DE102018132441A1 - Process for processing a surface with energetic radiation - Google Patents

Abstract

In a method for processing a surface with energetic radiation, in particular for selective laser melting, at least one energetic beam is guided along one or more regions of the surface along a plurality of scan vectors running at a distance from one another. If the length of the scan vectors in one or more of the areas varies, the distance between adjacent scan vectors is selected depending on the length of at least one of the adjacent scan vectors. The process can be used to increase the remelting rate in powder bed-based jet melting processes and thus reduce the construction time for the components to be manufactured.

Description

Technical application area

The present invention relates to a method for processing a surface with energetic radiation, in particular for selective laser melting, in which at least one energetic beam is guided along one or more regions of the surface along a plurality of scan vectors running at a distance from one another.

With such processing methods, surface layers can be melted or removed. In additive manufacturing processes, especially powder bed-based beam melting processes such as With selective laser melting (SLM), components are generatively produced directly from 3D CAD models. A repetitive process of layer-by-layer application of a thin powder layer with a thickness of less than 100 µm, for example, and subsequent selective melting of certain areas of this powder layer by laser radiation, in accordance with the geometry information of the 3D CAD model, creates three-dimensional components of almost unlimited complexity. A substrate plate on which the powder layers are applied one after the other serves as the basis for powder bed-based jet melting processes. The laser radiation is then guided over the substrate plate or the construction field, for example with the aid of galvanometer scanners.

During the construction process, certain areas of the powder bed are remelted in each layer using the laser beam. The areas to be remelted are in turn divided into subordinate areas depending on the process control strategy used. These sub-areas can be strips or fields, for example. 1 shows an example of an exemplary component cross section of any layer of such a manufacturing process with a corresponding division into strips 2nd . The solid line represents the component contour 1 of the component to be manufactured. The width of the individual strips 2nd is firmly defined. Each of these strips 2nd with individual, parallel scan vectors 3rd filled. These scan vectors 3rd represent the path along which the laser beam is guided to melt or expose the layer. 2nd shows an example of a strip 2nd of the component cross section 1 with the associated scan vectors 3rd . During the exposure process, the scan vectors 3rd of a strip 2nd departed one after the other. The laser beam then jumps to the beginning of the next stripe and scans the vectors of this stripe until all stripes of the respective layer have been scanned. In alternative process control strategies, the area of each layer to be exposed can also be divided into square fields, similar to a chessboard, instead of strips. The individual fields are then filled with scan vectors that are parallel to one another and identical to the strip exposure. Other patterns for dividing the areas of a layer to be exposed are also possible. The strip-shaped exposure is considered as an example below.

An essential property of conventional process control strategies for beam melting processes is to define a constant distance between the scan vectors lying within the strip. This distance is also called the track distance or hatch distance. The amount of track spacing is typically constant for all vectors within the stripes of a component. In such a process, the process parameters in general or the track spacing in particular are typically determined on cube-shaped test specimens with a constant scan vector length before the process is carried out, the scan vector length corresponding to the strip width.

State of the art

From the DE 196 06 128 A1 A method for producing a three-dimensional object is known, in which the object is produced by successively solidifying individual layers of a solidifiable material at the locations corresponding to the object cross section by the action of a beam of electromagnetic radiation. In this method, the scanning speed with which the beam is guided over the respective layer is adjusted as a function of the scan vector length. A higher scan vector is assigned a higher speed than a long scan vector. This procedure is intended to achieve an equalization of the temperature distribution within the component during production, which leads to a homogeneous density distribution within the respectively solidified layer. This strategy is particularly relevant for plastic components, for example to prevent overheating effects.

A fundamental problem with such additive manufacturing processes is the construction time required to manufacture a component. Shortening the construction time by choosing larger layer thicknesses or beam diameter leads to an undesirable loss of accuracy or resolution, as is required in particular with more complex components. A similar problem also exists with other methods of surface processing with energetic radiation, in which the energetic beam is guided along corresponding scan vectors over one or more areas of the surface.

The object of the present invention is to provide a generic method for machining a surface with energetic radiation, which enables an increase in the machining or construction speed without loss of machining accuracy. The method is intended in particular to enable the construction speed to be increased in a powder bed-based beam melting process for the production of three-dimensional components.

Presentation of the invention

The object is achieved with the method according to claim 1. Advantageous embodiments of the method are the subject of the dependent claims or can be found in the following description and the exemplary embodiment.

In the proposed method for processing a surface with energetic radiation, in particular for selective laser melting, at least one energetic beam is guided in a known manner along a plurality of scan vectors running alongside one another over one or more regions of the surface. The method is characterized in that, with a varying length of the scan vectors in one or more of the areas, the distance between adjacent scan vectors is selected in each case as a function of the length of at least one of the respectively adjacent scan vectors. The distance between adjacent scan vectors is preferably selected depending on the length of the scan vector along which the energetic beam is first guided.

