Abstract
In the current research, the production of composites, characterization, mechanical and wear behavior of 5 and 10 wt.% of nano composites have been investigated by reinforcing the nano B4C ceramic particulates into Al2024 alloy. The composites were prepared by using two stage stir casting technique containing Al2024 alloy as a matrix phase and nano B4C particulates as reinforcement. After the composite preparation, the prepared composite material was examined by using various techniques such as SEM, EDS and XRD for characterizing the chemical elements and microstructures of reinforced and unreinforced material. Later, the mechanical properties and wear behavior of as cast Al2024 alloy and Al2024 −5 and 10 wt.% nano B4C composites were studied. Different mechanical properties such as hardness, percentage elongation, ultimate and yield strength were evaluated as per the ASTM standards. The dry sliding wear tests were conducted by using pin on disc equipment. The experiments were conducted for the sliding distance of 3000 m by varying the sliding speed and load. It was found that due to the addition of nano B4C ceramic particles in the Al2024 alloy matrix, the hardness, ultimate tensile strength and yield strength of the prepared composites were increased and the percentage elongation was decreased. Furthermore, there was an improvement in the volumetric wear loss with respect to the speed, load, and sliding distance for all the prepared composite materials. However, with the addition of nano B4C ceramic particulates in the base Al2024 matrix, the volumetric wear loss was decreased. The scanning electron microscope was used to study and analyze the fractography and different wear mechanisms for various test conditions of different compositions, tensile fractured surfaces and worn surfaces.
Introduction
In modern technology, nano metal matrix composites play a crucial role in various applications. Nano ceramic particles are often used to enhance the ultimate strength or better yield the metals. However, the ductility of nano metal matrix composites deteriorates with respect to strength by the absorption of high ceramic particles. Therefore, nano ceramic particles can be substituted by micro particles [16].
Compared to other alloys and conventional metals, aluminum alloys are used as a matrix phase in various ranges of domains such as automobile, aerospace, and marine industries due to their different physical properties such as low density, good corrosion resistance, low thermal coefficient of expansion and relative low cost of production [12,14]. Due to these characteristics, aluminum alloy is a very strong and good competitor in a broad range of applications. However, some of the major mechanical properties such as elastic modulus, strength and wear resistance are not very satisfactory for industrial applications; therefore, to meet the requirements, they are reinforced by various ceramic reinforcements such as B4C, SiC, graphite. Hence, compared to other micro particles, nano B4C ceramic particles used as reinforcement are a better choice and have recently started being used in aluminum matrix composites because they offer very high hardness, enhanced strength, chemical stability and also increase thermal stability [11,15].
Nano metal matrix composites are gradually becoming distinctly appealing materials for cutting edge aviation applications and yet their properties can be custom-made by the proper choice of reinforcement. Among three different composites, particulate strengthened MMCs as of late discovered unique intrigue on account of their quality and firmness at normal room and elevated temperatures. It is important to note that the properties of the nano metal matrix composites are unequivocally affected by few secondary parameters of the reinforcement, for example shape, size, distribution and volume.
Nano B4C is one of the hard-ceramic reinforcement materials which tend to possess a low specific gravity, outstanding hardness and good high temperature melting point hence they are the best choice of reinforcement material for nano metal matrix composites. Different applications such as pulleys and linkages in automobiles are prepared by using Al–B4C nano composites. Because of the hard-nano particles, it acts as an interface for wear resistant applications [7]. For the manufacturing of nano metal matrix composites various fabrication techniques are commonly available, such as powder metallurgy, high-energy ball milling, mechanical alloying, stir casting, nano-sintering, and spray-deposition. However, the mechanical stir casting process is considered one of the finest techniques due to the formation of vortex which leads to disperse nano sized B4C particles in molten aluminium without clustering and agglomeration and is also relatively low-cost. With respect to the fabrication processes, the proper selection of several parameters are much needed, such as stirring speed in rpm, time in minutes, preheating temperature of the mould, temperature of molten matrix in °C, along with the continuous uniform feed rate of the nano ceramic reinforcement in steps of two stages into the molten matrix to acquire good wetting property. By using the stir casting technique, components with complex geometry can be produced in huge volumes at a lower cost of production. However, there are several disadvantages such as small blowholes and porosity because of the improper distribution of the nano ceramic reinforcing particles between the metal matrixes which lead to deterioration in various mechanical properties. However, this type of case occurs when the calculated volume of the reinforcement is more than the matrix phase [1]. As revealed so far by performed research, even at the superior temperatures the nano B4C particulates contribute in enhancing various mechanical properties. The matrix deformation is successfully prevented by the existence of nano B4C particulates, which hold the load and lock up the micro cracks that often build up along the friction direction.
