**2. Materials processing**

**1. Introduction**

physical vapor deposition and so on.

34 Advances in Functionally Graded Materials and Structures

through spark plasma sintering (SPS).

this method in our previous studies [3, 4].

New processing routes for metal-matrix functionally graded materials (FGMs) and structures through combinations of powder metallurgy and casting are described in this chapter. FGMs are well known as a relatively new class of inhomogeneous composite materials having property gradient. The property gradient in the FGMs is caused by a position-dependent chemical composition, microstructure, or atomic order [1]. These FGMs are generally fabri‐ cated based on powder metallurgy, melt-processing technique, chemical vapor deposition,

**Figure 1.** A schematic illustration showing a typical fabrication process of FGMs by the powder metallurgy method

Figure 1 shows a schematic illustration of a typical fabrication process of FGMs by the powder metallurgy method through spark plasma sintering (SPS). At first, mixed powders with various ratios of materials A and B are prepared. Predesigned mixed powders are stacked inside a die for the SPS as shown in Figure 1. The case of six-graded composition layers is shown in Figure 1. The number of graded layers can be freely chosen. Then, FGMs with stepwise graded structure can be obtained by sintering these powders with an SPS machine. Ti–ZrO2 FGMs with stepwise graded structure were fabricated by this method in our previous study [2]. A continuous graded structure can also be obtained by this method with a green body having continuous graded composition. For example, Ti–ZrO2 FGMs were fabricated by

### **2.1. Centrifugal mixed-powder method**

Many attempts to fabricate FGMs have been done by the centrifugal casting [5–7]. Generally, the finer dispersed particle size becomes, the more difficult to disperse them into molten matrix. The equation for velocity of a solid particle in a viscous liquid can be written as:

$$\frac{\mathbf{dx}}{\mathbf{dt}} = \frac{\left|\rho\_{\mathrm{p}} - \rho\_{\mathrm{m}}\right| \mathrm{G} \mathrm{g} \mathrm{D}\_{\mathrm{p}}^{2}}{18\eta} \tag{1}$$

where *ρ*p is density of particles, *ρ*m density of molten matrix, *g* gravitational acceleration, *D*<sup>p</sup> particle diameter, and *η* viscosity of melt [7]. Since the velocity of a solid particle in a viscous liquid is dependent on the square of the particle diameter *D*p, it is quite difficult to control graded distributions of dispersion nanoparticles in FGMs in the case of the conventional centrifugal casting. As a new processing technique for metal-matrix FGMs, the centrifugal mixed-powder method is proposed by Watanabe et al. [12] for overcoming these problems. The centrifugal mixed-powder method could give us fine particle-dispersed FGMs by using a combination of high centrifugal force and mixed powder. This new method is a devel‐ oped technique of the centrifugal casting by setting predesigned mixed powder in a mold in advance [12].

**Figure 2.** A schematic illustration showing the process of the centrifugal mixed-powder method [12].

Figure 2 shows the experimental procedure of the centrifugal mixed-powder method. At first, a predesigned mixed powder is prepared. This mixed powder consists of metal-matrix particles and dispersion particles. Basically, the melting point of dispersion particles should be higher than that of metal-matrix particles to form FGMs. Particles such as ceramics, metals, and alloys that have higher melting points compared with metal matrix can be chosen as dispersion particles for metal-matrix FGMs. The mixed powder including metal-matrix particles and dispersion particles is inserted into a spinning mold as shown in Figure 2(a). After that, a metal-matrix ingot is melted in a crucible. This molten metal matrix is poured into the spinning mold as shown in Figure 2(b). The poured molten metal matrix penetrates into the space between the particles due to the applied centrifugal force as shown in Figure 2(c). The heat from the poured molten matrix melts the metal-matrix particles as shown in Figure 2(d). Finally, ring- or disc-shaped FGMs or structures having dispersion particles distributed in the outer part of the cast sample can be obtained as shown in Figure 2(e). FGMs, such as Cu/ SiC [12], Al/TiO2 [12], and Al/Al3Ti/Ti [15], were obtained with this processing method in our previous studies.

The centrifugal mixed-powder method can also be performed by using centrifugal casting machines which are commercially available. Figure 3 shows a typical appearance of vacuum centrifugal casting machine supplied by Yasui & Co, Japan. This centrifugal casting machine has a heating coil, a straight arm, a crucible, a mold, and a balancer inside the casting chamber [18]. By setting predesigned mixed powder in the mold in advance, FGMs can be obtained. By using this processing method, Cu/diamond [13], Al alloy/diamond [14], and the other FGMs New Processing Routes for Functionally Graded Materials and Structures through Combinations of... http://dx.doi.org/10.5772/62393 37

**Figure 3.** A typical appearance of vacuum centrifugal casting machine.

were fabricated in our previous studies. Detailed processing method and microstructural characterization of fabricated Cu/diamond FGMs are shown below.

