ITMO20090295A1 - PROCEDURE FOR SINTERING THE DUST ASSISTED BY PRESSURE AND ELECTRICITY - Google Patents

Description

â € œPROCESS FOR THE SINTERING OF DUST ASSISTED BY PRESSURE AND ELECTRIC CURRENTâ €.

DESCRIPTION

The present invention relates to a process for the sintering of powders assisted by pressure and electric current.

It is known that powder metallurgy represents a family of processes capable of obtaining a finished object starting from metal powders of different sizes that are pressed and sintered (Press & Sinter). This technology is widely used for the production of large-scale objects, usually in steel, and allows the production of materials and / or composites (Metal Matrix Composites) that would otherwise be difficult to obtain. If, on the other hand, one wishes to use easily oxidizable metal powders (in particular aluminum, magnesium, and related alloys), the simple sintering process is not feasible.

In fact, it must be borne in mind that any metal powder is always coated with a thin layer of oxide which must be removed to obtain the necessary cohesion between the particles and, therefore, to manufacture sintered pieces with good mechanical properties.

In the case of sufficiently noble metals such as copper and its alloys, for example, the removal of this oxide occurs simply by exposing the metal to a reducing atmosphere.

In the case of steel, it is possible to resort to reducing atmospheres and / or modifying the carbon content in the ferrous alloy.

For less noble metals such as aluminum and magnesium, on the other hand, the removal of the oxide would require the application of oxygen partial pressures so low as to make it impossible to remove them simply by means of thermochemistry.

For the classic sintering of aluminum, the standard Press & Sinter process can be adopted using, for example, commercial EckaGranules® powders; in this process it is however necessary to sinter at very high temperatures (about 600 ° C) for long times (2 hours) in dry nitrogen atmospheres, even with the presence of alloying elements that can form liquid phases (copper, magnesium, manganese , silicon, tin ...).

The adoption of this process allows to obtain sintered pieces with rather poor mechanical properties, in particular the toughness and elongation at break (less than 2%).

For the sintering of aluminum or magnesium alloys, therefore, it is preferred to resort to other processes that usually involve a stage of plastic deformation of the powders, necessary to break the surface oxide layer to ensure metal / metal contact and therefore perfect junction between the different dust particles.

This stage of plastic deformation is even more necessary if you want to obtain a Metal Matrix Composites based on aluminum or magnesium.

One of these processes, for example, involves the succession of different phases: mixing of aluminum and silicon carbide powders, degassing and compaction (Hot Isostatic Pressing, Canning ...), obtaining the bloom, plastic deformation of the material (extrusion and / or forging), machine tool processing.

It is therefore evident that the whole of the aforementioned phases is very complex and therefore expensive: in particular the degassing and canning phases (filling a tube with powder and subsequent hot extrusion) are very slow, technologically demanding and economically inconvenient.

Another sintering method consists of the so-called â € œpowder forgingâ €, in which the sintered pieces are obtained starting from a green body, meaning by green body a compacted powder not yet sintered or partially or totally sintered, which is subjected to forging through one or more shocks of a mallet.

Typically the green body is obtained in a preliminary way after pressing the powder at room temperature to about 400 MPa, after which it is heated to a temperature of 450-500 ° C for a time sufficient for degassing, and then it is forged by applying repeated blows of a hammer with pressures of the order of about 800 MPa in a mold that is at a temperature of 200 ° C, determining the required plastic deformation of the dust particles.

However, this process has some drawbacks including the limited plastic deformation that can be achieved and the low level of detail that can be obtained in the sintered pieces.

The high deformation speed imposed by the hammer, in fact, requires that the preliminary shaping of the green body substantially corresponds to that of the piece to be obtained in order not to incur a faulty shaping. It is therefore frequent the case in which some areas of the material have not reached a sufficient level of plastic deformation such as to guarantee the necessary breakage of the superficial oxide layers and the consequent sintering cohesion.

Moreover, sometimes the objects obtained present inconvenient and dangerous internal fractures.

