1 Introduction

Modern advancements in vital industries of manufacturing such as aerospace, automotive, defense, and naval require an increasing number of lightweight materials to be used in the construction of their structural parts [1]. Aluminum alloys are widely utilized in numerous industries because of their many potential applications and advantages, including their low density (approximately one-third that of steel), superior formability, and resistance to corrosion [2, 3]. Their application has, however, occasionally been restricted due to their low wear resistance, low hardness and strength, and vulnerability to pitting corrosion [4, 5].

Many scientists are now focused on improving these qualities because of these shortcomings. Equal-channel angular pressing is one of the severe plastic deformations (SPD) techniques used to enhance the mechanical characteristics of Al alloys [6], accumulative roll-bonding (ARB) [7], high-pressure torsion (HPT) [8, 9], and constrained groove pressing (CGP) [10]. These techniques involve obtaining a notable level of grain refinement in the structure, wherein the strength can be significantly increased by the fine or ultrafine grain structure. However, because of a sharp rise in dislocation density during the operations, using SPD techniques may result in a noticeable decline in the formability of the Al alloys [11]. The latest additive manufacturing (AM) method known as friction stir additive manufacturing (FSAM) can create metal matrix composites using various grades of aluminum alloys [12, 13]. Three-layered construction made of 3 mm thick sheets of AA5083 (upper and bottom layer) and AA7075-T6 (middle layer) alloys was created using FSAM technique [14].

FSP is a solid-state method that has been employed recently to modify the matrix microstructure, mechanical and corrosion behavior to create particle-reinforced metal matrix composites (MMCs) [15, 16]. Friction stir welding is the basis for FSP, a useful treatment for microstructural refinement [17]. This procedure involves inserting a non-consumable rotating tool into an integrated workpiece to provide frictional heating and mechanical mixing [18]. FSP tools are primarily utilized for the objective of heating the workpiece, inducing material flow, limiting heated metal flow beneath tool shoulder. When the tool rotates, the material escapes from its front to its back [19]. The FSP procedure is an excellent SPD technique for manufacturing surface composites because it only modifies the surface, leaving the material's properties unaltered. Furthermore, casting flaws may be eliminated, and secondary particulates dissolved using FSP. Change the granular structure, replace tiny, equiaxed grains with dendritic ones, and homogenize the microstructure of an alloy [20,21,22,23,24,25,26]. The creation of surface nanocomposites is significantly hampered by the homogeneous distribution of nanoparticles within aluminum matrix because these particles have high surface area, a tendency to agglomerate, and high energy. One method to create surface nanocomposites that can disperse nanoparticles uniformly and remove their agglomerates is FSP, which involves multiple passes [27,28,29,30,31]. The most common kind of particles applied in compositing are probably ceramic micro and nanoparticles. This is because these composites offer better strength, wear, and creep resistance together with appropriate ductility [32].

Increasing resilience to wear and producing a more consistent distribution of hardening particles have been demonstrated using ceramic nanoparticles [33]. Friction stir spot welding was utilized to successfully create a joint between copper and aluminum sheets with an interlayer of SIC nanoparticles [34, 35]. Moreover, to improve optical and dielectric characteristics, carbon nanotubes, for instance, can also increase electrical and heat conductivity [36]. The synthesis of MMCs has also been done via FSP [16], particularly for light metals like Al/SiC [37, 38], CNT [39,40,41], TiC [42, 43], and Al2O3 [44,45,46]. Numerous processing parameters, including the pin's geometry, rotational and linear speeds, and the number of FSP cycles, influence the FSP procedure [47, 48].

