Introduction

Metal matrix composites (MMCs) are frequently used materials in industrial applications due to their advantages over polymer matrix composites (PMCs) and ceramics matrix composites (CMCs). MMCs exhibit mechanical and physical properties superior to that of PMCs. Aluminium is one of the most popular and commonly used metals because of its simplicity in handling, lightweight with good corrosion resistance, high strength and low production and manufacturing cost. Aluminium-based MMCs are widely used in automotive and aerospace applications, since their unique properties like greater strength, reduced density, controlled thermal expansion and better wear resistance.1 In aluminium-based MMCs, base constituent is an aluminium alloy (i.e. Al–Si, Al–Cu, Al–Si–Mg), which forms percolating network and is termed as matrix phase. The other constituent is embedded in this aluminium/aluminium alloy matrix and serves as reinforcement, which is usually non-metallic and common ceramic such as SiC, Al2O3, C, B, B4C, AlN and BN. The properties like tensile strength, compressive strength, impact strength, hardness, wear resistance and corrosion resistance were found to be positively improved with reinforcement addition thereby making them a suitable material than the unreinforced base aluminium alloy. It is also found that B4C reinforcements exhibit higher hardness than TiC, TiB2, SiC, Al2O3 and Gr.2 The enhancement of mechanical properties was achieved for Al7075 composites attained by reinforcing micro- and nanoparticles of Al2O3 and B4C.3 The demand for aluminium hybrid composite has been increased due to the enhanced processing methods, reliable production cost and desired high mechanical and tribological properties. The structural and functional application of the aluminium hybrid composites and the selection of parameters for synthesizing are mostly influenced by the selection of reinforcements.4 The addition of reinforcement particles not only increases the hardness but also the brittle nature of the composites. These restrictions can be rectified by adding secondary reinforcement to achieve further improvements in the mechanical properties of the hybrid composites.5 Various kinds of reinforcement and co-reinforcement have been used to improve the behaviour on mechanical properties of aluminium composites for different engineering applications.6,7,8,9 MMCs can be processed using three main methods they are, solid-state processing, liquid-state processing and in-situ processing. Fabrication techniques usually vary according to the reinforcements. However, there are various techniques such as stir casting, liquid metal infiltration, squeeze casting, spray decomposition and powdered metallurgy10,11 for the fabrication of composites. In our method, the stirring-squeeze casting method techniques was used. The squeeze casting methods provide less porosity due to the elimination of gases; during the pressing process, desired surface texture, expected microstructures and less wastage with more strength of the casting composites could be achieved.11 The technical challenges are that the need of great attention for: achieving homogeneous distribution of reinforcements in the MMCs. The wettability of the particles in the matrix, porosity developed from casting and chemical interaction between matrix and the reinforcements are the main parameters.11,12,13 This problem is common to most production routes, including stirring-squeeze casting. To achieve the desired properties, these problems must be optimized. The method of supersonic vibration removes the gas formation during the process of fabrication and reduces the level of porosity in the cast.14 In a quasi-crystalline state, the preservation of Cu–Cr–Fe particles in the Al7075 composites formed the strong mechanical interlocking with the Al matrix. Li et al studied the effect of incorporation of these particles into the Al matrix and concluded a significant improvements in the wear resistance and modifications in the mechanisms of wearing.15 The interface contact and interfacial adhesion between the CNTs and Cu Cr matrix improve the strength and ductility of the composites. Zhao et al., reported that the strengthening behaviour of Al7075 MMCs by CNTs reinforcement.16 By observing the wear behaviour exhibited by the composites studied by Alaneme et al., there was a considerable increase in wear resistance by adding silicon carbide and rice husk ash as the reinforcements.17 Even though hybrid AMMCs show better performance on mechanical and tribological behaviour than the pure alloys, still study under various corrosion surroundings to establish their corrosion performance is still lacking.18 The improved performance on corrosion resistance of aluminium composites was achieved with the reinforcements of sub-micron B4C particles in the matrix of aluminium alloy19 However, the effect of B4C and BN reinforcements on the tribological and corrosion properties of Al7075 MMCs has not been investigated in detail. Moreover, the stirring-squeeze casting process and comparative study on mechanical, tribological and corrosion behaviour of B4C/BN co-reinforced Al7075 alloy with B4C-reinforced Al7075 alloy was not reported. Therefore, in the present work, different weight percent of B4C and constant weight percentages of BN particles have been added to synthesis Al7075/B4C/BN hybrid composites to analyse the microstructure, mechanical, tribological and corrosion properties for the composites, synthesized through the route of squeeze casting method.

