Plastic deformation characteristics of cross-equal channel angular pressing

For the first time, the plastic deformation characteristics of cross-equal channel pressing (cross-ECAP), a modified equal channel angular pressing, using a cross-shaped channel instead of a conventional l-shaped channel, was analyzed by using finite element analysis. The deformation in the cross-ECAP is more complicated and the strain induced is much more severe than that in the conventional ECAP. However, the plastic strain is localized in the central linear region of the workpiece and very small strain is developed in the edge regions, which is in good agreement with experimental results in the literature showing nonuniformity in microstructure and hardness distribution. The load requirements of cross-ECAP are much higher in comparison to conventional ECAP and T-ECAP processes.     

Comparison of deformation and microstructural evolution between equal channel angular pressing and forward extrusion using the dislocation cell mechanism-based finite element method

In order to compare plastic deformation and microstructural evolution behavior deformation between equal channel angular pressing (ECAP) and forward extrusion (FE) processes, finite element analyses in associated with the mechanism of dislocation glide and cell formation have been employed. It was found from the simulation results that the ECAP process is superior to the FE process, in terms of strength, grain refinement and deformation homogeneity as well as repeatability due to the equal channels of entry and exit.     

等径角挤压Cu-10%Cr合金过程塑性

等径角挤压(EACP)能够制备具有超细晶粒的致密材料,而且其材料具有优良的机械性能.所以,等径角挤压是目前材料研究的热点之一.在本文中,建立了Eshelby等效夹杂方法的模型,并采用有限元方法对Cu-10%Cr合金的等径角挤压过程进行了模拟.研究了等径角为90°和120°时的两种模型,并对两种模型中材料的等效应变分布,瞬时应变和变形形状变化,以及截面硬度分布进行了分析.研究结果表明,等径角为120°的模型中材料的应变分布比等径角为90°的模型更加均匀;此外,两种模型中材料的硬度分布与应变分布具有较强的联系,等径角为90°的模型中,材料具有较高的硬度值和更加剧烈的硬度变化.两模型硬度最大值差别达到了8.5%.     

Properties of AA6061 Processed by Multi-Axial Compressions/Forging (MAC/F)

Multi-axial compressions/forgings (MAC/F) were conducted at room temperature to obtain severe plastic deformation (SPD) of AA6061 alloy. Microhardness measurements taken across the cross-section of each of the MAC/F processed samples indicated that the hardness distribution is nonuniform during initial compressions/forgings and becomes uniform with subsequent compressions. Microhardness and tensile testing showed that the hardness, yield strength, and elongation of the MAC/F processed material followed similar trends to that for AA6061 processed by equal channel angular pressing (ECAP). The MAC/F material exhibited high strain rate sensitivity and high percentage of elongation to failure in the temperature range of 300-350°C. This observation is in agreement with the high formability of SPD-processed AA6061 over the same temperature range.     

Effect of die design parameters on the deformation behavior in pure shear extrusion

Influences of die design parameters in terms of diameter ratio and length of the deformation zone on the distribution of effective strain, filling fraction of the die exit channel and pressing load in pure shear extrusion (PSE) are studied using finite element method (FEM). Dimensional stability, pressing load and hardness measurements are used to validate the predictions of the simulation. Acceptable agreements between the predictions of simulation and experimental results are observed. It is found that strain is inhomogeneously distributed which increases from the center to the corners. Effective strain, inhomogeneity of strain, filling fraction of the die exit channel and pressing load are increased with increasing diameter ratio. In addition, the work-piece is deformed more homogeneously at lower pressing load by increasing the length of deformation zone. However, filling fraction of the die exit channel initially increases by the length of the deformation zone up to 60 mm after which it reduces. The optimum die design parameters covering a range of acceptable effective strain and strain homogeneity, filling fraction of the die exit channel and pressing load are proposed as being 60 mm and 2 for length of the deformation zone and diameter ratio, respectively.     

Finite element analysis of rotary-die equal channel angular pressing

In this paper, the finite element method (FEM) was applied to analyze the plastic flow and strain hardening behavior of pure copper, subjected to rotary-die equal channel angular pressing (RD-ECAP) up to four passes. The die was rotated 90° counter clockwise between the passes in the simulation. The effective strain distribution and load–stroke curves were investigated. The load was increased with the number of rotary-die equal channel angular pressing passes. The results show that, plastic deformation becomes inhomogeneous with the number of passes due to an end effect, which was not found seriously in conventional equal channel angular pressing (ECAP). Especially, decreasing corner gap with increasing the number of passes was observed and explained by the strain hardening effect.     

摩擦力在ECAP成形时作用的有限元模拟

应用有限元方法对一种全新的可应用于板料的等通道角挤压(ECAP)方法中摩擦力的作用进行了有限元模拟.计算模拟结果表明,不同于常规的等通道角挤压中摩擦力的负面影响,在板料等通道角挤压方法中一定的摩擦力和尺寸精确的模具结构却是保证此类等通道角挤压正常进行的关键.     

