Study On Removal Mechanisms And Surface Damage In Micro-scale Machining Of Network-structed TiBw/TA15 Composites

The high-end aerospace equipment has an urgent demand for high-performance metal matrix composites.Through the idea of microstructure optimization,the titanium matrix composites with good plasticity and strength,such as network-structured Ti Bw/TA15(Ti-6.5Al-2.5Zr-1Mo-1V-0.5Si)composites,have been developed and prepared.These materials can meet the needs of research institutes for materials with high strength,low density,wear resistance,corrosion resistance and oxidation resistance.The networkstructured titanium matrix composites are thought to have a great deal of application potential because of the improved comprehensive performance.The technical issue with preparing Ti Bw/TA15 composites has mostly been resolved as of late.However,the absence of machining mechanisms and guidelines for processing parameters makes it difficult to promote the practical application of such materials.Therefore,it is of practical value and relevance to cover the cutting mechanisms of Ti Bw/TA15 composites and investigate the excellent processing conditions.Ti Bw/TA15 composites have multi-scale and multi-phase structural features.The deformation response of the composites at macroscopic scale is determined by the changes in the fine and microscopic structure and properties.To fill the gap of its research content,it is necessary to provide theoretical and technical guidance from the material removal process to machining quality assessment.And the response mechanism of multiscale material structures should be considered.The main research contents are as follows:An in-situ imaging system is built to present the deformation behavior of the workpiece microstructures during cutting.And a method of image analysis to characterize the plastic deformation in the primary shear zone of segmented chips is proposed and validated.These offer an intuitive and effective technical tool for follow-up studies.In accordance with the requirement of high-quality imaging for micro-scale cutting conditions,an in-situ imaging system is designed to acquire pictures of the kinematic and temperature fields in the cutting zone.And the natural texture of the workpiece is used as the scattering pattern.Combined with the digital image correlation(DIC)technique,the evolution of physical quantities of plastic deformation(i.e.,the equivalent strain rate,equivalent strain,and temperature)in the primary shear zone are statistically and analytically evaluated.Based on the deformation characteristics of segmented chips,a grouping analysis method for sequence pictures is proposed.The results between the empirical model and the developed in-situ image analysis method are compared for the physical quantities of plastic deformation.And the effectiveness of this method is illustrated.The influence of reinforcements and network structure on the material removal mechanism is revealed based on in-situ images during cutting.This analysis is conducted through the fine/microscopic deformation mechanism of Ti Bw/TA15 composites.The investigation laid the foundation for modeling the deformation behavior of the material at the macroscopic scale.The images of the machining process are obtained using the developed in-situ imaging system.And the plastic deformation in the primary shear zone is assessed quantitatively and qualitatively.The study combines the mechanical properties of the material under the multi-scale mechanism,the microstructural morphology of the chips and the finite element modeling considering the microstructure characteristics.In view of the matrix microstructure and the fine/microscopic deformation mechanism,the impact of the reinforcements and network structure on the removal mechanism of Ti Bw/TA15 composites is thoroughly investigatedConsidering the effects of reinforcements and network structure,a constitutive model of Ti Bw/TA15 composites was established,which reflects the relationship between the material structures and flow stress.And the stress field in the workpiece is constructed by combining the in-situ imaging analysis and the deformation response of component materials.In this way,the constructed stress field is more comparable with the realistic deformation characteristics of Ti Bw/TA15 composites.Combining the microstructure and properties of Ti Bw/TA15 composites,a constitutive model including the internal physical parameters of reinforcements and network structure is proposed.It can quantitatively characterize the effective strengthening effect generated by the network-structured reinforcements.And the equivalent flow stress of the reinforcements and the matrix as well as the overall flow stress are quantitatively characterized and qualitatively analyzed using the proposed model.The results of in-situ imaging analysis are also used as input quantities.And the establishment of stress field is based on the slip line field theory and material removal mechanism.As a result,the stress field in workpiece under different machining conditions is constructed and analyzed by deducing the deformation in the primary shear zone.The types of machining damage in the workpiece are investigated,and the effects of microstructures on surface damage morphology and microhardness in the subsurface are analyzed.The processing parameters for obtaining a low-damage surface layer in the workpiece by micro-milling are explored.Considering the effect of reinforcements and network structure on the macroscopic cutting performance under the fine/microscopic mechanisms,the distribution of machining damage and formation reasons are revealed.Considering the thermoplastic deformation characteristics of the matrix during machining,the variation of microhardness along the depth direction is predicted and analyzed from the mapping relationship between depth-theoretical yield strengthmicrohardness.The established mathematical relation is based on the constructed stress field in the workpiece.The effects of microstructures and processing parameters on the formation of surface damage in micro-milling are taken into account.The results preliminarily explore the processing conditions for micro-milling to obtain a low-damage surface layer.