Finite element analysis of planar twist channel angular extrusion (PTCAE) as a novel severe plastic deformation method

A new severe plastic deformation (SPD) method based on equal channel angular pressing (ECAP) is introduced for producing ultrafine grains in bulk alloys, entitled as “Planar twist channel angular extrusion (PTCAE)”. In PTCAE method, there is additional angle, α, (plus φ and ψ angles in ECAP method) which represents angle associated with the lateral reversal arc of curvature in deformation zone. Three dimensional finite element method (FEM) simulations of both ECAP and PTCAE processes were performed in order to investigate the plastic deformation state of processed samples and, moreover, the effect of different die geometry (in terms of variation of planar twist angle) on plastic strain distribution and magnitude. Results revealed that PTCAE process related with ECAP process can impose higher strain values in different shear planes simultaneously in one deformation zone. Therefore, PTCAE can produce UFG or nanostructured materials better than ECAP method which has simpler design and significantly higher efficiency compared with other new SPD processes.     

Employing severe plastic deformation to the processing of electrical discharge machining electrodes

The tool electrode has a significant role in electrical discharge machining (EDM) performance, as it affects machining efficiency, surface quality and the geometrical accuracy of the machined component. This study presents a new approach for developing a pure copper electrode using severe plastic deformation (SPD) to enhance the machining characteristics during EDM. Equal channel angular pressing (ECAP) is selected because it is the most successful SPD method of processing bulk materials. Finite element analysis, microstructural assessment as well as nanoindentation tests are carried out to determine the behavior of pure copper after one and two ECAP passes. The effectiveness of EDM when using ECAP-treated electrodes is evaluated by introducing new techniques of measuring the volumetric overcut (VOC) and corner sharpness. In addition, tool wear rate (TWR), material removal rate (MRR), electrode wear ratio, surface roughness, surface crack density and the critical crack zone are studied. The results emphasize that an electrode subjected to one pass of ECAP can enhance the workpiece accuracy by decreasing the VOC and increasing corner sharpness by 13 and 66%, respectively. It is also revealed that the nanohardness enhancement following ECAP leads to lower TWR and electrode wear ratio. An investigation of the surface characteristics indicates a thinner recast layer is achieved when using one ECAP pass-treated electrode, which leads to 26% lower surface crack density.     

多模式超声振动等径角挤压超细晶纯铝成形机理研究

超细晶金属材料由于具有优异的力学性能,特别适合微小金属零件的塑性成形。大塑性变形法是制备超细晶金属材料的常用方法,等径角挤压法被认为是最具有发展前景的大塑性变形方法之一。传统等径角挤压需要通过多道次的应变量累积来获得超细晶材料,制备效率较低。将超声振动与等径角挤压过程相结合可以有效减小挤压成形载荷,提高等径角挤压制备超细晶的性能和效率。现有研究主要采用工具辅助超声振动模式,提出并研发基于工件辅助超声振动模式的等径角挤压成形工艺,并对不同超声振动模式1070纯铝等径角挤压成形机理进行对比研究,研究工具超声振动和工件超声振动两种不同振动方式对晶粒道次细化能力的影响规律。结果表明,随着超声功率的增大,工具超声振动和工件超声振动的超声软化效应逐渐增强,能更大幅度降低等径角挤压成形力,并提高晶粒道次细化能力。工件超声振动比工具超声振动更有利于吸收超声能量,从而能更有效提升超细晶金属的制备效率。     

