Mechanistic modeling of cutting forces in micro-end-milling considering tool run out,minimum chip thickness and tooth overlapping effects

This work proposed an improved mechanistic model for prediction of cutting forces in micro-milling process. The combined influences of tool run out, trochoidal trajectory of the tool center, overlapping of tooth, edge radius and minimum chip thickness are incorporated in this model to realize the exact cutting phenomenon. Moreover, an improved undeformed chip thickness algorithm has been presented by considering tool run out, minimum chip thickness and trajectory of all passing teeth for one complete revolution of the tool instead of only the current tooth trajectory. For estimation of tool run out, a model based on the geometry of the two fluted end mill cutter has been developed. Effects of trochoidal trajectory of the tool center and tool run out are found to be significant as each tooth has a different chip load. Further, the effect of minimum chip thickness is found to be significant at lower feed value. The proposed model has been validated by micro-milling experiments on Ti6Al4V-titanium alloys using uncoated flat end mill cutter. The predicted cutting forces were found to be in good agreement with the experimental cutting forces in both feed and cross feed directions.     

高速切削钛合金Ti6Al4V切屑的形成及其数值模拟

应用Hopkinson压杆实验装置,确定了航空用钛合金Ti6Al4V高应变和高温条件下的应力-应变关系,结合Ti6Al4V合金准静态试验数据,建立了适合高速切削仿真的Johnson-Cook本构模型;通过有限元数值模拟,仿真了高速切削Ti6Al4V合金的锯齿状切屑形成过程,分析了整个锯齿状切屑形成过程的切削力、切削温度、等效塑性应变的变化,深入探讨了锯齿状切屑的形成机理;将模拟计算得到的切削力和切削温度与试验结果进行了比较,两者具有较好的一致性。
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Suppression of forced vibration due to chip segmentation in ultrasonic elliptical vibration cutting of titanium alloy Ti–6Al–4V

Ultrasonic elliptical vibration cutting of titanium alloy Ti–6Al–4V is investigated in this research. Because products made of Ti–6Al–4V alloy are usually designed for possessing low-rigidity structures or good-quality cut surfaces, machining requirements such as low cutting forces and slow rate of tool wear need to be fulfilled for realization of their precision machining. Therefore, the ultrasonic elliptical vibration cutting is applied as a novel machining method for those products. Machinability of Ti–6Al–4V alloy by the ultrasonic elliptical vibration cutting with cemented carbide tools is examined to figure out suitable cutting conditions for precision machining of Ti–6Al–4V alloy. As experimental results, generated chips, cutting forces, and profiles of cut surfaces are indicated. A forced vibration problem occurred due to the segmented chip formation, which is also well-known in the ordinary non-vibration cutting. Therefore, characteristics of the forced vibration due to the chip segmentation are investigated in this research. Through the experiments, it is found that the frequency and magnitude of the forced vibration have relation with the average uncut chip thickness and cutting width. Especially, it is found that the averaging effect can suppress the forced vibration, i.e. the chip segmentation tends to occur randomly over the large cutting width, and hence the force fluctuations with random phases tend to cancel each other as the cutting width increases relatively against the average uncut chip thickness. Based on the investigations, a new practical strategy to suppress the forced vibration due to chip segmentation is proposed and verified. Using the proposed method significantly decreased cutting forces and good quality of surfaces are obtained when the forced vibration is suppressed compared to the ordinary non-vibration cutting results. Therefore, the results suggest that the precision machining can be realized without sacrificing the machining efficiency by increasing the width of cut and decreasing the average uncut chip thickness.     

Study on orthogonal turning of titanium alloys with different coolant supply strategies

In this paper, the effects of different coolant supply strategies (using flood coolant, dry cutting, and minimum quantity of lubricant [MQL]) on cutting performance in continuous and interrupted turning process of Ti6Al4V are investigated. Based on the observation of the cutting forces with the different coolant supply strategies, the mean friction coefficient in the sliding region at the tool–chip interface has been obtained and used in a finite element method (FEM) to simulate the deformation process of Ti6Al4V during turning. From the FEM simulation and Oxley’s predictive machining theory, cutting forces have been estimated under different coolant supply strategies and verified experimentally.     

