Energy beam exposure in powder bed fusion

Abstract

A powder bed fusion additive manufacturing method includes exposing layers of a powder bed to an energy beam to selectively melt regions of each layer, melting at least a proportion of the regions using pulse exposure. The method may further include commanding the energy beam source to generate at least one pulse of a pulse exposure having a pulse duration of less than 200 microseconds. The step of commanding may include specifying a plurality of elevated power levels higher than a reference power level for a power waveform of the at least one pulse.

Description

Technical Field

[0001] The present invention relates to energy beam exposure in powder bed fusion, and in particular to pulse exposure. Background Art

[0002] In powder bed fusion, a layer of powder is deposited on a powder bed in a build chamber, and a portion of the powder layer corresponding to the cross-section (cut plane) of the workpiece being constructed is scanned with an energy beam (such as a laser beam or electron beam). The energy beam melts the powder to form a solidified layer. After selective solidification of the layer, the powder bed is reduced to the thickness of the newly solidified layer, and another layer of powder is spread on the surface and solidified as required. In a single build, more than one part can be built, and these parts are spaced apart in the powder bed. It is known to melt powder layers simultaneously using more than one energy beam.

[0003] Typically an energy beam is scanned along a scan path. The scanning of the energy beam along the scan path may be in a continuous mode in which the intensity of the energy beam is increased to an intensity level sufficient to melt the powder and the points directed along the scan path have the intensity maintained at this level (hereinafter referred to as a "continuous exposure"). In an alternative pulsed mode, the energy beam is emitted as a train of pulses, with each pulse being directed to a different area along the scan path (hereinafter referred to as a "pulsed exposure"). The pulses of the pulsed exposure may be synchronized with the control of beam manipulation components that direct the energy beam to the powder bed, with the aim of moving the beam manipulation components only when the energy beam is turned off, so that the energy beam is stationary during the exposure. In practice, this is difficult to achieve for the scanning speeds typically used in powder bed fusion due to the inertia of the beam manipulation components, and some movement of the energy beam over the surface of the powder bed will occur during the pulses.

[0004] The RenAM 500Q machine from Renishaw enables scanning of a powder bed using pulsed exposure with a pulse duration of about 80 microseconds. The equipment includes four 500W continuous wave (CW) lasers. The lasers are redPOWER PRISM module fiber lasers supplied by SPI. The controller for each CW laser includes a class A power amplifier for generating control signals to the pump diode of the laser. The square wave control signal modulates the output of the CW laser between on (single increase in power level) and off to generate laser pulses such as Figure 1As shown. The maximum laser power of each laser pulse is preset by the user, for example. The rise time (from 10% to 90% of the maximum laser power) and the fall time (from 90% to 10% of the maximum laser power) of the pulse are about 8 microseconds. A change in the maximum power of the laser pulse requires several milliseconds, i.e., several tens of pulses. The arrival time of the first pulse can vary by as much as 7 microseconds, while the arrival times of subsequent pulses vary less. The lag time from the start of the control signal to the arrival of the pulse is typically about 17 microseconds. It has been observed that the first laser pulse may exhibit a spike in power at the beginning of the laser pulse and then stabilize at the maximum laser power preset for the laser pulse.

[0005] WO 2018 / 029478 A1 discloses that the pulse shape (waveform) of the laser pulse may include a plurality of sub-shapes, wherein each of the sub-shapes may have a different duration, energy, ramp-up or ramp-down. A waveform with a slowly increasing intensity and three plateaus compared to a relatively rapid decrease in intensity is shown. The total duration of the laser pulse may vary between 200 microseconds and 1000 microseconds.

[0006] "Selective Laser Melting of thin wall parts using pulse shaping" by KAMumtaz, N. Hopkinson investigated pulse shaping in selective laser melting. The system enables the user to tailor the energy profile to as close as 0.5ms within a single laser pulse. Various ramp-up and ramp-down pulse shapes were generated and used to process four layers of Inconel 625. The ramp-up pulses varied from 1.7ms to 10ms. The ramp-down pulses varied between 1ms and 10ms. Summary of the invention

[0007] According to a first aspect of the invention, there is provided a powder bed fusion additive manufacturing method, the method comprising exposing layers of a powder bed to an energy beam to selectively melt regions of each layer, wherein pulse exposure is used to melt at least a certain proportion of the region, the method further comprising commanding an energy beam source to generate at least one pulse of the pulse exposure, preferably each of a plurality of pulses.

[0008] The step of commanding may include specifying a plurality of increased power levels above a reference (e.g., zero) power level for the power waveform of at least one pulse. In the present invention, the step of commanding includes specifying each of the plurality of increased power levels, for example, in a control signal sent to or driving the energy beam source. In this way, a desired pulse shape may be achieved that is different from a pulse generated by a simple on / off command (i.e., a command that specifies only one ("on") increased power level during a pulse).

[0009] The step of commanding may include specifying a pulse shape, pulse profile and / or pulse form for the power waveform. The power waveform may be a non-rectangular pulse shape / profile / form. It has been found that a non-rectangular pulse shape / profile / form may cause a change in how the material consolidates, which may improve the part compared to a part produced using a rectangular pulse. The pulse shape / profile / form may be triangular. The pulse shape / profile / form may include multiple power plateaus at specified increased power levels.

[0010] The pulse duration (the time between the laser power of the pulse rising to above the maximum specified boosted power level and then returning to below 10% of the maximum specified boosted power level) of at least one of the pulses of the pulsed exposure (preferably each of the plurality of pulses) may be less than 200 microseconds, and optionally less than 150 microseconds. It has been found that at the energy beam powers required to melt powders, particularly metal powders, longer pulse durations result in evaporation of the material. Reducing the power of the energy beam below this threshold will cause the powder to not be melted even if it is exposed to the energy beam for a longer period of time, because for such low energy beam powers, the heat is dissipated too quickly to allow the temperature of the powder to rise above the melting temperature. Accordingly, pulsed exposures comprising pulse durations longer than 200 microseconds are undesirable.

[0011] The pulse duration may be greater than 1 microsecond, preferably greater than 5 microseconds, and optionally greater than 10 microseconds.

