The impact of zirconia nanoparticles on the mechanical characteristics of 7075 aluminum alloy

1 Introduction

In the last two decades, a lot of attention has been given to composites of base metal due to their remarkable mechanical characteristics, including hardness and strength. Aluminum is considered the most reliable material that can be used to get these superior properties [1,2]. Recently, a ceramic additive to reinforce the metallic matrix has played a significant role in various engineering applications. Its impact appears in the aerospace industries and automotive applications owing to its corrosion resistance, high strength, and low density. These achievements are accomplished by mixing various materials, such as metallic and ceramic alloys [3,4]. Due to its excellent properties [5], aluminum alloy 7075 metal matrix is considered the most promising material used in the aeronautics industry for various applications such as shafts, gears, and aircraft fittings. This aluminum nanocomposite displays a decreased tribological behavior since it is difficult to produce a complete homogeneous microstructure of Al matrix composites [6,7,8]. Powder metallurgy [9,10,11], spray deposition [1,12], squeeze casting [13], mechanical milling [14], and stir casting [4,15–17] are some of the ways that metal matrix composites can be made. Numerous studies have been conducted to study the impact of ceramic nanoparticles (NPs) on the Al matrix to show the tribological and mechanical characteristics. Titanium dioxide (TiO₂) (NPs) was added to pure aluminum using powder metallurgy. This revealed an increase in the power of tensile and porosity with increasing TiO₂ (NPs) concentration. Similarly, this composite showed excellent anti-wear in contrast to metal matrix alloys [18]. Ravikumar et al. [19] examined the mechanical characteristics and wear behavior of AA7075 reinforced by Al₂O₃. Abbas and Rasheed reported the mechanical characterization of the Cu:TiO₂ nanomaterial [20].

The AA7075 reinforced by Al₂O₃ was produced using stir casting. The wear resistance and mechanical characteristics of adding Al₂O₃ particulates have improved significantly. Also, an improvement in mechanical properties has been gained with 25 nm Al₂O₃ size addition to AA6061 alloy [21]. Al-Zuhairi and Alshalal [22] examined the silicon influence of carbide on the mechanical behavior of the aluminum nanocomposite. The finding showed an enhancement in ultimate compressive strength (UTS), compression test, and the values of hardness [23]. Numerical methods can be utilized to examine the mechanical characteristics of the materials [24–42].

The hardness and wear resistance of Al enhanced with ZrO₂ NPs were examined. This alloy was manufactured using the powder metallurgy technique. A pin-on-disc wear test apparatus was utilized for the evaluation. The findings proved that the wear resistance and hardness of reinforced composites were significantly enhanced with increasing ZrO₂ NP percentage.

In this investigation, Al 7075 composite materials reinforced with 0.8, 1.6, and 2.4% of ZrO₂ NP ratios were synthesized using a stir-casting process. Certain mechanical characteristics of composites, such as structural morphology, hardness, composition of the elements, young modulus, thermal conductivity, and roughness values of the samples, have been examined.

2 Experimental procedure

2.2 Nanocomposite preparation

Figure 1 shows melting the Al 7075 alloy in a gas furnace lined with the isolated material at 750°C for 5 min using a graphite crucible. The temperature was measured using a laser temperature measurement device. Before adding nano-ZrO₂ particles, they were covered gently with AL particles to ensure that the particles do not oxidize and prevent them from floating. When the melting process is completed, the slag is removed using aluminum hydrochloride.

Figure 1

Gas furnace.

The structure is homogenized by stirring, and the ZrO₂ NP is added to the liquefied ingredient. Mechanical stirring is performed for 3–4 min at a speed of 600 rpm on a stirrer. The molten metal is poured into a cast iron mold to reduce heat leakage, as shown in Figure 2, and then cooled in air.

Figure 2

Casting mold with a cooling system. (a) Aschematic mold, (b) Section A_A in the schematic mold, (c) The section metal mold with cooling tubes and (d) Metal mold assembly.

