FR3085609A1 - LASER SPOT WELDING PROCESS - Google Patents

Abstract

A laser spot welding process includes a step of irradiating superimposed metal plates (11, 12, 13) with a laser (L1, L2) with a change in the irradiation diameter of the laser gradually or in stages between a first irradiation diameter (φ1) and a second irradiation diameter (φ2) in a state in which an optical axis of the laser is fixed on a predetermined surface of the metal plates, in which: one of the first and second irradiation diameter is the smallest irradiation diameter (φ1) during the step and the other is the largest irradiation diameter (φ2), providing a point diameter (S2), at stage course; and the change in irradiation diameter is provided by changing a defocus value (from d1 to d2) of the laser. Figure of the abstract: Figure 1

Description

Description

Title of the invention: LASER SPOT WELDING PROCESS

Technical Field [0001] The present invention relates to laser spot welding methods.

PRIOR ART [0002] Laser welding, in which the optical energy caused by laser irradiation on a workpiece heats and melts the material at the level of the irradiated part, has the advantage of large-scale welding. contactless speed, and therefore tends to replace arc welding and resistance spot welding. In laser spot welding as an alternative to resistance spot welding, the junction resistance is obtained by scanning the laser beam circularly or spirally in the spot area as described, for example, in the Patent Document 1.

Unfortunately, such a welding process poses problems in that the welding requires an agile scanning operation for scanning by the beam in the area of the point, which makes the checking operation complicated and increases the time of beam sweep duration cycle.

Technical problem The present invention has been made in view of the above situation, and an object thereof is to provide a laser spot welding process which provides a resistance of the junction which is stable with a simple operation and which does not require complexity of control or increase in cycle time.

Technical solution [0005] To solve the above problems, a laser spot welding method according to the present invention comprises:

a step of irradiating metal plates superimposed with a laser by changing an irradiation diameter of the laser gradually or in stages between a first irradiation diameter and a second irradiation diameter in a state in which an optical axis of the laser is fixed on a predetermined area of the metal plates, in which: one of the first and second irradiation diameters is the smallest irradiation diameter during the step and the other is an irradiation diameter the largest, providing a point diameter, during the stage; and the change in the irradiation diameter is provided by changing a defocus value of the laser. Advantages provided [0006] In the laser spot welding method according to the present invention, since the laser irradiation diameter is modified with the laser optical axis fixed as described above, the laser irradiation with the the smallest irradiation diameter provides the melting depth, and laser irradiation with the largest irradiation diameter provides the point diameter. Thus, the laser spot welding method according to the present invention, which is a simple operation not involving scanning of the optical axis of the laser, provides a desired junction resistance and also has the advantage of improving the productivity, because no complexity of control and no increase in cycle time is required. In addition, in the case where metal plates are arranged one on top of the other with gaps between them, the laser spot welding method according to the present invention has the advantage of significantly improving the ranges of deviations. admissible.

According to one aspect of the present invention above, in which the first irradiation diameter is the smallest irradiation diameter during the step; the second irradiation diameter is the largest irradiation diameter during the step; and the step includes irradiation with the laser with a gradual or stepwise increase in the irradiation diameter from the smallest irradiation diameter to the largest irradiation diameter, the laser irradiation with the first irradiation diameter melts and joins superimposed metal plates, and the molten part is extended to a desired point diameter by increasing the irradiation diameter from the first irradiation diameter to the most irradiation diameter large, while the laser is irradiating. This allows stable spot welding to be performed with a high degree of deviation tolerance.