[0012] The energy beam source for generating the energy beam may have a response time (the time between a change in the control signal and a change in the output energy beam corresponding to the desired power) of 10 microseconds or less, preferably 5 microseconds or less. Controlling the shape of the power waveform may include specifying one or more increased power levels between 10% and 90% of the maximum increased power level. The method may include specifying one or more increased power levels as having a duration of less than 15 microseconds, optionally less than 10 microseconds, and further optionally less than 5 microseconds. The method may include controlling the shape of the power waveform, increasing or decreasing in multiple steps. At least one of the steps may have a duration of less than 15 microseconds, optionally less than 10 microseconds, and further optionally less than 5 microseconds. At least one of the steps may have a duration greater than 1 microsecond, and further optionally greater than 2 microseconds.

[0013] The method may include specifying a pulse shape to control the cooling rate of the melted and / or resolidified material. In powder bed fusion, metallic materials typically solidify within tens of microseconds, so the melted material rapidly resolidifies after the intensity of the energy beam drops below an intensity sufficient to maintain the temperature of the material above the melting point. It has been found that extending the fall time of the energy beam pulse to greater than an uncontrolled fall time of 8 microseconds (implemented in Renishaw's RenAM 500Q machine) can improve the material properties of the solidified material. (The fall time in Renishaw's RenAM 500Q machine is uncontrolled / unspecified because the fall time is not defined by control signals (which are square wave pulses with vertical rise and fall times), but rather is generated by the non-ideal response of the laser to these control signals). The method includes controlling the shape of the power waveform of the pulses of the pulsed exposure so that the fall time is longer than the rise time. The method includes controlling the shape of the power waveform of the pulses of the pulsed exposure so that the fall time of at least one pulse is greater than 10 microseconds, preferably greater than 20 microseconds, and most preferably about 30 microseconds. The fall time may be less than 100 microseconds, preferably less than 50 microseconds, and most preferably less than 40 microseconds. The fall time of at least one pulse may be between 10 microseconds and 100 microseconds, between 10 microseconds and 50 microseconds, between 10 microseconds and 40 microseconds, between 20 microseconds and 100 microseconds, between 20 microseconds and 50 microseconds, or between 20 microseconds and 40 microseconds. The maximum increase in power may be between 200W and 1000W. The average gradient of the fall time may be between 2MW / s and 20MW / s, preferably between 4MW / s and 20MW / s, and more preferably between 6MW / s and 20MW / s. Extending the fall time of the pulse may affect the cooling rate during the solidification of the molten material, thereby changing the resulting microstructure. The fall time should be of the order of magnitude of the time required for the molten material to solidify, such as tens of microseconds, so as to change the cooling rate by the extended fall time, but avoid unnecessary retention of the material at an elevated temperature. Lower cooling rates can reduce solidification cracking in certain materials such as steels, for example tool steels (eg H13 tool steel), W360 or nickel-based super alloys.

[0014] The method may include controlling the shape of the waveform so that the rise time of at least one pulse is greater than 10 microseconds, preferably greater than 20 microseconds, and most preferably about 30 microseconds. The rise time may be less than 100 microseconds, preferably less than 50 microseconds, and most preferably less than 40 microseconds. The rise time of at least one pulse may be between 10 microseconds and 100 microseconds, between 10 microseconds and 50 microseconds, between 10 microseconds and 40 microseconds, between 20 microseconds and 100 microseconds, between 20 microseconds and 50 microseconds, or between 20 microseconds and 40 microseconds. The maximum power set may be between 200W and 1000W. The average gradient of the rise time may be between 2MW / s and 20MW / s, preferably between 4MW / s and 20MW / s, and more preferably between 6MW / s and 20MW / s. A slower rise time than the uncontrolled rise time of the RenAM 500Q machine may result in a wider melt pool and / or a melt pool with a lower aspect ratio. It is desirable to form a wide, shallow melt pool for obtaining rapid cooling rates and oriented grain formation. Further details of this oriented grain formation method are disclosed in WO 2020 / 249932 A1 and PCT / GB2021 / 051193, the disclosures of which are incorporated herein by reference in their entirety.

[0015] The method may comprise controlling the shape of the power waveform to combine a rise time and a fall time of at least one pulse as defined above.

[0016] The method may comprise controlling the shape of the power waveform so that at least one of the pulses comprises a plurality of maxima. A plurality of the maxima may be higher than 90% of the set maximum boost power level for the pulse. A plurality of the maxima may have a power higher than the power required to melt the powder. Between each pair of maxima are intermediate local minima of the pulse. At least one of the intermediate local minima may be higher than 10%, preferably higher than 90%, of the set maximum boost power level for the pulse. At least one of the intermediate local minima may have a power higher than the power required to melt the powder. The time between the intermediate local minimum and the adjacent maximum may be between 1 microsecond and 50 microseconds, between 5 microseconds and 50 microseconds, between 10 microseconds and 50 microseconds, between 1 microsecond and 40 microseconds, between 5 microseconds and 40 microseconds, between 10 microseconds and 40 microseconds, between 1 microsecond and 30 microseconds, between 5 microseconds and 30 microseconds, between 10 microseconds and 30 microseconds, between 1 microsecond and 20 microseconds, between 5 microseconds and 20 microseconds, between 10 microseconds and 20 microseconds, between 1 microsecond and 17 microseconds, between 5 microseconds and 17 microseconds, or between 10 microseconds and 17 microseconds. Oscillation of the power of the pulses during exposure may agitate the molten pool to improve grain size uniformity and / or improve the penetration depth of the pulses.

[0017] According to a second aspect of the invention, a powder bed fusion additive manufacturing method is provided, the method comprising exposing layers of a powder bed to an energy beam to selectively melt regions of each layer, wherein pulse exposure is used to melt at least a certain proportion of the region, and at least one pulse of the pulse exposure (preferably each of a plurality of pulses) has a pulse duration of less than 20 microseconds.

[0018] Shorter pulse durations than the 80-microsecond pulses used in the RenAM 500Q machine can produce finer microstructures and / or higher build rates.

[0019] The second aspect of the invention may be used in combination with the pulse shaping of the first aspect of the invention.

[0020] According to a third aspect of the invention, a powder bed fusion additive manufacturing method is provided, the method comprising exposing layers of a powder bed to an energy beam to selectively melt at least one region of each layer, wherein the energy beam travels along a scanning path to melt material of at least one region using pulse exposure, an initial pulse and / or an end pulse of the pulse exposure having a pulse duration shorter than the pulse duration of an intermediate pulse between the initial pulse and the end pulse.