Another way to sinter aluminum or magnesium powders in an economical way is the use of sintering techniques assisted by pressure and electric current (Spark Plasma Sintering, Pulsed Electric Current Sintering, Pressure Assisted Sintering, ...): in this case the powder to be sintered is poured and distributed in a conductive mold (usually made of graphite) and, after reaching a vacuum level of a few pascals, electric current and pressure are applied independently through two punches.

It is therefore possible to obtain aluminum and magnesium alloys with good mechanical properties thanks to the combined action of the passage of current and mechanical pressure which are thus able to break the oxide layer present on the surface of the dust particles.

This technique, summarized for brevity in the acronym SPS, is particularly effective in the sintering of so-called hard-to-sintermaterials, including aluminum and magnesium.

The major problem in using the SPS process concerns the creation of objects with a complex geometric shape; in this case, in fact, the use of isotropic graphite as a material for the construction of the molds into which the powder to be sintered is poured is highly limiting. In fact, isotropic graphite has some particularly advantageous peculiar characteristics, such as high electrical resistance (8Ã · 20 pOm), high thermal conductivity (50Ã · 100 W / mK), low density (1.5Ã · 1.85 g / cm3) and self-lubricating capacity, but has negative factors such as low flexural strength (30Ã · 60 MPa), low hardness (40Ã · 90 Shore) and poor wear resistance.

The low flexural strength makes it impossible to apply pressures through the punches higher than 50-60 MPa in order to avoid the fracture of the molds and in any case the impossibility of having geometries with squared profiles or low radius fittings (for example less than 4 mm). The low resistance to wear and the low hardness of graphite makes it necessary to replace the molds frequently in order to avoid dust leakage or the obtaining of objects out of tolerance.

On the other hand, the use of steel, superalloy or WC / Co molds, which would allow to obtain much more complex objects, apply very high loads and reduce wear and tolerance problems, is not compatible with traditional SPS technology as such materials have low electrical resistivity (around 1 pOm).

The low electrical resistivity of the mold leads to poor sintering of the aluminum powder. This is determined by the resistivity values of the starting sinterable powder, typically around 1 Om, and by the resistivity of the material after sintering, around 0.01Ã · 10 pOm.

The wide range of resistivity values of the sintered material (i.e .: three orders of magnitude) depends on the actual breakage of the surface oxide layer (which is a perfect insulator): values of 0.01 pOm represent a perfectly sintered material and therefore conductive, while values around 10 pOm represent a poorly sintered material, not very conductive and with poor mechanical properties.

The use of steel molds does not allow the achievement of low resistivity values in the sintered product as the current prefers to pass into the mold instead of into the powder to be sintered, thus eliminating the beneficial effects of electric current assisted sintering.

To obviate the fact that by using metal molds the electric current tends to pass through the mold and not into the powder to be sintered, it is possible to resort to suitable insulating coatings, such as that illustrated in the patent document WO 2008/085947.

In this document it is envisaged to interpose a sheet of mica between the walls of the mold and the powder to be sintered, thus forcing the electric current to pass through the powder.

Even in this case, however, there are some drawbacks, including the reduced degassing offered by mica and the difficulty in applying the coating sheets on molds of complex shape.

Another way to try to induce a greater passage of electric current in the metal powder is to coat the metal mold with PVD deposits of intermetallic AlTiN or AlTiSiN with semiconductor properties.

However, even this method is not very feasible since the coating must be perfectly homogeneous and continuous in order to prevent an intensification of electric current from occurring in one point and, therefore, melting and / or damage to the coating.

The main task of the present invention is to devise a process for the sintering of powders assisted by pressure and electric current which allows the metal powders to be sintered in a practical, easy and functional way, including the so-called hard-to-sintermaterials, to obtain sintered pieces with high mechanical properties, including, in particular, high toughness and elongation at break, even in the case of pieces with complex geometry.

Another object of the present invention is to devise a process for the sintering of powders assisted by pressure and electric current which allows to overcome the aforementioned drawbacks of the prior art in the context of a simple, rational, easy and effective use and low cost solution. .

