Laser spot welding process

Abstract

Laser spot welding process for overlapping metal plates (11, 12, 13), comprising in sequence: a first step (21) of beginning to irradiate the overlapping metal plates (11, 12, 13) with a laser (L1) having a first irradiation diameter (φ1) in a state in which an optical axis of the laser is set to a predetermined area of ​​the metal plates (11, 12, 13) to heat the metal plates (11, 12, 13) to melt them;and a second step (22) to continuously irradiate the metal plates (11, 12, 13) with the laser (L1-L2) after the first step (21), with a gradual or stepwise increase of an irradiation diameter of the laser from the first irradiation diameter (φ1) to a second irradiation diameter (φ2) in the state in which the optical axis of the laser is set to the predetermined surface, in order to expand a molten area (W1-W2) of the metal plates (11, 12, 13), wherein the increase of the irradiation diameter (φ1-φ2) is provided by increasing a defocusing component (d1-d2) of the laser.

Description

[Technical field] The present invention relates to a laser spot welding process. Laser welding, in which the optical energy generated by a laser beam heating and melting the material in the irradiated area of ​​a workpiece, offers the advantage of non-contact, high-speed welding and has therefore replaced arc and resistance spot welding. In laser spot welding, as an alternative to resistance spot welding, the joint strength is achieved by circular or spiral scanning of the laser beam within the spot area, as described, for example, in patent document 1. Unfortunately, such a welding process has problems in that the welding requires an agile scanning process for beam scanning within the spot area, which complicates the control process and increases the cycle time by the time of the beam scanning. [State of the art] [Patent document] [Patent Document 1] JP 2012-115876 A DE 102 54 917 A1 relates to a method for melting a specific section of a material, such as a plastic or a metal, by means of a laser, and to a device that operates according to this method. DE 11 2016 005 576 T5 discloses a method for laser spot welding of a stack of workpieces which contains at least two overlapping workpieces made of steel, at least one of which has a surface coating. [Summary of the invention] [Problems to be solved by the invention] The present invention was made in view of the above situation, and one object of it is to provide a laser spot welding process which provides stable joint strength with a simple process and does not require complexity of control or increase in cycle time. [Means of solving the problems] The invention is defined in the independent claims. The dependent claims specify embodiments of the invention. A laser spot welding process is disclosed, comprising: a first step of irradiating overlapping metal plates with a laser having a first irradiation diameter in a state in which an optical axis of the laser is set to a predetermined area of ​​the metal plates in order to heat the metal plates to melting; and a second step of continuously irradiating the metal plates with the laser after the first step, with a gradual or stepwise increase in an irradiation diameter of the laser from the first irradiation diameter to a second irradiation diameter in the state in which the optical axis of the laser is set to the predetermined area in order to extend a molten area of ​​the metal plates, wherein the increase in the irradiation diameter is provided by increasing a defocusing component of the laser. [Advantageous effects of the invention] In the disclosed laser spot welding process, the overlapping metal plates are heated and melted by irradiating the laser with the first irradiation diameter and the fixed optical axis. The molten area is then expanded to a desired spot diameter by irradiating the laser with a gradual or stepwise increase in the irradiation diameter to the second irradiation diameter, in a state where heat can be transferred to all metal plates, as described above. Therefore, the laser spot welding process according to the disclosure, which is a simple process that does not involve scanning the optical axis of the laser, provides the desired joint strength and also offers an advantage in improving productivity, as no complexity of control and no increase in cycle time are required.In addition, the laser spot welding process according to the disclosure has the advantage of being able to perform spot welding stably with a high tolerance to gaps between the metal plates. In a preferred mode of disclosure, the second step includes a first section in which the laser's irradiation diameter is increased from the first irradiation diameter at a first rate, and a second section following the first section in which the laser's irradiation diameter is increased to the second irradiation diameter at a second rate, and the first rate is greater than the second rate. In conjunction with the foregoing, in a preferred mode of disclosure, in the second step, as the irradiation diameter of the laser is increased from the first irradiation