Influence of Al₂O₃ and SiO₂ nano-particles on hardness and corrosion resistance of aluminum alloy (Al-4.5%Cu-1.5%Mg)

The route materials to prepare the compacted aluminum alloy (Al–4.5%Cu-1.5%Mg) are powder of aluminum (Al), powder of copper (Cu), powder of magnesium (Mg) and two kinds of nano-particles (SiO₂ and Al₂O₃). The properties of these materials are given in table 1.

Blending or mixing or blending is mostly known as a pre-compaction stage in the powder technology. Theoretically, every composition is prepared commencing from the powders in form of elements. In the experimental procedure, (9.4 gm) from powder of Al was blended with (0.45 gm) from Cu powder, (0.15 gm) Mg powder, Al₂O₃ nano particles having particle size of (50 nm), and SiO₂ having particle size of (25 nm) in 2.5 wt%. For each one, (0.5%) zinc stearate was added as a lubricant and binder. Blending was conducted using a ball mill kind (using a motor to rotate the jar) that has balls made of alumina. The number of balls number 20 with a proportion (20) to (1) weight of powder to weight of balls for a (6 hr) time of blending at (650 rpm) speed for getting a consistence blend and prevent the agglomeration of nanoparticles. The uniform blending will enhance the ability of sintering, compact ejection, and compact strength compact. The process of a uniaxial cold compaction was conducted on the blended powders for obtaining an adequate compaction and for producing 'Green Billet' having less porosity. Then, powders were compressed at (450 MPa) for the whole specimens utilized in the present investigation. A single direction action cylindrical die having a hole diameter of (20 mm), and an electric hydraulic press with a capacity of 20 ton, for 15 min incubation time in every exerted pressure, were employed. The process of sintering was performed in a furnace having an electric tube, 'type MITI Corporation GSL', with a maximum temperature of (1600 °C) for sintering utilizing the argon (Ar) gas having (99.99%) purity. A process of sintering was done in a furnace using (2 L min⁻¹) (Ar) flow and at temperature (560 °C) for 60 min . The analysis of the prepared sintered specimen was conducted using the x-ray fluorescent (XRF) device (type 'Spectro X-Lap) that operated at (45 KV), (0.5 mA) and (25 KV) range of energy. The chemical analysis of the sintered parent alloy is shown in table 2. Also XRD analysis was performed for the best sintered specimen using x-ray diffractometer type (Shimadzu- XRD-6000) Japan using Cu- tube with wavelength 1.540 A°, current 30 mA, voltage 40KV and scan rate was 5° min⁻¹ to identify the main phases in the specimen.

The final measurement actual density and porosity for the whole specimens was achieved in accordance with standard (ASTM D792) depending on the law of Archimedes. Equation (1) gives the material specific gravity, while equation (2) yields the porosity [14]. The process of measurement was done at the room temperature, and water with (1 gm cm⁻³) density was utilized as the auxiliary liquid.

$$\begin{eqnarray*}&&Sp{.}_{.}G{r}_{(forsample)}=\displaystyle \frac{{\boldsymbol{W}}{\bf{1}}}{\left({\boldsymbol{W}}{\bf{1}}-{\boldsymbol{W}}{\bf{2}}\right)}\times {\boldsymbol{Sp}}{\boldsymbol{.Gr}}(1)\end{eqnarray*}$$

Where,

$$\begin{eqnarray}&&{\rm{P}}=\displaystyle \frac{({\bf{W3}}-{\bf{W1}})}{({\bf{W3}}-{\bf{W2}})}\times {\bf{Sp}}.{\bf{Gr}}\end{eqnarray} \tag{ 2 }$$

Where, P is the porosity sample, W1 is the weight of sample (gm) in the air, W2 is the weight of suspended sample in the liquid (gm), and W3 is the weight of the wet (soaked in air) sample (gm).

