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STRUCTURE AND PROPERTIES OF HYPEREUTECTIC
Al-Si ALLOY METAL MATRIX COMPOSITES

  
     
 

Research Scholar: J.Sudheer Reddy, Mech Engg Dept.

  
 

Abstract:
In recent years the need for high specific strength, light weight, high specific stiffness, good wear resistance & many other properties has resurrected much interest in the development of Metal Matrix Composites (MMC).

Over the years, several metallic systems such as Aluminum, Magnesium, Titanium, copper etc have been considered for use as a matrix material for MMC. Among the MMC systems, The Aluminum alloy matrix composites (AMC) have elicited a great deal of interest in automotive, aerospace etc. industries

The present study was motivated by the need to identify a light weight Al based MMC with suitable combination of reinforcement and processing route  to obtain precise balance of Mechanical, physical properties, including the tribological properties for large range of engineering applications.

Nature of the Work
Among the different MMC’s, the selection of a particular combination of matrix, reinforcement & processing method is a challenging task..The present work focus on Hypereutectic Al-Si alloy (12% and 17%Si) as matrix element and  SiC & Al203 as reinforcement material in particulate form with the objective of evaluating the effect of both the reinforcements individually on hypereutectic Al-Si alloy.
 
Among the different processing techniques, the liquid state process in particular stir casting technology is considered to be the most potential method in terms of production cost and efficiency. The selection of processing route (Stir casting) was motivated by the need to produce MMC in an economical way.

The cast MMC’S Mechanical & Tribological properties are to be evaluated and compared with unreinforced material.

Work Accomplished
Cold Model Study
Although stir casting is generally accepted as an economical & viable process for production of PMMC’s, there are however technical challenges associated with the process such as 1) The difficulty of achieving uniform/homogeneous distribution of reinforcement in the matrix 2) poor wettability and chemical reaction between matrix & reinforcement materials.3) porosity in cast MMC’s .

In an effort to understand stir casting process, a work of cold model study was taken up in order to analyse the factors that influence the process such as suitable impeller geometry and its positioning in melt to form a vortex without much air entrapment. The vortex formation, its height in a few liquids of different viscosity with respect to impeller size and position in liquid medium were carried out. Homogeneous distribution of dispersion in liquid medium was noted by visual observation.

Manufacture of MMC
The stir casting equipment as shown in Fig.1 & Fig.2 has been indigenously developed to prepare MMC.
 
 
Fig.1 3-D Model of Stir casting Equipment Fig.2 Stir Casting Equipment
   
 
 

The chosen four blade stirrer from the cold model study is connected to a 15 mm diameter hollow steel shaft. Inside the hollow shaft a rod of diameter 5 mm is concentrically placed The bottom end of the rod is attached to a tapered graphite plug. The mating hole is drilled in the crucible. Initially the shaft is placed at a level above the top surface of the crucible and the rod consisting plug closes the hole. After the Matrix material melts, the melt will be stirred at 500 rpm and then the reinforcement will be introduced in the vortex. The stirring is continued for about 3-4 min. By lifting the stirrer shaft together with inside rod connected to plug the slurry passes through the hole into the mould placed beneath the crucible.

The Al-Si alloy reinforced with Sic and Al203 particulates were manufactured by Stir casting and are compared with the unreinforced Al-Si alloys. The presence of Reinforcement in matrix element was clearly identified using optical microscope. The samples manufactured were subjected to hardness test and the results were found better.

WORK TO BE CARRIED OUT
Mechanical & Tribological properties of the cast specimens need to be evaluated and should be compared with the unreinforced alloy.

A preliminary survey has been done with Goetz (India), Bangalore regarding the suitability of this material for I.C engine Cylinder sleeve application and the material developed is to be tested for the requirements of the application.

LIST OF PUBLICATIONS IN CONFERENCES/JOURNALS
Sudheer Reddy, H.Suresh Hebbar and P G Mukunda “An attempt to understand Stir Casting Process”, International Conference on Advanced Materials and Composites (ICAMC) – Thiruvananthapuram – 2007.

  
 
 
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