基于Bauschinger效应的逆向精切削变形机理

基于工件材料Bauschinger效应,从已加工表面变质层的形成入手,建立了逆向精切削法的切削变形模型。利用金相显微试验技术对比研究了正、逆向精车削条件下工件材料晶粒流动情况,用位错理论对可逆向精切削的变形机理进行了论述。研究表明,可逆向切削时,可以采用首切时正向切削,复切(半精加工或精加工)时逆向切削的工艺路线,这样既可提高加工效率和加工精度,又可减小工件表面变质层。     

不同冷却润滑条件下TB6钛合金高速切削表面完整性研究

通过TB6钛合金高速铣削试验,测量观察加工表面粗糙度、表面三维形貌和表层微观组织等表面完整性特征,利用极差法分析切削参数对表面粗糙度影响的显著性,探讨冷却润滑条件对加工表面形貌和表面变质层的影响。研究表明:工艺参数对表面粗糙度影响程度依次为径向切深、切削速度、进给量和轴向切深;相比低温冷风加,微油雾润滑加工时钛合金表面粗糙度低,且表面无明显晶粒变形,表明加工表面塑性变形是影响粗糙度的主要因素。     

A NUMERICAL INVESTIGATION OF THE CHIP TOOL INTERFACE IN ORTHOGONAL MACHINING

A plane-strain thermo-elasto-viscoplastic finite element model has been developed and used to simulate orthogonal machining of 304L stainless steel using a ceramic tool. Simulations were carried out employing temperature-dependent physical properties. The model is used to investigate the effect of process parameters, tool geometry and edge preparation on the contact mechanics at the chip/tool interface. Stress and strain within the chip and the elastic tool are presented. Variables at the chip/tool interface such as contact length, sticking and sliding regions, normal and shear stresses, and frictional heat are investigated. Plastic deformation beneath the machined surface is compared for sharp and chamfered tools.     

A NUMERICAL INVESTIGATION OF THE CHIP TOOL INTERFACE IN ORTHOGONAL MACHINING

A plane-strain thermo-elasto-viscoplastic finite element model has been developed and used to simulate orthogonal machining of 304L stainless steel using a ceramic tool. Simulations were carried out employing temperature-dependent physical properties. The model is used to investigate the effect of process parameters, tool geometry and edge preparation on the contact mechanics at the chip/tool interface. Stress and strain within the chip and the elastic tool are presented. Variables at the chip/tool interface such as contact length, sticking and sliding regions, normal and shear stresses, and frictional heat are investigated. Plastic deformation beneath the machined surface is compared for sharp and chamfered tools.     

SURFACE INTEGRITY OF HIGH-SPEED FACE MILLED Ti-6Al-4V ALLOY WITH PCD TOOLS

This study is focused on the machined surface integrity of Ti-6Al-4V alloy using polycrystalline diamond (PCD) tools under wet milling condition. The surface integrity in terms of surface roughness, surface topography, microhardness, microstructure, and metallurgical alternations is investigated. The observations and conclusions are primarily focused on the effect of cutting speed (250–2,000 m/min) on the surface and subsurface of the machined Ti-6Al-4V. Experimental results show that machined surface integrity of Ti-6Al-4V alloy is sensitive to the variation of cutting speeds. Obvious machining (feed) marks can be found on the machined surfaces. Micro hardness examinations showed 5–20% hardening of the top machined surfaces than the bulk material. The analyses of microstructure and metallurgical alternations reveal that slight subsurface microstructure alteration such as plastic deformation on the subsurface and no phase transformation were observed. The evolution of crystallographic texture induced by the intense plastic deformation of the machined surface should be responsible for the modifications of the peak intensity radios in XRD patterns as well as higher peak broadening crystal structures.     

Microlubrication machining of 1018 steel: the effect of a biodegradable lubricant on the microstructural integrity

Microlubrication is a green machining technique that reduces the amount of cutting fluid during the machining process. Its effect on subsurface microstructural integrity is a very important aspect for the functionality of a machined component, but it is often neglected and requires advanced characterisation techniques. The focus of this study is to investigate the influence of microlubrication using a biodegradable vegetable oil‐based cutting fluid to characterise the subsurface microstructural integrity during end milling of AISI 1018 steel. Vickers microhardness was measured along the cross section of the machined component from the surface edge to inside the bulk, while transmission electron microscopy was conducted inside the subsurface deformation zone to quantify the dislocation densities. It was determined that increased dislocation densities near the workpiece edge resulted in increased microhardness with reduced tool wear. Thus, microlubrication machining is a sustainable green machining process that does not significantly compromise the subsurface integrity of the workpiece material. Copyright © 2017 John Wiley & Sons, Ltd.     

