Metallurgical Assessment of Graphite Reinforced Al-Cu-Mg Nanocomposites Produced by the Shaker Mill Method

1. D. Kartika , H. Rudianto ^*

Advance Materials Laboratory, Department of Mechanical Engineering, Gunadarma University, Jalan Margonda Raya Number 100, Depok 16424, West Java, Indonesia

1.     Introduction

Nanocomposites refer to materials that consist of a nanocrystalline structure of small nanocrystals incorporated within or between the nanoscale matrix grains [1]. Nanoscale second-phase particles such as borides, carbides, nitrides, oxides, and hydrides are commonly incorporated in these nanocomposites [2]. Nanocomposite materials are usually classified on the basis of the physical or chemical nature of the matrix phase, e.g., polymer matrix, metal-matrix, and ceramic nanocomposites [3,4].

Aluminum matrix nanocomposites are widely used because of their excellent combination of properties [5,6]. Applications of such materials take place in automobile, aerospace, defense, and other related sectors. In the automobile sector, aluminum composites are used to produce various components such as brake drums, cylinder liners, cylinder blocks, and driveshafts [7,8]. Among the various matrix materials available, aluminum and its alloys are widely used in the fabrication of metal matrix nanocomposites and have reached the industrial production stage. The emphasis has been given to developing affordable aluminum-based metal matrix nanocomposites with various hard and soft reinforcements (SiC, Al₂O₃, graphite, and mica) because of the likely possibilities of these combinations informing highly desirable composites.

There are many ways to prepare these nanocomposites, among which an economical way of producing metal matrix nanocomposites is mechanical alloying [1]. In the 1960s mechanical alloying (MA) technique was used for the production of an oxidation-resistant alloy. Mechanical alloying (MA) is a powder processing technique that allows the production of homogeneous materials starting from blended elemental powder mixtures [9,10]. The MA is a unique process in that a solid-state reaction takes place between the fresh powder surfaces of the reactant materials at room temperature [11]. Consequently, it can be used to produce alloys and compounds which are difficult or impossible to be obtained by the conventional melting and casting technique [12].

At atmospheric pressure, graphite is more stable form of carbon, because of the very stable bonds between the atoms, graphite is very difficult to bond with another element [13-15]. Many researchers have been investigating the manufacturing process of carbon-based materials reinforced aluminum matrix nanocomposites. CNT, graphene, graphite, and diamond have been used as reinforcements to produce aluminum metal matrix nanocomposites. Agglomeration due to its strong affinity between particles has been a challenge to manufacture nanocomposites [16-18]. High energy ball mill is commonly used to produce graphite-reinforced aluminum matrix nanocomposites. But it leads to some unnecessary disadvantages such as manufacturing time.

The shaker mill technique is able to produce homogenous nanoparticles in such a short time because of the continuous process of fracturing and welding of powder by ball mill at high speed. Agglomeration of graphite or any other carbon-based reinforcement materials is the main challenge in manufacturing this kind of nanocomposites due to its very high stability. The Shaker mill technique is expected to successfully produce graphite-reinforced aluminum matrix nanocomposites in this research. Intermetallic compounds are also expected to form as a result of the milling process such as Al₂Cu, Al₃Mg₂, and Al₄C₃ in aluminum alloys cases [19-23].

In this study, Al-4.5Cu-1.5Mg powder was used as a matrix material and graphite was used as reinforcement material. AA2024 is one of the aluminum alloy series that have high mechanical properties and high thermal stability. The purpose of this research was to investigate the shaker mill technique to manufacture graphite-reinforced Al-4.5Cu-1.5Mg metal matrix nanocomposite. Sintering and mechanical properties were also investigated to determine the effects of the graphite and shaker mill technique.

