WO2022214538A1

WO2022214538A1 - Process and apparatus for additive manufacturing of three-dimensional parts

Info

Publication number: WO2022214538A1
Application number: PCT/EP2022/059107
Authority: WO
Inventors: Valentin BLICKLE; Frederik SCHAAL
Original assignee: Trumpf Laser- Und Systemtechnik Gmbh
Priority date: 2021-04-08
Filing date: 2022-04-06
Publication date: 2022-10-13
Also published as: DE102021203453A1

Abstract

Disclosed is a process for the additive manufacturing of three-dimensional parts (2), in which a feedstock (6) in powder form is locally melted using a processing beam, in particular a laser beam (8), the melting operation resulting in a molten bath (15), the method being characterized in that in order to acquire at least one additive manufacturing process parameter (T, v), a spectrum (22) of electromagnetic radiation (18) emitted by a plasma (17) forming in surroundings (16) of the molten bath (15) during the melting operation is measured and analyzed.

Description

Process and device for the additive manufacturing of three-dimensional components

Background of the Invention

The invention relates to a method for the additive manufacturing of three-dimensional components, in which a starting material in powder form is melted locally by means of a processing beam, in particular a laser beam, and a molten pool is formed as a result of the melting. The invention also relates to a device for the additive manufacturing of three-dimensional components, comprising: a beam source that emits a processing beam, in particular a laser beam, and processing optics that are designed to locally melt a starting material in powder form by irradiation with the processing beam, wherein forms a molten pool as a result of melting.

The manufacture of complex three-dimensional components using additive manufacturing has become increasingly important in research and industry in recent years. A particularly forward-looking process for additive manufacturing is powder bed fusion. In powder bed fusion, a three-dimensional component is built up layer by layer from a powder. In each step, a thin layer of powder is first applied to the part of the component that has already been manufactured. The powder is then locally irradiated with a processing beam. As a result, the powder is melted locally and firmly bonded to the lower-lying layers of the component. This procedure is now repeated until all layers of the component have been manufactured. As a result of the irradiation, a locally limited melt pool is initially formed from the melted powder. The high temperature gradients also create a thermal flow above the melt pool. Of particular importance for the process are the processes in the spatially limited process zone, in which different phases (powder, melt bath, gas flow) interact with each other during the irradiation.

In order to achieve high component quality, process parameters can be monitored during additive manufacturing. For example, systems for monitoring the molten pool are known which rely on optical emissions from the molten pool, which are measured by diodes or cameras. Such systems can be used, for example, to measure the temperature of the molten bath, which has a significant impact on component quality. However, only a relative temperature determination is possible with this, since the emissivity of the weld pool and powder is generally unknown. The flow conditions within the process zone, which also have an influence on the component quality, cannot be recorded in this way either.

object of the invention

In contrast, the object of the invention is to provide a method and a device for the additive manufacturing of three-dimensional components, which allow a reliable detection of meaningful process parameters during the additive manufacturing and thus the reproducible production of three-dimensional components of high quality.

subject of the invention

This object is achieved according to a first aspect by a method of the type mentioned, in which for the detection of at least one Process parameters of additive manufacturing, a spectrum of electromagnetic radiation emitted by a plasma that is formed in the vicinity of the melt pool during melting is measured and evaluated.

The processing beam is a beam of charged particles, in particular an electron beam, or electromagnetic radiation, in particular a laser beam, preferably a pulsed laser beam, particularly preferably a pulsed laser beam with pulse durations in the picosecond or femtosecond range or less. If the processing beam is a laser beam, the additive manufacturing process is also referred to as laser powder bed fusion (L-PBF). When the starting material, which is in powder form, is a metal, the additive manufacturing process is also known as laser metal fusion (LMF). However, the starting material present in powder form can also be a ceramic, in particular a ceramic containing at least one metal, or another material.

The processing beam is usually focused for the irradiation. The melting of the starting material, which is in powder form, forms a melt pool consisting of a melt that is locally heated (e.g. by the laser spot), which continues in a liquid state both laterally and in depth due to heat conduction. In the vicinity of the melt pool, typically directly above the melt pool, a plasma is formed, which consists of protective gas (e.g. argon or nitrogen) and melt pool components.