According to one aspect of the present invention above, in which the first irradiation diameter is the largest irradiation diameter during the step, the second irradiation diameter is the irradiation diameter the smallest during the step, and the step comprises irradiation with the laser with a decrease in the irradiation diameter gradually or in stages from the largest irradiation diameter to the largest irradiation diameter small, the laser irradiation with the first irradiation diameter forming a molten part corresponding to a desired spot diameter from the highest surface, and the laser irradiation performed with a decrease in the irradiation diameter from the first irradiation diameter up to the smallest irradiation diameter promotes heat transfer to the lowest surface and thus provides a desired melting depth at the center of the bottom part eu. In particular, in the case where the deviation from the side of the uppermost surface is large, this aspect of the present invention has the advantage of being able to stably perform spot welding having a high degree of tolerance because laser irradiation with the largest irradiation diameter is first performed on the metal plate on the uppermost surface, so that a junction by thermal deformation in the maximum range progresses before penetration, promoting heat transfer to the second or lower plate.

According to one embodiment, a power of the laser is substantially constant during the step

BRIEF DESCRIPTION OF THE DRAWINGS Other characteristics, details and advantages of the invention will appear on reading the detailed description below, and on analysis of the appended drawings, in which: Fig.l [0011] [ fig.l] shows views in lateral cross section (a) and from above (b) laser spot welding according to a first embodiment of the present invention and a graph (c) schematically illustrating a variation of the irradiation diameter ;

Fig. 2a [fig.2a] shows a graph (a) illustrating the change in the irradiation diameter during laser spot welding according to a comparative example,

Fig. 2b [fig.2b] shows a graph (b) among graphs (b) to (d) illustrating the change in irradiation diameter during laser spot welding according to first to third examples of the first mode of the present invention, FIG. 2c [fig.2c] shows a graph (c) among the graphs (b) to (d) illustrating the change in irradiation diameter during laser spot welding according to first to third examples of the first mode of the present invention, FIG. 2d [fig.2d] shows a graph (d) among the graphs (b) to (d) illustrating the change in irradiation diameter during laser spot welding according to first to third examples of the first mode of the present invention, FIG. 2e [0016] [fig.2e] shows a graph (e) illustrating the upper and lower gaps and the weldable range during laser spot welding of the comparative example (a), Fig. 2f [fig.2f] shows a graph (f) among graphs (f) to (h) illustrating the upper and lower deviations and the weldable range during laser spot welding of the first to third examples;

Fig. 2g [fig.2g] shows a graph (g) among the graphs (f) to (h) illustrating the upper and lower gaps and the weldable range during laser spot welding of the first to third examples;

Fig. 2h [fig.2h] shows a graph (h) among the graphs (f) to (h) illustrating the upper and lower deviations and the weldable range during laser spot welding of the first to third examples;

Fig. 3a [fig.3a] shows a graph (a) among graphs (a) and (b) illustrating the variation of the irradiation diameter

Fig. 3b [fig. 3b] shows a graph (b) among the graphs (a) and (b) illustrating the variation of the irradiation diameter

Fig. 3c [fig.3c] shows a graph (c) among graphs (c) and (d) illustrating the upper and lower gaps and the weldable range, during laser spot welding according to the fourth and fifth modes of carrying out the present invention;

Fig. 3d [fig.3d] shows a graph (d) among the graphs (c) and (d) illustrating the upper and lower gaps and the weldable range, during laser spot welding according to the fourth and fifth modes of carrying out the present invention;

Fig. 4 [Fig. 4] shows a graph (a) illustrating the change in the irradiation diameter and a graph (b) illustrating the upper and lower deviations and the weldable range, during laser spot welding of the 'Example 6 according to a second embodiment of the present invention;

Fig. [Fig. 5] is an enlarged cross-sectional view of the weld portion of the laser spot welding according to the first embodiment of the present invention;

Fig. 6 [Fig. 6] shows side cross section views (a) and from above (b) laser spot welding according to the second embodiment of the present invention and a graph (c) schematically illustrating a variation of the irradiation diameter. Description of the Embodiments Hereinafter, embodiments of the present invention are described in detail with reference to the drawings.

