FR2491056A1 - SINKING METHOD AND APPARATUS - Google Patents

Abstract

THE INVENTION CONCERNS A PROCESS AND AN APPARATUS FOR SINTING A FRITTABLE MASS 1 OF CORPUSCLES BY SIMULTANEOUS APPLICATION OF THERMAL ENERGY 11, 12A AND PRESSURE 3, 4. ACCORDING TO THE INVENTION, 13, 15 OF ACOUSTIC WAVES ARE APPLIED A THE FRITTABLE MASS OF CORPUSCLES SUBJECT TO THERMAL ENERGY AND PRESSURE; 17 THE LEVEL OF SAID ACOUSTIC WAVES SPREADING THROUGH THE SAID GROUND IS DETECTED, TO PRODUCE A CONTROL SIGNAL 17B, 17C, 17D, AND AT LEAST THE THERMAL ENERGY 17C OR THE PRESSURE 17B, 17D, AS A FUNCTION OF THE SAID SIGNAL.

Description

Process and apparatus for frittaae

  The invention relates generally to the technical

  that consists of sintering a mass of corpuscles, metalli-

  whether or not metallic, by simultaneously applying thermal energy and pressure so that the corpuscles unite and deform and that the mass contracts to provide a sintered product of desired or sufficient density. More particularly, the invention

  a sintering method and apparatus of the type described in

  crit in which the progressions of the coalescence and the deformation of the corpuscles, the contraction of the mass

  and the increase in density are checked beforehand.

  the sintering process being controlled according to

an optimal mode.

  The sintering process in which thermal energy and pressure are simultaneously applied to a sinterable mass of corpuscles is commonly referred to as hot pressing and may involve a high intensity electrical current conducted directly through the mass of corpuscles. which may be a metal, an alloy or a non-metallic body such as carbon or certain nitrides, hides or silicates. In the sintering device of this type, a mass of corpuscles is loaded into a cavity or space defined by a mold and one or more punches. The mass is compressed by a punch against the mold or between a pair of punches in the mold, while simultaneously being heated by the heating current passing directly through the mass. Alternatively, the

  heating current can pass into the mold or a bo-

  Induction heating coil arranged to surround the mass and the mold. The hot pressing process

  is sometimes performed isostatically or semi-isostatically

  with a compressive force uniformly applied to the periphery of the mass towards a central point or axis of

it.

  In hot pressing, it has been recognized that it is desirable

  table that the pressure applied to the mass of corpuscles is initially kept at a low level, for example

  0.5 to 10 kg / cm. When the particles join together

  or when the mass has shrunk to a certain extent, the pressure must be increased to a final or high compression level, for example 50 to 5000 kg / cm It is also common to stop or decrease

  the heating current concurrently, before or after the

  increasing compression pressure depending on the particular material to be sintered. Sometimes,

  it is necessary to use an extreme camiande protocol -

  sophisticated method in which the heating current or the pressure, or both, are modified sequentially over a multiplicity of steps, to obtain a sintered product

the quality sought.

  In the prior art, however, the camera protocol, whether relatively simple or sophisticated, has only been determined on an empirical basis for each material or sintering operation, and indeed difficulties have been encountered in accurately capturing the timing of the variation in heat or thermal energy and pressure that is truly optimal for each material and each sintering operation. As a result, the quality of a conventionally sintered or hot-pressed product is not only very often much lower than desired, but it is extremely difficult to reproduce a given sintering quality by means of

conventional technique.

  Accordingly, an important object of the invention is to provide

  an improved sintering process of the type described by

  when hot, which allows to automatically produce a sintered product of excellent quality,

  reproduce without difficulty a desired sintering quality.

  A specific object of the invention is to provide an improved method of hot pressing and, in particular, the electric sintering of a mass of corpuscles under pressure, in which the progression of coalescence and deformation

  corpuscles, the contraction of the mass or the increase

  density, is observed precisely to

  put in a de-sintering operation to be controlled according to

an optimal mode.

