Method for additively manufacturing a workpiece having a core of pure copper or copper alloy

Abstract

A method is described for manufacturing a workpiece having a core of pure copper or copper alloy by powder nozzle laser deposition welding, the method including the steps of providing a core workpiece of pure copper or copper alloy, providing a meltable metal powder, and applying the meltable metal powder to at least a portion of the core workpiece by powder nozzle laser deposition welding such that a plurality of metal layers are formed on at least a portion of the core workpiece, wherein control of a laser power used for the powder nozzle laser deposition welding is activated when a predetermined limit value is exceeded.

Description

[Technical Field]

[0001] The present invention relates to a method for producing workpieces with a core of pure copper or a copper alloy by powder nozzle laser deposition welding. By activating the control of the laser power used for powder nozzle laser deposition welding when a predetermined limit value is exceeded, the method makes the production process more stable and thus makes it possible to produce particularly high-quality products. [Background technology]

[0002] The object of the present invention is to provide a method that allows for the additive production of high-quality components with a core of pure copper or a copper alloy, in particular, the method being intended to allow a stable production process.

[0003] In tool manufacturing, the application of multiple materials plays a major role, especially in the cooling circuits. In die-casting tools, injection molding tools or hot forming tools, the removal of generated heat by a cooling medium is crucial in the manufacture of the product. Efficient cooling can, for example, shorten process times and thus enable higher throughput.

[0004] For heat dissipation or cooling, cooling channels are generally provided in the tool or mold insert, through which a corresponding coolant can flow to cool the material placed in the tool. Copper or copper alloys are distinguished by good thermal conductivity and provide correspondingly good cooling properties for such multi-material applications.

[0005] The tools required for this, as well as the products being manufactured, have complex geometries, and therefore manufacturing such tools with cooling channels extending therethrough is complex and difficult.

[0006] Continuous coating of metal strips, including copper strips, by powder bed-based melting is known. For example, WO 2018 / 192 865 A1 describes a method for applying a material layer to a metal strip, the method comprising the steps of providing a metal strip, providing a meltable and / or sinterable powder, generating a layer of the meltable and / or sinterable powder on a portion of the moving metal strip while obtaining the powder bed, and heating at least a portion of the powder bed with a laser or electron radiation to at least the sintering or melting temperature of the powder while generating the material layer.

[0007] The base material or core part for manufacturing a tool is generally a so-called volume part. In contrast to a strip, which provides a continuously flat surface, due to the complex geometry of the tool, often only a small portion of the surface is flat enough to form even a powder bed. Metal layers with a constant layer thickness are difficult to produce in this way, if at all.

[0008] An additive manufacturing method for producing an injection mold having at least one coolant channel is known from DE 10 2004 040 929 A1, which describes the cohesive application of a wear protection layer, preferably of steel, to a carrier material, preferably of copper or a copper alloy, by a DMD method.

[0009] The applicant has already described in DE 10 2020 116 037 A1 a method for producing a workpiece with a cooling channel system. The cross sections of the workpiece are formed by additive cohesive application of a build-up material. The introduction of cavities into each workpiece cross section and the formation of connecting channels allows for the production of cooling channel systems with similarly complex shapes running through the workpiece. Here, the build-up material can consist of copper or a copper alloy. This method also allows the workpiece to be provided with a wear-resistant outer layer of tool steel.

[0010] A common feature of both methods is that a wear layer with a low layer thickness is applied to a build-up or carrier material of relatively large dimensions, but such methods are not always applicable to the production of workpieces with a copper or copper alloy core intended to be provided with multiple metal layers.

[0011] Of course, in the case of additive manufacturing, heat is also dissipated into the copper or copper alloy core, so if the good thermal conductivity of pure copper or copper alloys is advantageous for use in cooling circuits, this property now presents a serious drawback precisely in additive manufacturing.

[0012] If a high energy input is still required during application of the first layer, overheating may occur during the manufacturing process. In turn, the temperature of the workpiece changes as the metal layer surrounding the core is formed. The temperature of the workpiece during the manufacturing process may change rapidly, if appropriate, which may have a negative impact on the stability of the manufacturing process and therefore on product quality.

[0013] A further drawback is the high reflectivity of pure copper, especially in the infrared range, which leads to insufficient energy input in the case of laser-based methods and at the same time high power consumption for melting.

[0014] It would therefore be desirable to provide a stable additive manufacturing method that can overcome the above-mentioned drawbacks and that allows parts with a core of pure copper or copper alloy to be produced with high quality. Summary of the Invention [Means for solving the problem]

[0015] The above mentioned object is achieved by the features of independent claim 1. The dependent claims are directed to particular embodiments of the invention.

[0016] The method according to the invention for producing a workpiece with a core of pure copper or a copper alloy by powder nozzle laser deposition welding first comprises the step a) of providing a core workpiece of pure copper or a copper alloy.

