STUDIES ON THE MICROSTRUCTURE, MECHANICAL PROPERTIES AND WEAR BEHAVIOUR OF SQUEEZE CAST A141203-Mg0 PARTICULATE MMCs

Abstract

The present thesis comprises of mainly five chapters. Chapter - 1, Introduction, outlines the achievements and problems associated with the fabrication of cast particulate MMCs through the liquid metallurgy route. The major problem of considerable amount of porosity in the cast structure, which seriously deteriorates the performance of composites in tension, has been pin-pointed and emphasized. It has been reasoned in this chapter, as to how the process of squeeze-casting can be a possible means of overcoming this problem satisfactorily. This chapter also details out the entire scope of present investigation. Chapter - 2 deals mainly with a comprehensive study of different composite systems, role of squeeze casting operation in improving the performance levels of different alloys & composite systems, and, the wear behaviour of aluminium alloys and different composite systems under mainly dry sliding conditions. The objective of this study was to critically examine the findings of other workers and identify the information gaps. This chapter is divided into three major sections. The first section is devoted to a discussion of the characteristics of different composite systems. In the beginning, the composite systems are classified into different categories and then a major discussion on the Metal Matrix Composites (MMCs) is taken-up. Again in this sub-section, major emphasis is laid on the kinds of cast particulate composites, liquid metallurgy route of their fabrication, and, problems associated with vortex liquid metallurgy route of their preparation vi in particular. The second major section deals mainly with the squeeze casting process. In this section are discussed various operating steps, control parameters and the theoretical aspects related with the influence of squeeze pressure on the phase diagrams of different alloy systems. Considerable amount of work done by different workers on the influence of squeeze pressure on the properties of different alloy systems and composites has been critically analysed in this section. The third major section of this chapter is devoted to a study of the fundamental aspects of wear. This section includes a discussion on the types of wear, techniques of measuring wear and an account of recent investigations into the wear behaviour of aluminium alloys and composites. A critical appraisal of various aspects mentioned above finally leads to the formulation of a specific strategy to be followed in the present investigation. Chapter - 3 deals with various experimental techniques adopted in the present investigation. Starting from the preparation of Al203 and submicron Mg0 ceramic powders with respect to the dehydroxylation of their surfaces, their blending and the development of 'modified Mg0 coating technique' is explained. What different parameters are required to be controlled during the creation of vortex and the preparation of the stirred slurry are then explained. Process parameters and their limits are then set out for the gravity chill casting and squeeze casting of the stirred slurry. This chapter details out various techniques employed for the characterisation of cast particulate composites in respect of %Vf retention of A1203, density, mean dendritic arm vii spacing, Vicker's hardness and microhardness and tensile property evaluation upto 300°C (573K). This chapter also details out the techniques adopted for evaluation of the wear behaviour of above composites under dry sliding conditions. Chapter - 4 is devoted to a critical examination of the results of present study. The strategy adopted is that the entire set of results are first divided into three main sections dealing with three specific aspects. The first section deals with the preparation and initial characterisation of gravity chill cast and squeezed Al-A1203-MgO and Al-MgO particulate composites. The second section deals with the mechanical property evaluation of above composites upto 300°C (573K). This section also deals with a detailed SEM examination of the tensile fracture surfaces of different composites. This study was undertaken with a specific objective of establishing the nature and mode of fracture in case of gravity chill cast and squeezed Al-A1203-MgO particulate composites. The third major section of this chapter is devoted to a study of the wear behaviour of above composites under dry sliding conditions. The major objective of this study was to ascertain as to how the squeeze pressure influences the wear behaviour of Al-Al2 03 -Mg0 particulate MMCs. The common strategy adopted in this chapter is that the results are presented first and summarised and then these results are discussed also in the same sub-section. Like this, the entire set of results are fully compartmentalised. In chapter - 5, the conclusions arising from the above investigation are listed systematically. The later part of this viii chapter is devoted to a listing of suggestions for future studies. In the Appendix, some Xerox copies of research publications emanating from the present investigation are presented.