Study On The Preparation Of New Al-Ti-C Grain Refiner And Its Refining Ability And Chemical Stability

Grain refinement is not only the main means of strengthening and toughening aluminum alloy materials,but also the necessary premise of aluminum alloy application.Generally,the development of grain refiner for aluminum alloy has experienced three generations:Al-Ti refiner,Al-Ti-B refiner and Al-Ti-C refiner.According to the principle analysis of heterogeneous nucleation,TiC_x particles in Al-Ti-C refiner belong to face-centered cubic structure with similar lattice constants,which accords with lattice matching principle.Therefore,the interface energy between aluminum crystal nucleus and TiC_x particles is the smallest,and the nucleation resistance is also the smallest.However,the main problem of Al-Ti-C refiner at present is the poor chemical stability of TiC_x particles in aluminum melt.In addition,the traditional"bottom-up"preparation method of TiC_x particles is easy to produce harmful phases such as impurities and component segregation,which will affect the properties of refiners.In this paper,firstly,Ti₂AlC in MAX phase material was used as the precursor of TiC particles,and in the process of high-temperature reaction between Ti₂AlC and Al,tiny TiC_x particles with sub-micron scale were formed by in-situ decomposition of Ti₂AlC,which was used as grain refiner.The effects of key process parameters such as initial Ti₂AlC content and reaction temperature on the phase composition,morphology and distribution were systematically studied,and the optimal preparation process of master alloy was determined based on the results of grain refining experiment.In order to further improve the stability of the master alloy,the X-site N-doped MAX solid solution material(Ti₂AlC_1-yN_y)was designed and prepared,and the N-doping and doping amount of TiC_xwere precisely controlled,and the control effect of N-doping on the chemical stability of TiC_x was studied.The master alloy with different N-doped TiC_x was diluted into ZL101A cast aluminum alloy melt,and the practical application effect of the master alloy was verified.The main conclusions are as follows:（1）Using the mixed powder of 15 vol.%Ti₂AlC-85 vol.%Al as raw material,it is found that the mixed powder has reacted at 700℃,and the Al in Ti₂AlC diffuses outward to decompose Ti₂AlC into TiC_x;When the sintering temperature is between 700℃and850℃,the reaction products are mainly Al₃Ti and a small amount of TiC_x and Al₄C₃phases,as well as a small amount of Al₂O₃;When the sintering temperature is above900℃,Al₃Ti and less Al₄C₃ phase in the reaction product will further react to form TiC_xphase.With the increase of sintering temperature,Al₃Ti in master alloy 15Ti₂AlC/Al gradually changes from block or short rod shape to large needle rod shape.The dispersion of TiC_x in aluminum matrix is more uniform,but the size of TiC_x particles also gradually increases,and the C vacancy concentration in TiC_x particles gradually decreases with the increase of sintering temperature.（2）In the grain refinement experiment,adding master alloy 15Ti₂AlC/Al-900℃to aluminum melt can effectively refine the grainα-Al grain,and has good refining stability.When 0.7 wt.%15Ti₂AlC/Al-900℃master alloy is added to the aluminum melt at 760℃for 15 min,theα-Al grain size changes from the initial 5500μm,refined to 98μm;After holding for 60 min,theα-Al grain size remained at 132μm,the fading rate f₆₀ of15Ti₂AlC/Al-900℃master alloy is maintained at about 34%;The higher the sintering temperature of master alloy 15Ti₂AlC/Al（between 900℃and 1200℃）,the higher the refining efficiency of master alloy,but the stability of master alloy gradually decreases,and the fading rate f₆₀ of master alloy 15Ti₂AlC/Al-1200℃is about 47%.（3）The Ti₂AlC_1-yN_y powder is used as raw material to prepare master alloy 15Ti₂AlC_1-yN_y/Al,which can effectively realize the N-doping of TiC_x in master alloy and form a new nucleation particle TiC_xN_0.5-x;The master alloy 15Ti₂AlC_0.9N_0.1/Al-900℃was used to refine pure aluminum,and the influence of trace N-doping on refining efficiency and stability of the master alloy was studied.It was proved that TiC_0.45N_0.05 can effectively refineα-Al grains and has good chemical stability,and the grain size will not increase when it is kept in the aluminum melt at 760℃for 60 min.Excessive doping of N element will seriously affect the refining efficiency of master alloy.（4）Adding 0.5 wt.%master alloy 15Ti₂AlC_0.9N_0.1/Al-900℃to refine ZL101A,the grain size of ZL101A decreases from 3900μm to 90μm;After heat treatment,the Vickers hardness of ZL101A aluminum alloy increases by 19.7%.