**3. Applications of FGMs processed by the centrifugal sintered-casting**

In this section, applications of metal-matrix FGMs processed by the centrifugal sinteredcasting method are introduced. As described in Section 2.2, Al–Si alloy/diamond and Cu/ diamond FGMs were fabricated by the centrifugal sintered-casting method. Diamond plays important role as abrasive in the field of mechanical machining. Recently, CFRP is widely used as main structural parts for aircraft due to its high strength, stiffness, and lightweight [19]. However, some issues about occurring defects, such as fiber pullout, delamination, burrs, and splintering, have become problems in machining CFRP by common drills. These technical issues have led numerous researchers to seek solutions for precision machining of CFRP [20– 23]. The key issue of precision machining of CFRP has been obtaining good hole quality in the aircraft industry. In addition, tool change is frequently required in drilling CFRP in many practical situations. Therefore, the improvement of tool life for machining CFRP is important subject since high-priced diamond grains have been widely used as abrasive.

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

**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

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

**3. Applications of FGMs processed by the centrifugal sintered-casting**

In this section, applications of metal-matrix FGMs processed by the centrifugal sinteredcasting method are introduced. As described in Section 2.2, Al–Si alloy/diamond and Cu/

of about 34 G at 1393 K [17].

42 Advances in Functionally Graded Materials and Structures

samples [16, 17].

way to fabricate metal-matrix FGMs.

**Figure 11.** A schematic illustration of the gyro-driving grinding wheel system [24].

A novel CFRP machining equipment, that is, gyro-driving grinding wheel system for machin‐ ing CFRP was recently developed [24–26]. The gyro-driving grinding wheel system was developed for overcoming problems related to defect issues during mechanical machining. Figure 11 shows a schematic illustration of the gyro-driving grinding wheel system [24]. In the gyro-driving grinding wheel system, a grinding wheel is used instead of drill bits for drilling CFRP. The equipped grinding wheels required toughness as a desirable mechanical property as the grinding wheel was subjected to the torsion force in the gyro-driving grinding wheel system. In the previous studies [17, 24], holes with good quality in CFRP plates have been obtained without defects by this machining system equipped with our fabricated metal-matrix FGMs.

CFRP drilling tests were performed with the gyro-driving grinding wheel system equipped with fabricated Cu/diamond FGMs as a grinding wheel. Cu was selected as metal matrix for its mechanical properties and high thermal conductivity. Diamond particles were used as abrasive in Cu matrix for machining CFRP. CFRP drilling tests were carried out with feed rate of 5 mm/min, peripheral wheel speed of 7000 rpm, spindle speed of 2800 rpm, and dry machining. Bidirectional CFRP composite laminates having thickness of 5 mm were used as workpiece material. Photographs of a hole having diameter of 20 mm drilled by the gyrodriving grinding wheel system equipped with fabricated Cu/diamond FGMs as a grinding wheel (Fig. 12a) and the one having diameter of 10 mm drilled by a conventional drill bit (Fig. 12b) in CFRP plates are shown in Figure 12. Delamination and burrs were seen in the drilled CFRP plate in the case of the conventional drill bit. It should be noted that precision drilling of CFRP plate without burring and delamination were achieved by the gyro-driving grinding wheel system equipped with fabricated Cu/diamond FGMs as grinding wheel. In this way, FGMs fabricated by the centrifugal sintered-casting method have been attempted to apply for the practical use.

**Figure 12.** Drilled hole having diameter of 20 mm made by the gyro-driving grinding wheel system equipped with fabricated Cu/diamond FGMs as a grinding wheel (a) and the one having diameter of 10 mm made by a conventional drill bit (b) in CFRP plates [17].

As the other possibilities for application of FGMs, materials for heat sink can be considered as candidate. Nowadays, thermal management materials such as heat sink for microelectronics and semiconductors have been investigated, extensively [27–30]. The materials currently used for heat sinks are Al and Cu due to their high thermal conductivity in metals and alloys. The thermal conductivities of Al and Cu are about 250 and 400 W m−1 K−1, respectively. On the other hand, diamond is well known as the material having the highest thermal conductivity in materials. To enhance the thermal conductivity of heat sink materials, Al/diamond and Cu/ diamond composites are promising materials. Our Al alloy/diamond and Cu/diamond FGMs fabricated by the centrifugal sintered-casting method might work as well in this field.

### **4. Summary**

Two kinds of new processing routes for metal-matrix FGMs through combinations of powder metallurgy and casting were developed: the centrifugal mixed-powder method and the centrifugal sintered-casting method. Metal-matrix FGMs were obtained by these two methods. These processing methods enable us to overcome existing problems in the conventional fabrication process of FGMs. Fabricated FGMs were also applied to machining CFRP as an attempt for the practical use. Continued studies for fabrication processes of FGMs and the gyro-driving grinding wheel system are still required to put them into practical use in the future. Further investigations should open up a new field of and a market for FGMs.
