## 1 Introduction  
Graphite-aluminum composites (G/Al) present good machinability [1], self-lubrication [1] and low friction coefficient [2], good friction and wear resistance [2, 3], improved mechanical properties [4], high specific strength and high specific rigidity [5, 6], as well as low thermal expansion and high damping capacity [7]. These characteristics provide potential applications of the G/Al, especially in the fields where excellent mechanical properties and wear resistance are required.  
In the past decades, preparation of the G/Al attracted considerable attention. As is known, graphite is easy to float in molten aluminum and the wettability between graphite and Al or Al alloy is very low [8], which increases the tendency of inhomogeneous distribution of graphite and of the formation of porosity at the interface of graphite and the matrix. Keeping the aluminum matrix solid during preparation (powder metallurgy, including pressure-free sintering [9, 10], hot pressing sintering [11, 12, 13, 14], spark plasma sintering [7, 15], electrical resistance heat assisted pressing [16]) may reduce these defects and provide more superior performances. However, there are still porosities formed during the powder metallurgy process, which has a detrimental effect on the property [17, 18]. Researchers believe it is important to decrease the porosity and increase the relative density, which can be achieved by using coated graphite [15], hot deforming after sintering [9], optimizing sinteringtemperature [7] and sintering under pressure [19]. The last two are important sintering parameters that have been verified to have a great influence on relative density. [PERSON] [7] reported that relative density was raised from 89.5% to 97.1% when sintering temperature was optimized. The literature [14] documented that the relative density was higher than 97% when sintering at a high temperature (500 \({}^{\circ}\)C) and high loading (150 MPa). [PERSON] [3] reported that the relative density higher than 99% could be generated by sintering at 500 MPa. It seemed that both the sintering temperature and the sintering pressure were helpful for densification, but not enough attention was focused on the G/Al sintered at a high load and the consequent properties. Furthermore, the mechanical performances of the G/Al composites still need to be improved, although high relative density was obtained in previous investigations [3; 5; 9].  
Thus, preparation of the graphite-aluminum alloy composite by using ultra-high pressure (UHP) sintering at pressure of 5.2 GPa was investigated in order to achieve the compact composite and its consequent excellent properties. Considering that the properties of the composite could be further improved by heat treatment in later application, the commonly used 6061 Al was chosen to be the matrix because of its good comprehensive performance, good formability and good heat treatability. To obtain homogeneous dispersion of graphite in the composite, graphite flakes coated with nano-aluminum (NAGF) powders were used. Correspondingly, microstructure, relative density, hardness, tensile strength and wear resistance of the compact graphite-6061 Al composite (G/6061 Al) were investigated and compared with that of the G/6061 Al hot pressing (HP)- sintered.