Patent Publication Number: US-2020276667-A1

Title: Systems and methods for direct laser melting of metals using non-diffracting laser beams

Description:
BACKGROUND 
     Additive manufacturing machines perform additive manufacturing processes that use laser energy to fuse successive layers of powder bed material to build a three-dimensional (3D) part. A direct metal laser melting (DMLM) is one example of an additive manufacturing machine. For purposes of this disclosure, the terms “direct metal laser melting”, “DMLM”, and “additive manufacturing machine” are used interchangeably. Traditional spherical optics focuses the laser energy to a collimated, Gaussian beam shape, with an in-plane transverse profile and a sagittal focus intensity described by gaussian distribution functions. The physical extent of the distribution is determined by the curvature and fill-factor of the optical elements. This conventional approach to forming a Gaussian-focused beam for DMLM operations results in the beam&#39;s most stable part being limited to typically just a few millimeters in the beam propagation direction. 
       FIG. 1  schematically depicts conventional DMLM machine  100 . Collimator  110  forms a parallel beam from laser light provided by laser source  105 . Dynamic optics unit  112  contains optical elements that can are mechanically movable in relation to one another under the control of optical control and driver unit  120 . Dynamic movement of the optical elements alters their focal length to refocus the laser beam as scanner  125  scans the beam in an x-y plane across powder bed  130 . This refocusing during scanning operation is necessary in conventional additive manufacturing machines because of inherent error sources that singly or cumulatively can move the focus spot from the powder bed. F-Theta lens  127  is incorporated into conventional DMLM machines to provide a flatter field at the powder bed plane by adjusting the focal length during beam scanning to retain in-focus operation as the beam is swept across the scanner&#39;s field of view.  FIG. 2  illustrates a depiction of laser beam  200  generated by conventional DMLM machines. Dynamic optic  115  can be repositioned under the control of driver unit  120  along the beam propagation (z) axis. Changes in the distance between dynamic optic  115  and focusing optic  118  causes a change in the beam width impinging on the focusing optic. This change in beam width alters the position of beam waist  210 , which is the depth-of-focus for melting powder. Beam-waist length  220  along the z-axis is on the order of a few millimeters (in the range of about 2-5 mm). Dynamic movement of the optical elements can reposition the beam waist along the z-axis for consistent welding. This dynamic movement is necessary in conventional DMLM machines to negate errors introduced by thermal changes in the optical path, imprecise build plate movement, and inconsistent powder height. These errors can cumulatively move the laser spot along the z-axis about the length of the beam waist (e.g., about 2-5 mm), which moves the focus off the powder surface. 
     Providing a consistently sized laser spot over the entire powder bed (transverse to the beam) is a significant challenge in manufacturing, installing, and maintaining a laser-based powder bed consolidation machine (or DMLM 3D printer). The powder bed is typically in the range of 100 mm to 500 mm transverse to the beam and positioned distal from the laser scanner system in about a similar scale. Conventional approaches to maintain a focused spot size across the powder bed requires considerable tuning and software-based calibration to control lens position, beam width. To maintain an accurate, focused spot calibration files need to anticipate the physical extent of the build plate to apply adjustments to build plate height, fore/aft positions. 
     During manufacture and installation, technicians often make careful adjustments by hand to optomechanical components to correctly locate the laser beam relative to the build plate location. Despite best efforts, under conventional techniques the location of a laser beam&#39;s focal region relative to the build plate position will ultimately drifts due to mechanical manipulations of the system, temperature, run time, age of components, and vibration. As a result, conventional approaches require considerable regular calibration and/or adjustment is required to ensure that build quality is good. 
     What is missing from the art is an ability to generate a laser beam for DMLM additive manufacturing processes that has a focal spot range sufficiently deep enough to be invariant to these mechanical variations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically depicts a conventional DMLM machine that includes a conventional dynamic optics unit; 
         FIG. 2  depicts a laser beam generated by the conventional DMLM machine of  FIG. 1 ; 
         FIG. 3  schematically depicts a DMLM machine equipped with an axicon focusing unit in accordance with embodiments; 
         FIG. 4  depicts a laser beam generated by an axicon focusing unit having a single axicon optical element in accordance with embodiments; 
         FIG. 5  depicts a laser beam generated by an axicon focusing unit having a first set of optical elements in accordance with embodiments; and 
         FIG. 6  depicts a laser beam generated by an axicon focusing unit having a second set of optical elements in accordance with embodiments. 
     
    
    
     DESCRIPTION 
     Embodying systems and methods generate a type of non-gaussian beam that can be used to melt and process powder in an additive manufacturing machine. Conventional approaches to providing a laser beam suitable for additive manufacturing generates a traditional gaussian optics using beam shaping methods. Embodying systems and methods convert a traditional gaussian beam (the product of spherically shaped optics) into a non-diffracting optical beam profile pattern that is created by the superposition of two or more collimated beams. 