This variable adaptation of the distance between the scan vectors or the track spacing as a function of the scan vector length reduces the number of required scan vectors in the case of corresponding components or surfaces and thus increases the remelting rate. In addition, this results in a reduced number of non-productive times within the beam guidance system, so-called scanner delays. The method thus enables the construction speed to be increased in the area of additive manufacturing, for example in selective laser melting. The method can be used without any modification of the technical design of the construction or processing system. Adjustments are primarily necessary in the software for data preparation and possibly in the software for controlling the system.

According to tests carried out to date, the method enables, depending on the component geometry, an increase in the construction speed in selective laser melting of 10% to 30% while maintaining the desired material density, for example in the range of 99.5% relative density. This density results from the proportion of the pores remaining in the material after the surface layer has been melted and solidified. In particular in the case of the filigree component structures with correspondingly short scan vectors, which are preferably manufactured by means of the SLM method, the proposed method can be used in particular. The shorter the average vector length for the respective component, the stronger the advantages of the method compared to conventional exposure strategies with a constant distance between the scan vectors.

In addition to the above-described advantage of an increased construction speed in additive manufacturing processes, it is also possible to achieve a homogenization of the temperature field during the remelting process. This can result in advantages in terms of process stability, for example reducing welding spatter, and component quality, for example due to reduced internal stresses within the component.

The proposed method takes advantage of the fact that the length of the individual scan vectors for most components varies within one of the strips or fields selected for processing in accordance with the component contour. The shorter the length of a scan vector, the shorter the cooling time of the associated melting trace until the laser beam in turn heats up the neighboring trace. This local preheating effect widens the weld pool. Accordingly, with shorter scan vectors and a correspondingly shorter cooling time or a wider weld pool, a larger track spacing or spacing of the scan vectors can be set and a connection of the tracks to one another can also be ensured. This variation of the distance between the scan vectors or track distance as a function of the length of at least one of the adjacent scan vectors is used in the present method carried out. The result is an increase in the remelting rate, a reduction in the number of vectors and a homogenization of the temperature distribution within the strip or field to be exposed.

The proposed method can be used for all beam-melting additive manufacturing processes in which the component areas are traversed vector by vector by means of electromagnetic radiation. With the help of appropriately adapted software, the process can be implemented on existing as well as on future available systems. In addition to additive manufacturing, the method can also be used for other areas in which the surface is processed, in particular melted, with at least one energetic beam along a plurality of scan vectors running at a distance, and the length of the scan vectors varies over the respective area.

Figure list

• 1 eine Darstellung einer Aufteilung eines beispielhaften Bauteilquerschnitts einer beliebigen Schicht bei der generativen Fertigung in mehrere zu belichtende Streifen;
• 2 ein Beispiel für eine Anordnung der Scanvektoren innerhalb eines Streifens;
• 3 einen Vergleich zwischen der Länge der Scanvektoren bei einem würfelförmigen Probekörper und einer anwendungsspezifischen Bauteilgeometrie;
• 4 einen Vergleich der konventionellen Belichtungsstrategie mit konstantem Spurabstand (Teilabbildung A) und der Belichtungsstrategie mit variablem Spurabstand beim vorgeschlagenen Verfahren (Teilabbildung B); und
• 5 ein Diagramm, das einen beispielhaften Zusammenhang zwischen Scanvektorlänge und Spurabstand beim vorgeschlagenen Verfahren zeigt.

The proposed method is explained in more detail below using an exemplary embodiment in conjunction with the drawings. Here show:

• 1 a representation of a division of an exemplary component cross-section of any layer in additive manufacturing into several strips to be exposed;
• 2nd an example of an arrangement of the scan vectors within a strip;
• 3rd a comparison between the length of the scan vectors in a cube-shaped test specimen and an application-specific component geometry;
• 4th a comparison of the conventional exposure strategy with constant track spacing (partial image A) and the exposure strategy with variable track spacing in the proposed method (partial image B); and
• 5 a diagram showing an exemplary relationship between scan vector length and track spacing in the proposed method.