However, insufficient information exists with regards to the tribological and mechanical properties of nano B4C particulates reinforced with Al2024, which is processed by the two stage mechanical stir casting method. The aluminum–nano B4C composites play an important role in the industry because of the increased demand of advanced lightweight materials in different industrial applications. Keeping the above observations in mind, it is proposed to develop Al2024 composites with 5 and 10 wt.% of nano B4C ceramic particulates.
Experiments
Materials
The aluminum alloys are classified into two categories: cast aluminum and wrought aluminum. In the current research, Al2024 alloy is used as the matrix material, which is one type of wrought aluminium alloy designated by 4 numbers. Copper is the primary element and is combined with various other elements such as zinc, magnesium, silicon and others, which are listed in Table 1. The melting point of Al2024 is 660 °C and the density is 2.81 g/cm3.
Chemical composition of Al2024 alloy by weight%
Chemical composition of Al2024 alloy by weight%
The main benefit of integrating the ceramic reinforcement material to the matrix material is to enhance the tribological and mechanical properties. In the current research, nano B4C ceramic particulates have been used with a density of 2.52 g/cm3, which is less than that of Al2024 alloy [5]. Because of this, the nano reinforcement material is added in steps of two stages during the preparation of the nano composites to have proper bonding between the matrix and reinforcement and to avoid agglomeration. The nano B4C particles also have a high hardness, as well as a good dimensional and phase stability, which improves the fracture toughness, creep resistance and fatigue resistance of the nano composite materials.
The stir casting method is used in the current research work for the preparation of nano composites by using Al2024 alloy along with 5 and 10 wt.% of nano B4C particulates. The crucible is made from graphite and is made to be placed in an electrical furnace by introducing the pre-weighed Al2024 billet, which was made into small pieces. The electric furnace was heated to 750 °C. The nano B4C ceramic particulates were introduced in a small graphite crucible and were preheated to max. 400 °C. The digital temperature controller was used to check the temperature of the aluminum molten melt inside the graphite crucible, which was placed in an electrical resistance furnace. The degassing agent known as solid hexachloro-ethane (C2Cl6) was added to remove the unwanted gases present in the molten melt [10]. Furthermore, to increase the wettability between nano reinforced particulate to the metal matrix, 5 to 10 grams of magnesium was added. By using a zirconium coated stirrer, the mechanical stirring was done for the molten metal to the speed of 400 rpm for about 2 minutes before adding the reinforced particles to achieve a vortex. Once the formation of vortex is achieved then the preheated nano B4C ceramic particles were added at a constant feed with an equal interval into the molten metal of a cast alloy in two step addition process. The melted molten liquid is poured into a preheated die made from cast iron and finally allowed to cool at room temperature to obtain the desired samples for further processing, as shown in Fig. 1.
Al2024-B4C nano composites after casting.
The prepared samples obtained from the casting are cut into an appropriate size of 5 mm × 15 mm, and then subjected to mirror polishing at different levels for microstructure study. Initially, with 1000 grit size emery paper, the samples were polished and then polished with Al2O3 suspension on a polishing disc by using a soft cloth made from velvet. Furthermore, the diamond paste of 0.3 microns was used for polishing. Finally, the Keller’s reagent chemical was used for etching the polished surface and subjected to microstructure study by using the scanning electron microscope (SEM).
