Production of Functionally Graded AlB 2 / Al-4 % Mg Composite by Centrifugal Casting

In this study, production of functionally graded AlB2 reinforced Al-4%Mg composite has been investigated. In-situ high-aspect-ratio flake AlB2 reinforcement was obtained by the addition of boron oxide to the liquid Al-Mg at 1400 °C. It has been observed that there were two distinct regions without a smooth grading: one is rich of and the other is poor of AlB2. The results showed that the highest AlB2 content has been found to be around 10% in external zones, whereas the lowest AlB2 content has been observed to be 0.02% in the internal zones. Additionally, results showed that, depending on the increase in the reinforcement phase in external zones, up to 20 % increase in the hardness of the composite has been achieved.


Introduction
Metal matrix composite have been gaining importance in engineering applications, because of their high specific strength, high modulus and high wear resistance [1][2][3][4][5][6].Functionally graded metal matrix composites have a great significance for the use in automobile, aerospace and defence industries.A functionally graded metal matrix composite can be defined as its reinforcement particle's volume fraction varies continuously from the inner to the outer sections of the part.Therefore, their mechanical properties are different from the inner region and outer region accordingly [7,8].
Until recently, a number of studies on functionally graded metal matrix composites produced by centrifugal casting had SiC, Al2O3 and AlB2 reinforcement particles in various aluminium alloys [7][8][9].Production of functionally graded materials under centrifugal force is an effective method.Advantage of centrifugal casting is good mould filling combined with good microstructural control.Aluminium boron master alloys have been used commercially to scavenge transition-metal elements during the production of aluminium electrical wires [10] and is also used for grain refiner as Al-Ti-B in aluminium castings [11].It is known that the crystal structure of borides is a hexagonal close packed (HCP).Depending on the cooling rate they can be in fine flake shaped structure having few micron thickness with aspect ratio ranging from 30 to 400 [12,13].
In the present work, in-situ AlB2 flake reinforced Al-Mg matrix composites were in-situ prepared by the reaction of boron oxide (B2O3) with liquid Al at a reaction temperature of 1400ºC.Centrifugal casting was applied to the composite to produce functionally graded material FGM.

Experimental Procedure
For the fabrication of AlB2 flake reinforced Al-4%Mg composites were prepared by the reaction of boron oxide (B2O3) with Al at a reaction temperature of 1400ºC as explained in detail elsewhere [13].The composites were placed in a die and the die was heated at 800°C in an electric resistance furnace to bring the composites to a semi-solid state (Al (liquid) + AlB2 (solid)).Centrifugal action was then employed with the die containing semi-solid composite to drive the solid AlB2 particles towards the outer region to produce a functionally graded AlB2/Al-Mg composite with even higher volume per cent reinforcement.The centrifugation process was carried out under rotation speed of 600 rpm at 800 ºC.A schematic representation of the in-situ production and the following centrifugation process of the functionally graded AlB2/Al-Mg composite were shown in Fig. 1.Since boron cannot be quantified by Energy Dispersive Spectrometer (EDS), measurement of total weight per cent of boron in the AlB2/Al-Mg composites were carried out by a wet chemical analysis method, as explained elsewhere [13].For examination, Small samples were extracted from the AlB2/Al-Mg composites.For the microstructure analysis the samples were ground up to 1200 grid by using SiC paper followed by polishing, using 0.2 µm diamond paste.For detailed evaluation of the boride structure, the samples of composites were deep etched using % 10 HCl solution followed by an examination with JEOL JSM 6060LV scanning electron microscope (SEM).Brinell hardness (BHN) of the composites was measured after polishing to a 1 µm finish.Hardness of composites was measured with 2 mm ranges on the composites from outside to inside as seen Fig 1.
The hardness values of the samples were measured using a 2.5 mm diameter ball at load of 31.25 kgf for 10 sec.The composites were solutionized at 540 °C for 4 h followed by water quenching at 60 °C, waiting at room temperature for 12 h, and aging was carried out at 190 °C for 10 h.

