Effect of ball milling on graphene reinforced Al6061 composite fabricated by semi-solid sintering
Introduction
Since the second half of the twentieth century, metal matrix composites (MMCs) have been considered as one of the important materials. They are favored with superior properties compared with unreinforced metals and potentially offer ways to provide materials of higher strength-to-weight ratio, lower thermal expansion coefficient, and higher resistance to thermal fatigue and creep [1], [2]. MMCs have made their ways into various applications in aerospace, electronic packaging, and automotive industries [3], [4]. Recently, MMCs reinforced with nano-elements have attracted the interest of many researchers [5], [6], [7], [8], [9], [10]. Graphitic structured materials like carbon nanotubes (CNTs), graphite, and graphene have been among the more widely researched materials due to their exceptional mechanical [11], thermal [12], [13], electrical properties [14], and tribological behavior [15], [16]. Moreover, improved manufacturing techniques have made these materials more affordable [17], [18], [19], [20], [21].
Various studies can be found in which CNTs were used as a reinforcement medium with different base metals like copper [8], [22], [23], aluminum [5], [6], [15], [19], [20], [24], [25], [26], and their alloys [17], [18], [21], [27]. Although some studies report significant mechanical property enhancement, CNT reinforced MMCs face various challenges. The uniform dispersion of CNTs and wetting between the CNT and the metal have been among the major concerns. Various researchers have used mechanical alloying (ball milling) as an effective means to disperse the CNTs [19], [24], [28], [29], [30], [31], [32]. Wu and Kim et al. [21], Esawi et al. [24], [25], and Wang et al. [33] have investigated the effects of the mechanical alloying time on the dispersion of CNTs in ball milling. Kim et al. has investigated the effects of milling time on the CNT structure, and it was reported that the length of CNTs shortened significantly with increasing milling time [21], [34]. Graphene, being the basic structural element for the CNT, also has a great potential as a reinforcing material but with a different form factor. The graphene is favored by excellent mechanical properties and high electrical and thermal conductivities [33]. Not much research, however, has been found on synthesis of metal–graphene composites and on understanding of graphene dispersion on mechanical properties.
In this study, a semi-solid processing technique has been used to synthesize an aluminum alloy composite reinforced by few-layer graphene oxide that has been manufactured by the modified Brodie’s method [35]. The aluminum–graphene composite was synthesized in the semi-solid state of the aluminum alloy by pressure-assisted sintering. The technique has given good results for the aluminum–CNT composite in earlier studies performed by the authors [21], [34], [36], [37]. A mechanical milling process was used to disperse the graphene in the matrix phase and its influence on mechanical properties and microstructure has been investigated.
Section snippets
Materials and methods
Graphite was expanded to exfoliate graphene according to the modified Brodie’s method. First, 10 g of graphite, 160 ml of nitric acid, and 85 g of sodium chlorate were mixed at room temperature. The mixture was kept for 24 h under continuous stirring. Then it was washed with 5% hydrochloric acid and distilled water for four times. The intercalated graphite was achieved through sedimentation and finally was dried at 60 °C. With the aid of ultrasonication, the intercalated graphite was exfoliated to
Results and discussion
With the aid of sonication, the intercalated graphite was exfoliated to few-layer graphene, while some of them were in the monolayer state. A transmission electron microscopy (TEM) image of few-layer graphene is shown in Fig. 2.
A portion of the mechanically alloyed powder was extracted at the specified milling times (30, 60, and 90 min) for analysis under the SEM. As shown in Fig. 3, the alloyed particle size increased with longer milling times. However, the graphene size decreased as the
Conclusions
In this study, the Al6061-1.0 wt.% graphene composites were fabricated by ball milling Al6061 particles and graphene, followed by pre-compaction at room temperature, and finally by hot compaction in the semi-solid regime. The ball milling time varied from 10 min to 90 min. The 10- and 30-min ball milling times were not enough to homogeneously disperse the graphene into the Al6061 matrix, which resulted in degradation of the flexural strength for the 10-min milling time sample and no enhancement
Acknowledgments
The authors greatly appreciate the financial support from the United States National Science Foundation (CMMI-1030120) and Defense Advanced Research Projects Agency (N66001-12-1-4257). Finally, the authors would like to thank the valuable discussions and support from Warren Straszheim, Michael Martin, and Jie Wang.
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