Development of a new filler for discrete reinforcement of cast aluminum-matrix composites by titanium carbide

Abstract

In order to improve the method for obtaining cast aluminum-matrix composites dispersion-filled by titanium carbide and to increase characteristics of these materials were investigated the structure and properties of the composites reinforced by in-situ and ex-situ methods. For this aim were used the synthesis of the TiC under the melt layer by the introduction of compacts of pressed mixture of powders of titanium and graphite, titanium and aluminum carbide and mechanical mixing into the aluminum melt of the pre-synthesized powders of TiC and TiC in the Al₃Ti matrix. It was found that the in-situ method of reinforcing aluminum and aluminum alloys with discrete particles of titanium carbide has low efficiency, because of intensive wetting of titanium powders by melt in the volume of compacts, when they are introduced. As a result, the actively formed Al₃Ti phase prevents the flowing self-propagating high-temperature synthesis (SHS) between titanium and carbon or aluminum carbide. It was established, that this method is not suitable for reinforcing aluminum alloys, doped with silicon and magnesium, in view of the almost complete cessation of the synthesis of titanium carbide and the active formation of titanium alumino-silicides and aluminides, which is accompanied by spattering of the melt. The SHS-reaction between Al₄C₃ and Ti under heating conditions at a rate up to 6000 °C per hour leads to the formation of dispersed TiC and Ti₃AlC₂ of globular shape in an Al₃Ti matrix. With rise of the heating rate and the heat dissipation, the number of non-equilibrium phases formed during the reaction increases. The formation of metallic aluminum found under such conditions allows suggest that the reaction between titanium and aluminum carbide is going by stages. The introduction of briquettes into the aluminum melt after initiation of the SHS-reaction in them does not ensure the dissolution of the matrix and the distribution of titanium carbide in the volume of the melt. This requires their preliminary grinding. Ex-situ aluminum reinforcement with TiC-Al₃Ti powders is high efficiency due to good wetting of their surface by aluminum melts and subsequent active dissolution of the Al₃Ti matrix. In this case, the Al₃Ti phase is recrystallized, and the released titanium carbides are distributed in the form of clusters in the melt volume. The resulting materials are superior to the characteristics of the composites reinforced with ex-situ TiC powders of same dispersity. This allows recommend the usage of TiC-Al₃Ti powders as fillers to obtain discretely reinforced aluminum-matrix composite materials.