FIELD OF THE INVENTION

This invention relates to additive manufacturing. More particularly, the invention relates to a control system for controlling an energy beam in an additive manufacturing apparatus and to a method of controlling an energy beam in an additive manufacturing apparatus. The invention also relates to an additive manufacturing apparatus.

BACKGROUND OF THE INVENTION

Additive manufacturing (AM) refers to various processes used to synthesise a three- dimensional object (hereafter simply referred to as an "object" or "part"). Certain AM techniques are sometimes referred to as "3D printing". In AM, parts are typically manufactured by digitally slicing a three-dimensional computer-aided design (CAD) model into two-dimensional layers or images. These layers are then manufactured by curing, consolidating, melting or otherwise forming these layers from a raw material, typically in the form of a powder or fluid. For the sake of convenience, the terms "consolidating" or simply "forming" will hereinafter be used to refer to the forming of such layers, irrespective of the specific manner in which the layers are formed.

Parts can be produced from various raw materials, such as metals, polymers, ceramics, resins and gypsum. Further, various techniques are used to consolidate layers, including lasers, electron beams, other high energy beams, binders and thermal modules. In the Applicant's experience, AM provides a number of advantages over traditional manufacturing methods. These advantages include the ability to manufacture highly complex parts which allows for weight reduction, integration of more functionality into parts and part count reduction. The process also ensures relatively low material wastage due to the reusability of raw material, the freedom in part design and the obviation of the need for tooling.

A number of AM processes employ a laser or electron beam to consolidate material in a material bed in layers ultimately to form a desired part. Such processes will hereinafter be referred to as "powder bed fusion processes". In powder bed fusion processes, a laser is typically directed by a number of optical components to a scanning unit. The laser is then switched on and off based on the geometry of a CAD model to ensure that the desired layers are consolidated in the correct manner. The material bed is supported on a build platform which is incrementally lowered as each new layer of the object is consolidated. A fresh layer of material is then added to the material bed before the next layer is scanned.

The energy beam travels along an optical path from its energy source (e.g. laser) ultimately to reach a surface of the material bed. This optical path may include a number of different optical components such as a collimator, a beam reducing telescope (BRT), a high-speed modulation unit or optical switch, a beam expanding telescope (BET), one or more mirrors and one or more laser windows. The energy beam then reaches the scanning unit which directs the energy beam onto the material bed.

The Inventors have found that it is important to maintain a constant power density, spot size and profile on the surface of the material bed (within certain tolerances) in an AM apparatus to ensure consistency and thereby ensure that parts of the desired quality are manufactured. To this end, it is equally important that the energy beam maintains its diameter and beam divergence (or collimation) at an entrance of the scanning unit (within specified tolerances). Materials from which optical components are made and their anti-reflection (AR) coatings, e.g. fused silica, sapphire and other optical glasses and crystals, may absorb and/or scatter some of the radiation of the energy beam. Absorption and scattering tends to heat the optical components themselves, resulting in a phenomenon known as "thermal lensing". Thermal lensing may cause changes in focusing properties of an affected optical component, namely a focus shift and a deterioration of the beam wave front due to the onset of optical aberrations.

The Inventors have found that properties of the energy beam, such as its focused spot size, spatial profile, power density and focus location, may change significantly over time (to values outside permitted tolerances) due to thermal lensing. This is likely to have an adverse impact on the quality of manufactured parts and the degree of accuracy achievable in AM. The Inventors are aware of techniques that have been developed to address some of the issues associated with thermal lensing when the optical elements have uniform temperatures. However, these techniques were developed for conventional laser machining applications, such as laser welding and cutting. The Inventors have found that the techniques of which they are aware are not suitable for use in the relatively high power and high temperature environments associated with AM, where the temperature of an optical element may vary both radially and axially over time.

A need thus exists for a system and method which permits the correction of or compensation for effects of thermal lensing in optical components of an AM apparatus. The Inventors believe that the present invention will address this need, at least to some extent.

