**2. Magnetic pulse compaction**

In the commercial sector there is a constant demand for high density, net shape parts that come with an affordable price. At the moment automotive parts, such as powertrain gears for high- performance applications, are typically machined from forged and wrought blanks. Due to high machining costs, these components are much more expensive than conventional press and sintered powder metallurgy (PM) parts. Although PM hot pressing and extrusion techniques can be used effectively to produce bulk materials, these processes are costly due to numerous intermediate steps involved in the process (Kim, 1998; Seaok et al., 2004). Beside, in static compaction methods like cold pressing, hot isostatic pressing etc., the particles often have slow movement with the range of 10-3 to 10-4 m/s of velocity, resulting in grain growth of nanopowders during the static process (Lee et al., 2004). One of the ways to produce high relative density powder product is magnetic pulse compaction (MPC), which is a kind of dynamic magnetic compaction (DMC) technique. Having superiority to other compaction methods in energy control and forming efficiency, DMC is a kind of high-energy rate method which can be used to produce composite powder, ceramic, metal etc. (Min et al., 2010). While using MPC, it is possible to apply a high pressure (up to 5 GPa) to powders within a short period of time (~500μs). This ultra high pressure in 500µs brings about an enhanced deformation and microstructural rearrangement to these compacted powders, resulting in the improved green density (J.G. Lee et al., 2010).

In the static compaction methods, in general, the particles move slowly with the range of 10- 3 to 10-4 m/s of velocity, whereas in MPC process it is possible to achieve the high speed movement of particle over 10-100m/s owing to very high pressure, more than 1GPa (Lee et al., 2004). During MPC, since effect time of the magnetic force is too short, it can be considered that there is an impulsive force acting in MPC and it is well known that impulsive forces are very high, which are capable of breaking the activation energy of the compacted particle to move them from their places in the bulk.

Magnetic Pulse Compaction (MPC) is a new PM process that can produce components like these at costs comparable to the single-sinter, single- sinter route. In addition, MPC technology is also expected to find use in applications where special shapes, characterized by radially symmetric geometries and/or long length/diameter (L/Ds), are required.

Magnetic forces have been used for more than two decades in high rate metal forming and powder compaction. The same electro-magnetic based pulse forces are used in MPC to realize net shape powder consolidation. The powder-pressing route makes use of transmitted impact energy in a process analogous to driving a nail into a board with a hammer. While almost any material can be compacted to full density using a sufficiently large impact pressure, the important benefits of magnetic powder compaction when compared to die pressing are higher green densities and higher aspect ratios.

The basic working principle of the Magnetic Pulse Compaction process is shown in Figure 1. Powders are filled in a conductive container (armature) placed in the bore of a high field coil. The coil is pulsed with a high current to produce a magnetic field in the bore using the Biot-Savart law, which in turn, induces currents in the armature using Ohm's law.

Biot-Savart Law:

160 Sintering of Ceramics – New Emerging Techniques

contribution compared to other conventional methods. Several experimental results and reports have been discussed, especially on ceramics and hard-to-compress materials. In each case, we have assayed to delineate the differences that this process is introducing, the contributions it has, or the necessity to incorporate this in the chain of processes. Finally, we made some concluding remarks based on real term experimental results about the advantages of this process, which may aid certain streams of material research. Applications of MPC, both as a stand-alone process and prior to sintering have been highlighted

In the commercial sector there is a constant demand for high density, net shape parts that come with an affordable price. At the moment automotive parts, such as powertrain gears for high- performance applications, are typically machined from forged and wrought blanks. Due to high machining costs, these components are much more expensive than conventional press and sintered powder metallurgy (PM) parts. Although PM hot pressing and extrusion techniques can be used effectively to produce bulk materials, these processes are costly due to numerous intermediate steps involved in the process (Kim, 1998; Seaok et al., 2004). Beside, in static compaction methods like cold pressing, hot isostatic pressing etc., the particles often have slow movement with the range of 10-3 to 10-4 m/s of velocity, resulting in grain growth of nanopowders during the static process (Lee et al., 2004). One of the ways to produce high relative density powder product is magnetic pulse compaction (MPC), which is a kind of dynamic magnetic compaction (DMC) technique. Having superiority to other compaction methods in energy control and forming efficiency, DMC is a kind of high-energy rate method which can be used to produce composite powder, ceramic, metal etc. (Min et al., 2010). While using MPC, it is possible to apply a high pressure (up to 5 GPa) to powders within a short period of time (~500μs). This ultra high pressure in 500µs brings about an enhanced deformation and microstructural rearrangement to these

compacted powders, resulting in the improved green density (J.G. Lee et al., 2010).

compacted particle to move them from their places in the bulk.

In the static compaction methods, in general, the particles move slowly with the range of 10- 3 to 10-4 m/s of velocity, whereas in MPC process it is possible to achieve the high speed movement of particle over 10-100m/s owing to very high pressure, more than 1GPa (Lee et al., 2004). During MPC, since effect time of the magnetic force is too short, it can be considered that there is an impulsive force acting in MPC and it is well known that impulsive forces are very high, which are capable of breaking the activation energy of the

Magnetic Pulse Compaction (MPC) is a new PM process that can produce components like these at costs comparable to the single-sinter, single- sinter route. In addition, MPC technology is also expected to find use in applications where special shapes, characterized

Magnetic forces have been used for more than two decades in high rate metal forming and powder compaction. The same electro-magnetic based pulse forces are used in MPC to realize net shape powder consolidation. The powder-pressing route makes use of transmitted impact energy in a process analogous to driving a nail into a board with a

by radially symmetric geometries and/or long length/diameter (L/Ds), are required.

throughout the entire chapter.

