| Dynamic constitutive model and data of materials are the integral parts of high-end-manufacturing-oriented advanced databases.Modern cutting technologies are characterized by high-speed,high-precision,and new types of difficult-to-machine materials,especially with the widespread use of third-generation high-strength steels and ultra-high-strength steels,and high specific strength composites such as SiC-reinforced aluminum matrix(SiC_p/Al)composites.The cutting mechanics,surface quality and tool wear in high-speed and precision machining of these difficult-to-cut advanced materials need to be further investigated.The material removal in cutting process is often accompanied by extreme deformation under large strain,high strain rate and high temperature.And hence for realizing the high-speed and high-precision machining technology of these difficult-to-cut materials and promoting the development of advanced machining theories and technologies,it is of importance to bring the deep investigation on the material dynamic mechanical behaviors into the further exploration of cutting mechanisms.This paper is focused on the SiC_p/Al composites,which are used as the important parts in aerospace and automotive industries.And the research aspects include parameters identification of high-speed-machining-oriented material constitutive model,dependence-based integrated phenomenological constitutive modelling,multiscale mechanical behavior of SiC_p/Al composites based on realistic microstructure,high-speed milling and drilling of strongly discontinuous SiC_p/Al composites,and tool wear mechanisms in cutting SiC_p/Al composites.The main research contents are as follows:1.A weighted multi-objective optimization method for identifying the parameters of high-speed-machining-oriented constitutive model was proposed to solve the existing problems of high experimental cost and low accuracy in using the traditional parameter identification methods,as well as the data fluctuation caused by poor signal-to-noise ratio at high strain rates.In this method,the quasi-static and dynamic mechanical data of composite materials are used to establish a multi-objective optimization function with the measurement error-based weighting factors under different loading conditions,and by combing the Levenberg-Nielsen algorithm,the constitutive model of Al6063/SiC_p/65p composites is identified reversely.Finally the fast and accurate parameter identification of the constitutive models is arrived at.The reliability of the weighted multi-objective parameter identification method of constitutive model based on the measurement errors is verified by the comparison between the experimental and simulation results of cutting forces and chip morphologies in small-hole drilling.2.A dependence-based integrated methodology,together with an improved weighted multi-objective parameter identification strategy was presented for the development of phenomenological constitutive model and the parameter identification under the criterion of fitting quality,and the instantaneous strain rate variations and plastic strain-induced temperature changes in the material deformation were considered and compensated.This method can lower the uncertainty and non-uniformity in the development of the material constitutive model.The feasibility of this methodology was verified by the development of a constitutive model with damage evolution in compression for SiC_p/Al6061 composites.And the corresponding material subroutine was developed and successfully applied in 2D turning simulation of Al6061/SiC_p/30p composites.The effectiveness and reliability of Al6061/SiC_p/30p composites’constitutive model developed by this dependence-based integrated methodology was verified through the comparison of the numerical and experimental results of chip morphologies and cutting forces at various cutting speeds.3.A hierarchical multi-scale micromechanics model from discrete atomic scale to continuum scale was developed for the inverse determination of the relationship between the microstructure and constitutive properties of strongly discontinuous SiC_p/Al composites.To this end,a real-microstructure-based finite element modeling methodology by mapping the image pixel of the microstructure into the finite element mesh was proposed and realized in the real-microstructure-based modelling of SiC_p/Al composites.The interfacial temperature-related mechanical behavior of Mode I tensile failure and Mode II shear failure was studied for SiC_p/Al composites by molecular dynamics simulations.And a cohesive zone model for characterizing interfacial debonding was defined to correlate the interfacial stress to the opening displacement.Based on Taylor’s nonlocal theory of plasticity,the strengthening of Al matrix caused by thermal-expansion mismatch in quenching and elastic-modulus mismatch in plastic deformation were considered.The comparison of stress-strain curves from the dynamic mechanical experiments and micromechanics simulations shows that the hierarchical-multiscale-micromechanical real-microstructure-based model of SiC_p/Al composites can accurately predict the dynamic mechanical behaviors.On this basis,the micro-damage evolution at different strain rates are analyzed,and this laid the foundation for the theoretical analysis about the dominating role of the non-compatibility mechanism in the formation of machined SiC_p/Al composites surface by high-speed machining.4.The experimental study,numerical simulations,and theoretical analysis were carried out for high-speed milling and drilling of Al6063/SiC_p/65p composites.Based on the milling process parameters and tool geometry parameters,a milling force prediction model was established for high-speed cutting of Al6063/SiC_p/65p composites by incorporating the high-speed-machining-oriented plastic constitutive model and combining Waldorf’s slip-line field model and the improved Armarego oblique cutting model.It is assumed that the formation of the machined surface and subsurface damage and the effects of cutting parameters are caused by the change of the hydrostatic pressure and SiC-particles removal mode,the high-strain-rate-induced localization of strains and damages,and geometrical non-compatibility in high-speed-milling.Compared to PCD brazed drills,CVD diamond coated drills are more suitable for the drilling of strongly-discontinuous SiC_p/Al composites,due to their stable cutting forces,less tool wear,and stable drilling quality.And the hole-edge defects were analyzed based on the drilling FE simulations and fracture mechanics.5.The main wear mechanisms of diamond tools in machining Al6063/SiC_p/65p composites containing 1.51 wt%Cu was studied.The abrasion induced by high-frequency scratching of SiC particles and the graphitization of diamond activated by cutting thermodynamics are the common wear forms of PCD brazed and CVD diamond coated drills.The graphitization mechanism of diamond tools in cutting SiC_p/Al composites is assumed that under the combined action of Copper catalysis and machining-induced temperature,the desorption of chemisorbed hydrogen on the diamond surfaces promotes the transformation of diamond into graphite.Under the hard SiC particulates’high-frequency scratch and impact,the newly formed thin graphite layer is quickly scraped off.This reciprocating actions of the oxidation of hydrogen chemisorbed on diamond surface with copper oxides and SiC particulates’high-frequency scrape on graphite flakes led to the continuous graphitization of diamond.Under the catalysis of Copper,the temperature over 500°C together with pressure below 15 GPa is a key requisite for diamond-graphite phase transformation.An attempt was made to predict the wear evolution of PCD brazed and CVD diamond coated drills in drilling Al6063/SiC_p/65p composites by using a 3D thermo-mechanical drilling FE model,together with a coupled abrasive-graphitization wear rate model based on the dominant wear mechanisms. |
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