${\rm{Sp}}.{{\rm{Gr}}}_{({\rm{for}}{\rm{sample}})}={\rm{material}}\,{\rm{specific}}\,{\rm{gravity}}$

Specimens were first prepared, then ground and finally polished to remove the scratches from the surface and to obtain a mirror surface. After that the etching process was carried out on the specimens by utilizing etching solution Keller's reagent (composed of 95 ml H₂O, 2.5 ml HNO₃, 1.5 ml HCL, 1.0 ml HF). They were washed and dried. Microstructure examination was performed by using optical microscope (Type MBL3300) (model KRUSS OPTRONIC-Germany ) connected to computer. Scanning electron microscope (SEM) type (VEGA3LM TESCAN COMPANY) provided with EDS (type Oxford) was used to examine the microstructure and topography of specimen surface.

Specimens micro-hardness measurement was conducted via a tester of Vickers hardness 'type (LECO) Micro-hardness Tester (LM248AT)', using (1.96 N) applied load for (15 s) indentation time. The microhardness readings were taken at (5) equivalent positions for very specimen for evaluating the average value micro-hardness.

In the present work, the parameters of the corrosion of base material (Al-4.5%Cu-1.5%Mg) and nano composites reinforced by Al₂O₃ or/and SiO₂ nanoparticles specimens after immersion in the solution 3.5%NaCl at the room temperature were evaluated. A specimen having (1 cm²) surface area was placed in an electro-chemical cell of (1000 ml) consisting of three electrodes. The 'working electrode' is the corroding metal specimen having (1 cm²) that exposed to the solution, while the 'auxiliary or the counter electrode' is, in general, a platinum rod, which is an inert conductor having dimensions of (10 × 100) mm. The 'reference' electrode' or (standard H₂ electrode) was utilized to measure the working electrode potential. The cyclic polarization test range was from (−250) to (+1000 mV + 1500) under and above the potential of the open circuit, respectively. Through the test of the cyclic polarization, the rate of scan was (3 mV s⁻¹). The tests of the electrochemical corrosion via the method of 'Tafel extrapolation' and the cyclic polarization tests were conducted via employing 'Wenking MLab multi channels' as well as 'SCF-MLab'. The measuring system of corrosion is from 'Bank Electronics-Intelligent controls GmbH, Germany 2007'. The corrosion parameters were determined, like (Icorr) and (Ecorr) to compute the rate of corrosion (CR) using the following formula [15].

$$\begin{eqnarray}&&{\bf{CR}}\left({\bf{mpy}}\right)=0.13\times {\bf{Icorr}}\times \left(\displaystyle \frac{{\bf{EW}}}{{\boldsymbol{\rho }}}\right)\end{eqnarray} \tag{ 3 }$$

Where, (CR) is the rate of corrosion in mils/year (mpy), (Icorr) is the corrosion current density in (μA/cm²), (EW) is the equivalent weight in (gm/equivalent), and (ρ) is the density of the corroding specimen (gm.cm⁻³).

Pitting energy was measured by using the mostly common standard model of 'Koizuimi Placom' N-Series that induced the stimulatingly greatest reaching in the 'Placom Digital Planimeter KP-90N' history which ever done. The capacity of measuring is noticeably expanded. The measurement via a 6-digit pulse count helps to record (100) times bigger heaped area than the usual planimeters.

The wetability or hdyrophobicity of the base alloy (Al-4.5%Cu-1.5%Mg) and nano composites reinforced by Al₂O₃ or/and SiO₂ nanoparticles specimens before corrosion were evaluated and measured with static drop of water by using the liquid and solid contact angle meter type CAM 110-O4W made in China which provided with CCD camera.