2.4 Hardness test

The measurements of all specimens were performed by using the Vickers hardness test with a diamond indenter to determine the hardness of the Al7075/ZrO₂ nanocomposite. The accuracy reading was determined by averaging the results of three indentations (HVS-1000 Digital Micro Hardness Tester, Laryee Technology Co., Ltd., from China). The hardness scale HV or HK can be selected on the operation panel. The hardness value tested will be automatically calculated and displayed. Various hardness values can be converted to one another. The hardness error can be corrected by software input to make the hardness more accurate. Test results can be automatically stored, processed, and printed. The HVS-1000 digital display microhardness tester has a manual switch between the head and the objective lens during the test, and the test point positioning is accurate. To determine the ZrO₂ NP content on strength, the Vickers microhardness test was performed on the samples using the micro hardness tester (HVS-1000 Digital, Figure 3). The test load (force) was fixed to be (300 gf = 2.942 N) for an indentation time of 0–60 s (5 s as a unit), measuring range = 5–3,000 HV, test microscope magnification = 400× for measuring and 100× for observation, maximum height of specimen = 65 mm, with the diagonal of impression in the range of 40–60 µm. All samples for hardness tests were polished to a surface roughness of around 0.0625 µm. The distance between the tip of the indenter and the plane of the specimen is about 0.4–0.5 mm. The operation system is as follows: press the input buttons measuring on the eyepiece, the LCD screen shows D1 and D2 values, and the system calculates hardness value automatically and shows testing results (HV value) automatically as well. At least five measurements were taken for each sample, the mean value was utilized to compute its hardness.

Figure 3

Micro hardness tester device.

3 Results and discussion

Table 3 reveals the toughness of the Al matrix strengthened with various quantities of ZrO₂ NPs. The rigidity of Al-ZrO₂ composites increases as the amount of ZrO₂ NPs increases, the greatest hardness was 177 HV, which resulted from the sample containing 2.4 wt% of ZrO₂ NPs. This figure shows that the hardness value improved after increasing the addition of ZrO₂ NPs from 0 to 2.4%; this can be attributed to ZrO₂ NPs, which have a high hardness value when compared with Al hardness.

Table 3 depicts the hardness values of stir-cast Al 7075 composites reinforced with 0.8, 1.6, and 2.4% ZrO₂. Owing to an increase in the ratio of ZrO₂ reinforcement, Table 3 demonstrates that the hardness value increases progressively. Hard-phase ZrO₂ reinforcement particles in the composite structure are primarily responsible for this increase in hardness. Good wetting between the matrix material of the composite, Al 7075, and the ZrO₂ reinforcing particulates increases both the mechanical strength and hardness of the structure. The 120 HV hardness of unadulterated Al 7075 alloy without ZrO₂ reinforcement increased progressively as the ZrO₂ reinforcement ratio increased. The highest hardness value was found to be 177 HV in the composite material with 2.4% ZrO₂ reinforcement, which increased the hardness of the Al 7075 alloy by 20%. In addition to increasing the composites’ hardness, the addition of harder reinforcement particles increased the formation of dislocations, which inhibit plastic deformation. The fluidity of the molten matrix, the density of the reinforcing particles, the rate of solidification, and the distribution of the reinforcing particles are the primary factors influencing the hardness of composites. Consequently, it was observed that the hardness values of ZrO₂-reinforced composites were significantly higher than those of the unadulterated Al 7075 alloy.

SEM images of stir-casted Al 7075 composites reinforced with 0, 0.8, 1.6, and 2.4% of ZrO₂ ratios are given in Figure 3. Energy-dispersive X-ray (EDX) spectra of Al-ZrO₂ surfaces were collected at three individual positions on a solo test section. The average values for the three locations are displayed in Table 3. It seems that the ZrO₂ substance rises with increasing the ZrO₂ NPs except for the specimen with 2.4% ZrO₂ amount, which has less concentration. It is attributed to the nonhomogeneous distribution of ZrO₂ NPs. Figure 4 depicts the distribution of ZrO₂ reinforcement particles in the composite structure using microstructure images. The images suggest that increasing the ZrO₂ reinforcement ratio enhanced the reinforcement distribution’s homogeneity.

