Friction behavior modeling and analysis in micro/meso scale metal forming process

Currently, a lot of know-how in conventional metal forming process cannot be directly applied to micro/meso forming processes due to so-called size effects. As a very important phenomenon in metal forming process, friction size effects are observed with an increasing degree of miniaturization. For microforming application, the input data of friction behaviors becomes critical to obtain accurate results for process simulation and traditional friction models are not reliable.     

Polycrystal models for the analysis of intergranular crack growth in metallic materials

In polycrystal materials the intergranular decohesion is one important damage phenomena that leads to microcrack initiation. The paper presents a mesoscale model, which is focused on the brittle intergranular damage process in metallic polycrystals. The model reproduces the crack initiation and propagation along cohesive grain boundaries between brittle grains. An advanced Voronoi algorithm is applied to generate polycrystal material structures based on arbitrary distribution functions of grain size. Therewith, the authors are more flexible to represent realistic grain size distributions. The polycrystal model is applied to analyze the crack initiation and propagation in statically loaded samples of aluminium on the mesoscale without the necessity of initial damage definition.     

Influences of temperature and grain size on the material deformability in microforming process

This paper investigated the influences of temperature and grain size on the deformability of pure copper in micro compression process. Based on the dislocation theory, a constitutive model was proposed taking into account the influences of forming temperature, Hall-Petch relationship and surface layer model. Vacuum heat treatment was employed to obtain various grain sizes of cylindrical workpieces, and then laser heating method was applied to heat workpieces during microforming process. Finite element (FE) simulation was also performed, with simulated values agreed well with the experimental results in terms of metal flow stress. Both the FE simulated and experimental results indicate that forming temperature and grain size have a significant influence on the accuracy of the produced product shape and metal flow behaviour in microforming due to the inhomogeneity within the deformed material. The mechanical behaviour of the material is found to be more sensitive to forming temperature when the workpieces are constituted of fine grains.     

Investigation of finite element mesh independence in rate dependent materials

Crystal plasticity theory is commonly used in finite element analyses to predict large strain ductility in single crystal and polycrystal deformation. In the rate-dependent formulation of the theory it is possible, for cases of simple deformation, to achieve an analytical solution that is independent of any effects due to the finite element mesh spacing. In this study single crystal and polycrystal models were subjected to alternative loading conditions. The effect of the mesh density on the generation of strain localisations and shear bands was investigated with regard to consistency of results. It was found that, prior to the initiation of a narrow shear band, it was possible to achieve a numerical result independent of mesh spacing. In the larger polycrystal analyses, an element size was identified that enabled the generation of a mesh independent solution. This allowed the accurate prediction of the mechanical behaviour of the model up to, and including, the failure point. The implications of this for small-scale metallic device design are discussed.     

CaCO3刚性粒子增韧HDPE的脆韧转变研究

研究了ＨＤＰＥ／ＣａＣＯ３增充体系中ＣａＣＯ３表面处理，粒径，含量及其体树脂分子量，结晶性与其材料缺口冲击强度，产生脆韧转变现象及其体晶态结构间的关系。结果表明，该共混体系中界面应力的应变诱导致结晶作用及其所引起的基体中伸展链晶体络结构的形态是该材料实现脆韧转变的重要原因。     

Fracture toughness of polycrystalline tungsten under mode I and mixed mode I/II loading

The fracture toughness of swaged polycrystalline tungsten was tested parallel and perpendicular to the swaging direction and under mixed mode I/mode II loading. The fracture mode is dominated by the microstructure and changed from all-transgranular cleavage in mode I to almost all-intergranular fracture in mode II. The mixed mode results can be related to two common failure criteria, the maximum tensile stress criterion (Maximum σ) and the maximum energy release rate criterion (Maximum G), but the large scatter in the data prohibits a clear distinction between the two criteria. Tests at 77 K show that the polycrystal is significantly tougher than the single crystal at this temperature. This is a consequence of the deflection of the crack into the grain boundaries and the imperfect texture (as compared to a single crystal) of the polycrystalline material.     

Computational micro–macro transitions and overall moduli in the analysis of polycrystals at large strains

This paper presents a numerical procedure for the computation of the overall moduli of polycrystalline materials based on a direct evaluation of a micro–macro transition. We consider a homogenized macro-continuum with locally attached representative micro-structure, which consists of perfectly bonded single crystal grains. The deformation of the micro-structure is assumed to be coupled with the local deformation at a typical point on the macro-continuum by three alternative constraints of the microscopic fluctuation field. The underlying key approach is a finite-element discretization of the boundary value problem for the fluctuation field on the micro-structure of the polycrystal. This results in a new closed-form representation of the overall elastoplastic tangent moduli or so-called generalized Prandtl–Reuss-tensors in terms of a Taylor-type upper bound term and a characteristic softening term which depends on global fluctuation stiffness matrices of the discretized micro-structure.     

