Background

Selective laser melting (SLM) and selective laser sintering (SLS) apparatus produce objects through layer-by-layer solidification of a material, such as a metal powder material, using a high energy beam, such as a laser beam. A powder layer is formed across a powder bed in a build chamber by depositing a heap of powder adjacent to the powder bed and spreading the heap of powder with a wiper across (from one side to another side of) the powder bed to form the layer. A laser beam is then scanned across areas of the powder layer that correspond to a cross-section of the object being constructed. The laser beam melts or sinters the powder to form a solidified layer. After selective solidification of a layer, the powder bed is lowered by a thickness of the newly solidified layer and a further layer of powder is spread over the surface and solidified, as required. An example of such a device is disclosed in US6042774. Layers can be formed through rotational motion of a wiper relative the build platform, as described in US8172562.

The strategy used for scanning a laser beam can affect the thermal loads generated during the build and accuracy of a resultant solidified line of material. Excessive, unrestrained thermal loads created during the build cause warping and/or curling of the part being built. Typically, the laser beam is scanned across the powder along a scan path. An arrangement of the scan paths will be defined by a scan strategy.

US5155324 describes a scan strategy comprising scanning an outline (border) of a part cross-section followed by scanning an interior (core) of the part cross-section. Scanning a border of the part may improve the resolution, definition and smoothing of surfaces of the part.

US5155324 and US2008/0241392 Al describe scanning an area in a plurality of parallel scan paths (hatches). (Referred to herein as a "meander scan"). The direction of the scan paths are rotated between layers to homogenise tensions generated during the build. US2008/0241392 Al extends this concept to scanning in a series of parallel stripes, wherein each stripe consists of a plurality of parallel scan path (hatches) running perpendicular to a longitudinal direction of the stripe. (Referred to herein as a "stripe scan"). US2008/0241392 Al also discloses the stripes covering partial regions (commonly squares) of the area to be solidified, wherein the hatches of adjacent partial regions are at 90 degrees to each other. (Referred to herein as a "checkerboard scan") US2005/0142024 discloses a scan strategy for reducing thermal loads comprising successively irradiating individual areas of a layer, which are at a distance from one another that is greater than or at least equal to a mean diameter of the individual areas. Each individual area is irradiated in a series of parallel scan paths (hatches). It is known to use a continuous mode of laser operation, in which the laser is maintained on whilst the mirrors move to direct the laser spot along the scan path, or a pulsed mode of laser operation, in which the laser is pulsed on and off as the mirrors direct the laser spot to different locations along the scan path. A melt pool generated by the laser is dependent upon the properties of the material and the state (powder or solidified) and temperature of material surrounding the volume being melted. The mode of laser operation can affect the state and temperature of the neighbouring material. For example, scanning of the laser spot along a scan path in continuous mode forms a large melt pool that is dragged along just behind the laser spot, resulting in larger, less detailed solidification lines. For some materials, such as tool steels and aircraft grade super alloys, it can be difficult to drag the melt pool across the layer in a continuous mode of operation of the laser. These problems can be mitigated by using the laser beam in the pulsed mode of operation. In particular, setting the time between pulses/ distance between exposure points to be long enough to allow a previously formed melt pool to cool before forming an adjacent melt pool can result in more accurate solidification lines, which may be particularly beneficial for border scans.

The laser beam is typically steered on to the powder bed using a pair of mirrors that can be tilted to a required angle by galvanometers. An example of such a scanner is described in WO2010/026397. The limited dynamic response of the galvanometers can result in significant divergence of the laser beam from the desired patterns (as described above). In particular, it has been found that, at the transition between hatch lines, the mirrors direct the laser beam in a curved path, visible in parts that are manufactured, rather than providing a sharp angular change in direction. For scanning of a laser in a pulsed mode, ideally the mirrors direct the laser beam at a fixed spot on the working surface when the laser is on, hopping to the next exposure point between pulses. However, the dynamic response of the galvanometers is too slow relative to desired point exposure times and time between exposures such that the mirrors will still be moving during an exposure. This results in the creation of elongate melt pools having melt characteristics similar to continuous scanning. Such problems can be overcome by increasing the time between exposures in order to give the mirrors time to move to and settle in the new position but this can significantly lengthen the build time. "Electro-optic and acousto-optic laser beam scanners", G.R. B. E. Romer, P. Bechtold, Physics Procedia 56 (2014), 29-39, discloses combining a mirror based scanner with an electro or acoustic optical deflector, which relies on a change in refractive index n of the material to control deflection of a laser beam. A problem with the electro and acoustic optical deflectors is that the deflectors absorb a significant proportion of the light travelling therethrough. Accordingly, such deflectors are unsuitable for use with high power laser beams because of the resultant heating of the deflector that would occur. Such heating would make it extremely difficult, if not impossible, to control the temperature of the crystals of the deflector to be just above the Curie temperature, as required. Summary of Invention

