FABRICATION AND CHARACTERISATION OF TIB? REINFORCED AL BASED COMPOSITES

Abstract

In the present study, an aluminium based metal matrix was reinforced with TiB2 particulates by an in-situ reaction process using K2TiF6 and KBF4 salts. The exothermic reaction period was varied from a minimum of 10 minutes to a maximum of 40 minutes to investigate the relationship between the degree of reaction, the growth behaviour of TiB2 and the weight percentage of TiB2 formed. The effect of reaction holding time on the microstructure and mechanical properties were investigated. An X-ray diffractometer was used to confirm the presence of TiB2 as well as to quantitatively estimate the weight percent of TiB2 particles in the composite for the various reaction holding times. X-ray spectra showed that the amount of TiB2 increased initially when the exothermic reaction period was prolonged. After a reaction holding time of 20 minutes, the amount of TiB2 started decreasing. Microscopic investigation carried out on the composite samples revealed the presence of hexagonally shaped submicron size TiB2 particulates with a reasonably clean interface with the aluminium matrix. Microstructural observation showed that the grain size of the composite was refined significantly at the beginning of the reaction. It however increased slightly at the later stage of the reaction Ambient temperature mechanical properties carried out on the as-cast in-situ MMCs show an increase in 0.2% YS, and the UTS and a decrease in the ductility of the reinforced composites compared to that of unreinforced material. Two different processing techniques namely squeeze-casting and extrusion were carried out on the TiB2 reinforced AI-Cu MMCs synthesized using in-situ technique. The effect of reaction holding time on the ageing behaviour, microstructure and mechanical behaviour of the in-situ MMCs were investigated after squeeze-casting and extrusion. The length of reaction holding time was found to have no significant effect on the peak ageing duration of the MMCs. The results of ambient temperature mechanical testing of squeeze-cast MMCs with 5 minutes reaction holding time show an increase in strength and degradation in ductility compared to those with the reaction holding times of 10 and 20 minutes. Extrusion of the MMCs was found to eliminate the porosity levels completely. No macro seggregation of reinforcements was observed in the composites after extrusion. The ambient temperature tensile testing results obtained from extruded MMCs and unreinforced alloy matrix indicated that the composite samples exhibited superior 0.2% YS, and the UTS values when compared to those of the matrix. The ductility values of the composites after extrusion were found to be comparable with that of the unreinforced alloy matrix. A comparison was made between the squeeze-cast and extruded MMCs based on the microstructure as well as mechanical behaviour. The mechanical properties of extruded MMCs was found to be superior than the squeeze-cast MMCs due to the elimination of porosities in the extruded MMCs. The effect of processing on the various strengthening mechanisms have been investigated. Mechanical properties of the composites as related to the fabrication and microstructural features are discussed. The microstructural results obtained in the present study were finally rationalized in terms of the processing parameters.