diameter to the second irradiation diameter, the rate of increasing the irradiation diameter is gradually or stepwise reduced. These features allow for a rapid and positive expansion of the spot diameter, which is advantageous for the stable execution of spot welding with a high tolerance to the gaps between the metal plates. [Brief description of the drawings] [Fig. 1] Fig. 1 shows a cross-sectional side view (a) and a top view (b) of a laser spot welding process according to an embodiment of the disclosure and a graph (c) schematically representing a change in the irradiance diameter. [Fig. 2] Fig. 2 shows a graph (a) representing the change in the irradiance diameter in the laser spot welding process according to a comparative example, a graph (e) representing the upper and lower columns and the weldable area of ​​the comparative example, graphs (b) to (d) representing the change in the irradiance diameter in the laser spot welding process according to examples of the disclosure, and graphs (f) to (h) representing the upper and lower columns and the weldable area of ​​the laser spot welding process of the examples. [Fig. 3] Fig.Figure 3 shows graphs (a) and (b) representing the change in the irradiation diameter, and graphs (c) and (d) representing the upper and lower columns and the weldable area in laser spot welding according to preferred examples of the disclosure. [Fig. 4] Figure 4 is a schematic graph showing the change in the irradiation diameter and the progress of a welding process in laser spot welding according to the embodiment of the disclosure. [Fig. 5] Figure 5 is an enlarged cross-sectional view of the welding area of ​​laser spot welding according to the embodiment of the disclosure. [Modes for carrying out the revelation] The following describes embodiments of the disclosure in detail with reference to the drawings. Parts (a) to (c) of Fig. 1 depict a laser spot welding process 10 according to an embodiment of the disclosure for three metal plates 11, 12 and 13. In part (a) of Fig. 1, the metal plates 11, 12 and 13, which have thicknesses of t1, t2 and t3, are arranged overlapping with gaps ga and gb between them. The gaps ga and gb are gaps whose intervals are adjusted by overlapping the metal plates 11, 12 and 13 over projections (not shown) that are formed in advance by pressing on some of the metal plates 11, 12 and 13 (usually the metal plates 12 and 13 that are located on the lower side of gaps ga and gb), or by overlapping the metal plates 11, 12 and 13 over spacers (not shown) that are inserted between the metal plates and hold them with clamps or other tools if necessary, and / or by gaps whose intervals are caused by springback at flange areas or the like of pressed parts and are therefore not adjusted. The metal plates 11, 12, and 13 are not restricted to specific thicknesses but are assumed to be thin steel plates with thicknesses ranging from 0.6 to 2.0 mm. In experiments described later, the thicknesses t1, t2, and t3 of the steel plates were 0.6 mm, 0.8 mm, and 1.2 mm, respectively. In cases where the metal plates have surface treatment layers of a low-melting-point metal, such as an electroplated layer, on their interfaces, gaps with adjusted intervals as described above are intentionally provided to discharge metal vapor. In cases where the metal plates do not have surface treatment layers of a low-melting-point metal, the metal plates can be stacked directly on top of each other without the intervening gaps ga and gb. When laser spot welding 10 is performed, a laser processing head is first positioned over the metal plate 11, which is located on the top surface, and a laser irradiation L1 is carried out with the fixed optical axis at a constant power with a defocusing fraction d1 to heat the three metal plates 11, 12 and 13 at a point S1, thereby forming a welding area W1 (a molten area at that time). This point S1 is an irradiation area with the smallest surface area (and the highest energy density) during a welding process. Point S1 penetrates the metal plates 11 and 12, which are the two closest to the uppermost surface of the three metal plates 11, 12, and 13 to be welded, with a minimum required laser power, and sufficient melting depth can also be achieved in the bottommost metal plate 13. The focus is then controlled by the laser welder's optical system, with the optical axis remaining fixed. The defocus portion is gradually increased from d1 to d2, as indicated by symbol Ws in part (c) of Fig. 1, to gradually increase the laser irradiation diameter to φ2, while maintaining constant power for laser irradiation (L1 to L2). The molten area is extended to W2, and laser irradiation L2 is terminated when point S2 is reached. This point S2 is an irradiation area with the largest surface area (and therefore the smallest energy density) during the welding process. Although the energy density of the laser irradiation gradually decreases as the laser irradiation diameter expands from φ1 to φ2, thus increasing the irradiation area from S1 to S2, the heat transfer from the central region to the surrounding areas, which