WO2011072961A1 - Process for sintering powders assisted by pressure and electric current - Google Patents

Process for sintering powders assisted by pressure and electric current Download PDF

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Publication number
WO2011072961A1
WO2011072961A1 PCT/EP2010/067269 EP2010067269W WO2011072961A1 WO 2011072961 A1 WO2011072961 A1 WO 2011072961A1 EP 2010067269 W EP2010067269 W EP 2010067269W WO 2011072961 A1 WO2011072961 A1 WO 2011072961A1
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WO
WIPO (PCT)
Prior art keywords
mould
fact
process according
green body
electric current
Prior art date
Application number
PCT/EP2010/067269
Other languages
French (fr)
Inventor
Francesco Casari
Luca Girardini
Mario Zadra
Original Assignee
K4Sint S.R.L.
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Publication date
Application filed by K4Sint S.R.L. filed Critical K4Sint S.R.L.
Publication of WO2011072961A1 publication Critical patent/WO2011072961A1/en

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Classifications

    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F3/00—Manufacture of workpieces or articles from metallic powder characterised by the manner of compacting or sintering; Apparatus specially adapted therefor ; Presses and furnaces
    • B22F3/10—Sintering only
    • B22F3/105—Sintering only by using electric current other than for infrared radiant energy, laser radiation or plasma ; by ultrasonic bonding
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F3/00—Manufacture of workpieces or articles from metallic powder characterised by the manner of compacting or sintering; Apparatus specially adapted therefor ; Presses and furnaces
    • B22F3/12—Both compacting and sintering
    • B22F3/14—Both compacting and sintering simultaneously
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C21/00—Alloys based on aluminium
    • C22C21/12—Alloys based on aluminium with copper as the next major constituent
    • C22C21/16—Alloys based on aluminium with copper as the next major constituent with magnesium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C23/00—Alloys based on magnesium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F3/00—Manufacture of workpieces or articles from metallic powder characterised by the manner of compacting or sintering; Apparatus specially adapted therefor ; Presses and furnaces
    • B22F3/02—Compacting only
    • B22F2003/026—Mold wall lubrication or article surface lubrication

Definitions

  • the present invention relates to a process for sintering powders assisted by pressure and electric current.
  • powder metallurgy represents a family of processes able to obtain a finished object starting with metal powders of different dimensions which are pressed and sintered (Press & Sinter).
  • Such technology is widely used to make large numbers of objects, usually in steel, and makes it possible to produce materials and/or composites (Metal Matrix Composites) otherwise hard to obtain.
  • any metal powder is always coated with a thin layer of oxide that has to be removed to obtain the necessary cohesion among the particles and, therefore, to fabricate sintered pieces with good mechanical properties.
  • the removal of such oxide occurs by simply exposing the metal to a reducing atmosphere.
  • One of these processes envisages the succession of different phases: mixing of the aluminium and silicon carbide powders, degassing and compacting (Hot Isostatic Pressing, Cauning...), obtaining bloom, plastic deformation of the material (extrusion and/or forging), machine tool working. It is therefore evident how all the above phases together are very complex and therefore costly: in particular the degassing and canning phases (filling a tube with powder and subsequent hot extrusion) are very slow, technologically demanding and economically unfavourable.
  • Another sintering method consists in so-called "powder forging", wherein the sintered pieces are obtained starting with a green body, meaning by green body a compacted powder body not yet sintered or partially or totally sintered, which undergoes forging by means of one or more blows of a hammer.
  • the green body is obtained on a preliminary basis after pressing the powder at room temperature at about 400 MPa, after which it is heated at a temperature of 450-500°C for time enough for degassing, and then forged by applying repeated blows of a hammer with pressures of around 800 MPa in a mould which is at a temperature of 200°C, bringing about the required plastic deformation of the powder particles.
  • This process does however have a number of confraindications including the limited plastic deformation achievable and the low level of detail that can be obtained in the sintered pieces.
  • the high speed of deformation produced by the hammer in fact requires that the preliminary shaping of the green body correspond substantially to that of the piece to be obtained so as not to incur in defective fonning.
  • Aluminium and magnesium alloys can thus be obtained with good mechanical properties thanks to the combined action of the transit of electric current and mechanical pressure which thus manage to break the layer of oxide on the surface of the powder particles.
  • This method is particularly effective in sintering so-called hard-to-sinter-materials, including aluminium and magnesium.
  • Isotropic graphite in fact has a number of particularly advantageous characteristics, such as high electrical resistance (8 ⁇ 20 ⁇ Om ), high thermal conductivity (50 ⁇ 100 W/mK), low density (1.5 ⁇ 1.85 g/cm 3 ) and self-lubricating capacities, but has negative factors such as low bending strength (30 ⁇ 60 MPa), low hardness (40 -90 Shore) and lack of resistance to wear.
  • the low bending strength results in the impossibility to apply pressures through the punches higher than 50-60 MPa so as to avoid the breakage of the moulds and in any case the impossibility -of having geometries' with square profiles or low radius connections (e.g., less than 4 mm).
  • moulds made of steel, super-alloys or WC-Co which would allow obtaining much more complex objects, applying much higher loads and reducing the wear and tolerance problems, is not compatible with traditional SPS technology because such materials have low electrical resistivity (around 1 ⁇ Om.).
  • the low electrical resistivity of the mould results in the scarce sintering of the aluminium powder. This is determined by the resistivity values of the initial sinterable powder, normally around 1 Qm, and by the resistivity of the material after sintering, around 0.01 ⁇ 10 ⁇ Om.
  • Such a wide fork of resistivity values of the sintered material depends on the actual breakage of the surface oxide layer (which is a perfect insulator): values of 0.01 ⁇ represent a perfectly sintered and therefore conductive material, while values around 10 ⁇ represent a not very sintered, not very conductive material with few mechanical properties.
  • suitable insulating coatings can be used, such as that illustrated in patent document WO
  • This document envisages placing a sheet of mica between the walls of the mould and the powder to be sintered, thus forcing the electric current to transit through the powder.
  • the main aim of the present invention is to provide a process for sintering powders assisted by pressure and electric current that allows sintering in a practical, easy and functional way the metal powders, including so-called hard- to-sinter materials, to obtain sintered pieces with high mechanical properties, including, in particular, high toughness and ultimate elongation, also in the case of complex-geometry pieces.
  • Another object of the present invention is to provide a process for sintering powders assisted by pressure and electric current which allows overcoming the mentioned drawbacks of the state of the art within the ambit of a simple, rational, easy and effective to use as well as low cost solution.
  • FIGS. 1 to 4 illustrate, in a sequence of section, schematic and partial views, a first embodiment of the process according to the invention
  • FIGS. 5 to 8 illustrate, in a sequence of section, schematic and partial views, a second embodiment of the process according to the invention
  • FIG. 9 to 12 illustrate, in a sequence of cutaway, schematic and partial views, a third embodiment of the process according to the invention.
