Thermo-mechanical modeling of orthogonal machining process by finite element analysis

A plane strain finite element method is used with a new material constitutive equation for 1020 steel to simulate orthogonal machining with continuous chip formation. Deformation of the workpiece material is treated as elastic–viscoplastic with isotropic strain hardening, and the numerical solution accounts for coupling between plastic deformation and the temperature field, including treatment of temperature-dependent material properties. To avoid numerical errors associated with large deformation of elements, automatic remeshing is used, with at least 15 rezonings required to achieve a satisfactory solution. Effects of the uncertainty in the constitutive model on the distributions of strain, stress and temperature around the shear zone are presented, and the model is validated by comparing average values of the predicted stress, strain, strain rate and temperature at the shear zone with experimental results. Parametric effects associated with cutting speed and initial work temperature are considered in the simulations.     

Continuous and ultra-fine grained chip production with large strain machining

In this study, orthogonal cutting technique as a severe plastic deformation (SPD) method for producing chips with an ultra-fine grained microstructure and fair mechanical properties is investigated; further, it has been suggested that by controlling the cutting velocity and contact length between tool and material, it is possible to produce a severely deformed and continuous chip with unrestored microstructure even at high cutting velocities. Solution treated Al-6061 samples in plane strain condition were severely deformed through applying various cutting velocities (from 50 to 2230 mm/s) for three different rake angles (−5°, −10° and −20°) in fixed cutting parameters. Chip thickness, contact length, shear strain, and Vickers microhardness variations were examined for different samples and chip formation mechanism was discussed for different processing conditions. In addition, the microstructure of especial produced chips was studied using transmission electron microscopy (TEM). The results showed that during the dominance of seizure mechanism at the contact surface, microhardness and shear strain (as well as contact length) have inverse dependency upon the variation of the cutting velocity. The results are discussed by considering the heat-time effect contribution in the final microstructure and mechanical properties.     

Severe plastic deformation (SPD) processes for metals

Processes of severe plastic deformation (SPD) are defined as metal forming processes in which a very large plastic strain is imposed on a bulk process in order to make an ultra-fine grained metal. The objective of the SPD processes for creating ultra-fine grained metal is to produce lightweight parts by using high strength metal for the safety and reliability of micro-parts and for environmental harmony. In this keynote paper, the fabrication process of equal channel angular pressing (ECAP), accumulative roll-bonding (ARB), high pressure torsion (HPT), and others are introduced, and the properties of metals processed by the SPD processes are shown. Moreover, the combined processes developed recently are also explained. Finally, the applications of the ultra-fine grained (UFG) metals are discussed.     

Mechanics of large strain extrusion machining and application to deformation processing of magnesium alloys

An analysis of the mechanics of large strain extrusion machining (LSEM), a constrained chip formation process, is presented for deformation processing of bulk alloys. The deformation field is shown to be narrowly confined and controllable, with attributes ranging from conventional deformation processing to severe plastic deformation. Controllable deformation parameters include strain/strain rate, hydrostatic pressure, temperature and deformation path. These attributes are highlighted in deformation processing of Mg AZ31B, an alloy of commercial significance but noted for its poor workability, into sheet and foil forms. Noteworthy features of the processing are suppression of segmentation, realization of a range of strains and deformation rates, engineering of microstructures ranging from conventional to ultrafine grained, and creation of sheet/foil from the bulk in a single step of deformation without pre-heating. Guidelines for realizing specific sheet attributes, and scalability of LSEM for production are analyzed and discussed.     

FEA modeling and simulation of shear localized chip formation in metal cutting

The finite element analysis (FEA) has been applied to model and simulate the chip formation and the shear localization phenomena in the metal cutting process. The updated Lagrangian formulation of plane strain condition is used in this study. A strain-hardening thermal-softening material model is used to simulate shear localized chip formation. Chip formation, shear banding, cutting forces, effects of tool rake angle on both shear angle and cutting forces, maximum shear stress and plastic strain fields, and distribution of effective stress on tool rake face are predicted by the finite element model. The initiation and extension of shear banding due to material's shear instability are also simulated. FEA was also used to predict and compare materials behaviors and chip formations of different workpiece materials in metal cutting. The predictions of the finite element analysis agreed well with the experimental measurements.     