  Another object of the invention is to provide an apparatus for implementing the described sintering method, in which new means are provided for precisely observing the behavior of the corpuscles and the mass during a sintering operation, and which may include operative control means in response to the means of

  detection to vary under control at least the

their sintering or pressure.

  According to a first aspect, the invention provides a method for sintering a sinterable mass of corpuscles by subjecting it simultaneously to the action of thermal energy and pressure,

  characterized in that it comprises the steps of

  to apply a beam of acoustic or elastic waves

  (ie, sound or ultra-sound) to the frit

  particle of corpuscles subjected to thermal energy and pressure; detecting a level of acoustic waves propagating through the sinterable mass to produce a control signal; and control at least thermal energy or pressure in

response to the control signal.

  Acoustic waves can be sound waves or

  ultrasonic sound of a relatively low frequency, for example

  between 1 and 100 kHz, but they are preferably

  the presence of ultra-sound waves with a frequency

  between 0.1 and 300 MHz, preferably between 1 and 150 MHz.

  According to a second aspect, the invention also provides a

  sintering electrode of a sinterable mass of corpuscles, which is characterized in that it comprises:

  power supply means for applying energy

  thermal energy to the sinterable mass; pressure means for applying pressure to the sinterable mass; means for applying an acoustic or elastic wave (ie sonic or ultrasonic) beam to the sinterable mass simultaneously subjected to thermal energy and pressure by the current supply means and the pressure means, respectively; detection means for detecting the level of acoustic waves propagating through the sinterable mass to produce a control signal; and control means responsive to the control signal for acting at least on the current supply means or the pressure means, for modifying at least the energy

thermal or pressure.

  Embodiments of the invention will now be described, by way of example, in conjunction with the accompanying drawings, in which Fig. 1 is a schematic view, partly in section, showing an embodiment of the invention, and FIG. 2 is a schematic view, partly in section,

  representing another embodiment of the invention.

  Referring to Figure 1 which shows a mass M of corpuscles 1 loaded into a cylindrical mold

  2 in a heat resistant material such as graphite.

  The mass M in the mold 2 is held between a pair of punches, a lower punch 3 and an upper punch 4, each of them being also heat-resistant and being here an electrically conductive material such as

  than graphite. Hallmarks 3 and 4 are trained respectively

  by hydraulic cylinders 5 and 6, to compress

  the mass M. A slave valve 7 is associated with the lower cylinder

  5 while an electromagnetic switching valve 8 is associated with the upper cylinder 6. The valves 7 and 8 are respectively controlled by servocontrol circuits.

and control 9 and 10.

  A current source 11 is electrically connected to the punches 3 and 4 by a switch S and a resistor 12a to produce heat or heat energy sufficient to sinter the mass M. When the corpuscles 1 are in

  metal or alloy, the heating current is directly

  from the current source 11 through the mass M

  to electrically sinter the corpuscles 1. When the cor-

  1 are non-metallic, or electrically insulated, or of a relatively low electrical resistance, the mold 2 must be electrically conductive and the heating current from the current source 11 passes through the mold 2 to produce the heat necessary to sinter the corpuscles 1. The resistance 12a can be conmected to

  punches 3 and 4 in parallel with the power source 11.

  A voltage detector 12 is connected to the resistor 12a to provide a detection signal representing a voltage or current between the punches 3 and 4. The detection signal is supplied to the slave circuit 9. The slave circuit 9 responds to the detection signal and it provides a servo signal that it applies to the slave valve 7 by a terminal 7a. The slave valve 7 acts on the jack 5 for

  order the position of the lower punch 3.

  To implement the features of the invention, a sound or ultrasonic transmitter 13 is shown coupled

  with the lower punch 3 and it can be constituted by a trans-

  conventional electromechanical conductor maintained in contact therewith and excited by a current source 14. At the upper punch 4 is coupled a sound or ultrasonic receiver. The receiver 15 may be a transducer

  electro-mechanical, and it is connected to a detector of ni-

  caliber 17- by an amplifier 16 and a link 17a.

