DE102019208836B4 - Process for the production of complex three-dimensional components - Google Patents

Abstract

Method for producing complex three-dimensional components (6), in which powder layers are formed one above the other on a stepwise lowerable platform and are applied to a respective uppermost powder layer (13)
by means of a rotating axis aligned parallel to the surface of the uppermost powder layer (13) and thereby
a roller (4) consisting of graphite or formed with graphite, which is moved translationally along the surface of the uppermost powder layer (13), compresses the respective uppermost powder layer (13) in a contact area (18) which is arranged between the mutually facing surfaces of the surface of the respective uppermost powder layer (13) and the roller (4) with a contact force of 10 ⁰ N to 10 ² N per cm width of the uppermost powder layer (13) and in the process reduces the layer thickness of the respective uppermost powder layer (13) and
at the same time, the uppermost powder layer (13) within the contact area (18) is heated by an electric current with an electrical voltage of 0.01 V to 10 V and an electrical current intensity of 10 ¹ A to 10 ³ A per cm width of the uppermost powder layer (13) flowing through the roller (4), which is connected to an electrical voltage source, and
during the translational movement of the roller (4) after the contact area (18) a locally defined deflected laser or electron beam (5.1)
for carrying out a selective laser beam or electron beam melting process onto the surface of the compacted and preheated respective uppermost powder layer (13); wherein an electric current flow from the roller (4) through the contact area (18) and the powder layers is achieved by connecting the platform on which the powder layers are successively formed one above the other to a pole of the electrical voltage source or to
connected to earth potential and the powder layers are formed with an electrically conductive material and
the electrical current flow is carried out in a pulsed manner, with a pulse duration and pause times in between.

Description

The invention relates to a method for producing three-dimensional components with complex shapes.

New methods for producing three-dimensional components from metal powder are the laser beam melting (LBM) and electron beam melting (EBM) processes. What both processes have in common is that a computer model of the component is first divided into individual slices, and these slices are then produced successively by spreading a layer of powder of a defined height onto a substrate and then melting it locally using a laser or electron beam. The unirradiated powder in the surrounding area remains unaffected or is only very weakly bonded and can therefore be easily removed later. The process is continued with the further application of a powder layer and the melting of the surface, which may be modified to suit the workpiece geometry, and is repeated as often as necessary until the entire three-dimensional component is built up layer by layer.

• Speziell für das EBM-Verfahren ist das Vorheizen des Pulverbettes ein notwendiger Schritt für die Prozessstabilität und abhängig vom zu verarbeitenden Werkstoff sehr zeitaufwendig. Wenn der Elektronenstrahl auf eine lose Pulverschicht trifft, kommt es zur Ladungskonzentration in den Pulverpartikeln und als Folge dessen zu deren Abstoßung untereinander. Dies führt dazu, dass in der Anlage eine Pulverwolke (sog. „smoke“) entsteht und der Aufbauprozess abgebrochen werden muss.

Although LBM and EBM offer a multitude of new possibilities in terms of resource efficiency and geometric diversity and have great potential for cost reduction in complex structured components (especially when they are made of materials that are difficult to machine), these powder bed-based jet melting processes have the following limitations, among others:

• Preheating the powder bed is a necessary step for process stability, especially for the EBM process, and depending on the material being processed, it can be very time-consuming. When the electron beam hits a loose layer of powder, the charge concentrates in the powder particles, causing them to repel each other. This leads to a powder cloud (so-called "smoke") forming in the system, and the build-up process must be aborted.

In US 8,187,521 B discloses a solution to the problem of smoke formation. Based on the fact that the charge distribution in the powder particles depends on the choice of process parameters, such as beam current, scan speed, electron speed (specified by the acceleration voltage), as well as material properties such as electrical conductivity, an additional process step is described during which the powder layer is heated across the surface and as homogeneously as possible using an electron beam. This reduces the electrical conductivity as a function of temperature. However, this is offset by a strong increase in electrical conductivity when slight sintering occurs between the powder particles, which prevents electrostatic charging. This preheating step is an integral part of the process control in commercially available EBM systems to ensure the stability of the manufacturing process. However, this type of preheating increases the process time and is energy-intensive.

In the LBM process, preheating the powder bed with the laser beam is not possible and is currently implemented exclusively by heating the build plate. This creates a large temperature difference in the build direction as the build height increases. The limited preheating temperature leads to very high cooling rates after the selective melting process and, consequently, to residual stresses in the component.