The aforementioned purposes are achieved by the present process for the sintering of powders assisted by pressure and electric current, comprising the steps of:

- preparing a sinterable material in the form of powder and at least in part electrically conductive inside a mold having a predetermined conformation to generate the geometry of the final object;

sintering said sinterable material in said mold by passing electric current through said sinterable material and applying pressure on said sinterable material;

characterized by the fact that said preparation includes the phases of:

compacting said sinterable material in the form of powder externally of said mold to form at least one green body having an initial shape different from said predetermined shape of the mold; And

introducing said green body into said mold;

pressure replication during said sintering causing the deformation of said green body from said initial conformation to said predetermined mold conformation.

Other characteristics and advantages of the present invention will become more evident from the description of some preferred, but not exclusive, embodiments of a process for the sintering of powders assisted by pressure and electric current, illustrated by way of indication, but not of limitation, in the accompanying tables of drawings in which:

Figures 1 to 4 illustrate, in a succession of schematic and partial sectional views, a first embodiment of the method according to the invention;

Figures 5 to 8 illustrate, in a succession of schematic and partial sectional views, a second embodiment of the method according to the invention;

Figures 9 to 12 show, in a succession of schematic and partial cut-away views, a third embodiment of the method according to the invention;

Figures 13 to 17 show, in a succession of schematic and partial cut-away views, a fourth embodiment of the method according to the invention.

With particular reference to Figures 1 to 4, the reference numeral 1 generally designates a mold having a predetermined shape.

The mold 1, in particular, comprises a first shaped element or first punch 2, and a second shaped element 3, consisting for example of a second punch 30 inserted in a shaped die 40. The second punch 30 and the die 40 are made separately and reciprocally movable to facilitate the opening and closing of the mold 1 but, however, alternative embodiments are not excluded in which, instead, the second shaped element 3 consists of a single body, or of three or more separate pieces.

The first punch 2 and the second punch 30 can be approached and removed from each other along a pressing direction, while the direction transverse to the pressing direction defines a so-called deformation direction.

In figure 1, the pressing direction is identified by the arrows P while the deformation direction is identified by the arrows D.

In the particular embodiment illustrated in Figures 1 to 4, for example, the first punch 2 and the second punch 30 are arranged, inside the shaped die 40, one above the other and can be approached / removed along a direction of pressing P substantially vertical, while the direction of deformation D is substantially horizontal.

The assembly of the first punch 2, of the second punch 30 and of the shaped die 40 defines an internal forming cavity 4 suitable for shaping a sinterable material in the form of powder to obtain a sintered piece.

Conveniently, in the present discussion it will be possible to refer equally to the mold 1 and to the internal molding cavity 4, since this cavity constitutes the useful part of the mold 1.

The mold 1 is made of steel and, preferably, is at least partially coated with graphite spray, boron nitride, silicone based lubricant, PTFE lubricant or molybdenum disulfide, to reduce the friction coefficients.

However, the possibility that it is made of stainless steel, superalloys and / or electrically conductive ceramics is not excluded.

The sinterable material in powder form is at least partially electrically conductive such as, for example, metal powders based on aluminum, chromium, magnesium, Al-MMC (Aluminum Metal Matrix Composite), Mg-MMC (Magnesium Metal Matrix Composite ); it is not excluded, however, that the present invention may also be used for the sintering of powders other than those listed above. Referring again to Figures 1 to 4, 5 indicates a green body obtained by compacting the sinterable material in the form of a powder.

Advantageously, the green body 5 is shaped with an initial conformation different from the predetermined conformation of the mold 1.

The initial conformation of the green body 5 has at least a portion whose bulk along the direction of deformation D is substantially less than the bulk of the predetermined shape of the mold 1 along the same direction of deformation D.

In other words, by sectioning the mold 1 along a plane parallel to the pressing direction P, for example analogous to the section plane of Figures 1 to 4, then the horizontal dimensions of the green body 5 are significantly lower than the dimensions of the internal molding cavity 4. In this way, relative approximation of the shaped elements 2, 3 allows to deform the sinterable material which constitutes the green body 5, pushing it to slide in the direction of deformation D to fill every point of the internal molding cavity 4.