SUMMARY OF THE INVENTION According to one aspect of the invention, there is provided a control system for controlling an energy beam in an additive manufacturing apparatus, including: at least one beam sensor configured to monitor at least one property of an energy beam directed onto a working surface by an optical arrangement; and

a control unit in electronic communication with the beam sensor, the control unit being configured to

receive, from the beam sensor, a value associated with one of the at least one monitored properties;

analyse the received value or a processed received value to determine whether the received value or the processed received value is equal to a predefined value or within a predefined range of values; and

if the received value or the processed received value is not equal to the predefined value or is not within the predefined range of values, cause adjustment of at least one variable parameter associated with the energy beam. The system may further include at least one beam actuator. Adjustment of the at least one variable parameter associated with the energy beam may be effected through actuation of the at least one beam actuator. The term "actuator" should throughout this specification be interpreted so as to include a set of actuators. Adjustment of the at least one variable parameter may include adjustment of an optical component of the optical arrangement and/or adjustment of an energy source of the energy beam.

The beam sensor may be configured to monitor the at least one property at or near a monitoring point along an optical path of the energy beam. The monitoring point may be between an energy source and a scanning unit of the optical arrangement. The energy source may be a high energy or high power laser.

The monitoring point may be between one or more primary optical components of the optical arrangement and the scanning unit. The primary optical components may include one or more of: a collimator, a beam reducing telescope (BRT), a high- speed modulation unit or optical switch, a beam expanding telescope (BET), one or more mirrors and one or more laser windows.

Alternatively or additionally, the beam sensor may be configured to monitor the at least one property at or near an entrance of the scanning unit. The monitoring point may thus be in the region of the entrance of the scanning unit.

Alternatively or additionally, the beam sensor may be configured to monitor the at least one property at or near a focus zone on the working surface or in a conjugated plane. The beam sensor may be configured to monitor energy beam spot size and/or the size of a melt pool.

In some embodiments, the control system may include at least two beam sensors, one of the beam sensors being configured to monitor the at least one property at or near the monitoring point and the other beam sensor being configured to monitor the at least one property at or near the focus zone or its conjugated plane.

A plurality of beam sensors may be provide to monitor the at least one property at or near the monitoring point. A plurality of beam sensors may be provided to monitor the at least one property at or near the focus zone or its conjugated plane. A plurality of monitoring points may be provided, each monitoring point having one or more different beam sensor associated therewith.

The control unit may be configured to cause adjustment of the at least one optical component of the optical arrangement by transmitting a movement command to at least one beam actuator associated with the optical component. The actuator may be configured to receive the movement command and, in response to receiving the movement command, cause adjustment of the optical component. Adjustment of the optical component may include axial movement of the optical component or a subcomponent thereof or a change in a shape of the optical component or a subcomponent thereof. The actuator or actuators may form part of the control system. In some embodiments, the optical component which is adjusted is a telescope, which may be a zoom telescope. Adjustment of the telescope may include movement of at least one lens of the telescope. The telescope may be a motorised optical zoom telescope and movement of the at least one lens of the telescope may be axial displacement of the at least one lens.

In some embodiments, the control unit is configured to transmit movement commands to at least one beam actuator associated with either a BET and/or a BRT, depending on the received value or values.

In some embodiments, the optical component which is adjusted is at least one deformable mirror. The shape of mirror surfaces may be adjusted by applying voltages to an actuator supporting the mirror. The control unit may be configured to analyse the received value and/or the processed received value by applying a compensation algorithm in order to determine movement commands to be transmitted to the at least one beam actuator. The control unit may be configured to process the received value to obtain the processed received value.

The beam sensor or the system may include one or more sensing components, a beam splitter, a focusing lens, filters, and other optical components. The sensing component may include a camera or slit-scanner. The sensing component may include a "beam waist analyser" camera which is configured to monitor a plurality of axially spaced slices of the beam diameter.

The at least one property of the energy beam may include one or more of: a beam spot size, a spatial profile, a power density, a focus location, a beam waist size, a beam position, divergence, beam quality (M ² -value) and wave front shape. Wave front shape may be required for adaptive optics and may be measured using a Shack- Hartmann sensor.

