Patent Publication Number: US-10315274-B2

Title: Laser marking method and system and laser marked object

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 13/236,949, entitled “LASER MARKING METHOD AND SYSTEM” and filed Sep. 20, 2011, which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Generally, the field of the present invention is the marking of objects. More particularly, the present invention relates to a laser marking method and system and laser marked object. 
     2. Background 
     Objects have been marked with indicia since the dawn of civilization. Some of the earliest human rock carvings date back 12,000 years. Such petroglyphs were created by removing a portion of a rock surface to expose the underlying material. In some glyphs, significant contrast between the carved portion and the host rock was created by the removal of a shallow outer layer have different properties than the inner layers. Even as an oxide forms at the exposed section, the difference between the two materials causes the contrast to be retained over extended periods of time. However, in general glyphs in drier climates and glyphs on walls that were not otherwise exposed to weather effects were better suited to withstand many thousands of years of weather degradation. In the last few thousand years as innovation yielded dye technology and marking abilities associated therewith, pictographs emerged as a common alternative to petroglyphs. Again, drier climates and locations insulated from weather effects allowed some pictographs to remain intact to the present day. 
     In the last few centuries, advances in metallurgy and other material technology have expanded the set of markable objects to include pure metals and sophisticated alloys. Such objects are marked using traditional carving and marking methods and results tend to have several undesirable attributes. For example, mechanical carving methods tend to take a long time and are therefore cost prohibitive. Additionally, these methods can leave behind sharp shavings and edges that can injure a person interacting with the mark. Chemical marking does not fare much better as they frequently use toxic compounds to provide the permanence to the mark. And even then, the marks tend to wear away with repeated contact by a person or with repeat exposure to different elements. 
     More recently, the development of lasers and laser systems has provided an alternative to mechanical carving methods. Laser systems can be configured to provide quick and repeatable beam paths making them particularly suited for manufacturing. Consequently, lasers are now used to carve into the surface layers of materials to provide markings thereon. For example, a laser beam impinges on a metal surface and ablates away a shallow amount of material by locally superheating the portion of the object where the beam hits while leaving the bulk substrate unaffected. Unfortunately, the touch, appearance, and permanence aspects of such prior art markings are less than desirable, and the act of carving can cause long-term deleterious effects on the object itself by removing a protective oxidation layer, such as an anodization layer, or otherwise compromising the integrity of the material. Thus, there is a general need to have a marking process that can produce indelible marks that do not rub off after exposure to the elements and that do not themselves expose any underlying material to the elements. 
     In U.S. Pat. No. 6,590,183 a method is described for providing a mark in an aluminum element&#39;s surface by using a laser to penetrate an anodized outer layer in order to produce a mark substantially exclusively locally in a zone where the anodized layer adjoins the underlying aluminum substrate. To create such a mark, it is suggested that a laser be used having an operating wavelength of 1064 nm and a pulsed output of less than 20 ns such that material is locally melted and solidified. However, these procedures still fail to produce a mark that is very dark, smooth to the touch, and to do so very quickly. Thus, there remains a need for further innovation that will overcome these failings. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an innovation that overcomes the shortcomings of prior procedures by providing a superior and long-lasting sub-oxidized layer mark in an object using an enhanced process for making such a mark within a very short amount of time. 
     According to one aspect of the present invention, a laser marking process for producing high quality marks is provided. A pulsed laser beam operating at 10-1000 kHz and an average power of less than 20 W is directed to a target object for marking such that a sub-surface interface between an oxidized surface layer and underlying non-oxidized substrate of the target object is in a mark zone of the pulsed laser beam. The mark zone can be defined through identification of an ablation threshold and a mark threshold, and suitable tuning of laser parameters, including power, spot size, and scan rate. The pulsed laser beam and target object are scanned relative to each other in a predetermined pattern such that a mark is created below the surface of the object and into the underlying substrate. 
     According to another aspect of the present invention, a marked object is provided that includes an oxidized outer layer, non-oxidized underlying substrate, and an interface therebetween, wherein the object has a surface roughness substantially similar to adjacent unmarked regions and has a LAB standard for color characterization darkness value of less than 40 irrespective of viewing angle. 