不同硬化模型对铝合金板冲压成形模拟结果的影响

研究不同塑性变形硬化模型对汽车5182-O铝合金板材冲压成形模拟结果的影响。采用材料单向拉伸试验得到应力应变关系曲线,基于Hollomom、Krupskowsky与Power方程对曲线进行拟合,建立材料室温下塑性变形硬化模型,对厚度为1.5 mm和0.85 mm的5182板材进行冲压试验和有限元模拟分析,对比分析冲压试验与模拟结果。试验与模拟结果显示,当板料厚度为1.5 mm时,板料冲压试验的成形力最大为42.95 kN,板料拉深深度为30.58 mm,基于Power方程计算得到的最大成形力为41.5kN与试验结果比较接近,Hollomom方程计算得到的拉深深度为30.546 mm,板材成形厚度分布与试验结果比较接近;当板料厚度为0.85 mm时,板料冲压试验的成形力最大为34.47kN,板料拉深深度为33.792 mm,基于Power方程计算得到的最大成形力为34.27 kN与试验结果比较接近,Hollomom方程计算得到的拉深深度为33.636 mm,板材成形厚度分布与试验结果比较接近。基于三种硬化模型铝合金冲压成形过程的计算模拟分析结果,并通过与试验对比得到不同硬化模型对铝合金板材冲压成形计算模拟的影响,进一步为汽车铝合金覆盖件在成形工艺的研究分析提供理论指导。     

Analytical modeling of work hardening of duplex steel alloys in the milling process

Severe plastic deformation in cutting operations such as milling might change mechanical properties (especially the strength and hardness) of the machined surface and its underlying layers. This phenomenon called work hardening and reduces machinability. This study presents an analytical solution to calculate the work hardening of the upper layers of the workpiece in the milling process of 2205 duplex stainless steel. In this regard, the stresses in the cutting regions are calculated to find the stress and temperature fields in the workpiece. Then the strain and strain rate values are calculated for each point of the surface and subsurface layers using the determined stress field. Finally, the Johnson-Cook material model is used to calculate flow stress and work hardening. Experimental results of the different machining conditions have been used to validate the proposed model. However, comparisons of subsurface microhardness and resultant cutting force obtained by an analytical model with experimental tests showed that the model properly predicts the amount of work hardening.

     

等通道转角挤压下变形体长度对应力的影响及开裂判据分析

利用有限元软件ANSYS对2024铝合金等通道转角挤压(Equal-channel angular pressing,ECAP)过程进行计算模拟,得到等效正应力(Von Mises应力)、切应力随变形体长度及下行位移的变化,结合ECAP试验,研究ECAP形变开裂的判据.结果表明,无论变形体长度及下行位移如何变化,等效正应力都小于材料的抗拉强度,而切应力则随着变形体长度的降低总体上呈下降趋势,当变形体的长度小于某一临界值时,计算模拟表明最大切应力小于材料的抗剪强度,样品开裂倾向大幅降低,二者相当吻合.研究结果不仅说明"最大切应力不大于材料抗剪强度"可以作为材料ECAP形变过程中不出现开裂现象的判据,而且也为材料的ECAP加工提供理性研究思路,即先实测材料的应力-应变曲线,然后基于上述判据对模具结构、材料预处理工艺及样品尺寸等进行优化设计.     

Rigid viscoplastic finite element analysis of the gas-pressure constrained bulging of superplastic circular sheets into cone disk shape dies

This paper describes the development of a rigid viscoplastic finite element formulation for analysing the gas-pressure constrained bulging processes of superplastic circular sheets in cone disk shape dies. In this formulation, the effects of strain hardening and strain-rate sensitivity of materials are included, and the boundary friction condition is introduced into the formulation in the form of friction functional. The finiteelement model based on the membrane theory is developed, and then applied to simulate superplastic constrained bulging processes. The solutions by the rigid viscoplastic finite element method are compared with existing experimental data. The influences of the geometrical parameters of the dies, the friction factor in the friction functional, the strain hardening and the strain-rate sensitivity on the inhomogeneity of thickness distribution are studied in detail.     

An investigation on manufacturing process parameters of CNG pressure vessels

Mass production of CNG pressure vessels requires an accurate understanding of process effective parameters. In this paper, the finite-element method has been used to study the vessel manufacturing parameters. The FE model has been verified by experimental results. The entire manufacturing process, including deep drawing, redrawing and ironing, of an aluminum liner sample of CNG pressure vessels (without spinning) have been simulated. The deep drawing process has been modeled by using three types of dies: flat, conical and tractrix; then drawing force and wall thickness variations have been compared. In order to achieve the final diameter of the liner, the redrawing process has been implemented in a conical die. To obtain a uniform wall thickness, the ironing process has been simulated in two stages, and the required force and die angle for each process have been extracted. The result of this work presents an integrated perspective for decision-making on the manufacturing of CNG liners.     