First Embodiment [0029] Parts (a) to (c) of Eig. 1 illustrate laser spot welding 10 according to a first embodiment of the present invention for three metal plates 11, 12 and 13. In part (a) of Eig. 1, the metal plates 11, 12 and 13 having thicknesses of tl, t2 and t3 are placed one on the other with gaps ga and gb between them.

The gaps ga and gb are gaps whose intervals are adjusted by placing the metal plates 11, 12 and 13 on top of each other with projections (bosses not shown) between them, the projections being formed in advance. by pressing on some of the metal plates 11, 12 and 13 (in general, the metal plates 12 and 13 located on the lower side of the gaps ga and gb), or by placing the metal plates 11, 12 and 13 on top of each other with non-illustrated spacers inserted between the metal plates and holding them with pliers or other tools if necessary, and / or gaps whose intervals are caused by the elastic return at the level of parts of flanges or the like of working parts of the press and are therefore not adjusted.

The metal plates 11, 12 and 13 are not limited to particular plates, but are assumed to be thin steel plates of thicknesses between 0.6 and 2.0 mm. In experiments, described later, the thicknesses t1, t2 and t3 of steel plates used were 0.6 mm, 0.8 mm and 1.2 mm, respectively. In the case where metal plates have surface treatment layers made of a metal with a low melting point, for example a galvanized layer, on joining surfaces, the gaps at adjusted intervals as described above are intentionally provided to evacuate the metal vapor. In the case where the metal plates do not have a surface treatment layer of a metal with a low melting point, the metal plates can be placed directly on top of each other without gaps ga and gb between them.

When performing laser spot welding 10, a laser processing head is first positioned above the metal plate 11 located on the uppermost surface, and laser irradiation L1 is produced, with the optical axis fixed, at a constant power with a defocus value dl (the smallest irradiation diameter φΐ) to form a weld part W1 (a part melted at this stage) which penetrates the three metal plates 11, 12 and 13 at a point SI.

This point SI is an irradiation area of an area which is the smallest (with an energy density which is the highest) during a welding operation. The point SI enters the metal plates 11 and 12, which are the two plates closest to the highest surface, among the three metal plates 11, 12, and 13 to be welded, at a minimum required laser power , and a sufficient depth of fusion can also be obtained in the lowest metal plate 13.

Then, the focus is controlled by the optical system of the laser welder with the optical axis kept fixed. The defocus value is gradually increased from dl to d2, as indicated by the symbol Ws in part (c) of the

Fig. 1 to gradually increase the laser irradiation diameter to φ2 while performing the laser irradiation (L1 to L2) at constant power. The molten part is extended to W2, and the laser irradiation L2 is complete when the point is S2.

This point S2 is an irradiation zone with an area which is the largest (with an energy density which is the lowest) during a welding operation. Although the energy density of laser irradiation gradually decreases during the expansion of the laser irradiation diameter from φΐ to φ2 to increase the irradiation area from SI to S2, a transfer of heat accompanying this process from the central part to the neighboring parts promotes stable fusion in the irradiation zone S2, generating the final weld part W2 corresponding to the laser irradiation diameter φ2.

It will be noted that in the case in which the metal plates 11, 12 and 13 have surface treatment layers of a metal with a low melting point, the metal vapor caused at the level of the molten part and its vicinity is diffused and evacuated through the gaps ga and gb at the same time as the transfer of heat from the central part to the neighboring parts described above and the expansion of the diameter of the laser irradiation.

As described above, in laser spot welding 10, the laser irradiation diameter is changed with the laser optical axis fixed so that the laser irradiation L1 with the diameter smallest irradiation φΐ offers sufficient melting depth to the central part (SI, Wl), and laser irradiation L2 with the largest irradiation diameter φ2 offers the desired point diameter (S2, W2 ). Consequently, laser spot welding 10, which is simple to operate, does not involve scanning the optical axis of the laser, offers a desired junction resistance and also offers the advantage of significantly increasing the admissible ranges. deviations ga and gb between the metal plates 11, 12 and 13.