Ways of Carrying Out the Invention

The conventional exposure strategy for powder bed-based beam melting processes was described in the introduction to the 1 and 2nd already explained. 3rd shows a comparison between the scan vector length for a cube-shaped specimen selected for the parameter determination (right part of the figure) and the scan vector length for an application-specific component (left part of the figure). Such cube-shaped test specimens are used to determine the process parameters, in particular the distance between the scan vectors or track spacing, in advance during the production of a component. The test specimens consist of the same material as the component to be manufactured. From the 3rd can be seen that within the individual strips 2nd , into which the area to be melted for processing is divided, a constant scan vector length occurs in the cube-shaped test specimen, while it occurs in the application-specific component geometry due to the component contour 1 inside the strips 2nd can vary. This variation results in cooling times of different lengths, before the laser beam on the adjacent track provides for heating, and thus melting baths of different widths. This is used in the present method by setting a larger distance between the scan vectors (track spacing) in the case of shorter scan vectors and a correspondingly shorter cooling time or a wider weld pool, which further ensures that the material of adjacent tracks is connected to one another. The distance between the scan vectors or the track distance is adjusted by setting the distance between two tracks as a function of the length of the track first exposed by these two tracks. 4th shows this procedure schematically for a triangular component contour.

In 4A the exposure strategy typical of SLM up to now is shown. The distance is characteristic D1 between the individual scan vectors 3rd which is constant for all vectors, regardless of their length. 4B on the other hand schematically shows the novel exposure strategy with variable track spacing used in the proposed method. The distance between the scan vectors or the track spacing is adjusted depending on the scan vector length. In the present case, D2> D1 applies, since for shorter scan vectors 3rd the track spacing is increased.

The enlargement or variation of the track spacing is carried out in such a way that, even with a correspondingly enlarged track spacing, a predetermined relative minimum density, for example 99 99.5%, of the components produced thereby is achieved.

The relationship between the scan vector length and the distance between the scan vectors or track spacing can be determined experimentally, for example, by producing test specimens of different sizes and thus vector lengths. In this way, a table of values with assignments between the scan vector length and track spacing can be created for the respective material and the respective system. Alternatively, the relationship between scan vector length and track spacing can also be determined by means of simulation.

An exemplary value table with five discrete data points for five different scan vector lengths is shown below. Vector length [mm] Track spacing [µm] 1 140 2nd 135 3rd 125 4th 110 5 90

5 shows the graphic relationship between the scan vector length and the distance between the scan vectors and the track spacing. For each scan vector or its specific length, the associated distance is taken into account in the data preparation for production in accordance with the value table. You can interpolate between the individual discrete values.

By increasing the track spacing, the remelting rate is increased and the number of vectors required is reduced. The preparation of the corresponding component data with variable track spacing is preferably carried out during path planning with the aid of special software that can process the experimentally determined value tables accordingly.

In the present description and the exemplary embodiment, reference was made above all to methods for the selective laser melting of metallic materials, in which galvanoscanners are generally used as steel deflection units. However, it is also a transfer to other beam sources, for example electron beam sources, other beam deflection units such as for example XY plotter systems or other materials such as e.g. Plastics easily possible. The present method is not limited to selective laser melting, powder bed based beam melting or generative manufacturing in general.

Reference symbol list

1

Component contour

2nd

Stripes

3rd

Scan vectors

D1 / D2

Distance of the scan vectors

QUOTES INCLUDE IN THE DESCRIPTION

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

Patent literature cited

• DE 19606128 A1 [0005]

Claims (7)

Process for processing a surface with energetic radiation, in particular for selective laser melting, in which at least one energetic beam is guided along one or more regions (2) of the surface along a plurality of scan vectors (3) running at a distance from one another, characterized in that at varying Length of the scan vectors (3) in one or more of the areas (2), a distance between adjacent scan vectors (3) is selected depending on the length of at least one of the adjacent scan vectors (3). Procedure according to Claim 1 , characterized in that the distance between adjacent scan vectors (3) is chosen depending on the length of the scan vector (3) along which the energetic beam is first guided during processing. Procedure according to Claim 1 or 2nd , characterized in that the distance between adjacent scan vectors (3) is selected such that it increases as the length of the scan vectors (3) decreases and decreases as the length of the scan vectors (3) increases. Procedure according to one of the Claims 1 to 3rd , characterized in that the distance between adjacent scan vectors (3) is taken from a table in which different distances were previously assigned to different scan vector lengths. Procedure according to one of the Claims 1 to 4th , characterized in that the dependence of the distance between adjacent scan vectors (3) on the length of at least one of the respectively adjacent scan vectors (3) is determined beforehand experimentally or by simulation. Procedure according to one of the Claims 1 to 5 , characterized in that the distance between adjacent scan vectors (3) is maximized in the manufacture of three-dimensional components by melting a powdery material layer by layer, in each case observing the condition that a predetermined density of the manufactured components is achieved at the respectively selected distance. Procedure according to one of the Claims 1 to 6 , characterized in that the respective distance between the adjacent scan vectors (3) is determined before processing as part of a path planning using software.

Images