Experimental Investigation of Mechanical and Wear Behaviour of AZ91 Magnesium Hybrid Composite Materials

Abstract

In recent years, emerging requisite for advanced materials gave a path for hybrid composites. Magnesium metal matrix composites are gaining more interest and a better substitute for heavier steel, aluminium, titanium and even for plastic based materials. At present the AZ91 magnesium alloy is most widely in transport vehicle industry. However, the application of AZ91 magnesium alloys are limited due to several negative effects such as poor creep resistance, wear resistance and inferior corrosion resistance when it is exposed to atmospheric conditions. Future to improve the strength, better corrosion resistance and wear resistance are important for their extend applications of exciting alloy AZ91. The main objective of the present investigation is to achieve above mentioned properties. The AZ91 alloy was reinforced with titanium dioxide/0.5% graphene and with titanium/0.5% graphene in varying weight percentage (1%, 2%) by stir casting technique. These combinations are called hybrid metal matrix composite of materials such as AZ91 + 1%Ti +0.5% Gr (A1), AZ91 + 2%Ti +0.5% Gr (A2), AZ91 + 1%TiO2 + 0.5% Gr (B1) and AZ91 + 2%TiO2 + 0.5% Gr (B2) alloys. The following experiments such as tensile, compressive, hardness and wear tests have been carried out to find all the properties from the newly developed hybrid metal matrix composite of materials and compared with AZ91. Wear tests have been carried out by pin on disc tribometer for both dry and wet sliding condition under 20 N,40 N,60 N, and 80 N. The results indicated the AZ91–1%TiO2–0.5%Gr having high wear resistance compared to other three combinations as well as AZ91. The present experimental investigations of hybrid metal matrix composite of materials have wear resistance in the order of B1 > A2 > A1 > B2 > AZ91 and AZ91–2%TiO2–0.5% Gr showed good tensile strength and hardness. The enhanced these properties were discussed in this paper.

1. Introduction

Materials are the key stuff of engineering to household applications which creates pursuit for commercialization. The present world is a competitive customer driven one. The need to save our mother Earth, limited primary resources and stringent norms paved the way to use of the newer materials in engineering sectors. Quality, comfort, safety and reliability of product are the major concern of survival of the industries. Product’s quality is influenced by its design features, material selection and processing techniques. This emphasised the innovation of newer materials to tailor the needs of ever growing demand product for better engineering properties as well as aesthetics. Materials are playing the most vital part from the past till the current scenario as well as in upcoming future. These are substances out of which anything can be prepared. They have contributed to the development of various fields such as space, communication, transportation, medicine, agriculture and food processing industries. Light weight magnesium and its alloys are the most promising next generation materials. Magnesium and its alloys find unique place in the automotive sector due to better solidification and fluidity, high strength to weight ratio, good damping behaviour and high recycling potential in automotive, aerospace, defence and electronics for better radiation shielding effect as well in biomedical sectors as properties of bone is similar to magnesium.

1.1 Classification of materials

Most of the engineering materials are broadly classified into one of the following such as Metals and alloys (ferrous & non-ferrous), Ceramics, Organics, Semi-conductors, Biomaterials and Composites.

1.2 Metals and alloys

Metals are the class of materials characterised by their high electrical conductivity as well by thermal conductivity. Metals are crystalline solids which are closely packed and symmetric in nature. Based on the crustal abundance, metals are arranged as - aluminium, iron, calcium, sodium, titanium, copper, brass, magnesium. New materials were developed from predates by intrinsic modification that is by addition of alloying elements to enhance the mechanical and wear properties of the existing materials.

1.3 Ceramic materials

Any inorganic, non-metallic solid substances that has chemical inertness, high strength, hardness, high melting temperature, low thermal and electrical conductivity as their characteristics but brittle in nature may be termed as ceramics. Alumina, calcia are few examples.

1.4 Composite materials

New materials are developed to meet the customer driven competitive environment by extrinsic modification. Composite materials fall under this category. Composite materials constitute two or more distinct type of materials. Reinforcements are added to the parent material to improve the properties. Figure 1 show the classification of composite materials.

Figure 1.

Classification of composite materials based on reinforcement and matrix types.

2. Need for composite materials

Pollution has become the major consideration by legislation to save our mother earth, in particular, the emission of carbon dioxide by combustion of fossil fuels. Globally, the transportation industries face two major issues - CO₂ emission and fuel consumption. Moreover, stringent rules framed by the environment pollution act [1] to reduce CO₂ emission and for energy conservation, have paved the way for automotive sectors to turn their attention towards energy efficient vehicles. On the other hand, consumers demand for better aesthetic appearance, comfort, use of electronic system like image processing, navigators and extra safety measure etc., adds weight to the existing system [2].

Some of the possible ways to resolve the issues faced by automobile manufacture sector are: Innovation in use of green energy, Improvising the design and Substitution of advanced material with the existing ones. The use of green energy includes high technology incubation and high cost. Design improvement again involves a lot of research and development from the point of industry. So to meet out the growing demands and challenges, automotive sectors are focusing on use of advanced lightweight materials as a better solution.

Enhancing use of light weight material in current scenario is not only most significant way to reduce the overall weight of the vehicles but also cost effective one. Another important criterion to be considered by reduction in weight of vehicle results in reduction in greenhouse gases emission. It is evident that 100 kilogrammes of self -weight of the vehicles reduced gives us 0.5 litres per 100 kilometres as fuel saving [3].

To achieve weight reduction, the most promising lightweight materials were used to directly substitute heavier steel. Among the various materials, designers found magnesium has potential to substitute heavier steel and aluminium [4]. Gradually magnesium and its alloys usage has increased greatly in the last few decades [5]. It is evident from various researches that magnesium has growing demand in automotive sectors, biomedical, electronics due to its unique characteristics [6]. Another major issue is regarding the radiation by using of mobile phones apart from energy efficiency, stringent norms of pollution control and use of limited resources. Due to new regulation concerning the electromagnetic radiation effect of electronic gadgets paved the way for higher demand for magnesium die cast parts since magnesium has excellent electromagnetic shielding effect [7].

Magnesium and its alloys has low density, high strength to weight ratio, good castability, better solidification due to low latent heat, good vibration damping effect (high speed application), recyclable, better noise dampening, good manufacturability than the conventionally used metals [8]. Among the various magnesium based alloys, AZ (Aluminium and Zinc) and AM magnesium alloy finds wide spread applications.

