DE102019208836A1 - Process for the production of complex three-dimensional components - Google Patents

Abstract

In the process for the production of complex three-dimensional components, powder layers are formed one above the other on a platform that can be lowered step by step and an uppermost powder layer (13) is rotated around an axis of rotation parallel to the surface of the uppermost powder layer (13) and translated along the surface of the uppermost Powder layer (13) moving roller (4) compresses the respective top powder layer (13) in a contact area (18) and thereby reduces the layer thickness of the top powder layer (13). At the same time, the uppermost powder layer (13) within the contact area (18) is heated in that an electric current flows through the roller (4), which is connected to an electric voltage source. During the translatory movement of the roller (4), after the contact area (18), a locally defined deflected laser or electron beam is directed onto the surface of the compressed and preheated respective uppermost powder layer (13) for performing a selective laser or electron beam melting process.

Description

The invention relates to a method for producing three-dimensional components with complex shapes.
Laser beam melting (LBM) and electron beam melting (EBM) are new methods of manufacturing three-dimensional components from metal powder. Both methods have in common that a computer model of the component is first broken down into individual wafers and these wafers are then successively produced by spreading a powder layer of a defined height on a base and then melting it on in a locally defined manner using a laser or electron beam. The non-irradiated powder in the area remains unaffected or is only very weakly bound and can therefore be easily removed later. The process is continued with the renewed application of a powder layer and the melting of the surface, which may have been changed in accordance with the workpiece geometry, and is repeated until the entire three-dimensional component is completely built up in layers.

• 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.

Even if 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 if they are made of materials that are difficult to machine), the following limitations exist for these powder-bed-based beam melting processes:

• Especially for the EBM process, preheating the powder bed is a necessary step for process stability and, depending on the material to be processed, very time-consuming. When the electron beam hits a loose powder layer, the charge concentration in the powder particles occurs and, as a result, they repel each other. As a result, a powder cloud (so-called “smoke”) is created in the system and the build-up process has to be terminated.

In U.S. 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, scanning speed, speed of the electrons (specified by the acceleration voltage) and material parameters such as electrical conductivity, an additional process step is described during this the powder layer is heated flat and as homogeneously as possible by means of the electron beam. This reduces the electrical conductivity as a function of temperature. On the other hand, however, there is a sharp increase in electrical conductivity if there is a slight sintering between the powder particles, which avoids electrostatic charging. This preheating process step is an integral part of the process control in the commercially available EBM systems in order to ensure the stability of the manufacturing process. However, such preheating extends the process time and is energy-intensive.

With the LBM process, preheating of the powder bed with the laser beam is not possible and is currently only implemented by heating the building board. With increasing height, there is a large temperature difference in the direction of construction. The limited preheating temperature leads to very high cooling rates after the selective melting process and consequently to internal stresses in the component.

In the case of alternative heating by thermal radiation from infrared or resistance heaters, it is disadvantageous that heating by radiant heating, in particular due to the initially very low thermal conductivity of the powder, is extremely slow. This leads to long process times and poor reproducibility in the individual layers.

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

According to the invention, this object is achieved with a method which has the features of claim 1. Advantageous refinements and developments of the invention can be implemented with features identified in the subordinate claims.

In the method according to the invention, powder layers are formed in layers on top of one another on a platform that can be lowered in steps. The respective uppermost powder layer is pressed together by means of a roller rotating about an axis of rotation aligned parallel to the surface of the uppermost powder layer and thereby moving translationally along the surface of the uppermost powder layer in a contact area which is arranged between the mutually facing surfaces of the respective uppermost powder layer of the roller and thereby the layer thickness of the powder layer is reduced. At the same time, the respective uppermost powder layer within the contact area is heated in that an electrical current is carried out through the roller, which is connected to an electrical voltage source.

A local is defined in the direction of movement of the roller after the contact area deflected laser or electron beam for carrying out a selective laser beam or electron beam melting process directed onto the surface of the compressed and preheated respective uppermost powder layer.