  • FIGS 13 to 17 illustrate, in a sequence of cutaway, schematic and partial views, a fourth embodiment of the process according to the invention.
  • the mould 1 in particular, comprises a first shaped element or first punch 2, and a second shaped element 3, composed e.g. of a second punch 30 fitted in a shaped matrix die 40.
  • the second punch 30 and the matrix die 40 are made separate and reciprocally moving to favour the opening and closing of the mould 1, but nevertheless, other embodiments cannot be ruled out wherein, on the other hand, 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 reciprocally moved closer and away along a direction of pressing, while the direction transversal to the direction of pressing defines a so-called direction of deformation.
  • the first punch 2 and the second punch 30 are arranged, inside the shaped matrix die 40, one above the other and can be moved near/away along a substantially vertical direction of pressing P, while the direction of defomiation D is substantially horizontal.
  • the first punch 2, the second punch 30 and the shaped matrix die 40 together define an internal forming cavity 4 suitable for shaping a sinterable material in powder form to obtain a sintered piece.
  • the mould 1 is made of steel and, preferably, is coated at least partially with graphite spray, boron nitride, silicone-based lubricant, PTFE lubricant or molybdenum bi-sulphide, to reduce the coefficients of friction.
  • the sinterable material in powder form is at least in part electrically conductive such as, e.g., metal powders with aluminium, chrome, magnesium, Al-MMC (Metal Matrix Composite of Aluminium), Mg-MMC (Metal Matrix Composite of Magnesium) base; it cannot however be ruled out that the present invention can be also used to sinter other powders than those previously listed.
  • electrically conductive such as, e.g., metal powders with aluminium, chrome, magnesium, Al-MMC (Metal Matrix Composite of Aluminium), Mg-MMC (Metal Matrix Composite of Magnesium) base; it cannot however be ruled out that the present invention can be also used to sinter other powders than those previously listed.
  • the green body 5 is shaped with an initial conformation different to the preset conformation of the mould 1 ,
  • the initial conformation of the green body 5 has at least a portion whose overall dimensions along the direction of deformation D are substantially less than the overall dimensions of the preset conformation of the mould 1 along the same direction of deformation D.
  • the horizontal dimensions of the green body 5 are considerably lower than the dimensions of the internal forming cavity 4.
  • the relative approach of the shaped elements 2, 3 allows deforming die sinterable material which makes up the green body 5, pushing it to flow in the direction of deformation D to go and fill every point of the internal forming cavity 4.
  • the ratio between the overall dimensions of the mould 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%.
  • the mould 1 has a shape of the forming cavity 4 different to that shown in the figures from 1 to 4 and such as to cause 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 for the plastic deformation of the material to always remain inside the above 20 ⁇ 500% range to ensure the necessary breakage of the oxide layers.
  • the mould 1, furthermore, is designed to be crossed by electric current to make it pass through the green body 5 and heat the sinterable material up to a preset sintering temperature.
  • the difference in potential required to obtain electric current is usually conveyed to the mould 1 by means of isotropic graphite.
  • the making electric current to pass through or, in otlier words, the transit of electric current is obtained by applying a difference in electric potential between the first punch 2 and the second punch 30 and placing the green body 5 in electric contact with the punches 2, 30; by this is meant that the green body 5 is intended to be arranged between the punches 2, 30 and to come into contact with both to allow the electric current to pass through it.
  • the mould 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 matrix die 40 before touching the green body 5,
  • the punches 2, 30 come into reciprocal electric contact through the matrix die 40 before the green body 5 is placed in electric contact with the punch 2 and the punch 30, and this affects the transfer of the electric current through the mould 1.
  • 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 mould 1 and can in any case even be very high, considering that at sintering temperature re-crystallization and superp!asticity phenomena occur which eliminate the risk of material defects and/or breakage.
  • the pressure applied by means of the punch 1 could be minimum at the start of the process and then have increased at the end, varying e.g. between 5 MPa and 200 MPa, but it cannot however be ruled out that this remain constant for the entire sintering process.
  • the adjustment of the transit of electric current on the other hand can be such as to cause the temperature of the green body 5 to increase at the start of the process, e.g. with a variation of 5 ⁇ 500oC/min, and then keep it at a preset temperature towards the end of the process.
  • the process according to the invention comprises the folio wing phases: - arranging the sinterable material inside the mould 1, More in detail, this step comprises a preliminary phase of compacting the sinterable material in powder form outside the mould 1, to form the green body 5, and then introducing the green body 5 into the mould 1;
  • the application of pressure is achieved by moving the punches 2, 30 closer together and determines the deformation of the green body 5 from the initial conformation to the preset conformation of the mould 1.
  • the transit of electric current is obtained by applying a difference in electric potential between the first punch 2 and the second punch 30 and placing the green body 5 in electric contact with the punches 2, 30.
  • the placing of the punches 2, 30 in reciprocal electric contact through the matrix die 40 occurs before the placing in electric contact with the green body 5.
  • the mould 1 is composed of a first punch 2, of a second punch 30 and of a matrix die 40 substantially similar to those of the previous embodiment and will not be the subject of a further detailed explanation as regards all the characteristics which make it similar to the above description.
  • this embodiment differs from the previous one due to the fact that the mould 1 and the green body 5 are shaped so that the placing in reciprocal electrical contact of the punches 2, 30 occurs after the placing in electrical contact of the green body 5 with the punches 2, 30.
  • the moving of the first punch 2 nearer to the second punch 30 brings the first punch 2 into contact first with the green body 5 (figure 6) and only secondly with the matrix die 40 (figure 7).
  • the electric current is forced to pass only through the green body 5 producing the breakage of the oxide and the start of the sintering of the green body 5 before its plastic deformation.
  • the transit of a high intensity of electric current in the green body 5 also determines the fast and effective degassing of the sinterable material.
  • the further forward movement of the punches 2, 30 determines the plastic deformation of the sintered material and the entry of the punch 2 in the matrix die 40 (figure 7), so as to obtain the final desired geometry (figure 8).
  • the latter can have connected or chamfered corners, or else external guides can be used made of non-conducting material winch maintain the aligmnent between the punch 2 and the matrix die 40.
  • the process according to the invention comprises the following phases: - arranging the sinterable material inside the mould 1, in a similar way to the solution of the figures from 1 to 4, that is compacting the sinterable material in powder form outside the mould 1 to form die green body 5. and then introducing the green body 5 into the mould 1 ;
  • sintering the sinterable material in t!ie mould 1 by making electric current to pass through and applying a pressure.
  • the application of pressure is achieved by moving the punches 2, 30 closer together to detenriine the deformation of the green body 5 from the initial conformation to the preset conformation of the mould 1.
  • the transit of electric current is obtained by applying a difference in electric potential between the first punch 2 and the second punch 30 and placing the green body 5 in electric contact with the punches 2, 30.