However, the application of magnesium alloys is limited due to some negative effects like poor creep resistance, and inferior corrosion resistance when exposed to sea and road salt [9].

Composite materials provide an opportunity to enhance particular property along with the advantages of base material. New spot in advanced innovation is hybrid metal matrix composite materials [10] which tailors the need of the functional requirement. Selection of proper material is a vital part of design team in automobile companies. They focus on performance as well as cost. Hybrid composite materials are combination of composite and hybrid materials. They possess two or more reinforcements having different properties. The performance of the hybrid composite material is the joint effect of all the individual constituents. They are usually employed for low density and high strength applications particularly in the field of automotive sector and aerospace sector.

Most of the magnesium alloys possesses excellent fluidity, good machinability and processability which enables the production of complicated die cast parts easier [11]. AZ91(Mg-9Al-0.8Zn-0.2Mn) has less susceptibility to hydrogen porosity and less reactive iron hence can be poured in steel mould. Among the various alloys of magnesium, AZ91 is conferred as one of the best magnesium alloy suitable for thin walled applications. Use of AZ91 instead of aluminium reduced the weight of about 25% but the geometry and production tools remained the same compared to AZ [4]. In addition, literatures also reveal that most of published work were focused on magnesium based composites owing to their good mechanical properties, good damping behaviour which arrests the vibration and reduces the sonic emission and good radiation shielding effect [11]. The SiC, Graphene Nanoplates, Boron carbide and titanium reinforced with Magnesium alloy to improve the mechanical properties.

3. Scenario of composite materials and magnesium alloys

Light weight material still plays a significant role in structural application in the field of automotive and aerospace sectors. Demand for newer materials increases every year which is evident in various scientific researches carried all over world by researchers. It has been possible because of growing technologies, processing techniques, advent of newer phase in materials and its applications in this customer’s demand driven world. Composite materials have brought a revolution in the world of material. These materials are unique and different from the monolithic or heterogeneous materials. Composite materials constitute two or more distinct type of material which is not the by-product of chemical reaction between the constituents. It has two phases (constituent material), one of the constituent is reinforcing phase which are discontinuous and are embedded in continuous phase called matrix.

Matrix is continuous phase whose work is to retain the shape of the composite, share the stress to other phase. They provide better finish to the product.

Reinforcement phase is strong and may not be less dense. Reinforcing phase may be in the form of fibres, particles or flakes. They contribute the desired properties and also transfer the strength to the matrix.

3.1 Types of composites

The composites materials are classified into three major types.

Polymer matrix composites.

Ceramic matrix composites.

Metal matrix composites.

3.2 Polymer matrix composites

Polymer Matrix Composites (PMCs) has organic polymer as matrix and reinforcing phase are a variety of short or continuous fibres. Organic polymers are long chain of carbon. Fibres are used to enhance the mechanical properties and to carry loads. Matrix’s purpose is to bond the fibres and protect reinforcing phase. Polymer matrix composites are categorised into two types based on mechanical properties (strength and stiffness): reinforced plastics and advanced composites. Reinforced plastics are polymer matrix composites that impart additional strength by adding embedded fibrous material into plastics. These type of composites are usually cheap and consist of polyester resins reinforced with glass fibres (low stiffness). Advanced composites consist of matrix and fibres which facilitates superior stiffness and strength. Fibres in advanced composite type are mostly high performance like graphite, aramid. Advanced composites are relatively expensive but possesses high strength and superior stiffness.

3.3 Ceramic matrix composites

Ceramics are solid materials which exhibit very strong ionic bonding and in few cases covalent bonding. Ceramic matrices such as silicon nitride, silicon carbide surround the fibres. They are the good choice for high temperature applications. Ceramic matrix composites possess principal characteristics like resilience to oxidation and high compressive strength. Production errors or scratches may lead to cracks in conventional ceramics. However in ceramic matrix composite, by embedding the fibres, fracture toughness values is enhanced. Even crack propagation was hindered by the presence of fibres (reinforcement) in matrix of ceramic composites.

3.4 Metal matrix composites

Metals play significant and versatile role extending itself from house hold to engineering application. Metal Matrix Composite (MMC) as the name implies has reinforcement (fibres or particles) surrounded by matrix of metal. The matrix is the monolithic material. The main matrix materials used are aluminium, titanium, magnesium and copper. Reinforcement phase not only carries the structural task but also used to enhance the property like wear resistance, resistance to degradation by fluid and so on. Metal matrix composite finds its wide spread opportunities as it tailors the need to achieve a better property beyond those unattainable by monolithic materials. These composites potentially have high stiffness, strength and high temperature resistance over polymer matrix composites. However metal matrix composites can operate in varied temperatures and has better electrical and thermal conductivity.

In recent years, metal matrix composite materials are gaining more attention due to their greater wear resistance, higher thermal conductivity when compared to unreinforced metals and alloy. Apart from these advantages, Metal matrix composites are environmentally suitable as they do not absorb water like PMC. They use low density materials like aluminium and magnesium as matrix.

3.5 Hybrid composites

Hybrid composite materials are consisting of a matrix and possess two or more reinforcements having different properties. The performance of the hybrid composite material is the joint effect of all the individual constituents. They are usually employed for low density and high strength applications particularly in the field of automotive sector and aerospace sector.

3.6 Magnesium based composites

Magnesium alloys and its composites find a significant place as next generation materials as a substitute for aluminium and steel. Magnesium offer better mechanical and physical properties like high strength, good vibration dampening capacity, better castability, ease of machinability, good recyclability, environmental friendly and above all low density 1.74 g/cc (pure magnesium) than the conventionally used materials in the automotive sectors like iron and aluminium [2]. Pure Magnesium though light in weight but has limited use due to its insufficient strength and corrosion resistance [12]. Alloying magnesium with one or more element like aluminium, zinc, manganese, zirconium, rare earths and so on gave rise to new phase of magnesium alloys which enhanced the properties of pure magnesium. Magnesium alloys not only stand for their better properties but also as environmental friendly material [13].

