Experimentation and Optimization of Multilayered Aluminum-Based Functionally Graded Materials

Abstract

According to current industrial and societal demands, product manufacturing is now highly competitive. The current research is primarily focused on the creation of functionally graded materials that are essential for automotive cylinders and their internal components. Since aluminum plays a significant role in automobile components, layerwise deposition of the matrix and reinforcements is used. Aluminum alloy (Al 356) was investigated in weight proportions of 100, 95, and 90%, while the reinforcement varied from 0 to 7.5%. The particulate reinforcements were chosen to be silicon carbide (SiC) and nickel (Ni). Zinc stearate is used as lubricating agent to enhance the free-flow compaction process and to avoid the wastage in synthesis. The compressed specimens were examined for various mechanical and microstructural characterization. An ultimate compressive strength of 328 MPa and 68 BHN was achieved at 85% Al, 5% SiC, and 7.5% Ni, as per research criterion. Scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDAX), and X-ray diffraction analysis (XRD) images of the inclusions and matrix are compatible and compact due to the excellent bonding. The process variables were adjusted using Taguchi optimization, which shows that the sintering duration and compaction pressure are crucial for the validation of manufacturing and characterization.

1. Introduction

Functionally graded materials (FGMs) are a distinct and varied class of materials with a wide range of uses, from home to commercial. The introduction of these materials in the field of mechanical and materials engineering aims to provide a novel and resilient material that satisfies mechanical, microstructural, and tribological requirements. In order to provide solutions with this kind of materials, these types of materials are designed and developed. Materials with continuous material properties in all directions and some modifications to their microstructure are unable to withstand all temperature variations and gradients within a short period of the material thickness.

Even if they are successful, researchers with their creativity and knowledge are working hard to produce these materials to a greater level, but the production and characterization processes have flaws and limitations. This is frequently accomplished by progressively shifting the volume fraction of two components with various thermomechanical properties in a certain direction, resulting in a compound with various volume ratios [1]. For spacecraft with one side exposed to extremely high temperatures and the other exposed to extremely low temperatures, FGMs are the best choice because they distribute material functions throughout the material body for best heat resistance and mechanical qualities. FGM is used in the development of most industrial sectors’ products today, including those in the automotive, information technology, computer science, and other related fields [2].

1.1 Development and progression of FGM

Most of the research investigations are progressed with the development of functionally graded aluminum composites by the aid of centrifugal casting analyzed the influence of mechanical and wear properties of pure aluminum, boron carbide, silicon carbide, alumina, and titanium boride. Radhika et al. [3] in her research experimentations with FGM stated that the outer peripheries of the FGM exhibit higher hardness except in AlB₄C and the outer zone exhibits tensile strength at its maximum. Chandrappa et al. [4] have synthesized with the aid of conventional powder metallurgy method at 436^oC sintering temperature and depicted that high volume fraction of SiC caused clustering of carbide phase at grain boundaries, which restricts interparticle contacts and further becomes a wall for densification in their study titled, “manufacturing and characterization of Al-SiC FGM developed through powder metallurgy.” For preparation of Al/Si functionally graded materials using ultrasonic separation method, Zhang Zhong tao et al. [5, 6] have successfully synthesized the standardized Al-SiC FGM and also depicted that increasing the composition of SiC makes the FGM harder and at a limiting value, it becomes brittle and crack formation is observed, with the highest compressive strength obtained at 7.5% of SiC insertion.

A research article designated as synthesis of hydroxyapatite in combination with titanium alloy to prepare FGM composites by powder methodology ultimately used as implant materials depicted that microhardness is increased by 28% when compared with steel alloys. Amir Arifin et al. [7] successfully processed HA/Ti FGMs and depicted that increasing the titanium percentage increases the hardness and compaction capability of the synthesized FGM. He has also illustrated the elements’ flawless cohesion and excellent microstructural characteristics. Madhusudan et al. [8] successfully formulated a procedure to determine the optimal thickness of the mold material in the production of FGM in centrifugal casting, as well as inferred various parameters affecting FGM production, starting with preheating, molten metal heating, and solidification rate.