**2. Magnetic pulse compaction** 

$$
\overrightarrow{B}(t) = \frac{\mu\_0 I(t)}{4\pi} \mathbf{J} \frac{d\vec{l} \times \vec{r}}{r^2} \tag{1}
$$

where µ0 is the permeability of free space, r is the distance from the element to the point *P*, and *r*ˆ is a unit vector pointing from dl toward point *P*. We find the total field at *P* by integragrating this expression over the entire current distrubition. Now, Faraday's law of induction states that the emf (ε) induced in a circuit is directly proportional to the time rate of change of magnetic flux through the circuit.

Faraday's Law:

$$
\varepsilon = -\iint \frac{d\vec{B}}{dt} dyd\mathbf{x} \tag{2}
$$

Hence, current (I) is induced using the Ohm's Law

Ohm's Law:

$$I(t) = \frac{1}{R}(V - \varepsilon(t))\tag{3}$$

where *V* is terminal voltage, *R* is resistance of external resistor and *ε* is the induction magnetic field. The induced currents interact with the applied magnetic field to produce an inwardly acting magnetic force that collapses the tube, thereby compacting the powder, using Lorentz force.

Lorentz Force:

$$
\overrightarrow{F} = I(t) \int d\mathbf{x} \times \overrightarrow{B} \tag{4}
$$

where I(t) and *B* are induction current and magnetic field, respectively. The armature is launched into the powders with a large kinetic energy within a few microseconds of the compaction cycle. The powders are pressed to full density via the transmitted impact energy, with the entire compaction occurring in less than one millisecond.

The process steps of MPC are similar to conventional PM compaction technique, and include tooling for compaction, powder filling, part extraction and sintering, as well as the optional steps of sizing and finishing. In most commercial applications, the powder filling and compaction are done in air medium at room temperature. The filling can also be carried

Effect of Magnetic Pulsed Compaction (MPC) on Sintering Behavior of Materials 163

The Al2O3 powder was formed into the shape of a disk by magnetic pulsed compaction (MPC). Table 1 demonstrates the compaction arrangements, the apparent features and final density of the bulks fabricated by the combination of precompaction, MPC, and sintering processes. In order to improve the density and properties, the starting powder was precompacted in a die under 110 MPa, 220 MPa, and 330 MPa, and then each precompacted sample without separation from the die was MPC-ed at room temperature. The obtained density of the MPC-ed specimen increased with increasing MPC pressure. The pressure required to consolidate the nanopowders is related to the force required to push the particles together. In order to push two particles together, the applied force must be equal to or greater than the resisting force. The ceramic powders are not expected to plastically deform during compaction like metals. In hard ceramics, a plastic deformation of the particles or the formation of a particle-to-particle contact is so difficult that the stored strain energy by the compaction pressure could not be readily relaxed, which results in the

**Uniaxial static compaction (110 MPa) + Sintering (1450 oC for 3 h) 90.0** 

**Pre-compaction (110 MPa) + MPC (1.25 GPa)** 

**Pre-compaction (220 MPa) + MPC (1.25 GPa)** 

**Pre-compaction (330 MPa) + MPC (1.25 GPa)** 

of MPC and precompaction processing.

Table 1. Consolidation conditions and relative densities of sintered bulks by a combination

In addition, it is clear that density also increases with increasing precompaction pressures from 110 MPa to 220 MPa and then becomes stagnant at 94.5 % for precompaction pressure beyond 330 MPa. The highest density of 94.5 % was achieved in the sample precompacted at 220 MPa, while the density of the sample without precompaction was 92%. The means that precompaction of the powder before MPC improves the final density of the sintered bulk. This may be due to the higher initial packing density introduced by precompaction pressure, as well as the higher MPC pressure. The increased green density may be due to better packing associated with small particles filling the voids between the larger ones. Therefore, the pressure may play a role in the initial stage through particle rearrangement and distribution of agglomerates as well as in the later stages of densification. Additionally, it was reported that a high initial density is not only effective to enhance the subsequent densification during sintering, but also capable of limiting the

**MPC (0.5 GPa) + Sintering (1450 oC for 3 h) 90.0** 

**MPC (1.25 GPa) + Sintering (1,450 oC for 3 h) 92.0** 

**MPC (1.8 GPa) + Sintering (1,450 oC for 3 h) 90.0** 

**+ Sintering (1450 oC for 3 h) 93.0** 

**+ Sintering (1450 oC for 3 h) 94.5** 

**+ Sintering (1450 oC for 3 h) 94.5** 

**Experimental conditions Density (%)** 

formation of cracks in the materials compacted at high pressure.

out in special environments, such as in an inert gas or other cover gases, for special applications. The powders can also be compacted at elevated temperatures with suitable system modifications. The *MAGNEPRESSTM* system used in the DMC process consists of four subsystems – pulsed power system, electromagnetic field coil, material handling capabilities, and programmable logic controller (PLC) systems. The power supply has a modular design and has been designed to accommodate expanding energy needs. High production rates up to ten parts per minute can be achieved with this system. Material handling capabilities, such as powder filling and transportation of filled cassettes, can be easily integrated into the system for a given application.

Fig. 1. Working principle of Magnetic Pulse Compaction