The received value may be a value of the property of the energy beam and the processed received value may be a thermal lensing value. In such cases, the predefined value or predefined range of values may be thermal lensing value(s).

The working surface may be a surface of a working area defined by a build platform or a surface of a material bed deposited on a build platform of the additive manufacturing apparatus.

According to another aspect of the invention, there is provided a method of controlling an energy beam in an additive manufacturing apparatus, the method including the steps of:

monitoring, typically by a beam sensor, at least one property of an energy beam directed onto a working surface by an optical arrangement;

transmitting a value associated with one of the at least one monitored properties to a control unit in electronic communication with the beam sensor;

receiving, by the control unit, the transmitted value;

analysing, by the control unit, the received value or a processed received value to determine whether the received value or the processed received value is equal to a predefined value or within a predefined range of values; and

if the received value or the processed received value is not equal to the predefined value or is not within the predefined range of values, causing, by the control unit, adjustment of at least one variable parameter associated with the energy beam.

The step of causing adjustment of the at least one variable parameter may include causing adjustment of an optical component of the optical arrangement and/or adjustment of an energy source of the energy beam. The method may include continuously or periodically monitoring the at least one property and incrementally adjusting the at least one optical component of the optical arrangement each time the received value is found to be not equal to the predefined value or to be not within the predefined range of values.

The step of monitoring the at least one property may include monitoring the at least one property at or near a monitoring point along an optical path of the energy beam and/or monitoring the at least one property at or near a focus zone on the working surface or at a conjugated plane thereof. Different properties may be monitored at the monitoring point and focus zone, respectively.

A plurality of monitoring points may be provided, each monitoring point having one or more different beam sensors associated therewith. The step of causing adjustment of the at least one optical component may include applying a compensation algorithm in order to determine movement commands to be transmitted to at least one beam actuator. The step of causing adjustment of the at least one optical component may further include transmitting the movement command to the at least one beam actuator.

The method may include processing the received value to obtain the processed received value. The compensation algorithm may be applied using the received value or the processed received value. According to a further aspect of the invention, there is provided an additive manufacturing apparatus which includes an optical arrangement for generating and/or directing an energy beam onto a material bed and control means configured to control one or more components of the optical arrangement in order to compensate for thermal lensing.

The control means may include a control system as described above for controlling an energy beam in the apparatus. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be further described, by way of example, with reference to the accompanying drawings.

In the drawings:

FIG. 1 is a three-dimensional partially sectional view of an exemplary additive manufacturing (AM) apparatus in which the system and method of the present invention may be implemented;

FIG. 2 is a diagrammatic illustration of a first exemplary optical layout which may be used in an AM apparatus in accordance with the invention;

FIG. 3 is a diagrammatic illustration of a second exemplary optical layout which may be used in an AM apparatus in accordance with the invention;

FIG. 4 is a diagrammatic illustration of components of an AM apparatus, including an embodiment of a control system according to the invention;

FIG. 5 is a diagrammatic illustration of components of an AM apparatus, including an embodiment of a control system according to the invention;

FIG. 6 is a diagrammatic illustration of components of an AM apparatus, including an embodiment of a control system according to the invention; and

FIG. 7 is a flow diagram illustrating an exemplary method of controlling an energy beam in an AM apparatus. DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS

The following description of the invention is provided as an enabling teaching of the invention. Those skilled in the relevant art will recognise that many changes can be made to the embodiments described, while still attaining the beneficial results of the present invention. It will also be apparent that some of the desired benefits of the present invention can be attained by selecting some of the features of the present invention without utilising other features. Accordingly, those skilled in the art will recognise that modifications and adaptations to the present invention are possible and can even be desirable in certain circumstances, and are a part of the present invention. Thus, the following description is provided as illustrative of the principles of the present invention and not a limitation thereof.

An example of a known additive manufacturing (AM) apparatus 10 is shown in FIG 1. The AM apparatus 10 is a conventional laser sintering (powder bed fusion) apparatus. FIG. 1 also shows a coordinate system (axes X-Y-Z) with reference to which the structure and function of the apparatus 10 is described below.