According to a first aspect of the invention there is provided a scanner for directing an energy beam in an additive manufacturing apparatus comprising a first beam steering component for deflecting the energy beam over a first range of angles in a first dimension and a second beam steering component for deflecting the energy beam over a second range of angles in the first dimension, wherein the second beam steering component provides a faster dynamic response but a smaller range of movement of the energy beam than the first beam steering component. In this way, the scanner can deflect the energy beam over a large range of angles in the first dimension whilst achieving rapid changes in direction unachievable using the first beam steering component alone. The dynamic capability of the scanner may allow the energy beam to be scanned along paths closer to the ideal scanning strategies, such as those described above.

The scanner may comprise a controller arranged to control the first and second beam steering components to accelerate/decelerate the energy beam in the first dimension using the faster dynamic response of the second beam steering component synchronously with moving the energy beam in the first dimension using the first beam steering component. The controller may be arranged to control the first and second beam steering components such that synchronously with operating the first beam steering component to deliver continuous motion in a direction, the second beam steering component is operated to deliver an oscillating motion such that the energy beam moves along a path in a series of a staccato movements. The resultant motion may be the energy beam performing a series of moves, wherein the energy beam is held relatively stationary to expose a region of the path to the energy beam (for example, when the action of the first beam steering component is to move the energy beam in an opposite direction in the first dimension to the action of the second beam steering component, with the possibility that the opposed actions cancel to provide a stationary energy spot) before the energy beam undergoes a short, sharp movement to "hop" the energy beam to another region of the path (for example, when the first and second beam steering components act to move the energy beam in the same direction in the first dimension). Such control of the beam steering components may be synchronised with a pulsed energy beam such that during the "hop" the energy beam is switched off. This may have particular application in additive manufacturing, wherein "hopping" the energy spot across the powder bed to expose a series of points may produce finer detail because of a reduction in or elimination of balling caused by dragging of the energy beam across the surface.

The controller may be arranged to control the first and second beam steering components such that synchronously with operating the first beam steering component to change a direction of the energy beam in the first dimension, the second beam steering component is operated to provide a sharper change in direction of the energy beam in the first dimension. In particular, the slower dynamic response of the first beam steering component may result in a longer stopping and accelerating distance in the first dimension than desired. The controller may be arranged to control the second beam steering component to compensate for divergence from a desired scanning path that would occur if an attempt was made to scan along the desired path only using the first beam steering component. The energy beam may comprise a laser beam. The first beam steering component may comprise an optical component for reflecting the laser beam, the optical component operable under the control of a first actuator, and the second beam steering component may comprise the or a further optical component for reflecting the laser beam, the or the further optical component operable under the control of a second actuator. The second actuator may provide a faster dynamic response but a smaller range of movement of the laser beam than the first actuator. By reflecting the laser beam, significant absorption of the laser beam in the optical component can be avoided. The optical component may be arranged to absorb less than 1% of the radiant flux of the laser beam. The optical component may be a mirror or a Bragg grating reflector. Such a reflector may be suitable for use with high power laser beams, such as laser beams of greater than 50W, and preferably greater than 100W, used in certain additive manufacturing processes.

The second actuator may comprise a piezoelectric actuator. Piezoelectric actuators typically provide a fast dynamic response but a smaller range of movement than a galvanometer. Alternatively, the second actuator may be one or more voice coils. In a further embodiment, the second actuator comprises one or more normal-stress actuators and, in particular, one or more hard-linearized normal-stress actuators.

The first actuator may be a galvanometer. Galvanometers typically provide a slower dynamic response than piezoelectric motors but a greater range of movement.