[0021] Using pulses at the beginning and / or end of the scan path may improve penetration of the material in these areas compared to initiating or ending continuous exposure in these areas of the scan path.

[0022] The pulse duration of the intermediate pulses may have a length similar to scanning in continuous mode.The pulse duration of the intermediate pulses may be greater than 80 microseconds, more preferably greater than 100 microseconds, even more preferably greater than 150 microseconds, yet more preferably greater than 200 microseconds.

[0023] Shorter pulse durations may be less than 200 microseconds, less than 100 microseconds, less than 80 microseconds, less than 50 microseconds, less than 30 microseconds, or less than 20 microseconds.

[0024] For all initial pulses and / or end pulses, the shorter pulse duration may be constant. Alternatively, for different pulses in the initial pulses and / or end pulses, the shorter pulse duration may vary. The shorter pulse duration of the first pulse and / or the last pulse may be shorter than the duration of other initial pulses or end pulses, respectively. For the initial pulses, the shorter pulse duration may gradually increase from the first initial pulse to the intermediate pulses. For the end pulses, the shorter pulse duration may gradually decrease from the intermediate pulses to the last end pulse.

[0025] The time between pulses may be constant. Alternatively, the time between pulses may vary. The time between the first pulse and the second pulse or the time between the second to last pulse and the last pulse may be longer than the time between other pulse pairs of the pulse exposure. The time between pulses may gradually decrease from the first initial pulse to the intermediate pulses. The time between pulses may gradually increase from the intermediate pulses to the final end pulse.

[0026] The dot pitch between pulses may be constant. Alternatively, the dot pitch between pulses may vary. The dot pitch between the first pulse and the second pulse or the dot pitch between the second to last pulse and the last pulse may be shorter than the dot pitch between other pulse pairs of pulse exposure. The dot pitch between pulses may gradually increase from the first initial pulse to the middle pulse. The dot pitch between pulses may gradually decrease from the middle pulse to the last end pulse.

[0027] The method may include synchronizing the pulses with control of at least one beam manipulation component that directs the energy beam to the powder bed. Synchronizing the pulses with control of at least one beam manipulation component may include moving the at least one beam manipulation component (such as a mirror) relatively quickly between pulses compared to the speed at which the at least one beam manipulation component moves during the pulses. Synchronizing the pulses with control of at least one beam manipulation component may include moving a target on the powder bed of the beam manipulation component (such as a mirror) relatively quickly between pulses compared to the speed at which the target moves during the pulses. As used herein, the term "target on the powder bed of the beam manipulation component" refers to a location on the powder bed to which the energy beam is directed or would be directed if the energy beam were generated.

[0028] According to a fourth aspect of the present invention, there is provided a laser comprising a gain medium, a pump for pumping the gain medium, and a controller for controlling the pump.

[0029] The controller may be arranged to control the pump so that the response time of the laser is less than 17 microseconds. The response time may be less than 10 microseconds, preferably less than 5 microseconds. The response time may be about 3 microseconds. The response time is the time between a control signal requesting a change in power and a change in the power of the laser beam output from the laser. The term "control signal" as used herein refers to a signal received by a power amplifier of the laser.

[0030] Such response times are desirable for applications such as powder bed fusion, where the pulse exposure has a duration of tens of microseconds.

[0031] The controller may be arranged to control the pump so that, in response to a corresponding control signal, a plurality of stable non-zero laser beam powers may be output within one millisecond, preferably within 500 microseconds, and more preferably within 200 microseconds. The term "stable" laser beam power refers to a power at which the laser remains stable for a period of time, rather than a power through which the laser beam transiently transitions when rising or falling between zero power and non-zero power. The plurality of stable non-zero laser beam powers may occur within a single pulse or across multiple pulses. The time period of the stable non-zero laser beam power may be at least 1 microsecond, and optionally at least 3 microseconds.

[0032] The controller may include a power circuit arranged to generate a pulse width modulated signal to an inertial load located between the power circuit and the pump. The pulse width modulated signal is generated by the power circuit in response to the control signal. The inertial load converts the digital pulse width modulated signal into a smoother waveform (drive signal) for driving a pump (such as one or more laser diodes). In the absence of the inertial load, the laser diode will pulse with the pulse width modulated signal. The inertial load may include an inductor. The inertial load may include a capacitor. The inertial load may include an inductor and a capacitor. The inductor and the capacitor may be provided in series. The inertial load may be an electronic filter, such as a second order filter. The inductor acts to smooth the current, and the addition of the capacitor forms an electronic filter.

[0033] The power supply circuit may include a switching power amplifier (class D amplifier) ​​for generating a pulse width modulated signal. Class D power amplifiers are more efficient than class A power amplifiers, which may produce significant advantages in the case of high power lasers (200 W or more) when high electrical power is applied to the pump. The laser may be capable of generating a laser beam having a power higher than 200 W, preferably higher than 300 W, more preferably higher than 400 W.

[0034] The switching power amplifier may include two switching transistors. Each switching transistor may be a GaN transistor. Each switching transistor may be a high electron mobility transistor. Conventional MOSFET transistors have a switching frequency of about 100KHz (switching once every 10 microseconds), which is too slow to achieve the response time required for the laser of the present invention. GaN transistors / high electron mobility transistors can achieve higher switching frequencies. For example, a GaN transistor can be driven at 2.5MHz (switching once every 400 nanoseconds). Accordingly, the use of GaN transistors / high electron mobility transistors enables the use of switching power amplifiers while still achieving the response time required for the laser. GaN transistors / high electron mobility transistors are also suitable for high voltage, high temperature and high efficiency applications.

[0035] The laser may comprise a plurality of controllers, each controller being arranged to generate a drive signal to at least one laser diode so that the at least one laser diode pumps the gain medium. Each controller may be arranged to generate a drive signal to two or more laser diodes. In this way, by varying the pulse width modulation of each controller and / or by activating / deactivating some or all of the controllers, the power obtained by the laser may be varied.

[0036] The gain medium may be a doped optical fiber. The optical fiber may be doped with neodymium. The laser may be an NG:YAG fiber laser. The laser may be a continuous wave laser.

[0037] According to a fifth aspect of the present invention, a powder bed fusion device is provided, which includes an energy beam irradiation device for generating an energy beam and guiding the energy beam to a powder bed, the energy beam irradiation device includes an energy beam source and a controller, and the controller is arranged to control the energy beam source to implement a method according to any one of the first aspect, the second aspect and the third aspect of the present invention.