A study of the cutting performance of poly-crystalline oxygen free copper with single crystalline diamond micro-tools

A study was carried out to investigate the crystallographic effects on the performance of cutting poly-crystalline oxygen free copper C10200 (OFC) with single crystalline diamond (SCD) micro-tools. At both large cutting depth and cross-feed rate, as the micro-tool traversed a grain with a crystallographic orientation less favorable for a stable machining process, the work material in front of the rake face was found to be severely deformed. This may lead to a reduced shear angle, thick chip, striation at the back of the chip, high cutting forces, degraded machined surface and the possibility of burr formation. The results showed minimal variations in the machined surface integrity and cutting forces compared to cut amorphous NiP plating with micro-tools. For a high cutting depth, burrs were also observed due to material deformation and pile-up occurring at the groove edges since the localized stress probably built up in front of the rake face. Cutting strategies were demonstrated to improve the performance of cutting OFC with micro-tools and to generate high aspect ratio micro-pillar arrays.     

Chip perforation and ‘burnishing–like’ finishing of Al alloy in precision machining

In precision machining, a high quality machined surface cannot be achieved without using a diamond tool, due to the deterioration of workpiece surface integrity. Burnishing process is often used to improve the surface integrity by minimizing the roughness of the machined surface. For any given cutting tool-workpiece combination, the surface roughness depends on a parameter known as relative tool sharpness (RTS), which is quantified as the ratio of undeformed chip thickness (a) to tool edge radius (r). To achieve burnishing-like surface quality from precision machining, it is necessary to understand the material deformation behaviour in machining. Moreover, the quality of the machined surface is also directly related to the formation of μ-chip and its geometry. Thus, in this study, an attempt has been undertaken to develop the behavioural chip formation mechanics for the transition from unstable to the stable regime. Orthogonal microcutting experiments have been conducted with Al alloy (Al 6082) workpiece to investigate the micro-mechanics of chip perforation and to develop the chip stability mapping. Furthermore, the quantitative assessment criterion has been adopted to determine the material flow stress, which augmented the investigation of the burnishing-like deformation behaviour. By appraising the factors like machined surface integrity, compressive flow stress and improvement of surface roughness (R_a) profile, a ‘burnishing-like’ finishing zone has been identified. Additionally, SEM and EDX analyses have been performed to study the elemental composition of μ-chips, which allowed to validate the transition phenomena of chip perforation from incomplete (unstable) to complete (stable) chip formation. The applicability of this novel study lies in its ability to produce superior quality machined surface without requiring a secondary finishing operation and thus, improving the performance of precision machining.     

ASSESSING THE POSSIBILITY OF MACHINING OF THE SELF-REINFORCED BIORESORBABLE POLYMER P(L/DL)LA 70:30

This new study has sought to assess the machining possibility of the self-reinforced bioresorbable polymer P(L/DL)LA 70:30, used in healthcare. To verify the viability of machining of the components and, consequently, the influence of the cutting parameters on the surface quality of the machined samples, experiments were performed with different values for speed, feed, applications of cutting fluid, material and tool geometry. After machining, the roughness was measured and the generated surfaces were analyzed qualitatively. The interaction of the machining parameters and the surface quality were particularly analyzed on the search for surfaces with the characteristics used in the area of implants that interact with bone tissue, to decide on the viability of machining. With these results, it was determined that the material properties have played a decisive role in the resoluteness of the surface quality of the machined samples. To achieve the necessary characteristics for an implant, the machining parameters should be selected based on the material properties, such as glass transition temperature, molecular mobility and mechanical strength. This work has shown that it is possible to establish parameters for an appropriate machining of the material.     

An internal state variable plasticity-based approach to determine dynamic loading history effects on material property in manufacturing processes