2. Selection of materials

The materials used in the experimentation process were chosen based on functional requirements, as we intend to develop an electromechanical aeronautical application product, and thus, aluminum is the best product, with silicon carbide for high toughness and hardness and nickel for good wear and corrosive resistance, and all materials were obtained from Raghavendra spectro metals LTD, Hyderabad, and are in powdered form as shown in Figure 1.

Figure 1.

Powdered form of (a) Al 356 (98% purity, 100 mesh size), (b) SiC (150 mesh size), and (c) Nickel (180 mesh size).

2.1 Blending of powders by weight ratio

The matrix and the reinforcements are successfully blended by considering weight percentages with the aid of digital weighing balance with least count of 1 milligram, and the uniform mixing of matrix and the reinforcements are successfully blended with ball milling machine with 1000 kg capacity as per ASTM D7152 standardization, which is depicted in Figure 2 and Table 1.

Figure 2.

SEM morphological images of Al 356, SiC, and Nickel.

Layer	Composition (%)	Weight (g)	Matrix	Reinforcement
1	100	26	Pure Al	NIL
2	90/10	22.4/2.6	Pure Al	SiC
3	80/10/10	20.8/2.6/2.6	Pure Al	SiC, Ni

Table 1.

Depiction of layerwise composition of the matrix and the reinforcements in FGM.

3. Compaction process

After uniform blending of the powders that consist of matrix and reinforcements, the next most crucial process is the compaction where the blending mixture as per rule of mixtures is needed to attain the solid shape, which means we need to convert from powder to solid form that is a complicated and cumbersome process. This is achieved by powder compaction machine having capacity of 1Ton supplied from Instron equipments limited, which is suitable for all kinds of metallic power metallurgical process. The compaction and ejection process is controlled automatically, and the application of the loading can be visualized on monitor and display of crucial parameters with a graphical interface. The compaction load is maintained at 480KN, and the ejection at 80KN that is treated as nominal and safe standards and any variations of loading conditions and the material failure are indicated with the aid of a buzzer or by alarming system. Cylindrical specimens are generated, which have 40 mm diameter and 10 mm thickness depicted in Figures 3 and 4. After the necessary and sufficient conditions, the green compacted specimen is manufactured and is digitally weighed and checked its manufacturing defects visually and also by the microscope according to specified standards of ASTM B925-15.The compacted specimens can be viewed in Figure 5.

Figure 3.

Microbalance weighing of green-compacted FGM specimens.

Figure 4.

Sectional views (front and side) of the dry-compacted specimen.

Figure 5.

Compaction testing machine for the preparation of green compact FGM.

4. Sintering process

The compacted specimens are sintered at a temperature of 472 degrees centigrade in muffle furnace depicted in Figure 6. The sintering process is mainly developed to increase the hardness of the green compacts as they are in very low strength and cannot be used for the material testing and also to enhance the hardness of the component. It is taken to be 70% of the melting temperature of the pure aluminum melting point (673 degree centigrade) [9].

Figure 6.

Sintering process of green-compacted FGM in high tubular furnace.

5. Material characterization

5.1 Density

Table 2 defines the comparison between the experimental measured density and the density achieved through theoretical calculations of the aluminum alloy and the manufactured FGM composites. By clearly visualizing the spectral images, we can clearly depict that the composites are prepared successfully with low porosity and microstructural imperfections. However, during the calculation we depicted that the measured values have attained lower values than those of the theoretical values. The depiction of low pores in the composite, low interstitial voids, agglomerations, and discontinuities, which are due to the impingement of reinforcements in proper weight percentages, is the main source during the compaction process. The theoretical and measured densities have been depicted in Figure 7

Material	Theoretical density(g/cc)	Measured density(g/cc)
Pure Al 356	2.78	2.68
Al 356+10%SiC	2.84	2.79
Al 356+10%SiC+10%Ni	2.92	2.84
Al 356+5%SiC	2.85	2.75
Al 356+5%SiC+5%Ni	2.87	2.82

Table 2.