The apparatus 10 includes a housing 12 which houses a platform 14, two material containers 16, a material deposition arrangement 18 and an optical arrangement 20 spaced above the platform 14 along the Z-axis of the apparatus 10.

The housing 12 is a sealed enclosure which provides a processing environment operatively containing an inert gas, such as argon, helium or nitrogen.

The platform 14 is generally planar and extends in a generally horizontal X-Y plane defined by the X-axis and the Y-axis of the apparatus 10. A vertically displaceable (along the Z-axis) working area 22 is provided in a central region of the build platform 14.

The material deposition arrangement 18 is configured to travel across the build platform 14 in the direction of the Y-axis to deposit layers of powder material onto the working area 22 to form a material bed, in use. The thickness of a deposited layer is typically of the order of 30μιη to ΙΟΟΟμιη.

The apparatus 10 further includes a moving arrangement in the form of a piston and cylinder arrangement 26 which is configured to move the working area 22 incrementally downwardly in order for each layer of material to be deposited to maintain the surface of the material bed at a constant level, in use, as will be well understood by those of ordinary skill in the art. The optical arrangement 20 directs an energy beam "B" produced by an energy source in the form of a laser 24 onto the working area 22, in use, so as to consolidate powder material forming part of the material bed.

The optical arrangement 20 includes a number of optical components which direct and focus the energy beam B produced by the laser 24 ultimately to reach a desired point or zone (typically referred to as a "spot") on the working area 22. The optical components are arranged along an optical path of the energy beam.

It will be appreciated that the apparatus 10 of FIG. 1 is shown as an example of an apparatus in which the system and method of the invention may be implemented and that the invention may find application in apparatuses making use of other scanning and consolidating arrangements as well as other laser material processing methods, such as cutting, welding and the like. FIGs 2 and 3 illustrate two exemplary optical layouts which can be employed in accordance with the invention.

In the exemplary layout 30 of FIG. 2, the AM apparatus includes a fibre laser 32 and a movable scanning unit 48. The scanning unit 48 includes a post-objective galvanometer scanner for consolidating material deposited in the working area 50. A number of optical components are provided to direct the energy beam B along its optical path, from the laser 32 to the scanning unit 48: a collimator 34 which is coupled to the fibre laser 32 by an optical fibre 33, a first motorised optical zoom telescope in the form of a beam reducing telescope (BRT) 36, an optional optical switch 38, a second motorised optical zoom telescope in the form of a beam expanding telescope (BET) 40, a first 45° mirror 42, a horizontally extending window 44 and a second 45° mirror 46.

The scanning unit 48 is movable along the Y-axis. The BRT 36 and BET 40 each include multiple lenses, two of which are movable by spindles which are coupled to actuators in the form of stepper motors, and one of which is fixed.

FIG. 2 also shows a beam sensor 52 and a control unit 54. The beam sensor 52 is configured to monitor at least one property of the energy beam (e.g. diameter) at a predefined monitoring point and transmit measured values to the control unit 54. The control unit 54 is configured to analyse a received value it obtains from the beam sensor 52. If the value, or a processed value associated therewith, is not equal to a predefined value or is not within a predefined range of values, the control unit 54 causes adjustment of the BRT 36 and/or the BET 40. In this way, thermal lensing can be compensated for through adjustment of the telescopes 36 and 40. These aspects will be described in greater detail below.

In the exemplary layout 60 of FIG. 3, the AM apparatus includes a fibre laser 62 and a movable scanning unit 64. The scanning unit 64 includes a post-objective galvanometer scanner for consolidating material deposited in the working area 66.

A number of optical components are provided to direct the energy beam B along its optical path, from the laser 62 to the scanning unit 64: a collimator 68 which is coupled to the fibre laser 62 by an optical fibre 70, a fixed BRT 72, an optional optical switch 74, a fixed BET 76, a deformable mirror 78, a horizontally extending window 80 and a 45° mirror 82. The scanning unit 64 is movable along the Y-axis. The BRT 72 and BET 76 do not necessarily have adjustable lenses.