  • the placing of the punches 2, 30 in reciprocal electric contact through the matrix die 40 occurs after the placing of the green body 5 in electric contact with the punches 2, 30.
  • the mould 1 is composed of a first shaped element 2 and of a second shaped element 3 shaped for making the rough-shaped piece of a piston.
  • the second shaped element 3 consists of a single body, but other alternative embodiments similar to those of the figures from 1 to 4 and from 5 to 8 cannot be ruled out wherein this is made, e.g., in two separate pieces defined by a bottom punch and by an external matrix die.
  • Fabrication of the object must advantageously start with two green bodies 5a, 5b that can be introduced into the mould 1 substantially side by side, the initial conformation of the assembly of green bodies 5a, 5b positioned side by side being different to the preset conformation of the mould 1; alternative embodiments cannot however be ruled out that envisage starting with a different number of green bodies, such as just one, three or more.
  • the two green bodies 5a, 5b shown in the embodiment in the figures from 9 to 12 have a cylindrical shape with identical base and are meant to be introduced into the mould 1 side by side along the direction of pressing P, i.e., one on top of the other.
  • the green bodies 5a, 5b are formed starting with sinterable materials in powder form identical to one another, e.g., aluminium powder AA2124 mixed dry with SiC using a Turbula® mixer and a number of steel balls.
  • the sinterable material of the green body 5a unites intimately with the sinterable material of the other green body 5b, obtaining an end product without any discontinuity at all.
  • the green bodies 5a, 5b can be replaced by a single green body with dimensions identical to those of the assembly of green bodies 5a, 5b.
  • the use of two green bodies 5a, 5b instead of just one is particularly useful when difficulties are found in compacting a single green body, e.g., because this has a particularly slim and elongated shape; in fact, rather than form a single green body with very elongated shape and, therefore, difficult to handle without the risk of deforming it or crushing it, it could be preferable to shape two or more green bodies of compact shape which, once positioned side by side inside the mould 1 , recompose the desired initial configuration.
  • the shaped elements 2, 3 and tlie green bodies 5a, 5b are shaped so that during the sintering process, all the current is forced, at least initially, to transit in the green bodies 5a, 5b as already described in the embodiment of the Figures from 5 to 8.
  • the figure 9 shows the situation at tlie start of the process after the green bodies 5 a, 5b have been introduced into the mould 1.
  • the figure 10 represents tlie situation after about 3 minutes from the start of the process wherein the increase in temperature and the application of 5 MPa has determined the obtaining of two green bodies 5a, 5b perfectly densified but where plastic deformation has still not occurred.
  • the figure 11 represents the situation after about 5 minutes at 500°C: the first shaped element 1 has moved further downwards, determining the plastic deformation of the sinterable material which has reached the value of 50% and the separation edge of the green bodies 5a, 5b has almost completely disappeared.
  • the figure 12 shows the final situation wherein the punch 1 has moved to the desired point with die sinterable material of a green body 5a which has intimately united with tlie other green body 5b; the maximum plastic deformation of the sinterable material is substantially equal to 200%.
  • the process according to tlie invention can be summed up in tlie following phases; arranging the sinterable material inside the mould 1, compacting the sinterable material in powder form to form the two green bodies 5a, 5b and then introducing the green bodies 5a, 5b into the mould 1 being careful to place them side by side along the direction of pressing P, i.e., one on top of the other ;
  • the placing in reciprocal electric contact of the shaped elements 2, 3 occurs after the placing in electric contact of the green bodies 5a, 5b with the shaped elements 2, 3.
  • Sintering comprises a first phase, wherein electric current is made to pass through to increase the temperature of the green bodies 5a, 5b substantially by 1G0°C a minute substantially up to 400°C and a pressure is applied substantially equal to 5 MPa, a second phase, wherein electric current is made to pass through to increase the temperature of the green bodies 5a, 5b substantially by 50°C a minute up to substantially 550°C and a pressure is applied substantially equal to 60 MPa, and a third phase, wherein electric current is made to pass through to maintain the temperature of the green bodies 5a, 5h substantially at 550°C for substantially one minute and a pressure is applied substantially equal to 60 MPa.
  • the mould 1 is composed of a first shaped element 2 and of a second shaped element 3 shaped for making a toothed pinion.
  • the second shaped element 3 consists of a single body, but other alternative embodiments cannot be ruled out wherein this is made, e.g., in three separate pieces defined by a bottom punch, an external matrix die and an internal core. Taking into account the axial symmetry of the object, for simplicity of representation, in these illustrations only a schematic and partial angular sector of the mould 1 has been shown.
  • Fabrication of the obj ect must advantageously start with two green bodies 5a, 5b that can be introduced into the mould 1 substantially fitted one into the other, the initial conformation of the assembly of green bodies 5a, 5b positioned side by side being different to the preset conformation of the mould 1.
  • the two green bodies 5a, 5b shown in the embodiment in the figures from 13 to 17 have tubular shape of the same height and complementary diameters and are meant to be introduced into the mould 1 side by side along the direction of deformation D, i.e., one inside the other coaxially.
  • the green bodies 5a. 5b are formed starting with sinterable materials different the one from the other; the internal green body 5a is obtained, e.g., from AA2124 powder while the external green body 5b is obtained starting from a mixture of AA2124 and SiC.
  • the shaped elements 2, 3 and the green bodies 5a, 5b are shaped so that during the sintering process, all the current is forced, at least initially, to transit in the green bodies 5a, 5b.
  • the figure 13 shows the situation at the start of the process after the green bodies 5a, 5b have been introduced into the mould 1.
  • the figure 14 represents the situation after about 3 minutes from the start of the process wherein the increase in temperature and the application of 5 MPa has determined the obtaining of two green bodies 5a, 5b perfectly densified but where plastic deformation has still not occurred.
  • die punch 2 starts to crush the green bodies 5a, 5b and, in combination with the transit of current which increases their temperature, deforms them.
  • the figure 15 shows the situation in correspondence to the entry of the punch 2 In the second shaped element 3; the electric current therefore starts to pass not only from the punch 2 to the. green bodies 5a, 5b and from the green bodies 5a, 5b to the second shaped element 3, but also directly from the punch 2 to the second shaped element 3.
  • the figures 16 and 17 represent the situation at the end of the process wherein the first shaped element 2 has moved further downwards, determining the gradual plastic deformation of the sinterable material up to the complete filling of the internal forming cavity 4.
  • the sinterable material of a green body 5a, 5b intimately unites with the sinterable material of the other green body 5 a, 5b but on the outer part of the end piece, corresponding to the teeth of the pinion, the material enriched with SiC, which performs better in terms of wear resistance, remains concentrated,
  • Sintering comprises a first phase, wherein electric current is made to pass through to increase the temperature of the green bodies 5a, 5b substantially by 100°C a minute substantially up to 400°C and a pressure is applied substantially equal to 5 MPa, a second phase, wherein electric current is made to pass through to increase the temperature of the green bodies 5a, 5b substantially by 50 D C a minute up to substantially 550°C and a pressure is applied substantially equal to 60 MPa, and a third phase, wherein electric current is made to pass through to maintain the temperature of the green bodies substantially at 550°C for substantially one minute and a pressure is applied substantially equal to 60 MPa.

