Introduction

Aluminium metal matrix composites have emerged as an important class of materials due to its wide application in aerospace, automobile, structural and marine engineering. In recent decades the particle reinforced metal matrix composites have been considered as a high-performance material and shows outstanding physical and mechanical properties. Nowadays these composites are replacing the applications of conventional aluminium alloys due to its superior properties such as high strength to weight ratio, high wear resistance and low thermal expansion.1,2 Aluminium alloys are most commonly used metallic materials as a matrix for the production of composites. When compared with lightweight alloys such as magnesium and titanium, aluminium alloys are cheap and can be easily processed. The important properties of aluminium alloys are high strength, good ductility and high corrosion resistance in conjunction the properties can be tailor made to satisfy various requirements.3,4 Considering the various aluminium alloys, age hardening grades (2xxx, 6xxx and 7xxx) are preferred as matrix material because it allows further improvement in mechanical properties by various age hardening treatment.5,6

The mechanical and tribological properties can be enhanced by reinforcing the ceramic particles.7 The various manufacturing techniques available for producing metal matrix composites are stir casting; powder metallurgy; squeeze casting and friction stir process, etc. Among the various available techniques stir casting is the most widely used manufacturing process because of its uniqueness in homogeneous mixing and also it can be produced economically in large scale.8 Reinforcing the ceramic particles in aluminium alloys activates the strengthening mechanisms such as grain refinement; increase in dislocation density caused by thermal incompatibilities between the matrix and reinforcement particles; facilitate load sharing from matrix to reinforcement particles.9,10 The commonly reinforced ceramic particles in aluminium matrix composites are SiC, Al2O3, B4C, TiC, TiB2, etc.11,12,13,14,15,16 These ceramic particles increase the mechanical properties (tensile, hardness and wear resistance) but there arises incompatible in processing the composites. So it is difficult to produce the morphologically defect-free composites to harvest the maximum increase in properties. When SiC is reinforced with aluminium matrix it reacts with aluminium to form a reaction layer at the particle matrix interface. This reaction layer is intermetallic Al4C3 that weakens the mechanical properties of the composites.17 When Al2O3 is reinforced with aluminium matrix, the magnesium present in the form of solid solution in the aluminium matrix reacts with reinforcement particles to form Al2MgO4.18,19

TiB2 is an attractive ceramic material that can be used to reinforce the aluminium matrix to improve the mechanical, wear and corrosion characteristics.20,21,22,23 TiB2 is thermodynamically stable in molten aluminium thus it facilitate the liquid state processing of Al-TiB2 composites production. So TiB2 will not react with molten aluminium to form any brittle products that impairs the mechanical properties of the composites.24 When compared with other common reinforcement particles TiB2 is captured by the molten aluminium thereby activates the dispersion strengthening mechanism.25 Aluminium TiB2 composites have excellent properties and most commonly used in cutting tools, crucibles, wear and impact resistant applications. However it is mainly used for structural and functional applications in aerospace, defence, marine and automotive industries.26 Recently the tensile, hardness, wear and corrosion characteristics of aluminium metal matrix composites have been emerged as an important research fields due to its wide applications in various industries. So, in this study the different weight percentage of micron size TiB2 particles were reinforced in Al7075 by stir casting process. The effects of addition of TiB2 particles in the tensile properties, hardness, wear and corrosion characteristics of Al7075-TiB2 composites were investigated.

Materials and Methodology

Materials

Al7075 was used as the matrix material due to its excellent mechanical properties, wear and corrosion resistance applications. The major alloying elements are zinc, magnesium and copper with chemical composition shown in Table 1.

Table 1 Chemical Composition of Aluminium Alloy 7075 (wt%)
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TiB2 is a ceramic material with high strength used as a reinforcement material with chemical composition shown in Table 2. The physical and mechanical properties are shown in Table 3.27 The SEM micrograph and EDS (Energy dispersive spectroscopy) of TiB2 particles are shown in Figures 1 and 2.

Table 2 Chemical Composition of TiB2 Particles (wt%)
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Table 3 Physical and Mechanical Properties of Titanium Diboride
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Figure 1
figure 1

SEM image of TiB2 particles.

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Figure 2
figure 2

EDS pattern of TIB2 particles.