[0017] The term "core workpiece" is to be understood herein as a basic material or part that is coated with multiple metal layers according to the method described herein so that the core workpiece forms the inner core of the finished workpiece. The size and shape of the core workpiece are limited only insofar as it is a volume part. The volume part can be, for example, a cube, a rectangular parallelepiped, a pyramid, or a rod, or it can have a more complex three-dimensional shape that is oriented according to the complex shape of the workpiece to be manufactured. However, a volume part within the meaning of the present invention is not a strip or a similar planar part.

[0018] As already mentioned, powder bed-based additive manufacturing methods have disadvantages when it comes to processing volumetric parts. In contrast, "powder nozzle laser cladding" is an additive manufacturing method or 3D printing technology that works on the principle of directed energy deposition (DED).

[0019] In the DED process, metal powder is extruded through a feed nozzle (powder nozzle), such as an annular gap nozzle, and melted or welded to the base material by a laser to form a so-called molten pool. Both the laser and the feed nozzle are typically attached to a multi-axis machine tool or a robot arm. The process is typically carried out under a local protective gas atmosphere or in a sealed chamber under protective gas to better control the material properties and protect the material from unwanted oxidation.

[0020] Pure copper has a resistance of 400Wm at 20°C. -1 K -1 , and therefore provides very good temperature dissipation in the components. Due to faster heat dissipation, therefore, the cooling performance can be improved.

[0021] As used herein, the term "pure copper" refers to copper that is present in elemental form and may contain trace amounts of impurities. Preferably, the total amount of impurities is 1 weight percent or less, particularly preferably 0.1 weight percent or less, and most particularly preferably 0.01 weight percent or less, based on the total weight of copper.

[0022] As an alternative to pure copper, copper alloys can also be used in the method according to the invention. According to the invention, the composition of the copper alloy is not limited in principle. In contrast to pure copper, copper alloys generally have a lower thermal conductivity, so a high copper content is advantageous, the term "high" generally depending on the type and composition of the alloy. In any case, the thermal conductivity of copper alloys is 65 Wm at 20°C. -1 K -1 , preferably 166Wm at 20°C -1 K -1 In this way, the temperature dissipation in the component is reduced, but still improved compared to materials without copper content.

[0023] Copper-tin alloys (bronze), copper-aluminium alloys, in particular aluminium bronze, copper-zinc alloys (brass), copper-nickel alloys, copper-nickel-zinc alloys, copper-tin-zinc alloys, copper-manganese alloys, low-alloy copper alloys or copper-lead-tin alloys are conceivable for use.

[0024] It should be noted here that the alloys mentioned above are not limited to alloys of two or three materials, but may also contain further alloying elements or additives, as long as the thermal conductivity does not fall below a certain lower limit. However, copper-aluminum alloys with an aluminum content of 1% to 10%, such as CuAl1 or CuAl10, are preferably used. Unless otherwise stated, data in % relate herein to mass percent.

[0025] According to the present invention, the method further comprises a step b) of providing a meltable metal powder. According to the present invention, the composition and particle size of the meltable metal powder are not limited in principle, as long as it can be processed by powder nozzle laser deposition welding.

[0026] In a preferred embodiment, the meltable metal powder can be a tool steel, and in this way, a multi-material workpiece can be produced from the tool steel and pure copper or copper alloy, with the pure copper or copper alloy forming a core surrounded by a layer of tool steel.

[0027] Alternatively, according to a further preferred embodiment, the meltable metal powder can consist of a nickel-based alloy. The term nickel-based alloy, as used herein, refers to an alloy containing nickel as the main component and at least one other alloying component (other element). Nickel-based alloys include, for example, nickel-copper, nickel-iron, nickel-iron-chromium, nickel-chromium, nickel-molybdenum-chromium, nickel-chromium-cobalt, low-alloy nickel alloys (nickel content up to 99.9%), and other multi-material alloys. Examples include Alloy 718 (NiCr19NbMo) or Alloy 600 (NiCr15Fe). In this way, a multi-material workpiece can be produced, consisting of a nickel-based alloy and pure copper or a copper alloy. The pure copper or copper alloy forms a core, which is surrounded by a layer of nickel-based alloy.

[0028] According to the invention, the method further comprises a step c) of applying a meltable metal powder by powder nozzle laser deposition welding to at least a portion of the core work so that a plurality of metal layers are formed on at least a portion of the core work, wherein control of the laser power used for the powder nozzle laser deposition welding is activated when a predetermined limit value is exceeded.

[0029] As mentioned above, the coating process is complicated for copper or copper alloy volume parts. As the metal layer surrounding the core workpiece increases, the temperature of the workpiece, i.e., the core workpiece including the metal layer already formed, may change rapidly, which may result in overheating of the workpiece during the manufacturing process. This may have a negative impact on the stability of the manufacturing process and therefore on the product quality.

