SELECTIVE LASER SOLIDIFICATION APPARATUS AND METHOD

Abstract

Selective laser solidification apparatus is described that includes a powder bed onto which a powder layer can be deposited and a gas flow unit for passing a flow of gas over the powder bed along a predefined gas flow direction. A laser scanning unit is provided for scanning a laser beam over the powder layer to selectively solidify at least part of the powder layer to form a required pattern. The required pattern is formed from a plurality of stripes or stripe segments that are formed by advancing the laser beam along the stripe or stripe segment in a stripe formation direction. The stripe formation direction is arranged so that it always at least partially opposes the predefined gas flow direction. A corresponding method is also described.

Claims

1. Selective laser solidification apparatus, comprising; a powder bed onto which a powder layer can be deposited, a gas flow unit for passing a flow of gas over the powder bed along a predefined gas flow direction, and a laser scanning unit for scanning a laser beam over the powder layer to selectively solidify at least part of the powder layer to form a required pattern, the required pattern being formed from a plurality of stripes or stripe segments, each stripe or stripe segment being formed by advancing the laser beam along the stripe or stripe segment in a stripe formation direction, wherein the stripe formation direction is always at least partially opposed to the predefined gas flow direction.

2. An apparatus according to claim 1, wherein the stripe formation direction subtends an angle (α) of more than 30° to the normal to the gas flow direction.

3. An apparatus according to claim 1, wherein the flow of gas over the powder bed originates from a first side of the powder bed and the plurality of stripes are formed in reverse order of their proximity to the first side of the powder bed.

4. An apparatus according to claim 1, wherein the gas flow unit comprises at least one gas outlet and at least one gas exhaust, the at least one gas outlet and the at least one gas exhaust being placed either side of the powder bed such that gas pumped from the at least one gas outlet to the at least one gas exhaust provides a substantially planar flow of gas along the predefined gas direction.

5. An apparatus according to claim 1, wherein the required pattern is formed from a plurality of elongate stripes.

6. An apparatus according to claim 5, wherein each of the plurality of elongate stripes comprises a linear stripe having an elongate axis and the stripe formation direction is parallel to the elongate axis.

7. An apparatus according to claim 1, wherein the required pattern is formed from a plurality of stripe segments.

8. An apparatus according to claim 1, wherein the laser scanning unit generates a laser spot that is moved across the stripe or stripe segment to form a hatch line, each of the plurality of stripes or stripe segments being formed using a plurality of hatch lines that are advanced along the stripe formation direction.

9. An apparatus according to claim 8, wherein the laser scanning unit forms the series of hatch lines by moving the laser beam back and forth across the stripe or stripe segment.

10. An apparatus according to claim 8, wherein all hatch lines are formed by moving the laser beam in the same line direction across the stripe, the line direction being at least partially opposed to the gas flow direction.

11. An apparatus according to claim 10, wherein the line direction is selected based on the orientation of the stripe formation direction relative to the gas flow direction.

12. An apparatus according to claim 1, wherein the apparatus deposits and selectively solidifies a plurality of powder layers to form a three dimensional object, the required pattern of each powder layer being formed from a plurality of stripes or stripe segments, wherein the stripe formation direction is always at least partially opposed to the predefined gas flow direction for each stripe or stripe segment of each layer.

13. An apparatus according to claim 12, wherein different stripe formation directions are used for adjacent powder layers.

14. An apparatus according to claim 1, wherein the laser unit is arranged to selectively melt the powder layer.

15. A method of selectively solidifying a powder layer deposited on a powder bed, comprising the steps of; passing a flow of gas over the powder bed along a predefined gas flow direction, and scanning a laser beam over the powder layer to selectively solidify at least part of the powder layer to form a required pattern, the required pattern being formed from a plurality of stripes or stripe segments, each stripe or stripe segment being formed by advancing the laser beam along the stripe or stripe segment in a stripe formation direction, wherein the stripe formation direction always being at least partially opposed to the predefined gas flow direction.