The and/or the further optical component may comprise a tiltable optical component. Alternatively, the and/or the further optical component may be a deformable component that can be deformed by the actuator to reflect the laser beam over a range of angles.

The energy beam may comprise an electron beam. The first beam steering component may comprise a first electromagnetic coil and the second beam steering component may comprise a second electromagnetic coil, the second magnetic coil arranged to generate a smaller magnetic field for deflecting the electron beam through a smaller range of angles then the first electromagnet.

The second electromagnet has a smaller inductance allowing faster changes in the magnetic field (faster dynamic response) than the first electromagnet. However, the resultant smaller magnetic field deflects the electron beam through a smaller range of angles. The larger magnetic field generated by the first electromagnet enables the electron beam to be deflected through a larger range of angles in the first dimension.

The scanner may comprise a third beam steering component for deflecting the energy beam over a third range of angles in a second dimension orthogonal to the first dimension. In this way, the scanner can steer the energy beam over a 2- dimensional area. The scanner may comprise a fourth beam steering component for deflecting the energy beam over a fourth range of angles in the second dimension, wherein the fourth beam steering component provides a faster dynamic response but a smaller range of movement of the energy beam than the third beam steering component.

In the case of the energy beam being a laser beam, the third beam steering component may comprise the or an additional optical component for reflecting the laser beam, the or the additional optical component operable under the control of a third actuator. The fourth beam steering component may comprise the, the further or the additional optical component for reflecting the laser beam, the, the further or the additional optical component operable under the control of a fourth actuator to deflect the energy beam in the second dimension. Alternatively, the fourth beam steering component may comprise a further additional optical component operable under the control of a fourth actuator to reflect the laser beam in the second dimension. In the case of the energy beam being an electron beam, the third beam steering component may comprise a third electromagnetic coil and the fourth beam steering component may comprise a fourth electromagnetic coil, the fourth magnetic coil arranged to generate a smaller magnetic field for deflecting the electron beam through a smaller range of angles in the second dimension then the third electromagnet.

In this way, the scanner has an increased dynamic response in both the first and second dimensions.

The controller may be arranged to control the combination of the third and fourth actuators in a like manner to the combination of the first and second actuators, as described above.

According to a second aspect of invention there is provided an additive manufacturing apparatus comprising a build chamber, a build platform lowerable in the build chamber such that layers of flowable material can be successively formed across the build platform, a radiation generator for generating an energy beam and a scanner for directing the energy beam onto each layer to selectively solidify the material, the scanner comprising a beam steering component operable for deflecting the energy beam over a first range of angles in a first dimension and a second beam steering component for deflecting the energy beam over a second range of angles in the first dimension, wherein the second beam steering component provides a faster dynamic response but a smaller range of movement than the first beam steering component.

According to a third aspect of the invention there is provided a method of scanning a laser beam comprising operating a scanner according to the first aspect of the invention to accelerate/decelerate the energy beam in the first dimension using the faster dynamic response of the second beam steering component synchronously with moving the energy beam in the first dimension using the first beam steering component.

The method may be used in scanning the laser beam across layers of material in a layer-by-layer additive manufacturing process, wherein successive layers of flowable material are formed across a build platform and the laser beam is scanned across selected areas of each layer to consolidate the material in the selected areas.

According to a fourth aspect of the invention there is provided a data carrier having instructions stored thereon, which, when executed by a controller of a scanner according to the first aspect of the invention, causes the controller to control the first and second actuators of the scanner to carry out the method of the third aspect of the invention. The data carrier of the above aspects of the invention may be a suitable medium for providing a machine with instructions such as non-transient data carrier, for example a floppy disk, a CD ROM, a DVD ROM / RAM (including - R/-RW and +R/ + RW), an HD DVD, a Blu Ray(TM) disc, a memory (such as a Memory Stick(TM), an SD card, a compact flash card, or the like), a disc drive (such as a hard disc drive), a tape, any magneto/optical storage, or a transient data carrier, such as a signal on a wire or fibre optic or a wireless signal, for example a signals sent over a wired or wireless network (such as an Internet download, an FTP transfer, or the like). According to a fifth aspect of the invention there is provided a method of manufacturing a part using a layer-by-layer additive manufacturing process, the method comprising depositing successive layers of flowable material across a build platform and operating a scanner according to the first aspect of the invention to scan an energy beam across selected areas of each layer to consolidate the material in the selected areas, wherein the energy beam is accelerated/decelerated in the first dimension using the faster dynamic response of the second beam steering component synchronously with moving the energy beam in the first dimension using the first beam steering component.

