EP3184211A1 - Material obtained by compacting and densifying metal powder(s) - Google Patents

Abstract

The present invention relates to a compacted and densified metal material comprising one or more phases formed of an agglomerate of grains, the cohesion of the material being provided by bridges formed between grains, said material having a relative density greater than or equal to 95% and, preferably, 98%.

Description

OBJECT OF THE INVENTION

The present invention relates to a material and its method of manufacturing powder metallurgy. An area of application targeted with this new material is that of mechanics and, more specifically, micromechanics. It is even more specifically adapted for components having complex geometries with severe tolerances, as in watchmaking, for example.

TECHNOLOGICAL BACKGROUND AND STATE OF THE ART

Powder metallurgy materials are of considerable technological importance and are used in a wide range of fields from nuclear to biomedical.

By way of example, mention may be made of the documents US 5,294,269 and US 2004/0231459 respectively disclosing a sintering process of tungsten-based alloys and a cermet. Without going into details, the interactions between powder particles (surface and volume diffusion) during sintering drastically modify the microstructure and the distribution of the initially mixed powders. The result is a product with properties specific to this new microstructure.

SUMMARY OF THE INVENTION

The present invention proposes to select the composition of the starting powders according to the desired properties on the final product and to adapt the parameters of the process to limit the interactions between the powders and thus obtain the expected properties on the basis of the initial choice of the powders.

For this purpose, a method and a product according to the appended claims are provided.

BRIEF DESCRIPTION OF THE FIGURES

The features and advantages of the present invention will become apparent on reading the detailed description below with reference to the following figures.

The figure 1 represents the microstructure of a three-phase material obtained with the method according to the invention. The densification was carried out at a temperature close to 500 ° C on a compacted mixture of nickel, brass and bronze. The figure 2 represents this same microstructure after image processing to reveal the different phases. The Figures 3 and 4 represent the microstructure of the same triphasic material when the densification is operated at a temperature close to 700 ° C.

The Figures 5 and 6 represent, by way of comparison, the microstructures of materials of the prior art obtained by powder metallurgy. To the figure 5 it is a bi-phased sintered solid ( US 5,294,269 ). White represents the heavy phase consisting mainly of tungsten. The black phase is the metallic binder phase consisting essentially of a nickel, iron, copper, cobalt and molybdenum alloy. To the figure 6 it is a sintered cermet ( US 2004/0231459 ). Binder is the binder phase made of stainless steel 347SS. The ceramic phase is composed of TiC (titanium carbide). The last phase consists of precipitates M ₇ C ₃ where M contains chromium, iron and titanium.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method of manufacturing a material by powder metallurgy and to the material resulting from the process. The method is adapted so that the microstructure of the material is perfectly homogeneous through its volume and that it is an image as faithful as possible of the microstructure of the mixed powders and their initial distribution in the mixture. The material resulting from the process may be a finished product or a semi-finished product requiring a subsequent machining step.

The material is a metallic material obtained from a three-step process.

The first step consists of selecting one or more metal powders and dosing them when several powders are present. It can be powders of a pure metal or an alloy. The number of starting powders, their compositions and their respective percentages depend on the physico-mechanical properties desired on the consolidated product. Preferably, the powders are at least two in number to combine the properties of different compositions. Each powder is formed of particles having a particle size chosen to guarantee the quality of the material. Although depending on the targeted properties, their average diameter d ₅₀ is preferably chosen in a range between 1 and 100 microns.

The metal powders are chosen from the non-exhaustive list comprising pure metals or alloys based on titanium, copper, zinc, iron, aluminum, nickel, chromium, cobalt, vanadium and zirconium. , niobium, molybdenum, palladium, silver, tantalum, tungsten, platinum and gold. By way of example, the mixture comprises three powders: a nickel powder, a bronze powder and a brass powder. The proportion of bronze powder is between 2 and 20% by weight, the proportion of nickel powder is between 3 and 40% by weight, the proportion of brass powder being the remaining proportion (= 100% - the sum of the% of nickel and bronze powders). For bronze and brass, the percentages Cu, Sn and Cu, Zn can be respectively modulated. For example, for brass, the content of Cu and Zn can be respectively 60 and 40% and for bronze, the content of Cu and Sn can be respectively 90 and 10%.