Effect of tilted plate vibration on solidification and microstructural and mechanical properties of semisolid cast and heat-treated A356 Al alloy

To optimize the machining process, finding the minimum uncut chip thickness is of paramount importance in micro-scale machining. However, strong dependency of the minimum uncut chip thickness to the tool geometry, workpiece material, tool-work friction, and process condition makes its evaluation complicated. The paper focuses on determination of the minimum uncut chip thickness experimentally during micro-end milling of titanium alloy Ti-6Al-4V with respect to influences of cutting parameters and lubricating systems. Experiments were carried out on a CNC machining center Kern Evo with two flute end mills of 0.8 and 2 mm diameters being used in the tests for micro- and macro-milling, respectively. It was found that the micro-milling caused more size effect than macro-milling due to higher surface micro-hardness and specific cutting forces. The specific cutting force depended strongly on feed rate (f _z) and lubricating system, followed by depth of cut (a _p) and cutting speed (v _c), mainly in the micro-scale. All output parameters were inversely proportional to the specific cutting force. Finally, depending on different process parameters during micro-milling of Ti-6Al-4V, the minimum uncut chip thickness was found to vary between 0.15 and 0.49 of the tool edge radius.     

Cutting performance of nano-crystalline diamond (NCD) coating in micro-milling of Ti6Al4V alloy

Hard coatings are an important factor affecting the cutting performance of tools. In particular, they directly affect tool life, cutting forces, surface quality and burr formation in the micro-milling process. In this study, the performance of nano-crystalline diamond (NCD) coated tools was evaluated by comparing it with TiN-coated, AlCrN-coated and uncoated carbide tools in micro-milling of Ti₆Al₄V alloy. A series of micro-milling tests was carried out to determine the effects of coating type and machining conditions on tool wear, cutting force, surface roughness and burr size. Flat end-mill tools with two flutes and a diameter of 0.5 mm were used in the micro-milling process. The minimum chip thickness depending on both the cutting force and the surface roughness were determined. The results showed that the minimum chip thickness is about 0.3 times that of the cutter corner radius for Ti₆Al₄V alloy and changes very little with coating type. It was observed from wear tests that the dominant wear mechanism was abrasion. Maximum wear occurred on NCD-coated and uncoated tools. In addition, maximum burr size was obtained in the cutting process with the uncoated tool.     

Multi-material metallic structure fabrication using electron beam melting

The fabrication of multi-material structures using Ti–6Al–4V and copper was explored with the additive manufacturing (AM) technology of electron beam melting (EBM). A new method was developed that included multiple build sequences to accommodate both materials. The process was enabled by machining a start plate so that the parts built with the first material could be press fit into the plate, providing a flat surface on which the second material fabrication would occur. This method provided the ability to fabricate simple multiple metallic material components built in the Z and X directions [1]. Registration of the electron beam was performed manually resulting in slight misalignment for the shift of diameters of specimens built in the Z direction, and along the width and length for specimens built in the X direction. Microstructures observed and hardness values measured for copper and Ti–6Al–4V were different to those observed in normally fabricated EBM parts. These observations might be explained by the different processing conditions required for multi-material fabrication in contrast to the regular EBM process where parts are built in a single machine run. The hardness profiles for as-fabricated and HIPed multi-material parts depicted an increase in hardness for both materials close to the interface with values leveling off to those of single material EBM fabricated parts as measurements proceeded away from the interface. As the benefits of EBM processing are exploited, the method introduced in this research can have profound implications in many technological applications including metal extraction, energy production and for the repair of metallic components.     

EFFECT OF CUTTING CONDITIONS ON THE SERRATED CHIP FORMATION IN HIGH-SPEED CUTTING

Titanium alloy Ti6A14V has been widely used in many engineering fields due to its attractive specific strength and corrosion resistance. A deep understanding of the material's machinability is of primary importance. This article investigates the serrated chip formation mechanism of Ti6Al4V alloy under high-speed cutting by finite element analysis. The effect of the cutting conditions on the serrated chip formation is analyzed comprehensively. The study found that when the initial chip thickness becomes small or when the rake angle becomes large, the size of sawtooth decreases and the number of sawtooth increases. The serrated chip morphology is more sensitive to the initial chip thickness. The severe fluctuation of cutting forces is caused by the formation of sawtooth in chipping. To minimize the serrated chipping in high-speed machining, the initial chip thickness is the most important factor to consider.     