Research has not focused much on how the number of passes affects the size, dispersion, and mechanical attributes of particles produced by FSP in MMCs [49, 50]. Using a modified friction stir manufacturing technique, AZ91 composite layers' mechanical properties, wear, and corrosion have all been studied in relation to vibration [51, 52]. Agglomeration of the hardening particles during the producing process is a significant issue that reduces the mechanical performance and durability of these composites. The aggregated nanoparticles size reduces from 650 to 70 nm when the number of FSP cycles is increased from one to four, according to Sharifitaba et al. [53]. One of these unique and appealing reinforcing particles is ZrO2, which has greater mechanical properties, low thermal expansion, and good thermal conductivity. Better physical, chemical, and wear qualities are possessed by C particles, another reinforcing ingredient. AMMCs are often created using techniques for both solid and liquid states. The AMMCs materials were produced by stir casting, which improved the homogenies distribution of reinforcement materials within matrix [54, 55].

Several reinforcing particles are integrated during the creation of surface material matrix composites (MMC) utilizing FSP. The significance variety of reinforcing particles during FSP and their effects on surface characteristics like hardness, tribological behavior, and corrosion behaviors are covered in numerous of recently published works. Many industries, including the aerospace, automotive, defense, naval, and biomedical sectors, may find use for the manufacturing of functionally nanocomposite materials with graded properties and microstructure. The objectives of the present research are to identify which of the pass numbers 1, 2, 3, 4, and 5 is ideal for creating a surface hybrid aluminum matrix nanocomposite (AMNC) reinforced with ZrO2 using (FSP). The fabrication of the hybrid nanocomposite has undergone extensive analysis and comparison with BM in terms of microstructural development, hardness, tensile properties, wear, and fracture behavior.

2 Materials and procedures

2.1 Materials

Table 1 indicates the chemical composition of the as-received AA1050, which was utilized to be the base material. Zirconium oxide (ZrO2) nano particles were supplied by US Research Nanomaterials, Inc. With size range of < 100 nm was utilized as the reinforcement particles. Figure 1 demonstrates the size and shape of reinforcement nano particles as identified by scanning electron microscopy (SEM).

Table 1 Chemical composition (wt. %) of AA1050 sheet
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Fig. 1
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SEM micrograph and EDX of the as-received ZrO2 reinforcing particulates

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2.2 Friction stir processes (FSP)

Aluminum alloy sheets (AA1050) were wire-cut with dimensions of 200 mm × 50 mm × 6 mm. FSP was conducted using a milling machine. Rectangular grooves with dimensions of 3 mm in depth and 1 mm in width appeared on specimen’s surface. The processing conditions for each prepared sample are displayed in Table 2. Based on the optimum parameters discovered through previous research, the FSP process window was chosen [56, 57]. As illustrated in Fig. 2a, a special fixture was used to securely position the specimens. After creating the groove in the middle of the sheet, fill it with ZrO2 particles, and seal it is using a pin-less tool to prevent ZrO2 particles from escaping Throughout the FSP procedure. Reinforcement particles were inserted into the matrix using a hardened H13 steel tool, as demonstrated in Fig. 2b, which had an M6 threaded pin profile, a 5 mm height, and an 18 mm shoulder diameter. A wire electrical discharge machine (EDM) was used to cut FSP specimens, both with and without ZrO2 particles, from the middle of the pass in a direction perpendicular to the processing method to perform a mechanical, wear, and microstructural examination.

Table 2 FSP process parameters with designations
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Fig. 2
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a Macrograph of FSP procedure and b developed FSP tool and its dimensions

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2.3 Microstructural evaluations

Microstructural evaluations were conducted at the Tabbin Institute for Metallurgical Studies (TIMS), Egypt, exploiting optical microscopy (Leco LX 31-USA) and SEM. A Model Quanta 250FEG (Field Emission Gun) is proceeding to analyze the reinforcement particles dispersion following FSP. At the friction stir processing cross section, transverse cuts were made in samples measuring 20 mm × 10 mm × 6 mm. The surfaces of all the specimens were polished and ground to a mirror-like sheen prior to displaying the surface morphology of each one. The specimens were polished mechanically, and then Keller's reagent (190 mL distilled water, 5 mL nitric acid, 3 mL hydrochloric acid, and 2 mL hydrofluoric acid) was observed to etch them.