     According to another aspect of the present invention, a laser system includes a laser source for providing a pulsed laser beam, a galvo-scanner for scanning the pulsed laser beam in a pre-determined pattern, and a focusing optic for focusing the pulsed laser beam to a mark zone, wherein the properties of the pulsed laser beam are sufficient to create a mark having a darkness value of less than 40 below the surface of a metallic target such that the surface roughness of the metallic target in the marked area is substantially unchanged. 
     The foregoing and other objects, features, and advantages will become more apparent from the following detailed description, which proceeds with reference to the accompanying figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a laser system for marking objects in accordance with an aspect of the present invention. 
         FIG. 2  is a perspective view of a laser beam in accordance with an aspect of the present invention. 
         FIG. 3  is a side view of an object for marking by the laser system of  FIG. 1 . 
         FIG. 4  is a plan view of a laser beam path taken on an object. 
         FIG. 4A  is an expanded view of a portion of  FIG. 4 . 
         FIG. 5  is a side view of a portion of the laser system and object being marked thereby in accordance with an aspect of the present invention. 
         FIG. 6  is a depiction of an exemplary object of the present invention showing marked and unmarked areas in accordance with an aspect of the present invention. 
         FIG. 7  is a chart of marked linewidth and beam diameter with respect to Z position of the laser system in accordance with an aspect of the present invention. 
         FIG. 8  is a chart of Gaussian intensity distributions with respect to spot position. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , a laser marking system  10  in accordance with the present invention is shown to include a laser  12 , a scanner  14 , and one or more optics  16  optically coupled to the scanner  14 . A laser beam  18  is emitted from the laser  12  and propagates through the system  10  before becoming incident on a target object  20  which is preferably aluminum having an oxidized surface, such as anodization. Herein, “object” has a broad meaning to include various workpieces that are capable of being marked. The target objects are preferably metallic in nature, such as pure elemental metals, and alloys thereof. Additionally, metals with oxidized surfaces are preferred, such as anodized aluminum, whether rough or polished. Referring back to  FIG. 1 , the laser  12  has a pulsed output whereby laser light is generated only intermittently or otherwise allowed through an output only intermittently. A pulse of light transmitted through an output of the laser  12  has a characteristic shape of intensity with respect to time, and therefore has a characteristic peak intensity, peak power, pulse width, and pulse repetition rate. The beam also has spatial characteristics that are important, particularly in conjunction with the aforementioned intensity characteristics. As will be discussed hereinafter, the beam characteristics are important and must be selected and adjusted carefully to achieve the desired mark attributes. A careful balance between pulse energy, peak power, thermal crosstalk, and run time must be achieved such that superior marks can be obtained in as short a time as possible. This balance can also manifest as a careful harmonization of average power, peak power, and scan speed. 
     Laser  12  preferably operates such that it emits a diffraction limited laser beam  22  substantially in the fundamental mode. One such system includes a solid state gain medium emitting a seed signal, a fiber amplifier for amplifying the seed signal provided by the solid state medium, and diode sources for pumping the solid state medium and fiber amplifier. Such a system can generate 1 kW of peak power while having a pulse width of less than about 2 ns. In an alternative embodiment, a diode-pumped solid state laser is used having similar system peak power, pulse width, and peak intensity. The fiber amplifier can comprise one or more fiber amplifier stages for increasing the output power of the laser beam  22 . In other embodiments, a mode-locked fiber laser is used having a pulse picker to control pulse repetition and multiple fiber amplifier stages for amplifying the pulse picked signal. Some embodiments utilize a diode-pumped microchip as a pulsed seed source. 
       FIG. 2  shows a section of a laser beam  22  propagating in the fundamental mode. On the top side of a beam waist  24 , beam  22  has a lower portion  26  and an upper portion  28 . Interposed between the upper and lower portions  26 ,  28  is a marking zone  34  bounded below by an ablation threshold plane  30  and above by a marking threshold plane  32 , the threshold planes  30 ,  32  lying generally perpendicular to the propagation path  48  of the beam  22 . As beam  22  propagates along path  48  downward from the top of  FIG. 2 , the spot size of beam  22  slowly decreases to a minimum at the waist  24 . Because the waist radius W 0  is inversely related to the beam divergence θ, the characteristics of diffraction limited beam  22  can be adjusted to match the needs of the system or mark. For example, a very small waist  24  can be achieved only at the expense of substantial divergence. In a system with large divergence, the spot size can vary quickly and consequently the range of suitable focus positions, or Z positions, along the propagation path becomes more limited making system design more difficult. 
