Microstructure and mechanical properties of Al-7075 alloy processed by equal channel angular pressing combined with aging treatment

In order to examine the combined effect of plastic deformation and aging process, the Al 7075 alloy was subjected to equal channel angular pressing (ECAP) deformation by route B_C in various ECAP and aging conditions: pre-ECAP aging, post-ECAP aging and dynamic aging during ECAP at 393 K and 423 K. Followed by ECAP and aging treatment, Vickers microhardness and tensile test were performed and microstructural observations were undertaken using transmission electron microscopy (TEM) and X-ray diffractometer (XRD). TEM investigation showed that ultrafine-grained (UFG) materials with grain size less than 500 nm could be obtained after three or four passes of ECAP. Precipitates characterization revealed that maximum mechanical properties are achieved when the microstructure mainly consists of fine dispersion of small η^′ precipitates and minor quantities of GP zones. Dynamic aged specimens at 393 K and 423 K represented maximum and minimum mechanical properties, respectively, due to formation of fine η^′ precipitates plus GP zones and η^′ plus η precipitates, respectively. Dynamic aging during ECAP at 393 K appeared preferable to other procedures for attaining maximum mechanical properties as well as saving time and energy.     

Fatigue crack growth resistance of 7075 Al alloy after equal channel angular pressing

This work presents experimental results on effects of severe plastic deformation (SPD) and subsequent natural ageing on tensile mechanical properties and fatigue crack growth resistance of fine‐grained 7075 Al alloy. The alloy was subjected to equal channel angular pressing (ECAP) after solution treatment. Fatigue crack propagation tests were conducted in room condition, at load ratio R = 0.1 and different load ranges on small disk shaped compact tension specimens. Fatigue fracture surface is also investigated using scanning electron microscopy observations and showed more ductile fatigue crack growth in the unECAPed specimen. Despite the increased tensile strength after ECAP, the ductility that controls low‐cycle fatigue behaviour has decreased. It is found that ECAP has resulted in a remarkable change in Paris regime parameters and a significant increase in fatigue crack growth rate. The decrease in fatigue crack growth resistance and ΔK_c after ECAP can be attributed to the decrease in alloy's ductility.     

Effects of equal channel angular pressing on the strength and toughness of Al–Cu alloys

Tensile and impact tests were performed on Al–0.63 wt%Cu and Al–3.9 wt%Cu alloys subjected to equal channel angular pressing (ECAP) with different number of passes. Besides the tensile properties, data about the static toughness and the impact toughness were obtained. The strength and the toughness of the Al–Cu alloys were ameliorated and upgraded to a high level collectively. In addition, fracture surface observations show that the fracture behavior of the Al–Cu alloys changes from brittle mode to ductile mode after multi-pass ECAP.     

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.     

连续等通道转角挤压对Al-Ti-C合金组织与性能的影响EI北大核心CSCD

采用连续等通道转角挤压工艺,以连续的方式对Al-Ti-C合金进行多道次挤压,通过观察微观组织演化,探讨晶粒细化机理和力学性能变化。结果表明:连续等通道转角挤压工艺可有效细化Al-Ti-C合金微观组织,晶粒尺寸减小至1μm左右,形变诱导是变形过程中最主要的晶粒细化机制;高密度位错堆积引起Al基体和TiAl_(3)界面的裂纹以及TiAl_(3)内部的空洞产生,裂纹进一步扩展贯穿整个TiAl_(3)颗粒,最终导致第二相TiAl_(3)组织的细化,同时细小的第二相TiAl_(3)组织的钉扎机制和剪切机制促进了Al基体细化;连续等通道转角挤压1道次后,合金硬度提升最明显,与原始态相比提高59.2%;之后随挤压道次的增加,硬度提升的趋势变缓,合金塑性下降,韧性提高。     

Effect of initial microstructure on the processing of titanium using equal channel angular pressing

Equal channel angular pressing (ECAP) is a metal processing technique that is used to produce materials with ultrafine (<1 μm) grain sizes. In this work, the effect of the initial microstructure on ECAP of commercially pure titanium (CP Ti), a material used in many industrial applications, was investigated. To produce different initial microstructures, samples of CP Ti were exposed to different annealing conditions: no annealing (Material 1), annealed at 1033 K for 2 hr (Material 2), or annealed at 1173 K for 4 hr (Material 3). Each material was subjected to one pass of ECAP and the resulting microstructures were analyzed using XRD, SEM, and TEM, and compared to the microstructures before ECAP. It was found that each material developed a unique microstructure after one pass of ECAP, which was attributed to the varying microstructural characteristics before ECAP. Microhardness values before and after ECAP varied with each microstructure.     

