RU2789410C1 - Method for creating a rainbow effect on the surface of a material and apparatus for implementing the method - Google Patents

Abstract

FIELD: laser technology.

SUBSTANCE: inventions relate to a method and apparatus for creating a rainbow effect on the surface of an item (1). Laser beams (9) with a pulse duration below one nanosecond are directed at said surface in adjacent optical fields of the focusing systems (14) of at least two stationary apparatus or in the optical region of at least one mobile apparatus. Each of said apparatus comprises a laser emission source (8), a scanner (13), and said focusing system (14) for applying a structure in the form of small waves onto said surface to the width of said pulse. Said surface is scanned with at least one scanner (13) using said laser beams (9) along a group of lines (5, 6; 16, 17, 18, 16’, 17’, 18’) arranged in a sequence in the direction (7) of relative movement of the item (1) and at least one scanner (13), and along a group of lines arrange so as to continue each other in a direction perpendicular to said direction (7) of relative movement, with each of said lines belonging to the optical field of one or multiple apparatus. Each line (5, 6; 16, 17, 18, 16’, 17’, 18’) has a width equal to the diameter of said pulse. The optical fields overlap in the overlap region and have a width in the range between double the diameter of the laser beam pulse (9) and 2 cm, so that two lines arranged so as to continue each other overlap at the articulation (2). Between two groups of lines (5, 6; 16, 17, 18, 16’, 17’, 18’) arranged so as to continue each other in the direction (7) of relative movement of the item (1) and at least one scanner (13), said articulation areas (2) are positioned in a random arrangement or in a periodic manner according to a random pattern inside said optical field overlap region.

EFFECT: processing of a larger surface area (in most cases, the entire surface) than would be possible in the case of processing using a single optical field, leading to an increase in the productivity of processing items with a large surface area.

18 cl, 3 dwg

Description

The present invention relates to laser treatment of the surfaces of a sheet of stainless steel or other materials, designed to impart an iridescent appearance to these surfaces.

Rainbow effect treatment, also referred to as "LIPPS" (LIPPS - laser-induced periodic surface structure) or "ripples" (ripples - small waves), refers to the processing of the surface of a material by exposing it to pulsed laser radiation with a short pulse duration (less than one nanosecond). The diameter of each pulse at the point of impact on the material to be processed is approximately 10 to several hundred microns. If the energy of the incident beam is high enough, this exposure to radiation causes a modification of the structure and/or a transformation of the surface of the material, on which a periodic structure is formed. However, if the beam energy is too high, an ablation phenomenon may occur due to the occurrence of evaporation/sublimation/shock wave, preferably together with the formation of a periodic surface structure. The range of the amount of energy to be used can be easily determined experimentally for a given material in order to produce the desired iridescent effect without changing the state or gloss of the surface.

The indicated processing, in particular, but non -excitingly, is performed on stainless steels of all types. The purpose of processing can be purely aesthetic, but processing also provides a change in wettability, as well as the resistance of friction and a decrease in bacterial adhesion. This processing can be performed directly on the surface of an object that has a passivating layer on stainless steel, without the need for preliminary activation/depassivation.

Other materials on which this processing is carried out are, in particular, metals, polymers, such as PVC, ceramics, glass.

In the remainder of this document, priority is given to stainless steels on the basis that the invention relates to all metallic or non-metallic materials that are currently known or may become known in the future, with the aim of creating an iridescent effect after laser processing, performed as above, according to discretion by adjusting the exact operating parameters of the setup (laser power and frequency), which are known to play a role in creating the rainbow effect as a result of the formation of a periodic surface structure.

Although the exact mechanism for the formation of this periodic surface structure has not yet been determined, tests and characterizations by various laboratories show that, depending on the number of laser passes and/or pulse energy and/or scan parameters, the surface structure can be one of four the following structures, depending on the total radiation energy per unit surface area, these structures are classified in order of increasing energy, and their name is commonly used even by non-English speakers in this field:

1) The so-called HSFL structure (High Spatial Frequency LIPSS):

This structure consists of small waves (ripples), which, as applied to stainless steels, are oriented in the direction of polarization of the laser beam incident on the surface. The spatial frequency of these waves is less than the wavelength of the laser used for processing.

2) The so-called LSFL structure (LIPSS with low spatial frequency):

This structure consists of larger waves (larger ripples) than in the previous structure and they are oriented in relation to stainless steels in the direction perpendicular to the polarization of the laser beam incident on the surface. The spatial frequency of these waves is slightly less than or greater than or equal to the wavelength of the laser. With regard to stainless steel surface treatment with a 1064 nm laser, the frequency of the waves is approximately 1 µm. HSFL structure can still be visible in the grooves of the LSFL structure.

It should be noted that for some materials, the respective orientations of the HSFL structure and LSFL structure may be reversed compared to structures for stainless steels.

3) The so-called "Grooves"-structure or "Bumps"-structure:

This structure consists of micrometer-sized protrusions covering the entire machined surface. These protrusions are formed into a structure resembling "snakeskin" in appearance.

4) "Spike"-structure:

This structure consists of spikes ranging in height from a few microns to several tens of microns. The distance separating the spikes depends on the processing parameters.

Additional characteristics of structures and the mechanism of their occurrence are described, in particular, in the article "Evolution of nano-ripples on stainless steel irradiated by picosecond laser pulses", Journal of Laser Applications 26, February 2014, by B. Liu et al. The article states that for an equal number of pulses, an increase in the radiation density leads to the creation of an HSFL structure, and not an LSFL structure (as noted above), while for an equal density, a greater number of pulses leads to the creation of an LSFL structure, and not an HSFL -structures, up to the moment when the number of pulses becomes too high to observe waves. The precise configuration of the surface after exposure to radiation is achieved by a mechanism involving both a certain number of received pulses and the energy supplied by each of them, for a given material. This mechanism is complex, but for a given material, the user can experimentally determine reliable conditions for obtaining one or the other of the above configurations.

