WO2023001500A1

WO2023001500A1 - Surface forming process for additively produced structures

Info

Publication number: WO2023001500A1
Application number: PCT/EP2022/067635
Authority: WO
Inventors: Christoph Haberland; Michael Ott
Original assignee: Siemens Energy Global GmbH & Co. KG
Priority date: 2021-07-20
Filing date: 2022-06-28
Publication date: 2023-01-26
Also published as: EP4334059A1; DE102021207722A1; CN117677456A

Abstract

The invention relates to a process for treating the surface of a structure (10) that has been additively produced from a powder bed, the additively built structure (10) being exposed to an impulse (1) and the internal tensile stresses in surface areas (10) of the structure (10) being reduced by the impulse (1). The invention also relates to an apparatus (100) for carrying out the process, a corresponding process for additively producing a structural component, and to a structural component thereby produced from a conventionally non-weldable alloy.

Description

description

Surface reshaping for additively manufactured structures

The present invention relates to a method for surface treatment, in particular for superficial reshaping or surface modification of a structure produced additively, preferably from a powder bed. Furthermore, a device for carrying out this method, a manufacturing method for a component, including the method for surface treatment and a correspondingly manufactured component are specified.

The component is preferably intended for use in the hot gas path of a gas turbine. Accordingly, the component can be made of a superalloy and/or a (hardenable) nickel- or cobalt-based material. For example, the component relates to a component to be cooled with a thin-walled or filigree design. Alternatively or additionally, the component can be a component for use in automobiles or in the aviation sector.

High-performance machine components are the subject of constant improvement, in particular to increase their efficiency in use. In the case of heat engines, in particular gas turbines, however, this leads, among other things, to ever higher operating temperatures. The metallic materials and the component design of heavy-duty components such as turbine rotor blades, especially in the first stages, are constantly being improved in terms of their strength, service life, creep resistance and thermomechanical fatigue.

Due to technical developments, generative or additive manufacturing is becoming increasingly interesting for the series production of the above-mentioned components, such as components that can withstand high thermal loads. Additive manufacturing processes (AM: "additive manufacturing"), colloquially also referred to as 3D printing, include, for example, powder bed processes such as selective laser melting (SLM) or laser sintering (SLS), or electron beam melting (EBM). Other additive processes are for example "Directed Energy Deposition (DED)" methods, in particular laser deposition welding, electron beam or plasma powder welding, wire welding, metallic powder injection molding, so-called "sheet lamination" method, or thermal spraying methods (VPS LPPS, GDCS).

A method for selective laser melting with pulsed radiation is known, for example, from EP 3022 008 Bl.

Additive manufacturing processes have also proven to be particularly advantageous for complex or delicately designed components, such as labyrinthine structures, cooling structures and/or lightweight structures. In particular, additive manufacturing is advantageous due to a particularly short chain of process steps, since a manufacturing or manufacturing step of a component can be carried out largely on the basis of a corresponding CAD file and the selection of corresponding manufacturing parameters.

The production of gas turbine blades using the described powder bed-based process (LPBF for “Laser Powder Bed Fusion”) advantageously enables the implementation of new geometries, concepts, solutions and/or designs that reduce the manufacturing costs or the construction and throughput time reduce, optimize the manufacturing process and, for example, improve a thermo-mechanical design or durability of the components.

Components manufactured in a conventional way, e.g. by casting, stand up to the additive manufacturing route, for example with regard to their freedom of design and also with regard to the required throughput time and the with associated high costs and the manufacturing effort, significantly.

However, the powder bed process inherently creates high thermal stresses in the component structure. In particular, radiation paths or vectors that are too short lead to severe overheating, which in turn leads to distortion or cracking of the structure. Severe distortion during the assembly process also easily leads to structural detachments, thermal deformation or geometric deviations outside of an allowable tolerance.

