METHOD OF SINTERING ELECTRICALLY CONDUCTING POWDERS AND AN APPARATUS FOR CARRYING OUT SAID METHOD

Abstract

A method of sintering electrically conducting powders in an air atmosphere for obtaining a sintered product includes the following step sequence: placing the powders in an electrically isolating mold, applying a pressure to the powders between 100 and 500 MPa, and applying to the powders a sintering current at a sintering voltage during a sintering time, for sintering the powders. Before applying the sintering current density to the powders, an activation current density is lower than the sintering current density at an activation voltage greater than the sintering voltage during an activation time lower than the sintering time, to reduce the electrical resistance of the powders.

Claims

1. A method of sintering electrically conducting powders in an air atmosphere for obtaining a sintered product, the method including the following sequence of steps: a) placing the powders in an electrically isolating mold, b) applying a pressure to the powders between 100 and 500 MPa, and c) applying to the powders a sintering current at a sintering voltage during a sintering time, for sintering the powders, wherein applying to the powders, between step b) and step c), an activation current density lower than the sintering current density at an activation voltage greater than the sintering voltage during an activation time lower than the sintering time, to reduce the electrical resistance of the powders.

2. The method according to claim 1, wherein the activation current density is greater than 0.5 kA/cm.sup.2, the activation voltage is greater than 10V, and the activation time is lower than 300 ms.

3. The method according to claim 1, wherein: the application of the sintering current density and sintering voltage are carried out by employing a first electrical power unit; the application of the activation current density and activation voltage are carried out by employing a second electrical power unit; and the first and second electrical power units operate independently.

4. The method according to claim 1, wherein: the activation current density is comprised in the range of 0.5 to 5 kA/cm.sup.2; the activation voltage is comprised in the range of 10 to 100 V; the activation time is comprised in the range of 50 to 300 ms; the sintering current density is comprised in the range of 3 to 15 kA/cm.sup.2; the sintering voltage is lower than 15 V; the sintering time is comprised in the range of 500 to 1500 ms, and wherein the activation current density is lower than the sintering current density and the activation voltage is greater than the sintering voltage.

5. The method according to claims 1, wherein: the powders are WC-6Co powders or WC-10Co; the activation current density is comprised in the range of 2 to 4 kA/cm.sup.2; the activation voltage is comprised in the range going from of 15 to 50 V; the activation time is comprised in the range going from of 100 to 200 ms; the sintering current density is comprised in the range of 6 to 10 kA/cm.sup.2; the sintering voltage is lower than 10 V; and the sintering time is comprised in the range of 500 to 1000 ms.

6. The method according to claim 1, wherein: the powders are titanium powders; the activation current density is comprised in the range of 1 to 3 KA/cm.sup.2; the activation voltage is comprised in the range of 10 to 50 V; the activation time is comprised in the range of 90 to 110 ms; the sintering current density is comprised in the range of 4 to 7 kA/cm.sup.2; the sintering voltage is lower than 10 V; and the sintering time is comprised in the range of 500 to 1000 ms.

7. The method according to claim 1, wherein: the powders are a mixture of TiC-25Ni powders and TiC-25Fe powders; the activation current density is comprised in the range of 1 to 3 kA/cm.sup.2; the activation voltage is comprised in the range of 15 to 50 V; the activation time is comprised in the range of 100 to 200 ms; the sintering current density is comprised in the range of 6 to 9 kA/cm.sup.2; the sintering voltage is lower than 10 V; and the sintering time is comprised in the range of 500 to 1000 ms.

8. The method according to claim 1, wherein: the powders are an aluminium powders; the activation current density is comprised in the range of 0.5 to 2 kA/cm.sup.2; the activation voltage is comprised in the range of 30 to 80 V; the activation time is comprised in the range of 100 to 200 ms; the sintering current density is comprised in the range of 3 to 4 kA/cm.sup.2; the sintering voltage is lower than 10 V; and the sintering time is comprised in the range of 500 to 1000 ms.

9. An apparatus for carrying out the sintering of electrically conducting powders in an air atmosphere, comprising an electrically isolating mold which can be filled with the powders, two opposite electrodes coupleable to the mold for applying a current density to the powders in the mold, means for applying a pressure to the powders in the mold and means for providing current and voltage through the electrodes wherein the means for providing current density and voltage comprise: a first electrical power unit arranged to provide to the powders a activation current density at an activation voltage; a second electrical power unit arranged to provide to the powders a sintering current density at a sintering voltage; the activation current density being lower than the sintering current density and the activation voltage greater than the sintering voltage; the apparatus further comprising: means for switching between the first and the second electrical power unit; means for controlling the duration of the activation current density and activation voltage provided by the first power unit; means for controlling the duration of the sintering current density and sintering voltage provided by the second power unit; and the activation time being lower than the sintering time.