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Abstract

The process for sintering powders assisted by pressure and electric current comprises the phases of: arranging a sinterable material in powder form and at least in part electrically conductive inside a mould (1) having a preset conformation. Such phase in turn comprises the phases of: compacting the sinterable material in powder form outside the mould (1) to form at least a green body (5, 5a, 5b) having an initial conformation different to the preset conformation of the mould (1); and introducing the green body (5, 5a, 5b) into the mould (1); sintering the sinterable material in the mould (1) by making electric current to pass through the sinterable material and applying a pressure on the sinterable material; the application of pressure during the sintering phase causing the deformation of the green body (5, 5a, 5b) from the initial conformation to the preset conformation of the mould (1).

Description

PROCESS FOR SINTERING POWDERS ASSISTED BY PRESSURE AND ELECTRIC CURRENT
Technical Field
The present invention relates to a process for sintering powders assisted by pressure and electric current.
Background Art
It is known that powder metallurgy represents a family of processes able to obtain a finished object starting with metal powders of different dimensions which are pressed and sintered (Press & Sinter).
Such technology is widely used to make large numbers of objects, usually in steel, and makes it possible to produce materials and/or composites (Metal Matrix Composites) otherwise hard to obtain.
In the case, on the other hand, of wanting to use easily oxidable metal powders (in particular aluminium, magnesium and relative alloys) the simple sintering process is not viable.
In fact, it must be taken into account that any metal powder is always coated with a thin layer of oxide that has to be removed to obtain the necessary cohesion among the particles and, therefore, to fabricate sintered pieces with good mechanical properties.
In the case of sufficiently noble metals such as copper and its alloys, for example, the removal of such oxide occurs by simply exposing the metal to a reducing atmosphere.
In the case of steel, it is possible to make use of reducing atmospheres and/or modify the carbon content contained in the ferrous alloy.
In the case of metals that are not very noble such as aluminium and magnesium, on the other hand, the removal of the oxide would require the application of partial pressures of the oxygen so low as to make impossible their removal in a simple thermo-chemical way.
For classic sintering of aluminium, the standard Press & Sinter process can be adopted using e.g. commercial EckaGranules® powders; in this process it is however necessary to sinter at very high temperatures (about 600°C) for long periods (2 hours) in dry nitrogen atmospheres, including with the presence of alloy elements which can form liquid phases (copper, magnesium, manganese, silicon, tin...).
The adoption of this process allows obtaining sintered pieces with mechanical properties in any case rather low, in particular their toughness and ultimate elongation (below 2%).
To sinter aluminium or magnesium alloys, other processes are preferred which usually envisage a stage of plastic deformation of the powders, needed to break the layer of surface oxide to ensure metal/metal contact and consequently the perfect joining of the different powder particles.
Such stage of plastic deformation is even more necessary in order to obtain Metal Matrix Composites based on aluminium or magnesium.
One of these processes, e.g., envisages the succession of different phases: mixing of the aluminium and silicon carbide powders, degassing and compacting (Hot Isostatic Pressing, Cauning...), obtaining bloom, plastic deformation of the material (extrusion and/or forging), machine tool working. It is therefore evident how all the above phases together are very complex and therefore costly: in particular the degassing and canning phases (filling a tube with powder and subsequent hot extrusion) are very slow, technologically demanding and economically unfavourable.
Another sintering method consists in so-called "powder forging", wherein the sintered pieces are obtained starting with a green body, meaning by green body a compacted powder body not yet sintered or partially or totally sintered, which undergoes forging by means of one or more blows of a hammer.
Typically, the green body is obtained on a preliminary basis after pressing the powder at room temperature at about 400 MPa, after which it is heated at a temperature of 450-500°C for time enough for degassing, and then forged by applying repeated blows of a hammer with pressures of around 800 MPa in a mould which is at a temperature of 200°C, bringing about the required plastic deformation of the powder particles.
This process does however have a number of confraindications including the limited plastic deformation achievable and the low level of detail that can be obtained in the sintered pieces. The high speed of deformation produced by the hammer in fact requires that the preliminary shaping of the green body correspond substantially to that of the piece to be obtained so as not to incur in defective fonning.
The case is consequently frequent in which some areas of the material have not achieved a high enough level of plastic deformation to ensure the necessary breakage of the oxide surface layers and the consequent sintering cohesion.
Sometimes, fuilliennore, the obtained objects have inconvenient and dangerous internal fractures.
Another way of sintering aluminium or magnesium powders in an inexpensive way is to use sintering methods 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 mould (usually made of graphite) and, after reaching a vacuum level of a few pascals, by means of two punches, electric current and pressure are independently applied.
Aluminium and magnesium alloys can thus be obtained with good mechanical properties thanks to the combined action of the transit of electric current and mechanical pressure which thus manage to break the layer of oxide on the surface of the powder particles.
This method, summarised for the sake of brevity in the acronym SPS, is particularly effective in sintering so-called hard-to-sinter-materials, including aluminium and magnesium.
The biggest problem in using the SPS process concerns the manufacture of objects of complex geometric shape; in this case in fact, the use of isotropic graphite as material to make the moulds in which the powder to be sintered is poured is highly limiting.
Isotropic graphite in fact has a number of particularly advantageous characteristics, such as high electrical resistance (8÷20 μOm ), high thermal conductivity (50÷100 W/mK), low density (1.5÷1.85 g/cm3) and self-lubricating capacities, but has negative factors such as low bending strength (30÷60 MPa), low hardness (40 -90 Shore) and lack of resistance to wear.
The low bending strength results in the impossibility to apply pressures through the punches higher than 50-60 MPa so as to avoid the breakage of the moulds and in any case the impossibility -of having geometries' with square profiles or low radius connections (e.g., less than 4 mm).
The lack of resistance to wear and the low hardness of the graphite results in the need to frequently replace the moulds in order to prevent the powder overflowing or obtaining out-of-tolerance objects.
Conversely, the use of moulds made of steel, super-alloys or WC-Co, which would allow obtaining much more complex objects, applying much higher loads and reducing the wear and tolerance problems, is not compatible with traditional SPS technology because such materials have low electrical resistivity (around 1 μOm.).
The low electrical resistivity of the mould results in the scarce sintering of the aluminium powder. This is determined by the resistivity values of the initial sinterable powder, normally around 1 Qm, and by the resistivity of the material after sintering, around 0.01÷10 μOm.
Such a wide fork 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 μθπι represent a perfectly sintered and therefore conductive material, while values around 10 μθιη represent a not very sintered, not very conductive material with few mechanical properties.