     Referring to  FIG. 3 , a cross-section of an exemplary target object  20  is shown. Object  20  has a surface  36  and a non-oxidized underlying substrate  40 . For anodized aluminum, object  20  will include an oxidized layer  38  as well as an interface  42  between the oxidized layer  38  and underlying non-oxidized layer  40 . The addition of an oxidation layer  38 , or anodization layer, has several benefits including corrosion resistance, enhanced surface hardness, and non-conductivity. Oxidized layer  38  is generally thin, being measurable in microns, and is typically formed according to various electrolytic methods that are known in the art. While illustrated with straight lines for convenience, surface  36  and interface  42  actually have considerable contour when under magnification, e.g., in  FIG. 6 . In  FIG. 5 , beam  22  from  FIG. 2  is shown to be transmitted through a portion of focusing optic  16  and to be directed towards target object  20  of  FIG. 3 . Optic  16  causes a focusing of the beam  22  such that the waist  24  is disposed hypothetically below the interface  42  of the object  20 . Also disposed hypothetically below the interface  42  are ablation threshold  30  and the continued beam shape  46 . 
     Laser beam  18  has photons of a particular wavelength. The wavelength should be selected such that that the photons interact with the metallic material of object  20 . For aluminum, a laser emitted wavelength near 1 micron, such as that generated by Neodymium or Ytterbium doped optical gain media, is suitable. Other wavelengths may be used, for example, frequency doubled 532 nm, as well as many others. Since the oxidized layer  38  is less electro-active than the underlying substrate  40 , beam  22  can propagate through layer  38  without detrimentally interacting therewith. As shown in  FIG. 5 , the ablation and marking planes  30 ,  32  lie above the hypothetical beam waist  24  and are positioned such that interface layer  42  lies between the respective thresholds  30 ,  32 . Under a similar laser system setup, a larger propagation distance may separate optic  16  and target object  20  such that waist  24  lies above interface layer  42 , e.g., using opposite zone  33  shown in  FIG. 2 , thereby utilizing the symmetric shape of Gaussian beam  22 . Thus, herein symmetric zone  33  may be substituted for mark zone  34 . 
     It is also possible to select the beam parameters such that the waist  24  is coterminous with the ablation threshold  30  while the marking threshold  32  lies somewhat above the waist  24 . To obtain a coterminous waist  24  and ablation threshold  30 , typically the average power of the laser  12  is decreased while leaving other adjustable parameters, such as repetition rate, unchanged. As the average power decreases, so does the pulse energy and peak power. Accordingly, the ablation and marking thresholds  30 ,  32  gradually shift towards the waist  24 . The marking zone  34  in such a configuration would include the waist  24  and extend up to marking threshold  32  thereabove. 
     Referring again to  FIG. 5 , pulses  56  penetrate the surface oxidized layer  38  and cause a dark marked portion  44  to be formed at the interface  42 , below the interface  42  and into the substrate  40 , and above the interface  42  in the oxidized layer  38 . Using the methods and systems of the present invention, objects of the present invention can be marked so as to obtain mark LAB color characterization darkness levels, or “L” values, of less than 40. In some embodiments the L value achieved is 30 or less. Moreover, marked portion  44  tends to maintain substantially the same darkness level across many viewing angles, including shallow angles. For example, the L value may not change by more than 20% across viewing angles, though discerning whether the darkness value changes is more easily determined by simple visual observation. In some embodiments, even lower L values are achieved, such as L values less than 30 and even less than 25. 