[0030] The cause is heating of the metal powder or molten pool during welding. This, in turn, heats the core workpiece by thermal conduction. As a result of the encasing of the core workpiece in at least one section, heat from the core workpiece can no longer be easily dissipated to the surroundings. The core workpiece then heats up, and the temperature within the workpiece can rise rapidly, resulting in imminent overheating. The increase in the absorption coefficient with temperature further contributes to the temperature rise of the core workpiece.

[0031] According to the invention, these effects can be stabilized by activating the control of the laser power when a predetermined limit value is exceeded, whereby the laser power can be controlled in the range of 100 W to 2500 W, preferably 100 W to 2000 W, particularly preferably 100 W to 1500 W, with a maximum control speed of, for example, 50 W / s or more, preferably 100 W / s or more.

[0032] Such control of laser power in DED-based methods has not been known in this context until now. This issue has not yet arisen. Only the complex workpiece geometries, combined with the challenging characteristics of pure copper and copper alloys for applying and processing multiple metal layers, have made such considerations necessary. In this case, the final method of controlling the laser power is not initially important. To minimize the aforementioned adverse effects, it is important that the laser power control be activated even slightly during powder nozzle laser deposition welding when certain limit values ​​are exceeded to avoid overheating of the workpiece.

[0033] According to a preferred embodiment, the predetermined limit value can be a predetermined number of metal layers to be formed. Therefore, the laser power control can be activated, for example, after the formation of the first, second, or third metal layer. The predetermined number of metal layers in this case depends on the layer thickness of the first metal layer, and alternatively or additionally on the geometric shape and size of the volumetric component and / or the initial laser power. However, preferably, the laser power control is activated after the formation of the first metal layer to ensure high stability of the manufacturing process.

[0034] According to a preferred embodiment, the predetermined limit value may also be the maximum temperature of the workpiece during powder nozzle laser deposition welding. The maximum temperature is essentially unlimited in this case, since it also depends on the layer thickness of the metal layer to be formed, and alternatively or additionally on the geometric shape and size of the volumetric part and / or the initial laser power. Preferably, the maximum temperature may be in the range of 200°C to 700°C, particularly preferably in the range of 300°C to 600°C, and very particularly preferably 400°C. The maximum temperature may also correspond to the preheating temperature described herein.

[0035] According to a further preferred embodiment, the method may then further comprise monitoring the temperature of the workpiece during the powder nozzle laser deposition welding by means of a thermal imaging camera.

[0036] Therefore, preferably, at the start of powder nozzle laser deposition welding in step c) of the method according to the present invention, the temperature of the workpiece can be monitored at a predetermined sampling rate, for example, at a narrow mesh interval of 1 to 10 seconds, or even continuously by a thermal imaging camera. When a previously defined maximum temperature is exceeded, the laser power can then be controlled. For this purpose, the thermal imaging camera is particularly advantageously coupled to the laser control device, and the control is automatically activated via feedback of the temperature value of the thermal imaging camera to the laser control device. Thus, when a predetermined maximum temperature is exceeded, the laser power can be automatically reduced, for example.

[0037] According to the invention, the way in which the control of the laser power is carried out is not limited in principle, as long as it is possible to avoid overheating of the workpiece during powder nozzle laser deposition welding.

[0038] However, according to a preferred embodiment, the laser power can be controlled as a function of the size of the melt pool. The melt pool formed during powder nozzle laser deposition welding is directly correlated to the laser power used. If the melt pool is large, e.g., 17 mm 2, the laser power is high. If the laser power is subsequently reduced, this is also reflected in a reduction in the size of the melt pool. To control the laser power after operation, for example, a characteristic curve relating the laser power to the size of the melt pool can be stored in the laser control device. Alternatively or additionally, the laser power can be controlled based on a comparison with a predetermined target melt pool size. In this case, the target melt pool size can be predetermined according to the geometry and size of the core workpiece and the number and thickness of the metal layers to be applied. The target melt pool size can be, for example, 6 mm. 2 from 10mm 2 , preferably 7 mm 2 From 9mm 2 It can be said that:

[0039] According to a preferred embodiment, the size of the melt pool can be determined by a melt pool camera. Such a melt pool camera can be, for example, integrated into or coupled to the laser optics. The laser power can then be automatically controlled via feedback of the actual values ​​measured by the melt pool camera to the laser's controller.

[0040] According to a particularly preferred embodiment, controlling the laser power as a function of the melt pool size can include determining at least one actual melt pool size and controlling the laser power based on a comparison of the at least one actual melt pool size with a predetermined target melt pool size. To this end, the melt pool size can be measured, for example, at a narrow mesh interval, for example, 1 s to 10 s, monitored at a predetermined sampling rate, or continuously monitored by a melt pool camera.

