CA2926749A1

CA2926749A1 - Process for obtaining tight components by powder metallurgy

Info

Publication number: CA2926749A1
Application number: CA2926749A
Authority: CA
Inventors: Roberto Binder; Kaline Pagnan FURLAN; Cristiano Binder; Aloisio Nelmo Klein; Jose Daniel Biasoli De Mello
Original assignee: Whirlpool SA; Universidade Federal de Santa Catarina
Current assignee: Whirlpool SA; Universidade Federal de Santa Catarina
Priority date: 2013-10-07
Filing date: 2014-10-07
Publication date: 2015-04-16
Also published as: SE1650471A1; CN105745044A; JP2016540114A; BR102013025874A2; US20160236276A1; WO2015051432A1; DE112014004626T5

Abstract

The process comprises: mixing a metallic powder defined by iron or nickel or copper or mixtures of two or three elements thereof and provided that the sum thereof defines at least 55% in mass of the metal matrix of the component to be produced, and a molybdenum disulfide powder as a densification agent and as a solid lubricant with a content from 3 to 30% in volume; filling a mold with the mixture and obtaining a compact presenting from 5 to 25% in volume of primary pores (P1); and subjecting the compact to a temperature and for a time sufficient to allow the reaction of the molybdenum disulfide with the matrix, forming iron sulfide, nickel sulfide or copper sulfide, and the diffusion of the molybdenum in the matrix; and subjecting the compact to a temperature sufficient to transform the sulfide into a liquid phase, filling the primary pores (P1) before finishing the sintering step of the compact.

Description

2 PROCESS FOR OBTAINING TIGHT COMPONENTS BY POWDER
METALLURGY
Field of the Invention The present invention refers to an improved process for molding powders by compaction and sintering, with low cost and having a large scale capacity of producing components in tight material, to be applied in the restriction of gas or liquid flows, without requiring subsequent operations after sintering said components.
Description of the Prior Art For several applications, the process of powder metallurgy may be an alternative for the conventional processing, such as casting or machining, of ferrous and non-ferrous materials. It is a process which consists, basically, in obtaining components through raw materials in the powder form. The powder is then mixed, molded and then subjected to a thermal process, known as sintering, in order to provide physical and mechanical properties to the component, by means of the consolidation of the previously molded powder particles.
The powder metallurgy process presents as advantages and in comparison to other productive processes such as, for example, casting or machining, a precise control, chemical composition flexibility, minimum loss of material, surface finishing, dimensional precision and micro-structural control during processing.
Among the properties defined by the micro-structural control one can mention the density of the components.
Density is a factor that directly influences the properties of the components obtained by powder metallurgy. When 'porosity does not have a specific engineering function on the component, such as, for example, filtering or fluid flow control, it is usually harmful to the properties of the components. In such way, several-processes have been employed aiming at increasing the final density of the components.

However, the increase in the final density of a component produced by powder metallurgy is usually followed by an increase in the processing cost, said relation usually being nonlinear and even being exponential when aiming to achieve a final density close to 100% (totally dense material, free of pores).
In relation to the type of pores, the materials may be divided into materials with closed pores, which may be applied as a structural support element, or materials with open pores, which find application mainly where fluids transportation is necessary such as, for example, in flow control, filtering, catalyst support, thermal and acoustic insulation, lubricant reservoir, among others.
The process used to produce the porous materials defines the properties of the latter, such as the type of porosity (open or closed), volumetric percentage of pores in the component volume, size and shape of the component, uniformity of the distribution and interconnectivity of the pores.
A totally dense material is that in which all the pores have been eliminated. However, it is possible to obtain a material which is tight to liquid and gas fluids, presenting closed pores in an acceptable degree depending on the structural resistance to be demanded from the component, but having the open and communicating pores totally eliminated. Thus, the passage of fluids through the component presenting the density characteristics demanded for the use thereof is totally blocked, that is, there is no fluid flow through the component. Figure 1 illustrates the green density variation curves of different compositions of particulate material, depending on the compaction force to which said compositions are subjected.
The most common methods used for increasing the density of the final component, aiming for the latter to become tight or totally dense, are: metallic infiltration,