     Referring now to  FIG. 4 , an exemplary scanning path  52  is shown depicting a typical path for pulsed laser beam  22  to scan across target object  20 . The path  52  has a scan spacing distance D 1  between parallel path segments  54 . As shown, path segments are bi-directional, i.e., each subsequent path segment  54  has an opposite direction. However, other paths are possible, such as uni-directional. In uni-directional scanning, the movement of the laser beam is in the same direction for each parallel segment. Because the laser beam is operating in a pulsed regime and scanning across object  20  at one or more scan speeds, a pattern of laser pulses impinges object  20 . As shown more clearly in the sectional close-up of  FIG. 4A , pulses  56  overlap one another both in the direction of the scanning path  52  as well as perpendicular thereto, i.e., across the scan spacing distance D 1  direction. Suitable scan spacing distances D 1  are typically around 20 μm or less, and it may be preferred for the distance to be significantly closer, such as between 5 and 10 μm, depending on the parameters of the laser system used. The overlap in the direction of scanning is dependent in part on the scanning speed along scanning path  52  and the repetition rate of the laser  12 . The scanning speed divided by the repetition rate determines distance D 2  between successive pulses along the scanning path  52 . The overlap in the direction of scanning is also dependent on the size and shape of the laser pulse  56 . The overlap in the direction of scanning is typically 80% or more and it may be preferred for the overlap to be significantly higher, such as 95% or more, depending on the parameters of the laser system used. While  FIG. 4A  shows laser pulses  56  having a circular shape, other shapes fall within the scope of the invention. For example, in some embodiments laser pulses  56  may have an oval or other non-circular shape. In other embodiments, the intensity distribution of the laser pulse  56  is closer to uniform across the width of the pulse instead of having a Gaussian distribution. Such a uniform distribution can be achieved by using various means, including a top-hat type filter disposed in the path of the pulsed laser beam. In some embodiments the uniformity may occur across only one axis transverse to the beam propagation path  48  while in others uniformity occurs across two transverse spatial axes. In general the significantly spatial uniformity achieved by the filter can vary by about 20% of a particular intensity value and include even more deviation from that value near the edges. 
     As described earlier, the laser system  10  generally includes a scanning system for scanning the laser beam in two dimensions. A suitable scanner is of the galvo-type, which typically directs the input laser beam to motion controlled mirrors. The mirrors direct the laser beam  18  to an optic  16 , such as an Fθ lens, which then directs the beam to the target object  20 . While other lens may be used, an Fθ lens is preferred so that the laser beam  18  is projected substantially uniformly across the lateral dimensions of the scanning area. As shown in  FIG. 5 , upper portion  28  of laser beam  18  projects nearer a side of the optic  16  and emerges with a Gaussian shape. In other configurations, the object  20  is translated in a predetermined pattern with respect to a fixed beam  18 . In such configurations, a less expensive lens may be used since the beam  18  is not scanned. 
       FIG. 8  shows several Gaussian distributions with differing spreads and normalized to unity, such as larger intensity distribution  66  and smaller intensity distribution  68 . The increase in spread and concomitant decrease in peak intensity is representative of beam defocusing. For Gaussian distributions, several factors must be weighed to achieve optimum conditions for marking. One factor that must be weighed carefully is the peak power of the laser beam  18 . As the peak power of a pulse increases for the same spot size, due to, for example, larger pulse amplification in an amplifier section of the laser system  10 , the peak intensity similarly increases as well. The peak power is also strongly related to ablation level of the material being irradiated. Keeping other factors constant, such as repetition rate and pulse width, doubling pulse energy will result in a doubling of peak intensity. When directed at a target object, pulses  56  having doubled intensity will ablate away or damage the oxidized surface and underlying material. Example ablation and marking thresholds  70 ,  72  show regions within which peak intensity must be kept to prevent ablation and to provide a superior mark. 
     One way to mitigate the increased intensity of higher peak power is to defocus the pulsed laser beam  18 . Again, referring to  FIG. 8 , a decrease in peak power and peak intensity can be achieved while spreading out the shape of the beam  18  laterally to mark a larger area. However, as the peak intensity increases by increasing the pulse energy, a larger amount of energy is introduced into the vicinity of pulse target. More energy per pulse produces a larger heat affected zone  50 , an example of which being depicted in cross-section in  FIG. 5 , and can introduce additional undesirable effects on the target object  20 . For example, often the oxidized layer  38  and the underlying material  40  have differing coefficients of thermal expansion. This variation can enhance cracking, surface changes, and angular color variation in the marked area  62 , all caused by the pulsed beam  18 . As pulse energy increases, the negative effects introduced by the coefficient of thermal expansion mismatch become enhanced. Consequently, thermal communication between pulses and scan path segments  54  is another factor to consider for making superior marks according to some aspects of the present invention. 
