A damage related thermo-viscoplastic constitutive model of metallic materials under high rate deformation

Abstract

A constitutive model considering the effects of strain hardening, strain rate hardening, thermal softening and material damage softening is suggested. In order to take the effect of material damage into account, a strain softening term is added in Johnson–Cook flow stress law. The model can predict the overall deformation process of metallic materials at high strain rates and a simple way is provided to determine the coefficients of softening term.     

Influence of heat treatment and cutting speed on chip segmentation of age hardenable aluminium alloy

Abstract

A change in chip shape has been observed as a function of age hardening and cutting speed during high speed milling of the aluminium alloy 7075. In order to study this effect systematically, the aluminium alloy was heat treated to produce different precipitation states and machined under carefully controlled conditions at cutting speeds between 1000 and 7000 m min^-1. The underaged state shows local shearing producing segmented chips. The degree of segmentation increases with cutting speed. In contrast, the overaged state shows continuous chips up to the highest cutting speeds. The chips obtained with the peak aged state show a fluctuation between segmented and continuous parts. These results can be understood in terms of the differing work softening/hardening behaviour of the under- and overaged states owing to the specific interactions between dislocations and precipitates during chip formation.     

Burr formation during drilling of mild steel at different machining conditions

This paper investigates burr generation while drilling of mild steel grade 350. The influences of feed, speed and point angle on burr height, thrust force, torque and chip ratio are investigated to correlate with burr height. It was found that, the burr height reduces gradually with the rise of speed at minimum feed and point angle. At maximum feed and point angle, initially the height rises with speed and then reduces as speed rises further. The maximum burr is 720 μm at 584 rpm. At maximum point angle and speed, the height rises initially and then reduces as the feed rises where the trend is opposite at minimum feed and speed. The maximum burr is 1223.15 μm at 0.25 mm/rev feed. The variation of burr height with point angle is similar to that with the variation of feed. The maximum burr is 1230 μm at 125°point angle. The trends of thrust forces, torques and chip ration with the variation of different parameters are not similar to that of burr height in most of the cases. The complex interaction between strain hardening and thermal softening plays the main role in burr formation for the considered material.     

基于离散单元法的刨煤机刨削性能分析及试验研究

为研究刨煤机的刨削性能,以BH38/2×400型刨煤机为研究对象,对其应用煤层煤样性质进行测定,确定出煤壁材料本征参数并构建出煤壁三维黏结接触模型。通过对刨煤机刨削过程进行数值模拟,得到刨削过程中刨头的受力信息、煤炭颗粒运动规律以及刨削后煤壁表面残留状态,分析了不同刨削深度和刨速条件下刨刀受力的变化规律。研究结果表明:刨刀受力的线性累加使得刨头刨削阻力和煤壁挤压力较大,刨头受力接近高斯分布;刨削过程中刨削深度过大会造成能量积聚与释放急剧增加,刨削速度改变对刨刀受力影响较小。通过动力学仿真发现刨刀合金头镶嵌处和刀体背脊处的受力较为恶劣,刨刀前侧合金头位置出现严重的变形;对不同条件下刨煤机刨削过程进行了试验测试,测试得到刨刀三向力与仿真结果最大误差分别为7.21%,7.91%和9.41%,通过追踪刨刀的工作状态,发现刨刀损坏形式与仿真结果一致,验证了离散元数值模拟的准确性;研究结果为刨煤机刨削性能评价及其结构优化提供一种新的方法。     

Broaching Performance of Superalloy GH4169 Based on FEM

The nickel-based superalloy GH4169 is an important material for high temperature applications in the aerospace industry. However, due to its poor machinability, GH4169 is hard to be cut and generates saw-tooth chips during high speed machining, which could significantly affect the dynamic cutting force, cutting temperature fluctuation, tool life, and the surface integrity of the parts. In this paper, the saw-tooth chip formation mechanism of superalloy GH4169 was investigated by the elasto-viscoplastic finite element method (FEM). Using the finite element software of ABAQUS/Explicit, the deformation of the part during high speed machining was simulated. The effective plastic strain, the temperature field, the stress distribution, and the cutting force were analyzed to determine the influence of the cutting parameters on the saw-tooth chip formation. The study on broaching performance has great effect on selecting suitable machining parameters and improving tool life.     