According to the invention, it is proposed to measure and evaluate a spectrum of electromagnetic radiation emitted by the plasma in order to record at least one process parameter of additive manufacturing. The spectrum of the electromagnetic radiation emitted by the plasma is preferably measured in the visible spectral range, but can also be in the ultraviolet or infrared range. The at least one process parameter is a physical variable that provides information about the processes during additive manufacturing and/or that correlates with the component quality. the Determining the at least one process parameter thus allows the reproducible manufacture of three-dimensional components of high quality.

In a further variant of this method, the process parameter or one of the process parameters is a temperature of the plasma in the vicinity of the molten bath, the temperature being determined using intensities of at least two photoemission lines in the spectrum of the electromagnetic radiation emitted by the plasma. The spectrum of the electromagnetic radiation emitted by the plasma can be measured using a spectrometer, for example. The photoemission lines in the spectrum are material-specific and very sharp. The position and intensity of these photoemission lines is known for the most common elements (Al, Ti, Cr, Fe, ... ), see for example the electronic Handbook of Basic Atomic Spectroscopic Date in the NIST database "www.nist.gov/pml /handbook-basic-atomic-spectroscopic-data".

The temperature of the plasma results from the intensity l _t and the transition coefficient g _t of a first photoemission line, the intensity I _j and the transition coefficient g _j of a second photoemission line and the energy difference AE _tj of the first and second photoemission line

where k _B is the Boltzmann constant, see the dissertation "Spectroscopic temperature measurements of flames and their physical significance" (1971), Retrospective Theses and Dissertations, 4422, Iowa State University. In the notation of equation (25) of the dissertation, the respective intensity is denoted by B or B' and the transition coefficient by 1 / ( g _q A _qp vo ) or 1 / (g _q A _q'P' v 'o ). An application of the method is described in the article "Determination of Plasma Temperature by a Semi-Empirical Method", FO Borges et al., Brazilian Journal of Physics, Vol. 34, No. 4B, December 2004. By measuring and comparing the intensities of several lines, the statistical accuracy can easily be increased by determining the temperature for two of the lines in the above manner and forming an average from the temperatures calculated in this way. Based on the intensities of at least two photoemission lines, an absolute determination of the temperature in the vicinity of the melt pool and thus directly within the process zone is possible. This is particularly advantageous in comparison with the melt pool monitoring from the prior art described above, which is based on the measurement of the optical emissions, since the latter only enables a relative temperature determination. As already discussed, the temperature structure within the process zone has a direct influence on the component quality. The measurement or control of the absolute temperature is therefore advantageous for production with consistently high component quality.

In a further variant of this method, the process parameter or one of the process parameters is a flow rate of the plasma in the vicinity of the molten bath, the flow rate being determined using at least one Doppler shift of at least one photoemission line in the spectrum of the electromagnetic radiation emitted by the plasma.

The local melting by means of the processing beam is necessarily accompanied by strong local heating. This strong local heating leads to a thermal convection flow. Inert gas is sucked in and ejected upwards. The movement of the particles of the plasma due to the convection current leads to a Doppler shift of the photoemission lines in the spectrum of the electromagnetic radiation emitted by the plasma. The measured wavelength l' of a photoemission line thus deviates from the wavelength A ₀ of the photoemission line, which would be measured without any relative movement. The wavelength shift DL caused by the Doppler effect is:

Al = lh - l' = _lh - c where it _is assumed that the flow velocity v prevailing in the direction of detection is small compared to the speed of light c. From the wavelength shift DL, the prevailing flow velocity v in the direction of detection in the vicinity of the melt pool can be determined according to determine. As in the case of recording the temperature, measurement uncertainties can also be reduced here by averaging the values for the flow velocity v i determined from the Doppler shifts of a large number of photoemission lines.

Like the temperature, the flow situation within the process zone also has a direct impact on component quality. Analogously to the temperature, the measurement or control of flow velocities in the vicinity of the melt pool is therefore advantageous for manufacturing with consistently high component quality.

The temperature of the melt pool can also be a process parameter of the additive manufacturing process. The temperature of the molten bath can be determined in particular from the temperature of the plasma, which is determined in the manner described above. A flow rate of the plasma in the vicinity of the melt pool can also indirectly provide information on the temperature of the melt pool. In particular, the convection-related flow speed of the plasma, in particular also of the protective gas components of the plasma, in the vicinity of the molten bath is higher, the higher the temperature of the molten bath.

