Inventors: Farzad Vakili Farahani, Chris Janker, Michael Miiller, Michael Bemdt

PRIORITY

This application claims priority to U.S. Patent Application Serial No. 62/854,208, filed May 29, 2019, the disclosure of which is incorporated herein in its entirety.

TECHNICAL FIELD OF THE INVENTION

The present invention relates in general to laser drilling holes in a workpiece. The invention relates in particular to laser drilling of a plurality of holes that are larger than a focused laser beam.

DISCUSSION OF BACKGROUND ART

Lasers are increasingly used for cutting and drilling a wide range of materials including metals, glasses, and polymers. Traditional mechanical processing produces rough surfaces and unwanted defects, such as micro cracks that may propagate when a processed workpiece is stressed, thereby degrading and weakening the processed workpiece. Laser material processing produces more precise cuts and holes, which have higher-quality edges and walls, while

minimizing the formation of unwanted defects. A laser is used in conjunction with an optical system to generate a laser beam and direct the laser beam onto a workpiece that is to be machined. The optical system may focus the laser beam to increase its fluence on the workpiece. The interaction of the focused laser beam with the workpiece locally melts and/or vaporizes the workpiece to produce or extend a hole or cut in the workpiece. The focused laser beam may be moved relative to the workpiece to control the geometry of the hole or cut. Laser drilling has become a cost-effective alternative to mechanical drilling that may be used to process almost any kind of workpiece material. In laser drilling, a through or blind hole is formed in a workpiece by directing the focused laser beam to a preselected location on the workpiece. The beam waist and Rayleigh range may be selected according to the desired diameter and depth of the hole. The location of the beam waist may be translated along the optical axis of the beam during the drilling to drill a deep hole relative to the Rayleigh range. Three common techniques for laser drilling are: single pulse drilling (shown in FIG. 1 A), which uses a beam waist diameter that is comparable to the desired hole diameter; percussion drilling (shown in FIG. IB), which uses the aforementioned beam waist diameter and a series of laser pulses to drill through the workpiece thickness; and trepanning drilling (shown in FIG. 1C), which uses a smaller beam waist and laser pulses applied to form spatially-overlapping holes or nearly-overlapping holes in the workpiece. In trepanning drilling, these holes perforate an outline of a larger hole having a desired size and shape, e.g., a circle or an irregular polygon.

Disadvantages of percussion drilling include“recast layer”, where the re solidified material remains at the wall of the hole and“bellow shaping”, where there is a local increase of the hole diameter. In laser trepanning drilling, there is a compromise between the desired edge quality and throughput (production time). Less spatial overlap of the individual holes increases the throughput, but also increases the surface roughness in the edges of the completed hole.

In laser trepanning, the three-dimensional profile of a hole may be controlled by selecting an incident angle of the laser beam on the workpiece to form, for example, a conical, funnel-shaped, or double-conical hole. Various optical configurations have been proposed for rapid trepanning, such as a tilted objective lens offset from a rotation axis, directing the beam through rapidly rotating wedges, or directing the beam through a rapidly rotating prism. A disadvantage of these configurations is that the profile of the hole is fixed by the hardware. Another disadvantage is that the processing speed is limited by the maximum practical rotation rate of a substantial mechanical assembly that includes lenses, wedges, or a prism. There is a need for a laser drilling system that provides fast precision drilling with a high duty cycle. Preferably, the drilling system would be configurable to drill holes of different diameters and profiles without modifying the hardware.

SUMMARY OF THE INVENTION

In one aspect, an apparatus in accordance with the present invention for laser machining a plurality closed forms in a workpiece comprises a laser source for providing a beam of laser pulses. Each laser pulse has a shorter duration first portion followed by a longer duration second portion. The first portion has a first power and the second portion has a second power. The first power is greater than the second power. A lens is provided that focuses the beam of laser pulses and a beam waist of the focused beam is located on or close to a surface of the workpiece. A scanner is provided for moving the focused beam across the surface of the workpiece. The focused beam traces a complete outline of a closed form during each laser pulse. The lens and the scanner are combined in a laser head. The first power is sufficient for the focused beam to pierce a full thickness of the workpiece during the first portion of each laser pulse. The second power is sufficient to continue machining through the full thickness of the workpiece during the second portion of each laser pulse. The laser head is continuously laterally translated with respect to the workpiece while machining the plurality of closed forms. Each closed form is machined by one laser pulse.