During a subsequent heat treatment of certain alloys (superalloys) after the additive build-up, superimposition of residual stresses occurs - on the one hand resulting from the high thermal gradients in the AM process, on the other hand due to the precipitation of the gamma prime phase (g') in the heat treatment to crack formation. This phenomenon is referred to as "strain-age-cracking" (SAC) and means that certain alloys are considered unweldable or difficult to weld and therefore pose a major challenge for welding processes, especially the LPBF process, or even pose a challenge The SAC, otherwise also referred to as "post-weld heat treatment" (PWHT) cracking, has the consequence, particularly in the precipitation of the gamma prime phase, that internal stresses are caused by the different lattice constants of g and g' , which lead to the tendency of the corresponding material to crack.

This problem is countered by different approaches; however, to date, no satisfactory solution has been available that solves the problem without inevitably degrading the material properties - and in particular the high-temperature strength and/or creep rupture strength - of the corresponding material. For example, attempts are already being made to adjust a chemical composition of the material to be welded. As a rule, the content of alloying elements that contribute to the formation of the gamma prime phase is reduced. However, this is disadvantageously also associated with a devaluation of the material properties in terms of strength or thermal and/or mechanical resistance.

Another approach concerns the adaptation or integration of the heat treatment by a so-called HIP cycle (Hot Isostatic Pressing). The problem of poor weldability or susceptibility to cracking can thus be kept within certain limits or suppressed. In the case of particularly high-strength alloys (e.g. CM247), however, this approach only has a partial effect. If there are also surface-open defects in particular, the crack cannot be closed even as a result of the pressurization in the HIP cycle, nor can it be prevented from propagating (crack propagation).

It is also possible to subject component surfaces to a shot peening process (cf. "peening") before an initial heat treatment, in order to reduce cracking and/or stress-causing residual tensile stresses in areas close to the surface and, ideally, even to generate residual compressive stresses. This can effectively reduce the SAC. However, this measure is quite complex and only insufficiently reproducible.

Furthermore, shot peening coverage is limited. The accessibility of the blasting process is insufficient, especially in narrow gaps or notches. Because it is precisely in these hard-to-reach and/or filigree areas of complicated additively manufactured geometries that stress concentrations and/or notch effects often occur, which significantly increase the SAC susceptibility. In addition, internal areas, such as cooling channels in turbine blades, cannot be reached at all by the shot peening, so that the residual tensile stresses cannot be reduced here. Another challenge in shot peening is to prevent the peening material from penetrating into internal structures (e.g. cooling channels), which then sinters or even melts in the subsequent heat treatment and can thus close the cavity or channel. Said closure can then (economically) no longer be eliminated, which leads to the component being unusable for its intended use. Furthermore, there is a risk in the shot peening process that - due to the locally limited effective zone - component areas can be excessively deformed. This can apply in particular to the additively manufactured exit edges of turbine blades, but also to any other delicately designed component.

It is therefore an object of the present invention to solve the stated problem in a different, more effective way, without impairing other relevant properties of the component.

This object is solved by the subject matter of the independent patent claims. Advantageous configurations are the subject matter of the dependent patent claims.

One aspect of the present invention relates to a method for surface treatment or surface shaping of a structure additively produced from the powder bed, the additively built structure being subjected to a pulse or energy pulse, with residual tensile stresses in surface areas of the structure being reduced by the pulse.

The pulse can in particular mean a pressure wave or a pressure pulse.

By the method described here can be advantageously a uniform loading of the entire construction part surface, preferably in all surface areas can be achieved, whereby the risk of SAC uniform and can be significantly reduced. In other words, the method described for surface treatment or surface modification is uniformly accessible for all relevant component areas, including notch bases and internal surfaces such as cooling channel surfaces.

Furthermore, there is advantageously no risk of closure or clogging of internal cavities of the component by blasting material or (other) sintering effects.

It is also advantageously inherently prevented that blasting material residues remain in the component and impair the functionality (physically or chemically). In particular, the risk of the formation of intermetallic phases (in the case of metallic blasting material) or low-melting eutectic phases in the case of vitreous blasting material can be advantageously prevented.

Furthermore, deformation of the component as a whole can advantageously be prevented by the isotropic or uniform pressure or pressure pulse on the component or its surface area. This is particularly important for filigree designs.

