Research On Fabrication And Processing Of In-situ TiB₂ Particulates Reinforced 7055 Matrix Composites

In situ TiB₂ particulates reinforeced 7055 composites from 7055Al-K₂TiF₆-KBF₄ system was studied in this paper.Adding minimal Mg and Ce element can promote in-situ reaction greatly;By electromagnetic stirring,ceramics particles are well dispersed and eutectic phases are greatly refined.The comprehensive mechanical properties of materials are improved by hot extrusion,quenching and thermal processing.The solidification microstructure,patterns of in-situ particulate,composition of phase were investigated with the help of optical microscope(OM),scanning electron microscope(SEM),X-ray diffraction(XRD),electronic universal testing machine and other modern equipments.The results show that TiB₂ particles with radius of 0.4～1μm are synthesized in 7055Al-K₂TiF₆-KBF₄ at 850℃.The morphology of TiB₂ are six-rowed granular,parts of them are spheroidized.The grain size of 7055 eutectic phases under electromagnetic stirring are more refined than the grain size of 7055 eutectic phases without influence of electromagnetic stirring.We optimize the grain size and morphology in electromagnetic parameters:current,150A,frequency,4Hz.while,electromagnetic stirring promotes the reaction,the speed of mass and heat conduction.the contact area between high temperature liquid aluminum and mixed salt is expend.the quantity of TiB₂ particles is increasing and well dispersed.In the fabrication of in situ TiB₂ particles reinforced composites,The results indicate that adding microscale Mg(0.3%) or 3%(wt%) Al-10%Ce can well improve the wettability between ceramic and aluminum melt then promotes the reaction and restrain the TiB₂ particles assemble,and also the sizes and morphology of TiB₂ are refined largely.The primitive grain boundary of as-deposited material vanished and the reticular phases was disrupted and the grain broke up.The microstructure was more compact and more tiny crowded second-phases precipitated after a series of hot working treatment on cast TiB₂/7055 composites:homogenization under 450℃×24h,hot extrusion,quenching process under 480℃×60min,aging under 120℃×24h.Processed after technics above,the tensile strengthσ_b of 2%(vol%) TiB₂/7055 composites achieve 630MPa,which is enhanced than processed under technics conditions obviously,and elongationδis 4.0%.