The disadvantage of alternative heating methods such as thermal radiation from infrared or resistance heaters is that heating by radiant heating is extremely slow, particularly due to the initially very low thermal conductivity of the powders. This leads to long process times and poor reproducibility in the individual layers.

So from WO 2011/ 001 270 A2 a device and a method for layer-by-layer production of a three-dimensional object are known.

WO 87/ 07 538 A1 relates to an apparatus and a method for layer-by-layer production of three-dimensional integral objects.

Possibilities for additive manufacturing with direct resistance heating of a workpiece are WO 2018/ 194 481 A1 described.

DE 10 2015 201 796 B3 discloses a powder application unit for applying and processing, in particular, dry powder.

It is therefore an object of the invention to provide possibilities for the production of three-dimensional components by means of LBM or EBM, which lead to a reduction in the required production time while at the same time maintaining good quality.

According to the invention, this object is achieved by a method having the features of claim 1. Advantageous embodiments and further developments of the invention can be realized with features defined in subordinate claims.

In the method according to the invention, powder layers made of an electrically conductive material are formed layer by layer on top of one another on a gradually lowerable platform. The respective uppermost powder layer is pressed together by means of a roller made of graphite or with graphite, which rotates about an axis of rotation aligned parallel to the surface of the uppermost powder layer and moves translationally along the surface of the uppermost powder layer, in a contact area arranged between the mutually facing surfaces of the respective uppermost powder layer of the roller, with a contact force of 10 ⁰ N to 10 ² N per cm width of the uppermost powder layer, thereby reducing the layer thickness of the powder layer. At the same time, the respective uppermost powder layer is heated within the contact area by an electrical current flowing with an electrical voltage of 0.01 V to 10 V and an electrical current of 10 ¹ A to 10 ³ A per cm width of the uppermost powder layer through the roller, which is connected to an electrical voltage source.

In the direction of movement of the roller after the contact area, a locally defined deflected laser or electron beam is directed onto the surface of the compacted and preheated respective uppermost powder layer to carry out a selective laser beam or electron beam melting process.

During each translational movement of the roller in at least one axial direction around its rotational axis, a top layer of powder can be applied, and a component with a three-dimensional surface can be successively built up. The powder layers can be applied using a conventional powder conveying device and, if necessary, a doctor blade. The thickness of each powder layer should be maintained as consistently as possible.

• - Die Walze kann gleichzeitig mit Pulvervorrat oder Rakel bewegt werden oder der Walzvorgang findet separat statt
• - Der Pulvervorrat kann mitgeführt werden oder das Pulver wird von der Rakel aus Vorratsbehältern an den Rändern abgeholt
• - Die Walze kann auch ohne eine Rakel verwendet werden

Powder application variants:

• - The roller can be moved simultaneously with the powder supply or doctor blade or the rolling process can take place separately
• - The powder supply can be carried along or the powder is picked up by the doctor blade from storage containers at the edges
• - The roller can also be used without a squeegee

An electrical current flow from the roller through the contact area and the powder layers is achieved by connecting the roller to a pole of the electrical voltage source or to ground potential, and by forming the powder layers from or with an electrically conductive material. This allows electrical current to flow from the roller through the powder layers compacted in the contact area. This can lead not only to the direct heating of the respective uppermost powder layer, but also to further heating of the compacted uppermost powder layer through the heat release in the contact area and to improved bonding, even to the point of sintering, of the powder particles to one another.

The electrical current flow is pulsed. This allows for preferential heating of the contact area between powder particles, thus achieving sintering at a reduced average temperature of the powder layer and the component.

In pulsed operation, a pulse duration and pause time are maintained, preferably in the range of 1 ms to 500 ms.

In addition, a pause time longer than 10 ms can be maintained after a specified number of pulses, for example, at least 10 with appropriate pause times in between. A longer pause time should preferably be at least 50 ms.

The preferably pulsed electric current creates a bond between the powder particles and between the powder particles and the substrate/previous powder layer. This prevents movement of the powder particles (e.g., due to convection of heated gases). Furthermore, electrical conductivity increases, preventing charging during EBM by the electron beam. Plastic deformation and potentially molten volume in the contact between the powder particles are minimal, allowing the powder to be reused later.

Especially when the pulse duration is extended, instead of limiting heat release to the contact area between the powder particles, the powder bed can be heated throughout its entire volume. This reduces the cooling rate after selective melting using EBM/LBM, enabling the creation of isotropic microstructures and microstructure fineness, and reducing or preventing residual stresses in the component.