The outputs of measurement of the mean size of particles for the (SiO₂) or (Al₂O₃) nanopowders were obtained beyond preparing a single drop that was placed on a glass slide for only nano alumina or silica. (50 nm) and (25 nm) from nanopowder (Al₂O₃) and (SiO₂), respectively were utilized as depicted in figures 1 and 2 that indicate the topography, and the particles distribution. The obtained images via the (AFM) microscope resemble topographical maps. The image color indicates to the material height. The image of lighter parts is higher. Figure 3 shows the micrographs of sintered sample (Al-4.5 wt%Cu- 1.5%Mg) base alloy at the best sintering temperature of (560 °C) and time of 60 min. It is believed sufficient for appreciable densification through conventional mechanisms such as particles rearrangement and enable Cu and Mg to diffuse into Al matrix. There are different grain sizes and clear grain boundaries of α-Al phase. Figure 4 indicates the analysis chart of (XRD) for the sintered parent alloy. Such results match with analysis of (XRD) of the sintered (Al-4.5%Cu-1.5%Mg) specimen at the best sintering temperature of (560 °C) for 60 min from reference [16]. It was found, when comparing with the peaks of diffraction from the aluminum powder, that the peaks of the copper powder and magnesium powder phases (Al₂CuMg), (Al₂Cu) and (Mg₂Cu₆Al₅) are formed and distributed uniformly in α-Al-matrix. These phases possess less intensity in the pattern of diffraction, and this is ascribed to the weight low percentages of such phases in the parent alloy. Such resullts corroborated the alloying elements diffusion of copper and magnesium in the matrix of aluminum through the process of sintering and the intermetallic compounds formation as phases of strengthening.

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Figures 5 and 6 show SEM-EDS elemental mapping analysis in the composites reinforced with 5 wt% nano alumina and silica respectively after sintering at 560 °C for 60 min. EDS analysis reveals the presence of oxygen and Al ,Si , Cu and Mg which confirms the presence of Al₂O₃ and SiO₂ nanoparticles distributed homogenously in Al -matrix, in addition to intermetallic compounds of phases (Al₂CuMg), (Al₂Cu) and (Mg₂Cu₆Al₅) as 2nd phase precipitates were observed as mentioned in above paragraph(4.1).

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The density and porosity measurement outputs beyond the process of sintering at 560 °C for 60 min for the parent alloy and composites that reinforced by Al₂O₃ and/or SiO₂ nano particles are given in table 3. The base alloy density was noted to increase with the addition of 5% nano powder for each Al₂O₃ or SiO₂, while the porosity percentage reduces in comparison with the parent alloy. This table also reveals the increased density from (2.63 g cm⁻³) to (2.68) g cm⁻³ and (2.66) g cm⁻³ with the addition of 5%Al₂O₃ and 5% SiO₂ nanoparticles, respectively. These values are within the standard one (10%–20%) [17]. The density beside the less pores causes to increase the bonding of particles between the nanoparticles and the microparticles of the alloy (Al-4.5%Cu-1.5%Mg). The mismatch in the coefficient of thermal expansion between the alloy (Al-4.5%Cu-1.5%Mg) matrix and the nano Al₂O₃ or nano SiO₂ could produce residual stresses about the scattered nano-particles which could cause microcracks and decrease in the mechanical characteristics. Furthermore, the nano Al₂O₃ or SiO₂ present at the grain boundaries are assumed to act like barriers to avoid the grains close up. It has been found that the hybrid nano composities with 2.5%Al₂O₃ and 2.5% SiO₂ (sample D) exhibited higher densities than the base sample (A) and samples (B and C). Such outputs agree with the researcher Cooke et al [18]. The density increment is associated with the porosity reduction for nano composities, and the pores are going to close with raising the nanoparticles addition. Such results are in conformity with the [19]. In the present study, the nanoparticles addition acts as micro filling and makes enhancement in the filling of pore such that the nano Al₂O₃ or SiO₂ particles will fill the (Al-Cu-Mg) structure voids. This structure becomes compact and denser, resulting a densification influence that improves the sintered microstructure.

