Experimental Analysis on the Frictional Behaviour of Drawbeads in Sheet Metal Forming

In sheet metal forming, drawbeads are commonly used to control uneven material flow, which may cause defects such as wrinkles, fractures, surface distortion and springback. Although friction may not directly change the limiting strain of steel sheets, the tribological conditions in the contact zone between the sheet surface and the tool surface play an important role in determining the limits of the forming process. Friction in the drawbead contact zones affects the flow of the material in the tool and is used deliberately to control the stamping process. Therefore in this study, the frictional behaviour of drawbeads is experimentally investigated by the drawbead friction test. To characterize the effect of processing variables on the friction coefficients, tests are performed for various sheets, lubricants and bead materials suffering different surface treatments. The results obtained from the drawbead friction test show that the friction and drawing characteristics of deforming sheets were strongly influenced by the strength of sheet, viscosity of lubricant and hardness of bead surface.     

Roll Forming Strategies for Welded Tubes

Tubes are used as semi‐finished products as well as final components in almost all areas of the engineering industry. Roll forming of tubes with longitudinally oriented welding lines is one of the most efficient and economic tube production processes. However, numerous roll forming strategies already exist. Each strategy involves a characteristic change of the material properties from the initial slit strip to the final tube. A classification of the different roll forming strategies, which is given in this paper, aims to provide a systematic overview. A finite element analysis of the roll forming process is presented to identify specific forming loads and property changes.     

Sand Plasticity Model Accounting for Inherent Fabric Anisotropy

A sand plasticity constitutive model is presented herein, which accounts for the effect of inherent fabric anisotropy on the mechanical response. The anisotropy associated with particles’ orientation distribution, is represented by a second-order symmetric fabric tensor, and its effect is quantified via a scalar-valued anisotropic state variable, A. A is defined as the first joint isotropic invariant of the fabric tensor and a properly defined loading direction tensor, scaled by a function of a corresponding Lode angle. The hardening plastic modulus and the location of the critical state line in the void ratio?mean effective stress space, on which the dilatancy depends, are made functions of A. The incorporation of this dependence on A in a pre-existing stress-ratio driven, bounding surface plasticity constitutive model, achieves successful simulations of test results on sand for a wide variation of densities, pressures, loading manners, and directions. In particular, the drastic difference in material response observed experimentally for different directions of the principal stress axes with respect to the anisotropy axes, is well simulated by the model. The proposed definition and use of A has generic value, and can be incorporated in a large number of other constitutive models in order to account for inherent fabric anisotropy effects.     

半固态金属制备原理与应用

综述了半固态金属加工的特点以及主要制备方法。从金属凝固原理出发，结合实验结果论述了半固态金属组织的几种生成机制以及这些生成机制在工程实施中将遇到的困难。     

Modelling of Incremental Bulk and Sheet Metal Forming

In incremental forming operations, forming is accomplished by a number of single, local forming steps with simple, generic tools, and in some cases even by laser or plasma beams. This makes incremental forming processes very flexible. However, complex tool kinematics with a large number of degrees of freedom can cause severe problems for process planning and optimization. In addition, new questions regarding the material behaviour under cyclic plastic deformation arise. This article gives an overview of current problems in the finite element modelling of incremental forming processes, focusing on two examples, hammer forging and incremental CNC sheet metal forming. Based on this, insight is given into current research at the Metal Forming Institute (IBF) regarding adapted simulation methods for incremental forming processes.     

半固态金属加工工艺过程的模拟进展

论述了半固态金属加工技术,即流变铸造,二次加热与触变成形等工艺过程数值模拟技术的研究进展和现状,给出了各工艺过程的模拟实例,并对半固态金属加工过程模拟的发展进行了评价。     

金属板材成形性能的力学分析

根据金属材料拉伸试验所获得的强度、塑性指标、成形性能如Rp0.2、Rm、A、n、r等,并结合成形极限图(FLD)试验实测的金属板材拉胀成形性能,介绍了一整套用于评价金属板材塑性成形性能的实验室分析技术.文章以两种厚薄规格的冷轧板材为例,根据拉伸应变硬化指数动态跟踪技术实测的n值随应变量变化的趋势,揭示了上述两种材料拉伸性能与成形性能异同特点的力学本质.     

