength laser light (e.g., blue laser light) or high-power laser light. It is also effective to reduce reflectivity by roughening the metal surface beforehand or by applying a coating.

[0072] Problems with joining dissimilar metals: When joining different metals, there is a concern that cracks or delamination may occur at the joint due to their differing melting temperatures and thermal expansion coefficients. There is also a concern that undesirable, brittle alloys may be formed during the melting of dissimilar metals. Therefore, it is important to find and set the optimal welding parameters according to the characteristics of the metals to be joined, and design measures such as using materials that suppress the reaction between dissimilar metals at the interface are also effective.

[0073] • Concerns about porosity formation: In overlap laser welding, gas can become trapped in the molten metal, causing porosity in the welded area. This can reduce the strength of the joint, so measures to mitigate it are sometimes taken. Specifically, this involves maintaining the stability of the molten pool by setting the appropriate laser power and adjusting the welding speed, preventing gas from dissolving, and preventing porosity formation by cleaning the metal surface beforehand to remove dirt and oil.

[0074] • Problems with gap management during welding: If there is a large gap between overlapping metal plates, the molten metal may spread unevenly during laser welding, potentially reducing its strength. Therefore, it is necessary to confirm that the overlapping parts are properly in contact before welding, and to take measures to prevent the metal plates from shifting during welding by using jigs or fixtures.

[0075] • Problems with balancing welding speed and power: If the laser welding speed is too fast, sufficient melting will not occur, while if the speed is too slow, there is a concern about excessive melting or overcoating. These welding parameters also affect the weld depth and the size of the molten pool. Therefore, it is necessary to find the optimal conditions through experiments and simulations, depending on the type and thickness of the metal, by balancing the appropriate welding speed and laser power.

[0076] • Concerns regarding the condition of the metal surface: If there is an oxide film or dirt on the surface of the metal to be welded, the laser will not be absorbed efficiently, resulting in a decrease in weld quality. If the oxide film is thick, the joint strength will decrease, and there is a concern that delamination or uneven bonding may occur. For this reason, it is effective to clean the metal surface in advance to remove oil, oxide films, and foreign matter, and to use an inert gas such as argon to prevent oxidation during welding.

[0077] • Points to note regarding focus adjustment: If the laser is not properly focused, the laser energy will not be efficiently concentrated on the welding area, resulting in welding defects or insufficient strength. Therefore, it is necessary to precisely set the focal length and beam shape, and to perform optimal focus adjustment according to the thickness and shape of the metal being welded.

[0078] • Points to note regarding cooling and strain management: It is important to note that the rapid heating and cooling process of laser welding can leave residual stress within the metal, potentially resulting in distortion or cracks after welding. However, the occurrence of stress and distortion can be suppressed by slow cooling after welding and by controlling the cooling process. In addition, post-treatment to relieve stress may be performed as needed.

[0079] • Safety Considerations: Laser welding uses high-energy laser light, posing a risk to the eyes and skin. Reflected light also poses a significant danger, requiring appropriate protection. It is crucial that laser operation is performed by well-trained workers, using appropriate protective equipment (such as laser goggles) and protective screens, and operating in a sealed room where other laser light cannot leak out.

[0080] As mentioned above, overlap laser welding requires attention to various aspects, including managing welding heat, countermeasures for highly reflective metals, joining dissimilar metals, and managing pores and gaps. By properly understanding each of these points and optimizing the welding conditions and environment, high-quality and reliable joining becomes possible.

[0081] [Influence of gaps between samples on overlapping laser welding] In overlap laser welding, the gap between samples is a crucial factor that significantly affects weld quality. The presence of a gap negatively impacts the flow of molten metal and the uniformity of the joint, making it more likely for defects to occur in the welding result.

[0082] • Impact on reduced weld strength: If there is a gap between the metal plates, the molten metal irradiated by the laser will flow into the gap, resulting in insufficient joining of the weld and a decrease in strength. Furthermore, if the gap is large, it may lead to poor joining or partial non-jointing (defects), potentially resulting in locally weak joints. When a gap exists, the laser energy is not efficiently transmitted to the contact surface, making the molten pool unstable, which in turn results in an uneven joint and insufficient mechanical strength.

