Development of metal-matrix composite materials by rapid solidification process

Abstract

The current demand of industries to develop high performance lightweight alloys for fuel saving is increasing day by day. Light weight Al–alloys which are used in structural applications like automotive, aerospace, defense industry and other fields of engineering requires further modification for better properties. Al–Si alloys which are used for structural purposes have wide range of composition and varies from hypoeutectic to hypereutectic compositions. However, eutectic and near eutectic Al–Si alloys with Si contents (≈12.6 wt% Si) exhibit better physical properties. Moreover, other reinforced particles like oxides, nitrides and carbides in various volume fractions are used in combination with Al–alloys to enhance their properties. The manufacturing methods of these composites are also important factor which causes variation in their properties. There are two ways to modify the microstructures i.e. chemically or by rapid solidification processes. In the present study, the spray forming process has been adopted to develop MMC which is different from other rapid solidification processes. After studying the literature of Al–Si alloys, it has been found that a little information regarding particulate size and volume fraction related to abrasive or dry sliding wear processes is available. The data collected from the work done indicate that only two reinforcement types, mainly silicon carbide and alumina have been tried. The wear behavior of spray deposited alloys with other potential reinforcement materials such as TiC, TiB, ZrSiO4, Si3N4 and SiO2 are less studied systems. Only few publications are on the combined approach of immiscible elements like Pb, Bi, Sn and particle reinforced rapidly solidified AMCs i.e. spray forming techniques. Limited work has been reported on fabrication and characterization of zircon sand reinforced with aluminum metal matrix composites and none has used spray deposition technique to manufacture zircon sand reinforced AMCs. Considering these aspects we have planned to work on spray forming. The main objective is to prepare the cost-effective AMCs by taking Al–Si alloy as matrix with immiscible elements like Sn and Bi and also some ceramic particulate like silicon carbide and zircon sand as reinforced phase using spray deposition technique. Further the wear behavior of the developed composites has been analyzed with variation in load and temperature. The 1st chapter presents in brief the background of the composites and their classification. Various fabrication and processing techniques for their preparation with superiority of composites over monolithic alloys is also described. Further the advantages of rapid solidification process viz. spray forming are discussed. The fundamental characteristics of light weight Al–Si alloys and reinforcements are explained in light of their application for better mechanical properties. The concepts of rapid solidification have also been summarized. Chapter also describes about wear and related mechanisms for loss of materials during wear. Chapter 2 gives a detailed account of the literature review. It was necessary to review the work done at national and international level before selecting any particular composition and reinforcement material. The near eutectic Al–Si composition (commercially grade LM13) and zircon sand, SiC as reinforcement are selected on the basis of gaps in the literature. The 3rd chapter contains the information about designing of the nozzle. The spray formed setup has been designed and fabricated in our metallurgical laboratory. In this chapter, the designs of convergent-divergent nozzle used for atomization has been described. The problem faced with the initial designs and details of modified nozzle is also documented. The chapter reveals different parts of spray set-up assembly and their drawings. On the basis of literature study different samples have been prepared and detailed description is listed in chapter 4. This chapter describes the materials specifications used in present work and procedure adopted to prepare composites from these materials. A lot of experiments have been done to optimize the parameters before the fabrication of composites. The various characterization techniques; X–ray diffraction (XRD), Scanning electron microscopy (SEM), Energy dispersive spectroscopy (EDX), Rockwell hardness testing and dry sliding wear with their parameter details are also elaborated. Further the whole methodology and the step-wise wear testing plan is shown in the form of flow charts. In chapter 5, the experimental results of as received and spray formed LM13 alloy (containing around 12 wt.% Si) are presented. The samples were analyzed by optical and SEM along with EDS to study the difference in morphology of the alloy after spray forming. Further the samples were wear tested under various loads and temperatures. The worn surfaces and debris obtained were also analyzed by SEM and EDS to predict the mode of wear under different set of conditions. The results and discussions part are presented in chapter 6 and chapter 7 for zircon sand and silicon carbide reinforcement respectively. In the chapter 6, the experimental results of zircon sand reinforced composites are presented. The chapter is subdivided in two parts on the basis of the size range of zircon sand particulates. The zircon sand reinforced composites have been prepared by spray deposition process where immiscible element Sn and Bi in varying quantity is added to achieve dense product. The prepared samples have been characterized by XRD, Optical microscope, SEM and EDS. Wear tests of spray formed samples have been done at different loads and temperatures. The analysis of worn surfaces has been done to predict the mode of wear for the prepared samples. Chapter 7 describes the details of the experimental results of silicon carbide (SiC) reinforced composites. This chapter is also subdivided in two parts on the basis of the size range of SiC particulates. The characterization of the prepared samples has been done by XRD, SEM and EDS. The microstructural analysis of worn surfaces and wear debris has been done to predict the mode of wear in various tested specimens. Chapter 8 summarizes the results of various experiments described in previous chapters. At the end, suggestions for future work are given on the basis of results obtained in this work. At the end, the list of papers published, presented and communicated are also appended.