Experimental Methodology

Materials Selection

Zinc-based Al7075 aluminium alloy with copper and magnesium provide the highest strengths compared to any other commercial series of alloys. However, the Al7075 alloys possess relatively poor corrosion resistance compared to other aluminium alloys. Therefore, it is highly imperative to improve the corrosion resistance of Al7075 to widen its applications in the marine environment and further improve mechanical behaviour for the automobile industry applications.20 Hence, in the present work, Al7075 alloy has been chosen as the base matrix to improve mechanical, tribological and anticorrosion properties by reinforcing with B4C and BN particles. The constituents of the base matrix are specified in Table 1.

Table 1 Constituents of Al7075 Aluminium Alloy
Full size table

Density is the most important factor for selecting reinforcement particles. Priority should be given to the low-density material in weight-sensitive sectors such as the automotive and marine applications. In the present work, 1-µm size B4C and 10-micron size BN particles have been selected as reinforcement particles with densities of 2.5 g/cm3 and 2.1 g/cm3, respectively, which are relatively less than that of Al7075 alloy (2.8 g/cm3). Since the reinforcements are of lower density than the base matrix, by the law of mixtures the overall density of the hybrid composites will be less than that of the base matrix. From the literature study, it is observed that the weight percentages of reinforcement for aluminium matrix composite are mostly kept at below 10% with very few experiments exceeding 10%.21,22 In the present study, three aluminium metal matrix composites (AMMCs) and three aluminium hybrid metal matrix composites (AHMMCs) have been prepared with compositions (Al7075 + x%B4C) and (Al7075 + x%B4C + 3%BN) where x = 3, 6, 9 (measured in wt.%). Since B4C is a harder substance, exhibits desired wettability, good thermal stability and exceptional chemical inertness, it is chosen to be primarily used as reinforcement in aluminium composites.21 Properties of BN such as lamellar crystal structure, good lubrication, high thermal conduction, low thermal expansion, superior shock resistance and excellent workability23 make it a good reinforcement for the usage in high temperature and high wear applications. BN forms stable chemical bonds with aluminium resulting in a uniform and stable microstructure. Thus, BN is selected as secondary reinforcement with a fixed weight percentage of 3%.

Stirring-Squeeze Casting Processes

The stirring-squeeze casting method can provide a significant interface between reinforcements and is thereby well suited for fabricating hybrid composites.11 In the present work, the pure and hybrid composites of Al7075 alloy were prepared by the conventional stir casting method using a resistive heating furnace (graphite crucible) with an impeller. Approx. 1/3 of melt height has been maintained to the bottom of the crucible. Initially, the alloy was melted at the temperature of 700 °C. Further, the temperature was raised to 750 °C and maintained for 20 min to ensure a homogeneous melt. The melting process was carried out in an inert atmosphere containing a continuous flow of the mixture of Argon and SF6 at the rate of 3.5 l/min to limit the reaction between MMC and air. The preheated B4C and BN reinforcements particles were gravity fed into the molten alloy. The stirring is started and then reinforcements were added. Particles entered into the vortex created by the stirring in the melt and the temperature was further increased to 850 °C. Stirring of the melt was carried out at the speed of 500 RPM for 15 min to obtain a homogeneous mixture. The particles distribution may be affected by several factors like rheological behaviour, the method of incorporation of particles, interactions, distribution during solidifications and density of matrix and particles.24 Thus, careful attention has been given to the above factors. The particles with respective concentrations were added to the molten matrix. Since at relatively lower temperature B4C particles exhibit lower wettability with molten aluminium matrix,25,26 the temperature and stirring speed further increased to 950 °C and 600 RPM, and this state was maintained for 20 min. To avoid rapid cooling of the composite while casting, a pressure of 100 MPa was maintained at each pressing and this pressure level is maintained till the end of the process of solidification. The graphite coated die temperature was maintained at 250 °C. The molten matrix has been poured into it and allowed to cool under ambient temperature after being punched by hydraulic press. The same procedures have been repeated for all composites. Figure 1 shows the (a) image of squeeze casting setup (b) image of die used for casting with casted specimen (c) the dimensional sketch of die used. Figure 2 shows the schematic diagram of the fabrication process. The synthesized AMMCs and HAMMCs and their compositions are shown in Table 2

Figure 1
figure 1

(a) Image of squeeze casting setup, (b) image of die used for casting with casted specimen, c the dimensional sketch of die used.

