Preheat the build plate for additive manufacturing

Abstract

In summary, the present invention relates to a method for preheating a build plate (10) for additive manufacturing using an energy beam (50) with at least one emission power (P), wherein the method comprises controlling the power distribution (P XY ) over the dimensions (X, Y) of the build plate (10), comprising: - determining the power (P) of the beam (50) based on the target temperature (T SET ) of the substrate (10), and - determining the distribution of the power (P) over the dimensions (X, Y) based on the temperature distribution (ΔT) in the substrate (10). The present invention also relates to a method for manufacturing an object and an additive manufacturing device.

Description

Technical Field

[0001] The present invention relates to a method for preheating a build plate for additive manufacturing, in particular powder bed additive manufacturing. The present invention also relates to a method for manufacturing a product using the preheated build plate and an apparatus. Background Art

[0002] The method according to the present invention can be applied to energy beam additive manufacturing, such as selective laser sintering process (SLS) / selective laser melting process (SLM), electron beam melting process (EBM) and similar additive manufacturing techniques.

[0003] For processing brittle alloys using powder bed-based additive manufacturing techniques, it may be necessary to preheat the build plate (also referred to as the substrate) to a temperature on the order of the melting point of the alloy being processed. This can be achieved by repeatedly scanning the top surface of the substrate with a defocused beam before depositing the first layer of powder. The target temperature of the preheating process can be specified in advance, but it is difficult to determine in advance the beam power required to reach and maintain the target temperature. The beam power depends not only on the substrate material and beam efficiency, but is also very sensitive to the surface emissivity of the substrate and to the environment (build chamber) which is vulnerable to oxidation and metal evaporation. In addition, the substrate also conducts heat into the surrounding powder bed. Although metal powders exhibit low thermal conductivity, significant temperature drops can be observed at the boundaries and edges of the substrate due to the elevated temperature levels. Such non-uniformity is undesirable because the parts built in the edge regions of the substrate have a different preheating temperature compared to the parts built in the center of the plate. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to improve the preheating of the build plate in energy beam additive manufacturing.

[0005] To solve this technical problem, a method for preheating a build plate for additive manufacturing is proposed. The preheating is carried out by at least one energy beam of emission power. The method includes controlling the power distribution of the energy beam over the size of the build plate, including:

[0006] - determining a central temperature indicative of the temperature in the central region of the substrate,

[0007] - determining an edge temperature indicative of the temperature in the edge region of the substrate,

[0008] - determining the power of the beam based on the target temperature of the substrate, and

[0009] - determining the distribution of the power over the size based on the temperature distribution in the substrate,

[0010] Among them, the power distribution is controlled based on a target temperature difference that is a set value regarding the temperature distribution, and

[0011] wherein, the first controller is configured to control the power and the second controller is configured to control the power distribution.

[0012] The target temperature is the set value of the temperature that the build plate has after the preheating process. The power distribution for the build plate size causes a temperature distribution in the build plate. The temperature distribution is a measure of the degree of uniformity of the temperature distribution over the build plate. Additive manufacturing of an object is generally carried out layer by layer in a powder bed, wherein the object is built on a preheated build plate by selectively melting and / or sintering the powder bed. The present invention can automatically and quickly preheat the substrate without user interaction. In addition, the present invention can also achieve a uniform preheating temperature, thereby better controlling the microstructure of the component built in the boundary region of the plate. Depending on the material, the component can be built first in the boundary region of the substrate, thereby improving the overall productivity of the high-temperature additive manufacturing process.

[0013] The central temperature and the edge temperature can be measured by one or more thermocouples in each region. When multiple edges are heated similarly, the edge temperature can be determined for one edge. Multiple temperatures can also be determined to more accurately measure the temperature distribution.

[0014] The temperature distribution is the difference between the central temperature and the edge temperature. This difference provides a measure of the remaining temperature difference within the build plate and can be effectively calculated and processed. It has been shown to be advantageous to use the temperature difference between the edge temperature and the central temperature because this temperature difference is an accurate enough control variable to achieve an acceptable uniform temperature distribution within the build plate.

[0015] In another embodiment of the method, the beam power is limited such that the melting temperature of the substrate is not locally exceeded. This can be achieved by applying a maximum power value per region related to the build plate and / or the material and / or the temperature. In another embodiment of the method, the beam power is limited such that the melting temperature of the substrate is not locally exceeded by applying a limiting function to control the power distribution and / or the beam power. As a first measure to avoid damaging the substrate, the melting temperature of the substrate as the temperature set value of the controller input cannot be exceeded. However, due to the dynamic characteristics of the controller, the melting temperature may be locally exceeded because too much power is applied at one location within a defined time span. This should be avoided due to the negative impact on the substrate. To prevent the local exceeding of the melting temperature of the build plate, a limiting function can be implemented. It has been shown that in addition to the limitation of the set value, it is preferably also possible to implement a limiting function. The limiting function can be implemented directly using the control function of the power distribution.

