Modified material constitutive models for serrated chip formation simulations and experimental validation in machining of titanium alloy Ti–6Al–4V

Titanium alloys present superior properties such as high strength-to-weight ratio and resistance to corrosion but, possess poor machinability. In this study, influence of material constitutive models and elastic–viscoplastic finite element formulation on serrated chip formation for modeling of machining Ti–6Al–4V titanium alloy is investigated. Temperature-dependent flow softening based modified material models are proposed where flow softening phenomenon, strain hardening and thermal softening effects and their interactions are coupled. Orthogonal cutting experiments have been conducted with uncoated carbide (WC/Co) and TiAlN coated carbide cutting tools. Temperature-dependent flow softening parameters are validated on a set of experimental data by using measured cutting forces and chip morphology. Finite Element simulations are validated with experimental results at two different rake angles, three different undeformed chip thickness values and two different cutting speeds. The results reveal that material flow stress and finite element formulation greatly affects not only chip formation mechanism but also forces and temperatures predicted. Chip formation process for adiabatic shearing in machining Ti–6Al–4V alloy is successfully simulated using finite element models without implementing damage models.     

钛合金铣削加工表面残余应力有限元仿真

为了分析铣削工艺参数对钛合金已加工表面残余应力的影响,根据金属切削有限元分析的相关理论,以钛合金Ti6Al4V为工件材料,建立了铣削加工的有限元模型。采用正交试验设计法对钛合金Ti6Al4V铣削仿真的工艺参数进行优化,并用极差法分析不同的铣削速度、铣削深度、铣削路径对钛合金Ti6Al4V工已加工表面残余应力的影响。研究表明:在钛合金Ti6Al4V铣削过程中,对工件已加工表面残余应力影响因素由小到大依次为:铣削深度<铣削路径<铣削速度,切削深度对已加工表面残余应力影响较小,铣削速度对已加工表面残余应力影响最大;在研究范围内,随着铣削速度的增大,已加工表面残余应力逐渐增加。     

钛合金高速切削切屑形成机理的有限元分析

钛合金在切削加工时容易产生锯齿状切屑，周期性的锯齿状切屑会引起切削力高频波动，从而影响加工表面质量和刀具寿命。然而其切屑形成的机理尚无统一的结论。本研究采用刚塑性有限元模型以及正交化Cockroft—Latham断裂准则，对钛合金Ti6A14V高速正交切削进行了仿真。仿真结果显示，周期性断裂理论能很好地解释钛合金锯齿状切屑形成的机理，主剪切变形区应力状态的变化是裂纹萌生与扩展的主要原因。研究结论与相关试验切屑显微照片特征相吻合，可以为实现钛合金高速切削提供理论依据和技术支持。     

DEVELOPMENT OF CONSTITUTE EQUATIONS FOR Ti-6Al-4V ALLOY UNDER HOT-WORKING CONDITION

1.~nonNumericalmodelingofindustrialplasticdeformationprocesseshasbecomeafieldofveryactiveresearchinthepastfewyears.ForthefullpotentialeXPloitingofthismethod,itisessentialtogetthepreciseknowledgeofconstitutivebehaviorofthematerial.Severalpapershavedevotedtotheestablishingofgeneralconstitutiveeqllationsfordescribingtheflowstressofthematerialasafunctionoftheprocessparameterssuchasstrain,strainrate,andtemperature['--7).Theseparametersareoftencalculatedforpeakstressvalueonly,becausemanymodelsassum…     

钛合金TC4切削过程流动应力模型研究

运用有限元技术对切削过程进行仿真可以预测切削力、切削温度、应力分布,优化刀具参数和切削条件。建立适合于切削条件中大应变、高应变率条件下材料的流动应力模型,是切削过程有限元仿真的关键技术。文章通过正交切削实验和有限元迭代的方法,修正了难加工材料TC4在大应变、高应变率条件下的J-C流动应力模型,使修正模型能够适应切削仿真中的大应变、高应变率要求。计算结果表明,采用新的J-C流动应力模型进行计算,所得主切削力值与实验测量值的平均误差从36.28%降为12.06%,进给力的平均误差由原来的61.03%降为现在的25.57%。该修正的流动应力模型比用霍普金森实验所得到的流动应力模型更适合于切削过程的有限元仿真,可以提高切削仿真的计算精度。     

基于ABAQUS有限元仿真的硬质合金刀具磨损机制研究

钛合金在切削过程中会产生严重的加工硬化现象,导致切削性下降、刀具磨损加剧,直接影响工件的加工质量。为研究钛合金切削性能和刀具磨损机制,利用ABAQUS软件建立了钛合金的有限元模型,对其切削过程进行仿真分析,研究硬质合金刀具磨损机制;设计Ti6Al4V钛合金车削实验,研究不同加工参数对刀具磨损程度的影响规律。研究结果表明:在切削钛合金时,刀具的磨损主要发生在刀尖和后刀面位置,刀具的磨损长度随车削速度的增加而变大,随车削深度的增加而减小,随进给量的增加呈现出先减小后变大的情况,实验和仿真结果趋于一致,平均误差在6%以内。     

