CN118871233A - Energy beam exposure in powder bed fusion - Google Patents

Abstract

A powder bed fusion additive manufacturing method comprising exposing a layer 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 scan path to melt material of the at least one region using pulsed exposure. The initial pulse and/or the end pulse of the pulse exposure may have a pulse duration that is shorter than the pulse duration of the intermediate pulse between the initial pulse and the end pulse.

Description

Energy beam exposure in powder bed fusion

Technical Field

The present invention relates to energy beam exposure in powder bed fusion, and in particular to pulsed exposure.

Background

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

Typically scanned along a scan path with an energy beam. 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 intensities maintained at this level (hereinafter referred to as "continuous exposure"). In an alternative pulse mode, the energy beam is emitted as a pulse train, wherein each pulse is directed to a different region along the scan path (hereinafter referred to as "pulse exposure"). The pulses of the pulsed exposure may be synchronized with the control of beam manipulation members that direct the energy beam to the powder bed, the purpose of which is to move the beam manipulation members only when the energy beam is 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 member, and some movement of the energy beam over the powder bed surface will occur during the pulse. In both continuous scanning and pulsed scanning, a continuous consolidated material is formed from the beginning to the end of the scan path.

A RenAM Q machine from Renishaw enables scanning of powder beds with pulse durations of about 80 microseconds (microsecond) using pulsed exposure. The apparatus includes four 500W Continuous Wave (CW) lasers. The laser is a redPOWER PRISM module fiber laser provided by the SPI. The controller of each CW laser includes a class a power amplifier for generating a control signal to the pump diode of the laser. The square wave control signal modulates the output of the CW laser between on (single boost power level) and off to generate laser pulses, as shown in fig. 1. The maximum laser power per laser pulse is set in advance, for example, by a user. The rise time (from 10% to 90% of the maximum laser power) and fall time (from 90% to 10% of the maximum laser power) of the pulse are about 8 microseconds. The variation in maximum power of the laser pulses requires several milliseconds, i.e. several tens of pulses. The arrival time of the first pulse may vary by up to 7 microseconds while the arrival time of the subsequent pulse varies 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 plateau at the maximum laser power preset for the laser pulse.

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

K.A.Mumtaz, N.Hopkinson, "SELECTIVE LASER MELTING of THIN WALL PARTS using pulse shaping (selective laser melting of thin-walled parts using pulse forming)" studied pulse forming in selective laser melting. The system enables a user to tailor the energy distribution to the nearest 0.5ms within a single laser pulse. Various ramp up and ramp down pulse shapes are generated and used to process the four-layer Inconel 625. The ramp pulse varies from 1.7ms to 10ms. The ramp down pulse varies between 1ms and 10ms.

Disclosure of Invention

According to a first aspect of the present invention there is provided 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 a pulsed exposure is used to melt at least a proportion of the regions, the method further comprising commanding an energy beam source to generate at least one pulse of pulsed exposure, preferably each pulse of a plurality of pulses.

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

The step of commanding may include specifying a pulse shape, a pulse profile, and/or a pulse form for the power waveform. The power waveform may be non-rectangular pulse shape/profile/form. It has been found that non-rectangular pulse shapes/contours/forms can cause variations in how the material solidifies, which can improve the part compared to a part produced using rectangular pulses. The pulse shape/profile/form may be triangular. The pulse shape/profile/form may include multiple power plateaus at a given elevated power level.

The pulse duration (the time between the laser power of the pulse rising above the maximum specified elevated power level and then returning below 10% of the maximum specified elevated 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 longer pulse durations result in material evaporation at the energy beam power required to melt the powder, especially the molten metal powder. Lowering the power of the energy beam below this threshold will result in the powder not being melted even if exposed to the energy beam for a longer period of time, since the heat dissipates too quickly for such low energy beam power that the temperature of the powder does not rise above the melting temperature. Accordingly, pulse exposure comprising a pulse duration longer than 200 microseconds is undesirable.

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

The energy beam source for generating the energy beam may have a response time (time between a change in the control signal and a change in the output energy beam corresponding to the required power) of 10 microseconds or less, preferably 5 microseconds or less. Controlling the shape of the power waveform may include designating one or more elevated power levels between 10% and 90% of the maximum elevated power level. The method may include designating one or more elevated power levels as having a duration of less than 15 microseconds, alternatively less than 10 microseconds, and further alternatively less than 5 microseconds. The method may include controlling the shape of the power waveform, increasing or decreasing in a plurality of 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 of greater than 1 microsecond, and further optionally greater than 2 microseconds.

