DE102023206901A1 - Process for thermal processing of processing areas on surfaces of workpieces, semi-finished products or powder beds - Google Patents

Abstract

In this case, an energy beam with a base power P _Basis is directed at the surface and its focal spot is moved two-dimensionally along a predetermined starting contour within the processing area in such a way that the focal spot moves from a position P _xi,yi to a subsequent position P _xi+1,yi+1 , which is arranged at a predetermined distance Δl _i from the previous position P _xi,yi , and predeterminable holding times Δt _Basis are maintained there. The temperature on the surface of the processing area is measured with spatial resolution and fed to an electronic evaluation and control unit. Then, depending on the spatially resolved temperatures, control is carried out in such a way that a predetermined starting contour that is to be traversed is changed accordingly into adapted contours that take the spatially resolved temperatures into account and/or the predetermined distances Δl _i+x between the individual positions P _xi+x,yi+x traversed one after the other and/or the holding times Δt _i+x at the individual positions P _xi,yi , are adjusted. This is repeated until a predetermined locally defined temperature distribution in the processing area has been achieved.

Description

The invention relates to a method for thermally processing processing areas on surfaces of workpieces, semi-finished products or powder beds using a deflectable energy beam. Thermal processing can be a primary forming, forming, joining, coating or a method for influencing the material properties.

A laser or electron beam can be used as an energy beam (ES), for example, with which locally defined thermal energy can be introduced into the workpiece surface, which leads to the heating of the surface in the respective specified surface area of a workpiece. Since the impact zone of the ES projected onto the processing area (hereinafter the focal spot size) is limited for technical reasons, the energy input (E) can be distributed over the surface of the processing area in a time interval (Δt) by means of a (highly dynamic) deflection (scanning) of the ES. The deflection movement of the focal spot of the ES can, on the one hand, take place continuously at the speed v. On the other hand, a time-defined stop at predetermined positions is possible (holding time = Δt. The time interval for the jump (Δt _jump ) between the individual positions at which no further movement of the focal spot occurs during a holding time Δt should be significantly smaller than the holding time at one position. (Δt _jump « Δt _point ). If this condition is met, the speed along a linear movement of the focal spot is $$\text{v}=\text{l}/\left(\text{N}\ast \Delta t_{\text{Punkt}}\right)$$

For both variants of the deflection movement, the ES can be moved along different geometric shapes in order to follow certain contours. In addition to a static position, these can be followed in a linear fashion, but particularly preferably over a surface. This movement of the ES's focal spot can be achieved, depending on the technology, e.g. via magnetic fields (electron beam), mechanical axes or mirrors (laser beam), etc. The arrangement of these positions at which the movement of a focal spot is stopped is almost arbitrary within the technology-related scanning limits, which enables the heating of a wide variety of contours.

For an energy beam of constant power P, v or Δt = const. results in a constant energy input within the contour, i.e. each position is subjected to the same energy. The resulting temperature distribution is influenced by many variables, e.g. different heat dissipation conditions, varying geometry, etc. and therefore cannot be defined and/or reproducibly set. Local control of the energy input is therefore desirable. The present invention enables any two-dimensional temperature distribution to be set within the limits of the physical laws without using any other heat sources apart from the energy beam. To this end, the power of the energy beam and the holding time Δt at the positions, hence the scanning speed v, can be controlled during the processing process. Control of the contour to be traveled (e.g. adding/removing positions) is also possible.

Up to now, influence has been exerted by repeatedly scanning a treatment area with defined cooling intervals, regulating the beam power based on the measured surface temperature of a pyrometer (measurement on two wavelengths) averaged over a limited measuring area within the scanned treatment area with constant contour and constant holding time (Δt) or scanning speed (measuring area of the pyrometer < scanned area), regulating from a previous position with adjusted holding time at defined positions of the scanned area based on a numerical simulation, and temperature-related adjustment of the energy distribution on a linear scan path (one-dimensional, oscillating mirror optics with adjustment of the oscillation function (adjustment of the continuous speed v)).

A desired predetermined temperature distribution on the surface of a processing area can only be achieved inadequately and with increased effort.