Instead of a pulsed direct current, a constant direct current or alternating current can also be used.

The pulse patterns of the pulsed electrical current can also be combined into pulse blocks. For example, 2 ms pulse, 2 ms pause, Repeat the whole thing 10 times and then take a 50 ms break.

The invention will be explained in more detail below by way of example.

Features can be combined independently of the respective example and independently of a graphic representation in the figures.

• 1 in schematischer Darstellung eine Möglichkeit zur Durchführung des erfindungsgemäßen Verfahrens;
• 2 eine Detaildarstellung von Stromfluss und Temperaturverteilung im Kontaktbereich;
• 3 eine vergrößerte Detaildarstellung eines Kontaktbereichs und
• 4 ein Beispiel für eine bei der Erfindung einsetzbare Walze.

Showing:

• 1 a schematic representation of a possibility for carrying out the method according to the invention;
• 2 a detailed representation of current flow and temperature distribution in the contact area;
• 3 an enlarged detailed view of a contact area and
• 4 an example of a roller that can be used in the invention.

For the production of components 6, as in 1 As shown, the powder is applied from a powder storage container 1 by means of a doctor blade 2 as a uniformly thick uppermost powder layer 13 onto a surface of a platform or a surface of a previously formed, already compacted, heated, and locally defined powder layer with a laser or electron beam 5.1 from a laser or electron beam source 5. By means of a rotating and translationally moved roller 4, which consists, for example, of graphite or is formed with graphite, the pressing force F applied to the uppermost powder layer 13 by means of the roller 4 results in a pre-compaction of the uppermost powder layer 13 from the height h ₀ 14 to h ₁ 15, as in 2 shown. With the preferably pulsed direct current I flowing through the roller 4, a preferably partial sintering of particles with which the uppermost powder layer 13 is formed takes place. The roller 4 can be driven by the frictional forces between the surfaces of the roller 4 and the uppermost powder layer 13 as a result of the pressing force F or by an additional motor.

The roller 4 is connected to an electrical voltage source (not shown). A defined, locally deflectable laser or electron beam 5.1 is directed from the laser or electron beam radiation source 5 onto the surface of the pre-compacted and heated uppermost powder layer 13 in the direction of movement of the roller after the contact area 18 in order to implement an additive laser/electron melting process. The focal spot of the laser or electron beam 5.1 migrates along a defined contour on the surface of the uppermost powder layer 13 in a region that is located behind the contact area 18 in the direction of movement of the roller 4 during its translational movement.

2 shows the heating of the uppermost powder layer 13 and the roller 4 as a result of the electric current flowing through the roller 4 and the uppermost powder layer 13 in the contact area 18. The area 17 is preferentially heated because, firstly, the flowing electric current 16 has to pass through a narrow point here and the electric current density is consequently increased, and secondly because the uppermost powder layer 13 has a high specific electrical resistance depending on the pressing pressure. The extension of the area 17 in the direction of the rotational axis of the roller 4 is determined by the propulsion (circumferential speed 19 of the current-carrying roller 4 and the feed rate of the translational movement of the roller 4 along the surface of the uppermost powder layer 13) and the thermal conductivity of the roller 4 or the component 6.

The extension of the heated contact area 18 in the plane of the uppermost powder layer 13 and in the rolling direction 20 in 2 is controlled via the radius of the current-carrying roller 4, the powder layer thickness h ₀ 14 and the pressing pressure acting between the surface of the uppermost powder layer 13 of the roller 4 on the uppermost powder layer 13.

The generation of the pulsed electrical current, preferably direct current, occurs as described in the field-activated sintering (FAST, also referred to as Spark Plasma Sintering, SPS) process. Within the uppermost powder layer 13, the heat is released preferentially at the contact points of the powder particles 21 (see 3 ). This means that heating takes place where contact is to be formed and sintered between the powder particles 21 in order to increase electrical conductivity, thermal conductivity and to create adhesion between the powder particles 21. The heat release is proportional to the electrical resistance of the material through which the current flows and the square of the electrical current intensity. The advantage of a pulsed electrical current here is that a very high electrical current can flow during the short pulse duration, which determines the heating of the powder particles 21 in their microscopic contact areas 25 where the powder particles 21 are in contact. The extent of the area 25 is determined by the pulse pattern and the thermal conductivity of the powder particles 21. The heating of the entire system, on the other hand, remains limited because this is determined by the equivalent direct current intensity I _RCS . $$\left(I_{RCS}=\sqrt{\frac{Pulsdauer}{Pulsdauer+Pausendauer}I}\right)$$