铍铝合金挤压成形技术研究

文章研究了铍铝合金挤压成形技术,分析了挤压成形的影响因素,并观测了挤压组织微观结构,测试了铍铝合金挤压力学性能。研究发现,铍铝合金铸锭经过热挤压成形之后,力学性能有明显改善。但由于影响因素较多,较难获得理想的挤压工艺。     

Simple Model for Directional Distortional Hardening in Metal Plasticity within Thermodynamics

Directional distortion, observed in many experiments on various types of metals, refers to the formation of a region of high curvature (sharpening) on the yield surface approximately in the direction of loading, and a region of low curvature (flattening) approximately in the opposite direction. Constitutive modeling of directional distortion was recently presented by the writers where an evolving fourth-order tensor-valued internal variable was introduced. In the current paper a much simpler mathematical formulation describing directional distortional hardening is presented without the use of a fourth-order tensor, in conjunction with kinematic and isotropic hardening. Two versions of the model in ascending level of complexity follow similar lines of development, which include derivation of all hardening rules on the basis of conditions sufficient to satisfy the thermodynamic dissipation inequality. As a tradeoff for its simplicity the present model does not fit experimental data as well as the model with the evolving fourth-order tensor, but it still captures the salient features of directional distortion in a rather satisfactory way.     

Modelling and Numerical Simulation of Ductile Damage in Bulk Metal Forming

A complete set of fully coupled constitutive equations accounting for both combined isotropic and kinematic hardening as well as the ductile damage under anisothermal conditions at finite (visco)plastic strain is developed and implemented into the general purpose Finite Element code for metal forming simulation. First, the fully coupled anisotropic constitutive equations in the framework of Continuum Damage Mechanics are presented. Attention is paid to the strong coupling between the main thermomechanical fields as thermal effects, elasto‐viscoplasticity, mixed hardening, ductile isotropic damage and contact with friction. The associated numerical aspects concerning both the local integration of the coupled constitutive equations as well as the (global) equilibrium integration schemes are presented. The local integration is outlined thanks to the Newton iterative scheme applied to a reduced system of two differential equations. For the global resolution of the equilibrium problem, the classical dynamic explicit (DE) scheme with an adaptive time step control is used. A fully adaptive 2D methodology with mesh and loading sequences adaptation based on some appropriate error estimates is used. For 3D simulations only a constant appropriately refined 3D mesh is used. Various 2D and 3D examples are given in order to show the capability of the methodology to predict the ductile damage initiation and growth during metal forming processes.     

Evaluation of Friction Coefficient by Simulation in Bulk Metal Forming Process

With the objective of evaluating the accuracy of the upper-bound theory for calculating the average Tresca friction coefficient m in the hot forging process, we performed simulations using different values of m in each compression process to high strain levels. It was found that the upper-bound theory is not applicable at high strain levels, because the contact surface of the cylindrical sample is composed of an originally flat end surface and the annular portion formed by the contact of the lateral surface with the anvil surface. The relation among P = \fracR_m HR_t H₀ , P = {\frac{{R_{m} H}}{{R_{t} H_{0} }}}, true strain, and m could be expressed by ( a^￠ + a^￠￠e+ a^￠￠￠e² - P ) + ( b^￠ + b^￠￠e+ b^￠￠￠e² )m + ( c^￠￠e+ c^￠￠￠e² )m² = 0. \left( {a^{\prime} + a^{\prime\prime}\varepsilon + a^{\prime\prime\prime}\varepsilon^{2} - P} \right) + \left( {b^{\prime} + b^{\prime\prime}\varepsilon + b^{\prime\prime\prime}\varepsilon^{2} } \right)m\,+ \left( {c^{\prime\prime}\varepsilon + c^{\prime\prime\prime}\varepsilon^{2} } \right)m^{2} = 0. Here, the m values obtained were in good agreement with the actual ones used in the simulations. The value of m of the arbitrary geometry cylindrical sample could also be directly read from a contour map with a relationship among nominal strain, parameter B of the corresponding standard sample, and m.     