Among various grades of magnesium alloys, A-Z magnesium alloy are used for structural applications. It has good castability and high specific strength. AZ91 magnesium alloy finds numerous applications in building structural components in aerospace and automotive sector like chassis, wheel, and indoor frame so on [14]. But its negative effect is poor corrosion resistance, low wear resistance in some service environments. It exhibits poor corrosion resistance property in sea salt environment but generally has good corrosion resistance during atmospheric exposure condition [15]. However, its application cannot extend beyond the limit due to some of its negative effects. Reinforcement of AZ91 magnesium alloy with suitable particles can overcome the shortfalls of alloy. Due to growing demand and to meet out the requirements, hybrid composite material is more advantageous than the conventional and composites materials. There are various researches on magnesium metal matrix composite reinforced with SiC, Al₂O₃, Ti, TiO₂, Carbon nanotubes, Graphene, TiC to establish the correlation between mechanical properties and microstructure characteristics of composites, corrosion resistance, wear resistance property [16] etc. It was observed that there were not much research reports on the influence of titanium oxide and graphene and titanium & graphene reinforced with AZ91 magnesium alloy as hybrid composites.

3.7 Selection of matrix material: AZ91 magnesium alloy (Mg-9Al-0.8Zn-0.2Mn)

Constituents of composite materials are matrix and reinforcement. Matrix plays a major role in binding and protecting the reinforcement, distributing the load, enhancing the final property and giving better finish to final product. Matrix is usually aluminium, magnesium and titanium for structural applications.

AZ91 magnesium alloy possesses good mechanical properties. It has less susceptibility to hydrogen porosity and less reactive iron that can be poured in steel mould. Among the various AZ magnesium alloy, AZ91 is conferred as one of the best magnesium alloy suitable for thin walled applications. Usage of AZ91 magnesium alloy instead of aluminium alloy reduced the weight by about 25%, but the geometry and production tools remained the identical [17].

3.8 Reinforcement

Reinforcement can enhance desired properties of the conventional materials. The desired properties depend upon the working environment in which the material is exposed like strength, stiffness, corrosion resistance, wear resistance, reduced weight, dimensional stability, fatigue and extends up to attractiveness and cost. Reinforcements can be classified into two groups: continuous and discontinues. Metal matrix Composites processed by them are termed as continuously reinforced metal matrix composite and discontinuously reinforced composite. They are further sub divided into categories like short fibres, whisker, particulates, continuous fibre and preform/wire (fibre solid). Some important parameters considered are as follows for the selection of reinforcement [15].

• Geometry-Size

• Shape –spherical, flake like, needle

• Content –Volume

• Poly or single crystal

• Inherent properties – Strength, density, hardness

In order to attain the desired properties in (AZ91 magnesium alloy) like improved wear resistance and corrosion resistance, right choice depends upon the types of reinforcement, production method and compatibility with matrix. Most widely used reinforcement in Metal matrix composites is ceramics [18]. Usually ceramics such borides, oxides and carbides are widely used reinforcements.

One of the best advantage of Magnesium/Magnesium alloys based composites over aluminium composites is that the wettability of liquid magnesium with ceramics reinforcement phase is better. Use of titanium reinforcement in Magnesium MMCs gave better results which had the potential to create a new material with excellent mechanical properties, wear and corrosion resistance [19]. Carbon nanotube and graphene are also gaining high interest in the field of materials. Graphene possesses better mechanical, thermal and electrical properties and also light weight with density of 1.06 g/cc. Addition of graphene as reinforcement in magnesium composites has enhanced the strength of metals. Titanium oxide are usually used for coating purpose to enhance corrosion resistance property. Titanium oxide used as reinforcement with magnesium matrix showed improved hardness and strength [20].

3.9 Selection of reinforcement

Selection of reinforcement plays a crucial role as improving the desired properties of final material. As reinforcement (discontinuous or continuous) is embedded in the matrix, they share the load and transfer strength to the matrix. Discontinues metal matrix composites are mostly isotropic in nature and also cost effective. The following reinforcements are selected for this work.

Graphene.

Titanium and Titanium dioxide.

Most widely used and recommended reinforcement were titanium carbide, boron carbide, MWCNT for melting metallurgical methods. Yet the studies discloses the other types of reinforcement like titanium dioxide and graphene are usually used for coating purpose to enhance the corrosion resistance property on magnesium alloy and results obtained were positive. But titanium dioxide and graphene used as reinforcement in magnesium matrix hybrid composites are limited and yet to be explored. Similarly, titanium and graphene as reinforcements embedded in magnesium matrix are yet to be explored.

4. Wear

Tribology is the branch of science dealing with contacting surfaces in relative motion. It deals with friction, lubrication and wear of interacting surfaces. Wear is recognised as the phenomenon of undesirable removal of materials from the surface due to interaction with the counter surface. Wear resistance is not the basic property of any material, but depends on environment. It varies even if it is not exposed to the same work environment. Almost all moving parts of machines durability, reliability and life depend upon one main parameter: wear. Therefore, wear property demands its analysis as strong need for advanced and reliable material property for application oriented environment. Wear can be controlled by the selection of right material and proper operating conditions [21]. Wear is a critical factor composing of dynamic parameters, environmental parameters, and material parameters. So wear resistance property is usually analysed for purpose of lubrication, to screen material & its surface treatments, sometime to establish relationship between finishing methods (processing) and its performance. The different type of wear is sliding wear, impact wear and rolling contact wear. Sliding wear is a relative movement between two solid surfaces acting tangential to each other.

5. Materials

The AZ91 magnesium alloy is the base material. AZ91 magnesium alloy used in the present work was fabricated by stir casting process. This alloy was prepared by commercial magnesium ingots, aluminium ingot and zinc ingot. AZ91 magnesium alloy has main composition of 9% Aluminium, 1% Zinc and 0.3% Manganese. Aluminium of 9% by weight is added to enhance the castability, hardness and as well the freezing range. Zinc is added upto 1% by weight to magnesium as alloy to increase harmful corrosive effects. Zinc when exceed more than 1% will cause hot shots. Manganese (0.3% by weight) was added to increases the resistance against corrosion. The remaining was pure magnesium. Chemical composition of AZ91 magnesium alloy is listed in Table 1. Titanium, titanium dioxide and graphene were the reinforcements used for the preparation of hybrid composite materials. Titanium (99.9% supplied by Nextgen steel and suppliers, Mumbai) graphene nanoparticles of 99% purity (supplied by AdNano Technologies, Karnataka) and titanium dioxide of 250 nm (99.97% purity supplied by Sisco Research laboratory (SRL) Maharashtra). The AZ91 magnesium alloy and hybrid composites are manufactured by stir casting technique.