等径角挤压对Al-Cu-Mg-Ag合金组织性能的影响

为研究大塑形变形对耐热铝合金的作用,采用铸冶金工艺制备了新型的Al-Cu-Mg-Ag耐热铝合金,通过显微组织观察、差热分析及硬度测试等方法,研究了等径角挤压对耐热铝合金显微组织与力学性能的影响.结果表明:通过对挤压态的Al-Cu-Mg-Ag耐热铝合金在固溶淬火后时效前进行等径角挤压变形,可获得晶粒尺寸低于2μm的块体超...     

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.     

Modelling of equal channel angular pressing using a mesh-free method

Severe plastic deformation (SPD) processes are widely recognised as efficient techniques to produce bulk ultrafine-grained materials. As a complement to experiments, computational modelling is extensively used to understand the deformation mechanisms of grain refinement induced by large strain loading conditions. Although considerable research has been undertaken in the modelling of SPD processes, most of the studies have been accomplished using mesh-based methods, such as the finite element method (FEM). Mesh-based methods have inherent difficulties in modelling high-deformation processes because of the distortions in the mesh and the resultant inaccuracies and instabilities. As an alternative, a mesh-free method called smoothed particle hydrodynamics (SPH) is used. The effectiveness of this technique is highlighted for modelling of one of the most popular SPD techniques, equal channel angular pressing. A benchmark between SPH and FE calculation is performed. Furthermore, a number of simulations under different processing conditions are compared to existing literature data. A satisfactory agreement is found, which indicates that SPD processes can be approached by mesh-free methods, such as SPH.     

Analyses of route Bc equal channel angular pressing and post-equal channel angular pressing behavior by the finite element method

Equal channel angular pressing (ECAP) process provides an efficient procedure for achieving ultrafine grained microstructures with excellent mechanical properties in metallic materials. In this article, a simulation scheme for predicting the mechanical behavior during and after ECAP was proposed. The proposed scheme was applied for interstitial-free (IF) steels, which are widely used for the automobile body applications. Plastic deformation behavior during several passes of ECAP in route Bc, including such aspect as deformed geometry, corner gap, forming load and strain uniformity, was predicted. Tensile testing responses of the ECAP-processed IF steel, including strain hardening, onset of necking, and post-necking behavior, were analyzed using the finite element method and compared with the experimental results. The predicted tensile curves, ultimate tensile strength, and elongation varying with the number of ECAP passes were in good agreement with experimental results. The computational scheme developed was demonstrated to successfully predict not only the plastic deformation behavior during ECAP but also the mechanical properties of the ECAP-processed material.     

Texture simulation of aluminum rod during equal channel angular pressing

A general procedure for texture simulation in multi-pass equal channel angular pressing (ECAP) with capability of applying different processing routes is proposed. The program inputs are the initial texture and the loading condition and the output is texture after deformation. Deformation texture in ECAP of aluminum rod was predicted based on simple shear model for deformation and Visco plastic self consistent model for texture simulation. The simulation was done for two consecutive passes of ECAP and the results were compared with experimental texture measurements. The initial texture of the sample before ECAP was found to play a key role in formation of the final texture and a good agreement between the simulated and the experimental texture was obtained.     

An investigation on the effects of equal channel angular pressing on the mixed‐mode fracture toughness and mechanical properties of 6063 aluminium alloy

In this study, effects of equal channel angular pressing (ECAP) on the mixed‐mode fracture toughness of Al‐6063 were investigated. The ECAP process continued up to 5 passes without failure. Grain refinement was obvious after 5 passes of the ECAP process. The average grain size reduced from 45μm to less than 1μm, and textural studies shows aligning the grains in known directions. After 4 passes, yield and ultimate strengths increase respectively from 100 and 209 MPa to 300 and 375 MPa and reduction in elongation was also observed. The microhardness improved after the process. The fracture toughness for different orientations was measured. For pure mode I (opening mode), its value decreased after the first pass from 18.4 to 15.71  ; however, it increased to about 18.8  after the fifth pass. For mixed‐mode loading condition, different orientations were investigated. The results revealed different fracture toughness reductions after the first passes of the process for specimens with different orientations. The fracture surfaces were studied by using scanning electron microscope, and refined equiaxed dimples were observed after the ECAP process.     

Pure copper processed by extrusion preceded equal channel angular pressing

High purity copper (99.99%) was severely plastically deformed by miniaturized extrusion preceded ECAP at 200 °C. A partially recrystallized microstructure was detected by TEM observations due to the relative high processing temperature. Isochronal annealing was performed to investigate the material's behavior by measuring the development of Vickers microhardness. As copper is a metal with a relative low stacking fault energy, recovery does not occur readily and only recrystallization takes place.     