Example according to the First Embodiment and Comparative Example Below, experiments have been carried out to verify the advantageous effect of laser spot welding 10 according to the first embodiment. During the experiments, laser spot welding was carried out by modifying the distances ga and gb between the metal plates 11, 12 and 13 and the combinations of these for each new pattern of change in irradiation diameter at the laser, and the ranges of allowable deviations were compared. During the experiments, steel plates with a thickness of tl = 0.6 mm, t2 = 1.2 mm, and t3 = 0.8 mm were used from the side of the highest surface (the side laser irradiation) as metal plates 11, 12 and 13. The laser was irradiated at a laser power of 6 kW for 0.4 seconds, changing the defocus value in the range between 30 and mm and the laser irradiation diameter in the range of 1.8 to 5.0 mm. Comparative Example First of all, by way of comparative example, the laser spot welding was carried out so that the laser was irradiated for 0.2 seconds with a defocus value dl = 30 mm, then the defocus value is increased to d2 = 90 mm, and the laser has irradiated for 0.15 seconds, as illustrated in part (a) of FIG. 2. The deviations ga and gb between the metal plates and the combinations thereof have been modified to study the ranges of admissible deviations.

Part (e) of FIG. 2 shows the result, in which the hatched combinations in the diagram show the admissible ranges of deviations in which a good welding result has been obtained. In the case where the upper deviation ga is equal to 0, the lower deviation is allowed up to 1.0 mm = gb. In combinations in which neither the ga deviation nor the gb deviation is zero, the sum of the deviations is approximately 0.9 mm. Although in some combinations extending the laser irradiation time has shown some improvement, it can be seen that there is a difference between the range in which the lower deviation gb is large and the allowable deviation range of l 'Example 1 (described later) designated by the thick lines in the figure.

Example 1 Hereinafter, as Example 1 according to the first embodiment of the present invention, the laser spot welding was carried out so that the laser irradiated for 0.4 seconds with an increase in the defocus value from dl = 30 mm to d2 = 90 mm at a constant speed as illustrated in part (b) of FIG. 2. The deviations ga and gb between the metal plates and the combinations thereof have been modified to study the ranges of admissible deviations.

Part (f) of FIG. 2 shows the result of Example 1. Compared to the Comparative Example described above, the lower deviation up to 1.0 to 1.1 mm is allowed in the range in which the lower deviation gb is important, and the admissible range is extended to the area in which the sum of the upper and lower deviations is 1.2 to 1.3 mm.

Example 2 Hereinafter, as Example 2 according to the first embodiment of the present invention, the laser spot welding was carried out so that the laser irradiated for a total period 0.4 seconds, during which the defocus value was increased from dl = 30 mm to 40 mm in 0.2 seconds at a relatively low speed, then the defocus value was increased to d2 = 90 mm in 0 , 2 seconds which followed at a relatively high speed, as illustrated in part (c) of FIG. 2. The deviations ga and gb between the metal plates and the combinations thereof have been modified to study the ranges of admissible deviations.

Part (g) of FIG. 2 shows the result of Example 2. Although the allowable deviation range is widened and larger than in the above Comparative Example, the allowable range is less than that of Example 1 described above by about 0.2 mm in the range in which the lower gap gb is large.

Example 3 Then, as Example 3 according to the first embodiment of the present invention, the laser spot welding was carried out so that the laser irradiated for a total duration of 0 , 4 seconds, during which the increase in the defocus value was increased from dl = 30 mm to 50 mm in 0.1 seconds at a relatively high speed, then the defocus value was increased to d2 = 90 mm in the 0.3 seconds that followed at a relatively low speed, as illustrated in part (d) of FIG. 2. The deviations ga and gb between the metal plates and the combinations thereof have been modified to study the ranges of admissible deviations.

Part (h) of FIG. 2 shows the result of Example 3. In contrast to Example 2 above, Example 3 showed a slightly better result than Example 1 in the area where the sum of the upper and lower deviations is large .

