Study On Process Optimization And Properties Of Composite Modified Shell For Single Crystal Directional Solidification

With the increase of refractory element added into the nickel-based single crystal superalloy,the initial melting temperature has been being improved,and the ceramic shell used for the nickel-based single crystal blade is put forward higher requirements.Therefore,the preparation of the "thin-walled and high-strength" ceramic shell is the key technology to restrict the development of new generation single crystal turbine blades.In this paper,composite modified shells were prepared by using mineralizer and short-carbon fiber,the effects of the content of mineralizer,the content and length of short-carbon fiber on green strength,sintering strength and high temperature gravitational distortion of shell sample were studied by orthogonal experiment.The effects of sintering temperature on the sintering strength and resistance to deformation of shells sample were investigated,the bonding strength between the shell sample and the ceramic cores with different formulations and sintering processes were investigated.Finally,the temperature field at the edge of composite modified shell is simulated by ProCAST software,and the effect of the composite modified shell on the macro-structure and the micro-structure of single crystal casting of the new Ni-based superalloy with low Re was investigated by directional solidification experiment.Study on the mechanical properties of the shell showed: The content of short-carbon fiber was the greatest,the length of short-carbon fiber was medium,the content of mineralizer contributed minor in the three factors affecting green strength of shell sample,optimization of the formulation were obtained successively 0.5g/ml,3mm and 2%,compared with the ordinary fused corundum shell,the green strength of the composite modified shell sample was increased from 1.02 MPa to 1.58 MPa.The content of mineralizer was the greatest,content of short-carbon fiber was medium,and length of short-cut carbon fiber contributed minor in the three factors affecting sintering strength of specimen,optimization of the formulation were obtained successively 6%,0.5g/ml and 3mm,compared with the ordinary fused corundum shell,the sintering strength of the composite modified shell sample was increased from7.79 MPa to 11.63 MPa.The content of short-carbon fiber was the greatest,length ofshort-carbon fiber was medium,and content of mineralizer contributed minor in the three factors affecting the high temperature gravitational distortion,optimization of the ratio were obtained successively 6%,5mm and 0.3g/ml,compared with the ordinary fused corundum shell,high temperature gravitational distortion of the composite modified sample was decreased from 1.02 MPa to 1.58 MPa.Study on the sintering temperature of the shell and the bonding strength between the shell and the ceramic core showed: the shell samples were prepared by the optimized formulation of the mineralizer and short-cut carbon fiber,the sintering strength,high temperature gravitational distortion and linear shrinkage of the samples increased with the increase of sintering temperature of 1000?,1100? and 1200?,respectively.When the sintering process was the same,since both sides of the interface matrix properties compatible retained low residual stress,interface bonding strength of alumina ceramic core and sample was greater intensity.From the sintering process we can see that after been secondly sintered,the interface structure was densification and the porosity was lower,interface bonding strength of the of the ceramic core and sample with secondary sintering got greater.Numerical simulation and directional solidification experiments showed:Compared with the normal shell,in the numerical simulation,the composite modified shell did not appear super-cooling phenomenon at the platform of the blade,which reduces the possibility of the formation of the mixed crystal.Wall thickness of Composite modified shell is thinner and more uniform,speed up the cooling effect of shell.Macroscopically,mixed crystal had decreased on platform of the blade;microscopically,the dendrite spacing of primary dendrite and secondary dendrite were 274.2?m and 88.7?m,respectively.Dendrite of single crystal casting presented more fine,Eutectic phase ?-??of single crystal casting located in dendrite with smaller size and discontinuous.