5.1 Fabrication of base material AZ91 magnesium alloy (matrix)

Initially the ingots are cut and weighed for right proportion. The proportions are illustrated in Table 2.

Element	Al	Mn	Zn	Si	Fe	Others	Mg
Composition	8.3–9.1%	0.15%	0.35–1.0%	0.01%	<0.005%	<0.052%	Balance

Table 1.

Nominal composition of AZ91 magnesium alloy.

Material	% weight	Weight
Magnesium	89.7%	897 grams
Aluminium	9%	90 grams
Zinc	1%	10 grams
Manganese	0.3%	3 grams

Table 2.

Proportion of base material (matrix) AZ91.

5.2 Hybrid composites by stir casting process

A recent development in stir casting is a two-step process. The effectiveness of the two-step method of processing is mainly attributed to its ability to break the gas layer around the particle surface. Particles have a thin layer of gas being absorbed on their surface, which hinders wetting between the particles and molten metals.

Magnesium alloys has unique feature in solidification. In this process, the metal is heated to above its liquids temperature with inert argon gas atmosphere, because Magnesium is flammable in molten state. Then the metal is fully melted. The melt is then cooled to a temperature between the liquids and solidus points. It is now in a semi-solid state. At this stage, the preheated particles are added and stirred well by mechanical stirrer for certain period of time to ensure proper mixing. The slurry is again heated and mixed thoroughly. This technology is relatively simple, easy adaptable and cost effective.

The test samples were cut into desired size (10 x10x 5) mm and polished with silicon carbide paper up to 1200 grit. Alumina powder polishing was done to ensure mirror image on the specimens. The prepared specimens are washed with acetone and dried. The specimens are then etched with acetic picral (5 mL acetic acid, 6 g picric acid, 10 mL water, 100 mL ethanol (95%) a universal etchant). The test sample was immersed gently until the face turns brown. It was then washed with ethanol and air dried at room temperature.

6. Mechanical testing

The utility of metals/composites in different fields of engineering depends on its capacity to accomplish design and service requirements of the current scenario. The competence of composites to satisfy these requirements is determined by the mechanical, thermal and physical properties of the metal [22]. Mechanical testing like tensile test, compression test and hardness test were done on the hybrid composites with varying composition and varying weight % of reinforcements as shown in Table 3.

Sr.No	Hybrid composite Sample	AZ91 (%)	Ti (%)	TiO2 (%)	Gr (%)
1	AZ91 + 1%Ti +0.5% Gr (A1)	98.5	1	—	0.5
2	AZ91 + 2%Ti +0.5% Gr (A2)	97.5	2	—	0.5
3	AZ91 + 1%TiO2 + 0.5% Gr (B1)	98.5	—	1	0.5
4	AZ91 + 2%TiO2 + 0.5% Gr (B2)	97.5	—	2	0.5

Table 3.

Proportions of matrix and reinforcements for hybrid composites.

6.1 Tensile test

The tension test is one of the most common tests performed on the casted samples for evaluating materials. The tension test was done by gripping opposite ends of a tensile test sample within the load frame of a tension machine. A tensile force was applied by the machine, resulting in the gradual elongation and then results in fracture of the test sample. The mechanical properties found from tension test are yield strength, ultimate tensile strength, ductility properties, such as elongation and reduction in area. Figure 2 shows the dimension of test specimen as per ASTM and Figure 3 shows the tensile test sample of hybrid Composite specimen.

Figure 2.

Dimensions of test specimen-ASTM.

Figure 3.

Tensile test sample of hybrid composite prepared as per standards.

7. Results and discussion

7.1 Microstructural characterisation

Microstructural studies conducted on the casted hybrid composites sample revealed the equiaxial grain morphology and monolithic showed coarse grain structure. The unreinforced AZ91 showed coarse dendrites and SEM observation indicated the presence of the α-Mg matrix which was surrounded by inter dendrite and β phase (Mg₁₇Al₁₂). Further SEM image with EDS analysis was used to verify the micro-composition of the unreinforced AZ91 magnesium alloy. The EDS analysis shows the presence of magnesium, aluminium and zinc peaks but manganese cannot be detected due to low content of addition (0.3%) as shown in Figure 4.

Figure 4.

AZ91/1% Ti/0.5% Gr (a) optical micrograph (b) SEM image (c) EDS spectrum.

Figure 5.

AZ91/2% Ti/0.5% Gr (a) optical micrograph (b) SEM image (c) EDS spectrum.

Addition of titanium and graphene to AZ91 magnesium has positively refined the grain size. The AZ91 magnesium alloy, AZ91 + 1% Ti + 0.5%Gr and AZ91 + 2% Ti + 0.5%Gr showed the intermetallic compounds appeared brighter surrounding the grains. The titanium combines with aluminium during initial stage of solidification. Hence at eutectic temperature there is less content of aluminium available to form β phase (Mg₁₇Al₁₂) [23]. The EDS analysis proves the presence of graphene of the hybrid composites [24] depicted in Figure 4. The other combination of hybrid composite AZ91 magnesium alloy was with titanium dioxide and graphene also shows the discontinuous β precipitates are found around the grains. The EDS spectrum clearly signifies in the Figures 4–7 and reveals the presence of magnesium, aluminium, zinc, graphene and titanium dioxide. The titanium dioxide as overlapped with Ti and oxygen. It was observed that the reinforcements are uniformly distributed in the AZ91 magnesium matrix for all the hybrid composites. It was also observed no voids or cracks from the micrographs which is assisted by density measurement.

Figure 6.

AZ91/1% TiO₂/0.5% Gr (a) optical micrograph (b) SEM image (c).

Figure 7.

AZ91/2% TiO₂/0.5% Gr (a) optical micrograph (b) SEM image (c) EDS spectrum.

7.2 Density measurement

The results of density test are illustrated in the Table 4. The results showed that denser hybrid composites produced among them was AZ91 + 2%Ti +0.5% Gr. The theoretical density was found to be higher than actual density of the hybrid composites.