10. The apparatus according to claim 9, wherein: means for controlling the duration of the activation current density and activation voltage provided by the first power unit are able to control an activation time comprised in the range of 50 to 300 ms; and means for controlling the duration of the sintering current density and sintering voltage provided by the second power unit are able to control a sintering time comprised in the range of 500 to 1500 ms.

11. The apparatus according to claim 9, wherein the first power unit is able to provide a activation current density greater than 0.5 kA/cm.sup.2 and an activation voltage greater than 10 V.

12. The apparatus according to claim 9, wherein the first power unit is able to provide an activation current density between 0.5 and 5 kA/cm.sup.2 and an activation voltage between 10 and 100 V, and the second power unit is able to provide a sintering current density between 3 and 15 kA/cm.sup.2 and a sintering voltage lower than 15 V.

13. The apparatus according to claim 9, wherein each power unit comprises a transformer and an inverter.

14. The apparatus according to claim 9, wherein the two power units and the means for applying a pressure to the mold are controlled by a control unit, preferably a programmable logic controller.

15. The apparatus according to claim 14, wherein the control unit includes: means for switching between the first and the second electrical power unit; means for controlling the duration of the activation current density and activation voltage provided by the first power unit; and means for controlling the duration of the sintering current density and sintering voltage provided by the second power unit.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0064] To complete the description and in order to provide for a better understanding of the disclosure, a set of drawings is provided. Said drawings form an integral part of the description and illustrate an embodiment of the disclosure, which should not be interpreted as restricting the scope of the disclosure, but just as an example of how the disclosure can be carried out. The drawings comprise the following figures:

[0065] FIG. 1 is a block diagram of an apparatus according to a preferred embodiment; and

[0066] FIG. 2 is a time plot of the pressure and the voltage/current when the inventive method is applied to a WC—Co powder.

DETAILED DESCRIPTION OF THE DRAWINGS

[0067] In FIG. 2 the time pressure/current/voltage diagram corresponding to the implementation of the method according to the disclosure for obtaining a sintered WC—Co is shown.

[0068] The process starts with the step comprising placing an electrically conducting powder in an electrically insulating mold.

[0069] Then a pressure between 100 and 500 MPa is applied inside the mold, preferably with two pistons, in this case around 300 MPa.

[0070] Then an activation step is carried out, comprising applying an activation current density at an activation voltage for an activation time and carried out by employing a first electrical power unit (2). As shown, in this step a low current density (around 2 kA/cm2) and a high voltage (around 30V) are applied. The pulse is about two tenths of a second.

[0071] Then a waiting step is carried out wherein no current and/or voltage are applied. This step comprises switching of the power units, that is, to switch from a power unit (2) to another power unit (3). The waiting time is that needed for carrying out said switching by the control unit (4), in the present case a PLC. In FIG. 2 the switching time is about 2 tenth of a second. A technical possibility would be to use a single power unit but with instantaneously variable current and voltage. However, the control requirements for the current and voltage levels and the discharge times would imply very sophisticated equipment that would make the method uneconomic at industrial level.

[0072] Then, the proper sintering step is performed, which comprises applying a sintering current density at a sintering voltage during a sintering time carried out by employing the second electrical power unit (3). In this case the intensity is higher (around 10 kA), but the voltage is reduced to 5 V.

[0073] The current density is applied using two opposite electrodes. In an embodiment the pistons can be used as opposite electrodes.

[0074] As shown in FIG. 1, and according to a preferred embodiment, the disclosure also relates to an apparatus (1) for carrying out the inventive method.

[0075] The apparatus comprises: [0076] means for applying current and voltage to the powders, represented by the power units (2, 3); [0077] an electrically insulating mold (5) containing the conductive powders (6), which is closed in its ends by two pistons for applying mechanical pressure and which form the electrodes (7) as well.

[0078] As shown in FIG. 1, the means for providing current and voltage for an activation step is a first electrical power unit (2) and the means for providing current and voltage for a sintering step is a second electrical power unit (3).

[0079] The first power unit (2) is arranged to provide through the electrodes (7) an activation current density comprised between 0.5 and 5 KA/cm.sup.2 and an activation voltage comprised between 10 and 100 V whereas the second power unit (3) is arranged to provide through electrodes (7) a sintering current density comprised between 3 and 15 kA/cm.sup.2 and a sintering voltage lower than 15 V. These ranges allow to sinter most of the commercially interesting conductive powders for typical applications, with a single machine, which parameters have to be set prior to the sintering.

[0080] The apparatus further comprises: [0081] Means for switching between the first 2 and the second 3 electrical power unit; [0082] Means for controlling the duration of the current density and voltage provided by the first power unit (2); [0083] means for controlling the duration of the current density and voltage provided by the second power unit (3); [0084] connections (23, 33) between each of the power units (2, 3) and the electrodes (7) of the mold (5). [0085] means for controlling the pistons that apply pressure in the mold.

