| The titanium alloy Ti-Al-4V has been extensively used to manufacture critical components,such as engine blades,impellers,fuselage,wings and landing gear,employed in the aviation and aerospace industries because of its high strength-to-weight ratio,high fracture toughness,good casting properties,excellent corrosion and fatigue resistance,etc.Yet these critical components are often used in extreme environments,and the quality of the component processing directly affects the performance of the entire aircraft structure.In order to meet tolerance requirements or improve surface integrity,ultrasonic-assisted machining techniques have been developed to meet part requirements for surface quality and surface properties.However,similar to other titanium alloys,Ti-6Al-4V is not easy to machine due to its inherent physical and mechanical properties such as poor thermal conductivity and low modulus of elasticity.The thermomechanical load in cutting process has a great influence in the formation of microstructure on the machined surface of titanium alloy,which in turn affects the service performance of Ti-6Al-4V parts.Unlike ordinary cutting technology,ultrasonic-assisted machining technology applies high frequency and low amplitude vibration to the tool or workpiece,which makes the material exhibit stronger plastic activity during machining.However,the plastic effect results in a complex evolution of the microstructure of the processed surface material that differs from the matrix and the formation of the surface metamorphic layer.The microstructure characteristics of the surface metamorphic layer directly affect the service performance of the material.In this paper,longitudinal torsional ultrasonic vibration-assisted milling and general milling tests are conducted on titanium alloy Ti-6Al-4V as the research object.The microstructure of Ti-6Al-4V surface layer of titanium alloy and the generation mechanism and interconnection of surface formation features are investigated to provide data support for ultrasonic vibration-assisted cutting process optimization with the goal of high surface integrity research.(1)Theoretically,the trajectories of tool tip motion during axial and torsional vibrations of milling cutters and torsional vibrations of tools are derived respectively,and the unique periodic separability characteristics in ultrasonic machining are analyzed.Based on the K-M model framework,the physical processes in ultrasonic vibration-induced plastic deformation are clearly reflected by considering the unique acoustic softening effect that can cause a decrease in the forming load during metal forming by applying ultrasonic vibrations.And other physical field parameters are introduced and lead to partial mechanisms or parameter changes to modify the model to obtain an acoustic plasticity theory that is consistent with ultrasonic processing conditions.(2)The two-dimensional ultrasonic finite element model for milling was established based on the theory of trochoid and the effective front angle.And the following analysis focuses on the multi-physical field distribution of the material in the cutting process,including strain,strain rate and temperature.Ultimately,the effect law of ultrasonic amplitude on stress,strain and temperature fields during milling is revealed to assist in the analysis of subsequent experimental phenomena.(3)General milling and longitudinal torsion ultrasonic vibration-assisted milling tests were conducted on the titanium alloy Ti-6Al-4V.Subsequently,scanning electron microscopy(SEM)and electron backscatter diffraction(EBSD)inspection methods were used to examine the microstructure of milled surface metamorphic layers,focusing on the effect of ultrasonic amplitude on the plastic deformation,grain boundaries,grain size and grain orientation of surface metamorphic layers.The experimental results showed the grains in surface metamorphic layer were severely stretched and distorted by the violent action of the tool,resulting in refinement.When the ultrasonic amplitude is increased,the plastic strain on the surface metamorphic layer increases,which leads to an increase in the depth of the strain layer.The special tool-tip trajectory of ultrasonic machining results in a lower cutting heat than the level of ordinary milling,and the surface grains only have dynamic reversion without recrystallization.(4)The surface formation characteristics were examined in detail by means of an ultra-depth-of-field microscope,a surface roughness tester and a Vickers microhardness tester.The orthogonal test results show that the influence of each factor on the surface roughness during the milling process is in the order of feed per tooth f_z>longitudinal torsional amplitude A_l>axial depth of cut ap>cutting speed Vt.The single-factor tests of surface microstructure show Ti-6Al-4V surface forms a regular crater-like vibrational microstructure after milling with longitudinal vibration and torsional virbation.When the longitudinal torsional amplitude reaches 6μm,the surface texture of the machined surface decreases,and the high cutting speed causes serious interference in the tool edge trajectory,resulting in the machined surface no longer showing the fish scale bionan structure.The machined surface after ultrasonic milling is generally machine-hardened,and the depth of the machine-hardened layer obtained within the parameters of this paper is up to 50μm.And the degree of machine-hardening and the depth of the hardened layer of the machined surface after ultrasonic milling are generally higher than those of the machined surface after ordinary milling. |
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