[0041] According to the invention, the wavelength of the laser radiation with which the powder nozzle deposition welding is performed is not limited in principle. However, according to a preferred embodiment, the powder nozzle laser deposition welding can be performed with laser radiation having a wavelength in the range of 380 to 560 nm or in the range of 780 to 1400 nm, particularly preferably 450 nm or 1030 nm.

[0042] According to the invention, the laser used is also not limited in principle: for example, conventional lasers for DED-based methods, such as diode, CO2 or fiber lasers, can be used.

[0043] As already mentioned, the good thermal conductivity of pure copper or copper alloys is advantageous for use in cooling circuits, but it also poses a serious drawback in additive manufacturing, since heat is dissipated into the copper or copper alloy core. When applying the first metal layer, high thermal conductivity makes the welding process more difficult due to the rapid dissipation of heat into the core due to direct contact between the metal layer and the core workpiece. This can result in poor molten pool stability. In the case of copper alloys, heat dissipation from the molten pool is also present, but less pronounced than in the case of pure copper due to the reduced thermal conductivity.

[0044] This effect can make application of the first metal layer more difficult. To counteract this effect, according to a preferred embodiment, applying the meltable metal powder to at least a portion of the core work by powder nozzle laser deposition welding can include applying the first metal layer at maximum laser power and a low powder mass flow rate.

[0045] The powder mass flow rate indicates how many grams of metal powder are extruded through the feed nozzle per minute. The higher the powder mass flow rate, the more metal powder is extruded through the feed nozzle. When applying the first metal layer, the metal powder has not yet been heated by the welding process, which can lead to additional cooling and therefore destabilization of the molten pool. Therefore, a low powder mass flow rate is advantageously used when applying the first metal layer.

[0046] For example, when applying the first metal layer, the maximum laser power can be in the range of 2000 W to 2500 W and the powder mass flow rate can be in the range of 4 g / min to 6 g / min. This is especially advantageous when using laser radiation in the infrared (IR) range, as pure copper has a high degree of reflectivity in the IR range, which makes the input of energy more difficult.

[0047] Alternatively, the above-mentioned adverse effects of forming the first metal layer can be mitigated by preheating the core work by reducing heat dissipation into the core work.

[0048] According to a preferred embodiment, the method can therefore further comprise preheating the core workpiece to a preheat temperature of 150°C or higher, preferably 250°C or higher, particularly preferably 400°C or higher, before forming the metal layer in step c).

[0049] According to a preferred embodiment, the preheating of the core workpiece before forming the metal layer in step c) can be carried out by laser radiation, induction, or an external furnace. Particularly preferably, preheating is carried out by laser radiation having a wavelength in the blue-green range of 380 nm to 560 nm or a wavelength in the infrared range of 780 nm to 1400 nm, with wavelengths of 450 nm or 1030 nm being particularly preferred. In contrast to infrared laser radiation, blue-green laser radiation has the advantage during preheating that pure copper has a better absorption behavior in the case of laser radiation having a wavelength in the green-blue range. In this case, the absorption rate is 44 percent compared to 2 percent for infrared light.

[0050] In order to further stabilize the additive manufacturing method and further improve product quality, according to a preferred embodiment, the application of meltable metal powder to at least a portion of the core work by powder nozzle laser deposition welding can include the following two stages: i) applying one or more metal layers with controlled laser power at a low powder mass flow rate; and ii) applying one or more further metal layers with controlled laser power at a high powder mass flow rate.

[0051] In step i), the focus here is on maximum mixing between the pure copper or copper alloy of the core workpiece and the metal powder. This is particularly useful for creating a load-bearing cohesive bond between the core workpiece and the metal layer. Likewise, as already explained, overheating of the workpiece should be controlled to avoid it.

[0052] In contrast, in stage ii) the focus is now on maximum material application, as well as on avoiding overheating of the workpiece as already explained.

[0053] In step i), the laser power can be controlled, for example, in the range of 2500 W to 100 W (upper and lower power limits) at a control rate of 50 W / s or more, and the powder mass flow rate can be in the range of 4 g / min to 6 g / min.

[0054] In step ii), the laser power may be controlled, for example, in the range of 1500 W to 100 W at a control rate of 50 W / s or more, and the powder mass flow rate may be in the range of 8 g / min to 12 g / min.

[0055] Combined with the previously described application of the first metal layer at maximum laser power and low powder mass flow rate, a three-stage process can also be realized: In the first stage, upstream of the application of the first metal layer at maximum laser power and low powder mass flow rate, the focus is on preheating the core workpiece, increasing absorption in the core workpiece, and stabilizing the melt pool as previously described.

[0056] As also previously mentioned, preheating of the core work may alternatively be achieved by laser radiation, induction or an external furnace.