Table 3 manifests the results of Vickers micro-hardness for the whole specimens at the sintered at 560 °C for (60 min). It was noticed that the specimens of (MMNCs) possess importantly higher micro-hardness than the base material. The micro-hardness increased with the addition of nanoAl₂O₃ or/and nano SiO₂ powders in the aluminum matrix. That is owing to the higher micro-hardness of nano silica and alumina. Also, it was observed that the specimen of (MMNCs) reinforced by the hybrid addition of (2.5 wt% SiO₂ + 2.5%wt Al₂O₃) has the highest value of micro-hardness in comparison with the parent alloy and composites specimens reinforced by a single addition. This is due to the good densification with a high density and decreasing porosity. Additionally, the particles of nano Al₂O₃ or SiO₂ work as a filling medium that possesses has a high diffusivity and capability for entering the inter-spacing of Al phase crystal structure and closing the micro-porosity in the matrix of alloy. The hardness enhancement was (68.5%) for the hybrid addition of Al₂O₃ and SiO₂. Several investigators confirmed such results in their research works, like Muna and Mohammed [19] and Ali Hubi et al [20]. Abdur Rahman et al [21] showed the improvement in hardness was 44.11% for Al 7150 alloy reinforced with 15 vol% of Al₂O₃ nanoparticles when compared to the unreinforced base alloy .

In the current research, the existence of Mg and Cu, in the Al-α phase of base alloy (Al-4.5%-1.5%Mg) and also the intermetallic phases Al₂CuMg and Mg₂Cu₆Al₅ (as identified via the XRD) act like cathodic locations that promote the protective oxide film dissolution and improve the corrosion of pitting in the (3.5%NaCl) solution. These outputs are corroborated via the researcher of reference [22], which indicated that the intermetallic compounds reactively of copper and magnesium takes a role in the consistent dissolution of such compounds and re-deposition of Cu pursued via the localized dissolution of the encompassing aluminum matrix in accordance with the subsequent reactions:

$$\begin{eqnarray}&&{\rm{Al}}\to {{\rm{Al}}}^{+3}+3{{\rm{e}}}^{-}\end{eqnarray} \tag{ 4 }$$

$$\begin{eqnarray}&&{\rm{Mg}}\to {{\rm{Mg}}}^{+2}+2{{\rm{e}}}^{-}\end{eqnarray} \tag{ 5 }$$

$$\begin{eqnarray}&&{{\rm{Al}}}_{2}{\rm{Cu}}\to {{\rm{Cu}}}^{\cdot }+2{{\rm{Al}}}^{+3}\end{eqnarray} \tag{ 6 }$$

$$\begin{eqnarray}&&{{\rm{Al}}}_{2}{\rm{Cu}}\to 2{{\rm{Al}}}^{+3}+{{\rm{Cu}}}^{+2}+8{\rm{e}},\,{\rm{and}}\,{{\rm{Cu}}}^{+2}+\,2{\rm{e}}\to {{\rm{Cu}}}^{\cdot }\end{eqnarray} \tag{ 7 }$$

The above clarified results corroborate that the (Al–Cu–Mg) inter-metallics are the desired locations for the pitting within the alloy (Al-4.5%Cu-1.5%Mg) or aluminum alloy (2024). Therefore, to realize the Al alloy corrosion behavior, it is necessary to study the passive films formed on the base alloy (Al–Cu–Mg) together with the alloys having the nanoparticles (SiO₂ or Al₂O₃) for relating the electro-chemical behavior of corrosion of nano composites materials and base alloy to the passive films structure and chemical composition above their surface and for realizing the (Al–Cu–Mg) inter-metallics reactivity in the Al alloy.

Dimitrov et al [23] manifested that the potential of corrosion of the alloy (Al2CuMg) specimen was highly negative than the aluminum alloy (2024-T3) in a chloride solution. These results explained that one can assume that the discrepancies in the crystallographic structure can be noted between the obtained phases within the (Al–Cu–Mg) alloy matrix.

The anodic trend of Al₂CuMg (S‐phase) specimens in (0.1 M) NaOH solution was studied. The distinctive critical potential trend of the de-alloying was noticed whilst sweeping the potential from the open circuit in the anodic direction.

Brown and Kobayashi [24] demonstrated that the electro-chemical behavior of the (Al–Cu–Mg) particles changed from the anodic to the cathodic towards the aluminum matrix owing to a change in their surface chemical composition. In this work, a similar phenomenon was noticed in studying the behavior of the electro-chemical corrosion of the sintered alloy (Al-4.5%Cu-1.5%Mg) in the solution of 3.5% NaCl.