45CrNiMoVA高速切削条件下本构关系建模技术研究

为建立连续介质材料高速切削的材料本构关系模型,以45Cr Ni Mo VA材料为研究对象,通过准静态扭转试验和直角自由切削试验相结合的方法,建立了满足高速切削仿真要求的45Cr Ni Mo VA材料的Johnson-Cook本构模型.采用建立的Johnson-Cook本构模型参数,利用ABAQUS有限元分析软件建立了直角自由切削的有限元模型,对切削过程中的切屑厚度、主切削力、进给抗力进行了仿真,并将仿真预测值同试验测量值进行了对比.结果表明:由于切削仿真过程中刀具不存在磨损,进给抗力的仿真误差较大;主切削力和切屑厚度的仿真预测值与试验测量值的误差在10%之内,模型的准确度较好.最后,利用VB和C语言,开发了Johnson-Cook材料本构集成建模系统,并验证了其使用效果的实用性.     

A Study of High-Speed Milling Characteristics of Nitinol

High-speed milling (HSM) has many advantages over conventional machining. Among these advantages, the lower cutting force associated with the machining process is of particular significance for Nitinol alloys because their machined surfaces show less strain hardening. In this article, a systematic study has been carried out to investigate the machining characteristics of a Ni_50.6Ti_49.4 alloy in HSM. The effects of cutting speed, feed rate, and depth of cut on machined surface characteristics and tool wear are studied. It is found that an increase in cutting speed has resulted in a better surface finish and less work hardening. This is attributed to the reduction of chip cross-sectional area or chip thickness, which thus leads to a lower cutting force or load.     

Finite element simulation of the micromachining of nanosized-silicon-carbide-particle reinforced composite materials based on the cohesive zone model

A finite element method based on the cohesive zone model was used to study the micromachining process of nanosized silicon-carbide-particle(SiCp) reinforced aluminum matrix composites. As a hierarchical multiscale simulation method, the parameters for the cohesive zone model were obtained from the stress-displacement curves of the molecular dynamics simulation. The model considers the random properties of the siliconcarbide-particle distribution and the interface of bonding between the silicon carbide particles and the matrix.The machining mechanics was analyzed according to the chip morphology, stress distribution, cutting temperature, and cutting force. The simulation results revealed that the random distribution of nanosized SiCp causes non-uniform interaction between the tool and the reinforcement particles. This deformation mechanics leads to inhomogeneous stress distribution and irregular cutting force variation.     

Spanende Bearbeitung von Hartlegierungen – Drehen und Schleifen. Teil II: Drehen von Hartlegierungen

Metal Cutting of Hard Alloys – Turning and Grinding. Part II: Turning of Hard Alloys Turning tests were carried out on selected hard alloys on iron (FeCr12C2.1, FeCr13Nb9MoTiC2.3, FeCr14Mo5WVC4.2) and cobalt basis (CoCr29W5C1.3) in a cutting speed range of between v_c = m/min and 180 m/min. Polycrystalline cubic boron nitride (PCBN) turned out to be a suitable tool material. Subsequent examinations focused on evaluating the mechanisms of chip formation, cutting tool wear and surface integrity of the workpiece. During turning of hard alloys the formation of chips is primarily influenced by the ductility and fracture toughness of the work material. While a ductile matrix enables the formation of highly deformable chips, the chips stemming from martensitically hardened alloys show low deformation. As the cutting depth increases shear and segmented chips are chiefly produced. Type and arrangement of the hard phases play a significant role. Adhesion is the main wear mechanism impacting the cutting face of the tool. Particularly, strong adhesion effects will arise during the machining of the work hardening alloy on cobalt basis. A high cobalt content of the metallic bonding phase of the PCBN cutting tool appears to be a disadvantage with this type of work material. When machining alloys on iron basis adhesion is promoted by the mechanical linking of alloy-specific hard phases to the cutting material binder. Abrasion primarily acts on the flank. The hard carbides of the work material produce typical grooves in the cutting edge zone of the tool. The flank wear increases as the carbide content goes up. As the cutting speed rises the tool wear ascertained passes through a minimum. Whereas the formation of built-up cutting edges predominates at lower speeds, a thermal softening of the PCBN binder takes place and is dominating at high cutting speeds. The location of the wear minimum depends not only on the cutting temperature but also on the strain hardening capability of the metal matrix. Raising the cutting speed will cause the cutting force to continuously reduce. The highest cutting forces are found for the Co-based alloy. The passive forces develop in line with cutting tool wear and vary with content and hardness of the hard phases involved. The selected process parameters also affect the surface near zone. With low cutting speeds and process temperatures the surface is mainly stressed mechanically. Carbides break or detach from the surrounding matrix. If the cutting speed and process temperature are increased the eutectic carbides (M₇C₃) are deformed together with the metal matrix. Microhardness profiles are indicative of near-surface strain-hardened zones after cutting of the Co-based alloy. Fe-based matrices do not show hardness changes worth mentioning. Although there are no new hardened zones noticeable even at maximum cutting speed, the matrix is nevertheless influenced thermally so that residual stresses will develop in the machined surface layer. In the lower cutting speed range the surface quality is characterized by flakes and material squeezing (Co-based alloy) and by spalling (Fe-based alloy). Only if the cutting speed is raised, a minor roughness is detected due to a potential deformation of eutectic hard phases.     

