Preparation And Properties Of Al₂O₃ Crucible For Smelting Superalloy

Crucible is employed as a melting vessel of superalloy,and its performance and quality play a vital role in the development of superalloy.At present,the Al₂O₃crucible for smelting of domestic superalloy mainly depends on the import,but the import crucible is expensive and easy to be affected by the international situation.The thermal shock resistance of domestic Al₂O₃crucible is poor,and the crucible surface often cracks and falls off after once use leading to short service life,which increases the production cost of superalloy.In this paper,based on the preparation technology of Al₂O₃crucible,the Al₂O₃crucibles were systematically studied by slipping casting along with pressureless sintering,attempting at improving the mechanical properties and thermal shock stability of the Al₂O₃crucible,prolonging the service life,and promoting the domestic development of the Al₂O₃crucible for smelting superalloy.By studying the effects of dispersant and solid content on slurry stability and particle segregation,the slurry with high solid content and low viscosity was prepared with controlled particle segregation in slipping casting,so as to relieve the adverse effect of particle segregation on the crucible.The results showed that,when 0.3 wt%ammonium citrate and 0.3wt%arabic gum were added as dispersants with solid content of 66 vol%,the particle segregation can be effectively controlled in slipping casting,and the Al₂O₃crucible samples with uniform structure were prepared.The bending strength at room temperature of Al₂O₃crucible was 19.72 MPa and the strength retention was rate up to 71.6%after thermal shock for five times at 1300℃.The electrostatic and steric stabilization formed by dispersant are conducive to the stable existence of particles in suspension.Moreover,the number of particles in unit volume increases with high solid content,and the interaction force between particles increases.It is difficult for individual particles to break away from the surrounding binding force,which further inhibits particle segregation.Therefore,the crucible specimens exhibited uniform microstructure and excellent thermal shock resistance.The structural properties of crucible samples were optimized by regulating the particle gradation of powders.The experiment showed that the optimal particle gradation was 18.6wt%of fine Al₂O₃,41.4 wt%of coarse Al₂O₃,10 wt%of fine Si O₂and 30 wt%of coarse Si O₂.The particle accumulation mode is the coarse particles as the skeleton and the fine particles fill the gaps between the coarse particles.The optimal particle gradation obtained by experiments was in good agreement with the theoretical ratio calculated by Dinger-Funk model.The Al₂O₃crucibles without cracks were obtained after the green body was sintered at1300℃,and the crucible exhibits density of 2.37 g/cm³,porosity of 18.72%,low linear shrinkage of less than 2%,high bending strength at room temperature of 21.81 MPa,low thermal expansion coefficient of 2.62～2.94(×10^-6/℃),and superior thermal shock resistance.The strength retention rate up to 73.5%after thermal shock for five times,which can well meet the requirement of Al₂O₃crucible recycling in superalloy smelting.Based on Al₂O₃ crucibles with the optimal particle gradation,reinforcement phases was added to further improve the performance of Al₂O₃crucibles.Zr O₂,mullite fiber and in-situ growth whiskers were introduced as reinforcement phases,respectively,and the effect of different reinforcement phases on the performance of Al₂O₃crucibles was studied.The results showed that the in-situ growth whiskers has a good strengthening effect,Al F₃was added as a catalyst to form gaseous Al OF and Si F₄at high temperature,which promote the formation of mullite whiskers.The whiskers prevented the initiation and propagation of cracks through the bridging and pulling out effects,which played a strengthening effect on the crucible.When the amount of Al F₃was 5 wt%,the highest bending strength at room temperature was 23.65MPa and the strength retention rate up to 75.2%after thermal shock for five times.The results showed that the in-situ growth mullite whiskers improved the thermal shock resistance of the crucible,which is beneficial to prolong the service life of the crucible in the smelting superalloy.