3 iron oxidation, powder forging and double compaction/double sintering. However, these processes present some drawbacks.
In the case of metallic infiltration and iron oxidation, it is necessary to carry out the treatment after sintering the pieces, that is, it is an additional operation. Both of these methods act only on the pores that are open and communicating with the surface of the piece or component. In the first method, the component should also present pores of certain geometry and dimensions to act as capillaries, in order that the molten metal is sorbed by the component. In the second case, it is formed a layer of oxides on the surface and in the pores. When the component is under stress this layer may fracture, generating abrasive particles during contact with other components, which may cause premature wear or even failure under operation thereof. Furthermore, this process presents strict control variables. A small change in the dew point of the furnace, for example, may cause a layer of oxides having inadequate composition and morphology, thus generating a layer which is more fragile or even more porous. In 'these two methods the material becomes tight, but not 100% dense.
The methods of forging and of double compaction/double sintering, although being carried out simultaneously with the processing, that is, do not require an additional step, invariably imply in a greater processing cost, added to the fact that the first method is limited in relation to the geometry of the components.
In the first case, it is necessary to carry out the forging of the pre-sintered piece, which is usually carried out at high temperatures, which implies increased costs with tooling and limitations related to productivity--caused-by- thedifficulties in producing components in large scale. This same difficulty is

4 faced by the double compaction/double sintering method, since every component, after being pre-sintered, should be further compacted and then sintered. Both methods can provide tight components, but they can only be applied to materials which can be easily deformed, since the principle used for reducing the amount of pores is the plastic deformation of the material.
The drawbacks mentioned above limit the application of the components produced by such methods, either due to the high cost and difficulties of large scale implementation or due to geometric or dimensional control limitations.
Objectives of the Invention Due to the limitations and drawbacks of the known techniques, the present invention has as one of its objectives to provide a process, by using powder metallurgy, to produce, without requiring additional operations and without being subjected to undesirable limitations regarding geometry and dimensional control, tight components which are capable to totally limit the flow of fluids in general, by controlling the chemical composition, the properties of the powders to be mixed, and the processing parameters, mainly temperature, time, heating rate and sintering atmosphere.
Summary of the Invention The process object of the present invention is designed to produce a tight component to be used in mechanical systems which demand tightness, such as, for example, in fluid compaction systems and in hydrodynamic bearings.
The present process, using the powder metallurgy -technique, comprises the steps of:
i) mixing a metallic powder selected from any of the elements defined by iron, nickel, copper and mixtures of two or three of these elements, defining at least 55% of the mass -of¨the-metal matrix of-the-component, and a molybdenum disulfide powder as a densification agent and as a solid lubricant with a content varying between 3 and 30% in volume;
ii) homogenizing the mixture obtained in the previous step;

5 iii) filling the cavity of =a mold, by compacting the homogenized mixture, until achieving a compact which is resistant to handling and presenting between 5 to 25%
in volume of primary pores;
iv) subjecting the compact to a temperature and period of time sufficient to allow the reaction of the molybdenum disulfide with the metallic powder which forms the matrix of the component, forming iron sulfide, nickel sulfide or copper sulfide, as well as and the diffusion of the molybdenum in the metallic powder which forms the matrix;
v) subjecting the compact, with the already reacted molybdenum disulfide, to a temperature sufficient to transform the sulfide formed during the reaction into a liquid phase, filling the primary pores before finishing the sintering step of the compact.
The present process may further comprise the additional steps of: adding at least one additional powder solid lubricant to the mixture of the element which forms the metal matrix with the molybdenum disulfide, before the step of homogenizing the mixture; and subjecting the compact to a single thermal sintering cycle, using a reduction atmosphere for eliminating possible oxides on the powder surface, keeping a temperature sufficient to cause the complete vaporization of the additional solid lubricant, and for a period of time necessary to promote the extraction of the additional solid lubricant and the formation of respective secondary pores in the compact, before subjecting the compact, already free of the additional solid lubricant, to the steps of reaction of the molybdenum disulfide, of liquefying the iron sulfide, copper sulfide or nickel sulfide into a liquid phase of filling the primary and