In the present work, the added 5 wt% of (Al₂O₃) or 5% of (SiO₂) to the base alloy (Al-4.5%Cu-1.5%Mg) led to shift the open circuit potential (Eocp) to (−551.2 mV) or (−620 mV), respectively to lower negative magnitudes than the parent alloy of −630 mV, as depicted in figure 7 and table 4.

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Table 4 lists the corrosion parameters results from the Tafel and cyclic polarizations in (3.5%NaCl) solution for composites reinforced by single and hybrid addition of (nano Al₂O₃) or/and (nano SiO₂). Figures 8–11 evince the cyclic polarization curves for the parent material as well as the composites reinforced by 5 wt% Al₂O_3, 5 wt% SiO₂ and hybrid addition of (2.5%Al₂O₃ + 2.5%SiO₂), respectively in the (3.5%NaCl) solution. The curves of cyclic polarization for the base material and all nano composites are summarized in figure 12. It was found that the corrosion rate decreases with the addition of Al₂O₃ or/and SiO₂ nanoparticles, whereas the resistance of corrosion of the composites reinforced by the addition of hybrid (Al₂O₃ and SiO₂) nanoparticles samples showed a noticeable increase and became the greatest in comparison with the parent alloy and other composites. The corrosion potential (Ecorr.) of the hybrid composite reinforced was shown to be less negative (more noble) than that of the parent alloy and reinforced composites with the single addition of nanoparticles of Al₂O₃ or SiO₂. Such results are in conformity with those of the researchers, Ramachandra et al [25] who found that the resistance of corrosion of nano composite of Al-alloy (AA6061) reinforced with 7.5% ZrO₂ was higher than the parent alloy in the solution of 3.5% NaCl, and the rate of corrosion decreased significantly via increasing the percentage of ZrO₂ nanoparticles in the nano composite.

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4.6.1. Micrographs characterization

Figures 13 and 14 illustrate the micrographic images of the sintered parent alloy and the hybrid reinforced composite by (2.5% Al₂O₃ + 2.5% SiO₂) after the corrosion test, respectively. The Al₂O₃ or/and SiO₂ nano particles addition increases the oxide film adherence to the metal surface, reduces the number of pits, and varies the shape and size of the pits. That is owing to the pits covering via the Al₂O₃ or/and SiO₂ nano particles. Muna and Basma [26] confirmed these results in Al-4.5% Cu-1.5% Mg alloy reinforced with nano Al₂O₃ composites fabricated by powder metallurgy route.

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4.6.2. Pitting energy measurement

Table 5 shows the water contact angles of the base alloy (Al-4.5%Cu 1.5%Mg) and the reinforced nano composities with %Al₂O₃ or/and SiO₂ nanoparticles at the best sintering temperature (560 °C) before corrosion test in (3.5%NaCl) gives an indication of the effect of nanoparticles in an enhancement of stability and continuous of protective oxide layer on the specimen surface and led to increase contact angles of composite specimens as compared to the base alloy. The contact angles obtained with specimens of the base alloy (A) and all nanocomposites (B,C&D) are 60.089°, 79.678°, 71.135°,and 75.596° respectively as shown in figures 15(a)–(d). It was observed that there are increasing in contact angles of all nanocomposites as ccompared to the base alloy. This is due to hydrophobic behavior of protective oxide film on the nanocomposite surface. Chien-TeHsieh et al [27] demonstrated the hydrophobic coating of silica nanoparticles onto microscaled carbon fabrics (CFs) and studied the superhydrophobic behavior of nanocomposite microstructures. The two composite surfaces are based on regularly ordered carbon fibers (8–10 μm in diameter) that are coated with SiO₂ nanoparticles with an average size of 300–500 nm. The results showed that the increasing the density of silica on CFs showed significant effects on the enhancement of static contact angle, decrease of contact angle hysteresis, and superhydrophobic stability.

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