Study On The Effect Of Thermally-Controlled Polycrystalline Solidification Process On The Microstructure And Properties Of K417G Superalloy

K417G superalloy,with favorable high-temperature mechanical properties and good corrosion resistance,is widely used in the aviation,aerospace,and transportation industries mainly for the manufacture of turbine blades and vane blade components of aviation engines.However,due to the size effect and thin-wall effect brought about by its casting structure,there is no ideal solution to such metallurgical defects as porosity and incomplete casting in the casting process.In recent years,researchers have reported promising results in controlling the metallurgical defects in K4169,IN792,and other superalloys via thermally-controlled polycrystalline solidification.Despite these accomplishments,the technique has not yet been applied to K417 G alloy processing.Against this background,the current study analyzed K417 G using optical microscopy(OM),scanning electron microscopy(SEM),and energy dispersive spectroscopy(EDS),carried out the tensile experiment at 900°C and the stress rupture experiment under 645 MPa at 760°C and investigated the influence of thermally-controlled polycrystalline solidification on the microstructure and properties of K417 G via the casting simulation software Pro CAST based on Finite Element Technology.In thermally-controlled polycrystalline solidification,the pouring temperature was found to produce a significant impact on the porosity of K417 G.A relatively high level of porosity was observed at the pouring temperature of 1360°C.In contrast,as the pouring temperature increased between 1380-1420°C,porosity gradually decreased from 0.56% to 0.18%.As the pouring temperature rose,marginal changes were observed in the precipitated phase γ′ and the(γ+γ′)eutectic level,and MC carbides remained unchanged.At the pouring temperature of 1360°C,the tensile strength was656 MPa at 900°C,and the stress rupture life was 18 h under 645 MPa at 760°C.At the pouring temperature between 1380°C and 1420°C,the tensile and stress rupture strengths were very close.However,the tensile strength and stress rupture life increased by 761 MPa and 81 h,respectively,as compared with those at 1360°C.Mold temperature in thermally-controlled polycrystalline solidification had a greater impact on the grain size of K417 G than on its dendritic morphology.As the mold temperature rose from 1300°C to 1340°C,the precipitated phase γ′ increased from0.50μm to 0.69μm.MC carbides transformed into sticks from blocks,while the(γ+γ′)eutectic content grew from 5.68% to 6.92%.The average tensile strength was 779 MPa at 900°C,showing no clear association with mold temperature.Plasticity increased from 4.5% to 7.4% as the mold temperature rose.Additionally,when the mold temperature was 1320°C,the stress rupture life was up to 94 h exposed to 645 MPa at760°C,followed by 76 h at the mold temperature of 1340°C,and merely 36 h at 1300°C.As to the association between the withdrawal rate of thermally-controlled polycrystalline solidification and the microstructure and properties of K417 G,a significantly stronger impact was found on the dendritic morphology compared with that on the grain size.As the withdrawal rate increased,the precipitated phase γ′ was reduced from 0.72μm to 0.42μm,the eutectic content decreased from 8.47% to 5.09%,and MC carbides changed from blocks to sticks;the tensile strength at 900°C improved slightly,with an increase in the ultimate tensile strength from 774 MPa to 798MPa;however,plasticity declined from 7.5% to 6.2%.In the stress rupture experiment under645 MPa at 760°C,the stress rupture life was 94 h at a withdrawal rate of 10mm/min,followed by 71 h at 5mm/min,and 58 h at 15mm/min.