6 secondary intercommunicating pores, and of sintering the metallic powder of the matrix.
It is the possible to obtain a sintered component having the matrix formed from metallic powders containing one of the matrix elements selected from iron, nickel and copper, or also from a mixture thereof, either of two or of three elements, by means of simple operations of compaction and sintering of the compact during a single thermal cycle, making use of the densification agent to form, during sintering, a reaction product which, from a temperature lower than the sintering temperature, forms a liquid phase which is able to fill the primary and secondary pores, in case there are any, thus assuring, in comparison with samples which do not contain the densification agent, a greater density of the component upon finishing the sintering thereof, with the molybdenum left from the reaction of the molybdenum disulfide with the matrix being diffused in the matrix or forming intermetallic phases.
The proposed invention provides a process for obtaining tight components without the need for additional operations. Tests indicate that the process of the present invention is of low cost, producing components having high final density and enhanced mechanical properties, which may be applied to a range of metallic materials, for example, ferrous materials. The process allows the production of large batches of equal pieces, with high productivity and with easily controllable parameters.
Brief Description of the Drawings The present invention will be described below in more detail with reference to the attached drawings, in which:
Figure 1 represents a graph with the green density variation curves of the compacts formed: of pure iron;
of iron with the solid lubricant zinc stearate or

7 amides; and of iron with the densification agent defined by molybdenum disulfide, as a function of the compaction pressure to which they are subjected;
Figure 2 represents a graph with porosity variation curves of components which are compacted and formed of:
pure iron; iron with the solid lubricant zinc stearate or amides; iron with two distinct percentages of the densification agent defined by molybdenum disulfide, as a function of the compaction pressure to which they are subjected;
Figure 3A is a schematic metallographic representation of the structure of the component being formed, still in the condition of a compact defined by the metal matrix, in the example a ferrous matrix, maintaining the islands of densification agent and presenting the primary pores;
Figure 3B is a metallographic representation similar to that of figure 3A, however illustrating the component under the sintering thermal cycle, with the densification agent starting its reaction with the metal matrix which, in the example, is represented by a ferrous matrix;
Figure 3C is a metallographic representation similar to that of figure 3B, however illustrating the component under the sintering thermal cycle, during a more advanced phase of the reaction of the densification agent with the metal matrix;
Figure 3D is a metallographic representation similar to that of figure 30, however illustrating the component by the end of the sintering thermal cycle and of the reaction of the densification agent with the metal matrix, with the product of the reaction forming, in the respective islands, a liquid phase whose expansion fills the primary pores which communicate with each other and with said islands;
Figure 4A is a schematic metallographic representation of the structure of the component being formed, still

8 in the condition of a compact defined by the metal matrix, in the example a ferrous matrix, containing the particle islands of solid lubricant and of densification agent and presenting the primary pores;
Figure 4B is a metallographic representation similar to that of figure 4A, however illustrating the component under the sintering thermal cycle, by the end of the vaporization of the solid lubricant, with the formation of secondary pores;
Figure 40 is a metallographic representation similar to that of figure 4E, however illustrating the component under the sintering thermal cycle, with the densification agent starting its reaction with the metal matrix;
Figure 4D is a metallographic representation similar to that of figure 40, however illustrating the component under the sintering thermal cycle, in a more advanced phase of the reaction of the densification agent with the metal matrix;
Figure 4E is a metallographic representation similar to that of figure 4D, however illustrating the component by the end of the sintering thermal cycle and of the reaction of the densification agent with the metal matrix, with the product of the reaction forming, in the respective islands, a liquid phase whose expansion fills the primary and secondary pores which communicate with each other and with said islands;
Figure 5 represents a graph illustrating the different tightness levels, as a function of the time (for the same volume of nitrogen gas and same pressure), of sintered components obtained from: pure iron; iron oxide (process also used for obtaining a tight microstructure); iron + zinc stearate; iron +
molybdenum disulfide.
It is important to note that the figures present one example of a possible composition containing iron, the densification agent defined by the molybdenum disulfide