Microstructure – A Dominating Parameter for Chip Forming During High Speed Milling

High speed milling experiments with cutting speeds of up to 7000 m/min have been performed on an aluminium alloy (AlZnMgCu1.5, AA 7075) and a steel (40CrMnMo7). Both materials were heat treated to produce a variety of microstructures in order to investigate the influence of microstructure on segmentation during chip formation. In case of the aluminium alloy it has been shown that chip formation is governed by the precipitation state, but not by the hardness of the material. In contrast, hardness can be used for a qualitative prediction of chip shape of the steel. In all cases, the chip formation mechanisms remain essentially unaffected by cutting speed, i.e. no transition from continuous to segmented chips occurred.     

硅晶圆磨削减薄工艺的磨削力模型

硅晶圆磨削减薄是一种有别于传统磨削的材料加工方式。磨削减薄过程中,硅晶圆和砂轮同时绕旋转轴旋转,砂轮沿垂直方向连续进给去除材料,其中磨削力是磨削质量的决定性因素。目前,尚缺少一个用于硅晶圆磨削减薄工艺的磨削力预测模型。为了得到磨削力模型,分析了磨削减薄过程中的硅晶圆材料去除机理,将磨削力分为摩擦力和切屑力,考虑了磨粒运动轨迹,分别计算了单颗磨粒在法向和切向上的摩擦力和切屑力,最后基于有效磨粒总数建立了总磨削力模型。模型综合考虑了磨削参数、砂轮和硅晶圆的几何参数和材料性质对磨削力的影响。讨论了砂轮进给速度、晶圆转速和砂轮转速三个主要磨削参数对磨削力的影响,讨论了硅晶圆上晶向对磨削力的影响,给出了磨削力在硅晶圆面上沿径向的分布情况。     

Hardening and softening mechanisms of pearlitic steel wire under torsion

The mechanical behaviors and microstructure evolution of pearlitic steel wires under monotonic shear deformation have been investigated by a torsion test and a number of electron microscopy techniques including scanning electron microscopy (SEM) and transmission electron microscopy (TEM), with an aim to reveal the softening and hardening mechanisms of a randomly oriented pearlitic structure during a monotonic stain path. Significantly different from the remarkable strain hardening in cold wire drawing, the strain hardening rate during torsion drops to zero quickly after a short hardening stage. The microstructure observations indicate that the inter-lamellar spacing (ILS) decreases and the dislocations accumulate with strain, which leads to hardening of the material. Meanwhile, when the strain is larger than 0.154, the enhancement of dynamic recovery, shear bands (SBs) and cementite fragmentations results in the softening and balances the strain hardening. A microstructure based analytical flow stress model with considering the influence of ILS on the mean free path of dislocations and the softening caused by SBs and cementite fragmentations, has been established and the predicted flow shear curve meets well with the measured curve in the torsion test.     

THE EFFECT OF MATERIAL MICROSTRUCTURE ON MICROCUTTING PROCESSES

In the machining of mirror-like surfaces, a typical cutting depth of a few micrometers is common. With such a small depth of cut, chip formation takes place within individual grains of polycrystalline materials. In this article, orthogonal cutting of single copper crystals was performed in order to investigate the dependency of cutting deformation and surface quality on the crystallographic orientation of the substrate material. The experimental results show that the crystallographic orientation of the workpiece exerts a significant influence on the shear angle and the machined surface roughness. Cutting force variation with crystallographic orientation was analyzed on the basis of a microplasticity model. The trend in the variation of theoretical values of an effective Taylor factor (the shear strength) compares well with that of published experimental data on cutting forces.     