16. Selective laser solidification apparatus, comprising; a powder bed onto which a powder layer can be deposited, a gas flow unit for passing a flow of gas over the powder bed along a predefined gas flow direction, and a laser scanning unit for scanning a laser beam over the powder layer to selectively solidify at least part of the powder layer to form a required pattern, the required pattern being formed from a plurality of stripes or stripe segments, wherein the flow of gas over the powder bed originates from a first side of the powder bed and the plurality of stripes or stripe segments are formed in reverse order of their proximity to the first side of the powder bed.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The invention will now be described, by way of example only, with reference to the accompanying drawings in which:

[0032] FIG. 1 is a schematic illustration of a selective laser melting machine,

[0033] FIG. 2 shows a prior art technique for solidifying a layer of powder using hatch lines advanced along twelve stripes,

[0034] FIG. 3 is a photomicrograph that shows the variation in oxidisation that occurs using the prior art technique of FIG. 2,

[0035] FIG. 4 illustrates the deposition of ejected material that occurs when the gas flow is in the same direction as the stripe formation direction (the “tail-wind” situation),

[0036] FIG. 5 illustrates the deposition of ejected material that occurs when the gas flow direction opposes the stripe formation direction as per the present invention,

[0037] FIG. 6 shows a first example of a stripe scan of the present invention,

[0038] FIG. 7 shows a second example of a stripe scan of the present invention,

[0039] FIGS. 8A, 8B and 8C show three options for forming hatch lines by rapid scanning of a laser beam across the width a stripe,

[0040] FIG. 9 illustrates how the stripe formation direction can be varied between layers, and

[0041] FIG. 10 illustrates how multiple stripe segments may be formed within a layer in a checkerboard pattern.

DETAILED DESCRIPTION

[0042] Referring to FIG. 1, a known selective laser melting machine 2 is schematically illustrated.

[0043] The laser melting machine 2 comprises a build chamber or housing 4 in which there is provided a powder bed 6. The powder bed 6 can be raised and lowed (i.e. moved in the z-direction) by a piston mechanism 8. A powder dispensing and roller system 10 is provided for depositing a thin (e.g. 10-100 m) powder layer onto the top of the powder bed 6. The powder used to form the powder layer is preferably a metal powder (e.g. 1.2709 grade steel powder).

[0044] A laser scanning unit 20 is also provided that comprises a high power continuous wave (CW) laser and scanning optics to direct a laser beam 22 towards the powder bed 6. The scanning optics also allow the laser beam 22 to be moved rapidly over the surface of the powder bed 6. The laser scanning unit 20 also includes an optical modulator to enable the laser beam 22 that impinges on the powder layer to be turned on and off as required.

[0045] A gas flow unit 30 is also provided. The gas flow unit 30 comprises a gas outlet bar 32 having a plurality of nozzles 34 for ejecting gas. A gas exhaust bar 36 is also provided for collecting gas. A pump 38 is used to draw in gas from the gas exhaust bar 36 and to pump gas to the nozzles 34 of the gas outlet bar 32. Suitable gas tubing 40 is provided to connect the gas outlet bar 32 and gas exhaust bar 36 to the pump 38. In use, gas flows from the gas outlet bar 32 to the gas exhaust bar 36. There is thus a predefined gas flow within the machine; i.e. gas is passed over the powder bed along the gas flow direction G.

[0046] The laser melting machine 2 is operated, under the direction of a controller 50, as follows. Firstly, a substrate plate is affixed to the piston mechanism. The substrate plate, which is preferably formed from the same material as the powder to be deposited on it, forms the base of the powder bed. The powder dispensing and roller system 10 is then used to dispense a powder layer of a certain thickness (e.g. 80 μm) onto the substrate plate. The laser scanning unit 20 then directs the laser beam 22 onto the powder layer and melts selected parts of the powder layer; i.e. selected regions of the powder layer are melted to the substrate plate. The path over the powder bed that is used by the laser is scanned is described in more detail below. Once the required pattern (e.g. cross-section) has been written into the powder layer, the piston 8 drops the powder bed 6, another powder layer is deposited on top of the existing (partly solidified) layer and the laser scanning unit then selectively melts the newly deposited powder layer. This process is then repeated, layer-by-layer, to form the required three dimensional object. During this fabrication process, a continuous supply of gas is passed over the powder bed along the gas flow direction G by the gas flow unit 30.