Sr. No.	Test Samples	Actual density (g/cc)	Theoretical density (g/cc)
1	AZ91	1.81	1.813
2	AZ91 + 1%Ti +0.5% Gr (A1)	1.824	1.826
3	AZ91 + 2%Ti +0.5% Gr (A2)	1.846	1.847
4	AZ91 + 1%TiO2 + 0.5% Gr (B1)	1.823	1.8237
5	AZ91 + 2% TiO2 + 0.5% Gr (B2)	1.841	1.843

Table 4.

Density of hybrid composite material.

It was noted from the result that the densities of each hybrid composites were higher than the base matrix clearly signifying that porosity will be reduced by increase the weight percentage of the reinforcement as shown in Figures 8 and 9. The increase in density attributes to increase in hardness and decrease in porosity.

Figure 8.

Density measurement.

Figure 9.

Tensile strength of AZ91 magnesium alloy and hybrid composites.

7.3 Tensile test

The tensile tests are carried out at room temperature and results of all the hybrid composites are depicted in Table 5 and Figure 9. It revealed that the addition of reinforcement titanium and graphene have gradually increased the strength of the material than the monolithic AZ91. The addition of titanium shows improved ductility than the base matrix. The diffused magnesium and aluminium with titanium yielded higher tensile strength due to the addition of reinforcement graphene, inspite of its high thermal conductivity yet has no chemical reaction with the matrix [25].

Sr.No.	Test Sample	Ultimate Tensile Strength(MPa)	Yield strength(MPa)	% Elongation
1	AZ91	83.3	76.55	3.0
2	AZ91 + 1%Ti +0.5% Gr (A1)	131.08	109.74	4.52
3	AZ91 + 2%Ti +0.5% Gr (A2)	137.33	105.36	4.56
4	AZ91 + 1%TiO2 + 0.5% Gr (B1)	138.9	123.98	3.5
5	AZ91 + 2% TiO2 + 0.5% Gr (B2)	149.1	131.2	2.94

Table 5.

Tensile strength of AZ91 magnesium hybrid composite material.

The titanium dioxide and graphene hybrid composites exhibited higher strength but probably low ductility. It was noticed that the decrease in ductility may be due to the increase in weight percentage of the reinforcement creating a strained lattice and also void nucleation. The strength of hybrid composites depended on the bonding between the AZ91 magnesium matrix and reinforcements. In both the cases, the graphene acts diffusion barrier for aluminium and magnesium forming intermetallic bond which have poor stability.

7.4 Compression test

Compression test have been conducted at room temperature. The results indicated that significant improvement in ultimate compressive strength of AZ91 magnesium hybrid composite than the unreinforced AZ91 magnesium alloy. Within the hybrid composites sample, the increase in compressive properties was only marginal due to the addition of reinforcements. The test samples and recorded test results are, illustrated in Table 6 and Figure 10. It was clearly evident that from the compressive testing that hybrid composites and monolithic AZ91 magnesium alloy test samples are split into two halves at angle of 45°. It was observed that reinforcement’s combination titanium and graphene with AZ91 have higher compressive strength when compared to the other hybrid combination.

Sr. No.	Test Sample	Ultimate Compressive; Strength (MPa)	Elongation; %
1	AZ91	289.54	2.88
2	AZ91 + 1%Ti +0.5% Gr (A1)	349.9	3.7
3	AZ91 + 2%Ti +0.5% Gr (A2)	366.1	3.29
4	AZ91 + 1%TiO2 + 0.5% Gr (B1)	335.56	3.4
5	AZ91 + 2% TiO2 + 0.5% Gr (B2)	351.46	3.36

Table 6.

Compressive strength of hybrid composites.

Figure 10.

Ultimate compressive strength.

7.5 Hardness

Table 7 shows the test result of micro hardness of hybrid composites and monolithic AZ91 magnesium alloy.

Sr. No.	Test Samples	Location(1 HV)
		1	2	3	4	Average
1	AZ91	62.4	62.7	64	66.7	64.0
2	AZ91 + 1%Ti +0.5% Gr (A1)	75	72.3	68.4	74.1	72.5
3	AZ91 + 2%Ti +0.5% Gr (A2)	73.8	80.1	80.7	80.8	78.9
4	AZ91 + 1%TiO2 + 0.5% Gr (B1)	73.5	71.0	75.2	69	72.2
5	AZ91 + 2% TiO2 + 0.5% Gr (B2)	70.1	72.2	75.7	75.2	73.3

Table 7.

Hardness.

The results revealed that increase in micro hardness of the reference alloy AZ91 magnesium had increased due to the addition of reinforcements of titanium/graphene and titanium dioxide/graphene. Among the two reinforcement’s combinations, it was evident that the hybrid composites titanium/graphene combination exhibited better hardness than the other combination of reinforcement.

The increase in hardness of hybrid composites was due to presence of harder titanium and graphene particles in the reference which assist in load bearing capacity of matrix [26]. Addition of graphene to AZ91 significantly improved the hardness of the materials. Within the hybrid composites samples, increase in hardness was very significant as the attribute to uniform distribution of reinforcements. The AZ91/2% Ti/0.5% Gr hybrid composite showed the highest hardness and compressive strength among the entire hybrid composite. Figure 11 shows the micro hardness of all the hybrid composite samples and AZ91 reference alloy.

Figure 11.

Hardness.

8. Summary

From the experimental investigation the following results were obtained.

• The yield strength, ultimate tensile strength and compressive strength have significantly increased compared with base alloy with addition of titanium in the range of 1% -2%. The compressive strength has increased by addition of Ti and graphene nano particulates.

Hardness of the entire hybrid composites were increased which attributed to uniform distribution of the reinforcements and load bearing capacity. Titanium and graphene has higher hardness than titanium dioxide and graphene.

9. Wear

9.1 Weight loss

The pin on disc was proposed to study and evaluate the tribological behaviour. The amount of wear loss due to dry sliding and wet sliding conditions of the novel hybrid composites fabricated by using stir casting technique and conducted the experiments. The results of the weight loss of wear test of all the hybrid composites and base alloy AZ91 magnesium are illustrated in the Tables 8–11. W1–initial weight of the sample before test(g) and W2-final weight of the sample after test(g). The test results obtained from the dry sliding wear test and wet sliding wear test were thoroughly analysed to assess the effect of reinforcements and effect of load on the test samples. The sliding distance and velocity were maintained constant. The coefficient of friction is the ratio of frictional force to normal applied load. The coefficient of friction purely depends upon the materials properties and also on surface roughness, temperature of exposure, normal load and environment.