With each translational movement in at least one axial direction of the roller around its axis of rotation, an uppermost layer of powder can be applied and a component with a three-dimensional surface can be built up successively. The powder layers can be applied with a powder conveying device known per se and possibly with a doctor blade. The layer thicknesses of the individual powder layers should be kept as constant 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

Variants of powder application:

• - The roller can be moved at the same time as the powder supply or doctor blade, or the rolling process takes place separately
• - The powder supply can be carried along or the powder is picked up by the squeegee from supply containers at the edges
• - The roller can also be used without a doctor blade

An electrical current flow from the roller through the contact area and the powder layers can advantageously also be achieved by connecting the roller to a pole of the electrical voltage source or to earth potential and by forming the powder layers from or with an electrically conductive material. As a result, electrical current can flow from the roller through the powder layers compacted in the contact area, which, in addition to direct heating of the respective top powder layer, can lead to further heating of the respective top powder layer due to the release of heat in the contact area and a better connection to the sintering of powder particles on one another .

It is also favorable to carry out the electrical current flow in a pulsed manner. As a result, preferred heating of the contact area between powder particles can be achieved and thus sintering can be achieved at a reduced mean temperature of the powder layer and the component.

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

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

The preferably pulsed electrical current creates a material connection between the powder particles and between the powder particles and the base / previous powder layer. This prevents the powder particles from moving (e.g. through convection of heated gases). In addition, the electrical conductivity increases so that the EBM is not charged by the electron beam. Plastic deformation and possibly melted volume in the contact between the powder particles are minimal, so that the powder can be used again later.

Especially when the pulse duration is lengthened, instead of limiting the heat release to the contact area between the powder particles, the entire volume of the powder bed can also be heated. This reduces the cooling rate after selective melting by means of EBM / LBM, which enables the setting of isotropic structures and the structural fineness and reduces or prevents internal 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 electric current can also be combined into pulse blocks. E.g. 2 ms pulse, 2 ms pause, the whole 10 times and then 50 ms pause can be observed.

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

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

Show:

• 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
• 4th an example of a roller that can be used in the invention.

For the manufacture of components 6th , as in 1 shown, the powder is from a powder reservoir 1 using a squeegee 2 than even thick top layer of powder 13 onto a surface of a platform or a surface of a previously formed, already compressed, heated and with a laser or electron beam 5.1 coming from a laser or electron beam source 5 locally defined processed powder layer applied. By means of a rotating and translatory over the surface of the top powder layer 13 moving roller 4th , which consists for example of graphite or is formed with graphite, takes place by means of the roller 4th on the top powder layer 13 applied pressing force F precompaction of the top powder layer 13 of height h ₀ 14th on h ₁ 15th , as in 2 shown. With the preferably pulsed direct current I flowing through the roller 4th flows, a preferably partial sintering of particles with which the top powder layer takes place 13 is formed. The roller 4th can be caused by the frictional forces between the surfaces of the roller 4th and the top powder layer 13 be driven as a result of the pressing force F or by an additional motor.

The roller 4th is connected to an electrical voltage source, not shown. From the laser or electron beam radiation source 5 is a defined locally deflectable laser or electron beam 5.1 onto the surface of the pre-compacted and heated top powder layer 13 in the direction of movement of the roller after the contact area 18th directed to realize an additive laser / electron melting process. The focal point of the laser or electron beam 5.1 moves along a defined contour on the surface of the top powder layer 13 in an area behind the roller in the direction of movement 4th during their translational movement behind the contact area 18th is arranged.

