EP3393701B1 - Material obtained by compacting and densifying nickel, bronze and brass powders and method thereof - Google Patents

Description

OBJECT OF THE INVENTION

The present invention relates to a material and to its manufacturing process by powder metallurgy. One field of application targeted with this new material is that of mechanics and, more specifically, micromechanics. It is even more specifically suited for components with complex geometries with tight tolerances, as in watchmaking for example.

TECHNOLOGICAL BACKGROUND AND STATE OF THE ART

The materials obtained by powder metallurgy have considerable technological importance and are used in a wide range of fields ranging from nuclear to biomedical.

For example, we can cite the documents US 5,294,269 and US 2004/0231459 respectively disclosing a method for sintering tungsten-based alloys and a cermet. Without going into details, the interactions between powder particles (diffusion on the surface and in volume) during sintering drastically modify the microstructure and the distribution of the initially mixed powders. This results in a product with properties specific to this new microstructure. CN1 04959609A and WO02/38315A1 disclose powder metallurgy processes using nickel, bronze and tin powders. US4147 568A discloses alloys based on bronze, nickel and brass used for the manufacture of mechanical watch elements.

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 to obtain the expected properties on the basis of the initial choice of the powders.

To this end, a material, a part comprising this material, a use of this material and a method of manufacturing this material according to the appended claims are proposed.

BRIEF DESCRIPTION OF FIGURES

The characteristics and advantages of the present invention will appear 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 process 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 picture 2 shows this same microstructure after image processing to show the different phases. The figures 3 and 4 represent the microstructure of the same three-phase material when the densification is carried out 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 two-phase sintered solid ( US 5,294,269 ). The white represents the heavy phase consisting mainly of tungsten. The black phase is the metallic binder phase composed 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 347SS stainless steel. The ceramic phase is composed of TiC (titanium carbide). The last phase consists of M ₇ C ₃ precipitates where M contains chromium, iron and titanium.

DETAILED DESCRIPTION OF THE INVENTION

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

The material is a metallic material obtained from a process comprising three stages.

The first step consists of selecting several metal powders and dosing them when several powders are present. 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 in order to combine the properties specific to different compositions. Each powder is made up of particles with a particle size chosen to guarantee the quality of the material. Although depending on the targeted properties, their mean diameter dso is preferably chosen in a range between 1 and 100 μm.

The metallic powders consist of 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 % of nickel and bronze powders). For bronze and brass, the Cu, Sn and Cu, Zn percentages can also be modulated respectively. For example, for brass the Cu and Zn content can be 60 and 40% respectively and for bronze the Cu and Sn content can be 90 and 10% respectively.

In a second step, the different powders are mixed. Mixing is carried out in a standard commercial dry mixer. The setting of the mixer and the duration of the mixing are chosen so that at the end of this stage, the mixture is perfectly homogeneous. Generally, the mixing time is greater than 12 hours to guarantee 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 are carried out using impact compaction technology as described in the application. WO 2014/199090 . Thus, the mixed powders are placed in an imprint made in a die and the mixture is compacted using a punch. Then, the compacted mixture is densified hot by subjecting the punch to impacts. Unlike the process described in the application WO 2014/199090 , the pressure cooling step 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 still greater than or equal to 98%, while limiting the interactions between the different powders. The objective is to produce a micro-weld 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 densification by impacts of the mixture of powders thus makes it possible to weld the grains of the powders together while preserving a microstructure with high-energy interfaces between the different constituent 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 basic particles is preserved after compaction and densification. Similarly, in the presence of a single starting powder, the morphology of the grains of the material obtained is an image of the morphology of the particles of the initial powder, which is advantageous for guaranteeing mechanical properties based on the initial choice of morphology. powder.

To obtain this particular microstructure, the mixture of powders is at a temperature lower than the melting point of the powder with the lowest melting point 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 into the press or into the press. The time indicated above includes the heating time to reach the given temperature and the holding time at this temperature. During densification, the mixture is subjected to a number of impacts comprised between 1 and 50 with an energy level comprised between 500 and 2000J, this level being preferentially 10 to 30% higher than the energy level required during compaction. The product thus obtained has a relative density greater than or equal to 95% and preferably 98%, measured conventionally by Archimedean weighing. After this densification step, a metallurgical section reveals a very specific microstructure due to the material shaping process. The material comprises a number of phases corresponding to the initial number of 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 totally preserved with an interface between phases of irregular shape, which is can also qualify as non-spherical. The consolidated phases thus retain a high specific surface.

For example, the figures 1 and 2 reveal the microstructure obtained starting from a mixture of three powders: nickel, bronze, brass as presented 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 at figures 3 and 4 this same homogeneity of microstructure with three distinct phases. However, an interdiffusion between the two couples 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%.