[0083] • Impact on weld defects (porosity and voids): The presence of gaps increases the risk of molten metal flowing irregularly into the joint, leading to the formation of voids and bubbles (porosity). These defects not only reduce weld strength but also impair fatigue resistance and durability after welding. When molten metal flows into gaps, air and gases can easily become trapped, raising concerns about the formation of pores and voids when the metal solidifies during cooling.

[0084] • Impact on weld bead shape defects: Gaps can cause the shape of the weld bead (the raised metal layer remaining after welding) to become unstable, resulting in inconsistencies with surrounding areas and making it difficult to form a uniform and aesthetically pleasing bead. This can lead to cosmetic defects and increased post-processing costs, especially when appearance is important. It is known that molten metal can be drawn into gaps or, conversely, spread excessively, disrupting the shape of the weld bead and making it prone to bumps and unevenness.

[0085] • Impact on weld depth instability: Gaps can cause dispersion of laser energy, leading to concerns about uneven weld depth, potentially resulting in an undesirable weld depth and insufficient joint strength. There is also concern about over-melting or under-melting in certain areas. This is likely due to the laser beam not striking the metal surface evenly when there are gaps, reducing energy concentration and causing uneven welding.

[0086] • Impact on uneven heating and cooling: The presence of gaps causes uneven stress during the cooling of molten metal, leading to the accumulation of residual stress inside. This stress can later cause cracks and deformation. Because the heating and cooling processes during welding are locally uneven, the cooling rate and heat distribution become unbalanced, which can weaken the joint.

[0087] • Impact of light transmission on energy loss: If the gap is too large, the laser light will pass through the gap, resulting in energy loss. As a result, the bonding may be incomplete, and melting may be insufficient. The laser light that passes through the gap is not sufficiently absorbed by the metal, so the energy is not used efficiently.

[0088] • Influence of welding speed on gap size: Depending on the size of the gap, proper joining may not be achieved without adjusting the welding speed. If the gap is large, a welding speed that is too fast may result in insufficient melting, while a speed that is too slow may cause excessive melting. This can sometimes be resolved by setting the welding speed and laser power according to the size of the gap, but it is important to check for gaps before welding using appropriate fixtures and jigs to minimize the gap and ensure that the overlapping metal plates are tightly joined.

[0089] • Impact on setting appropriate laser parameters: Even if the gap is not completely zero, welding quality can sometimes be ensured by adjusting the laser power and welding speed appropriately. In particular, when there is a gap in thin plates, controlling the molten pool is important. As a multi-layer welding method, even if the gap is large, welding in multiple passes can be performed to fill the gap and obtain a uniform joint, and surface treatment such as removing oxide films and contaminants from the metal surface beforehand can be effective in preventing deterioration of welding quality in the gap.

[0090] In lap laser welding of metals, gaps between samples can cause various problems, including reduced weld strength, defect formation, poor bead shape, and energy loss due to light transmission. Therefore, minimizing gaps and optimizing welding conditions is crucial. If gaps are found to exist, countermeasures such as promptly classifying the product as defective or minimizing its impact through post-processing are required.

[0091] [Method for detecting gaps between samples during overlap laser welding] Several techniques and approaches are known for detecting gaps between samples during overlap laser welding. Understanding the presence and size of these gaps helps ensure welding quality and prevent defects. Therefore, several methods have been tested, and their outlines are described below.

[0092] 1) Optical measurement (laser measurement) This method uses laser light to non-contactly detect gaps between metal plates. It precisely measures the height and distance of the metal surfaces before or during welding to determine if a gap exists. Laser triangulation is a technique that irradiates a sample surface with laser light and measures the distance from the angle of the reflected light. If there is a gap between stacked metal plates, the laser reflection pattern and angle change, allowing for highly accurate gap detection. Laser interferometers utilize the phenomenon of light interference to detect minute surface displacements, enabling accurate gap measurement and confirmation of gap presence with nanometer-level precision. These methods offer high-precision, non-contact, real-time detection and are easily integrated into laser welding equipment. However, introducing high-precision equipment incurs significant costs.

