Patent Application
20220032370
The invention relates to a method for manufacturing a part of complex shape by pressure sintering starting from a preform produced by means of an additive technique.
The invention relates to the field of manufacturing parts, in particular industrial mechanical parts, by densifying porous or pulverulent materials. This field covers various techniques for densification by means of sintering under uni- or multiaxial, isostatic load, for example hot, in particular isostatic, pressing, high-pressure spark sintering known as SPS (spark plasma sintering), or selective laser sintering.
More particularly, the SPS sintering technique consists in consolidating a volume of ceramic, polymer or metal powders in a conductive mold in order to rapidly obtain finely microstructured dense materials. This consolidation is achieved by means of the simultaneous application of a load (under high uniaxial pressure exerted on the mold, for example of the order of 100 MPa and heating provided by a high-intensity pulsed direct current in the mold, of the order of 500 to 10000 A, the total sintering of the powders then being obtained in only a few minutes.
The main advantage of this SPS sintering technique is that, given the high rate of temperature rise and the relatively short residence times at high temperature, the densification of the material is accompanied with no, or very little, crystal growth.
The uniaxial pressure exerted by the SPS sintering technique has the drawback of causing heterogeneity of densification, in particular for parts of complex shape having substantial differences in thickness leading to a removal of material that is non-uniform, with respect to the limited space and time, between the regions of different thicknesses. In general, a complex shape refers to a shape of variable curvature with or without variation in thickness, or with a great variation in thickness with or without variation in curvature, and/or with geometric break.
In order to overcome this drawback of inhomogeneity of densification, patent document FR 3 042 992 provides for adding a deformable interface layer between the pulverulent (or porous) material and the counterform face of the mold tailored for producing the part.
However, sintering techniques also have a problem related to the use of molds or mold counterforms which do not allow easy removal from the mold when the part has a complex shape, in particular forming undercuts which are difficult to access and, therefore, tapers which are difficult to remove from the mold without partially destroying the part.
Counterforms may be prints obtained by pressing a bed of ceramic powder agglomerated by a binder or by binding ceramic powder on a polymer preform, the counterform being covered with an inert interface. These techniques have substantial limitations with regard to accessible geometries, due to the problems of undercuts of complex shapes, and of the surface state of these counterforms which requires covering by an interface.
In order to produce counterforms compatible with undercuts that can be removed from the mold, it is then necessary to multiply the number of counterforms in order to avoid problems of removal from the mold. The tools tailored for each counterform then need to be manufactured, resulting in additional design and machining steps. The assembly of these many counterforms is also a source of defects (mismatched geometry of the final part, incipient cracks, loss of material, etc.).
In order to produce parts of complex shape by densifying material, it is also known practice to use the multiaxial isostatic loading technology known as HIP (hot isostatic pressing). This technology is illustrated for example by document WO 2016/189312 which provides for the use of a densifying furnace filled with a pressure transfer material completely surrounding the outer skin of an object preformed by means of an additive printing technique so as to have a skin and an inner volume of different densities. Such an approach implements oversized, complex and time-consuming means, in particular in the context of HIP technology.
Alternatively, the mold of document WO 2016/030654, which relates to the production of components by means of HIP technology, is produced using a sacrificial model of the component surrounded with ceramic. The model is removed to form a cavity and a metal layer is applied to the ceramic to constitute the mold which, filled with metal powder, is then placed in a HIP technology container. This solution is still complex and implements HIP technology.
The invention aims to overcome this problem, and to that end the invention provides for the use of a preform of the part produced by means of a digitally controlled additive technique combined with uniaxial densification.
More specifically, the subject of the present invention is a method for manufacturing a part of complex shape by successive deposition of layers according to a technique of 3D additive printing and pressure sintering, this method comprising the following steps: an initial step of producing a model from a material chosen from porous or pulverulent materials based on a metal alloy, a ceramic, a composite material and a lost material by formation of successive layers deposited according to the from a material chosen from a porous or pulverulent material based on a metal alloy, a ceramic, a composite material and a lost material digitally controlled three-dimensional (3D) additive printing technique, followed by a step of introducing a preform made of porous or pulverulent material to be densified, derived from the model, into a mold filled with sacrificial porous or pulverulent material in addition to the preform. The uniaxial densifying pressure sintering is then applied to the mold in order to form the part which is finally extracted from the mold.