Once the current-carrying roller 4 has passed a compacted and heated surface area of the uppermost powder layer 13 during its rotation and translational movement, sufficient adhesion and electrical conductivity are achieved for the powder particles 21 in the compacted area 15 with the reduced layer thickness h _1. The respective material with which the component 6 is to be manufactured can then be selectively melted using the electron or laser beam 5.1 (see Figure (1)).

Variants

• • Instead of a pulsed direct current, a constant direct current or alternating current can also be used.
• • The pulse patterns of the pulsed electrical current can also be combined into pulse blocks (e.g. 2 ms pulse, 2 ms pause, the whole thing repeated 10 times and then a 50 ms pause is observed).
• • From a storage container 1, a respective uppermost powder layer 13 with a powder layer thickness of 10 µm to 1000 µm, preferably 50 µm to 150 µm, is applied via the doctor blade 2. In this specific example, a layer thickness of 150 µm is applied.
• • The powder used is preferably spherical. The average particle diameter d ₅₀ should be between 10 µm and 200 µm, preferably between 50 µm and 150 µm. In this specific example, powdered tool steel 1.2343 with an average particle size d ₅₀ of 50 µm was used to form powder layers 13.

• • Abmessungen der Walze 4: Radius R = 10 mm bis 1000 mm beim konkreten Beispiel: Radius 50 mm, Breite 10 mm bis 1000 mm beim konkreten Beispiel 50 mm. Die Breite der Walze 4 entspricht im Beispielfall (aber nicht zwingend) der Breite der obersten Pulverschicht 13
• • Anpresskraft: 10⁰ N bis 10² N je cm Breite der obersten Pulverschicht 13 (im Beispiel 20 N mal 50 mm = 100 N)
• • Die Vorschubgeschwindigkeit der translatorisch bewegten Walze 4 soll im Bereich 0,1 cm/s bis 1 cm/s liegen (im Beispiel 0,2 cm/s)
• • Nach 4 erfolgt die Übertragung der Presskraft F auf die Walze 4 bevorzugt über die Wälzlager 24. Die Umfanggeschwindigkeit mit der die Walze 4 rotiertentspricht der Translation der Walze auf der Pulverschicht 13

The following characteristics apply to the current-carrying roller 4:

• • Dimensions of roller 4: Radius R = 10 mm to 1000 mm in the specific example: radius 50 mm, width 10 mm to 1000 mm in the specific example 50 mm. The width of roller 4 in the example corresponds (but not necessarily) to the width of the top powder layer 13
• • Contact pressure: 10 ⁰ N to 10 ² N per cm width of the top powder layer 13 (in the example 20 N times 50 mm = 100 N)
• • The feed speed of the translationally moving roller 4 should be in the range of 0.1 cm/s to 1 cm/s (in the example 0.2 cm/s)
• • After 4 The transmission of the pressing force F to the roller 4 is preferably carried out via the roller bearings 24. The peripheral speed with which the roller 4 rotates corresponds to the translation of the roller on the powder layer 13

• • Elektrische Spannung 0,01 V bis 10 V, bevorzugt 0,1 V bis 1 V, beim konkreten Beispiel: 1 V
• • Elektrische Stromstärke: 10¹ A bis 10³ A je cm Breite der obersten Pulverschicht 13, bevorzugt 10 A bis 100 A je cm Breite der Pulverschicht 13, beim konkreten Beispiel 500 A (bei 5 cm breiter Pulverschicht)
• • Die Einleitung des elektrischen Stroms erfolgt bspw. über Schleifkontakte 4.4 bzw. 4.5 in die Walze 4 selbst oder in der unmittelbaren Umgebung der Walze 4. Letztere Variante bietet den Vorteil einer größeren Kontaktfläche, so dass die Erwärmung bzw. der Verschleiß der Schleifkontakte 4.4 bzw. 4.5 reduziert werden kann.
• • Pulsmuster: Puls- und Pausenzeiten des elektrischen Stromflusses durch die Walze 4 von 1 ms bis 500 ms, vorzugsweise 3 ms bis 15 ms Pulsdauer mit 2 ms bis 10 ms Pausenzeit (im Beispiel Pulsdauer 10 ms, Pausendauer 5 ms).
• • Die nicht miteinander ver- oder angesinterten Pulverpartikel können nach Abschluss des Verfahrens mechanisch (z.B. mittels Druckluft) einfach voneinander getrennt werden und stehen ohne messbare Eigenschaftsveränderungen (Fließfähigkeit, Schüttdichte, Sauerstoffgehalt) für eine erneute Verwendung zur Verfügung.