In general, in the first two cases, this formation of a periodic surface results in a phenomenon well known to operators performing laser surface treatment, which is the diffraction of light by creating an optical grid when the workpiece is placed under a light source. Depending on the orientation and position of the user and the light, rainbow colors may be visible on the sample. This is what is known as the "rainbow effect"

This effect no longer exists when the surface of the sample has a distinct appearance related to the third or fourth above cases, because in these two cases the energy supplied by the laser source to the surface of the sample reaches a level that is excessive, at least locally. , causing surface deformations that no longer allow the rainbow effect to be created, as the surface structuring is no longer periodic.

This iridescent coloring should not be confused with the surface coloring of stainless steels, which is obtained whether it is intentional or not, through plasma treatment or surface oxidation through furnace treatment or welding torches. The iridescent effect considered in the present invention is not achieved by coloring the material in the strict sense, but due to the appearance of colors on the surface under certain viewing conditions. The lack of periodicity in the surface structure of the painting processes, collectively referred to as appropriate, is a significant difference between the iridescence of the surface of the present invention and the painting of stainless steels using plasma and furnace treatment or welding torch treatment.

However, it should be noted that the observation or non-observation of this iridescence is usually associated with high directivity, i.e. the observation of this iridescence and the intensity of the observed iridescence is largely dependent on the angle at which the surface of the material is observed.

The problem faced by operators during surface iridescence is the following.

It is currently possible to obtain homogeneous samples in the laboratory with iridescence processing, using either only a system that connects a laser and a scanner to each other, creating both an axis of fast movement of the laser beam (using a polygonal wheel or a galvanometric mirror) and an axis of slow moving a laser beam (using a galvanometric mirror), or a system of laser and scanner connected to a mechanical arm that moves the scanner along a slow axis relative to the object to be processed.

Moving the scanner along the slow axis can be replaced by moving the object to be processed, such as a metal sheet, along the slow axis in front of the laser, which remains stationary on the slow axis and moves along the fast axis. It can also be provided that the laser remains stationary along both axes (slow axis and fast axis) and the object to be processed moves along both axes. What is essential is the relative movement between the object to be processed and the laser sequentially along both axes.

The mechanism of structure formation described above depends on the total energy transferred to the surface of the material and on the spatial and temporal distribution of this energy. Thus, the "intensity" of iridescence obtained with LSFL structures increases between each new laser pass on already processed passes until a maximum is reached, after which it decreases as the LSFL structures gradually become "protrusions" under the action of additional applied energy.

This means that there is an energy optimum that must be transferred to the surface of the material, i.e. the optimum for which the rainbow effect is most intense, and this optimum must be determined and applied to the entire surface under consideration.

However, such processed objects are typically small in size and/or manufactured at low throughput.

The limitation on the size of objects is mainly due to the limited size of the optical fields of the nodes formed by the laser, the scanner and the focusing system, the latter being a lens or a converging mirror. Ensuring a uniform finish requires proper control of the finish at every point on the surface. However, regardless of the focusing systems used, they all have an optical field in which they have a stable effect within the optimal region. But shortly after leaving this optimal area, the system initiates distortion and attenuation of the laser beam power. They lead to inhomogeneous processing between the optimal region of the optical field and regions outside this optimal region.

Therefore, for processing large surfaces of stainless steel sheet, wide field focusing systems are required, which are large and assembled only on request. In addition, they must be used in conjunction with lasers with ultra-short pulse duration and high power and are not yet widely available on the market.

To eliminate this double drawback, the known technical solutions use traditional focusing systems and lasers currently on the market, and:

- or place several devices containing these focusing and laser systems, side by side, for linear processing of the moving strip;

- or perform processing for several times (on successive strips distributed over the surface, to set the group processing);

- or combine these two technical solutions.

However, this solution requires particularly careful regulation of the interface areas between the optical fields of the two serial devices, which, if improperly regulated, can cause a phenomenon known to those skilled in the art as "crosslinking", which is described below.

This mechanism eliminates the need for a very large field overlap to couple two consecutive laser processing fields to each other.

In the case of a very large field overlap on the order of the resolution of the human eye, this means that the overlap region receives twice as much energy as is transmitted to the rest of the surface. This doubling of energy transmitted during processing causes a local change in the structure and thus appearance of the surface compared to areas that receive only a nominal amount of energy during processing, and this change is visible to the naked eye. This phenomenon is more commonly known as "crosslinking" and can make the junction between two fields visible.

Conversely, a distance between the fields of laser processing, which could reliably eliminate this phenomenon, would entail the formation between the two fields of an untreated zone or a zone that would in any case be less processed than in normal mode. This zone would also be visible to the naked eye.

Thus, an almost perfect articulation is required between successive processing fields.

In addition, performing this type of processing with high performance involves scanning at high speeds (at least up to km/s). The scanning systems used for this type of processing are in most cases scanners having at least one polygonal wheel. At high laser frequencies and high scan speeds, these systems generally tend to have synchronization problems between the laser electronics and the scanner electronics. These timing differences cause a shift in the position of the first pulse relative to its target position and thus the entire line. Even though this difference is estimable and calculable (because it is obtained from the difference between the control frequencies of two equipment components), it occurs in most existing systems and can correspond to a difference of several tens of microns between the beginning of the processing lines (lines related to the movement of a polygonal wheels). This difference depends on the number of revolutions of the polygon and the natural frequency of the laser, and experience has shown that overlapping fields with this difference is quite sufficient to create a zone in which double processing is performed to obtain the iridescent effect of the metal sheet.

Some systems in development include internal means to partially correct for this offset through the action of an additional reflective mirror, referred to as a "galvo mirror", which acts like a galvanometer located upstream of the polygon in the process flow. For example, RAYLASE presented the concept of this system at the SLT 2018 congress in Stuttgart on June 5 and 6, 2018: “New Generation of High-Speed Polygon-Driven 2D Deflection Units and Controller for High-Power and High-Rep. Rate Applications" (presentation by E. Wagner, M. Weber and L. Bellini).

However, this improvement by itself is not sufficient to permanently eliminate the negative effects of field shifting. There is a risk that the start and end of each line is not treated with the same energy as the rest of the line. The corresponding lines of two adjacent fields must also be strictly aligned.

To address a local processing disadvantage, it is possible to increase the energy supply to the rest of the line, but this would run the risk of exceeding the maximum amount of energy configured to create LSFL structures, and reducing or even incapacitating iridescence. For all other systems, this lack of synchronization entails the need for an "imaginary" overlap of the order of at least a twofold spread in the position of a line that starts between different optical fields. This overlap therefore translates into a patchy swath that has no untreated areas between fields, but may have an overlap of two times the specified spread at some points.