9 and also, optionally and only in certain cases, a solid lubricant (zinc stearate or amides). However, as already mentioned and illustrated in figures 3A to 3D, it is also possible to produce components containing only iron (as an example of metallic material) +
densification agent (molybdenum disulfide), without producing secondary pores, since there is no removal of the solid lubricant by vaporization and extraction. In this case, the molybdenum disulfide also functions as a solid lubricant during the compacting step of the homogenized mixture, and there is no need for extracting it during the sintering cycle, since it will react with the metal matrix, forming one or more sulfides upon reacting with the metallic material(s) of the matrix and a liquid phase of said sulfide(s), which will fill the primary pores before the end of the sintering step of the compact.
It should be understood that when using metal matrices formed only by Ni or only by Fe or only by Cu, the densification agent will react with the materials of the matrix, forming sulfides with said materials.
However, when using binary metal matrices, formed by Fe+Cu or Fe+Ni or Cu+Ni, or also ternary metal matrices, formed by Fe+Ou+Ni, the reaction of the densification agent with the matrix will form the sulfide of the element which has the greater stability (copper is more stable than nickel which is more stable than iron). Thus, in the case the element of greater stability is exhausted (all of its contents has reacted with the densification agent), it will be also formed a sulfide of the next element with a second stability level, until the entire densification agent is consumed. In the case the second material of the matrix is also exhausted before the densification agent is consumed, the latter will react with the third element of the matrix,--forming a third sulfide.
For example, if a matrix containing only iron is used, the densification agent will form only iron sulfide. In the case the matrix contains Fe + 5%wt Ni, the densification agent will preferably form nickel sulfide, until all of the nickel has reacted. After 5 this, it will be formed iron sulfide until all of the MoS2 has reacted. In case the matrix contains Ni + 5%
Fe, it will be formed only nickel sulfides, since the sulfide of this element is more stable than iron sulfide.

10 Detailed Description of the Invention The process object of the present invention includes the selection of powders, the known step of compacting the powder material into a mold, before said compacted material is subjected to the sintering step. In the present case, the process uses compaction of the uniaxial type which has been optimized to allow obtaining, by the end of sintering, a tight microstructure, presenting the required characteristics for the specific intended use, that is, for obtaining a tight component to be used to restrain the flow of liquid or gas fluids, sealing the passage thereof under different assembly conditions of the component.
The tight component to be obtained may be formed from a powder material, which forms the metal matrix of the compact, comprising a metallic powder selected from any of the elements defined by iron, nickel, copper and mixtures thereof, binary or ternary, as long as the sum of the contents of these elements in the total mixture is greater than 55% in mass of the metal matrix, and it being possible to add other alloy elements.
To the powder material which forms the metal matrix is added, in a homogeneous mixture, a densification agent with a minimum content of 3 in volume and, according to convenience, it may be also added at least one additional solid lubricant to improve the technological properties¨of¨the -mixture and/or facilitate the compaction step for forming the green compact.

11 Preferably, the mixture comprises powders for forming the matrix which present, in relation to the densification agent, a ratio between 95/05% and 90/10%
in volume.
The present process starts with a step of mixing the metallic powder (pure or alloy), which forms the matrix of the component to be produced, with a powder of a densification agent defined by the molybdenum disulfide, according to the volume percentages mentioned above and considering the quality definitions of the powder materials related to the formation of the component matrix and which may be defined by iron, nickel, copper or by either of these containing one or both of the others as alloy elements, presenting percentages lower than that of the element which defines the metal matrix.
Subsequently, the mixture of the powders of the matrix with the powders of the densification agent (molybdenum disulfide) is homogenized, preferably using mixers with a low shearing rate, for example, Y type mixers.
The addition only of the densification agent or of the densification agent and of one or more additional solid lubricants makes easier the compaction of the powder mixture and to obtain compacts with a higher green density when compared with those obtained by compacting only the powder which forms the metal matrix, without adding any component which presents solid lubricant characteristics.
The curves illustrated in figure 1 of the drawings show.
a comparison between a mixture containing only iron and the densification agent (molybdenum disulfide) and a mixture containing iron and zinc stearate or amides (additional solid lubricant). In this case, the molybdenum disulfide is what provides a higher green density value of the compacts, for the compaction pressure range of 200 to 690 MPa.--For ¨the ¨pressure __________________________ range lower than 200 MPa the curves are equivalent to