Results And Discussion
The mixture of boron oxide and Al-4%Mg alloys was melted and heated to 1400 °C for 1.5 h to maximize the boron solubility in the liquid Al-Mg alloy as explained in detail in a previous work [10].During synthesizing a portion of boron oxide, chemical reaction of Equation (Eq.) 1 occurs at the interface between the molten alloy and boron oxide resulting in boron in liquid solution and aluminium oxide mixed with remaining (unreacted) boron oxide on top of the melt [11,12].In Eq. ( 1), α represents aluminium boron solid solution at 298 degree K. ∆G and ∆H are the Gibbs free energy and the enthalpy of the reaction respectively.
298°0 = −416.9/  298°0 = −402.7 / Results from wet chemistry analyses showed that the amount of boron in the as-cast (unfiltered) casting was 1.2 wt % as all the boron atoms within the Al-B alloy can be considered to be boride compounds as reported in previous studies [9,14].The volume content of the boride phase within the matrix was calculated as 2.6 %.The microstructure of 2.6 vol.% AlB2/Al-Mg composite material is given in Fig. 2. In Fig. 2, it can be observed that the in-situ AlB2 boride particles with flake crystals and the -Al3Mg2 phases with needle-shape form are distributed uniformly in the aluminium matrix.
A sample of 2.6 vol.%AlB2 reinforced aluminium composite was deep-etched to extricate boride flakes from the aluminium matrix.Fig. 3 shows a SEM image of %2.6 vol.AlB2/Al-Mg composite.As seen in the SEM image, AlB2 structures have a fine flake shape and a high aspect ratio (width/thickness).This result is in good agreement with previous reports on AlB2/Al [13,14,16].These results show that all the AlB2 flakes have been segregated by the centrifugal force at 800 °C as also reported in previous work [9,18].
Results from wet chemistry analyses showed that volume fraction of AlB2 flake in the external zone increased significantly.The mean volume fraction of AlB2 flake in this region has been measured as 10 vol.%.On the other hand in the internal region it was 0.02 vol.%.Examination with optical microscope shows that the average thickness of AlB2 is less than 1 m.The measured average AlB2 width distribution histogram is seen in Fig. 6.As seen from these histograms that, the average width of AlB2 flakes is 170 m, the width of the flakes vary between 40 µm and 350 µm and has demonstrated a normal (Gaussian) distribution within the matrix.

Figure 3 .
Figure 3. SEM image of the deep etched AlB2/Al-Mg compositeThe 2.6 % vol.AlB2/Al-Mg composite was heated for centrifugal casting to 800 o C to produce functionally graded (FG) composites material.After the centrifugal casting process the composite was cooled to room temperature for examinations.A cross section of the FG

Figure 4 .
Figure 4.A cross section of the functionally graded AlB2/Al-Mg composites The microstructures of the darker and the lighter regions are shown in Fig 5.As the seen in Fig. 5a that there are numerous of AlB2 reinforcement particles embedded in the aluminium matrix, whereas, there are almost no AlB2 particles within the Al-Mg matrix in the internal zone as seen Fig 5b.These results show that all the AlB2 flakes have been segregated by the centrifugal force at 800 °C as also reported in previous work[9,18].

Figure 5 .
Figure 5. Microstructure of composite of a) external zone and b) internal zone.

Figure 6 .
Figure 6.AlB2 flake width histogramMeasured Brinell hardness as a function of radial distance along the centrifugally cast composite is given in Fig.7for the functionally graded AlB2/Al-Mg composites.A significant increase in hardness of external zone was observed.It can be seen that the Brinell hardness value increased with solution heat treated composites.The functionally graded AlB2/Al-Mg composites the external zone has the highest hardness with 90 HB.These results show that the hardness of Al-Mg alloys were significantly increased by the addition of 10 % of AlB2 flake.This result is in good agreement in a number of experimental works reported for AlB2/Al type composites[9,15,[18][19][20][21][22].

Figure 7 .. Conclusions 1 - 2 -
Figure 7. Measured Brinell hardness (HB) as a function of radial distance along the centrifugally casting composite 4. Conclusions 1-AlB2 Reinforcing particles were produced by chemically reacting aluminium alloys with boron oxide at 1400 °C for 1.5 h.It was observed that the AlB2 flakes in Al-Mg alloy have average 170 micron width, fine flake shape and volume fraction of AlB2 flake is 2.6 vol.% in as cast condition.