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1. (WO2019005602) Large Scale High Speed Precision Powder Bed Fusion Additive Manufacturing
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Large Scale High Speed Precision Powder Bed Fusion Additive Manufacturing

RELATED APPLICATIONS

[0001] This Application claims priority to U.S. Provisional Application No. 62/527,448, filed June 30, 2017, which is hereby incorporated by reference in its entirety.

BACKGROUND

[0002] Powder Bed Fusion (PBF) systems for Additive Manufacturing (AM) began with Deckard (US 5597589 A) creating Selective Laser Sintering (SLS) as a means for manufacturing three dimensional parts using a focused laser to selectively sinter an exposed layer of powder material in the pattern of a cross-sectional slice of the desired geometry, covering that sintered layer with a fresh powder layer, and repeating the process with the successive adjacent cross-sectional slices until the desired geometry is constructed. Meiners et al. (US 6215093 Bl) introduced Selective Laser Melting (SLM) by adding the requirement that the powder temperature be brought to the melting point in a manner such that it reaches a fully molten state. Many systems and variants have been developed. Most systems employ a single beam scanning over a particular area. Using this methodology the build rate tends to be slow, the dimensional accuracy and resolution are limited, and metallurgical integrity can be a concern. Greater speed, dimensional accuracy, and assured material quality are desired.

[0003] A scanned single beam generates a melt pool geometry that is somewhat like a distorted portion of a sphere from the combined effects of temperature distribution from the moving beam, and surface tension in the molten material subject to the gravity field, having a liquid-gas interface with the surrounding gas and liquid-solid interface with unmelted material. For the most part, the process degrees of freedom available in such systems are beam size, beam power, scan speed, hatch spacing (the overlap of adjacent scans), scan path strategy, and perhaps a level of preheating. While these parameters can be optimized to generate a useful version of the previously described melt pool, they are insufficient to manipulate and control the temperature distribution and melt pool geometry in three dimensional space and time with any significant independence in these dimensions.

[0004] Temperature distributions in these single beam scanning melt pools have large gradients in all three dimensions providing a highly curved solidification interface, and also have maximum temperatures often exceeding the boiling temperature and significantly vaporizing material. While often pushed to operate in this high temperature regime by the desire to melt more material faster, this can generate a number of defects as vapor recoil pressure produces depressions in the melt pool, "spatter" is ejected from the melt pools, vapor streams disrupt and displace the surrounding powder bed creating denuded zones, and gas bubbles can be introduced into the molten pool prior to solidification. Further, the presence of large thermal gradients in undesirable directions during solidification and melt pool geometries that promote the instantiation of high angle grain boundaries can lead to cracking of the solidified material. Increasing power beyond optimized parameters can produce cracking, and attempts to simultaneously increase scanning speed to accommodate the higher power can lead to "balling," which is a surface tension induced separation of the melt pool into spherical features that solidify on the surface of the previous layer. Thus, the processing rate of systems scanning individual beams is severely limited.

[0005] To increase the processing rate, higher laser power levels are necessary; but the powder bed cannot accept more energy concentrated at the single spot location without causing the problems described above. Therefore, while more power is desired, its application should be distributed over space and time in a manner so that more energy can be delivered while avoiding the constraining issues. Diaz et al. (WO 2016026706 Al) provided an approach to this still using a single beam, but superimposing a smaller beam motion pattern on top of the independently controlled larger (scanning) beam motion.

[0006] To provide a larger working volume or increased build rates, systems have been devised using multiple beams where each beam is independently positioned by its own scanner. Each of these beams is still subject to the same limitations previously discussed as each isolated beam still uses the standard process.

[0007] Multiple beams with different sizes and powers were used sequentially passing over the same region by Benda et al. (US 5393482 A, US5508489 A) in SLS to preheat with one or more larger beams prior to sintering with a focused beam. Other incarnations have also used multiple beams that may pass sequentially over the same region. Typically, these utilize fiber lasers with ends arranged in an optical head in one or two dimensional arrays and

translated with an x-y stage. Scan speeds are inherently limited by this approach due to the inertial limitations of performing the scanning with translation stage motion. Practicalities involving space, mass, and fiber bundle stiffness also limit the number of beams that can be utilized with this approach.

[0008] Material cracking can occur either during beam exposure if the thermal stresses from high thermal gradients in the solid material surrounding the melt pool exceed the fracture strength, or the cracking can occur later during the cooling phase if high residual

solidification stresses are introduced by large thermal gradients at the solidification boundary. This is further exacerbated when grain boundaries form with high angles relative to the generated surface. The benefits of a larger area low aspect ratio molten pool were demonstrated by Marcin et al. (US 6103402 A) when larger beam diameters and lower powers were utilized to avoid cracking during the repair of single crystal turbine blades with laser deposition.

[0009] Some AM systems utilize the concept of "voxels" to represent the three dimensional structure to be fabricated. A voxel is a geometrical construct that is a discrete unit of three dimensional space in a three dimensional grid that is used to define a three dimensional structure. It is a three dimensional analog to a "pixel" which is a familiar concept in two dimensional display systems. Just as undesirable "pixilation" occurs in two dimensions as individual pixels become discemable when the pixel resolution is insufficiently fine, so also is the potential for discrete three dimensional artifacts associated with the voxels. As higher fidelity is desired in the fabricated structures, systems and methods utilizing alternate constructs for representation without three dimensional discretized grids can be beneficial.

[0010] Outside the realm of AM, higher speed multiple beam laser scanning has been utilized in other fabrication processes. Tamkin et al. (US 6084706 A) devised a high efficiency laser scanner and optical relay employing a polygon scanner and primarily reflective optics that was utilized for photolithography laser direct imaging of printed circuit board patterns.