12 each other. In such way, the densification agent molybdenum disulfide acts not only during sintering, but as early as in the compaction step, allowing to achieve a higher green density value (lower volume of pores in the component) when compared with other conventional solid lubricants used in powder metallurgy.
Following the step of homogenizing the powder mixture, the process comprises the step of filling the cavity of a mold, for example by compaction under a pressure from 300 to 800 MPa of the homogenized mixture, until obtaining a compact resistant to handling and presenting from 5 to 25% in volume of primary pores Pl.
Figure 2 of the drawings also illustrates the increased ease in obtaining a lower percentage of primary pores P1 in the compact, for the same value of compaction pressure, when the densification agent is the molybdenum disulfide in a percentage of 5% in volume of the compact material.
The compaction molding of the powders mixture inside the mold is usually carried out by presses with uniaxial strength application (ambient temperature), and it may further be carried out through cold isostatic pressing (ambient temperature) or by using hot isostatic pressing. By the end of this step, the prepared material is compacted in the form of a porous component which presents the form of the mold in which it was compacted.
Finally, the porous compact, comprising the primary pores Pl, is subjected to a sintering step in which the temperature to which it is subjected is increased, usually to the range of 1050 C to 1250 C and maintained in said increased value and for a time sufficient to cause the sintering of the metallic powder or powders of the matrix, the reaction of said powder(s) with the densification¨agent, and the filling of the communicant and open primary pores P1 of the compact with a liquid

13 phase resulting from the product of said reaction, as described below. This step of the process may be observed in the sequence of figures 3A to 3D. The sintering time is usually from 5 to 180 minutes, varying depending on the lower or higher amount (%
content in volume) of the densification agent added to the initial mixture of powders.
The chemical composition of the powder material which forms the metal matrix of the compact may be defined by a metallic powder comprising only one of the matrix elements selected from iron, nickel, copper or also comprising any of said matrix elements, in a preponderant amount and mixed to one or both the remaining matrix elements which function as alloy elements present in percentages lower than that of the matrix element which characterize the metal matrix.
Besides, the metal matrix characterized for containing only one of the matrix elements or a preponderant amount of the latter, may also include, as alloy elements, at least one of the elements selected from:
at least one of said other two matrix elements; chrome;
molybdenum; niobium; manganese; phosphorus; carbon;
vanadium; silicon; and sulfur.
According to the invention, the powder material which forms the metal matrix of the compact contains at least 55% in mass of any of the matrix elements defined by a metallic powder selected from iron, nickel and copper, or mixtures of two or three thereof, and optionally at least one of the other alloy elements mentioned above, in individual contents varying between 0,01% and 20% in mass of the material of the metal matrix. One example of material of the metal matrix is the stainless steel AISI 316 L powder.
In order to obtain a component with a high tightness level, without the need for the undesirable procedures of the known solutions and using the compaction molding of the powders, it is necessary that the powders

14 selected for forming the metal matrix present specific characteristics, such as high particle packing, good compressibility and capacity of helping the retention of the shape of the component to be molded in the compaction step. The addition of a densification agent, which also presents solid lubricant characteristics, improves packing and compressibility in comparison to a matrix without said addition.
One of the factors that influence the final microstructure of the tight component to be obtained is the particle size of the powders used for forming said component.
In case it is used a metal matrix (pure or alloy, as previously described) defined by matrix elements in the form of powders selected from iron, nickel and copper, or mixtures of two or three thereofT and also molybdenum disulfide as the densification agent and solid lubricant, it is desirable to use matrix elements (metallic powders) with grain size between lOpm and 180pm and molybdenum disulfide particles between 1,0pm and 60pm, with said grain sizes corresponding to d90 particle sizes measured by laser granulometry.
The process described so far uses the molybdenum disulfide also as a solid lubricant, allowing the process for obtaining the component to present the sintering steps illustrated in the sequence of the figures 3A to 3D.
The use of the molybdenum disulfide also as solid lubricant makes possible to obtain a compact with higher density (lower percentage of primary pores P1) for a certain value of compaction pressure, making easier to obtain compacts of complex geometries without demanding sophisticated and expensive compaction equipment which operate at high pressures (see figure 2).
However, even_with ¨the molybdenum disulfide being a densification agent with lubricant properties, it may be desirable, as a function of the geometric characteristics of the compaction mold and of the final piece to be formed, of the compressibility requirements and of the morphological characteristics of the 5 powders, of the formability of the compact and also of the mechanical resistance and tightness requirements of the piece to be formed, within certain limits of compaction pressure, to include an additional solid lubricant to the mixture of powders which form the 10 compact.
In these cases in which it is used an additional solid lubricant, the present process comprises the additional step of adding a solid lubricant powder to the mixture of matrix element and densification agent, before the