If the edges of each field are defined as "straight lines", and therefore if the length of each line of each field is consistently the same, as would be expected to be apparent to those skilled in the art, the area of overlap will be in the form of a thin rectangular strip covering the article, in in the direction of relative movement of the product and laser scanners, and having a width substantially equal to the width of the processing lines and thus essentially equal to twice the pulse diameter, on which the appearance of the processing will not be the same as on the rest of the surface. Similarly, if the edges of the treatment area are defined by a periodic pattern, the overlap area will reproduce that pattern and be visible to the naked eye for all that.

The object of the present invention is to provide a laser treatment method, with ultra-short pulses, for surface treatment of a product such as a stainless steel sheet, but not limited to, making the junction of several successive optical fields invisible to the naked eye, and these fields arranged in such a way that together they provide processing of a large area of the surface (in most cases the entire surface) than would be possible in the case of processing using a single optical field. This method must have adequate productivity so that it can be applied in an economically viable manner on an industrial scale to process articles with a large surface area.

To this end, the subject of the invention is a method for creating a visual iridescent effect on the surface of a product, according to which laser beams having a pulse duration of less than one nanosecond are directed to the specified surface in adjacent optical fields of focusing systems of at least two stationary devices or in the area of at least one of a movable device, wherein said device(s) comprise a laser source, a scanner and said focusing system for depositing a structure in the form of small waves on said surface for a specified pulse width, and said surface is scanned by said scanner(s) using said laser beams along the group consecutive lines in the direction of the relative movement of the product and the scanner (scanners) and along a group of lines lying on the continuation of each other in the direction perpendicular to the indicated direction of relative movement, and related to the optical field device (devices), and each line has a width equal to the diameter of the specified pulse; the method is characterized in that said optical fields of two fixed adjacent devices or optical fields corresponding to two successive positions of said moving device overlap in an overlap region with a width between twice the laser beam pulse diameter and 2 cm, so that the two lines located on continuation of each other, overlap at the junction, and between two groups of serial lines in the direction of the relative movement of the product and the scanner (scanners), these junction zones are scattered or organized periodically in an arbitrary pattern with a frequency equal to at least ten times the maximum value of the displacement between places joints on two consecutive lines in the specified direction of relative movement within the specified region of overlap of the optical fields.

Between scanning along two consecutive lines in the specified direction of relative movement of the product and the scanner (scanners), it is possible to change the polarization of the laser beam in such a way as to create waves of different orientations on these two consecutive lines and with a common pairwise orientation in a direction perpendicular to the direction of relative movement.

The polarization of the laser beam can be changed according to a periodic pattern, said periodic pattern extending for M successive lines in the direction of the relative movement of the product and scanners, and M is at least 2, preferably at least 3.

Two consecutive lines in the direction of relative movement of the product and scanners have polarization angles that differ by at least 20°.

The polarizations of two lines of two adjacent fields that lie on each other's extension can be identical.

Said relative movement of said surface of said article and devices emitting said laser beams can be achieved by placing said article on a movable support.

Said relative movement of said surface of said article and devices emitting said laser beam(s) can be achieved by placing said device(s) on a movable support.

Said article may be a metal sheet.

Said surface of said article may be three-dimensional, in which case the distance between the focusing system and the surface of the article is measured, and the focusing system is adjusted to provide substantially the same pulse diameter and laser beam energy density, regardless of the actual distance between the focusing system and the surface of the article. product.

Said surface of said article may be three-dimensional, in which case the distance between the focusing system and the surface of the article is measured, and the relative position of said device and said surface is adjusted so that when said method is carried out, the distance between said focusing system and said surface remains the same. same.

The specified product can be made of stainless steel.

Another object of the invention is a device for giving a rainbowing surface of the product by forming small waves on the specified surface using a laser ray pulse, and the indicated device is formed at least two motionless related individual devices or at least one mobile separate device, each of which contains The source of laser radiation generating a laser beam with a pulse duration is shorter than 1 ns, the optical system of forming a beam, a scanner made with the possibility of a linear scanning of an optical field on the surface of the product with a laser beam with a pulse of a laser beam after passing it through a focus system, and the above optical fields of two adjacent individuals are blocked in width by the size in the range between the double diameter of the laser beam and 2 cm pulse and contain the joints of two lines, each of which is created by a separate device, and the means to create a relative one sternation in a given direction between the specified device and the specified product to perform processing at least on one section of the surface of the specified product; characterized by the fact that these scanners of the indicated individual devices are made with the ability to form the specified joints in such a way that together they form an arbitrary scheme or periodically organized arbitrary scheme with a frequency of at least tenfold maximum displacement between the joints on two sequential lines in the indicated direction of the relative direction movement within the specified area of overlap of the optical fields.

The optical systems of said separate devices may comprise an optical polarization system that provides a given polarization of said beam, and means for changing this polarization so that adjacent lines are obtained on said surface in a certain direction by means of pulses with different polarizations.

These separate devices can provide pulses of two adjacent lines having a difference in polarization of at least 20°.

These separate devices may include means for measuring the distance between the focusing system and the surface of the product, connected to the controls of the focusing system, so that the latter maintains a constant pulse diameter and a constant energy density on the specified surface, regardless of the specified distance.

Said separate devices may include means for measuring the distance between the focusing system and the surface of the product, connected to the focusing system controls that control the relative position of said device and said surface, ensuring that a constant distance is maintained between said focusing system and said surface.

Said means for providing relative movement in a given direction between said device and said article may comprise a movable support for the article.

Said means for providing relative movement between said device and said article may comprise a movable support for said individual devices.

As should be understood, the object of the invention is to make invisible or practically invisible joints between two opposite and facing surface lines formed by at least two stationary adjacent laser scanning devices, the respective laser scanners of which move along an axis referred to as " slow axis”, and the fields of these devices overlap slightly to eliminate the risk of non-processing or insufficient processing of these articulation zones. To this end, the points of articulation of said opposing and facing surface lines, each of which is formed by a laser scanning device, are randomly distributed (i.e., the respective fast axes of the devices are located on each other's extension) if two sets of lines that follow each other are considered. behind the other in the direction of relative movement of the product and laser scanners, referred to as the "fast axis", which is essentially perpendicular to the slow axis.