occurs during this process, promotes stable melting within the irradiation area S2. This results in the production of the final weld area W2, which corresponds to the laser irradiation diameter φ2. It should be noted that in the case where the metal plates 11, 12 and 13 have surface treatment layers made of a low-melting-point metal, the metal vapor caused at the molten area and its surroundings is scattered and discharged through the gaps ga and gb, along with the heat transfer from the central area to the surrounding areas described above and the expansion of the laser irradiation diameter. As described above, in laser spot welding 10, the laser irradiance diameter is varied with the fixed optical axis so that the laser irradiance L1 with the smallest irradiance diameter φ1 provides the sufficient melting depth at the center area (S1, W1), and the laser irradiance L2 with the largest irradiance diameter φ2 provides the desired spot diameter (S1, W2). As a result, laser spot welding 10, which is a simple process that does not involve scanning the optical axis of the laser, provides the desired joint strength and also offers the advantage of a significant increase in the permissible gap areas ga and gb between the metal plates 11, 12, and 13. Next, experiments were conducted to verify the advantageous effect of laser spot welding 10 according to the embodiment. In these experiments, laser spot welding was performed, with the gaps ga and gb between the metal plates 11, 12, and 13, and a combination thereof, being varied for each different pattern of laser irradiation diameter, and the permissible ranges of the gaps were compared. Steel plates with thicknesses of t1 = 0.6 mm, t2 = 1.2 mm, and t3 = 0.8 mm were used as the metal plates 11, 12, and 13 in the experiments, with the uppermost surface (the laser irradiation side) being used. The laser was applied at a laser power of 2 kW for 0.4 seconds, with the defocus ratio being varied within the range of 30 to 90 mm and the laser irradiation diameter within the range of 1.8 to 5.0 mm. comparative example First, as a comparative example, laser spot welding was performed by shining the laser for 0.2 seconds with a defocus area d1 = 30 mm. Then, the defocus area was increased to d2 = 90 mm, and the laser was shine for 0.15 seconds, as shown in part (a) of Fig. 2. The gaps ga and gb between the metal plates and their combinations were varied to investigate the permissible gap widths. Part (e) of Fig. 2 shows the result, where the hatched combinations in the diagram indicate the permissible gap sizes in which a good welding result was achieved. In the case where the upper gap ga is 0, the lower gap is permissible up to gb = 1.0 mm. In the combinations where both ga and gb have some gaps, the sum of the gaps is approximately 0.9 mm. Although in some combinations increasing the laser irradiation time showed some improvement, it can be seen that there is a difference in the range where the lower gap gb is large, compared to the permissible gap range of Example 1 (described later), which is indicated by the thick lines in the figure. Example 1 Next, as Example 1 according to the disclosure, laser spot welding was carried out such that the laser was applied for 0.4 seconds, with the defocus portion increasing from d1 = 30 mm to d2 = 90 mm at a constant rate, as shown in part (b) of Fig. 2. The gaps ga and gb between the metal plates and combinations thereof were varied to investigate the permissible gap ranges. Part (f) of Fig. 2 shows the result of Example 1. In comparison to the comparison example described above, the lower gap is permissible up to 1.0 to 1.1 mm in the area where the lower gap gb is large, and the permissible area is extended to the area where the sum of the upper and lower gaps is 1.2 to 1.3 mm. Example 2 Next, as Example 2 according to the disclosure, laser spot welding was performed such that the laser was applied for a total of 0.4 seconds, during which the defocus portion was increased from d1 = 30 mm to 40 mm in 0.2 seconds at a relatively shallow rate, and then the defocus portion was increased to d2 = 90 mm in the next 0.2 seconds at a relatively steep rate, as shown in part (c) of Fig. 2. The gaps ga and gb between the metal plates and combinations thereof were varied to investigate the permissible gap ranges. Part (g) of Fig. 2 shows the result of Example 2. Although the permissible range of the gap is extended and larger than the preceding comparative example, the permissible range is smaller than in the above described Example 1 by about 0.2 mm in the area where the lower gap gb is large. Example 3 Next, as Example 3 according to the disclosure, laser spot welding was performed such that the laser was applied for a total of 0.4 seconds, during which the defocus portion was increased from d1 = 30 mm to 50 mm in 0.1 seconds at a relatively steep rate, and then the defocus portion was increased to d2 = 90 mm in the next 0.3 seconds at a relatively shallow rate, as shown in part (d) of Fig. 2. The gaps ga and gb between the metal plates and combinations thereof were varied to