15 step of homogenizing the mixture, respecting the volume percentages already previously defined and as long as the final mixture contents of molybdenum disulfide is between a minimum of 3% and a maximum of 30% of the total volume of the powder mixture, in order to function as a densification agent as already previously described. This content of 3% in volume is the minimum content to form a liquid phase sufficient to make the material tight.
Next, the mixture of the matrix powders with the powders of the densification agent and additional solid lubricant is homogenized preferably using low shear rate mixers, such as, for example, Y type mixers.
The additional solid lubricants to be used in the present process may be selected, for example, from:
zinc stearate, amides, manganese sulfide, graphite and hexagonal boron nitride (h-BN).
In the alternative solution which comprises the additional solid lubricant, the present process also comprises the step of filling the cavity of a mold by the compaction, for example under a pressure of 300 to 800MPa, of the homogenized mixtUre, until it is obtained a compact resistant to handling and presenting

16 from 05% to 25% in volume, of primary pores Pl.
The compaction characteristics of the powders mixture inside the mold are those already mentioned when using the densification agent and the only solid lubricant.
After the compaction step, the compact is subjected to a single sintering thermal cycle, using a reducing atmosphere, in order to eliminate possible oxides on the surface of the powders, maintaining a temperature sufficient to cause the vaporization of the additional solid lubricant(s), and for a time necessary to promote the extraction of the additional solid lubricant(s) and the formation of respective secondary pores P2 in the compact.
The zinc stearate and, in a larger scale, the amides, define a type of additional solid lubricant widely used in the industry to assist in the molding step, during the compaction of the powder mixtures which form the compact, the additional solid lubricant being later extracted during a initial phase of the sintering thermal cycle, which is carried out for an extraction time of about 30 minutes at a temperature between about 300 C to 500 C. Upon extraction of the compact in the sintering step, said additional solid lubricant forms, in the compact, empty spaces which, in the conventional techniques, form the secondary pores P2, increasing the porosity of the component, thus making it harder to obtain pieces having a low content of residual porosity.
Still in the same single sintering thermal cycle, the porous compact, comprising the primary pores P1 and secondary pores P2, is subjected to a sintering step, in which the temperature to which it is subjected is increased, usually in the range from 1050 C to 1250 C
and maintained in said high value and for sufficient time to cause the sintering of the metallic powder of the matrix, the reaction of the latter wi1H--the densification agent and the filling of the

17 intercommunicating and open primary pores P1 and secondary pores P2 of the compact, with a liquid phase resulting from the product of said reaction. This step of the process may be observed in the sequence of the figures 4A to 4E.
The sintering time is, usually, from 5 to 180 minutes, varying in function of the lower or higher amount (%
mass contents) of the densification agent and of the solid lubricant added to the powders which form the metal matrix.
In a particular form of the invention, using a matrix of pure iron and the molybdenum disulfide, the sintering step itself comprises increasing the temperature of the compact to a value sufficient to cause the sintering of the metallic powder of the matrix and the reaction of the iron thereof with the densification agent (molybdenum disulfide), thus producing a liquid phase of iron sulfide which fills the primary pores P1 of the compact, which communicate with each other and with the densification agent powder.
As it may be observed, there is no need for extracting the densification agent, that is, the molybdenum disulfide of the compact during the sintering step, since said agent is totally used for filling the primary pores Fl, guaranteeing that a component having a tight microstructure is obtained.
The present invention presents the typical advantages of the processing via powder metallurgy techniques, such as: reducing to a minimum of raw material losses, easy and exact control of the chemical composition of the material, good surface finish, productive process of easy automation, final products with high purity and easy microstructural control.
During the compaction there occurs the shearing of the molybdenum disulfide. This shearing occurs in an easier manner for this element than for the additional solid