2.4 Mechanical characterization

Vickers microhardness tester was employed to evaluate the microhardness. of the FSP samples, with a 100-g load and a 10-s dwell time. The tensile sample is made with dimensions of 2.5 mm thick, 4 mm broad, 58 mm long, and a gauge length of 26 mm, parallel to the composite direction by wire cut EDM consequently, ASTM E8M-04 standards [58].

2.5 Wear test

The pin-on-disk method is observed to evaluate wear. Wear tests on BM surface and the FSP surfaces were carried out in compliance with ASTM G99 standard [59]. The test is achieved using a K110 steel disc with 58 HRC hardness. The disc speed, load, and sliding velocity were all set at 230 rpm 30 N and 0.6 m/s, respectively. Sliding velocity and applied load were fixed to allow comparison of the active wear mechanism under similar conditions. An electronic weighting balance was used to measure and record the samples' weight loss in grammes every 400 m before and after the wear test. The wear rate was calculated by dividing the weight loss by the distance. SEM was performed to evaluate wear track at small magnifications to conduct discussions about wear behavior.

3 Results and discussion

3.1 Effects of multi-pass FSP AA1050/ZrO2 on microstructure, and particle dispersion investigations

Investigating microstructure was conducted across the FSP region by using optical microscope. An elongated and rough grain structure was identified in the optical microstructure of base metal AA1050, as depicted in Fig. 3a. The microstructure of stir zone (SZ) for various FSP passes with reinforcement particles is depicted in Fig. 3b–f. A fine and equiaxed grain structure was found in the SZ, and heat input had valuable influence on the microstructure due to frictional heat and strong plastic deformation [60, 61]. As illustrated in Fig. 3a, lower FSP passes during the first pass (Z1P sample) and inappropriate stirring tool action caused ZrO2 particles to aggregate in multiple areas [62]. A majority of the ZrO2 particles stayed in the middle of the SZ due to problematic flow because of their concentration inside the groove and lower formability than the base material. ZrO2 particles were found to be more uniformly distributed and to exhibit less agglomeration as the number of passes increased [63, 64]. FSP passes increased along with the refinement and uniform distribution of the primary ZrO2 nanoparticles. As illustrated in Fig. 3b, grain size was drastically reduced following the second pass FSP (Z2Psample), improving material mixing and reinforcement particle dispersion. Consequently, with an increase in FSP stir effects and material flow, as shown in Fig. 3c (Z3P sample), the ZrO2 particles are more thoroughly mixed with AA1050 matrix.

Fig. 3
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Microstructure for specimens a base metal (BM), b Z1P, c Z2P, d Z3P, e Z4P and f Z5P

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Grain equiaxed are finely visible in the micrograph. Finely equiaxed grains are produced by dynamic recrystallization (DRX) and have the largest heat input in SZ regions [46, 47]. In the Z4P sample, grain size is more refined, and distribution of reinforced particles is more uniform (Fig. 3e). Since the extreme plastic deformation, and high temperature throughout FSP, the DRX is the primary cause of the refining grain of composites. Furthermore, the DRX is supported by the pinning and refining action of ZrO2 particles [65]. Finer recrystallized grains and a higher proportion are seen in the Z5P sample (Fig. 3f). This is a result of FSP pass being increased, which increases the materials' cumulative heat input and enhances the DRX effect to refine the grains [66, 67]. Furthermore, Fine grains form in SZ during FSP as a result of severe plastic deformation increasing dislocations density, which prevents grain boundary from slipping [68, 69]. As FSP number of passes increased, grain size decreased, the reinforcement particle clustering decreased, and the powder dispersion within the matrix became more homogeneous [70]. Following the fourth and fifth passes, respectively, ZrO2 particles are dispersed gradually and noticeably. The ZrO2 area fraction rises with number of FSP passes, demonstrating a direct correlation between FSP passes and particle dispersion because of reduction in ZrO2 particle dispersion. The distribution of various elements is spread out in the ZrO2 nanoparticle-reinforced AA 1050 surface nanocomposite is illustrated in Fig. 4 from the SEM and EDS analysis. There was no discernible clustering among the evenly dispersed reinforcing ZrO2 nanoparticles in the SZ. As a result, the Zr element is evenly distributed throughout the AA 1050 matrix and there are no noticeable aggregations or vacancies.