It is therefore an object of the invention to provide possibilities for thermal processing of tools with which a predetermined temperature distribution can be achieved within a processing area on a workpiece surface, wherein the heating is to be achieved with a single deflectable energy beam.

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

In the process for thermal processing of processing areas on workpiece surfaces, in which an energy beam with a base power P _Basis is directed onto the surface, its focal spot is formed two-dimensionally within the bear along a predetermined initial contour. processing area is moved in such a way that the focal spot is moved from a position P _xi,yi to a subsequent position P _xi+1,y+1 , which is arranged at a predetermined distance Δl _i from the previous position P _xi,yi . The respective starting contour can be specified, for example, on the basis of empirical values or simulation calculations.

At the respective positions P _xi,yi the focal spot is moved further over a predeterminable base holding time Δt _base . This is carried out until the predefined initial contour has been traversed with the focal spot.

The temperature on the surface of the processing area is measured in a spatially resolved manner and the respective spatially resolved temperature measurement values are fed to an electronic evaluation and control unit. The appropriately designed electronic evaluation and control unit then carries out a control depending on the spatially resolved temperatures so that
during control, the specified initial contour that is to be traversed is changed according to the spatially resolved temperature measurement values and/or the specified distances Δl _i+x between the individual positions P _xi+1,yi+1 traversed one after the other and/or the holding times Δt _i+x at the individual positions P _xi,yi are adjusted and this is repeated until a specified locally defined temperature distribution in the processing area has been achieved.

In addition, at least one of the following control options can be changed depending on the spatially resolved temperature measurement signals. These can be the power P _i with which the energy beam is operated, the power density in the focal spot,
the speed v _i with which the focal spot is moved from position P _xi,yi to each subsequent position P _xi+1,yi+1
and/or
the pulse frequency, pulse length and pulse duration of a pulsed energy beam.

Where i = 1 to N can be selected and N in particular a number of complete descents of contours until a predetermined target temperature distribution within a respective processing area of a workpiece has been achieved by achieving a certain heating with possible subsequent cooling,

An external contour or contour can be understood as the path or the direction of advance of the focal spot of the energy beam along the respective surface during a single traverse. A respective initial contour can be specified before the start of processing and the contours to be traversed subsequently can then also be regulated and the focal spot covers different paths when traversing a contour as a result of the regulation.

A given initial contour can, for example, be spiral-shaped, meander-shaped, circular, or annular with changing radii, a row and column arrangement of positions P _x,y, but also completely arbitrary or randomly selected. This contour can then be changed as a result of the control by moving the focal spot to other positions P _xi,yi than those selected for the initial contour. If contours are traversed multiple times in a controlled manner, the contour geometry can also be changed. It can also be the case that each time a contour is traversed, the respective contour can be adapted to the currently measured temperature distribution within the respective processing area, so that the individual contours used when carrying out the process differ from one another on at least some of the traverses of the focal spot of the contours.

As is already practiced elsewhere, a relative movement of the focal spot of the energy beam and the respective surface can be achieved by two-dimensional movement of a workpiece, semi-finished product or powder bed and/or by a correspondingly controlled or regulated deflection of the energy beam with which the impact position of the focal spot on the respective surface can be influenced.

Accordingly, the distances Δl _i between positions P _x,y at which the focal spot movement is stopped can be approached one after the other and/or the respective holding times Δt _i during a stop can be regulated. If larger distances Δl _i are selected, a smaller energy input per area occurs. With longer holding times Δt _i, more energy and thus a greater temperature increase can be achieved in the area of the respective position P _xi,yi .

The speed v of the feed movement can also be controlled depending on the temperature.

If no continuously operated ES is used, the pulse duration and/or the pulse pause length can also be controlled depending on the spatially resolved measured temperatures.

The power of the respective ES as well as the power density in the focal spot can also be regulated. When adjusting the power density The respective size of the area of the focal spot can be adjusted as required by adjusting the focus accordingly. Changes in the geometry of a focal spot due to larger deflection angles at which the ES hits a surface can also be taken into account.

This makes it possible to run an unchanged initial contour or a subsequently run contour with the respective focal spot several times before an adjustment to a changed temperature distribution on the surface of the respective processing area is required and carried out. This can be done without having to change any of the control options mentioned above, which must be carried out when running several times, before an actual control of at least one of the other parameters mentioned is carried out.

