POWDER BED FUSION ADDITIVE MANUFACTURING METHODS AND APPARATUS

Abstract

A powder bed fusion additive manufacturing method in which an object is built in a layer-by-layer manner. The method includes, for each layer of a plurality of successively fused layers, melting material of the layer by irradiating the layer with one or more energy beams a first time using a first set of irradiation parameters and allowing the melted material to solidify to define a fused region of the layer and reheating the fused region by irradiating the layer a subsequent time with one or more of energy beams using a second set of irradiation parameters. The first set of irradiation parameters includes at least one different irradiation parameter to the second set of irradiation parameters.

Claims

1. A powder bed fusion additive manufacturing method in which an object is built in a layer-by-layer manner, the method comprising, for each layer of a plurality of successively fused layers, melting material of the layer by irradiating the layer with one or more energy beams a first time using a first set of irradiation parameters and allowing the melted material to solidify to define a fused region of the layer and reheating the fused region by irradiating the layer a subsequent time with one or more of energy beams using a second set of irradiation parameters, wherein the first set of irradiation parameters comprises at least one different irradiation parameter to the second set of irradiation parameters.

2. A powder bed fusion additive manufacturing method according to claim 1, wherein the reheating increases a temperature of the fused region above a temperature at which grain refinement occurs, wherein the grain refinement may reduce an amount of epitaxial and/or columnar grains.

3. A powder bed fusion additive manufacturing method according to claim 2, wherein the temperature is a tempering temperature at which tempering of the fused material occurs, an annealing temperature at which annealing of the fused material occurs, a solution heat treatment temperature at which a solution heat treatment of the fused material occurs, a sintering temperature at which sintering of the fused material occurs or a melting temperature at which melting of the fused material occurs.

4. A powder bed fusion additive manufacturing method according to claim 1, wherein the reheating increases a temperature of the fused region by at least 100° C., 200° C., 300° C., 400° C. or 500° C.

5. A powder bed fusion additive manufacturing method according to claim 1, wherein reheating of the fused region is carried out after the fused region has cooled to below 350° C.

6. A powder bed fusion additive manufacturing method according to claim 1, comprising reheating the fused region more than one subsequent time.

7. A powder bed fusion additive manufacturing method according to claim 1, wherein a separation between the first time and the subsequent time may be greater than 250 microseconds.

8. A powder bed fusion additive manufacturing method according to claim 1, wherein the same irradiation pattern is used for irradiating the material the first time and the or each subsequent time.

9. A powder bed fusion additive manufacturing method according to claim 1, wherein irradiating material of each layer the first time comprises progressively irradiating a predefined irradiation path with a first, leading energy beam and irradiating the fused region the or each subsequent time comprises progressively irradiating the predefined irradiation path with a trailing energy beam.

10. A powder bed fusion additive manufacturing method according to claim 9, wherein the leading energy beam has a different power to the trailing energy beam and/or a spot size the same as or larger than the leading energy beam and/or the trailing energy beam irradiates an area the same width or wider than a fused line of material formed by the progression of the leading energy beam along the irradiation path.

11. A powder bed fusion additive manufacturing method according to claim 1, wherein a different irradiation pattern is used for irradiating the layer the first time and the or each subsequent time.

12. A powder bed fusion additive manufacturing method according to claim 11, wherein the further irradiation pattern is arranged to produce a preferential direction of grain formation.

13. A powder bed fusion additive manufacturing method according to claim 1, comprising the reheating comprises melting of the fused region the or each subsequent time such that the melt pool(s) formed extend deeper than the melt pool(s) formed when melting material of the layer the first time to form the fused region.

14. A powder bed fusion additive manufacturing method in which an object is built in a layer-by-layer manner, the method comprising, for each layer of a plurality of successively fused layers, melting material of the layer using a leading energy beam by progressing the energy beam over the material along an irradiation path, allowing the melted material to solidify and reheating the solidified material by progressing a trailing energy beam along the irradiation path.

