Finite element modeling of 3D turning of titanium

The finite element modeling and experimental validation of 3D turning of grade two commercially pure titanium are presented. The Third Wave AdvantEdge machining simulation software is applied for the finite element modeling. Machining experiments are conducted. The measured cutting forces and chip thickness are compared to finite element modeling results with good agreement. The effects of cutting speed, a limiting factor for productivity in titanium machining, depth of cut, and tool cutting edge radius on the peak tool temperature are investigated. This study explores the use of 3D finite element modeling to study the chip curl. Reasonable agreement is observed under turning with small depth of cut. The chip segmentation with shear band formation during the Ti machining process is investigated. The spacing between shear bands in the Ti chip is comparable with experimental measurements. Results of this research help to guide the design of new cutting tool materials and coatings and the studies of chip formation to further advance the productivity of titanium machining.     

The investigation of mechanism of serrated chip formation under different cutting speeds

Chip type is determined by the coupled effects of workpiece material property, cutting speed, uncut chip thickness, feed rate, and tool edge geometry. The understanding of chip formation plays a critical role in studying surface integrity and optimization of machining process variables. Serrated chip, one of the major important chip type, is usually formed in hard cutting at high speed. In this study, a new analytical model has been proposed to better understand the formation of serrated chip, and the simulations have been acquired using ABAQUS/Explicit in machining AISI 1045 during different speeds (from 60 to 6000 m/min). The workpiece material property is modeled with the Johnson-Cook model, and the experiments have been conducted with AISI 1045 during speeds from 60 to 1200 m/min. It has been shown that flow stress is influenced simultaneously by the strain rate hardening and temperature softening. When the speed reaches very high, the temperature softening will fail, and the strain rate hardening will play a more important role. Also, it can be found that the hardening ratio increases when the cutting speed rises. The results of the simulations and experiments correlated well. The cutting force and thrust force both decrease as the cutting speed increases, and the difference between them will shrink when the machining speed reaches a high level.     

A comparative study of the tool–chip contact length in turning of two engineering alloys for a wide range of cutting speeds

Tool chip contact length is an important parameter in machining, as it provides an indication of the size of area of interaction between the hot chip and the tool surface and hence the interface heat transfer zone. Heat transfer and thermally activated wear modes usually dominate tool wear in the high speed machining of steels and machining of titanium alloys at most cutting speeds. In this study, existing models for the prediction of tool–chip contact length are reviewed and examined for their suitability in high speed machining of two widely used engineering alloys. Orthogonal turning tests for AISI 1045 steel and Ti6Al4V titanium alloy are conducted for a range of cutting speeds from conventional to high speeds. New contact length models are presented for both materials covering a wide range of cutting speeds. More significantly, these contact length models are appropriate for high speed machining where thermal loads significantly influence process performance. Additionally, the work discusses how the machinability of engineering materials influences the ability to predict contact length.     

Characterization of chip formation during machining 1045 steel

A deep understanding of the generation and characterization of chip formation can result for practical advices of chip type controlling in engineering applications. The chip formation is divided into the continuous chip and the serrated one in this study. The characterization of the continuous chip formation is expressed as the chip deformation and that of the serrated chip formation is expressed as the frequency of serration, the degree of segmentation, and the deformation of serrated chip. The chips of 1045 steel under different cutting speeds (100–3,600?m/min) are collected during machining. After inlay and polishing of the collected chips, the chip morphology is observed with VHX-600 ESO digital microscope. It is found that at the cutting speeds of 100–400?m/min, the chip type is continuous, at the cutting speeds of 600–2,200?m/min the chip type is serrated, and at the cutting speeds of 2,500–3,600?m/min the chip type is segmented. The quantitative relations between the characterization parameters of chip formation and the cutting speed are obtained. The chip deformation increases with the cutting speed, and the influence of the cutting speed on the shear strain rate is more sensitive than that on the shear strain during the continuous chip formation. All the characterization parameters including the shear strain rate, the frequency of serration, the degree of segmentation, and the shear strain increase with the cutting speed during the serrated chip formation. The sensitivity of influence of the cutting speed on these parameters is in the following: the shear strain rate, the degree of segmentation, the frequency of serration, and the shear strain.     