Für eine Ausgangskontur gilt demnach Δt_i = Δt_Basis und P = P_Basis Initially, an initial contour is specified. The distances Δl between the positions P _xiyi cannot be significantly larger than the diameter of the energy beam focal spot. The relative energy applied (E _relative ) at each position is determined based on a previously selected base dwell time (Δt _base ), the current dwell time Δt _i at the respective individual position (for N points i = 1 ... M), a previously selected base beam power (P _base ) and the current beam power (P) according to equation 2. $$E_{\text{relativ}}=\left(\text{P}\ast \Delta t_i\right)/\left({\text{P}}_{\text{basis}}\ast \Delta t_{\text{Basis}}\right)$$

For an initial contour, Δt _i = Δt _Basis and P = P _Basis

The input variables can be passed to an actuator and to a controller of the energy beam ES. The ES projects the output contour onto the surface of the processing area (this can also be one or more parts of a larger total area) and scans this repeatedly (initially with the output power P _Basis ). The projection of the contour onto the surface of the processing area is referred to below as the heating field. The temperature distribution that occurs in the heating field is the controlled variable for the associated control loop of the electronic evaluation and control device. The radiation emitted from the component surface can be received by a measuring element as spatially resolved measurement signals. A thermal camera that can measure in the infrared range can be used as a sensor for this purpose. However, measurement in a different wavelength range is also conceivable.

The temperatures at at least 50, preferably at least 100 and most preferably at least 10,000 positions should be recorded simultaneously and taken into account in the control system.

The digitized measurement signals can be used to determine a two-dimensional actual temperature distribution. The number of pixels of the temperature measurements measured by the thermal camera and the position distances of the respective contour can be adjusted in advance so that the xy coordinates of the positions of the contour match those of a respective pixel. The actual temperature distribution determined in this way is compared with a specified target temperature distribution. For each position P _xi,yi, the difference between the determined actual temperature and the respective target temperature is calculated. This results in a matrix with the target-actual deviations for each pixel of the measured temperature distribution. These deviations are used as the basis for a control that can be carried out, for example, with a PID controller (proportional-integral-derivative controller). On the one hand, Δt _i can be controlled for each position with an individual control curve. On the other hand, the control of P* Δt _i can be used to compensate for temperature differences within a traced contour, while P is identical for each position P _xi,yi and therefore only regulates the level of energy input proportionally. As a result, there is no need for temporal synchronization between the current beam position and the beam power. Control of the beam power using another contactless measuring method in a separate control loop or based on a section of the actual temperature distribution is also conceivable.

Based on the differences, a contour is created in which individually adjusted holding times for each position P _xi,yi . as well as an adjusted beam power can be maintained or set. Both are fed back to the actuator, the energy beam. Scanning then takes place along a new contour to be followed subsequently and, if necessary, adjusted power. Then another run of the control loop begins until the specified target temperature distribution within the processing area has been achieved.

It should be remembered that the energy beam repeatedly scans the surface during the entire control cycle and follows the contours of its focal spot. The number of scan cycles completed with an individual contour and beam power also depends on the control or scanning frequency. The control method can be used both for a statically held processing area and for relative movement between the heating field and the processing area.

During the processing/heating process, the specified starting area, the processing/heating area, the specified temperature distribution and/or the parameters of the electronic control and evaluation unit can also be modified.

During processing, the spatially resolved temperatures and associated parameters with which the controlled processing is carried out can be transferred to a memory and control strategies for future and at least similar processing can be determined and made available using an electronic evaluation unit. Using stored values and the control strategies determined in this way, subsequent identical or similar processing can be carried out in the future, so that the control can also be improved over time.

• • die Lösung dieses Problems mittels eines einzigen Energiestrahls, ohne zusätzliche Heizvorrichtungen, kombiniert mit der Tatsache, dass
• • die Regelung u. a. mittels des relativen Energieeintrags an definierbaren Positionen des Heizfelds erfolgt: d.h. die Regelung der 2D-Temperaturverteilung erfordert keine unmittelbare zeitliche Synchronisation zwischen Strahl-Leistung und Strahl-Bewegung, da die Lokalisierung des Energieeintrags inhärent zu einer Stellgröße der Regelung ist und
• • jede Koordinate im Heizfeld mit individuellem Regelungsverlauf geregelt werden kann.
• • Weiterhin ist die Auflösung der Unterteilung der Behandlungsfläche aufgrund der Nutzung eines Energiestrahls mit kleinem Brennfleckdurchmesser hoch.