Prediction and experimental validation of micro-milling cutting forces of AISI H13 steel at hardness between 35 and 60 HRC

This paper presents prediction and validation of micro-milling cutting forces of AISI H13 steel at hardnesses between 35 and 60 HRC. The cutting forces are predicted based on an approach considering the full kinematics of the cutting tool including the run-out effect, effects of the cutting velocity and tool geometry, ploughing and chip formation phenomena and the hardness of the AISI H13 steel. A plane strain dynamic thermo-mechanical finite element (FE) model of orthogonal cutting is used to predict the cutting forces where the geometry of the cutting tool edge is modelled based on scanning electron microscope measurements. A constitutive elastic–plastic isotropic material model describing the relationship between stresses, strains, strain rates and hardnesses is modelled and implemented into ABAQUS/Explicit FE code by the user-defined subroutine VUMAT. Finite element analyses (FEA) are employed to obtain the relationship between cutting forces, uncut chip thickness, cutting velocity and material hardness. Numerous FEA are performed at different uncut chip thicknesses (0–20?μm), cutting velocities (104.7–4,723?mm/s) and hardnesses (35–60 HRC) using the FE model of orthogonal cutting. The full kinematics of the cutting tool including the run-out effect and the FE-predicted cutting forces are incorporated to predict the micro-milling cutting forces. The predicted micro-milling cutting forces have been experimentally validated at hardness of 43.2 HRC at different feed rates and spindle speeds. The result showed that the cutting forces and cutting temperatures increase by increasing the hardness of the AISI H13 while the stability limits of the process decrease by increasing the hardness.     

钛合金旋转超声辅助钻削的钻削力和切屑研究

针对难加工材料钛合金在采用普通麻花钻传统钻削过程中存在钻削力和扭矩较大使得钻孔困难,刀具使用寿命低,连续长切屑易缠绕刀具、划伤孔加工表面、增大刀具-切屑-工件孔壁之间的摩擦以及排屑差引起堵屑和卡刀具的问题,引入一种新刃型刀具（即八面钻）,并结合超声振动钻削技术,进行了钛合金旋转超声辅助钻削试验。分析了旋转超声辅助钻削和普通钻削中切屑形成原理,采用文中所设计的旋转超声振动钻削主轴结合BV100立式加工中心平台、测力系统和非接触激光测量系统进行了无冷却条件下基于八面钻的钛合金旋转超声辅助钻削和普通钻削试验以及钻削力、扭矩和切屑形态的研究。试验结果表明：相比于普通钻削,超声钻削明显降低钻削力和扭矩分别为19.07%~20.09%和31.66%~34.3%,明显增强了钻头横刃和主切削刃的切削能力,获得了良好的断屑和排屑效果,提高了切削过程的稳定性,能够极大改善钛合金钻孔过程钻削困难、刀具使用寿命低和孔加工质量差的问题。     

Surface Integrity of Ultrasonically-Assisted Milled Ti6Al4V Alloy Manufactured by Selective Laser Melting

The Ti6Al4V parts produced by the existing selective laser melting (SLM) are mainly confronted with poor surface finish and inevitable interior defects,which substantially deteriorates the mechanical properties and performances of the parts.In this regard,ultrasonically-assisted machining (UAM) technique is commonly introduced to improve the machining quality due to its merits in increasing tool life and reducing cutting force.However,most of the previous studies focus on the performance of UAM with ultrasonic vibrations applied in the tangential and feed directions,whereas few of them on the impact of ultrasonic vibration along the vertical direction.In this study,the effects of feed rate on surface integrity in ultrasonically-assisted vertical milling (UAVM) of the Ti6Al4V alloy manufactured by SLM were systemically investigated compared with the conventional machining (CM) method.The results revealed that the milling forces in UAVM showed a lower amplitude than that in CM due to the intermittent cutting style.The surface roughness values of the parts produced by UAVM were generally greater than that by CM owing to the extra sinusoidal vibration textures induced by the milling cutter.Moreover,the extra vertical ultrasonic vibration in UAVM was beneficial to suppressing machining chatter.As feed rate increased,surface microhardness and thickness of the plastic deformation zone in CM raised due to more intensive plastic deformation,while these two material properties in UAVM were reduced owing to the mitigated impact effect by the high-frequency vibration of the milling cutter.Therefore,the improved surface microhardness and reduced thickness of the subsurface deformation layer in UAVM were ascribed to the vertical high-frequency impact of the milling cutter in UAVM In general,the results of this study provided an in-depth understanding in UAVM of Ti6Al4V parts manufactured by SLM.     