Figure 4

Micrograph images and EDX for different amounts of ZrO₂ (a) 0 wt%, (b) 0.8 wt%, (c) 1.6 wt%, and (d) 2.4 wt% adding to Al 7075.

However, increasing the ZrO₂ ratio caused the reinforcement to aggregate. It appears that the displacement of ZrO₂ particles during the stir-casting process causes agglomeration in certain regions of the molten Al 7075 matrix. Figure 4, which depicts the microstructure of a composite material reinforced with 2.4% ZrO₂, suggests that dissolving the matrix material in the graphite crucible caused the ZrO₂ particles to migrate upward.

However, this study, which employed the stir-casting method, revealed that the matrix material and reinforcement were sufficiently wetted, and there was no significant porosity in the composite structure. It is possible that the addition of 0.8% ZrO₂ to the molten aluminum during the stir-casting process contributed to hydration. The procedure of preheating the graphite crucible prior to its immersion in the molten metal could have had a positive effect on the absorption. Therefore, it can be concluded that the parameters for the stir-casting procedure used in this study (750°C for the molten metal matrix temperature and 5 min) were suitable. In the production of such particle-reinforced aluminum composites, the construction of a porous composite structure is a significant obstacle. The composite structure’s porosity increases as the number of reinforcing particles increases.

In this investigation, the stir-casting technique appears to have yielded superior matrix-reinforcement compatibility than other production techniques. Figure 4 depicts the interface between the ZrO₂ reinforcing element and the Al 7075 matrix material with greater clarity. The images of the microstructure reveal that there is no porous structure between the aluminum matrix (Al 7075) and the ZrO₂ reinforcing particle, and that the bond between the two phases is strong. This depicts that this excellent hydration will increase the mechanical strength of the composite material, and that the stir-casting method used to produce composites in this study is fruitful.

Additionally, the surface images reveal that the cavities are irregular, with some being larger or deeper than others. The ZrO₂ particles removed from the composite structure by fracture must have been relatively small, as particles with smaller surfaces have weaker adhesion between the matrix and reinforcement interface. Therefore, smaller ZrO₂ particles must have created the microscopic voids on the eroded surfaces.

The surface morphology of all four samples is shown as (a) for Al 7075 alloy, (b) for 0.8% ZrO₂, (c) for 1.6% ZrO₂, and (d) for 2.4% ZrO₂. All images presented have specifications of 10 HV, KV, Mag. 3,500, 30 µm, and 11.3 mm WD. It indicates that there is an aggregation of ZrO₂ particles as a result of a non-regular allocation of particles in the nanocomposite. This may be attributed to stirring speed or stirring time. EDS analyses indicate that the peaks represent the presence of Al, Zr, and O. This confirms that the composite contains ZrO₂ particles. This analysis serves to comprehend the elemental composition of the Al7075/ZrO₂ composite sample (Table 4).

In casting procedures, nano-ZrO₂ NPs typically do not perform a nucleation role. They are utilized as reinforcement particulates to enhance the mechanical properties of the cast material, including grain refinement in solidified materials such as Al 7075.

Nucleation typically refers to the initial formation of solid crystals (nuclei) from a molten state in casting processes. In conventional casting techniques, nucleation occurs naturally as the molten metal solidifies, and the presence of impurities or particular alloying elements can influence the nucleation process. However, adding nano-ZrO₂ NPs to the liquid to promote nucleation in casting is not a prevalent practice. In order to refine the particle size in a cast material like Al 7075, other techniques include the following

1. Grain refiners: In conventional casting, titanium or boron-based grain refiners are added to the molten metal to promote nucleation and control particle development during solidification.

2. Ultrasonic treatment: During the casting process, ultrasonic treatment can be used to induce nucleation and facilitate the formation of nanoscale granules.

3. Controlling the chilling rate during solidification can also have an effect on particle size. Typically, faster chilling rates result in finer granules.

Concerning the assessment of grain size variations in Al7075 with nano-ZrO₂ NPs added, it would indeed be necessary to conduct experiments to observe the effect of reinforcement. Grain refinement in NPs is expected to occur via the anchoring effect, where nano-ZrO₂ NPs act as obstacles to grain growth, resulting in reduced grain diameters.

