ES2738627T3 - Method for sintering electrically conductive powders - Google Patents

Abstract

Method for sintering electrically conductive powders in an air atmosphere to obtain a sintered product, which comprises the following sequence of stages: a) placing the powders in an electrically insulating mold, b) applying a pressure of between 100 and 500 MPa to the powders , c) applying to the powders a sintering current at a sintering voltage during a sintering time, to sinter the powders, characterized by applying to the powders, between stage b) and stage c), a current density of activation lower than the sintering current density at an activation voltage higher than the sintering voltage for an activation time shorter than the sintering time, to reduce the electrical resistance of the powders, the activation current density and the sintering current density.

Description

DESCRIPTION

Method for sintering electrically conductive powders

Technical field

The present invention pertains to the field of methods for obtaining sintered parts, which consist of the application of heat and pressure to the powders to finally obtain dense parts, in particular in which heat is obtained through electrical currents that are forced to pass through conductive dusts.

State of the art

Electric current assisted sintering (ECAS) brings together a family of consolidation methods in which mechanical pressure is combined with electrical and thermal fields to improve bonding and densification between particles. The starting materials may be in the form of powders or compact green. The main purpose of the electrical currents imposed is to provide the required amount of resistive heat.

ECAS techniques can be classified with respect to download time. Conventionally, a discharge time of 0.1 s can be assumed as the threshold between fast and ultrafast ECAS. However, confusion between the fast term (fast in English) described herein and the acronym FAST (field activated / field-assisted sintering technique) frequently found in the scientific literature should be avoided. Here, fast simply refers to high processing speed or low processing time. On the one hand, the fast ECAS techniques are known with different acronyms that depend mainly on the waveform of the electric current: SPS (spark plasma sintering; SPS for its acronym in English, spark plasma sintering), PECS (sintering Pulsed electric current sintering), RS (resistance sintering; RS resistance sintering), etc. The bases of the process were developed by INOUE (US3241956 A). Electric current is normally applied as pulses and the possibility of applying several superimposed currents with different frequencies and two pressure phases to increase densification was also reported. The application of the charge was also described in United States Patent 3508029 A. These two processes are characterized by the low intensity of the electrical pulses (<1 kA / cm2) and the high duration of the cycles (from seconds to minutes ). In addition, the manufacturing of fast ECAS equipment is complex with the need for a protective (or empty) atmosphere.

On the other hand, ultrafast ECAS techniques usually employ one or up to three repeated (capacitor) discharges. Each download lasts less than 0.1 s. The current pulse density can be of the order of 10 kA / cm2. Ultrafast ECAS is generally referred to as electric discharge compaction (EDC) or electroforming sintering (EFS) (Electro Forging Sintering). Representative examples of these methods were explained in the following patents: EP 2198993 A1 of Fais, US4929415 and US4975412 of Okazaki.The main problem of these methods is that the capacitors discharge the stored energy suddenly and uncontrollably and, therefore, did not allow the adaptation of the input power to the mass of dust The use of high current and high voltage resulted in an inconsistent densification and non-homogeneity of the parts manufactured using these consolidation processes, because the resistance of the powders is not homogeneous due to the porosity, oxidation of the surface, compaction or union between the particles, and it is well known that the current always follows the path resis Lower tivo Other problems are the small size of the samples that can be produced and the sparks in the electrodes produced by high current and high voltage.

Other ultrafast processes used low voltage equipment such as patents developed by Cremer (US2355954), Knoess (US5529746) and Bauer (US7361301). Knoess and Bauer obtained a good densification with highly conductive powders such as iron and copper. Knoess used various pulses of very high current (> 100 kA / cm2) and Bauer an intensity less than 10 kA / cm2, voltage less than 10 V using a sintering time of approximately 1 s. The problem can occur when samples of high electrical resistance are manufactured (due to the high resistivity of the dust or due to the large size of the parts), it will not be possible to close the electrical circuit so that the current can pass all the material of the parts. For this reason, with these techniques it will not be possible to obtain larger samples or the consolidation of powders with higher resistivity due to the low voltage used.

JP H03 236402 discloses an apparatus for sintering electrically conductive powders.

Description of the invention

To overcome the drawbacks of the prior art, the present invention proposes a method for sintering electrically conductive powders in an air atmosphere to obtain a sintered product, comprising The following sequence of stages:

a) place the powders in an electrically insulating mold,

b) apply a pressure to powders between 100 and 500 MPa,

c) apply to the powders a sintering current density at a sintering voltage for a sintering time, to sinter the powders.

