Preparation Of Al-Ti-C-Ce Master Alloy And Study On Its Refinement Mechanism For Commercial Pure Aluminum

Grain refinement of aluminum and its alloys can significantly improve the mechanical properties,casting properties,deformation processing properties,and surface quality of the materials.As compared to Al-Ti-B master alloys,Al-Ti-C master alloys have the advantage of small size of TiC particles,low aggregation tendency in aluminum melt,and difficulty in poisoning with element such as Zr and Cr.Unfortunately,the key issue in the production of Al-Ti-C alloy is that the wet-ability between carbon and Al melt is poor,which makes the formation of TiC difficult.The rare earth metals have certain refining ability and catalytic effect,which play an important role in promoting the synthesis of fine TiC particles in the preparation of Al-Ti-C alloy.In this paper,Al-Ti-C-Ce master alloy was prepared by the aluminum melt in-situ reaction with CeO₂as additive.X-ray diffraction(XRD),optical microscope(OM),scanning electron microscope(SEM),electron probe microanalyzer(EPMA),transmission electron microscope(TEM)and differential scanning calorimetry(DSC)were employed to study the influence of CeO₂contents and preparation process on the microstructure and refinement performance of Al-Ti-C-Ce master alloy,and clarify the relationship between the microstructure and refinement performance of Al-Ti-C-Ce master alloy.Meanwhile,the quenching experiment was used to study the effect of CeO₂content and size on the phase transformation and microstructure transformation of Al-Ti-C-Ce master alloy during the preparation process,and the reaction thermodynamics and macro-dynamics process were analyzed.In addition,the refinement effect of Al-5Ti-0.62C,Al-5Ti-0.62C-0.41Ce and Al-5Ti-0.41Ce master alloys on commercial pure aluminum was studied,the evolution behavior of Ti₂Al₂₀Ce in the refinement process was analyzed,and the refinement mechanism of Al-Ti-C-Ce master to commercial pure aluminum alloy were explored.The main research works and conclusions as follows:(1)The effect of CeO₂additions on the microstructure and refinement performance of Al-Ti-C-Ce master alloy were studied.The results show that the Al-Ti-C-Ce master alloy is mainly composed of?-Al,Al₃Ti,TiC and Ti₂Al₂₀Ce.Adding CeO₂is beneficial to refine the size of Al₃Ti,promote the synthesis of Al₃Ti and TiC,and reduce the aggregation tendency of TiC.With the increase of CeO₂addition,the shape of Al₃Ti is changed from long to short rod,the size of Ti₂Al₂₀Ce is gradually increased,the amount of TiC is increased and the distribution is more uniform.When the amount of CeO₂is 8%,the number and the size of TiC particles in Al-Ti-C-Ce master alloy is the largest and uniform,the size of TiC and Al₃Ti are refined to 0.3?m and 106?m.Al-5Ti-0.62C-0.83Ce master alloy has the best uniform organization and optimal refinement effect on commercial pure aluminum.(2)The effect of holding time,synthesis temperature and holding temperature on the microstructure and refinement performance of Al-5Ti-0.62C-0.83Ce master alloy were analyzed.The results show that prolonging the synthesis holding time properly can reduce the size of Al₃Ti and promote the uniform distribution of TiC.Moreover,the flocculated Ce-rich exists around TiC,which hinders the growth of TiC and avoids the merge growth up of TiC due to agglomeration.Ti₂Al₂₀Ce is mainly distributed around Al₃Ti,which arrest Al₃Ti from growing or fusing Al₃Ti that has grown up.when the synthesis temperature of master alloy is 1100?,the preferred orientation of Al₃Ti of grows into needles or strips,Ti₂Al₂₀Ce is in the form of small blocks,and the amount of TiC is large,with a size of about 0.3?1?m.However,At a synthesis temperature of 800?,the Al₃Ti and Ti₂Al₂₀Ce grow into blocks in each direction,and the size of the TiC is about 3?m.When holding at 1100?,TiC particles are mainly segregated on the grain boundaries or evenly distributed in the form of particle clusters.When holding temperature is reduce to 800?,TiC is mainly uniformly distributed in the form of single particles in the Al matrix.As compared to A₁?C₁master alloy,A₃master alloy synthesizes and keeps at 1100?for 15 min has the best refinement effect on commercial pure aluminum.(3)The formation process of the second phase in Al-Ti-C system and Al-Ti-C-Ce system were compared and analyzed,and the influence of CeO₂contents and sizes on the phase transformation and microstructure transformation of Al-Ti-C system were studied.The results show that the quenching samples under complete reaction in Al-Ti-C system is mainly composed of?-Al matrix,irregular bulk Al₃Ti of 6?12?m and TiC particles of 1?3?m.CeO₂has little effect on the synthesis method of Al₃Ti particles in Al-Ti-C system,but a significant effect on the synthesis of TiC particles.In the Al-Ti-C system,TiC is formed by the reaction of Ti atoms provided by the dissolution of Al₃Ti with solid C.In the Al-Ti-C-Ce system,CeO₂first reacts with solid C to form Ce C₂,then Ce C₂reacts with Ti atoms to form TiC.In addition,the addition of CeO₂improves the wettability between Al and C,reduces the formation temperature of Al₃Ti and TiC,and promotes the synthesis of TiC particles.The smaller the size of CeO₂,the lower the formation temperature of Al₃Ti and TiC.When the content of CeO₂is 4%and the size is 2-4?m,the reaction promotion effect on the Al-Ti-C system is the most obvious,and the amount of Al₃Ti and TiC synthesized is the largest and the distribution is relatively uniform.When the CeO₂size is 30 nm,the formation temperatures of Al₃Ti and TiC are the lowest,783.2°C and 926.1°C,respectively,and the average size is the smallest,4?m and 1.1?m,respectively.(4)The refinement effect of Al-5Ti-0.62C,Al-5Ti-0.62C-0.41Ce and Al-5Ti-0.41Ce master alloy on commercial pure aluminum was studied,the evolution behavior of Ti₂Al₂₀Ce in the refinement process was analyzed,and its refinement mechanism was explored.The results show that Al-5Ti-0.62C-0.41Ce master alloy has significantly better refinement performance than the other two master alloys.The refinement of Al-5Ti-0.62C master alloy mainly depends on Al₃Ti assisting the nucleation of?-Al by TiC particles.Al-5Ti-0.41Ce master alloy mainly relies on Ti₂Al₂₀Ce to assist the nucleation of Al₃Ti.The dissolved Ce atoms segregate around Al₃Ti and react with Ti around Al₃Ti to form Ti₂Al₂₀Ce again.Ti₂Al₂₀Ce in the Al-Ti-C-Ce master alloy will evolve into AlCe and AlCe₃during the refinement process.AlCe is mainly distributed around the TiC particles,hindering the diffusion of Ti atoms,preventing TiC particles from failing,and assisting TiC nucleation.AlCe₃is mainly distributed around the impurity phase,which hindering the growth of the impurity phase.The Ce atoms released by Ti₂Al₂₀Ce reacts with Fe,Si elements to form Fe Si Ce phase,which refine the Al₃Fe phase on the grain boundary,and pinning at the grain boundary hinders the growth of aluminum grains.