Laser marking method and system

A laser marking method and system, and laser marked object are disclosed. The method includes directing a pulsed laser beam towards an object such that an interface between an oxidized layer and non-oxidized substrate is in a mark zone of the pulsed laser beam, and scanning the pulsed laser beam across the object in a predetermined pattern to create a mark having an L value of less than 40 and a surface roughness that is substantially unchanged compared to adjacent unmarked areas. The system includes a fiber laser generating amplified pulses that are directed towards a galvo-scanner and focusing optic, while the object includes an oxidized surface layer, an underlying non-oxidized substrate, and a mark having an L value of less than 40 with substantially unchanged roughness features.

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.

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'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.

DETAILED DESCRIPTION OF THE INVENTION

Referring toFIG. 1, a laser marking system10in accordance with the present invention is shown to include a laser12, a scanner14, and one or more optics16optically coupled to the scanner14. A laser beam18is emitted from the laser12and propagates through the system10before becoming incident on a target object20which 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 toFIG. 1, the laser12has 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 laser12has 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.

Laser12preferably operates such that it emits a diffraction limited laser beam22substantially 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 over 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 beam22. 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. 2shows a section of a laser beam22propagating in the fundamental mode. On the top side of a beam waist24, beam22has a lower portion26and an upper portion28. Interposed between the upper and lower portions26,28is a marking zone34bounded below by an ablation threshold plane30and above by a marking threshold plane32, the threshold planes30,32lying generally perpendicular to the propagation path48of the beam22. As beam22propagates along path48downward from the top ofFIG. 2, the spot size of beam22slowly decreases to a minimum at the waist24. Because the waist radius W0is inversely related to the beam divergence θ, the characteristics of diffraction limited beam22can be adjusted to match the needs of the system or mark. For example, a very small waist24can 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 toFIG. 3, a cross-section of an exemplary target object20is shown. Object20has a surface36and a non-oxidized underlying substrate40. For anodized aluminum, object20will include an oxidized layer38as well as an interface42between the oxidized layer38and underlying non-oxidized layer40. The addition of an oxidation layer38, or anodization layer, has several benefits including corrosion resistance, enhanced surface hardness, and non-conductivity. Oxidized layer38is 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, surface36and interface42actually have considerable contour when under magnification, e.g., inFIG. 6. InFIG. 5, beam22fromFIG. 2is shown to be transmitted through a portion of focusing optic16and to be directed towards target object20ofFIG. 3. Optic16causes a focusing of the beam22such that the waist24is disposed hypothetically below the interface42of the object20. Also disposed hypothetically below the interface42are ablation threshold30and the continued beam shape46.

Laser beam18has photons of a particular wavelength. The wavelength should be selected such that that the photons interact with the metallic material of object20. 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 layer38is less electro-active than the underlying substrate40, beam22can propagate through layer38without detrimentally interacting therewith. As shown inFIG. 5, the ablation and marking planes30,32lie above the hypothetical beam waist24and are positioned such that interface layer42lies between the respective thresholds30,32. Under a similar laser system setup, a larger propagation distance may separate optic16and target object20such that waist24lies above interface layer42, e.g., using opposite zone33shown inFIG. 2, thereby utilizing the symmetric shape of Gaussian beam22. Thus, herein symmetric zone33may be substituted for mark zone34.

It is also possible to select the beam parameters such that the waist24is coterminous with the ablation threshold30while the marking threshold32lies somewhat above the waist24. To obtain a coterminous waist24and ablation threshold30, typically the average power of the laser12is 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 thresholds30,32gradually shift towards the waist24. The marking zone34in such a configuration would include the waist24and extend up to marking threshold32thereabove.