Study of channel angle influence on material flow and strain inhomogeneity in equal channel angular pressing using 3D finite element simulation

Equal channel angular pressing (ECAP) is currently being widely investigated because of its potential to produce ultra-fine grained microstructures in metals and alloys. A sound knowledge of the plastic deformation and strain distribution is necessary for understanding the relationships between strain inhomogeneity and geometry of die. Considerable research has been reported on finite element analysis of this process, assuming 2D plane strain condition. The 2D models are not suitable due to the component geometry, especially for workpiece with cylindrical cross-section. In the present work 3D simulation of ECAP process was carried out, using ABAQUS/Standard software, for different channel angles for a strain hardening aluminium alloy (AA6101). Strain inhomogeneity is presented and discussed for all cases. Pattern of variation of strain along some selected radial lines in steady-state zone is presented.     

等通道球形转角挤压过程中工业纯铝的微观组织演变与力学性能

突破传统ECAP变形全过程通道等截面思路,提出一种耦合剪切应变和正应变于一体的新型等通道球形转角挤压(equal channel angular extrusion with spherical cavity,ECAE-SC)工艺。在自行研制的模具上对工业纯铝进行室温单道次ECAE-SC挤压实验,采用OM、EBSD和TEM等技术手段,研究了ECAE-SC变形过程中工业纯铝微观组织的演变规律,并测试了变形后试样的显微硬度。结果表明,在ECAE-SC工艺剧烈简单剪切变形诱导下,工业纯铝仅需1道次挤压变形即可获得等轴、细小、均匀的超细晶组织,平均晶粒尺寸约为400 nm;工业纯铝室温ECAE-SC变形以位错滑移为主并伴有不完全连续动态再结晶,其微观组织经历了剪切带→位错胞→小角度亚晶→大角度等轴晶粒等动态演化过程。1道次ECAE-SC变形后,工业纯铝组织以{110}001高斯织构为主,同时存在部分{111}112铜型织构;材料显微硬度值大幅提升,由初始289.4 MPa提高到565.3 MPa,增幅高达95.33%,且分布均匀性良好。     

等通道转角挤压过程塑性变形行为的研究

等通道转角挤压工艺是一种利用纯剪切变形获得块状超细晶材料的新技术,本文采用坐标网格法进行实验,获得了力一行程曲线和试样网格的变化,并应用Deform-3D有限元软件数值模拟了5052铝合金挤压塑性变形过程,将挤压后的实验结果同模拟结果进行比较,两者吻合较好,以此为基础,分析了挤压变形力和等效应变的分布规律,探讨了塑性变形的行为.     

Mode I crack tip fields in amorphous materials with application to metallic glasses

In this work, stationary crack tip fields in amorphous materials such as metallic glasses under mode I loading are studied to understand the factors that control crack tip plasticity and in turn impart toughness to those materials. For this purpose, finite element simulations under plane strain, small scale yielding conditions are performed. A continuum elastic–viscoplastic constitutive theory, which accounts for pressure sensitivity of plastic flow as well as the localization of plastic strain into discrete shear bands, is employed to represent the material behavior. The influence of internal friction and strain softening on the plastic zone, stress and deformation fields and notch opening profile is examined. It is found that higher internal friction leads to a larger plastic zone. Also, it enhances the plastic strain ahead of the notch tip but leads to a substantial decrease in the opening stress. Thus, it appears that a higher friction parameter promotes toughening of amorphous solids. The shear band patterns within the plastic zone and brittle crack trajectories around the notch root generated from the simulations match qualitatively with those observed in experiments.     

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通过对传统孔型的讨论和模拟的结果分析,基于大塑性变形原理的晶粒超微细化方法,提出了均匀大应变棒材轧制的新型孔型设计思想,建立了以多向大塑性变形加工为特点的扁椭圆系列孔型.采用非线性有限元法,建立了孔型棒材温轧过程数值分析模型,对比分析了新型孔型系列与传统孔型系列轧件断面内塑性应变分布规律,得出了在满足形状尺寸精确的条件下新型扁椭孔型能更好地将大塑性应变引入到断面中心,并使断面塑性应变趋于均匀,其中心处最大累积应变超过5.0,达到产生超微细晶粒的大塑性变形制备条件.     