Ti-6Al-4V热变形行为及利用多项耦合进行本构关系修正

采用热模拟试验机对钛合金Ti-6Al-4V进行高温压缩试验,研究其在850~1100 ℃温度下,0.001~0.1 s?1应变速率的流动应力行为。结果表明:钛合金Ti-6Al-4V具有应变速率、温度敏感性;随着温度的升高和应变速率的减小,流动应力逐渐降低,加工硬化速率与动态软化速率达到动态平衡。通过分析工艺参数对材料参数的影响,发现合金的材料参数(N、A、Q)随变形条件的变化而变化。在传统双曲正弦函数型Arrhenius方程的基础上提出一种考虑应变速率、温度和应变耦合修正的双曲正弦本构方程,并用相关系数R和平均相对误差(AARE)来评价所建立的本构模型的准确性,定量分析结果表明,修正的本构方程能够较准确地预测钛合金Ti-6Al-4V的流动应力。     

离心式滚磨光整加工钛合金的数值模拟分析

目的研究离心式滚磨光整加工对钛合金表层残余应力场的影响。方法利用有限元分析软件ABAQUS建立了加工过程中磨块碰撞TC4钛合金材料的三维有限元模型,对碰撞过程中的能量变化、材料应变及不同光整参数对工件表面残余应力场分布规律的影响进行分析,对比碳化硅磨块与氧化铝的加工效果。结果磨块参数对残余压应力峰值影响显著,但对其出现的位置影响不大。与碳化硅磨块相比,氧化铝磨块更适合于加工钛合金。结论有限元模拟可以探讨磨块碰撞作用下钛合金材料残余应力场的分布规律,优化滚磨光整加工钛合金的工艺参数。碰撞模拟证明,氧化铝磨块更适用于钛合金材料的滚磨光整加工。     

Machinability of titanium alloys (Ti6Al4V and Ti555.3)

Near-beta titanium alloys like Ti555.3 are increasingly being used in aeronautics replacing in some critical applications the most common Ti6Al4V. However, these near-beta titanium alloys have a poor machinability rating which needs to be overcome so as to maintain at least the same productivity levels as in Ti6Al4V.This paper presents the machinability results carried out for Ti555.3 compared with the commonly used Ti6Al4V. The aim of this research work is to understand tool wear mechanisms when machining Ti555.3. Analysis of variables such as cutting forces, chip geometry and tool wear shows that: (I) greater difficulty is encounterd when machining Ti555.3 alloy compared with Ti6Al4V alloy which can be machined at higher speeds up to 90 m min^?1; (II) there was a correlation between the mechanical properties of work material, tool wear, and component forces; (III) the occurrence of the diffusion process leads to the formation of a layer of adhered material composed of Ti and TiC on the tool's rake face for both Ti alloys.     

Experimental investigation on the effect of the material microstructure on tool wear when machining hard titanium alloys: Ti–6Al–4V and Ti-555

An experimental investigation was conducted in this work to analyze the effect of the workpiece microstructure on tool wear behavior and stability of the cutting process during marching difficult to cut titanium alloys: Ti–6Al–4V and Ti-555. The analysis of tool–chip interface parameters such as friction, temperature rise, tool wear and workpiece microstructure evolution under different cutting conditions have been investigated. As the cutting speed increases, mean cutting forces and temperature show different progressions depending on the considered microstructure. Results show that wear modes of cutting tools used for machining the Ti-555 alloy exhibit contrast from those obtained for machining the Ti–6Al–4V alloy. Because of the fine-sized microstructure of the near-β titanium Ti-555, abrasion mode was often found to be the dominate wear mode for cemented cutting tools. However, adhesion and diffusion modes followed by coating delamination process were found as the main wear modes when machining the usual Ti–6Al–4V alloy by the same cutting tools. Moreover, a deformed layer was detected using SEM–EDS analysis from the sub-surface of the chip with β-grains orientation along the chip flow direction. The analysis of the microstructure confirms the intense deformation of the machined surface and shows a texture modification.     