The method may include specifying a pulse shape to control the cooling rate of the melted and/or resolidified material. In powder bed fusion, the metallic material typically solidifies within tens of microseconds, and therefore, after the intensity of the energy beam drops below an intensity sufficient to maintain the temperature of the material above the melting point, the melted material rapidly resolidifies. It has been found that extending the fall time of the energy beam pulse to an uncontrolled fall time of greater than 8 microseconds (implemented in the RenAM Q machine of Renishaw) can improve the material properties of the cured material. (the fall time in the Renishaw RenAM Q machine is uncontrolled/unspecified, since the fall time is not defined by control signals (which are square wave pulses with vertical rise and fall times) but 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 such 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 such that the fall time of at least one pulse is greater than 10 microseconds, preferably greater than 20 microseconds, most preferably about 30 microseconds. The fall time may be less than 100 microseconds, preferably less than 50 microseconds, most preferably less than 40 microseconds. The fall time of the 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 boost 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, more preferably between 6MW/s and 20 MW/s. Extending the fall time of the pulse may affect the cooling rate during solidification of the molten material, thereby altering the resulting microstructure. The fall time should be of the order of the time required for the melted material to solidify, for example tens of microseconds, in order to change the cooling rate by an extended fall time, but avoid unnecessary maintenance of the material at elevated temperatures. Lower cooling rates may reduce solidification cracking in certain materials such as steels, for example tool steels (e.g., H13 tool steels), W360, or nickel-based superalloys.

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

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

The method may include controlling the shape of the power waveform such that at least one of the pulses includes a plurality of maxima. A number of the maxima may be higher than 90% of the maximum elevated power level set 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 is the middle local minimum of the pulse. At least one of the intermediate local minima may be higher than 10%, preferably higher than 90% of the set maximum elevated 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 microsecond, between 5 microsecond and 50 microsecond, between 10 microsecond and 50 microsecond, between 1 microsecond and 40 microsecond, between 5 microsecond and 40 microsecond, between 10 microsecond and 40 microsecond, between 1 microsecond and 30 microsecond, between 5 microsecond and 30 microsecond, between 10 microsecond and 30 microsecond, between 1 microsecond and 20 microsecond, between 5 microsecond and 20 microsecond, between 10 microsecond and 20 microsecond, between 1 microsecond and 17 microsecond, between 5 microsecond and 17 microsecond, or between 10 microsecond and 17 microsecond. The oscillations in the power of the pulses during exposure may agitate the melt pool to improve grain size uniformity and/or to improve the penetration depth of the pulses.

According to a second aspect of the present invention there is provided 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 proportion of the regions are melted using pulsed exposure, at least one pulse of the pulsed exposure (preferably each pulse of a plurality of pulses) having a pulse duration of less than 20 microseconds.

Shorter pulse durations than the 80 microsecond pulses used in RenAM Q machines can result in finer microstructures and/or higher build rates.

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

According to a third aspect of the present invention there is provided a powder bed fusion additive manufacturing method comprising exposing a layer 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 scan path to melt material of the at least one region using a pulsed exposure, an initial pulse and/or an end pulse of the pulsed exposure having a pulse duration shorter than a pulse duration of an intermediate pulse between the initial pulse and the end pulse.

The use of pulses at the beginning and/or end of the scan path may improve the penetration of material in these areas compared to initiating or ending a continuous exposure in these areas of the scan path.

It should be understood that a "scan path" as used herein refers to a path along which an energy beam is scanned to form a continuous consolidated material from the start point to the end point of the path. The scan path begins where the consolidated material begins (in the direction of travel of the energy beam along the scan path) and ends where the consolidated material ends (in the direction of travel of the energy beam along the scan path). Two or more scan paths are formed when there is an interruption in the continuity of the consolidated material. The initial pulse is a pulse for forming a consolidated material comprising consolidated material at the beginning of the scan path and the end pulse is a pulse for forming a consolidated material comprising consolidated material at the end of the scan path.

The scan path may be a straight scan path. The scan path may be one of a plurality of parallel scan paths. The scan path may be one of a plurality of fill lines, wherein each fill line constitutes a separate scan path. The energy beam may be turned off or reduced in power such that no material is consolidated when the energy beam is directed from the end of a first scan path (e.g., a fill line) to the start of another second scan path (e.g., another fill line).

The initial pulse may consolidate the material along at least 200 μm of the scan path. The initial pulse may consolidate material along a scan path of less than 1mm, preferably less than 500 μm. The ending pulse may consolidate material along the scan path less than 1mm, preferably less than 500 μm. The ending pulse may consolidate material less than 500 μm along the scan path.

The pulse duration of the intermediate pulse may have a length similar to scanning in continuous mode. The pulse duration of the intermediate pulse 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.

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.

The shorter pulse duration may be constant for all initial and/or end pulses. Alternatively, the shorter pulse duration may vary for different ones of the initial pulse and/or the end pulse. The shorter pulse duration of the first pulse and/or the last pulse may be shorter than the duration of the other initial pulse or the end pulse, respectively. For the initial pulse, the shorter pulse duration may gradually increase from the first initial pulse to the intermediate pulse. For an ending pulse, the shorter pulse duration may taper from the middle pulse to the last ending pulse.

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 penultimate pulse and the last pulse may be longer than the time between the other pulse pairs of the pulse exposure. The time between pulses may decrease gradually from a first initial pulse to an intermediate pulse. The time between pulses may gradually increase from the middle pulse to the last end pulse.