**Figure 4.** A cross-sectional drawing of a mold for centrifugal casting.

**Figure 2.** A schematic illustration showing the process of the centrifugal mixed-powder method [12].

36 Advances in Functionally Graded Materials and Structures

previous studies.

Figure 2 shows the experimental procedure of the centrifugal mixed-powder method. At first, a predesigned mixed powder is prepared. This mixed powder consists of metal-matrix particles and dispersion particles. Basically, the melting point of dispersion particles should be higher than that of metal-matrix particles to form FGMs. Particles such as ceramics, metals, and alloys that have higher melting points compared with metal matrix can be chosen as dispersion particles for metal-matrix FGMs. The mixed powder including metal-matrix particles and dispersion particles is inserted into a spinning mold as shown in Figure 2(a). After that, a metal-matrix ingot is melted in a crucible. This molten metal matrix is poured into the spinning mold as shown in Figure 2(b). The poured molten metal matrix penetrates into the space between the particles due to the applied centrifugal force as shown in Figure 2(c). The heat from the poured molten matrix melts the metal-matrix particles as shown in Figure 2(d). Finally, ring- or disc-shaped FGMs or structures having dispersion particles distributed in the outer part of the cast sample can be obtained as shown in Figure 2(e). FGMs, such as Cu/ SiC [12], Al/TiO2 [12], and Al/Al3Ti/Ti [15], were obtained with this processing method in our

The centrifugal mixed-powder method can also be performed by using centrifugal casting machines which are commercially available. Figure 3 shows a typical appearance of vacuum centrifugal casting machine supplied by Yasui & Co, Japan. This centrifugal casting machine has a heating coil, a straight arm, a crucible, a mold, and a balancer inside the casting chamber [18]. By setting predesigned mixed powder in the mold in advance, FGMs can be obtained. By using this processing method, Cu/diamond [13], Al alloy/diamond [14], and the other FGMs

A cross-sectional drawing of a mold for centrifugal casting is shown in Figure 4. The mold has a cylindrical casting pattern with 40 mm width and 22.8 mm diameter. A cylindrical core with 15 mm width and 12 mm diameter is also attached in the mold as shown in Figure 4. Since fabricated FGMs can be applied to grinding wheel for mechanical machining as described in Section 3, these pattern and core are required. Dendritic-shaped Cu particles in the mean particle diameter of approximately 22 μm and 100/120 mesh diamond particles (149 μm in JIS B 4130) were used. Both the particles were mixed in a mortar. The volume fraction of diamond to Cu was chosen as 25 vol.%. The mixed powder was inserted into the mold as shown in Figure 4. Then, molten Cu was cast into the spinning mold by applying centrifugal force with the vacuum centrifugal casting machine in vacuum at 1473 K and 1573 K. The mold was spun for 99 s. The calculated applied G number (ratio of centrifugal force to gravity) at the top of mold along the direction of centrifugal force was about 36 G.

Figure 5 shows Cu/diamond FGMs cast at 1473 K (Fig. 5a) and 1573 K (Fig. 5b) [13]. As these samples were fabricated for an application as grinding wheel, these cast samples have hollows for attaching pulley. It was observed that consolidated mixed-powder area kept leaning to the

**Figure 5.** Cu/diamond FGMs fabricated by the centrifugal mixed-powder method. Casting temperatures were 1473 K (a) and 1573 K (b) [13].

right side of the cast sample, that is, the position of maximum centrifugal force as shown in Figure 5. Since density of diamond (3.52 Mg/m3 ) was smaller than that of molten Cu (8.00 Mg/ m3 ), a little amount of diamond particles were distributed around surface at sprue side due to the molten metal flow. A graded structure should be made by this difference of density between diamond and Cu.

Cross-sectional observations were carried out with scanning electron microscope (SEM) to investigate diamond dispersion behavior after casting at 1473 K. Figure 6 shows a backscat‐ tered electron compositional image showing a cross section of the Cu-based diamond graded cast sample fabricated without pulley hollow [13]. Diamond particles were distinguished from Cu matrix as black colored area in the sample. It should be noted that the distribution of diamond particles in the inner part of the cast sample also biased to the top side (right side) as shown in Figure 6. It was also confirmed that obvious traces or boundaries of Cu particles were not observed although voids were seen around some diamond particles. Therefore, Cu particles in the predesigned mixed powder were fully melted and fused each other due to heat transfer from the poured molten Cu.