investigate the permissible gap ranges. Part (h) of Fig. 2 shows the result of Example 3. In contrast to the preceding Example 2, Example 3 showed a slightly better result than Example 1 in the area where the sum of the upper and lower columns is large. The results of Examples 1 to 3 described above show that, when welding in cases where both upper and lower gaps exist, it is advantageous to apply the laser irradiation L1 with the smallest irradiation diameter (φ1) for a very short time and then gradually increase the irradiation diameter to cover larger permissible gap areas and to stably form a preferred weld spot. In particular, the comparison between Example 2 and Example 3 indicates that a relatively rapid increase in the irradiation diameter in the first half of the welding process and a relatively slow increase in the irradiation diameter in the second half of the welding process yields a more favorable result.To verify this trend, additional experiments were conducted to compare the permissible range of the gap in which laser spot welding was performed, changing only the pattern of change of the laser irradiation diameter. Example 4 First, as Example 4 according to the disclosure, laser spot welding was performed such that the laser was applied for a total of 0.4 seconds, during which the defocus portion was increased from d1 = 30 mm to 60 mm in 0.1 seconds at a steeper rate than in Example 3, and then the defocus portion was increased to d2 = 90 mm in the next 0.3 seconds at a shallower rate than in Example 3, as shown in part (a) of Fig. 3. The gaps ga and gb between the metal plates and combinations thereof were varied to investigate the permissible gap ranges. Part (c) of Fig. 3 shows the result of Example 4. Compared to the preceding Example 3, the combination of an upper gap ga of 0.3 mm and a lower gap gb of 0.9 to 1.0 mm proved to be faulty. However, in the case where the upper gap ga is less than or equal to 0.2 mm, the permissible range of the lower gap gb was extended to 1.3 to 1.4 mm, which shows that Example 4 is advantageous in the case where the lower gap gb is large. Example 5 Next, as Example 5 according to the disclosure, laser spot welding was performed such that the laser was directed for a total of 0.4 seconds, during which the defocus portion was increased from d1 = 30 mm to 70 mm in 0.1 seconds at a steeper rate than in Example 5, and then the defocus portion was increased to d2 = 90 mm in the next 0.3 seconds at a shallower rate than in Example 5, as shown in part (b) of Fig. 3. The gaps ga and gb between the metal plates and combinations thereof were varied to investigate the permissible ranges of the gaps. Part (d) of Fig. 3 shows the result of Example 5. Although, as in the preceding Example 4, the combination of an upper gap ga of 0.3 mm and a lower gap gb of 0.9 to 1.0 mm was faulty, it was found that in the case where the upper gap ga is 0.6 to 0.7 mm, the permissible range was extended. Furthermore, it was found that in the case where the upper gap ga is less than or equal to 0.2 mm, the permissible range of the lower gap gb was extended to 1.3 to 1.5 mm, and therefore Example 5 is advantageous in the case where the lower gap gb is large. Discussion about an optimal design Fig. 4 represents an optimal change pattern of an irradiation diameter in laser spot welding according to the disclosure, which results from the findings of Examples 1 to 5. The optimal embodiment, which focuses on this pattern, is discussed below with reference to Fig. 4. First, as indicated by the continuous lines in Fig. 4, a first step 21 is carried out and completed, in which the laser irradiation L1 is started from the smallest irradiation diameter φ1 (defocusing fraction d1) in the process that can melt the three metal plates 11, 12 and 13 by heating, and then the process proceeds to a second step 22 of gradually increasing the irradiation diameter. The second step 22 includes a first section 22a in which the laser irradiation diameter is increased from the smallest irradiation diameter φ1 at a first rate v1 and a second section 22b in which the laser irradiation diameter is increased to the largest irradiation diameter φ2 at a second rate v2 which is smaller than the first rate v1. Between this first section 22a and the second section 22b, one or more intermediate sections can be set in which the irradiation diameter is increased at a rate between the first rate v1 and the second rate v2. Therefore, the second step 22 is a section in which the rate of increasing the irradiation diameter (v1 to v2) is gradually or stepwise decreased, while the laser irradiation diameter is increased from the smallest irradiation diameter φ1 to the largest irradiation diameter φ2. It should be noted that, since the first section 22a is the section immediately following the first step 21, it is preferable to proceed to the second section 22b in the shortest possible time, and it is advantageous for the first section 22a to be shorter than the second section 22b with respect to the expansion of the molten region. The energy density of a laser irradiation is