The use of steel moulds prevents achieving low resistivity values in the sintered material considering the current prefers to pass in the mould rather than in the powder to be sintered, cancelling the beneficial effects of sintering assisted by electric current.
To overcome the fact that by using metal moulds, the electric current tends to transit in the mould and not in the powder to be sintered, suitable insulating coatings can be used, such as that illustrated in patent document WO
2008/085947.
This document envisages placing a sheet of mica between the walls of the mould and the powder to be sintered, thus forcing the electric current to transit through the powder.
In this case too however, a number of drawbacks are encountered including the reduced degassing offered by the mica and the difficulty in applying the coating sheets on moulds of complex shape.
Another way of trying to induce greater transit of electric current in the metal powder is to coat the metal mould with PVD deposits of intennetallics such as AlTiN or AlTiSiN which have semi-conducting properties.
This method is however not very viable either, because the coating has to be perfectly uniform and continuous to prevent the intensification of electric current at any one point and, therefore, melting and/or damaging the coating.
Description of the Invention
The main aim of the present invention is to provide a process for sintering powders assisted by pressure and electric current that allows sintering in a practical, easy and functional way the metal powders, including so-called hard- to-sinter materials, to obtain sintered pieces with high mechanical properties, including, in particular, high toughness and ultimate elongation, also in the case of complex-geometry pieces.
Another object of the present invention is to provide a process for sintering powders assisted by pressure and electric current which allows overcoming the mentioned drawbacks of the state of the art within the ambit of a simple, rational, easy and effective to use as well as low cost solution.
The above objects are achieved by the present process for sintering powders assisted by pressure and electric current, comprising the phases of:
arranging a sinterable material in powder form and at least in part electrically conductive inside a mould having a preset conformation;
sintering said sinterable material in said mould by making electric current to pass through said sinterable material and applying a pressure on said sinterable material;
characterised by the fact that said arranging comprises the phases of:
compacting said sinterable material in powder form outside said mould to form at least a green body having an initial conformation different to said preset conformation of the mould; and
introducing said green body into said mould;
the application of pressure during said sintering causing the deformation of said green body from said initial conformation to said preset conformation of the mould.
Brief Description of the Drawings
Other characteristics and advantages of the present invention will become more evident from the description of some preferred, but not sole, embodiments of a process for sintering powders assisted by pressure and electric current, illustrated purely as an example but not limited to the annexed drawings in which:
figures 1 to 4 illustrate, in a sequence of section, schematic and partial views, a first embodiment of the process according to the invention;
figures 5 to 8 illustrate, in a sequence of section, schematic and partial views, a second embodiment of the process according to the invention;
figures 9 to 12 illustrate, in a sequence of cutaway, schematic and partial views, a third embodiment of the process according to the invention;
figures 13 to 17 illustrate, in a sequence of cutaway, schematic and partial views, a fourth embodiment of the process according to the invention.
Embodiments of the Invention
With particular reference to the figures 1 to 4, globally indicated by 1 is a mould having a preset conformation.
The mould 1, in particular, comprises a first shaped element or first punch 2, and a second shaped element 3, composed e.g. of a second punch 30 fitted in a shaped matrix die 40.
The second punch 30 and the matrix die 40 are made separate and reciprocally moving to favour the opening and closing of the mould 1, but nevertheless, other embodiments cannot be ruled out wherein, on the other hand, 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 reciprocally moved closer and away along a direction of pressing, while the direction transversal to the direction of pressing defines a so-called direction of deformation.
In figure 1, the direction of pressing is indicated by the arrows P while the direction of deformation is shown by the arrows D.
In the particular embodiment shown in the figures from 1 to 4, e.g., the first punch 2 and the second punch 30 are arranged, inside the shaped matrix die 40, one above the other and can be moved near/away along a substantially vertical direction of pressing P, while the direction of defomiation D is substantially horizontal.
The first punch 2, the second punch 30 and the shaped matrix die 40 together define an internal forming cavity 4 suitable for shaping a sinterable material in powder form to obtain a sintered piece.
Usefully, in this treatise, reference can be made equally to the mould 1 and to the internal forming cavity 4, inasmuch as such cavity represents the useful part of the mould 1.
The mould 1 is made of steel and, preferably, is coated at least partially with graphite spray, boron nitride, silicone-based lubricant, PTFE lubricant or molybdenum bi-sulphide, to reduce the coefficients of friction.
The possibility of this being made of stainless steel, superalloys and/or electrically conductive ceramics cannot however be ruled out.
The sinterable material in powder form is at least in part electrically conductive such as, e.g., metal powders with aluminium, chrome, magnesium, Al-MMC (Metal Matrix Composite of Aluminium), Mg-MMC (Metal Matrix Composite of Magnesium) base; it cannot however be ruled out that the present invention can be also used to sinter other powders than those previously listed.
Making further reference to the figures from 1 to 4, by 5 is indicated a green body obtained by compacting the sinterable material in powder form.
Advantageously, the green body 5 is shaped with an initial conformation different to the preset conformation of the mould 1 ,
The initial conformation of the green body 5 has at least a portion whose overall dimensions along the direction of deformation D are substantially less than the overall dimensions of the preset conformation of the mould 1 along the same direction of deformation D.
In other words, by sectioning the mould 1 along a plane parallel to the direction of pressing P, e.g., similar to the section plane of the figures from 1 to 4, the horizontal dimensions of the green body 5 are considerably lower than the dimensions of the internal forming cavity 4. This way, the relative approach of the shaped elements 2, 3 allows deforming die sinterable material which makes up the green body 5, pushing it to flow in the direction of deformation D to go and fill every point of the internal forming cavity 4.
In detail, the ratio between the overall dimensions of the mould 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 must be stressed however that alternative embodiments of the present invention are possible wherein the mould 1 has a shape of the forming cavity 4 different to that shown in the figures from 1 to 4 and such as to cause 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 for the plastic deformation of the material to always remain inside the above 20÷500% range to ensure the necessary breakage of the oxide layers.
The mould 1, furthermore, is designed to be crossed by electric current to make it pass through the green body 5 and heat the sinterable material up to a preset sintering temperature.
The difference in potential required to obtain electric current is usually conveyed to the mould 1 by means of isotropic graphite.
The making electric current to pass through or, in otlier words, the transit of electric current is obtained by applying a difference in electric potential between the first punch 2 and the second punch 30 and placing the green body 5 in electric contact with the punches 2, 30; by this is meant that the green body 5 is intended to be arranged between the punches 2, 30 and to come into contact with both to allow the electric current to pass through it.
Furthermore, the mould 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 matrix die 40 before touching the green body 5,
In this phase, therefore, the punches 2, 30 come into reciprocal electric contact through the matrix die 40 before the green body 5 is placed in electric contact with the punch 2 and the punch 30, and this affects the transfer of the electric current through the mould 1.
