Mechanical Performance and Corrosion Behaviour of Aluminum7075 Reinforced by Nano-Titanium dioxide

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Introduction
Aluminium and aluminium alloys are considered important materials in the manufacture of aircraft and spacecraft, as well as in the manufacture of vehicles.Aluminium A7075 in particular, despite its wide applications, suffers from many defects, such as structural cracks.Structural cracks (internal or/and surface) appear during the casting process or when aluminium is exposed to external mechanical stress 1 .Casting is one of the most widespread manufacturing processes, as it is widely used in the generation of various metal alloys to obtain the desired properties 2 .Casting has several types, the most common of which is used in this work, stir casting, which is Published Online First: February, 2024 https://doi.org/10.21123/bsj.2024.9690P-ISSN: 2078-8665 -E-ISSN: 2411-7986 Baghdad Science Journal good hardness, a low specific gravity, and a high melting point, they are a popular choice among hard-ceramic reinforcing materials.The addition of nanoparticles is one alternative for producing homogenous, fine, and equiaxed microstructures of additively manufactured aluminium composites due to grain refinement resulting from a change in the nucleation mechanism during solidification 5,6 .Al-TiO2 nano composites are applied in a variety of industries, such as automobiles, aircraft, and aerospace applications.
Kamaal Haider et al. 7 designed aluminium 6061 alloy-based ceramic-reinforced composites with silicon carbide and alumina using SCT.The total amount of composite was 100% by weight.Some mechanical properties were examined, like tensile strength, hardness value, wear, and impact strength, on two types of composites to compare with aluminium alloy.The results showed an increase in most mechanical properties, like hardness, tensile strength, and impact strength, of prepared composites containing silicon carbide and alumina particulates compared to aluminium 6061-based alloys.Del Real Romero et al. 8 investigated the mechanical characteristics of an aluminium matrix composite prepared using powder metallurgy in addition to stir casting.The metal matrix of aluminium, which involved some nanomaterials like graphene, graphite, zinc and magnesium, is studied using microscopic and spectroscopic methods.The wear properties were determined by varying the load, velocity, and distance using the pin-on-disc wear machine, and the wear surface was examined by a microscope.The results showed that the pouring temperature and the percentage of magnesium and zinc play a key role in the strength of the cast.The increase in the percentage of magnesium during casting and sintering leads to the vaporisation of other reinforcements because magnesium tends to catch fire.They found that the wear properties of the cast are better than those of sintering.They also concluded that a higher percentage of graphite lubrication is better, but it tends to reduce the strength of composites.Cardoso et al. 9 carried out solution heat treatments on a commercial-based Al7050 aluminium alloy as a pure alloy and with the addition of titanium at different temperatures.The effects resulting from the addition of mechanical alloys and titanium on the stability of sediments were examined through the formation of specimens in two ways, the first is mechanical mixtures, and the second is the method of hot extrusion.The results showed that the addition of Ti and mechanical alloying augmented the hardness of the alloy under heat treatment conditions.Radhika et al 10 used SCT to prepare an AlSi₁₀Mg alloy consisting of 3 wt.%graphite and reinforced with three different concentrations of alumina 3, 6, and 9 wt.%.They studied the microstructures and mechanical properties like hardness, double shear strength, and tensile strength of non-reinforced specimens and composites.They concluded that hybrid composites have better mechanical characteristics than unreinforced alloys.
In view of the aforementioned, several investigations have employed nano-TiO2 as a molecular structure supporter, particularly to enhance mechanical properties at concentrations greater than 0.1% 11 .Looking back at the literature that covered this topic, it is important to remember that the research skipped over low concentrations, or concentrations lower than 0.1%.Thus, this work investigates the behaviour of a composite material based on low concentrations of the nano-supporting material Nano-TiO2 and the aluminium alloy A7075.

