| FGH96 powder metallurgy superalloy is a significant material for turbine disks of high performance aeroengines and has strict requirement for inclusions.Inclusions mainly come from the preparation process of the master alloy.In the present study,the formation mechanism and control methods of inclusions in FGH96 master alloy was discussed,and the master alloy with high purity was successfully fabricated.The main research contents are listed as follows:(1)The effects of crucible materials(MgO,Al2O3 and MgO-Spinel)and adding sequence of active elements(Al and Ti)on inclusions during vacuum induction melting(VIM)have been studied.In the case of MgO crucible,Al in the alloy reacts with the crucible,and the inclusions are near spherical MgO-Al2O3 with smaller size and number.The formation of continuous and dense MgAl2O4 layer in the inner of MgO crucible can resist the physical erosion by liquid alloy.In the case of Al2O3 crucible,oxides are Al2O3 with larger size,and the crucible suffers severe physical erosion.In the case of MgO-Spinel crucible,the chemical reaction and physical erosion are in between.The adding sequence of active elements has significant impact on the size and number of inclusions.The optimized process is to add Ti before Al.Inclusions are small spherical TiOx oxides when Ti is added before Al,and the average size is smaller than 1 ?m.And then they are transferred to Al2O3 after the addition of Al.In the industrial VIM ingot,nitrides occupies 80.4%of inclusions and has the size range of 1-5 ?m.The number of oxides is smaller,but the size is in the range of 1?25.5 ?m?(2)The effects of atmosphere and slag compositions on inclusions have been studied during electroslag remelting.The number density of inclusions are lower when the superalloy was remelted under vacuum because vacuum can prevent the oxidation and nitriding of electrodes and the penetration of N2 and O2 into the molten slag and alloy.The addition of CeO2 is effective to reduce the number of oxides and change the composition of oxides.When CeO2 content is 1.0 wt.%,the oxides are changed to Al2O3-MgO-Ce2O3.When CeO2 content is 3.0?10.0 wt.%,the oxides are changed to Al2O3-Ce2O3.The optimized CeO2 content is 3.0 wt.%.Master alloy with high purification is obtained via vacuum induction melting and vacuum electrosalg remelting and vacuum induction remelting before atomization,and the inclusion content is 0.399 mg/kg by slime method.(3)Inclusion collision,agglomeration and floatation during electromagnetic levitation melting have been investigated.Electromagnetic levitation melting promotes the agglomeration and floatation of inclusions.When the liquid alloy solidifies directly in the copper mold,the size and number of clusters are large.However,the size and number of clusters decrease after pouring.The different behavior of oxides and nitrides during melting and solidification were investigated by in-situ confocal laser scanning microscopy method and the mechanism was discussed.Oxides are stable at melting temperature and they agglomerate to a cluster,while nitrides dissolve during melting and precipitate during solidification.(4)The effect of inclusions on fatigue crack growth(FCG)rate of FGH96 superalloy was investigated.The lower the content of inclusions is,the lower the FCG rate is.When the nitrogen content is higher than 12 ppm,many inclusions are observed across the main and secondary cracks.The nitrogen content should be under 12 ppm if the lower FCG rate is needed. |
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