The dot spacing between pulses may be constant. Alternatively, the dot spacing between pulses may vary. The dot spacing between the first pulse and the second pulse or the dot spacing between the penultimate pulse and the last pulse may be shorter than the dot spacing between other pulse pairs of the pulse exposure. The dot spacing between pulses may gradually increase from a first initial pulse to an intermediate pulse. The point spacing between pulses may decrease gradually from the middle pulse to the last end pulse.

The method may include synchronizing the pulse with control of at least one beam manipulation member that directs the energy beam to the powder bed.

Synchronizing the pulses with the control of the at least one beam manipulation member may comprise: the at least one beam manipulation member (such as a mirror) is moved relatively fast between pulses compared to the speed of movement of the at least one beam manipulation member during the pulses. Synchronizing the pulses with the control of the at least one beam manipulation member may comprise: the target on the powder bed of the beam manipulation member, such as a mirror, is moved relatively fast between pulses compared to the speed at which the target is moved during the pulses. The term "target on the powder bed of the beam manipulation member" as used herein refers to a location on the powder bed to which the energy beam is or would be directed if it were generated.

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.

The controller may be arranged to control the pump such 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 the control signal requiring the change in power and the change in 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 a laser.

Such response times are required for applications such as powder bed fusion, where the pulsed exposure has a duration of several tens of microseconds.

The controller may be arranged to control the pump such that in response to the corresponding control signal, a plurality of stable non-zero laser beam powers may be output within one millisecond, preferably within 500 microseconds, more preferably within 200 microseconds. The term "stabilized" laser beam power refers to the power at which the laser is stationary for a period of time, rather than the power at which the laser beam transitions instantaneously as it rises or falls between zero and non-zero power. Multiple stable non-zero laser beam powers may occur within a single pulse or across multiple pulses. The period of time for which the non-zero laser beam power is stabilized may be at least 1 microsecond, and optionally at least 3 microseconds.

The controller may comprise a power supply circuit arranged to generate the pulse width modulated signal to an inertial load located between the power supply circuit and the pump. The pulse width modulated signal is generated by the power supply 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 the pump (such as one or more laser diodes). In the absence of an inertial load, the laser diode would pulse with a pulse width modulated signal. The inertial load may include an inductor. The inertial load may comprise a capacitor. The inertial load may include an inductor and a capacitor. The inductor and 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.

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

The switching power amplifier may comprise 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 (once every 10 microseconds) and are 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, gaN transistors may be driven at 2.5MHz (switching every 400 nanoseconds). Accordingly, the use of GaN transistors/high electron mobility transistors enables the use of switching power amplifiers while still achieving the required response time of the laser. GaN transistors/high electron mobility transistors are also suitable for high voltage, high temperature and high efficiency applications.

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

The gain medium may be a doped fiber. The optical fiber may be doped with neodymium. The laser may be a NG-YAG fiber laser. The laser may be a continuous wave laser.

According to a fifth aspect of the present invention there is provided a powder bed fusion apparatus comprising an energy beam irradiation device for generating and directing an energy beam to a powder bed, the energy beam irradiation device comprising an energy beam source and a controller arranged to control the energy beam source to carry out a method according to any of the first, second and third aspects of the present invention.

The energy beam irradiation device may comprise an energy beam source, such as a laser, and at least one beam manipulation member for directing the energy beam to a selected location on the powder bed. The energy beam source may be a laser according to the fourth aspect of the invention.

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

The data carrier may be a suitable medium for providing instructions to a machine, such as a non-transitory data carrier, for example a floppy disk, a CD ROM, a DVD ROM/RAM (including-R/-RW and +R/+RW), an HDDVD, a Blu-Ray (TM) optical disc, a memory (such as a memory stick (TM), an SD card, a compact flash card, etc.), a disk drive (such as a hard disk drive), a magnetic tape, any magnetic/optical memory; or a transitory data carrier such as a signal on a wire or an optical fiber or a wireless signal, for example a signal sent over a wired or wireless network (such as internet download, FTP transmission, etc.).

Drawings

FIG. 1 shows control signals generated by a laser used in a RenAM Q powder bed fusion device of Renisshaw and the resulting laser pulses;

FIG. 2 is a schematic view of a powder bed fusion apparatus according to an embodiment of the invention;

FIG. 3 is a perspective view of a galvanometer system of an optical scanner of the powder bed fusion apparatus shown in FIG. 1;

FIG. 4 is a schematic diagram of scan parameters according to an embodiment of the invention;

FIG. 5 is a schematic view of a laser for use in a powder bed fusion apparatus according to an embodiment of the invention;

FIG. 6 is a circuit diagram of the power supply circuit and pump diode of the laser;

Fig. 7A to 7H show power waveforms of pulses of pulse exposure according to an embodiment of the present invention; FIG. 8A shows a microscope image of a region of a sample constructed using an initial burst pulse; FIG. 8B shows a microscope image of a region of a sample constructed using rectangular pulses; and FIG. 8C shows a microscope image of a region of a sample constructed using progressive cooling pulses;

Fig. 9A-9D are Scanning Electron Microscopes (SEM) of samples formed using progressive cooling pulses;

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

FIG. 11A shows a cross-sectional image of a cube constructed with H13 tool steel using different progressive cooling pulses with (1) a step size of 5 μs, (2) a step size of 10 μs, (3) a step size of 15 μs, (4) a step size of 10 μs with corrective power, and (5) a step size of 15 μs with corrective power; FIG. 11B is a table of the resulting density measurements;

fig. 12A-12D are images of bare substrate exposed to rectangular pulses (fig. 12A), to triangular pulses (fig. 12B), to initial burst pulses (fig. 12C), and to progressive cooling pulses with a step size of 5 mus (fig. 12D).