2 shows the heating of the top powder layer 13 and the roller 4th as a result of the electric current flowing through the roller 4th and the top powder layer 13 in the contact area 18th . The area 17th is preferably heated because firstly the flowing electrical current 16 a bottleneck has to pass here and the electrical current density is consequently increased, and secondly there is the uppermost powder layer 13 has a high specific electrical resistance depending on the pressing pressure. The expansion of the area 17th in the direction of the axis of rotation of the roller 4th is about the advance (peripheral speed 19th the current-carrying roller 4th and the feed rate of the translatory movement of the roller 4th along the surface of the top powder layer 13 and the thermal conductivity of the roller 4th or the component 6th certainly.

The extent of the heated contact area 18th in the level of the top powder layer 13 and in the rolling direction 20th in 2 is over the radius of the current-carrying roller 4th , the powder layer thickness h ₀ 14th and the pressure applied between the surface of the top powder layer 13 the roller 4th on the top powder layer 13 acts, controlled.

The pulsed electrical, preferably direct current, is generated as described in the field-activated sintering (FAST, also referred to as Spark Plasma Sintering, SPS) method. Inside the top powder layer 13 the heat is preferred at the contact points of the powder particles 21st released (see 3 ). This means that heating takes place where contact is formed, where the powder particles are sintered 21st to increase the electrical conductivity, the thermal conductivity and to establish the adhesion between the powder particles 21st should be achieved. The heat release is proportional to the electrical resistance of the current-carrying material and the square of the electrical current. The advantage of a pulsed electric current is that a very high electric current can flow during the short pulse duration, which heats the powder particles 21st in their microscopic contact areas 25th , in which there are powder particles 21st touch, definitely. The expansion of the area 25th is determined by the pulse pattern and the thermal conductivity of the powder particles 21st certainly. The heating of the entire system, however, remains limited, as this is determined by the equivalent direct current I _RCS . $$\left({\mathrm{I.}}_{\mathrm{R.}\mathrm{C.}\mathrm{S.}}=\sqrt{\frac{Pulsdauer}{Pulsdauer+Pausendauer}\mathrm{I.}}\right)$$

Has the current-carrying roller 4th a compacted and heated surface area of the uppermost powder layer during their rotation and translational movement 13 happened is with the powder particles 21st in the compacted area 15th With the reduced layer thickness h _1, sufficient adhesion and electrical conductivity are achieved. Then you can use the electron or laser beam 5.1 the respective material with which the component 6th is to be produced, are selectively melted (see Figure (1)).

variants

• • Anstelle eines gepulsten Gleichstromes kann auch ein konstanter Gleichstrom oder Wechselstrom eingesetzt werden.
• • Die Pulsmuster des gepulsten elektrischen Stromes können auch zu Pulsblöcken zusammengefasst werden (z.B. 2 ms Puls, 2 ms Pause, das ganze 10 mal wiederholt und danach 50 ms Pause eingehalten werden).
• • Aus einem Vorratsbehälter 1 wird über die Rakel 2 eine jeweilige oberste Pulverschicht 13 mit einer Pulverschichtdicke 10 µm bis 1000 µm, bevorzugt 50 µm bis 150 µm aufgetragen. Bei dem konkreten Beispiel mit einer Schichtdicke von jeweils 150 µm aufgetragen werden.
• • Das verwendete Pulver ist bevorzugt sphärisch. Der mittlere Partikeldurchmesser d₅₀ sollte 10 µm bis 200 µm, vorzugsweise 50 µm bis 150 µm betragen. Bei dem konkreten Beispiel wurde pulverförmiger Werkzeugstahl 1.2343 mit einer mittleren Partikelgröße d₅₀ von 50 µm eingesetzt, um Pulverschichten 13 auszubilden.

Für die stromführende Walze 4 gelten folgende Kennwerte:

• • 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: 10° 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

Die Leistungseinheit mit der elektrischen Spannungsquelle kann folgende Kennwerte aufweisen:

• • Elektrische Spannung 0,01 V bis 10 V, bevorzugt 0,1 V bis 1V, beim konkreten Beispiel: 1V
• • 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.

