Synthesis of aluminium (Al) and alumina (Al 2 O 3 )- based graded material by gravity casting

: In the present work, aluminium alloy-based composites were developed for automobile applications. Aluminium-based functionally graded material (FGM) was developed by adding 2.5, 5 and 7.5 wt% of nanoparticles of alumina. The composites were made using gravity casting in an open hearth furnace with the surface temperature of FGM maintained at 750, 850 and 1,000°C. The microstructure of the samples was studied using scanning electron microscopy and particle distribution. The particle distribution was higher at the bottom in all compositions, which can be attributed to more solidi ﬁ cation time. As the wt% of the Al 2 O 3 increased, the volume fraction of particles also increased from the top surface to the bottom surface of the samples. After adding 7.5 wt% of Al 2 O 3 and heating up to 1,000°C led to the grain re ﬁ nement of the alloy. The increase in hardness from the top surface to the bottom surface of the sample veri ﬁ ed the development of FGM. Due to increase in the solidi ﬁ cation temperature, better reinforcement was observed in the developed FGM.


Introduction
From aerospace to leisure products, functionally graded materials (FGMs) have been used in a wide range of applications.A spacecraft's exterior body and different engine components can be composed of a metal matrix composite, which has a higher superheat resistance [1,2].Because of the increase in the demand for enhanced lightweight mechanical solutions, leading to energy-efficient goods, there is a growing emphasis on the development of FGMs.Lightweight alloys can be strengthened by incorporating reinforcement into their composition, which can be in the form of particles or fibres, increasing the concentration ratio [3].As compared to a metal matrix, FGMs provide enhanced thermal, mechanical, and tri-biological performance at either high or low temperatures.High thermal conductivity, low density, great abrasion and wear resistance, and acceptable fatigue responses are only some of the essential qualities that are enhanced by FGMs.Vehicle components such as connecting rods, pistons, and nozzles, as well as spacecraft, helicopters, missile noses, and satellites, all make use of FGMs in their production.Different manufacturing processes, such as solid freeform (SFF) fabrication [4,5], powder metallurgy [6,7], physical vapour deposition [8], and centrifugal casting [9], are used to create FGMs.Many categorization methods used in the study of FGMs have been critically examined by researchers.[10][11][12].FGMs produced by centrifugal methods have been investigated by researchers [13].Gravity casting has not been researched for the manufacture of FGMs, which has been attempted in this research.
Currently, cast aluminium alloys and micron-sized particle reinforcements are the principal subjects of domestic and international research on particle-reinforced aluminium matrix composites.Nonetheless, when the composite strength is increased, the micron-sized particles typically cause the elongation of the composites to decrease.However, the nano-SiC particles (SiCnp) have the ability to enhance the strength of the composite materials while preserving their high plasticity when compared to micronsized SiC particles.The SiCp/Al6061 composite was made by stir casting [14].
Particle-reinforced aluminium matrix composites (PAMCs) offer a wide range of applications in the automotive, aerospace, electronic circuit, and military weapons industries, among other fields.They also have excellent electrical and thermal conductivity, high-temperature resistance, low coefficient of thermal expansion, good wear resistance, and dimensional stability [15].