[0047] The illustration and description of FIG. 1 shows only the basic operation of known laser melting machines. The skilled person would be aware of further details of the machine construction and operation. It should be noted that the above schematic illustration is based on the M280 model of laser melting machine that is made by EOS GmbH.

[0048] Referring next to FIG. 2, a prior art process for selectively melting a deposited powder layer using the machine described above with reference to FIG. 1 will be described. This process is implemented as standard on the EOS M280 machine mentioned above.

[0049] FIG. 2 illustrates a powder layer 100 that is to be selectively melted to form the solidified layer pattern 102. The powder layer 100 is deposited on the powder bed using the powder dispensing and roller system 10 that is described with reference to FIG. 1. Also shown is the gas outlet bar 32 and the gas exhaust bar 36 that provide a planar flow of gas along a gas flow direction G.

[0050] In order to solidify the powder layer 100 to form the solidified layer pattern 102, a plurality of stripes (labelled S1-S12 in FIG. 2) are defined. The stripes S1-S12 together define a square region that contains the area on the powder bed where the solidified layer pattern 102 is to be written. The laser scanning unit 20 generates a laser spot that is rapidly scanned across the width of the stripe (i.e. along a direction perpendicular to the elongate axis or length of the stripe) to form a so-called hatch line. In order to selectively melt powder along the length of the stripe, successive hatch lines are moved along the stripe in the direction L. In other words, the stripe is formed by movement of the hatch line along the stripe formation direction L. It should be noted that the hatch line formed by the laser may be the width of the stripe or it may be shorter than the width of the stripe if melting is not required across the whole stripe width at that particular position.

[0051] In the prior art example shown in FIG. 2, the stripe S12 is addressed first. This involves the laser scanning unit forming a hatch line that is moved from left to right along the stripe formation direction L to solidify the part of the layer pattern 102 falling with the S12 stripe. After stripe S12 has been written, the pattern of stripe S11 is written, which involves moving the hatch line along the stripe S11 in the stripe formation direction L. The stripe formation direction L for stripe S11 is opposite to the stripe formation direction L for stripe S12. It can thus be seen that the stripe formation direction L alternates between stripes as the stripes are written in turn (i.e. in the order S12 to S1). In the present example, all even numbered stripes (S2, S4, S4 etc) are formed using a hatch line that is moved from left to right whereas all odd numbered stripes (S1, S3, S5 etc) are formed using a hatch line that is moved from right to left.

[0052] The present inventors have found that this prior art technique has a number of disadvantages. As shown in FIG. 3, it has been found by the present inventors that stripes formed by hatch line movement in opposed stripe formation directions have different surface discoloration and surface roughness. In particular, the present inventors have found that oxidization and high surface roughness occurs for even numbered stripes (S2, S4, S4 etc) that have been formed using a hatch line that is moved (in FIG. 2) from left to right. These even numbered stripes are identified by the label 140 in the photomicrograph of FIG. 3.

[0053] Referring now to FIGS. 4 and 5, the effect of the gas flow direction relative to the stripe formation direction on layer formation will be described.

[0054] FIG. 4 shows a powder bed 200 that carries a layer of melted metal 202 and a powder layer 204 that has yet to be melted. The dashed lines 206 illustrate the hatch lines that were generated by the laser scanning unit to melt the powder that now forms the layer of melted metal 202. In this example, the gas flow direction G is the same as the stripe formation direction L. In other words, there a component of the gas flow direction that is in the same direction as the stripe formation direction L; this could be thought of as there being a “tail-wind”.