It is important to note that the specific processing conditions, such as the quantity and size of nano-ZrO₂ NPs introduced, the casting technique used, and the overall composition of the alloy, can all affect the final results. In order to determine the actual effects on grain size and other mechanical properties of the Al 7075 alloy with nano-ZrO₂ NPs added, experimental testing is required.

I can, however, provide some general information regarding the addition of ZrO₂ to composite materials.

In many instances, the performance of composite materials tends to improve as the amount of reinforcing particulates (such as ZrO₂ NPs) increases up to a certain threshold. Beyond this threshold, the performance enhancements may begin to diminish, indicating that the gains become less significant with each additional addition of ZrO₂ NPs. This is due to factors such as particulate agglomeration, limited matrix adhesion, and other potential material property trade-offs. Cost and practicality: When determining the optimal content of ZrO₂ NPs or any other reinforcement, the cost and practicality of the material should also be considered. High concentrations of ZrO₂ NPs or other additives may increase the overall cost of the composite material, rendering it uneconomical. Additionally, some processing techniques may have restrictions on the greatest quantity of ZrO₂ NPs that can be effectively incorporated. Material compatibility: The addition of an excessive quantity of ZrO₂ NPs (or any other reinforcement) may result in compositional imbalances or phase separation, altering the material’s overall properties and potentially decreasing its performance. Experimental validation: In research and development, determining the optimal amount of ZrO₂ NPs or any other additive is typically a portion of the inquiry. Researchers conduct a series of experiments with varying concentrations of the reinforcement to determine the optimal concentration at which the material exhibits the desired properties while avoiding potential disadvantages. In conclusion, the decision to add more ZrO₂ NPs (or any other additive) to a composite material is determined by the application’s specific objectives, intended properties, and limitations. Typically, researchers evaluate the trade-offs and conduct experimental tests to determine the optimal composite material composition for attaining the desired performance characteristics.

To characterize Al-ZrO₂ nanocomposite, a compression test was carried out for all specimens, and the results are shown in Figure 5. The outcomes demonstrate that the yield strength (YS) and maximum strength rise with growing ZrO₂ NP concentration. The change in the composite strength also benefits from the improved distribution and stronger packing of the hard metal into the pure aluminum matrix, which contributes to better interfacial characteristics and this agrees with that of previous study [23]. The variation of yield and maximum strength with ZrO₂ concentration is shown in Figure 6. By incorporating ZrO₂ reinforcement particles, the yield and UTSs have been enhanced. As the quantity of ZrO₂ particles increases, the composite’s YS and UTS increase, while elongation (%) decreases. In comparison to the Al7075 matrix material, the Al 7075/2.4ZrO₂ composite exhibits greater YS and UTS with less elongation. The difference in thermal expansion coefficient is responsible for the composite material’s enhanced compression strength. During solidification, the difference in the coefficient of thermal expansion between matrix material and reinforcements tends to reduce dislocations surrounding ZrO₂ particles. This reinforces the composite under stress conditions.

Figure 5

Stress–strain curve for (a) Al 7075, (b) 0.8% ZrO₂, (c) 1.6% ZrO₂, and (d) 2.4% ZrO₂ nanocomposite.

Figure 6

Young’s modulus yield and maximum strength with ZrO₂ particle concentration.

The composite’s compression strength is enhanced as a result of the increase in the wt% of ZrO₂ particles due to the close interaction between reinforcements and matrix, which reduces dislocation and increases the composite’s tensile strength.

The uniform distribution of ZrO₂ particles with spherical morphology also contributes significantly to the composite’s increased tensile strength. Due to the distinct interface between reinforcements and matrix material, the tensile strength of the composite increases further. The stir-casting technique utilized in the synthesis of composite allows for the homogenous distribution of ZrO₂ reinforcement particles and a distinct interface between reinforcements and matrix material. This reduces the initiation of fracture propagation under conditions of high load. The absence of defects in the uniform distributions of ZrO₂ particles on YS, UTS, and % elongation of AA7075/ZrO₂ particles enhances the YS and UTS of the composite. The refinement of the composite’s particle structure also increases its compressive strength. Overall, the addition of ZrO₂ particles increased the composite’s hardness and compression strength, which decreased its elongation.