• • Instead of a pulsed direct current, a constant direct current or alternating current can also be used.
• • The pulse pattern of the pulsed electrical current can also be combined into pulse blocks (eg 2 ms pulse, 2 ms pause, repeated 10 times and then followed 50 ms pause).
• • From a storage container 1 gets over the squeegee 2 a respective top powder layer 13 With a powder layer thickness of 10 µm to 1000 µm, preferably 50 µm to 150 µm. In the specific example, a layer thickness of 150 µm is applied.
• • The powder used is preferably spherical. The mean particle diameter d ₅₀ should be 10 μm to 200 μm, preferably 50 μm to 150 μm. In the specific example, powdery tool steel 1.2343 with an average particle size d ₅₀ of 50 μm was used to form powder layers 13 to train.

For the live roller 4th the following parameters apply:

• • Dimensions of the roller 4th : 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 the roller 4th corresponds in the example (but not mandatory) to the width of the top powder layer 13
• • Contact pressure: 10 ° N to 10 ² N per cm of width of the top powder layer 13 (in the example 20 N times 50 mm = 100 N)
• • The feed rate of the roller moving in a translatory manner 4th should be in the range 0.1 cm / s to 1 cm / s (in the example 0.2 cm / s)
• • After 4th the pressing force F is transferred to the roller 4th preferably over the roller bearings 24 . The peripheral speed with which the roller 4th rotated corresponds to the translation of the roller on the powder layer 13

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 1V, in the specific example: 1V
• • Electric current: 10 ¹ A to 10 ³ A per cm width of the top powder layer 13 , preferably 10 Å to 100 Å 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 4th itself or in the immediate vicinity of the roller 4th . 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 4th 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 have not been sintered or sintered together can easily be separated from each other mechanically (eg 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

4th

roller

4.1

Core of the roller made of CuCrZr1

4.2

Graphite foil

4.3

roller bearing

4.4

Sliding contact

4.5

Sliding contact

5

Laser or electron beam source

5.1

Laser or electron beam

6th

Powder bed / component

7th

Installation space

8th

Squeegee (if adhering powder particles are to be wiped off)

13

top powder layer

14th

Height of the top powder layer before compaction by the roller

15th

Height of the top powder layer after compaction by the roller

16

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

17th

macroscopically heated area

18th

Contact area

19th

Peripheral speed of the roller

20th

21st

Powder particles

24

a current path through the compacted top powder layer

25th

a microscopically heated area

QUOTES INCLUDED IN THE DESCRIPTION

This list of the documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent literature cited

• US 8187521 [0003]

Claims (7)

Process for the production of complex three-dimensional components, in which powder layers are formed one on top of the other on a platform that can be lowered in steps and on a respective top powder layer (13) by means of an axis of rotation aligned parallel to the surface of the top powder layer (13), rotating and thereby roller (4) moved translationally along the surface of the uppermost powder layer (13) the respective uppermost powder layer (13) in a contact area (18) between the mutually facing surfaces of the surface of the respective uppermost powder layer (13) and the roller (4 ) is arranged, pressed together and thereby the layer thickness of the respective uppermost powder layer (13) is reduced and at the same time the uppermost powder layer (13) within the contact area (18) is heated in that an electric current flows through the roller (4), which is connected to an electric voltage source, and During the translational movement of the roller (4) after the contact area (18), a locally defined deflected laser or electron beam is directed onto the surface of the compressed and preheated respective uppermost powder layer (13) to carry out a selective laser beam or electron beam melting process. Procedure according to Claim 1 , characterized in that an electrical 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 one pole of the electrical voltage source or to earth potential and the powder layers are formed with an electrically conductive material. Method according to one of the preceding claims, characterized in that the electrical current flow is carried out in a pulsed manner. 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 observed. Method according to one of the three preceding claims, characterized in that pulse patterns of the pulsed electrical current are combined to form pulse blocks. Method according to one of the preceding claims, characterized in that the component (6) is removed from the platform after its completion.

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