     A laser beam profile, suitable for use in the laser consolidation of a powder bed, generated by embodiments does not require extensive calibration and is insensitive over a large range of positional variations between the laser scanning system, the build plate, and the powder height. The energy distribution of this non-diffracting beam is tailored to provide an optimal peak-to-sidelobe intensity ratio using a set of non-gaussian optical elements, such as right circular prisms, which results in optimal powder melting and consolidation. 
     The non-diffracting beam profile is truncated to provide an extended outer portion of the beam of enough intensity to preheat a region of the powder bed prior to exposing that region to a central beam portion having greater intensity sufficient to cause actual melting. This ability to preheat the powder prior to melting is instrumental in the processing of crack prone alloys to mitigate the cracking. The laser beam generated by embodying systems and methods is insensitive to position once tuned and requires no calibration procedures. This embodiment-generated laser beam ensures that all points across the build plate receive the same energy distribution, and therefore melt uniformly. 
     Conventionally-generated collimated beams can be achieved with a single element, or an optical train of multiple elements placed a focal distance from the exit aperture of a fiber-optic laser source. A well-collimated beam can propagate long distances without changing its distribution of energy either in transverse or sagittal planes. In accordance with embodiments, this beam profile can be dynamically adjusted by altering the spacing between elements of the optical train. To create an embodiment-generated beam shape, portions of the collimated beam are bent to constructively and destructively interfere with other portions of the overall beam. 
     In accordance with embodiments, these interference patterns can be achieved with a right-circular prism (or axicon lens). The prism can be refractive, reflective, or some combination of the two. The resulting interference pattern generates an intensity profile that can be described mathematically as the superposition of Bessel beams. In implementation, the finite extent of the beam can be tuned. A true Bessel beam has infinite extent and would require tremendous energy to produce. Embodiments can combine two, three, or more prism elements in cascade to achieve tuning of the beam&#39;s spatial extent. An embodiment-generated beam can have its ratio of central peak energy to peripheral energy tailored to be optimal for melting powder. For example, a pair of steeply-angled prisms can convert a collimated beam into a collimated ring; then as the collimated ring passes through a small angle prism element it is converted to an overlapping Bessel beam. 
     An embodiment-generated non-diffracting laser beam provides a unique way of melting powder. The Bessel-type beam is comprised of a series of concentric rings, with each ring representing a fraction (about 5-25%) of the total energy level in the laser beam. In accordance with embodiments, the energy of the distribution function central lobe can be tuned to provide an intensity to melt the powder. The surrounding concentric rings can be tuned to preheat the powder prior to being exposed to the central lobe. This preheating can reduce spatter by reducing thermal gradients, which can prevent cracking in hard to weld alloys. Further the preheat regions can be used to sinter powder to prevent movement during subsequent melting processing. 
     Embodiment-generated Bessel beams can be generated by controlling the energy level distribution between the beam&#39;s central portion and sidelobes. This control can be attained by controlling velocity (dwell time) across the powder bed in combination with beam forming techniques. Embodiments are not limited to any particular type of laser source or method of generating the original beam. A DMLM machine that includes an embodiment-generated non-diffracting laser beam does not require the extensive tuning, software-based calibration, and regular adjustments of conventional DMLMs because of the beam&#39;s insensitivity to spatial variances present in DMLMs. 
     An embodiment-generated beam is non-diffracting and does not require focusing to achieve correct energy distribution. Elimination of a focusing system increases DMLM throughput, resulting in decreased expenditure of time and money during the manufacture, installation, calibration, and maintenance of a DMLM machines. Further, elimination of a focusing stage reduces the overall cost to produce the DMLM machine. 
     An embodiment-generated beam is insensitive to position across the build plate (x, y) and along its z-axis. The same beam generation system can be used on large and small machine sizes. The large spatial extent and ratio-centric nature of the energy distribution of an embodiment-generated beam provides advantages for difficult to weld materials, grain size and structure control and spatter control—thus, producing cleaner welds than conventionally-generated beams. 
       FIG. 3  schematically depicts DMLM machine  300  equipped with axicon focusing unit  315  in accordance with embodiments. Collimator  310  forms a parallel beam from laser light provided by laser source  305 . Scanner  325  scans the beam in an x-y plane across powder bed  330 . In accordance with embodiments, axicon focusing unit  315  contains optical elements that are in fixed relation to each other and immoveable at a predetermined displacement between the optical elements. An embodying axicon focusing unit replaces both dynamic optics unit  112  and F-Theta lens  127 , which are required in conventional DMLM machines to produce proper welds. 
