Laser processing

Abstract

A method of laser processing including generating a laser beam having, at different longitudinal positions in a propagation direction, first and second transverse beam profiles of energy density. The first transverse beam profile is different to the second transverse beam profile and is non-Gaussian. The method includes carrying out a scan of the laser beam across a working surface, wherein, during the scan, the laser beam and/or working surface is adjusted such that, for a first part of the scan, the first transverse beam profile is located at the working surface and, for a second part of the scan, the second transverse beam profile is located at the working surface.

Claims

1. A method of laser processing comprising: generating a laser beam having, at different longitudinal positions in a propagation direction, first and second transverse beam profiles of energy density, the first transverse beam profile at a focal point of the laser beam having a flatter-top than the second transverse beam profile away from the focal point of the laser beam; and carrying out a scan of the laser beam across a working surface, wherein, during the scan, at least one of the laser beam and the working surface is adjusted to adjust a relative position of the working surface to the focal point such that, for a first part of the scan, the first transverse beam profile is located at the working surface and, for a second part of the scan, the second transverse beam profile is located at the working surface.

2. The method according to claim 1, wherein the first transverse beam profile is flatter than the second transverse beam profile in accordance with a measure of flatness.

3. The method according to claim 2, wherein the measure of flatness is one or more of a flatness factor, beam uniformity, plateau uniformity and edge steepness as defined within the EN ISO 13694-2001 standard.

4. The method according to claim 1, wherein the first transverse beam profile is flatter than a corresponding Gaussian profile having the same total and peak energies as the first transverse beam profile.

5. The method according to claim 1, wherein, in a plane transverse to the propagation direction, the first transverse beam profile comprises a ring about a central spot.

6. The method according to claim 1, wherein the laser processing comprises an additive manufacturing process, wherein the laser beam is used to solidify material to form a component, the method comprising using the second transverse beam profile to solidify material to form component surfaces and using the first transverse beam profile to solidify material to form a core of the component.

7. A controller for controlling a laser processing apparatus to carry out a method of laser processing according to claim 1.

8. A non-transient data carrier having instructions thereon, which, when executed by a processor of an apparatus, causes the apparatus to carry out a method of laser processing according to claim 1.

9. A method of laser processing comprising: generating a laser beam having, at different longitudinal positions in a propagation direction, first and second transverse beam profiles of energy density, the first transverse beam profile at a focal point of the laser beam being a super-Gaussian shape and the second transverse beam profile away from the focal point of the laser beam being a Gaussian shape; and carrying out a scan of the laser beam across a working surface, wherein, during the scan, at least one of the laser beam and the working surface is adjusted to adjust a relative position of the working surface to the focal point such that, for a first part of the scan, the first transverse beam profile is located at the working surface and, for a second part of the scan, the second transverse beam profile is located at the working surface.

10. The method according to claim 9, wherein the super-Gaussian shape is a second order super-Gaussian.

11. A controller for controlling a laser processing apparatus to carry out a method of laser processing according to claim 9.

12. A non-transient data carrier having instructions thereon, which, when executed by a processor of an apparatus, causes the apparatus to carry out a method of laser processing according to claim 9.

13. A method of laser processing comprising: generating a laser beam having, at different longitudinal positions in a propagation direction, first and second transverse beam profiles of energy density, the first transverse beam profile at a focal point of the laser beam being a super-Gaussian shape and having a flatter-top than the second transverse beam profile away from the focal point of the laser beam; and carrying out a scan of the laser beam across a working surface, wherein, during the scan, at least one of the laser beam and working surface is adjusted to adjust a relative position of the working surface to the focal point such that, for a first part of the scan, the first transverse beam profile is located at the working surface and, for a second part of the scan, the second transverse beam profile is located at the working surface.

14. The method according to claim 13, wherein the super-Gaussian shape is a second order super-Gaussian.

15. The method according to claim 13, wherein the first transverse beam profile is flatter than the second transverse beam profile in accordance with a measure of flatness.

16. The method according to claim 15, wherein the measure of flatness is one or more of a flatness factor, beam uniformity, plateau uniformity and edge steepness as defined within the EN ISO 13694-2001 standard.

17. The method according to claim 13, wherein the first transverse beam profile is flatter than a corresponding Gaussian profile having the same total and peak energies as the first transverse beam profile.

18. A controller for controlling a laser processing apparatus to carry out a method of laser processing according to claim 13.

19. A non-transient data carrier having instructions thereon, which, when executed by a processor of an apparatus, causes the apparatus to carry out a method of laser processing according to claim 13.