ROLE OF MICROSTRUCTURAL SOFTENING EVENTS IN METAL CUTTING

Oxley's model for predicting equilibrium shear angle turns out to be the most comprehensive approach that incorporates both the mechanics of metal cutting and dynamic behavior of metal during metal cutting. Oxley's prediction of equilibrium shear angle for flow chip morphology is validated in metal cutting at low cutting speeds. However, the domain of flow chip is limited by major microstructural softening events that occur at high cutting speeds particularly if the matrix is hardened by heat treatment or there is a large volume fraction of second phase particles. Dynamic recrystallisation and phase transformation are identified as major microstructural softening events occurring in the hardened matrix that cause shear localisation. Incompatibility of deformation between the matrix and second phase particles causes shear localisation due to geometric softening.

Quantitative modeling to predict the critical speed for the onset of shear localised chip morphology requires quantitative database on the dynamic flow stress behavior of materials that duly incorporates the microstructural softening events, which is the critical path. Quantitative analysis of phenomenological database in model alloys has shown that shear localisation can be suppressed by engineering glassy oxide inclusions that lubricate in-situ the tool-chip interface. This concept underlies the development of self-lubricating steel designed to suppress chemical wear in high speed machining.     

An association rule mining and maintaining approach in dynamic database for aiding product–service system conceptual design

Many previous researches on high-speed machining have been conducted to pursue high machining efficiency and accuracy. In the present study, the characteristics of cutting forces, surface roughness, and chip formation obtained in high and ultra high-speed face milling of AISI H13 steel (46–47 HRC) are experimentally investigated. It is found that the ultra high cutting speed of 1,400?m/min can be considered as a critical value, at which relatively low mechanical load, good surface finish, and high machining efficiency are expected to arise at the same time. When the cutting speed adopted is below 1,400?m/min, the contribution order of the cutting parameters for surface roughness Ra is axial depth of cut, cutting speed, and feed rate. As the cutting speed surpasses 1,400?m/min, the order is cutting speed, feed rate, and axial depth of cut. The developing trend of the surface roughness obtained at different cutting speeds can be estimated by means of observing the variation of the chip shape and chip color. It is concluded that when low feed rate, low axial depth of cut, and cutting speed below 1,400?m/min are adopted, surface roughness Ra of the whole machined surface remains below 0.3?μm, while cutting speed above 1,400?m/min should be avoided even if the feed rate and axial depth of cut are low.     

基于ABAQUS的高速切削切屑形成过程的有限元模拟

基于有限元分析软件ABAQUS的Johnson-Cook材料模型以及断裂准则模拟高速切削淬硬钢锯齿状切屑形态,并讨论刀具前角和锯齿状切屑形态对切削力的影响.研究表明仿真结果和试验结果是一致的,文中介绍的有限元模拟方法可以准确地模拟并预测高速切削淬硬钢时的切屑形成过程.     

基于最小能量法的高速切削锯齿状切屑变形分析

以高速切削典型变形特征为研究对象,建立了锯齿状切屑的几何模型。根据高速切削绝热剪切理论,通过对剪切面相对滑移失稳瞬间的切屑块受力平衡分析建立力学模型和运动学模型,获得切屑摩擦力和剪切力及剪切速度和切屑流动速度,并确定了切削能量方程。在考虑应变、应变率、切削温度和变形硬化因素条件下按最小能量原理求变形方程,结合变形曲线的分析和切削理论确定了高速切削变形方程及影响变形的因素。分析结果表明,切屑锯齿化是塑性变形超过临界失稳条件的结果。     

高速切削锯齿形切屑形成机理的研究现状与发展

概述高速切削锯齿形切屑的形成机理,总结高速切削切屑形态的分类,分析锯齿形切屑形成的两大理论:绝热剪切理论和周期性断裂理论,并对两种理论进行比较,认为绝热剪切理论适用于塑性材料或者是在切削过程中由于力热化学耦合作用而转化为塑性材料的脆性材料,周期性断裂理论适用于脆性材料,两种理论的主要区别在于在材料自由表面产生破坏之前剪...     