Full size image
Figure 2
figure 2

Schematic diagram of the fabrication process.

Full size image
Table 2 Manufactured AMMCs and AHMMCs and Its Compositions
Full size table

Measurement of Properties

The microstructure analysis of prepared hybrid composites has been performed using an optical microscope. The distribution of reinforcement particles has been confirmed by SEM analysis. The micro-hardness of the hybrid composites was studied by Vickers micro-hardness tester. The specimens with the dimensions of 32 mm length, 6 mm width and 3.5 mm thickness were prepared as per the standard of ASTM E-8 for the tensile test. To minimize the error, an interpolation technique was adapted through averaging of tested values of five specimens. The prepared specimens were subjected to a compression test as per ASTM E9 standard. The specimen's dimensions for the compression test were 10 mm in diameter and length of 25 mm. Salt spray test has been carried out as per the ASTM B-117 standard on the composites and unreinforced alloy to study the anticorrosion properties of composites. Rectangular samples of 25 × 25 mm and 3.5 mm thickness were prepared for all the hybrid composites to conduct salt spray tests. The prepared samples were pre-polished and pre-cleaned before the test. Pin on disc wear testing machine was used to study the wear properties of synthesized hybrid aluminium composites under dry sliding conditions as per ASTM G99-04. The steel material EN31 of surface roughness 0.3 μm was used as a complement of rotating disc. The wear specimens have been prepared with 8 mm diameter, 32 mm length and spherical end nose radius of 4 mm. The wear track width at the composite contact surface was measured using a universal measuring microscope. The volume of wear loss was calculated as per ASTM G77-83. The effect of formation of the intermetallic phase on microstructure, hardness, tensile strength, compressive strength and the corrosion resistance of hybrid composites has been investigated, and the experimental results have been comparatively discussed with the Al/B4C composites and base alloy.

Salt spray test was conducted on the prepared composites as per ASTM B117 standards. This test was conducted in a controlled corrosion environment and the result of corrosion resistance value for the composite specimens in the given test chamber was recorded. One side of the specimens was polished using SiC 1200 grade sheet. Marking is done on the unpolished side of the specimens and the initial weight of each composites specimen was recorded using a weigh balance of accuracy 0.001 g. Then the specimens were exposed to a chamber for salt spray test and were positioned at an angle of 25° to the vertical. The periodical salt spray was done especially on the polished side for a period of 1.5 h then off period of 1.5 h, for 2 days. The 3.5 wt% of salt solution was employed for the wetting cycle and the chamber temperature was maintained at 25 °C.18 Two specimens for each composite were taken for corrosion tests.

Results and Discussion

Structural Analysis

Microstructure Analysis

Studies on microstructure were conducted for all composites with the help of an optical microscope on the specimens’ etched surface. The dark-colour intermetallic phases like Al3BC, AlB12 and AlN are shown in the bright metal matrix for all samples. However, due to the squeeze casting a greater number of grains are observed in the microstructure. As can be seen from Figure 3a for Al7075 alloy, relatively smaller grain sizes (10–15 µm) than the AHMMC 1, 2 and 3 as shown in Figure 3b–d (18–23 μm, 23–25 μm and 24–35 μm). Moreover, the increasing the % of reinforcements or increasing the volume of intermetallic phases lead to increasing size of the grains in the AHMMC3. The formation of dendrite pattern by the fine grains due to the deposition and precipitation at the intersection of grain boundaries as well as the elevated temperature does not affect the homogeneity of materials as shown in Figure 3. Increasing the area of intermetallic phases in the grain boundaries are improving the performance of the AHMMCs. Both reinforcement particles have been placed in boundaries and lead to a thicker one in the AHMMCs.

Figure 3
figure 3

Microstructure of (a) Al7075, (b) AHMMC1, (c) AHMMC2 and (d) AHMMC3 at etched conditions.