In detail, the ratio between the overall dimensions of the mold 1 along the direction of deformation D and the overall dimensions of the green body 5 along the direction of deformation D is substantially between 1.2 and 6, which corresponds to deformations of the green body 5 varying between 20% and 500%.

It should be pointed out, however, that alternative embodiments of the present invention are possible in which the mold 1 has a shape of the forming cavity 4 different from that illustrated in Figures 1 to 4 and such as to induce the plastic deformation of the material not only along the direction of deformation D but also in other directions; in this case it is preferable that the plastic deformation of the material always remains within the aforementioned range of 20 ÷ 500% to ensure the necessary breakage of the oxide layers.

Furthermore, the mold 1 is intended to be crossed by an electric current to make it pass through the green body 5 and heat the sinterable material up to a predetermined sintering temperature.

The potential difference necessary to obtain electric current is brought to the mold 1 usually by means of isotropic graphite.

The passage of the electric current is obtained by applying an electric potential difference between the first punch 2 and the second punch 30 and putting the green body 5 in electrical contact with the punches 2, 30; by this we mean that the green body 5 is intended to be placed between the punches 2, 30 and to come into contact with both to allow the electric current to pass through it.

Furthermore, the mold 1 is shaped so that, during the approach of the first punch 2 to the second punch 30, the punch 2 comes into contact with the die 40 before touching the green body 5.

In this phase, therefore, the punches 2, 30 are put into mutual electrical contact through the die 40 before the green body 5 is put into electrical contact with the punch 2 and the punch 30, and this affects the transfer of the electric current through the mold 1.

In this regard, it should be emphasized that in figures 2 to 4 the passage of electric current of great intensity is schematically indicated with a solid line and with a dashed line the passage of electric current of lesser intensity.

At the beginning of the process (figure 2), almost all the electric current released by the difference in electric potential applied to the two ends of the mold 1 passes through the steel matrix 40, as the sinterable material which constitutes the green body 5 is still covered by the traditional oxide layer which isolates it electrically.

Following the application of the pressure exerted by the approaching punches 2, 30 and the increase in temperature that is transmitted by the walls of the internal molding cavity 4, a plastic deformation occurs in the green body 5 which is characterized by shear stresses capable of breaking the surface oxide layer of the sinterable material.

This determines a progressive increase of the electrical conductivity through the green body 5, with consequent reduction of the electric current that crosses the matrix 40 and an increase of that through the green body 5, which can therefore sinter perfectly (figures 3 and 4).

The plastic deformation of the sinterable material substantially depends on how much the shape of the green body 5 differs from the geometry of the mold 1 and can in any case also be very high, given that at the sintering temperature there are phenomena of recrystallization and superplasticity which avoid the risk of defects and / or breakage of the material.

More in detail, the methods of applying pressure and passing the current may vary from case to case.

For example, the pressure applied by the punch 1 could be minimal at the beginning of the process and then increased at the end, varying for example between 5 MPa and 200 MPa, but it is not excluded that it will remain constant throughout the process. of sintering.

The regulation in the passage of electric current, on the other hand, can be such as to increase the temperature of the green body 5 at the beginning of the process, for example with a variation of 5 ÷ 500 ° C / min, and therefore to keep it at a temperature predetermined towards the end of the process.

In accordance with the embodiment of the invention illustrated in Figures 1 to 4, the method according to the invention comprises the steps of:

prepare the sinterable material inside the mold 1. More in detail, this step includes a preliminary phase of compacting the sinterable material in the form of powder externally to the mold 1, to form the green body 5, and then Î "introduce the body green 5 in mold 1;

sintering the sinterable material in the mold 1 by passing electric current and applying pressure. In this phase, Î "application of pressure is obtained by approaching the punches 2, 30 and determines the deformation of the green body 5 from the initial conformation to the predetermined conformation of the mold 1. The passing of electric current, on the other hand, is obtained by applying a electric potential difference between the first punch 2 and the second punch 30 and putting the green body 5 in electrical contact with the punches 2, 30. In this phase, the mutual electrical contact of the punches 2, 30 through the die 40 takes place before electrical contact with the green body 5.