If necessary, the at least two fixed adjacent devices can be replaced by a single movable laser scanner that moves along the fast axis to successively obtain opposite and facing lines in optical fields corresponding to two consecutive positions of the movable device, which is technically equivalent to simultaneous use of several stationary adjacent devices, but the processing time increases.

In other words, the articulation points of the opposing and facing lines generated by two adjacent laser scanners (or a laser scanner that is moving at the same time) are not on a straight line oriented substantially along the slow axis, and thus in a manner substantially perpendicular to the scanning direction (fast axis) of the laser devices. They form a broken line of arbitrary shape or a periodic shape but built in an arbitrary pattern that repeats periodically (thereby eliminating a regular periodic pattern such as a sine wave) and remains within the overlapping area of the fields of two laser scanners with a common orientation, which is essentially , perpendicular to the specified scanning direction. Thus, the junctions between the optical fields of two sets of serial lines in this general orientation do not jointly form a linear pattern, and the pattern is less visible to the naked eye than if it formed a substantially straight line. This pattern is not, and preferably is not, a short period periodic pattern, which would also risk being visible to the naked eye. If the pattern is periodic, it is preferred that the length of said distance be at least ten times the maximum displacement between joints on two consecutive lines.

The width of the area within which these joints are located should preferably not exceed 2 cm. If this width is too small (less than twice the pulse diameter), there is a risk of forming a broken line, very similar to a straight line, and the risk of visibility of the joint may remain. If the width is too large, the effective optical field of the devices is reduced, thereby reducing the performance of the installation.

It is clear that if there is a need to use more than two laser scanning devices for processing the entire surface of the product, the invention is used sequentially for all pairs of next devices.

As an advantage, this method and related device can be used in conjunction with a method and related device related to the elimination or at least a very significant reduction in problems associated with excessive iridescence viewing directionality on the surface of stainless steel treated with the device. containing a laser scanner. Using this method, different polarizations of the radiation emitted by the laser are performed to form LIPSs on two lines arranged in series in the direction of the relative movement of the product and laser scanning devices. To obtain a popular effect, it is recommended to use at least three different polarizations used for a group of at least three consecutive lines.

The invention will become more clear after reading the following description with reference to the attached drawings:

fig. 1 shows the surface of a metal sheet on which the iridescence treatment according to the invention has been carried out using two adjacent laser devices forming lines extending each other along the rapid axes of the devices, with areas of overlap between the two lines arranged according to the preferred embodiment of the invention. , namely, on an arbitrary broken line whose general orientation is that of the slow axis rather than a straight line, which would essentially correspond to the slow axis, or on a periodic broken line generally oriented along the slow axis.

fig. 2 is a diagram of a device according to the invention, which provides the implementation of the method corresponding to the invention, in the optical field of the laser processing device, and also provides observation of the iridescence of the surface of a metal sheet, regardless of the viewing angle, according to a preferred embodiment of the invention;

fig. 3 shows the surface of a metal sheet resulting from a preferred embodiment of the method of the invention, which improves on the method used in the case of FIG. 1.

Thus, it is an object of the present invention to perform processing, the defects of which are difficult to detect by the human eye, which can easily detect short-pitch linear or periodic defects. In this case, if we assume that the optimal processing of the surface of the metal sheet 1 requires N successive laser passes along the same line corresponding to the fast axis of a given optical field to supply a certain amount of energy necessary and sufficient to obtain the required waves, for N superimposed lines offset randomly from the slow axis is identical from one pass of the laser to another.

In FIG. 1 shows this structure, made on a metal sheet 1. It can be seen that for a group of two passes (scan lines) using a scanner 13, corresponding to two fields of adjacent scanners 13, which are located one after the other and slightly overlap to avoid unprocessed areas on the surface metal sheet 1, points 2 of the junction of the corresponding optical fields of two groups of lines 3, 4, obtained using one of the two scanners, respectively, are not shifted linearly, but randomly between two lines 5, 6 or two groups of N superimposed lines, successively obtained along a slow axis 7, which is the direction of relative movement of the metal sheet 1 and the laser scanners 13. In other words, the respective articulations 2 of two opposite lines facing each other, each of which belongs to the group 3, 4, together form an irregular line or a short-pitch periodic pattern , and an arbitrary broken line, which is less easily distinguishable, h We eat a straight line or a broken line in which the displacements of the joints 2 are periodic and small.

It should be noted that between the two consistent lines 5, 6 formed in the same optical field and, thus, displaced in direction 7 of the movement (in other words, in the direction of the slow axis) of laser scanners 13 or in the direction of moving the metal sheet 1, If it is the sheet that moves along the slow axis, while the scanners are motionless, this problem, in general, does not arise with the same intensity, if the overlap between the lines of the two groups 3, 4 in series along the slow axis 7 is not obviously insufficient.

In fact, the relatively slow relative motion of the metal sheet 1 and the scanners 13 must be taken into account, as indicated by the arrow 7 defining the slow axis.

As already indicated, various lines 5, 6 of each group 3, 4 have a width, which, in essence, is equal to the diameter of the pulse, i.e. approximately in general 30 - 40 microns. In order to prevent the unprocessed zones between the two sequential lines 5, 6 of the same group 3, 4 along the slow axis 7, you can adjust the scanner and/or the device for moving the metal sheet on the surface of the metal sheet, so that the sequential lines 5, 6 along the slow axis 7 overlapped.

In other words, the lines 5, 6 of the same group 3, 4 are formed after a shift in the relative positions of the pulses of each scanner 13 and the metal sheet 1, the shift being slightly less than the diameter of the pulses. In the area of overlap of lines 5, 6, in fact, double surface treatment of the metal sheet 1 can take place, but since the displacement of lines 5, 6 along the slow axis 7 can be controlled with appropriate accuracy, and with much greater accuracy than the overlap of adjacent optical fields along the fast axis, the width of these areas, if any, is in any case sufficiently narrow (a few µm) so that the double treatment is not visually perceived as a distortion of the iridescent effect with respect to the effect obtained on the rest of the surface of the metal sheet 1. The choice can be deliberately made in this way , so as not to provide for an overlap between two successive lines 5, 6 along the slow axis 7, but to give a much smaller offset of the order of a few µm, in any case sufficiently reduced so that it does not cause unprocessed lines that would be visible to the naked eye in the direction , perpendicular to the slow axis 7.

