NL2025698B1 - Additive manufacturing method of making a workpiece using selective laser melting of a metal powder and metal powder for use in said method - Google Patents

Abstract

The invention relates to an additive manufacturing method of making a workpiece using selective laser melting of a metal powder and metal powder for use in said method. The method of Hakim; a workpiece comprises 5 the steps of: providing' a layer of metal powder in a build chamber; directing one or more energy beams to selectively irradiate the layer of metal powder in a 10 pattern. corresponding' to a cross—sectional layer‘ of the workpiece; wherein the metal powder having a particle size distribution in which a ratio DL/Ds between a first diameter of the largest particles DL and a second diameter 15 of the smallest particles Ds in the metal powder is in a range such that l í DL/Ds < 5.

Description

No. P138498NL00 Additive manufacturing method of making a workpiece using selective laser melting of a metal powder and metal powder for use in said method.

BACKGROUND The invention relates to an additive manufacturing method of making a workpiece using selective laser melting of a metal powder. The invention further relates to a powder for use in such an additive manufacturing method.

Selective laser melting (SLM) is one of the most promising metal additive manufacturing technologies, offering a unigvue capability of printing geometrically complex products. Selective laser melting is also referred to by terms as ‘Laser powder bed fusion (LPBF)’, ‘direct metal laser melting (DMLM)’, and \‘3-D printing’, which terms are treated as synonyms for purposes of the present invention.

One known type of an additive manufacturing machine includes a build chamber that encloses a powder bed, and a laser for providing a beam of radiant energy, wherein beam of radiant energy is directed onto the powder bed to selectively melt and/or fuse at least a part of the powder particles to form a workpiece.

As described in Us 2018/0178286 Al the interaction of the radiant energy beam with the metal powder causes vaporization of the powder, generating a plume which originates in the vicinity of the melt pool.

The plume is the condensate of this vaporized metal powder. As indicated in US 2018/0178286, the presence of the condensate can have detrimental effects on the additive manufacturing process, for example by blocking Oor scattering of the radiant energy beam from the laser. These effects can prevent rapid beam scanning or the use of multiple beams.

Furthermore, part of the plume may precipitate on the powder bed or workpiece adjacent to the melt pool. This may contaminate the powder bed and/or result in the presence of undesirable material on the workpiece.

SUMMARY OF THE INVENTION It is an object of the present invention to at least partially solve at least one of the above identified disadvantages and/or to provide an alternative powder material and application of this powder material, which at least reduces the formation of a plume in the vicinity of the melt pool.

According Lo a first aspect, the present invention provides an additive manufacturing method of making a workpiece comprising the steps of: providing a layer of metal powder in a build chamber; directing one Oor more energy beams to selectively irradiate the layer of metal powder in a pattern corresponding to a cross-sectional layer of the workpiece; wherein the metal powder having a particle size distribution in which a ratio D:/Ds between a first diameter of the largest particles D; and a second diameter of the smallest particles Ds in the metal powder is in a range such that 1 £ D:/Ds < 5.

The inventor has found that when using metal powder with a ratio D:/Ds in this range, the creation of a plume can at least be reduced, preferably the creation of a plume can at least substantially be prevented.

The present invention is based on the insight that the energy level of the one or more energy beams must be high enough in order to melt the largest particles of the metal powder during the scanning of the one or more energy beams over the layer of metal powder. Smaller particles in the layer of metal powder require less energy to melt, but they are irradiated with at the same energy level as the largest particles. When the smaller particles are much smaller than the largest particles, these smaller particles may receive so much energy that they reach the boiling point and at least partially evaporate, which is believed to result in the formation of the plume.

By using a metal powder with a relatively narrow particle size distribution, an energy level which is high enough to melt the largest particles, will also melt the smallest particles, but the smallest particles do not receive so much energy that evaporation occurs, and the formation of the plume can at least be reduce or can be prevented.

A further advantage of the method according to the present invention is, that spatter generation can at least partially be reduced, preferably that spatter generation can at least substantially be prevented. Spatter generation is a by-product of evaporation.

It is noted that the preferred range is also dependent on the metal used. The above range is in particular suitable for a stainless steel metal powder.