Model Describing Material-Dependent Deformation Behavior in High-Velocity Metal Forming Processes

A constitutive model for rate-dependent and thermomechanically coupled plasticity at finite strains is presented. The plasticity model is based on a J2 model and rate-dependent behavior is included by use of a Perzyna-type formulation. Adiabatic heating effects are handled in a consistent way and not, as is a common assumption, through a constant conversion of the internal work rate into rate of heating. The conversion factor is instead derived from thermodynamic considerations. The stored energy is assumed to be a function of a single internal variable which differs from the effective plastic strain. This allows a thermodynamically consistent formulation to be obtained which, as shown, can be calibrated by use of simple procedures. Choosing 100Cr6 steel in two differently heat treated conditions as prototype material, experimental tests are performed, enabling the model to be calibrated. Significant differences in deformation behavior are noted as the differently heat treated specimens are compared. In addition, the local stress-updating procedure is reduced to a single scalar equation, permitting a very efficient numerical implementation of the model. The constitutive formulation proposed was employed in an explicit finite element solver, illustrative simulations of a high-velocity metal forming process being performed to demonstrate the capabilities of the model and certain characteristic traits of the materials that were studied.     

SPD 法及P/M 法制备金属纳米体材料进展

综述了金属纳米体材料的深度塑性变形法 (SPD)和粉末冶金法 (P/M)制备方法及最新进展 ,讨论了目前金属纳米体材料制备中存在的问题 ,提出了今后的发展方向 ,中温高压成型技术是未来的纳米材料工业化制备技术发展的重要方向     

Transformation Induced Martensite Evolution in Metal Forming Processes of Stainless Steels

Because of their high corrosion resistance and deformation characteristics, the industrial application of stainless steels is of high importance. During deep drawing processes, phase transformation of austenite to martensite occurs, which leads to an increased strain hardening of the material. The phase transformation depends on alloying constituents, transformation temperatures, stresses and strains. Consequently, in the design of deep drawing processes of stainless steels the phase transformation has to be considered. This paper presents a mathematical model for the calculation of the martensite evolution depending on temperatures, stresses and strains. The precise simulation of deep drawing processes of stainless steels can be enabled by the implementation of this model into commercial FE‐programs.     

多功能金属薄板成形试验系统及其在汽车板成形研究中的应用

介绍了一个等于先进的金属薄板形试验系统，该试验系统设计先进，控制灵活，采用先进的数码技术，能满足大多数成形试验研究的需要，并利用此设备针对汽车板在胀形＋弯曲复合形成方式下的成形极限进行了研究。     

Reynolds Stress Anisotropy in Open-Channel Flow

This paper reports the results of an experimental study characterizing turbulence and turbulence anisotropy in smooth and rough shallow open-channel flows. The rough bed consists of a train of two-dimensional transverse square ribs with a ratio of the roughness height (k) to the total depth of flow (d) equal to 0.10. Three rib separations (p/k) of 4.5, 9, and 18 were examined. Here, p is the pitch between consecutive roughness elements and was varied to reproduce the classical condition of d- and k-type roughness. For each case, two-component velocity measurements were obtained using a laser Doppler velocimetry system at two locations for p/k = 4.5 and 9: on the top of the rib and above the cavity, and an additional location for p/k = 18. The measurements allow examination of the local variations of the higher-order turbulent moments, stress ratios as well as turbulence anisotropy. Large variations of the turbulence intensities, Reynolds shear stress, turbulent kinetic energy and turbulence production are found for y1<3k. In this region, the flow is more directly influenced by the shear layers from the preceding ribs. The higher-order moments appear to be similar for all rough surfaces beyond y1 ≈ 7k. In the outer layer (y1>3k), all higher-order turbulent moments for the k-type roughness show a substantial increase due to the complex interactions between the roughness and the remnants overlying shear layers shed from succeeding ribs. Analysis of the components of the Reynolds stress anisotropy tensor shows that at p/k = 18, the flow at y1<5k tends to be more isotropic which implies that for this particular case, the effect of the roughness density could also be important. On the smooth bed, at the shallower depths, the correlation coefficient near the free surface increases and turbulence tends to become less anisotropic.     

