Study On Coatings For Al-Li Alloy DC Casting

This work is a part of the key project of the National Natural Science foundation of China-Fundamental research on processing of the purity,fine grain and homogeneous ingots of Al-Li alloys(U1608252),to develop new coatings for melting Al-Li alloys for meeting the need of development of Al-Li alloys.321 stainless steel was selected as the base material of steel crucible.Phosphate was selected as adhesive.And BN and Y2O3 were selected as aggregate.The composition,sintering method and ability of anti-thermal fatigue,corrosion behavior in liquid Al-(1.5-2.5)wt.%Li alloys were worked out.Refractory No.l and calcium silicate board were used as the hot top of DC casting.BN and MgO were selected as main component of coatings.The corrosion behavior of different coatings in molten Al-1.5-2.0 wt%Li was studied.According to the surface quality and microstructure of DC casting ingot,the application scope of various coatings was determined.The main results are as followings:1.Good bonding was achieved when stainless steel 321 stainless steel was coated with phosphate coating(9.00g phosphoric acid,1.18g Al(OH)3,MgO 0.88g,5.90g H2O,1.77g zinc chrome,0.66g CrO3,17.69g Al powder,5.62g BN,5.62g Y2O3)and sintered at 850?for 1h.2.The sintered coating of 321 stainless steel with phosphate coating has good thermal shock resistance;3.If the phosphate was chosen as adhesive and BN and Y2O3 was chosen as aggregate coating,which were coated on stainless steel.No corrosion occurred in the interfaces of stainless steel/coating/Al-Li alioys,and the interfaces of steel/coating/Al-Li alloy were intact and plain,which means that this coating is perfect.In molten pure aluminum and Al-(1.5-2.5)wt.%Li alloys,the coating has not been corroded and has high chemical stability,which ensured that Al-Li alloy has a high purity and no Fe element intermingled into the liquid of Al-Li alloy according to electron probe testing.4.Refractory No.1 and Calcium silicate refractories coated with BN,SiC and BN mixed coating or MgO and BN mixed coating.The coating were not be corroded in molten Al-1.5wt.%Li alloy,but corrosion taked place in molten Al-2.0wt.%Li alloy.No corrosion occurred in molten 2.0wt%Li Al-Li alloy when Refractory No.l and Calcium silicate refractory coated with MgO.5.When alloys with Li content,1.5wt%used the hot top coated by BN coatings and SiC and BN mixed coatings,the surface quality of DC casting ingots is good and the bound edge microstructure is not obviously polluted.However,when the alloys with Li content?2.0wt%used the hot top coated by MgO coatings the surface quality of DC casting ingot is good and the bound edge microstructure is not obviously polluted.