The method of the present invention comprises, between step b) and step c), applying to the powders an activation current density lower than the sintering current density at an activation voltage greater than the sintering voltage during a activation time less than sintering time, to reduce the electrical resistance of the powders, the activation current density and the sintering current density being constant.

The application of the activation current density and the sintering current density is carried out while pressure is applied to the powders.

The activation current density applied to the powders at a voltage greater than the voltage used for sintering in stage c) for a shorter time than in stage c) produces a current discharge that breaks the oxide layer on the surface of the powders and creates bridges between the particles of the powders, obtaining a more uniform and cleaner surface of particles that reduces the electrical resistance to the flow of the current through the powders, so that the sintering current density applied in step c) is distributed more homogeneously through the powders in the mold. In this way, it is possible to sinter large pieces and pieces of material with a high electrical resistivity.

Preferably, the activation current density is greater than 0.5 kA / cm2, the activation voltage is greater than 10 V and the activation time is less than 300 ms, to generate a low intensity current discharge at high voltage in a very short time, to ensure a homogeneous surface deoxidation of the powders and the formation of bridges between the particles.

According to the invention, the time between the elimination of the activation current density and the application of the sintering current density is less than 20 ms to ensure an optimal distribution of the sintering current density. More preferably, the sintering current density is applied immediately after application of the activation current density, that is, just after the activation time is used up.

According to a preferred aspect of the invention:

- The activation current density is applied using a first unit of electrical power.

- The sintering current density is applied using a second unit of electrical power.

- The first and second electric power units are operated independently.

The method of the invention comprises the control of the two electric power units that allows to optimize the processing time and energy consumption, in conjunction with the installation costs.

In addition, it allows to program and monitor at all times the power that is being introduced into the powder, thus allowing the process to be controlled very accurately and repetitively, both in the application of the activation current density and in the application of sintering current density.

In addition, it has been discovered that the precise control enabled by this method allows a considerable increase in the size of the pieces, their geometry and the types of materials that can be sintered.

According to a preferred embodiment of the invention:

- the activation current density is in the range of 0.5 to 5 kA / cm2;

- the activation voltage is in the range of 10 to 100 V;

- the activation time is in the range of 50 to 300 ms;

- the sintering current density is in the range of 3 to 15 kA / cm2;

- the sintering voltage is less than 15 V;

- the sintering time is in the range of 500 to 1500 ms.

but these parameters must meet the conditions that the values selected for the sintering current density applied to the powders must be greater than the values selected for the activation current density and the values selected for the activation voltage must always be greater than the sintering voltage applied.

According to a more preferred embodiment of the invention:

- the activation current density is in the range of 0.5 to 4 kA / cm2;

- the activation voltage is in the range of 10 to 100 V;

- the activation time is in the range from 90 to 200 ms;

- the sintering current density is in the range of 3 to 10 kA / cm2;

- the sintering voltage is less than 10 V;

- the sintering time is in the range of 500 to 1500 ms.

but these parameters must meet the conditions that the values selected for the sintering current density applied to the powders must be greater than the values selected for the activation current density and the values selected for the activation voltage must always be greater than the sintering voltage applied.

The expert will select the precise values of these parameters for each conductive powder, but always taking into account that the activation current density applied to a conductive powder must be less than the applied sintering current density and the activation voltage greater than the sintering voltage Therefore, it is not possible to choose a value of 4 kA / cm2 for the activation current density and a value of 3 kA / cm2 for the sintering current density. The same applies to the activation and sintering voltages where it is not possible to apply an activation voltage of 10 V and a sintering voltage of 15 V. Some examples of parameter selections are disclosed in the description of the preferred embodiments.

Brief description of the drawings

To complete the description and in order to provide a better understanding of the invention, a set of drawings is provided. These drawings form an integral part of the description and illustrate an embodiment of the invention, which should not be construed as restricting the scope of the invention, but as an example of how the invention can be carried out. The drawings comprise the following figures:

Figure 1 is a block diagram of an apparatus according to a preferred embodiment.

Figure 2 is a time graph of pressure and voltage / current when the method of the invention is applied to a WC-Co powder.

Description of a way of carrying out the invention

Figure 2 shows the time-pressure-current / voltage diagram corresponding to the implementation of the method according to the invention to obtain a sintered WC-Co.

The process begins with the stage that consists of placing an electrically conductive powder in an electrically insulating mold.

Subsequently, a pressure between 100 and 500 MPa is applied inside the mold, preferably with two pistons, in this case approximately 300 MPa.

An activation step is then carried out, which consists in applying an activation current density to an activation voltage during an activation time and is carried out using a first electric power unit (2). As shown, at this stage a low current density (approximately 2 kA / cm2) and a high voltage (approximately 30 V) are applied. The pulse is approximately two tenths of a second.