In a second step, the different powders are mixed. Mixing is carried out in a standard commercial dry blender. The setting of the mixer and the duration of mixing are chosen so that at the end of this step, the mixture is perfectly homogeneous. Generally, the mixing time is greater than 12 hours to ensure this homogeneity and less than 24 hours. It should be noted that in the presence of a single starting powder, the mixing step is optional.

In a third step, the homogeneous mixture is shaped, i.e. compacted and densified at a temperature below the melting temperature of the respective powders. Hot compaction and densification is performed using impact compaction technology as described in the application. WO 2014/199090 . Thus, the mixed powders are placed in an impression made in a matrix and compaction of the mixture is carried out by means of a punch. Then, the compacted mixture is hot densified by subjecting the punch to impacts. Unlike the process described in the application WO 2014/199090 the cooling step under pressure can be omitted.

The process parameters are chosen to obtain a consolidated body with a relative density greater than or equal to 95% and better than or equal to 98%, while limiting the interactions between the different powders. The objective is to achieve microstrain between particles to consolidate the material without significantly altering the microstructure of the various powders present. More precisely, the consolidation parameters are chosen to limit the degree of sintering to surface bond formation and not to volume bond formation as observed during actual sintering. Microstructurally, this intergranular bond results in the formation of bridges between particles. Limiting the interactions between particles makes it possible to maintain a distribution of the powders within the consolidated material close to that observed after mixing the powders. Compaction and impact densification of the powder mixture thus makes it possible to weld the grains of the powders together while preserving a microstructure with high energy interfaces between the different constitutive phases. In other words, the material resulting from the process has the characteristics that the constituent elements of the different powders do not mix and that the morphology of the base particles is retained after compaction and densification. Likewise, in the presence of a single starting powder, the morphology of the grains of the obtained material is an image of the morphology of the particles of the initial powder, which is advantageous for guaranteeing mechanical properties on the basis of the initial choice of morphology. powder.

To obtain this particular microstructure, the powder mixture is at a temperature below the melting temperature of the lower melting point powder during hot densification. The mixture is brought to this temperature for a time of between 3 and 30 minutes and, preferably, between 5 and 20 minutes. It can be brought to this temperature before introduction in the press or in the press. The time indicated above includes the heating time to reach the given temperature and the maintenance at this temperature. During densification, the mixture is subjected to a number of impacts of between 1 and 50 with an energy level of between 500 and 2000J, this level being preferably 10 to 30% higher than the energy level required when compaction. The product thus obtained has a relative density greater than or equal to 95% and, preferably, 98%, measured conventionally by weighing Archimedes. After this densification step, a cut metallurgical reveals a very specific microstructure due to the process of shaping the material. The material comprises a number of phases corresponding to the number of initial powders with a distribution of the phases substantially the same as that of the powders within the starting mixture. Another very specific characteristic of this microstructure is that the surface energy of the phases thus consolidated is conserved at high levels. The native morphology of the powder particles remains almost completely preserved with an interface between irregularly shaped phases, which can also be described as non-spherical. The consolidated phases thus retain a high specific surface area.

For example, Figures 1 and 2 reveal the microstructure obtained from a mixture of three powders: nickel, bronze, brass as shown in Table 1. The mixture was compacted and densified at a temperature close to 500 ° C. The microstructure has three distinct phases respectively consisting mainly of nickel, bronze and brass. The homogeneity of the microstructure obtained is that obtained after the step of mixing the three kinds of powder. The product thus obtained has a relative density greater than 95%. Starting from the same mixture but with a densification temperature close to 700 ° C, we observe Figures 3 and 4 this same homogeneity of microstructure with three distinct phases. However, an interdiffusion between the two pairs nickel-bronze and bronze-brass is observed, the phase rich in nickel being surrounded by the phase rich in bronze. This interdiffusion makes it possible to increase the relative density to a value greater than or equal to 98%.