According to a sixth aspect of the invention there is provided a part manufactured in accordance with the method of the fifth aspect of the invention.

Description of the Drawings

Figure 1 is a schematic of a selective laser solidification apparatus according to an embodiment of the invention;

Figure 2 is a schematic of the selective laser solidification apparatus from another side; Figure 3 is a schematic diagram of a scanner according to one embodiment of the invention;

Figure 4a is a diagram illustrating the scanning sequence of a laser beam according to an embodiment of the invention;

Figure 4b shows the scan path of the galvanometer driven mirrors compared to a path followed by the laser spot resulting from the additional motion imparted on the laser beam by the piezoelectric driven mirror; Figure 4c is a graph showing a typical displacement of the laser spot on the powder bed during the scanning sequence;

Figure 5 shows a scanning strategy according to a further embodiment of the invention;

Figure 6a is a perspective view of a steerable mirror and associated actuator according to a further embodiment of the invention; Figure 6b is a cross-sectional view of the steerable mirror and associated actuator shown in Figure 6a; Figure 7 is a perspective view of a steerable mirror and associated actuator according to a further embodiment of the invention.

Description of Embodiments Referring to Figures 1 and 2, a laser solidification apparatus according to an embodiment of the invention comprises a main chamber 101 having therein partitions 115, 116 that define a build chamber 117 and a surface onto which powder can be deposited. A build platform 102 is provided for supporting an object 103 built by selective laser melting powder 104. The platform 102 can be lowered within the build chamber 117 as successive layers of the object 103 are formed. A build volume available is defined by the extent to which the build platform 102 can be lowered into the build chamber 117.

Layers of powder 104 are formed as the object 103 is built by dispensing apparatus 108 and an elongate wiper 109. For example, the dispensing apparatus 108 may be apparatus as described in WO2010/007396.

A laser module 105 generates a 500W laser for melting the powder 104, the laser directed as required by a scanner, in this embodiment an optical module 110, under the control of a computer 130. The laser enters the chamber 101 via a window 107.

The optical module 110 comprises beam steering components 106 for directing the laser beam to the desired location on the powder bed 104 and focussing optics, in this embodiment a pair of movable lenses 111, 112, for adjusting a focal length of the laser beam. Actuators of the beam steering components 106 and focussing optics 111, 112 are controlled by a controller 139. Referring to Figures 3a and 3b, in one embodiment, the beam steering components 106 comprise two movable mirrors 106a, 106b driven by galvanometers 121a, 121b and a third movable mirror 106c driven by a piezoelectric actuators 120a, 120b. Mirror 106a is rotatable about an axis A perpendicular to an axis B about which mirror 106b can be rotated, and the third movable mirror 106c is steerable about two perpendicular axes C, D. The mirrors 106a, 106b and 106c are arranged such that the laser passes through the focussing optics 111, 112 to mirror 106c, which deflects the laser on to mirror 106a. Mirror 106a deflects the laser onto mirror 106b and mirror 106b deflects the laser out of the optical module through a window or opening to the powder bed 104.

The piezoelectric actuator 120a is operable to rotate the mirror 106c by a few degrees in a direction about axis C and piezoelectric actuator 120b is operable to rotate the mirror 160c by a few degrees in a direction about an axis D. The piezoelectric actuators 120a, 120b provides a faster dynamic response (acceleration) than the galvanometers 121a, 121b but a smaller range of movement. The mirror 106c can be used to deflect the laser beam through a range of angles in the same dimensions as can be achieved with mirrors 106a and 106b. Typically, each galvanometer 121a, 121b will be capable of moving the associated mirror 106a, 106b through a range of angles about axis A, B of +/- 10 degrees, although a range of angles of up to +/- 20 degrees could be used. The piezoelectric actuators 120a, 120b will typically be capable of steering mirror 106c through a range of angles about axes C and D that is approximately 1% of the range of mirrors 106a, 106b.