In this respect, it is underlined that in the figures from 2 to 4, schematically indicated by an unbroken line is the transit of high-intensity electric current and by a broken line the transit of lower-intensity electric current.
At the start of the process (figure 2), nearly all the electric current produced by the difference in electrical potential applied at the two extremities of the mould 1 transits through the steel matrix die 40, because the sinterable material which makes up the green body 5 is still coated with the traditional layer of oxide that insulates it electrically .
Following the application of the pressure applied by the punches 2, 30 moving closer to one another and the increase in the temperature transmitted by the walls of the internal fomiing cavity 4, in the green body 5 a plastic deformation is determined which is distinguished by shearing stresses able to break the surface oxide layer of the sinterable material ,
This determines a gradual increase in electrical conductivity through the green body 5, with consequent reduction of the electric current that crosses the matrix die 40 and increase of that through the green body 5, which can therefore perfectly sinter (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 mould 1 and can in any case even be very high, considering that at sintering temperature re-crystallization and superp!asticity phenomena occur which eliminate the risk of material defects and/or breakage.
More in detail, the procedures for applying pressure and for the transit of current can vary from case to case.
For example, the pressure applied by means of the punch 1 could be minimum at the start of the process and then have increased at the end, varying e.g. between 5 MPa and 200 MPa, but it cannot however be ruled out that this remain constant for the entire sintering process.
The adjustment of the transit of electric current on the other hand can be such as to cause the temperature of the green body 5 to increase at the start of the process, e.g. with a variation of 5÷500ºC/min, and then keep it at a preset temperature towards the end of the process.
In agreement with the embodiment of the invention shown in the figures from 1 to 4, the process according to the invention comprises the folio wing phases: - arranging the sinterable material inside the mould 1, More in detail, this step comprises a preliminary phase of compacting the sinterable material in powder form outside the mould 1, to form the green body 5, and then introducing the green body 5 into the mould 1;
sintering the sinterable material in the mould 1 by making electric current to pass through and applying a pressure. In this phase, the application of pressure is achieved by moving the punches 2, 30 closer together and determines the deformation of the green body 5 from the initial conformation to the preset conformation of the mould 1. The transit of electric current on the other hand is obtained by applying a difference in electric potential between the first punch 2 and the second punch 30 and placing the green body 5 in electric contact with the punches 2, 30. In this phase, the placing of the punches 2, 30 in reciprocal electric contact through the matrix die 40 occurs before the placing in electric contact with the green body 5.
With reference to the embodiment of the present invention shown in the figures from 5 to 8, the mould 1 is composed of a first punch 2, of a second punch 30 and of a matrix die 40 substantially similar to those of the previous embodiment and will not be the subject of a further detailed explanation as regards all the characteristics which make it similar to the above description.
In particular, this embodiment differs from the previous one due to the fact that the mould 1 and the green body 5 are shaped so that the placing in reciprocal electrical contact of the punches 2, 30 occurs after the placing in electrical contact of the green body 5 with the punches 2, 30.
In particular, after loading the green body 5 inside the mould 1 (figure 5), the moving of the first punch 2 nearer to the second punch 30 brings the first punch 2 into contact first with the green body 5 (figure 6) and only secondly with the matrix die 40 (figure 7). This way, in the position shown in figure 6, the electric current is forced to pass only through the green body 5 producing the breakage of the oxide and the start of the sintering of the green body 5 before its plastic deformation.
The transit of a high intensity of electric current in the green body 5 also determines the fast and effective degassing of the sinterable material.
Subsequently, the further forward movement of the punches 2, 30 determines the plastic deformation of the sintered material and the entry of the punch 2 in the matrix die 40 (figure 7), so as to obtain the final desired geometry (figure 8). To favour the entry of the punch 2 in the matrix die 40, the latter can have connected or chamfered corners, or else external guides can be used made of non-conducting material winch maintain the aligmnent between the punch 2 and the matrix die 40.
In agreement with the embodiment of the invention shown in the figures from 5 to 8, the process according to the invention comprises the following phases: - arranging the sinterable material inside the mould 1, in a similar way to the solution of the figures from 1 to 4, that is compacting the sinterable material in powder form outside the mould 1 to form die green body 5. and then introducing the green body 5 into the mould 1 ;
sintering the sinterable material in t!ie mould 1 by making electric current to pass through and applying a pressure. The application of pressure is achieved by moving the punches 2, 30 closer together to detenriine the deformation of the green body 5 from the initial conformation to the preset conformation of the mould 1. The transit of electric current, on the other hand, is obtained by applying a difference in electric potential between the first punch 2 and the second punch 30 and placing the green body 5 in electric contact with the punches 2, 30. In this phase, the placing of the punches 2, 30 in reciprocal electric contact through the matrix die 40 occurs after the placing of the green body 5 in electric contact with the punches 2, 30.
With reference to the embodiment of the present invention shown in the figures from 9 to 12, the mould 1 is composed of a first shaped element 2 and of a second shaped element 3 shaped for making the rough-shaped piece of a piston. The second shaped element 3 consists of a single body, but other alternative embodiments similar to those of the figures from 1 to 4 and from 5 to 8 cannot be ruled out wherein this is made, e.g., in two separate pieces defined by a bottom punch and by an external matrix die.
Taking into account the axial symmetry of the object, for simplicity of representation, in these illustrations only a schematic and partial angular sector of the mould 1 has been shown.
Fabrication of the object must advantageously start with two green bodies 5a, 5b that can be introduced into the mould 1 substantially side by side, the initial conformation of the assembly of green bodies 5a, 5b positioned side by side being different to the preset conformation of the mould 1; alternative embodiments cannot however be ruled out that envisage starting with a different number of green bodies, such as just one, three or more.
The two green bodies 5a, 5b shown in the embodiment in the figures from 9 to 12 have a cylindrical shape with identical base and are meant to be introduced into the mould 1 side by side along the direction of pressing P, i.e., one on top of the other.
The green bodies 5a, 5b are formed starting with sinterable materials in powder form identical to one another, e.g., aluminium powder AA2124 mixed dry with SiC using a Turbula® mixer and a number of steel balls.
During the sintering process, the sinterable material of the green body 5a unites intimately with the sinterable material of the other green body 5b, obtaining an end product without any discontinuity at all.
Alternatively, the green bodies 5a, 5b can be replaced by a single green body with dimensions identical to those of the assembly of green bodies 5a, 5b.
The use of two green bodies 5a, 5b instead of just one is particularly useful when difficulties are found in compacting a single green body, e.g., because this has a particularly slim and elongated shape; in fact, rather than form a single green body with very elongated shape and, therefore, difficult to handle without the risk of deforming it or crushing it, it could be preferable to shape two or more green bodies of compact shape which, once positioned side by side inside the mould 1 , recompose the desired initial configuration. The shaped elements 2, 3 and tlie green bodies 5a, 5b are shaped so that during the sintering process, all the current is forced, at least initially, to transit in the green bodies 5a, 5b as already described in the embodiment of the Figures from 5 to 8.