Compared with the materials obtained by powder metallurgy in the documents US 5,294,269 and US 2004/0231459 ( Figures 5 and 6 respectively), there is a clear difference in the interfaces separating the different phases. In these documents, the interfaces are smooth and, more precisely, essentially spherical in shape, unlike the material according to the invention having irregular interfaces, i.e. high energy interfaces, between phases.

A detailed example below illustrates the process according to the invention.

• mise en forme facile du demi-produit par un procédé d'usinage par enlèvement de copeaux avec absence de bavure,
• stabilité dimensionnelle, pour éviter une déformation du matériau après l'opération d'usinage ;
• soudable, notamment par laser.

In the first step, the powders have been selected to produce a material having a set of properties:

• easy shaping of the semi-finished product by a chip removal machining process with no burrs,
• dimensional stability, to avoid deformation of the material after the machining operation;
• weldable, especially by laser.

To meet these criteria, three metal powders listed in Tables 1 and 2 below were selected in step 1) of the process. The function of each powder is detailed in Table 1. The compositions and percentages of the various powders are detailed in Table 2. <u> Table 1 </ u> Selected powders Function and / or characteristic Pure nickel (Ni) metal powder Providing consolidated and densified material with good welding behavior, especially laser welding Brass alloy metal powder, with a nominal chemical composition of 60% copper (Cu) and 40% zinc (Zn). Offering good machinability Bronze alloy metal powder, with a nominal chemical composition of 90% copper (Cu) and 10% tin (Sn). Offering better behavior to consolidation and densification Type of powder Powder content (by weight) Granulometry (μm) (supplier data) Nominal chemical composition of the material (by weight) Or Cu Zn Sn Nickel powder (100% Ni) * 25% Fisher size: 1.8-2.8 25% Brass powder (60% Cu, 40% Zn) ** d10: 2 48% 26% 1% 65% d50: 6 d90: 20 Bronze powder (90% Cu, 10% Sn) *** d10: 6 10% d50: 11 d90: 20 * Ni2800A powder from Eurotungstene
SF-BS6040 10μm Powder from Nippon Atomized Metal Powders Corp.
*** SF-BR9010 10μm Powder from Nippon Atomized Metal Powders Corp.

In the second step, the powders were mixed in a commercial Turbula T10B mixer. The mixing speed is an average speed of the order of 200 rpm for 24 hours.

In the third step, the shaping was carried out using a high speed and high energy press of the manufacturer Hydropulsor. The formatting was performed in two phases:

Cold compaction:

The dosing of the powders in the impression is done volumetrically with a given filling height. In the example, this filling height is 6 mm to reach a compacted thickness of about 2 mm. This parameter - filling height - can vary between 2 mm and 50 mm depending on the desired final thickness on the compacted solid. The quantity of powders thus dosed is compacted between the punch above and the punch below, surrounded by a matrix to form a washer of a given diameter. This compaction is done in the example with 25 impacts. The goal of this step is to get a solid enough dense for subsequent densification at hot. This compaction also serves to ensure that the solid thus compacted is sufficiently solid for handling operations during hot densification. The relative density obtained at this stage is greater than 90%.

Hot densification

The compacted washer is brought to a temperature close to 700 ° C in an oven preheated to this temperature. The compacted puck is placed in the oven for at least 5 minutes and preferably 15 minutes. The thus heated washer is transported and placed in the cavity of diameter slightly larger than the diameter of the washer. The duration of the transport of the preheated washer of the oven to the press, put in the matrix, is between 2 and 5 seconds. The preheated disc is then hot densified between the top punch and the bottom punch with 25 impacts. In the absence of heating means, a decrease in temperature is observed during the densification by impact. The final thickness in the example of the densified washer is about 1.8 mm. The relative density of the washer is greater than 98%. The microstructure is similar to that obtained at figure 3 .

Thanks to compaction and hot densification as described above, the solid obtained is a multiphase material comprising phases having different functions. In addition, the solid thus obtained has a homogeneous microstructure throughout its volume. As a result, there is no internal stress gradient across the solid. This provides geometric stability to the machined part.