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Stir Casting Process

TiB2 particles were reinforced with Al7075 matrix materials with the help of the stir casting process. Al7075 were placed in the graphite crucible and heated up to 800°C to get the molten metal using electric resistance furnace.24 TiB2 particles with average size of 3–5 µm were preheated at 850°C. Laser scattering analyses were used to find the average size of TiB2 particles as shown in Figure 3. The stir casting facility consist of various parts such as powder injection system, agitator made of titanium, resistant furnace, thermocouples and graphite crucible as shown in Figure 4. TiB2 particles with different weight percentage of 2, 4, 6 and 8% were added to get the different composites. Stirrer with speed of 350 rpm is used to mix the reinforcement particles with molten metal for about 30 minutes to obtain the homogeneous mixture. The mixture has been poured into the mould cavity with cavity dimensions 100 × 100 × 12 mm as shown in Figure 5. Then it is allowed to cool by keeping the mould at room temperature. A sample specimen is shown in Figure 6 after solidification.

Figure 3
figure 3

Laser scattering analysis of TiB2 particle size distribution.

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Figure 4
figure 4

Stir casting set-up.

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Figure 5
figure 5

Mould used for casting aluminium-TiB2 composites.

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Figure 6
figure 6

Cast of aluminium-TiB2 composites.

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Material Characterization

The properties of new materials can be characterized by conducting microstructural analysis. The specimens were prepared for Al7075 and Al7075-TiB2 composites as per the standard metallographic procedure. The specimen surfaces were prepared by polishing with different grades of emery sheet with grit size ranging from 400 to 1200. Fine polishing was carried out by polishing machine using velvet cloth. Finally, the specimens were etched by Keller’s reagent (95 mL H2O, 2.5 mL HNO3, 1.5 mL HCl, 1.0 mL HF). The effects of reinforcement particles on porosity content of the composites were studied. The SEM analyses were carried out to find the dispersion of the TiB2 particles. The optical microstructures were observed to reveal the TiB2 particle distribution and grain size. The mechanical properties were characterized to evaluate the tensile, hardness and wear characteristics of the composites. In addition to that corrosion test were also conducted to find the effect of addition of TiB2 particles in corrosion rate.

Results and Discussion

Density Characteristics

The density is an important physical property that may influence the mechanical properties of materials. The density gets affected by casting process and reinforcement particles. Thus, the porosity of composites is comparatively more with respect to non-reinforced alloy. The experimental density is measured using the densitometer apparatus as shown in Figure 7. The theoretical density of composites is calculated by rule of mixture using the Eqn. (1)

$$ \rho t = \left( {\rho m \times {\text{wt}}\% } \right) + \left( {\rho r \times {\text{wt}}\% } \right) $$
(1)

where ρt, theoretical density of composite; ρm, theoretical density of matrix material; ρr, theoretical density of reinforcement particle.

Figure 7
figure 7

Density measurement using densitometer apparatus.

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The theoretical vs experimental density for pure aluminium and various TiB2 reinforced composites is shown in Figure 8. The experimental density of aluminium gets affected due to the manufacturing defects associated with casting approach even though the degassing process was carried out by adding the wetting agents like borax, magnesium and TiK2F6 to the melt. The high temperature for making the molten metal introduces porosity, shrinkages and slag inclusion in the aluminium. The porosity in the composites increased due to various sources. The stirring process is used to distribute the reinforcement particle homogeneously in the aluminium matrix. This process also controls the agglomeration of particles during the increase in reinforcement contents. But this stirring process introduces gas inside the molten metal. This entrapped gas during solidification is the main reason for the porosity.14 The other reasons are increase in the contact surface area due to the presence of reinforcement particle in the molten metal, creation of pore nucleation near the ceramic particle locations.

Figure 8
figure 8

Theoretical versus experimental density.

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Microstructure of Al7075-TiB2 Composites

The SEM micrograph of as-cast base material (Al7075) is shown in Figure 9. In general there has been no indication of casting defects like porosity, shrinkages or slag inclusion in the SEM micrograph.

Figure 9
figure 9

SEM images of Al7075.

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The SEM micrograph of composites (Al7075-TiB2) with different amount of TiB2 reinforcements (2%, 4%, 6% and 8%) is shown in Figure 10. The reinforcement particles are homogeneously distributed in Al7075-TiB2 composites without showing any sign of particle clusters. The wettability of the matrix material and reinforcement particles hinders the dislocation of the TiB2 particles. This makes the particles to stay on suspended for long time during solidification to get homogeneous distribution.28,29 This particle also prevents the dislocation movement of matrix material by dispersion strengthening mechanism which in turn increases the mechanical properties of Al7075-TiB2 composites.

Figure 10
figure 10

SEM micrograph of Al7075-TiB2 composites.