The results of Examples 1 to 3 and of the Comparative Example described above show that it is more advantageous, during welding in the case where the two upper and lower deviations are present, to perform the laser irradiation L1 with the smallest irradiation diameter (φΐ) for a very short period and then gradually increase the irradiation diameter, in order to obtain a larger range of admissible deviations and to form stably an advantageous welding point. In particular, the comparison between Example 2 and Example 3 suggests that increasing the irradiation diameter relatively quickly during the first half of the welding process and increasing the irradiation diameter relatively slowly during the second half of the welding process. welding process provides a more favorable result. In order to verify this trend, other experiments were carried out to compare the allowable deviation range, in which laser spot welding was carried out by changing only the pattern of change of the laser irradiation diameter.

Example 4 First of all, as Example 4 according to the first embodiment of the present invention, the laser spot welding was carried out so that the laser irradiated for a period of time 0.4 seconds total, during which the defocus value was increased from dl = 30 mm to 60 mm in 0.1 seconds at a higher speed than in Example 3, then the defocus value was increased to d2 = 90 mm within 0.3 seconds at a lower speed than in Example 3, as illustrated in part (a) of FIG. 3. The differences ga and gb between the metal plates and their combinations have been modified to study the ranges of admissible deviations.

Part (c) of FIG. 3 shows the result of Example 4. Compared to Example 3 above, the combination of an upper gap ga of 0.3 mm and a lower gap gb of 0.9 to 1.0 mm was found to be defective. However, in the case where the upper deviation ga is less than or equal to 0.2 mm, the admissible range for the lower deviation gb has been extended to 1.3 to 1.4 mm, which shows that the Example 4 is advantageous in the case where the lower difference gb is large.

Example 5 Hereinafter, as Example 5 according to the first embodiment of the present invention, the laser spot welding was carried out so that the laser irradiated for a total period 0.4 seconds, during which the defocus value was increased from dl = 30 mm to 70 mm in 0.1 seconds at a higher speed than in Example 5, then the defocus value was increased to d2 = 90 mm within 0.3 seconds at a lower speed than in Example 5, as illustrated in part (b) of Fig. 3. The differences ga and gb between the metal plates and their combinations have been modified to study the ranges of admissible deviations.

Part (d) of FIG. 3 shows the result of Example 5. Although, as in Example 4 above, the combination of an upper gap ga of 0.3 mm and a lower gap gb of 0.9 to 0.6 mm has been defective, it has been observed that, in the case where the upper deviation ga is 0.7 to 1.7 mm, the admissible range has been widened. Furthermore, it has been found that in the case where the upper deviation ga is less than or equal to 0.2 mm, the admissible range for the lower deviation gb has been extended to 1.3 to 1.5 mm, and thus, Example 5 is advantageous in the case where the lower gap gb is large.

Example of Portion of Soldering according to the First Embodiment [0060] FIG. 5 is a cross-sectional view of a weld portion in which three metal plates 51, 52 and 53 are welded by laser spot welding. Metal plates 51, 52 and 53 have thicknesses of 0.8mm, 1.2mm and 0.6mm, respectively, and the top gap was 0.5mm, and the bottom gap was 1 , 6 mm. The laser was irradiated with a defocus value dl = 10 mm for 0.2 seconds, 20 mm for 0.05 seconds, and 40 mm for 0.2 seconds, then, the laser irradiated continuously for 0.8 seconds , with a gradual increase in the defocus value at d2 = 90 mm. As a result, the welding portion 50W having an effective spot diameter was obtained. Although this example is a special case in which the lower gap is greater than the thicknesses of the plates, it has been confirmed that welding is possible even in the case where there is such a gap.

