Hot pressing
Encyclopedia
Hot pressing is a high-pressure, low-strain-rate powder metallurgy
process for forming of a powder or powder compact at a temperature high enough to induce sintering
and creep
processes. This is achieved by the simultaneous application of heat and pressure.
Hot pressing is mainly used to fabricate hard and brittle
materials. One large use is in the consolidation of diamond-metal composite cutting tool
s and technical ceramic
s. The densification works through particle rearrangement and plastic flow at the particle contacts. The loose powder or the pre-compacted part is in most of the cases filled to a graphite
mould that allows induction or resistance heating up to temperatures of typically 2400 °C (4,352 °F). Pressures of up to 50 MPa (7,251.9 psi) can be applied.
Within hot pressing technology, three distinctly different types of heating can be found in use: induction heating
, indirect resistance heating, and direct hot pressing.
coupled to an electronic generator. The mould is made out of graphite or steel, and pressure is applied by one or two cylinders onto the punches. The mould is positioned within the induction coil. The advantage here is that the pressure and the inductive power are completely independent. Even powders with a liquid phase are amenable to this process and low pressures are possible, too. Among the disadvantages are the expense of a high-frequency generator and the need for proper alignment. If the mould is placed off centre, the heat distribution is uneven. But the main disadvantage is the dependence of the process on good inductive coupling and thermal conductivity
of the mould. The magnetic field can penetrate the mould only 0.5mm to 3mm. From there on, the heat has to be "transported" into the mould by the thermal conductivity of the mould material. Uniform heating is much more difficult if the air gap between the mould and the inductive coil is not the same all along the mould profile. Another potential problem is heating rate. Too high a heat up rate will result in high temperature differences between the surface and core that can destroy the mould.
. As the electrical energy heats the heating elements that then heat the mould in a secondary manner, the process is called indirect resistance heating.
Advantages are high achievable temperatures, independent from the conductivity of the mould and independent from heat and pressure. Main disadvantage is the time that it takes to heat up the mould. It takes relatively long for heat transfer to take place from the furnace atmosphere to the mould surface and subsequently throughout the cross-section of the mould.
powders was patented by Tayler as early as 1933. This method is currently undergoing renewed interest. When applying a standard (unpulsed) AC or DC current, it is referred to as Direct Hot-Pressing (DHP) which is a common term in many industries. When applying a pulsed DC current, it is referred to as Spark Plasma Sintering
(SPS) or Field Assisted Sintering Technique(FAST). The compelling reason for shortening the cycle time then was to avoid grain growth and also save energy. In direct hot pressing, the mould is directly connected to electrical power. The resistance of the mould and the powder part generates the heat directly in the mould. This results in very high heating rates. Additionally, this leads to significant increase in the sintering activity of fine metal powder aggregates which makes short cycle times of a few minutes possible. Further, this process lowers the threshold sintering temperature and pressure compared to that required in conventional sintering processes. The previous two methods are both closely dependent on the an intrinsic property of the mould material, i.e., its thermal conductivity. With direct resistance heating, however, the heat is generated where it is needed.
Recently, the manufacture of such critical items as sputtering targets and high-performance ceramic components, such as boron carbide
, titanium diboride, and sialon
, have been achieved. Using metal powder, the conductivity of the mould is ideal for fast heating of the work-piece. Moulds that have a big diameter and relatively small height can be heated up very fast. The process is especially suitable for applications that need high heating rates, e.g. for materials that should not be kept at high temperatures too long or for processes that require fast heating rates for high productivity.
With the direct hot pressing technology, materials can be sintered to their final density. The near net-shape precision achieved is very high and saves in many cases mechanical reworking of the high grade materials that are often difficult to process.
Powder metallurgy
Powder metallurgy is the process of blending fine powdered materials, pressing them into a desired shape , and then heating the compressed material in a controlled atmosphere to bond the material . The powder metallurgy process generally consists of four basic steps: powder manufacture, powder...
process for forming of a powder or powder compact at a temperature high enough to induce sintering
Sintering
Sintering is a method used to create objects from powders. It is based on atomic diffusion. Diffusion occurs in any material above absolute zero, but it occurs much faster at higher temperatures. In most sintering processes, the powdered material is held in a mold and then heated to a temperature...
and creep
Creep
Creep may refer to:* CREEP, the Committee for the Re-Election of the President, associated with the Watergate scandal of U.S. president Nixon's administration....
processes. This is achieved by the simultaneous application of heat and pressure.
Hot pressing is mainly used to fabricate hard and brittle
Brittle
A material is brittle if, when subjected to stress, it breaks without significant deformation . Brittle materials absorb relatively little energy prior to fracture, even those of high strength. Breaking is often accompanied by a snapping sound. Brittle materials include most ceramics and glasses ...
materials. One large use is in the consolidation of diamond-metal composite cutting tool
Cutting tool
In the context of machining, a cutting tool is any tool that is used to remove material from the workpiece by means of shear deformation. Cutting may be accomplished by single-point or multipoint tools. Single-point tools are used in turning, shaping, plaining and similar operations, and remove...
s and technical ceramic
Ceramic
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
s. The densification works through particle rearrangement and plastic flow at the particle contacts. The loose powder or the pre-compacted part is in most of the cases filled to a graphite
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...
mould that allows induction or resistance heating up to temperatures of typically 2400 °C (4,352 °F). Pressures of up to 50 MPa (7,251.9 psi) can be applied.