[0086] The means for controlling the duration of the current density and voltage provided by the first power unit (2) are able to control a predetermined discharge time (activation time) comprised in the range going from 50 to 300 ms and the means for controlling the duration of the current and voltage provided by the second power unit (3) are able to control a predetermined discharge time (sintering time) comprised in the range going from 500 to 1500 ms.

[0087] Each power unit (2, 3) comprise a transformer (21, 31) and an inverter (22, 32), and the two power units (2, 3) are controlled by a single control unit (4), which is preferably a programmable logic controller.

[0088] This PLC includes: [0089] means for switching between the first (2) and the second (3) electrical power unit, [0090] means for controlling the duration of the current and voltage provided by the first power unit (2), [0091] means for controlling the duration of the current and voltage provided by the second power unit (3); and [0092] means for controlling the pistons that apply pressure in the mold.

[0093] Now, specific examples of application of the method of the disclosure to different metal powders are described.

EXAMPLE 1

WC-6Co/WC-10Co

[0094] A WC-6Co or WC-10Co disk is produced with the disclosed apparatus with a thickness of 16 mm and a diameter of 22 mm. The agglomerated powder was spherical with an agglomerate size of less than 100 microns.

[0095] In the activation step a current density between 2 and 4 kA/cm.sup.2 during 100-200 ms was applied in order to activate the powder. A voltage between 15-50 V is needed for this activation step.

[0096] In the subsequent sintering stage a current density between 6-10 kNcm.sup.2 was applied to obtain a densified sample with a voltage lower than 10 V during 500-1000 ms. Between stages, activation and sintering, a minimum time of 10 ms was established. Pressure, from 100-500 MPa, was applied from the beginning of the process.

[0097] The density of the final disk, measured by the Archimedes method, is around 13-14.8 g/cm.sup.3. It is possible to obtain fully dense samples with hardness around 1800-2100 HV30.

EXAMPLE 2

Titanium

[0098] A titanium disk is produced with the disclosed apparatus with a thickness of 10 mm and a diameter of 22 mm. The shape of the particles of the powder was irregular with a maximum particle size around 75 microns.

[0099] In the activation step a current density between 1-3 kNcm.sup.2 was applied during 90-100 ms in order to activate the powder. A voltage between 10-50 V is needed for the activation stage.

[0100] In the sintering stage a current density between 4-7 kA/cm.sup.2 was applied during 500-1000 ms to obtain a densified sample with a voltage lower than 10 V. Between stages, activation and sintering, a minimum time of 10 ms was established. Pressure, from 100-500 MPa, was applied from the beginning of the process.

[0101] The density of the final disk, measured by the Archimedes method, is around 3.5-4.4 g/cm.sup.3. It is possible to obtain fully dense samples.

EXAMPLE 3

TiC-25Ni and TiC-25Fe

[0102] A TiC-25Ni and TiC-25Fe disks are produced with the disclosed apparatus with a thickness of 16 mm and a diameter of 22 mm. The agglomerated powder was irregular with a particle size of less than 30 microns.

[0103] In the activation step a current density between 1-3 kA/cm.sup.2 was applied during 100-200 ms in order to activate the powder. A voltage between 15-50 V is needed for this activation stage.

[0104] In the subsequent sintering step a current density between 6-9 kA/cm.sup.2 was applied during 500-1000 ms to obtain a densified sample with a voltage lower than 10 V. Between stages, activation and sintering, a minimum time of 10 ms was established. Pressure, from 100-500 MPa, was applied from the beginning of the process.

[0105] The density of the final disk, measured by the Archimedes method, was around 5.1-5.5 g/cm.sup.3 for TiC-25Ni and 5.1-5.4 g/cm.sup.3 for TiC-25Fe. It is possible to obtain fully dense samples with hardness around 1600-2000 HV30.

EXAMPLE 4

Aluminium

[0106] An aluminium disk was produced with the disclosed apparatus with a thickness of 12 mm and a diameter of 12 mm. The powder was irregular with a particle size of less than 150 microns.

[0107] In the activation step a current density between 0.5-2 kNcm.sup.2 was applied during 100-200 ms in order to activate the powder. A voltage between 30-80 V is needed for this activation stage.

[0108] In the subsequent sintering stage a current density between 3-4 kNcm.sup.2 was applied during 500-1000 ms to obtain a densified sample with a voltage lower than 10 V. Between stages, activation and sintering, a minimum time of 10 ms was established. Pressure, from 100-300 MPa, was applied from the beginning of the process.

[0109] The density of the final disk, measured by the Archimedes method, was around 2.5-2.7 g/cm.sup.3.

[0110] The disclosure is obviously not limited to the specific embodiments described herein, but also encompasses any variations that may be considered by any person skilled in the art.