Referring again toFIG. 5, pulses56penetrate the surface oxidized layer38and cause a dark marked portion44to be formed at the interface42, below the interface42and into the substrate40, and above the interface42in the oxidized layer38. 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 portion44tends 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 toFIG. 4, an exemplary scanning path52is shown depicting a typical path for pulsed laser beam22to scan across target object20. The path52has a scan spacing distance D1between parallel path segments54. As shown, path segments are bi-directional, i.e., each subsequent path segment54has 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 object20at one or more scan speeds, a pattern of laser pulses impinges object20. As shown more clearly in the sectional close-up ofFIG. 4A, pulses56overlap one another both in the direction of the scanning path52as well as perpendicular thereto, i.e., across the scan spacing distance D1direction. Suitable scan spacing distances D1are 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 path52and the repetition rate of the laser12. The scanning speed divided by the repetition rate determines distance D2between successive pulses along the scanning path52. The overlap in the direction of scanning is also dependent on the size and shape of the laser pulse56. 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. WhileFIG. 4Ashows laser pulses56having a circular shape, other shapes fall within the scope of the invention. For example, in some embodiments laser pulses56may have an oval or other non-circular shape. In other embodiments, the intensity distribution of the laser pulse56is 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 path48while 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 system10generally 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 beam18to an optic16, such as an Fθ lens, which then directs the beam to the target object20. While other lens may be used, an Fθ lens is preferred so that the laser beam18is projected substantially uniformly across the lateral dimensions of the scanning area. As shown inFIG. 5, upper portion28of laser beam18projects nearer a side of the optic16and emerges with a Gaussian shape. In other configurations, the object20is translated in a predetermined pattern with respect to a fixed beam18. In such configurations, a less expensive lens may be used since the beam18is not scanned.

FIG. 8shows several Gaussian distributions with differing spreads and normalized to unity, such as larger intensity distribution66and smaller intensity distribution68. 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 beam18. 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 system10, 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, pulses56having doubled intensity will ablate away or damage the oxidized surface and underlying material. Example ablation and marking thresholds70,72show 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 beam18. Again, referring toFIG. 8, a decrease in peak power and peak intensity can be achieved while spreading out the shape of the beam18laterally 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 zone50, an example of which being depicted in cross-section inFIG. 5, and can introduce additional undesirable effects on the target object20. For example, often the oxidized layer38and the underlying material40have differing coefficients of thermal expansion. This variation can enhance cracking, surface changes, and angular color variation in the marked area62, all caused by the pulsed beam18. 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 segments54is another factor to consider for making superior marks according to some aspects of the present invention.

As a laser pulse56impacts the target object, energy is transferred and a localized zone50of heat is created. The heat rapidly conducts into the surrounding material of the object20. Each successive pulse56in the scanning path54produces another adjacent heat zone50. As was described earlier, the scan speed and the repetition rate determine in part the overlap of each successive pulse56in the propagation path54. 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 zone50together with thermal expansion coefficient mismatch can result in ablation and damage effects. Similarly, when the distance D1between 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.

InFIG. 6, a depiction of a magnified surface of an exemplary object20is shown. A marked area62contrasts with a neighboring unmarked area64at 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 inFIG. 6, the surface topology remains substantially similar between the marked and unmarked areas62,64. In other embodiments, the substantial similarity in surface roughness between marked and unmarked areas62,64can 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 pulses56to continue material processes in and beneath the oxidized layer38and into the underlying substrate40in order to maximize the darkness of the marked area62. 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 area62. For a particular set of beam parameters, an excessive number of beam passes in the mark zone34can cause material degradation in the mark area62. Consequently, in some embodiments, the boundaries of mark zone34are 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 toFIGS. 2 and 5, laser pulses56from beam18are configured to impinge the interface42in marking zone34to create a marked area62. While the spatial relation between the beam18and target object20in the mark zone34provides a beam18having 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 toFIG. 7, a chart compares the beam width58and the marked width60on object20with respect to Z position for a cross-section of beam18for a particular set of laser system parameters. The marked linewidth60is about the same as the spot width58at the waist (Z=0) but gradually decreases, crossing the laser spot width58, 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 widths60are similarly smaller than beam widths58in the marking area74of the mark zone34, though the relative amount of difference between widths58,74varies upon the laser system configuration and the parameters selected for the laser beam18.

Together the ablation and marking thresholds30,32hereinbefore described and the Gaussian beam shape depicted in perspective inFIG. 2define a mark zone34having a shape of a frustum76, or conic-section. The shape76has outer gradient surfaces78forming the bounds of the laser beam18and with top and bottom boundaries forming the respective ablation and marking thresholds30,32. The shape of frustum76defining mark zone34can vary depending on the beam parameters selected or achieved by the laser system10in relation to object20. The portion of the frustum76that contributes significantly to the formation of marked area62is defined by the marking area74, shown inFIG. 7. The shape of the marking area mark zone74may 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.