The sintering of tlie green bodies 5a, 5b occurs in the following way;
application of a pressure of 5 MPa with respect to the final dimensions of the piece with increase in the temperature of the green bodies 5a, 5b by 100°C/min up to 400°C;
application of the external load of 60 MPa with respect to the final dimensions of the piece and heating by 50°C/min up to 550°C;
maintaining of the load of 60 MPa and 550°C for 1 minute;
free or forced cooling.
The evolution of the green bodies 5a, 5b inside the mould 1 is shown in the figures from 9 to 12.
The figure 9 shows the situation at tlie start of the process after the green bodies 5 a, 5b have been introduced into the mould 1.
The figure 10 represents tlie situation after about 3 minutes from the start of the process wherein the increase in temperature and the application of 5 MPa has determined the obtaining of two green bodies 5a, 5b perfectly densified but where plastic deformation has still not occurred.
The figure 11 represents the situation after about 5 minutes at 500°C: the first shaped element 1 has moved further downwards, determining the plastic deformation of the sinterable material which has reached the value of 50% and the separation edge of the green bodies 5a, 5b has almost completely disappeared.
The figure 12 shows the final situation wherein the punch 1 has moved to the desired point with die sinterable material of a green body 5a which has intimately united with tlie other green body 5b; the maximum plastic deformation of the sinterable material is substantially equal to 200%.
In agreement with the embodiment of tlie invention shown in the figures from 9 to 12, therefore, the process according to tlie invention can be summed up in tlie following phases; arranging the sinterable material inside the mould 1, compacting the sinterable material in powder form to form the two green bodies 5a, 5b and then introducing the green bodies 5a, 5b into the mould 1 being careful to place them side by side along the direction of pressing P, i.e., one on top of the other ;
sintering the sinterable material in the mould 1 by making electric current to pass through and applying a pressure on the green bodies 5a, 5b, Usefully, the placing in reciprocal electric contact of the shaped elements 2, 3 occurs after the placing in electric contact of the green bodies 5a, 5b with the shaped elements 2, 3. Sintering comprises a first phase, wherein electric current is made to pass through to increase the temperature of the green bodies 5a, 5b substantially by 1G0°C a minute substantially up to 400°C and a pressure is applied substantially equal to 5 MPa, a second phase, wherein electric current is made to pass through to increase the temperature of the green bodies 5a, 5b substantially by 50°C a minute up to substantially 550°C and a pressure is applied substantially equal to 60 MPa, and a third phase, wherein electric current is made to pass through to maintain the temperature of the green bodies 5a, 5h substantially at 550°C for substantially one minute and a pressure is applied substantially equal to 60 MPa.
With reference to the embodiment of the present invention shown in the figures from 13 to 17, the mould 1 is composed of a first shaped element 2 and of a second shaped element 3 shaped for making a toothed pinion.
The second shaped element 3 consists of a single body, but other alternative embodiments cannot be ruled out wherein this is made, e.g., in three separate pieces defined by a bottom punch, an external matrix die and an internal core. Taking into account the axial symmetry of the object, for simplicity of representation, in these illustrations only a schematic and partial angular sector of the mould 1 has been shown.
Fabrication of the obj ect must advantageously start with two green bodies 5a, 5b that can be introduced into the mould 1 substantially fitted one into the other, the initial conformation of the assembly of green bodies 5a, 5b positioned side by side being different to the preset conformation of the mould 1.
The two green bodies 5a, 5b shown in the embodiment in the figures from 13 to 17 have tubular shape of the same height and complementary diameters and are meant to be introduced into the mould 1 side by side along the direction of deformation D, i.e., one inside the other coaxially.
The green bodies 5a. 5b are formed starting with sinterable materials different the one from the other; the internal green body 5a is obtained, e.g., from AA2124 powder while the external green body 5b is obtained starting from a mixture of AA2124 and SiC.
The shaped elements 2, 3 and the green bodies 5a, 5b are shaped so that during the sintering process, all the current is forced, at least initially, to transit in the green bodies 5a, 5b.
The sintering of the green bodies 5a, 5b occurs in the way already described in relation to the figures from 9 to 12, that is:
~ application of a pressure of 5 MPa with respect to the final dimensions of the piece with increase in the temperature of the green bodies 5a, 5b by
100°C/min up to 400°C;
application of the external load of 60 MPa with respect to the final dimensions of the. piece and heating by 50°C/min up to 550°C;
- maintairiing of the load of 60 MPa and 550°C for 1 minute;
free or forced cooling.
The evolution of the green bodies 5a, 5b inside the mould 1 is shown in the figures from 13 to 17.
The figure 13 shows the situation at the start of the process after the green bodies 5a, 5b have been introduced into the mould 1.
The figure 14 represents the situation after about 3 minutes from the start of the process wherein the increase in temperature and the application of 5 MPa has determined the obtaining of two green bodies 5a, 5b perfectly densified but where plastic deformation has still not occurred.
From now on, die punch 2 starts to crush the green bodies 5a, 5b and, in combination with the transit of current which increases their temperature, deforms them. The figure 15 shows the situation in correspondence to the entry of the punch 2 In the second shaped element 3; the electric current therefore starts to pass not only from the punch 2 to the. green bodies 5a, 5b and from the green bodies 5a, 5b to the second shaped element 3, but also directly from the punch 2 to the second shaped element 3.
The figures 16 and 17 represent the situation at the end of the process wherein the first shaped element 2 has moved further downwards, determining the gradual plastic deformation of the sinterable material up to the complete filling of the internal forming cavity 4.
During the sintering process, the sinterable material of a green body 5a, 5b intimately unites with the sinterable material of the other green body 5 a, 5b but on the outer part of the end piece, corresponding to the teeth of the pinion, the material enriched with SiC, which performs better in terms of wear resistance, remains concentrated,
In agreement with the embodiment of the invention shown in the figures from 13 to 17, therefore, the process according to the invention can be summed up in the following phases:
arranging the sinterable material inside the mould 1, compacting the sinterable material in powder form to form the two green bodies 5a, 5b and then introducing the green bodies 5 a, 5b into the mould 1 being careful to place them side by side along the direction of deformation D, i.e., one inside the other, coaxially;
sintering the sinterable material in the mould 1 by making electric current to pass through and applying a pressure on the green bodies 5a, 5b. Sintering comprises a first phase, wherein electric current is made to pass through to increase the temperature of the green bodies 5a, 5b substantially by 100°C a minute substantially up to 400°C and a pressure is applied substantially equal to 5 MPa, a second phase, wherein electric current is made to pass through to increase the temperature of the green bodies 5a, 5b substantially by 50DC a minute up to substantially 550°C and a pressure is applied substantially equal to 60 MPa, and a third phase, wherein electric current is made to pass through to maintain the temperature of the green bodies substantially at 550°C for substantially one minute and a pressure is applied substantially equal to 60 MPa.