[0093] 2) Ultrasound examination Ultrasonic testing uses ultrasound to detect gaps between metal plates. Because ultrasound reflects and refracts at the interface of materials, it can detect the presence of air (gaps) between metal plates. Contact ultrasonic testing uses a probe to send ultrasound waves to the sample and detects gaps by analyzing the differences in reflected waves and propagation times. Air coupler ultrasonic measurement is a non-contact method that emits ultrasound waves from the air and determines the presence or absence of gaps by analyzing the reflection from the metal surface. It can detect hidden internal gaps and joining defects, and offers high inspection accuracy. A key feature is its ability to detect differences in thickness and dissimilar material joining. However, when the probe is in direct contact with the sample, it is necessary to maintain the sample's position, which can make it difficult to use in real time during welding.

[0094] 3) Eddy current testing This method uses eddy currents to detect gaps. It generates an electromagnetic field on the metal surface and measures the changes in electromagnetic reactions caused by gaps or defects. By measuring the changes in eddy currents generated in the metal using electromagnetic induction, a change in current flow occurs when a gap is present, and detecting this change reveals the presence of the gap. It is a non-contact method, enabling high-speed and high-precision inspection, and is considered particularly effective for electrically conductive metals. Equipment installation is also relatively easy. However, its detection range is limited, meaning it may only be usable under specific conditions, and it may not be applicable to metals with surface treatments or coatings.

[0095] 4) Infrared thermography An infrared camera is used to measure the temperature distribution of a sample in real time and determine the presence or absence of gaps. Because heat transfer differs in areas with gaps, temperature variations appear in the infrared image. For temperature distribution analysis, an infrared camera is used to detect areas where heat transfer is poor (areas with gaps) by observing the temperature distribution of the metal plate during welding. Real-time monitoring is possible even during welding, allowing for visual confirmation of gaps and abnormalities, and inspection from a distance is also possible. However, it is sensitive to temperature changes, easily affected by welding speed and ambient temperature, and requires a high-precision camera, resulting in high costs, which are drawbacks.

[0096] 5) Vision system (camera surveillance) This method involves visually inspecting gaps between metal plates using high-resolution cameras or 3D scanners, and some systems utilize AI to automatically detect gaps. Image processing and analysis technologies are used to detect the relative positions of the metal plates and the size of the gaps in real time. 3D scanning allows for three-dimensional inspection of the gaps. This enables the creation of systems that monitor the entire welding process and automatically detect anomalies. While it is non-contact and relatively easy to implement, detection accuracy depends on camera performance and image processing algorithms, making it difficult to detect minute gaps.

[0097] 6) Use of acoustic signals This method monitors acoustic signals generated during welding and detects the presence or absence of gaps from changes in these signals. Since the transmission of sound waves changes when gaps are present, the difference in signals is analyzed. It utilizes acoustic emission (AE) technology, where minute cracks and gaps that occur in the material during welding emit specific acoustic patterns, and these sound waves are captured and analyzed. It allows for non-contact, real-time detection and is considered particularly suitable for inspecting large structures and long materials. On the other hand, it is said to be sensitive to noise and ambient sounds and vibrations, which is considered a drawback.

[0098] 7) Mechanical measurement (probes and pins) This method physically measures whether there are gaps between metal plates using mechanical probes or pins. It involves measuring the distance between the metal plates with mechanical sensors or probes to confirm the presence of gaps. A technique of mechanically pressing the sample to fill the gaps can also be used in conjunction with this method. It is a relatively simple method and clearly identifies gaps. However, it is not suitable for real-time inspection during welding and is mainly used for pre-welding inspections.

[0099] As mentioned above, various techniques exist for detecting gaps between samples in overlap laser welding, including optical measurement, ultrasonic testing, and eddy current testing. However, each has its advantages and disadvantages, so it is important to select the optimal inspection method according to the purpose and situation. Furthermore, non-contact technologies such as optical sensors (laser measurement), infrared thermography, and vision systems (image analysis) are effective for monitoring gaps between samples in real time during laser welding. In particular, when high accuracy and real-time performance are required, combining these technologies can improve welding quality.

[0100] [Experiment details / Supplementary explanation and data... Laser scanning welding equipment] Figure 14 is a diagram illustrating the configuration of the laser scanning welding apparatus 2000. In the experiment described above, welding can also be performed using laser scanning welding. In Figure 14, parts corresponding to or identical to those in the laser welding apparatus 1000 shown in Figure 1(a) are denoted by the same reference numerals, and their descriptions are omitted.