Table 5 and Figure 7 depict that the thermal analysis is carried out of stir-casted Al 7075 composites reinforced with 0, 0.8, 1.6, and 2.4% ZrO₂ to find out the thermal behavior of composite materials. In thermal analysis, the thermal conductivity of samples is found with respect to the composition ratio. The thermal conductivity of Al 7075/0.8ZrO₂ composite is higher than other composite materials, in which thermal conductivity will increase by adding zirconium content in composite material. In Al 7075/0.8ZrO₂, the zirconium content is 0.8%, so the thermal conductivity of the material gets increased compared with pure Al 7075.

Figure 7

Thermal conductivity of samples after adding ZrO₂ (0, 0.8, 1.6, 2.4) wt% nanocomposite samples.

The values of the surface roughness of samples R _ave are listed in Table 6, one can see that the sample with the greatest average surface roughness value obtained for the pure sample, and this value decreased by adding ZrO₂ NPs.

Due to the presence of ZrO₂ NPs, the surface roughness of an Al7075 alloy can decrease when more ZrO₂ is added. These NPs can also improve the material’s mechanical properties and surface polish. Here are some explanations for why this occurs:

Fine grain structure: During the solidification of the Al7075 alloy, ZrO₂ NPs serve as nucleation sites, promoting the formation of a fine-grained microstructure. Smaller granules minimize surface irregularities, so fine-grained materials typically have cleaner surfaces than coarse-grained materials. Refinement of grain size: The addition of ZrO₂ NPs can refine the grain size of Al7075 alloy, thereby improving its mechanical properties, such as hardness and strength. A material with enhanced mechanical properties is less susceptible to plastic deformation and attrition, which reduces surface roughness. The dispersion of ZrO₂ NPs within the matrix of Al7075 alloy can enhance the homogeneity of the material. Homogeneous materials have a tendency to exhibit more uniform surface properties, resulting in a decrease in surface roughness.

Reduced porosity: ZrO₂ NPs may act as effective grain refiners and reduce the formation of porosity during solidification. As pores produce cavities and irregularities on a material’s surface, porosity can be a significant factor contributing to increased surface roughness.

Enhanced wear resistance: The presence of ZrO₂ NPs can enhance the Al7075 alloy’s wear resistance. Materials with greater resistance to wear are less susceptible to surface damage and abrasion, resulting in surfaces that become smoother over time.

Surface refinement (smoothing) during processing: In some instances, the incorporation of ZrO₂ NPs may contribute to processing techniques that further enhance the surface finish of the Al7075 alloy, resulting in reduced surface roughness. Notably, the precise effect of adding ZrO₂ to Al7075 can vary depending on the processing parameters, chemical composition, and particle size of the ZrO₂ utilized. Therefore, the reduction in surface roughness should be evaluated experimentally in each instance to corroborate the actual results.

4 Conclusion

After evaluating the microstructures and mechanical characteristics of stir-casted Al 7075 composites reinforced with 0, 0.8, 1.6, and 2.4% ZrO₂, one can reach the following conclusions:

1. Al 7075 matrix composites reinforced with 0, 0.8, 1.6, and 2.4% ZrO₂ were effectively produced using the stir-casting technique, and the manufacturing parameters were determined.

2. In the composite structure, ZrO₂ reinforcement distribution was only partially homogeneous. Effective condensation between the matrix-reinforcement led to the formation of strong bonds.

3. The ZrO₂ reinforcing elements increased the composite structure’s hardness, compressive strength, and Young’s modulus while decreasing its thermal conductivity and surface roughness.

4. The photos of the micrographs refer to the consistent allocation of the soft matrix hard reinforcement. The weight ratio selection of ZrO₂ NPs was optimistic since the mechanical characteristics of the Al-ZrO₂ is better than the pure Al alloy.

5. The findings of this work suggest that the composite’s mechanical characteristics are enhanced by the strong bonds formed between the matrix and reinforcing phases as a result of excellent hydration.