Tribological properties of the AZ91D magnesium alloy hardened with silicon carbide and by severe plastic deformation

Results of investigation of the tribological contact characteristics of R18 tool steel in interface with AZ91D magnesium alloy hardened with SiC disperse powder filler and by severe plastic deformation (SPD)—specifically, equal-channel angular pressing (ECAP)—are presented. It is established that introduction of the SiC powder filler into the magnesium alloy increases the friction coefficient and reduces the wear rate. The size and volume of the powder filler particles, the normal load, and the relative sliding velocity influence these tribological characteristics. SPD of the original material leads to reduction of the molecular component of the friction coefficient.     

忽略误差时三环减速器的参数对受力和均载状况的影响

建立了考虑三环传动系统过约束特性的弹性静力分析模型，计算分析了三环减速器的受力情况。并以某型号减速器为例，分析了减速器结构参数时其受力状态的影响，探讨了改善三环减速器均载情况的措施。     

Theoretical and experimental study on the effects of explosive forming parameters on plastic wrinkling of annular plates

Wrinkling is increasingly becoming one of the most common and troublesome modes of unacceptable deformation in sheet metal-forming prediction that is very important on the design of die geometry and processing parameters. In an effort to provide a reliable and efficient tool to predict the critical blank holding force for prevention of wrinkling, an analytical model for flange wrinkling in high-velocity forming processes, such as explosive forming, is presented here. With consideration of constant blank holder force and using a combination of energy method and plastic bending theory, the critical radial displacement and number of wrinkling waves are obtained. For validation, some experimental tests have been performed that their results have adequate agreements with the analytical ones. Moreover, the effects of process parameters such as blank holding force, radii ratio, and material mechanical properties on wrinkling behavior has been discussed.     

Determination of the yield <Emphasis Type="Italic">loci</Emphasis> of four sheet materials (AA6111-T4, AC600, DX54D+Z,and H220BD+Z) by using uniaxial tensile and hydraulic bulge tests

In sheet metal forming simulation, a flow curve and a yield criterion are vital requirements for obtaining reliable numerical results. It is more appropriate to determine a flow curve by using biaxial stress condition tests, such as the hydraulic bulge test, than a uniaxial test because hardening proceeds higher strains before necking occurs. In a uniaxial test, higher strains are extrapolated, which might lead to incorrect results. The bulge test, coupled with the digital image correlation (DIC) system, is used to obtain stress–strain data. In the absence of the DIC system, analytical methods are used to estimate hardening. Typically, such models incorporate a correction factor to achieve correlation to experimental data. An example is the Chakrabarty and Alexander method, which uses a correction factor based on the n value. Here, the Chakrabarty and Alexander approach was modified using a correction factor based on normal anisotropy. When compared with DIC data, the modified model was found to be able to better predict the hardening curves for the materials examined in this study. Because a biaxial flow curve is required to compute the biaxial yield stress, which is an essential input to advanced yield functions, the effects of the various approaches used to determine the biaxial stress–strain data on the shape of the BBC2005 yield loci were also investigated. The proposed method can accurately predict the magnitude of the biaxial yield stress, when compared with DIC data, for all materials investigated in this study.     