Essentially, the invention enables the control of a 2D temperature distribution at definable coordinates using an energy beam in a closed control loop. The main advantage over the prior art is that

• • the solution of this problem by means of a single energy beam, without additional heating devices, combined with the fact that
• • the control is carried out by means of the relative energy input at definable positions of the heating field: ie the control of the 2D temperature distribution does not require immediate temporal synchronization between beam power and beam movement, since the localization of the energy input is inherent to a control variable and
• • each coordinate in the heating field can be controlled with an individual control process.
• • Furthermore, the resolution of the subdivision of the treatment area is high due to the use of an energy beam with a small focal spot diameter.

• • eine Vorwärmung, Versinterung sowie das Fusionieren bei der Additiven Fertigung/ bei Beschichtungsverfahren,
• • das Randschichthärten oder -umschmelzen von bewegten oder statischen Bauteilen mit unterschiedlichsten Geometrien,
• • weitere lokale Wärmebehandlungen (z. B. Glühen) an nahezu beliebigen Oberflächenkonturen,

für Fügetechnologien (z. B. Halten der Temperatur beim Löten oder Vor- bzw. Nachwärmen beim Schweißen) genutzt werden.The invention can be used for

• • preheating, sintering and fusion in additive manufacturing/coating processes,
• • surface hardening or remelting of moving or static components with different geometries,
• • further local heat treatments (e.g. annealing) on almost any surface contour,

for joining technologies (e.g. maintaining the temperature during soldering or preheating or postheating during welding).

Claims (5)

Method for the thermal processing of processing areas on surfaces of workpieces, semi-finished products or powder beds, in which an energy beam with a base power P _Basis is directed at the surface and its focal spot is moved two-dimensionally along a predetermined starting contour within the processing area in such a way that the focal spot is moved from a position P _xi,yi to a subsequent position P _xi+1,yi+1 , which is arranged at a predetermined distance Δl _i from the previous position P _xi,yi , and is not moved any further at the respective positions P _xi,yi for a predeterminable holding time Δt _Basis and this is carried out until the predetermined starting contour has been traversed with the focal spot and the temperature on the surface of the processing area is measured in a spatially resolved manner and fed to an electronic evaluation and control unit and then a control is carried out with the appropriately designed electronic evaluation and control unit depending on the spatially resolved recorded temperatures in such a way that a predetermined starting contour that is to be traversed is converted into the spatially resolved recorded temperatures taking into account adapted contours is changed and/or the specified distances Δl _i+x between the individual successively traveled positions P _xi+x,yi+x and/or the holding times Δt _i+x at the individual positions P _xi,yi , are adjusted and this is repeated until a predetermined locally defined temperature distribution in the processing area has been achieved. procedure according to claim 1 , characterized in that in addition to changing the initial contour, at least one further of the following control options is changed during thermal processing as a function of the spatially resolved temperatures in the processing area, and for this purpose the power P _i with which the energy beam is operated, the power density in the focal spot, the speed v _i with which the focal spot is moved from position P _xi,yi to a subsequent position P _xi+1yi+1 and/or the pulse frequency, the pulse duration and the pulse pause duration of a pulsed energy beam are/are controlled. Method according to one of the preceding claims, characterized in that a predefined initial contour is spiral-shaped, meander-shaped, circular, circular-ring-shaped with alternating Radii, a row and column arrangement of positions P _xi,yi but also completely arbitrary or randomly selected. Method according to one of the preceding claims, characterized in that a modification of the predetermined starting surface, the processing/heating area, the predetermined temperature distribution and/or the parameters of the electronic control and evaluation unit is carried out during the temporal processing/heating process. Method according to one of the preceding claims, characterized in that during the processing, the spatially resolved recorded temperatures and associated parameters with which the controlled processing is carried out are transferred to a memory and control strategies for future and at least similar processing are determined and made available by means of an electronic evaluation unit.