Claims

1) Process for sintering powders assisted by pressure and electric current, comprising the phases of:
arranging a sinterable material in powder form and at least in part electrically conductive inside a mould ( 1 ) having a preset conformation; sintering said sinterable material in said mould (1) by making electric current to pass through said sinterable material and applying a pressure on said sinterable material;
characterised by the fact that said arranging comprises the phases of:
- compacting said sinterable material in powder form outside said mould (1) to fonn at least a green body (5, 5a, 5b) having an initial conformation different to said preset conformation of the mould (1); and
introducing said green body (5, 5a, 5b) into said mould (1);
the application of pressure during said sintering causing the deformation of said green body (5, 5a, 5b) from said initial conformation to said preset conformation of the mould (1).
2) Process according to the claim 1, characterised by the fact that said mould (1) comprises at least a first shaped element (2) and a second shaped element (3), that can be moved nearer and away from each other along a direction of pressing (P) and having at least a direction of deformation (D) substantially transversal to said direction of pressing (P), the application of pressure during said sintering comprising the approaching of said shaped elements (2,3).
3) Process according to the claim 2, characterised by the fact that said first shaped element (2) comprises a first punch and said second shaped element (3) comprises a second punch (30) fitted in a matrix die (40).
4) Process according to the claim 2 or 3, characterised by the fact that said initial conformation of the green body (5, 5a, 5b) has at least a portion whose overall dimensions, along said direction of deformation (D), are substantially less than the overall dimensions of said preset conformation of the mould (1) along said direction of deformation (D).
5) Process according to the claim 4, characterised by the fact that the ratio between said overall dimensions of the mould (1) along said direction of deformation (D) and said overall dimensions of the green body (5, 5a, 5b) along said direction of deformation (D) is substantially between 1.2 and 6.
6) Process according to one or more of the preceding claims, characterised by the fact that the total plastic deformation of said green body (5, 5a, 5b) in said mould (1) is substantially between 20÷500%.
7) Process according to one or more of the preceding claims, characterised by the fact that said by making electric current to pass through comprises the application of a difference in electric potential between said first and second shaped element (2, 3) and placing said green body (5, 5a, 5b) in electric contact with said shaped elements (2, 3).
8) Process according to the claim 7, characterised by the fact that said making electric current to pass through comprises placing said shaped elements (2, 3) in reciprocal electric contact before said placing the green body (5, 5a, 5b) in electric contact with said shaped elements (2, 3).
9) Process according to the claim 7, characterised by the fact that said making electric current to pass through comprises placing said shaped elements (2, 3) in reciprocal electric contact after said placing the green body (5, 5a, 5b) in electric contact with said shaped elements (2, 3).
10) Process according to one or more of the preceding claims, characterised by the fact that said mould (1) is made of an electrically conductive material chosen from the list comprising: steel, stainless steel, superalioys, WC/Co, titanium-based alloys, electrically conductive ceramics.
11) Process according to one or more of the preceding claims, characterised by the fact that said mould (1) is at least partially coated with graphite spray, boron nitride, silicone-based lubricant, PTFE lubricant or molybdenum bi-sulphide.
12) Process according to one or more of the preceding claims, characterised by the fact that said sintering comprises making electric current to pass through 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, characterised by the fact that said sintering comprises applying a pressure of 5÷200 MPa.
14) Process according to one or more of the preceding claims, characterised by the fact that said sintering comprises a first phase in which electric current is made to pass tlirough to increase the temperature of said green body (5, 5a, 5b) substantially by 100°C a minute substantially up to 400°C, and a pressure is applied substantially equal to 5 MPa,
15) Process according to one or more of the preceding claims, characterised by the fact that said sintering comprises a second phase in which electric current is made to pass through to increase the temperature of said green body (5, 5a5 5b) substantially by 50°C a minute substantially up to 550°C, and a pressure is applied substantially equal to 60 MPa.
16) Process according to one or more of the preceding claims, characterised by the fact that said sintering comprises a third phase in which electric current is made to pass through to maintain the temperature of said green body (5, 5a, 5b) substantially at 550°C for substantially 1 minute, and a pressure is applied substantially equal to 60 MPa.
17) Process according to one or more of the preceding claims, characterised by the fact that said compacting said sinterable material in powder form comprises the forming of at least two of said green bodies (5a, 5b) that can be introduced in said mould (1) positioned side by side, the initial conformation of the assembly of said green bodies (5a5 5b) positioned side by side being different to said preset conformation of the mould (1).
18) Process according to the claim 17, characterised by the fact that said introducing comprises placing said green bodies (5a, 5b) side by side along said direction of pressing (P).
19) Process according to the claim 17, characterised by die fact that said introducing comprises placing said green bodies (5a, 5b) side by side along said direction of deformation (D).
20) Process according to one or more of the preceding claims, characterised by the fact that said green bodies (5a, 5b) are formed starting with sinterable materials in powder form identical to one another.
2.1) Process according to one or more of the preceding claims, characterised by the fact that said green bodies (5a. 5b) are formed starting with sinterable materials in powder form different the one from the other.
PCT/EP2010/067269 2009-12-16 2010-11-11 Process for sintering powders assisted by pressure and electric current WO2011072961A1 (en)

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WO2018177453A1 (en) * 2017-03-27 2018-10-04 Schaeffler Technologies AG & Co. KG Hydraulic camshaft adjuster
IT201800004594A1 (en) * 2018-04-17 2019-10-17 Process for the realization of targets for the production of radioisotopes
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CN112921336A (en) * 2020-12-30 2021-06-08 河南科技大学 Preparation method of heterostructure-loaded self-supporting catalytic electrode
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EP3643431A1 (en) * 2018-10-27 2020-04-29 Hamilton Sundstrand Corporation Components having low aspect ratio
US11701707B2 (en) 2018-10-27 2023-07-18 Hamilton Sundstrand Corporation Components having low aspect ratio
US11084092B2 (en) 2018-10-27 2021-08-10 Hamilton Sunstrand Corporation Components having low aspect ratio
US20210242385A1 (en) * 2020-01-31 2021-08-05 Nichia Corporation Method of producing heat dissipation substrate and method of producing composite substrate
US20230086662A1 (en) * 2020-01-31 2023-03-23 Nichia Corporation Method of producing composite material
US11799065B2 (en) * 2020-01-31 2023-10-24 Nichia Corporation Method of producing heat dissipation substrate and method of producing composite substrate
CN112921336B (en) * 2020-12-30 2022-02-18 河南科技大学 Preparation method of heterostructure-loaded self-supporting catalytic electrode
CN112921336A (en) * 2020-12-30 2021-06-08 河南科技大学 Preparation method of heterostructure-loaded self-supporting catalytic electrode

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