[0057] Optionally, according to a preferred embodiment, the method may further comprise, before step c), applying a buffer layer of copper alloy to at least a portion of the core work. In this way, for example, the very good thermal conductivity of pure copper in the core work can be obtained, and the manufacturing method can take advantage of the reduced thermal conductivity and reduced reflectivity in the infrared range of copper alloys.

[0058] According to a particularly preferred embodiment, the buffer layer may consist of a copper-aluminum alloy, which may contain aluminum in a proportion of 1% to 10%.

[0059] The method described herein can in principle be used to produce volume parts with a copper or copper alloy core, in particular volume parts with complex geometries. If the workpiece is a tool, the method can further comprise step d) of drilling one or more cooling channels in the core workpiece, as explained at the beginning.

[0060] Additionally, it should be noted that the methods described herein may be part of a hybrid method that may further include, for example, material removal and shaping steps, to obtain a final product of desired geometry and function. [Brief explanation of the drawings]

[0061] [Figure 1] FIG. 1 shows a flow diagram of one embodiment of a method for producing a workpiece having a core of pure copper or copper alloy by powder nozzle laser deposition welding.

[0062] [Figure 2] 1 shows a control circuit to illustrate the control of laser power used for powder nozzle laser deposition welding according to one embodiment.

[0063] [Figure 3] FIG. 1 shows a flow diagram of a method for producing a workpiece having a core of pure copper or copper alloy by powder nozzle laser deposition welding according to one embodiment with preheating of the core workpiece.

[0064] [Figure 4] 1 shows a flow diagram of a method for manufacturing a workpiece having a core of pure copper or copper alloy by powder nozzle laser deposition welding according to one embodiment with two-stage metal layer deposition.

[0065] [Figure 5] 1 shows a schematic representation of a system suitable for carrying out the method;

[0066] [Figure 6] 10A-10C illustrate schematically the manufacture of a mandrel according to an exemplary embodiment of the present method.

[0067] [Figure 7a] 10 shows a photograph of the production of a mandrel according to a further exemplary embodiment of the method. [Figure 7b] 10 shows a photograph of the production of a mandrel according to a further exemplary embodiment of the method. [Figure 7c] 10 shows a photograph of the production of a mandrel according to a further exemplary embodiment of the method. [Figure 7d] 10 shows a photograph of the production of a mandrel according to a further exemplary embodiment of the method. [Figure 7e] 10 shows a photograph of the production of a mandrel according to a further exemplary embodiment of the method.

[0068] [Figure 8a] 7a-7e show profiles of process parameters as a function of time during the manufacture of a mandrel according to the exemplary embodiment of FIGS. [Figure 8b] 7a-7e show profiles of process parameters as a function of time during the manufacture of a mandrel according to the exemplary embodiment of FIGS. [Figure 8c] 7a-7e show profiles of process parameters as a function of time during the manufacture of a mandrel according to the exemplary embodiment of FIGS. [Figure 8d]7a-7e show profiles of process parameters as a function of time during the manufacture of a mandrel according to the exemplary embodiment of FIGS.

[0069] [Figure 9] 7a-7e show the relationship between the laser power profile and the melt pool size as a function of time in the first metal layer during the manufacture of the mandrel according to the exemplary embodiment from FIGS. 7a-7e.

[0070] Examples or exemplary embodiments of the present invention are described in detail below with reference to the accompanying figures, in which identical or similar elements may be designated herein by the same, but sometimes different, reference numerals.

[0071] However, it should be emphasized that the present invention is in no way limited or restricted to the exemplary embodiments and features of the embodiments described below, but rather also includes modifications of the exemplary embodiments, in particular modifications constituted by modifications of the features of the described examples or by combinations of individual or multiple features of the described examples, within the scope of the claims.

[0072] 1, a flow diagram of one embodiment of a method for manufacturing a workpiece having a core of pure copper or copper alloy by powder nozzle laser deposition welding is shown. In step S101, a core workpiece of pure copper or copper alloy is first provided. As previously described, the copper alloy may be a copper-aluminum alloy having an aluminum content of 1% to 10%, such as CuAl1 or CuAl10.

[0073] In step S102, a meltable metal powder is provided. According to a preferred embodiment, the meltable metal powder can be made of tool steel. Alternatively, according to a more preferred embodiment, the meltable metal powder can be made of a nickel-based alloy.

[0074] Finally, in step S104, a meltable metal powder is applied to at least a portion of the core workpiece by powder nozzle laser deposition welding so that multiple metal layers are formed on at least a portion of the core workpiece, and control of the laser power used for the powder nozzle laser deposition welding is activated when the laser power exceeds a predetermined limit value. In this case, the laser power can be controlled, for example, in a range from 100 W to 2500 W, preferably from 100 W to 2000 W, particularly preferably from 100 W to 1500 W, with a maximum control speed of 50 W / s or more, preferably 100 W / s or more.

