Device and a method for consolidation of powder materials

Abstract

The object of the invention is a device intended for powder materials consolidation, provided with an operating chamber, press connected to high-current discharge electrodes top and bottom, with arranged therebetween the sintered powder subjected to the pressure exerted by the press. To the top and bottom electrode there is connected a capacitive circuit with a power supply unit, closed by a high-current switch being a transistor switch. The object of the invention is also a method of powder materials consolidation in the device according to the invention, wherein the powder material is subjected to simultaneous operation of pressure in the range of 1-200 MPa and consolidation by electric current pulses with intensity of 1-80 kA, repeated with frequency from the range of 0.1 Hz to 100 Hz, generated by opening and closing the transistor switch.

Claims

1. A method of powder materials consolidation, comprising locating a sintered powder in a die between two electrodes connected to press exerting pressure thereon, applying voltage to the electrodes through a capacitive circuit with a power supply unit closed by a high-current switch, subjecting the sintered powder to simultaneous operation of pressure in the range of 1-200 MPa and consolidation by pulses of electric current with an intensity of 1-80 kA, repeated with frequency of 0.1 Hz to 100 Hz, generated by opening and closing a high-current transistor switch connected in series to the sintered powder and discharging a battery of capacitors in a capacitive circuit charged to the voltage of 0.5-15 kV with rectangular pulses of electric current.

2. The method according to claim 1, wherein the consolidation is performed in temperature in the range of 0.5 to 0.8 of the melting temperature of the consolidated material or melting temperature of the consolidated material's matrix.

3. The method according to claim 1, wherein sintered powder is selected from a group of powder materials being metallic, ceramic, intermetallic and composites comprising a metallic matrix and dispersed non-metallic particles or mixtures of thereof.

4. The method according to claim 3, wherein the powder material is selected from a group including in particular diamond, cubic boron nitride, Al.sub.2O.sub.3, SiC, Si.sub.3N.sub.4, WC, Ta, ZrO.sub.2, TiC, TiN, and mixtures thereof, in a matrix of hard material selected from group including in particular sintered carbides or high thermal conductivity materials selected from a group including in particular tungsten, molybdenum, aluminium, copper, and mixtures thereof.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The object of the invention is shown in embodiments in the drawings, wherein

(2) FIG. 1a is a block diagram of the sintering device with a capacitor battery;

(3) FIG. 1b is a block diagram of a modified device;

(4) FIG. 2 is a plot showing the current pulse waveform in a state-of-the-art device;

(5) FIG. 3 is a block diagram of the drive according to the invention;

(6) FIG. 4 shows in tabular form parameters of the device according to the invention;

(7) FIG. 5 is a graph showing current pulse waveform in the device according to the invention;

(8) FIG. 6 is a block diagram of a circuit according to an embodiment of the invention; and

(9) FIG. 7 illustrates exemplary waveforms of signals according to an embodiment of the invention.

DETAILED DESCRIPTION

(10) FIG. 3 is a block diagram of the powder sintering device. The sintering cycle is as follows: the power supply charges capacitors, which are subsequently discharged through the sintered powder placed in a graphite die between two punches connected to the capacitor battery. For controlled capacitor battery discharging a transistor switch is used. The sintering process requires a defined number of capacitor discharging cycles, with specified frequency and their charging voltage. Intensity of current flowing through the powder being sintered during discharging of capacitors achieves the value of a few to tens of kA, and its duration is of hundreds of microseconds. A very short current pulse duration with respect to separation of subsequent pulses from a fraction of a second up to a few seconds creates specific conditions of heating and cooling the sintered powder. During the current flow the powder being sintered is heated to a high temperature, and after it ceases it is cooled very quickly to defined sintering temperature. Duration of capacitor battery discharging current pulse is defined and results directly from parameters of the equivalent circuit of the system. The possibility of reduction of separation of pulses with respect to pulse duration depends on frequency at which the switch can be turned on and off.

(11) The device according to the invention is provided with a hydraulic press 1 exerting pressure in the sintering process and in the process of cooling the sintered powder, wherein between the punches of the press the sintered powder 6 is placed. The sample 6 is between the top electrode 4 and the bottom electrode 3 inside the graphite die 9. The graphite die 9, punches 12a, 12b and the sintered powder 6 are closed in the operating chamber 2, providing a possibility of conducting the sintering process at atmospheric pressure or at lowered pressure (1.Math.10.sup.8 Pa), in a neutral gas or other working gas continuously supplied to the chamber. Conducting the sintering processes at atmospheric pressure allows obtaining nanocrystalline sinters with pure grain boundaries without a layer of oxides or adsorbed gasses from powders with nanocrystalline size. Conducting the sintering processes in a working gas, e.g. in hydrogen, allows obtaining a strongly reducing atmosphere.