Materials and Methods
An aluminium alloy of grade Al7075 was selected as the base matrix material,l and nanoparticles of titanium dioxide (nano-TiO2) were added in powder form to the reinforcement material for the present study.Table 1 shows the chemical composition of the aluminium alloy Al7075 12 .Titanium dioxide (TiO2) nanoparticles of chemical grade from Changsha Santech Co., China, have a particle size of 30±5 and a purity of ≥99.8%.
The aluminium (Al7075)-based matrix composites have been produced by the SCT with varied weight https://doi.org/10.21123/bsj.2024.9690P-ISSN: 2078-8665 -E-ISSN: 2411-7986 Baghdad Science Journal percentages of TiO2 nanoparticles (0, 0.01, 0.02, 0.03, and 0.04 wt.%).To synthesize the Al7075-TiO₂ nanocomposite, the weights of TiO₂ nanoparticles were initially calculated by means of a four-digit sensitive balance.Then, Al7075 alloy ingots were charged into the crucible and superheated in an electrical resistance furnace for 60 minutes at a temperature of 750 °C.A digital temperature controller was used with an accuracy of ±30 °C to manage the furnace temperature until the aluminium ingot totally melted.At that time, the specified weight percentages of nanotitanium dioxide powder were added to melt with stirring until the powder was homogeneous with the molten alloy.To study the effect of reinforcement, one specimen was left without reinforcements.The molten composite was then poured into a cylindrical mould with a diameter of 22 mm and a length of 100 mm and allowed to solidify.Later, the casting process was completed and cooled.Specimens were taken out of the mould.The distinctive features of prepared nanocomposites were studied by using scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy.The XRD pattern of the base matrix Al7075 and 0.01, 0.02, 0.03, and 0.04 wt.% of nano-TiO2-reinforced aluminium matrix nanocomposites was recorded using an X-ray diffractometer.For microstructural analyses, specimens with a diameter of 22 mm and a length of 10 mm were cut from the central portion of the casting.
The hardness test of nanocomposites was carried out according to ASTM E384 13 by using a Vickers microhardness tester.The surface of each nanocomposite specimen was polished before the test.Then, a 0.5 kg load was applied with a 20second dwell time at room temperature 25 °C, and the readings were taken on each nanocomposite at various locations in order to calculate the average value of hardness.
A tensile test was conducted in order to determine the act of a nanocomposite under an axial stretching load.The specimens were carried out according to the specifications of the ASTM E8-04 standard 14 .Similarly, tensile tests were conducted before and after the addition of TiO₂ nanoparticles, and for each specimen, the indentation test was conducted with three values, and the averaged value was obtained.Tensile specimen Potentiodynamic polarization measurements were conducted for the specimen exposed to the 0.5 M HCl solution to investigate the corrosion behavior of the specimens.The specimens for the test were cut from each kind of composite with a diameter of 22 mm and a height of 10 mm, after which the specimen surfaces were mechanically polished with emery paper starting from 200 grit down to 1000 grit.A three-electrode electrochemical cell was employed in the experiment with a platinum plate as the auxiliary electrode, a saturated calomel electrode (SCE) as the reference electrode, and the specimen as the working electrode, which consisted of an Al7075 alloy and Al7075 with TiO₂ nanoparticles reinforced aluminium matrix composites with a diameter of 22 mm.The specimens were cold-mounted in epoxy resin, following a similar procedure as elsewhere 3 and shown in Fig 3 .Different grades of sandpaper were used to polish the specimen, which was then polished to a mirror.After that, the electrodes were cleaned using double-distilled water and ethanol.The specimen was immersed in the test solution for 100 seconds to achieve a steady open circuit potential (OCP) at ambient temperature.The potentiodynamic polarization proceeded with a 0.5 mV/min scanning rate in a 0.5 M HCl solution.The Tafel formula was used to fit all data polarization tests.

Results and Discussion
In the current investigation, hardness, tensile, and corrosion tests were conducted for pure aluminium and nanocomposite specimens, in addition to studying the structure of the prepared samples using XRD and SEM.