18 lubricant defined by the zinc stearate or by amides. In such way, the presence of the densification agent makes easier the compaction of the powder mixture and to obtain compacts with a higher green density (g/cm3), mainly when compared with the compaction of pure iron powder and, to a lower extent, with the compaction of mixtures comprising only this type of lubricants (zinc stearate or amides).
For the matrices of iron, nickel or copper, there occurs a reaction process between the molybdenum disulfide and the matrix in temperatures higher than 750 C, resulting in iron sulfide, nickel sulfide or copper sulfide and in the diffusion of the excess molybdenum in the metal matrix.
After passing the necessary time for the reaction of the entire molybdenum disulfide with the matrix element, during the sequence of the sintering step, the temperature is increased enough so as to form a liquid phase with the iron sulfide, nickel sulfide or copper sulfide. The liquid phase of the sulfide resulting from the reaction fills the voids defined by the primary pores P1 or by the secondary pores P2 (in the case it is used the additional solid lubricant for the compaction step), before completing the sintering of the piece to be obtained. The molybdenum of the densification agent, diffused in the metal matrix, influences in the mechanical behavior of the sintered piece, making possible to obtain components presenting increased values of tensile strength.
Considering that the molybdenum disulfide has reaction temperatures with the iron, the nickel and the copper higher than 750 C and that the sintering of the metal matrix occurs under temperatures usually higher than 1050 C, during the sintering itself of the compacted powder material, the molybdenum disulfide particles progressively- react with the iron, nickel or copper matrix, until they are totally consumed, forming a new

19 phase defined by iron sulfide or copper sulfide or nickel sulfide which will form a liquid phase after increasing the temperature to the value which allows the formation of this material, thus filling the intercommunicating pores of the material, making it tight, as illustrated in figures 30, 3D, 3E, 40, 4D e 4E. Thus, during the sintering of the compact, the densification agent particles are progressively consumed, becoming other sulfides which will form the liquid phase, allowing their expansive migration into the primary pores P1 or secondary pores P2, which communicate with each other and with each respective particle of the product of the reaction between the densification agent and the matrix, under a liquefying process, resulting in the filling of said pores and in obtaining a tight microstructure with the internal sealing of all porosity which directly or indirectly communicates with the particles of the densification agent homogeneously distributed in the compact structure.
In order to guarantee the formation of a microstructure which originates a tight component, it is necessary that the latter is sintered under a temperature higher than the temperature of formation of the liquid phase of the sulfide resulting from the reaction between the metallic matrix and the densification agent.
An example of a sintering step of a compact formed from pure iron powder containing a homogeneous addition of the densification agent molybdenum disulfide, may be carried out under temperatures in the range of 1050 C
to 1200 C, the reaction between the densification agent and the pure iron matrix, producing iron sulfide, starting from 850 C and the formation of the liquid phase of the resulting sulfide occurs at about 990 C.
The molybdenum which is left in the reaction is then diffused in the ferrous matrix and the liquefied iron sulfide seals the communicating and superficial pores, making the material to be tight, that is, totally restricting the passage of liquid or gas fluids. A
respective curve of the tightness test carried out in said sample may be observed in figure 5.
5 As already mentioned, one of the factors that influence the final microstructure of the tight component to be obtained is the particle size of the powders used in the formation of said component.
In the case of a mixture of powders containing iron and 10 also molybdenum disulfide as the densification and lubrication agent, it is desirable to use iron or ferrous alloys particles that form the matrix having a grain size between 10 and 180pm and particles of the solid lubricant and of the densification agent between 15 1,0 and 60pm, with said grain sizes corresponding to particle sizes d90 measured by laser granulometry.
The compaction pressure directly influences the green density of the compacted component, such density influencing the existent contacts between the powder

20 particles of the compact, which influences the sinterability of the components. In such way, it is preferable to use a compaction pressure between 300 and 800 MPa during the uniaxial compaction of the components.
Typically, the final porosity of the components obtained by means of the compaction process followed by sintering results from the sintering itself, as a function of the thermally activated mass transport, resulting in the reduction of specific free surfaces due to the growth of contacts between the particles by their coalescence, by reduction of the volume and also by the modification of pore geometry until they are densified.
The sintering process of the present invention may be carried out, for example, in a conventional resistive furnace or in a vacuum furnace with or without plasma assistance, inside which the tightness properties are

21 achieved.
The process of the present invention, as described above, allows achieving a result different than that of the conventional sintering process, producing tight components through a single sintering process, without requiring subsequent operations, allowing for the reduction of energy expenses and processing time. As additional advantages, the proposed process further presents the possibility of producing materials with medium and high geometric complexity, besides the easy control of the final microstructure during processing, facilitating its industrial implementations and wide market use.