15. A powder bed fusion additive manufacturing method in which an object is built in a layer-by-layer manner, the method comprising, for each layer of a plurality of successively fused layers, melting material of the layer a first time by irradiating the layer in a first irradiation pattern with one or more energy beams, allowing the melted material of the pattern to solidify to define a fused region of the layer and melting the fused region a subsequent time in a second irradiation pattern with one or more energy beams.

Description

DESCRIPTION OF THE DRAWINGS

[0055] FIG. 1 is a schematic diagram of a powder bed additive manufacturing apparatus according to an embodiment of the invention;

[0056] FIG. 2 is a plan view of the powder bed additive manufacturing apparatus shown in FIG. 1;

[0057] FIG. 3 is a schematic illustration of a scanning strategy according to a first embodiment of the invention;

[0058] FIG. 4 is a cross-section of fused layers illustrating a core, upskin and downskin regions of an object;

[0059] FIG. 5 is a schematic illustration of a scanning strategy according to a second embodiment of the invention;

[0060] FIG. 6 is a schematic illustration of a scanning strategy according to a third embodiment of the invention;

[0061] FIG. 7 is a schematic illustration of melt pools formed using the scanning strategy of the third embodiment;

[0062] FIG. 8 is a back-scattered electron image of a cross-section of a cube formed from H13 tool steel using a single laser beam;

[0063] FIG. 9 is a back-scattered electron image of a cross-section of a cube formed from H13 tool steel using leading and trailing laser beams, wherein a 200 W trailing laser beam trials the leading laser beam by 2500 μs;

[0064] FIG. 10 is a back-scattered electron image of a cross-section of a cube formed from H13 tool steel using leading and trailing laser beams, wherein a 100 W trailing laser beam trials the leading laser beam by 2500 μs;

[0065] FIG. 11 is a back-scattered electron image of a cross-section of a cube formed from H13 tool steel using a leading laser beam and three trailing laser beams, wherein each trailing laser beam follows the preceding laser beam by 500 μs;

[0066] FIG. 12 is a back-scattered electron image of a cross-section of a cube formed from H13 tool steel using a leading laser beam and three trailing laser beams, wherein each trailing laser beam follows the preceding laser beam by 1000 μs;

[0067] FIG. 13 is a back-scattered electron image of a cross-section of a cube formed from H13 tool steel using a leading laser beam and three trailing laser beams, wherein each trailing laser beam follows the preceding laser beam by 1500 μs;

[0068] FIG. 14 are tables showing the scan parameters used for the laser beams of Example 4;

[0069] FIGS. 15a and 15b are histograms showing the number of differently sized cracks found in the cubes built in the manner described in Example 4;

[0070] FIGS. 16a and 16b are a back-scattered electron image of cross-sections of a cube formed from H13 tool steel using a single laser beam, wherein the powder bed has been heated to 500° C.;

[0071] FIGS. 17a and 17b are a back-scattered electron image of cross-sections of a cube formed from H13 tool steel using leading and trailing laser beams (without heating the powder bed to 500° C.);

[0072] FIGS. 18a and 18b are a back-scattered electron image of cross-sections of a cube formed from H13 tool steel using a single laser beam (without heating the powder bed to 500° C.);

[0073] FIG. 19 is a table showing the scan parameters used for the laser beams of Example 6 in which cubes and tracks were formed from M2 high-speed steel;

[0074] FIGS. 20-1 to 20-7 show solidified hatch lines formed with laser beams having the scan parameters set out in FIG. 19;

[0075] FIGS. 21a and 21b are a back-scattered electron images of top surfaces of the cubes formed from M2 high-speed steel using leading and trailing laser beams (FIG. 21a) and a single laser beam (FIG. 21b); and

[0076] FIG. 22 is a back-scattered electron image of a cross-section of a cube formed from M2 high-speed steel using leading and trailing laser beams parallel to the build direction, wherein a hatch formation direction is normal to the page.