In an embodiment, the metal powder has a particle size distribution in which a ratio D:;/Ds is in a range such that 1 £ D;/De S 4, preferably in a range such that 1 £ D:/Ds £ 2, more preferably in a range such that 1 £ D:/Ds £ 1,5. A smaller difference between the diameter of the largest particles D; and the smallest particles Ds results in a more evenly heating of the powder particles, and thus allows a more controlled fusion of the powder particles for forming the workpiece.

In an embodiment, the particles of the metal powder have a diameter in a range from 5 micrometers to 100 micrometers, preferably in a range from 10 micrometers to 50 micrometers. Within this diameter range, a metal powder suitable for the method of the present invention preferably comprises a particle size distribution having a width equal or smaller than 20 micrometers, preferably equal or smaller than 10 micrometers, more preferably equal or smaller than 5 micrometers.

A disadvantage of metal powder with such a narrow distribution is, that they are very expensive. Commonly, metal powders for 3D-printing are manufactured in a production process which yield metal powders with particle sizes ranging from approximately 0 to approximately 150 micrometers. When using metal powders with a particle size ranging from 10 to 50 micron for the additive manufacturing, only 30 to 50% of the metal powder from the production process can be used. When using metal powders with a very narrow distribution, this yield is even further reduced. Accordingly, in an embodiment, the method comprises the steps of: providing a raw metal powder material, separating the raw metal powder material in a series of different fractions, each fraction having a particle size distribution with a ratio Ds/D;, according to the present invention and/or having a particle size and a particle size distribution with a width according to the invention, and using one fraction of said series of different fractions individually for providing the layer of metal powder in the build chamber. Accordingly, less expensive metal powder with a large difference between the smallest and largest powder particles can be used, which less expensive metal powder must be separated in different fractions by using a separation method, for example using an assembly of screening devices.

Preferably, the raw metal powder material is separated in the series of different fractions 5 using an assembly of screening devices as described in the Dutch Patent Application 2025437, which is incorporated herein by reference.

By using only one fraction from said series of different fractions, this part of the less expensive metal powder can be used in the method of the present invention.

One of the other fractions from said series of different fractions can individually be used for the making of a subsequent layer of the workpiece or for the making of a different workpiece.

By using the different fractions individually, a large percentage of the less expensive metal powder can be used in the method of the present invention.

In an embodiment, the method further comprises the step of: adjusting the energy level of the one or more energy beams when changing from a first fraction of said series of different fractions to a second specific fraction of said series of different fractions, wherein the first fraction is different from said second fraction.

By adjusting the energy level to a specific fraction of said series of different fractions the energy can be selected to be suitable for this specific fraction, that is that the particles melt as desired and at least substantially do not reach the boiling point.

In an embodiment, the method further comprises the step of: adjusting the energy level of the one or more energy beams to a level such that the particles of the metal powder melt only partially.

By adjusting the energy lever such that the particles of the metal powder only melt partially, a material with an open and/or porous structure can be obtained. By using a metal powder with a particle size distribution with a ratio Dg/D, according to the present invention and/or having a particle size and a particle size distribution with a width according to the invention, the arrangement and/or packing of the particles in the layer of metal powder in the build chamber is much more regular and predictable. Accordingly, the method of the present invention allows to obtain a better defined porous material, in particular wherein the porosity is a direct consequence of the selected particle size(s) and the packing or stacking of said particles in the layer of metal powder in the build chamber.

According to a second aspect the present invention provides a metal powder for use in an additive manufacturing method, wherein the metal powder having a particle size distribution in which a ratio D;/D«s between a first diameter of the largest particles D; and a second diameter of the smallest particles Ds in the metal powder is in a range such that 1 £ Dy/Dg < 5. In an embodiment, the metal powder has a particle size distribution in which a ratio D:/Ds is in a range such that 1 £ D:;/Ds £ 4, preferably in a range such that 1 S D:/Ds S 2, more preferably in a range such that 1 S D:/Ds £ 1,5. In an embodiment, wherein the particles of the metal powder have a diameter in a range from 5 micrometers to 100 micrometers, preferably in a range from 10 micrometers to 50 micrometers. In an embodiment, the metal powder for use in the method comprises a particle size distribution having a width equal or smaller than 20 micrometers, preferably equal or smaller than 10 micrometers, more preferably equal or smaller than 5 micrometers The various aspects and features described and shown in the specification can be applied, individually, wherever possible. These individual aspects, in particular the aspects and features described in the attached dependent claims, can be made subject of divisional patent applications.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be elucidated on the basis of an exemplary embodiment shown in the attached drawings, in which: Figure 1 is a schematic cross-section of a part of an additive manufacturing machine, and Figure 2 schematically shows a distribution of particles PD of a metal powder as typically obtained by a powder production process.