BRIEF SUMMARY OF THE INVENTION

[0011] According to one embodiment disclosed herein, a system for power bed fusion additive manufacturing is disclosed. The system comprises a platen having an additive manufacturing powder material spread thereon, a plurality of energy beams, an optical relay system comprising: fixed and moving optics that maintain focus of the plurality of energy beams at the surface of the additive manufacturing powder material , beam steering system that actively corrects the flight of the plurality of energy beams, a rotating polygon mirror that reflects the plurality of energy beams for fast scanning motion, and post scan relay optics that provide planar focus of the plurality of energy beams at a predetermined portion of the additive manufacturing powder material, and control electronics that selectively control the motion of the optical system and the power of the plurality of energy beams.

[0012] According to another embodiment disclosed herein, a method of additive

manufacturing is disclosed. The method comprises directing, through optical relay components, an arrangement of a plurality of energy beams, toward a platen having an additive manufacturing powder material spread thereon, wherein the relayed image of the arrangement of a plurality of energy beams scans a region of the additive manufacturing powder material to provide selective melting of the powder material under prescribed melt pool conditions involving a limited temperature range and constraints on the melt pool geometry.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Fig. 1 is a schematic presentation of an example device for large scale high speed powder bed fusion additive manufacturing, according to an embodiment of the present invention.

[0014] Fig. 2 is an example of a beam pattern. This particular beam pattem is appropriate for titanium fabrication in conjunction with a scan speed of 34 m/s.

[0015] Fig. 3A is an example time history graph of the incident intensity and corresponding temperature at the top and bottom of the fabricated layer throughout a scan of the beam pattern example provided in Fig. 2 when scanning over titanium at a scan speed of 34 m/s.

[0016] Fig. 3B is an example time history graph of the incident intensity and corresponding temperature at the top and bottom of the fabricated layer throughout a scan of single beam in the conventional manner over titanium using process parameters appropriate for that material. Parameters: 100 μηι beam diameter, 110 W, scan speed 0.8 m/s, 45 μηι layer depth, 60 μηι hatch spacing.

[0017] Fig. 4 provides example details in the two dimensional plane at time 25.1 μββΰ of a scan with the beam pattern depicted in Fig. 2.

[0018] Fig. 5 provides example details in the two dimensional plane at time 32.6 μββΰ of a scan with the beam pattern depicted in Fig. 2.

[0019] Fig. 6 provides example details in the two dimensional plane at time 50.4 μββΰ of a scan with the beam pattern depicted in Fig. 2.

[0020] Fig. 7 provides example details in the two dimensional plane at time 75.6 μββΰ of a scan with the beam pattern depicted in Fig. 2.

[0021] Fig. 8 provides example details in the two dimensional plane at time 107.1 μββΰ of a scan with the beam pattern depicted in Fig. 2.

[0022] Fig. 9 provides example details in the two dimensional plane at time 123.2 μββΰ of a scan with the beam pattern depicted in Fig. 2.

[0023] Fig. 10A shows example melt pool geometry according to an embodiment of the present invention.

[0024] Fig. 10B shows example melt pool geometry of the conventional single beam process.

[0025] Fig. 11 compares the process parameters and the material processing rates for titanium fabrication between an example embodiment of the present invention and typical appropriate values for the conventional single beam process.

DETAILED DESCRIPTION OF THE INVENTION

[0026] To provide processing rates and part sizes significantly exceeding the current state of the art in PBF while also improving resolution, a method and apparatus are described. The process utilizes multiple (typically thousands) energy beams (e.g. laser) that cumulatively provide high power (typically tens to hundreds of kilowatts) while locally delivering relatively moderate intensity doses of energy (peak intensity of 25 kW/mm2 for example on titanium powder). The beam power is distributed over space using the plurality of beams, each having their power individually controlled, with the energy input to a particular powder bed location spread over time through sequential brief interactions with multiple beams of various sizes and powers. A controlled geometry large area melt pool with a low depth aspect ratio is developed with the temperature distribution, thermal gradients, and solidification surface geometry managed in a manner that enables higher speed scanning without defects such as cracking, balling, spatter, or powder bed denudation, such as are encountered in the conventional single beam process when beam power and scanning speed are increased.

[0027] The controlled melt pool conditions that enable high scanning speeds free of defects include, but are not limited to: maintaining maximum melt pool temperatures between the point of complete melting and initiation of boiling for the given powder material, lateral surface temperature gradients that manage the capillary flow due to thermal gradient driven surface tension variations, and a solidification surface boundary during cooling that is nearly parallel to the top of the layer being formed and is fairly constant in slope everywhere during final solidification.

[0028] By keeping the maximum melt pool temperature from exceeding the powder material's boiling point, the vaporization of material is largely avoided so that vapor streams do not generate melt pool depressions, eject spatter, displace the particles in the surrounding powder bed, and gas bubbles are not generated within the melt pool. The melt pool geometry is managed through manipulation of the lateral surface temperature gradients by exploiting the functional relationship between temperature and the surface tension of the molten material. By timing the re-melting of the adjacent previously melted interfaces (in each of the three dimensions) relative to the particle melting and joining in that vicinity, the joining interfaces to formations from previous scans and layers are methodically managed.