It should be noted that in Fig. 1, each group of lines 3, 4, located one behind the other and meeting at joint 2, itself consists of an overlay of N superimposed lines, where, for example, N = 3. The number of superimposed lines for a given optical field depends on the amount of energy that must be be transferred to the surface of the metal sheet 1 to obtain the desired wave pattern, providing a rainbow surface effect. The greater this energy value, the greater the number of lines must be for the same amount of energy delivered for each pass of the laser.

To the extent possible, this structure is an LSFL structure of the type that has been seen to be more suitable for producing a rainbow effect under conditions that nevertheless depend on the viewing angle. The energy supplied along the given line should be in the range between a lower limit, below which the waves will not be sufficiently noticeable, and an upper limit, above which the probability of excessive presence of protrusions greatly increases. It is clear that the limits depend to a large extent on many factors, in particular, the correctly chosen material of the metal sheet 1, the condition of its surface, the energy supplied by pulses with each pass of the laser in a given zone. Those skilled in the art should carry out studies in due course to determine the specified limits depending on the equipment available and the material to be processed.

Although this first approach provides a significant reduction in the visibility of the overlap of two consecutive fields, depending on the material used and/or the target effect, since the overlaps between the fields (joint 2) are not located on a straight line, which corresponds to the slow axis 7, but on an arbitrary polyline line, only the general orientation of which essentially corresponds to the slow axis 7, and which corresponds to the offsets between overlaps, this may not be sufficient to obtain a proper uniform surface. In this case, the same approach can be used, but by changing the offset between different laser passes. This makes it possible to further increase the random nature of the overlap detection scheme compared to the previous case. In other words, the broken line connecting successive overlaps and forming the indicated pattern is a line of a non-periodic or random type, which is even less obvious. However, it should be ensured that adjacent processing fields have the same offsets as the first offset for each pass, since local accumulation of laser passes must be avoided to provide processing with a uniform appearance, so that ideally every point on the surface receives the same the amount of energy according to the same distribution, the same number of impulses and passes.

Thus, the use of a random overlay scheme for the edges of the fields ensures the distribution of points of inhomogeneity without the subsequent formation of a straight line or a broken line with a short step, which were very visible to the naked eye. When the pattern they create is identical for all passes of the laser along the fast axis forming a given line, these overlapping points are local points that have a high discontinuity, since the discontinuity of the line is noticeable on each pass.

As indicated, a certain periodicity of the articulation pattern 2 at the edge of the field is acceptable if it is obtained at a sufficient length along the slow axis 7, namely at least ten times the maximum displacement between the joints 2 on two consecutive lines 5, 6 along the slow axis 7.

Performing processing in the form of lines oriented along the fast axis allows you to take advantage of the high frequency of repetition of ultrashort pulses of the laser to increase processing performance. Thus, when a line is scanned once by a scanner along the fast axis, the line can be exposed to radiation N times if the distance between two pulses of two adjacent fields is equal to the pulse diameter divided by N. This eliminates the effect in which small power fluctuations could cause inhomogeneity surfaces. For example, to perform N successive exposures to treat the same point on the surface of the metal sheet 1, it is not necessary to perform N passes of the laser beam 9 on said point, and one pass of the laser beam 9 may be sufficient.

However, this mode of operation has a drawback, which consists in the formation of zones of inhomogeneity at the ends of the lines at distances equivalent to the pulse diameter (several tens of microns).

As indicated, the rainbow effect obtained by processing with ultrashort laser pulses 8 refers to the spontaneous formation on the surface of the metal sheet 1 of a periodic structure having a character similar to an optical grid in the light reflected from the surface. As described above, the mechanism for the formation of this structure in the form of waves periodically distributed over the treated surface has not yet been determined by the scientific community.

However, it was shown (see, for example, the document “Control Parameters in Pattern Formation Upon Femtosecond Laser Ablation”, Olga Varlamova et al., Applied Surface Science 253 (2007) PP. 7932–7936), which is mainly related beam radiating to the surface. For example, on stainless steel, the orientation of HSFL structures is parallel to polarization of the falling beam, while LSFL structures that are formed in the future, when a larger amount of energy is supplied to the surface, oriented perpendicular to polarization of the falling beam. On other materials, the opposite effect is observed, but this does not question the applicability of the invention to these materials.

In laser processing along lines, a surface treated without changing the polarization of the laser beam 9 during various passes along a given line of said surface would, at the end of processing, acquire a structure consisting of lines/waves oriented in the same direction. This means that the "optical grid" effect of the surface is also oriented.

The rainbow effect is maximum when the observation is made transverse to the wave orientation, and weakens when the observation orientation angle is aligned with the surface structure. Therefore, observing the surface in a straight line with the waves does not allow color to appear. This can be considered a disadvantage of the finished product, since an appropriate choice of wave orientation is required at the beginning of processing in order to obtain a product that has a rainbow effect under the required viewing conditions. In addition, the finished product has a full coloration only in the main viewing direction for a given light source.

The best embodiment of the invention, as shown in Fig. 2 and 3 eliminates this shortcoming. If two successive fields jointly forming the same line across the entire width of the metal sheet 1 (direction perpendicular to the slow axis 7) have the same polarization on this line, the visual effect of double processing of the articulation zone between these two fields tends to to be much less noticeable than when the two fields have different polarizations with a difference in polarization angle, said difference being preferably 20° - 90°.

In addition, in a preferred embodiment of the invention, having polarizations that are clearly sufficiently different between two successive lines in the direction 7 of the relative movement of the product and the laser scanners, the directionality of the observation of the rainbow effect is eliminated. The combination of the phenomena described means that the rainbow effect of the treated metal sheet 1 appears to be much more uniform in all viewing directions than when there is no polarization alternation between adjacent lines.