In another aspect, a method in accordance with the present invention for laser machining a plurality of closed forms in a workpiece comprises generating a beam of laser pulses. Each laser pulse has a shorter duration first portion followed by a longer duration second portion. The first portion has a first power and the second portion has a second power. The first power is greater than the second power. The beam of laser pulses is focused using a lens. A beam waist of the focused beam of laser pulses is located on or close to a surface of the workpiece.

The beam waist is moved across the surface of the workpiece using a scanner. The focused beam traces a complete outline of a closed form during each laser pulse. The lens and the scanner are combined in a laser head. The laser head is translated continuously and laterally with respect to the workpiece while machining the closed forms. The first power is sufficient for the focused beam to pierce a full thickness of the workpiece during the first portion of each laser pulse. The second power is sufficient to continue machining through the full thickness of the workpiece during the second portion of each laser pulse. Each closed form is machined by one laser pulse.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate a preferred embodiment of the present invention, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain principles of the present invention.

FIGS 1 A - 1C schematically illustrate common prior-art techniques for laser drilling a workpiece.

FIG. 2 schematically illustrates an apparatus for laser drilling a plurality of closed forms in a workpiece according to the present invention, including a laser source providing a beam of laser pulses, a lens, and a scanner.

FIG. 3 schematically illustrates further details of the workpiece in FIG. 2, including the outlines of closed forms.

FIG. 4A is a timing diagram schematically illustrating the power in one pulse of FIG. 2 vs. time.

FIG. 4B schematically illustrates a cross-section along the outline of a closed form in FIG. 3, through the thickness of the workpiece, which is projected onto a plane. DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, wherein like components are designated by like numerals, FIG. 2 schematically illustrates one embodiment of a laser apparatus 10 for drilling a workpiece 22 according to the present invention. A laser source 12 provides a beam of laser pulses that are delivered through an optical fiber 14 to a scanner 16 and a lens 18. By way of example, laser source 12 could be a pulsed fiber laser. A suitable commercial pulsed fiber laser is the StarFiber 150-P, available from Coherent Inc. of Santa Clara, California. The StarFiber 150-P provides a beam having a wavelength of 1070 nanometers (nm), a pulse energy of up to 15 joules (J), a pulse duration between 20 microseconds ( ms) and 50 milliseconds (ms), and a pulse repetition rate of up to 50 kilohertz (kHz).

Scanner 16 preferably uses galvanometer-actuated mirrors to deflect the beam of laser pulses. The deflected beam is focused by lens 18. Scanner 16 and lens 18 are combined in a laser head 24, which may be rotated or translated with respect to the workpiece. A suitable commercial laser head is the LASAG-FLBK series, configured for SmartDrill+ laser drilling, also available from Coherent Inc.

A beam waist 28 of the focused beam 26 is located on or close to a surface of the workpiece 22. Workpiece 22 is mounted on a translation stage 20 that supports and translates the workpiece. The location of beam waist 28 may be moved with respect to workpiece 22 by translating the laser head or by translating the translation stage and the workpiece thereon. Translation of the translation stage is indicated by double-headed arrows T. The location of the beam waist may also be moved and/or the angle-of-incidence of the focused beam onto the workpiece may be changed by deflecting the beam in scanner 16.