Figure 3b shows mirror 106c and the piezoelectric actuators 120a, 120b in more detail. Each piezoelectric actuator 120a, 120b comprises two pairs 122, 123; 124 125 of piezoelectric stacks 122a, 122b, 123a, 123b, 124a, 124b, 125a, 125b. Each piezoelectric stack 122a, 122b, 123a, 123b, 124a, 124b, 125a, 125b comprises a stack of piezoelectric elements that are connected together to provide, as a combined structure, a sufficient range of movement of the mirror 106c. The piezoelectric stacks 122a, 122b, 123a, 123b, 124a, 124b, 125a, 125b may be held in place by supporting substrates (not shown), the supporting substrates applying a compressive force to the piezoelectric stacks 122a, 122b, 123a, 123b, 124a, 124b, 125a, 125b.

The two piezoelectric stacks 122a, 122b; 123a, 123b; 124a, 124b; 125a, 125b of each pair 122, 123, 124, 125 are located either side of the mirror 106c and aligned to apply a force to the same section of the mirror 106c. The stacks 122a, 122b; 123a, 123b; 124a, 124b; 125a, 125b of each pair 122, 123, 124, 125 work in tandem, with one of the piezoelectric stacks 122a, 123 a, 124a, 125 a operated to contract/extend when the other 122b, 123b, 124b, 125b is operated to perform the opposite motion (extend/contract). In this way, the piezoelectric stacks 122a, 122b; 123a, 123b; 124a, 124b; 125a, 125b of each pair 122, 123, 124, 125 apply a force to the mirror 106c in the same direction.

The two pairs 122, 123; 124, 125 of piezoelectric stacks of each actuator 120a; 120b are each attached to a periphery of the mirror 106a at a location spaced from the other pair 123, 122; 125, 124 either side of the corresponding axis C, D. The two pairs 122, 123; 124, 125 of piezoelectric stacks work in tandem to apply a force in opposite (linear) directions to cause rotation of the mirror 106c about the axis C, D.

Appropriate circuitry is connected to the piezoelectric stacks 122a, 122b, 123a, 123b, 124a, 124b, 125a, 125b to apply appropriate voltages to the stacks to control extension and contraction of the stacks.

A low powered laser may generate a monitoring laser beam 126 directed towards the mirror 106c and a sensor 127 is provided for detecting an angle at which the laser beam 126 is reflected from the mirror 106c. The sensed position of the laser beam 126 may be used as feedback to determine an angle of the mirror 106c about axes C and D. The actuators, 120a, 120b, 121a and 121b are controlled by controller 139 of the optical module 106, in manner as now described with reference to Figures 4a to 4c. Typically, the laser beam 118 is scanned across an area to be solidified in a series of hatch lines. For example, the laser beam 118 may be scanned across an area using a stripe, checkerboard or meander scan strategy. The mirrors 106a and 106b are controlled to move smoothly such that the mirrors 106a, 106b would direct the laser beam 118 along a path 140 if no additional steering of the laser beam 118 was provided by mirror 106c. Rapid changes in velocity and direction of rotation of the mirrors 106a to 106b are avoided. The piezoelectric actuators 120a, 120b are controlled by controller 139 to add perturbations to the path 140 that result in the laser beam 118 being "hopped" between static exposure points 141. In particular, the piezoelectric actuators 120a, 120b are controlled to maintain the laser beam spot substantially stationary for a set period of time (exposure time) at a point 141 on the powder bed 104 counteracting the continuous movement of the mirrors 106a, 106b. At the end of the exposure time, the piezoelectric actuators 120a, 120b move the mirror 106c rapidly to direct the laser beam 118 to the next point 141 along the hatch. In this way, the mirrors 106a and 106b provide the large scale movement of the laser beam 118 across the powder bed 104 and the mirror 106c oscillates to impart a small scale perturbation to the large scale movement.

At the end of the path 140 at least one of the mirrors 106a, 106b will have to undergo a change in direction to direct the laser beam 118 along the next hatch. Because of the relatively low dynamic response of the galvanometers 121a, 121b, it may not be possible for path 140 to follow the more sudden change is direction required by the points 141 at the ends of each hatch. However, the controller 139 may control the piezoelectric actuators 120a, 120b such that mirror 106c compensates for the slow change in direction provided by the mirrors 106a, 106b to move the laser beam spot between the points 141 at the end of the hatches with the required exposure time. Compensation may still be applied even when scanning in a continuous mode of operation (rather than the point scanning mode shown in Figure 4a) in order that the laser beam 118 scans a path with a sharper change in direction than that which can be achieved with the mirrors 106a, 106b alone.

