Optimisation of Process Parameters and Assessment of Mechanical Properties of In-Situ AA6082-TiB2 Particle Reinforced Composites

Abstract

The specific properties exhibited by aluminium matrix composites make them attractive for different applications, especially in the automotive and aerospace industries. The main objective of this research work was to produce in-situ AA6082- TiB2 particle reinforced composite using flux assisted synthesis technique. The in-situ TiB2 ceramic reinforcements were prepared by adding K2TiF6 and KBF4 fluoride salts to the AA6082 alloy melt. Addition of salts caused severe exothermic reaction in the melt, where Ti and B are reduced from their respective salts leading to the formation of TiB2 particles in the melt. However, incomplete reaction will always lead to the formation of unwanted Al3Ti and AlB2 intermediate intermetallic particles along with TiB2. Hence, in order to supress the formation of intermetallic particles and to result in the complete formation of TiB2 particles, several preliminary experiments were carried out. Initially, the AA6082-TiB2 composites with 5wt.% of TiB2 particles were prepared by optimising the process parameters such as melt holding time, melt holding temperature, stoichiometry of the added fluoride salts and intermittent melt stirring interval. For the optimisation of composites, characterisation techniques such as XRD, SEM, EDS, Optical microscopy and chemical analysis using ICP-OES were used. The mechanical properties were assessed using hardness and tensile tests. Experiments on melt holding time and temperature indicated that, melt holding time of 60 minutes and a temperature of 850ºC is essential during the processing of AA6082-5wt.%TiB2 composite. The experimental results showed that the addition of stoichiometric mixture of K2TiF6 and KBF4 resulted in the formation of Al3Ti and AlB2 intermetallic particle along with TiB2 particles in the matrix alloy. Studies carried out by varying the percentage of KBF4 salts in the stoichiometric mixture confirmed that 10% of excess addition of KBF4 salts to the stoichiometric mixture is necessary to eliminate the formation of intermetallic particles. The excess addition of KBF4 salts to the stoichiometric mixture compensated the loss of B which occurred during the addition of fluoride salts to the aluminium melt thereby forming only TiB2 particles in the matrix alloy. Investigations on the intermittent stirring interval timerevealed that an interval of 8 minutes intermittent stirring time for 20 seconds is essential for the homogeneous distribution of TiB2 particles in the composite. In spite of adding TiB2 reinforcements, the composites processed during the melt holding time showed deterioration in hardness and tensile properties, when compared with the unreinforced alloy. Hence to understand this peculiar behavior observed among the processed composite, studies were focused to understand the influence of matrix alloy. Hence a thorough investigation on the influence of major alloying elements (Mg, Mn and Si) present in AA6082 matrix alloy during the processing of composites was carried out. Chemical compositional analysis on the processed composites using ICP-OES confirmed the complete loss of Mg in the matrix alloy of the processed composites. Investigations confirmed that the loss of Mg has occurred due to the reaction of Mg with KAlF4 and AlF3 fluxes which were formed in the melt by the dissociation of K2TiF6 and KBF4 salts. The loss of Mg in the matrix melt was compensated by adding the required amount of Mg to the matrix melt, soon after the removal of slag, thereby restoring the overall properties of composite. Influence of Mg in the AA6082-5wt.%TiB2 composite was investigated by varying the Mg percent in the matrix alloy. Compared to the unreinforced alloy, composite with the addition of Mg showed 13% increase in hardness and the Y.S and U.T.S were found to increase by 26.5 and 19.2%. However, addition of Mn showed minor improvement in hardness and tensile properties. Presence of Mn in some of the TiB2 particles indicates the diffusion of Mn atoms into the TiB2 lattice. During the investigation on the influence of Si, no excess or varying of Si was done. The influence of Si on AlTiSi particles formed during processing of composites was studied with respect to time. Considerable amount of Si from the matrix alloy is seen to diffuse into the Al3Ti lattice forming AlTiSi particles. However, during the dissolution of AlTiSi particles with increase in melt holding time, Si tends to diffuse out thereby leaving fine Al3Ti particles in the matrix. AA6082-TiB2 particles with different weight percent (2.5, 5, 7.5 and 10wt.%), with and without the addition of Mg was prepared and analysed. Composites processed with the addition of Mg showed better properties, than the composites processed without the addition of Mg.Ageing treatment on unreinforced AA6082 alloy and AA6082-TiB2 composites at 180ºC after solution treatment at 560ºC show significant reduction in the time for peak ageing and increase in peak age hardness for the as cast composites with respect to those of as cast unreinforced alloy. Ageing kinetics is enhanced with increase in density of particles in the composites. The faster kinetics of ageing is attributed to the higher density of dislocations formed due to the presence of TiB2 particles and fine precipitates formed during ageing treatment.