[0029] The scanning beam partem overlaps a portion of the region melted by the previous scan in a manner such that a portion of the adjoining previously solidified material is re-melted and forms one edge of the current melt pool. The overlapping region of the beam pattern may extend beyond the zone of re-melting further into the region solidified on the previous scan providing heating to temperatures below the melting point for a prescribed temperature distribution that helps to manage the geometry of the thermal gradients at and around the solidification interface. This overlapping region of the beam partem may extend beyond the last melting beams so that the solidification gradients can be managed throughout the entire solidification process. The overlap re-melts the adjacent material solidified on the previous scan where the solidification interface deviated in slope from the rest of the melt pool, and re-solidifies with an orientation matching the rest of the melt pool. In this manner a contiguous and smooth transition is provided between the adjacent scans, while managing the solidification stresses and surrounding thermal stresses.

[0030] Because high scan speeds are used, balling may occur at the end of the melt pool first engaged by the beam pattern. This is permissible since it is only temporary, with the beams to follow providing more heating and redistribution of material through further capillary flow, eventually providing a uniform melt pool.

[0031] An example embodiment utilizes a polygon mirror mounted on an x-y motion stage with the pattern of laser beams as described above constructed off of the motion stage with the pattern image relayed to the final image plane at the powder bed surface through the stage mounted scanner and relay optics, potentially undergoing some reduction or other basic optical transformation. A translating reflector mounted on an additional single axis motion stage is present within the beam path and is moved in a manner coordinated with the x-y stage, so that a constant beam path distance between the constructed beam partem image and the final image plane can be maintained throughout the range of x-y stage motion.

Additionally, as the beam pattern reaches the x-y stage, it first encounters an active beam steering system that maintains precise location and pointing into the polygon. The majority of the beam partem reflected off of the rotating polygon mirror facets is further conditioned by the post-scan relay optics and passed on to the final image plane, while a beam or a portion thereof may be redirected over a grating to an optical sensor to provide an optical timing signal used to coordinate the beam modulation and stage motion with the scanning motion of the beam pattern.

[0032] The beam partem scans through a fixed distance with the reflection off each facet of the rotating polygon. As this occurs with a constant polygon speed, a constant y-stage speed is utilized with a velocity so that the scanning pattern advances with each scan by the amount that provides the desired overlap in the beam partem. A series of such successive scans of common length may cover only a portion of the powder bed in the scan direction (particularly when used in conjunction with a large powder bed), and will be collectively referred to as an imaging stripe. When this is the case, multiple imaging stripes are used utilizing the x-y motion capability to fabricate a layer. Adjacent stripes will overlap by at least a portion of the beam partem so that features formed in the previous stripe can be joined to the features in the current stripe.

[0033] According to one aspect of the invention, there is the selection of the pattern of energy beams in combination with the beam power levels and the scanning speed for the material being fabricated so that the described melt pool characteristics are achieved and the usual PBF defects are avoided. The already superior melt pool and solidification characteristics are further enhanced by the selection of the scan overlap so that an even more uniform solidification orientation is achieved. Additionally, the prescribed beam pattern enables much higher speed scanning, which leads to the exploitation of non-traditional PBF scanning optics such as rotating polygon mirrors. Embodiments utilizing an x-y translation stage to carry a polygon scanner provide for the use on much larger parts in powder beds that are much wider than the scan length of the optics chosen. The high material processing rates achieved with the described invention involve high speed scanning at smaller layer depths, which yields finer vertical resolution. Because the large area melt pool is achieved through the plurality of beams in the beam pattern, the individual beams may be focused to a small diameter enabling finer lateral resolution. As a result, finer resolution is achieved in all dimensions while also producing defect free faster volumetric processing, and uniform crystallographic orientation. A finer microstructure may also be achieved due to the enablement of higher velocities of the solidification interface in combination with the prescribed thermal gradients at that interface.

[0034] With the above in mind, this disclosure relates to a method for powder bed fusion additive manufacturing for the fabrication of three dimensional structures through a succession of multiple powder bed build layers utilizing a succession of parallel scans of a pattern of energy beams where the length and width of the beam pattern are more than 5 times the depth of the powder bed build layers and each build layer consists of a union of parallel channels of melted or sintered volume having lengths corresponding to the distances between structure surfaces as measured in the plane of the build layer in the direction of the scanning motion where the lengths of the channels are not necessarily limited to discrete values, and the combination of the scan speed and the power settings for the beams generate and maintain powder bed surface temperatures between the point of complete melting and the initiation of boiling for the fabrication material during the scan engagement.

[0035] This disclosure is also related to a method for powder bed fusion additive

manufacturing of wherein the lengths of the channels of melted or sintered volumes are limited to discrete values.

[0036] This disclosure is also related to a method for powder bed fusion additive

manufacturing of wherein the parallel channels of melted or sintered volume are generated within the bounds of the potential melt pool which travels across the powder bed in the scan direction with the scanning beam partem, forming channels and not forming channels by selectively having the power of the beams on or off as they engage the specific locations within the powder bed.

[0037] This disclosure is also related to a method for powder bed fusion additive

manufacturing wherein the power amplitude of some of the energy beams are modulated in a manner that shifts relative maxima in the surface temperature distribution relative to the boundaries of the melt pool, which may include providing an AC component in the power amplitude that is nominally at a frequency near the quotient of the scan speed and the typical beam spacing and is frequency modulated at a frequency that is a fraction of the quotient of the scan speed and the width of the beam partem.

[0038] This disclosure is also related to a method for powder bed fusion additive

manufacturing of wherein the fabrication of an individual build layer includes one or more imaging stripes, each of which are characterized by a succession of scans of the pattern of energy beams with each scan being equal in length and parallel to one another and the successive scans offset in at least the direction perpendicular to the scan direction and overlapping to some degree in the direction perpendicular to the scan direction.