Full size image

SEM Analysis

SEM images for the 6% and 9% B4C added reinforced hybrid composites are shown in Figure 4. It has been observed that the reinforcing particles (B4C and BN) are embedded in the MMCs as the particles are observed in the SEM images of synthesized hybrid aluminium composites. In general, the reinforcement particles have higher hardness and have higher melting points than a matrix material. A needle-shaped formation of Al3BC, AlB12 and AlN particles precipitated in the grain boundaries of the composites. The dispersion of reinforcement particles prevents the motion due to dislocation and improves the strength of materials which gives the better behaviour on the mechanical and corrosion properties. The enhancement of compressive strength is due to the dispersed reinforcement particles in the matrix, which significantly play an important role in the cushioning effect improvements.

Figure 4
figure 4

SEM image of (a) AHMMC2, (b) AHMMC3.

Full size image

XRD and EPMA Analysis

Figure 5 shows the B4C and BN particles XRD patterns with varying weight percentages of B4C (3, 6, and 9%). The XRD patterns confirm the dominant effect of Al in the hybrid composites with high crystalline as the aluminium-related peaks are very sharp and high intensity. In addition, the secondary phases of Al3BC, AlB12, and AlN are observed in the composites. Owing to the interfacial reaction of reinforcements with aluminium, the secondary phases are formed in the hybrid composite. Such kind of interfacial reaction improves the mechanical properties of composites.27 The element of pure aluminium alloy and the presents of B4C and BN reinforcements in the hybrid composites are shown in Figure 6.

Figure 5
figure 5

XRD patterns of AHMMCs.

Full size image
Figure 6
figure 6

Elements presents of AHMMCs.

Full size image

Mechanical Properties

Hardness of Alloy and Composites

The two different ceramic reinforcements B4C and BN having higher hardness than Al7075 alloy contributes to enhancement in the hardness of synthesized hybrid composites. Figure 7 shows the comparison of the hardness of alloy and composites. The AMMCs exhibited relatively high hardness compared to the AHMMCs and AAl7075. Furthermore, the hardness of the AMMCs increased with the percentage of B4C reinforcements. The higher hardness is due to the resistance to the motion of dislocation in the composites than the monolithic alloy.17 The uniform distribution and the intermetallic phase like Al3BC, AlB12 and AlN and grain enrichments are the factors for improving the hardness of composites.28

Figure 7
figure 7

Vickers hardness of AA7075, AMMCs and AHMMCs.

Full size image

Tensile Strength of AA7075, AMMCs and AHMMCs

It is observed that the tensile and yield strength of MMC's and AHMMCs have been increased with the increasing percentage of B4C particles when compared with the monolithic Al7075 alloy as shown in Table 3. Moreover, the tensile strength of AHMMCs is superior to AMMCs. This is owing to the formation of interfacial reaction, fine grain size, and strengthening effect of strain gradient of BN particles.29 The increasing percentage of reinforcements in the matrix alloy is reducing the percentage of elongation in the hybrid composites.17

Table 3 Tensile Strength, Yield Strength and % of Elongation of AA7075, AMMCs and AHMMCs
Full size table

The presents of nucleation sites in the aluminium matrix lead to the composites’ recrystallization owing to the forming of grains with the least sizes and this provides a resisting mechanism for the dislocation movements and provides the comprehensive strength of composites. The properties of the brittle nature of reinforced are playing a dominant role decreasing in the ductility of composites and increasing the % of reinforcements control the flow-ability of the matrix and thus % of elongation of composites are reduced.28

Compressive Strength of AA7075, AMMCs and AHMMCs

The Compressive strength exhibited by the composites and the unreinforced alloy is shown in Figure 8a. A significant increase in the compressive strength is observed in the composite with the increase of B4C content and this can be attributed to the increasing area of interface boundaries between B4C and Al as B4C particle concentration in the composite. It is observed that the co-reinforcement of BN further increases the compressive strength of the composites. The homogeneous distribution of B4C and BN and their strengthening effect support to the composites against compression load. Whereas, raise in % of B4C would be increasing the strength of interface due to the piling of displacement between boundaries and interfaces. The grain enrichments offered by squeezing and regular geometrical shape of grains (Figure 8b, c) are the influencing factors for the cushioning effect against compressive load.11 Especially, BN particles in the hybrid composites accelerating for more intermetallic phases as shown in Figure 8c and the improved mechanical properties of synthesized AHMMCs are confirmed through the comparative investigations of various Al hybrid composites as shown in Table 4.