[0038] The energy beam irradiation device may include an energy beam source (such as a laser) and at least one beam manipulation component for directing the energy beam to a selected position on the powder bed. The energy beam source may be a laser according to the fourth aspect of the invention.

[0039] According to a sixth aspect of the present invention, there is provided a data carrier comprising instructions stored thereon, which instructions, when executed by a controller of a powder bed fusion apparatus, comprise an energy beam irradiation device for generating an energy beam and directing the energy beam to a powder bed, the energy beam irradiation device comprising an energy beam source, the controller being arranged to control the energy beam source to implement a method according to any one of the first, second and third aspects of the present invention.

[0040] The data carrier may be a suitable medium for providing instructions to the machine, such as a non-transitory data carrier, for example a floppy disk, CD ROM, DVD ROM / RAM (including -R / -RW and +R / +RW), HDDVD, Blu Ray (TM) disc, memory (such as Memory Stick (TM), SD card, Compact Flash card, etc.), disk drive (such as hard disk drive), magnetic tape, any magnetic / optical storage; or a transient data carrier, such as a signal on a wire or optical fiber or a wireless signal, such as a signal sent on a wired or wireless network (such as Internet download, FTP transfer, etc.). BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 shows the control signals and resulting laser pulses generated by the laser used in Renishaw's RenAM 500Q powder bed fusion tool;

[0042] Figure 2 is a schematic diagram of a powder bed fusion apparatus according to an embodiment of the present invention;

[0043] Figure 3 yes Figure 1 A perspective view of a galvanometer system of an optical scanner of a powder bed fusion apparatus shown in;

[0044] Figure 4 is a schematic diagram of scanning parameters according to an embodiment of the present invention;

[0045] Figure 5 is a schematic diagram of a laser for use in a powder bed fusion apparatus according to an embodiment of the present invention;

[0046] Figure 6 It is the circuit diagram of the laser's power supply circuit and pump diode;

[0047] 7A to 7H shows a power waveform of a pulse of pulse exposure according to an embodiment of the present invention;

[0048] Fig. 8A A microscope image of a region of a sample constructed using an initial burst pulse is shown; Figure 8B shows a microscope image of a region of a sample structured using rectangular pulses; and Figure 8C shows a microscope image of a region of a sample constructed using a progressive cooling pulse;

[0049] 9A to 9D is a scanning electron microscope (SEM) of a sample formed using a progressive cooling pulse;

[0050] Fig. 10A is an image of the surface of a cube constructed using rectangular pulses, and Fig. 10B is the corresponding image of the surface of a cube constructed using a progressive cooling pulse.

[0051] Fig.11A shows cross-sectional images of a cube built with H13 tool steel using different progressive cooling pulses having (1) 5 μs step size, (2) 10 μs step size, (3) 15 μs step size, (4) 10 μs step size with modified power, and (5) 15 μs step size with modified power; and Fig. 11B is a table of the resulting density measurements;

[0052] FIG. 12A to FIG. 12D The bare substrate is exposed to rectangular pulses ( Fig. 12A ), exposure to triangular pulses ( Fig. 12B ), exposure to the initial blast pulse ( Fig. 12C), and exposure to a gradual cooling pulse with a 5 μs step size ( Fig.12D ) images.

[0053] FIG. 13A to FIG. 13D is an image of exposing a bare substrate to a pulse exposure comprising a plurality of the following: (i) rectangular pulses ( Fig.13A ), (ii) a gradual cooling pulse with a step size of 5 μs ( Fig. 13B ); (iii) initial blast pulse ( Fig. 13C ), and (iv) triangular pulse ( Fig.13D );

[0054] Fig.14 is a graph of hardness measurements performed on samples formed using rectangular pulses and progressive cooling pulses;

[0055] Fig.15 is a table showing parameters used to construct multiple samples using short rectangular pulses and density measurements of these samples;

[0056] FIG. 16A to FIG. 1 6D shows a power waveform of hybrid pulse exposure according to an embodiment of the present invention;

[0057] Fig.17 is a table showing the powers for different progressive cooling pulses used in Example 3; and Fig.18A and Fig.18B shows the use of rectangular pulses ( Fig.18A ) and the initial burst pulse ( Fig.18B ) Cross-sectional image of a cube constructed with IN718. DETAILED DESCRIPTION

[0058] refer to Figure 2 and Figure 3 , a powder bed fusion apparatus according to an embodiment of the present invention includes a build chamber 101 having: a tooling plate 115 having a hole therein; and a build sleeve 116 extending downward from the hole. The build platform 102 is lowerable in the build sleeve 116, so that the build sleeve 116 and the build platform 102 together define a build volume 117. When a workpiece is built by selective laser melting of powder, the build platform 102 supports a build substrate 102a, a powder bed 104, and a workpiece (object) 103. As successive layers of the workpiece 103 are formed, the platform 102 is lowered within the build volume 117 under the control of a drive mechanism (not shown).

[0059] When the workpiece 103 is built by the dispensing device 108 and the spreader 109, a powder layer 104 is formed. For example, the dispensing device 108 can be a device as described in WO2010 / 007396. The dispensing device 108 distributes the powder onto the upper surface 115a defined by the processing plate 115, and spreads it across the powder bed by the spreader 109. The position of the lower edge of the spreader 109 defines a working plane 110, where the powder is consolidated.

[0060] A plurality of laser modules 105a, 105b generate laser beams 118a, 118b for melting the powder 104, and each module 105a, 105b is arranged to deliver the laser beam to a corresponding optical scanner 106a, 106b. The optical scanner 106a, 106b steers the laser beam 118a, 118b to a selected area of ​​the powder bed 104 in order to build an object. The laser beams 118a, 118b enter through a common laser window 107.

[0061] Each optical scanner 106a, 106b includes a movable steering optical device 121 (such as two mirrors 141a, 141b mounted on a galvanometer 124a, 124b (see Figure 3 )), steering components for steering the laser beam 118 in perpendicular directions (X and Y) across the working plane 110, and focusing optics 120 (e.g., two movable lenses for changing the focus of the laser beam 118). The optical scanner is controlled so that the focus position of the laser beam 118 remains in the same plane 110 as the laser beam 118 moves across the working plane 110. An angular position sensor 125a, 125b is integrated into each galvanometer 124a, 124b for measuring the angular position of the corresponding mirror 121a, 121b.