     As a laser pulse  56  impacts the target object, energy is transferred and a localized zone  50  of heat is created. The heat rapidly conducts into the surrounding material of the object  20 . Each successive pulse  56  in the scanning path  54  produces another adjacent heat zone  50 . As was described earlier, the scan speed and the repetition rate determine in part the overlap of each successive pulse  56  in the propagation path  54 . When the scan speed increases while the repetition rate remains fixed, the pulse overlap decreases. Similarly when the repetition rate decreases but the scan speed remains fixed, the pulse overlap also decreases. Once the overlap reaches a critical separation distance with respect to the pulse energy and spot size, the transient temperature profile mismatch between the adjacent pulse zone  50  together with thermal expansion coefficient mismatch can result in ablation and damage effects. Similarly, when the distance D 1  between adjacent scan paths increases to a critical separation distance ablation effects occur. Consequently, minimum adjacent scan path separation and pulse overlap produce marks with superior appearance and unaltered tactile feel. 
     In  FIG. 6 , a depiction of a magnified surface image of an exemplary object  20  is shown. A marked area  62  contrasts with a neighboring unmarked area  64  at a micrometer scale. In some embodiments the surface topology includes bumps and dimples producing a characteristic roughness while in other embodiments the surface topology is very smooth, for example, with a polished surface. Notably in the embodiment illustrated in  FIG. 6 , the surface topology remains substantially similar between the marked and unmarked areas  62 ,  64 . In other embodiments, the substantial similarity in surface roughness between marked and unmarked areas  62 ,  64  can be confirmed by tactile observation using fingertips, fingernails, small objects, or conventional instruments such as a profilometer or 3D imaging microscope. 
     It is observed that a single pass of pulses is generally insufficient to create a high quality mark that is very dark and visually consistent at different angles. Multiple passes of the beam over the same mark area allows successive pulses  56  to continue material processes in and beneath the oxidized layer  38  and into the underlying substrate  40  in order to maximize the darkness of the marked area  62 . Moreover, to achieve the desired darkness in a single-pass requires more intensity which can cause damage to the oxidation and which can change the surface roughness features in the marking area. However, multiple passes can also have detrimental effects on the marked area  62 . For a particular set of beam parameters, an excessive number of beam passes in the mark zone  34  can cause material degradation in the mark area  62 . Consequently, in some embodiments, the boundaries of mark zone  34  are adjusted to account for such detrimental effects. One way to optimize the mark zone defining parameters, including marking and ablation thresholds and laser process parameters, including how quickly a mark is created, is by performing a multivariate statistical analysis, such as through a design of experiments. 
     As was described earlier in relation to  FIGS. 2 and 5 , laser pulses  56  from beam  18  are configured to impinge the interface  42  in marking zone  34  to create a marked area  62 . While the spatial relation between the beam  18  and target object  20  in the mark zone  34  provides a beam  18  having a particular width varying consistently as a function of Z position, the width of the area marked for a particular set of laser and beam parameters is a separate function of Z position or beam width. Referring to  FIG. 7 , a chart compares the beam width  58  and the marked width  60  on object  20  with respect to Z position for a cross-section of beam  18  for a particular set of laser system parameters. The marked linewidth  60  is about the same as the spot width  58  at the waist (Z=0) but gradually decreases, crossing the laser spot width  58 , and begins to decrease as the beam spreads out and the beam intensity decreases. The chart data shown were produced with a pulsed fiber laser operating at 1064 nm wavelength and 10 W of average power. The pulses were generated at a repetition rate of 120 kHz and had a pulse duration of around 1 ns. For other sets of parameters, marked widths  60  are similarly smaller than beam widths  58  in the marking area  74  of the mark zone  34 , though the relative amount of difference between widths  58 ,  74  varies upon the laser system configuration and the parameters selected for the laser beam  18 . 
     Together the ablation and marking thresholds  30 ,  32  hereinbefore described and the Gaussian beam shape depicted in perspective in  FIG. 2  define a mark zone  34  having a shape of a frustum  76 , or conic-section. The shape  76  has outer gradient surfaces  78  forming the bounds of the laser beam  18  and with top and bottom boundaries forming the respective ablation and marking thresholds  30 ,  32 . The shape of frustum  76  defining mark zone  34  can vary depending on the beam parameters selected or achieved by the laser system  10  in relation to object  20 . The portion of the frustum  76  that contributes significantly to the formation of marked area  62  is defined by the marking area  74 , shown in  FIG. 7 . The shape of the marking area mark zone  74  may have a non-frustum shape and again depends on selected beam and laser system parameters. 
     It is thought that the present invention and many of the attendant advantages thereof will be understood from the foregoing description and it will be apparent that various changes may be made in the parts thereof without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the forms hereinbefore described being merely exemplary embodiments thereof.