With reference to the embodiment of the present invention illustrated in Figures 5 to 8, the mold 1 consists of a first punch 2, a second punch 30 and a die 40 substantially similar to those of the previous embodiment, and therefore it will not be the subject of a further detailed explanation for all the characteristics that add it to the above description.

In particular, this embodiment differs from the previous one in that the mold 1 and the green body 5 are shaped so that the mutual electrical contact of the punches 2, 30 takes place after the electric contact of the green body 5. with punches 2, 30.

In particular, after having loaded the green coipo 5 inside the mold 1 (figure 5), approaching the first punch 2 towards the second punch 30 brings the first punch 2 into contact first with the green body 5 (figure 6) and only later with the matrix 40 (Figure 7).

In this way, in the position illustrated in figure 6, the electric current is only forced to pass through the green body 5, causing the oxide to break and the start of sintering of the green body 5 before its plastic deformation.

The passage of a high intensity of electric current in the green body 5 also determines a rapid and effective degassing of the sinterable material.

Subsequently, the further advancement of the punches 2, 30 determines the plastic deformation of the sintered product and the entry of the punch 2 into the die 40 (figure 7), so as to obtain the desired final geometry (figure 8).

To facilitate the entry of the punch 2 into the die 40, the latter can be equipped with rounded or rounded edges, or external guides in non-conductive material can be used to maintain the alignment between the punch 2 and the die 40 .

According to the embodiment of the invention illustrated in Figures 5 to 8, therefore, the method according to the invention comprises the steps of:

prepare the sinterable material inside the mold 1 in a similar way to the solution of Figures 1 to 4, ie by compacting the sinterable material in the form of powder externally to the mold 1 to form the green body 5, and then introducing the green body 5 in mold 1;

sintering the sinterable material in the mold 1 by passing electric current and applying pressure. The application of pressure is obtained by approaching the punches 2, 30 to determine the deformation of the green body 5 from the initial conformation to the predetermined conformation of the mold 1. The passing of electric current, on the other hand, is obtained by applying a potential difference between the first punch 2 and the second punch 30 and putting the green body 5 in electrical contact with the punches 2, 30. In this phase, the mutual electrical contact of the punches 2, 30 through the die 40 takes place after in electrical contact of the green body 5 with the punches 2, 30.

With reference to the embodiment of the present invention illustrated in Figures 9 to 12, the mold 1 is constituted by a first shaped element 2 and by a second shaped element 3 shaped for making a piston blank.

The second shaped element 3 consists of a single body, but alternative embodiments similar to those of Figures 1 to 4 and 5 to 8 in which it is made, for example, in two separate pieces defined by a punch are not excluded. bottom and an external matrix.

Taking into account the axial symmetry of the object, for simplicity of representation in these figures only a schematic and partial angular sector of the mold 1 is shown.

For the manufacture of the object, one advantageously starts from two green bodies 5a, 5b which cannot be translated into the mold 1 substantially next to each other, the initial conformation of the set of green bodies 5 a, 5b placed next to each other being different from the predetermined configuration of the mold 1; however, alternative forms of implementation are not excluded which envisage starting from a different number of green buildings, such as one, three or more.

The two green bodies 5a, 5b illustrated in the embodiment of figures 9 to 12 have a cylindrical shape of the same base and are intended to be introduced into the mold 1 side by side along the pressing direction P, i.e. one on top of the other. .

The green bodies 5 a, 5b are formed starting from identical powder sinterable materials, for example aluminum powder AA2124 dry mixed with SiC using a Turbula® mixer and some steel balls.

During the sintering process, the sinterable material of the green body 5a is intimately bonded with the sinterable material of the other green body 5b, obtaining a final product which does not present any discontinuity.

Alternatively, the green bodies 5a, 5b can be replaced by a single green body having dimensions equal to those of the set of green bodies 5a, 5b.

The use of two green bodies 5 a, 5b instead of just one is particularly useful when there are difficulties in compacting a single green body, for example because of a particularly slender and elongated shape; in fact, rather than forming a single green body with a very elongated shape and, therefore, difficult to handle without the risk of deforming or crumbling it, it may be preferable to shape two or more compact green bodies which, once placed together inside the mold 1, recompose the desired initial configuration.