[0101] The laser scanning welding apparatus 2000 is equipped with a scanner processing head 1110 that additionally includes a galvanometer mirror 1111 for X-axis drive and a galvanometer mirror 1112 for Y-axis drive, enabling high-speed scanning of the laser beam irradiation position on the welding material 1300 without driving or moving the focusing lens 1800 or the welding material 1300.

[0102] Laser scanning welding (laser scan welding) is a technique that performs welding while moving a laser beam at high speed. It uses a high-speed driven mirror called a "galvanometer mirror" or scanner to move the laser beam along a predetermined path. Its greatest advantage is its high-speed processing capability; because the laser beam can be moved at high speed, welding speed is improved, and extremely high productivity can be achieved.

[0103] Furthermore, because the beam position and path can be precisely controlled, accurate control is achieved, enabling welding of complex shapes and intricate details. In addition, because welding is performed in a short time, heat input control is easy, minimizing the thermal impact on the base material and reducing distortion and deformation. It can also handle a variety of materials such as stainless steel, aluminum, and copper, and welding of dissimilar materials is relatively easy. Laser scan welding is most commonly used in welding automotive parts (battery packs, engine parts), joining precision equipment (electronic devices, sensors), manufacturing medical devices, and welding thin metal sheets and parts requiring high precision, making it applicable to a wide range of industries. Moreover, because scan welding can achieve both efficiency and precision, it has become an indispensable technology in manufacturing and precision machining.

[0104] [Experiment details / Supplementary explanation and supplementary data... Galvano mirror of laser scanning welding equipment] A galvanometer mirror (galvanometer scanner) is a mechanical device used in laser processing and welding to move the laser beam quickly and precisely. Its role and function are crucial for improving the efficiency and accuracy of laser processing. The galvanometer mirror controls the direction of the laser beam by reflecting it in different directions, thereby shifting the laser's focal point. This enables welding and processing to be performed at precise locations, ranges, and sizes, even at a distance from the fixed laser head.

[0105] Furthermore, by driving the mirror with a precision motor at high speed and with high accuracy, high-speed scanning is possible, allowing the laser beam to move very quickly. This high-speed scanning significantly improves the speed of welding and processing, increasing production efficiency. Moreover, by using a galvanometer mirror to freely move the laser beam in a two-dimensional plane, complex shapes and patterns can be created, enabling the generation of diverse patterns. For example, paths such as circles, straight lines, and waves can be easily drawn in a short time, making complex welding and processing possible.

[0106] A galvanometer mirror uses one or more mirrors to rapidly adjust the reflection angle of a laser beam. The mirrors are precisely driven by servo motors, and one of the main functions of a galvanometer mirror is to change the beam direction to illuminate a specific area with the laser. Furthermore, galvanometer mirrors typically operate in conjunction with dedicated control software, allowing for the programming of processing patterns and beam movements to automate complex welding and machining operations. The ease with which this integration with control software is achieved is also a key feature.

[0107] Furthermore, because the mirror controls the beam position quickly and accurately, extremely high-precision welding and processing are possible, enabling highly accurate positioning. It is particularly suitable for welding fine structures and parts, and for processing in narrow areas. In summary, the advantages of galvanometer mirrors include high-speed processing that allows the beam to move much faster than when the laser head is moved mechanically, extremely fine positioning through servo motor control of the mirror, precision processing ideal for micro-processing and processing of complex shapes, and improved production efficiency that balances processing speed and quality through high-speed and precise operation. Galvanometer mirrors are now an indispensable element for streamlining laser processing and welding, and are recognized as playing a crucial role, especially in tasks requiring complex shapes and high precision.

[0108] [Experiment details / Supplementary explanation and supplementary data...Analysis of the vibration frequency of spectral intensity] Figure 9(b) is a table showing the integrated spectral intensity values ​​corresponding to the oscillation frequencies of the 1300 nm thermal radiation light when laser scan welding was performed using the laser scan welding apparatus 2000, for each range of gap t: 400-600 Hz, 600-800 Hz, 800-1000 Hz, and 1000-1200 Hz, for each case where the upper plate thickness of welding sample 1300(1) was 1.2 mm, 1.6 mm, 2.0 mm, and 2.6 mm, respectively, for gap t of 0 mm, 0.5 mm, and 1.0 mm. In the experiment shown in Figure 9(b), the lower plate thickness of welding sample 1300(2) was uniformly 1.6 mm.