DESCRIPTION OF EMBODIMENTS

[0077] Referring to FIGS. 1 and 2, a powder bed fusion additive manufacturing apparatus according to an embodiment of the invention comprises a build chamber 101 having therein partitions 115, 116 that define a build volume 117. A build platform 102 is lowerable in the build volume 117. The build platform 102 supports a build substrate plate 102a, a powder bed 104 and workpiece 103 as the workpiece is built by selective laser melting of the powder. The platform 102 is lowered within the build volume 117 under the control of a motor (not shown) as successive powder layers are formed.

[0078] Layers of powder 104 are formed as the workpiece 103 is built by dispensing apparatus 108 and a wiper 109. For example, the dispensing apparatus 108 may be apparatus as described in WO2010/007396. The dispensing apparatus 108 dispenses powder onto an upper surface 115a defined by partition 115 and is spread across the powder bed by recoater, in this embodiment in the form of wiper 109. A position of a lower edge of the wiper 109 defines a working surface/plane 110 at which powder is consolidated.

[0079] A plurality of laser modules 105a, 105b, 105c and 105d generate laser beams 118a, 118b, 118c, 118d for irradiating the powder bed 104, the laser beams 118a, 118b, 118c, 188d steered as required by a corresponding optical module 106a, 106b, 106c, 106d. The laser beams 118a, 118b, 118c, 118d are steered by the corresponding optical module to enter the build chamber 101 through a common laser window 107. Each optical module comprises steering optics 121, such a two mirrors 141a, 141b mounted on galvanometers 124a, 124b (see FIG. 3), for steering the laser beam 118 in perpendicular directions, X and Y, across the entire working plane 110 and focusing optics 120, such as two movable lenses for changing the focus of the laser beam 118. The optical modules 106a, 106b, 106c, 106d are shown here as separate but they may be combined together into a single-piece housing, as is the case in Renishaw's RenAM 500Q additive manufacturing machine. Each scanner is controlled such that the focal position of the laser beam 118 remains in the same plane 110 as the laser beam 118 is moved across the working plane 110. Rather than maintaining the focal position of the laser beam in a plane using dynamic focusing elements, an f-theta lens may be used.

[0080] A controller 140, comprising processor 161 and memory 162, is in communication with modules of the additive manufacturing apparatus, namely the laser modules 105a, 105b, 105c, 105d, optical modules 106a, 106b, 16c, 106d, build platform 102, dispensing apparatus 108 and wiper 109. The controller 140 receives build instructions from an external computer having build preparation software thereon and controls each of the modules based upon the build instructions in order to build one or more objects using an additive manufacturing process. The build preparation software may be as described in WO2014/207454 but further adapted to generate build instructions implementing the scanning strategies as described below.

[0081] Referring to FIG. 3, a scanning strategy for consolidating powder material to form a cross-section 200 of an object comprises melting material of a powder layer L.sub.5 by progressively irradiating a predefined irradiation path 201 with a first, leading laser beam 203a and reheating the fused region 205 by progressively irradiating the predefined irradiation path 201 with a trailing laser beam 203b. In this embodiment, the leading laser beam 203a and trailing laser beam 203b are continuously scanned along the parallel irradiation paths 201 (also called hatch lines) at the same speed. The hatch lines are spaced a hatch distance HD apart such that adjacent solidified lines form a continuous fused region with no powder therebetween. Parallel hatch lines 201 are typically used as a fill scan to solidify a core of a cross-section and one or more border scans 202 are carried out to provide a hull around the core.

[0082] An example of a core of a layer L.sub.2 of fused material is illustrated in FIG. 4 by the shaded areas 510a and 510b. Shaded areas 510a and 510b are formed directly on fused material of the layer L.sub.1 below and have fused material formed thereon when the layer L.sub.3 above is fused. Accordingly, region 511 is not part of a core of layer L.sub.2 because it is a downskin region not directly formed on fused material (but fused on powder), region 512 is not part of a core of layer L.sub.2 because it is an upskin region on which no fused material of layer L.sub.3 is formed and region 513 is a border region fused through irradiating this region using a border scan.