It will be noted that in this example, since the parameter of the difference was constraining, the control was carried out so that the defocus value (irradiation diameter) was increased in stages. However, in real laser spot welding which ends in 0.2 to 0.4 seconds as in the examples above, there is no significant difference in the welding result between the check that the defocus value (irradiation diameter) is increased in stages via fixed intermediate defocus values (irradiation diameters) and the control according to which the defocus value (irradiation diameter) is continuously increased. This is only a difference in setting. In addition, although according to the specification of the laser welder (processing machine), laser irradiation can be stopped for a very short period, when the defocus value is changed, it has been confirmed that even in this In this case, no significant difference in the welding result is obtained.

Second Embodiment Below are the parts (a) to (c) of FIG. 6 illustrate laser spot welding 20 according to a second embodiment of the present invention for three metal plates 11, 12 and 13. The difference with the first embodiment lies only in the pattern of change in the irradiation diameter laser illustrated in part (c) of Fig. 3, and the basic configuration, according to which the material and the thicknesses t1, t2, and t3 of the metal plates 11, 12 and 13 and the gaps ga and gb, is the same as that of the first embodiment.

When performing laser spot welding 20, a laser processing head is first positioned above the metal plate 11 located on the uppermost surface, and laser irradiation L1 is performed, with the optical axis fixed on a predetermined area, at a constant power with a defocus value dl (the smallest irradiation diameter φΐ), so that the metal plate 11 located on the most surface high is heated and melted at point S1 corresponding to the final diameter of the point, and the metal plate 11 hangs down and is joined to the metal plate 12 located below. The molten metal in the metal plate 11 is joined to the metal plate 12, promoting heat transfer throughout the area of the point SI. The metal plate 12 which is below the metal plate 11 and has been joined to the metal plate 11 also softens, forming a molten part W1.

Then, the focus is controlled by the optical system of the laser welder with the optical axis kept fixed. The defocus value is gradually reduced from dl to d2 as indicated by the symbol Ws in part (c) of Fig. 6 to gradually reduce the laser irradiation diameter to φ2 (point S2), while the laser irradiation (from L1 to L2) is carried out at constant power, so that the center of the melted part W1 is caused to develop in the metal plate 13 located below before the laser irradiation L2 is completed. This forms the melted part W2 which penetrates the three metal plates 11, 12 and 13.

In this laser spot welding 20 of the second embodiment, after the laser irradiation Ll with the largest irradiation diameter φΐ corresponding to the end point diameter (SI, Wl) has been carried out with the laser optical axis fixed, the laser irradiation diameter is reduced to the smallest irradiation diameter φ2 to increase the energy density, providing a sufficient melting depth in the central part (S2, W2 ). Consequently, as in the first embodiment, laser spot welding 20, which is a simple operation not involving scanning of the optical axis of the laser, provides a desired junction resistance and also offers the advantage to increase the admissible ranges of the ga and gb deviations between the metal plates 11, 12 and 13.

Example according to the Second Embodiment Below, the experiments were carried out under the same conditions as in the first embodiment to verify the advantageous effect of laser spot welding 20 according to the second mode of achievement. The laser spot welding was carried out so that the laser irradiated for 0.1 seconds with a defocus value dl = 90 mm, then the defocus value was lowered to d2 = 20 mm at 0, 2 seconds at a constant speed, and the laser irradiated for another 0.1 seconds as illustrated in part (a) of Fig. 4. The deviations ga and gb between the metal plates 11, 12 and 13 and the combinations thereof have been changed to compare the ranges of permissible deviations.

Part (b) of FIG. 4 shows the result of Example 6. The ranges of permissible deviations are significantly enlarged on both the upper and lower sides compared to the Comparative Example above (part (e) of Fig. 2). Compared to Examples 1 to 5 (parts (f) to (h) of Fig. 2, and parts (c) and (d) of Fig. 3) of the first embodiment, the example of the second embodiment embodiment is characterized in that the lower deviation is allowed up to 0.7 mm in the range in which the upper deviation ga is large, and the range of permissible deviations of the sum of the upper and lower deviations is extended up to '' at 1.4 to 1.5 mm.