DETAILED DESCRIPTION OF THE INVENTION Figure 1 shows a schematic cross-section of a part of an additive manufacturing machine 1. In order to produce an workpiece, in particular a metallic workpiece, of the present invention, a metal powder 2 is provided in a layer 3 with a substantially flat upper side 4. The metal powder may for example be a Stainless Steel powder comprising powder particles with a diameter in a range of 10 - 50 um. In order to selectively melt said Stainless Steel metal powder 2, the additive manufacturing machine 1 comprises a source 5 which produces an energy beam 6 which is focussed down to a spot size of approximately 100 micrometres. The focus position is arranged at or near the upper side 4 of the layer 3 of metal powder.

It is noted that the source 5 may comprise any source which can produce an energy beam 6 for heating the metal powder 2. For example, the source 5 may be a charged particle beam source, for generating for example an electron beam or an ion beam. Preferably, the source 5 comprises a laser for generating a laser beam, for example a Nd-YAG laser or a diode laser selected in order to provide a laser beam with an energy level suitable for melting the metal powder 2. When the source 5 comprises a laser, the process is referred to as selective laser melting (SLM).

The source 5 and/or the energy beam 6 is scanned XY over the flat upper side 4 of the layer 3 of Stainless Steel metal powder 2 and may be switched on and off in order to selectively irradiate the layer 3 of metal powder 2 in a pattern corresponding to a cross-sectional layer of the workpiece 9. It is noted that the energy beam 6 can be scanned over the plane spanned by the upper side 4 of the layer 3.

As schematically shown in figure 1, the layer 3 of metal powder 2 is arranged on top of a building platform 7 arranged in a build chamber 10. In a first step of the production process, the building platform 7 is covered by a first layer of metal powder 2. In a second step, the energy beam 6 is scanned over the layer to selectively expose the layer of metal powder at the positions where the first layer of the workpiece 9 needs to be created. In a third step the building platform 7 is lowered, in particular along a vertical direction Z, and a new layer of metal powder 2 is arranged on top of the first layer. By subsequently repeating the second and third steps, the workpiece 9 is built up layer by layer, until the workpiece is completely printed. The completely printed workpiece and the building platform 7 are then removed from the build chamber 10 and the workpiece is separated from the building platform 7. If necessary, the workpiece can be subjected to a post-treatment and/or a finishing process.

When using, for example, a laser power of 150 Watt, a scanning speed of 1000 mm/s, a distance between adjacent scanning lines as scanned by the laser of 0,1 mm, and positioning the upper side 4 of the layer 3 in the focus of the laser beam 6, a substantially impervious or solid Stainless Steel workpiece can be produced. Typically, the inventor has found that when providing an energy density of approximately 70 Joule/mm: or higher to a bed of Stainless Steel powder with a particle diameter in a range between 10 pm and 65 um, substantially impervious or solid Stainless Steel material is obtained. However, during the irradiation of the metal powder, typically a vapour plume and spatter generation at or near the position where the energy beam 6 irradiates the upper side 4 of the metal powder 3.

During an SLM process the metal powder particles are irradiated by a laser beam, which heats up the particles such that these particles melt and fuse together. However, the particles of the metal powder typically have different diameter sizes as indicated above. Since the volume of a spherical particle scales with the diameter to the power of three, a particle with a diameter of 65 um has about 275 times the volume compared to a particle with a diameter of 10 um. If we assume both particle have the same density, the particle with a diameter of 65 um has about 275 times the mass of a particle with a diameter of 10 pm. Accordingly, the smaller particles with much less mass, will heat up much faster than the larger particles.

In order to manufacture a solid work piece, the energy level of the laser must be sufficiently high so that even the largest particles substantially melt during irradiation by the laser. However, the smaller particles receive so much energy that they not only melt, but such that their temperature reaches the boiling point and (at least partially) evaporate. The Plume, which is visible above the position where the laser irradiates the metal powder, is not burned material, but is a condensate of the evaporated metal.

