Influence of Trace Element and Melt Treatment on Microstructure and Property of K4169 Superalloy

Author:Jie Zi Qi

Supervisor:zhang jun


Degree Year:2018





The integral superalloy castings are widely used in aerospace,aviation industries and other fields.Due to the demand of a structural weight reduction and a high reliability,the integral superalloy casting,represented by the casing in the turbine engine,has been aimed at size maximization,structural complication and light weight,which has brought great challenges to the design of the casting process.These structural features render the large complex thin wall superalloy casting to be prone to misrun,shrinkage and coarse grains during the filling and solidification process.Thus,the qualified rate and mechanical properties of castings are difficult to fulfill their requirements well.Therefore,the control of solidification process and microstructure under the premise of good filling has become a major technical problem for precision casting of large complex thin wall castings.In this thesis,K4169 superalloy is used as the research object.The influence of trace elements and melt properties on the solidification process,microstructure and properties of the alloy is investigated by means of the combination of trace elements regulation and casting process optimization.A new method of precision casting is established by combining the thermally controlled solidification,chemical refining method and melt superheating temperature treatment is proposed to improve filling ability and refine grain size of superalloy castings in this work.The main results of this thesis are as follows:(1)The fluidity test model with variable cross section sizes was designed and established the normalization calculation method.This fluidity test model has the advantages of small size,wide test range,good reproducibility and reflects fluidity at different thickness.(2)The fluidity is greatly improved by adding trace elements B and Zr,subsequently,the casting performance is enhanced.As the boron content in K4169superalloy increases from 48 ppm to 70 ppm,the fluidity is improved by 0.18-0.51times than the master alloy.With the increase of zirconium content from 350 ppm to490 ppm,the fluidity is increased by 0.25-0.53 times than the master alloy.At a pouring temperature of 1470°C,when the content of boron or zirconium is 59 ppm or 420 ppm,respectively,the optimum fluidity is obtained,which is equivalent to raising the alloy pouring temperature to 1550°C.Therefore,the filling ability of the thin wall casting can be improved under the low pouring temperature by adding a proper amount of boron and zirconium to K4169 alloy.(3)Adding a proper amount of B and Zr elements in K4169 alloy can reduce the amount of harmful phases,improve the distribution of carbides,increase the volume fraction of theγ′′phase and improve the mechanical properties of the alloy.As compared with master alloy,the stress rupture life at 650°C/620 MPa can be improved by 1.3 times,reached to 231 h with a boron content of 59 ppm.In addition,the alloy contains 420 ppm zirconium,the stress rupture life at 650°C/620 MPa is 208 h which is increased by 1.1 times of the master alloy.(4)A new thermally controlled solidification process with a low temperature pouring,high mold temperature and sequential solidification is proposed,which solves the problem of complicated structural casting and defect control.Under the conditions of the pouring temperature of 1380°C,the mold temperature of 1290°C and the withdrawal rate of 400μm/s,the characteristic casting of the superalloy with the minimum wall thickness of 1.8 mm was filled perfectly.(5)After adding the mixed refiner of Co3FeNb2 and CrFeNb ternary intermetallic compounds,the grain size under the thermally controlled solidification process is quite finer to than that of the convention casting process.Under the convention casting process,the grain size is refined from 4560μm to 1230μm and the stress rupture life at650°C/620 MPa is increased by 0.54 times.Moreover,using the thermally controlled solidification process with the mixed refiner,the grain size is refined from 3340μm to126μm and the stress rupture life at 650°C/620 MPa improved from 89 h to 201 h which is increased by 1.3 times.(6)A thermally controlled solidification using the melt superheating treatment was designed to achieve a grain refinement and improved the mechanical properties of K4169 alloy.At the superheating temperature of 1680 oC,the grain size is refined to 89μm and the stress rupture life at 650°C/620 MPa reached 242 h,which was approximate two times larger than that of 1550oC.The grain refinement mechanism is attributed to the increase in the nucleation rate after the melt superheating treatment.