SYNTHESIS AND MECHANICAL PROPERTIES OF IN-SITU AL-TIB2 COMPOSITE

Abstract

Of the Al-base MMCs, Al-TiB2 composite has a promising potential for advanced applications. Being microstructurally superior, it can withstand relatively high temperature strength and fatigue. The exothermic formation of TiB2 in AI and the high thermal stability of TiB2 permit the suitability of AI-TiB2 composite to be synthesised by in-situ technique. Such a technique of synthesising produces a uniform dispersion of fine reinforcement of TiB2 particulates in the matrix together with a clean interfacial zone between the reinforcement and the Al matrix. These attractive features lead to a remarkable improvement in tensile properties. Several techniques have been utilised to produce in-situ AI-TiB2 composite. These techniques include the proprietary processes such as XD™ and Lanxide™, mechanical alloying and salt route of synthesising. Notwithstanding these existing techniques of synthesising, there was no attempt to use the elemental powders to create the composite directly by stir casting technique. The present project focuses on the possibility of using this technique to produce the Al-TiB2 composite effectively. In this research, it was found that the in-situ TiB2 reinforcement was produced within the Al-matrix when elemental (Ti+B) mixture was added to molten aluminum at above 1 000°C under normal atmospheric condition and with a simple experimental set-up. Characterisation of the synthesised composite shows a fine dispersion of TiB2 particulates of sizes between 1-3 µm in the AI matrix. Through tailoring the control of the process variables, viz. the reaction temperature and the holding time, an Al-15vol.%TiB2 composite produced was able to exhibit a tensile strength and yield strength as high as 224 and 171MPa respectively and a E-modulus value of 91GPa. These values are comparable to the performance of a similar composite synthesised by the other aforementioned techniques. Modification of the matrix with an addition of 4.5wt. %Cu further increased the tensile and yield strengths up to 333 and 248MPa respectively. However, these improvements were accompanied by a loss of ductility attributed to the premature fracturing of the Al3Ti flakes present in the composite. In wear performance, the Al-15vol.%TiB2 composite was on par to a much harder medium carbon steel. This wear improvement was ascribed to the presence of the TiB2 particulates in the composite. In synthesising the Al-TiB2 composite by the Al-Ti-B system, Al3Ti, which degrades the mechanical properties of the composite, is formed as an unavoidable byproduct. The formation of this brittle compound was also reported in the well-known XD process. To counter the problem, incorporation of foreign elements such as FeSi and Zn was made to poison the nucleation of Al3 Ti. However, the approach had not deterred the formation of the compound. The formation of Al3Ti was eliminated when C was incorporated into the (Ti+B) mixture and introduced simultaneously into the melt. This elimination was the result of the formation of the TiC in addition to TiB2 in the composite. For commercial production of AI-TiB2 composites by stir casting, Al-Cu matrix material is recommended. Cu promoted the nucleation of TiB2 reinforcement and improved the composite yield.