IMPULSIVE CHIP BREAKING IN METAL MACHINING: A PROOF-OF-CONCEPT STUDY

An innovative non-conventional technique, called impulsive chip breaking, is developed in the present study to break difficult-to-break chips that are often generated in machining high toughness or soft gummy materials, such as pure aluminum, pure copper, aluminum alloys, copper alloys, low carbon steels, and stainless steels. These materials have a wide variety of engineering applications. In impulsive chip breaking, the machine tool spindle rotational speed periodically increases to a prescribed higher speed within a set short period of time and then resumes to its normal constant speed to continue machining operations. The experimental investigations covering a range of cutting conditions on a selected work material are preformed to confirm the feasibility of impulsive chip breaking and study its basic mechanism as well as the characteristic variations of machining performances, including the chip morphology, the cutting forces, the machining vibrations, and the surface roughness of the machined workpiece. It is demonstrated that as long as the impulsive rotational speed of the machine tool spindle is appropriately selected or optimized, both requirements of breaking chips and maintaining the machined surface quality can be simultaneously satisfied.     

IMPULSIVE CHIP BREAKING IN METAL MACHINING: A PROOF-OF-CONCEPT STUDY

An innovative non-conventional technique, called impulsive chip breaking, is developed in the present study to break difficult-to-break chips that are often generated in machining high toughness or soft gummy materials, such as pure aluminum, pure copper, aluminum alloys, copper alloys, low carbon steels, and stainless steels. These materials have a wide variety of engineering applications. In impulsive chip breaking, the machine tool spindle rotational speed periodically increases to a prescribed higher speed within a set short period of time and then resumes to its normal constant speed to continue machining operations. The experimental investigations covering a range of cutting conditions on a selected work material are preformed to confirm the feasibility of impulsive chip breaking and study its basic mechanism as well as the characteristic variations of machining performances, including the chip morphology, the cutting forces, the machining vibrations, and the surface roughness of the machined workpiece. It is demonstrated that as long as the impulsive rotational speed of the machine tool spindle is appropriately selected or optimized, both requirements of breaking chips and maintaining the machined surface quality can be simultaneously satisfied.     

3D FINITE ELEMENT MODELLING OF CHIP FORMATION PROCESS FOR MACHINING INCONEL 718: COMPARISON OF FE SOFTWARE PREDICTIONS

Many efforts have been focused on the development of Finite Element (FE) machining models due to growing interest in solving practical machining problems in a computational environment in industry. Most of the current models are developed under 2D orthogonal plane strain assumptions, or make use of either arbitrary damage criterion or remeshing techniques for obtaining the chip. A complete understanding of the material removal process together with its effects on the machined parts and wear behaviour of the cutting tools requires accurate 3D computational models to analyze the entire physical phenomenon in materials undergoing large elastic-plastic deformations and large temperature changes as well as high strain rates. This work presents a comparison of 3D machining models developed using commercially available FE softwares ABAQUS/Explicit^© and DEFORM?3D Machining. The work material is chosen as Inconel 718, a difficult-to-cut nickel-based alloy material. Computational results of temperature, strain and stress distributions obtained from the FE models for the effect of cutting speed are presented in comparison with results obtained from experimental tests. In addition, modified material model for Inconel 718 with flow softening is compared with the Johnson-Cook model. The predictions of forces and chip formation are improved with the modified material model.     

An experimental study on the machining characteristics in ductile-mode milling of BK-7 glass

Glass is considered as one of the most challenging materials to machine because of its high hardness coupled with high brittleness. The challenge, in machining such a brittle material, lies in achieving the material removal through plastic deformation rather than characteristic brittle fracture. It has already been established that every brittle material, no matter how brittle it is, can be machined in ductile mode under certain critical conditions. The critical conditions are material specific, and hence, every material tends to show unique behavior in terms of critical conditions during machining process. This paper outlines the results of an experimental study to determine the critical chip thickness for ductile–brittle transition, chip morphology, and the effect of cutting speed on the critical conditions in peripheral milling process of BK-7 glass. It is established experimentally that the cutting speed affects the chip morphology, machined surface quality, and critical conditions due to possible thermal effects in such a way that ductile–brittle transition phenomenon is facilitated at high cutting speeds.     