Experimental and Numerical Study of Micro Deep Drawing of Copper Single Crystal

One of the problems in a micro-forming process is the grain size effect, which means the formed part consists of a single grain or several grains sometimes, so the material shows anisotropic or heterogeneous. Under these conditions, a conventional method, which based on the isotropic and homogeneous material hypothesis, is not suitable. In this paper, Experimental investigations into micro deep drawing of the copper single crystal were carried out and the pattern of the micro-cup and the drawing force were observed. Using crystal plasticity theory, a user material subroutine (VUMAT) was built and linked to ABAQUS, and the micro deep drawing was simulated according to the experimental configuration. The results show that earing occurs at mouth of the micro-cup. The profile, quantity, and location of ears depend on the crystalline orientation in the blank. The simulations are in good agreement with the experiments, which demonstrate that the crystal theory has the rationality and validity in micro-forming simulations.     

Size effects on process performance and product quality in progressive microforming of shafted gears revealed by experiment and numerical modeling

As one of the indispensable actuating components in micro-systems, the shafted microgear is in great production demand. Microforming is a manufacturing process to produce microgears to meet the needs. Due to the small geometrical size, there are uncertain process performance and product quality issues in this production process. In this study, the shafted microgears were fabricated in two different scaling factors with four grain sizes using a progressively extrusion-blanking method. To explore the unknown of the process, grain-based modeling was proposed and employed to simulate the entire forming process. The results show that when the grains are large, the anisotropy of single grains has an obvious size effect on the forming behavior and process performance; and the produced geometries and surface quality are worsened; and the deformation load is decreased. Five deformation zones were identified in the microstructures with different hardness and distributions of stress and strain. The simulation by using the proposed model successfully predicted the formation of zones and revealed the inhomogeneous deformation in the forming process. The undesirable geometries of microgears including material unfilling, burr and inclination were observed on the shaft and teeth of gear, and the inclination size is increased obviously with grain size. To avoid the formation of inclination and material unfilling, the punch was redesigned, and a die insert was added to constraint the bottom surface of the gear teeth. The new products had then the better forming quality. The full text can be downloaded at https://link.springer.com/article/10.1007/s40436-022-00414-0     

Synergistic Effects of Novel Superparamagnetic/Upconversion HA Material and Ti/Magnet Implant on Biological Performance and Long‐Term In Vivo Tracking

To solve the clinical challenges presented by the long‐term tracking of implanted hydroxyapatite (HA) bone repair material and to investigate the synergistic effects of superparamagnetic HA and a static magnetic field (SMF) on the promotion of osteogenesis, herein a new type of superparamagnetic/upconversion‐generating HA material (HYH‐Fe) is developed via a two‐step doping method, as well as a specially‐designed titanium implant with a built‐in magnet to provide a local static magnetic field in vivo. The results show that the prepared HYH‐Fe material maintains the crystal structure of HA and exhibits good cytocompatibility. The combined use of the superparamagnetic HYH‐Fe material and SMF can effectively and synergistically promote osteogenesis/osteointegration surrounding the Ti implants. In addition, the HYH‐Fe material exhibits distinct advantages in terms of both long‐term fluorescence tracking and microcomputed tomography (micro‐CT) tracking. The new material and tracking strategy in this study provide scientific feasibility and will have important clinical value for long‐term tracking and evaluation of implanted materials and the bone repair effect.     

晶体塑性模型描述多晶体循环加载中的Bauschinger效应

为了研究循环加载过程中织构对多晶材料Baushinger效应的影响,利用经典晶体塑性模型及含随动硬化的晶体塑性模型模拟AA6104铝合金循环加载力学行为.研究了多晶体中晶粒取向差异对材料宏观塑性行为的影响.详细分析了经典晶体塑性模型可描述多晶体循环加载Bauschinger效应机理,定量分析了多晶有限元模型中晶体取向差异对模拟结果的影响.结果表明多晶体中由于晶粒取向差异而造成的晶粒间相互作用力使得多晶体模型宏观卸载时晶粒内的残余应力是产生Bauschinger效应的主要原因,采用含随动硬化的晶体塑性模型能够较好地模拟具有织构的AA6014铝合金的循环加载过程.     