The shaped elements 2, 3 and the green bodies 5 a, 5b are shaped so that during the sintering process all the current is forced, at least initially, to pass through the green bodies 5a, 5b as already described in the embodiment of the figures 5 to 8.

The sintering of the green bodies 5a, 5b takes place according to the following method:

application of a pressure equal to 5 MPa with respect to the final dimensions of the piece with an increase in the temperature of the green bodies 5a, 5b of 100 ° C / min up to 400 ° C;

application of the external load equal to 60 MPa with respect to the final dimensions of the piece and heating at 50 ° C / min up to 550 ° C;

maintaining the load of 60 MPa and 550 ° C for 1 minute;

free or forced cooling.

The evolution of the green bodies 5 a, 5b inside the mold 1 is illustrated in figures 9 to 12.

Figure 9 shows the situation at the beginning of the process after the green bodies 5a, 5b have been introduced into the mold 1.

Figure 10 represents the situation after about 3 minutes from the beginning of the process in which the increase in temperature and the application of 5 MPa determined the obtainment of two green bodies 5 a, 5b perfectly densified but not plastic deformation still occurred.

Figure 11 represents the situation after about 5 minutes at 500 ° C: the first shaped element 1 has moved further downwards, causing the plastic deformation of the sinterable material which has reached the value of 50% and the separation edge of the green coipi 5 a, 5b has almost completely disappeared.

Figure 12 shows the final situation in which the punch 1 has moved to the desired point with the sinterable material of a green body 5a which is intimately welded to another green body 5b; the maximum plastic deformation of the sinterable material is substantially equal to 200%.

According to the embodiment of the invention illustrated in Figures 9 to 12, therefore, the process according to the invention can be summarized in the following steps:

prepare the sinterable material inside the mold 1 by compacting the sinterable material in the form of powder to form the two green bodies 5a, 5b and then introducing the green bodies 5a, 5b into the mold 1 taking care to approach them along the pressing direction P , that is, one above the other;

sintering the sinterable material in the mold 1 by passing electric current and applying pressure on the green bodies 5 a, 5b. Conveniently, the mutual electrical contact of the shaped elements 2, 3 takes place after the electrical contact of the green bodies 5a, 5b with the shaped elements 2, 3. The sintering comprises a first phase, in which electric current is passed through increase the temperature of the green bodies 5a, 5b substantially by 100 ° C per minute up to substantially 400 ° C and a pressure substantially equal to 5 MPa is applied, a second phase, in which electric current is passed to increase the temperature of the green bodies 5 a, 5b substantially of 50 ° C per minute up to substantially 550 ° C and a pressure substantially equal to 60 MPa is applied, and a third phase, in which electric current is passed to maintain the temperature of the green bodies 5a , 5b substantially at 550 ° C for substantially one minute and a pressure substantially equal to 60 MPa is applied.

With reference to the embodiment of the present invention illustrated in Figures 13 to 17, the mold 1 consists of a first shaped element 2 and a second shaped element 3 shaped for the realization of a toothed pinion.

The second shaped element 3 consists of a single body, but alternative embodiments are not excluded in which it is made, for example, in three separate pieces, defined by a bottom punch, an external die and an internal core.

Taking into account the axisymmetry of the object, for simplicity of representation in these figures only a schematic and partial angular sector of the mold 1 is shown.

For the manufacture of the object, one advantageously starts from two green bodies 5a, 5b which can be introduced into the mold 1 substantially inserted one into the other, the initial conformation of the set of green bodies 5a, 5b placed next to each other being different from the predetermined conformation of the mold 1.

The two green bodies 5a, 5b illustrated in the embodiment of Figures 13 to 17 have a tubular shape of equal height and complementary diameters and are intended to be introduced into the mold 1 side by side along the direction of deformation D, i.e. one inside the € ™ other coaxially.

The green bodies 5 a, 5b are formed starting from different sinterable materials; for example the internal 5a green body is obtained from AA2124 powder while the external 5b green body is obtained from a mixture of AA2124 and SiC.