Materials and methods
Aluminium (99.8%) and alumina (mesh size-70-230) particles were procured from Sigma Aldrich and aluminium FGM was prepared by conventional gravity casting with 2.5, 5 and 7.5% mass compositions of alumina in an openhearth furnace using a graphite mould.The casting temperature had to be high enough to guarantee the bonding of the components without causing the mixture to solidify too quickly.A number of factors, including the difference in liquidus-solidus temperature between the two compounds and the casting temperature, impact the time taken between successive pours of nanoparticles in castings.Bonding at interfaces while preventing the components from mixing required a time period of this length.Degassing was influenced by the mould temperature; greater mould temperatures result in more rapid removal of moisture.The mould was preheated to 400°C to eliminate humidity and prevent thermal shock.A refractory coating was applied to the inside of the mould chamber to keep the casting from sticking.Aluminium was melted in the mould, and nanoparticles of alumina were poured slowly.Pouring was carried out for 10 min with slow stirring and allowed to solidify in the furnace for 8 h after the surface temperatures varied from 735 to 760°C.Cast samples were prepared by maintaining the surface temperature ranges of 740-760, 840-860, and 1,000-1,060°C, as shown in Table 1.
The prepared cylindrical samples were machined in the lathe to remove the excess material, and samples of the required sizes were prepared for various investigations, as shown in Figure 1.Investigation of the bonding region and gradations in the distribution of particles along the depth was very important to validate the formation of suitable FGM.

Material characterization 2.1.1 Scanning electron microscopy (SEM)
The microstructures of all samples were analysed using standard metallographic procedures.SEM and mirror-polished samples were used for this.

Hardness test
A Vickers hardness testing machine was used to determine the hardness of the samples.Coarse polishing was done by emery papers and fine polishing was executed by cloth polishing.After that, the mirror polished sample was kept below a diamond indenter of the Vickers hardness machine; five indentations for each sample were taken to find the exact data of hardness.Then, 1 kgf load was applied for 10 s of dual time.

Tensile testing
A universal testing machine was used to find the stressstrain curve of the samples.The sample for this purpose was prepared with the help of an EDM wire using ASTM D638.

Microstructure
Figure 1 shows the stages of the sample preparation from casting to machining.The machined samples are then sliced to the required sample size for microstructural investigations.Microstructure studies were carried out for all the samples to validate the formation of FGM.The sample is first cast by gravity casting and then machined on a lathe and then sliced vertically for the microstructural study.Sample sizes of 10 × 10 × 5 mm 3 were prepared for all the compositions subjected to SEM.
Figures 2-4 show the SEM images of the 2.5, 5, and 7.5% alumina in an aluminium matrix, respectively.Black spots represent alumina, and their intensity increased as the composition of alumina increased.The structure is more prevalent in higher composition with better dispersion.It is noted that particle distribution varied along the depth and, at the bottom, the particle distribution was higher.This can be attributed to the solidification time being given   under furnace cooling and because of gravity the particles get more time to travel down the depth of the mould.This causes a gradation of dispersion of alumina nanoparticles in the aluminium matrix along the depth.This gradation is more visible in higher surface temperatures and higher mass percentage, i.e. 7.5% at 1,000°C samples.The microstructure also reveals that a honeycomb structure is very clearly formed without much rupture and porosity.To further validate the sample as FGM, the sample with a good honeycomb structure is chosen for further structural studies.Sample XI was selected and was prepared for further SEM studies.