Theoretical and measured densities of the alloy and FGM composites.

Figure 7.

Theoretical density vs. practical density.

5.2 Evaluation of material hardness

The FGM shows a macro-hardness of 34.65% greater than the pure aluminum base metal when conducted in Brinell hardness tester. The process is done as per ASTM E384 since these results are to be validated with a standard process and then only the results will serve the purpose.

The compressive strength is been characterized by using UTM with ASTM E-38 standardized specifications. Upon application of force on both sides, the FGM composites are failed at point and when calculating the stress-strain diagrams, the compressive strength is depicted to be 258.5 MPa, which has enhanced from 215.8 MPa for the base metal Al356 compression strength, which is depicted in Figures 7–9 for Al356/10%SiC/10%Ni composition attained the maximum value.

Figure 8.

Compression strength vs. composition.

Figure 9.

Microhardness vs. composition.

5.3 Vicker’s microhardness test

The microhardness of FGM is found to be 2.85 times the base metal when conducted on Vicker’s hardness. Where VHN is known as Vicker’s hardness number, the hardness value for each of the layer is designated in Tables 3 and 4 with VHN units.

Material	Compressive strength(MPa)
Pure Al 356	189
Al 356+10%SiC	195
Al 356+10%SiC+10%Ni	258
Al 356+5%SiC	190
Al 356+5%SiC+5%Ni	220

Table 3.

Depiction of compressive strength of FGM composites with composition.

Material	Microhardness (VHN)
Pure Al 356	65
Al 356+10%SiC	79
Al 356+10%SiC+10%Ni	102.5
Al 356+5%SiC	92
Al 356+5%SiC+5%Ni	94.5

Table 4.

Microhardness results by Vickers hardness tester.

5.4 Scanning electron microscopy (SEM)

The scanning electron microscopy (SEM) characterization shows the development of grains and grain limits of grid and building up materials, and there are no arrangement of voids, breaks, and surface flaws [10]. This process is done as per ASTM E986, which clearly shows how the bonding of constituents and blunt interfaces with good amount of contact between them. The observation of bonding is not only done at the bonding region of the matrix and the reinforcement’s level but also we have observed at the intermolecular level, that is, at the middle region where the matrix and the reinforcements bond together with the equal weight percentages (50%) as depicted in Figures 10–12.

Figure 10.

SEM micrographs of Al356/0.6SiC_np/0.6Ni FGM. (a) Al356/0.05Ninp/0.05CrnpHybrid nano metal matrix composite (HNMMC). (b) Al356/0.1Ninp/0.1CrnpHybrid nano metal matrix composite (HNMMC).

Figure 11.

EDAX micrographs of Al356/0.6SiC_np/0.6Ni FGM.

Figure 12.

SEM micrographs, quantitative results, and mapping of Al6061/0.6SiC_np/0.4Cr_np.

5.5 Energy-dispersive X-ray analysis (EDAX)

The energy-dispersive X-ray analysis (EDAX) shows that there is no presence of any foreign material in the microstructure of the final developed material and also there are no traces of chemical reaction within the elements and nonexistence of carbides and oxides [11, 12] as shown in Figures 13 and 14.

Figure 13.

EDAX Analysis of Al-SiC-Ni FGM bottom most layer.

Figure 14.

EDAX test results for bottom layer of Al-SiC-Ni FGM.