Claims

22

1. A process for obtaining tight components by powder metallurgy, characterized in that it comprises the steps of:
i) mixing a metallic powder selected from any of the elements defined by iron, nickel, copper and mixtures of two or three of these elements, as long as the content of the element or mixture thereof defines at least 55% of the mass of the metal matrix of the component, and a molybdenum disulfide powder as a densification agent and as a solid lubricant with a content varying between 3 and 30% in volume;
ii) homogenizing the mixture obtained in the previous step;
iii) filling the cavity of a mold, by compaction of the homogenized mixture, until achieving a compact resistant to handling and presenting between 5 to 25% in volume of primary pores (P1);
iv) subjecting the compact to a temperature and period of time sufficient to allow the reaction of the molybdenum disulfide with the metallic powder which forms the matrix of the component to be produced, forming at least one of iron sulfide, nickel sulfide and copper sulfide, and the diffusion of the molybdenum left from the reaction in the metallic powder which forms the matrix;
v) subjecting the compact, with the already reacted molybdenum disulfide, to a temperature sufficient to transform the sulfide into a liquid phase filling the primary pores (P1) before finishing the sintering step of the compact.

2. The process, as set forth in claim 1, characterized in that the powder material which forms the metal matrix of the compact further comprises at least one of the alloy elements selected from iron, nickel, copper, chrome, molybdenum, niobium, manganese, phosphorus, carbon, vanadium, silicon and sulfur.

3. The process, as set forth in claim 2, characterized in that the powder material which forms the metal matrix of the compact contains at least one of the alloy elements in individual contents varying between 0,01 and 20% in mass of the material of the metal matrix.

4. The process, as set forth in any of the claims 1 to 3, characterized in that the powder material which forms the metal matrix of the compact presents a particle size (d90 measured by laser granulometry) between 10 and 180µm.

5. The process, as set forth in any of the claims 1 to 4, characterized in that the powder of the densification agent presents a particle size (d90 measured by laser granulometry) between 1,0 and 60 µm.

6. The process, as set forth in any of the claims 1 to 5, characterized in that the step of compacting the powder mixture inside the mold is carried out under a pressure of 300 to 800 MPa).

7. The process, as set forth in any of the claims 1 to 6, characterized in that the temperature of reaction of the molybdenum disulfide with the element of the metal matrix is greater than 750°C, but lower than the sintering temperature of the metal matrix.

8. The process, as set forth in any of the claims 1 to 7, characterized in that the sintering time is from 5 to 180 minutes, as a function of the lower or higher amount (%
content in mass) of the densification agent added to the initial powder mixture.

9. The process, as set forth in any of the claims 1 to 8, characterized in that it further comprises the additional steps of: adding at least one additional solid lubricant to the mixture of the element which forms the metal matrix with the molybdenum disulfide, before the step of homogenizing the mixture; and subjecting the compact to a single sintering thermal cycle, using a reducing atmosphere, in order to eliminate possible oxides on the surface of the powders, maintaining a temperature sufficient to cause the vaporization of the additional solid lubricant, and for a time necessary to promote the extraction of the additional solid lubricant and the formation of respective secondary pores (P2) in the compact, before subjecting the latter, already devoid of the additional solid lubricant, to the steps of reaction of the molybdenum disulfide, of liquefying the iron sulfide, the copper sulfide or the nickel sulfide into a liquid phase of filling the intercommunicating primary pores (P1) and secondary pores (P2), and of sintering the metallic powder of the matrix.

10. The process, as set forth in claim 9, characterized in that the solid lubricant powder presents a particle size (d90 measured by laser granulometry) between 1,0 e 60 µm.

11. The process, as set forth in any of the claims 9 and 10, characterized in that the additional solid lubricant used for forming the compact comprises at least one of the elements selected from zinc stearate, amides, manganese sulfide, graphite and hexagonal boron nitride.