The surface treatment process is even scalable to the simultaneous treatment of a number of (small or large) additively manufactured components, provided that all components can be pulsed in a common mold.

In one embodiment, the impulse is not or not exclusively thermal impulse.

In one embodiment, the structure of the component is subjected to the impulse in such a way that the surface regions are acted upon isotropically and/or uniformly, or are deformed. In one embodiment, the process induces compressive stresses in the surface areas of the structure. This configuration can advantageously not only reduce the intrinsic tensile stress of the component, but also counteract the SAC inclination of the component structure even more effectively by the compressive stresses.

In one embodiment, the impulse causes high-energy or high-speed deformation of the surface areas, which is locally limited, so that there is no plastic deformation of the structure as a whole. With this configuration, the energy expenditure for the surface treatment is also moderate, in particular, since it is locally limited.

In one configuration, the surface areas include an inner surface or internal surface and/or an outer surface of the component structure.

In one embodiment, the surface areas of the structure have a fine or sharp-edged geometry (see above).

In one embodiment, an incompressible or virtually incompressible medium or active medium that is in contact with the surface areas to be treated is used for the application of the pulse. This advantageously achieves a uniform pulse impingement on the surface areas, and internal surfaces of the component structure are also impinged on evenly and effectively.

In one embodiment, the named active medium includes water or oil or consists of these substances.

By using the active medium, for example, a medium for the generation of pressure waves and a means for the transmission of the pulse can be specified at the same time. In one embodiment, the impulse is applied by hydro-impulse conversion. This can be accomplished, for example, by or via an explosion, a blast, a projectile, or a drop weight.

In one embodiment, the pulse is applied by an electrical discharge, an electrohydraulic transformation, or a hydroelectric transformation.

In one embodiment, the pulse is applied by electrodynamic or electromagnetic conversion, in particular a magnetic pressure pulse.

In one embodiment, a pulse strength of the applied pulse corresponds to an Almen intensity of more than 0.1 mmA.

In one embodiment, a pulse strength of the applied pulse corresponds to an Almen intensity of more than 0.14 mmA.

These configurations allow the surface to be acted on in a particularly effective or impulsive manner, as is the case with related blasting or shot-peening processes.

In one configuration, the structure is or is made from a nickel or cobalt-based alloy.

A further aspect of the present invention relates to an apparatus for carrying out the method described, the apparatus further comprising a mold and a device and a medium for applying the impulse (as described above).

A further aspect of the present invention relates to a method for additively manufacturing a component, comprising additively building up the component structure by means of a powder bed processes such as selective laser melting or electron beam melting.

This method also includes subjecting the structure to the impulse, with a cracking tendency of the structure, in particular by so-called “strain age cracking”, being reduced and, preferably, a heat treatment of the structure.

A further aspect of the present invention relates to a component which is produced or can be produced according to the method or methods described, the component being produced from a (conventionally) non-weldable alloy.

Configurations, features and/or advantages, which in the present case relate to the method for surface treatment or the corresponding device, can also directly relate to the described additive manufacturing method and the correspondingly manufactured component, and vice versa.

As used herein, the term "and/or" when used in a series of two or more items means that each of the listed items can be used alone, or any combination of two or more of the listed items can be used .

Further details of the invention are described below with reference to the figures.

FIG. 1 shows a schematic sectional view of an additively manufactured component structure and a device for carrying out the method described here.

FIG. 2 shows a schematic flow chart indicating method steps according to the invention.

In the exemplary embodiments and figures, elements that are the same or have the same effect can each be given the same reference numbers. be provided. The elements shown and their size ratios to one another are not to be regarded as true to scale; rather, individual elements may be shown with exaggerated thickness or large dimensions for better display and/or better understanding.

Manufacturing systems for powder bed-based additive manufacturing, for example by selective laser sintering, se lective laser melting or electron beam melting, are well known in the prior art.