[0055] The inventors have found that when the gas flow direction G and stripe formation direction L are aligned in the manner shown in FIG. 4, debris from the melting process (powder particles, partially melted clumps of powder particles and other residue/condensate from the melting process etc) are carried by the gas flow towards the part of the powder layer that has yet to be melted. This debris forms a surface region or bulge of contaminant 208 that moves along the stripe as the melt progresses. This not only results in a layer of non-uniform thickness being formed because of the different sized particles being deposited on the top of the unmelted powder, but it also reduces the laser power that reaches the powder layer thereby altering the melting conditions of the underlying powder layer. In particular, it has been found that sub-optimum oxidisation of the melted powder occurs and that the process generates a relatively high level of surface roughness and introduces defects etc. The effect shown in FIG. 4 accounts for the poorer quality of the even numbered stripes (S2, S4, S4 etc) shown in the photomicrograph of FIG. 3.

[0056] FIG. 5 shows a powder bed 300 that carries a layer of melted metal 302 and a powder layer 304 that has yet to be melted. The dashed lines 306 illustrate the hatch lines that were generated by the laser scanning unit to melt the powder that now forms the layer of melted metal 302. In this example, the gas flow direction G is opposed to the stripe formation direction L. In other words, there is no component of the gas flow direction that is in the same direction as the stripe formation direction L; i.e. there is no “tail-wind”.

[0057] In this example, the flow of gas in the gas flow direction G acts to blow debris from the melting process away from the powder layer of the stripe that has yet to be melted. This has been found to prevent the formation of a bulge of contaminant as illustrated in FIG. 4. It should be noted that although FIG. 5 illustrates the use of gas flow direction G that is fully opposed to the stripe formation direction L, there may instead be an oblique angle between the gas flow direction G and the stripe formation direction L. Providing such an oblique angle also ensures that the debris is not deposited onto regions of the stripe that have just been melted.

[0058] It should also be noted FIGS. 4 and 5 illustrate embodiments in which the whole width of the stripe is melted by a plurality of full width hatch lines that are formed at successive points along the stripe formation direction L. It is, of course, possible to only melt selected parts of each stripe in order to construct the desired cross-section or pattern of melted material.

[0059] FIG. 6 illustrates the powder layer 400 that is to be selectively melted to form the solidified layer pattern 402. The powder layer 400 is deposited on the powder bed using the powder dispensing and roller system that is described with reference to FIG. 1. Also shown are the gas outlet bar 32 and gas exhaust bar 36 described with reference to FIG. 1 that provide a flow of gas along a gas flow direction G.

[0060] In order to solidify the powder layer 400 to form the solidified layer pattern 402, a plurality of stripes are melted in turn; these stripes are labelled as S1-S12 in FIG. 6. Unlike the prior art process described above with reference to FIG. 2, the stripes illustrated in FIG. 6 formed by moving the hatch line along each stripe in 5 the same direction. In other words, the same stripe formation direction L is used for each of the stripes S1 to S12. In addition, the stripe formation direction L is arranged to differ from the gas flow direction G by the angle θ, which in this example is about 125°. Also shown in FIG. 6 is the angle α between the normal to the gas flow direction G and the stripe formation direction L. In this example, α has a value of around 35°.

[0061] Providing such an angle between the gas flow direction G and the stripe formation direction L means that any debris ejected during the melting process is carried by the flow of gas away from the part of the powder layer that is yet to be melted and also away from any material of that stripe that has just been melted. For example, debris ejected from the surface when melting the point P shown in FIG. 6 is carried along the vector d and away from the stripe S1. This helps ensure that the majority of the debris does not cover powder yet to be sintered and also does not adhere to powder that has recently been melted. The solidified layer pattern 402 formed by the melting process thus has a more uniform (less rough, fewer defects) surface than layers produced using alternating stripe formation directions as per the prior art process described with reference to FIG. 2.