[0109] Furthermore, Figures 10 and 11 show the time-amplitude relationship, i.e., the change in amplitude of the thermal radiation spectrum intensity fluctuation over time, for each condition, based on the measurement data of the analysis shown in the table in Figure 9(b). Figure 10(a) shows the case where the upper plate thickness of the welding sample 1300(1) is 1.2 mm and the gap t is 0 mm, (b) shows the case where the upper plate thickness of the welding sample 1300(1) is 1.2 mm and the gap t is 0.5 mm, and (c) shows the case where the upper plate thickness of the welding sample 1300(1) is 1.2 mm and the gap t is 1.0 mm. Furthermore, Figure 10(d) shows the case where the upper plate thickness of welding sample 1300(1) is 1.6 mm and the gap t is 0 mm, (e) shows the case where the upper plate thickness of welding sample 1300(1) is 1.6 mm and the gap t is 0.5 mm, and (f) shows the case where the upper plate thickness of welding sample 1300(1) is 1.6 mm and the gap t is 1.0 mm. In the graph shown in Figure 10, all welding samples used were uniformly those with a lower plate thickness of 1.6 mm.

[0110] Furthermore, Figure 11(a) shows the case where the upper plate thickness of welding sample 1300(1) is 2.0 mm and the gap t is 0 mm, (b) shows the case where the upper plate thickness of welding sample 1300(1) is 2.0 mm and the gap t is 0.5 mm, and (c) shows the case where the upper plate thickness of welding sample 1300(1) is 2.0 mm and the gap t is 1.0 mm. Also, Figure 11(d) shows the case where the upper plate thickness of welding sample 1300(1) is 2.6 mm and the gap t is 0 mm, (e) shows the case where the upper plate thickness of welding sample 1300(1) is 2.6 mm and the gap t is 0.5 mm, and (f) shows the case where the upper plate thickness of welding sample 1300(1) is 2.6 mm and the gap t is 1.0 mm. In the graphs shown in Figure 11, the lower plate thickness of welding sample 1300(2) was uniformly set to 1.6 mm for all samples.

[0111] Furthermore, Figures 12 and 13 plot the analysis results from the table shown in Figure 9(b) on a graph. Figure 12(a) is a graph plotting the relative spectral intensity with respect to the gap t between welded samples 1300, assuming the top plate thickness of each welded sample 1300(1) is 1.2 mm, for each vibration frequency range. Figure 12(b) is a graph explaining the differences due to each gap t, with the horizontal axis representing the vibration frequency and the vertical axis representing the spectral intensity, corresponding to (a) when the top plate thickness of each welded sample 1300(1) is 1.2 mm. Figure 12(c) is a graph plotting the relative spectral intensity with respect to the gap t between welded samples 1300, assuming the top plate thickness of each welded sample 1300(1) is 1.6 mm, for each vibration frequency range. Figure 12(d) is a graph explaining the differences due to each gap t, with the horizontal axis representing the vibration frequency and the vertical axis representing the vibration frequency, corresponding to (c) when the top plate thickness of each welded sample 1300(1) is 1.6 mm.

[0112] Furthermore, Figure 13(a) is a graph plotting the relative spectral intensity with respect to the gap t between welded samples 1300 when the top plate thickness of welded sample 1300(1) is 2.0 mm for each vibration frequency range, and Figure 13(b) is a graph explaining the differences due to each gap t, with the horizontal axis representing the vibration frequency and the vertical axis representing the spectral intensity when the top plate thickness of welded sample 1300(1) is 2.0 mm, corresponding to (a). Furthermore, Figure 13(c) is a graph plotting the relative spectral intensity with respect to the gap t between welded samples 1300 when the top plate thickness of welded sample 1300(1) is 2.6 mm for each vibration frequency range, and Figure 13(d) is a graph explaining the differences due to each gap t, with the horizontal axis representing the vibration frequency and the vertical axis representing the spectral intensity when the top plate thickness of welded sample 1300(1) is 2.6 mm, corresponding to (c). In the results shown in Figures 12 to 13, the bottom plate thickness of all welded samples 1300(2) was uniformly 1.6 mm, and the raw data for the analysis results shown in Figures 9(b) and 10 to 13 are all based on the same measurement data.

