The Optimization Of BaZrO₃/Al₂O₃ Composite Mould And Its Application In Directional Solidification Of Ti-46Al-8Nb Alloy

TiAl-based alloy is a new lightweight engineering material with broad application prospects,its density is less than 50%of the nickel-based alloy.It has light weight,high specific strength,wear resistance,high temperature resistance,excellent oxidation resistance and creep resistance,and its engineering application prospects are good.However,its low room temperature ductility,poor room temperature processing performance,and insufficient oxidation resistance and poor stability above 850°C greatly limit its practical application.The introduction of TiAl-based alloy directional solidification technology,the greatest possible reduction of the existence of transverse grain boundaries,can effectively improve its high temperature stability.Therefore,the directional solidification technology has been developed.The cold crucible directional solidification technology is a new type of alloy preparation technology,because of its cold wall material,the crucible is not in contact with the alloy melt.It has the advantages of zero pollution.And it is widely used in the directional solidification of high melting point and high activity TiAl-based alloys However,cold crucible directional solidification technology can only produce a crucible shape product,which is a technical problem for the preparation of complex shape products.The directional solidification technology of the mould not only has the advantages of water-cooled copper bismuth to prepare TiAl-based alloy,but also can efficiently prepare a specific shape alloy.Because of the high temperature activity of the TiAl-based melt,some interactions between the alloy and the mould materials always occur during the directional solidification process.Therefore,choosing a suitable material is a key to realize the production of TiAl-based alloy or TiAl single.BaZrO₃ is a new type of refractory for the preparation of titanium alloys.The BaZrO₃ refractory material was successfully applied to the melting of Ti6Al4V,TiAl,TiNi and TiFe alloys.By using BaZrO₃refractory as the mould surface layer,EC95?Al₂O₃?is used as the back layer in the field of casting.Based on this,this paper uses the refractory material to prepare a composite mould and attempts to prepare a TiAl single crystal by directional solidification technology.In the preliminary work,the research group used the BaZrO₃ composite mould to solidify the TiAl-based alloy,and casting with five columnar crystal structure was obtained.However,the mould needed to be in direct contact with the high temperature melt for a long time during the solidification process,which inevitably caused the melt penetration and erosion.In order to solve the leakage phenomenon,the mould needed to be optimized.The main research contents of the thesis include the following aspects:1)The BaZrO₃ composite mould was prepared by using self-synthesized BaZrO₃ powder as the surface layer?yttrium sol as a binder?,Al₂O₃ as a back layer?silica sol as a binder?.On the basis of research of the group,the surface layer was optimized and the effect of different particle size on the compactness of the mould surface was investigated.The results showed that the thickness of the surface layer with fine-grained particles?100-200 mesh?could reach 3500?m,the number of pores between particles was reduced and the size decreased,the porosity of the surface layer decreased from 17.3%to 13.5%.The thickness of the optimized adhesive layer between the BaZrO₃ surface layer and the Al₂O₃ back layer was increased from 300?m to 1200?m,and the adhesion was also enhanced.After the sintering,the corrugations on the inner surface of the mould were changed,the roughness Ra was 3.62?m,and it was in the range of 6 grades in 14 grades of roughness.2)The interface penetration layer between the optimized mould and alloy was reduced from 900?m to 300?m,the penetration of the alloy melt was limited to the surface layer,and no exudate was observed in the back layer.In addition to BaZrO₃,BaAl₂O₄ was formed after the surface layer material was in contact with the alloy liquid.3)At the end of the crystal selection,a large grain can be obtained.The spiral part showed an excellent effect on grain selection.A large columnar grain covered the directional area and the columnar grains grew continuously along the grain growth direction.However,three small grains can be seen at the edges and in the mid zone due to the existence of heterogeneous nucleation sites,forming a columnar crystal with three misoriented grains.The angle between the lamellar direction and growth direction was 45°.No BaZrO₃ particles were detected in the microstructures.The Ba,Zr contents in the sample were about 24,4130ppm.The maximum oxygen content?3730ppm?was below the critical value of about 4800ppm for growth of the hcp-?phase.Compared with the amount of contamination in the alloy using pre-optimized mould,the content of Ba,Zr,O in the alloy were reduced from 89,16800,7580ppm to 24,4130,3690ppm.