Fig. 13A to 13D are images of exposing a bare substrate to pulse exposure including a plurality of: (i) Rectangular pulses (fig. 13A), (ii) progressive cooling pulses with a step size of 5 μs (fig. 13B); (iii) An initial burst pulse (fig. 13C), and (iv) a triangular pulse (fig. 13D);

FIG. 14 is a graph of hardness measurements made on samples formed using rectangular pulses and progressive cooling pulses;

FIG. 15 is a table showing parameters for constructing a plurality of samples using short rectangular pulses and density measurements for the samples;

FIGS. 16A-16C illustrate power waveforms for a mixed pulse exposure according to an embodiment of the present invention; FIG. 17 is a table showing the power for the different progressive cooling pulses used in example 3; and fig. 18A and 18B show cross-sectional images of cubes constructed IN718 using rectangular pulses (fig. 18A) and initial blast pulses (fig. 18B).

Detailed Description

Referring to fig. 2 and 3, the powder bed fusing apparatus according to an embodiment of the present invention includes a build chamber 101 having therein: a tooling plate 115 having a hole therein; and a build sleeve 116 extending downwardly from the bore. Build platform 102 may be lowered in build sleeve 116 such that build sleeve 116 and build platform 102 together define build volume 117. When a work piece is built by selective laser melting of powder, the build platform 102 supports the build substrate 102a, the powder bed 104, and the work piece (object) 103. As successive layers of the workpiece 103 are formed, the platen 102 is lowered within the build volume 117 under the control of a drive mechanism (not shown).

The powder layer 104 is formed while the work piece 103 is being built up by the dispensing apparatus 108 and the spreader 109. For example, the dispensing device 108 may be a device as described in WO 2010/007496. The dispensing apparatus 108 dispenses powder onto an upper surface 115a defined by a tooling plate 115 and spreads across a powder bed by a spreader 109. The position of the lower edge of the spreader 109 defines a work plane 110 where the powder is consolidated.

The 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 beams to a corresponding optical scanner 106a, 106b. The optical scanners 106a, 106b steer the laser beams 118a, 118b to selected areas of the powder bed 104 in order to build objects. The laser beams 118a, 118b enter through the common laser window 107.

Each optical scanner 106a, 106b includes a steering component for steering the laser beam 118 in the vertical directions (X and Y) across the work plane 110, in the form of movable steering optics 121, such as two mirrors 141a, 141b (see fig. 3) mounted on galvanometers 124a, 124b, and focusing optics 120, such as two movable lenses for changing the focus of the laser beam 118. The optical scanner is controlled such that the focal position of the laser beam 118 remains in the same plane 110 as the laser beam 118 moves across the work plane 110. An angular position sensor 125a, 125b is integrated into each galvo 124a, 124b for measuring the angular position of the respective mirror 121a, 121 b.

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 may be used to provide feedback to laser 105 to ensure proper synchronization between the movement of mirrors 141a and 141b and the emission of laser 105. Fig. 4 shows the scanning parameters used to define the pulsed exposure. Fig. 4 shows a scan path of a raster scan section including a core for forming a bonding layer surrounded by boundary scan sections 21 and 22 for forming edges of the bonding layer. The distance between the scanning paths (fill lines) 24 of the raster scan section is defined by a fill distance 25, and the distance between the boundary scan section and the scanning paths of the raster scan section is defined by boundary offsets 26 and 27. One or more or all of the scan paths may be formed using pulsed exposure. The pulse exposure is defined by the dot pitch 23 and the exposure time (pulse duration) of each exposure. The system may also implement a jump delay that defines the time that the mirror is allowed to stabilize after moving between points before laser firing. The jump delay may vary depending on the point distance. The use of jump delays may be eliminated if mirror movement during exposure is deemed acceptable, or if a scanning system is used that compensates for such mirror movement (e.g., as disclosed in WO 2016/156824, which is incorporated herein in its entirety).

Referring to fig. 5 and 6, the laser 105 includes a main laser controller 200 (e.g., a programmable integrated circuit) programmed with firmware 201, and a plurality of laser diode controllers 201a, 201b, each for controlling a different set of laser diodes 202a, 202b to pump a gain medium 203, such as a neodymium-doped fiber. In this embodiment, each set 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 may be used to pump the doped fiber 203.