The hardness test was done on the polished surface of the specimens by using Brinell hardness for both reinforced and unreinforced materials, as per the ASTM E10 standard. The hardness test machine is used for conducting the hardness test has a ball indenter of 5 mm diameter and 250 kg load for a dwell period of 30 seconds and 5 different sets of readings were taken at different locations of the specimen’s polished surface and the average was considered. As per the ASTM E8 standard [13], the tensile study was carried out on the prepared specimens with the use of electronic universal testing machine at room a temperature to study different tensile properties such as percentage of elongation, yield strength, and ultimate tensile strength. Figure 2 shows the size of tensile specimen as per the ASTM standards used for testing.
Dimensions of tensile test specimen in mm.
The pin on the disc machine (DUCOM, TR-20LE) was used to conduct the wear test for the study of wear behavior. The dry sliding wear tests were performed on both reinforced and unreinforced materials which had a diameter of 8 mm and length of 30 mm, as per the ASTM G99 standards. The wear machine counter disc was made of EN32 steel material. Prior to the testing process, the acetone liquid was used for cleaning the disc and test pin surface. The various investigations were led at 3000 m sliding distance and 400 rpm steady sliding velocity through varying loads of 20 N, 30 N and 40 N. Similarly, tests were conducted at 40 N constant load through varying speeds of 200, 300 and 400 rpm. During testing the pin was kept opposite and stationary to the spherical steel disc while the circular plate was pivoted. The initial weight of the test pins was measured by using a digital electronic machine with the accuracy of 0.0001 g. After each test, the acetone liquid was used for cleaning of the worn surface. To measure the wear misfortune, the test pin was weighed prior and after the surface was worn. The measured weight reduction was further changed and calculated into volumetric wear misfortune. Figure 3 demonstrates the wear specimens utilized for the wear investigation.
Wear test specimen.
SEM microstructure photographs of (a) cast Al2024 alloy and (b) Al2024-10 wt.% nano B4C composite.
Microstructural study
To examine SEM images, the samples were preferred from the middle segment from the cylindrical specimens. Figure 4a and b show the SEM microstructures of cast Al2024 alloy and the composite of 10 wt.% of nano B4C reinforced with Al2024 alloy. The microstructure of cast Al2024 alloy comprises of fine grains of solid solution of the aluminium along with an ample distribution of inter-metallic precipitates. In addition, the prepared nano composite shows the great bonding among the matrix and the reinforcement alongside the uniform homogenous circulation of nano estimated B4C particulates without any agglomeration and bunching in the composites. This is essentially because the practical mixing activity is accomplished all through by two stage addition process of nano B4C. By the uniform distribution of nano particle in the matrix, the grain limit of the lattice obstructs the grain improvement and opposes the separation development of grains amid stacking.
Energy dispersive spectrum analysis of Al2024-10 wt.% nano B4C composite.
XRD analysis of Al2024-10 wt.% nano B4C composite.
Energy dispersive spectrum analysis (Fig. 5) confirmed the existence of nano B4C particulates in the form of B (Boron) and C (Carbon) elements in the Al2024 alloy base matrix.
Figure 6 shows the XRD (X-ray diffraction) pattern of the Al2024-10 wt.% nano B4C and the occurrence of Al and nano B4C phase are evidently seen. It can be observed that peak height increases and then decreases on 2-theta scale indicating the presence of different phases of material. In Fig. 6, it can be seen that X-ray intensities when peaks are higher than 38°, 45°, 65°and 78°, indicating the presence of the aluminium phase. Similarly, we observed peaks for different phases of boron carbide at 31°, 37°, 50° and 54°.
Hardness of Al2024 alloy and nano B4C composites.
Hardness is a mechanical parameter that demonstrates the capability of resisting of prepared materials to indentation under a static load. The addition of 5 and 10 wt.% of nano B4C particulates to the Al2024 alloy with respect to unreinforced alloy can lead to the variation in the hardness, which can be observed in Fig. 7. There is a noticeable increase in hardness from 66.3 BHN to 103.6 BHN for aluminum with 10 wt.% of nano B4C reinforced composites. This can be credited to the of harder nano B4C ceramic particles in the lattice than base alloy, and the higher constraint to the localized matrix deformation during indentation as an outcome of the presence of harder phase. Furthermore, the B4C, as other fortifications fortify the matrix by making higher density dislocations amid cooling to room temperature because of the distinction of co-efficient of thermal expansion developments between the nano B4C and grid Al2024 compound. The variance in the strain is developed between the nano reinforcement and the matrix alloy obstructs the movement of dislocations by resulting in the enhancement of the hardness of the prepared nano composites [4,9].