Hot deformation behavior of Ti—6Al—4V—0.1Ru alloy during isothermal compression

The hot deformation behavior of Ti—6Al—4V—0.1Ru titanium alloy was investigated by isothermal compression tests on a Gleeble—3500 thermal simulator over deformation temperature range of 1023—1423 K and strain rate of 0.01–10 s⁻¹. Arrhenius-type constitutive models were developed for temperature ranges of both α+β dual phase and β single phase at strain of 0.1. Afterwards, a series of material constants (including activation energy Q, material constants n, α and ln A) as polynomial functions of strain were introduced into Arrhenius-type models. Finally, the improved Arrhenius-type models in temperature field of α+β and β phase were constructed. The results show that the improved Arrhenius-type models contribute to the calculation of Zener—Hollomon (Z) parameter, and the microstructural evolution mechanism is uncovered by combining microstructure observations with Z-parameter. Furthermore, the improved Arrhenius-type models are also helpful to improve the accuracy of finite element method (FEM) simulation in the deformation process of Ti—6Al—4V—0.1Ru titanium alloy.     

Effect of micro-textured tools on machining of Ti–6Al–4V alloy: An experimental and numerical approach

In this work, an attempt is made to reduce the detrimental effects that occurred during machining of Ti–6Al–4V by employing surface textures on the rake faces of the cutting tools. Numerical simulation of machining of Ti–6Al–4V alloy with surface textured tools was employed, taking the work piece as elasto-plastic material and the tool as rigid body. Deform 3D software with updated Lagrangian formulation was used for numerical simulation of machining process. Coupled thermo-mechanical analysis was carried out using Johnson-cook material model to predict the temperature distribution, machining forces, tool wear and chip morphology during machining. Turning experiments on Ti–6Al–4V alloy were carried out using surface textured tungsten carbide tools with micro-scaled grooves in preferred orientation such as, parallel, perpendicular and cross pattern to that of chip flow. A mixture of molybdenum disulfide with SAE 40 oil (80:20) was used as semi-solid lubricant during machining process. Temperature distribution at tool–chip interface was measured using an infrared thermal imager camera. Feed, thrust and cutting forces were measured by a three component-dynamometer. Tool wear and chip morphology were captured and analyzed using optical microscopic images. Experimental results such as cutting temperature, machining forces and chip morphology were used for validating numerical simulation results. Cutting tools with surface textures produced in a direction perpendicular to that of chip flow exhibit a larger reduction in cutting force, temperature generation and reduced tool wear.     

一种新型亚稳β钛合金热变形的本构模型(英文)

基于新型亚稳β钛合金Ti2448在温度1023～1123K、应变速率63～0.001s-1下的等温热压缩流动应力曲线特征,构建能够完整描述该合金流动应力与应变、应变速率、变形温度之间关系的本构模型。在此过程中,通过基于统一黏塑形理论改进双曲正弦函数,构建合金在高应变速率(≥1s-1)下发生动态回复(DRV)的模型;通过对标准的Avrami方程进行简化,表征了Ti2448在低应变率(1s-1)下发生的动态再结晶(DRX)软化机制。最终通过应用全局优化求解非线性方程的新方法确定模型中的相关参数。根据所建模型得到的预测曲线和实验曲线吻合得较好,能够有效预测Ti2448在热变形过程中的流动应力,为构建亚稳β钛合金热变形本构模型提供一种有效的方法。     

Effects of temperature and initial microstructure on the equal channel angular pressing of Ti–6Al–4V alloy

Equal channel angular pressing of Ti–6Al–4V alloy was successfully carried out isothermally above 600 °C. The equiaxed microstructure presented more uniform material flow than the Widmanstätten microstructure, which was discussed in relation to flow softening behavior of the two microstructures.     

Experimental investigation and 3D finite element prediction of the heat affected zone during laser assisted machining of Ti6Al4V alloy

An experimental study was conducted to characterize the heat affected zone produced when laser heating a Ti6Al4V alloy plate workpiece. The emissivity and absorptivity of the Ti6Al4V alloy were determined experimentally. A 3D transient finite element method for a moving Gaussian laser heat source was developed to predict the depth and width of the heat affected zone on the Ti6Al4V alloy workpiece. There was a close correlation between the experimental data and the simulation results. It was found that the depth and width of the heat affected zone were strongly dependent on the laser parameters (laser power, laser scan speed, the angle of incidence and the diameter of the laser spot) and material properties (thermal conductivity, specific heat and density). Parametric studies showed that the depth and width of the heat affected zone increased with an increase in the laser power and decreased with an increase of the laser spot size and the laser scan speed. The thermal model can be used to determine the laser parameters for a given cut geometry that will yield no residual heat affected zone in the material after cutting. This provides the basis to optimize and improve laser assisted machining technique.     

