Study On Microstructure Evolution Induced By Grinding Machining Single Crystal Parts Of Aeroengine

Because of its high strength and excellent creep,fatigue and corrosion resistance at high temperature,nickel-based single crystal superalloys are often used in hot-end parts of aeroengines.The traditional advantages of grinding finish machining can well meet the needs of high precision,high quality and high machining efficiency of single crystal parts under special working conditions.During the whole process of single crystal material from blank to part,the surface damage caused by external factors is unavoidable,which directly affects the performance of the part.At present,the research on single crystal superalloy mainly focuses on the process test,casting and heat treatment process,and the effect of rare element content on the properties of the alloy,however,a series of microstructure evolution caused by machining and its effect on the properties of alloys are seldom reported.So in this paper,from the microscopic point of view,the relationship between the microstructure of single crystal DD5 and the grinding performance,the evolution of the sub-surface microstructure induced by grinding and the evolution of the sub-surface microstructure under high temperature environment were exploratory research.The research results of this paper will further improve the single crystal superalloy grinding process theory and technical system,and provide some guidance for the actual processing of single crystal parts.The main research contents are as following:(1)The single crystal superalloy exists in the form of γ’ reinforced phase coherent embedded on the y matrix phase,and the kinds and proportions of the constituent elements have a great influence on the properties of the alloy.The unique anisotropy of single crystal superalloy is reflected in the difference of elastic modulus and Shear Modulus,which leads to the difference of mechanical properties.The difficult workability of single crystal superalloy will lead to excessive grinding force,grinding temperature and tool wear.But the traditional"three-stage" grinding theory is still applicable to single crystal superalloy,which makes the study of single crystal superalloy grinding theory feasible.(2)The relationship between the microstructure and the machinability of single crystal superalloy is studied based on the influence of different grinding conditions.Firstly,the simulation and theoretical grinding models of grinding force and grinding temperature are established,and the empirical formulas of both are obtained based on the results of orthogonal experiments.Secondly,the effects of grinding parameters and single crystal orientation on the grinding performance are summarized according to the experimental and theoretical values.Based on the microstructure and atomic distribution of the orientation of DD5 single crystal,the variation of Elastic modulus and shear modulus of each orientation is analyzed.At last,the contrast test shows that the processability of as-cast DD5 and heat-treated DD5 is different because of the difference of original microstructure.(3)The subsurface microstructure of single crystal superalloy changed obviously after grinding.Firstly,the influence of grinding parameters on the thickness of the damaged layer and "white layer" is quantitatively characterized,and the correlative models of grinding force,grinding temperature and the thickness of the damaged layer and "white layer" are established.Then,the changes of sub-surface microstructure induced by other machining conditions were analyzed,including the orientation of single crystal,the original state of base metal and the wear degree of wear Rod,the effects of above factors on the thickness of damaged layer and"white layer" are summarized.Finally,the element distribution and hardening degree of the subsurface damage layer are analyzed.(4)The subsurface precipitates and recrystallization of single crystal superalloys after grinding were studied by heat treatment at high temperature to simulate its working environment.Firstly,the types and elements of carbides and surface eutectic on single crystal superalloys at high temperature were analyzed.Secondly,the effects of heat treatment temperature and time,grinding parameters,single crystal orientation and original microstructure on recrystallization type and thickness are discussed,and the nucleation position of recrystallization was explored.Finally,the control measures of recrystallization are put forward based on the experimental results.