The Effect of Incorporating Ceramic Particles with Different Morphologies on the Microstructure, Mechanical and Tribological Behavior of Hybrid TaC_ BN/AA2024 Nanocomposites

Abstract

Improving the mechanical durability and wear resistance of aluminum alloys is a research challenge that can be solved by their reinforcement with ceramics. This article is concerned with the improvement of the mechanical properties and wear resistance of the AA2024 aluminum alloy surface. Surface composites were prepared by incorporating a hybrid of heavy particles (tantalum carbide (TaC), light nanoparticles, and boron nitride (BN)) into the AA2024 alloy using the friction stir process (FSP) approach. Three pattern holes were milled in the base metal to produce the composites with different volume fractions of the reinforcements. The effects of the FSP and the reinforcements on the microstructure, mechanical properties, and wear resistance are investigated. In addition to the FSP, the reinforced particles contributed to greater grain refinement. The rolled elongated grains became equiaxed ultrafine grains reaching 6 ± 1 µm. The refinement and acceptable distribution in the reinforcements significantly improved the hardness and wear resistance of the produced composites. Overall, the hardness was increased by 60% and the wear resistance increased by 40 times compared to the base alloy.

1. Introduction

The hybridization of reinforcement nanoparticles on aluminum alloys has attracted the interest of many researchers in recent years due to the significant improvements in the properties of the resulting composites. Many conventional methods have been used to hybridize alloys with reinforcement particles, such as casting, powder metallurgy, laser melting, etc. [1,2,3]. Friction stir processing (FSP) is considered one of the most advanced techniques to improve the properties of metal surfaces [4,5,6,7,8]. It is used to fabricate nanocomposite materials because it is a clean, versatile, and effective method for generating nanocomposite surfaces [9,10,11,12,13,14,15,16]. Moreover, reinforced particles and ceramic materials, such as Al₂O₃, B₄C, SiC, VC, MWCNT, and TiB₂, can be used. This method offers a composite material with high strength, uniformity, and a considerable number of microstructure grains [17,18,19,20,21]. Hybrid nanocomposites have been used to improve the surface properties of aluminum matrices. For example, when Al/SiC + Al₂O₃ were integrated and blended with the AA6061 matrix, significant increases in wear resistance and microhardness were observed [22,23]. The choice of the best volume fraction of reinforcement particles is one of the most important parameters in the fabrication of metal matrix composites [24]. Importantly, when a single type of particle (Al₂O₃) was used, the best tribological and mechanical properties of the fabricated AMC (AA7075) were obtained with the best combination of nano-reinforcements (Al₂O₃) in the aluminum composite matrix (AA7075) [25].

FSP has been used to strengthen the Al alloys with nanoparticles of hexagonal boron nitride (HBN). Researchers discovered an improvement in mechanical properties and wear resistance, which they attributed to the solid lubrication process of the HBN particles; nevertheless, these particles had low electrical and thermal conductivity [26,27]. The refinement of the microstructure has a significant effect on the mechanical properties, which can be attributed to the use of equiaxed grains formed during the dynamic recrystallization process by the FSP approach or to the use of modifiers. The FSP method offers the possibility of introducing and dispersing different reinforcing particles with different properties and morphologies, allowing the fabrication of hybrid nanocomposites without limitations [28,29,30,31] due to the unique properties of each ingredient. Hybrid reinforcements have been used in friction stir treated composites to further improve the surface properties of the Al alloys matrix [32,33,34]. Hybrid nanocomposites were used to improve the surface properties by incorporating SiC, Al₂O₃, BN, TiB₂, among others, into the Al alloys matrix; there was an improvement in the wear resistance and hardness properties of the composites [35,36,37,38].

Thus, the main gap between the previous literature and the current work is the use of two different ceramic families and morphologies. Therefore, tantalum carbide (TaC, heavy ceramic particles) and boron nitride (BN, low-density relative nanoparticles) are used to prepare the hybrid nanocomposites surface using the FSP method. Three different patterns are prepared to determine the optimum percentage of the volume fraction of the hybrid reinforcement. In this study, the mechanical, tribological, and physical properties of the fabricated hybrid surface composite are investigated.