       FIG. 4  depicts laser beam  400  generated by axicon focusing unit  315  with axicon lens  340  in accordance with embodiments. Laser beam  400  is a simplified non-diffracting Bessel beam formation for laser micro welding in a DMLM machine process. Collimated beam  405  is provided to axicon focusing unit  315 , which has axicon lens  340  positioned in the collimated beam&#39;s path. The axicon lens bifurcates the collimated beam into deflected collimated beam sections  410 A,  410 B. These two collimated beam sections merge at a distance distal from the axicon lens to form interference pattern region  415 . 
     The axicon focusing unit depicted in  FIG. 4  can generate an interference pattern propagation area along the z-axis that has a length in the range of about 100 mm to 1000 mm. In accordance with embodiments, reduction of the interference pattern area&#39;s length can concentrate more energy into the interference pattern. For DMLM machine applications, the interference pattern area&#39;s length needs not to be in that range—typical application can benefit with a beam waist length in the range of about 5 mm to 25 mm. It should be readily understood that the beam waist length is not limited to any produced by embodiments is not so limited. Rather, the beam waist length can vary for different working distances. For example, for some implementations the beam waist length can be about 2-5% of the working distance for that machine implementation. 
       FIG. 5  depicts laser beam  500  generated by axicon focusing unit  315  having a first set of optic elements in accordance with embodiments. An embodying axicon focusing unit can produce a beam with a beam waist length in the range of about 5 mm to 25 mm, which results in a DMLM machine having a beam impervious to errors introduced by thermal changes in the optical path, imprecise build plate movement, and inconsistent powder height. By introducing a reduction of its beam waist length, beam  500  has more energy concentrated in its interference pattern region than beam  400 . This concentration of energy is a result of moving energy from the beam&#39;s sidelobes into its central region (i.e., center lobe and near-in sidelobes). 
     Collimated beam  505  is provided to axicon focusing unit  315 , which has a pair of lenses  350 ,  352  are in a fixed relation to each other and immoveable. The lenses are positioned in the collimated beam&#39;s path. This pair of lenses forms annular beam  510 . Lenses  350 ,  352  are displaced at a predetermined fixed distance D apart. Selection of the magnitude of the lens&#39; displacement changes the annular beam&#39;s diameter to result in a change in the beam waist length of the interference pattern. The annular beam is bifurcated by axicon lens  354  to generate two deflected collimated beam sections  512 A,  512 B. These two collimated beam sections merge at a distance distal from the axicon lens to form interference pattern region  515 . 
       FIG. 6  depicts laser beam  600  generated by axicon focusing unit  315  having a second set of optic elements in accordance with embodiments. Collimated beam  605  is provided to axicon focusing unit  315 , which includes beam splitter  360  in the optical beam&#39;s path to create two beams  208 A,  208 B that follow two separate optical paths within axicon focusing unit  315 . The beam split ratio can be varied to achieve different energy intensity levels in interference pattern region  615 . 
     The first optical path includes a pair of axicon lenses  370 ,  372  that are displaced apart by a predetermined, fixed distance. The lenses are positioned in the collimated beam&#39;s path. This pair of lenses forms annular beam  610 . Selection of the magnitude of the lens&#39; displacement changes the annular beam&#39;s diameter to result in a change in the beam waist length of the interference pattern. Axicon lens  374  is an annulus structure defining an aperture. This annulus axicon lens is in the path of annular beam  610 . The annular beam is bifurcated by axicon lens  374  to generate two deflected collimated beam sections  612 A,  612 B. These two collimated beam sections merge at a distance distal from the axicon lens to contribute to the energy level of interference pattern region  615 . 
     The second optical path can include three right angle reflectors  362 ,  364 ,  366  that direct beam  608 A along the second optical path so that beam  608 A and beam  608 B are isolated from each other within axicon focusing unit  315 . In accordance with implementations, the number of beam splitters, reflectors (and angle of reflection) can vary. Beam  608 A passes through the aperture defined by axicon lens  374 . Axicon lens  380  is in the second optical path, in a portion of the path that passes through the aperture of axicon lens  374 . Lens  380  focuses beam  608 A so that it converges with beams  612 A,  612 B in interference pattern region  615  to contribute to focus spot energy level. 
     Although specific hardware and methods have been described herein, note that any number of other configurations may be provided in accordance with embodiments of the invention. Thus, while there have been shown, described, and pointed out fundamental novel features of the invention, it will be understood that various omissions, substitutions, and changes in the form and details of the illustrated embodiments, and in their operation, may be made by those skilled in the art without departing from the spirit and scope of the invention. Substitutions of elements from one embodiment to another are also fully intended and contemplated. The invention is defined solely with regard to the claims appended hereto, and equivalents of the recitations therein.