A waiting stage is then carried out in which no current or voltage is applied. This stage consists of switching the power units, that is, switching from one power unit (2) to another power unit (3). The waiting time is necessary to perform said switching by means of the control unit (4), in this case a PLC (programmable logic controller). In Figure 2 the switching time is approximately 2 tenths of a second. A technical possibility would be to use a single power unit but with variable current and voltage instantaneously. However, the control requirements for current and voltage levels and discharge times would involve very sophisticated equipment that would make the method not profitable at the industrial level.

Subsequently, the appropriate sintering stage is performed, which consists in applying a sintering current density to a sintering voltage during a sintering time carried out using the second electric power unit (3). In this case, the intensity is higher (approximately 10 kA), but the voltage is reduced to 5 V.

The current density is applied using two opposite electrodes. In one embodiment, the pistons can be used as opposite electrodes.

As shown in Figure 1, and according to a preferred embodiment, the invention also relates to an apparatus (1) for carrying out the method of the invention.

The apparatus comprises:

- means for applying current and voltage to the powders, represented by the power units (2, 3);

- an electrically insulating mold (5) containing the conductive powders (6), which is closed at its ends by two pistons to apply mechanical pressure and which also form the electrodes (7).

As shown in Figure 1, the means for providing current and voltage for an activation stage is a first unit of electrical power (2) and the means for providing current and voltage for a sintering stage is a second unit of electrical power. (3).

The first power unit (2) is arranged to provide through the electrodes (7) an activation current density between 0.5 and 5 kA / cm2 and an activation voltage between 10 and 100 V, while the second power unit (3) is arranged to provide through the electrodes (7) a sintering current density between 3 and 15 kA / cm2 and a sintering voltage of less than 15 V. These intervals allow most sinters to be sintered. of commercially interesting conductive dusts for typical applications, with a single machine, whose parameters must be set before sintering.

The apparatus further comprises:

- means for switching between the first electric power unit (2) and the second (3);

- means for controlling the duration of the current density and voltage provided by the first power unit (2);

- means for controlling the duration of the current density and voltage provided by the second power unit (3);

- connections (23, 33) between each of the power units (2, 3) and the electrodes (7) of the mold (5);

- means for controlling the pistons that apply pressure in the mold.

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

Each power unit (2, 3) comprises 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 It is preferably a programmable logic controller.

This PLC includes:

- means for switching between the first electric power unit (2) and the second (3),

- means for controlling the duration of the current and voltage provided by the first power unit (2),

- means for controlling the duration of the current and voltage provided by the second power unit (3); Y

- means for controlling the pistons that apply pressure in the mold.

Specific examples of the application of the method of the invention to different metal powders are described below.

EXAMPLE 1: WC-6Co / WC-10Co

A WC-6Co or WC-10Co disc 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 micrometers.

In the activation stage a current density between 2 and 4 kA / cm2 was applied for 100-200 ms to activate the powder. A voltage between 15 and 50 V is required for this activation stage.

In the subsequent sintering phase, a current density between 6-10 kA / cm2 was applied, to obtain a densified sample with a voltage below 10 V for 500-1000 ms. Between the phases, activation and sintering, a minimum time of 10 ms was established. A pressure of 100-500 MPa was applied from the beginning of the process.

The density of the final disk, measured by the Archimedes method, is approximately 13-14.8 g / cm3. It is possible to obtain completely dense samples with a hardness of approximately 1800-2100 HV30.

EXAMPLE 2: Titanium

A titanium disc is produced with the apparatus disclosed with a thickness of 10 mm and a diameter of 22 mm. The dust particle shape was irregular with a maximum particle size of approximately 75 micrometers.

In the activation stage a current density between 1-3 kA / cm2 was applied for 90-100 ms to activate the powder. A voltage between 10 and 50 V is required for the activation phase.

In the sintering phase a current density between 4-7 kA / cm2 was applied for 500-1000 ms to obtain a densified sample with a voltage lower than 10 V. Between the phases, activation and sintering, a minimum time of 10 ms A pressure of 100-500 MPa was applied from the beginning of the process.

The density of the final disk, measured by the Archimedes method, is approximately 3.5-4.4 g / cm3. It is possible to obtain totally dense samples.

EXAMPLE 3: TiC-25Ni and TiC-25Fe

TiC-25Ni and TiC-25Fe discs 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 micrometers.

In the activation stage a current density between 1-3 kA / cm2 was applied for 100-200 ms to activate the powder. A voltage between 15 and 50 V is required for this activation phase.