Processing is done "line by line" with a distance separating the centers of pulses that is slightly less than the diameter of the pulse in the direction of the fast scan to exclude areas not treated with the pulse. The technical solution according to the preferred embodiment of the invention is to alternate lines for which the orientation of the waves changes from one line to another, due to the action of a polarizer or any other type of polarizing device located in the optical path of the laser beam 9.

Therefore, the treatment field is obtained either by an automatic system for changing the polarization of the incident beam between the lines, or the treatment field is obtained in a number of M times equal to at least two and preferably at least three, and thus M corresponds to the number orientations given to waves by periodic successive polarizations of the pulse of the laser beam that forms these waves.

In FIG. 2 schematically shows a typical arrangement of a part of a separate device for implementing the method of the invention and related to the above-mentioned preferred embodiment, for processing a part of a stainless steel metal sheet 1 in a given field. It is clear that this device is controlled by automatic means that synchronize the relative movements of the support 15 of the sheet 1 and the laser beam 9 and, if necessary, adjust the parameters of the laser beam and focus it. The programming of these automatic means is within the ability of those skilled in the art.

The separate device in Fig. 2 primarily contains a laser radiation source 8 of a traditionally known type for creating iridescent effects on metal surfaces, and accordingly, the source 8 generates a pulsed laser beam 9 with a short pulse duration of less than one nanosecond, and the diameter of each pulse is of the order of 30 - 40 μm. The amount of energy supplied to the surface of stainless steel using a pulse must be determined experimentally in order to generate LIPSS waves on the surface of the metal sheet 1, preferably of the LSFL type, and prevent the formation of protrusions, and even more so spikes, and the frequency and power of the laser beam 9 must be selected according to criteria known to those skilled in the art and related to the precise characteristics of the other elements of the device and the material to be processed. The laser beam 9 generated by the source 8 passes through the optical system 10 that forms the beam 9, which, in addition to the traditional components that provide control of the shape and size of the beam 9, if necessary, according to a preferred embodiment of the invention, a polarizing optical element 12, which ensures the polarization of the beam 9, selected by the operator or automatic means that control the device.

Next, the laser beam 9 passes through a scanning device (eg scanner) 13, which, as is known, allows the beam 9 to scan the surface of the metal sheet along a straight path (fast axis) in the processing area. At the output of the scanner 13, as usual, there is a focusing system 14, for example a focusing lens, with which the laser beam 9 is focused in the direction of the metal sheet 1.

In the example shown, the metal sheet 1 is supported by a support 15, which allows the metal sheet 1 to move in a plane in the direction 7 (slow axis) or, if necessary, in three-dimensional space relative to the device that generates, polarizes and scans the laser beam 9, so that the latter can process the surface metal sheet 1 along the new processing field line of the device shown.

Preferably, prior to this treatment of said new line according to an embodiment of the invention, the result of which is shown in FIG. 3, the adjustment of the optical polarizing device 12 has been changed to polarize the laser beam 9, which is different from its previous polarization when processing the previous line. With the optical polarizing device 10, at least two different polarization angles, and preferably at least three, can be obtained and preferably, but not necessarily, alternate with each line change. The periodicity of the polarization scheme is not essential and, as mentioned, it is sufficient that the polarization angles of two adjacent lines 16, 17, 18, 16', 17', 18' along the slow axis differ from each other, preferably differ by at least 20° but no more than 90°. However, the periodicity of the pattern, for example, as shown by the polarization angles that repeat every three lines 16, 17, 18, 16', 17', 18', is preferred because periodic programming of polarization change is simpler than random programming, in in particular, since two lines 16, 16' or 17, 17' or 18, 18', belonging to two different fields and lying on the continuation of each other along the fast axis and meeting at joint 2, must have the same wave orientation.

A sequence of random polarizations in a given optical field, preferably taking into account the above minimum difference in angles of 20° - 90°, would be acceptable.

According to the invention, an assembly for processing a metal sheet 1 comprises a plurality of individual devices described above, which are arranged in such a way that they face the metal sheet 1 and are placed next to each other, so that the respective fields for processing two separate devices placed side by side, i.e. .e. the optical fields of the focusing systems 14 of the respective scanners 13 overlap slightly. This overlap is approximately two pulse sizes plus a positioning error with respect to the pulse period of the laser 8 and the scanning speed of the laser beam along the fast axis. It must be experimentally confirmed that this overlap is sufficient to eliminate the untreated zone on the metal sheet 1 at the end of the operation. In addition, the lines 16, 16' or 17, 17' or 18, 18' generated by each of these fields must lie on each other's extension in two adjacent fields, and the control of the individual devices must be identical for the two adjacent fields at the time t, in particular in terms of the size, shape, power and polarization angle of the respective laser beams 9, so that the processing is uniform over the entire line having the width of the metal sheet 1, and so that the alternation of the polarization angles of the laser beam 9 between two successive other lines 16, 17, 18, 16', 17', 18' along the slow axis 7 was the same over the entire width of the metal sheet 1.

The controls for these individual devices are common to all individual devices so that they operate in complete synchronization with each other. Preferably, they also control the movements of the support 15 of the metal sheet 1 also in order to best synchronize the relative movements of the metal sheet 1 and the laser beams 9 of the individual nodes.

It is clear that the mobile support 15 could be replaced by a motionless support, and the relative movement of the metal sheet 1 and laser rays of 9 separate processing nodes could be ensured by placing them on a movable support. In addition, both options could be combined, in the sense that the device in the invention contains a movable support 15 for a metal sheet 1 and other movable support for individual processing devices, and at the request of the user, any of the two supports or both supports can be simultaneously actuated by a control device. If only one processing device is provided, it is moved along the fast axis after processing part of the metal sheet 1. In this case, the joint scheme received after processing the part of the metal sheet 1 with the specified device is stored in the memory, and the same scheme is obtained on the processed field (fields ) after moving(s) the device along the fast axis.

In addition, the relative movement of the metal sheet 1 and the laser beams 9 of the individual devices along the slow axis 7 can be provided by means of optical means built into the individual processing devices and acting on the points of the fast axis to move the laser beams 9 of these individual devices. These optical means replace or supplement the mechanical means for moving the movable support 15 of the metal sheet 1 and/or the movable support of the lasers 8 of the individual processing devices.