Effect of cutting speed on surface integrity and chip morphology in high-speed machining of PM nickel-based superalloy FGH95

High-speed machining is being recognized as one of the key manufacturing technologies for getting higher productivity and better surface integrity. FGH95 powder metallurgy superalloy is a kind of nickel-based superalloy which is produced by near-net-shape technology. With increasing demands for high precision and high performance of FGH95 components in aerospace industry, it is essential to recognize that the machined surface integrity may determine machined part service performance and reliability. Then, little is known about the machined surface integrity of this superalloy. Thus, the surface integrity in high-speed machining of FGH95 is investigated in this paper. Experiments are conducted on a CNC milling center with coated carbide tools under dry cutting conditions. The surface integrity is evaluated in terms of surface roughness, microhardness, and white layer. The influence of cutting speed on chip morphology is also investigated. Experiment results show that surface integrity and chip morphology of FGH95 are very sensitive to the cutting speed. When cutting speeds are below 2,400?m/min, the values of surface roughness have little variation, while when cutting speeds are in the range of 2,800–3,600?m/min, the values of surface roughness are higher than that of other cutting speeds. Severe work hardening is observed resulting from high-speed machining of FGH95 superalloy. The higher the cutting speed, the higher the surface hardness. When cutting speeds are in the range of 2,800–3,600?m/min, the white layer thickness is slightly higher than that of other cutting speeds. In high-speed machining of FGH95, the chip is segmented and has a typical sawtooth shape. The degree of serrated chip increases with the cutting speed. When the cutting speeds exceed 2,400?m/min, serrated chips change into fragment chips.     

Effect of very high cutting speeds on shearing, cutting forces and roughness in dry turning of austenitic stainless steels

Behavior of austenitic stainless steels has been studied at very high cutting speeds. Turning tests were carried out using the AISI 303 austenitic stainless steel. In particular, the influence of cutting speed on tool wear, surface quality, cutting forces and chip geometry has been investigated. These parameters have been compared when performing machining at traditional cutting speeds (lower than 350?m/min) versus high cutting speeds. The analysis of results shows that the material undergoes a significant change in its behavior when machining at cutting speeds above 450?m/min, that favors the machining operation. The main component of cutting forces reaches a minimum value at this cutting speed. The SEM micrographs of the machined surfaces show how at the traditional cutting speeds the machined surfaces contain cavities, metal debris and feed marks with smeared material particles. Surfaces machined at high cutting speeds show evidence of material side flow, which is more evident at cutting speeds above 600?m/min. Tool wear is located at the tool nose radius for lower cutting speeds, whereas it slides toward the secondary edge when cutting speed increases. An analysis of chips indicates also an important decrement in chip thickness for cutting speeds above 450?m/min. This study concludes that there is an unexplored range of cutting speeds very interesting for high-performance machining. In this range, the behavior of stainless steels is very favorable although tool wear rate is also significant. Nevertheless, nowadays the cost of tool inserts can be considered as secondary when comparing to other operation costs, for instance the machine hourly cost for high-end multitasking machines.     

Study of saw-tooth chip in machining of Inconel 718 by metallographic technique

The present research work aims to understand the chip formation mechanisms in Inconel 718 machining through metallographic technique. The influence of machining speed on shear band and chip shape have been analyzed. Characterization of the shear band has been conducted by analyzing the shear band thickness, strain, strain rate, stress and micro-hardness. Additionally, chip reduction coefficient parameters and chip segmentation ratio parameters have been analyzed to understand the type of chip formation at various machining speeds. The observations and analysis of the same indicated that the formation of the saw-tooth chip is due to high-strain localization within the chip and crack initiation at the free surface of the chip. Thinner shear bands are observed at higher machining speed (120 m/min), and thicker shear bands are observed at lower machining speed (40 m/min). The magnitude of chip segmentation parameters indicates that crack generation increases with increase in machining speed.     

