| In this dissertation,the grain continuators(GCs)with the transfer structure(the shape of cross-sections,the size of cross-sections,and the spatial angles of GCs),the extended structure(the flare angle and thickness of GCs),and the convergent structure between primary crystals and GC crystals(horizontal convergence span and directional solidification height of GCs)were designed based on the spatial growth paths of grains,the transfer pattern of crystallographic orientation,the convergent structure of crystals from different growth paths,and the characteristics of directional solidification process of the gas turbine single crystal(SX)blades.The GCs with transfer structure,extended structure,and convergent structure SX castings were prepared by a 1st generation Nibased SX superalloy DD413 and a 3rd generation Ni-based SX superalloy DD33 and cooperated with a Bridgman high-rate-solidification technology(HRS)with different withdrawal rates.Optical microscopy(OM),scanning electron microscopy(SEM),and electron backscattered diffraction(EBSD)were used to investigate the characteristics of crystallographic orientation evolution and solidification defect formation in SX castings.ProCAST software were used to simulate the solidification process of different structures castings,and further to analysis the evolution mechanism of crystallographic orientation and the formation mechanism of solidification defects in different characteristic castings.According to the results of relevant research,large SX turbine vane was successfully prepared by the GC process and the optimized directional solidification process parameters.The main conclusions in this dissertation are as follows:The crystallographic orientation of SX castings was equal to the statistical average value of all the individual dendrite orientations in the selected cross-section.The evolution rule of both the primary and secondary orientations of the SX GCs with transfer structure along the directional solidification direction fluctuated irregularly within a small range(by less than 4°)at the withdrawal rates of 3 and 6 mm/min.Compared to the withdrawal rate of 3 mm/min,at the withdrawal rate of 6 mm/min,the liquid isotherms in the mushy zone became more concave and inclined,the difference of the actual growth directions of dendrites at different locations increased,the competitive growth between dendrites became more intense,and the fluctuation range of crystallographic orientation increased slightly.In addition,the fluctuation range of the crystallographic orientation along the solidification direction can be controlled within 4° by designing the GCs with transfer structure reasonably(maximum φ10 mm for GC rod and 8 mm×10 mm for GC stick with the rectangular cross-section).The crystallographic orientation fluctuation range of the inclined GCs with transfer structure was slightly larger than that of the vertical GCs with transfer structure.As for the GC stick with the 8 mm×10 mm cross-section and the 15° deviation angle at the withdrawal rate of 6 mm/min,the undercooling nucleation occurred at the corner of cross-section,and further to form stray grain(SG)defect.For the extended structure GCs with different flare angles(46°,68°,86° and 99°),as the increase of the withdrawal rates,the liquid isotherms in the mushy zone became more concave,the more concave isotherms in the mushy zones enlarged the difference of actual dendrite growth directions on both sides of specimens,which led to the increase of the[001]misorientations between both sides of the specimens.Especially for the large flare angles(86°～99°)specimens,the curvature of liquid isotherms in the mushy zone was larger and the dendrite grown along both sides of the specimens branched more frequently,which increased the misorientation between both sides of specimens.As the thickness of GCs with extended structure increased from 2 mm to 10 mm,the crystallographic orientation on both sides of the specimens was different caused by the more concave liquid isotherms in the mushy zone.With the increase of the flare angle and thickness,the heights of mushy zones of specimens increased to promote the solute segregation between dendrite cores and interdendritic regions as well as the intensity of thermosolutal convection in the interdendritic regions of mushy zones,which increased the probability of the freckle defects formation at the start position of the GCs with extended structure.As for different thicknesses GCs with extended structure,at 1 mm/min withdrawal rate,the cooling rate in solidification front was lower,and the primary dendrite arm spacing(PDAS)was larger,which caused a strong thermosolutal convection in the interdendritic region of mushy zones.The freckle chains formed at the inside corner of triangular GCs with a low flare angle(46°)owing to the impact of lateral feeding flow on the weak primary dendrite in the mushy zone of solidification front.However,as for the large flare angle(68°～99°)specimens,the height of the mushy zone was larger and the isotherms in the mushy zone was more inclined,the broken dendrite arms caused by the lateral feeding flow might be remelt,which might be the reason that the freckle defect was not observed in the large flare angle specimens.According to the results of this work,the optimal process parameters of the GCs with extended structure could be obtained as follows:46°～68° flare angle,2 mm sample thickness,and 3 mm/min withdrawal rate.The SX rejoined platform with large span size(100 mm)at high solidification height(220 mm)provided more complex solidification conditions(the lower temperature gradient and the larger undercooling)for dendrites growth than that with small size span(40 mm)at low solidification height(40 mm)during the process of convergence between primary crystal and GC crystal in the SX rejoined platform,which reduced the stability of dendrite growth and increases the tendency of forming solidification defects.Under 1 mm/min withdrawal rate,low growth rate of solidification front,low cooling rate and large dendrite arm spacing(DAS)in the SX rejoined platform were obtained,which enhanced the intensity of the thermosolutal convection in interdendritic areas.The intense thermosolutal convection pinched off the dendrite arms and formed dendrite fragments,which further evolved to the freckles defect finally;Under 6 mm/min withdrawal rate,the interdendritic turbulent fluid in the initial lateral growth region of SX rejoined platform at a low solidification height occurred due to the local chaotic horizontal temperature gradient distribution.The turbulent fluid impinged on the slim and weak secondary dendrite arms,which resulted in the formation of deviation dendrites.Besides,the bending deformation of long lateral growing dendrites caused by the asymmetrical contraction stress reduced the stability of crystallographic orientation evolution,and finally formed the sliver and LABs defects in the SX rejoined platform.The large(300 mm length,150 mm width)SX double vanes without solidification defects can be prepared by the "transfer+extended" structure GCs with the optimized size(the width of transfer structure GC was 3 mm,the thickness of transfer structure GC was 2 mm,the flare angle of extended structure GC was 68°,the height of extended structure GC was 20 mm,and the thickness of extended structure GC was 2 mm)and the reasonable directional solidification process(3 mm/min withdrawal rate).Moreover,the misorientation of convergent interface between primary crystal and GC crystal was less than 3°. |
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