Figure 8
figure 8

Compressive strength of AA7075 alloy, AMMCs and AHMMCs.

Full size image
Table 4 Comparative Investigation of Hybrid Composites Mechanical Behaviours
Full size table

Wear Properties of AA7075, AMMCs and AHMMCs

The wear track width at the composite contact surface was used to measure the wear parameter using a universal measuring microscope. The wear track width and calculated wear rate of synthesized aluminium hybrid composites at dry sliding conditions of varying sliding speeds and applied loads are shown in Table 5. The wear rate of AMMCs has been significantly improved for all the combinations sliding speeds and applied loads. This is due to the increase in load tends to increase the width of synthesized composites and hence the wear rate also increases. Normally the wear rate and wear width of composites are increasing with increasing applied load and speed. Whereas, the wear width and wear rate of the composites were significantly reduced for the same load and same speed for the hybrid composites. Increasing the % of B4C is decreased the wear rate. The presence of stiffer and stronger reinforcement in the matrix alloy and more grains refinement by squeeze casting is restricting the motion of dislocation movement thus hindering the wear rate. Figure 9 shows the graphical representation of wear rate of AMMCs (Figure 9a, c, d) and AHMMCs (Figure 9b, d, f) specimens for the respective sliding velocities of 0, 25 m/s, 0.5 m/s and 1 m/s for the normal load of 10 N, 20 N and 30 N.

Table 5 Wear Track and Wear Rate of AA7075, AMMCs and AHMMCs.
Full size table
Figure 9
figure 9

Wear rate of AMMCs (a, c, d) and AHMMCs (b, d, f) specimens for the respective sliding velocities of 0.25m/s, 0.5m/s and 1m/s for the normal load of 10 N, 20 N and 30 N.

Full size image

Friction Coefficient

The coefficient of friction (COF) with different velocities (0.25 m/s, 0.50 m/s and 1.0 m/s) under three different loads 10 N, 20 N and 30 N have been shown in Figure 10. The COF of B4C and BN particles reinforced hybrid composites increased with increasing reinforced particles and the values are 0.45, 0.50 and 0.60 for AHMMC1, 0.58 0.60 and 0.70 for AHMMC2, 0.65, 0.70 and 0.80 for AHMMC3 for the applied loads of 10 N, 20 N and 30 N, respectively. It can be seen that the friction coefficient of the hybrid composites has been significantly increased. This is due to the strengthening effect of matrix and reinforcements, the dislocations in the alloy are ineffective which causes the increase in hardness and it is the most important factor to enhance the wear resistance of the reinforced composites.

Figure 10
figure 10

Coefficient of friction of aluminium hybrid composites under varying its weight percentage.

Full size image

SEM Study on Wear Out Surfaces of the AMMCs and AHMMCs

Figure 11a–f reveals that the images of worn-out surfaces of the composite specimens for load 30 N. The SEM images of plastic deformation and the presence of grooves parallel with the sliding directions. The furrows and grooves are the typical topographic accompanying abrasive and adhesive wear. Other than grooving, local plastic distortion and particles of material spreading were shown and which indicates abrasive wear. For all the applied load, the wear surface of the AHMMCs specimen (Figure 9a) is smoother than AMMCs (Figure 11d) and shows fine grooves with smaller steps and a plastic stream. Moreover, the stiffer and stronger reinforcements restrict the motion of dislocation movement and hinder the wear rate. BN inclusion leads to less shear stress on the composite and lesser detached particles on the wear process and is beneficial due to self-lubrication properties. This improvement continues for all loads and all composites. As significance, the wear resistivity of the B4C- and BN-reinforced particles was increased due to the load-bearing capacity of the particles and the creation of transfer layers that shield the surface from abrasion. As the load increases from 10 to 30 N, the morphology of worn-out surfaces of AMMCs (Figure 11a–c) and the AHMMCs (Figure 11d–f) gradually become rougher. Moreover, the adhesive zones have become more, especially for the hybrid composites. The worn-out surface observation suggests that the dominant wear mechanism for all three specimens is particulates micro-fracture. BN particles suppress the wear due to the formation of debris and thus the portion with adhesive wear is larger, thereby the worn-out surface is comparatively smoother in hybrid composites.