Sample	LOAD (N)	WET TEST			DRY TEST
		W1	W2	Weight loss (g)	W1	W2	Weight loss (g)
AZ91	20	3.2308	3.22	0.0108	2.0953	2.0733	0.022
A1	20	3.8759	3.8699	0.006	3.1271	3.1061	0.021
A2	20	3.366	3.3629	0.0031	1.3662	1.3475	0.0187
B1	20	3.1879	3.1867	0.0012	2.226	2.206	0.02
B2	20	4.2149	4.2115	0.0034	2.512	2.5007	0.0113

Table 8.

The weight loss of the hybrid composites and AZ91 alloy (dry and wet test) with load of 20 N.

Sample	LOAD (N)	WET TEST			DRY TEST
		W1	W2	Weight loss (g)	W1	W2	Weight loss (g)
AZ91	40	3.6082	3.596	0.0122	2.0733	2.0412	0.0321
A1	40	3.3945	3.3872	0.0073	3.1061	3.0812	0.0249
A2	40	3.603	3.5864	0.011	1.4202	1.3908	0.0294
B1	40	3.5085	3.5054	0.0031	2.2032	2.1782	0.025
B2	40	4.098	4.0873	0.0107	2.5007	2.4727	0.028

Table 9.

The weight loss of the hybrid composites and AZ91 alloy (dry &wet Test) with load of 40 N.

Sample	LOAD (N)	WET TEST			DRY TEST
		W1	W2	Weight loss (g)	W1	W2	Weight loss (g)
AZ91	60	3.2304	3.2139	0.0165	2.1139	2.077	0.0369
A1	60	3.8701	3.8574	0.0127	2.8955	2.8645	0.031
A2	60	3.3629	3.3461	0.016	1.3669	1.3352	0.0317
B1	60	3.1876	3.1764	0.0112	2.2065	2.1777	0.0288
B2	60	4.2115	4.1985	0.013	2.549	2.517	0.032

Table 10.

The weight loss of the hybrid composites and AZ91 alloy (dry &wet Test) with load of 60 N.

Sample	LOAD (N)	WET TEST			DRY TEST
		W1	W2	Weight loss (g)	W1	W2	Weight loss (g)
AZ91	80	3.6027	3.5662	0.0365	2.077	2.0374	0.0396
A1	80	3.3927	3.3731	0.0196	2.8645	2.834	0.0305
A2	80	3.5844	3.5601	0.0243	1.3475	1.3175	0.03
B1	80	3.5074	3.4834	0.024	2.177	2.143	0.034
B2	80	4.0783	4.06	0.0183	2.517	2.486	0.031

Table 11.

The weight loss of the hybrid composites and AZ91 alloy (dry &wet Test) with load of 80 N.

The wear test results (dry sliding & wet sliding) depicted in Figure 12 (a)-(d) shows the weight loss of base alloy and all the hybrid composites fabricated by stir casting technique. Hybrid composites exhibited less weight loss under the load condition in both the dry and wet test condition than the reference alloy AZ91.

Figure 12.

(a). Samples vs weight loss of hybrid composites and AZ91 (20 N). (b). Samples vs weight loss of hybrid composites and AZ91 (40 N). (c). Samples vs weight loss of hybrid composites and AZ91 (60 N). (d). Samples vs weight loss of hybrid composites and AZ91 (80 N).

The lower weight loss may be attributed to the addition of reinforcements (Ti and Gr and TiO₂ and Gr). Among the reinforcement’s combinations, TiO₂ and graphene reinforced hybrid composites showed less weight loss than Ti and graphene hybrid composite.

It was also inferred that as normal load increases, the weight loss also increased. Within the hybrid composites the weight loss slightly decreased with increase amount of titanium dioxide (2%). However, titanium and graphene hybrid system, it was found that weight loss increased with increase in the amount of titanium. It was also inferred from the results, weight loss in any test condition of the hybrid composites was not as high as AZ91 magnesium alloy. The similar trend of weight loss was found in dry and wet sliding condition of each hybrid composites.

9.2 Wear rate

The wear rate was calculated for the hybrid composite samples and AZ91 magnesium alloy samples in both dry and wet sliding test conditions under the varying load condition of 20 N, 40 N, 60 N, and 80 N. The results are tabulated in the Tables 12–15. The AZ91 magnesium alloy and hybrid composites wear test results of the samples under load 20 N,40 N,60 N, and 80 N in both dry and wet condition revealed that the wear rate, wear mechanism, coefficient of friction and wear resistance of AZ91, A1, A2, B1 and B2 hybrid composite materials.

Sample	Load (N)	Wet/Wear rate (mm^3/m) x10^−3	Dry/Wear rate (mm^3/m)x10^−3
AZ91	20	5.295	10.786
A1	20	2.921	10.123
A2	20	1.492	9.500
B1	20	0.585	9.748
B2	20	1.640	7.450

Table 12.

Wear rate of hybrid composites and AZ91 alloy (dry and wet 20 N).

Sample	Load (N)	Wet/Wear rate (mm^3/m)x10^−3	Dry/Wear rate (mm^3/m)x10^−3
AZ91	40	5.981	15.738
A1	40	3.554	12.121
A2	40	5.989	10.251
B1	40	1.511	12.185
B2	40	5.161	13.505

Table 13.

Wear rate of hybrid composites and AZ91 alloy (dry and wet 40 N).

Sample	Load (N)	Wet/Wear rate (mm^3/m)x10^−3	Dry/Wear rate (mm^3/m)x10^−3
AZ91	60	8.090	18.092
A1	60	6.182	14.59
A2	60	7.085	15.256
B1	60	5.459	14.437
B2	60	6.270	15.434

Table 14.

Wear rate of hybrid composites and AZ91 alloy (dry and wet 60 N).

Sample	Load (N)	Wet/Wear rate (mm3/m)x10–3	Dry/Wear rate (mm3/m)x10–3
AZ91	80	17.895	19.415
A1	80	9.541	14.847
A2	80	11.695	14.438
B1	80	11.698	14.572
B2	80	8.826	14.951

Table 15.