等径弯曲通道变形的有限元模拟现状

等径弯曲通道变形(Equal Channel Angular Pressing 简称ECAP)由于能直接制备块状超细晶材料而备受关注.介绍了等径弯曲通道变形(ECAP)及有限元数值模拟的基本机理,并在此基础上讨论了有限元模拟在ECAP变形中的研究及发展现状.随着ECAP的深入研究和工业化的进一步发展,有限元数值模拟必然在该领域中得到越来越广泛的应用.     

A combination of severe plastic deformation and ageing phenomena in Al–Mg–Si Alloys

In this paper static and dynamic strain ageing behavior in Al–Mg–Si alloys related to equal channel angular pressing (ECAP) was investigated. In order to examine the combined plastic deformation, solution treatment and ageing effects on strengthening characteristics, experimental results of ageing without ECAP, pre-ECAP ageing, post-ECAP ageing and dynamic ageing inside of ECAP die were compared. In particular, the effects of ageing temperature, ageing time, strain rate in ECAP, and sequence of heat treatment and ECAP on Vickers hardness were discussed. To achieve a higher hardness, an optimum ageing cycle combined with ECAP process is presented based on the results of current study. By employing the proposed schedule the hardness value was increased from 86 HV (as-solution treatment) to 138 HV (peak hardness of the current schedule).     

Finite element analysis of the effect of back pressure during equal channel angular pressing

There is an increasing interest in applying back pressure during equal channel angular pressing (ECAP) to improve the process for better control of microstructure and property. The effect of increasing back pressure on deformation characteristics during ECAP such as the plastic deformation zone (PDZ) size and strain rate distribution in the PDZ, size of the corner gap and strain distribution on the longitudinal section were analysed by finite element analysis for both the quasi-perfect plastic and strain hardening materials. This investigation revealed that the back pressure influence very differently the PDZ of the quasi-perfect plastic and strain hardening materials. Many beneficial effects of back pressure were observed in the strain hardening material, with reduced PDZ size, dramatically reduced corner gap, and more uniform strain distribution. For the quasi-perfect plastic material, however, the application of increasing back pressure leads to broadening of PDZ and a decrease in strain rate homogeneity.     

Three-dimensional numerical analysis and experimental investigation of grain refinement in multi-pass equal channel angular pressing for round-workpieces

Equal channel angular pressing (ECAP) has the capability of producing ultra fine-grained (UFG) materials bellow the dimension of 1 μm. At present, it is one of the most important methods to get bulk UFG materials. Multi-pass ECAP processes for round workpieces are investigated by using numerical simulations and experimental studies in this paper. The deformation mechanism of ECAP for grain refinement is obtained. Three processing routes A, B and C are simulated in order to study the influence of the processing routes to the deformation uniformity of the workpiece. The finite element (FE) analysis results of the multi-pass ECAP process show that the different processing routes result in the different deformation distributions. The grain in the workpiece is refined obviously after multi-pass pressing. The microstructures of the processed material are more different than that of the microstructure of the annealing initial equiaxed grains. The microstructure evolution of the workpiece can be changed via different processing routes. It is found that route B can get a high angle grain boundaries distribution in the workpiece than other routes. The results of the analysis show that the process of grain refinement can be described as a continuous dynamic recovery and recrystallization. The microstructure evolutions of the grain refinement mechanisms and micro-structural characteristics for different multi-pass ECAP processing routes are verified by using OM (optical model) and TEM (transmission electron microscope) analysis. In addition, the experimental microstructure results are also consistent with FE analysis results.     

Finite element modelling of plastic instability during ECAP processing of flow-softening materials

A finite element analysis of the equal channel angular pressing (ECAP) of flow-softening materials is presented in this paper. A very fine mesh was used in the simulations, allowing a detailed analysis of the development of localized shear phenomena. Two different flow curves were used in the simulations; one displayed an initial flow-softening followed by perfect plastic behavior, whereas the other followed a constant flow-softening behavior. The flow-softening rate affects the intensity of shear localization. The deformation zone, that is usually concentrated around a fixed shear plane during processing of perfect plastic or strain hardening materials, splits into two parts and its position varies cyclically during the process, leading to oscillations in the punch load during processing. A comparison of the finite element predictions with those from the slip line field theory is also presented.     

Investigation of texture and mechanical properties of copper processed by new route of equal channel angular pressing

The evolution of crystallographic texture and the mechanical properties of copper subjected to severe plastic deformation (SPD) using equal channel angular pressing (ECAP) were investigated. Samples were subjected to ECAP under two different processing routes: B₆₀ and B_C. As the cross sections of the samples were circular, a new route with a rotation angle of 60° in the same direction between consecutive passes was introduced. The material exhibited texture development similar to the simple shear texture in both routes and the most significant changes in texture strength in both processing routes took place after the second pass. Microstructure of ECAP processed samples were investigated using electron backscatter diffraction (EBSD) analysis. Comparison of the EBSD data with optical micrograph of the initial sample confirmed that ECAP process has led to a significant decrease in grain size. Significant increases in hardness and tensile strength were observed after the first pass of ECAP. Variations of tensile strength as a function of the number of passes were related to the dislocation densities and the average boundary spacing.     