Fig. 4
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SEM and EDS evaluation in SZ with ZrO2/AA 1050 reinforcement

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3.2 Mechanical performance of AA1050/ZrO2 surface nanocomposites

3.2.1 Effects of multi-pass FSP AA1050/ZrO2 on the microhardness

Figures 5 and 6 show the microhardness profiles of the investigated specimens perpendicular to the FSP direction and the average Vickers microhardness number (VMHN) for the SZ of processed specimens. The FSP joints' hardness increased caused by the increasing number of passes, with hardness values of 47.7 HV, 56.1 HV, 70.1 HV, 77 HV, and 86.5 HV found in SZ for Z1P, Z2P, Z3P, Z4P, and Z5P, respectively. The microhardness of base metal AA1050 (BM) is observed to be 30.1 HV. Whereas the Z5P sample exhibits an improvement of 188%, which is attributable to its fine, homogenous, and recrystallized microstructure. The presence of hard reinforcing particles, along with larger dislocation density and finer size of grain, is the primary factor contributing to the FSP samples' increased hardness [35, 71]. High hardness of ZrO2 particles, homogenous distribution reinforcement, and grain refinement may have an influence on significance of microhardness during multi-pass FSP. Furthermore, consistent distribution of reinforcement at the nanoscale is thought to pinning dislocations and slowing grain growth [72, 73].

Fig. 5
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Microhardness profile of BM and FSP AA1050/ZrO2 surface nanocomposites

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Fig. 6
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Average micro-hardness values of BM, Z1P, Z2P, Z3P, Z4P, and Z5P

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3.2.2 Multi-pass FSP effect on tensile strength of AA1050/ZrO2

The stress–strain diagram of multi-pass FSP of AA1050/ZrO2 is represented in Fig. 7. The tensile attributes of aluminum composites that are being evaluated are illustrated in Fig. 8. The AA1050 alloys (BM) were found to have yield strength, ultimate strength, and elongation to break values of 24 MPa, 59.2 MPa, and 34%, respectively. As illustrated by Fig. 8a, b, the ultimate strength and yield strength increased from 56.3 to 94.5 MPa and 23.3 MPa to 55 MPa, respectively, as passes number increases. The malleable qualities of the SZ material were also being impacted by the restriction of grain growth development. One important factor that improved the metal's superplastic behavior was grain refinement. Due to material mixing, which demonstrated extraordinary grain refinement, the FSP AA1050/ZrO2 stirred structure equiaxed fine grains [74]. Conversely, as the number of processing passes increases, the fine-grain size of nano-sized FSP composites results in better controllable properties. Furthermore, it has revealed that as the FSP passes through more stages, it affects the Al-matrix surface composite's ability to refine its grains and strengthen its molecules [75]. The tensile results, including tensile strength (56.3, 65.7, 74.8, 86, and 94.5 MPa), yield strength (23.3, 27, 40.6, 48, and 55 MPa), and elongation (12.6, 16.8, 20.2, 22.3, and 23.8%), were thus observed following one pass (Z1P), two (Z2P), three (Z3P), four (Z4P), and five passes (Z5P). A lower enhancement of malleable properties was identified in the first FSP pass of AA 1050/ZrO2 compared to BM. This could be explained by ZrO2 agglomeration in some regions and less homogenization of matrix grains [76].

