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.[1] 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 tools and technical ceramics. 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 2,400 °C (4,350 °F). Pressures of up to 50 MPa (7,300 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.
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In this process heat is produced within the mould when it is subjected to a high frequency electromagnetic field, generated by using an induction coil 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.
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 convection. 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.
The basic idea of sintering with electric current going through the mould is quite old. Resistance heating of cemented carbide powders was patented by Tayler[2] 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.