inversely proportional to the irradiation area, and the irradiation area is proportional to the square of the irradiation diameter. Therefore, if the irradiation diameter is increased at a constant rate, the energy density decreases along with the expansion of the irradiation area at a progressive rate.Thus, the molten area is expected to be efficiently expanded by rapidly increasing the irradiation diameter in the first half of the second step 22 and gradually decreasing the rate of increasing the irradiation diameter in the second half, since the rate of expanding the irradiation area and the rate of decreasing the energy density can be made nearly constant. After the second step 22, the laser irradiation L2 can continue while maintaining the largest irradiation diameter φ2, and this section is referred to as the third step 23. The third step 23 serves the purpose of an adjustment process to homogenize and stabilize the molten metal by heating the molten area W2, which has already reached the desired spot diameter (φ2). This is also evident from the fact that extending the irradiation time favored the weld area in the comparative example. However, as can also be clearly seen from the results of Examples 4 and 5, the third step 23, which does not contribute to the enlargement of the spot, is either unnecessary or a very short time is sufficient, since the preceding second section 22b of the second step 22 fulfills an identical function. In the case where preferential melting cannot be achieved by heating in the first step 21, in which the laser irradiation L1 is initiated, due to a high thermal capacity, such as a case where the metal plates 11 and 12 closer to the uppermost surface have large thicknesses, it is more advantageous than increasing the laser power to set the initial defocus fraction smaller than d1 and close to a focused state, and to initiate the laser irradiation with a smaller irradiation diameter, as indicated by 21' in Fig. 4. In this case, the defocus fraction is increased to enlarge the laser irradiation diameter immediately after the laser irradiation is initiated. Therefore, instead of performing the first step 21, which involves a short-term continuous irradiation with the smallest irradiation diameter φ1 (defocusing fraction d1), it is advantageous to perform a first step 41 of heating and melting the metal plates with a gradual increase in the irradiation diameter immediately after the start of the laser irradiation with the smallest irradiation diameter φ1 (defocusing fraction d1) and then to proceed to a second step 42 in which the rate of increase of the irradiation diameter is small, as indicated by the dashed lines in Fig. 4. Although, in addition to the first step 21 and the third step 23, the second step 22 may involve continuous irradiation with a constant irradiation diameter, or in the second step 22 the irradiation diameter (defocusing component) may be gradually increased by alternating continuous irradiation with a constant irradiation diameter and increasing the irradiation diameter, it is preferable to continuously change the irradiation diameter in order to efficiently expand the molten area along with heat transfer. Example of a welding area Fig. 5 is a cross-sectional view of a welding area where three metal plates 51, 52, and 53 are welded by laser spot welding. The metal plates 51, 52, and 53 have thicknesses of 0.8 mm, 1.2 mm, and 0.6 mm, respectively, and the upper gap was 0.5 mm and the lower gap was 1.6 mm. The laser was applied with a defocus of d1 = 10 mm for 0.2 seconds, 20 mm for 0.05 seconds, and 40 mm for 0.2 seconds. The laser was then applied continuously for 0.8 seconds, with the defocus gradually increasing to d2 = 90 mm. As a result, a welding area with a 50W effective spot diameter was achieved. Although this example is a special case where the lower gap is larger than the thickness of the plates, it has been confirmed that welding is possible even in cases where such a gap exists. It should be noted that in this example, due to the challenging gap conditions, the control was implemented to gradually increase the defocus percentage (irradiation diameter). However, in practical laser spot welding, which, as in the previous examples, takes 0.2 to 0.4 seconds to complete, there is no significant difference in the welding result between a control that gradually increases the defocus percentage (irradiation diameter) via preset average defocus percentages and a control that continuously increases the defocus percentage (irradiation diameter). It is simply a matter of setting the parameters.Additionally, although depending on the specifications of the laser welder (processing machine) the laser irradiation can be stopped for a very short time when the defocus ratio is changed, it has been confirmed that even in this case it makes no significant difference in the welding result. Although the foregoing embodiment provides a description for the case in which the defocusing component d1 to d2 is changed by controlling the optical system of the laser, the defocusing component can be changed by