In a further variant of this method, the photoemission line or at least one of the photoemission lines is a photoemission line of protective gas components of the plasma. As described above, the plasma is made up of protective gas and melt pool components. Deviating from the variant described here, the photoemission lines from which the temperature is determined can also be photoemission lines from melt pool components of the plasma. There may also be photoemission lines due to traces of water in the starting material in powder form.

In a further variant of this method, the recorded process parameter or the recorded process parameters are used to regulate the method for additive manufacturing. In this variant, the process parameters, for example via a suitable adjustment of the power of the processing beam or another Manipulated variable by means of a controller, are regulated to the desired target values, with which a reproducible high component quality is achieved. In the case of several recorded process parameters, preferably at least one of the recorded process parameters can be used to regulate the method for additive manufacturing. In particular, the recorded process parameter or the at least one recorded process parameter can represent the controlled variable.

As a result of the regulation, the method for additive manufacturing can be carried out in particular in a border area between deep welding and heat conduction welding or close to this border. Heat conduction welding is characterized by low exposure depths. The width of the melt pool, i.e. the extent of the melt pool perpendicular to the processing beam and perpendicular to its direction of movement, is therefore typically greater in heat conduction welding than the depth of the melt pool. Furthermore, heat conduction welding produces smooth and non-porous weld seams that do not require any post-processing. Due to very high power densities, deep penetration welding creates a deep and narrow vapor capillary, also known as a keyhole, which is surrounded by molten metal. As a result, in deep penetration welding, the depth of the weld pool is greater than its width. Deep penetration welding is also characterized by high efficiency, high speed and a small heat-affected zone. Control in the border area or near the border between deep penetration welding and heat conduction welding is particularly advantageous because it allows the advantages of both types of process to be combined with one another within certain limits, and because it allows an aspect ratio of close to 1:1, i.e. a ratio of width to depth of the weld pool close to 1:1, is feasible.

A corresponding control can be implemented, for example, by detecting a flow rate of the plasma in the vicinity of the molten bath. As soon as a vapor capillary forms, spatter occurs with an ejection speed that is higher than in heat conduction welding, which can be detected via a corresponding Doppler shift of at least one photoemission line in the plasma.

In a further variant of this method, the recorded process parameter or at least one of the recorded process parameters is above by the regulation a minimum value and/or below a maximum value. The recorded process parameter or the at least one of the recorded process parameters is preferably the temperature of the plasma or the temperature of the molten bath.

In particular, the temperature of the plasma and/or the molten bath can be controlled above a minimum temperature at which complete melting of the starting material in powder form can no longer be achieved and/or below a maximum temperature at which the starting material in powder form can overheat , solidification that is too slow or a weld pool that is too large.

In a further variant of this method, the ratio between a width of the molten pool and a depth of the molten pool is between 4:1 and 1:2, preferably between 2:1 and 1:1.5, particularly preferably between 1.5: 1 and 1:1.2. With the specified values of the ratio of width to depth of the melt pool, additive manufacturing takes place close or very close to the borderline between heat conduction welding and deep penetration welding. Such a control is preferably implemented via a calibration, in which at least one manipulated variable (e.g. the power of the processing beam) is varied and at the same time the width and depth of the melt pool (or the formed part of a test component to be manufactured) and at least one process parameter are recorded. The relationship determined from this between the at least one process parameter and the ratio of width to depth of the molten pool can then be used for the control. The depth of the melt pool means the maximum depth of the melt pool. The width of the melt pool is understood to mean the maximum width of the melt pool in the lateral direction (transverse to the direction of movement of the machining beam) on the upper side of the starting material present in powder form.

On the other hand, a deviation from the limitation to these areas, ie a greater asymmetry of the melt pool, is disadvantageous. Even with a ratio of width to depth of the melt pool of 4:1, a considerable overlap of successive exposure vectors is necessary. A significant proportion of the solidified material in a layer is therefore melted twice. will this If the ratio is even greater, then the necessary overlap also increases further. The process becomes more inefficient and the resolution drops.

On the other hand, a certain number of pores is already formed at a ratio of width to depth of the molten pool of 1:2, which increases further with a further reduction in this ratio. The formation of spatter also increases, which can lead to further pores, for example, if larger powder agglomerates of a spatter on the powder bed have to be melted at a later point in time. A reduction in the ratio of width to depth of the melt pool to a value of less than 1:2 thus impairs the component quality.