高速切削30CrNi3MoV淬硬钢切屑形成机理的试验研究

通过30CrNi3MoV淬硬钢的高速切削试验,观察和测量不同切削条件下切屑形态的演变过程、锯齿状切屑形成的临界切削条件、切削力.结果表明,切削速度和刀具前角是影响切屑形态和切削力的主要因素,随着切削速度的提高,在某一临界切削速度下,切屑形态由带状屑转变为锯齿状切屑,随着刀具前角由正前角逐渐变为负前角,临界切削速度明显减小,当锯齿状切屑形成时,切削力大幅度降低.使用金属切削过程中绝热剪切临界切削条件判据对锯齿状切屑形成临界切削速度预测的结果表明,锯齿状切屑形成的根本原因是主剪切区内发生周期性的绝热剪切断裂.     

PTFE材料正交切削切屑成形特性研究

为了研究PTFE材料在切削过程中的成屑机理,设计了单因素正交切削实验以及准静态力学实验。首先,基于剪切平面理论和断裂理论,以切削速度、切削厚度为变量,提出了PTFE材料的连续切屑及切屑毛边成形机理;然后通过分析毛边高度和间距,探究切削参数对毛边形态的影响规律;最后,利用分形理论对切削力稳定性进行分析,揭示了毛边数量、出现频率与切削力稳定性的关系。结果表明,PTFE连续切屑的产生是因为切削过程中材料真实压缩应变未超过断裂屈服应变,毛边的产生则是因为切屑边缘片层状结构处裂纹的形核与扩展。此外,切削参数会影响毛边生成,在切削速度100 mm/min、切削厚度0.3 mm工况下,切屑毛边数量最少、出现频率最小,切削力最稳定。研究结果可为PTFE材料切削过程毛边抑制和表面加工质量优化提供参考。     

RECENT PROGRESS AND SOME DIFFICULTIES IN COMPUTATIONAL MODELING OF MACHINING

A computer simulation approach to predicting chip formation, tool damage, and distribution of residual stress in metal machining is reviewed based on the authors' recent work. It is stressed that material characteristics should be included in the analysis of a practical machining process, since machining phenomena are intimately associated with deformation and fracture of work materials at high strain rates and high temperatures. Various simulated results and some comparisons with experiments are shown. Finally, the future scope of computational modeling of machining is envisaged.     

Understanding and Characterizing the Drilling of Recycled Plastics

Recycled plastics are increasingly being used to manufacture planks used in large-volume applications, including decks, garden, and cloakroom chairs. These products, although manufactured near-to-net shape, often require drilling for assembly purposes. There are very limited data on the machining of plastic material. Manufacturers often rely on data and models established for metals. The machining of plastics, although limited to assembly purposes, or to the removal of excess materials, requires an understanding of the behavior of these materials during the machining in order to obtain better quality parts. It is even more important for recycled plastics, which are inhomogeneous, contain pores, and most often, are made with more than one type of product. This work analyzes the machining of recycled plastics in order to establish and compare their machining models with those traditionally used for metals, and to better understand the behavior of the plastics during machining. The workpiece is drilled at different process conditions and at different temperatures. The process performance indicators such as cutting forces, chip formation, and chip form are analyzed. The originality of this work resides in its study of chip formation and the effects of the preset workpiece temperature on the drilling mechanisms. It is found that there is a range of critical temperatures of transition for plastics similar to the Charpy impact ductile-brittle temperature separating the domain of low cutting force and long and spiral chip from that of high cutting force corresponding to the accordion-type of chip. A parameter describing this phenomenon is defined. It is also found that for low- to moderate-speed operations, the cutting speed has very little effect on the cutting forces, which depend mainly on the feed rate and the workpiece temperature. The relationship between the drilling forces and the feed rate established for metals remain valid, but the exponent of the feed rate for the thrust force is lower. The thrust force and the tangential force are proportional to the feed rate exponent 0.4 compared to 0.8 for metals when drilling workpiece at room temperature or below.