FIG. 3 also shows a beam sensor 84, a scanning unit position sensor 86 and a control unit 88. The beam sensor 84 is configured to monitor at least one property of the energy beam at a monitoring point and transmit measured values to the control unit 88. The scanning unit position sensor 86 is configured to sense the position of the scanning unit 64 along the Y-axis and transmit feedback to the control unit 88 regarding this position. The control unit 88 is configured to analyse the received values and feedback. The control unit 88 is configured to cause adjustment of the mirror 78 based on the received values and feedback if one or more measured values or processed values associated with the measured values do not equal predefined values or are not within predefined ranges. The shape of surfaces of the mirror 78 are adjustable by applying voltages to actuators supporting the mirror 78. In this way, thermal lensing can be compensated for by way of so-called "adaptive optics".

FIGs 4-6 are diagrammatic illustrations of components of an AM apparatus, each including an embodiment of a control system according to the invention. These figures also illustrate an optical path followed by the energy beam B.

In FIGs 4-6, like reference numerals refer to like components. In each case, in addition to the control system, the AM apparatus includes a laser 90, an adjustable BRT 92, an optional optical switch 94, an adjustable BET 96 and a movable scanning unit 98, which directs the energy beam B onto a working surface 100.

The control system 110 of FIG. 4 includes first and second beam sensors 112 and 114, a control unit 116 and first and second actuators 118 and 120. The first actuator 118 is a stepper motor configured to actuate the movable lenses of the BRT 92 and the second actuator 120 is a stepper motor configured to actuate the movable lenses of the BET 96. The control system 110 further includes a scanner controller 122 and a scanning unit position sensor 124.

The beam sensors 112 and 114 are configured to monitor one or more properties of the energy beam along the optical path. The first beam sensor 112 monitors a property at a point near the BET 96 and the second beam sensor 114 monitors a property near the entrance of the scanning unit 98.

Examples of properties which can be measured are beam spot size, a spatial profile, a power density, a focus location, a beam waist size, a beam position, divergence and beam quality. It will be understood that any suitable property or properties may be measured and that any suitable number of beam sensors may be employed. It will also be understood that the beam sensors 112 and 114 may measure the same or different properties of the energy beam.

In the example of FIG. 5, a beam splitter 128 is included between the BET 96 and the scanning unit 98.

The control system 130 of FIG. 5 includes first and second beam sensors 132 and 134, a control unit 136 and first and second actuators 138 and 140. The first actuator 138 is a stepper motor configured to actuate the movable lenses of the BRT 92 and the second actuator 140 is a stepper motor configured to actuate the movable lenses of the BET 96. In the example of FIG. 5, the first beam sensor 132 is a beam waist analyser camera which characterises the energy beam's spatial profile, beam quality and waist location and the second beam sensor 134 includes a camera and an imaging lens, permitting it to monitor beam spot size at a focus zone Z on the working surface 100. A scanning unit position sensor 142 is also shown in FIG. 5. Thermal lensing is compensated for by adjusting the BRT 92 and the BET 96 based on the values received from the sensors 132 and 134 by the control unit 136. The control system 150 of FIG. 6 is substantially similar to the control system 130 of FIG. 5. However, in this example, the scanning unit 98 has an internal beam actuator 152 and position sensor 154. Thermal lensing inside the scanning unit 98 is addressed by adjusting an internal BET (not shown) of the scanning unit 98 based on values obtained from the beam sensor 134 and/or the position sensor 154.

In FIGs 4 to 6, the control unit 116, 136 is electronically coupled to the beam sensors 112, 114, 132, 134 and the actuators 118, 120, 138, 140. The control unit is provided with circuitry and software enabling the control system 110, 130, 150 to control the energy beam B of the additive manufacturing apparatus in real time. More specifically, the control system 110, 130, 150 is capable of monitoring the energy beam and adjusting variable properties of the energy beam in order to compensate for thermal lensing. FIG. 7 illustrates some of the steps conducted in a continuous process whereby the control system 110 of FIG. 4 compensates for thermal lensing.

It should be appreciated that the stages referred to below are typically carried out continuously or at a suitable frequency and may overlap at least to some extent.

At a first stage 160, the beam sensors 112 and 114 are used to monitor the energy beam as described above while the AM apparatus is in operation.