In the subsequent sintering stage a current density between 6-9 kA / cm2 was applied for 500-1000 ms to obtain a densified sample with a voltage lower than 10 V. Between the phases, activation and sintering, a minimum time was established of 10 ms. A pressure of 100-500 MPa was applied from the beginning of the process.

The density of the final disk, measured by the Archimedes method, was approximately 5.1-5.5 g / cm3 for TiC-25Ni and approximately 5.1-5.4 g / cm3 for TiC-25Fe. It is possible to obtain completely dense samples with a hardness of approximately 1600-2000 HV30.

EXAMPLE 4: Aluminum

An aluminum disc was produced with the apparatus disclosed with a thickness of 12 mm and a diameter of 12 mm. The dust was irregular with a particle size of less than 150 micrometers.

In the activation stage a current density between 0.5-2 kA / cm2 was applied for 100-200 ms to activate the powder. A voltage between 30 and 80 V is required for this activation phase.

In the subsequent sintering stage, a current density between 3-4 kA / cm.2 was applied for 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. The pressure, from 100-300 MPa, was applied from the beginning of the process.

The density of the final disk, measured by the Archimedes method, was approximately 2.5-2.7 g / cm3.

In this text, the term "comprises" and its derivations should not be understood in an exclusive sense, that is, these terms should not be construed as an exclusion from the possibility that what is described and defined may include other elements, stages, etc. .

The invention is obviously not limited to the specific embodiments described herein, but also encompasses any variation that any person skilled in the art may consider, within the general scope of the invention as defined in the claims.

Claims (8)

1. Method for sintering electrically conductive powders in an air atmosphere to obtain a sintered product, comprising the following sequence of steps: a) place the powders in an electrically insulating mold, b) apply a pressure of between 100 and 500 MPa to the powders, c) apply a sintering current to the powders at a sintering voltage during a sintering time, to sinter the powders, characterized by applying to the powders, between stage b) and stage c), an activation current density lower than the sintering current density at an activation voltage greater than the sintering voltage during an activation time less than sintering time, to reduce the electrical resistance of the powders, the activation current density and the sintering current density being constant. 2. A method according to claim 1, wherein the activation current density is greater than 0.5 kA / cm2, the activation voltage is greater than 10 V and the activation time is less than 300 ms. 3. Method according to any of the preceding claims, wherein: - the application of sintering current density and sintering voltage is carried out using a first electric power unit (2); - the application of the activation current density and the activation voltage is carried out using a second electric power unit (3); - the first and second electric power units (2, 3) operate independently. 4. Method according to any of the preceding claims, wherein: - the activation current density is in the range of 0.5 to 5 kA / cm2; - the activation voltage is in the range of 10 to 100 V; - the activation time is in the range of 50 to 300 ms; - the sintering current density is in the range of 3 to 15 kA / cm2; - the sintering voltage is less than 15 V; - the sintering time is in the range of 500 to 1500 ms, and in which the activation current density is less than the sintering current density and the activation voltage is greater than the sintering voltage. 5. Method according to claims 1 to 4, wherein: - the powders are WC-6Co or WC-10Co powders; - the activation current density is in the range of 2 to 4 kA / cm2; - the activation voltage is in the range of 15 to 50 V; - the activation time is in the range of 100 to 200 ms; - the sintering current density is in the range of 6 to 10 kA / cm2; - the sintering voltage is less than 10 V; - the sintering time is in the range of 500 to 1000 ms. 6. Method according to claims 1 to 4, wherein: - the powders are titanium powders; - the activation current density is in the range from 1 to 3 - kA / cm2; - the activation voltage is in the range of 10 to 50 V; - the activation time is in the range from 90 to 110 ms; - the sintering current density is in the range of 4 to 7 kA / cm2; - the sintering voltage is less than 10 V; - the sintering time is in the range of 500 to 1000 ms. 7. Method according to claims 1 to 4, wherein: - the powders are a mixture of TiC-25Ni powders and TiC-25Fe powders; - the activation current density is in the range of 1 to 3 kA / cm2; - the activation voltage is in the range of 15 to 50 V; - the activation time is in the range of 100 to 200 ms; - the sintering current density is in the range of 6 to 9 kA / cm2; - the sintering voltage is less than 10 V; - the sintering time is in the range of 500 to 1000 ms. 8. Method according to claims 1 to 4, wherein: - powders are aluminum powders; - the activation current density is in the range of 0.5 to 2 kA / cm2; - the activation voltage is in the range from 30 to 80 V; - the activation time is in the range of 100 to 200 ms; - the sintering current density is in the range of 3 to 4 kA / cm2; - the sintering voltage is less than 10 V; - the sintering time is in the range of 500 to 1000 ms.