Wear rate of hybrid composites and AZ91 alloy (dry and wet 80 N).

From the experimental test observation, it was inferred the wear rate was lower for all the hybrid composites than AZ91 magnesium alloy. Also the wear rate was lower for wet sliding than dry sliding condition due to the lubrication effect. The wear rate of the test samples are depicted in Figure 13 (a) and (b). AZ91 magnesium alloy showed high wear rate both in dry sliding and wet sliding conditions.

Figure 13.

(a). Samples vs Wear rate for wet sliding Wear test. (b). Samples vs Wear rate for dry sliding wear test.

9.3 Effect of reinforcement and load

The variation of wear rate as the function of load for all the test samples AZ91, A1, A2, B1, and B2 were plotted as shown in Figures 14 and 15. It was noticed that the addition of reinforcement decreased the wear loss of the contacting specimen (pin surface) against hardened steel disc, under all conditions of normal load applied in air. It was found the wear resistance of the hybrid composites showed improved performance than the unreinforced base alloy. This behaviour in the hybrid composites was attributed due to two main reasons are (i) the addition of reinforcements and (ii) refinement in the grain size of the hybrid composites.

Figure 14.

Load/samples vs Wear rate(x10⁻³) for dry wear test.

Figure 15.

Load/samples vs Wear rate (x10⁻³) -wet sliding wear test.

The addition of titanium and graphene had added enough hardness to the novel material to resist the wear due to sliding. At higher loads, AZ91/Ti/0.5% Gr showed more evident result in difference in wear rate than AZ91 reference alloy. Within the hybrid composite, varying weight percentage in titanium exhibited better wear resistance. Similar results were reported in the previous work [27]. But the addition graphene along with titanium in the AZ91 base alloy had better results.

The other system AZ91 reinforced with titanium dioxide and graphene revealed less wear loss even at higher load conditions. The presence of titanium dioxide in the matrix along with graphene, acts as solid lubricants protecting the pin sample from the direct contact with the disc surface and also the material loss [28]. Within the hybrid composites, varying weight percentage of titanium dioxide exhibited better wear resistanceAZ91/1% TiO₂/0.5%Gr. At higher loads, it was inferred that the both the combination of reinforcements with base alloy showed similar behaviour of wear loss.

The wear resistance and specific wear resistance each of hybrid composites and AZ91 magnesium alloy are illustrated in the Tables 16–19 under different load condition. It was also inferred that the wear loss was remarkably lower in wet sliding condition than dry sliding as depicted in Figure 16 (a) and (b). Hence the wear resistance was greatly improved in wet sliding condition.

Sample	Load (N)	Wet/Wear resistance; x10^3	Specific wear rate; (mm^3/Nm)x10^−3	Dry/Wear resistance; x10^3	Specific wear rate; (mm^3/Nm)x10^−3
AZ91	20	0.1889	0.2648	0.0927	0.539
A1	20	0.3423	0.1461	0.0988	0.506
A2	20	0.6702	0.0746	0.1053	0.475
B1	20	0.7094	0.0293	0.1026	0.487
B2	20	0.6098	0.0820	0.1342	0.373

Table 16.

Wear resistance and specific wear rate of hybrid composites and AZ91 alloy (dry and wet 20 N).

Sample	Load (N)	Wet/Wear resistance; x10^3	Specific wear rate; (mm^3/Nm)x10^−3	Dry/Wear resistance; x10^3	Specific wear rate; (mm^3/Nm)x10^−3
AZ91	40	0.1672	0.1495	0.0635	0.3935
A1	40	0.2814	0.0889	0.0825	0.3030
A2	40	0.1670	0.1497	0.0976	0.2563
B1	40	0.6618	0.0378	0.0821	0.3046
B2	40	0.1938	0.1290	0.0740	0.3376

Table 17.

Wear resistance and specific wear rate of hybrid composites and AZ91 alloy (dry and wet 40 N).

Sample	Load; (N)	Wet/Wear resistance; x10^3	Specific wear rate; (mm^3/Nm)x10^−3	Dry/Wear resistance; x10^3	Specific wear rate; (mm^3/Nm)x10^−3
AZ91	60	0.1236	0.1348	0.0553	0.3015
A1	60	0.1618	0.1030	0.0685	0.2432
A2	60	0.1411	0.1181	0.0655	0.2543
B1	60	0.1832	0.0910	0.0693	0.2406
B2	60	0.1595	0.1045	0.0648	0.2572

Table 18.

Wear resistance and specific wear rate of hybrid composites and AZ91 alloy (dry and wet 60 N).

Sample	Load; (N)	Wet/Wear resistance; x10^3	Specific wear rate; (mm^3/Nm)x10^−3	Dry/Wear resistance; x10^3	Specific wear rate; (mm^3/Nm) x10^−3
AZ91	80	0.0559	0.2237	0.0515	0.2427
A1	80	0.1048	0.1193	0.0674	0.1856
A2	80	0.0855	0.1462	0.0693	0.1805
B1	80	0.0855	0.1462	0.0686	0.1822
B2	80	0.1133	0.1103	0.0669	0.1869

Table 19.

Wear resistance and specific wear rate of hybrid composites and AZ91 alloy (dry and wet - 80 N).

Figure 16.

(a). Samples vs Wear resistance(x10³) for wet sliding Wear test. (b). Samples vs Wear resistance(x10³) for dry wear test.

The reports of earlier researches represent that increase in load, increases the wear loss. In sight about the results clearly depicts that presence of titanium or titanium dioxide with 0.5% graphene as reinforcements for the hybrid composites as hard phase in the magnesium matrix results in strengthening. As a result, it provides improved resistance against plastic deformation.

9.4 Surface analysis of Wear samples

In order to explore the wear behaviour of the novel hybrid composites under different test condition and also the presence of reinforcements nanoparticles effect, wear track of test samples are recorded using optical microscope for dry sliding and wet sliding wear test under the load conditions of 20 N, 40 N, 60 N, and 80 N. As well as SEM analysis was done for clear understanding of wear mechanism. The Figures 17–24 depicts the wear track of AZ91, hybrid composites A1, A2, B1, and B2in the dry and wet sliding wear condition respectively.