The main 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. The instructions in the form of control signals (data packets) sent by the master laser controller 200 may be addressed to the respective laser diode controller 201a, 201b or broadcast to all laser diode controllers 201a, 201b. Accordingly, the laser diode controllers 201a, 201b may be controlled individually or as a group by the main controller 200. The instructions sent to each laser diode controller 201a, 201b define the desired output of a set of laser diodes 202a, 202b controlled by that laser diode controller 201a, 201b. Each laser diode controller 201a, 201b is also connected to the main controller 200 by a transmit (FIRE) and ENABLE (ENABLE) communication line. The user interacts with the main controller via a user interface UI to set the control and read states.

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 connected in series to form a second order filter. The inertial load 211 converts the digital pulse width modulated signal into a smoother (or "average") waveform (drive signal) +v_ld for driving the laser diode.

The power supply circuits 210a, 210b include programmable devices (in this embodiment field programmable gate arrays 209) and switching amplifiers 212.

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

Switching amplifier 212 includes a half H-bridge 214 connected across a high voltage power supply. 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. GaN driver 213 receives output signals from programmable device 209 and converts these output signals to corresponding higher voltage and higher current switching signals suitable for driving GaN transistors of half H-bridge 214.

The programmable device 209 controls the switching of the transistors to generate the pulse width modulated signal such that the output from the 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 diode is monitored across resistor 215.

When the ENABLE (ENABLE) parameter is set and the ENABLE (ENABLE) signal is high, the laser diode controllers 201a, 201b are enabled to drive current into the laser diode. The amount of current will depend on the state of the transmit (FIRE) signal and the values of the GAN-LO and GAN-HI parameters. When the emission (FIRE) is low, the current demand is set by the GAN-LO parameter. When the emission (FIRE) is high, the current demand is set by the GAN-HI parameter.

The main controller 200 drives the transmit (FIRE) signals and sets the GAN-HI parameters and GAN-LO parameters of the laser diode controllers 201a, 201b as needed to instruct the power supply circuits 210a, 210b to generate drive signals according to the desired pulse shape. The master controller 200 may be preprogrammed with a library of pulse sequences and/or pulse shapes so that the user/powder bed fusion device may select a pulse sequence and/or pulse shape from the library as desired. Additionally or alternatively, however, the master controller 200 may be programmed to generate the pulse sequence and/or pulse shape based on aspects of the pulse sequence(s) and/or pulse shape(s) encoded in the received command. In this way, the laser may be controlled to generate pulse sequences and/or pulse shapes that exceed the pulse sequences and/or pulse shapes contained in the library.

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

Fig. 7A to 7E illustrate pulse shapes obtainable with a laser.

In fig. 7A, the pulse shape is essentially a rectangular pulse that 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 the laser according to the invention the power rise and fall can be achieved within 5 microseconds of the control signal used to generate such pulses. When such pulses are used for pulse exposure in powder bed fusion, any delay between the emission of the laser and the generation of the laser beam can be reduced compared to prior art systems, allowing for closer and/or improved synchronization of the laser with the scanner, as less room must be provided for variability in the emission time of the laser.

In another embodiment, a pulse shape with stepwise decreases in increasing power level is provided. The pulse includes 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 elevated power remains stationary in power for a first period of time of 5 microseconds or more and then decreases to one or more second elevated powers less than the first elevated power at which the or each second elevated power stationary. The or each second boost power may remain stationary in power for a (second) period of time of 5 microseconds or more. 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 pair may be less than 15 microseconds, preferably less than 10 microseconds, more preferably less than 5 microseconds. Finally, the laser pulse is returned to the reference power (power below 10% of the first boost power or zero power).

Fig. 7B shows an example of a pulse shape with a stepwise decrease in power, with a first increased power plateau 301 and a single second increased power plateau 302 (initial burst pulse). In fig. 7B, the first boost power plateau 301 has a longer duration than the second boost power plateau 302, although it is understood that the duration of the first boost power plateau 301 and the second boost power plateau 302 may be the same, or the duration of the second boost power plateau 302 may be longer than the duration of the first boost power plateau 301. In this embodiment, the first elevated power plateau 301 lasts 60 microseconds at 280W and the single second elevated power plateau 302 lasts 20 microseconds at 240W. The power of both the first elevated power plateau 301 and the second elevated power plateau 302 will be sufficient to melt the laser spot size powder, in this embodiment 60 microns to 80 microns of 1/e ² spot size powder.

Fig. 7C is another example of a pulse shape (progressive cooling pulse) with a stepwise decrease in power, with a first increased power plateau 401 and a plurality of second increased power plateaus 402 at progressively lower power. In fig. 7C, the first elevated power plateau 401 has a longer duration than the second elevated power plateau 402, although it is understood that the duration of the first elevated power plateau 401 and the second elevated power plateau 402 may be the same, or the duration of at least one of the second elevated power plateaus 402 may be longer than the duration of the first elevated power plateau 401. In this embodiment, the first elevated power plateau 401 lasts 80 microseconds at 200W and each second elevated power plateau 402 lasts 5 microseconds at a power that is reduced by about the same amount for each step (a reduction of 33.3/33.4W in this example). However, it will be appreciated that other steps of other sizes in power and other durations may be used for the second settling period 402. In addition, all second stationary phases 402 do not have to have the same duration. This example differs from the example of fig. 7B in that at least some, if not all, of the second settling periods 402 provide insufficient energy density at the powder bed to melt laser spot size powder, in this embodiment 60 to 120 microns of 1/e ² spot size powder at the plane of the powder bed.