[0062] In one scanning scheme, the movement of the mirrors 141a, 141b is synchronized with the laser beam pulses generated by the laser 105 to expose the powder bed 104 using pulsed exposure. Angular position sensors 125a, 125b can be used to provide feedback to the laser 105 to ensure proper synchronization between the movement of the mirrors 141a and 141b and the firing of the laser 105. Figure 4 Scan parameters used to define the pulse exposure are shown. Figure 4A scanning path of a raster scan portion 24 including a core for forming a consolidation layer surrounded by boundary scan portions 21 and 22 for forming the edges of the consolidation layer is shown. The distance between the scanning paths of the raster scan portions is defined by a fill distance 25, and the distance between the scanning paths of the boundary scan portion and the raster scan portion is defined by boundary offsets 26 and 27. One or more or all of the scanning paths can be formed using pulse exposure. The pulse exposure is defined by the point pitch 23 and the exposure time (pulse duration) of each exposure. The system can also implement a jump delay that defines the time allowed for the mirror to stabilize after moving between points before the laser is fired. The jump delay can vary depending on the point pitch. If mirror movement during exposure is considered acceptable, or a scanning system that compensates for such mirror movement is used (e.g., as disclosed in WO 2016 / 156824, which is incorporated herein in its entirety), the use of jump delays can be eliminated.

[0063] refer to Figure 5 and Figure 6 , the laser 105 includes a master laser controller 200 (such as a programmable integrated circuit) programmed with firmware 201, and a plurality of laser diode controllers 201a, 201b, each laser diode controller being used to control a different group of laser diodes 202a, 202b to pump a gain medium 203, such as a neodymium-doped fiber. In this embodiment, each group of laser diodes 202a, 202b consists of two laser diodes. For simplicity, only two laser diode controllers 201a, 201b are shown, but more than two laser diode controllers 201a, 201b are typically provided. In this embodiment, six laser diode controllers 201a, 201b are provided. Accordingly, a total of twelve laser diodes can be used to pump the doped fiber 203.

[0064] The master controller 200 communicates with the laser diode controllers 201a, 201b via a communication interface 204. In this embodiment, the communication interface 204 is a serial peripheral interface (SPI) synchronous bus. Instructions in the form of control signals (data packets) sent by the master laser controller 200 can be addressed to individual laser diode controllers 201a, 201b, or broadcast to all laser diode controllers 201a, 201b. Accordingly, the laser diode controllers 201a, 201b can be controlled by the master controller 200 individually or as a group. The instructions sent to each laser diode controller 201a, 201b define the desired output of a group of laser diodes 202a, 202b controlled by this laser diode controller 201a, 201b. Each laser diode controller 201a, 201b is also connected to the master controller 200 via FIRE and ENABLE communication lines. The user interacts with the master controller via a user interface UI to set control and read status.

[0065] Each laser diode controller 201a, 201b includes a power supply circuit 210a, 210b that generates a pulse width modulated signal to an inertial load 211. In this embodiment, the inertial load includes an inductor 211a and at least one capacitor 211b, which are connected in series to form a second order filter. The inertial load 211 converts the digital pulse width modulated signal into a smoother (or "averaged") waveform (drive signal) +V_LD for driving the laser diode.

[0066] The power supply circuits 210 a , 210 b include a programmable device (field programmable gate array 209 in this embodiment) and a switching amplifier 212 .

[0067] The programmable device 209 processes the control signals (instructions) received from the main controller 200 and generates a low voltage output signal and a low current output signal (described below) corresponding to the desired switching state of the GaN transistor.

[0068] The switching amplifier 212 includes a half H-bridge 214 connected across a high voltage power supply. The half H-bridge 214 includes two GaN transistors. The switching amplifier 212 further includes a GaN driver 213 connected to the gates of the GaN transistors of the half H-bridge 214. The GaN driver 213 receives the output signals from the programmable device 209 and converts these output signals into corresponding higher voltage and higher current switching signals suitable for driving the GaN transistors of the half H-bridge 214.

[0069] Programmable device 209 controls the switching of the transistors to generate a pulse width modulated signal so that the output from a set of laser diodes 202 corresponds to the desired output as encoded in the received control signal, as described in more detail below. The drive current delivered to the laser diodes is monitored across resistor 215.

[0070] When the ENABLE parameter is set and the ENABLE signal is high, the laser diode controller 201a, 201b is enabled to drive current into the laser diode. The amount of current will depend on the state of the FIRE signal and the values ​​of the GAN-LO parameter and the GAN-HI parameter. When FIRE is low, the current demand is set by the GAN-LO parameter. When FIRE is high, the current demand is set by the GAN-HI parameter.

[0071] The master controller 200 drives the FIRE signal and sets the GAN-HI and GAN-LO parameters of the laser diode controllers 201a, 201b as needed to command the power circuits 210a, 210b to generate drive signals according to the desired pulse shape. The master controller 200 can be pre-programmed with a library of pulse sequences and / or pulse shapes so that the user / powder bed fusion device can select a pulse sequence and / or pulse shape from the library as needed. However, additionally or alternatively, the master controller 200 can be programmed to generate a pulse sequence and / or pulse shape based on aspects of the (one or more) pulse sequences and / or (one or more) pulse shapes encoded in the received command. In this way, the laser can be controlled to generate pulse sequences and / or pulse shapes beyond the pulse sequences and / or pulse shapes contained in the library.

[0072] The response time of a laser (the time between a change in the control signal and a corresponding change in the output laser beam corresponding to the desired power) is typically 5 microseconds or less.

[0073] 7A to 7E Demonstrates the pulse shapes that can be obtained using the laser.

[0074] exist Fig. 7AIn the embodiment of the present invention, the pulse shape is essentially a rectangular pulse which rises to the maximum power of the pulse, remains at the maximum power for a set duration (typically longer than 5 microseconds), and then falls to a minimum reference power within 5 microseconds. With a laser according to the present invention, the rise and fall in power can be achieved within 5 microseconds of the control signal used to generate such a pulse. When such a pulse is used for pulsed exposure in powder bed fusion, any delay between the firing of the laser and the laser beam being generated can be reduced compared to prior art systems, thereby allowing closer and / or improved synchronization of the laser with the scanner, as less room must be provided for variability in the firing time of the laser.