Figure 4b shows a loop 140a of path 140 traced by the galvanometers when changing direction between hatch lines and a path 142, along which points (not shown) are scanned. The length of the loop 140a and the length 2Li are selected such that the time it takes the galvanometers to trace path 140a is equivalent to the time it takes the galvanometers to trace a linear path of length 2Li (The laser beam is typically "hopped" in a single jump across length L ₂ such that the length of this section can be ignored. However, if L ₂ is to be scanned in the same manner as lengths Li, then the length of loop 140a and 2Li+ L ₂ are selected such that the time it takes the galvanometers to trace path 140a is equivalent to the time it takes the galvanometers to trace a linear path of length 2Li+ L ₂ ). As at least one of galvanometers slows in order to change direction along path 140a, path 140a will be shorter than the length 2Li . In this way the point exposure time and time between point exposures for the end of the hatches can be maintained consistent with the same for the middle of the hatches.

Figure 4c shows a graph of displacement, D, with time, t. To scan a middle portion of a hatch, mirrors 106a and 106b are moved such that the laser spot would be displaced on the powder bed 104 at a constant velocity, as shown by line G, if mirror 106c was kept static. Mirror 106c is moved such that the laser spot would be displaced on the powder bed 104 in accordance with the saw-tooth profile, shown by line P, if mirrors 106a and 106b were kept static. Line L indicates the actual displacement of the laser spot on the powder bed 104 as a result of the synchronous movement of the mirrors 106a, 106b and 106c. The negative gradient of the sawtooth profile has the same magnitude as the positive gradient of line G. Accordingly, during this time period 142, the actions of the mirrors 106a, 106b and 106c on movement of the laser spot on the powder bed 104 cancel such that the laser spot is maintained stationary to expose a point 141 to the laser beam. When, during period 143, the movement of mirror 106c is to displace the laser beam in the same direction as the movement of mirrors 106a, 106b, the laser spot is "hopped" to the next point location on the powder bed 104. During period 143, the laser/laser beam may be switched off such that the laser spot is not scanned across the powder bed. In this way, a series of discrete points 141 are exposed to the laser beam.

These methods of providing a faster dynamic change in direction than that which can be achieved using the mirrors 106a, 106b alone may also be applied to other types of scans other than hatches, such as border scans. Mirror 106c may also be used to compensate for settling time of the galvanometers. For example, when there is a gap between scan lines and/or areas to be scanned, the galvanometers may be moved quickly to redirect the laser beam 1 18 to the start of the subsequent scan line. For example, this may occur when moving from the last hatch line in a square of a checkerboard pattern or a stripe to the first hatch line of a subsequent square or stripe or between border scan lines. A further example is when forming a fine lattice structure that involves a series of single exposures or very small scan lines interposed with a gap. In such scenarios, the galvanometers can "wobble" at the start point of the scan line/at the exposure point after the galvanometers are operated to traverse the gap. It is known to provide a settling time to allow mirrors in a scanner to settle to a desired position after a deceleration.

With the scanner of the invention, mirror 106c can be used to compensate for undesired movement of the galvanometers that can occur during a deceleration. In this way, the settling time can be eliminated or at least reduced. This may be particular advantageous in the scanning of geometries having many gaps, as the elimination/reduction in settling time can significantly reduce the duration of the build. The galvanometers may comprise encoders for providing a measurement of the angle of the galvanometer. The piezoelectric mirror 106c may be controlled by controller 139 in response to the readings from the encoder to compensate for undesired movement of the mirrors 106a, 106b. Rather than exposing the areas to be solidified as a series of point exposures 141 with a static laser spot, the areas may be exposed to a moving laser spot that is moved at least in a direction perpendicular to a hatch/scan line direction using the faster dynamic response of the mirror 106c. The exposed region could be a line or a spot larger than the laser spot. An example of such an embodiment is shown in Figure 5. In this embodiment, for each exposure region 241 of a hatch line, the mirror 106c moves the laser spot 245 perpendicular to the hatch line (as indicated by the arrow) to expose a substantially line shaped region 241 to the laser spot 245. The laser spot 245 is then hopped to the next exposure region 241 in the hatch (in the manner as described above with reference to Figures 4a to 4c) to expose a further line shaped region 241 to the laser spot 245, for example, by scanning the laser spot 245 perpendicular to the hatch direction in the opposite direction to the previous line shaped region 241. Such a scanning strategy may allow higher power lasers to be used as the energy of the laser spot can be spread over a larger area within a set time period than when using the galvanometers alone to direct the laser spot.