Note that according to the change in the irradiation diameter (defocus value) in laser spot welding 20 according to the second embodiment above, the change in the irradiation diameter (defocus value) of laser spot welding 10 according to the first embodiment above can be performed.

More precisely, after the defocus value is gradually reduced, going from dl to d2 (after the diameter of the laser radiation is gradually decreased to φ2 (spot S2)), the defocus value can be gradually increased to Dl (or more / less) (the laser radiation the diameter can be gradually enlarged to 01 (or more / less)) before the completion of the laser irradiation.

Conversely, after the irradiation diameter (defocus value) has increased as in laser spot welding 10 according to the first embodiment above, the irradiation diameter (defocus value) can be reduced as in laser spot welding according to the second embodiment above before the completion of the laser irradiation.

In addition, although in the above embodiments, the description has been provided for the case in which the defocus value dl to d2 is changed by controlling the laser optical system, the defocus value can be changed by mechanically moving the position of the laser treatment head up and down (linear movement).

In addition, although the above embodiments have illustrated the case in which laser spot welding is carried out on two or three superimposed metal plates, four or more superimposed metal plates can be used for the laser spot welding. Although it has not been confirmed experimentally that the present laser spot welding can be applied to the total thickness of plates up to 4.2 mm, the present laser spot welding can probably be implemented over a thickness greater than this, depending on conditions, such as the power of the laser.

In addition, although the above embodiments have illustrated the case in which the laser irradiates from the vertical above the metal plate 11 at the highest surface, the same level of properties treatment can be obtained in the case of an irradiation angle of up to 40 degrees. Furthermore, the present laser spot welding can be applied for welding not only of horizontally placed metal plates, but also for those placed to be inclined at any angle.

Although the description has been provided for certain embodiments of the present invention, the present invention is not limited to the above embodiments. Various types of other modifications and changes may be made based on the technical idea of the present invention.

List of reference signs - 10, 20 laser spot welding

- 11, 12, 13 metal plate

- dl, d2 defocus value

- ga, gb deviation

- LI, L2 laser irradiation

- IF, S2 point

- φΐ, φ2 laser irradiation diameter

- Wl, W2 welding part

List of cited documents Patent documents For any useful purpose, the following patent document is cited:

- [Patent Document 1] JP 2012-115876 A.

Claims (1)

Claims [Claim 1] A laser spot welding process (10, 20) comprising a step of irradiating metal plates (11, 12, 13) superimposed with a laser (LI, L2) with a change in irradiation diameter of the laser gradually or in stages between a first irradiation diameter (φΐ) and a second irradiation diameter (φ2) in a state in which an optical axis of the laser is fixed on a predetermined area of the metal plates, in which: one of the first and second irradiation diameters is the smallest irradiation diameter during the step and the other is the largest irradiation diameter, providing a point diameter (S2), during the 'stage; and the change in the irradiation diameter is provided by changing a defocus value of the laser (from dl to d2). [Claim 2] The laser spot welding method (10) according to claim 1, wherein: the first irradiation diameter (φΐ) is the smallest irradiation diameter during the step; the second irradiation diameter (φ2) is the largest irradiation diameter during the step; and the step comprises irradiation with the laser (LI, L2) with an increase in the irradiation diameter gradually or in stages from the smallest irradiation diameter to the largest irradiation diameter. [Claim 3] The laser spot welding method (20) according to claim 1, wherein: the first irradiation diameter (φ 1) is the largest irradiation diameter during the step; the second irradiation diameter (φ2) is the smallest irradiation diameter during the step; and the step comprises irradiation with the laser (LI, L2) with a reduction in the irradiation diameter gradually or in stages from the largest irradiation diameter to the smallest irradiation diameter. [Claim 4] The laser spot welding process (10, 20) according to any one of claims 1 to 3, wherein a power of the laser (LI, L2) is substantially constant during the step. 1/10