数值模拟技术在微成形研究中的应用

微成形技术具备高生产效率、高材料利用率和优异的成形质量,是一种极具发展前景的高精度加工技术。数值模拟技术作为一种先进的研究手段,可以在塑性加工中对材料的变形和工艺可行性等进行评估和预测,达到节约生产成本、缩短研发周期的作用。主要综述了数值模拟技术在微成形研究中的典型应用。介绍了数值模拟技术在研究材料性质和材料变形方面的应用,包括利用Voronoi方法和晶体塑性方法建立金属多晶体模型,研究了微成形过程中材料的变形机制和尺寸效应,建立了材料摩擦函数、构建了零件粗糙表面,研究了微成形过程中的摩擦行为;将晶粒大小、晶体取向与板料模型相关联,研究了微成形过程中薄板的回弹行为和成形极限。除此之外,也介绍了近年来微成形领域的许多新成形技术,如激光辅助微成形、水射流增量微成形、超声辅助微成形,以及数值模拟方法在这些新微成形技术方面的应用。最后,总结了数值模拟技术在微成形研究中所起的作用,并展望了该领域的未来发展趋势。     

Anisotropic trivalent ion conducting behavior in single crystals of aluminum tungstate-scandium tungstate solid solution

An aluminum tungstate-scandium tungstate solid solution was successfully grown in a single crystal form by modified Czochralski (CZ) method. The crystal grown was transparent and a satisfactory quality was examined by a polarizing microscope measurement. The Al³⁺ ion conductivity was considerably improved by forming the solid solution and these by expanding the crystal lattice size. The lattice expansion contributes greatly to enhancing the Al³⁺ ion conducting behavior especially along c-axis among the three axes in the orthorhombic symmetry. By comparing the ion conducting characteristics between each axis of the single crystal and the polycrystal, the enhancement of the Al³⁺ ion conductivity is ascribed to the cooperative effects of both ion conductivity increase along c-axis and the conductivity enhancement in the grain boundaries.     

特种能场辅助微塑性成形技术研究及应用

特种能场辅助微塑性成形技术是利用声、光、电、磁等特殊能量源对微型零件变形过程进行调控的先进制造技术。特种能场已被证明在宏观尺度下对于降低零件加工难度、提高尺寸精度、改善材料微观组织、提升构件力学性能、提高表面质量等存在促进作用。然而,在微塑性成形过程中,材料的变形特性在尺寸效应的影响下与宏观情况存在一定差异。梳理了特种能场辅助微塑性成形技术的研究进展,总结了微型零件在特种能场辅助下的成形特点。其中,着重综述了超声场辅助微成形中体积效应和表面效应的宏观表现及微观机理,展示了多种微成形工艺中超声场对微型零件成形质量的提升效果。同时,重点概述了电场辅助微成形时材料力学性能及微观组织演变规律,剖析了电致塑性效应产生的本质原因。此外,列举了激光、电磁、高压流体等其他特种场辅助微成形的原理及作用效果。最后,对特种能场辅助微成形的发展趋势进行了展望。     

Modeling of strengthening and softening in inelastic nanocrystalline materials with reference to the triple junction and grain boundaries using strain gradient plasticity

The work presented here provides a generalized structure for modeling polycrystals from micro- to nano-size range. The polycrystal structure is defined in terms of the grain core, the grain boundary and the triple junction regions with their corresponding volume fractions. Depending on the size of the crystal from micro to nano, different types of analyses are used for the respective different regions of the polycrystal. The analyses encompass local and nonlocal continuum or crystal plasticity. Depending on the physics of the region dislocation-based inelastic deformation and/or slip/separation is used to characterize the behavior of the material. The analyses incorporate interfacial energy with grain boundary sliding and grain boundary separation. Certain state variables are appropriately decomposed into energetic and dissipative components to accurately describe the size effects. This new formulation does not only provide the internal interface energies but also introduces two additional internal state variables for the internal surfaces (contact surfaces). One of these new state variables measures tangential sliding between the grain boundaries and the other measures the respective separation. Additional entropy production is introduced due to the internal subsurface and contacting surface. A multilevel Mori–Tanaka averaging scheme is introduced in order to obtain the effective properties of the heterogeneous crystalline structure and to predict the inelastic response of a nanocrystalline material. The inverse Hall–Petch effect is also demonstrated. The formulation presented here is more general, and it is not limited to either polycrystalline- or nanocrystalline-structured materials. However, for more elaborate solution of problems, a finite element approach needs to be developed.     