In particular, the 3D additive printing technique is chosen from stereolithography, binder jetting, controlled extrusion, fused filament fabrication, inkjet printing and aerosol jet printing.
In addition, the uniaxial densification allows simplified densification with respect to three-dimensional isostatic densification of HIP type, while making it possible to obtain features of physico-chemical homogeneity and precision of part dimensions.
According to some advantageous features:
the sacrificial material is a material chosen from a ceramic, a silica, a metal silicate and a composite material;
the ceramic may be chosen from powdered YSZ (yttria-stabilized zirconia), ATZ (alumina-toughened zirconia), ZTA (zirconia-toughened alumina), and alumina exhibiting degrees of densification that may range from 40 to 80%.
an interface layer may be arranged on at least one face of the preform before inclusion thereof in the mold, in order to prevent any interaction between the preform and the sacrificial material;
the materials of the preform, of the interface layer and of the sacrificial material exhibit the same densifying behavior at the sintering start and end temperatures.
A step of removing the binder from the preform may advantageously be performed at the output of the 3D additive printing by means of a heat treatment at temperatures of between 200 and 600° C. and rates of temperature rise of between 0.1 and 1° C./min, depending on the material of the counterform. This step makes it possible to remove organic compounds introduced into the material during the production of the counterform.
In addition, the step of binder removal may be followed by a step of pre-sintering which consists in heat-treating the counterform at higher temperatures, of between 600 and 1500° C. depending on the material of the counterform, this step making it possible to start the densification of the counterform, giving it mechanical strength and promoting the application of the interface.
According to a first implementation, the model directly provides the preform before densification and the mold is filled with pulverulent or porous sacrificial material before and after introduction of the preform.
According to another implementation, the method provides for the model being produced by means of additive printing from lost material—for example from wax or resin with a low melting point—and then a counterform made of porous or pulverulent material is formed around said model. The lost material being removed, the counterform is then filled with the material to be densified which constitutes the preform corresponding to the model, before being introduced into the pressure sintering mold, the sacrificial material being added to the mold. After application of the pressure sintering, the part is extracted from the counterform.
In the figures, identical elements are identified by the same reference sign which refers to the one or more passages of the description in which it is mentioned.
Further information, features and advantages of the present invention will become apparent from reading the following non-limiting description given with reference to the attached figures which show, respectively:
The main steps 1A to 1F of one example of the method for manufacturing a part of complex shape according to the invention are illustrated in
The material of the part is initially in the form of a pulverulent or porous material, based on a metal alloy, in particular, in the examples illustrated, based on a titanium alloy, a titanium-aluminium alloy for example, a stainless steel or a nickel-based superalloy in the René family.
Advantageously, removal of the binder from the preform in the form of a model 1 is implemented by heat treatment at temperatures of between 200 and 600° C., 400° C. in the example, with a rate of temperature rise of between 0.1 and 1° C./min, 0.5° C./min in the example. This step makes it possible to remove organic compounds which may be introduced into the ceramic powder during the 3D printing for producing the counterform portions.
Preferably, pre-sintering is also performed after removal of the binder. This pre-sintering consists in treating the preform 1 at even higher temperatures, for example of between 600° C. and 1500° C. depending on the materials used, at 1200° C. in the example. This heat treatment makes it possible to start the densification of the counterform portions in order to give them mechanical strength and thus facilitate the application of the one or more interface layers, as described below.
In step 1B, a graphite interface layer 11 is advantageously deposited uniformly by spraying onto the outer face 1e (see step 10) or the inner face 1i (see step 1A) of the preform 1. Alternatively, a graphite layer is sprayed onto both faces 1i, 1e of the preform 1. This graphite layer 11 makes it possible to prevent any interaction between the model 1 and the sacrificial pulverulent material—ceramic in the example—which will be deposited later in order to facilitate final removal from the mold.