2. Materials and Experimental Work

In this work, surface composites were fabricated on sheets of a standard alloy AA2024. The reinforcements employed were BN and TaC supplied by Nanoshel, Ltd. (Congleton, UK). The densities of the AA2024 alloy and the reinforcements, BN and TaC, were 2.78, 2.1, and 14.30 g/cm³, respectively. The reinforcements were introduced into the base alloy using the friction stir process (FSP). The FSP was performed using an automatic milling machine (Bridgeport, Elmira, NY, USA). The design and dimensions of the tool used for FSP are shown in Figure 1a—before the FSP process. The AA2024 alloy plates were perforated in three groups as shown in Figure 1b to obtain three different volume fractions of the reinforcements, denoted as AA2024D1, AA2024-D2, and AA2024-D3. After milling the holes, 50% BN + 50% TaC (in volume percentage) were mixed, and then the milled holes were filled. The FSP was performed under the following conditions: rotation speed—1120 rpm, traverse speed—25 mm/min, and fixed tilt angle—1°, which were chosen according to [17]. Two methods were used to introduce the reinforcing nanoparticles into the grooves; in the first method, two mixed reinforcing particles were used (dual hybrid composites).

Transmission electron microscopy (TEM, type JEOL JSM-200F, Tokyo, Japan) was used to examine the BN and TaC particles. The average sizes of the BN and TaC particles were 100 ± 5 nm and 300 ± 10 nm, respectively, as shown in Figure 2. Optical light microscopy (Olympus BX51, Miami, FL, USA) and scanning electron microscopy (SEM) Philips XL30 (FEI/Phillips, Hillsboro, OR, USA) were used to analyze the microstructure of the fabricated surface composites. The samples were ground and polished for microstructure analysis and then etched according to [14]. Vickers microhardness was measured using a Rockwell hardness tester (True Blue United Testing Systems, Fullerton, CA, USA) based on the ASTM E-384-17 specification. The pin-on-disk wear test was performed using a TNO tester (TNO, Delft, The Netherlands) based on the ASTM G99-04A standard at room temperature. The test was performed under the following conditions: rotation speed of the counter cylinder—256 RPM, applied pressure—0.3 bar for 10 min. The counter cylinder is made of 316 stainless steel with a diameter of 200 mm. Prior to the wear test, the samples were ground and polished for microstructure analysis to ensure the high quality of the test. In addition, the counter cylinder was cleaned with acetone to remove surface impurities.

2.1. Calculating the Wear Loss, Wear Resistance, and Wear Rate of the Manufactured Composites

The wear loss, wear resistance, and wear rate can be calculated by Equations (1)–(3):

$$\mathrm{Wear} \mathrm{loss} \left(\mathrm{g}\right)=\mathrm{mass} \mathrm{loss}=\left(\mathrm{befor} \mathrm{test} \mathrm{weight}-\mathrm{after} \mathrm{test} \mathrm{weight}\right)$$

(1)

$$\mathrm{Wear} \mathrm{resistance} \left({\mathrm{g}}^{-1}\right)=\frac{1}{\mathrm{wear} \mathrm{loss} }$$

(2)

$$\mathrm{Wear} \mathrm{rate} \left(\raisebox{1ex}{$\mathrm{g}$}\!\left/ \!\raisebox{-1ex}{$\mathrm{s}$}\right.\right)=\frac{\mathrm{wear} \mathrm{loss}}{\mathrm{time}}$$

(3)

2.2. Calculating the Manufactured Composites Density and Volume Fraction of the Reinforcements

The volume of the manufactured composite can be calculated using Equations (4) and (5):

$$\mathrm{volume} \mathrm{of} \mathrm{composite} \left(V_C\right)= \mathrm{Volume} \mathrm{of} \mathrm{particles} (V_p)+\mathrm{Volume} \mathrm{of} \mathrm{matrix} \left(V_m\right)$$

(4)

$$V_C= \mathrm{projected} \mathrm{area} \mathrm{of} \mathrm{tool}\times \mathrm{length} \mathrm{of} \mathrm{the} \mathrm{processed} \mathrm{composite}$$

(5)

In this study, the projected area of 44 mm² was calculated from the penetration of the FSP tool pin (Figure 2a). For AA2024-D1, the total length of the fabricated composite is 102 mm (Figure 2b). Thus, the fabricated AA2024-D1 composite volume is equal to 4488 mm². In the same way, the volume of the fabricated AA2024-D2 and AA2024-D3 composites can be calculated. Equations (6) and (7) can be used to calculate the volume of the nano reinforcements and base matrix:

$$V_p=№ \mathrm{of} \mathrm{holes} \mathrm{along} \mathrm{the} \mathrm{processed} \mathrm{composite}\times \mathrm{volume} \mathrm{of} \mathrm{each} \mathrm{hole}$$