[0075] As already explained, in the case of copper or copper alloy volume parts, the coating process is complicated, therefore, for the reasons already explained, control of the laser power during powder nozzle deposition welding is used for the first time.

[0076] According to a preferred embodiment, the predetermined limit may be a predetermined number of metal layers to be formed, as previously explained.

[0077] Alternatively, according to a further preferred embodiment, the predetermined limit value may also be a maximum temperature of the workpiece during the powder nozzle laser deposition welding. The method may then include monitoring the temperature of the workpiece during the powder nozzle laser deposition welding by means of a thermal imaging camera.

[0078] Thus, preferably at the start of powder nozzle laser deposition welding, the workpiece temperature can be monitored at a predetermined sampling rate, for example, at a narrow mesh interval of 1 to 10 seconds, or even continuously by a thermal imaging camera. When a previously defined maximum temperature is exceeded, the laser power can then be controlled. For this purpose, the thermal imaging camera is particularly advantageously coupled to the laser control system, and the control is automatically activated via feedback of the temperature value from the thermal imaging camera to the laser control system. Thus, when a predetermined maximum temperature is exceeded, the laser power can be automatically reduced, for example.

[0079] Referring to FIG. 2, a control circuit is shown to illustrate the control of laser power used for powder nozzle laser deposition welding according to one embodiment.

[0080] In step S201, the actual melt pool size is first preferably determined by a melt pool camera, which can be, for example, integrated into or coupled to the laser optics.

[0081] In step S202, the measured actual melt pool size is then compared to a predetermined target melt pool size.

[0082] A given tolerance, e.g. ±0.5mm 2 Based on this, it can then be determined in step S203 whether the target melt pool size is exceeded or undershot and whether the laser power needs to be controlled accordingly.

[0083] If there is an overshoot / undershoot in step S204, control of the laser output begins in step S205. If there is no overshoot / undershoot, the cycle starts from the beginning.

[0084] The control circuit described can preferably be performed automatically via feedback of the actual values ​​measured by the melt pool camera to the laser's controller.

[0085] Depending on the frequency of determination of the actual melt pool size, the cycle can be carried out with a correspondingly narrow mesh spacing, for example, from 1 s to 10 s.

[0086] As already mentioned, the good thermal conductivity of pure copper or copper alloys is advantageous for use in cooling circuits, but it also poses a serious drawback in additive manufacturing, since heat is dissipated into the copper or copper alloy core. When applying the first metal layer, high thermal conductivity makes the welding process more difficult due to the rapid dissipation of heat into the core due to direct contact between the metal layer and the core workpiece. This can result in poor molten pool stability. In the case of copper alloys, heat dissipation from the molten pool is also present, but less pronounced than in the case of pure copper due to the reduced thermal conductivity.

[0087] This effect can make application of the first metal layer more difficult. To counteract this effect, according to a preferred embodiment, applying the fusible metal powder to at least a portion of the core workpiece by powder nozzle laser deposition welding can include applying the first metal layer at maximum laser power and a low powder mass flow rate. This is shown in step S103a of FIG. 3. For example, when applying the first metal layer, the maximum laser power can be in the range of 2000 W to 2500 W, and the powder mass flow rate can be in the range of 4 g / min to 6 g / min. This is particularly advantageous when using laser radiation in the infrared (IR) range, because pure copper has a high reflectivity in the IR range, making energy input more difficult.

[0088] Alternatively, the above-mentioned adverse effects of forming the first metal layer can be mitigated by preheating the core workpiece by reducing heat dissipation into the core workpiece. According to a preferred embodiment, the method can therefore further comprise preheating the core workpiece to a preheat temperature of 150°C or higher, preferably 250°C or higher, particularly preferably 400°C or higher, prior to forming the metal layer in step c). This is shown in step S103b of FIG. 3.

[0089] According to a preferred embodiment, preheating of the core workpiece prior to forming the metal layer in step S104 can be performed by laser radiation, induction, or an external furnace. Particularly preferably, preheating is performed by laser radiation having a wavelength in the blue-green range (380 nm to 560 nm) or a wavelength in the infrared range (780 nm to 1400 nm), with wavelengths of 450 nm or 1030 nm being particularly preferred. In contrast to infrared laser radiation, blue-green laser radiation has the advantage during preheating that pure copper has a better absorption behavior than laser radiation having a wavelength in the green-blue range. In this case, the absorption rate is 44 percent compared to 2 percent for infrared light.

[0090] 4, a flow diagram of a method for manufacturing a workpiece having a core of pure copper or copper alloy by powder nozzle laser deposition welding according to one embodiment with two-stage metal layer deposition is shown. To further stabilize the additive manufacturing method and further improve product quality, according to a preferred embodiment, the application of a fusible metal powder to at least a portion of the core workpiece by powder nozzle laser deposition welding can include two stages: Step S104a of applying one or more metal layers while controlling the laser power at a low powder mass flow rate; and Step S104b of applying one or more additional metal layers while controlling the laser power at a high powder mass flow rate.