Mechanical characteristics
The hardness test of the graded material was conducted on a microhardness tester machine.A diamond indenter with a diameter of 1/16″ and a load of 60 kgf was specified for the measurement.A red dial on scale C was used for measuring the readings, and the samples obtained from the graded material had a dimension of 20 mm × 20 mm × 5 mm.
The product was gravity cast.Specimens of FGMs with varying volume percentages of reinforcement Al 2 O 3 particles (2.5, 5.0, and 7.5 wt%) were subjected to a Micro Rockwell hardness test.Samples were cut along the depth, and surface locations selected for testing are top, middle, and bottom.All the samples were tested for their hardness, and variation due to the composition of alumina reinforcement and surface temperature along the depth of cast was studied.The results of the study reveal that, as expected, gravity causes the reinforcement particles to be more densely packed on the underside of the cast than on the top.As a result, all materials show increased hardness in the bottom zone of the cast product, which gradually diminishes in the top zone.However, in the case of MMC, the effect was much more noticeable due to the different ceramic particles [16].According to Watanabe et al. [17], the presence of the compositional gradient is the primary reason why the hardness of a composite material increased.The gradient of the composite has the greatest impact on the Young's modulus and thermal expansion coefficient [18].The study of the FGM results revealed that the percentage of alumina particles in their composition correlates inversely with the difference in hardness values.Figures 5-7 show that the hardness of the top, middle, and bottom portion of fabricated cast products with a higher solidification temperature differ more than that of products with a lower solidification temperature.This is because the top surface experiences more cancellation of reinforcement particles than the bottom surface.
Figure 8 and Table 2 show the plot of tensile strength (YS and UTS) for various Al alloy-based FGM samples with     an increased percentage of reinforce starting from 2.5 to 7.5 wt% at different casting temperatures.The 2.5 wt% sample with a solidification temperature of 750°C had a yield strength (YS) of 19 MPa, ultimate tensile strength (UTS) of 35 MPa, and ductility of 14%.The YS and UTS in the FGM sample were reinforced at 2.5 wt% at a casting temperature of 850°C are smaller, i.e., 19 and 36.5 MPa, respectively.Also, a decrease in ductility (13%) was observed compared to the 750°C solidification temperature sample.However, for the 2.5 wt% sample at a solidification temperature of 1,000°C, the YS and UTS in the FGM sample are increased to 28 and 40 MPa, respectively, and the ductility also increased up to 16%.This might be because the reinforcing particles are accelerated downward due to the gravitational force to the bottom surfaces, while congregated Al 2 O 3 particles that are present in FGM samples show reduced ductility and increase the YS and UTS.In addition, these deformities can infatuate the microstructural continuity of FGMs.The stress-strain curve of FGM samples with different weight percentages of reinforcement particle compositions is shown in Figure 8.It was observed that higher values of UTS were found in the higher wt% Al 2 O 3 for higher solidification temperature specimens due to the dispersion-strength effect.The results indicate that the tensile characteristics like UTS and YS increase with the increase in Al 2 O 3 content [19,20].It may be noted that internal oxidation, which prevents the movement of dislocation and pin-up dislocation lines, is the method used to create reinforcing particles with increased dispersion strength.Additionally, as described in the literature [21], these are essential for increasing the dislocation density in the metal matrix, which leads to enhanced yield and tensile strength values.In general, in the casting process, the cast metal is deposited in a metal mould and solidified under certain conditions.This process causes some changes in the microstructure that lead to changes in the mechanical properties of the material, such as the increase in the microhardness of the processed alloy.

Conclusions
In this experimental study, FGM were produced via gravity casting process with different solidification temperatures.The microstructural and mechanical characteristics of Al/ Al 2 O 3 FGM were investigated by SEM, microhardness test and tensile test.The following conclusions are drawn: 1.Most of the Al 2 O 3 particles in 7.5 vol%.Al/Al 2 O 3 FGM were found in the bottom zone of the cast product due to the gravitational force, and less density of Al 2 O 3 particles congregated to the upper zone of the cast product.2. Variations in microhardness were seen between the top and bottom zones of the casted product at a larger percentage of reinforcing particles at 7.5 wt% and 750°C solidification temperature.As the ceramic particles' solidification temperature is increased by 7.5 wt%, the microhardness variations between the top and bottom surfaces also increased.3. Due to the presence of clustered Al 2 O 3 particles, increasing the weight percentage of these reinforced particles increased the mechanical characteristics of FGM at the price of ductility.

Figure 5 :
Figure 5: Variation of hardness along the depth in 2.5 wt% alumina samples at different surface temperatures.

Figure 7 :
Figure 7: Variation of hardness along the depth in 7.5 wt% alumina samples at different surface temperatures.

Figure 8 :
Figure 8: Stress-strain curve for all material conditions.

Figure 6 :
Figure 6: Variation of hardness along the depth in 5 wt% alumina samples at different surface temperatures.

Table 1 :
Design of experiments

Table 2 :
Mechanical properties S. No. Material condition (wt% of Al 2 O 3)