The number of diamond particles and the mean diameter of diamond particles at each divided area along the direction of centrifugal force were measured. The results are shown in Figures 7 and 8, respectively. These data were taken from the cross-sectional image of the cast sample New Processing Routes for Functionally Graded Materials and Structures through Combinations of... http://dx.doi.org/10.5772/62393 39

**Figure 6.** A backscattered electron compositional image showing a cross section of the Cu-based diamond graded cast sample obtained by the centrifugal mixed-powder method [13].

right side of the cast sample, that is, the position of maximum centrifugal force as shown in

**Figure 5.** Cu/diamond FGMs fabricated by the centrifugal mixed-powder method. Casting temperatures were 1473 K

Cross-sectional observations were carried out with scanning electron microscope (SEM) to investigate diamond dispersion behavior after casting at 1473 K. Figure 6 shows a backscat‐ tered electron compositional image showing a cross section of the Cu-based diamond graded cast sample fabricated without pulley hollow [13]. Diamond particles were distinguished from Cu matrix as black colored area in the sample. It should be noted that the distribution of diamond particles in the inner part of the cast sample also biased to the top side (right side) as shown in Figure 6. It was also confirmed that obvious traces or boundaries of Cu particles were not observed although voids were seen around some diamond particles. Therefore, Cu particles in the predesigned mixed powder were fully melted and fused each other due to heat

The number of diamond particles and the mean diameter of diamond particles at each divided area along the direction of centrifugal force were measured. The results are shown in Figures 7 and 8, respectively. These data were taken from the cross-sectional image of the cast sample

), a little amount of diamond particles were distributed around surface at sprue side due to the molten metal flow. A graded structure should be made by this difference of density

) was smaller than that of molten Cu (8.00 Mg/

Figure 5. Since density of diamond (3.52 Mg/m3

38 Advances in Functionally Graded Materials and Structures

between diamond and Cu.

(a) and 1573 K (b) [13].

transfer from the poured molten Cu.

m3

**Figure 7.** The number of diamond particles as a function of distance from the top of Cu/diamond cast sample [13].

as shown in Figure 6. In Figure 7, the number of diamond particles was drastically decreased at around 3 mm from the top, where the mixed powder was inserted before the centrifugal casting. The result indicates that the mixed powder was compressed and immobilized by pressure of molten metal due to centrifugal force. Whereas diamond particles were sufficiently

**Figure 8.** The particle diameter as a function of distance from the top of Cu/diamond cast sample [13].

immobilized by Cu particles between diamond particles, graded distribution of diamond particles was successfully obtained. Within the diamond particles densely dispersed region between 0 and 3 mm from the top of the Cu/diamond cast sample, the number of diamond particles increased with approaching to the top of the Cu/diamond cast sample. Thus, the Cu/ diamond FGMs were successfully fabricated by the centrifugal mixed-powder method. On the other hand, the particle diameter distribution between 0 and 3 mm is almost homogeneous as shown in Figure 8. The mean diameters of diamond particles in this range are 80–100 μm. In the distance from the top, between 3 and 5 mm, particle diameter distribution is also homo‐ geneous. However, the mean diameters of diamond particles in this range are 30–40 μm. These results may suggest that collision of Cu molten metal with the mixed powder at surface of the powder area washed away part of the diamond particles, and molten Cu flow sent it to the surface at the sprue side. This phenomenon is not appropriate for production of FGMs. To overcome this problem, a modified processing method is described in the next section.

#### **2.2. Centrifugal sintered-casting method**

As the latest processing method for metal-matrix FGMs developed by our research group, centrifugal sintered-casting method is shown in this section. The centrifugal sintered-casting method is a modified processing technique of the centrifugal mixed-powder method. In the centrifugal sintered-casting method, FGMs are processed by the combination of centrifugal sintering and centrifugal casting [16, 17]. As described in Section 2.1, the centrifugal mixedpowder method enables us to fabricate metal-matrix FGMs. Especially, the centrifugal mixedpowder method is an effective way to fabricate metal-matrix FGMs reinforced with nanoparticles [12]. However, predesigned powder mixtures tended to flow away during the centrifugal casting in the case of some combinations of powders in the centrifugal mixedpowder method. As an attempt to overcome this problem, the centrifugal sintered-casting method is developed through the combination of centrifugal sintering and centrifugal casting.