Each phase of the solid obtained and, upstream, each powder, is chosen to fulfill a specific function. One of the phases may be chosen to improve the weldability, for example, by laser. This function is fulfilled by the phase consisting mainly of nickel in the example. Another phase may be chosen to facilitate hot densification without sintering itself. In the example, one of the phases of solid consists essentially of bronze which has the lowest melting range of the three constituents. The third phase which is, as an example, the majority phase, is composed of consolidated brass powder. This phase thus mixed with the other two makes it possible to guarantee better machining aptitude by chip removal.

In the presence of a single starting powder, the process according to the invention also has advantages. It is thus observed that the morphology of the grains within the material is an image of the morphology of the particles of the starting powder. As the grain size plays an important role in the mechanical properties of the material, it is particularly advantageous to be able to predict the final properties on the basis of the choice of the morphology of the starting powder.

Thanks to the process according to the invention, the morphology of the base powder or powders is maintained while obtaining a product with a high relative density, unlike the known sintering process where the consolidation at relative density values greater than or equal to 95, or even 98% is accompanied by a drastic change in morphology.

Claims (18)

Compacted and densified metal material comprising one or more phases formed of an agglomerate of grains, the cohesion of the material being provided by bridges formed between grains, said material having a relative density greater than or equal to 95% and, preferably, 98% . Material according to Claim 1, in which the phase or phases are mainly composed of an element chosen from the list consisting of Ni, Cu, Zn, Ti, Al, Fe, Cr, Co, V, Zr, Nb, Mo, Pd. , Ag, Ta, W, Pt, Au and an alloy thereof. Metal material according to claim 1 or 2, wherein the phases are separated by irregularly shaped interfaces. Material according to any one of the preceding claims, comprising three phases, a first phase being predominantly composed of nickel, a second phase being predominantly composed of bronze and a third phase predominantly consisting of brass. Material according to claim 4, wherein the mass fraction of the first phase is between 3 and 40%, the mass fraction of the second phase is between 2 and 20% and the mass fraction of the third phase corresponds to the percentage remaining for to do 100%. Part comprising the material according to any one of claims 1 to 5. Part according to claim 6 being a watch component. Use of the material according to any one of claims 1 to 5 in the field of micromechanics. A method of manufacturing a powder metallurgy material comprising the steps of: - provision of one or more metal powders, compaction of the one or more metal powders to form a compacted assembly, - densification by impact of the compacted assembly at a temperature below the melting temperature of the powder having the lowest melting temperature, the assembly being before or during the densification brought to said temperature for a time between 3 and 30 minutes and preferably between 5 and 20 minutes. Process according to claim 9, comprising a step of mixing the powder or powders before compaction. Process according to claim 9 or 10, in which the powder or powders are chosen from a list consisting of alloys or pure metals of Ni, Cu, Zn, Ti, Al, Fe, Cr, Co, V, Zr, Nb, Mo, Pd, Ag, Ta, W, Pt and Au. Process according to any one of claims 9 to 11, wherein at least two powders of different compositions are provided. Process according to any one of claims 9 to 12, wherein three powders are provided, a first powder being a nickel powder, a second powder being a brass powder and a third powder being a bronze powder. A process according to claim 13, wherein the percentage of nickel powder is between 3 and 40%, the percentage of bronze powder is between 2 and 20% and the percentage of brass powder is the percentage remaining to make 100% ; the percentages being expressed by weight. A process according to claim 13 or 14, wherein the Cu and Zn content of the brass powder is 60% and 40%, respectively, and wherein the Cu and Sn content in the bronze powder is 90% and 10%, respectively. . The method of claim 15 wherein the impact densification is at a temperature of 500 ° C or higher and preferably 700 ° C or higher. Process according to any one of claims 9 to 16, wherein the compaction is performed cold. A method according to any one of claims 9 to 17, wherein the number of impacts during densification is between 1 and 50 with an energy of between 500 and 2000J.

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