2.     Experimental

2.1. Material Preparation

Elemental powders as matrices were prepared with composition ratio as shown in Table 1 below:

Table 1. Al-Cu-Mg Matrix Composition

Graphite was used with compositions of 0.1, 0.5, 1, and 3wt%. All powders were produced by Zhok Material Technical Co., Ltd in China with specifications in Table. 2 as follows:

Table 2. Powder specifications

The theoretical density of compounds was calculated by using the formula below:

                                                              (1)

where m_a and m_b are the mass of each element, ρ_a and ρ_b are the theoretical density of each element in the alloy.

And the morphology of graphite is shown below;

Fig. 1 Morphology of graphite

The powders were milled in a shaker mill with the ball-to-powder weight ratio of 10:1 under argon gas for 8 hours at 750 rpm. A ceramic ball mill and Teflon or polytetrafluoroethylene jar were used during milling.

Milled powder was compacted with single action press at 500 MPa. Stearic acid was added for 1.5wt% of the total weight to the powder specimen as a lubricant to reduce friction during the compaction process. Green density was measured by measuring dimension and mass. Sintering was carried out under ultra-high purity argon gas at 550^oC, 580^oC, 600^oC, and 620^oC. Sintering density was obtained by using the Archimedes method.

2.2. Microstructure and Characterization

Particle size analyzing (PSA) test was used to measure the particle size distribution of nanocomposite powder after the shaker mill process. This process used isopropanol as a solvent and is carried out at room temperature using PSA equipment.

A scanning electron microscope (SEM) was used to characterize the morphology of the powder. SEM instrument with vacuum measurement parameters of 20 kV and 50 intense spots.

To characterize the thermal properties of the powder and estimate the optimal temperature for the sintering process, DSC-TGA was used using 99.999% ultra-high purity argon gas with a temperature of 20-800°C and a heating rate of 10°K / minute.

2.3. Density and Hardness Test

Vickers hardness tests were carried out to investigate mechanical properties. using Vickers microhardness tool with ASTM E92-16 standard. The sintering density samples were measured by Archimedes principle according to ASTM B961 standard and calculated by the following formula:

3.     Result and Discussion

3.1. Powder Characterization

Fig. 2. Particle morphology of Al-4.5Cu-1.5Mg with 0.5wt% graphite after shaker mill process.

The powder was characterized to identify its morphology as shown in Fig 2. The Figure shows the irregular shape of the powder. The irregular shape of the particles has the advantage during the compacting process. Because when compressing, the powder is able to fill almost all voids between them leading to lower porosity meaning it is easier for sintering. Damaging, fracturing, and cold welding happened during the shaker mill at such a shorter time. Due to high-speed milling, the cold welding process is applied when all elementals will be diffused from each other to form intermetallics.

According to particle size analyzer results shown in Table 3, it shows that the average particle diameter after 8 hours of shaker mill was 112.521 nm. This result shows that the shaker mill technique was able to produce graphite-reinforced Al-4.5-1.5Mg nanocomposite. But the range between D 10%, D 50%, and D 90% shows non-homogenous particle size. Agglomeration of nanoparticles is still remained a challenge to characterize the particle size of the nanoparticle.

Table 3. particle size distribution of Al-4.5Cu-1.5Mg with 0.5wt% graphite nanocomposite

Investigating the thermal properties of this powder is very important in order to be able to estimate the optimal temperature for sintering and to know the delubrication temperature.

Figure 3 shows DSC-TGA curves of Al-4.5Cu-1.5MG with 0.5wt% graphite nanocomposites. From the DSC curve (blue line), it can be seen when the phase transformation occurs from the exothermic and endothermic reactions. At a temperature of 477°C is the exothermic reaction where oxide removal reaction occurred.