Grain refinement of commercial purity zirconium by equal channel angular pressing

Abstract

To develop an ultrafine grain structure in commercial purity zirconium, equal channel angular ECA pressing was carried out at room temperature by varying the number of passes and the rotation methods. It was possible to reduce the grain size to 0.2 μm without fracture of the specimen by the imposition of severe plastic straining on commercial purity zirconium via ECA pressing. Grain size decreased and misorientation between grains increased with increasing number of ECA pressing passes, using processing route B_A in which the specimen was rotated by 90° in the same direction between each consecutive pass through the ECA pressing die.     

Achieving superplasticity at high strain rates using equal channel angular pressing

Abstract

Equal channel angular pressing (ECAP) is a processing procedure in which a sample is pressed through a die containing a channel bent into an L shaped configuration. This procedure introduces a high strain into the sample without any change in the cross-sectional area and it may be used to attain an ultrafine grain size with dimensions lying typically within the submicrometer range. This paper describes a series of experiments where ECAP was applied to a commercial Al–Mg–Li–Zr alloy having an initial grain size of ～400 µm. The results demonstrate a refinement in the grain size of this alloy to a size of ～1 µm and it is shown that these small grains are stable up to temperatures >600 K because of the presence of β′-Al₃Zr particles. The stability of these ultrafine grains at elevated temperatures provides an opportunity to achieve superplastic ductilities in this alloy at very high strain rates: for example, the measured elongations to failure under optimum pressing conditions exceed 1000% at a strain rate of 10^-1 s^-1 when testing at temperatures above 600 K.     

Microstructural evolution of bainitic steel severely deformed by equal channel angular pressing

High Si bainitic steel has been received much of interest because of combined ultra high strength, good ductility along with high wear resistance. In this study a high Si bainitic steel (Fe-0.22C-2.0Si-3.0Mn) was used with a proper microstructure which could endure severe plastic deformation. In order to study the effect of severe plastic deformation on the microstructure and properties of bainitic steel, Equal Channel Angular Pressing was performed in two passes at room temperature. Optical, SEM and TEM microscopies were used to examine the microstructure of specimens before and after Equal Channel Angular Pressing processing. X-ray diffraction was used to measure retained austenite after austempering and Equal Channel Angular Pressing processing. It can be seen that retained austenite picks had removed after Equal Channel Angular Pressing which could attributed to the transformation of austenite to martensite during severe plastic deformation. Enhancement of hardness values by number of Equal Channel Angular Pressing confirms this idea.     

Thermal stability study on two aluminum alloys processed with equal channel angular pressing

Equal channel angular pressing was used to produce sub-micrometer size grain structures in two aluminum alloys (commercially pure 1200 and Al-Mn-Si 3103). ECAP was conducted at room temperature following the process via route C, which involves a 180° angular rotation between passes and strongly affects material microstructure by reversing the shear strain every second pass. This unique characteristic of route C induced limited build-up of new high-angle boundaries, at least for the first three passes. The equal channel angular pressing was extended to six passes for both alloys; three passes by route C throughout the die were sufficient to produce a very fine-structured material for both alloys. High-resolution electron back-scattered diffraction pattern analysis was carried out to measure boundary misorientation within the deformed structures. Measurements of subgrain and grain spacing revealed a more effective microstructure refining effect in the 3103 than the 1200 alloy. Thermal stability of the severely deformed materials was studied at temperatures of 130, 240, 330°C, corresponding to 0.2, 0.35, 0.5 of the melting temperature. The results showed considerable grain growth in both materials solely at temperatures from and above half of the melting one.     

Flow properties of an aluminum alloy processed by equal channel angular pressing

Equal Channel Angular Pressing (ECAP) process is an important process for producing ultrafine-grained microstructures in bulk metals and alloys. In the present work, aluminum alloy AA 6063 samples were subjected to ECAP for up to three passes using an ECAP die with a die angle of 105°. The strain imparted to the specimen after three passes was approximately 2.64. Compression testing of the ECAP specimens was carried out to determine the subsequent flow behavior. Two types of compression test specimen orientations, one parallel to the axis of pressed sample and the other at 45° to the axis of the pressed sample, were used for the study. The strain path change (SPC) parameter was used to quantify the strain path change involved in straining by ECAP followed by straining by uniaxial compression. Higher flow strength values were observed in compression in specimens machined at 45º to the axis of the ECAP specimens. Flow softening and anisotropic behavior have been studied with respect to the number of passes and processing routes.     

On the effect of acute angles on deformation homogeneity in equal channel angular pressing

Finite element calculations for strain development and deformation homogeneities under equal channel angular pressing (ECAP) considering the channel angle, friction and the channel thickness show that general phenomena characteristic of ECAP still hold when the channel angle is acute ( = 75°), in contrast to the conclusion of the recent paper on acute channel angles [A.V. Nagasekhar, Y. Tick-Hon, S. Li, H.P. Seow, Mater. Sci. Eng. A410–A411 (2005) 269–272] that formation of corner gap was not a factor for the acute channel angles.