[0083] The fill scan may comprise a meander scan pattern in which all of the hatch lines are in the same direction (although they be scanned bi- or unidirectionally), a chequerboard scan pattern, wherein the cross-section to be solidified is split into a plurality of squares, each square comprising a plurality of hatch lines, wherein a direction of the hatch lines between squares may differ, for example by 90° or a stripe scan pattern, wherein the cross-section to be solidified is split into a plurality of parallel stripes, each stripe comprising a plurality of hatch lines. The chequerboard and stripe scan patterns may provide equal length hatch lines, except in circumstances wherein a square or stripe is curtailed due to a border of the cross-section 200 to be solidified.

[0084] The border scans 202 may use a different set of parameters to the fill scans, for example such that a desired surface finish is achieved. The border scans may also be scanned with leading and trailing laser beams or may be scanned with only a single laser beam.

[0085] The leading and trailing laser beams 203a, 203b are separated by a delay, d, such that material 204 melted by the leading laser beam 203a is allowed to solidify before the fused material 205 is reheated by the laser beam 203b. In this embodiment, the delay. d, is greater than 250 microseconds and preferably 2500 microseconds. The set of irradiation parameters for the leading and trailing laser beams are also different. In this embodiment, the leading laser has a smaller (1/e.sup.2) spot size, S.sub.1, and high power than the trailing laser beam 203a. The larger spot size S.sub.2 of the trailing laser beam 203b may be achieved by defocusing the laser beam, for example by focusing the laser beam to a plane above or below a working surface of the powder bed 104.

[0086] An energy density provided by the trailing laser beam 203a may be insufficient to melt the material but may heat the material above a temperature at which a grain refinement occurs.

[0087] Alternatively, an energy density provided by the trailing laser beam 203a may be sufficient to melt the material resulting in a grain refinement. In this alternative embodiment, the energy density of the trailing laser beam alone may be insufficient to melt the material to form a continuous hatch line of solidified material. However, in conjunction with the leading laser beam, the delay between the leading and trailing laser beam is long enough for the material melted by the leading laser beam to solidify but shorter enough that sufficient heat remains within the locality such that the trailing laser beam remelts the material.

[0088] It has been found that this scanning strategy reduces a number of cracks in the resultant object compared to using only a single laser beam. It is believed the reheating of the fused material by the trailing laser beam 203b refines the grains to reduce an amount of equiaxed and/or columnar grains, thus reducing boundaries between differently oriented equiaxed and columnar grains. The reduction in such grain boundaries, reduces an amount of solidification cracking in the metal material. It has been found that such a scanning strategy is capable of reducing cracking to such an extent that builds that previously failed due to cracking can now be built.

[0089] Referring to FIG. 5, a further scanning strategy is shown. This embodiment differs from the embodiment described with reference to FIG. 3 in that an additional trailing laser beam 303c is provided. The additional trailing laser beam 303c reheats the fused region 205 a third time by progressively irradiating the predefined irradiation path 201 a set delay, d.sub.2, after the trailing laser beam 303b. The delay, d.sub.2, may be the same of different from delay, d.sub.1.

[0090] It has been found that the addition of a further trailing laser beam further reduces cracks in the resultant object compared to using a leading laser beam and one trailing laser beam. In this embodiment, the set of irradiation parameters used for the additional trailing laser beam 303c is different to that used for the trailing laser beam 303b.

[0091] Referring to FIGS. 6 and 7, a scanning strategy for consolidating powder material to form a cross-section 400 of an object comprises melting material a first time with a first scan pattern comprising a first set of exposures, in this embodiment hatch lines 401a, allowing the melted material to solidify to form an entire fused cross-section 400 of the object and then melting the material of the fused cross-section 400 a second time using a second scan pattern comprising a second set of exposures, in this embodiment a second set of hatch lines 401b. The hatch lines of the second set of exposures may be in the same direction as the first set of exposures or may be in a different direction to the first set of exposures, as shown in FIG. 6.