(6)

For example, for AA2024-D3, the number of holes is 22 in a 105 mm composite. The volume of one hole is equal to $(\pi {(1.5)}^2)\times 4.5$ = 31.77 mm², thus the total volume of the reinforcements for AA2024-D3 is equal to 31.77 × 22 = 698.94 mm³.

$$V_m=V_c-V_p$$

(7)

Thus, Equations (8) and (9) can be used to determine the volume fraction of the nano-reinforcements and base matrix:

$$\mathrm{Volume} \mathrm{fraction} \mathrm{of} \mathrm{particles} (V{F}_p)=\frac{V_p}{V_C}$$

(8)

$$\mathrm{Volume} \mathrm{fraction} \mathrm{of} \mathrm{particles} (V{F}_p)=\frac{V_p}{V_C}$$

(9)

After determining the volume of nano-reinforced particles and the base matrix, the theoretical density of the produced composite can be calculated using Equations (10) and (11):

$$\mathrm{Density} \mathrm{of} \mathrm{composite} \left({\rho }_c\right)= \frac{\mathrm{Mass} \mathrm{of} \mathrm{composite} (M_c)}{\mathrm{Volume} \mathrm{of} \mathrm{composite} \left(V_c\right)}= \frac{\mathrm{Mass} \mathrm{of} \mathrm{particles} \left(M_p\right)+ \mathrm{Mass} \mathrm{of} \mathrm{matrix} (M_m)}{V_c}$$

(10)

$${\rho }_c= \frac{\left({\rho }_p\times V_p\right)+ ({\rho }_m{V}_m)}{V_c}$$

(11)

The theoretical densities and volume fractions of the reinforcements of the manufactured composites are summarized in

Table 1. Theoretical density and volume fraction of the reinforcements.

Composite	${\mathit{\rho }}_{\mathit{p}},$ g/cm3	${\mathit{V}}_{\mathit{p}},$ cm3	${\mathit{\rho }}_{\mathit{M}}$ g/cm3	${\mathit{V}}_{\mathit{m}}$ cm3	${\mathit{V}}_{\mathit{c}}$ cm3	${\mathit{\rho }}_{\mathit{c}}$ g/cm3	$\mathit{V}\mathit{F}$ %
AA2024-D1	8.2	1.10	2.78	3.39	4.49	4.11	24
AA2024-D2	8.2	0.85	2.78	3.73	4.58	3.79	19
AA2024-D3	8.2	0.69	2.78	3.93	4.62	3.59	14

. To calculate the density of the reinforcements, as mentioned in the experimental section, 50% BN and 50% TaC (by volume) were mixed; therefore, the density of the mixture can be calculated by adding both densities and dividing by 2. The hole pattern design on the AA2024 alloy sheet shown in Figure 2b resulted in reinforcement volume fractions of 24%, 19%, and 14% for AA2024-D1, AA2024-D2, and AA2024-D3, respectively (Table 1, last column). The increase in the volume fraction of the reinforcements increased the composite density produced (Table 1, seventh column).

3. Results and Discussion

3.1. Microstructure Observations

Figure 3 shows the microstructure of alloy AA2024 as rolled and after friction stir processing. The microstructure of the sheets of alloy AA2024 in the rolled condition exhibits elongated, non-crystallized grains due to the cold rolling process (Figure 3a). It can be observed that the intermetallic phases [39] formed in the aluminum matrix are unevenly distributed after the rolling process (Figure 3a). The microstructure is significantly altered after the friction stirring process. Typical microstructure regions include the heat-affected zone (HAZ), thermomechanical heat affected zone (TMAZ), and stirred or nugget zone (SZ) that are formed after the friction stirring process, as shown in Figure 3b. During the friction stirring process, additional heat is generated in the stirred zone due to extreme plastic deformation, leading to the development of a dynamically recrystallized (DRX) microstructure. This hypothesis explains why the stirred zone consists of uniformly equiaxed grains, unlike the base metal (BM), shown in Figure 3c due to plastic deformation and heat flux generated. The material was subjected to thermal cycling without plastic deformation in the heat-affected zone, resulting in only a negligible change in microstructure (Figure 3b). According to the FSP of the studied alloy, the grain size was 15 ± 2 µm.