5.6 X-ray diffraction analysis (XRD)

The X-ray diffraction analysis (XRD) analysis is exclusively done to determine the grain size, and it is found to be 6.28 Å and the calculated maximum interplanar spacing of the atoms is 3.13 Å [13, 14]. Table 5 clearly visualizes the effect of the diffraction angle and the full width at half maximum (FWHM) parameter on the crystalline size, thereby emphasizing the bonding strength between the matrix and the reinforcement constituents and also the size of the ultimate molecules formed after powder metallurgical process [15, 16, 17](Figure 15).

Peak position (2θ) (degrees)	FWHM (beta)	Crystalline size (D)
28.44487	0.16022	8.93493
38.51731	0.21445	6.85444
44.77116	0.23935	6.27019
43.34172	0.25013	5.96976
65.15187	0.29904	5.50677
78.29161	0.36344	4.92322
82.50489	0.36068	5.11749

Table 5.

XRD values of the dry compacted FGM specimens.

The average crystalline size is 6.224 Angstroms.

Figure 15.

XRD graph of Al-Sic-Ni FGM.

5.7 Optimization of the process parameters

Taguchi optimization is used to discover the best fabrication conditions for achieving the best microhardness and compressive strength. We employed the larger is better condition (1) of Taguchi optimization, since the material required great hardness and compacting strength in 95% of the applications [18, 19, 20, 21]:

$$LargerisBetter=-10{\log }_{10}\left[\left(\frac{1}{n}\right)\ast \sum \left(\frac{1}{y_i^2}\right)\right]$$E1

The SN (signal-to-noise) ratios for compressive strength measurements are generated based on the aforesaid equation, and the values are utilized to plot the curves, which are shown in Figure 16. Figure 16 shows the SN ratio plots for compressive strength, which shows the effect of each parameter utilized in manufacturing and blend production of the dry-compacted specimens [22, 23, 24]. High compressive strength was achieved by using parameters such as compacting pressure, sintering pressure, and sintering duration. When compared with compacting pressure and sintering time, sintering temperature causes high deviations in compressive strength, but sintering duration improves compressive strength as depicted in Tables 6 and 7 and Figure 17 [25, 26, 27].

Figure 16.

SN curves for compressive strength.

Level	Sintering time	Sintering temperature	Compaction pressure (KN)
1	374	472	447.25
2	398.78	489.36	452.69
3	408.65	498.57	487.25
Delta	34.65	26.57	40
Rank	2	3	1

Table 6.

Response table for the means of the SN ratio of compressive strength.

Level	Sintering time	Sintering temperature	Compaction pressure (KN)
1	1.025	0.985	1.325
2	1.058	1.157	452.69
3	1.154	1.3547	487.25
Delta	34.65	26.57	40
Rank	2	3	1

Table 7.

Response table for the means of the SN ratio of microhardness.

Figure 17.

SN curves for microhardness.

6. Conclusion

• FGM prepared through powder metallurgical process has attained ratios and the thorough blending with the aid of tumbling mechanism.

• The FGM is handled effectively by utilizing compaction machine with programmed load controlling limit where the compaction pressure is kept up with at 450KN and ejection load at 50KN.

• The sintered specimens are created at a sintering temperature of 472 degrees centigrade. The FGM shows a full-scale hardness of 34.65% more noteworthy than the Al 356 base metal when led in Brinell hardness tester that is extensively used for macro-hardness and the microhardness is generated with the help of Vicker’s hardness analyzer and observed that the FGM hardness is 2.85 times the base metal.

• The SEM characterization shows the arrangement of grains and grain limits of network and building up materials, and there are no developments of voids, breaks, and surface blemishes. The EDAX shows that there is no synthetic response inside components and furthermore with unfamiliar material and no presence of carbides and oxides.

• The XRD investigation is only done to decide the grain size and its observed to be 6.28 angstroms. The approval of the took on powder metallurgical cycle is confirmed effectively by the pictures shaped in miniature underlying characterization.

• Taguchi optimization conferred that the compressive strength and the microhardness have been highly dependent on the sintering temperature, and the microstructural properties are greatly been influenced by the compaction pressure.