Fig. 7D is another example of a pulse shape with a power-wise step down (which is similar to the power-wise step down shown in fig. 7B) but with a first plateau 501 and a second plateau 502 of the same duration (40 microseconds in this embodiment).

In another embodiment shown in fig. 7E, the pulse shape includes at least a portion having a triangular shape (triangular pulse). In this embodiment, the pulse comprises: a first rising portion 601 that rises from the reference power to a first increased power (200W in this example) in less than 5 microseconds, for example; a second rising portion 602, wherein the power rises relatively gradually to a second peak elevated power (280W in this example) compared to the first rising portion; a first falling portion 603 in which the power is reduced to a third increased power (in this example, the same as the first increased power); and a second falling portion 604, wherein the power drops relatively rapidly (e.g., within less than 5 microseconds) to a reference level as compared to the first falling portion. The first and second increased powers may provide an energy density at the powder bed sufficient to melt the powder for a given laser spot size. In this embodiment, the laser beam has a 1/e ² spot size of 60 microns to 120 microns at the plane of the powder bed.

Referring to fig. 7F and 7H, in another embodiment, a pulse shape (agitation pulse) configured to agitate the molten pool is provided. In such embodiments, the pulse shape includes a plurality of peaks (maxima) that oscillate between providing an energy density at the powder bed sufficient to melt the powder. The pulse may be regarded 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 shorter duration pulse superimposed on the pulse wave. In this embodiment, the power initially rises 701, 801 to a first elevated power. The power may then be maintained at the first elevated power (plateau in fig. 7F) and then reduced to the second elevated power 702, or the power may be immediately reduced to the second elevated power 802 (fig. 7H). Then, the power may be maintained at the second boost power (plateau 702 in fig. 7F) and then increased to the third boost power 703, or the power may be immediately increased to the third boost power 803 (fig. 7H). The third power 703, 803 may be the same as or different from the first boost power 701, 801. In the example shown in fig. 7F, the power remains at a third elevated power (plateau 706) and is then reduced to reference power/zero power. In fig. 7H, the power is immediately reduced to reference power/zero power. Accordingly, FIG. 7F illustrates a crenellated (castellation) pulse shape comprising a rectangular superimposed pulse wave oscillating between a first increased power and a second increased power. Fig. 7H illustrates a pulse shape comprising triangular superimposed pulse waves oscillating between a first increased power and a second increased power. It should be appreciated that other shapes of superimposed pulse waves may be used, such as saw tooth or sinusoidal pulse waves oscillating between a first elevated power and a second elevated power. In addition, the initial rise to the first boost power and the fall to the reference power/zero power (for any of the differently shaped pulse waves) may be gradual (possibly in a stepwise manner), such as over a period longer than 5 microseconds (as shown in fig. 7H), or rapid, such as over a period less than 5 microseconds (as shown in fig. 7F). It will be appreciated that the superimposed pulse wave of pulses may comprise more than two peaks.

Example 1

10Mm x 10mm x 11.75mm cubes were constructed with H13 tool steel in a RenAM E powder bed fusion machine modified to replace PRISM lasers with lasers according to the present invention. The cubes were formed using different pulse exposures including (i) an initial burst pulse (fig. 7B), (ii) a rectangular pulse (fig. 7A), and (iii) a progressive cooling pulse (fig. 7C). The construction parameters are: the laser spot size was 80 μm, the spot distance was 65 μm, the filling distance was 80 μm and the layer thickness was 40. Mu.m. Fig. 8A, 8B and 8C show cross sections of cubes at different magnifications. Fig. 8A shows an image of an initial burst pulse, fig. 8B shows a corresponding image of a rectangular pulse, and fig. 8C shows a corresponding image of a progressive cooling pulse.

The initial burst samples and rectangular samples resulted in a greater number of cracks than samples formed using progressive cooling pulses. In addition, as can be seen from the image, the progressive cooling pulse results in smaller cracks than the rectangular pulse and the initial burst pulse. Most of these smaller cracks have a smooth surface, meaning that the cracks are not caused by cure cracking.

SEM images of cubes formed using progressive cooling pulses shown in fig. 9A-9D further support the presence of these smaller cracks with smooth surfaces. Very few cracks were observed and most cracks were less than 20 μm.

Example 2

Fig. 10A is an image of the surface of a cube constructed using rectangular pulses, and fig. 10B is a corresponding image of a cube constructed using progressive cooling pulses. The highest z-plane is located at the right part of the image (top of the page). As can be seen from the image, the bath depth is clearly visible for progressive cooling pulses, while for cubes built using rectangular pulses, there are no clearly visible corresponding features.