Additive manufacturing of the component structure, as is described here, preferably relates to the methods just mentioned. Accordingly, a component structure 10 as described in FIG. 1 can be manufactured in an LPBF system (not explicitly identified in the figures). Alternatively, the system can also relate to a system for electron beam melting.

What these methods have in common is that a corresponding production plant usually has a construction platform on which the component 10 is irradiated in layers by selectively irradiating a powder in a powder bed.

The powder is distributed in layers on the construction platform by a coating device.

After the application of each powder layer, according to the predetermined geometry of the component 10, selective regions of the layer are melted with an energy beam, for example a laser or electron beam, from an irradiation device and then solidified.

After each layer, the construction platform is preferably lowered by an amount corresponding to the layer thickness L. The thickness is usually only between 20 and 40 gm, so that the whole process easily involves the selective irradiation of can range from thousands to tens of thousands of layers.

High temperature gradients of, for example, 10 ⁶ K/s or more can occur due to the only very locally acting energy input. Of course, there is a correspondingly large amount of stress on the component during assembly and afterwards, which considerably complicates the additive manufacturing processes. Said state of stress results on the one hand from the high thermal gradients in the AM process. On the other hand, in a precipitation heat treatment subsequent to the additive build-up, in which, for example, phase precipitations, in particular the gamma-ray phase, are caused, stresses are built up which are caused by the phase precipitation itself. This phenomenon is referred to as "strain-age cracking" (SAC) and means that certain alloys are considered to be hardly or not at all weldable and are therefore unworkable with AM welding.

The component 10 described here can, in particular, be a component of a turbomachine, for example a component for the hot gas path of a gas turbine. In particular, the component can be a blade or vane, a ring segment, a combustion chamber or a burner part, such as a burner tip, a skirt, a shield, a heat shield, a nozzle, a seal, a filter, an orifice or lance, a Designate a resonator, a stamp or a swirler, or a corresponding transition, a set, or a corresponding aftermarket part.

In particular, the component 10 is preferably produced additively and from a (precipitation-hardenable) superalloy and/or nickel- or cobalt-based alloy, particularly preferably from a conventionally or regularly non-weldable alloy (additive). FIG. 1 schematically shows a device 100 for carrying out the method for surface treatment described here. For this purpose, the device 100 comprises in particular a device 101 and a medium 102 for applying an energy or pressure pulse or a corresponding pressure wave, as well as a forming tool or forming die identified by the reference number 103 .

The component structure 10 that is identified in Figure 1 and is produced additively can be produced by the device 100 as uniformly, isotropically and/or homogeneously as possible with the said pulse (cf. arrow between the device for generating the pulse 101 and the component structure 10 in Figure 1 ) be applied, whereby residual tensile stresses in surface areas of the structure can be effectively reduced. Preferably, the treatment described can even induce compressive stresses in the surface areas, which further significantly reduce or even completely prevent the SAC tendency to crack.

With the method described, the impulse 1 can also be used particularly advantageously only on the surface areas or in a locally limited manner for forming the component structure, with plastic deformation of the structure as a whole also being able to be avoided, for example.

The uniform effect of the pulse can preferably be effected via the medium 102, which medium can preferably be an active medium such as water and/or oil. The active medium 102 mentioned is expediently in direct contact with surface areas or surfaces of the component structure 10 when the surface treatment is used. For the application or transmission of the impulse, the active medium 102 is expediently not, quasi-incompressible or hardly compressible in order to efficiently also transmit a mechanical impulse or to bring about a densification of the component structure at least in the surface areas. Chen in the surface areas can be all near-surface areas, ie inner Surfaces 12 and outer surfaces 13 of the component or particularly filigree areas 11 or edge areas of the component 10 act.

FIG. 1 also shows that the component has a cavity 14, for example a functional hollow space or a cooling channel, the inner surfaces 12 of which can therefore also be reliably modified by the method described, and a tensile stress state that triggers SAC can thus be efficiently counteracted can.

The pulse 1 is preferably not or not exclusively of a thermal nature. A pure heat treatment cannot solve the problem addressed by the invention; although heat treatment may be required for further or general stress relaxation, as well as for precipitation hardening (component and material dependent).

