**3.3 Hard materials: Effects of MPC on sintered tungsten carbide-cobalt (WC-Co)**

A number of alloy powders, namely ceramics, have gone through the combined MPC and sintering process. At this point, it is necessary to know how hard-to-compress materials react to similar processing conditions. Hard metal is the term used to signify a group of sintered, hard, wear-resisting materials based on the carbides of one or more of the elements; tungsten, tantalum, titanium, molybdenum, niobium and vanadium, bonded with a metal of lower melting point, usually cobalt. Tungsten carbide is however the most widely used one. These materials are commonly referred to as cemented carbides or simply as carbides as, for example, carbide tools. By varying the carbide particle size, the amount of binder metal, and the sintering conditions, the properties such as wear resistance, impact strength, resistance to cratering, and hot hardness may be optimized for a given application.

For example, in the case of a wire drawing die, wear resistance is a major requirement, but for a cutting tool, especially if subject to intermittent loading, high impact strength is required. Figure 10 shows the as-pressed density achieved in WC-Co. In most cases, high initial densities were achieved after MPC, with results getting higher when consecutive pressures were applied. The process parameters, used for some alloys, indicated in Figure 10, are not optimized; and higher densities than shown are possible.

The WC-Co MPC preforms were sintered at conventional PM sintering temperatures and were easily heat treatable. Their densities and hardness after sintering are illustrated in Figure 11 (a) and (b), respectively. It can be seen that hardness values greater than 1450 Hv were achieved in MPC processed WC-Co alloys. Figure 11 (a) illustrates the changes in

Effect of Magnetic Pulsed Compaction (MPC) on Sintering Behavior of Materials 173

Fig. 11. Changes in (a) relative density, (b) hardness of WC-7.5wt%Co and WC-12wt%Co

Fig 12 shows the changes in microstructure of sintered samples with varying MPC-ed pressure and Co content. It can easily be comprehended that these fine grained WC got more homogenously distributed in Co binder phase with increased pressure and Co content, at an elevated sintering temperature of 1450OC. It may have as well resulted in having these samples display a higher relative density and hardness for high magnetic pulsed compaction, especially after sintering, even at a lower operating temperature. Now, it is comparatively easier for finer grained WC material to sinter at lower temperature and become more homogeneous. The first stage in this mechanism is the spreading of binder phase on the WC grains. This happens within minutes after the temperature reaches 1000OC for fine-grained WC. For coarse-grained WC it happens at ~1100OC, even though the W–Co–

samples with MPC-ed pressure.

relative density for both WC-7.5wt%Co and WC-12wt%Co samples after magnetic pulsed compaction and sintering. Before and after sintering, samples of both compositions showed similar pattern with increasing pressure, and successfully reached a maximum of 97% dense state for WC-12wt% sample after sintering, which is higher than most commercial bulks. Here, the conventional explanation of the sintering mechanism of WC-Co comprises of some of the WC dissolving into the cobalt binder phase, migrating and re-precipitating on the surface of the original WC (Schwartzkopf & Kiefer, 1960). The final product consists of a three-dimensional skeleton of WC grains with cobalt as a binder phase matrix (Silva et al., 2001; Cha et al., 2001). In hard alloys such as WC-Co or TiC, even a slight difference in hardness value with respect to their constituents' content, pressure or grain size can mean a change to their mechanical behavior.

Fig. 10. Changes in initial density with varying MPC-ed pressure on a) WC-7.5wt%Co and b) WC-12wt%Co at room temperature.

relative density for both WC-7.5wt%Co and WC-12wt%Co samples after magnetic pulsed compaction and sintering. Before and after sintering, samples of both compositions showed similar pattern with increasing pressure, and successfully reached a maximum of 97% dense state for WC-12wt% sample after sintering, which is higher than most commercial bulks. Here, the conventional explanation of the sintering mechanism of WC-Co comprises of some of the WC dissolving into the cobalt binder phase, migrating and re-precipitating on the surface of the original WC (Schwartzkopf & Kiefer, 1960). The final product consists of a three-dimensional skeleton of WC grains with cobalt as a binder phase matrix (Silva et al., 2001; Cha et al., 2001). In hard alloys such as WC-Co or TiC, even a slight difference in hardness value with respect to their constituents' content, pressure or grain size can mean a

Fig. 10. Changes in initial density with varying MPC-ed pressure on a) WC-7.5wt%Co and

change to their mechanical behavior.

b) WC-12wt%Co at room temperature.

Fig. 11. Changes in (a) relative density, (b) hardness of WC-7.5wt%Co and WC-12wt%Co samples with MPC-ed pressure.

Fig 12 shows the changes in microstructure of sintered samples with varying MPC-ed pressure and Co content. It can easily be comprehended that these fine grained WC got more homogenously distributed in Co binder phase with increased pressure and Co content, at an elevated sintering temperature of 1450OC. It may have as well resulted in having these samples display a higher relative density and hardness for high magnetic pulsed compaction, especially after sintering, even at a lower operating temperature. Now, it is comparatively easier for finer grained WC material to sinter at lower temperature and become more homogeneous. The first stage in this mechanism is the spreading of binder phase on the WC grains. This happens within minutes after the temperature reaches 1000OC for fine-grained WC. For coarse-grained WC it happens at ~1100OC, even though the W–Co–

Effect of Magnetic Pulsed Compaction (MPC) on Sintering Behavior of Materials 175

varying the pressures. The process can be carried out both at room and elevated temperatures, making it possible to employ this method on a wide range of materials.

A number of applications have already been discussed in this chapter. There are however, more fields of application that MPC can cover. For example, even transmission ring gear of AGMA 9 (American Gear Manufacturers Association) rating for automotive powertrain applications has recently found interest in using MPC. The objectives of these efforts are to replace the cast and machined gear with a lower cost net shape PM gear and to increase the power density through enhanced material properties. In addition, a large number of industrial drilling parts, made of hard materials like WC-Co or WC-Ni-Co, such as drill bits, go through an initial phase of MPC. Initial development efforts have focused on achieving the target density; developing tooling that can survive the dynamic compaction conditions, and producing the desired geometry. With a number of applications now almost ready for commercial implementation, new applications are continuously being sought and

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C system has a ternary eutectic at ~1275OC. The second stage is that Co coated WC particles agglomerate into the Co liquid matrix; this happens faster for the fine-grained WC. The next stage is the forming of a network of agglomerates. The capillary forces close the pores and contract the sample towards an equidimensional shape in competition with the gravitational force, which forces the WC particles to sediment vertically. The last stage is the slight growth of the original WC grains from the tungsten and carbon dissolved in the cobalt.

Fig. 12. Changes in microstructure of sintered samples (a) WC-7.5wt%Co at 1 GPa, (b) WC-7.5wt%Co at 3 GPa, (c) WC-12wt%Co at 1 GPa and (d) WC-12wt%Co at 3 GPa.