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The optical micrograph of as-cast base material Al7075 alloy used as matrix material is shown in Figure 11. The micrograph consists of dendritic structure as formed due to the high rate of cooling (super cooling) during solidification. The dendritic structure consists of elongated α-Al dendritic arms and average spacing between the arms is 40–50 µm. The presence of major alloying elements of Al7075 such as Zn, Mg and Cu is found to be higher than their solubility limit which results in formation of intermetallic phases.

Figure 11
figure 11

Optical microstructure of as stir cast Al7075.

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The optical micrographs of stir casted composites with different amount of TiB2 reinforcement are shown in Figure 12. The contents of reinforcement particles increases in accordance with the addition of TiB2 particles in molten aluminium during stir casting process. Average grain size is measured using the intercept technique. A random straight line is drawn though the micrograph. The numbers of grain boundaries intersecting the line are counted. The average grain size is found by dividing the number of intersections by the actual line length.

$$ {\text{Average}}\;{\text{grain}}\;{\text{size}} = {1}/\left( {{\text{number}}\;{\text{of}}\;{\text{intersections}}/{\text{actual}}\;{\text{length}}\;{\text{of}}\;{\text{the}}\;{\text{line}}} \right) $$

where actual line length = measured length divided by magnification

It is also clear that the reduction in grain size is observed for increase in weight percentage of reinforcement particles. The average grain size for 2%, 4%, 6% and 8% TiB2 particles reinforced is 45, 36, 26 and 20 µm. This could be due to the resistance offered to the growth of α-Al by TiB2 particles during solidification process. This leads to grain refinement in dendritic structure of as-cast Al7075-TiB2 particles. The TiB2 particles act as a nucleus surrounded by α-Al grains on solidification. The entire micrograph consists of larger region of soft α-al and the presence of few hard TiB2 particles. When the TiB2 particles content is increased more nucleation sites are created during solidification. This creates enhanced resistance to grain growth of α-Al resulting in very fine grains28 (Figure 12).

Figure 12
figure 12

Optical microstructure of as stir cast Al7075-TiB2 composites.

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Tensile Properties

Aluminium materials possess lower tensile strength and very good ductility. When aluminium matrix is reinforced with TiB2 particles the tensile properties like yield strength and ultimate tensile strength gets enhanced without significant drop in elongation percentage. The tensile test has been conducted using Instron universal testing machine with load capacity of 50 kN. The dog bone-shaped specimens were prepared for Al7075 and various TiB2 reinforced composites as per ASTM Standard E8.30 Load vs deflection curve has been recorded for the cross head deflection of 2 mm/min. For each composites five specimens were tested and the average values of tensile properties are listed in Table 4.

Table 4 Tensile Properties of Al7075-TiB2 Composites
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The tensile properties of the Al7075-TiB2 composites enhances in accordance with the addition of reinforcement particles.15 The yield strength, ultimate tensile strength increases linearly for all the compositions as shown in Figure 13. But the percentage elongation gets affected with minimum degradation for 2% TiB2 and then it increases marginally for 4%, 6% and 8% as shown in Figure 14.

Figure 13
figure 13

Tensile properties for various Al7075-TiB2 composites.

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Figure 14
figure 14

Percentage elongation for various Al7075-TiB2 composites.

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The increase in tensile properties is due to the reinforcement of TiB2 particles. The TiB2 particles in the aluminium matrix refine the grains during solidification based on the Hall–Petch relation.31 This refined grain is the main cause for the improvement in strength, percentage elongation and toughness of the composites. These grain boundaries act as obstacles for dislocation movement. The other reasons are orowan strengthening mechanism. The residual stresses caused by the difference in thermal expansion co-efficient of reinforcement particles and aluminium matrix creates high dislocation densities in the interface region.32

Micro Hardness

The micro Vickers hardness test was conducted by making indentation in the composites. A dead weight of 300 gram with diamond shape indenter was used to make the indentation for about 30 seconds. The hardness value was calculated by measuring the diagonals of the impression made on the composites by the indenter as shown in Figure 15. For each composite five values were taken for reliability in the measured values.

Figure 15
figure 15

Sample shows indentation in micro Vickers hardness test.

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Aluminium matrix is soft but TiB2 particles are harder. Due to the reinforcement of the hard TiB2 particles into ductile matrix and strong interface between the matrix and reinforcement particles strengthen the composite. During the hardness test the indenter load gets shared between the matrix and the hard reinforcement particles is the cause for the increases the hardness. The average micro hardness values of five samples for each different composite are shown in Figure 16. The hardness of the Al7075-TiB2 composites increases in accordance with the addition of TiB2 particles. There is a significant increase in hardness for Al7075-8% TiB2 composites and it shows hardness can be drastically improved for the higher weight percentage of TiB2 particles.

