Magnesium Alloy (AZ91D) Matrix Syntactic Foams Reinforced by Dispersion Particles of Silicon Carbide

Magnesium Alloy (AZ91D) Matrix Syntactic Foams Reinforced

by Dispersion Particles of Silicon Carbide

MME 3314 Composite Materials

October 5, 2015

Magnesium Alloy (AZ91D) Matrix Syntactic Foams Reinforced

by Dispersion Particles of Silicon Carbide

1. Introduction

A composite material is a result of a combination of different types of materials which together will provide better properties to the main material. This is needed for many applications that involve high technology such as aerospace, naval, bioengineering and transportations industries. A combination of components can be controlled and made artificially and most of the times can be just distinguished using a microscope, even so it is possible to find natural composite materials, such as bone and wood, that show how composite material is something common [1][2][3][10].

The common type of composite material, which has just two phases, the matrix, that can be polymeric, metallic or ceramic and the other phase that is the reinforcement done by different processes such as particle dispersed, fibers or structural [2][3].

A metal matrix composite (MMCs) implies that the matrix made by ductile metal reinforced with other type of component is responsible to improve characteristics to include stiffness, strength, thermal conductivity and dimensional stability. This type of matrix shows advantages over polymer matrix composites when the material needs to operate at high temperatures and when dimensional stability is required [4]. The challenges for the application of this type of matrix is that it is expensive to produce, has limited compatibility and new techniques of processing that provided better control during the reinforcement distribution that make the use of this material less restricted. However, there are still many studies to improve this technology into other areas including military applications that has shown a more interest and opportunities for development of metal matrix composites [9][10].

The potential for industrial applications of Aluminum and Magnesium Matrix Composite are many due to the fact that these materials have the lowest density. The Aluminum has density (~2,7g/cc) and has been used to replace steel (~7,8 g/cc). However, the magnesium has a lower density (~1,74g/cc) which provides reduced weight. Magnesium alloys show an important way to provide components with less dense weight for applications to increased fuel efficiency and payload capacity [6].

A magnesium alloy consists in to metals mixtures to create a material with better mechanical properties with adding metals as aluminum, zinc, manganese, silicon, copper, rare earth elements and zirconium. Magnesium has the lowest density of the metals because of its hexagonal lattice structure that causes more complicated plastic deformation, which is a different from aluminum, copper and steel that have cubic structure which is easy to deform plastically. Therefore the application for the magnesium alloys goes farther, currently it has been used as a composite materials based on magnesium alloys reinforced by dispersion particles of silicon carbide (SiC) [6][9][10].

Magnesium even as a promising low-density metal, had shown limitations in their applications due to issues of flammability, corrosion and difficult processing. However, many studies have been made in order to overcome such barriers, and the AZ91D matrix syntactic foams reinforced by dispersion particles of silicon carbide. In particular it has great interest for the application because it has very low density which increases 30–40% of its mechanical properties. The basic composition of AZ91D is a Mg-alloy cast, mainly containing 9 wt.% Aluminum, 1 wt.% Zinc and 0.3 wt.% Manganese [5][10].

2. Syntactic foams

Syntactic foams can be considered a new class of composite materials which have a special classification, called metal matrix syntactic foams (MMSFs). They consist of a continuous metal matrix embedded with hollow or porous particles. This new type of structure provide to this material with a higher yield strength, resistance to higher temperatures and harsh environments and the mechanical properties present a more homogeneous behavior. Another important point is the density which is greater than in other materials due the porosity that improve the ability to absorb impact energy due to extensive strain accumulation. The syntactic foams made of magnesium alloy and silicon carbide have been studied, but still do not have enough available literature [5][6][11]. This new type of material consists of hollow particles that will be dispersed into the matrix of the composite. The ratio of inner to outer radius of hollow particles is over 0.9 and the volume fractions can be around 60%. The density of this materials is in the range 1–1.5 g/cc, which directly compete with polymer matrix composites [5][6][11].

The properties of MMSFs depend on a number of parameters: particle shell material, shell wall thickness to diameter ratio, matrix alloy, processing parameters, entrapped voids, and heat treatment conditions. The properties of MMSFs can be normalized with respect to the properties of the matrix alloy processed under similar conditions. However, the effect of porosity is the most significant aspect to understand. In most cases, the densities of aluminum and magnesium matrix MMSFs occur within a narrow range or 1–2.2 g/cc and they are expected to compete with each other for applications based on their lower density at the same level of desired mechanical properties.[5][6][7][11]

Cenospheres are the most widely used particles in syntactic foams due to their low cost; they are recovered from ash generated in thermal power plants. The first part of the process they go through is the beneficiation process to remove impurities and select only the low density intact cenospheres particles. However, even with this quality process some particles may come with defects that cannot be noticed before the manufacturing process of the matrix which can cause damage to the structure of the matrix due to unwanted particles proprieties. To solve this problem has been studied the use of hollow particles of alumina, silica, and silicon carbide that can provide advantage in improving interfacial bonding with the matrix [5][7].

Hollow particles are now available in a range of large size from