mechanically moving the position of the laser processing head up and down (linear movement). Additionally, although the preceding embodiment described the case where laser spot welding is performed on two or three metal plates, four or more metal plates can be used for laser spot welding. Although it has only been experimentally confirmed that the present laser spot welding process can be applied to plates with a total thickness of up to 4.2 mm, it is likely that the present laser spot welding process can be applied to a thickness greater than this, depending on the conditions, such as the laser power. Additionally, although the preceding embodiment depicts the case in which the laser is directed perpendicularly from above onto the metal plate 11 at its uppermost surface, the same process characteristics can be achieved in the case of an incidence angle of up to 40 degrees, in which case the welding spot is elliptical. Furthermore, the present laser spot welding process can be used not only on a horizontal plane but at any angle. Although the description has been provided for some embodiments of the disclosure, the present invention is not limited to the embodiments mentioned above. [List of reference symbols] 10 Laser spot welding 11, 12, 13 Metal plate 21, 21', 41 First step (heating and melting) 22, 42 Second step (expanding a molten area) 22a First section 22b Second section 23, 43 Third step (adjustment) d1, d2 Defocusing fraction ga, gb Gap L1, L2 Laser irradiation S1, S2 Spot φ1, φ2 Laser irradiation diameter W1, W2 Molten area

Claims

Laser spot welding process for overlapping metal plates (11, 12, 13), comprising in sequence: a first step (21) of beginning to irradiate the overlapping metal plates (11, 12, 13) with a laser (L1) having a first irradiation diameter (φ1) in a state in which an optical axis of the laser is set to a predetermined area of ​​the metal plates (11, 12, 13) to heat the metal plates (11, 12, 13) to melt them;and a second step (22) to continuously irradiate the metal plates (11, 12, 13) with the laser (L1-L2) after the first step (21), with a gradual or stepwise increase of an irradiation diameter of the laser from the first irradiation diameter (φ1) to a second irradiation diameter (φ2) in the state in which the optical axis of the laser is set to the predetermined surface, in order to expand a molten area (W1-W2) of the metal plates (11, 12, 13), wherein the increase of the irradiation diameter (φ1-φ2) is provided by increasing a defocusing component (d1-d2) of the laser. Laser spot welding method according to claim 1, wherein the second step (22) comprises a first section (22a) in which the irradiation diameter of the laser is increased from the first irradiation diameter (φ1) at a first rate (v1), and a second section (22b) which lies after the first section and in which the irradiation diameter of the laser is increased to the second irradiation diameter (φ2) at a second rate (v2), and wherein the first rate (v1) is higher than the second rate (v2). Laser spot welding method according to claim 1, wherein in the second step (22), when the irradiation diameter of the laser is increased from the first irradiation diameter (φ1) to the second irradiation diameter (φ2), a rate of increase of the irradiation diameter is gradually or stepwise reduced. Laser spot welding method according to claim 2, wherein a laser irradiation time in the first section (22a) is shorter than a laser irradiation time in the second section (22b). Laser spot welding process for overlapping metal plates (11, 12, 13), comprising in sequence: a first step (41) of beginning to irradiate the overlapping metal plates (11, 12, 13) with a laser (L1) with gradual or stepwise increase of an irradiation diameter of the laser from a first irradiation diameter (φ1) in a state in which an optical axis of the laser is set on a predetermined area of ​​the metal plates (11, 12, 13) to heat the metal plates (11, 12, 13) to melt;and a second step (42) to continuously irradiate the metal plates (11, 12, 13) with the laser (L1-L2) after the first step (41), with a gradual or stepwise increase of the irradiation diameter of the laser to a second irradiation diameter (φ2) in the state in which the optical axis of the laser is set to the predetermined surface, in order to expand a molten area (W1-W2) of the metal plates (11, 12, 13), wherein the increase of the irradiation diameter (φ1-φ2) is provided by increasing a defocusing component (d1-d2) of the laser. Laser spot welding method according to claim 5, wherein a rate (v1) of increase of the irradiation diameter in the first step (41) is higher than a rate (v2) of increase of the irradiation diameter in the second step (42). Laser spot welding method according to one of claims 1 to 6, wherein a laser irradiation time in the first step (21, 41) is shorter than a laser irradiation time in the second step (22, 42). Laser spot welding method according to one of claims 1 to 7, wherein the power of the laser is substantially constant during the first and second steps.

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