Example 3

Fig. 11A shows a cross-sectional image of 10mm x 10mm x 11.75mm cubes T1 to T4 constructed with H13 tool steel using a modified RenAM E powder bed fusion machine with (1) a 5 μs step, (2) a 10 μs step, (3) a 15 μs step, (4) a 10 μs step with corrective power, and (5) a different progressive cooling pulse with corrective power (as shown in fig. 17). A dot spacing of 65 μm, a fill distance of 80 μm, and a layer thickness of 40 μm were used. The same number of steps is used for each pulse. As the step duration increases, the correction power decreases the step power to offset the increased power density supplied to the powder. The construction parameters are: the laser spot size was 80 μm, the spot distance was 65 μm, the filling distance was 80 μm, and the layer thickness was 40. Mu.m.

As can be seen from fig. 11B, the progressive cooling pulse with a step size of 5 μs achieves the highest density, although the density achieved by a step size of 10 μs is also acceptable. A step size of 15 mus does not produce a good density of cubes and also does not produce pulses with modified power. This indicates that a step size of less than 15 mus (or a continuous (non-stepwise) decrease in power) is preferred, with steps of 10 mus and 5 mus providing improved results. There appears to be no obvious correlation between step duration and power, since for a 15 mus step, correction of power consistent with the target energy density does not yield a good density fraction.

Example 4

To explore the pool shapes formed by the different pulse exposures, points on the bare metal substrate are exposed to different ones of the pulse shapes. Fig. 12A is an image of a puddle feature formed by a rectangular pulse, fig. 12B is an image of a puddle feature formed by a triangular pulse, fig. 12C is an image of a puddle feature formed by an initial burst pulse, and fig. 12D is an image of a puddle feature formed by a progressive cooling pulse with a step size of 5 mus. As can be seen, each pulse shape creates a unique puddle feature, and this is expected to have an impact on the track shape and the creation of gas-borne condensate. The puddle formed by the rectangular pulse and the initial burst pulse appears more chaotic, with material ejected from the exposed location, while the triangular pulse and progressive cooling pulse create a more uniform puddle shape. The pool of rectangular pulses is smaller than the pool of progressive cooling pulses.

Example 5

Metal tracks are formed on a bare metal substrate using different pulse exposures. The track includes a plurality of fill lines and a boundary scan portion. Fig. 13A to 13D are images of these scanning sections. Fig. 13A shows the end and start of the fill line and the boundary scan portion of the rectangular pulse, fig. 13B shows the end and start of the fill line and the boundary scan portion of the progressive cooling pulse, fig. 13C shows the end and start of the fill line and the boundary scan portion of the initial burst pulse, and fig. 13D shows the end and start of the fill line and the boundary scan portion of the triangular pulse. The melt track shape, size, and noise level (balling) were different for the different shaped pulses (as expected from the puddle characteristics of example 4). From the image, the melting of the boundary trajectories appears more uniform for progressive cooling pulses than for rectangular pulses. For progressive cooling pulses and rectangular pulses, no start and end fill line defects were observed.

Example 6

Samples were constructed from unsieved H13 tool steel using rectangular pulses and progressive cooling pulses in a modified RenAM E powder bed fusion machine. The hardness of the sample was measured. Samples constructed using progressive cooling pulses have an average hardness that is 5% greater than the average hardness of samples constructed using rectangular pulses, as shown in fig. 15.

Example 7

Five samples were constructed with titanium alloy Ti-6Al-4V using a rectangular pulse with a total pulse duration of 10 mus using a modified RenAM E powder bed fusion machine. A 10 mus gap is provided between the laser pulses. Parameters for constructing the pulse exposure for each sample are provided in the table of fig. 15. These parameters are power in watts (watts) (P), spot distance in μm (PD), exposure time/pulse duration in mus (EXP), and fill distance in μm (HD). The Layer Thickness (LT) was 60. Mu.m. The delay between pulses is set by setting a variable called Jump Delay (JD), and the exposure time plus the jump delay is set to 20 mus.

The 2D energy density (2 DED), velocity (PD/exp+jd), and build rate have been determined from other parameters. As can be seen from the table, for a 10 μs pulse exposure, a density greater than 99.9% of 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 than longer pulse durations, and correspondingly, different microstructures due to steeper thermal gradients generated across the melt pool. Such finer microstructures can improve the properties of the part, particularly aluminum and aluminum alloys. Such a short pulse may be advantageously used in a distributed point scanning method as described in WO 2016/079496.

Example 8

Samples were constructed at IN718 IN a modified RenAM E powder bed fusion machine using a rectangular pulse as shown IN fig. 7A and an initial burst pulse as shown IN fig. 7B. As can be seen from fig. 18A and 18B, the rectangular pulse results in epitaxial grains, while fewer epitaxial grains are observed in the material melted using the initial burst pulse, e.g., grains having a reduced aspect ratio (grain uniformity). This may be due to a more chaotic puddle generated using the initial burst pulse (as shown in fig. 12C).