[0075] In another embodiment, a pulse shape with a step-wise decrease in boost power level is provided. The pulse comprises an initial rise to a first (preferably higher) boost power. The rise time to the first boost power may be less than 15 microseconds, preferably less than 10 microseconds, more preferably less than 5 microseconds. The first boost power remains plateaued in power for a first time period of 5 microseconds or longer, and then decreases to one or more second boost powers less than the first boost power, the power being plateaued at the second boost power or each second boost power. The second boost power or each second boost power may remain plateaued in power for a (second) time period of 5 microseconds or longer. The fall time between the first boost power and the second boost power or one of the second boost powers, and between the second boost power pairs, may be less than 15 microseconds, preferably less than 10 microseconds, more preferably less than 5 microseconds. Finally, the laser pulse returns to a reference power (a power less than 10% of the first boost power or zero power).

[0076] Figure 7B An example of a pulse shape with a step-wise decrease in power, with a first increased power plateau 301 and a single second increased power plateau 302 (initial burst pulse) is shown. Figure 7B , the first increased power plateau 301 has a longer duration than the second increased power plateau 302, although it will be appreciated that the duration of the first increased power plateau 301 and the second increased power plateau 302 may be the same, or the duration of the second increased power plateau 302 may be longer than the duration of the first increased power plateau 301. In this embodiment, the first increased power plateau 301 lasts for 60 microseconds at 280 W, and the single second increased power plateau 302 lasts for 20 microseconds at 240 W. The powers of both the first increased power plateau 301 and the second increased power plateau 302 will be sufficient to melt the powder of the laser spot size, which in this embodiment is 1 / e of 60 microns to 80 microns. 2 Spot-sized powder.

[0077] Figure 7C is another example of a pulse shape with a step-wise reduction in power, with a first increased power plateau 401 and a plurality of second increased power plateaus 402 at decreasing powers (progressive cooling pulse). Figure 7C , the first increase power plateau 401 has a longer duration than the second increase power plateau 402, although it will be appreciated that the duration of the first increase power plateau 401 and the second increase power plateau 402 may be the same, or that the duration of at least one of the second increase power plateaus 402 may be longer than the duration of the first increase power plateau 401. In this embodiment, the first increase power plateau 401 lasts for 80 microseconds at 200 W, and each second increase power plateau 402 lasts for 5 microseconds at a power reduction of approximately the same amount for each step size (a reduction of 33.3 / 33.4 W in this example). However, it will be appreciated that other sized steps in power and other durations may be used for the second plateaus 402. Additionally, all second plateaus 402 do not necessarily have to be of the same duration. This example is similar to the example of FIG. 4 . Figure 7B The example of FIG. 4 differs in that at least some, if not all, of the second plateau 402 provide insufficient energy density at the powder bed to melt the powder of the laser spot size, in this embodiment a 1 / e of 60 microns to 120 microns at the plane of the powder bed. 2 Spot-sized powder.

[0078] Fig.7D is a step-by-step reduction in power (similar to Figure 7B Another example of a pulse shape with a stepwise reduction in power as shown in FIG. 1 ), but with a first plateau 501 and a second plateau 502 having the same duration (40 microseconds in this embodiment).

[0079] exist Fig. 7EIn another embodiment shown in , the pulse shape includes at least a portion having a triangular shape (triangular pulse). In this embodiment, the pulse includes: a first rising portion 601, which rises from a baseline power to a first boosted power (200W in this example) in, for example, less than 5 microseconds; a second rising portion 602, in which the power rises relatively gradually compared to the first rising portion to a second peak boosted power (280W in this example); a first falling portion 603, in which the power decreases to a third boosted power (in this example, the same as the first boosted power); and a second falling portion 604, in which the power decreases relatively quickly (such as in less than 5 microseconds) to the baseline level compared to the first falling portion. For a given laser spot size, the first boosted power and the second boosted power can provide an energy density at the powder bed sufficient to melt the powder. In this embodiment, the laser beam has a 1 / e value of 60 to 120 microns at the plane of the powder bed. 2 Spot size.

[0080] refer to Figure 7F and Figure 7H In another embodiment, a pulse shape (stirring pulse) configured to stir the molten pool is provided. In such an embodiment, the pulse shape includes multiple peaks (maxima) oscillating between powers that provide an energy density sufficient to melt the powder at the powder bed. The pulse can be viewed as a rectangular or triangular pulse (having a pulse duration of between 20 microseconds and 200 microseconds, more preferably between 20 microseconds and 100 microseconds, and typically about 80 microseconds) with a (superimposed) pulse wave on which pulses of shorter duration are superimposed. In this embodiment, the power is initially raised 701, 801 to a first increased power. The power can then be maintained at the first increased power ( Figure 7F The power may then be reduced to the second increased power 702, or the power may be immediately reduced to the second increased power 802 ( Figure 7H ). The power can then be maintained at a second increased power ( Figure 7F The power may then be increased to a third increased power 703, or the power may be increased immediately to a third increased power 803 ( Figure 7H ). The third power 703, 803 may be the same as or different from the first increased power 701, 801. Figure 7F In the example shown in FIG. 7 , the power remains at the third increased power (plateau 706) and then is reduced to the reference power / zero power. Figure 7H In the process, the power is immediately reduced to the reference power / zero power. Figure 7F A castellation pulse shape comprising a rectangular superimposed pulse wave oscillating between a first boosted power and a second boosted power is demonstrated. Figure 7HA pulse shape is shown that includes a triangular superimposed pulse wave oscillating between a first increased power and a second increased power. It should be understood that other shapes of superimposed pulse waves may be used, such as sawtooth or sine pulse waves oscillating between a first increased power and a second increased power. In addition, (for any of the different shapes of pulse waves) the initial rise to the first increased power and the fall to the reference power / zero power may be gradual (possibly in a stepwise manner) (e.g., over a period longer than 5 microseconds (e.g., Figure 7H ), or quickly (e.g., within a period of less than 5 microseconds (e.g., Figure 7F As shown)). It will be appreciated that the superimposed pulse wave of pulses may include more than two peaks.