Ultimate tensile strength of Al2024 alloy and nano B4C composites.
Figure 8 presents the plot of ultimate tensile strength (UTS) with 5 and 10 wt.% of nano B4C dispersoids in metal lattice composite. When compared to base Al2024 alloy with 10 wt.% of nano composites, there is an increase of 44% in UTS. Because of legal contact between the framework mixture and the supporting materials, there is a major increase in strength. The better the grains estimate, the better the hardness and additionally the better the quality of composites prompting to enhance the ultimate strength [6]. The improvement in UTS is credited by the hard nano ceramic B4C particulates, which confer value to the framework mixture by giving improved solid rigidity. The expansion of these hard-nano particles may have offered a rise to huge lasting compressive unease created along with cementing because of a contrast in the developed coefficient between flexible matrix and brittle nano particles.
Yield strength of Al2024 alloy and nano B4C composites.
The character of the prepared composites is highly dependent on the volume or weight percentage of the reinforcement, which leads to the increase in yield quality. Figure 9 shows the variation in yield strength (YS) of Al2024 alloy matrix with 5 and 10 wt.% of nano B4C particulates reinforced composites. By adding 10 wt.% of nano B4C particles, the yield strength is improved from 169.4 MPa to 251.4 MPa. The expansion in yield strength of the nano composite is clear because of the hard nano B4C ceramic particles, which contribute to the quality by delectating the Al alloy network and bringing more quality resistance of the composite against the connected ductility load. On account of nano particle strengthened composites, the uniformly distributed hard ceramic particles in the grid make a limitation until the plastic stream, in this way giving upgraded quality to the composite [8].
The percentage elongation of Al2024 alloy and nano B4C composites.
Figure 10 represents the impact of nano B4C content on the elongation (ductility) of the composites and the flexibility of the nano composites reduces essentially with the 10 wt.% B4C prepared nano composites, as can be seen from the chart. This diminishing in rate prolongation in association with the matrix and reinforcement is the most frequently occurring disadvantage in particulate prepared MMCs. The decreased malleability in nano composites can be assigned to the closeness of nano B4C ceramic particulates which may break into small dendrites due to the stirring process, as they have a sharp corner that makes the prepared composites distorted to limited part initiate and increase [2]. The delicate collision that happens because of the contact of the hard nano B4C ceramic particles bringing on expanded locality stretch focus locales may likely be the reason.
Sample of fracture surfaces of the tensile test of (a) Al2024 matrix alloy and (b) Al2024-10 wt.% nano B4C composite.
After tensile testing the fractured surfaces (Fig. 11a and b) of cast alloy and nano composite samples were characterized by using SEM images to study the mechanism of the fracture. The cast Al2024 alloy fracture manner is a ductile fracture manner which can be seen in Fig. 11a and a huge number of hollow shaped structures and grains are visible. Figure 11b shows that structures have less ductile failure because of reinforcing the 10 wt.% nano B4C. The reason for failure of composites during the tensile test is because nano particles crack all along with matrix material get fractures, and debond between the boron carbide particulates and Al matrix alloy interface. Small voids are observed in the case of 10 wt.% B4C nano composites, fractured surfaces showed more limited stresses at the interfaces and small cracks at reinforcement particles mechanism are observed.
The varying load at 400 rpm along x axis for Al2024 alloy and nano B4C composites volumetric wear loss along y axis is represented in Fig. 12. Since the load is increased from 20 N to 40 N, the volumetric wear loss is also increased but it is lower in the case of nano B4C ceramic reinforced composite.
Volumetric wear loss of Al2024 alloy and nano B4C composites at varying loads and 400 rpm constant speed.