In another embodiment of the invention illustrated in fig. 16A-16C, the pulse exposure comprises a "mixed pulse exposure" in which the initial pulse and/or the end pulse 901, 1001, 1101 of the pulse exposure is used; 902. 1002, 1102 (which have a pulse duration shorter than the pulse duration of the at least one intermediate pulse 903, 1003, 1103 between the initial pulse and the end pulse). Typically, a start pulse and/or an end pulse 901, 1001, 1101; 902. 1002, 1102 have a pulse duration of less than 200 mus, more typically less than 100 mus, and the intermediate pulses 903, 1003, 1103 have a pulse duration (continuous exposure) of more than 200 mus and will depend on the length of the scan path and the scan speed. Start and/or end pulses 901, 1001, 1101; 902. 1002, 1102 may be similar to the pulses described with reference to fig. 7A-7H. In this embodiment, start and/or end pulses 901, 1001, 1101; 902. 1002, 1102 all have the same pulse duration. However, it will be appreciated that the start pulse and/or the end pulse 901, 1001, 1101; 902. 1002, 1102 may vary.

Fig. 16A illustrates a hybrid pulse exposure in which the pulse has an initial burst pulse shape. Fig. 16B illustrates a hybrid pulse exposure in which the pulses have a triangular pulse shape. Fig. 16C illustrates a hybrid pulse exposure in which the pulses have a progressive cooling pulse shape. The length of the step sizes shown in fig. 16C are illustrative only, and the first plateau may have a longer duration than the subsequent lower power step sizes.

It is believed that the use of such a hybrid exposure can reduce the defects observed at the beginning and end of the fill line using conventional continuous mode scanning while benefiting from the faster processing that can be achieved with scanning in continuous mode (because the laser is on for a longer period).

The hybrid exposure may be used for the scan path (fill line) 24 illustrated in fig. 4.

It will be appreciated that variations 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. Additionally, the build up of the part may include consolidating material using pulsed exposure for some areas and scanning in a continuous mode for other areas to consolidate the material.

Claims (19)

1. A powder bed fusion additive manufacturing 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 the material of the at least one region using pulse exposure, wherein an initial pulse and/or an end pulse of the pulse exposure has a pulse duration shorter than a pulse duration of an intermediate pulse between the initial pulse and the end pulse. 2. The powder bed fusion additive manufacturing method according to claim 1, wherein the pulse duration of the intermediate pulse is greater than 80 microseconds, more preferably greater than 100 microseconds, even more preferably greater than 150 microseconds, and even more preferably greater than 200 microseconds. 3. A powder bed fusion additive manufacturing method according to claim 1 or claim 2, wherein the shorter pulse duration is 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. 4. The powder bed fusion additive manufacturing method according to any one of claims 1 to 3, wherein the shorter pulse duration is constant for all initial pulses and/or end pulses. 5. The powder bed fusion additive manufacturing method according to any one of claims 1 to 3, wherein the shorter pulse duration varies for different pulses in the initial pulse and/or the end pulse. 6 . The powder bed fusion additive manufacturing method according to claim 5 , wherein, for the initial pulse, the shorter pulse duration gradually increases from the first initial pulse to the intermediate pulse. 7. The powder bed fusion additive manufacturing method according to claim 5 or claim 6, wherein, for the end pulse, the shorter pulse duration gradually decreases from the intermediate pulse to the last end pulse. 8. The powder bed fusion additive manufacturing method according to any one of claims 1 to 7, wherein the time between the pulses is constant. 9. The powder bed fusion additive manufacturing method according to any one of claims 1 to 7, wherein the time between the pulses varies. 10 . The powder bed fusion additive manufacturing method of claim 9 , wherein the time between pulses gradually decreases from a first initial pulse to the intermediate pulse. 11. The powder bed fusion additive manufacturing method according to claim 9 or claim 10, wherein the time between pulses gradually increases from the middle pulse to the last end pulse. 12. The powder bed fusion additive manufacturing method according to any one of claims 1 to 11, wherein the point spacing between the pulses is constant. 13. The powder bed fusion additive manufacturing method according to any one of claims 1 to 11, wherein the point spacing between the pulses varies. 14 . The powder bed fusion additive manufacturing method according to claim 13 , wherein the point spacing between pulses gradually increases from a first initial pulse to the intermediate pulse. 15. The powder bed fusion additive manufacturing method according to claim 13 or claim 14, wherein the point spacing between pulses gradually decreases from the middle pulse to the last end pulse. 16. A powder bed fusion additive manufacturing method according to any one of claims 1 to 15, comprising synchronizing the pulses with control of at least one beam manipulation component directing the energy beam to the powder bed. 17. The powder bed fusion additive manufacturing method of claim 16, wherein synchronizing the pulses with control of the at least one beam manipulation component comprises moving the at least one beam manipulation component relatively quickly between pulses compared to a speed at which the at least one beam manipulation component is moved during the pulses. 18. A powder bed fusion device, comprising 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 and a controller, the controller being arranged to control the energy beam source to implement the method described in any one of claims 1 to 17. 19. A data carrier comprising instructions stored thereon, when the instructions are run by a controller of a powder bed fusion device, the powder bed fusion device comprising 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 claims 1 to 17.