Analytical modeling of adiabatic shear band spacing for serrated chip in high-speed machining

Theoretical prediction of adiabatic shear band spacing is beneficial to understand the mechanism of the serrated chip formation. The momentum equation, energy equation and compatibility equation in orthogonal cutting are established in this paper. Using perturbation analysis by regarding cutting speed and uncut chip thickness as basic disturbance, an analytical solution of adiabatic shear band spacing is developed. Adiabatic shear band spacing of serrated chip is related to the wave number of the perturbation when the growth rate reaches to maximum. It is found that adiabatic shear band spacing decreases with the increase of cutting speed but increases with uncut chip thickness. The experiment of orthogonal cutting Ti6Al4V under different cutting speeds (50 m/min-1800 m/min) and uncut chip thicknesses (0.02 mm-0.16 mm) verifies the validity of the theoretical prediction.     

Fabrication and in vivo evaluation of Ti6Al4V implants with controlled porous structure and complex shape

Electron beam melting process was used to fabricate porous Ti6Al4V implants. The porous structure and surface topography of the implants were characterized by scanning electron microscopy (SEM) and digital microscopy (DM). The results showed that the pore size was around 600 and the porosity approximated to 57%. There was about±50 μm of undulation on implants surfaces. Standard implants and a custom implant coupled with porous sections were designed and fabricated to validate the versatility of the electron beam melting (EBM) technique. After coated with bone-like apatite, samples with fully porous structures were implanted into cranial defects in rabbits to investigate the in vivo performance. The animals were sacrificed at 8 and 12 weeks after implantation. Bone ingrowth into porous structure was examined by histological analysis. The histological sections indicated that a large amount of new bone formation was observed in porous structure. The newly formed bone grew from the calvarial margins toward the center of the bone defect and was in close contact with implant surfaces. The results of the study showed that the EBM produced Ti6Al4V implants with well-controlled porous structure, rough surface topography and bone-like apatite layer are beneficial for bone ingrowth and apposition.     

Prediction of cutting forces in helical milling process

The prediction of cutting forces is important for the planning and optimization of machining process in order to reduce machining damage. Helical milling is a kind of hole-machining technique with a milling tool feeding on a helical path into the workpiece, and thus, both the periphery cutting edges and the bottom cutting edges all participated in the machining process. In order to investigate the characteristics of discontinuous milling resulting in the time varying undeformed chip thickness and cutting forces direction, this paper establishes a novel analytic cutting force model of the helical milling based on the helical milling principle. Dynamic cutting forces are measured and analyzed under different cutting parameters for the titanium alloy (Ti–6Al–4V). Cutting force coefficients are identified and discussed based on the experimental test. Analytical model prediction is compared with experiment testing. It is noted that the analytical results are in good agreement with the experimental data; thus, the established cutting force model can be utilized as an effective tool to predict the change of cutting forces in helical milling process under different cutting conditions.     

Modeling and analyzing the effects of air-cooled turning on the machinability of Ti–6Al–4V titanium alloy using the cold air gun coolant system

This study provides the mathematical models for modeling and analyzing the effects of air-cooling on the machinability of Ti–6Al–4V titanium alloy in the hard turning process. A cold air gun coolant system was used in the experiments and produced a jet of compressed cold air for cooling the cutting process. The air-cooling process seems to be a good environment friendly option for the hard turning. In this experimental investigation, the cutting speed, feed rate and cutting depth were chosen as the numerical factor; the cooling method was regarded as the categorical factor. An experimental plan of a four-factor (three numerical plus one categorical) D-optimal design based on the response surface methodology (RSM) was employed to carry out the experimental study. The mathematical models based on the RSM were proposed for modeling and analyzing the cutting temperature and surface roughness in the hard turning process under the dry cutting process and air-cooling process. Tool wear and chip formation during the cutting process were also studied. The compressed cooling air in the gas form presents better penetration of the lubricant to the cutting zone than any conventional coolants in the cutting process do. Results show that the air-cooling significantly provides lower cutting temperature, reduces the tool wear, and produces the best machined surface. The machinability performance of hard turning Ti–6Al–4V titanium alloy on the application of air-cooling is better than the application of dry cutting process. This air-cooling cutting process easily produces the wrinkled and breaking chips. Consequently, the air-cooled cutting process offers the attractive alternative of the dry cutting in the hard turning process.     