Simulation of slip band evolution in duplex Ti–6Al–4V

Formation of slip bands plays an important role in deformation and fatigue processes of duplex Ti–6Al–4V. In this study, shear-enhanced crystal plasticity constitutive relations are proposed to account for the slip softening due to breakdown of the short-range order between titanium and aluminum atoms. A hybrid strategy is developed which allows the softening to occur in slip bands only within the primary α phase, with the degree of localization depending on the specific polycrystalline initial-boundary-value problem and the requirements for compatibility of each grain or phase with its neighbors. The proposed model is calibrated by performing finite-element (FE) simulations on an experimentally studied Ti–6Al–4V alloy. The slip behavior of a Ti–6Al–4V sample subjected to an in situ (scanning electron microscopy (SEM)) tensile test is investigated. A two-dimensional (2-D) FE with 3-D crystal plasticity relations is constructed to represent the microstructure of the Ti–6Al–4V sample. Due to the lack of access to fully 3-D microstructure, a generalized plane-strain condition is used in the FE model which assumes columnar grains that are free of net traction in the direction normal to the surface. The assumption of columnar grains significantly reduces the computational cost. The contours of effective plastic strain are compared with the surface SEM micrographs from experiments at various strain levels. It is shown that the proposed approach for modeling slip bands qualitatively captures experimentally observed slip band behavior.     

Comparison of the machinabilities of Ti6Al4V and TIMETAL® 54M using uncoated WC–Co tools

Single-point turning tests of cylindrical bars were undertaken to analyse and compare the machinability of Ti6Al4V, the most common titanium alloy, and TIMETAL^® 54M, a newly developed alloy with similar mechanical properties as Ti6Al4V but with better machinability. Conventional cooling and uncoated WC–Co tool inserts were used in the study, because they are the most recommended for machining these materials. The feed and the depth of cut were maintained constant, and only the cutting speed was varied because it is the most affecting parameter. Adhesion of workpiece material in the form of a built-up edge appeared in all the cutting inserts after machining both alloys, which was removed for flank- and crater-wear measurements. Lower wear rates were observed for the Ti54M alloy, especially at high cutting speeds. In the same manner, cutting-force measurements showed lower specific cutting- and feed-force values for the Ti54M alloy. Adiabatic shear bands, a typical feature in the machining of titanium alloys, were observed in chips from both alloys under all cutting conditions. Finally, scanning electron microscopy observations were carried out to analyse the adhered material on the cutting edges of the worn tools where signs of diffusion and attrition were detected.     

A new material model for 2D numerical simulation of serrated chip formation when machining titanium alloy Ti–6Al–4V

A new material constitutive law is implemented in a 2D finite element model to analyse the chip formation and shear localisation when machining titanium alloys. The numerical simulations use a commercial finite element software (FORGE 2005^®) able to solve complex thermo-mechanical problems. One of the main machining characteristics of titanium alloys is to produce segmented chips for a wide range of cutting speeds and feeds. The present study assumes that the chip segmentation is only induced by adiabatic shear banding, without material failure in the primary shear zone. The new developed model takes into account the influence of strain, strain rate and temperature on the flow stress and also introduces a strain softening effect. The tool chip friction is managed by a combined Coulomb–Tresca friction law. The influence of two different strain softening levels and machining parameters on the cutting forces and chip morphology has been studied. Chip morphology, cutting and feed forces predicted by numerical simulations are compared with experimental results.     

厚度和层间界面对Ti6Al4V钛合金抗弹性能的影响

开展了Ti6Al4V钛合金的抗弹性能研究,通过对厚度为10～30 mm的均质Ti6Al4V钛合金靶板和总厚度为30 mm的(15+15)mm双层Ti6Al4V钛合金靶板的终点弹道侵彻实验,研究了厚度和层间界面对Ti6Al4V钛合金抗弹性能的影响规律。结果表明:Ti6Al4V钛合金的抗弹性能随着厚度的增加逐渐提高;在靶板厚度由15 mm增加到20 mm时,其抗弹性能出现了陡增,这与其损伤模式由脆性冲塞破坏转变为塑性扩孔破坏有关;层间界面不利于Ti6Al4V钛合金抗弹性能的提高,厚度为30mm的单层均质Ti6Al4V钛合金靶板的抗弹性能优于总厚度为30mm的(15+15)mm双层Ti6Al4V钛合金靶板,这与双层靶板的层间界面几乎无剪切强度有关。     

Tool life and wear mechanisms in high speed machining of Ti–6Al–4V alloy with PCD tools under various coolant pressures

Usage of titanium alloys has increased since the past 50 years despite difficulties encountered during machining. Many studies involving different tool materials, cutting parameters, tool geometry and cutting fluids when machining this important aerospace material have been published. However, there are relatively few literatures available on the application of ultra hard tools in the machining of titanium-alloys. The primary objective of this study is to investigate the behaviour of Polycrystalline Diamond (PCD) tools when machining Ti–6Al–4V alloy at high speed conditions using high pressure coolant supplies. Tool performance under different tribological conditions and the dominant wear mechanisms were investigated. Increase in coolant pressure tends to improve tool life and reduce the adhesion tendency, accelerated by the susceptibility of titanium alloy to gall during machining. Adhesion and attrition are the dominant wear mechanisms when machining at the cutting conditions investigated.