[0062] In addition to each stripe being formed by moving a hatch line along the same stripe formation direction L, the stripes S1 to S12 are preferably formed in a specific order. In particular, the stripes S1 to S12 are preferably formed in reverse order of their proximity to the gas outlet bar 32. In other words, the stripe Si nearest the gas exhaust bar 36 is formed first, then stripe S2 is formed, then stripe S3 etc. Forming the stripes in this order has the advantage that any debris ejected when writing one stripe does not disrupt the powder layer for stripes that have yet to be written. In particular, it can be seen that any debris ejected whilst melting the selected parts of the powder layer within stripe S1 does not get carried in the flow of gas over the stripes S2-S12. This means that a more uniform, substantially debris free, powder layer is present when each stripe is written.

[0063] Referring to FIG. 7, a powder layer 500 is shown that is to be selectively melted to form the solidified layer pattern 502. The powder layer 500 is deposited on the powder bed using the powder dispensing and roller system that is described with reference to FIG. 1. Also shown are the gas outlet bar 32 and gas exhaust bar 36 described with reference to FIG. 1 that provide a flow of gas along a gas flow direction G.

[0064] In this example, the stripes S1 to S12 are again preferably formed in reverse order of their proximity to the gas outlet bar 32. Each stripe is formed by moving the hatch line along each stripe in the same direction. In other words, the same stripe formation direction L is used for each of the stripes S1 to S12. It can also be seen that the stripe formation direction L of FIG. 7 is a reflection of the stripe formation direction L shown in FIG. 6 about the gas flow direction. In other words, the stripe formation direction L is arranged to differ from the gas flow direction G by the angle −θ in FIG. 7. The arrangement of 7 thus has similar benefits to that of FIG. 6.

[0065] In addition to optimising the stripe formation direction L, it should be remembered that each stripe is preferably formed using a series of hatch lines. These hatch lines a formed by scanning a laser spot across the stripe; i.e. the hatch line is formed by moving a laser spot along a line that is perpendicular to the stripe formation direction L. It has further been found that a further improvement to the uniformity of stripe formation can be obtained by altering the hatch line formation process. This will now be explained with reference to FIGS. 8A, 8B and 8C.

[0066] Referring to FIG. 8A, there is shown a prior art method for scanning a laser spot back and forth across the width of a stripe S to form a succession of hatch lines 600 along the stripe formation direction L. The dashed lines 602 illustrate the notional paths at the end of each hatch line 600 that are traversed (with the laser beam deactivated) in order to appropriately position the laser beam ready for formation of the next hatch line. For convenience, the series of hatch lines of FIG. 8A can be termed bidirectional hatch lines.

[0067] The technique of hatch line formation shown in FIG. 8A has the advantage that the successive hatch lines can be formed at high speed because the beam steering optics of the laser scanning unit only need to provide a small amount of (notional) laser beam movement between the end of one hatch line and the start of the next hatch line. It has been found, however, that the direction of hatch line formation relative to the gas flow direction can also affect the quality and uniformity of the layer that is generated from melting the powder layer within a stripe. It has also been found that the non-uniformity caused by this effect increases as the magnitude of the angle α (which is described above with reference to FIGS. 6 and 7) between the normal to the gas flow direction G and the stripe formation direction L reduces.

[0068] Forming the hatch lines by always scanning the laser beam in the same direction across the stripe can thus improve the uniformity of the melted layer. FIG. 8B shows how hatch lines 610 can be formed by always scanning the laser spot from the top to the bottom of a stripe S. FIG. 8C shows how hatch lines 620 can be formed by always scanning the laser spot from the bottom to the top of a stripe S. For convenience, the series of hatch lines of FIGS. 8B and 8C can be termed unidirectional hatch lines. Although the formation of unidirectional hatch lines can improve stripe quality, such an improvement is accompanied by an increase in the time it takes to form a series of hatch lines. In particular, there is an additional delay associated with the scanning optics of the laser scanning unit moving back across the stripe to allow the next hatch line to be formed. It is thus preferable to only use unidirectional hatch lines when they provide a sufficiently significant improvement; e.g. when the magnitude of the angle a between the normal to the gas flow direction G and the stripe formation direction L reduces sufficiently so that the formation of unidirectional hatch lines has a benefit.