[0039] This disclosure is also related to a method for powder bed fusion additive

manufacturing wherein the combination of the scan speed and the power settings for the beams provide a melt pool geometry whose solidification boundary has a slope that varies by less than 0.5 over 60% of its surface area.

[0040] This disclosure is also related to a method for powder bed fusion additive

manufacturing wherein successive scans overlap in a manner where the portion of the melt pool where the slope of the solidification boundary deviated by more than 0.5 is re-melted and re-solidified with a slope that varies by less than 0.5 from the portion of the previous scan that is not re-melted.

[0041] This disclosure is also related to a method for powder bed fusion additive

manufacturing wherein the melting or sintering in parallel channels through parallel scans of the beam pattern is augmented with independently controlled single beam scans which may occur along any path or trajectory within the plane of the build layer.

[0042] This disclosure is also related to a method for powder bed fusion additive

manufacturing for the fabrication of three dimensional structures through a succession of multiple powder bed build layers utilizing a succession of scans of a partem of energy beams where the length and width of the beam pattern are more than 5 times the depth of the powder bed build layers, and the combination of the scan speed and the power settings for the beams generate and maintain powder bed surface temperatures between the point of complete melting and the initiation of boiling for the fabrication material during the scan engagement while providing a melt pool geometry whose solidification boundary has a slope that varies by less than 0.5 over 60% of its surface area. Throughout this disclosure "length and width of the beam partem" is defined such that the length of the beam partem is the distance in the primary direction of scanning between the centers of the leading beam(s) and the trailing beam(s), and the width of the beam partem is the greatest distance between the centers of beams in the direction perpendicular to the direction of primary scanning. The "leading beam" and "trailing beam" may also be referred to as the "start of beam pattern" and "end of beam pattern" respectively.

[0043] This disclosure also relates to an apparatus for powder bed fusion additive manufacturing utilizing a pattern of energy beams where the length and width of the beam pattern are more than 5 times the depth of the powder bed layers, and the beam pattern can be scanned at speeds exceeding 2 m/s.

[0044] This disclosure also relates to an apparatus wherein the beam pattern image is constructed in a structure that remains fixed in space and the beam pattern image is optically relayed to the final focal plane at the surface of the powder bed, with the relaying optics capable of moving the beam pattern image in space throughout the final focal plane.

[0045] This disclosure also relates to an apparatus wherein the relaying optics include a rotating polygon mirror to provide the scanning motion of the beam pattern image.

[0046] This disclosure also relates to an apparatus wherein the polygon mirror is mounted on a translation stage that provides motion in one or more directions.

[0047] This disclosure also relates to an apparatus wherein a controlled translating reflector in combination with other beam delivery reflectors maintains a focal distance at the final focal plane throughout all motions of the translation stage.

[0048] This disclosure also relates to an apparatus wherein an active beam steering system maintains pre-scan location and pointing of the beam partem image through control of actuated reflectors with detector feedback.

[0049] This disclosure also relates to an apparatus wherein the scan length is less than the width of the powder bed and multiple overlapping imaging stripes are used to pattern a layer.

[0050] This disclosure also relates to an apparatus wherein the energy beam pattern includes one or more of the beams of a different frequency that can be diverted in the post scan optics to scan across a grating that is over a detector to provide an optical timing signal for coordinating beam modulations with the beam pattern motions.

[0051] This disclosure also relates to an apparatus wherein the energy beam pattern includes one or more beams of a different frequency that can be diverted within the active beam steering system and directed to the detectors as the feedback for that system.

[0052] This disclosure also relates to an apparatus wherein fiber lasers or fiber coupled lasers are used in the construction of the beam pattern image.

[0053] This disclosure also relates to an apparatus wherein one or more laser diode arrays are used in the construction of the beam partem image.

[0054] This disclosure also relates to an apparatus wherein a preheating device such as a radiant lamp is mounted on the translation stage and is directed just ahead of the beam pattern in the process direction.

[0055] This disclosure also relates to an apparatus wherein the temperature throughout the bulk of the powder bed is controlled utilizing heating and cooling elements included within the structure containing the bed of powder material and the build platen.

[0056] This disclosure also relates to an apparatus wherein the powder bed translates in the process direction during imaging.

[0057] An example embodiment comprises a pattern of energy beams 201 whose arrangement provides specific heating patterns in a bed of powder material 610 when combined with certain process parameters, a stationary module of beam pattern construction optics 200 containing a plurality (typically thousands) of lasers and optics that form an image of the desired partem of energy beams 201, a two degree of freedom (x-y) motion stage 320, a rotating polygon mirror 340 mounted on the x-y motion stage 320, an active beam steering module 330 mounted on the x-y motion stage, post scan relay optics 350 mounted on the x-y motion stage 320, a translating focusing reflector 525 which works with y stage mounted beam path mirrors 316 to maintain a fixed focal plane throughout the range of x-y stage motion, a powder bed module 600 containing a vertically moving build platen 605 on which

the bed of powder material 610 resides, and a control system module 100 that coordinates all motions and beam modulation in a manner which manufactures parts of the prescribed geometry through selective melting of the bed of powder material 610 in a series of layers.