In a further variant of this method, the beam path of the laser beam and a beam path of the electromagnetic radiation emitted by the plasma run at least partially coaxially. Such a procedure is particularly simple and reduces the number of optical elements required for the measurement.

To determine at least one process parameter, (at least) one measuring laser beam can also be radiated into the area surrounding the melt pool, with the measuring laser beam being scattered on the particles of the plasma, and a spectrum of the radiation scattered on the plasma being measured and evaluated. A suitable method for determining the velocity components of flowing media, for example plasmas, by means of irradiating light and measuring the Doppler shift of the scattered light can be found, for example, in DE 3815214 A1. Similar methods are also discussed in WO 2008/092129 A2. However, irradiating a measuring laser beam and measuring and evaluating the spectrum of the scattered radiation is more complex than measuring and evaluating the spectrum of the electromagnetic radiation emitted by the plasma itself, partly because the measuring laser beam has to be guided into the processing chamber in which the powder is locally melted . Existing systems for additive manufacturing can therefore be adapted much more easily to use the method described above. According to a further aspect, the above-mentioned object is also achieved by a device for the additive manufacturing of three-dimensional components of the type mentioned at the outset, further comprising: a collecting optics which is designed to collect one of a plasma which forms in an area surrounding the molten bath during melting, collect emitted electromagnetic radiation, a spectrometer for measuring a spectrum of the electromagnetic radiation collected and an evaluation unit for determining at least one process parameter by evaluating the spectrum of the electromagnetic radiation collected.

In general, not only the electromagnetic radiation emitted by the plasma is collected by the collecting optics, but also the thermal radiation of the melt pool. While the electromagnetic radiation emitted by the plasma shows photoemission lines that are characteristic of the material, the thermal glow of the melt pool corresponds to continuous blackbody radiation. In general, the spectrum of the superimposition of both components is measured in the spectrometer. Due to their different signatures, however, the parts can be separated from one another: At short wavelengths, the blackbody radiation is weaker, so that the part originating from the material-specific photoemission lines dominates there.

The collecting optics can have a lens or an objective, for example. The evaluation unit is typically designed to determine the at least one process parameter from a comparison of the measured spectrum with at least one reference spectrum, from a comparison of at least one variable derived from the measured spectrum with tabulated variables or variables determined in suitable reference measurements, or by calculation based on at least one of the measured ones Spectrum derived size to determine. With regard to the advantages achieved with the device and its embodiments described below, reference is made to the above statements in relation to the method and its variants.

In one embodiment, the process parameter or one of the process parameters is a temperature of the plasma in the vicinity of the molten bath and the Evaluation device is designed to determine the temperature based on intensities of at least two photoemission lines in the spectrum of the electromagnetic radiation collected. This can be done in the manner described above in connection with the method.

In a further embodiment, the process parameter or one of the process parameters is a flow rate in the vicinity of the melt pool, and the evaluation device is designed to determine the flow rate using at least one Doppler shift of at least one photoemission line in the spectrum of the electromagnetic radiation emitted by the plasma. In order to measure the very small Doppler shifts associated with typical flow velocities, the spectrometer should have a particularly high resolution; the wavelength resolution should be below 0.1 pm, preferably below 0.05 pm. Examples of suitable spectrometers are scanning Fabry-Perot interferometers. These are Fabry-Perot interferometers in which the resonator length can be continuously adjusted, for example via piezoelectric actuators.

In a further embodiment, the photoemission line or at least one of the photoemission lines is a photoemission line of protective gas components of the plasma.

In a further embodiment, the device comprises a control device for controlling the additive manufacturing based on the at least one process parameter. The control device is designed to change at least one parameter of the additive manufacturing as a function of the at least one process parameter. In the event that the process parameter is the temperature of the plasma, the control device can be designed to change the power of the processing beam or the heat output of a heating device for heating the powder bed as a manipulated variable in order to bring the temperature of the plasma to a target temperature regulate. In a further embodiment, the control device is designed to keep the recorded process parameter or at least one of the recorded process parameters above a minimum value and/or below a maximum value. As has been described above, the process parameter can be, for example, the temperature of the plasma or possibly the temperature of the molten bath.

In a further embodiment, the control device is designed to have a ratio between a width of the molten bath and a depth of the molten bath of between 4:1 and 1:2, preferably between 2:1 and 1:1.5, particularly preferably between 1.5:1 and 1:1.2. As described above, in this case the additive manufacturing process is carried out in the border area between deep penetration welding and heat conduction welding.