[0081] Example 1

[0082] A 10 mm x 10 mm x 11.75 mm cube was constructed in H13 tool steel in a RenAM 500E powder bed fusion machine modified to utilize a laser according to the present invention instead of a PRISM laser. The cube was formed using different pulse exposures including (i) an initial blast pulse ( Figure 7B ), (ii) rectangular pulse ( Fig. 7A ) and (iii) progressive cooling pulse ( Figure 7C ). The build parameters are: laser spot size 80 μm, spot pitch 65 μm, fill distance 80 μm, layer thickness 40 μm. Fig. 8A , Figure 8B and Figure 8C Cross-sections of the cube are shown at different magnifications. Fig. 8A An image showing the initial burst pulse, Figure 8B The corresponding image for a rectangular pulse is shown, and Figure 8C The corresponding images of the progressive cooling pulse are shown.

[0083] The initial burst sample and the rectangular sample resulted in a greater number of cracks than the sample formed using the progressive cooling pulse. In addition, as can be seen from the images, the progressive cooling pulse resulted in smaller cracks than the rectangular pulse and the initial burst pulse. Most of these smaller cracks had smooth surfaces, which means that these cracks were not caused by solidification cracking.

[0084] 9A to 9D The presence of these smaller cracks with smooth surfaces is further supported by the SEM images of cubes formed using progressive cooling pulses shown in . Very few cracks were observed and most were smaller than 20 μm.

[0085] Example 2

[0086] Fig. 10Ais an image of the surface of a cube constructed using rectangular pulses, and Fig. 10B is the corresponding image of the cube built using the progressive cooling pulse. The highest z-plane is at the right portion of the image (top of the page). As can be seen from the image, the melt pool depth is clearly visible for the progressive cooling pulse, while there is no corresponding feature clearly visible for the cube built using the rectangular pulse.

[0087] Example 3

[0088] Fig.11A The results are shown using a modified RenAM 500E powder bed fusion machine, using (1) 5 μs step size, (2) 10 μs step size, (3) 15 μs step size, (4) 10 μs step size with modified power, and (5) 15 μs step size with modified power (as shown in FIG. Fig.17 Figure 1. Cross-sectional images of 10 mm x 10 mm x 11.75 mm cubes T1 to T4 built in H13 tool steel with different progressive cooling pulses (shown in Figure 1). A spot distance of 65 μm, a fill distance of 80 μm, and a layer thickness of 40 μm were used. The same number of steps was used for each pulse. As the step duration increased, the correction power reduced the power of the step to offset the increased power density supplied to the powder. The build parameters were: laser spot size of 80 μm, spot distance of 65 μm, fill distance of 80 μm, and layer thickness of 40 μm.

[0089] As from Fig. 11B As can be seen in the graph, the highest density is achieved with the progressive cooling pulses with a 5 μs step size, although the density achieved with a 10 μs step size is also acceptable. The 15 μs step size did not produce a good density of the cube, and also did not produce a pulse with modified power. This suggests that step sizes less than 15 μs (or a continuous (non-stepped) decrease in power) are preferred, with step sizes of 10 μs and 5 μs providing improved results. There does not appear to be a clear correlation between step duration and power, as for the 15 μs step size, the modification of the power consistent with the target energy density did not produce a good density portion.

[0090] Example 4

[0091] To explore the molten pool shapes formed by different pulse exposures, spots on a bare metal substrate were exposed to different pulse shapes in the pulse shape. Fig. 12A is an image of the melt pool features formed by a rectangular pulse, Fig. 12B is an image of the melt pool features formed by the triangular pulse, Fig. 12C is an image of the melt pool features formed by the initial blast pulse, and Fig.12Dis an image of the melt pool features formed by progressive cooling pulses with a 5μs step size. As can be seen, each pulse shape creates a unique melt pool feature, and this is expected to have an impact on the trajectory shape and the creation of gas-carried condensates. The melt pool formed by the rectangular pulse and the initial blast pulse appears more chaotic, with material ejected from the exposed location, while the triangular pulse and progressive cooling pulse create a more uniform melt pool shape. The melt pool of the rectangular pulse is smaller than that of the progressive cooling pulse.

[0092] Example 5

[0093] Metal tracks are formed on a bare metal substrate using different pulse exposures. The tracks include multiple fill lines and boundary scan sections. FIG. 13A to FIG. 13D These are images of the scanned parts. Fig.13A shows the end and start of the fill line of the rectangular pulse and the boundary scan section, Fig. 13B The end and start of the fill line and boundary scan section of the progressive cooling pulse are shown, Fig. 13C shows the end and start of the fill line of the initial burst pulse and the boundary scan portion, and Fig.13D The end and start of the fill line for the triangular pulse and the boundary scan are shown. The melt track shape, size and noise level (balling) are different for the different shaped pulses (as expected from the melt pool characteristics of Example 4). From the images, the melting of the boundary track looks more consistent for the progressive cooling pulse compared to the rectangular pulse. No start and end fill line defects are observed for the progressive cooling pulse and the rectangular pulse.

[0094] Example 6

[0095] The samples were built in an unscreened H13 tool steel using rectangular pulses and progressive cooling pulses in a modified RenAM 500E powder bed fusion machine. The hardness of the samples was measured. The samples built using progressive cooling pulses had an average hardness 5% greater than the average hardness of the samples built using rectangular pulses, as shown in Figure 2. Fig.15 shown.

[0096] Example 7

[0097] Five samples were built in titanium alloy Ti-6Al-4V using a modified RenAM 500E powder bed fusion machine using rectangular pulses with a total pulse duration of 10 μs. A 10 μs gap was provided between the laser pulses. Fig.15The parameters used to construct the pulse exposure for each sample are provided in the table. These parameters are the power (P) in Watt, the dot pitch (PD) in μm, the exposure time / pulse duration (EXP) in μs, and the fill distance (HD) in μm. The layer thickness (LT) is 60 μm. The delay between pulses is set by setting a variable called jump delay (JD), and the exposure time plus jump delay is set to 20 μs.

[0098] The 2D energy density (2DED), speed (PD / EXP+JD) and build rate have been determined based on the other parameters. As can be seen from the table, for a 10μs pulse exposure, a density greater than 99.9% theoretical density is achieved. Such shorter pulses may be useful for providing finer fill lines. In addition, shorter pulses may result in higher cooling rates compared to longer pulse durations, and accordingly, different microstructures due to the steeper thermal gradients generated across the molten pool. Such finer microstructures may improve the properties of parts, particularly aluminum and aluminum alloys. Such short pulses may be advantageously used in distributed point scanning methods as described in WO2016 / 079496.

