Study On Al-5Ti-1B Master Alloy Prepared Under Ultrasound And α-Al/Diboride Interface

Grain refinement of aluminum and its alloys by the addition of Al-Ti-B master alloy has become the most popular practice in industry due to its simplicity and efficiency. However, the grain refining performance of Al-Ti-B master alloy, generally prepared by the reaction of halide salts with molten aluminum, cannot satisfy the requirements of high quality aluminum ingots because of large mean diameter and wide size spread of TiB₂ particles in the microstructure. Meanwhile, the theoretical hypotheses about the heterogeneous nucleation ofα-Al on borides are controversial and disagree with the experimental phenomena in grain refinement. Therefore, the exploration of new preparation technique of Al-Ti-B master alloy and heterogeneous nucleation mechanism ofα-Al on diborides is crucial to developing more efficient grain refiner and advancing grain refining theoryIn this thesis, with the application of high-intensity ultrasound in the reaction among halide salts and molten aluminum, a new preparation method of Al-5Ti-1B master alloy was put forward. The effects of high-intensity ultrasound on the preparation reaction were studied and the microstructure and grain refining performance of the prepared Al-5Ti-1B master alloy were quantitatively characterized. Using first-principles calculations on Al/TiB₂ and Al/AlB₂ interfaces, the surface properties of diborides and the stacking sequence of Al atoms on the diborides surface were revealed, and the nucleant potencies of TiB₂ and AlB₂ particles were also analyzed, which is beneficial to theoretically explain heterogeneous nucleation ofα-Al on diborides. The main results are listed as follows:The preparation of Al-Ti-B master alloy is mainly decided by the absorption of Ti and B of fluoride salts in aluminum melt and the transformation from AlB₂ to TiB₂. The acoustic streaming and locally high temperature and pressure generated by the high-intensity ultrasound greatly increase the interface area between fluoride and aluminum melt, and facilitate the remelting of AlB₂ phase, which remarkably accelerates the preparation reaction.In our experiments, with the application of continuous high-intensity ultrasonic vibration for three to five minutes, the Ti and B in fluoride salts are adquently absorbed by the aluminum melt and the AlB₂ phase is effectively transformed to TiB₂ particles. The preparation duration of sound Al-5Ti-1B master alloy is decreased from over 60min to less than 5min.In the new Al-5Ti-1B master alloy prepared by the new technique within the application of ultrasonic vibration for four minutes, the blocky TiAl3 particles with 10-20μm in size uniformly distribute and the isolated regularly polygonal TiB₂ particles do not cling to each other. Compared with that in the conventional master alloy, the mean size of TiB₂ particles is diminished from 2.13μm to 0.65μm, and the spread of size distribution is reduced from 0.2-7.5μm to 0.1-1.5μm in the new Al-5Ti-1B master alloy, which indicates that the TiB₂ particles in the new master alloy possess better properties.Owing to the optimization of TiB₂ particles in the new master alloy, the TiB₂ particles released by the new master alloy is 35 times as many as by the conventional master alloy. The settlement velocity of TiB₂ particles in aluminum melt is accordingly reduced to nine percent of that with the addition of conventional master alloy, which greatly retards the fading behavior of grain refining performance of master alloy.With the addition of new Al-5Ti-1B master alloy from 0.1 to 1.0%, the mean grain size of commercial purity aluminum refined by Reynolds standard golf Tee mode method could be diminished from 132 to 60μm. The ultimate value of grain size refined by the new master alloys reaches as small as 45μm, which is superior to the limiting level of the present commercial grain refiner, 120μm, under TP-1 standard experiment.First-principles calculations on TiB₂ and AlB₂ surface reveal that large relaxations are confined to within the top three layers and the relaxation of their B-terminated surfaces is larger than that of their metal-terminated surface, respectively. Compared with the interlayer distance in bulk AlB₂, the distance of outmost interlayer in Al-terminated AlB₂ (0001) surface is abnormally stretched after full relaxation. In a thermodynamic sense, the AlB₂ surface in any system and the TiB₂ surface in the Al-5Ti-1B master alloy are apt to terminate with their respective metal layers.It is shown that the interfacial Al atoms prefer to continue the natural stacking sequence of bulk diborides through first-principles calculations on Al/TiB₂ and Al/AlB₂ interfaces. The interfacial electronic redistribution is locally confined to interfacial region. The bonding energy of the mixed covalent/metallic Ti-Al bond for the Ti-terminated Al/TiB₂ interface is larger than that of the polar covalent Al-B bond with partial ionic-like feature for the B-terminated interface. Metallic bond between Al atoms forms across the Al/AlB₂ interface, which results in weaker interfacial adhesion than that for the Al/TiB₂ interface.The interfacial energy for Al/TiB₂ depends on the interfacial chemical potential. With only TiB₂ particles in aluminum melt, the larger Al/TiB₂ interfacial energy than that betweenα-Al and aluminum melt is responsible for the poor heterogeneous nucleation ofα-Al on TiB₂ surface. The spontaneous segregation of Ti solute on the TiB₂ particle surface could provide sufficient Ti chemical potential, where the Al/TiB₂ interfacial energy is smaller than that betweenα-Al and aluminum melt and the nucleant potency of TiB₂ particles is aroused. Furthermore, it is thermodynamically possible for the segregated Ti solute to form TiAl₃ or TiAl3-like layer on the TiB₂ surface, which theoretically supported the hypothesis of TiAl₃ layer on TiB₂ in hypernucleation theory and duplex nucleation theory.Different from that for the Al/TiB₂, the interfacial energy for Al/AlB₂ is a constant, 0.7615J/m～2, which is much larger than that betweenα-Al and aluminum melt. Consequently, compared with TiB₂, despite lower disregistry betweenα-Al and AlB₂, the AlB₂ particles could not serve as heterogeneous nucleation substrates in a thermodynamic sense, which powerfully clarifies the difference of nucleant potencies between AlB₂ and TiB₂.