Hardness Measurements
The Vickers hardness values were measured on polished specimens of Al7075 alloy, and nanocomposites reinforced with nanoparticles of TiO2 are shown in Fig 4 .It is seen that the hardness of the nanocomposites with the addition of nanoTiO₂ increased significantly compared to the Al7075 alloy.The increased hardness of the nanocomposite can be due to the hard nature of TiO2 nanoparticles compared to the aluminium base alloy, especially nanomaterial being hard, which contributes positively to the hardness of alloys.
Here the effect of the nanomaterials appears, as the good diffusion of the nanomaterial in the matrix prevents dislocation in the Al7075/nano TiO₂ matrix, and thus increases the hardness, which in turn increases the corrosion resistance, as will appear later in the corrosion section.after the addition of nano TiO₂ It is evident that the increased strength of nanoparticles controls the depth of penetration.The nano-TiO₂ particles provide support to improve contact stress, which limits surface deformation and surface scratches.Also, the dislocations (line defects) inside the nano-TiO₂ under compression are the main factor in resisting high pressures, so nano-TiO₂ forms an intragranular structure that improves the grain boundary and promotes hardness 11 .However, the hardness of the Al7075/nano-TiO₂ nanocomposite has remained Nanoparticles provide reinforcement and support for stress, which resists deformation and corrosion between mating surfaces.Therefore, hardness increases with the reinforcement weight percentage, but at 0.04%, the value of hardness becomes practically stable due to the presence of a tougher phase 11,15 .

Tensile strength
The highest value of engineering stress, or what is known as tensile strength, was calculated.Since aluminium is ductile, it was noted that the deformation was uniform along the length of the measurement section, and this is because the tensile strength corresponds to the point from which the deformation began to be centred, thus the fracture strength is tensile strength.Tests were carried out in an environmentally conditioned room at 25°C and 40% relative humidity.The tensile load was applied to three duplicate specimens for each concentration of the prepared specimens.The tensile curves of pure Al7075 and 0.01, 0.02, 0.03, and 0.04 wt.% of Al7075/nano TiO₂ are shown in Fig 5 .The nano TiO₂-free models showed plastic deformation, and this represents a characteristic yielding route.In contrast, the nano TiO₂-filled composite specimens exhibit semi-brittle behaviour under tensile deformation.SEM images of the spacemen exhibited that there is a good diffusion of nano TiO₂ within the Al7075 structure, which may explain the semi-brittle behaviour.Back to Fig 5, it is clear that after increasing the tensile strength value, there is constancy at the three highest concentrations, 0.02, 0.03, and 0.04 wt.%,where the values are 0.766, 0.773, and 0.788 MPa, respectively.So, at a concentration of 0.04 wt.%, the increase is 60%.This percentage is higher than Nagaral et al. 11 achieved (47%), so low concentrations of nano TiO2 (0.04) can achieve higher tensile values by 60%.The increase in tensile strength of the specimens is due to the transfer of the loading to the hardest material (nano-TiO₂).In practice, nano TiO₂ bears more of the load than the Al7075 alloy.This is due to the fact that the small size of nano TiO₂ allows a larger surface area for storage or interaction, so the rigidity of the nanomaterials is greater if compared to the mass; therefore, the nanosize prevents stress or strain more and thus leads to an increase in the rigidity of the particle 11,16 .From the above (mechanical properties), it was found that the 0.01% concentration did not show any distinction, so it was discarded in structural and corrosion studies.It was also noted that concentrations higher than 0.04% made no discernible difference in the measured values for the most part, so the concentration of 0.04% was sufficient.

XRD analysis of the nanocomposites
Among the several techniques available, X-ray diffraction is most commonly used to consolidate the phase analysis of the metal matrix composition and to determine the reaction between alloys and nanoparticles.The XRD patterns of pure Al7075 and nanocomposites reinforced with 0.02, 0.03, and 0.04 wt.% of nanoTiO2 are shown in Fig. 6.The XRD patterns confirmed the presence of Al7075 and TiO₂ nanoparticles in composite specimens.Four peaks in the X-ray pattern were found in the two span ranges from nanoparticles are not diagnosed clearly because the weight percentage in the matrix is less than 0.04 wt.%.The sharpness of the peaks is due to a wellordered crystalline material 18 .