[0058] A schematic representation of the image path 202 throughout the system is provided in Fig. 1 represented as dashed lines. The image of the partem of energy beams 201

emanating from the beam pattern construction optics 200 is relayed to the final focal plane through a series of optical devices. The pattern image may first be preconditioned by mirrors or lenses in a fixed relay optics module 450 before being delivered to the x-y motion stage 320 through a series of beam delivery reflectors. The translating focusing reflector 525 is mounted on its own one-dimensional motion stage and moves in a manner coordinated with the x-y motion stage 320 movements so that the effective image distance to the final focal plane is constant. This may be done using planar reflectors and the true path distances, or with some non-unity magnification in at least one of the reflectors so that the focal distance changes to match the true beam path distance as it changes. Fine focus adjustments can be made through fine changes in the beam path distance. From the translating focusing reflector 525 the image travels to the y stage mounted beam path mirrors 316 and into the active beam steering system 330.

[0059] The active beam steering system 330 accommodates some degree of variation in the location of the image of the pattern of energy beams 201 as it reaches the x-y motion stage 320. The active beam steering system 330 actively adjusts the location and pointing of the image in both the scan and process direction. The active beam steering system may include, for example, four rotationally actuated mirrors and two detectors (each capable of two dimensional detection). Examples of appropriate hardware for the actuated mirrors include galvanometers or voice coil driven pivoting flexures. Quad cell detectors could be used, for example, for the detectors. Either an additional steering beam within the pattern of energy beams 201 or a portion of one of the other beams is diverted within the active beam steering system 330 and sent to each of the detectors, one in the near field and one in the far field, and the actuators are adjusted to keep the steering beam centered on the detectors. In this manner, precise location and pointing into the polygon mirror 340 scanning device is maintained. The last pre-scan mirror before the polygon mirror 340 may also provide a reduction so that the beam energy can be spread over a larger area throughout the pre-scan optics 450, 525, 316, 330 to avoid damage to the pre-scan optics 450, 525, 316, 330 since the high energy beam pattern is relatively fixed in position throughout those optical surfaces.

[0060] The scanned image from the rotating polygon mirror 340 passes into the post scan relay optics 350 which relays the image of the pattem of energy beams 201 to the final focal plane. The post scan relay optics 350 may provide scan linearization and image flattening. Also within the post scan relay optics 350, either an additional timing beam within the pattem of energy beams 201 or a portion of one of the other beams can be diverted to pass over a precision grating to a detector on the other side. A phase locked loop wave synched to this optical signal can be used to coordinate the timing of the beam modulation and stage motion. If a non-normal angle of incidence with the final focal plane is desired, the post scan relay optics 350 may also include optical elements that tilt the focused image relative to its direction of propagation. A non-normal configuration may be useful in managing the reflection and absorption of the pattern of energy beams 201 at the surface of the bed of powder material 610.

[0061] The beam pattem construction optics 200 may utilize one of a variety of laser types and may arrange the beams in the desired pattern using a variety of optical combinations and manufacturing methods. An example of suitable lasers includes fiber lasers that can be run in continuous wave or quasi-continuous wave modes at the appropriate power levels. Such lasers are available in visible through infrared wavelengths. While any wavelength in this band would be suitable, consideration may be given to the spectral absorptivity of the powder materials that will be processed. The beams can be coupled into an optical assembly that may form the desired pattern and be collimated as a whole, in groups, or individually. Individual collimation of each beam may also include individual location and pointing. Though this optical assembly could be manually constructed, it could also be constructed using automation methods employed in the opto-electronics manufacturing industry including automated active alignment. By constructing the pattern of energy beams 201 in a fixed module that is removed from the x-y motion stage 320 and optically relaying the image of the pattern of energy beams 201 to the x-y motion stage 320, several advantages are achieved over alternate design configurations. Significantly more beams can be incorporated into the pattern of energy beams 201 than would be practical if for example laser diodes were mounted on the x-y motion stage 320 or fiber lasers were connected directly to an optical head on the x-y motion stage 320. The mass and volume of thousands of diode lasers make mounting the devices on the motion stage untenable, as does provision of the power, cooling, and control signal framework. Similarly, thousands of fiber cables would not have space on the x-y motion stage 320 for connection and would add unacceptable mass and large varying stiffness to the precision high speed motion system. Having the waste heat from the laser systems far removed from the precision scanning system is advantageous in terms of minimizing errors induced from thermal expansion and vibrations associated with the cooling system are easily isolated.

[0062] The powder bed module 600 may utilize systems and methods consistent with current practice in PBF systems, though potentially on a bigger scale since the described invention enables the timely scanning in a much larger powder bed than is currently feasible with the prior art. This may include a hydraulic lift for the build platen 605 motion mechanism or another mechanism that provides sufficient lift capability and sufficiently precise motion. The recoater system 620 may be a blade or roller type fed from a hopper or a supply powder bed. To manage the temperature throughout the entire bed of powder material 610 over the entire build period, heating and cooling elements may be included in the structure containing the bed of powder material 610 and the build platen 605. The powder bed module 600 may be contained within an inert environment such as Nitrogen or Argon gas.

[0063] Some embodiments may also include a means to preheat the top layers of the bed of powder material 610 just prior to exposure to the partem of energy beams 201 by utilizing a device such as a radiant lamp mounted to the underside of the x-y motion stage 320 and directing it just ahead of the pattern of energy beams 201.