Figure 4 shows the microstructure of the fabricated nanocomposites with different volume fractions of the hybrid nano-reinforcements. In general, incorporating nano-reinforcements into the studied metal matrix resulted in significant grain refinement in the stirred zone. It is known that the presence of nano-reinforcements in the matrix during the stirring friction process increases the nucleation centers, resulting in a large number of new grains. Moreover, the nano-reinforcements prevent grain growth in the stirred zone after the dynamic recrystallization process [40]. Moreover, with a larger volume fraction (AA2024-D1), the microstructure has a greater number of fine equiaxed grains with an average grain size of 6 ± 1 µm (Figure 4a). Decreasing the volume fraction of nano reinforcements resulted in a larger average grain size, however, still below that of FSP (Figure 4b,c). The average grain size for AA2024-D2 and AA2024-D3 was 8 ± 2 and 11 ± 2 µm, respectively. The high-volume fraction in the case of AA2024-D1 resulted in a slight heterogeneity in the agglomeration of the nano improvements (Figure 4a).

Figure 5 shows the SEM micrographs of the fabricated composites with different hybrid volume fractions of the reinforcing materials. In general, all the reinforced composites containing TaC and BN nanoparticles were uniformly dispersed and scattered in the metal matrix after FSP. The SEM analysis revealed the distribution of the reinforcing materials and the undesirable effects of accumulation and nanoclusters on the reinforcing ceramic parts. In general, the reinforcing nanoparticles were homogeneously distributed in the SZ in all samples, and no strong clustering effect was observed.

3.2. Hardness Test Results

Figure 6 shows the Vickers microhardness of the samples studied: the base matrix, after FSP and the produced composites. The change in the microstructure altered the properties. The as-rolled sheets (base matrix) have an average hardness of 61.7 ± 4.2 HV. Performing FSP on this alloy resulted in softening of the worked zones, which led to a decrease in hardness in the worked zones (Figure 6a). The average hardness of the stirred zone after FSP was 53.2 ± 3.8 HV (Figure 6b). In general, the incorporation of hybrid reinforcements resulted in an increase in hardness in the stirred zone [41]. At a low reinforcement content, AA2024-D3, the hardness was slightly increased compared to the rolled sheets of AA2024 alloy as produced. When the content was increased to 24%, AA2024-D1, the hardness increased sharply, as shown in Figure 6a—this contribution is consistent with [42]. The improvement in hardness was 11%, 22%, and 60% for AA2024-D3, AA2024-D2, and AA2024-D1, respectively (Figure 6b). After the friction stir process, the thermomechanical heat affected zone and the heat-affected zone became the weakest areas (Figure 6b). After the FSP process, the microstructure of the as-rolled condition was partially maintained, and no elongated grains were formed. Moreover, these areas were free from the incorporated reinforcements; therefore, the hardness decreased.

3.3. Wear Characterizations

As mentioned in the Materials and Experimental Work Section, the wear test was conducted in the pin-on-disc machine under the following conditions: speed of the disc—265 rpm, pressure—0.3 bar, and test time—t = 10 min. The calculated wear resistance and wear rate of the tested samples are shown in Figure 7 and Figure 8. Firstly, it was found that the wear resistance was in accordance with the hardness results: the higher the hardness, the higher the wear resistance. The maximum wear resistance, 250 ± 9 g⁻¹, was observed for AA2024-D1, which contains the largest volume fraction of reinforcements in the matrix, 24%. AA2024 alloy, as prepared, exhibited the lowest wear resistance (6 ± 2 g⁻¹).

4. Conclusions

A hybrid composite metal matrix of Tac + BN/AA2024 was successfully fabricated using the FSP technique. The incorporation of two different particles into the surface of the aluminum alloy, AA2024, showed improvement in mechanical and tribological behavior. Increasing the volume fraction up to 24%—by reducing the distance between the groove holes filled with hybrid reinforcement particles—improves the hardness and wear resistance of the fabricated surface composite. The difference in morphology and type of reinforcement particles plays an important role in improving the fabricated surface composite. This can be attributed to the recrystallization processes mechanism and grain refinement of the base alloy in the presence of such particles. Lastly, the TaC and BN particles were uniformly diffused and distributed over the entire metal matrix surface.