With reference to step 1C, the preform 1 covered with the graphite layer 11 is introduced into the SPS pressure sintering mold 2 on a bed of sacrificial material of ceramic powder introduced beforehand (not visible). Next, the sacrificial powder 13 is also introduced into the mold 2 so that this mold 2 is filled with an assembly of materials to be densified (step 10). The sacrificial powder makes it possible to compensate for differences in densification during the subsequent sintering.
SPS sintering is then applied to the mold 2 (load 10 of 50 MPa, direct current intensity of 3000 A, which results in the densification of the assembly of material contained in this mold 2 (step 1E). The densified part 3 is then removed from the mold 2 by unmolding and removing the portions of sacrificial material 13 which surround the part 3 (step 1F).
According to another embodiment, as illustrated by the schematic views of steps 2A to 2G of
A conical counterform 5 is then applied around the wax 4, in the form of a slip in the example illustrated (step 2B). Alternatively, the porous material forming the counterform may be a ceramic, a silica, a metal silicate, a composite material or the like.
The wax 4 is then removed from the counterform 5 by way of suitable heating allowing this wax to be liquefied. The liquid escapes (arrow F1) through an opening 51 formed at the base of the counterform cone 5 which is thus completely emptied (step 2C).
Advantageously, the counterform 5 is covered on its inner face 5i with a graphite interface layer 7 in order to facilitate removal from the mold and to prevent any interaction between the counterform and the material to be densified. Alternatively, a graphite layer is sprayed onto at least one of the faces 5i, 5e of the counterform 5.
With reference to step 2D, the counterform 5 is filled with pulverulent material to be densified constituting the preform 6 of the part to be manufactured and corresponding to the wax model. The graphite layer 7 of the inner face 5i of the counterform 5 then covers the preform 6.
Steps of binder removal and of pre-sintering are then advantageously implemented under the conditions of temperature and of rate of temperature rise described above.
The counterform 5 and the preform 6 are then introduced into the SPS sintering mold 2 (step 2E) and sacrificial material 9 is additionally introduced into the mold 2. The sacrificial material is, in the examples illustrated, a material chosen from a ceramic, a silica, a metal silicate, a composite material or the like.
The SPS densification is implemented by applying a load 10 and a direct current of 3000 A (step 2F). The sacrificial powder makes it possible to compensate for differences in densification during the sintering.
The part 8 is then extracted from the mold 2 by unmolding, by removing the sacrificial material 9 and the counterform 5 (step 2G).
The invention is not limited to the examples described and shown. Thus, the interface layers may be formed by a porous or pulverulent material other than graphite, in particular an yttrium oxide, boron nitride or the like.
The first implementation is particularly suited to hollow parts, having convex and concave opposite faces, and the second implementation to solid, biconvex parts.
Regardless of the implementation, the materials of the sacrificial material, of the counterform, of the preform and of the interface layers exhibit similar behavior with respect to the sintering start and end temperatures. The ceramic used is in the form of powder based on YSZ (yttria-stabilized zirconia), ATZ (alumina-toughened zirconia), ZTA (zirconia-toughened alumina), and alumina exhibiting degrees of densification that may range from 40 to 80%.
In addition, the interface layers may be applied in a form chosen from a spray, a deposit of powder bound by a binder corresponding to the powder (aqueous solution, Rhodovil® solution, etc.), a sheet of suitable shape or the like.
Furthermore, in the first embodiment, the preform is directly the model in the example described. In general, this preform may also be derived from a base model by means of another 3D additive printing technique and/or by using another material, which makes it possible to change printing technique and printing material depending on the choice of the material for the part to be manufactured.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1859100 | Oct 2018 | FR | national |
This application is a national stage entry of PCT/EP2019/076550 filed Oct. 1, 2019, under the International Convention and claiming priority over French Patent Application No. 1859100 filed Oct. 2, 2018.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2019/076550 | 10/1/2019 | WO | 00 |