[0113] A closer examination of the table data in Figure 9(b) reveals that the thinner the top plate, the more clearly the difference in spectral intensity dependent on the gap size appears across a wider range of high frequencies. Conversely, the thicker the top plate, the more limited the high-frequency range in which the difference in spectral intensity dependent on the gap size is clearly apparent. For example, in Figure 9(b), for top plate thicknesses of 1.2 mm and 1.6 mm, there is more than double the difference in integrated spectral intensity between the case with no gap and the case with a 1 mm gap across the entire range of 400 to 1200 Hz. In this case, it can be seen that the presence and size of a gap can be accurately determined based on the integrated spectral intensity, regardless of the frequency used. In particular, focusing on the frequency range of 600 to 1200 Hz, for top plate thicknesses of 1.2 mm and 1.6 mm, the integrated spectral intensity for a 1 mm gap is less than 40% of that for a 0 mm gap, making it particularly useful for accurate gap detection.

[0114] On the other hand, in the case of the top plate thickness of 2.6 mm in Figure 9(b), there is more than double the difference in the integrated spectral intensity value between the case with no gap and the case with a 1 mm gap in the 600-1000 Hz range. In this case, it can be seen that using the 600-1000 Hz frequency range allows for good determination of the presence and size of the gap based on the integrated spectral intensity value. In particular, in this case, the largest difference is observed at 46% in the 600-800 Hz range, followed by 50% in the 800-1000 Hz range, 57% in the 400-600 Hz range, and 61% in the 1000-1200 Hz range. Therefore, it can be seen that the 600-800 Hz range is the most suitable for gap detection.

[0115] Furthermore, in the case of the top plate thickness of 2.0 mm in Figure 9(b), there is a difference of more than double in the integrated spectral intensity value between the case with no gap and the case with a gap of 1 mm across the entire range of 400 to 1200 Hz. In this case, it can be understood that by using any frequency within the range of 400 to 1200 Hz, the presence and size of the gap can be accurately determined based on the integrated spectral intensity value.

[0116] In summary, the analysis of Table 9(b) suggests that the most versatile frequency range for accurate gap detection across all plate thicknesses is 600-800 Hz. By extracting and monitoring only the 600-800 Hz frequency range of the intensity fluctuations of 1300 nm thermal radiation from the weld pool and processing point during laser scanning welding, it is possible to determine the presence and size of gaps with considerable accuracy. These analysis and evaluation results can also be seen from the trends in the graphed results in Figures 10 to 13.

[0117] As described above, the component with intensity fluctuations in the 1300nm thermal radiation in this frequency band is monitored by focusing only on a very small signal intensity portion that is in a completely different dimension from the peak value and satellite peak value of the thermal radiation. As long as you are monitoring the intensity of the normal peak and satellite peaks or analyzing their frequencies, this portion of the data will not be considered valid data and will be overlooked as noise, static, or meaningless data. According to the inventors' experiments and measurements, the frequency of the intensity fluctuations of the peak value and satellite peak value of the thermal radiation is almost limited to around 0Hz, and this is a completely different data portion from the 150Hz and above, 200Hz and above, 400Hz and above, and the 600-800Hz range that the inventors are focusing on.

[0118] The method for detecting gaps between samples during overlap laser welding, the apparatus / system for detecting gaps between samples during overlap laser welding, and related structures / methods, etc., according to the present invention are not limited to the shapes, structures / methods, etc., shown in the above description and drawings. Within the scope of the present invention, known or well-known methods, etc., accessible to those skilled in the art may be adopted, modified, arranged, combined, or arbitrarily modified and altered. [Explanation of Symbols]

[0119] 1000... Laser welding equipment, 1100... Scanner processing head, 1200... Calculation unit, 1300... Welding sample, 1400... Optical fiber, 1500... Collimating lens, 1600... Dichroic mirror, 1700... Sensor focusing lens, 1750... Interference filter, 1800... Focusing lens, 1900... Optical sensor.

Claims

1. During overlap laser welding, a process is performed to detect the intensity of thermal radiation from the processing point, A step of calculating the intensity of the thermal radiation light at which the oscillation frequency of the detected intensity fluctuation of the thermal radiation light is 150 Hz or higher, The process includes determining the presence and / or size of gaps between samples based on the calculated strength. A method for detecting the gap between samples during overlap laser welding, characterized by the above.