The use of optical means alone would be sufficient to process small sized products, but would very likely not be sufficient to process relatively large sized products with adequate accuracy. However, it is possible to combine optical means and mechanical means by placing the mechanical means at a predetermined position and allowing the relative movement of the laser beam 9 along the slow axis 7 with the optical means over a sufficiently short distance d so that the accuracy of the relative movement is sufficient, and then moving the mechanical means over a distance , equal to d, to continue processing the metal sheet 1, using optical means to ensure the subsequent relative movement of the laser beams 9 and the metal sheet 1 along the slow axis.

In a preferred embodiment of the invention, the number M corresponds to the number of different orientations that it is desirable to give the waves, providing a distance M times greater than in conventional processing, and shifting the lines by a conventional distance when making lines. In FIG. 3 shows an example of the appearance according to the above description with M = 3.

The metal sheet 1 has on its surface a periodic sequence of lines 16, 17, 18, 16', 17', 18', formed by two devices of the invention, which ensures the creation of this periodic pattern of three types of these lines on two adjacent optical fields 19, 20, and lines 16, 17, 18 of the given field lie in the continuation of similar lines 16', 17', 18', made in an adjacent optical field.

The lines 16, 17, 18, 16', 17', 18' of the scheme differ from each other by the effects of different polarizations that the polarizing device 12 performs in relation to the laser beam 9 at the time of formation.

As can be seen in the section of FIG. 3, which shows an enlarged portion of the surface of the metal sheet 1, in the non-limiting example shown, the polarization performed on the laser beam 9 when generating the first line 16, 16' leads to the orientation of the waves in a direction perpendicular to the relative direction of movement 7 (slow axis) of the metal sheet 1 regarding the laser processing device. Next, to generate the second line 17, 17' of the circuit, the polarization of the laser beam 9 was changed to obtain the orientation of the waves at an angle of 45° relative to the orientation of the waves of the first line 16, 16'. And finally, to generate the third line 18, 18', the polarization scheme of the laser beam 9 was changed to obtain a wave orientation at an angle of 45° relative to the wave orientation of the second line 17, 17', i.e. at an angle of 90° with respect to the orientation of the waves of the first line 16, 16': thus, the waves of the third line 18, 18' are oriented parallel to the direction 7 of movement of the metal sheet 1 relative to the laser processing device.

In the zone of connection of two lines 16, 16', 17, 17', 18, 18', lying on the continuation of each other, energy is supplied to the surface of the metal sheet 1, which is greater than the energy supplied to the rest of the surface, as in the main embodiment of the invention described higher. This is confirmed by the existence of zones 2 which have been over-treated and located at the junction of adjacent fields, the exact position of which within the zone of overlapping fields is an uncertainty supporting the invention. However, there are 2 junctions in these zones, since the connecting lines 16, 17, 18, 16', 17', 18' of each optical field were obtained with the same polarization of the laser beam 9, this further and more attenuates the visual rainbow deterioration. surface effect, which would be a problem in the absence of a controlled polarization of the laser beam 9. The lack of continuity in the orientation of the waves from one optical field to another would tend to increase the visibility of the field junction zone on a given line, creating an area of inhomogeneity on the surface. It is necessary to ensure that the lines 16, 17, 18, 16', 17', 18' of two adjacent fields made with the same polarizations are located in the appropriate continuation of each other, but this is a warning regarding the collinearity of the lines 16, 17, 18, 16', 17', 18' adjacent fields must also be taken into account when implementing the main variant of the method of the invention, as shown in FIG. 1, and the equipment used for this purpose can also be used for this embodiment of the invention. It is only necessary to ensure that the polarization changes of the laser beams of the 9 devices for each field are carried out with the same values for the mating lines 16, 17, 18, 16', 17', 18' fields.

The use of M = 2 orientations of different polarizations, shifted by, for example, 90°, is already sufficient to obtain a visible rainbow effect in most directions of observation. However, the intensity of the rainbow effect still varies greatly when viewed at 45°, and it can be considered that the problem of the lack of directionality of the rainbow effect is still not completely solved. This problem is solved as soon as M becomes greater than 2, and preferably if the angles between successive lines 16, 17, 18, 16', 17', 18' differ by more than 20°.

Experience has shown that by performing processing with at least three different polarization angles distributed between 0° and 90° and preferably having a polarization difference of at least 20° between two successive lines 16, 17, 18, 16', 17', 18' along the slow axis 7, the iridescent surface effect becomes visible with the same intensity in all directions. It is possible to use orientations M greater than 3, but care must be taken that the polarization angles of two adjacent lines are sufficiently different from each other to avoid directivity of the desired rainbow effect.

However, it is clear that the distribution of the surface structure in different orientations causes a decrease in the overall intensity of the rainbow effect compared to a surface treated in a single polarization direction and observed at the optimal angle (at an angle transverse to the structure). Thus, a compromise must be found between the intensity of the visual rainbow effect perceived by the observer and the omnidirectionality of this rainbow effect. However, the three directions of polarization (and thus the periodicity of the three lines of these directions, as shown in FIG. 3) is already said to be a proper compromise, at least in most common cases.

And finally, to achieve the effect of the most uniformity possible, it is recommended to alternate the orientations, preferably with periodicity, at the smallest possible distances. For M different orientations, it is preferable to periodically alternate only one line of each orientation, having a width equal to or preferably (to ensure the treatment of the entire surface of the metal sheet) slightly less than the pulse diameter.

It is possible to process metal sheets 1 that do not have perfect flatness by including in the processing apparatus means for measuring the distance between the focusing system 14 and the metal sheet 1 and connecting them to the controls of the focusing system 14 so that the latter can ensure that the pulse diameter and the density of the laser beam 9 are essentially the same regardless of the actual distance between the focusing system 14 and the metal sheet 1. Alternatively, said means for measuring the distance between the focusing system 14 and the metal sheet 1 can be servo-controlled by providing a relative movement of the device of the invention and metal sheet 1, which makes it possible to maintain a constant distance between the focusing system 14 and the surface of the metal sheet 1 throughout the processing of the metal sheet 1.

