Research On Cutting Mechanism Of In-situ TiB₂ Particle Reinforced Aluminum Matrix Composites

The aviation turbofan engine is consistently praised as the“crown jewel”of modern industrial technology.It is a strategic high-tech product related to national military security and national economic development,which is also one of the bottlenecks restricting the deveoplment of our aviation industry.The modern aviation turbofan engine is going towards the direction of high bypass ratio,large thrust and low fuel consumption.Particle reinforced aluminum matrix composites would probably play a vital role in weight-reduction for the next generation of aviation turbofan blades due to its superior properties such as low density,improved strength,wear resistance and so on.As a new kind of particle reinforced aluminum matrix composites,the in-situ TiB₂particle reinforced aluminum matrix composites have gradually become a research hotspot due to the advantages in particle size,particle distribution,particle-matrix interface properties and better mechanical properties.At present,to particle reinforced aluminum matrix composites,the researches on cutting mechanisms are mainly focused on ex-situ Si C particle reinforced aluminum matrix composites?Si C_p/Al MMCs?.Due to the difference of particle size and particle-matrix interface properties between in-situ TiB₂ and ex-situ Si C particles,these two materials,in-situ TiB₂/Al MMCs and ex-situ Si C_p/Al MMCs,show completely different machining performance.This means that the previous cutting mechanism of aluminum matrix composites could not be perfectly suitable for this new kind of in-situ TiB₂/Al MMCs.For this,this paper would carry out a set of researches on chip formation,modeling of cutting force and temperature,surface integrity and fatigue behavior in order to have a deep understanding of the machining performance and cutting mechanism of in-situ TiB₂/7050Al MMCs,which has been chosen for a type of turbine blade.The studies of this paper would be of great significance to bulid the cutting mechanism theory of in-situ TiB₂/Al MMCs.Meanwhile,it could provide favorable theoretical foundation and technical support for engineering application of this material.The main work,research conclusions and innovation points of this paper are listed as following:1)Chip formation mechanism of in-situ TiB₂/Al MMCs:the J-C constitutive equation of TiB₂/Al MMCs,which is needed for the cutting process simulation model,is reversely obtained from the basic mechanics of orthogonal cutting under very high strain rate.Then,the cutting process simulation model is built to investigate the stress,strain,strain rate and temperature in the shear zone,which could be hardly observed and measured during experimental process.Based on this,the chip formation mechanism of TiB₂/Al MMCs is studied and the effect of cutting parameters on chip geometric parameters is also analyzed.2)Modeling of cutting force and temperature of in-situ TiB₂/Al MMCs:based on chip formation conclusion,metal cutting theory and ex-situ Si C_p/Al MMCs cutting mechanism,an orthogonal cutting force model of TiB₂/Al MMCs is established to further study the cutting mechanism with taking into consideration the cutting performance of this material.Then,the dynamic milling force model is built for engineering application with calculated dynamic cutting force coefficients,detailed cutting conditions and complex tool geometric parameters.With this,a cutting temperature model for workpiece surface macnhined by the bottom cutting edge of flat end mill is presented based on the moving heat source theory.At last,the influence of cutting parameters on surface cutting force and temperature is deeply investigated.3)Surface integrity of in-situ TiB₂/Al MMCs:a set of surface integrity experiments is carried out to investigate the influence of cutting parameters on surface roughness,residual stress,surface hardness and machining defects.With test data,the surface integrity prediction model is proposed.The surface roughness,machined surface residual stress,residual stress along the subsurface depth direction,machining hardening effect is studied.Meanwhile,with analyzing the effect of cutting force and temperature,the thermal-mechanical coupled effect on the surface integrity is deeply analyzed.4)Fracture mechanism and fatigue life prediction modeling of in-situ TiB₂/Al MMCs:the function of surface roughness and topography on stress concentration and fatigue life is detailed studied with the help of surface integrity conclusion,surface topography reconstruction and finite element simulation.The stress concentration model is modified and a fatigue prediction model of TiB₂/Al MMCs is built to deeply investigate the stress concentration and fatigue life.The fracture morphology is observed and the fracture mechanism of TiB₂/Al MMCs is studied.