Worked materials in large deformation processes such as forming and machining experience a broad range of strain, strain rate, and temperatures, which in turn affect the flow stress. However, the flow stress also highly depends on many other factors such as strain path, strain rate and temperature history. Only a model that includes all of these pertinent factors is capable of predicting complex stress state in material deformation. In this paper, the commonly used phenomenological plasticity models (Johnson–Cook, Usui, etc.) to characterize material behavior in forming and machining were critically reviewed. Although these models are easy to apply and can describe the general response of material deformation, these models lack the mechanisms to reflect static and dynamic recovery and the effects of load path and strain rate history in large deformation processes. These effects are essential to understand process mechanisms, especially surface integrity (residual stress, microhardness, and microstructure) of the manufactured products.As such a dislocation-based internal state variable (ISV) plasticity model was used, in which the evolution equations enable the prediction of strain rate history and temperature history effects. These effects can be quite large and cannot be modeled by the equation-of-state models that assume that stress is a unique function of the total strain, strain rate, and temperature, independent of the loading path. The temperature dependence of the hardening and recovery functions results in the prediction of thermal softening during adiabatic temperatures rises, which are common in metal forming and machining.The dynamic mechanical behaviors of three different benchmark work materials, titanium Ti-6Al-4V, AISI 52100 steel (62 HRc), and aluminum 6061-T6, were modeled using the ISV approach. The material constants were obtained by using a nonlinear regression-fitting algorithm in which the stress–strain curves from the model were correlated to the experiments at different (extreme) temperatures. Then the capabilities of the determined material constants were examined by comparing the predicted material flow stress with the test data at different temperatures, strains, and strain rate history. The comparison demonstrates that the internal state variable plasticity model can successfully recover dynamic material behavior at various deformation states including the loading path effect. In addition, thermal softening due to adiabatic deformation was also captured by this approach.     

基于弹塑性理论的高速铣削表面粗糙度力学建模

基于分子—机械摩擦理论,提出了已加工表面残留高度的力学模型。并根据热—弹塑性大变形理论,对材料的流动应力方程(非线性)进行了分析,推导出了材料的应变硬化、应变率和温度与切削变形应力增量的关系,研究了不同材料(45钢和3Cr2Mo模具钢)已加工表面残留高度的实验结果,分析了材料物理性能对加工表面粗糙度的影响。     

精密加工表面完整性的研究及其进展

对国内、外精密切削加工中表面完整性韵研究工作进行了系统的概述和分析,给出精密加工表面完整性的概念, 指出精密切削加工表面完整性领域研究工作的重点及发展的主要方向。针对我国目前的研究现状和水平,提出了系统 开展精密加工表面完整性研究的对策。     

Analytical modeling of work hardening of duplex steel alloys in the milling process

Severe plastic deformation in cutting operations such as milling might change mechanical properties (especially the strength and hardness) of the machined surface and its underlying layers. This phenomenon called work hardening and reduces machinability. This study presents an analytical solution to calculate the work hardening of the upper layers of the workpiece in the milling process of 2205 duplex stainless steel. In this regard, the stresses in the cutting regions are calculated to find the stress and temperature fields in the workpiece. Then the strain and strain rate values are calculated for each point of the surface and subsurface layers using the determined stress field. Finally, the Johnson-Cook material model is used to calculate flow stress and work hardening. Experimental results of the different machining conditions have been used to validate the proposed model. However, comparisons of subsurface microhardness and resultant cutting force obtained by an analytical model with experimental tests showed that the model properly predicts the amount of work hardening.

     

Machining of NiTi-shape memory alloys-A review

The emerging trends in the development of advanced smart materials with better unique properties under different environments for a particular application fascinate the researchers and industrialists. Nickel-Titanium based shape memory alloys are exotic materials due to their unique properties such as SME, SE, high damping characteristics, high corrosion and wear resistance and biocompatibility. This article presents an overview of machining processes that can be used to machine the NiTi and its surface induced characteristics such as microhardness, surface roughness, topography, induced layer, residual stress, fatigue and phase transformation. The surface integrity characteristics are discussed for machining of NiTi-SMAs under the category of traditional, non-traditional and micro-machining with the effect of input parameters such as cutting speed, feed, depth of cut, type of lubricant and type of coating material on cutting tool. The conventional machining of NiTi alloys are quite complicated due to high toughness, severe strain hardening, fatigue hardening and distinctive property of NiTi-SMAs such as pseudoelastic and shape memory effect. From this study, non-traditional process is significantly used to machine the NiTi-SMAs due to its better results on surface integrity characteristics. Consequently, future trends are also identified for machining the NiTi-SMAs and to improve the surface integrity characteristics.     

高速硬切削加工表面白层形成机理研究

白层是高速硬切削的特有现象,对加工表面完整性和零件的服役性能有着重要影响。针对高速硬切削加工表面白层问题,进行了对GCrl5淬硬轴承钢高速硬切削试验和表面白层测试,研究了不同切削条件下的白层形成机理,分析了切削速度和刀具磨损状态对白层特征的影响规律。分析结果表明,白层厚度随切削速度和后刀面磨损的增大而增大,而其分布的均匀性和连续性也将变差;切削速度和后刀面磨损的增加引起切削温度升高,导致加工表面快速淬火效应,使得白层厚度增大,其中切削速度的影响较为显著;在切削速度较低(100 m/min左右)时白层的形成机理主要为塑性变形,切削速度超过300 m/min则主要是马氏体相变所致,而在中间切削速度(200 m/min左右)时为2种机理的混合作用结果。     