[0064] The control system module 100 includes the electronics and computational hardware required to drive and control the various subsystems, coordinating all motions and beam modulation to generate the geometry desired for each layer. For the geometry of the given layer being processed, the command position of the build platen 605 within the powder bed module 600, the polygon velocity or position profile, the motion profiles of the x-y motion stage 320, and the modulation profiles of each beam in the partem of energy beams 201 are provided to their respective subsystems with precise timing and coordination. These patterns can be generated within the control system module 100 as needed for each layer based on the geometry of the part, or can be calculated beforehand offline. In either case, the nature of the method and apparatus is such that the part resolution need not be limited by the geometric construct of a "voxel." A voxel is a discrete unit of three dimensional space in a three dimensional grid that is used to define a three dimensional structure. Such discretization limits the resolution of the structure and fosters a "stair-step" nature to sloped surfaces in the fabricated structures. In the present invention, the times at which the individual beams will be turned off and on always correspond to boundary surfaces of the structure being fabricated. By not constraining these times for switching on and off to discrete values determined by voxels, smoother surfaces that are closer to the desired geometry can be generated by selecting the switching times giving consideration to not only the location of the surface, but its spatial gradient relative to the scan direction, and understanding of the process itself. A geometrical construct description for fabrication in this manner is the union of parallel channels of melted or sintered material having varying lengths without the necessity for discretization of these lengths.

[0065] In an alternate embodiment, if the width of the imaging stripe is made to cover the entire width of the powder bed, then only a single axis (y-direction) motion stage may be used.

[0066] In another alternate embodiment, the x, y, and z motion are all achieved by moving the powder bed in the three dimensions. This allows the locations of the optical elements to be fixed in space and eliminates the need for the translating focusing reflector 525 in order to maintain a constant final focal plane. However, for large powder beds this will limit motion speeds and increase the machine footprint while also being susceptible to disruption of the powder bed from inertial forces.

[0067] While examples given here involve the processing of metal materials, the process can be similarly applied to non-metal materials.

[0068] The pattern of energy beams 201 is focused into the plane of the top surface of the powder bed 610 which will be referred to as the final focal plane. The motion of the pattern of energy beams 201 in the final focal plane throughout the time it is reflected off of a single facet of the polygon mirror 340 will be referred to as a scan. As indicated in Fig. 1 and Fig. 2, the scan direction refers to the direction of motion of the beam partem image in the final focal plane with each scan and is determined by the vector sum of the motion due to the polygon mirror 340 reflection and the motion due to the x-y stage 320, but is primarily generated by the polygon reflection. The process direction is perpendicular to the scan direction and represents the direction in which subsequent adjacent scans will sequentially occur as the process proceeds with many scans, each generated by the next facet of the polygon mirror 340. The slower process direction motion of the beam partem is provided primarily through the x-y motion stage 320. A series of scans executed sequentially in the process direction in one motion of the x-y motion stage 320 will be referred to as an imaging stripe. As each scan occurs, each beam is individually powered on and off as it encounters a portion of powder material in the final focal plane that is to be selectively melted or not melted. The portion of the powder bed that is in a molten state from the heating of the pattern of energy beams 201 at any given instant will be referred to as the melt pool. An imaging stripe or a series of multiple imaging stripes will be executed as needed to melt a pattern of material corresponding to the desired horizontal cross-section of the part or parts being manufactured, and constitutes the formation of a layer. After completion of a layer, the build platen 605 is lowered by the desired height of the next layer, which lowers the bed of powder material 610. The powder bed module 600 is equipped with a recoater system 620, which spreads a fresh layer of powder material on the bed of powder material 610, and trims the top surface of that fresh layer to the final focal plane. The process repeats until the entire part or parts have been constructed.

[0069] The pattern of energy beams 201 may utilize hundreds or thousands of laser beams of varying diameters and power capabilities in a 2 dimensional arrangement that is conducive to generating thermal distributions in and around the melt pool that adhere to a beneficial set of constraints. An example of an appropriate pattern of energy beams 201 is shown in Fig. 2. This particular pattern and indicated intensity levels are appropriate for manufacturing with titanium at a scan speed of 34 m/s. It is composed of 1546 lasers with beam diameters of

either 50 or 25 μηι with powers set to levels varying from 28 to 0.3 W. Each laser can have its power modulated independently from within the beam partem construction optics 200 as controlled by the control system module 100.

[0070] One of the constraints on the generated thermal profile in and around the melt pool involves complete melting without boiling. For materials that are alloys, this constraint requires that each volume of the powder bed material 610 that is to be melted exceeds the solidus temperature and remains below the value at which boiling is initiated. For materials that are a single element, the constraint can be stated simply as exceeding the melting point of the material for a minimum required amount of time without exceeding the boiling point of the material. For brevity, the terms "melting point" and "boiling point" will be used frequently in this specification with the understanding that the phrases "solidus temperature" and "temperature at which boiling is initiated" are implied if dealing with an alloy rather than a single element. For each particular scan, these conditions include the interface to adjacent previously solidified surfaces with penetration to a depth beyond the range of expected process variations. The minimum time for which every volume of material should exceed the melting point is the time needed to assure that capillary flow of the molten material will eliminate all voids. If the PBF process is SLS rather than SLM, then the melting point bound can be relaxed, but the boiling point constraint should be maintained to avoid spatter ejection and vapor streams which can disrupt and locally deplete the powder bed.

[0071] The diameters of the various beams in the pattern of energy beams 201 are selected to be less than or equal to the desired imaging resolution in the process direction. Possible exceptions to this would be beams that are added for general regional preheating. The spacing and power of the beams should be selected in combination with the scanning speed so that while there will be cyclic variations in temperature as a given material volume is sequentially irradiated by multiple beams, once the material temperature is raised above the melting point it generally remains above it until it is time for solidification. The majority of the pattern of energy beams 201 consists of beams whose function is to melt the powder material or re-melt adjacent previously solidified surfaces. In the example pattern of energy beams 201 provided in Fig. 2, such beams are indicated by the dashed parallelogram and the label "Melting Beams." The width of the pattern of melting beams across the scan direction is selected to provide enough irradiation time and cumulative energy to allow conduction for the given material's thermal diffusivity so that the powder particles have time to melt, join into a contiguous molten body, and propagate diffusive heat to a sufficient depth into the underlying surface of the previously solidified layer.