The power unit with the electrical voltage source can have the following characteristics:

• • Electrical voltage 0.01 V to 10 V, preferably 0.1 V to 1 V, in the specific example: 1 V
• • Electrical current: 10 ¹ A to 10 ³ A per cm width of the uppermost powder layer 13, preferably 10 A to 100 A per cm width of the powder layer 13, in the specific example 500 A (with a 5 cm wide powder layer)
• • The electrical current is introduced, for example, via sliding contacts 4.4 or 4.5 into the roller 4 itself or in the immediate vicinity of the roller 4. The latter variant offers the advantage of a larger contact surface, so that the heating or wear of the sliding contacts 4.4 or 4.5 can be reduced.
• • Pulse pattern: Pulse and pause times of the electrical current flow through the roller 4 from 1 ms to 500 ms, preferably 3 ms to 15 ms pulse duration with 2 ms to 10 ms pause time (in the example pulse duration 10 ms, pause duration 5 ms).
• • The powder particles that are not fused or sintered together can be easily separated from each other mechanically (e.g. using compressed air) after the process has been completed and are available for reuse without any measurable changes in properties (flowability, bulk density, oxygen content).

List of reference symbols

1

Powder storage container

2

Squeegee

3

Base plate

4

roller

4.1

Roller core made of CuCrZr1

4.2

Graphite foil

4.3

Rolling bearings

4.4

sliding contact

4.5

sliding contact

5

Laser or electron beam source

5.1

Laser or electron beam

6

Powder bed/component

7

Installation space

8

Squeegee (if adhering powder particles need to be removed)

13

top layer of powder

14

Height of the top layer of powder before compaction by the roller

15

Height of the top layer of powder after compaction by the roller

16

electrical current flow through the compacted top powder layer in the contact area

17

macroscopically heated area

18

Contact area

19

Peripheral speed of the roller

20 21

powder particles

24

a current path through the compacted top layer of powder

25

a microscopically heated area

Claims (5)

Method for producing complex-shaped three-dimensional components (6), in which powder layers are formed layer by layer on top of each other on a stepwise lowerable platform and the respective uppermost powder layer (13) is pressed together in a contact region (18) arranged between the mutually facing surfaces of the surface of the respective uppermost powder layer (13) and the roller (4) by means of a roller (4) consisting of graphite or formed with graphite, which rotates about a rotation axis aligned parallel to the surface of the uppermost powder layer (13) and thereby moves translationally along the surface of the uppermost powder layer (13), with a contact force of 10 ⁰ N to 10 ² N per cm width of the uppermost powder layer (13), thereby reducing the layer thickness of the respective uppermost powder layer (13) and simultaneously heating the uppermost powder layer (13) within the contact region (18) by applying an electric current with an electric voltage of 0.01 V to 10 V and an electrical current of 10 ¹ A to 10 ³ A per cm width of the uppermost powder layer (13) flows through the roller (4), which is connected to an electrical voltage source, and during the translational movement of the roller (4) after the contact region (18) a locally defined deflected laser or electron beam (5.1) is directed onto the surface of the compacted and preheated respective uppermost powder layer (13) to carry out a selective laser beam or electron beam melting process; wherein an electrical current flow from the roller (4) through the contact region (18) and the powder layers is achieved by connecting the platform on which the powder layers are formed one after the other to a pole of the electrical voltage source or to ground potential and the powder layers are formed with an electrically conductive material and the electrical current flow is carried out in a pulsed manner, a pulse duration and pause times in between are observed. Method according to the preceding claim, characterized in that a pulse duration and pause times in between in the range 1 ms to 500 ms are maintained. Method according to one of the two preceding claims, characterized in that after a predeterminable number of pulses with pause times in between, a pause time longer than 10 ms is maintained. Method according to one of the three preceding claims, characterized in that pulse patterns of the pulsed electrical current are combined into pulse blocks. Method according to one of the preceding claims, characterized in that the component (6) is removed from the platform after its completion.