A study on shear banding in chip formation of orthogonal machining

A simplified theory of instability of plastic flow is applied in this paper to analyze the formation of shear localized chips in orthogonal machining. A flow localization parameter is expressed in terms of associated cutting conditions and properties of the workpiece material. The analysis is used to investigate the effect of cutting conditions on the onset of shear localization and the formation of adiabatic shear banding in metal cutting. Comparisons are made between the analysis and experiments in which the flow localization parameter is obtained for several workpiece materials. The results of this investigation are thought to lend a strong justification for the analysis and its potential benefits in analyzing and/or remedying problems associated with chip formation and temperature generated in metal cutting.     

大应变切削制备的超细晶纯铜切屑热稳定性的研究

采用不同前角的刀具对纯铜进行大应变切削加工,对获得的超细晶纯铜切屑进行不同温度的退火处理.利用扫描电子显微镜(SEM)和维氏硬度测量仪进行检测,分析不同前角和退火温度对超细晶纯铜切屑微观组织和力学性能的影响.结果表明:用0°前角刀具进行大应变切削加工后,切屑晶粒平均尺寸为0.3μm、硬度为160HV,在200～280℃...     

Strain,strain rate and velocity fields determination at very high cutting speed

High speed imaging is used for the purpose of examining the strain and strain rate variations in the primary shear zone at cutting speed of 1020 m/min. Experimental investigation focused on flow pattern describing the severe plastic deformation zone where a general streamline model is employed to investigate the distribution of velocity. Strain and strain rate distribution are directly deduced from the experimental observation of a cross-section of chip obtained through a high-speed camera system. The strain and strain rate gradients were analyzed along several streamlines. A finite element method model based on a Lagrangian formulation has been used to corroborate the conclusion of the streamline model. It has been found that the simulation results are similar to the experimental observations with regards to the magnitude of the equivalent strain rate and cumulative plastic strain but slightly differ in the geometry of flow pattern.     

Thermomechanical modelling of oblique cutting and experimental validation

An analytical approach is used to model oblique cutting process. The material characteristics such as strain rate sensitivity, strain hardening and thermal softening are considered. The chip formation is supposed to occur mainly by shearing within a thin band called primary shear zone. The analysis is limited to stationary flow and the material flow within the primary shear zone is modelled by using a one-dimensional approach. Thermomechanical coupling and inertia effects are accounted for. The chip flow angle is determined by the assumption that the friction force on the tool face is collinear to the chip flow direction. At the chip–tool interface, the friction condition can be affected by the important heating induced by the large values of pressure and sliding velocity. In spite of the complexity of phenomena governing the friction law in machining, a reasonable assumption is to consider that the mean friction coefficient is primarily function of the average temperature at the tool–chip interface. Comparisons between model predictions and experimental results are performed for different values of cutting speed, undeformed chip thickness, normal cutting angle and inclination angle. A critical study is presented in order to show the influences of the input parameters of the model including the normal shear angle, the thickness of the primary shear zone and the pressure distribution at the tool–chip interface. The model permits to predict the cutting forces, the chip flow direction, the contact length between the chip and the tool and the temperature distribution at the tool–chip interface which has an important effect on tool wear.     

An analytical model to predict specific shear energy in high-speed turning of Inconel 718

Machining of Inconel 718 at higher cutting speeds is expected to provide some relief from the machining difficulties. Therefore, to understand the material behavior at higher cutting speeds, this paper presents an analytical model that predicts specific shearing energy of the work material in shear zone. It considers formation of shear bands that occur at higher cutting speeds during machining, along with the elaborate evaluation of the effect of strain, strain rate, and temperature dependence of the shear flow stress using Johnson–Cook equation. The model also considers the ‘size-effect’ in machining in terms of occurrence of ‘ploughing forces’ during machining. The theoretical results show that the shear band spacing in chip formation increases linearly with an increase in the feedrate and is of the order of 0.2–0.9 mm depending upon the processing conditions. The model shows excellent agreement with the experimental values with an error between 0.5% and 7% for various parametric conditions.     