2. In the method for detecting the gap between samples during overlap laser welding according to claim 1, The calculation step described above involves calculating the integrated intensity value of the thermal radiation light, where the vibration frequency is 150 Hz or higher. A method for detecting the gap between samples during overlap laser welding, characterized by the above.

3. In the method for detecting the gap between samples during overlap laser welding according to claim 2, In the aforementioned determination process, the gap between the samples is determined to be larger the smaller the cumulative strength value is. A method for detecting the gap between samples during overlap laser welding, characterized by the above.

4. A method for detecting the gap between samples during overlap laser welding according to any one of claims 1 to 3, The aforementioned thermal radiation is infrared radiation in the 1300 nm band. A method for detecting the gap between samples during overlap laser welding, characterized by the above.

5. A detection unit that detects the intensity of thermal radiation from the processing point during overlap laser welding, A calculation unit that calculates the intensity of the thermal radiation light at which the oscillation frequency of the detected intensity fluctuation of the thermal radiation light is 150 Hz or higher, The system includes a determination unit that determines the presence and / or size of gaps between samples based on the calculated strength. A device for detecting the gap between samples during overlap laser welding, characterized by the above features.

6. In the sample gap detection device during overlap laser welding according to claim 5, The calculation unit calculates the integrated intensity value of the thermal radiation light whose vibration frequency is 150 Hz or higher. A device for detecting the gap between samples during overlap laser welding, characterized by the above features.

7. In the sample gap detection device during overlap laser welding according to claim 6, The determination unit determines that the gap between the samples is larger as the cumulative intensity value decreases. A device for detecting the gap between samples during overlap laser welding, characterized by the above features.

8. In the sample gap detection device during overlap laser welding according to any one of claims 5 to 7, The aforementioned thermal radiation is infrared radiation in the 1300 nm band. A device for detecting the gap between samples during overlap laser welding, characterized by the above features.

9. In the method for detecting the gap between samples during overlap laser welding according to claim 1, The calculation step described above involves calculating the integrated intensity value of the thermal radiation light only when the vibration frequency is between 250 Hz and 1200 Hz. A method for detecting the gap between samples during overlap laser welding, characterized by the above.

10. In the method for detecting the gap between samples during overlap laser welding according to claim 9, The calculation process described above involves calculating the integrated intensity value of the thermal radiation light only when the vibration frequency is between 400 Hz and 1000 Hz. A method for detecting the gap between samples during overlap laser welding, characterized by the above.

11. In the method for detecting the gap between samples during overlap laser welding according to claim 10, The calculation process described above involves calculating the integrated intensity value of the thermal radiation light only when the vibration frequency is between 600 Hz and 800 Hz. A method for detecting the gap between samples during overlap laser welding, characterized by the above.

12. In the sample gap detection device during overlap laser welding according to claim 5, The calculation unit calculates the integrated intensity value of the thermal radiation light only when the vibration frequency is between 250 Hz and 1200 Hz. A device for detecting the gap between samples during overlap laser welding, characterized by the above features.

13. In the sample gap detection device during overlap laser welding according to claim 12, The calculation unit calculates the integrated intensity value of the thermal radiation light only when the vibration frequency is between 400 Hz and 1000 Hz. A device for detecting the gap between samples during overlap laser welding, characterized by the above features.

14. In the sample gap detection device during overlap laser welding according to claim 13, The calculation unit calculates the integrated intensity value of the thermal radiation light only when the vibration frequency is between 600 Hz and 800 Hz. A device for detecting the gap between samples during overlap laser welding, characterized by the above features.

15. A method for detecting the gap between samples during overlap laser welding according to any one of claims 1 to 3, The calculation target in the above calculation process is the graph of the intensity of the thermal radiation against the vibration frequency of the intensity fluctuation, where the intensity is 3% or less of the peak intensity. A method for detecting the gap between samples during overlap laser welding, characterized by the above.

16. In the sample gap detection device during overlap laser welding according to any one of claims 5 to 7, The target of calculation by the calculation unit is the graph of the intensity of the thermal radiation against the vibration frequency of the intensity fluctuation, where the intensity is 3% or less of the peak intensity. A device for detecting the gap between samples during overlap laser welding, characterized by the above features.

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