高速切削温度动态变化规律的数值模拟

切削温度与刀具磨损、工件加工表面完整性及加工精度密切相关,其变化规律反映出高速切削过程本质的重要方面。本文应用数值模拟,对高速切削加工过程中切屑、工件和刀具三方面的温度随切削速度、进给量、切削深度的动态变化进行了研究,探讨了其变化规律,其结论有助于优化高速切削工艺及建立高速切削数据库。     

Subsurface shear instability and nanostructuring of metals in sliding

Dry sliding wear conditions were used to obtain a 500 μm-thick layer of nanosize grains on copper samples. As shown, this layer reveals a flow behavior pattern similar to that of a viscous non-Newtonian fluid. Four structurally different zones were found in the longitudinal cross-sections of samples below the worn surface. Upper two of them are nanocrystalline and consist of many ～1 μm-thick sublayers, which show either laminar or turbulent flow behavior. These sublayers demonstrate different levels of elasticity as compared to each other and may be related to an interplay between work-hardening and thermal softening. Lower two zones undergo usual plastic deformation and severe fragmentation without viscous mass transfer. High level of Young's modulus in the fragmentation zone is evidence of insufficient thermal softening at that depth. We believe that viscous flow zones are the result of shear instability and subsequent shear deformation developed in subsurface layers due to thermal softening. Numerical study has been carried out to simulate friction-induced deformation and shear instability under conditions close to the experiment. As shown, such a situation is possible when deformation-generated heat is taken into account. Another interesting result relates to the sublayers’ strain rate distribution. It was found that 1 μm-thick sublayers may show either high strain rate gradient or zero strain rate as a function of depth below the worn surface. The latter case means that a pack of layers may exist and behave like an elastic body in ductile medium.     

H13淬硬模具钢精车过程的数值模拟

采用热力学耦合有限元方法研究了淬硬钢精车过程中切屑形成规律。运用H13 淬硬模具钢流动应力模型进行数值模拟,考查了H13淬硬模具钢精车过程中工艺参数对工件性能和刀具的影响。结果表明:切削速度愈高,进给量愈小,刀具刀尖半径愈大,则工件加工层上的静水拉应力愈小,表面质量愈好; 淬硬钢精车时径向力起主要作用,大于切削力;切削速度愈大,切削力和径向力则愈小,愈有助于改善工件加工层上的表面质量;切削速度、进给量和刀具刀尖圆角半径愈大,工件和刀具温度愈高,愈易导致刀具前刀面扩散磨损和刀具后刀面磨损。研究结论有助于优化H13淬硬模具钢精车过程中工艺参数选择和改进刀具镶片设计。     

Plastic deformation analysis in machining of Inconel-718 nickel-base superalloy using both experimental and numerical methods

Surface region plastic deformation of Inconel-718 nickel-base superalloy workpieces was evaluated when machined under orthogonal cutting conditions at various cutting speeds. Plastic deformation analysis was accomplished by determining the residual stress and plastic strain distributions in the surface region. The residual stresses were tensile and maximum near the surface and decreased in magnitude with an increase in depth beneath the machined surface. Similarly, the plastic strains were maximum near the surface and decreased with an increase in depth beneath the machined surface. In addition, a finite element simulation of orthogonal machining was carried out for predicting the residual stress and plastic strain distribution. In general, the trend of the curves predicted by the finite element model was similar to those found experimentally.     

UNDERSTANDING OF FINITE ELEMENT ANALYSIS RESULTS UNDER THE FRAMEWORK OF OXLEY'S MACHINING MODEL

We introduce an accurate coupled thermo-mechanical finite element analysis (FEA) of machining using the Arbitrary Lagrangian Eulerian (ALE) analysis capability of ABAQUS/Explicit. This analysis provides detailed information about the cutting forces, chip thickness, contact length, the extent of the primary and secondary shear zones as well as the distribution of strain, strain rate and temperature in the deformation zones. This information has to be viewed under the framework of an analytical model for it to lead to better understanding of the physics of machining. We use the best available analytical model, namely, Oxley's machining model, for this purpose and the FEA results are compared with the assumptions and predictions of Oxley's analysis. The strain rate in the primary shear zone, the hydrostatic pressure variation along the shear plane, the distribution of normal and shear stresses along the tool-chip interface and the shape of the secondary shear zone are the quantities compared. Due to the key role of temperature in the prediction of tool wear, the fraction of heat conducted away into the workpiece, the maximum temperature along the tool-chip interface and the maximum temperature along the flank face are also compared. The comparison reveals that Oxley's model captures the physics of machining quite well. However, some details such as the heat partition module and the assumptions on stress and temperature distribution at the tool-chip interface need to be revisited.