Research On Preparation Process And Grain Refining Performance Of Al-Ti-C-RE Grain Refiner

Aluminum and aluminum alloy, because of its excellent ductility, conductivity and thermal properties, excellent toughness, high specific strength, good corrosion resistance and other excellent performance, are widely used in aviation, marine, civil, electrical, medical, transport vehicles and other industrial fields. With the extensive use of aluminum, the industrial production presents strict requests to the microstructure and property of aluminum ingot. Grain refinement is one of the most important ways to improve the strength, plasticity and the comprehensive properties of aluminum alloy ingot. In the industrial production, adding grain refiner into the molten aluminum and refining the aluminum grain by heterogeneous nucleation during the solidification process is the most simple, cheap and effective way.Al-Ti-B refiner, as an efficient grain refiner, is widely used in aluminum and aluminum alloy around the world. But the easy agglomeration of TiB2 and easily lost refining effect with Zr, Cr, Mn and other elements by poisoning effect limited the application range of Al-Ti-B. With the research of Al-Ti-C grain refiner, it found that Al-Ti-C can make up many shortcomings of Al-Ti-B refiners in the industrial applications. But the poor wettability between carbon and aluminum melt make it difficult to generate sufficient number of TiC particles during the preparation of Al-Ti-C grain refiner, which greatly limited the Al-Ti-C industrial application. In this paper, a new type of Al-Ti-C-RE master alloy refiner was prepared by adding rare earth elements to improve the wettability of graphite and the melt in the preparation process with in-situ reaction method, and the synthesis mechanism, its refining effect on pure aluminum was also discussed.In this paper the microstructure characteristics and refining effect of Al-Ti-C and Al-Ti-C-RE were investigated by optical microscopy (OM), X-ray diffraction (XRD), energy dispersive spectrometry (EDS), scanning electron microscopy (SEM) and other methods. Combined the microstructure and the mechanical properties of refined aluminum, the synthesis mechanisms of Al-Ti-C-RE with different rare earth addition sequence and its refinement mechanisms were also discussed, which offers some theoretical reference and experimental basement for developing high efficient Al-Ti-C-RE grain refiner.The results showed that the Al-5Ti-0.25C-2RE consisted ofα-Al matrix, cluster-like or granular TiC, lath-like TiAl3, rim and blocky Ti2Al20Ce rare-earth phase.The synthesis mechanisms of Al-5Ti-0.25C-2RE(A group), which was prepared with RE adding in early stage, are as follows:TiAl3 is formed through the reaction between K2TiF6 and Al melt at 850℃; Ti2Al20Ce is formed by a peritectic reaction between TiAl3, Ce atoms and some free Ti atoms; when the temperature was heated up to 1200-1300℃, TiC is formed by graphite and Ti atoms through the reaction: Ti+C(s)→TiC(s) and Ti+C→TiC(s). The synthesis mechanisms of Al-5Ti-0.25C-2RE(B group), which was prepared with RE adding in late stage, are as follows:blocky TiAl3 is formed through the reaction between K2TiF6 and Al melt at 850℃; when the temperature was heated up to 1200-1300℃, TiC is formed by graphite and Ti atoms through the reaction:Ti+C(s)→TiC(s) and Ti+C→TiC(s); and then Ti2Al20Ce is formed by a peritectic reaction between TiAl3, Ce atoms and some free Ti atoms when the temperature was decreased to 850℃with rare earth elements was added.Because of the different addition order of rare earth elements, most of the TiAl3 phases are thin lath-like, and wrapped like Ti2Al20Ce phase is in thick size in Al-5Ti-0.25C-2RE of A group; but in B group Al-5Ti-0.25C-2RE, TiAl3 phases are mostly coarse lath, wrapped like Ti2Al20Ce phase is thinner than that of A group, and more blocky Ti2Al20Ce phases were formed. Adding the same amount of Al-5Ti-0.25C and A, B group Al-5Ti-0.25C-2RE into pure aluminum for refining, the results show that both two groups of Al-5Ti-0.25C-2RE refiner obtained better refinement than that of Al-5Ti-0.25C, and A group Al-5Ti-0.25C-2RE, which was prepared with RE adding in early stage, has more excellent refining effect.By changing the addition amount of A group Al-5Ti-0.25C-2RE (0.1 wt.%,0.3 wt.%,0.5 wt.% and 0.7 wt.%) and the refinement holding time (5min,15min,30min, 60min,120min), the relationship between microstructure and properties of aluminum are investigated. When adding 0.5% Al-5Ti-0.25C-2RE and holding for 5min,the results show that the grain size dropped from 340μm to 95μm, the tensile strength increased by 10.4% and the elongation increased by 19.2%. With the prolonged holding time, refining effect showed no apparent recession.Ti2Al20Ce phase is formed by a peritectic reaction between RE elements and TiAl3 phase. Ti2Al20Ce phase can decompose and release Ti atoms and rare earth atoms at about 630℃. Because of the decomposition, the nucleation of TiC particle can be promoted, still plenty of Ti atoms in the melt can be ensured after longer holding time, and thus to guarantee long effectiveness refining efficiency.