[0069] It should also be noted that the direction of formation of the unidirectional hatch lines relative to the stripe formation direction L will depend on the orientation of the stripe being formed relative to the gas flow direction G. For example, the stripes formed in FIG. 6 would benefit from being formed from the unidirectional hatch lines of FIG. 8B whilst the stripes formed in FIG. 7 would benefit from being formed from the unidirectional hatch lines of FIG. 8C. In both cases, the direction of beam movement during hatch line formation is arranged to at least partly oppose the gas flow direction G. This means that the majority of debris associated with powder melting is blown clear of powder within the hatch line that is yet to be melted. The formation of bidirectional or unidirectional hatch lines may thus be varied as required for different stripe orientations relative to the gas flow direction.

[0070] Referring to FIG. 9, the process of constructing a three dimensional object 710 from a plurality of melted layers (700a-700f) is illustrated. Each layer 700a-700f may be formed by a process that involves selectively melting each layer using multiple stripes, each stripe of one layer being formed along a common stripe formation direction L. The stripe formation direction L may vary between layers, but it is preferred that the gas flow direction is always at least partially opposed to the stripe formation direction for each layer. In this manner, the benefits of the present invention are obtained for each layer in a three dimensional object. The use of different stripe formation directions L for each layer may also mean that certain layers can be formed using bidirectional hatch lines whilst other layers are formed using unidirectional hatch lines. The benefits of altering the stripe formation direction between layers has also been described previously in U.S. Pat. No. 8,034,279 (EOS). A difference in stripe formation direction between adjacent layers of at least 30° is used in this example, but other different angles may be implemented. For example, the rotation angle between adjacent layers may be more than 40° or it may be 67°. The rotation angle between adjacent layers is preferably less than 80°. Again, it should be noted that it is preferred that each layer (or at least the majority of the layers) meets the requirement that the gas flow direction is always at least partially opposed to the stripe formation direction.

[0071] It should also be noted that although the melting process may take place by advancing hatch lines along a stripe, there may be other processing steps that do not requires such tight control over the stripe formation direction relative to the gas flow direction. For example, the laser scanning unit may perform a contour scan before and/or after a layer has been melted by advancing hatch lines along a stripe. The contour scan may simply scan the laser beam around the contour of an object to re-melt and solidify the metal to improve surface quality. In such a contour scan the path of the laser beam spot may take on any orientation relative to the gas flow direction. This has not been found to have a detrimental effect because the amount of debris generated by such a contour scan is minimal.

[0072] Referring to FIG. 10, it is also illustrated how the present invention can be applied to layer formation using stripe segments. In particular, FIG. 10 illustrates a powder layer 800 that is to be melted to form a desired pattern (not shown). The powder layer 800 may be deposited using the powder dispensing and roller system described with reference to FIG. 1. As described previously, a planar flow of gas is provided over the powder layer along the direction G.

[0073] The desired pattern is written to the powder layer 800 by dividing the layer into a plurality of stripe segments 802. Each stripe segment 802 is formed separately. The stripe segments may be written in any order. As also shown in FIG. 10, the stripe formation direction L may be different for the different stripe segments (although it may be the same). However, the gas flow direction G is always at least partially opposed to the stripe formation direction L of each stripe segment.

[0074] The stripe segments shown in FIG. 10 are equally sized and regularly arranged in a grid or checkerboard type pattern. It is, however, possible for the stripe segments or sections to be different shapes and/or sizes. The stripe segments may also be spaced irregularly. For example, stripe segments may be provided as shells or islands formed in local areas on the substrate. The stripe segments or sections may also comprise any of the arrangements described previously in US2005/0142024 (but with the gas flow direction controlled relative to the stripe formation direction in accordance with the present invention).

[0075] The skilled person would also recognise the other variations and additions to the technique of the present inventions that are possible.