Figures 6a and 6b show a voice coil actuator and mirror arrangement 200 for providing a steering capability with a faster dynamic response than the galvanometers. Such an arrangement 200 may be used in place of the piezoelectric actuator. The arrangement 200 comprises a coil of wire 201 wound round an iron core 202. A mirror 206 for steering the laser beam 118 is mounted to the iron core 202. Four horse-shoe shaped electromagnets 203a, 204a; 203b, 204b (only two of which are shown in Figure 6a) are circumferentially mounted around the centrally located iron core 202. The mirror 206 is biased to a central position by a planar spring 205. The planar spring 205 may be an "Isle of Man" spring, an example of which is described in US6683780, having three legs circumferentially and radially extending around the circular mirror 206.

In use, a substantially constant current is passed through coil 201 and the processor 141 controls each pair of electromagnets 203a, 204a; 203b, 204b to apply a magnetic field across the coil of wire 201 and iron core 202 to cause the iron core to pivot about axes C and D as required. Pivoting of the iron core 202 causes the mirror 206 to tilt against the biasing of the planar spring 205. The direction of the applied magnet field generated by each pair of electromagnets 203a, 204a; 203b, 204b can be used to control the direction in which the iron core pivots about the axes C and D.

The arrangement shown in Figures 6a and 6b can be controlled to carry out the scan strategies described with reference to Figures 4a to 4c. A larger version of the actuator shown in Figures 6a and 6b could be used as a replacement for the galvanometers.

Figure 7 shows a further mirror arrangement 300 for providing a steering capability with a faster dynamic response than the galvanometers. Movement of the mirror 306 is controlled by a first actuator 301 for rotating the mirror 306 about one axis and a second actuator (not shown) to rotate the mirror 306 about a perpendicular axis. Each actuator 301 comprises a pair of hard-linearized normal-stress actuators 302, 303 that can linearly displace armatures 304, 305 attached to opposite edges of mirror 306. Displacement of the armatures 304, 305 in opposite directions rotates the mirror 306.

Each hard linearized actuator 303, 304 comprises a stator core 307, 308 made of soft magnetic material, coil windings 309a, 309b, 310a, 310b, permanent magnet 311, 312 and the armature 304, 305. The armature 304, 305 may be movable on a pad 313, 314, such as a rubber pad, to allow the shear motion of the armature 304, 305 relative to the permanent magnet 311, 312. Dotted line 315, 316 shows the biasing flux generated by the permanent magnet and line 317, 318 the AC flux controllable through control of the current flowing through the coil windings 309a, 309b, 310a, 310b.

The current passed through the coil windings 309a, 309b, 310a, 310b can be controlled to generate differential flux on each side of the armature 304, 305 to produce a net actuating force on the armature 304, 305. The actuating force is proportional to the current.

Normal stress actuators potentially have higher force density and thus higher acceleration (dynamic response) than shear stress actuators.

A more detailed discussion of ultra-fast hard-linearized normal-stress actuators can be found in the thesis "Electromagnetically-Driven Ultra-Fast Tool Servos for Diamond Turning", Xiaodong Lu, Massachusetts Institute of Technology, September 2005.

It will be understood that alterations and modifications may be made to the above described embodiments without departing from the scope of the invention as defined herein.

Rather than a single mirror 106c, 203, 306 actuated to rotate about two perpendicular axes, separate mirrors may be provided, one for each axis, each mirror steered by an actuator that has a faster dynamic response compared to the galvanometers.

The mirror 106c, 203. 306 may reflect the laser wavelength but transmit other wavelengths, for example, wavelengths that could be used to monitor the build process as described in International patent application PCT/GB2014/052887.

The piezoelectric actuator and galvanometer may be combined to control a common mirror such that the mirror can undergo movements through a large range of angles under the control of the galvanometer and sharper, more dynamic movements under the control of the piezoelectric actuator.