This condensate of the metal vapour comprises extremely small particles (nano particles) which absorb light and which give the plume a dark or black appearance.

Due to the dark or black appearance, the plume is often mixed up with black smoke for a combustion.

However, the SLM process is commonly performed in an oxygen-free and/or inert atmosphere in the build chamber 10, which prevents any combustion taking place in the build chamber.

In addition to the plume, also spatter generation around the melt pool occurs.

The generation of a plume and of spatters result in a number of undesirable effects: a.

The condensated nano particles are very harmful for humans. b.

The nano particles are very incendiary and my not come into contact with oxygen, at least not in a concentrated form.

Such nano particles must be collect using special filters, which need to be removed and handled separated from oxygen, which makes the removal and handling cumbersome and dangerous. c.

The plume 1s commonly removed from the build chamber using extraction and an inert gas flow in the build chamber.

However, during this process, the plume also travels over a part of the building platform.

This may cause problems when using multiple energy beams to process different parts of the layer of metal powder simultaneously.

In order to substantially prevent that an energy beam crosses the path of a plume, complex algorithms are developed, which algorithms lead to very complex scanning patterns and scanning limitations. d.

Nano particles which have not been removed and stay floating in the build chamber, for example as an aerosol, may also lead to disturbing the energy beam,

even outside the plume.

e. A part of the plume material precipitates on the metal powder layer and on the already produced parts of the workpiece. The precipitated plume material contaminates the metal powder. According any non-used metal powder cannot be re-used, and must at least be cleaned to remove the precipitated plume material.

Also the generation of spatter is typically for the SLM process and may be an indicator of how the process proceeds, but is actually very much undesired. On the one hand, an operator can establish from the way the spatters Jump away from the powder layer how and how much energy is provided to the metal powder and how stable the process is running. On the other hand, the spatters produce particles which are much larger than the nano particles, and may be as large as the powder particles or even larger, precipitate on the surrounding metal powder and contaminates the metal powder and the already produced parts of the workpiece. These spatter of large particles may lead to disadvantages such as inclosing of such large particles in the workpiece, irregular thickness of a new layer of metal particles, inhomogeneous material structures in the workpiece, etc.

According to the present invention, the plume and/or spatter can at least largely be reduced or even circumvented by using metal powder having a particle size distribution in which the ratio Dy/Dg between a first diameter of the largest particles D: and a second diameter of the smallest particles Ds in the metal powder is in a range such that 1 S D:/Ds < 5. This can be obtained by using a metal powder in which the largest particles have a diameter of 50 micrometers and the smallest particles have a diameter of more than 10 micrometers. However, the effect of reducing the plume and/or the spatter generation, the difference between the diameter of the largest particles and the diameter of the smallest particles is preferably smaller. This result in a metal powder having a particle size distribution in which the ratio D:/Dsg is in a range such that 1 £ D./Dg < 4, preferably in a range such that 1 £ D:/Ds £ 2, more preferably in a range such that 1 < D:/Ds S 1,5.

When this ratio D:/Ds is 1, all particles of the metal powder have the same diameter. The laser can be set so that the energy density of the laser beam at the metal powder is such that the supplied energy is just sufficient in order to fully melt the particles of the metal powder and for the formation of a melt pool so that the melted metal powder can merge with the part of the workpiece underlying the layer of metal powder. Because the particles have the same size, they essentially reach the same temperature, well below the boiling temperature of the metal. Accordingly, the formation of a plume and the generation of spatter does not occur.

However, a metal powder comprising particles which all have substantially the same size are difficult to manufacture and accordingly are very expensive. Figure 2 schematically shows a distribution of particles PD of a metal powder as typically obtained by a powder production process.

Using an assembly of suitable screening devices, the raw metal powder material with the distribution PD can be separated in a series of different fractions Fl - F7, wherein the first fraction Fl comprises particles having a diameter in a range from 10 to 20 micrometer, the second fraction F2 comprises particles having a diameter in a range from 20 micrometers to 30 micrometers, … , a seventh fraction comprising particles having a diameter in a range from 70 micrometers to 80 micrometers.