Description

DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a powder bed fusion apparatus according to an embodiment of the invention;

(2) FIG. 2 shows an optical scanner according to an embodiment of the invention;

(3) FIG. 3 shows an optical scanner according to another embodiment of the invention;

(4) FIG. 4 shows an optical scanner according to yet another embodiment of the invention;

(5) FIG. 5 shows an optical scanner according to a further embodiment of the invention;

(6) FIG. 6 shows an optical scanner according to a yet further embodiment of the invention;

(7) FIG. 7 is a graph showing a cross-section of Gaussian and super-Gaussian beam distributions with varying n;

(8) FIG. 8a is a plot showing variation in beam intensity with longitudinal position for a super-Gaussian beam of the order n=2;

(9) FIG. 8b is a plot showing variation in beam intensity with longitudinal position for a super-Gaussian beam of the order n=3;

(10) FIG. 8c is a plot showing variation in beam intensity with longitudinal position for a super-Gaussian beam of the order n=5;

(11) FIG. 9 shows a variation in peak intensity with propagation distance for a super-Gaussian of the order n=2;

(12) FIG. 10 is a plot showing variation in beam intensity with longitudinal position for a laser beam that produces a ring-like intensity distribution at focus and Gaussian-like beam profiles either side of the focus;

(13) FIG. 11 shows a variation in peak intensity with propagation distance for the laser beam of FIG. 10; and

(14) FIG. 12 is a plot showing the ring-like profile generated at focus.

DESCRIPTION OF EMBODIMENTS

(15) Referring to FIGS. 1 and 2, a selective laser melting (SLM) apparatus according to an embodiment of the invention comprises a build chamber 101 having therein a partition 115 that defines a sleeve in which a build platform 102 is lowerable. A further partition 114 defines a surface onto which powder can be deposited for spreading across an upper surface of the build platform 102 and/or the powder bed 104 to form a powder layer in a working plane 104a. The sleeve 115 and travel of the build platform 102 defines a build volume 116 in which an object 103 is built by selective laser melting powder 104. The platform 102 can be lowered within the build volume 116 using mechanism 117 as successive layers of the object 103 are formed.

(16) Layers of powder 104 are formed as the object 103 is built by dispensing apparatus 109 and a wiper 110. For example, the dispensing apparatus 109 may be apparatus as described in WO2010/007396. A laser module 105 generates a laser for melting the powder 104, the laser directed onto a working surface 104a of the powder bed 104 as required by optical module 106 under the control of a computer 160. The laser beam 118 enters the chamber 101 via a window 107.

(17) Computer 160 comprises a processor unit 161, memory 162, display 163, user input device 164, such as a keyboard, touch screen, etc., a data connection to modules of the laser melting apparatus, such as optical module 106, laser module 105 and motors (not shown) that drive movement of the dispensing apparatus, wiper and build platform 102. An external data connection 166 provides for the uploading of scanning instructions to the computer 160. The laser unit 105, optical unit 106 and movement of build platform 102 are controlled by the computer 160 based upon the scanning instructions. Computer software is stored in memory 162 and execution of the computer program by processor 161 causes the computer to control the selective laser melting apparatus in accordance with the method described below.

(18) FIG. 2 shows an optical train of the optical module 106 in detail. The optical module comprises steering optics in the form of two mirrors 201 (only one of which is shown) rotatable under the control of a galvanometer for steering of the laser beam 118 to selected locations on the working surface 104a. The optical train further comprises movable focussing optics 202 under the control of a voice coil for adjusting a position of the focal point of the laser beam 118 relative to the working surface 104a. The laser beam is delivered to the optical module using a beam delivery optic 203, such as an optical fibre of a fibre laser.

(19) Located between the beam delivery optic 203 and the focussing optics 202 is a beam profile reshaping device 204 in the form of a refractive optical element. The refractive element 204 comprises one or more freeform surfaces shaped to reshape a phase and/or amplitude of the incident laser beam. The design of beam profile reshaping devices to provide a desired reshaping of the laser beam is described in “Laser Beam Shaping, Theory and Techniques, Fred M. Dickey, CRC Pres 2014”.

(20) For a single optic beam profile reshaping device, a high β factor is required. The β factor defines a quality of the shaped focal spot, where

(21) $\beta =\frac{2\sqrt{2\pi }{r}_0{y}_0}{f^2}$
and r.sub.0 is the 1/e.sup.2 radius of the incoming beam, y.sub.0 the half-width of the desired spot and f the focal length of the processing lens. The β factor should be >10 for good performance. The single optic beam profile reshaping device introduces divergence into the system, meaning that the performance in terms of Rayleigh range, spot size, etc. will be worse than that of the original Gaussian beam.

(22) Referring to FIG. 3, performance of the system can be improved through use of a double optic system as the beam profile reshaping device. In such an arrangement, a first optic 304a performs the reformatting of the beam and a second optic 304b, approximately 100 mm downstream of the first optic 304a, corrects the phase of the reformatted beam. Such a beam profile reshaping device can produce flat-top distributions with near single-mode diffraction limited performance.

(23) In the drawings, the first type of beam profile reshaping device comprising a single optic is called a Type I beam profile reshaping device and the second type of optic comprising multiple optics is called a Type II beam profile reshaping device.

(24) As an alternative to utilising bulk optic beam profile reshaping devices, it is possible to write the beam profile reshaping device into a surface of the end cap of the fibre laser 403. By direct writing of the shaping surface onto one of the existing optical components, the optical train comprises fewer surfaces, limiting the potential for failure and optical losses. Beam profile reshaping devices typically require very tight tolerances in an alignment of the laser beam with the beam profile reshaping device. Direct writing on the fibre optic circumvents this problem with the accurate alignment of separate optical components.