Rapid Finite Element Analysis of Bulk Metal Forming Process Based on Deformation Theory

 The one-step finite element method (FEM), based on plastic deformation theory, has been widely used to simulate sheet metal forming processes, but its application in bulk metal forming simulation has been seldom investigated, because of the complexity involved. In this paper, a bulk metal forming process is analyzed by using a rapid finite element simulation method based on deformation theory. The material is assumed to be rigid-plastic, strain hardening. The constitutive relationship between stress and total strain is adopted, whereas the incompressible condition is enforced by penalty function. The geometrical non-linearity in large plastic deformation is taken into consideration. Furthermore, the force boundary condition is treated by a simplified equivalent approach, considering the contact history. Based on constraint variational principle, the deformation finite element method is proposed. The one-step forward simulation of axisymmetric upsetting process is performed by this method. The results are compared with those obtained by the traditional incremental FEM to verify the feasibility of the proposed method.     

Effects of Cross Anisotropy on Three-Dimensional Behavior of Sand. II: Volume Change Behavior and Failure

The volume change behavior of cross-anisotropic sand is studied using results of a series of cubical triaxial tests. The relationships between the volumetric response, failure, and shear localization are addressed. Rates of dilation under various three-dimensional stress conditions are evaluated in conjunction with the peak shear resistance and initiation of shear banding in specimens of dense Santa Monica beach sand. The location of the line in principal stress space along which the tendency to deform changes from compressive to dilative (the characteristic line) is determined using two different methods. The uniqueness of this characteristic line for cross-anisotropic materials is analyzed.     

Effect of Tooling Temperature on the Transient Lubricant Behavior in Hot Metal Forming Processes

In hot metal forming processes, the temperature of the forming tool progressively increases under serial production conditions. Water-based two-phase lubricants may be applied to cool the forming tool and moderate temperature, in which the liquid agent would evaporate or decompose rapidly with dry matter deposited on the tooling surface during the dwelling time before the forming process commences. Herein, an interactive friction model for a two-phase lubricant is developed to predict the transient lubricant behaviors, i.e., predicting the effects of tool temperature and dwelling time on the friction coefficient evolution and lubricant breakdown. Friction tests between a warm pin and hot aluminum workpiece are conducted using the advanced friction testing system, TriboMate, to validate the modeling results.     

Mechanisms of Small-Strain Shear-Modulus Anisotropy in Soils

In this paper, experimental studies using a true triaxial apparatus and a bender element system, and numerical simulations based on the discrete element method (DEM) were used to investigate the stress- and fabric-induced shear-stiffness anisotropy in soils at small strains. Verified by experiments and DEM simulations, the shear modulus was found to be relatively independent of the out-of-plane stress component, which can be revealed by the indistinctive change in the contact normal distribution and the normal contact forces on that plane in the DEM simulations. Simulation and experimental results also demonstrated that the shear modulus is equally contributed by the two principal stress components on the associated shearing planes. Fabric-induced stiffness anisotropy, i.e., the highest Gxy or Ghh, can be explained by simulation findings in which more contact normals prefer to distribute along the horizontal direction. The experiments and simulations also reveal that the fabric-induced stiffness anisotropy increases with an increasing aspect ratio of the particles. The assumption of transversely isotropic fabric in soils is valid based on the DEM simulation results; however, this assumption is not completely supported by the experimental results.