The pulse 1 or a corresponding pressure wave can be applied according to the inventions, for example, by a hydro-Ipul transformation. For this purpose, the device 101 and the corresponding active medium 102 are preferably specially designed for this type of forming.

The same applies to an impulse caused by electrical discharge, electrohydraulic or hydroelectric transformation, which can also relate to an embodiment of the method according to the invention.

Electrodynamic or electromagnetic conversion (conversion by magnetic pressure pulse) can also be applied by way of the described invention.

According to one embodiment of the surface treatment according to the invention, an impulse strength of the applied impulse 1 preferably corresponds to an Almen intensity of more than 0.1 or 0.14 mmA. If, after the powder bed-based additive build-up of the component structure 10, depowdering or removal of powder residues in the cavity 14 is still required, the surface shaping described is preferably carried out after this depowdering, but at the same time before a subsequent heat treatment for the above-mentioned purposes.

FIG. 2 also uses a schematic flow chart to indicate a process step according to the invention for the production or completion of a component made of a high-performance alloy. The method described includes (a) the additive construction of the component structure 1 by a powder bed method, such as selective laser melting or electron beam melting.

As indicated above, method step aa) describes an optional method step of depowdering, which—depending on the geometry of the component—can be dispensable.

The method also includes, in (b), that the structure 10 is subjected to an impulse 1, as described above, where a tendency of the structure to crack, in particular by so-called "strain age cracking", is reduced, and in (c) ei ne heat treatment of the structure, which may be required for the purposes described above.

Claims

patent claims

1. Method for the surface treatment of a structure (10) produced additively from a powder bed, wherein the structure (10) built up additively is exposed to a pulse (1), wherein internal tensile stresses in surface regions (10) of the structure (10) are caused by the pulse ( 1) be reduced, wherein the surface areas comprise an inner surface (12) and/or an outer surface (13) of the structure (10), and wherein surface areas of the structure have a fine or sharp-edged geometry (11).

2. The method according to claim 1, wherein the pulse (1) is not or not exclusively of a thermal nature.

3. The method according to claim 1 or 2, wherein the structure (10) is subjected to the impulse in such a way that the surface regions (11, 12, 13) are acted upon isotropically and/or uniformly.

4. The method according to any one of the preceding claims, wherein - by the process - compressive stresses in the surface areas of the structure (10) are induced.

5. The method according to any one of the preceding claims, wherein the impulse causes a high-energy deformation of the surface areas that is locally limited, so that there is no plastic deformation of the structure (10) as a whole.

6. The method according to any one of the preceding claims, where - for the application of the pulse (1) - an incompressible or quasi-incompressible active medium is used (101), which is in contact with the surface areas.

7. The method according to claim 6, wherein the active medium comprises water or oil or consists of one of these substances.

8. The method according to any one of the preceding claims, where in the impulse (1) is applied by hydro-impulse forming.

9. The method as claimed in one of the preceding claims, where the pulse (1) is applied by an electrical discharge, an electrohydraulic or a hydroelectric transformation.

10. The method according to any one of the preceding claims, where in the pulse (1) is applied by an electrodynamic or electro-magnetic transformation.

11. The method according to any one of the preceding claims, where at a pulse strength of the pulse (1) corresponds to an Almen intensity of more than 0.14 mmA.

12. The method according to any one of the preceding claims, where the structure (1) is made of a nickel or cobalt base alloy.

13. Device (100) for carrying out the method according to any one of the preceding claims, comprising a molding tool (103) and a device (101) and a medium (102) for applying the pulse (1).

14. A method of additively manufacturing a component, comprising:

- (a) Additive build-up of the component structure (1) by a powder bed process, such as selective laser melting or electron beam melting,

- (b) subjecting the structure (10) to an impulse (1) according to one of the preceding claims, wherein a cracking tendency of the structure, in particular by so-called "strain age cracking", is reduced, and

- (c) heat treating the structure.

15. Component (10) which is produced according to the method according to claim 14, wherein the component (10) is made of a conventional non-weldable alloy.