[0099] Example 8

[0100] Use as Fig. 7A The rectangular pulses shown and Figure 7B The initial blast pulse shown was used to build the sample in IN718 in a modified RenAM 500E powder bed fusion machine. Fig.18A and 18B It can be seen that the rectangular pulses lead to epitaxial grains, whereas fewer epitaxial grains, such as grains with reduced aspect ratios (grain uniformity), are observed in the material melted using the initial burst pulse. This may be due to the more chaotic melt pool generated using the initial burst pulse (e.g. Fig. 12C shown).

[0101] exist FIG. 16A to FIG. 16CIn another embodiment of the present invention shown in , the pulse exposure includes a "hybrid pulse exposure" in which the scanning path is illuminated using an initial pulse and / or an end pulse 901, 1001, 1101; 902, 1002, 1102 of the pulse exposure (which has a pulse duration shorter than the pulse duration of at least one intermediate pulse 903, 1003, 1103 between the initial pulse and the end pulse). Typically, the start pulse and / or the end pulse 901, 1001, 1101; 902, 1002, 1102 has a pulse duration of less than 200μs, more typically less than 100μs, and the intermediate pulse 903, 1003, 1103 has a pulse duration of greater than 200μs (continuous exposure) and will depend on the length of the scanning path and the scanning speed. The start pulse and / or the end pulse 901, 1001, 1101; 902, 1002, 1102 can be similar to the reference 7A to 7H In this embodiment, all of the start pulse and / or end pulse 901, 1001, 1101; 902, 1002, 1102 have the same pulse duration. However, it will be understood that the start pulse and / or end pulse 901, 1001, 1101; 902, 1002, 1102 may vary.

[0102] Fig.16A A hybrid pulse exposure is demonstrated, where the pulse has an initial burst pulse shape. Fig. 16B A hybrid pulse exposure is demonstrated, where the pulses have a triangular pulse shape. Fig. 16C A hybrid pulse exposure is demonstrated, where the pulse has a progressively cooling pulse shape. Fig. 16C The lengths of the steps shown in are only illustrative, and the first plateau may be of longer duration than subsequent lower power steps.

[0103] It is believed that using this hybrid exposure can reduce defects observed at the beginning and end of fill lines using conventional continuous mode scanning, while benefiting from the faster processing achievable by scanning in continuous mode (because the laser is on for a longer period of time).

[0104] It should be understood that changes and modifications may be made to the described embodiments without departing from the scope of the invention as defined herein. For example, other pulse shapes may be used. In addition, the construction of the part may include consolidating the material using pulsed exposure for some areas and scanning in a continuous mode for other areas to consolidate the material.

Claims

1. A powder bed fusion additive manufacturing method, the method comprising exposing layers of a powder bed to an energy beam to selectively melt regions of each layer, wherein pulsed exposure is used to melt at least a certain proportion of the regions, the method further comprising commanding an energy beam source to generate at least one pulse of the pulsed exposure having a pulse duration of less than 200 microseconds, the commanding step comprising specifying a plurality of increased power levels above a baseline power level for a power waveform of the at least one pulse.

2. The powder bed fusion additive manufacturing method of claim 1 , comprising commanding the energy beam source to generate each of the plurality of pulses having the power waveform and a pulse duration of less than 200 microseconds.

3. The powder bed fusion additive manufacturing method according to claim 1 or claim 2, wherein: The step of commanding includes specifying each of the plurality of increased power levels in a control signal sent to or driving the energy beam source.

4. The powder bed fusion additive manufacturing method according to any one of claims 1 to 3, wherein: The step of commanding includes specifying a pulse shape for the power waveform.

5. The powder bed fusion additive manufacturing method according to claim 4, wherein: The power waveform is a non-rectangular pulse shape.

6. The powder bed fusion additive manufacturing method according to claim 4 or claim 5, wherein: The pulse shape includes a plurality of power plateaus at the increased power level.

7. The powder bed fusion additive manufacturing method according to any one of the preceding claims, wherein: The pulse duration is greater than 1 microsecond.

8. The powder bed fusion additive manufacturing method according to any one of the preceding claims, wherein: The energy beam source has a response time of 10 microseconds or less, preferably 5 microseconds or less.

9. The powder bed fusion additive manufacturing method according to any one of the preceding claims, wherein: The step of commanding includes specifying one or more increased power levels between 10% and 90% of a maximum increased power level.

10. The powder bed fusion additive manufacturing method of any preceding claim, comprising specifying one or more boosted power levels to have a duration of less than 15 microseconds, optionally less than 10 microseconds, and further optionally less than 5 microseconds.

11. The powder bed fusion additive manufacturing method according to any one of the preceding claims, wherein: The step of commanding includes specifying a shape of the power waveform that decreases in a plurality of steps.

12. The powder bed fusion additive manufacturing method according to claim 11, wherein: At least one step size of the plurality of steps has a duration of less than 15 microseconds, optionally less than 10 microseconds, and further optionally less than 5 microseconds.

13. The powder bed fusion additive manufacturing method according to claim 11 or claim 12, wherein: At least one step size of the plurality of step sizes has a duration greater than 1 microsecond, and further optionally greater than 2 microseconds.

14. The powder bed fusion additive manufacturing method according to any one of the preceding claims, wherein: The commanding step includes specifying a pulse shape to control the cooling rate of the melted and / or resolidified material.

15. The powder bed fusion additive manufacturing method according to any one of the preceding claims, wherein: The step of commanding includes specifying the power waveform such that a fall time is longer than a rise time.

16. The powder bed fusion additive manufacturing method according to claim 15, wherein: The fall time of the at least one pulse is greater than 10 microseconds, preferably greater than 20 microseconds, most preferably about 30 microseconds.

17. The powder bed fusion additive manufacturing method according to claim 15, wherein: The fall time of the at least one pulse is less than 100 microseconds, preferably less than 50 microseconds, most preferably less than 40 microseconds.

18. The powder bed fusion additive manufacturing method according to any one of the preceding claims, wherein: The step of commanding includes specifying the power waveform such that the at least one pulse includes a plurality of maxima.

19. A powder bed fusion additive manufacturing method comprising exposing layers of a powder bed to an energy beam to selectively melt regions of each layer, wherein at least a certain proportion of the regions are melted using pulsed exposure, at least one pulse of the pulsed exposure having a pulse duration of less than 20 microseconds.

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