[0072] While these melting beams will primarily scan over fresh powder, at least a portion of them will overlap the material melted on the previous scan so that features formed in the previous scan can be contiguously joined to features formed in the current scan. Adjacent to the melting beams, there may be other beams that also overlap the previous scan whose powers will generally be set to lower levels than the adjacent melting beams. When present, the function of these beams is to control the temperature distribution further into the previously solidified material surrounding the melt pool throughout both the melting and solidification phases of the process, managing the thermal gradients and the geometry of the solidification interface in a manner that minimizes residual stresses and the potential for cracking while generating more uniform crystallographic orientation.

[0073] The particular shape of the melt pool at any instant will vary depending upon the particular geometry of the desired part in that region. The beams will be turned off when passing over regions that will not be part of the structure being built. As a result there may be islands or peninsulas of unmelted powder around which the melt pool forms. Further, there may be smaller local melt pools within the islands or peninsulas if the part geometry dictates.

[0074] The principles and benefits of a pattern of energy beams 201 such as the example in Fig. 2 can be demonstrated through examination of the transient solution to the conduction heat equation in a semi-infinite body subject to the scanning beam partem in comparison to conventional single beam scanning. While the high temperatures and associated large thermal gradients in the single beam process can drive significant additional advective heat transfer mechanisms (often referred to as Marangoni convection), those mechanisms are far less prevalent in items manufactured according to the present disclosure. Also, limiting the analysis to conduction provides a conservatively bounded design, since convective heat transfer within the melt pool will only enhance the heat transfer permitting similar melt pool depths to be obtained at lower laser powers.

[0075] Figures 3A and 3B give comparative thermal time histories of a fixed location within a titanium powder bed with the center of each melt pool passing though the respective points. Fig. 3A does so for the present invention while Fig. 3B shows the conventional single beam case. The top portion of each figure is the intensity from the laser(s) on the powder bed location as a function of time, while the bottom portion of each figure gives the temperatures at the surface (solid line) and at the depth corresponding to the layer height (dash-dot line). The levels of the melting point and boiling point are indicated with the dotted lines and are labeled accordingly. For comparative purposes, Figures 3A and 3B are plotted on the same vertical scale, while the horizontal scale in 3B is much greater due to the slower nature of the conventional process. The present invention (Fig. 3A) quickly raises the surface temperature above the melting point, but does not ever drive it above the boiling point. The surface temperature cycles between the melting point and boiling point long enough so sufficient heat conducts down to raise the layer depth temperature beyond the melting point (and the depth of the melt pool extends beyond the layer depth). The conventional single beam process (Fig. 3B) drives the surface temperature beyond the melting point, but also well beyond the boiling point. It is also necessary for the single beam process to drive the layer depth temperature much further past the melting because of its highly curved melt pool geometry (as is shown in Fig. 10B) with the center of the melt pool much deeper than the edges.

[0076] Figures 4-9 sequentially show the melt pool geometry and thermal profiles at various stages of the scan for the present invention. At the top of each of these figures, the location of the beam pattern is shown relative to the x=0 location which is accentuated by a fat gray line. Below, a slice through that x=0 location is shown with the scan direction coming toward the viewer. The energy absorption into the powder bed from the beam partem is displayed in the center and the corresponding temperature distribution is shown below. The temperature is indicated both by the gray shading and by the labeled contour lines. Note the sharp transition used in the temperature scaling at the melting point and boiling point so that the currently molten portion has an obvious darker gray tone, the solid portion has a lighter tone, and any vaporized material would be white. The cumulative portion to be melted at any time during the scan up through the present time shown is indicated by the dashed line. The dotted lines indicate the cross-sectional geometry of the discrete addresses where channels of melted or sintered volumes may be fabricated. The vertical scaling is magnified in the two dimensional thermal profile to provided clearer visualization of the temperature distribution.

[0077] Figure 4 shows the exposure and melt pool profile approximately 25 μββΰ after the beam pattern started to engage the plane. At this point, the melting beams have engaged a little more than half of width of the beam pattern. Figure 5 shows about 7.5 μββΰ later that the width of the melt pool is growing in the process direction as a greater number of beams become simultaneously engaged with this plane, and the depth is also growing. During these early stages of melt pool development in the leading fraction of the growing melt pool, individual particles are melted and join with one another through capillary flow. In this early stage, it is allowable and potentially preferable for temporary surface tension induced "balling" to occur, so that localized initiation of the re-melting of the underlying layer can take place sooner in the regions temporarily vacated by capillary flow during ball formation. The balls soon join the re-melted underlying surface receding from their previous location and expose more underlying surface for re-melting. In this manner the transition from individual particles to a contiguous fluid body and the joining with the previously solidified surfaces can be accelerated. This balling and recession may be manipulated through the modulation of the beams in this region at a rate near the frequency at which adjacent beams pass over the region. Towards the overlapping portion of the beam pattern, more time with beam exposure has occurred, particles have been melted and joined, and the depth of the melt pool continues to grow.

[0078] Figure 6 shows the melt pool geometry after about 50 μββα The melt pool has reached its full width with most locations still being exposed by melting beams and growing in depth. Near the overlap portion of the beam pattern, the last melting beams have just passed and the melt pool in this region is near its full depth beyond the layer height. The solidification interface is now quite smooth throughout its entirety and becoming flatter (though there is still a slight slope upward in the process direction).