In a further embodiment, the processing beam is a laser beam and at least parts of the processing optics form the collecting optics. In this case, the beam path of the laser beam and the beam path of the electromagnetic radiation emitted by the plasma are at least partially coaxial. The processing optics typically include a focusing device for focusing the laser beam on the surface of the starting material, which is in powder form. The irradiation with the high intensity caused by the focusing then leads to the local melting of the starting material and the melt pool and the plasma form in the vicinity of the melt pool.

The focusing device, which can be a lens, for example, can now simultaneously serve to collect the electromagnetic radiation emitted by the plasma. If at least parts of the processing optics function as collecting optics, the measuring setup is usually particularly simple and manages with comparatively few optical elements, which reduces costs. Furthermore, such a design allows easy integration into existing systems. In principle, however, it is also possible to separate the collecting optics and the processing optics and thus also their beam paths. In this case, the collection optics are typically also external to a Arranged processing chamber in which the present in powder form starting material is melted.

Further features and advantages of the invention result from the following description of exemplary embodiments of the invention, based on the figures of the drawing, which show details essential to the invention. The individual features can each be implemented individually or together in any combination in a variant of the invention. drawing

Exemplary embodiments are shown in the schematic drawing and are explained below. 1 shows a schematic representation of a device for the additive manufacturing of three-dimensional components, which has collecting optics, a spectrometer and an evaluation unit for detecting at least one process parameter of a plasma, FIG. 2 shows a schematic sectional representation of the melting of an in

Starting material in powder form by irradiation with a processing beam, forming a molten bath and a plasma and causing a thermal convection flow, Fig. 3 shows a schematic representation of a spectrum of electromagnetic radiation emitted by the plasma with photoemission lines, on the basis of which a temperature and a flow rate of the plasma can be determined, FIG. 4 shows a schematic representation of the time course of the temperature of the plasma formed as a result of the irradiation for a variant of the method illustrated in FIG

FIG. 5 shows a schematic top view of the molten bath shown in FIG. 2. FIG. Fig. 1 shows a device 1 for the additive manufacturing of three-dimensional components 2, comprising a beam source 3, processing optics 4 and a processing chamber 5.

The three-dimensional component 2 is built up in the processing chamber 5 in a protective gas atmosphere G layer by layer from a starting material 6 present in powder form. The starting material 6 in powder form is a metal, but it can also be a ceramic or another material. In a first step, a thin layer of the starting material 6 in powder form is applied to a carrier 7 . For this purpose, the device 1 has a device (not illustrated) for applying layers of powder.

The beam source 3 emits a processing beam in the form of a laser beam 8. The laser beam 8 passes through a beam splitter 9 and the processing optics 4, which are designed to locally melt the starting material 6 in powder form by irradiation with the laser beam 8. For this purpose, the laser beam is focused by means of a focusing device 10 which is part of the processing optics 4 . The focusing device 10 shown in simplified form in FIG. 1 consists of a lens 11, but in deviation from this, the focusing device 10 can also have a plurality of lenses or an objective. The focused laser beam 8 enters the processing chamber 5 through a window 12 and hits the starting material 6 in powder form. Due to the local irradiation, the starting material 6 in powder form melts locally in the powder layer and solidifies as part of the three-dimensional component 2 to be manufactured.

The processing optics 3 also have a scanner device 13 for deflecting the laser beam 8 . The scanner device 13 shown here in simplified form consists of a scanner mirror 14 that can be rotated about two axes of rotation, but it can also have two scanner mirrors in the form of galvanometer mirrors, for example. By deflecting the laser beam 8 by means of the scanner device 13, almost any, initially still essentially two-dimensional,

shapes can be achieved. In each further step, a thin layer of the starting material 6 in powder form applied to the part of the three-dimensional component 2 that has already been manufactured. Proceed in the same way as in the first step. The applied layer is thus irradiated locally with the laser beam 8 , melted and bonded to the underlying layers of the three-dimensional component 2 . This procedure is repeated until all layers of the three-dimensional component 2 have been manufactured.