Fig. 3. DSC-TGA Curves of Al-4.5Cu-1.5Mg with 0.5wt% graphite nanocomposite

This reaction breaks the oxide layer on aluminum powder due to its high thermal expansion compared to the oxide layer. High internal pressure in aluminum at high temperatures is able to break the oxide layer on its surface. And this is beneficial for the sintering of aluminum powder, due to the high melting point of the oxide layer, it can prohibit the diffusion process during sintering between aluminum particles. With continued heating a strong endothermic peak appears at around 650 C, indicating the melting of the matrix alloy. At the TGA graph (black line), it seems the mass of powder decreased starting from 300°C to 600°C, indicated is a stearic acid that evaporates. The addition of stearic acid as a lubricant to the powder aims to reduce the friction in the compacting process so that no slip occurs which can affect diffusivity during sintering.

To see the phase transformation after the shaker mill process, x-ray diffraction was carried out as shown in Figure 4.

Fig. 4. X-Ray Diffraction Pattern of Al-4.5Cu-1.5Mg with 0.5wt% graphite nanocomposite after shaker mill

The graph shows the dominant peaks are Aluminum. In addition, the shaker mill process produces several intermetallic compounds such as Al₃Mg₂ and Al₂Cu which is advantageous due to its strong mechanical properties leading to better mechanical properties of the matrix [24]. AlF₃ was found as one of the impurities coming from shaker mill equipment. The jar is made with polytetrafluoroethylene material which expected fluorine will be one of the impurities for this nanocomposite powder but the effects of fluorine on the properties of nanocomposite powder is still a question.  Aluminum carbide, Al₄C₃ was found as result coming from the reaction between aluminum and graphite during the shaker mill. It was also expected to form Al₄C₃ after a high-energy ball mill which was shown by researchers. Al₄C₃ acts as an interface between the matrix and reinforcement and is able to receive load or stress coming to the matrix and leading to better mechanical properties of the matrix [16]. The formation reaction of Al₄C₃ is as follows;

4Al + 3C à Al₄C₃                                                       (3)

3.2. Green Compact Characteristic

From Figure 5 it can be seen that the best green density is in the graphite fraction of 0.5wt% which is equal to 73.7% relative. Green density started to low at 1 and 3wt% graphite. One of the main reasons is agglomeration between graphite particles. The challenge of dispersion of graphite at higher content will lead to limited application of graphite-reinforced aluminum matrix nanocomposite. In the sintering of aluminum powder, green density plays an important role in getting high sintering and mechanical properties. Some researchers showed that high green density leads to high sintering and mechanical properties. 

Fig. 5. Green density of graphite reinforced Al-4.5Cu-1.5Mg compacted at 500 MPa

3.3. Sintering Behavior

Sintering is the process of binding a compacted particle by using a thermal force below the melting point to produce high-density material. The sintering process of this powder was carried out at 550 , 580 , 600 , and 620  for 1 hour under ultra-high purity argon gas for 99.999%. Figures 6 a and b show the sintering behavior of graphite-reinforced Al-4.5Cu-1.5Mg matrix nanocomposite with variations of graphite and temperature respectively.  Comparable to Figure 5, in Figure 6a, the optimum sintering density was obtained with 0.5wt% graphite for 77% relative.

The sintering temperature is also one of the important factors in the success of the sintering of aluminum powder. For this nanocomposite, it seems the most suitable sintering temperature is 550^oC. According to DSC-TGA in Figure 3, the sintering temperature range can be predicted. Depending on chemical compositions, the optimum sintering temperature range is 550-600^oC. One of the main challenges in obtaining higher sintering properties of graphite-reinforced aluminum matrix nanocomposite is the agglomeration of graphite particles. This agglomeration will cause porosity which leads to lower sintering properties and it shows on microstructures in Figure 7.

Porosity remains visible throughout the surface of the cross-section of the nanocomposite. From Figures 7a and 7b also some intermetallics are visible with white color. The intermetallic dispersed homogenously throughout the surface, inside and in between grain boundaries. Sintering at such high temperatures can facilitate some reactions between elements. At 620^oC, there is grain growth with visible bigger grains. Too high sintering temperature will result in more porosity because of Ostwald ripening phenomena. This phenomenon occurs when particles diffuse each other at higher temperatures. Ostwald ripening leads to the dissolution of smaller solid grains, diffusion of the solute through the liquid, and the re-precipitation of the solid onto large grains [25].