FIG. 3 schematically illustrates further details of workpiece 22 shown in FIG. 2. A closed form 1 indicates a completed hole. A closed form 2 indicates a partially-drilled hole. A closed form 3 indicates the outline of a hole to be drilled. When machining a hole that is larger than beam waist 28, the focused beam is translated along the outline of the closed form. The focused beam traces the complete outline of the closed form during one laser pulse, as discussed below. FIG. 4A is a timing diagram schematically illustrating the power of the focused beam applied to workpiece 22 during a laser pulse while machining closed form 2, which is a partially drilled hole. FIG. 4B schematically illustrates a cross- section, along the outline of the closed form and through the thickness of the workpiece, which is projected onto a plane. The hatched portion indicates a partially-drilled portion while the dotted portion indicates a portion to be drilled. FIGS 4A and 4B are aligned horizontally to show the location of the beam waist along the outline of closed form 2 at each moment during the laser pulse. For example, the beam waist is located at E indicated in FIGS 3 and 4B at time TE in FIG. 4A.

Each laser pulse has a shorter-duration first portion A, followed by a longer- duration second portion B, and an optional third portion C. During the first portion A, the power increases from a low power P _L to a peak first power P _A , then decreases to a second power P _B . The first power P _A is greater than the second power P _B .

During the second portion the power is about constant at the second power P _B .

During the optional third portion C, the power ramps down from the second power P _B to the low power P _L . If the third portion is omitted, the power would simply be set directly to P _L at the end of the second portion B. The low power P _L could any power below a minimum power that would process the material workpiece 22 is made of. For example, the low power P _L could be zero watts.

In operation, the first power P _A is sufficient for the focused beam to pierce the full thickness of the workpiece 22 during the first portion of each laser pulse, while the second power P _B is sufficient to continue machining through the full thickness of the workpiece 22. The focused beam is translated around the outline of the closed form during the second portion of each laser pulse.

In the inventive apparatus and method, each closed form is machined by one laser pulse. For a plurality of holes, the time interval between the first portions A of successive pulses corresponds to the repetition rate of the laser pulses. Translating the laser head or the workpiece moves the focused beam laterally between holes. During each pulse the scanner rapidly moves the focused beam laterally around the outline of the hole. The motion around the outline of the hole is fast compared to the motion between holes. Indeed, the motion around the outline may be so fast that the translation between holes can be made continuous, without need to stop for the duration of each laser pulse. Deceleration and acceleration of a heavy laser head or a heavy workpiece takes time and considerably reduces throughput. If the desired holes have constant pitch, the focused beam may be continuously translated at a constant velocity between holes. This continuous translation while laser machining a plurality of holes“on the fly” enables rapid throughput compared to prior-art apparatus and methods.

Continuous lateral translation of the laser head with respect to the workpiece means the path traced by the focused beam on the workpiece during a laser pulse may be slightly distorted, unless the scanner motion is compensated to render the desired outline of the hole correctly. For example, a pure circular motion by the scanner may produce an elliptically shaped hole. However, scanners having galvanometer-actuated mirrors enable rapid motion, minimizing any distortion. For example, a scan speed of the focused beam with respect to the workpiece in a range between 500 millimeters per second (mm/s) and 2000 mm/s, compared to a translation velocity of the laser head with respect to the workpiece in a range between 400 millimeters per minute (mm/min) and 1000 mm/min.

It may be possible to machine holes having shapes that are acceptable without compensation. For example, a circular scanner motion may produce holes that are sufficiently circular for an application. In the absence of compensation, it is preferable to have a scan speed that is at least an order-of-magnitude greater than the translation velocity, and most preferably at least two orders-of-magnitude greater. Whether compensated or not, the translation of the laser head with respect to the workpiece, the delivery of pulses by the laser source, and the motion of the focused beam across the surface of the workpiece by the scanner all need to be synchronized. For example, synchronized by common firmware that controls and coordinates the translation of the laser head or translation stage, the triggering of pulses, and the motion of the scanner.