  17 shows one or more reference settings for

  to be settled in advance. When a reference setting in the detector 17 is crossed by the signal input from the terminal 17a, the detector 17 provides an output signal at one or each of its output terminals 17b and 17c. The output signal at the output terminal 17b is applied to the control circuit 10 which acts on the valve 8. The output signal at the output terminal 17c is adapted to act on the heating current source 11 or on the current switch S. In a typical sintering operation, after the mass M of corpuscles has been loaded as shown, the punch

  3 is driven to apply a relative pressure

  For example, the switch S is closed to allow the heating current to flow from the current source 11 through the mass M or the mold, for example, between 0.5 and 10 kg / cm 2. 2, or both,

  between punches 3 and 4. When closing the commu-

  Sator, or concurrently, the transducer 13 is activated

  and excited by the oscillator 14 to emit sound waves

  res or ultra-sound in the mass M of corpuscles 1. The transducer 15 is now operative to receive

  the sound or ultrasonic waves emitted by the transduc-

  13 and propagating in the mass M of corpuscles 1.

  The upper punch 4 remains stationary during this step. In this step, a relationship of slight mutual contact between

  the corpuscles 1 is advantageously maintained by means of

  under command the lower punch 3 and thereby controlling the position of this punch retaining the mass M. Pendant

  that the corpuscles are driven upwards by the point

  3, they may also tend to move downward by gravity. Thus, the corpuscles are forced to move dynamically relative to each other and

  they are therefore suitable for being uniformly compressed.

  mement throughout the mass M while being subject to the current

  of heating to destroy the thermal barriers between

  corpuscles, for example oxides, gases and impurities. To this end, the slave device 9, 7 is provided to respond to the voltage or the intensity in the mass M and thus to control in a controlled manner the external pressure applied to the mass M. This heating and this slight initial compression simultaneous

  used to activate the individual corpuscules 1 in

  thermal barriers or stinging dandruff

  oxide, impurities and gaseous substances which adhere

  rent, and away from the interfaces between corpuscles

  in the mass M. Intimate mutual contacts between

  1 are thus established, the corpuscles 1 unite

  and the contraction of mass M begins. With the

  the mass M becomes ready for plastic deformation, so that the compression supported by the punches 3 and 4 causes a sudden contraction of the mass M when a

  predetermined threshold condition is reached.

  It has now been determined that the achievement of such a condition

  threshold is accurately and accurately detected by detecting a change in the

  acoustic wave emitted by the transducer 13, propagates

  in the mass M and received by the transducer 15.

  The acoustic waves can be detected with regard to their intensity or their propagation speed in the mass M.

  Thus, the detector 17, which can advantageously be

  by a Schmitt trigger circuit, provides an output signal representing this change at each of the output terminals 17b and 17c. Thus, as the density of the sintered mass increases, the intensity or speed of transmission of sound or ultrasonic waves received by transducer 15 increases. When this increase reaches a pre-set threshold level in the detector 17, the signals are formed at the terminals 17b and 17c. The valve control circuit 10 responds to the output signal provided at the terminal 17b to open the valve 8, thereby operating the cylinder 6 and advancing the upper punch 4. The punch 4 is advanced to exert a high pressure, for example 50 at 5000 kg / cm, on the mass M. Simultaneously, the lower punch 3 can be advanced upwards by the jack 8 actuated by the total opening of the slave valve 7. This last action can be performed by another signal of command provided to

  the third output terminal 17d of the detector 17 and apply

  the slave valve 7 by the path shown in dashed line. The output present at terminal 17c is used to act on current source 11 to reduce the heating current or power, or to open the

  switch 7 to stop the heating current.

  Figure 2 shows another embodiment of the invention.

  tion, and the same reference figures are used to

  designate the same components of the device. In this

  position, a single transducer 18 is used to serve as both transmitter and acoustic receiver, and it is coupled to the upper punch 4. An oscillator 19 is provided to provide a sequence of clock pulses to periodically open a circuit of gate 20 connected between the oscillator 14 and the transducer 18. The latter is thus excited intermittently so as to emit repetitively

  a beam of sound or ultrasonic waves, with

  Thus, the acoustic wave beam is emitted by the transducer 18 only during each trigger period determined by the

  duration of each clock pulse provided by the oscilla-

  19. The acoustic wave beam emitted by the trans-

  conductor 18 propagates through the mass M, is reflected on the lower punch 3, and then propagates again through

  the mass M to return to the transducer 18. When the

  As the weight of mass M increases, its reflectivity of acoustic waves decreases and this is represented here by

  decreased intensity of reflected acoustic waves.

