Influence Of Laves Phase On The Mechanical Properties Of GH4169 Superalloy Fabricated By Laser Solid Forming

The improvement of the thrust-to-weight ratio of the aeroengine is of great importance to the design of the new type of fighter.In order to improve the thrust-to-weight ratio,the hot-end components of advanced aeroengines are usually made of nickel based superalloy with excellent comprehensive mechanical properties.GH4169,which is also known as Inconel 718,is one of the most widely used nickel based superalloys.It is typically employed to make integral parts with high performance and high value,such as blisks.However,when such an integral structure is mis-machined during the manufacturing process or damaged by a foreign object in serve,whole-rollscrapping is at risk,which causes massive losses in time and economy.Laser solid forming(LSF)technology,which has the advantages of controllable heat input,small heat affected zone,small deformation of parts and fine microstructure of laser cladding zone,etc,provides an effective way to restore both the geometrical and mechanical properties of the damaged componenets.The GH4169 component repaired by LSF technology is composed of laser cladding zone and forging substrate zone.Fracutre always occurs in the laser cladding zone when the laser repaired parts are under load.The large-scale Laves phase in the interdendritic region of laser cladding zone is the main reason causing failure during tensile,stress rupture and fatigue process.In generally,high temperature solution heat treatment is used to eliminate the hard and brittle Laves phase.However,this kind of heat treatment can lead to the deterioration of the forging substrate zone.In this case,Laves phase has to be preserved in the microstructure.Actually,Laves phase with excellent high temperature mechanical properties is not necessarily a deleterious phase.Its effect on the mechanical properties of GH4169 superalloy is closely related to the morphology and size.It is important to deeply understand the role of Laves phase in laser solid formed(LSFed)GH4169 for improving the comprehensive mechanical properties of laser repaired components.In the present study,the deformation behavior of Laves phase under different loads,the regulation of Laves phase and the influence mechanism of Laves phases with different characteristics are investigated.The results will promote the application of LSF technology in the field of repairing,and establish an important science and technology foundation for the development of new nickel based superalloy.The main conclusions are shown as follows:(1)The failure behaviors of double aged LSFed GH4169 superalloy during room temperature tensile,high temperature tensile and high temperature stress rupture process have been investigated.The deformation behaviors of Laves phase under different loads have been analysed.The results show that the deformation behavior of Laves phase mainly composes of fragmentation and debonding.And Laves phase has different deformation behaviors under different loads.During room temperature tensile process,Laves phase mainly breaks up into smaller ones.In the process of high temperature tensile test,both fragmentation and debonding are present.In addition,debonding holds a dominant position under high temperature stress rupture load.(2)The factors influencing the deformation behavior of Laves phase have been studied.Based on fiber loading mechanism,a fracture model about the fragmentation of Laves phase during room temperature tensile process has been established,i.e.,?min/Tmin=(1/A)(?)(where A represents the length-width ratio of Laves phase,L represents the length of Laves phase,? represents the angle between the long axis direction of the Laves phase and the applied load;?*min represents a referred shear stress when A=1,L=1 ?m,?=0°).According to the model,it can be seen that the morphology of Laves phase has important effect on its fracture.Long-striped Laves phase is much easier to break up than granular Laves phase.As for the Laves phases with the same characteristic,the fragmentation is closely related to the applied load.The higher the applied load,the more easily Laves phase breaks.On the other hand,the separation of Laves phase and the matrix can be affected by both the applied load and the test temperature.When the applied load holds the same,the higher the temperature,the more easily the debonding occurs.(3)The effect of solidification conditons on the formation of Laves phase has been investigated.The solidification rate R and the temperature gradient G affect the segregation degree of Nb element and the dendrite morphology,and then influence the size and morphology of Laves phase.Based on Kurz-Fisher model and the stable temperature distribution model in the molten pool,a selection map of Laves phase morphology has been established.According to the map,the formation interval of granular Laves phase has been obtained:G·R?104 K/s&G/R?107 K·s/m2.(4)The dissolution behavior of Laves phase during heat treatment has been studied.The results show that the initial dissolution of Laves phase is mainly a change in morphology.It changes from long-striped shape to granular shape.However,at the late stage of dissolution,the main manifestation is the reduction of the size.The dissolution kinetic mechanism of Laves phase has also been investigated.At the initial stage,both the long-range diffusion of Nb and the interfacial reaction control the dissolution process.With the degree of Nb segregation decreases,only the interfacial reaction holds the dominant position.(5)Laves phases with different characteristics(morphology,size,volume fraction,etc.)have been got by three kinds of heat treatments.The influence of Laves phases with different characteristics on the mechanical properties of LSFed GH4169 has been studied.The results show that LSFed GH4169 superalloy with a certain amount of granular Laves phase(S-15 sample)has the best room temperature tensile property,high temperature tensile property and high temperature stress rupture property.The high temperature stress rupture life(65 h)of this kind of sample exceeds the experimental value of forge GH4169(51.25 h).(6)Based on dislocation theory,a yield strength model suitable for LSFed GH4169 superalloy has been developed.The model is not only applicable to the case of homogeneous distribution of the strengthened phase,but also to the case of non-uniform distribution of the strengthened phase.According to the model,the effect of the Laves phases on the room temperature tensile property can be achieved through influencing the volume fraction,the size and the distribution of ?" phase.(7)The cooperative mechanism of Laves phase and ?" phase in the process of high temperature loading has been preliminarily revealed.The fine dispersed ?" phase may hinder the recovery softening(dynamic recovery and dynamic recrystallization)of LSFed GH4169 superalloy under high temperature loading conditions.The existence of appropriate granular Laves phase can decrease the density of ?" phase,induce the formation of subgrain and promote the occurrence of dynamic recrystallization.As a result,the high temperature mechanical properties of LSFed GH4169 can be improved.