[0092] The first set of exposures form melt pools 404a′, 404a″, 404a′″ and 404a″″ that are sufficiently deep to consolidate the fused material with fused material of the layer below but are shallower than then melt pools 404b′, 404b″, 404b′″ and 404b″″ formed by the second set of exposures. In this way, the second set of exposures “overwrite” the first set of exposures such that the resultant grain structure is primarily a consequence of the solidification rate and geometry of the melt pools formed by the second set of exposures. The fused material formed by the first set of exposures provides a uniform environment such that the second set of exposures form melt pools having the required shape and inter-relation to achieve a directional grain structure. This can be important when forming an object having a preferred grain direction, as is described in EP19179230.8.

[0093] In this embodiment, both the first and second sets of the exposures form melt pools in a conduction or transition mode.

Example 1

[0094] Eight 10 mm×10 mm×10 mm cubes were built from tool steel HS13 powder in a RenAM 500Q additive manufacturing machine. The cubes were built using a meander scan strategy, wherein the hatch direction was rotated every layer by 67 degrees. No preheating of the powder was carried out using the heater in the build platform. Seven of the cubes were built using leading and trailing laser beams and one of the cubes was built using a single laser beam. The scan parameters are set out below.

Single Laser

[0095] A cube was built using a single laser beam with the following scan parameters: [0096] Laser Power: 200 W [0097] Focal point relative to working plane: 0 mm [0098] Point distance: 20 μm [0099] Exposure time: 20 μs [0100] Hatch distance: 0.08 mm

[0101] FIG. 8 illustrates a back-scattered electron image of a cross-section of the cube formed using these scan parameters. As can be seen from the image, a number of large cracks are present in the fused material.

200 W Trailing Laser

[0102] A cube was built using leading and trailing laser beams, wherein the trailing laser beam trailed by 2500 μs. The following scan parameters were used:

Leading Laser Beam:

[0103] Laser Power: 200 W [0104] Focal point relative to working plane: 0 mm [0105] Point distance: 20 μm [0106] Exposure time: 20 μs [0107] Hatch distance: 0.08 mm

Trailing Laser Beam:

[0108] Laser Power: 200 W [0109] Focal point relative to working plane: +5 mm [0110] Point distance: 20 μm [0111] Exposure time: 20 μs [0112] Hatch distance: 0.08 mm

[0113] FIG. 9 illustrates a back-scattered electron image of a cross-section of the cube formed using these scan parameters. As can be seen from the image, the number and size of cracks present in the fused material have reduced compared to the cube formed using a single laser. A refinement of the grain structure can be observed in the image compared to building the cube using a single laser beam.

100 W Trailing Baser Beam

[0114] A cube was built using leading and trailing laser beams, wherein the trailing laser beam trailed by 2500 μs. The following scan parameters were used:

Leading Laser Beam:

[0115] Laser Power: 200 W [0116] Focal point relative to working plane: 0 mm [0117] Point distance: 20 μm [0118] Exposure time: 20 μs [0119] Hatch distance: 0.08 mm

Trailing Laser Beam:

[0120] Laser Power: 100 W [0121] Focal point relative to working plane: +5 mm [0122] Point distance: 20 μm [0123] Exposure time: 20 μs [0124] Hatch distance: 0.08 mm

[0125] FIG. 10 illustrates a back-scattered electron image of a cross-section of the cube formed using these scan parameters. As can be seen from the image, the number and size of cracks present in the fused material have reduced compared to the cube formed using a single laser beam and the cube formed using a 200 W trailing laser beam. Again, a refinement of the grain structure was apparent compared to the cube formed using single laser beam.