**Figure 9.** A schematic illustration showing the process of the centrifugal sintered-casting method [17].

immobilized by Cu particles between diamond particles, graded distribution of diamond particles was successfully obtained. Within the diamond particles densely dispersed region between 0 and 3 mm from the top of the Cu/diamond cast sample, the number of diamond particles increased with approaching to the top of the Cu/diamond cast sample. Thus, the Cu/ diamond FGMs were successfully fabricated by the centrifugal mixed-powder method. On the other hand, the particle diameter distribution between 0 and 3 mm is almost homogeneous as shown in Figure 8. The mean diameters of diamond particles in this range are 80–100 μm. In the distance from the top, between 3 and 5 mm, particle diameter distribution is also homo‐ geneous. However, the mean diameters of diamond particles in this range are 30–40 μm. These results may suggest that collision of Cu molten metal with the mixed powder at surface of the powder area washed away part of the diamond particles, and molten Cu flow sent it to the surface at the sprue side. This phenomenon is not appropriate for production of FGMs. To overcome this problem, a modified processing method is described in the next section.

**Figure 8.** The particle diameter as a function of distance from the top of Cu/diamond cast sample [13].

As the latest processing method for metal-matrix FGMs developed by our research group, centrifugal sintered-casting method is shown in this section. The centrifugal sintered-casting method is a modified processing technique of the centrifugal mixed-powder method. In the centrifugal sintered-casting method, FGMs are processed by the combination of centrifugal sintering and centrifugal casting [16, 17]. As described in Section 2.1, the centrifugal mixedpowder method enables us to fabricate metal-matrix FGMs. Especially, the centrifugal mixedpowder method is an effective way to fabricate metal-matrix FGMs reinforced with nanoparticles [12]. However, predesigned powder mixtures tended to flow away during the centrifugal casting in the case of some combinations of powders in the centrifugal mixed-

**2.2. Centrifugal sintered-casting method**

40 Advances in Functionally Graded Materials and Structures

Figure 9 shows a schematic illustration of the process of the centrifugal sintered-casting method [17]. In the centrifugal sintered-casting method, a ring-shaped metal-matrix preform with dispersed particles is produced by the centrifugal sintering at first. Predesigned mixed powder of dispersion particles and metal-matrix particles is inserted into a spinning mold as shown in Figure 9(a). Basically, the melting point of dispersion particles should be higher than that of metal-matrix particles to form FGMs in this method as well. Subsequently, the mixed powder is sintered under centrifugal force by heating coils to fabricate a preform as shown in Figure 9(b). Then, molten metal matrix is poured into the fabricated preform by the centrifugal casting to obtain metal-matrix FGMs as shown in Figure 9(c). The molten metal matrix penetrates into the space between the particles by the applied centrifugal force as shown in Figure 9(d). At the same time, the metal matrix particles are melted by the heat from the molten metal matrix. Finally, ring- or disc-shaped FGMs with dispersed particles distributed in the outer part of the samples can be obtained as shown in Figure 9(e).

In our previous studies, Al–Si and Cu were selected as metal matrix to fabricate Al–Si alloy/ diamond and Cu/diamond FGMs, respectively [16, 17]. Al–Si alloy particles and Cu parti‐ cles were uniformly mixed with diamond particles, respectively. The volume fraction of diamond particles in mixed powder was chosen as 10 vol.%. The predesigned mixed powder

was set in the cylindrical mold having a rotational axis of 20 mm diameter and 30 mm length, respectively. The mixed powders were sintered in the spinning cylindrical mold under the centrifugal force of about 280 G at 843 K in argon atmosphere for Al–Si alloy/diamond particles [16] and 1100 G at 1273 K in vacuum for Cu/diamond particles [17], respectively. Then, the centrifugal casting was performed under the centrifugal force of about 78 G at 1373 K with pouring molten Al in the case of Al–Si alloy/diamond preform [16]. In the same way, molten Cu was poured into the Cu/diamond preform in the mold under the centrifugal force of about 34 G at 1393 K [17].

**Figure 10.** Macrographs of Al–Si alloy based (a) and Cu-based (b) FGMs with dispersed diamond particles fabricated by the centrifugal sintered-casting method and SEM images showing the microstructures of the outer part of the cast samples [16, 17].

Figure 10 shows macrographs of Al–Si alloy and Cu-based FGMs with dispersed diamond particles fabricated by the centrifugal sintered-casting method. SEM images showing the microstructures of the outer part of the Al–Si alloy and Cu-based FGMs are also shown in Figure 10. It should be noted that the diamond particles were distributed at only outer part of the cast samples as shown in Figure 10. The centrifugal sintered-casting method is an effective way to fabricate metal-matrix FGMs.