Fig. 6. a) Sintering density of Al-4.5Cu-1.5Mg with content of graphite for 0.1,0.5,1 and 3wt% sintered at 550^oC and
 b) Sintering density of Al-4.5Cu-1.5Mg with 0.5wt%
graphite sintered at 550^oC, 580^oC, 600^oC, and 620^oC

To investigate intermetallic phases during sintering, X-Ray Diffraction was carried out as shown in Figure 8. Aluminum is the dominant peak followed by the formation of intermetallics such as Al₂Cu, Al₃Mg₂, and aluminum carbide (Al₄C₃). Al₂Cu is a common intermetallic found in the aluminum-copper alloy. Al₂Cu after sintering has a stronger presence compared to after milling. this indicates high sintering temperatures facilitate the reaction between aluminum and copper to form Al₂Cu. Al₃Mg₂ has little presence compared to Al₂Cu due to its lower content on the alloy. The Al₄C₃ particles which were the primary precipitates in the aluminum– graphite played a major role in improving the mechanical properties. The presence of aluminum carbide (Al₄C₃) in these nanocomposites has the advantage of transferring the load between the matrix to the reinforcement, because of the fragile nature of aluminum carbide, the formation of aluminum carbide can increase aluminum rigidity. AlF₃ was also formed which was identified as an impurity. These impurities generally come from friction between the ball and the Teflon jar during the milling process.

Vickers hardness test was carried out to investigate one of the mechanical properties of this nanocomposite. Optimum Vickers hardness was obtained at 550^oC sintering temperature for 159.4 HVN and decreased as the sintering temperature goes up as shown in Table 4. This indicates that hardness and sintering density have some correlation with determining the sintering properties of the aluminum nanocomposite. To determine how the Vickers hardness test was carried out, Figure 8 shows 5 indentation marks on this nanocomposite sintered at 550^oC. A high number hardness number will decrease the area of the indentation mark meaning it has high resistance to permanent deformation.

Fig. 7 Microstructure image of Al-4.5Cu-1.5Mg with 0.5wt% graphite nanocomposite sintered at
a) 550^oC and b) 620^oC

Fig. 8. X-Ray diffraction pattern of Al-4.5Cu-1.5Mg with 0.5wt% graphite nanocomposite after sintering at 550^oC

Fig. 9 Vickers hardness indentation of Al-4.5Cu-1.5Mg with 0.5wt% graphite nanocomposite sintered at 550^oC

Table 4. Vickers hardness value to compacting pressure of the nanocomposites

4.     Conclusions

In short, this research was successfully able to produce graphite-reinforced Al-4.5Cu-1.5Mg matrix nanocomposite by the shaker mil technique. 0.5wt% graphite seems to be the most suitable graphite content for Al-4.51.5Cu powder. Sintering under ultra-high purity argon gas for 99.999% was able to produce high-density nanocomposite which was able to reach 77% sintering density at 550^oC. Intermetallics were found before and after sintering such as Al₂Cu, Al₃Mg_2, and Al₄C₃. Al₄C₃ was found as a result of the reaction between aluminum graphite, it acts as an interface between matrix and reinforcement particles. AlF₃ was found as an impurity due to an unnecessary reaction during the shaker mill between jar, ball, and powder. And optimum Vicker hardness was also obtained at 550^oC sintering temperature and 0.5wt% graphite for 159.4 HVN.

Acknowledgments

This research project was funded by the Department of Mechanical Engineering, Gunadarma University.

Conflicts of Interest

The author declares that there is no conflict of interest regarding the publication of this manuscript. In addition, the authors have entirely observed the ethical issues, including plagiarism, informed consent, misconduct, data fabrication and/or falsification, double publication and/or submission, and redundancy.

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