Fig. 7
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Stress–strain curve for the AA1050 alloy and AA1050/ZrO2 surface nanocomposites

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Fig. 8
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Tensile properties of FSP surface nanocomposite and AA 1050 alloy

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Moreover, BM and ZrO2 interfacial compatibility improved and interparticle gaping was reduced as passes number increased. This feature increases yield strength and ultimate strength. When distance between interparticle increases, material strength decreases, and grain size increases [77, 78]. More grain boundary volume within the AMC can be accomplished through adjudicating the refined grains. The AMC's grain boundary must experience less dislocation pile up and driving force during axial loading to decrease the grain size. The recovery rate is lowered by the pinning effect of nanoparticles, which stops dislocations from moving to grain boundaries [79, 80]. This phenomenon strengthened the developed composite at 5-pass and reduces the possibility of cavitation or microcracks under axial loading circumstances throughout the composite Enhanced the composites' fracture resistance and tensile strength following multiple FSP passes [81, 82]. Strengthening through interfacial shear stress, the superior interfacial bonding facilitates the efficient transfer of tensile load to the ZrO2 particle (shear lag mechanism) [83, 84].

3.3 Morphology of fractures

The fractured surface was investigated with a SEM to ascertain how the microstructure affected the processed specimens' failure pattern. The fractographic image of the BM and multi passes AA1050/ZrO2 cracked tensile test specimens was evaluated. Six samples were tested, as Fig. 9 illustrates.

Fig. 9
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Morphology of tensile specimens fracture surfaces a BM, b Z1P, c Z2P, d Z3P, e Z4P and f)Z5P

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The fractured surface morphology of AA1050 (BM) as received is indicated in Fig. 9a. It turned out to have undergone a ductile failure with deep, large dimples. The ductile fracture in BM was experienced as a result of the specimen's fracture surface having several dimples. Consequently, during tensile test, voids that nucleate and join make up the dimples in fracture surfaces.

The material will neck as the plastic deformation increases, and dimples will form because of uniform growth of micro voids, which will cause the specimen to crack [70, 85, 86]. The sample (Z1P) experiences less plasticity under the axial loading condition, as indicated by the reduced number of dimples in Fig. 9b. The decreased ductility and fracture resilience of the composite after one pass are justified by this observation. The samples made with one tool pass formed large boundaries and sizes of grain because the non-fragmented ZrO2 particles were present. Because whole ZrO2 particles in AA1050 alloy matrix have been shown to function as void initiators, this could have reduced the sample's tensile strength and ductility.

As FSP number of passes rises, so does the area fraction of fracture surfaces with shallow dimples (Fig. 9c–f). This phenomenon is ascribed dispersion and fragmentation levels of ZrO2 additionally to a reduction in Al matrix's average grain sizes. The stability of the reinforcement-matrix under applied axial loading is established by the intact AA1050/ZrO2 interfaces on fracture surfaces [87]. The shallow dimples formation was encouraged by the fragmentation of ZrO2 particles and the decreased average grain sizes in the composites because of multiple FSP passes, which also increased tensile strength, and resilience to fracture.

3.4 Sliding wear of AA 1050/ZrO2 surface nanocomposites

Sliding distance and weight loss relationship in the AA1050/ZrO2 composites processed using varying numbers of passes is depicted in Fig. 10a. As FSP number of passes rises, significance of weight loss has decreased. In contrast to the AA1050 as received material (BM) and composites with predominant and sparsely distributed ZrO2particles (Z1P), the homogeneous dispersion of ZrO2 particles following multi-pass FSP provides enough hard and large surface area to counteract immediate deformation and material loss during wear test. The severity of grain refinement, hardness, and reinforcement dispersion were assessed to be the variables influencing composites resistance to wear exposed to several FSP passes [88].

Fig. 10
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a Weight loss for BM and the AA1050/ZrO2 nanocomposite varies with sliding distance. b Wear rate for BM and the AA1050/ZrO2 nanocomposite varies with sliding distance

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According to Fig. 10b, as FSP number of passes increased, composites wear rate that were processed consecutively was noticeably lower than that of their initial counterparts. Among the samples with fewer passes, the AA1050/ZrO2 nanocomposites that underwent a 5-pass (Z5P) exhibited the lowest wear rate. This is in line with some of the previous study on multi-pass FSP aluminum alloys [89, 90]. Since there are more FSP passes, the wear rate has decreased. This can be identified to high hardness that is uniformly distributed, the increased area fraction and fragmentation, and the load-bearing influence of the ZrO2 particles within Al matrix [91]. Furthermore, it has been proposed that the load-bearing capacity of hard ceramic particles will contribute less direct load within reinforced composite.