[0091] In step S104a, the focus here is on maximum mixing between the pure copper or copper alloy of the core workpiece and the metal powder. This is particularly useful for creating a load-bearing cohesive bond between the core workpiece and the metal layer. Similarly, as already explained, overheating of the workpiece should be avoided by control. Here, the laser power can be controlled, for example, in the range of 2500 W to 100 W (upper and lower power limits) at a control rate of 50 W / s or more, and the powder mass flow rate can be in the range of 4 g / min to 6 g / min.

[0092] In contrast, in step S104b, the focus is now on maximum material application, as well as avoiding overheating of the workpiece as previously discussed, where the laser power can be controlled, for example, in the range of 1500 W to 100 W at a control rate of 50 W / s or more, and the powder mass flow rate can be in the range of 8 g / min to 12 g / min.

[0093] When combined with step S103a or S103b from FIG. 3, a three-stage method can also be realized.

[0094] Referring to FIG. 5, an example of a system suitable for carrying out the present method is shown.

[0095] The system includes a powder nozzle 2 for generating a mass flow of meltable metal powder. The powder nozzle is designed as an annular gap nozzle, through whose center a laser beam 1 is guided. A melt pool camera 6 is integrated into the laser optics. Using mirrors, the size of the melt pool 4 generated on the core workpiece 3 can be determined by the melt pool camera during deposition welding. The temperature of the workpiece can be monitored by the thermal imaging camera 3. As already mentioned, diode, CO2, or fiber lasers can be used for powder nozzle laser deposition welding. The wavelength of the laser radiation 1 can be in the range of 380 to 560 nm or 780 to 1400 nm, preferably 450 nm or 1030 nm.

[0096] FIG. 6 illustrates a schematic representation of the manufacture of a mandrel according to an exemplary embodiment of the present method.

[0097] In this exemplary embodiment, the core work is a rod of pure copper. Optionally, in step S301, a buffer layer of copper alloy may be applied to at least a portion of the core work before the first metal layer of tool steel is formed in step S302. According to a particularly preferred embodiment, the buffer layer may comprise a copper-aluminum alloy containing 1% (CuAl1) to 10% (CuAl10) aluminum, as previously described.

[0098] As can be seen from steps S302 and S303, in this embodiment, a total of several metal layers are formed on the core workpiece portion, with one or more metal layers of different sizes being formed in each case. In this case, the size of the metal layer formed in each case on the core workpiece portion can be controlled, for example, by the feed speed at which the core workpiece is moved and / or by the movement of the powder nozzle above the core workpiece and by the powder mass flow rate. Additional rotations around the longitudinal axis allow the core workpiece to be covered with the respective metal layers.

[0099] The formed metal layer can then be finished by cutting off the tip in step S304, fabricating the cover in step S305, and lathing in step S306, as shown. Finally, additional channels are drilled into the bar-shaped core work to finish the mandrel (not shown).

[0100] Figures 7a-7e show photographs of the fabrication of corresponding mandrels according to an exemplary embodiment. Figure 7a shows a copper rod. The copper rod has a diameter of 12 mm and a length of 100 mm. However, the diameter of the copper rod can also be, for example, 8 mm or 10 mm. Figure 7b shows the copper rod after coating with tool steel, and in Figure 7c, the ground plane is indicated by the dashed line drawn. Figures 7d and 7e show the exemplary workpiece after finishing by turning and drilling. The cohesive bond between the copper core and the tool steel can be seen in the ground diagram.

[0101] Figures 8a-8d show time-dependent profiles of process parameters during deposition welding for the exemplary embodiment from Figure 7. The effect of controlling the laser power as described herein can be seen based on the relevant process parameter profiles in Figures 8a-8d. Corner data for the exemplary embodiment is listed again below. - Fix a pure copper rod, 12 mm in diameter and 100 mm in length, on the workbench. - Turn on the laser source, an IR laser with a wavelength ranging from 900nm to 1080nm, and turn on the powder conveyor for transporting the tool steel powder. - Preheat the copper rod by applying a first tool steel layer to a portion of the rod in a rotating motion with a maximum laser power of 2500 W and a low powder mass flow rate of 4 g / min, to produce a layer thickness of approximately 0.3 mm. - Limit value: After the maximum temperature of 400°C at the workpiece is reached, the laser power control is automatically activated based on the next target melt pool size and the control limit for the next laser power. Target melt pool size: 8.6mm 2 Control limit: 100W to 1500W (power that cannot be exceeded or exceeded during control) Maximum control speed: 100W / s (maximum allowable power change per time to control the system's reaction time) - To increase material coverage, the powder mass flow rate is increased from 8g / min to 12g / min and the feed rate is increased after the formation of multiple metal layers.