Influence of scaled undeformed sections of cut on strain rate,cutting force and temperature

In machining processes, a decreasing undeformed chip thickness leads to an increase in the specific machining forces. This effect is commonly known as the scaling effect in chip formation. In the literature, several reasons for this effect are discussed. One approach focuses on the increase in the strain rate due to a decrease in the undeformed chip thickness. The increase in the strain rate leads to a hardening effect of the machined material which results in higher specific cutting forces. However, it has not been definitely proven that this is the cause of the scaling effect in chip formation. This paper describes an approach for examining the influence of the strain rate on the scaling effect. Firstly, FE-simulations have been carried out to gain knowledge about the strain rates in the center of the shear zone. By means of these simulations, cutting speeds which lead to constant strain rates in the center of the shear zone have been determined for a broad range of chip thickness. In a second step, experimental investigations have been carried out using the simulated cutting speeds and chip thicknesses. The chip formation processes and the machining forces have been analyzed with constant strain rates and different chip thicknesses as well as with a constant cutting speed. The main result of these investigations is that the strain rate has only a minor influence on the specific cutting forces. It is shown that the temperature in the shear zone decreases with a decrease in the chip thickness. This leads to lower thermal softening of the material and thus to higher specific cutting forces.     

切削法制备超细晶材料研究进展与展望

综述了切削法制备超细晶材料时加工参数和工艺条件对晶粒细化的影响,分析了切削法制备超细晶材料的力学性能、耐腐蚀性能和热稳定性等,探讨了超声振动复合切削法制备超细晶材料的可能性。在超声振动加工中,材料受低应力高速、高频撞击的影响,会发生严重的塑性变形,表面大尺寸的晶粒得到细化,同时超声振动还可以在材料表面形成表面微结构,进一步改善材料性能。因而提出将切削法和超声振动相复合,高效制备具有功能微结构的超细晶材料,为微型零件超细晶材料制备提供新的工艺选择以及理论和技术支撑。     

TC4铣削切屑形态与表面形貌特征

目的 对TC4铣削过程中锯齿状切屑的形成与对应产生的加工表面形貌特征进行研究,掌握钛合金TC4高速铣削加工切屑形态随铣削速度的变化规律,从而提高加工表面质量和效率。方法 基于有限元软件,建立钛合金TC4二维变厚度切削模型,通过仿真和铣削试验分析铣削速度对切屑形态的影响规律。利用超景深显微镜和PS50表面轮廓仪对TC4铣削过程中形成的切屑形态及工件加工表面形貌进行观测和分析,确定铣削加工TC4过程中铣削速度与切屑形态、工件表面形貌和表面粗糙度之间的关系。结果 铣削试验验证得出铣削力仿真值与试验值最大误差为9.86%,验证了二维变厚度切削模型的准确性。随着铣削速度从40 m/min增大到120 m/min,切屑形态由带状转变为锯齿状,且铣削力逐渐减小。同时,铣削速度由80 m/min增大到240 m/min时,切屑的锯齿化系数和剪切带内的剪切角均增大,而剪切带间距减小,TC4加工表面波纹加深、波纹间距变宽,并且伴随有大量韧窝出现,导致表面粗糙度值增大。结论 掌握锯齿状切屑几何特征与工件表面形貌随铣削速度的变化规律,以便在铣削加工TC4过程中对锯齿状切屑进行控制,对于提高工件加工表面质量和加...     

Chip Formation, Acoustic Emission and Surface White Layers in Hard Machining

In hard machining, sawtooth chip formation is due to initiation of adiabatic shear within the lower region of the primary shear zone. Catastrophic failure within the upper region of the shear zone occurs through either of two different mechanisms and results in the rapid release of elastic strain energy. This periodic release of strain energy is the dominant source of acoustic emission during sawtooth chip formation. In addition to adiabatic shearing in the primary and secondary shear zones, there is evidence to suggest that it occurs in the tertiary shear zone also; namely the surface white layer.     

Mapping subgrain sizes resulting from severe simple shear deformation

Grain refinement under interactive effects of severe shear strains, strain rates and temperatures often follows complex trajectories. Encapsulating the process-structure linkages under these conditions is central to controlling product outcomes from processes involving severe plastic deformation. This paper uses in situ characterization of deformation in large-strain machining using high-speed digital image correlation and IR thermography to examine the microstructural consequences across a swathe of directly quantified thermomechanical conditions. Using electron microscopy, it is shown that the average subgrain sizes resulting from this deformation correlate with the strain and the theoretical limit of the subgrain-size that is achievable. From this, a suitably parameterized map-space is proposed for capturing average subgrain sizes resulting from severe shear deformation. The parameterization is characterized by the y axis as the strain and the x axis as a parameter R, which is a function of the strain-rate, temperature and material constants. Noting that the surfaces from machining processes are essentially derived from severe thermomechanical conditions akin to those explored here, the implications of subgrain size-maps for controlling surface microstructures on components manufactured by machining processes are discussed.