Finite element analysis of the effect of the inner corner angle in equal channel angular pressing

The inner corner angle (ICA) is one of the major factors affecting deformation homogeneity in workpieces during equal channel angular pressing (ECAP). In this study, the effect of the ICA on the plastic deformation behavior in ECAP was investigated using the finite element method. A round ICA induces highly inhomogeneous deformation in the head, tail, top and bottom regions of the workpiece due to increasing compressive and decreasing shear deformation components. It was found that a round inner corner with an angle up to 9° is acceptable in finite element simulations for reproducing a sharp inner corner. These results can serve as a design guide for processing and dies of ECAP.     

Effect of integrated extrusion–equal channel angular pressing temperature on microstructural characteristics of low carbon steel

Abstract

In the present research, a combined forward extrusion–equal channel angular pressing was developed and executed for the deformation of a plain carbon steel. In this method, two different deformation steps, including forward extrusion and equal channel angular pressing, take place successively in a single die. The deformation process was performed at different deformation start temperatures (800, 930 and 1100°C). Three-dimensional finite element simulation was used to predict the strain and temperature variations within the samples during deformation. With microstructural observations and the results of finite element simulation, the main grain refinement mechanisms were studied at different deformation temperatures. The results show that the forward extrusion–equal channel angular pressing is effective in refining the ferrite grains from an initial size of 32 μm to a final size of ～0·9 μm. The main mechanisms of grain refinement were considered to be strain assisted transformation, dynamic strain induced transformation and continuous dynamic recrystallisation, depending on the deformation temperature.     

Developing Ultrafine Grain Sizes Using Severe Plastic Deformation

Severe plastic deformation may be used as a processing tool to achieve a refinement in grain size in metallic alloys to the submicrometer or nanometer range. This paper describes recent developments using the procedure of equal‐channel angular pressing (ECAP) in which samples are pressed through a die containing a channel bent into an L‐shaped configuration. The shearing associated with passage through the die introduces bands of subgrains which evolve, with additional pressings, into arrays of grains separated by boundaries having high angles of misorientation. The process of ECAP is a useful tool for both increasing the strength and toughness of an alloy at ambient temperatures and achieving a potential for superplastic forming of the alloy at rapid strain rates at elevated temperatures.     

Deformation behavior of consecutive workpieces in equal channel angular pressing of solid dies

In this study, consecutive workpiece equal channel angular pressing (ECAP) in solid-dies, where the second workpiece is successively deformed without splitting and reassembling the die after the first workpiece processing, is employed to reduce the processing time in ECAP. The plastic deformation behavior of the two workpieces was investigated in terms of strain homogeneity, load, and defects using the finite element method (FEM). The experimental deformations of the consecutive workpiece during ECAP were compared with the FEM results, and it was found that the deformation was more heterogeneous in the second workpiece when compared with the first workpiece. The primary reason behind these findings is that the deformed geometry of the second workpiece was a back slant type and the first deformed workpiece provided back pressure to the second workpiece. Furthermore, the folding defect was less pronounced in the second workpiece because of the back slant head shape. Despite the less homogeneity in strain, the ECAP of the consecutive workpieces is an effective process for less defective materials and increases process efficiency.     

ECAP process capability in producing a power transmission bimetallic rod

In recent years, bimetallic rods have been considered by researchers due to the industrial applications in power transmission systems. In this paper, an Al-Steel rod is produced by equal channel angular pressing (ECAP) process. First, severe plastic deformation is applied to Al-Steel rod by ECAP process, and then some experiments such as microhardness measurement, interface strength evaluation, scanning electron microscope (SEM), and microstructure analysis have been accomplished to assess the capability of ECAP process in producing a power transmission bimetallic rod. Then, to evaluate significant parameters on strain distribution and pressing force, the FEM has been utilized. After verification of the numerical method, the effect of several parameters such as diameter, location, peripheral materials, and die angle on the applied strain and homogeneity are studied. The results indicate that the steel and aluminum hardness are increased about 59 and 61% after four passes ECAP, respectively. Also, the average grain size of steel core is reduced about 79% at the same conditions after four passes. In addition, the images received from SEM show that the interfaces between steel and aluminum have been improved significantly.     

Effect of friction model in numerical analysis of equal channel angular pressing process

During the last decade or so there has been a tremendous growth in the research and development of equal channel angular pressing (ECAP) process which was originally proposed by Segal et al. Numerical analyses are being used extensively to evaluate the effect of various die design and process parameters in ECAP. Friction is one such important parameter. Coulomb and shear friction models have been used in the numerical analysis of ECAP process and contradicting results have been reported. This study evaluates the effect of coulomb and shear friction models on the deformation pattern, strain distribution and load requirement during ECAP process and suggests which friction model should be used in the numerical analysis of ECAP process.     

镁合金双通道转角挤压有限元分析

基于有限元分析软件建立了双通道转角挤压模型,对AZ91镁合金挤压变形过程进行了模拟,并对挤压变形过程、等效应变、挤压力等模拟结果进行了分析。模拟结果表明:双通道转角挤压所需要的挤压力要大于传统的等通道转角挤压,通道夹角越大,挤压所需要的挤压力越小;双通道转角挤压获得的总的应变更大。