Figure 16
figure 16

Micro hardness values for Al7075-TiB2 composites.

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Wear Characteristics

Wear test was carried out using computerized wear testing pin on disc machine. Dry sliding wear test was conducted for as-cast Al7075 and Al7075-TiB2 composites with various wt% (2, 4, 6 and 8) of TiB2 reinforcement particles. The test samples with 8mm diameter and 32mm length cylindrical pins were prepared by machining from the cast blank. Prior to testing, the pin and disc surface were cleaned with acetone. Wear tests were carried out in the load range of 5, 10 and 15 N at a constant speed of 415 rpm, sliding speed 1 m/s and by varying (1000 m, 1500 m and 2000 m) the sliding distance. During testing the pin is kept stationary and perpendicular to the circular rotating disc The specimen were supposed to rub against the EN31 steel disc of hardness 70HRC (attached to the pin on disc wear test machine) by applying the load that acts as a counter weight and balances the pin as shown in Figure 17.33 The pin was weighted before and after testing to find the wear loss. For each composite three specimens were tested for reliability in the measured values.

Figure 17
figure 17

Pin on disc wear test apparatus.

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The wear test results for different loading conditions like 5 N, 10 N and 15 N for Al7075 and various Al7075-TiB2 composites is obtained. The wear vs time for Al7075 and Al7075-TiB2 composites is shown in Figure 18 to compare the wear due to reinforcement particles. The higher wear rate is observed during the initial stage of wear test due to the absence of roughness projections. After some time the wear rate decreases due to the worn of matrix materials and expose of reinforcement particles. The wear rate keep on reduces due to the formation of protective layer in between the pin and counter surface after crushing of reinforcement particles. Fe3O4, Fe and minute fractured particles of the TiB2 form a layer between the work hardened pin and the counter face and reduces the wear.34 From the graph it is clear wear increases with increase in applied load. In addition to that, the wear decreases with increase in reinforcement particles. It also confirmed that composites are better in wear resistance.

Figure 18
figure 18

Wear of various composites for various load.

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The SEM micrograph of worn surfaces of Al7075 for the applied load of 15 N is shown in Figure 19. The wear pattern is clearly exposed with the deformation and presence of grooves parallel to the sliding direction.35 These grooves are formed due to the ploughing action of hard asperities present on the steel counter face in-between the pin and disc. The Al7075 shows deep grooves and maximum deformation compared to the composites.

Figure 19
figure 19

Wear pattern of Al7075.

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The SEM micrograph of worn surfaces of aluminium composites for the applied load of 15 N is shown in Figure 20. The worn surface of the composites possesses smoother and shallow grooves. This shows that the composites are very good in wear resistance.

Figure 20
figure 20

Wear pattern of Al7075-TiB2 composites.

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Corrosion Characteristics

The corrosion characteristics of Al7075-TiB2 composites were studied by conducting two different corrosion test methods namely potentiodynamic polarization resistance and salt spray corrosion test. The potentiodynamic polarization resistance measurements were carried out for as-cast Al7075 and Al7075-TiB2 composites with various wt% (2, 4, 6 and 8) of TiB2 reinforcement particles. The test was performed in 3.5 wt.% NaCl mixed with distilled water medium. It is polished with different emery sheets with grit size ranges from 400 to 1200. Fine polishing was also performed with polishing machine. For each composite three specimens were tested for reliability in the measured values.

The polarization plot for Al7075 and Al7075-TiB2 composites is shown in Figures 21 and 22 respectively. The corrosion parameters like corrosion potential, current density for the samples are listed in Table 5. Faraday’s law was used to calculate the corrosion rate using the current density as shown in Table 5. From the table it is clear that corrosion rate were decreased for increase in the weight percentage of TiB2 particles.

Figure 21
figure 21

Polarization curve for Al7075.

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Figure 22
figure 22

Polarization curve for Al7075-TiB2 composites.

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Table 5 Polarization Results of Al7075 and Al7075-TiB2 Composites
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Corrosion test was also conducted using salt spray corrosion test for Al7075 and Al7075-TiB2 composites. The samples with dimensions 50 × 20 × 10 mm were prepared as per ASTM Standard B117M.36 Similarly the specimens were polished with different emery sheets with grit size ranges from 400 to 1200. Fine polishing was also performed with polishing machine. The specimens were cleaned by dipping in 70% nitric acid and 30% water in room temperature. Then the specimens were degreased with acetone and then rinsed in distilled water before it gets immersed in the solution. For each composite three specimens were tested for reliability in the measured values. Salt spray test was performed by 3.5% NaCl mixed with distilled water. The specimens were tied using nylon wire and then immersed in the closed salt spray chamber as shown in Figure 23. The salt water is sprayed continuously for 48 h and the fog developed inside the chamber.