灰铸铁和球墨铸铁前托架钻削性能评价

从导热系数、铁屑形貌、钻孔内表面粗糙度、钻头顶部温度、钻削负载(扭矩、进给力)等方面,评价了QT450和HT250材质拖拉机前托架的钻削性能,并分析了钻削性能与微观组织的关系。结果表明,对HT250和QT450前托架钻削加工时,两者钻削负载没有明显差别;与HT250相比,QT450的石墨对基体的分割作用小,其基体组织中较多的铁素体,造成钻削加工时断屑和排屑困难,导致钻孔内表面粗糙度大;QT450导热系数低、铁屑与钻头摩擦严重,使得钻头温度较高;QT450有较小的切削力和铁屑对钻头较大的摩擦力,在这2个力作用下,QT450钻削负载与HT250基本相同。     

Machining and surface integrity of fibre-reinforced plastic composites

The current focus of manufacturing research on fibre-reinforced plastics (FRP) is composed of the search for efficient processing techniques capable of providing high quality machined surfaces. Very limited work has been performed to identify the influence of manufacturing processes like edge-trimming and drilling on material performance. Recent reports suggest that process-induced damage may affect the mechanical behaviour of FRP materials. Therefore an experimental study of orthogonal cutting was conducted on the edge trimming of unidirectional and multi-directional graphite/epoxy composites with polycrystalline diamond tools. The effects of tool geometry and operating conditions were evaluated from an analysis of chip formation, cutting force, and machined surface topography. All aspects of material removal were found to be primarily dependent on fibre orientation. Discontinuous chip formation was noted throughout this study, regardless of machining parameters. Three distinct mechanisms in the edge trimming of fibre-reinforced composite material including a combination of cutting, shearing, and fracture along the fibre/matrix interface were observed. An investigation conducted on the compression, flexural and impact strength of graphite/epoxy composites machined by both traditional and non-traditional techniques, confirms that manufacturing characteristics may not only affect bulk properties but also influence the initiation and propagation of failure.     

Effect of Cold Deformation on the Machinability of a Free Cutting Steel

The effect of cold deformation on the machinability of a free cutting steel has been investigated through characterization of the variation in size and morphology of the sulfide inclusions. The machinability has been indexed in terms of cutting force, surface roughness, and chip characteristics. The possible interrelations between inclusion characterizing parameters and machinability indices have been examined. Increased cold deformation was found to increase interinclusion spacing as well as aspect ratio of the inclusions. These variations in inclusion characteristics, in turn, were found to reduce the cutting forces and the chip length up to a critical amount of deformation. The results highlight the influence of inclusion characterizing parameters on machinability characteristics of free cutting steels.     

Influence of wire-EDM textured conventional tungsten carbide inserts in machining of aerospace materials (Ti–6Al–4V alloy)

Importance of this present investigation is to identify the influence of modified tool (tool with texturing) on the process of orthogonal turning of Ti–6Al–4V work material. To achieve the enhanced turning conditions, four different types of textures (plain conventional, cross, perpendicularly textured and parallel textured tool to the chip flow direction) were fabricated on the rake face of the tool insert and the lubricant used during the machining process is molybdenum disulfide (MoS₂). Machining forces (the force of cutting and feed), angle of shear, chip morphology, temperature distribution between tool and chip were measured. Shear strain and strain rate were also computed and compared with all type of cutting tools. Experimental results revealed that the cross-textured cutting tool exhibit an effective reduction in cutting force, friction, shear strain and strain rate. The favorable metal removal condition of curling chip with low diameter was achieved through cross-textured tool.     

Micromechanical modelling of strain hardening and tension softening in cementitious composites

This paper will describe a procedure for modelling the complete macroscopic response (including strain hardening and tension softening) of two short fibre reinforced cementitious composites and show how their microstructural parameters influence this response. From a mathematical point of view it is necessary to examine how bridging forces imposed by the fibres alter the opening of multiple cracks in elastic solids under unidirectional tensile loading. The strain hardening is essentially due to elastic bridging forces which are proportional to crack opening displacements. After a certain critical crack opening displacement is reached, some fibres progressively debond from the elastic matrix and thereafter provide a residual bridging force by frictional pull-out, while others continue to provide full bridging. This results in a kind of elasto-plastic bridging law which governs the initial tension softening response of the composite. Besides the usual square-root singularity at crack tips, the elasto-plastic bridging law introduces a logarithmic singularity at the point of discontinuity in the bridging force. These singularities have been analytically isolated, so that only regular functions are subjected to numerical integration. Unbridged multiple crack problems have in the past been solved using double infinite series which have been found to be divergent. In this paper a superposition procedure will be described that eliminates the use of double infinite series and thus the problem of divergence. It is applicable to both unbridged and bridged multiple cracks. The paper will end by showing how the model of multiple bridged cracks can accurately predict the prolonged nonlinear strain hardening and the initial tension softening response of two cementitious composites.