In an illustrative example of the inventive apparatus and method, workpiece 22 in FIG. 3 is about 100 micrometer ( mm) thick and made of stainless steel. Each circular hole has a diameter of about 100 mih and the lateral separation between the holes is about 100 mm. Beam waist 28 is about 20 mm in diameter and the Rayleigh range is between about 150 mm and about 200 mm. In the illustrative laser machining process, each laser pulse melts the stainless steel in the focused beam and an assist gas blows the molten material out of the workpiece. PowerP _A is 400 watts (W) and the duration of the first portion is 20 ms . Power P _B is 160 W and the duration of the second portion is 300 ms. The scan speed during the second portion is about 950 mm/s. During the third portion, the duration of the ramp down from power P _B to power P _L is 80 ms. The laser head is translated continuously with respect to the workpiece. The pulse repetition rate and translation velocity are selected to achieve the desired hole-pitch of 200 mm. For example, a pulse repetition rate of 35 Hz corresponds to a translation velocity of about 420 mm/min (about 7 mm/s). Although the illustrative process produces melting, the inventive apparatus and method could also be applied to ablative processes.

First power P _A is selected to pierce the workpiece completely, which occurs at about the time of maximum power in first portion A, when the focused beam is located at a point D in FIGS 3 and 4B. Second power P _B is selected to maintain the material removal as the beam waist traces the outline of closed form 2 to make a hole. Second power P _B is sufficient to remove material through the full thickness of the workpiece, but is lower than first power P _A to minimize heating, unwanted damage, and heat affected zone. In the illustrative example, second power P _B is 40% of first power P _A . The optimum ratio of P _B to P _A will depend on material properties, such as the melting temperature and the thermal conductivity, and the thickness of the workpiece. For some workpieces, the optimum ratio may be more than 0.2 or even more than 0.4. For other workpieces, the optimum ratio may be less than 0.8 or even less than 0.6.

The hole is completed when the focused beam returns to point D at time T _D . The end of second portion B and the beginning of third portion C may be adjusted with respect to time T _D to optimize the drilling of a particular material and thickness of the workpiece. The ramp down in power could be begin either before or after time T _D , depending on the workpiece, but it is straight-forward to optimize this timing empirically. Ramping down the power during third portion C ensures complete material removal for clean separation of the closed form and also provides some annealing of the heated workpiece.

In the illustrative example, the beam waist has a relatively long Rayleigh range. The first power P _A applied initially is sufficient to pierce the workpiece. A reduced power may then be used as the beam has already penetrated the workpiece. Second power P _B needs to be sufficient to continue machining through the full thickness of the workpiece while the focused beam traces the complete outline of the desired outline. Since the focused beam has constant power and constant depth- of-focus while it moves laterally along the workpiece, the resulting hole has a uniform diameter and shape along the complete outline. This constant power produces uniform heating along most of the outline.

The focused beam may be applied to a location inside the closed form during the first portion A. During the second portion B, the focused beam would be translated from this inside location to the outline of the closed form and then translated along the complete outline of the closed form. For some workpieces, piercing at a location in the workpiece that is to be discarded may provide a more uniform edge in the completed hole, since an about constant second power P _B is applied along the whole outline of the closed form.

For some workpieces, first power P _A may need to be sustained for a time to fully pierce the workpiece, resulting in a longer first portion A than depicted in FIG. 4A. In the illustrative example, the duration of the first portion is 6.6% of the duration of the second portion. For some workpieces, the duration of the first portion may be less than 10% of the duration of the second portion. For other workpieces, the duration of the first portion may be less than 20% of the duration of the second portion or even less than 30%. For some workpieces, it may be advantageous to extend the second portion B, to trace the complete outline of the closed form a plurality of times. For a particularly thick workpiece, the focused beam may not pierce the full thickness during the first portion, and the focused beam traces the outline of the closed form a plurality of time during second portion B to make a through hole. The depth-of-focus may be changed from trace-to-trace. An advantage of the inventive method using a single pulse for laser machining a complete hole over prior-art laser trepanning using a series of laser pulses, is that the edges of the finished hole will be smoother. The prior-art method of FIG. 1C produces jagged edges having corrugations formed by the individual pulses.

The present invention is described above in terms of a preferred embodiment and other embodiments. The invention is not limited, however, to the embodiments described and depicted herein. Rather, the invention is limited only by the claims appended hereto.