The loading history and crystal orientation effects on the size-dependency of single crystal diamond properties

Much research has been conducted to study the size-dependency of material properties, as can be found from the open literature. Recent results on combined size, rate and thermal effects further demonstrate the dominant influence of specimen size on material strength, as compared with the loading rate and thermal effects. However, little has been done to understand the loading history and crystal orientation effects on the size-dependency of material properties. To evaluate the safety and integrity of MEMS devices under general loading conditions, a series of molecular dynamics simulations are performed to investigate the size-dependency of single crystal diamond properties with various crystal orientations under shear/tension and tension/shear loading conditions. It appears from the preliminary findings that the loading history and crystal orientation do have certain influence on the size-dependency of material properties. Specifically, the failure pattern is insensitive to the loading history, which provides useful information for formulating a multi-scale material model under general loading conditions.     

固态晶体生长及其在平面波导结构制备上的应用

研究了烧结温度、晶粒大小以及掺杂离子对固态晶体生长的影响。在热致固态晶体生长的过程中, 接近键合面的陶瓷会被单晶诱导而转变成取向一样的单晶, 同时形成稳固的键合。实验利用这种方法制备核层为12at%Yb:YAG陶瓷(100 mm), 包层为YAG晶体的平面波导结构。这种方法制备的波导结构没有Yb离子的浓度扩散现象。同时, 还测试了平面波导结构的激光性能: 通过设计平平腔实现了1.14 W, 斜率效率16.09%的激光输出; 在三镜腔中实现1.83 W, 斜率效率9.59%的激光输出。并且激光输出在1027.5 nm到1033.5 nm间连续可调。     

A new implementation of the spectral crystal plasticity framework in implicit finite elements

We present a new implementation of a computationally efficient crystal plasticity model in an implicit finite element (FE) framework. In recent publications, we have reported a standalone version of a crystal plasticity model based on fast Fourier transforms (FFTs) and termed it the spectral crystal plasticity (SCP) model. In this approach, iterative solvers for obtaining the mechanical response of a single crystal of any crystallographic orientation subjected to any deformation mode are replaced by a database of FFTs that allows fast retrieval of the solution. The standalone version of the code facilitates simulations of relatively simple monotonic deformation processes under homogeneous boundary conditions. In this paper, we present a new model that enables simulations of complex, non-monotonic deformation process with heterogeneous boundary conditions. For this purpose, we derive a fully analytical Jacobian enabling an efficient coupling of SCP with implicit finite elements. In our implementation, an FE integration point can represent a single crystal or a polycrystalline material point whose meso-scale mechanical response is obtained by the mean-field Taylor-type homogenization scheme. The finite element spectral crystal plasticity (FE-SCP) implementation has been validated for several monotonic loading conditions and successfully applied to rolling and equi-channel angular extrusion deformation processes. Predictions of the FE-SCP simulations compare favorably with experimental measurements. Details of the FE-SCP implementation and predicted results are presented and discussed in this paper.     

A micro‐mechanical damage model based on gradient plasticity: algorithms and applications

As soon as material failure dominates a deformation process, the material increasingly displays strain softening and the finite element computation is significantly affected by the element size. Without remedying this effect in the constitutive model one cannot hope for a reliable prediction of the ductile material failure process. In the present paper, a micro‐mechanical damage model coupled to gradient‐dependent plasticity theory is presented and its finite element algorithm is discussed. By incorporating the Laplacian of plastic strain into the damage constitutive relationship, the known mesh‐dependence is overcome and computational results are uniquely correlated with the given material parameters. The implicit C¹ shape function is used and can be transformed to arbitrary quadrilateral elements. The introduced intrinsic material length parameter is able to predict size effects in material failure. Copyright © 2002 John Wiley & Sons, Ltd.     

A texture component model for anisotropic polycrystal plasticity

There are many crystallographic textures which can be approximated by a small number of texture components [see, e.g., Int. J. Mech. Sci. 31(7) (1989) 549]. In some cases, such texture components can be described by central distributions. Central distributions are characterized by a mean orientation and a half width. The classical Taylor model for viscoplastic polycrystals assumes that a discrete set of single crystals deforms homogeneously. If the viscoplastic version of the Taylor model is numerically implemented then the crystallite orientation distribution function (codf) is usually discretized by a set of Dirac distributions, where each of the Dirac distributions represents a single crystal. Due to the specific discretization of the codf this approach requires usually a large number of discrete crystal orientations even if the texture can be described by a small number of texture components. In the present work, we consider face-centered cubic (fcc) polycrystals and compare the classical upper bound model with an approach based on texture components. The texture components are modeled by Mises–Fischer distributions, which are central distributions. The stress of the polycrystal is obtained by a numerical integration of the single crystal stress state over the orientation space.