[0016] To avoid local melting, the beam power can be limited by a limiting function of the form f LIM = min(P, P limit ), where P limit is the maximum beam power allowed without locally melting the substrate surface and can be a function of the substrate temperature, beam diameter, and scan speed.

[0017] According to the present invention, the first controller is configured to control the power and the second controller is configured to control the power distribution. This separation of power control and distribution control allows for a rigorous control design based on personalized requirements for power and power distribution. The first controller can be part of a first control loop for achieving an automatic increase in temperature, and correspondingly, the second controller can be part of a second control loop for achieving a uniform temperature distribution. In another embodiment of the method, the first controller is separated from the second controller. This can be implemented in software or by setting up the actual control hardware product.

[0018] According to the present invention, controlling the power distribution is based on a target temperature difference that is a setpoint for the temperature distribution. A setpoint can be provided for this difference. In an ideal case, the build plate has a uniform temperature distribution, with the temperature at the center being the same as the temperature at the edges. As a setpoint, the target temperature difference can be selected to have a negative value (temperature at the center is greater than the temperature at the edges), a zero value (temperatures at the center and edges are equal), and a positive value (temperature at the edges is greater than the temperature at the center). Depending on the beam source, different options for the power distribution on the build plate are available. If, for example, in laser-based powder bed additive manufacturing, the beam power dynamics are comparable to the deflection dynamics, the beam power can be adjusted according to its position on the plate.

[0019] Conversely, if the power dynamics are much lower than the deflection dynamics, another way to achieve non-uniform heat deposition is to adjust the heat strategy. By locally increasing the hatch-distance or decreasing the scan speed, an increase in the region-specific heat input can be achieved while keeping the beam power constant. Thus, the output of the internal controller regarding the power can be a scaling factor (e.g., range: 0 to 1) for a predefined heating strategy for distributing the beam power. For a scaling factor of zero, the beam power will be distributed uniformly. A scaling factor of one will result in a heating strategy where the heat input at the center of the plate is zero and the heat input increases towards the boundary. The exact heat distribution function may depend on the geometry and material properties of the substrate and can be pre-determined using simulations or empirical values.

[0020] In another embodiment of the invention, the power for the beam is determined based on the difference between the target temperature and the center temperature of the substrate. Alternatively, the average temperature of the center temperature and the edge temperature can be used. In a closed loop system, the difference between the target temperature and the center temperature of the substrate is used as a control difference for input to a closed loop controller. This is shown as an example in the accompanying drawings.

[0021] The object is also achieved by a method for the layer-by-layer production of an object in a powder bed, wherein the object is built on a building plate which is preheated by a method according to one of the preceding embodiments.

[0022] In another embodiment, the method includes determining a target temperature for the substrate based on the material to be processed. The step of determining may include looking up an ideal temperature for the build plate based on the material to be processed by a factoring method. Especially for highly brittle materials, it is advantageous to carefully select the substrate temperature for the material being used. A lookup table of target temperatures for different materials may be provided so that the target temperature can be automatically selected by selecting the material.

[0023] The technical problem is also solved by an additive manufacturing device, which comprises at least an energy beam source, a powder bed with a building plate and a control unit, which is designed and / or programmed to carry out the method for preheating the building plate.

[0024] In another embodiment, the additive manufacturing device includes an edge thermal sensor for determining the edge temperature and a central thermal sensor for determining the central temperature. The thermal sensors can be thermocouples attached to the corresponding positions. The advantages of thermocouples are that they are reliable and widely used. The change of heating power and power distribution can also rely on spatially resolved temperature measurement (thermal imaging).

[0025] In another embodiment, the additive manufacturing apparatus includes a temperature measurement system that provides core temperature and edge temperature. The system may be based on thermal imaging or other non-contact principles.

[0026] Additional methods of setting the beam power and distribution can also be based on thermal simulations of the substrate and different power settings and materials. A combination of two or more of these methods is also possible and can improve the thermal uniformity of the method through a more detailed understanding of the thermal distribution. An advantage of the invention is that manual adjustment of the preheating power and thus corresponding operator dependence of the process parameters can be avoided. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In the following figures, the invention is described and illustrated in more detail by means of the exemplary embodiments shown in the figures. DETAILED DESCRIPTION

[0028] The accompanying drawings show a schematic diagram of an exemplary control circuit that can be used to implement an embodiment of the present invention. The build plate 10, also known as the substrate, is shown as having dimensions X and Y. The central thermocouple TCC provides the central temperature T at the center of the build plate 10 C and the edge thermocouple TCE provides the edge temperature T at the edge of the build plate 10 E . It should be noted that the edge temperature T E can also be measured in one of the corners of the build plate 10. The build plate 10 is subjected to an energy beam 50 in order to preheat the build plate 10 and later build an object layer by layer.