Figure 11
figure 11

SEM images of AMMCs (a–c) and AHMMCs (d–f) worn-out surface at 1m/s and 30N load.

Full size image

Corrosion Properties

Corrosion Rate of AA7075, AMMCs and AHMMCs

The salt spray corrosion test was carried out on the unreinforced aluminium alloy and composites as per ASTM B117 standard. Observed results of unreinforced aluminium alloy and each composite are recorded and presented in Table 6. Aluminium alloys are generally treated as a more reactive material under a corrosive environment due to their high oxygen affinity.41 The oxidation reaction reduces the value of Ph. Also, the degradation of the matrix of aluminium spreads all over the cathode. But by incorporating suitable reinforcements in the matrix aluminium like B4C and BN, the rate of corrosion has been significantly reduced. It is recorded that the increased corrosion resistance of the hybrid composites is achieved by increasing the % of B4C particles in the synthesized composites. However, the formation of aluminium oxide on the surface acts as a protective layer preventing further corrosion due to its inertness.42,43 It is observed that the corrosion rate of the composites and hybrid composites is significantly decreased. The intermetallic phases like Al2Cu act as a reaction barrier and prevent the progression of undesired interfacial reactions.44,45

Table 6 Corrosion Behaviour of AA7075, AMMCs and AHMMCs
Full size table

SEM Study of the Corroded Surface of AHMMCs

Figure 12 shows the SEM images of the corroded surface of AHMMCs at different magnification. Corrosion started initially at localized sites, preferentially in both cases on primary and secondary phases. For alloy material, this is visible on the surface while these features are so clear for composites probably due to finer grain size. The localized attack invaded the entire surface with continued exposure to give a general corrosion pattern with several particles remaining on the surface unaffected while the entire matrix was dissolved. The degree of the attack revealed the aggressiveness of the corrosive medium. For alloy, corrosion was initiated at the primary phase. Generally, Al alloys possess’ high affinity towards oxygen. Hence, treated as the most reactive material. But it gets its inertness and protectiveness from the aluminium oxide formation on the surface of Al. The presence of metastable portion in components has been transferred to a great stable region due to the motion of the intermetallic phase. The available composition of composites is the same as the product composition. However, the variation in the structure of atoms on both sides of the interface has occurred. The quality and quality of attack on the surface of the composites show the speed rate of corrosion. Whereas, the increase in the resistance of corrosion due to the increasing % of B4C can be confirmed through the magnified images of corroded surfaces. An increased rate of corrosion resistance was observed on the surface of the Al7075/9% B4C/3%BN hybrid composites compared to other hybrid Al composites. Predominantly, the intermetallic phases form the metallization to the fierceness of Cl ions. A Logarithmic increment has been existing between Cl ions and the severity of corrosion. B4C and BN reinforcement particles provide high diffusivity to the Cl ions to line up the passivation. The formation of the metastable region in the composite material would change to a stable phase through a partition less transformation due to the movement of intermetallic materials in the interface. Moreover, the stable region provided a similar composition of the materials as the existing phase, there could be a difference in the configurations of the atoms on both sides of the interface.

Figure 12
figure 12

SEM images of the corroded surface of AHMMCs at lower (a) and higher (b) magnifications.

Full size image

Conclusion

In this study microstructure, mechanical, tribological and corrosion properties of B4C/BN-reinforced Al7075 hybrid composites have been studied and compared with that of B4C-reinforced Al7075 composites and Al7075 aluminium alloy.

The following conclusions are made.

  1. 1.

    The AHMMCs with various weight % (3%, 6% and 9% by weight) of B4C and fixed weight percentage (3%) of BN particulates were synthesized using the stirring-squeeze cast method.

  2. 2.

    The microstructural and SEM studies show the distribution of B4C, BN particles and sizes of the grains. The formation of intermetallic phases like Al3BC, AlB12 and AlN were confirmed by XRD. The composition of elements was further confirmed with EPMA test.

  3. 3.

    The hybrid composites (AHMMCs) exhibits improved properties such as hardness, tensile strength, compressive strength, wear resistance, and corrosion resistance compared to Al7075 and the AMMCs.