4 Surface evaluation of wear

The surfaces of wear tracks or surfaces following wear test are depicted in Fig. 11. Base metal (BM) displayed pits and deep grooves that suggested adhesive wear in wear surface's morphology (Fig. 11a). The worn surface of this specimen has deeper and wider grooves, which suggest adhesive wear. Where significant plastic deformation occurs before the friction temperature between the disc-steel plate and the material of matrix rises [92]. In the AA1050/ZrO2 composites treated with different numbers of FSP passes, delamination and wear debris were noticed to be most prevalent wear mechanisms. There are fewer grooves and pit sizes in Fig. 11b (Z1P). Particles of fine loose debris were noticed on the worn of composite surface after 2-passes as presented in Fig. 11c (Z2P). Figure 11d and e demonstrate that the worn track in the Z4P and Z3P samples had a few cracks associated with abrasive mechanisms subsequently to numerous shallow scratches. Ploughing and fine grooves, which are additionally recognized as observed forms of shallow grooves, are thought to have formed by an abrasive wear mechanism, whereas cavities and craters are thought to have formed by local adhesive wears brought on by the micro-weld breaking during sliding action [93]. Through their protective role against adhesive wear and dislocation movement along the wear tracks, the ZrO2 particles reduction plastic deformation. The greatest hardness of ZrO2 particles promotes abrasive wear mechanisms, which are assumed to impede the formation of more pronounced pit sizes or worn grooves in processed specimens as FSP number of passes increases (Z5P). As FSP number of passes increased, scratches disappeared (Fig. 11f), and a smooth track with a small crater was experienced. The wear mechanism resulted from the change from an adhesive to a delamination-abrasive mechanism, whereas the delamination was associated with the interaction and accumulation acting together [94].

Fig. 11
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Morphology of the worn out surfaces a Base metal (BM), b Z1P, c Z2P, d Z3P, e Z4P and f Z5P

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5 Conclusion

The current investigation conducted a comprehensive analysis to study the influence of multipass FSP ZrO2 particle reinforcement on the wear rate, microhardness profile, tensile properties, and macrostructure of the AA1050. According to results obtained, the following conclusion can be established:

  1. 1.

    A multi-phase hybrid AMNC with a pure AA1050 matrix and ZrO2 has been manufactured.

  2. 2.

    The hybrid AMNC reinforced by ZrO2 particles demonstrated a notable improvement in mechanical performance by raising the FSP passes. This improvement is primarily attributable to DRX, a rise in dislocation density, and grain refinement.

  3. 3.

    Particle homogeneity increased with FSP passes up to 5 scattering because of materials flowing through a tool's rotation and finer due to DRX particle mechanism.

  4. 4.

    As passes number increased, the microhardness improved. After 5 passes, the microhardness was found to be 86.5 HV, which is larger than the 30.1 HV of the AA1050 base metal.

  5. 5.

    AA1050 demonstrated ultimate tensile strength and yield strength of 59.2 MPa and 24 MPa, respectively. Tensile properties were improved concurrently with a rise in FSP pass after multi-pass FSP was implemented to the AA1050 using ZrO2 nanoparticles. One pass, two passes, three passes, four passes, and five passes were found to have ultimate tensile strengths of 56.3, 65.7, 74.8, 86, and 94.5 MPa and yield strengths of 23.3, 27, 40.6, 48, and 55 MPa, respectively.

  6. 6.

    Compared to AA1050 alloy as received, the specific resistance to wear of multi-pass FSP applied to the alloy using ZrO2 nanoparticles is significantly improved.

  7. 7.

    The innovation of metal matrix composites with ZrO2 additives using FSP has been gaining attention regarding the growing need for high-performance and lightweight materials in automotive, aerospace, and military applications, such as defensive armor.