[0079] Figure 7 shows the melt pool geometry after about 75 μββα The final melting beam has just passed, and the melt pool is close to its full depth all the way across with its bottom having a slope of nearly zero throughout its entirety. The overlap region is still being

exposed with some lower power beams to manage the slope of the solidification interface in that region and the solidification at that end of the pool is just starting.

[0080] Figure 8 shows the melt pool geometry after about 107 μββα Solidification has been progressing with the solidification surface maintaining a minimal slope on the left end as it moves from left to right while the depth of molten material continues to shrink. Thermal gradients are small at and around the surface of the solidification interface, in the lateral (non-depth) directions.

[0081] Figure 9 shows the melt pool geometry after about 123 μββα Solidification is almost complete. The left edge of the solidification interface continues to intersect the surface with a very small slope. Note that on the right edge of the melt pool, where the final solidification will have the greatest slope, this region will be overlapped by the next scan and re-melted so that the final solidification direction will be quite uniform everywhere allowing for uniform crystallographic orientation even across the boundaries of adjacent scans.

[0082] Fig. 10 compares the melt pool geometries and thermal profiles of the present invention to the conventional single beam process. Fig. 10A shows the same instant as Fig. 6 (approximately 3/4 of the way through the melting beams). Fig. 10B shows the single beam process at the instant when approximately 3/4 of the beam has passed. The left portion of the image contains the thermal profile plotted on the same scale as in Fig. 10A so that the shape and scale can be easily compared. A magnified view is provided on the right side of the image so that the densely grouped contour lines associated with the large thermal gradients can be discerned. Both Figures 10A and 10B use the same vertical magnification (about 3X). Some notable differences include the lower temperatures below the boiling point, the lower thermal gradients, the lower curvature at the solidification boundary, and the lesser angle where the solidification boundary meets the surface. All of these make the present invention less prone to the various defects discussed previously.

[0083] Specifically, by keeping the maximum melt pool temperature from exceeding the powder material's boiling point, the vaporization of material is largely avoided so that vapor streams do not generate melt pool depressions, ej ect spatter, displace the particles in the

surrounding powder bed, and gas bubbles are not generated within the melt pool. The lower thermal gradients reduce thermal stress and the potential for cracking as well as the residual stress. The almost constant low slope of the solidification boundary (nearly parallel to the layer surface) throughout the entire solidification process provides uniformity to the crystallographic orientation of the microstructure of the fabricated material. The lower curvature at the solidification boundary inhibits cracking by promoting a larger grain microstructure with fewer grain boundaries and grain boundaries that instantiate with far less severe angles relative to the top of the layer surface, if there are grain boundaries at all.

Quantifying this difference between the present invention and the conventional process, the slope of the solidification boundary throughout the entire solidification process is less than 0.2 over 90% of the solidification boundary in the present invention for the case discussed previously (the beam partem of Fig. 2 scanning at 34 m/s), with a maximum slope of less than 2 occurring briefly on one edge of the melt pool. In contrast, the single beam

conventional process discussed previously has a maximum slope of 13 occurring around almost all edges of the melt pool with a slope greater than 1 over about 50% of the solidification boundary throughout the solidification process. In the process of the present invention, the small portion of the slope that exceeds 0.2 is re-melted by the next scan and solidifies with a slope that is less than 0.2, generating the uniform microstructure everywhere. The conventional process only re-melts a portion of its highly sloped region, and solidifies with a similarly highly sloped solidification boundary. For other specific patterns of energy beams, beam power settings, and scanning speed combinations, moderately different solidification profiles can be achieved, but the general principles of the present invention are illustrated by the example provided. These benefits come from the large area low depth melt pool with nearly planar solidification interface together with the combination of the beam arrangement, beam power settings, and scan speed.

[0084] In the process comparisons shown in Fig. 3 and Fig. 10, the layer depth utilized in the conventional single beam process (Figs. 3B and 10B) is three times that used with the pattern of energy beams 201 (Figs. 3A and 10A). The smaller layer depths such as the 15 μιτι chosen (versus 45 μιτι in the conventional single beam process) have several advantages. The smaller layer depth used with the beam partem design allows finer resolution in the z-direction, requires fewer lasers to melt to that depth, and accentuates the benefits of a low depth aspect ratio melt pool with regard to residual stress and cracking. While the smaller layer depth requires more layers to build the same volume, the processing rate table in Fig. 1 1 shows that because the area processing rate is over 700 times that of the conventional single beam process, the volumetric processing rate is still well over 200 times faster than the conventional single beam process (even with the smaller layer size). The processing rate can be increased several times more by adding more melting beams since the hatch spacing (y-stage distance traveled per scan) will be proportional to the number of melting beams and is only limited by the maximum y-stage velocity and the size of the partem of energy beams that can be accommodated by the optical system. A typical y-stage velocity for imaging stripes in the example configuration is only 141 mm/s. As another example, approximately 6000 beams in a partem approximately 4 mm tall with a stage velocity of 570 mm/s will provide a volumetric process rate 1000 times that of the conventional single beam process while also having 3 times the resolution.

[0085] Similar to the manner in which overlapping scans provide the smooth union of adjacent scans, overlapping is also used between adjacent imaging stripes. When multiple imaging stripes are used to fabricate a large dispersed layer, the imaging stripes will overlap slightly so that features fabricated in the previous stripe can be joined to the continuation of those features fabricated in the current stripe.

[0086] Having described at least one of the example embodiments of the present disclosure with reference to the accompanying drawings, it is to be understood that the disclosure is not limited to those precise embodiments, and that various changes, modifications, and adaptations may be effected therein by one skilled in the art without departing from the scope or spirit of the disclosure as defined in the appended claims.