Although the raw material has a broad size distribution (D:;/D¢ is larger than 80/10 = 8) the inventor has realized that each fraction Fl — F7 individually, has a particle size distribution with a ratio D:/Ds according to the present invention and/or having a particle size and a particle size distribution with a width according to an embodiment of the present invention: F3 So7a0 = 125 80/70 = 1.14 Accordingly, using metal powder of only one fraction (for example F3) of said series of different fractions Fl - F7 individually for providing the layer of metal powder in the build chamber 10, will at least substantially prevent the generation of a vapor plume and/or spatters.

It is noted that the raw material can also be divided in factions with the same D;/Ds. For example, if we take a D:/Ds = 2, the raw material of figure 2 can be divided into three fractions Fl’, F2’, F3’ as follows: It is noted that an assembly of powder particles with substantially the same size can be arranged in the powder layer in a regular stacking arrangement. By adjusting the energy lever such that the particles of a selected one of the fractions Fl - F7 of metal powder only melt partially, a material 8 with an open and/or porous structure can be obtained. Accordingly, the method of the present invention allows to obtain a better defined porous material, in particular wherein the porosity is a direct consequence of the particle size(s) of the selected fraction Fl — F7 and the packing or stacking of said particles in the layer of metal powder in the build chamber.

It is to be understood that the above description is included to illustrate the operation of the preferred embodiments and is not meant to limit the scope of the invention. From the above discussion, many variations will be apparent to one skilled in the art that would yet be encompassed by the scope of the present invention.

Claims (10)

CLAIMS An additive manufacturing method for manufacturing a workpiece comprising the steps of: providing a layer of metal powder in a build chamber; directing one or more energy beams to selectively illuminate the layer of metal powder in a pattern corresponding to a cross-sectional layer of the workpiece; wherein the metal powder has a particle size distribution in which a ratio D i /Dg between a first diameter D : of the largest particles and a second diameter D s of the smallest particles in the metal powder is in a range such that 1 D : /Ds 5. The additive manufacturing method according to claim 1, wherein the metal powder has a particle size distribution wherein the ratio D:/Ds is in a range such that 1 D:/Ds S 4 , preferably in a range such that 1 SD:/Ds S2, more preferably, is in a range such that 1 D i /De S 1.5. The additive manufacturing method according to claim 1 or 2, wherein the particles of the metal powder have a diameter in a range from 5 micrometers to 100 micrometers, preferably in a range from 10 micrometers to 50 micrometers. The additive manufacturing method according to claim 3, wherein the metal powder for use in the method has a particle size distribution with a distribution width less than or equal to 20 micrometers, preferably less than or equal to 10 micrometers, more preferably less than or equal to 5 micrometers. The additive manufacturing method according to any one of claims 1-4, wherein the method further comprises the steps of: providing raw metal powder material, separating the raw metal powder material into a series of different fractions, each fraction having a particle size distribution with a ratio Dg/D:; according to claim 1 or 2 and/or having a particle size and a particle size distribution having a width of the distribution according to claim 3, and using one fraction of the set of different fractions individually to provide the layer of metal powder in the build chamber . The additive manufacturing method according to claim 5, wherein the method further comprises the step of: adjusting the energy level of the one or more energy beams when passing from a first fraction of the series of different fractions to a second specific fraction of the series of different fractions, the first fraction being different from the second fraction. The additive manufacturing method according to any one of claims 1 to 6, wherein the method further comprises the step of: adjusting the energy level of the one or more energy beams to a level such that the particles of the metal powder only partially melt. A metal powder for use in an additive manufacturing method, wherein the metal powder has a particle size distribution wherein a ratio D:/Ds between a first diameter Dp of the largest particles and a second diameter Ds of the smallest particles in the metal powder in a range lies such that 1 SD:/Ds < 5. The metal powder according to claim 8, wherein the metal powder has a particle size distribution wherein the ratio D:/Ds is in a range such that 1 D i/Ds 4, preferably in a range such that 1 D :/Ds < 2, more preferably is in a range such that 1 D 2 /Dg 1.5. The metal powder according to claim 8 or 39, wherein the particles of the metal powder have a diameter in a range from 5 micrometers to 100 micrometers, preferably in a range from 10 micrometers to 50 micrometers, wherein the metal powder for use in the method has a particle size distribution having a distribution width less than or equal to 20 micrometers, preferably less than or equal to 10 micrometers, more preferably less than or equal to 5 micrometers. -0-0-0-0-0-0-0-0-