径向磁化的双筒永磁轴承轴向磁力研究

为了解决径向磁化的双筒永磁轴承轴向磁力数值计算复杂及缺乏计算轴向磁力的工程化解析模型等问题,该文在分析磁筒气隙磁导的基础上,结合磁通连续性原理和稀土永磁材料特性,通过电磁场理论中的虚位移法得到了该型轴承工程化轴向磁力解析数学模型。模型表明:该型轴承轴向磁力与磁筒剩磁的平方成正比;轴向磁力随磁筒间隙半径、轴向长度及磁路总磁导的增大而增大,随磁筒间隙的增大而减小。在轴承正常轴向工作范围内,该模型计算值与实验值相当吻合。     

The influence of the cutting tool microgeometry on the machinability of hardened AISI 4140 steel

The aim of this work is to define the cutting conditions that allow the dry drilling of carbon fiber reinforced epoxy (CFRE) composite materials taking into consideration the quality of the drilled holes (the exit delamination factor and the cylindricity error) and the optimum combination of drilling parameters. A further aim is to use grey relational analysis to improve the quality of the drilled holes. The machining parameters were measured according to 3³ full factorial parameter designs (27 experiments with independent process variables). The experiments were carried out under various cutting parameters with different spindle speeds and feed rates. Drilling tests were done using WC carbide, high-speed steel (HSS), and TiN-coated carbide drills. The experiment design was accomplished by application of the statistical analysis of variance (ANOVA). Results show that the thrust force is mainly influenced by the tool materials and the feed rate, which has a strong influence on the exit delamination factor. On the other hand, the spindle speed particularly affects the cylindricity error of the holes. Correlations were established between spindle speed/feed rate and the various machining parameters so as to optimize cutting conditions. These correlations were found by quadratic regression using response surface methodology (RSM). Finally, tests were carried out to check the concordance of experimental results.     

The Effect of Grain Size on Stribeck Curve and Microstructure of Copper Under Friction in the Steady Friction State

In the present work, the effect of grain size on the friction and wear behavior of a copper (Cu) samples under different lubricant conditions was studied. The structural evolution of Cu subsurface layers under friction in different lubricant conditions was considered. All friction tests were conducted under laboratory conditions using a block-on-ring rig. The effects of sliding velocity and load on the friction coefficient and wear rate of Cu with different grain size (1, 30, and 60 μm) were analyzed. The Cu samples with the average grain size of 1 μm were obtained due to severe plastic deformation (SPD) by equal channel angular pressing (ECAP). The Stribeck curves for Cu samples with different virgin grain sizes were considered. Elasto-hydrodynamic lubrication (EHL) and boundary lubrication (BL) regions were mainly studied in the present work. Similar Stribeck curves were found out for Cu samples with different virgin grain size. A load of the transition from the EHL to BL region was increased with a decrease of the grain size. While the friction coefficients were similar in the EHL and BL regions for the samples with different grain sizes, the wear rate was increased remarkably with an increase the virgin grain size. Flow localization during friction in the BL region led to formation of the vortex structure in subsurface layers. Based on the dependence of the microhardness upon the depth, the degree of hardening (H) was evaluated. A correlation between the coefficient of wear and the deformation hardening of Cu samples with different virgin grain sizes was revealed. In order to take into account the effect of the grain size and to predict the Stribeck curve, a parameter, K, as the ratio between hardness of tested and annealed samples, was incorporated into the lubricant number. The theoretical values of the Stribeck curve calculated for preliminary deformed Cu samples (d = 1 μm) and annealed samples (d = 60 μm) were well coincided with the experimental results.     

The use of a shear instability criterion to predict local necking in sheet metal deformation

A shear instability criterion can provide a consistent approach to the onset of local necking in sheet metal forming under biaxial stretching. The neck is anticipated to initiate in the direction of pure shear when the shear stress attains some critical value. The yield function proposed by Hill is employed and the material is assumed to display only normal anisotropy. Empirically it is found that the additional material parameter required by this yield function is simply related to R, the coefficient of normal anisotropy, for a number of materials. This allows the limit strain to be predicted in terms of three well established plastic properties, viz. work hardening coefficient, coefficient of normal anisotropy and initial pre-strain. The influence of these on the limit strain curve is analysed and the coefficient of work hardening shown to play the most important role. Data available in the literature are employed in a comparison of the present theory with that due to Marciniak. In general the predicted limit strains are in reasonable agreement with the trend of experimental results for a wide range of materials. In the case of isotropic materials with work hardening coefficients in the range 0·2-0·6 predictions from the present theory are almost identical with those from that presented by Stören and Rice. The theory presented here exhibits a good correlation with experimental limit strains for materials with high work hardening coefficients, of approx. 0·4 or more. Generally, for low work hardening materials, with coefficients of 0·25 or less, the shear instability theory tends to an underestimate of limit strains and a Marciniak type of analysis may be more appropriate. However, bearing in mind the scatter of the experimental data the present theory constitutes a safe lower bound on limit strains and, in addition, has the advantage of simplicity in the mathematical calculation required.     