As is shown in a detailed representation in Fig. 2, when the starting material 6, which is in powder form, is irradiated with the laser beam 8, a molten pool 15 is formed and in an area 16 surrounding the molten pool 15, here directly above the molten pool 15, a plasma 17 forms electromagnetic radiation 18 emitted. The molten bath 15 formed during irradiation continues in the liquid state both laterally and in depth due to heat conduction. The depth D of the melt pool 15 depends, among other things, on the power of the laser beam 8 . In the example shown in FIG. 2, the depth D of the molten bath 15 is essentially constant, but it goes without saying that the depth D generally varies depending on the location. The plasma 17, which emits the electromagnetic radiation 18, includes protective gas G and molten bath components.

In the example shown, the focusing device 10 serves as a collecting optics 19 which collects the electromagnetic radiation 18 emitted by the plasma 17 . In the case shown, the beam paths of the laser beam 8 and the electromagnetic radiation 18 emitted by the plasma consequently run partially coaxially. However, the collecting optics 19 do not have to be part of the processing optics 4 . In this case, the beam path of the laser beam 8 and the beam path of the collected electromagnetic radiation 18 do not run coaxially.

The device 1 also has a spectrometer 20 and an evaluation unit 21 . The spectrometer 20 is used to measure a spectrum 22 of the collected electromagnetic radiation 18 that has previously passed through the scanner device 13 and the beam splitter 9 . The evaluation unit 21 serves to determine at least one process parameter by evaluating the spectrum 22 of the collected electromagnetic radiation 18. The spectrum 22 shown in FIG Intensities I, on. The position and strength of these photoemission lines P is known for a large number of materials from spectroscopy data. In particular, but not necessarily, the photoemission lines P can be photoemission lines of protective gas components of the plasma 17 . A temperature T of the plasma can be determined based on the intensities I, based on a Doppler shift of the photoemission lines P, and also a flow velocity v of the plasma 17 in the vicinity 16 of the melt bath 15. The evaluation of the spectrum 22 of the electromagnetic radiation 18 emitted by the plasma 17 is However, not limited to temperatures T and flow velocities v. The temperature T of the plasma 17 can be used, for example, to determine the temperature of the molten bath 15 . The flow velocity v of the plasma 17 in the vicinity 16 of the melt bath 15 can also indirectly provide information on the temperature of the melt bath 15 .

The device 1 also has a control device 23 in order to control the additive manufacturing based on the at least one process parameter, for example the temperature T or the flow rate v. For this purpose, the control device 23 can suitably adjust manipulated variables of the additive manufacturing. For example, the control device 23 can act on the beam source 3 in order to adapt the power of the laser beam 8 as a function of the measured actual temperature T of the plasma 17 in order to achieve a target temperature of the plasma 17 . A corresponding adjustment of one or more manipulated variables with the aid of the control device 23 can also take place as a function of the flow velocity v of the plasma 17 .

In the example shown, the flow velocity v of the plasma 17 runs essentially perpendicularly to the surface of the molten bath 15: Due to the strong local heating caused by the irradiation, a thermal convection flow 24 occurs (cf. FIG. 2). Shielding gas G is sucked in from the side and ejected upwards. In the example shown in FIG. 2, the direction of movement 25 of the laser beam 8 is from left to right, but the movement of the laser beam 8 has practically no influence on the direction of flow of the plasma 17, which is measured by the spectrometer 20. 3 shows an example of a spectrum 22 of the electromagnetic radiation 18 emitted by the plasma 17, more precisely by the metal ions contained in the plasma 17. The wavelength 1 is plotted on the abscissa axis and the measured intensity I or power on the ordinate axis. The spectrum 22 has sharp, material-specific photoemission lines P with corresponding intensities. Three photoemission lines P 1 , P 2 , P ₃ with three corresponding intensities h, I 2 , I ₃ are represented here, but typically the spectrum 22 has more than three photo emission lines P 1 . The process dynamics affect the photoemission lines P as follows:

(1) The strength (brightness of the emission) of the transitions depends on the temperature.

(2) The relative motion of the atoms due to convection leads to a Doppler shift in the wavelength l of the radiation emitted by the plasma 17 (blue shift).