Within hot pressing technology, three distinctly different types of heating can be found in use: induction heating
Induction heating
Induction heating is the process of heating an electrically conducting object by electromagnetic induction, where eddy currents are generated within the metal and resistance leads to Joule heating of the metal...
, indirect resistance heating, and direct hot pressing.
Inductive heating
In this process heat is produced within the mould when it is subjected to a high frequency electromagnetic field, generated by using an induction coilInduction coil
An induction coil or "spark coil" is a type of disruptive discharge coil. It is a type of electrical transformer used to produce high-voltage pulses from a low-voltage direct current supply...
coupled to an electronic generator. The mould is made out of graphite or steel, and pressure is applied by one or two cylinders onto the punches. The mould is positioned within the induction coil. The advantage here is that the pressure and the inductive power are completely independent. Even powders with a liquid phase are amenable to this process and low pressures are possible, too. Among the disadvantages are the expense of a high-frequency generator and the need for proper alignment. If the mould is placed off centre, the heat distribution is uneven. But the main disadvantage is the dependence of the process on good inductive coupling and thermal conductivity
Thermal conductivity
In physics, thermal conductivity, k, is the property of a material's ability to conduct heat. It appears primarily in Fourier's Law for heat conduction....
of the mould. The magnetic field can penetrate the mould only 0.5mm to 3mm. From there on, the heat has to be "transported" into the mould by the thermal conductivity of the mould material. Uniform heating is much more difficult if the air gap between the mould and the inductive coil is not the same all along the mould profile. Another potential problem is heating rate. Too high a heat up rate will result in high temperature differences between the surface and core that can destroy the mould.
Indirect resistance heating
With indirect resistance heating technology, the mould is placed in a heating chamber. The chamber is heated by graphite heating elements. These elements are heated by electrical current. The heat is then transferred into the mould by convectionConvection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....
. As the electrical energy heats the heating elements that then heat the mould in a secondary manner, the process is called indirect resistance heating.
Advantages are high achievable temperatures, independent from the conductivity of the mould and independent from heat and pressure. Main disadvantage is the time that it takes to heat up the mould. It takes relatively long for heat transfer to take place from the furnace atmosphere to the mould surface and subsequently throughout the cross-section of the mould.
Direct hot pressing
The basic idea of sintering with electric current going through the mould is quite old. Resistance heating of cemented carbideCemented carbide
Cemented carbide, also called tungsten carbide, hardmetal, or widia, is a hard material used in machining tough materials such as carbon steel or stainless steel, as well as in situations where other tools would wear away, such as high-quantity production runs. Most of the time, carbide will leave...
powders was patented by Tayler as early as 1933. This method is currently undergoing renewed interest. When applying a standard (unpulsed) AC or DC current, it is referred to as Direct Hot-Pressing (DHP) which is a common term in many industries. When applying a pulsed DC current, it is referred to as Spark Plasma Sintering
Spark plasma sintering
Spark plasma sintering , also known as field assisted sintering technique or pulsed electric current sintering , is a sintering technique....
(SPS) or Field Assisted Sintering Technique(FAST). The compelling reason for shortening the cycle time then was to avoid grain growth and also save energy. In direct hot pressing, the mould is directly connected to electrical power. The resistance of the mould and the powder part generates the heat directly in the mould. This results in very high heating rates. Additionally, this leads to significant increase in the sintering activity of fine metal powder aggregates which makes short cycle times of a few minutes possible. Further, this process lowers the threshold sintering temperature and pressure compared to that required in conventional sintering processes. The previous two methods are both closely dependent on the an intrinsic property of the mould material, i.e., its thermal conductivity. With direct resistance heating, however, the heat is generated where it is needed.
Recently, the manufacture of such critical items as sputtering targets and high-performance ceramic components, such as boron carbide
Boron carbide
Boron carbide is an extremely hard boron–carbon ceramic material used in tank armor, bulletproof vests, and numerous industrial applications...
, titanium diboride, and sialon
Sialon
Sialon ceramics are a specialist class of high temperature refractory materials, with high strength , good thermal shock resistance and exceptional resistance to wetting or corrosion by molten non-ferrous metals, compared to other refractory materials such as, for example, alumina. A typical use...
, have been achieved. Using metal powder, the conductivity of the mould is ideal for fast heating of the work-piece. Moulds that have a big diameter and relatively small height can be heated up very fast. The process is especially suitable for applications that need high heating rates, e.g. for materials that should not be kept at high temperatures too long or for processes that require fast heating rates for high productivity.
With the direct hot pressing technology, materials can be sintered to their final density. The near net-shape precision achieved is very high and saves in many cases mechanical reworking of the high grade materials that are often difficult to process.