Morphology Evolution Of TiAl₃ Particles In Al-4Ti Master Alloys

As typical grain refiner,Al-Ti master alloys have been frequently used in some special areas,such as aluminum foil,aluminum pressure tank,electronic accessories,etc.The second phases of Al-Ti master alloys exhibit thick flaky morphology easily,which will have a serious influence on grain refinement performance.A series of Al-4Ti master alloys were prepared via pouring the pure aluminum added with K2TiF6 or sponge titanium into four different molds made of graphite,copper,steel and sand.Optical microscope and scanning electron microscope?SEM?with energy dispersive spectroscopy?EDS?were employed to characterize the microstructure and morphology evolution of TiAl₃ particles on condition that different cooling rate and proportion of fluorides.Then the grain refinement efficiency of different Al-4Ti master alloys in commercial-purity was investigated.Through a series of experimental researches,the results can be got as follow:?1?Different morphologies of TiAl₃ particles in Al–4Ti master alloy could be acquired by adopting different cooling rates and proportion of fluorides.Al-4Ti master alloys are composed of ??Al?and TiAl₃ phase.The phase in Al-4Ti master alloys could hardly change whatever the cooling rate and proportion of fluorides were.?2?With the reducing of cooling rate,the morphology of TiAl₃ particles in the prepared master alloy changed from petal-like to blocky and,finally,to flake-like.It was also found that the petal-like TiAl₃ particles could be formed by thick plates during the solidification.When the proportion of Ti element from K2TiF6 in Al-4Ti master alloys reached to 40%,whatever the cooling rate was,those TiAl₃ particles in Al matrix showed blocky morphology,which meant that this proportion of fluoride had a great effect on the manufacture of high-quality Al-4Ti master alloys.?3?The three main types of TiAl₃ particles did not differ in composition and crystal structure.Moreover,the phase inversion temperature of Al–4Ti master alloy did not change with different particles morphologies.The transformation of morphologies of TiAl₃ particles was caused by atomic arrangement because of the difference of local titanium element concentration and diffusion rate.In addition,the addition of fluorides has a good ability to promote the formation of petal-like TiAl₃ particles.The tiny flaky TiAl₃ can be obtained from petal-like TiAl₃ particles by the means of brittle fracture.?4?The effect of cooling rate on morphologies of TiAl₃ crystal was realized by changing the diffusion rate of Ti atoms in solidification.While,the influence of proportion of fluorides on morphologies of TiAl₃ particles was achieved by altering the degree of saturation of Ti atoms in aluminum melt.?5?Within the ranges of this research,the grain size of the commercial-purity aluminum refined by the Al-4Ti master alloy with petal-like TiAl₃ particles increased firstly,and then dropped rapidly with the increase in the hold time.The Al-4Ti master alloy with flaky TiAl₃ particles could instantly achieve a fine grain refinement under a hold time of five minutes;however,the fine refinement efficiency went down as the hold time increased.The grain refinement efficiency of the Al-4Ti master alloy with blocky TiAl₃ particles was the best and did not fade away obviously with the change of the hold time.?6?The larger the average grain sizes of the original TiAl₃ particles were,the larger that of the refined commercial-purity aluminum was.