Influence Mechanism Of Material Deformation And Fracture Behavior On Chip Formation During High Speed Machining

With the rapid development of high speed machining tools and advanced cutting tools, high speed machining technology begins to be used in such fields as automobile manufacturing, aerospace, and national defense industry, etc. Compared with traditional speed machining, large strain rate loading caused by high speed machining leads to drastic variation of dynamic mechanical properties for workpiece materials, which will further induce the transition of chip formation mechanism and chip morphology. With the cutting speed increasing, the chip morphology for ductile metals will change from continuous chip to serrated chip, and then to completely separated serrated segments. The chip morphology will become fragmented like that of brittle materials for ductile metals under ultra high cutting speed. Previous researches about chip formation in high speed machining mainly concentrate on the critical condition for serrated chip formation and geometrical characterization of serrated chip, while it still lacks deep insights into the influence mechanism of material dynamic properties on serrated chip formation in addition to the material deformation and fracture behavior during fragmented chip formation. Metal cutting can be regarded as purposeful fracture between chip and workpiece material. Investigation of chip deformation and failure mechanism for serrated and fragmented chip formation during high speed machining can not only help to instruct the optimization of cutting parameters in order to realize the high efficiency and low energy consumption machining, but also lay a theoretical foundation for design of machining tools and cutting tools.In this thesis, such three metals as Ti6Al4V, Inconel 718 and 7050-T7451 are selected as the research objects to investigate the formation mechanism of serrated and fragmented chips. The research methods including analytical studies about material mechanics and metal cutting theory, finite element simulation, experimental researches about metal cutting and chip microstructure observation are carried out. The research emphasis is put on the influence mechanism of material dynamic mechanical properties, especially for the ductile-to-brittle transition of ductile metals, on the chip deformation and fracture behavior under ultra high cutting speeds. The main contents of the thesis include:the formation process of chips with different morphologies, the critical cutting condition for fragmented chip formation, the sensitivity analysis for the influence of workpiece material properties on serrated chip shear localization, the influence mechanism of stress state on fracture locus of serrated chip, and the analysis of cutting energy consumption characteristics for chip formation process during high speed machining. Through the research in this thesis, it is expected to build the bridge between the research of material dynamic mechanical properties under large strain rate and the research of high speed machining, which will lay a theoretical foundation for revealing the high speed machining mechanism and promoting the application of high speed machining technology.Firstly, the chip formation processes during high speed machining are investigated and the variations of chip morphology for different workpiece materials with the cutting speed increasing are researched. Then the chip deformation and fracture mechanism under different cutting speeds will be revealed. Meanwhile, the formation models for serrated and fragmented chips are developed. Through the microstructure observation for different locations on chips including the chip cross section, chip free surface and chip fracture surface, etc, the chip deformation and fracture mechanism for three workpiece materials can be researched. Based on the formation characteristics of serrated chip, the mixed mode of adiabatic shear and ductile fracture is proposed to explain the serrated chip formation mechanism. The formation mechanism of fragmented chip formation is brittle fracture. According to the ductile-to-brittle transition behavior for metals under large strain rate loading, the critical criterion for fragmented chip formation under ultra high cutting speed is developed with the application of stress wave propagation theory. Then the critical cutting condition for fragmented chip formation is determined. Based on the chip morphology and its formation mechanism, the cutting range can be divided into conventional speed machining, high speed machining, and ultra high speed machining.Secondly, the finite element simulation model for serrated chip formation during high speed orthogonal cutting is developed, based on which the influence sensitivity of material properties on serrated chip shear localization is investigated. The influence and control mechanism of intrinsic variables for workpiece material (including material mechanical properties and material damage parameters) on chip morphology variation during high speed machining is revealed. The evolution of chip morphology under different cutting speeds is simulated and characterized geometrically by chip serrated degree and chip serrated frequency. Meanwhile, the reliability of the finite element model is validated using high speed orthogonal cutting experiments. Through changing the constitutive model parameters and damage model parameters of workpiece material, the variation characteristics of chip morphology under different material properties are investigated. The sensitivity parameters of chip serration and chip curvature variation are proposed as the evaluation indexes to characterize the influence of material properties on serrated chip formation quantitatively. The results show that the initial yield stress and thermal softening coefficient are the most dominated constitutive parameters that affect the serrated chip shear localization, while the initial failure strain and exponential factor have the greatest influence on the chip serration among the damage model parameters.Thirdly, the distribution model of normal stress within the primary deformation zone for serrated chip formation during high speed machining is established, which aims to reveal the distribution law of stress triaxiality within the primary deformation zone. The chip deformation within the primary deformation zone of serrated chip can be regarded as the material deformation and failure problem under the combined loading of constant shear and gradient tension/compression. The modified Bao-Wierzbicki fracture strain model considering the effects of strain rate and temperature is developed. Through comparison between equivalent plastic strain during chip formation and material fracture strain, the fracture locus of serrated chip is determined. Furthermore, the evolution of serrated chip fracture loci under different cutting speeds is discussed. The results show that the normal stress within the primary deformation zone distributes heterogeneously. The upper part of primary deformation zone near the chip free surface belongs to the tensile stress zone leading to the positive stress triaxiality, and the stress triaxiality presents linear distribution pattern. The lower part of primary deformation zone near the tool tip belongs to the compressive stress zone leading to the negative stress triaxiality, and the stress triaxiality presents power function pattern. The upper part of primary deformation zone near the chip free surface deforms under the combined loading of shear and tensile, which results in the ductile fracture mode on the chip fracture surface. The lower part near the tool tip deforms under the combined loading of shear and compression, which results in the shear fracture and shear dimples can be seen on the chip fracture surface. With the cutting speed increasing, the expansion of tensile stress zone is the essential factor that causes the enhancement of crack propagation within the adiabatic shear band and the promotion of chip serration.Finally, the models of cutting energy dissipation for serrated and fragmented chip formation are developed. The variations of cutting energy dissipation due to different chip deformation behavior under different cutting parameters are explored. The cutting energy models are validated using the parameters of cutting force and acoustic emission signals. The results show that the cutting energy consumed for serrated chip formation mainly includes the plastic deformation energy within the primary deformation zone, the friction energy between tool-chip interface and the chip kinetic energy. Comparatively, the cutting energy consumed for fragmented chip formation mainly includes the chip fracture energy and the local kinetic energy. During the high speed machining period, the cutting tools with larger rake angles and larger uncut chip thicknesses are beneficial to reducing the cutting energy dissipation. The formation of fragmented chip helps to implement the brittle regime machining of ductile materials, which can cut down the cutting energy substantially. It demonstrates the advantages of high efficiency and low energy consumption for ultra high speed machining. The intensity of acoustic emission signal is affected by the cutting energy dissipation, which is determined by the mechanical properties of workpiece material and the cutting parameters simultaneously.