[0029] The first controller C1 is provided with a setpoint regarding the target temperature T SET , from which the central temperature T C is subtracted. This difference generates an input for the first controller C1. Based on the difference between the central temperature T C and the edge temperature T E , the temperature distribution ΔT is provided to the second controller C2. In this case, a target temperature difference ΔT SET is also provided to generate a control difference between the target temperature difference ΔT SET and the current temperature distribution ΔT as an input for the second controller C2.

[0030] The first controller C1 generates the power P required to reach the temperature T SET as an output. This power P can be directly fed into the control unit CU for the energy beam source 5. This will result in a local temperature according to the setpoint at the center, but not a uniform temperature distribution. Therefore, the second controller C2 outputs a scaling factor A indicating how the power P should be distributed over the dimensions X and Y of the build plate 10. The power P and the scaling factor A can be directly fed to the control unit CU in order to achieve a uniform temperature distribution. To further improve the preheating, the power P and the scaling factor A are subjected to an additional distribution function f LIM including a limiting function f LIM ). The distribution function f(P, A, f LIM ) is used to calculate a power distribution (P XY ) that can be implemented as position information in XY coordinates and the power to be introduced by the beam 50 into the corresponding positions. To prevent the build plate 10 from melting locally due to an excessive power P, the limiting function f LIM is applied to the power P to generate an output power distribution P XY . As an input to the beam control unit, the output power distribution P XY provides all the necessary information to control the additive manufacturing equipment to achieve a preheated build plate 10.

[0031] In summary, the present invention relates to a method for preheating a building plate 10 for additive manufacturing using at least one energy beam 50 emitting a power P, wherein the method comprises controlling the power distribution P of the power P over the dimensions X, Y of the building plate 10. XY ,include:

[0032] - Based on the target temperature T of the substrate 10 SET determining the power P of the beam 50, and

[0033] - Determining the distribution of the power P to the dimensions X, Y based on the temperature distribution ΔT in the substrate 10. The invention also relates to a method for producing an object and to an additive manufacturing device.

[0034] Reference numerals list

[0035] CU Beam Control Unit

[0036] C1 First Controller

[0037] C2 Second Controller

[0038] 5 Energy beam source

[0039] 10 Build Plate

[0040] X The X dimension of the build plate

[0041] Y The Y dimension of the build plate

[0042] 50 Energy Beam

[0043] TCE Thermocouple Edge

[0044] TCC Thermocouple Center

[0045] P Power

[0046] A Scale Factor

[0047] P XY Power Distribution

[0048] f(P,A,f LIM ) Distribution function

[0049] f LIM Limit Function

Claims

1. A method for preheating a build plate (10) for additive manufacturing, said preheating being carried out using at least one energy beam (50) with an emission power (P), wherein, The method includes controlling the power distribution (P XY ) of the power (P) over the dimensions (X, Y) of the build plate (10), and includes: - Determine the central temperature (T C ) indicating the temperature in the central region of the build plate (10) - Determine the edge temperature (T E ) indicating the temperature in the edge region of the building panel (10) - Determine the power (P) of the energy beam (50) based on the target temperature (T SET ) of the build plate (10), and control the power (P) by a first controller (C1). - Based on the difference between the central temperature (T C ) and the edge temperature (T E ), determine the temperature distribution (ΔT), and - Determine the distribution of power (P) to the dimensions (X, Y) based on the temperature distribution (ΔT) in the build plate (10). wherein, the second controller (C2) controls the power distribution (P SET ) based on a target temperature difference (ΔT XY ) that is a set value regarding the temperature distribution (ΔT). wherein, providing the target temperature difference (ΔT SET ) to generate a control difference between the target temperature difference (ΔT SET ) and the current temperature distribution (ΔT) as an input to the second controller (C2).

2. The method according to claim 1, comprising the step of limiting the power (P) such that the melting temperature of the build plate (10) is not locally exceeded.

3. The method according to claim 1, comprising the step of restricting said power (P) such that the melting temperature of said build plate (10) is not locally exceeded, by applying a restriction function (f LIM ) to control said power distribution (P XY ) and / or said power (P).

4. The method according to claim 3, wherein, The first controller (C1) is separated from the second controller (C2).

5. The method according to claim 1, wherein, Determine the power (P) based on the difference between the target temperature (T SET ) and the central temperature (T C ).

6. The method according to claim 1, comprising manufacturing an object layer by layer on a preheated build plate (10) in a powder bed.

7. The method according to claim 1, comprising determining a target temperature (T SET ) of the build plate (10) based on the material to be processed.

8. An additive manufacturing device, at least comprising an energy beam source (5), a powder bed having a build plate (10), and a control unit (CU), the control unit being programmed to perform the method for preheating the build plate (10) according to any one of claims 1 - 5.

9. The additive manufacturing apparatus according to claim 8, comprising an edge thermal sensor (TCE) for determining an edge temperature (T E ) and a central thermal sensor (TCC) for determining a central temperature (T C ).

10. The additive manufacturing device according to claim 9, comprising a temperature measurement system that provides a central temperature (T C ) and a peripheral temperature (T E ).

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