Figure 17.

Optical microscope image of worn surface of AZ91 for load 20 N, 40 N, 60 N, and 80 N (dry condition).

Figure 18.

Optical microscope image of worn surface of AZ91/1%Ti/0.5%Gr for load 20 N, 40 N, 60 N, and 80 N (dry condition).

Figure 19.

Optical microscope image of worn surface of AZ91/2%Ti/0.5%Gr for load 20 N, 40 N, 60 N, and 80 N (dry condition).

Figure 20.

Optical microscope image of worn surface of AZ91/1%TiO₂/0.5%Gr for load 20 N, 40 N, 60 N and 80 N (dry condition).

Figure 21.

Optical microscope image of worn surface of AZ91/1%Ti/0.5%Gr for load 20 N, 40 N, 60 N, and 80 N (wet condition).

Figure 22.

Optical microscope image of worn surface of AZ91/2%Ti/0.5%Gr for load 20 N, 40 N, 60 N, and 80 N (wet condition).

Figure 23.

Optical microscope image of worn surface of AZ91/1%TiO₂/0.5%Gr for load 20 N, 40 N, 60 N, and 80 N (wet condition).

Figure 24.

Optical microscope image of worn surface of AZ91/2%TiO₂/0.5%Gr for load 20 N, 40 N, 60 N, and 80 N (wet condition).

By Archard’s law, as the hardness of the material increases, the wear resistance property is greatly improved and wear rate is less. Considering all the test condition, it was found from the worn surface morphology of the test samples that there were three wear mechanism abrasion, delamination and oxidation. Adhesion was not very significantly found in the hybrid composites. This was mainly due to the presence of graphene which acts as solid lubricant. In addition to this, titanium dioxide not only imparts hardness to the materials but also act as lubricant. This may be one of the reasons for wear loss to be least in hybrid composites fabricated in this combination AZ91/TiO₂/0.5%Gr.

The SEM images of the worn surface were shown in Figures 25–29 in dry and wet condition to evalute the wear behaviour. It was inferred clearly that dry sliding test produced grooves on the pin surface incurring heavy material loss due to ploughing action of harder surface. It was also found that AZ91 magnesium alloy exhibited severe deformation with deep groove and cracks. The titanium and graphene reinforced hybrid composites showed grooves but at high weight percentage the grooves were deeper which depicts either detachment of reinforcement and breaking brittle layer formed. But in titanium dioxide and graphene reinforced hybrid composite showed only fine grooves in dry condition and very fine scratches in wet condition. It evident that hybrid composite reinforced with titanium dioxide and graphene possess better wear resistance.

Figure 25.

SEM image of worn surface of AZ91 Mg alloy in (a) dry & (b) wet.

Figure 26.

SEM image of worn surface of AZ91/1%Ti/0.5%Gr (a) dry and (b) wet.

Figure 27.

SEM image of worn surface of AZ91/2%Ti/0.5%Gr (a) dry and (b) wet.

Figure 28.

SEM image of worn surface of AZ91/1%TiO₂/0.5%Gr (a) dry and (b) wet.

Figure 29.

SEM image of worn surface of AZ91/2%TiO₂/0.5%Gr (a) dry and (b) wet.

10. Summary

From the experimental investigation the following results were obtained. The hybrid composites exhibited improved wear resistance due to the addition of the titanium and graphene reinforcements.

Among the combination of hybrid composites, AZ91/1% TiO₂/0.5%Gr and AZ91/2%TiO₂/0.5%Gr exhibited better wear resistance compared to AZ91/x%Ti/0.5%Gr. This due to the presence of stronger TiO₂ dispersed along with graphene act as solid lubricant during the sliding action between the contact surfaces.

11. Conclusion

In the present work, a new and cost effective magnesium based alloy hybrid material was developed successfully by stir casting technique. Addition of reinforcements like titanium, titanium dioxide and graphene has shown significant change in material’s property. It was inferred from the results obtained that hybrid composites exhibited improved properties than AZ91 base alloy. The main conclusion obtained from comparison of each combination of reinforcements system are listed below

• AZ91–2%TiO₂–0.5% Gr showed good tensile strength and hardness.

• AZ91–2%Ti-0.5% Gr showed high hardness and denser hybrid material among them.

• AZ91–2%Ti-0.5% Gr exhibited high compressive strength.

Wear resistance of the AZ91 magnesium alloy was greatly enhanced by addition of titanium dioxide and graphene. AZ91–1%TiO₂–0.5% Gr showed better wear property in all the test condition of dry and wet under varying load. Under low load conditions AZ91–2%Ti-0.5% Gr also disclosed better wear resistance and moderate difference at high load. AZ91–2%Ti-0.5% Gr showed high hardness but low wear resistance due to the hard Ti particle detachment may lead three body abrasion phenomenon. However, titanium dioxide has sufficient hardness and lubricant property to protective the material from wear. The order of wear resistance B1 > A2 > A1 > B2 > AZ91.

Corrosion resistance was very significant in titanium and graphene combination. Addition of reinforcement greatly prevented the material degradation by formation of stable protective layer. AZ91–2%Ti-0.5% Gr showed high corrosion resistance in acidic, neutral and alkaline environment of 3.5% NaCl. The order of corrosion resistance is A2 > A1 > B1 > B2 > AZ91.

Nevertheless all the hybrid composites exhibited better properties than base alloy. Among the four hybrid composites, based on mechanical properties, wear and corrosion behaviour. AZ91–2%Ti-0.5% Gr showed better properties in all aspects.

From engineering application point of view, materials play a key role in design, fabrication, quality, application and cost of the product.

Hence from the present investigation, hybrid composites developed may be considered for the varied field of engineering applications based on the need of usage in different environment like compressive strength or wear resistance or corrosion resistance. Recommended material with improved property based on the property requirement are listed in Table 20.

Properties requirement	Material Suggested
High compressive strength	AZ91–2%Ti-0.5%Gr; AZ91–2%TiO2–0.5% Gr
High Wear resistance	AZ91–1%TiO2–0.5%Gr
High Corrosion Resistance	AZ91–2%Ti-0.5%Gr
Moderate strength,Wear and Corrosion	AZ91–1%Ti-0.5%Gr

Table 20.

Material requirement.