By comparison with the materials obtained by powder metallurgy in the documents US 5,294,269 and US 2004/0231459 ( figures 5 and 6 respectively), a clear difference is observed at the level of the interfaces separating the different phases. In these documents, the interfaces are smooth and, more specifically, essentially spherical in shape, unlike the material according to the invention having irregular interfaces, ie. 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 were selected to produce a material with a set of properties:

• easy shaping of the semi-finished product by a machining process by chip removal with no burr,
• dimensional stability, to avoid deformation of the material after the machining operation;
• weldable, in particular 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 different powders are detailed in Table 2. <u>Table 1</u> Selected powders Function and/or characteristic Pure nickel (Ni) metal powder Offering the consolidated and densified material a good welding behavior, in particular 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 consolidation and densification behavior Powder type Powder content (by weight) Particle size (µm) (supplier data) Nominal chemical composition of the material (by weight) Neither Cu Zn sn Nickel powder (100% Ni)* 25% Fisher size: 25% 1.8-2.8 Brass powder (60% Cu, 40% Zn)** 65% d10: 2 48% 26% 1% d50: 6 d90: 20 Bronze powder (90% Cu, 10% Sn)*** 10% d10: 6 d50: 11 d90: 20 * Ni2800A powder from Eurotungsten
** 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 mixer of the Turbula T10B type. The mixing speed is an average speed of the order of 200 revolutions per minute for 24 hours.

In the third step, the shaping was carried out using a high-speed, high-energy press from the manufacturer Hydropulsor. The shaping was carried out in two phases:

Cold compaction:

The dosage of the powders in the cavity is done volumetrically with a given filling height. In the example, this filling height is 6 mm to arrive at a compacted thickness of approximately 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 upper punch and the lower punch, surrounded by a matrix to form a washer of a given diameter. This compaction is made in the example with 25 impacts. The objective of this step is to obtain a sufficiently dense solid for subsequent hot densification. This compaction also serves to ensure that the solid thus compacted is strong enough 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 washer thus heated is transported and placed in the cavity with a diameter slightly larger than the diameter of the washer. The duration of transportation of the preheated puck from the oven to the press, put in the die, is from 2 to 5 seconds. The preheated washer is then densified hot between the upper punch and the lower punch with 25 impacts. In the absence of heating means, a decrease in temperature is observed during densification by impact. The final thickness in the example of the densified washer is approximately 1.8 mm. The relative density of the puck is more than 98%. The microstructure is similar to that obtained with picture 3 .

Thanks to hot compaction and densification as described above, the solid obtained is a multi-phase 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 through 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 can be chosen to improve the weldability, for example, by laser. This function is fulfilled by the phase composed mainly of nickel in the example. Another phase can be chosen to facilitate hot densification without actual sintering. In the example, one of the phases of the solid consists essentially of bronze which has the lowest melting range of the three constituents. The third phase which is, again by way of example, the majority phase, is composed of the consolidated brass powder. This phase as well mixed with the other two makes it possible to guarantee better machinability by chip removal.

In the presence of a single starting powder, the method 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. The grain size playing 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(s) is preserved while obtaining a product with 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 (10)

1. A compacted and densified metal material consisting of three phases formed of an agglomerate of grains, the cohesion of the material being ensured by bridges formed between grains, said material having a relative density which is greater than or equal to 95% measured by Archimedean weighing, wherein the phases are separated by irregularly shaped interfaces and wherein the three phases comprise a first phase being composed of nickel, a second phase being composed of bronze and a third phase being composed of brass, and wherein the mass fraction of the first phase is comprised between 3 and 40%, the mass fraction of the second phase is comprised between 2 and 20% and the mass fraction of the third phase corresponds to the percentage remaining to make 100%.
2. A part comprising the material according to claim 1.
3. The part according to claim 2, being a timepiece component.
4. A use of the material according to claim 1 in the field of micromechanics.
5. A method for manufacturing a material by powder metallurgy comprising the following steps:

   - provision of three types of metal powders with grains having a non-regular random shape,

   - compaction of the three types of metal powders to form a compacted assembly in which the grains are linked together by entanglement of the respective valleys and peaks thereof and forms an intermediate product in the form of an agglomerate formed exclusively of the metal powder grains,

   - densification by impact of the agglomerate at a temperature lower than the melting temperature of the powder having the lowest melting temperature, the assembly being previously or during the densification brought to said temperature for a time period comprised between 3 and 30 minutes and, preferably, between 5 and 20 minutes,

   - the three types of powder comprising a first powder being composed of nickel, a second powder being composed of bronze and a third powder being composed of brass, and wherein the mass fraction of the first phase is comprised between 3 and 40%, the mass fraction of the second phase is comprised between 2 and 20% and the mass fraction of the third phase corresponds to the percentage remaining to make 100%.
6. The method according to claim 5, comprising a step of mixing the powders before compacting.
7. The method according to one of claims 5 to 6, wherein the Cu and Zn content of the brass powder in the third phase is respectively 60 and 40% and wherein the Cu and Sn content in the bronze powder in the second phase is respectively 90 and 10%.
8. The method according to one of claims 5 to 7, wherein the densification by impact is performed at a temperature greater than or equal to 500°C.
9. The method according to any one of claims 5 to 8, wherein the compaction is carried out under cold conditions.
10. The method according to any one of claims 5 to 9, wherein the number of impacts during the densification is comprised between 1 and 50 with an energy comprised between 500 and 2000J.

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