The application of the method can also be envisaged for materials other than flat metal sheets (eg shaped sheets, rods, pipes, parts having generally three-dimensional surfaces) by adapting the means for the relative movement of the lasers and the parts to be processed and/ or adjusting the focusing means, if adjustment of the distance between the laser emitter and the surface is required. For products having substantially cylindrical surfaces (e.g., bars, round tubes), the laser devices could be placed on a fixed support, and a support could be provided for the product to allow rotation of the product so that the surface of the product moved in the optical fields of lasers.

Finally, it should be appreciated that while stainless steels are prioritized as materials, the present invention also contemplates the use of other metal and non-metal materials on whose surface a rainbow effect can be obtained by laser processing.

Claims (18)

1. A method for creating a visual iridescent effect on the surface of a product, which includes directing laser beams with a pulse duration of less than one nanosecond to the specified surface of the product for applying a structure in the form of waves to the specified surface at the width of the specified pulse, while using at least two fixed devices or at least one movable device, each of the devices contains a source of laser radiation, a scanner and a focusing system, and the laser beams are directed in adjacent optical fields of the focusing systems of these devices, while the specified scanner or scanners scan the specified surface with the specified laser beams along a group of serial lines in the direction of relative movement of the product and the scanner or scanners and along a group of lines lying on the continuation of each other in a direction perpendicular to the specified direction of relative movement, each of which refers to the optical field of the device or devices, and each line has a width equal to the diameter of the specified pulse, characterized in that the specified optical fields of two fixed adjacent devices or optical fields corresponding to two successive positions of the specified mobile device overlap in the overlap area with a width of a value in the range between twice the pulse diameter of the laser beam and 2 cm, ensuring the overlap of two lines lying on the continuation of each other at the junction, and between two groups of consecutive lines in the direction of the relative movement of the product and the scanner or scanners, these junction zones are scattered or arranged periodically according to an arbitrary pattern on length in the direction of the axis of movement of laser scanners or in the direction of movement of a metal product, equal to at least ten times the maximum displacement between joints on two consecutive lines in the indicated direction of relative movement inside the specified region of overlapping optical fields. 2. The method according to claim 1, characterized in that between scanning along two consecutive lines in the indicated direction of relative movement of the product and the scanner or scanners, the polarization of the laser beam is carried out, which is changed to create waves of different orientations on these two consecutive lines and create a common pair orientation in a direction perpendicular to the direction of relative movement. 3. The method according to claim 2, characterized in that the polarization of the laser beam is changed in a periodic pattern, and the specified periodic pattern extends for M successive lines in the direction of the relative movement of the product and scanners, while M is equal to at least 2. 4. The method according to claim 2 or 3, characterized in that two consecutive lines in the direction of the relative movement of the product and scanners have polarization angles that differ by at least 20°. 5. The method according to one of paragraphs. 2 - 4, characterized in that the polarizations of two lines of two adjacent fields lying on the continuation of each other are the same. 6. Method according to any of the paragraphs. 1 - 5, characterized in that the specified relative movement of the specified surface of the specified product and devices emitting the specified laser beams is obtained by placing the specified product on a movable support. 7. The method according to any one of paragraphs. 1 - 6, characterized in that the specified relative movement of the specified surface of the specified product and devices emitting the specified laser beams is obtained by placing the devices emitting the specified laser beams on a movable support. 8. The method according to any one of paragraphs. 1 - 7, characterized in that the specified product is a metal sheet. 9. The method according to any one of paragraphs. 1 - 7, characterized in that the specified surface of the specified product is three-dimensional, while measuring the distance between the focusing system and the surface of the product and the focusing system is adjusted in such a way as to provide essentially the same pulse diameter and energy density of the laser beam, regardless of the actual distance between the focusing system and the product. 10. The method according to any one of paragraphs. 1 - 7, characterized in that the specified surface of the specified product is three-dimensional, while measuring the distance between the focusing system and the surface of the product and the relative position of the specified device and the specified surface is adjusted so that when implementing the method, the distance between the specified focusing system and the specified surface is maintained one and the same. 11. The method according to any one of paragraphs. 1 - 10, characterized in that the specified product is made of stainless steel. 12. Device for creating a visual iridescent effect of the surface of the product by forming waves on the specified surface using a pulse of a laser beam, consisting of at least two fixed adjacent separate devices or at least one movable separate device, each of the individual devices contains a source laser radiation to generate a laser beam with a pulse duration of less than 1 ns, an optical system for beam formation, a scanner configured to scan the beam with a pulse after it passes through the focusing system in the form of optical field lines over the surface of the product, and these optical fields of two adjacent separate devices overlap in a width between twice the laser beam pulse diameter and 2 cm and contain junctions of two lines, each of which is created by a separate device, while each separate device contains means for creating a relative movement in a predetermined direction between said device and said article to perform processing on at least one surface area of said article, characterized in that said scanners of said separate devices are configured to arrange said joints in such a way that together they form an arbitrary pattern or said joint zones are arranged periodically in an arbitrary pattern at a length in the direction of the axis of movement of laser scanners or in the direction of movement of a metal product, equal to at least ten times the maximum displacement between the joints on two consecutive lines in the specified direction of relative movement within the specified area of overlapping optical fields. 13. The device according to claim 12, characterized in that the optical systems of said individual devices comprise an optical polarization system that provides a given polarization of said beam, and means for causing this polarization to change so that two adjacent lines in the direction on said surface are obtained using pulses with different polarizations. 14. Device according to claim 13, characterized in that said separate devices are configured to form two adjacent lines with pulses whose polarization differs by at least 20°. 15. The device according to any one of paragraphs. 12 - 14, characterized in that these separate devices contain means for measuring the distance between the focusing system and the surface of the product, connected to the means for adjusting the focusing system so that the focusing system maintains a constant pulse diameter and a constant energy density on the specified surface, regardless of the specified distance . 16. The device according to any one of paragraphs. 12 - 14, characterized in that these separate devices contain means for measuring the distance between the focusing system and the surface of the product, while these measuring means are connected to the means for controlling the relative position of the specified device and the specified surface in such a way that a constant distance between the specified system is maintained focusing and specified surface. 17. The device according to any one of paragraphs. 12 - 16, characterized in that said means for creating relative movement in a given direction between said device and said article contain a movable support for the article. 18. The device according to any one of paragraphs. 12 - 17, characterized in that said means for creating a relative movement between said device and said article contain a movable support for said individual devices.

Images