Figure 23
figure 23

Salt spray test chamber.

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The corrosion results were evaluated by weight loss and the corrosion rate was calculated by the following Eqns. (2) and (3). The corrosion rate with respect to the addition of TiB2 particles is shown in Figure 24. This clearly shows the corrosion resistance gets increased by increase in TiB2 particles.

$$ {\text{Corrosion}}\;{\text{rate}}\;\left( {{\text{mm}}\;{\text{per}}\;{\text{year}}} \right) = \left( {{87}.{6} \times {\text{Weight}}\;{\text{loss}}} \right)/\left( {{\text{density}} \times {\text{area}} \times {\text{time}}\;{\text{of}}\;{\text{exposure}}} \right) $$
(2)
$$ {\text{Corrosion}}\;{\text{rate}}\;\left( {{\text{mils}}\;{\text{per}}\;{\text{year}}} \right) = {\text{corrosion}}\;{\text{rate}}\;{\text{per}}\;{\text{year}} \times {472} $$
(3)
Figure 24
figure 24

Salt spray test results for Al7075 various weight percentage of TiB2 and its composites.

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Corrosion resistance of composites increases with increase in TiB2 particles Even though there are various corrosion mechanisms available but these composites are largely affected by pitting corrosion. Pitting corrosion occurs when the cathode (damaged layer) is large and the anode (exposed metal) is small. Typically the surface protection layer or film becomes the cathode when it is damaged and cracked. A small area of metal is then exposed and becomes the anodic. The SEM micrograph of Al7075 corroded surface is shown in Figure 25.

Figure 25
figure 25

SEM micrograph of the corroded surfaces for Al7075.

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SEM micrograph of corroded surface of aluminium composites is shown in Figure 26. The NaCl reacts with aluminium metal and the white rust forms on the surfaces.37,38,39 The material surfaces undergo localized attack by chloride and at the same time show signs of nucleation and growth of a new layer of Al(OH)3. Aluminium hydroxide is not soluble and precipitates as a white gelatinous mass, normally called alumina although it is generally bayerite (α- Al(OH)3 β-Al(OH)3).40

Figure 26
figure 26

SEM micrograph of corroded surfaces for various Al7075-TiB2 composites.

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The formation of the oxide film on the alloy surface occurs gradually resulting in the initial increase in corrosion rate with respect to time. With the formation of the protective film, increase in corrosion rate ceases. At a certain stage the protective oxide film undergoes rupture, when the film breakage is faster than the rate of repassivation which leads to relatively constant corrosion rate with respect to time. This rupture phenomenon is assumed to have occurred due to residual tension created between the oxide film layer and aluminium matrix.

Conclusion

Al7075-TiB2 composites were synthesized by reinforcing the different weight percentage of TiB2 particles by using stir casting process. The following conclusions were made

  1. 1.

    The porosity of the composites increases due to the addition of TiB2 particles. This is due to the stirring action which introduce the gas during the casting process; increase in the contact surface area due to the presence of reinforcement particle in the molten metal and creation of pore nucleation near the ceramic particle locations.

  2. 2.

    Optical microstructure of Al7075-TiB2 composites shows the uniform distribution of reinforced TiB2 particles within the matrix material without any particles agglomeration. The reduction in grain size is observed for increase in weight percentage of TiB2 reinforcement particles.

  3. 3.

    The tensile properties like yield strength and ultimate tensile strength gets increased without any significant drop in the percentage elongation. This is due to the grain size reduction; increase in dislocation density makes the interaction between the matrix and reinforcement particle is the cause for increase in tensile properties.

  4. 4.

    The hardness increases due to the reinforcement of the hard TiB2 particles into ductile matrix and strong interface between the matrix and reinforcement particles strengthen the composite. During the hardness test the indenter load gets shared between the matrix and the hard reinforcement particles is the cause for the increases the hardness.

  5. 5.

    The wear decreases with increase in reinforcement particles. The wear rate keep on reduces due to the formation of protective layer in between the pin and counter surface after crushing of reinforcement particles. It is also confirmed that composites are better in resistance to wear.

  6. 6.

    From the corrosion test it is observed that the corrosion resistance of the composites gets improved by the formation of protective film avoids pitting formation in the surface of the Al7075-TiB2 composites.