[0102] Figures 8a and 8b show the automatically controlled laser power as a function of the measured melt pool size as a function of time. The dashed line indicates the target melt pool size. The correlation between the overshoot / undershoot of the target melt pool size and the resulting automatic control of the laser power can be seen here.

[0103] Figures 8c and 8d show the measured part temperature and powder mass flow rate as a function of time. The term "layer" corresponds to the term "layer." Thus, layer 1 refers to the first metal layer.

[0104] FIG. 9 shows the relationship between the laser power profile and the size of the melt pool as a function of time in the first metal layer.

[0105] Stage 1: Deposit welding at maximum power without power control active → large melt pool (here about 17.5 mm2 The focus is on preheating the core work → increasing the absorption in the core work → stabilizing the molten pool.

[0106] Stage 2: At a workpiece temperature of 400°C, power control is active. There is a large difference between the actual melt pool size and the target melt pool size. → Maximum reduction in laser power (here 100 W / s) → The melt pool size is smaller than the target melt pool size (8.6 mm). 2 , dashed line). Prevention of overheating of the core workpiece by power control → focus on maximum mixing between copper and tool steel.

[0107] Stage 3: Power control active, small difference between actual melt pool size and target melt pool size → minimal adjustment of laser power. Power control prevents overheating of core workpiece → focus on maximum material application. [Explanation of symbols]

[0108] 1. Laser light 2 powder nozzles 3 Core Work 4. Molten Pool 5. Thermal imaging camera 6. Melt Pool Camera

Claims

1. 1. A method for manufacturing a workpiece having a core of pure copper or a copper alloy by powder nozzle laser deposition welding, said method comprising the steps of: a) providing a core work of pure copper or copper alloy (S101); b) providing a meltable metal powder (S102); c) applying the meltable metal powder to at least a portion of the core workpiece by powder nozzle laser deposition welding so that a plurality of metal layers are formed on the at least a portion of the core workpiece, wherein control of a laser power used for the powder nozzle laser deposition welding is activated when a predetermined limit value is exceeded (S104); characterized in that it comprises method.

2. The predetermined limit value is a maximum temperature of the workpiece during the powder nozzle laser deposition welding. The method of claim 1.

3. The method further comprises determining the temperature of the workpiece during the powder nozzle laser deposition welding by means of a thermal imaging camera (5). The method of claim 2.

4. The predetermined limit value is a predetermined number of metal layers to be formed. The method of claim 1.

5. The laser output is controlled as a function of the size of the molten pool.

5. The method according to claim 1 .

6. wherein the controlling of the laser power as a function of the melt pool size includes determining (S201) at least one actual melt pool size and controlling (S205) the laser power based on a comparison (S202) of the at least one actual melt pool size with a predetermined target melt pool size. The method of claim 5.

7. the size of the molten pool is determined by a molten pool camera (6), 7. The method according to claim 5 or 6.

8. The powder nozzle laser deposition welding is performed with laser radiation having a wavelength in the range of 380 nm to 560 nm or in the range of 780 nm to 1400 nm.

8. The method according to claim 1 .

9. The application of the meltable metal powder to the at least one portion of the core work by powder nozzle laser deposition welding (S104) comprises applying a first metal layer at maximum laser power and a low powder mass flow rate (S103a).

9. The method according to at least one of claims 1 to 8.

10. The method further comprises, before step c), preheating the core workpiece to a preheating temperature of 150 ° C or higher (S103b); 9. The method according to at least one of claims 1 to 8.

11. The preheating is performed by laser radiation, induction or an external furnace. The method of claim 10.

12. The preheating is performed by laser radiation having a wavelength in the range of 380 nm to 560 nm or in the range of 780 nm to 1400 nm. The method of claim 11.

13. The application (S104) of the meltable metal powder to the at least a portion of the core work by powder nozzle laser deposition welding comprises: i) applying one or more metal layers while controlling the laser power at a low powder mass flow rate (S104a); and ii) applying one or more additional metal layers while controlling the laser power at a high powder mass flow rate (S104b); characterized in that it comprises 13. The method according to at least one of claims 1 to 12.

14. The method further comprises, before step c), applying a buffer layer of copper alloy to the at least a portion of the core work (S301), 14. The method according to at least one of claims 1 to 13.

15. The copper alloy is a copper-aluminum alloy containing aluminum in a ratio of 1% to 10%.

15. The method of claim 14.

16. The material of the fusible metal powder is made of tool steel.

16. The method according to at least one of claims 1 to 15.

17. The material of the fusible metal powder is a nickel-based alloy.

16. The method according to at least one of claims 1 to 15.

18. the method further comprising a step d) drilling one or more cooling channels in the core work; 18. The method according to at least one of claims 1 to 17.

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