These changes are used by the evaluation unit 21 to determine the temperature T and the flow rate v of the plasma 17 . The temperature T of the plasma results in pairs from two of the three intensities h, I2, I3 of the photoemission lines Pi, P2, P3, the energy differences DE ₁₂ ,DE ₁₃ ,DE ₂₃ of the transitions and the transition coefficients g ₁ ,g ₂ ,g ₃ according to

where k _B is the Boltzmann constant and i,j = 1,2,3; The three photoemission lines P1, P2, P3 therefore result in _three temperatures T. For further evaluation or for determining a single temperature T of the plasma 17, these three temperatures T are used by the evaluation device 21 to form an average value, for example educated. If further photoemission lines P i are used for the evaluation, the statistical accuracy can be increased. The flow velocity v of the plasma 17 prevailing in the direction of detection results from the Doppler shift of the photoemission lines P 1 . Due to the Doppler effect, the measured frequency f of a photoemission line P, deviates from the frequency / ₀ of the photoemission line P, which would be measured without any relative movement. The expression c + vf = /o C — V known from the special theory of relativity applies with the speed of light c. This results in a frequency shift D/ of

The Doppler or wavelength shift DC caused by the Doppler effect results from the frequency shift

For flow velocities v, which are small compared to the speed of light c, the approximation results

or for the velocity v as a function of the wavelength shift

As an example, consider the photoemission line P, of an iron plasma, which without relative movement has a frequency of / ₀ = 684 10 ¹² Hz, corresponding to a wavelength of X _Q = 438 nm. A typical flow velocity in the direction of detection due to convection of v = 50^ results in a frequency shift of 114 MHz,

corresponding to a wavelength shift of 0.073pm.

The detection of the very small wavelength shift DL or frequency shift D/ of less than 0.1 pm or less than approx. 120 MHz is a challenge. The use of a scanning Fabry-Perot interferometer as a spectrometer 20 offers a cost-effective solution, with which a frequency resolution of 1-2 MHz is achieved.

In summary, the method described above and the device 1 described above enable a direct, trouble-free measurement of process parameters such as the temperature T within the process zone and the flow rate v above the melt bath 15.

4 shows the course of the temperature T of the plasma 17 over time t for a variant of the method shown in FIG. 2, the temperature T of the plasma 17 being controlled above a minimum value T _min and below a maximum value T _max is held. If the minimum value T _min is not reached, complete melting of the starting material 6 present in powder form is no longer guaranteed. If the maximum value T _max is exceeded, the starting material 6 present in powder form overheats, with the result that solidification is too slow or the formation of a molten pool 15 that is too large. The minimum value T _min and the maximum value T _max of the temperature T of the plasma 17, where the effects described occur depend on various parameters, for example on the type of starting material 6 present in powder form, and can be determined in advance, for example by tests.

FIG. 5 shows a schematic plan view of the molten bath 15 shown in FIG. 2 with the surrounding starting material 6 in powder form and the part of the three-dimensional component 2 that has already been manufactured. The width w of the melt bath 15, which is essentially constant in Fig. 5 to simplify the illustration, designates the (maximum) extent of the melt bath 15 perpendicular to the direction of movement 25 of the laser beam 8 on the upper side of the starting material 6 present in powder form. The control device 23 shown in Fig. 1 can be designed in particular in such a way that the ratio w/D between the (maximum) width w of the melt pool 15 shown in Fig. 5 and the (maximum) depth D of the melt bath shown in Fig. 2 Melt bath 15 is between 4:1 and 1:2, between 2:1 and 1:1.5, or between 1.5:1 and 1:1.2. In the case of a melt pool 15 with a ratio w/D in the specified value range, the additive manufacturing process is carried out in the border area between deep welding and heat conduction welding, as a result of which the advantages of both types of process can be combined with one another.

In order to keep the production process in this limit range, the regulation of the temperature T of the plasma 17 between the minimum value Tmin and the maximum value Tmax can be carried out, for example, as described in connection with FIG. 4 . Since a keyhole is formed in the molten pool 15 during deep welding, spatters of molten pool components in the plasma 17 above the molten pool 15 move faster during deep welding than is the case with heat conduction welding. With the help of the determination of the flow velocity v of the plasma 17 or components of the plasma 17 using the Doppler shift AK described above in connection with FIG. The flow speed v of the plasma 17 thus represents a process parameter that is particularly well suited for controlling the method for additive manufacturing in the border area between heat conduction welding and deep welding.

Claims

patent claims

1. A method for the additive manufacturing of three-dimensional components (2), in which a starting material (6) in powder form is melted locally by means of a processing beam, in particular a laser beam (8), and a molten bath (15) forms as a result of the melting, characterized that for the detection of at least one process parameter (T, v) of additive manufacturing, a spectrum (22) of a plasma (17), which forms during melting in an area (16) of the melt pool (15), emitted electromagnetic radiation ( 18) is measured and evaluated.