The beam sensors 112 and 114 transmit values for the monitored properties to the control unit 116 at a next stage 162.

The control unit 116 receives, processes and analyses these values at a next stage 164. Thermal lensing manifests as measurable changes in the monitored properties of the energy beam. The aim of the processing and analysis is to determine whether or not the monitored properties are equal to or within a predefined range of acceptable values, at a next stage 166. If a property is found to be within an acceptable range, no adjustment is necessary, as indicated by stage 168. The directional arrow 170 indicates that monitoring is repeated or continues even when all monitored properties are found to have acceptable values at a certain point in time. On the other hand, if the control unit 116 determines that a value received from the first beam sensor 112 and/or the second beam sensor 114 is not equal to a predefined value or is not within a predefined range of values, it causes adjustment of one or more optical components of the optical arrangement. In this example, with reference to FIG. 4, the control unit 116 creates and transmits a movement command to the first actuator 118 and/or second actuator 120 at a next stage 171, which causes the BRT 92 and/or the BET 96 to be incrementally adjusted at a next stage 172. Again, the directional arrow 174 indicates that monitoring is repeated or continues during and after adjustment of optical components.

As an example, adjustment relating to monitoring carried out by the first beam sensor 112 will be described. The magnitude and sign of thermal lensing can be measured by monitoring the change in the spatial profile of the beam (e.g. its waist size and/or waist location) using the first beam sensor 112. The control unit 116 uses these values to determine corrective telescope lens positions for the BRT 92 and/or the BET 96 which can counteract a thermally-induced change in collimation.

The control unit 116 transmits appropriate motor movement commands to the actuators 118 and 120 associated with BRT 92 and/or BET 96, respectively, in order to position their inline lenses at the required positions. Adjustment of lenses may typically include axial displacement of motorised lenses relative to each other.

These actions are repeated continuously (or at a suitable frequency) in a feedback loop to ensure that desired energy beam properties are maintained during operation of the AM apparatus. Typically, incremental adjustments may be made until the difference between a monitored beam property and a reference beam property is sufficiently small. It will be understood that numerous forms of compensation algorithms may be employed in software of the control unit to acquire and process beam properties and determine corrective action. As an example, the compensation algorithm may conduct a systematic search through a four-dimensional input parameter space defined by the four variable lens positions of the BRT 92 and the BET 96 until optimal results are achieved. The search can be conducted in real-time and/or using an analytical predictive model. In the case of a collimated energy beam entering a BRT/BET, a trajectory can be defined in the input parameter space for which the energy beam size would be reduced or expanded while near field divergence would remain relatively constant. Searching orthogonally along this trajectory results primarily in a change in divergence with minor change in output beam size. Based on the changes observed using the beam sensor 112, a vector in parameter space can be calculated with proportional orthogonal components. The lenses are then moved along this vector and changes are again monitored using the beam sensor 112. If the correction is not satisfactory, the above steps are repeated until each measured parameter equals a reference value (within a certain margin of error).

The Applicant believes that the present invention provides numerous advantages. As mentioned above, the Applicant has found that the power density of the focused energy beam spot changes significantly over time after the source is switched on due to thermal lensing in the optical path or train. For instance, at a laser power of 3 kW, the Applicant has found that a steady state is reached only after tens of seconds resulting in a 2 to 3 fold increase in power density, while the allowed change in power density (for parts of acceptable quality) is typically approximately 10% and has to be maintained throughout the manufacturing process. The Applicant believes that the present invention may alleviate this problem through active control of beam properties. The Applicant has found that the present invention can be used to compensate for thermal lensing at the relatively high power levels associated with AM. Continuous adjustments of variable energy beam parameters will ensure that the high-powered beam entering the scanning unit and/or reaching the working surface remains satisfactorily controlled. For example, the diameter and beam divergence (or collimation) of the energy beam at an entrance of the scanning unit can be maintained at desired values or within a desired range. As a result, high quality AM parts can be manufactured.

The Applicant thus believes that the present invention can improve part accuracy and resolution by compensating for or counteracting thermal lensing.