The high volumetric wear loss is observed for matrix alloy and for the composites at higher loads varying from 20 N to 40 N. At maximum loads the pin exceeds the critical value at the temperature of sliding surface. As eventually the load increases on the pin there is also an increase in the volumetric wear loss of both the unreinforced alloy and reinforced composite. But it is practically observed that the volumetric wear loss of the composites reduces with 5 and 10 wt.% nano B4C ceramic reinforcements in the matrix alloy. This enhancement in the wear resistance of the nano composites is with wt.% of reinforcement is mainly due to the high hardness of nano B4C particulates which acts as a barrier for the material loss [3].
The dependence of all the wear loss of Al2024 matrix alloy along with B4C nano composites on sliding speed is shown in Fig. 13. As the sliding speed (rpm) is increased from 200 rpm to 400 rpm, the losses are also increased for both Al2024aluminium matrix alloy and its constituent composites due to wear. But although at every sliding speeds, the wear loss of the nano composites is much lesser, when compared with the matrix alloy.
Volumetric wear loss of Al2024 alloy and nano B4C composites at different speeds and at constant load (40 N).
Additionally, as sliding speed is kept on increasing there is noticeable increase in wear loss also because of softening of the nano composite at increased temperature due to rubbing action. The increase in temperature resulting due to higher sliding speeds also leads to plastic deformation of the test piece. Therefore, there is increased delamination contributing to enhanced wear loss.
The worn surface microphotographs studies of cast Al2024 alloy and nano B4C reinforced composites are examined with SEM. Figure 14 characterizes the worn surfaces of matrix material Al2024 alloy (Fig. 14a) and the nano composite surface which is tested at 40 N load and 400 rpm sliding speed by reinforcing 10 wt.% of nano B4C (Fig. 14b) particles in base materials.
Figure 14a shows the particular edges and depressions in the sliding direction that run parallel to each other. It can be seen that the cracks are deeper and wider spreading lattice combination Al2024 when compared with nano composites under similar conditions. Because of the sliding of oxide molecule in the reinforced composite, it might be seen from Fig. 14b that a break will likewise occur on the well-used outer surface of the Al2024-10 wt.% B4 C nano composite. On account of nano composites, a thick layer could be seen, which shields the basic matrix from being in contact with the sliding partner and along these lines minimize the volumetric wear misfortune. Therefore, the layer framed on the nano composites gives a self-protective cover to the hidden material and, as a result, represses the metal-metal contact.
SEM microphotographs of worn surfaces of (a) cast Al2024 alloy and (b) Al2024-10 wt.% B4C composites at 40 N load and 400 rpm speed.
Aluminium alloys such as Al2024, 2014, 2219 and 7075 are the most widely used materials for aerospace applications. They are particularly used in manufacturing aircraft components such as wing root fittings, hinges, bulkheads and the frames Al 2XXX and 7XXX series, due to their high strength properties. They play a crucial role in the aircraft design and manufacturing weight of the component as the weight of the aircraft directly affects fuel consumption. Since Al2024-B4C nano composites exhibit superior properties, these composites can be used for the fabrication of wing root fitting and hinge design. The major advantage of using these composites is weight saving due to the reduced cross sectional area of the components.
Conclusions
By using the stir casting fabrication technique, the nano B4C/Al2024 nano composites have been fabricated by considering 5 and 10 wt.% of reinforcement. The microstructures, mechanical properties such as hardness, yield strength, ultimate tensile strength, percentage elongation, and fractography and wear behavior of the prepared samples were studied as per the ASTM standards. The matrix is almost free from pores in cast alloy and uniformly distributed of nano particles in the prepared composite, which can be seen in the SEM microphotographs. The XRD and EDS analyses confirm the existence of nano B4C ceramic particles in the Al2024 alloy matrix. Compared to unreinforced material, the various mechanical properties of Al2024-10 wt.% nano B4C composite are superior and enhanced. Due to strain localization, the fracture surface of the nano composite material consists of small voids. These small voids were coalesced during tensile loading, ensuring a dimple appearance at the cracked area of surface in the structure. The wear resistance of Al2024-10 wt.% B4C nano composite is significantly superior to that of the unreinforced material. Furthermore, the volumetric wear loss of Al2024 alloy matrix and nano composites increased with the increase in load and speed.