  Another door 22 is mounted between the transducer 18 and the amplifier 16, and each pulse slot from the oscillator 19 is also applied to the gate 22 and the detector circuit 17d by a delay circuit 21 which differs the slot. pulse for a short period of 1 to 5 microseconds. So, the door 22 is open with

  this period of delay after the occurrence of the impulse

  oscillator 19, and the detector 21 has the ability to respond to the reflected acoustic waves or the transducer detection signal 18 for each clock pulse of the oscillator 19. When the level of the reflected acoustic waves falls below a predetermined threshold level in the detector 21, the latter provides an output signal at each of its output terminals 17b, 17c and 17d. The control circuit 10 is thus excited to open the valve 8 and allow a final pressure

  high to be applied to the mass M by the cylinder unit 8.

EXAMPLE

  A chromium-molybdenum steel part having a surface roughness of 15 V Rmax is housed in the mold 2 and is retained on the lower punch 3 according to the invention.

  position shown in Figure 2. On the part is distributed

  of iron powder with a particle size of 100

  microns forming a layer 0.2 mm thick on the

  what is placed a mass of corpuscles of tungsten-ccbalt carbide (WC-Co) of a particle size of 5 microns, retained by the upper punch 4. The mass is compressed initially

  under a slight pressure of 5 kg / cm by moving the pointer

  lower section 3, while being heated by the passage of an electric current with a density of 1000 amperes / cm2

  directly through the mass between the upper punches

  4 and 5. The transducer 18 is energized by the current source 14 so as to apply a

  succession of ultrasonic beams of

  frequency of 5 MHz and a power of 6 kW to ground.

  The reflection of each beam is detected with a

  delay period of 2 microseconds set at the circuit of

  delay 21 for the duration set at oscillator 19.

  The intensity of the reflection is initially 40% / o of the intensity of the waves emitted, but it is reduced to 30% when a period of two minutes has elapsed, after which a high pressure of 350 kg / cm2 is applied to the mass by moving the upper punch 4. It has been found that the mass of WC-Co has been sintered with excellent quality and also that it is firmly bonded to the chromium-molybdenum steel piece with resistance with traction of 350 / cm2

Claims (6)

Claims.   1. A method of sintering a sinterable mass of corpuscles according to which it is simultaneously subjected to the action of heat energy and pressure, characterized in that it comprises the steps of applying acoustic waves to the mass sinterable corpuscles subjected to thermal energy and pressure; detecting the level of said acoustic waves propagating in said mass, to produce a control signal; and control at least thermal energy or pressure in function of said signal.   2. Method according to claim 1, characterized in that the acoustic waves have a frequency between 0.1 and 300 MHz.   3. Method according to claim 2, characterized in that   the frequency is between 1 and 150 MHz.   4. Apparatus for sintering a sinterable body of particles, characterized in that it comprises:   current supply means for applying energy   heat to ground; pressure means for applying pressure to said mass; means for applying an acoustic wave beam to the sinterable mass of corpuscles simultaneously subjected to   thermal energy and pressure by means of furnace   current and pressure means; detector means for detecting the level of acoustic waves propagating in the mass, to produce a control signal; and control means responsive to the control signal for acting at least on the current supply means or the pressure means, for modifying at least the energy thermal or pressure.   5. Apparatus according to claim 4, characterized in that the application means the detection means comprise respectively a first and a second transducer, and in that said transducers are arranged on either side of   the mass for respectively emitting said beam and detecting   the acoustic waves propagating in the mass.   6. Apparatus according to claim 4, characterized in that the application means the detection means comprise a common transducer for emitting the beam and detecting a reflection of the acoustic waves propagating in the mass.