The shaped elements 2, 3 and the green strokes 5a, 5b are shaped so that during the sintering process all the current is forced, at least initially, to pass through the green bodies 5a, 5b.

The sintering of the green bodies 5 a, 5b takes place according to the method already described in relation to figures 9 to 12, namely:

application of a pressure equal to 5 MPa with respect to the final dimensions of the piece with an increase in the temperature of the green bodies 5a, 5b of 100 ° C / min up to 400 ° C;

application of the external load equal to 60 MPa with respect to the final dimensions of the piece and heating at 50 ° C / min up to 550 ° C; maintaining the load of 60 MPa and 550 ° C for 1 minute; free or forced cooling.

The evolution of the green bodies 5 a, 5b inside the mold 1 is illustrated in figures 13 to 17.

Figure 13 shows the situation at the beginning of the process after the green bodies 5a, 5b have been introduced into the mold 1.

Figure 14 represents the situation after about 3 minutes from the beginning of the process in which the increase in temperature and the application of 5 MPa resulted in Î "obtaining two green bodies 5a, 5b perfectly densified but not It still had plastic deformation.

From this moment on, the punch 2 begins to crush the green bodies 5 a, 5b and, in combination with the passage of current which raises their temperature, deforms them.

Figure 15 illustrates the situation at the entrance of the punch 2 into the second shaped element 3; the electric current therefore begins to pass not only from the punch 2 to the green bodies 5 a, 5b and from the green bodies 5 a, 5b to the second shaped element 3, but also directly from the punch 2 to the second shaped element 3.

Figures 16 and 17 represent the situation at the end of the process in which the first shaped element 2 has moved further downwards, causing the gradual plastic deformation of the sinterable material until the internal molding cavity 4 is completely filled.

During the sintering process, the sintering material of a green body 5a, 5b is intimately welded with the sintering material of the other green body 5a, 5b but on the external part of the final piece, corresponding to the teeth of the pinion, the material remains concentrated enriched with SiC, which is more performing from the point of view of wear resistance.

According to the embodiment of the invention illustrated in Figures 13 to 17, therefore, the process according to the invention can be summarized in the following steps:

prepare the sinterable material inside the mold 1 by compacting the sinterable material in the form of powder to form the two green coipients 5a, 5b and then introducing the green bodies 5a, 5b into the mold 1 taking care to approach them along the direction of deformation D, i.e. one inside the other, coaxially;

sintering the sintering material in the mold 1 by passing electric current and applying pressure on the green coipi 5 a, 5b. The sintering comprises a first step in which an electric current is passed to increase the temperature of the green bodies 5a, 5b substantially by 100 ° C per minute up to substantially 400 ° C, and a pressure substantially equal to 5 MPa is applied, a second phase in which an electric current is passed to increase the temperature of the green bodies 5 a, 5b substantially by 50 ° C per minute up to substantially 550 ° C, and a pressure substantially equal to 60 MPa is applied, and a third phase in which electric current is passed to maintain the temperature of the green bodies substantially at 550 ° C for substantially one minute, and a pressure substantially equal to 60 MPa is applied.

Claims (8)