[0126] An increase in the hardness of the material formed using leading and trailing laser beams over that formed using a single laser beam was also observed, which also is an indication that a refinement of the grain structure has occurred as a result of the heat treatment using the trailing laser beam.

Example 2

[0127] Three 10 mm×10 mm×3 mm cubes were built from tool steel HS13 powder in a RenAM 500Q additive manufacturing machine using a leading laser beam and three trailing laser beams. For a first one of the cubes, the delay time between each laser beam was 500 μs, for a second one of the cubes, the delay time between each laser beam was 1000 μs and for a third one of the cubes, the delay time between each laser beam was 1500 μs. The cubes were built using a meander scan strategy, wherein the hatch direction was rotated every layer by 67 degrees. No preheating of the powder was carried out using the heater in the build platform. The scan parameters are set out below.

Leading Laser Beam:

[0128] Laser Power: 240 W [0129] Focal point relative to working plane: 0 mm [0130] Point distance: 20 μm [0131] Exposure time: 20 μs [0132] Hatch distance: 0.08 mm

1.SUP.st .Trailing Laser Beam:

[0133] Laser Power: 150 W [0134] Focal point relative to working plane: +15 mm [0135] Point distance: 20 μm [0136] Exposure time: 20 μs [0137] Hatch distance: 0.08 mm

2nd Trailing Laser Beam:

[0138] Laser Power: 100 W [0139] Focal point relative to working plane: +15 mm [0140] Point distance: 20 μm [0141] Exposure time: 20 μs [0142] Hatch distance: 0.08 mm

3rd Trailing Laser Beam:

[0143] Laser Power: 50 W [0144] Focal point relative to working plane: +15 mm [0145] Point distance: 20 μm [0146] Exposure time: 20 μs [0147] Hatch distance: 0.08 mm

[0148] For a delay of 500 μs, a bulk density of 99.92% of the theoretical bulk density was achieved. For a delay of 1000 μs, a bulk density of 99.95% of the theoretical bulk density was achieved. For a delay of 1500 μs, a bulk density of 99.97% of the theoretical bulk density was achieved. As can be see from FIGS. 11, 12 and 13, a reduction in cracks can be observed compared to using a single laser beam to form a cube.

[0149] Again, an increase in hardness in these samples was observed compared to the cube formed using a single laser beam.

Example 3

[0150] Eight 10 mm×10 mm×10 mm cubes were built from tool steel W360 powder in a RenAM 500Q additive manufacturing machine. The cubes were built using leading and trailing laser beams, which traversed the cross-sections in a meander or stripe scan strategy. The hatch direction was rotated every layer by 67 degrees. No preheating of the powder was carried out using the heater in the build platform. Two cubes were built using a single laser beam. The scan parameters used for each cube is set out in FIG. 14.

[0151] A number of cracks in each cube was counted using a computer programme that automatically identifies cracks from an optical image of cross-sections of the cubes. FIG. 15 is a histogram showing the number of cracks identified as having a particular perimeter size of the cubes built using a single laser beam and FIG. 15b is a histogram showing the number of cracks identified as having a particular perimeter size of the cubes built using leading and trailing laser beams. As is apparent from the histograms, a number of cracks is lower in the cubes built using leading and trailing laser beams compared to the cubes built using a single laser beam.

Example 4

[0152] A 10 mm×10 mm×10 mm cube was built from tool steel HS13 powder in a Renishaw RenAM500Q HT additive manufacturing machine, wherein the powder bed was heated to 500° C. The cube was built using a single laser using a meander scan strategy, wherein the hatch direction was rotated every layer by 67 degrees.

[0153] FIGS. 16a and 16b is an electron backscattered image of two-cross-sections of the cube. As can be see, many cracks are present in the cube.

[0154] It is believed that Example 4 illustrates that reduced cooling rates do not prevent the formation of cracks in H13 tool steel.