Debye-Scherrer method
The pure Al7075 and nanocomposites reinforced with 0.02, 0.03, and 0.04 wt.% of nanoTiO2 crystallite sizes were determined using the Scherrer equation and demonstrated in the Tables 2-5, which is as follows 19 : Where is λ the X-ray wavelength, is the line broadening at FWHM, D is the average crystallite size, K (a constant) = 0.9, and is Bragg's angle.
The creation of nanocomposite materials has been greatly influenced by the crystal structure and particle size.The Scherrer plot method was used to analyse the broadening of peaks with lattice strain and crystallite size owing to dislocation from XRD data.The total of the sample-and instrument-related effects, as follows, determines the breadth of the Bragg peak:

SEM analysis of the nanocomposites
The SEM fractographic image of the fabricated Al7075 alloy matrix and also the Al7075 alloy matrix reinforced with TiO2 nanocomposites are illustrated in Figs 7(ad).The microstructure of the base matrix Al7075 alloy is presented in  b -d).The microstructures of the nanocomposites in Figs.7(bd) indicated that the distribution of TiO2 nanoparticles in the matrix Al7075 alloy is homogenous with no agglomeration.The fraction images for all concentrations show the structural homogeneity and the absence of aggregates in the added nano-TiO2.Choosing low concentrations is the important key here in this homogeneous distribution.Furthermore, cracks or pores have no appearance 11 .The TiO2 nanoparticle dispersion appeared to be uniform throughout the aluminium matrix due to the nanostructures having a higher surface area and providing enough absorption sites for all involved molecules in a small space.Moreover, this is possible due to the appropriate method factors used for casting manufacture, the efficient stirring action, and the use of proper process parameters 20 .The homogenous distribution of nanoparticles is required to improve the mechanical properties of the matrix 21 .Furthermore, the TiO2 nanoparticles are well bound to the aluminium matrix 22

Conclusion
For the purpose of improving the performance of the Al7075 matrix alloy used in gears, columns, and aircraft fittings, aluminium Al7075 nanocomposites were subjected to the stir casting method and evaluation of mechanical and corrosion properties with various weight percentages of reinforcement.The inclusion of TiO₂ as reinforcement was found to be useful in enhancing the mechanical characteristics of aluminium nanocomposites.In the tensile test, the results showed an increase of 60% at a concentration of 0.04%, while the hardness showed a noticeable increase of 22%.The XRD and SEM analyses of the fabricated nanocomposites demonstrated the uniform dispersal of the nanoparticles in the structure of the Al7075 alloy.Dispersion without agglomeration or clustering has to be ensured in the fabrication of aluminium nanocomposites.The nanostructural characteristics of the nanocomposites directly influence their mechanical properties.The corrosion resistance is found to be the maximum for the nanocomposites with TiO₂ and the least for the Al7075 alloy without TiO₂ nanoparticles.
Fig 1 shows the dimensions of the specimen used for tensile studies.The tensile specimen is shown in Fig 2.

Figure 1 .Figure 2 .
Figure 1.Dimension of the tensile testing specimen (all dimensions in mm)

Figure 4 .
Figure 4. Hardness results of Al 7075 before andafter the addition of nano TiO₂ It is evident that the increased strength of nanoparticles controls the depth of penetration.The nano-TiO₂ particles provide support to improve contact stress, which limits surface deformation and surface scratches.Also, the dislocations (line defects) inside the nano-TiO₂ under compression are the main factor in resisting high pressures, so nano-TiO₂ forms an intragranular structure that improves the grain boundary and promotes hardness11 .However, the hardness of the Al7075/nano-TiO₂ nanocomposite has remained

Fig 7 (
a), which exhibits the formation of an aluminium dendritic network structure caused by the supercooling of the composite during solidification.The scanning electron micrograph of the produced Al7075-TiO2 nanocomposites containing a different weight percentage of TiO2 nanoparticle reinforcement is shown in Figs 7(

Figure 7 .Fig 8
Figure 7. Scanning electron microscope images at 50X & 100X magnification of (a) pure Al7075 (b) Al7075+ 0.02 wt.% of Nano-TiO₂ (c) Al7075/ 0.03 wt.% of Nano-TiO₂ (d) with 0.04 wt.% of Nano-TiO₂ Corrosion rate Fig 8 depicts the potentiodynamic polarization curves of pure Al7075 alloy and nanocomposites specimens with various weight percentages of TiO₂ nanoparticles in a 0.5 M HCl solution at room temperature.Corrosion current density, corrosion potential, and cathodic and anodic slopes were determined from the polarization curves by the Tafel extrapolation method, and the results are presented in Table 6.The shapes of the polarization curves of pure Al7075 alloy and nanocomposites are similar.The polarization curves in Fig 8 exhibited that the