CLAIMS 1) Process for the sintering of powders assisted by pressure and electric current, including the steps of: providing a sinterable material in the form of a powder and at least in part electrically conductive inside a mold (1) having a predetermined conformation; sintering said sinterable material in said mold (1) by passing electric current through said sinterable material and applying pressure on said sinterable material; characterized by the fact that said preparation includes the phases of: compacting said sinterable material in the form of powder externally of said mold (1) to form at least one green body (5, 5a, 5b) having an initial shape different from said predetermined shape of the mold (1); And introducing said green body (5, 5a, 5b) into said mold (1); The application of pressure during said sintering causing the deformation of said green body (5, 5a, 5b) from said initial conformation to said predetermined conformation of the mold (1). 2) Process according to claim 1, characterized in that said mold (1) comprises at least a first shaped element (2) and a second shaped element (3), which can be approached and removed from each other along a pressing direction (P) and have at least a deformation direction (D) substantially transverse to said pressing direction (P), the application of pressure during said sintering comprising bringing said shaped elements (2, 3) close together. 3) Process according to claim 2, characterized in that said first shaped element (2) comprises a first punch and said second shaped element (3) comprises a second punch (30) inserted in a die (40). 4) Process according to claim 2 or 3, characterized by the fact that said initial conformation of the green body (5, 5 a, 5b) has at least a portion whose overall dimensions along said direction of deformation (D) is substantially less than encumbrance of said predetermined conformation of the mold (1) along said deformation direction (D). 5) Process according to claim 4, characterized in that the ratio between said size of the mold (1) along said direction of deformation (D) and said size of the green body (5, 5a, 5b) along said direction of deformation ( D) It is substantially between 1.2 and 6. 6) Process according to one or more of the preceding claims, characterized in that the total plastic deformation of said green body (5, 5 a, 5b) in said mold (1) is substantially comprised between 20-500%. 7) Process according to one or more of the preceding claims, characterized in that said passing electric current comprises applying an electric potential difference between said first and second shaped element (2, 3) and placing said green body (5, 5a, 5b) in electrical contact with said shaped elements (2, 3). 8) Process according to claim 7, characterized by the fact that said passing electric current comprises putting said shaped elements (2, 3) into mutual electric contact before said putting the green body (5, 5a, 5b) into electrical contact with said shaped elements (2, 3) 9) Process according to claim 7, characterized in that said passing electric current comprises putting said shaped elements (2, 3) into mutual electric contact after said putting the green body (5, 5a, 5b) in electrical contact with said shaped elements (2, 3). 10) Process according to one or more of the preceding claims, characterized in that said mold (1) is made of an electrically conductive material chosen from the list including: steel, stainless steel, superalloys, WC / Co, titanium-based alloys, electrically conductive ceramic. 11) Process according to one or more of the preceding claims, characterized in that said mold (1) is at least partially coated with spray of graphite, boron nitride, silicone-based lubricant, PTFE lubricant or molybdenum disulphide. 12) Process according to one or more of the preceding claims, characterized in that said sintering comprises passing an electric current to increase the temperature of said green body (5, 5a, 5b) substantially by 5-500 ° C per minute. 13. Process according to one or more of the preceding claims, characterized in that said sintering comprises Î "applying a pressure of 5Ã · 200 MPa. 14) Process according to one or more of the preceding claims, characterized in that said sintering comprises a first step in which an electric current is passed to increase the temperature of said green body (5, 5 a, 5b) substantially by 100 ° C at minute up to substantially 400 ° C, and a pressure substantially equal to 5 MPa is applied. 15) Process according to one or more of the preceding claims, characterized in that said sintering comprises a second phase in which an electric current is passed to increase the temperature of said green body (5, 5a, 5b) substantially by 50 ° C to minute up to substantially 550 ° C, and a pressure substantially equal to 60 MPa is applied. 16) Process according to one or more of the preceding claims, characterized in that said sintering comprises a third phase in which an electric current is passed to maintain the temperature of said green body (5, 5a, 5b) substantially at 550 ° C for substantially one minute, and a pressure substantially equal to 60 MPa is applied. 17) Process according to one or more of the preceding claims, characterized in that said compacting of said sinterable material in the form of powder comprises forming at least two of said green bodies (5a, 5b) which can be introduced into said mold (1) placed side by side , the initial conformation of the set of said green bodies (5a, 5b) placed side by side being different from said predetermined conformation of the mold (1). 8) Process according to claim 17, characterized by the fact that said introducing comprises approaching said green bodies (5a, 5b) along said pressing direction (P). 19) Process according to claim 17, characterized by the fact that said introducing comprises approaching said green bodies (5a, 5b) along said direction of deformation (D). 20) Process according to one or more of the preceding claims, characterized in that said green bodies (5a, 5b) are formed starting from identical powdered sinterable materials. 2 1) Process according to one or more of the preceding claims, characterized by the fact that said green bodies (5a, 5b) are formed starting from different materials that can be sintered in powder form.