3D FEM simulation of milling process for titanium alloy Ti6Al4V

Milling is used as one of the most important tools with the complex tool geometry in industry. However, the complex milling process cannot be simulated by 2D finite element method. Therefore, a more real 3D finite element model (FEM) for the complex milling process of titanium alloy Ti6Al4V is firstly developed using the finite element software ABAQUS. This model takes into account the dynamic effects, thermomechanical coupling, material damage law, and contact criterion. Firstly, the Johnson–Cook material constitutive equation was proposed, considering the effects of strain, strain rate, and temperature on material properties. Secondly, the damage constitutive law was adopted as the chip separation criterion. Then, the simulation for the milling process of Ti6Al4V was conducted through ABAQUS based on the established 3D FEM. Finally, chip formation, stress distribution, cutting force, and milling temperature were obtained. Further, a series of milling experiments of Ti6Al4V were carried out to validate the simulation results. It confirms the capability and advantage of 3D FEM simulation in the complex milling process of titanium alloy.     

硬质合金刀具高速切削Ti6Al4V合金时扩散磨损的数值模拟

应用通用商业有限元软件Deform-2D,对航空用钛合金Ti6Al4V进行了不同冷却润滑条件下的正交切削有限元模拟。在参考已有刀具扩散磨损率模型的基础上,利用有限元模拟出的刀具/工件接触区的切削温度与相对滑动速度等基本变量,对高速切削钛合金Ti6Al4V时的WC-Co类硬质合金刀具前刀面的扩散磨损率进行了预测,进而分析了切削介质的冷却与润滑作用对刀具扩散磨损率的影响。研究结果表明:切削介质的润滑作用对刀具前刀面的扩散磨损率具有较大影响,而切削介质的冷却作用则对刀具前刀面扩散磨损率无显著影响。     

基于ABAQUS的钛合金铣削力的模拟分析

为了研究钛合金在铣削过程中切削力随着切削参数的变化规律,建立了三维斜角切削有限元模型。通过对材料本构模型,刀—屑接触摩擦模型和切屑分离准则等关键环节建模,采用通用有限元求解器ABAQUS/Ex-plicit对钛合金Ti6Al4V的斜角切削过程进行了模拟,获得了切削速度v、切削深度ap和每齿进给量fz对切削力的变化趋势及影响程度。模拟结果表明:切削力随着切削深度ap和每齿进给量fz的增大而增大,而随着切削速度增大切削力波动很小。切削深度对切削力的影响最大,进给量次之,切削速度对切削力的影响最小。该模型可以为切削参数的合理选择提供参考。     

Correlation between Single Factor Material Constitutive Model Parameters and Temperature for Ti6Al4V Alloy

Accurate material constitutive model is considered highly necessary to perform finite element simulation and analysis.However,it is difficult to establish the material constitutive model because of uncertainty of mathematical relationship and constraint of existing experimental condition.At present,there exists considerable gap between finite element simulation result and actual cutting process.Particular emphases were put on investigating the correlation between "single factor" material constitutive model parameters and temperature for Ti6Al4V alloy,and also establishment of material constitutive model for this kind of material.Theoretical analyses based on dislocation theory and material functional relations showed that material model was deeply affected by variation temperature.By the least squares best fit to the available quasi-static and high-speed impact compression experiment data,material parameters at various temperatures were found.Experimental curves analyses and material parameters comparison showed that the "single factor" material constitutive model parameters were temperature dependent.Using the mathematical mapping between material parameters and temperature,"single factor" material constitutive model of Ti6Al4V alloy was established,which was proven to be right by comparing with experimental measurements.This work makes clear that the "single factor" material constitutive model parameters of Ti6Al4V alloy are temperature dependent.At the same time,an accurate material constitutive model is established,which helps to optimize cutting process and control machining distortion for Ti6Al4V alloy aerospace parts.     

The characteristics of cutting forces in the micro-milling of AISI D2 steel

The interaction effect of parameters to surface topography and cutting forces is investigated, and the magnitudes of these parameters are determined in the micro-milling of AISI D2 steel. The results show that the feed per tooth has a prominent impact on the surface topography. Due to the low feed per tooth to cutting edge radius ratio, a high surface roughness and a high amount of burrs are obtained in micro-milling. In micro cutting, the cutting forces present are small; in addition, the radial thrust cutting forces are greater than the principal cutting forces. This research proves that the micro-milling process can be applied to the manufacturing of AISI D2 steel micro parts and presents experimental evidence and possible solutions to the cutting parameters.