PRODUCTION OF MAGNESIUM COMPOSITE MATERIALS AND ITS CHARACTERIZATION

ABSTRACT

This project deals with the manufacturing of magnesium composites with three different composition of reinforcements such as Al₂O₃,Silica and Carbon nano tubes. Stir casting process has been employed for producing magnesium matrix composites.To understand and analyse the quality of Magnesium composites,experiments were conducted to find out Tensile strength,Hardness and Wear behaviour of magnesium composite materials.From this investigation we understand that the specimen fabricated by Carbon nano tubes have relatively higher Tensile strength and Micro Vickers hardness than other Mg composite materials.This is due to uniform distribution of nano particles through Stir casting and homogenous microstructure of the composite material.

INTRODUCTION

In the modern world there is a new era for materials having light weight applications. In automobile industries there is a huge demand for light weight materials .In the past few year aluminum alloys were the promising materials used for weight reduction in automobiles. In the recent years magnesium alloys have been increasingly used for weight reduction because of low density compared to aluminum alloys. The density of magnesium is approximately two thirds of that of aluminum, one quarter of zinc, and one fifth of steel.As a result, magnesium alloys offer a very high specific strength among conventional engineering alloys .Over the past decades magnesium casting production has been increased by 10% and 20%.

Composite Materials

Composites are considered to be mixture of materials of different composition. The materials retain their identities in the composite; that is, they do not dissolve or unite completely into each other though they perform. Normally, these materials can be physically identified and reveal an interface between one another.

Metal-Matrix Composites

Metal-Matrix composites structure is generally designated purely by the term metal alloy of the matrix and the material in the form of strengthening. The matrix is permeating soft part generally having excellent tensile strength, hardness, ductility and low thermal expansion.

SCOPE AND OBJECTIVE

The main purpose of our project is to develop strong light weighted metal matrix magnesium composite materials which are useful in the industrial sectors as well as advanced machineries. The most important part of our project is to fabricate magnesium metal matrix composites using Al2O3, silica, carbon nano tubes in powder form as reinforcements by stir casting. We adopt the stir casting method with high temperature in this project. These types of metal matrix composites have high specific strength, temperature resistance, stiffness properties that they are used in automotive and heavy goods vehicle, piston pins, breaking systems Etc. we conducted different tests such as Hardness test, density, tensile test, wear and friction test and micro structure of obtained composite materials.

EXPERIMENTAL PROCEDURE

Composition of base material

Magnesium base material represents the matrix magnesium is 99.94% and nearly 0.012% of aluminium, 0.0019% of zinc and 0.015% of manganese. It has excellent combination of mechanical properties, corrosion resistance and cast ability.

Raw material selection

For the preparation of magnesium matrix composites reinforcements like Aluminum oxide, Silica, Carbon nano tubes in powder form are used as raw materials.

Stir casting equipment details

• The important components of stir casting machine are stirrer, steel crucible and die.
• Steel crucible is the vessel which is heated to high temperature to melt the metal which is introduced in it(800˚C is needed for Mg).
• Stirrer is used to mix all the metals which are melted in the steel crucible.After the casting is done, the metal in molten state will be poured in the die and allow it to cool.
• This machine is suitable only for magnesium casting.

Preheating of reinforcement

Al₂O₃, silica are wrapped in aluminium foil and placed into the hot air oven to remove the moisture content and then the dried reinforcement will be introduced into the furnace which already has magnesium base material. Reinforcement will be kept inside the oven at 140 ˚C temperature for at least one hour. Because of the preheating treatment the moisture content is eliminated in reinforcement particles and are uniformly distributed in molten matrix.Argon gas is added to do the process in protective medium and also to avoid flammability of magnesium. SF₆ gas is added to the process to prevent reactions of magnesium.

Stir casting process

Firstly the graphite coating must be given to the furnace crucible, die and also to all the places where magnesium passes in the machine to avoid sticking of magnesium to the surface. Then the furnace temperature is raised to 800˚C to remove all the residues of previous casting from the furnace crucible.The die is also heated up to 250⁰C so that the molten matrix uniformly spreads in the die. Once the furnace is clean and empty without any residues then we can introduce the magnesium and Al₂O₃, Silica powder and carbon nano tubes are added as reinforcement. The metals which are melting in the furnace is blended together by the stirrer(500rpm) and it mixes them well. The reinforcement material is preheated in a hot air oven for removing the moisture content in it and then the dried reinforcement is added to the furnace. The magnesium with Al₂O₃, Silica powder and carbon nano tubes as reinforcement is solidified in the die and it takes around 30mins for conversion of molten metal into solid. The graphite coating is given to the surface of the die avoids the sticking of metal while solidifying. This magnesium composite which is obtained from stir casting process will be machined in lathe and depending on the requirements for testing, this metal will be cut into that dimensions. During machining in lathe, turning must be done slowly to avoiding temperature rise because magnesium is a flammable metal. It will burn at high temperature.

POST STIR CASTING

We produced 3 specimens using the above process namely stir casting. The following are the composites produced:

Mg + Al2O3

Firstly we took magnesium as base material and Al2O3 as reinforcement material. We added 900gm of magnesium with 18gm of Al2O3 and produced the composite material.

Mg + Al2O3 + Silica

Secondly we took magnesium as base material and Al2O3, Silica as reinforcement materials. We added 900gm of magnesium with 18gm of Al2O3 and Silica and produced the composite material.

Mg + Al2O3 + Silica + CNT ( Carbon Nano Tubes)

Finally we took magnesium as base material and Al2O3, Silica and CNT as reinforcement materials. We added 900gm of magnesium with 18gm of Al2O3, Silica and 3gm of CNT and produced the composite material.

RESULTS

MICRO STRUCTURE

Micro structure of material can strongly influence physical properties such as strength, toughness, hardness, ductility and wear resistance. Each specimen is polished with emery papers of grade 600, 800, 1000, 1200, 1500, 2000 to remove the major scratches. Then disc polishing is done for the specimen with diamond paste. Then the specimen is washed with water. It is allowed to dry by itself.

Micro Vickers Hardness

The Vickers test is often easier to use than other hardness tests since the required calculations are independent of the size of the indenter, and the indenter can be used for all materials irrespective of hardness. The basic principle, as with all common measures of hardness, is to observe the questioned material's ability to resist plastic deformation from a standard source. The Vickers test can be used for all metals and has one of the widest scales among hardness tests.

Tensile Test

Tensile test, also known as tension test, is probably the most fundamental type of mechanical test you can perform on material. By pulling on something, you will very quickly determine how the material will react to forces being applied in tension The specimens for tensile test were prepared according to ASTM E8 standard. This is used to determine the break load, peak load, ultimate tensile strength, elongation of the specimen.

CONCLUSION

The magnesium composites have been successfully synthesized using stir casting technique. It is observed that density value of the composite increases with increasing weight of carbon nano tubes. The micro structure has uniform distribution of reinforcement particles and the grain size of the particles are good and finer. The hardness increases with increase of carbon nano tubes as reinforcement. The tensile strength is highest for the specimen produced with carbon nano tubes as reinforcement.