Example 5

[0155] Cubes were built in H13 tool steel, one using leading and trailing laser beams, wherein the trailing laser beam trailed by 2500 μs and another with a single laser beam only. There was no preheating of the powder bed. The following scan parameters were used:

Leading Laser Beam:

[0156] Laser Power: 240 W [0157] Focal point relative to working plane: 0 mm [0158] Point distance: 20 μm [0159] Exposure time: 20 μs [0160] Hatch distance: 0.08 mm

Trailing Laser Beam:

[0161] Laser Power: 100 W [0162] Focal point relative to working plane: +5 mm [0163] Point distance: 20 μm [0164] Exposure time: 20 μs [0165] Hatch distance: 0.08 mm

[0166] For the single laser beam the same scan parameters used are those listed above for the leading laser beam.

[0167] FIG. 17a is a back-scattered electron image of a cross-section of the cube formed using the leading and trailing laser beams. FIG. 18a is a back-scattered electron image of a cross-section of the cube formed using the single laser beam. As can be seen from the images, the number and size of cracks present in the fused material has reduced using leading and trailing laser beams compared to the cube formed using a single laser beam. FIGS. 17b and 18b shows images of cross-sections of the cubes at higher resolution that have undergone a crack analysis using software. The crack area percentage of the cross-section formed using leading and trailing laser beams was 0.05%, the average crack length was 16+/−12 μm and the average crack width was 0.5+/−0.5 μm. The crack area percentage of the cross-section formed using a single laser beam was 0.26%, the average crack length was 25+/−15 μm and the average crack width was 2+/−1 μm.

Example 6

[0168] Cubes were built from M2 high-speed steel, four cubes built using leading and trailing laser beams and three with a single laser beam only. The scan parameters used are set out in FIG. 19. Hatch lines were also formed on top of each cube with the scan parameters to allow study of the melt region.

[0169] FIGS. 20-1 to 20-7 correspond to the sample number set out in the table of FIG. 19. As can seen from the hatch lines visible in FIGS. 20-1 to 20-4 the smoothness of the hatch lines increases with increase in laser power of the trailing laser from 50 W to 150 W, indicating an increase in remelting as the power in increased. FIGS. 20-5 and 20-6 show balling effects when using the single laser beam with lower power, illustrating these scan parameters of the trailing laser beam are insufficient without the leading laser beam to melt a continuous line of the material.

[0170] FIG. 21a show the top surface of a cube built using leading and trailing laser beams showing the refined microstructure. FIG. 21b show the top surface of a cube built using a single laser beam showing coarse dendrites and significantly more cracks.

[0171] The dotted box denoted in FIG. 22 encloses a single layer melted with leading and trailing laser beams in M2 high-speed steel. The hatch direction is normal to the page. As can be seen, the layer has been formed from deeper melt pools having a keyhole mode shape created by the leading laser beam and shallow melt pools having a conduction mode shape created by the trailing laser beam. This is an indication that remelting is being carried out by the trailing laser beam despite the scan parameters providing too low an energy density to melt the material without the heating carried out the leading laser beam.

[0172] It will be understood that alterations and modifications can be made to the above described embodiments without departing from the scope of the invention as defined herein. For example, rather than a continuous scanning of the irradiation paths 201, 401a, 401b by the laser beams, each or one or more of the laser beams may be modulated to irradiate a series of points or section along the irradiation path. Rather than applying the scanning strategy to only the fill scan or to both the fill scan and the border scan, the scanning strategy may be applied to the border scans only. Cracks during part failure tend to be initiated from microcracks at a surface of the part. Accordingly, reducing or eliminating cracks at the border may be sufficient to provide a part having the required mechanical properties. Furthermore, it may be more difficult to ensure that a melt of the material at the border is as required, whereas the required conditions may be easier to maintain within the core of a cross-section. Accordingly, a required mechanical property, such as grain orientation, may be achievable within the core on melting the material without grain refinement but a grain refinement may be required for the fused material at the borders.