MOLECULAR APPROACH TO MATERIAL DETACHMENT MECHANISM DURING CHEMICAL MECHANICAL PLANARIZATION

Mechanistic numerical analysis and molecular dynamics (MD) simulation are employed to understand the material detachment mechanism associated with chemical mechanical polishing. We investigate the mechanics of scratch intersection mechanism to obtain a characteristic length scale and compare the theoretical predictions with previous experimental observations on ductile copper discs at the micro-scale. First, an analytical model is developed based on mechanics of materials approach. The analytical model includes the effects of strain hardening during material removal as well as the geometry of indenter tip. In the next step, molecular simulations of the scratch intersection are performed at the atomistic scale. The embedded atom method (EAM) is utilized as the force field for workpiece material and a simplified tool-workpiece interaction is assumed to simulate material removal through scratch intersection mechanism. Both models are utilized to predict a characteristic length of material detachment related to material removal during scratch intersection. The predictions from two approaches are compared with experimental observations in order to draw correlations between experiment and simulation. The insights obtained from this work may assist in understanding the mechanism for chemical mechanical planarization (CMP), and even be applied to other different machining and polishing events.     

MOLECULAR APPROACH TO MATERIAL DETACHMENT MECHANISM DURING CHEMICAL MECHANICAL PLANARIZATION

Mechanistic numerical analysis and molecular dynamics (MD) simulation are employed to understand the material detachment mechanism associated with chemical mechanical polishing. We investigate the mechanics of scratch intersection mechanism to obtain a characteristic length scale and compare the theoretical predictions with previous experimental observations on ductile copper discs at the micro-scale. First, an analytical model is developed based on mechanics of materials approach. The analytical model includes the effects of strain hardening during material removal as well as the geometry of indenter tip. In the next step, molecular simulations of the scratch intersection are performed at the atomistic scale. The embedded atom method (EAM) is utilized as the force field for workpiece material and a simplified tool-workpiece interaction is assumed to simulate material removal through scratch intersection mechanism. Both models are utilized to predict a characteristic length of material detachment related to material removal during scratch intersection. The predictions from two approaches are compared with experimental observations in order to draw correlations between experiment and simulation. The insights obtained from this work may assist in understanding the mechanism for chemical mechanical planarization (CMP), and even be applied to other different machining and polishing events.     

缩口过程的刚塑性有限元分析

考虑到材料应变硬化和摩擦边界条件等,应用刚塑性有限元法分析窗口过程,得出用不同模角缩口时工件的应力、应变分布和变形情况,讨论了模角对变形力和等效应变速率、静水压力分布的影响。对刚塑性有限元计算的结果与试验结果进行比较,并提出缩口的最佳模角。     

Segregation of solute elements at grain boundaries in an ultrafine grained Al-Zn-Mg-Cu alloy

The solute segregation at grain boundaries (GBs) of an ultrafine grained (UFG) Al-Zn-Mg-Cu alloy processed by equal-channel angular pressing (ECAP) at 200 °C was characterised using three-dimensional atom probe. Mg and Cu segregate strongly to the grain boundaries. In contrast, Zn does not always show clear segregation and may even show depletion near the grain boundaries. Trace element Si selectively segregates at some GBs. An increase in the number of ECAP passes leads to a decrease in the grain size but an increase in solute segregation at the boundaries. The significant segregation of alloying elements at the boundaries of ultrafine-grained alloys implies that less solutes will be available in the matrix for precipitation with a decrease in the average grain size.