**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.