(12) The sintering chamber is made of low-magnetic stainless steel and is opened from one side. In the side part and in the rear part working gas inlets and connections to a vacuum system are located. Gas dosage means are not shown in the drawings. The vacuum pump system, not shown, comprises vacuum pumps adapted to operation in industrial conditions, resistant to sudden drops of high vacuum. The vacuum chamber with vacuum tightness of at least 10.sup.8 Pa. Electrodes 3, 4 pressing the sintered powder place in the graphite die are simultaneously high-current discharge electrodes, electrically isolated from the processing chamber, and moveable vacuum passages. Electrodes 3, 4 are cooled by a cooling medium and are isolated from the chamber 2 cooled by a cooling medium. The cooling medium is typically water or transformer oil. Electrodes 3, 4 are connected to the capacitor battery 8. Cooling the electrodes 3, 4 protects the vacuum sealing against the impact of high temperature. During sintering a pressure is imposed upon the punches 12a, 12b via the electrodes 3, 4 by means of a hydraulic press 1.

(13) The bottom electrode 3 has the possibility of travelling coarsely in order to define the initial height of the set being sintered (mechanical travel). During the sintering process a pressure is obtained by travelling of the top electrode 4 (hydraulic travel). On the electrodes there are located pads, usually made of steel, mounted by means of bolts (not shown in FIG. 3). This enables replacing them quickly. Electrodes 3, 4 are electrically isolated from the grounded operating chamber 2 by a set of ceramic-teflon sealings (not shown in FIG. 3).

(14) The electrodes bottom 3 and top 4 are connected to the power supply system comprising: capacitive circuit 8 with capacitor battery, and transistor switch 7 closing the capacitive circuit through the sample being sintered. In parallel to the capacitor battery a high-voltage power supply unit 5 is connected.

(15) The high-voltage power supply unit 5 provides an output of suitable current and voltage value for charging the capacitor battery. The high-voltage power supply unit 5 operates as an impulse high-voltage supply unit with current limit. The power supply unit 5 is equipped with a capacitor battery voltage measurement system, allowing their synchronous charging and discharging, and a number of protections including protection against short-circuit in internal circuit of power supply unit, a detector of temperature of inner heat sink with power components, protection against short-circuiting the power supply unit's output. A failure is signalled on a display of the device, which also serves for setting the power supply unit's operation parameters. These parameters can be set by means of a PLC program module.

(16) The capacitive circuit 8 is a capacitor battery with equivalent capacitance within the range of 50-1000 F, preferably equal to 250 F, and maximal operating voltage 15 kV. It comprises low-inductance capacitors in series-parallel connection, each adapted to operation with current intensity of tens of kA and steep rise slopes of a dozen or so kA/s.

(17) Electric pulse discharges applied to the sample 6 are initiated by the transistor switch 7 closing the electric circuit. The transistor switch 7 is built of eight transistors connected in parallel. The transistors are arranged in a multilayer structure providing a uniform pressure of the compressive force. Transistor switches usually are not used for switching such high currents and voltages like in the capacitive circuit according to the invention. It is caused mainly by a relatively low maximal current single transistor. Thus, construction of a transistor switch adapted to operate with voltage at the level of 15 kV and currents of tens of kiloamperes requires the use of a multiple transistor series-parallel circuit and construction of a dedicated control system. In result, the transistor switch has a slightly lower efficiency than alternative solutions available on the market. However, the inventor has observed that using rectangular pulses instead of oscillatory discharging the capacitor battery a better control over the sintering process is possible, due to a more precise setting of time, duration and the energy transferred to the sintered sample by current flow. Thus, paradoxically, a solution with lower efficiency and more complicated construction has proven advantageous.

(18) Taken into account in the construction of the transistor switch 7 are:

(19) selection of IGBT transistors, due to forward voltage characteristics in function of conducted current, mounting the transistors on a common liquid cooled heat sink (the heat sink in the form of a rectangular cuboid and the transistors mounted on both its greater sides. This construction allows serial connection of switch modules. each transistor has a dedicated surge suppression circuit (liquid cooled diode and resistor) and a diode clamping the inductance of the load, control signal transmitted from the control system to transistor control circuits by means of optical fibre links, transmit diodes of the link connected in series and controlled by one transistor, power supply of control circuits by means of a power converter (primary winding in the form of a HV conductor loop passing through secondary winding wound on ferrite toroidal cores, current leads of all transistors and diodes clamping the inductance of the load connected by liquid cooled Cu rails, between optical fibre links and control circuits of respective transistors, so-called drivers, electronic circuits allowing individual adjustment of transistor turn-on delay and adjusted delay of its turn-off are connected. This solution protects against transistor damage during turning off the load current (the slowest transistor turns off the whole current). Delays are set so that voltage waveforms on transistors during turn-on and turn-off mutually overlapped.
A block diagram of such circuit is presented in FIG. 6. The circuit is provided with an adjusted turn-on delay path 61 and an adjusted turn-off delay path 62. The adjusted turn-on delay path 61 responds to rising edge of signal S1 common for all transistors in the switch. In response to this rising edge generates a short negative pulse in signal S2. Signals S1 and S2 are fed to an AND gate, the output of which is fed through a diode to a transistor (not shown in FIG. 6) in the form of S4 signal. The output signal of AND gate is 0 when value of signal S2 equals 0. Because signal S2 is triggered by a rising edge of signal S1, it is synchronized to its beginning. Thus connecting these signals to an AND gate results in a signal with rising edge delayed with respect to rising edge of signal S1 by the duration of the negative pulse in signal S2. The adjusted turn-off delay path comprises a circuit responsive to falling edge of the signal S1. This circuit generates in its output section S3 a positive pulse, which added through a diode to the signal from the other path causes delay of the falling edge in signal S4 with respect to falling edge in the signal S1 by duration of the positive pulse in signal S3. Such configuration provides the possibility of individual transistor turn-on and turn-off delay. This possibility is used to compensate the production spread of their time of response to control signal. Exemplary waveforms of signals S1, S2, S3, S4 are shown in FIG. 7.

(20) Transistor switch comprising a serial connection of eight transistors rated for maximal pulse current of 5 kA can operate turning on and off current with maximal intensity of 32 kA.

(21) Transistor switch 7 is located directly by the rack of the capacitive circuit in the form of a capacitor battery 8 to minimize lead inductance, where a significant energy is accumulated during discharging pulse rise, with rising speed achieving a few thousand amperes per microsecond.

(22) Mounting the transistors on a common liquid cooled heat sink is preferable (heat sink in the form of rectangular cuboid and transistors mounted on both its greater sides, this construction allows serial connection of switch modules).

(23) The sintering device according to the invention is provided with systems measuring: pressing force, pressure, temperature, size changes of electrodes/punches/consolidated power set-up (measurement of shrinkage and expansion), and monitoring current pulse waveform using a Rogowski coil and oscilloscope, and monitoring the sintering process by application of a CCD camera. Measurement of temperature is implemented in two ways: using a thermoelement 11 located directly in graphite die 9, and/or using a pyrometer 10 on the surface of the graphite die 9 in which the sintering process is conducted. All process parameters, including temperature, pressure, pressing force, current waveform and the progress of the sintering process are recorded in real time and presented in graphic form during the sintering process.

(24) A waveform of the discharging pulse induced by closing and opening the transistor switch in the device according to the invention is shown in FIG. 5 in two embodiments. In the first one, the switch is opened and closed by the control waveform U1, shown in FIG. 5 as a binary logic waveform. The switch is open for the whole duration of the oscillatory fading capacitor battery discharging pulse. The sample is heated by current with waveform I1. In the second embodiment the switch is controlled by logic waveform U2. The capacitor battery discharging circuit is disconnected after the first positive half of discharging pulse oscillation. Then, the sintered sample is heated by current I2 with waveform close to rectangular. Also all parameters related to operating conditions of the device are continuously modified from the position of computer control panel. The parameters of the discussed sintering device are collected in the table shown in FIG. 4.

(25) The control system of the device according to the invention comprises a central programmable logic controller (PLC)Master, collecting data from a few secondary controllersSlave. Secondary controllers are responsible for monitoring and control of respective subsystems: autonomous high-voltage power supply unit, vacuum system automatics. The central controller (Master) supervises operation of respective slave-type controllers: Controller of power supply unit, providing the possibility of monitoring and real-time setting parameters of the power supply unit and is responsible for monitoring of technical condition of construction by application of a monitoring and control system for detection, localization, identification and prediction of development of damage, which can cause malfunction of the power supply unit. Vacuum system automatics, which is managed by a distinct PLC controller. The controller is directly responsible for digital control of elements, monitoring of parameters, and also ensuring the safety. Supervising subsystem, implemented in a distinct PLC controller. Its task is to monitor local sensors and alarming of emergency situations. The computer control panel allows to generate: timing diagrams, for analysing data correlated in time series, event charts, for searching and presentation of data according to criteria other than time, e.g. serial number, number of used setting, tabular data, for presentation of data from any source in form of a table, provided with possibility of filtering, comments, for adding, storing and sharing explanations of process anomalies or other production events.

(26) The method according to the invention has been presented by the way of examples of application.

(27) For a person skilled in the art it will be apparent, that the presented embodiments of the invention and examples of application of the method according to the invention are only a possible implementation of the invention. With further development of transistor technology it will be possible to replace the system of eight transistors by a smaller number of elements rated for higher voltage and higher operating current, both in IGBT and MOSFET technology.