2. The method according to claim 1, characterized in that the process parameter or one of the process parameters is a temperature (T) of the plasma (17) in the vicinity (16) of the molten bath (15), the temperature (T) being determined on the basis of intensities ( ) of at least two photoemission lines (Pi) in the spectrum (22) of the plasma (17) emitted electromagnetic radiation (18) is determined.

3. The method according to claim 1 or 2, characterized in that the process parameter or one of the process parameters is a flow rate (v) of the plasma (17) in the vicinity (16) of the molten bath (15), the flow rate (v) being determined on the basis of at least a Doppler shift (Dl) of at least one photoemission line (P,) in the spectrum (22) of the plasma (17) emitted electromagnetic radiation (18) is determined.

4. The method according to any one of claims 2 or 3, characterized in that it is the photoemission line (Pi) or at least one of Photoemission lines (Pi) is a photoemission line of protective gas components of the plasma (17).

5. The method according to any one of the preceding claims, characterized in that the recorded process parameter (T, v) or the recorded process parameters (T, v) are used to control the method for additive manufacturing.

6. The method according to claim 5, characterized in that the recorded process parameter (T, v) or at least one of the recorded process parameters (T, v) is kept above a minimum value (Tmin) and/or below a maximum value (Tmax) by the regulation .

7. The method according to claim 5 or 6, characterized in that the ratio between a width (w) of the melt pool (15) and a depth (D) of the melt pool (15) is preferably between 4:1 and 1:2 as a result of the regulation between 2:1 and 1:1.5, more preferably between 1.5:1 and 1:1.2.

8. The method as claimed in one of the preceding claims, in which a beam path of the laser beam (8) and a beam path of the electromagnetic radiation (18) emitted by the plasma (17) run at least partially coaxially.

9. Device (1) for the additive manufacturing of three-dimensional components (2), comprising

- A beam source (3) which emits a processing beam, in particular a laser beam (8), and

- Processing optics (4) which are designed to locally melt a starting material (6) in powder form by irradiation with the processing beam, with the melting forming a melt pool (15), characterized in that the device further comprises:

- A collecting optics (19), which is designed to receive a plasma (17) that is formed during melting in an area (16) of the molten bath (15) forms to collect emitted electromagnetic radiation (18),

- a spectrometer (20) for measuring a spectrum (22) of the collected electromagnetic radiation (18) and

- An evaluation unit (21) for determining at least one process parameter (T, v) by evaluating the spectrum (22) of the collected electromagnetic radiation (18).

10. The device according to claim 9, characterized in that the

Process parameters or one of the process parameters is a temperature (T) of the plasma (17) in the area (16) of the melt pool (15) and the evaluation device (21) is designed to determine the temperature (T) based on intensities (I,) of at least to determine two photoemission lines (P i ) in the spectrum (22) of the collected electromagnetic radiation (18).

11. Device according to claim 9 or 10, characterized in that the process parameter or one of the process parameters is a flow rate (v) of the plasma (17) in the vicinity (16) of the molten bath (15), and the evaluation device (21) is designed to determine the flow velocity (v) based on at least one Doppler shift (Dl) of at least one photoemission line (P,) in the spectrum (22) of the plasma (17) emitted electromagnetic radiation (18).

12. Device according to one of claims 10 or 11, characterized in that the photoemission line (Pi) or at least one of the photoemission lines (Pi) is a photoemission line of protective gas components of the plasma (17).

13. Device according to one of claims 9 to 12, characterized in that the device comprises a control device (23) for controlling the additive manufacturing based on the at least one process parameter (T, v).

14. The device according to claim 13, characterized in that the control device (23) is designed to use the detected process parameters (T, v) or at least one of the recorded process parameters (T, v) above a minimum value (Tmin) and/or below a maximum value (Tmax).

15. Device according to claim 13 or 14, characterized in that the control device (23) is designed to set a ratio between a width (w) of the molten bath (15) and a depth (D) of the molten bath (15) between 4:1 and 1:2, preferably between 2:1 and 1:1.5, particularly preferably between 1.5:1 and 1:1.2.

16. Device according to one of claims 9 to 15, characterized in that the processing beam is a laser beam (8) and at least parts of the processing optics (4) form the collecting optics (19).