(25) FIG. 4 shows a system with a Type I beam profile reshaping device 404 directly written on to the end cap of the fibre optic and FIG. 5 shows a Type II beam profile reshaping device comprising a freeform surface 504a directly written onto the end cap of the fibre optic and a second optic 504b that corrects the phase of the laser beam.

(26) FIG. 6 shows a further embodiment of an optical train. In this embodiment, an optical delay line in the form of a corner cube 606 has been added between the steering mirrors 601a, 601b. The corner cube is movable such that a path length for the laser beam between the two mirrors 601a, 601b can be varied. The corner cube 606 may be arranged to move back and forth by approximately 7.5 mm to provide an approximately 15 mm variable path length.

(27) Referring to FIGS. 7 to 9, the beam profile reshaping device is arranged to shape the Gaussian beam incident on the beam profile reshaping device into a super-Gaussian distribution. FIG. 7 shows how an order, n, of the super-Gaussian profile increases, a flatter topped transverse beam profile is achieved.

(28) Unlike a Gaussian beam, the super-Gaussian distribution does not remain constant during propagation. Using a beam profile reshaping device to alter the phase of a Gaussian beam to form a super-Gaussian beam will only result in a super-Gaussian transverse beam profile at a single z-plane (with a certain depth of focus). Outside of this z-range, the distribution changes. FIGS. 8a, 8b and 8c shows the distribution achieved at varying z (propagation direction) for super-Gaussian beams of varying n. A super Gaussian (first) transverse beam profile is present at focus but in regions centred around +/−15 mm a (second) transverse beam profile becomes more Gaussian-like. As can be seen, lower values of n give greater depth of field for the different regions of the beam.

(29) As n increases diffraction effects become more prominent, giving greater divergence and a less smooth transition from flat-top to Gaussian. Furthermore, the lower value of n, the fewer high frequency surface features are required for the beam profile reshaping device and, thus, the easier it is to form the beam profile reshaping device. Plotting the peak intensity with varying propagation distance, as shown in FIG. 9, shows the higher peak intensity in the Gaussian-like regions with a lower peak intensity in the flat-top, super-Gaussian region.

(30) In use, the laser beam is scanned across the working surface to solidify selected areas of each powder layer to form a component. Adjustment of the scanning mirrors, voice coil and, if present, the spatial delay line, is controlled by computer 160. For different areas of powder at the working surface 104a, different transverse beam profiles, the flat-top or Gaussian-like beam profiles, are used for solidifying the powder material. To form fine lines, such as may be required around a border of an area, the Gaussian-like transverse beam profile may be used, whereas within a core of an area, the flat-topped transverse beam profile may be used. Adjustment of the type of transverse beam profile (Gaussian or flat-topped) located at the working surface can be achieved using the focussing optics controlled by the voice coils and/or movement of the corner cube, if present. When switching between the two types of transverse beam profiles, the laser power may be adjusted.

(31) In this way, large regions of a powder layer may be quickly solidified using the flat-topped profile at higher power but without significantly increasing an amount of material vaporised by the laser beam, whereas regions that require fine detail/lines, such as at the surface of the component can be formed using the narrow, Gaussian-like transverse beam profile. Such a technique may speed up the build whilst still achieving near 100% density of the part and a good surface finish.

(32) Rather than reshaping the laser beam to form a super-Gaussian profile at the beam waist, a different, non-Gaussian profile may be formed. FIGS. 10 to 12 show a laser beam having a ring-shaped beam profile at the beam waist with Gaussian-like profiles either side of the beam waist. The ring-shaped beam profile may be desirable to reduce differential heating of powder material across the profile. In particular, with a circular spot (having a Gaussian or super-Gaussian beam profile), powder the is exposed, during scanning, to a central region of the spot may be heated more than powder heated only by an edge region of the beam profile, again potentially resulting in vaporisation of powder that is heated by the central region and/or powder that is heated only by the edge region not being melted. Using a ring-shaped profile may mitigate this problem.

(33) A further beam profile that may be useful in additive manufacturing apparatus is a ring about a Gaussian spot. A peak intensity of the Gaussian spot may be higher than a peak intensity of the ring. It is believed that the ring may help to stabilise a melt pool formed by the central Gaussian spot.

(34) In one embodiment, at least the shape of the laser “spot” formed by the laser beam on the powder at the waist of the laser beam is substantially rectangular, such as approximately square shaped (the shape of the spot being distorted as the laser beam is directed to be non-perpendicular with the powder layer). If a square or line shaped spot is used, then the scan paths may be selected based upon an orientation of the spot. For a square or line shaped spot, one would typically expect the spot to be scanned in a direction perpendicular to a side of the square or line shaped spot.

(35) It will be understood that alterations and modifications may be made to the above described embodiments without departing from the scope of the invention as described herein. For example, the laser may be a fibre-laser, or a non-fibre laser, such as a diode pumped solid state laser or a direct diode laser.