The role of back pressure in the processing of pure aluminum by equal-channel angular pressing

Samples of pure Al with a purity of 99.65% were processed by equal-channel angular pressing for up to four passes using different back pressures. Microhardness measurements were taken on cross-sectional planes and these data were supplemented by microstructural observations. The results show that the application of a back pressure leads to reasonable homogeneity within the billet except only in a narrow region of lower hardness lying adjacent to the bottom edge. For a constant number of passes through the die, the hardness distributions are independent of the magnitude of the imposed back pressure. It is shown that these results are consistent with theoretical predictions using finite element analysis. There is also evidence that the hardness values increase slightly when the pressing speed is increased by one order of magnitude.     

Using equal-channel angular pressing for the production of superplastic aluminum and magnesium alloys

Equal-channel angular pressing (ECAP) is a useful tool for achieving exceptional grain refinement in bulk metallic alloys. Typically, the grain sizes produced through ECAP are in the submicrometer range, and thus they are smaller by up to an order of magnitude than the grain sizes attained through typical thermomechanical treatments. As a consequence of these ultrafine grains, the as-pressed alloys may exhibit superplastic ductilities at faster strain rates than in conventional superplastic alloys. This work initially describes the application of ECAP to two different alloys. First, results are presented for a commercial Al-2024 alloy where this alloy was selected because it contains no minor additions of either zirconium or scandium to assist in restricting grain growth. The results show that superplasticity is achieved through the use of ECAP. Second, results are described for a Mg-0.6%Zr alloy where this alloy was selected because it is the optimum composition for achieving a high damping capacity. Again, processing by ECAP produces superplastic ductilities not attained in the cast alloy. The second part of this work demonstrates that processing by ECAP may be extended from conventional rod or bar samples to samples in the form of plates. This is a very attractive feature for industrial superplastic forming applications. This paper was presented at the International Symposium on Superplasticity and Superplastic Forming, sponsored by the Manufacturing Critical Sector at the ASM International AeroMat 2004 Conference and Exposition, June 8–9, 2004, in Seattle, WA. The symposium was organized by Daniel G. Sanders, The Boeing Company.     

Structure and mechanical properties of aluminum alloy 6061 subjected to equal-channel angular pressing in parallel channels

It is shown that a uniform ultrafine-grained structure in alloy 6061 can be formed already after four cycles of treatment by equal-channel angular pressing in parallel channels (ECAP-PC). Along with grain refinement, in the process of an ECAP-PC treatment there occurs in the alloy a dynamic strain aging, which results in the formation of nanodimensional particles of a strengthening phase Mg₂Si. It has been established that the alloy in the UFG state demonstrates a considerably higher level of strength and better plasticity in comparison with the material after a standard strengthening treatment.     

Effect of equal-channel angular pressing routes on high-strain-rate deformation behavior of ultra-fine-grained aluminum alloy

The effect of equal-channel angular pressing (ECAP) route on the high-strain-rate deformation behavior of ultra-fine-grained aluminum alloy was investigated. The 8-pass ECAPed specimens deformed via three different routes consisted of ultra-fine grains 0.5 μm in size, and contained a considerable amount of second-phase particles, which were fragmented and distributed in the matrix. In the torsion tests, the maximum shear stress significantly increased with increasing number of ECAP passes, while the maximum shear stress and fracture shear strain were lowest in the specimen deformed via route A among the three 8-pass ECAPed specimens. Observation of the deformed area beneath the fractured surface revealed the adiabatic shear bands of 100 μm in width in the specimen deformed via route A, which minimized the maximum shear stress and fracture shear strain, whereas they were hardly formed in the specimens deformed via route B or C. The formation of adiabatic shear bands was explained in terms of critical shear strain, deformation energy required for void initiation, and microstructural homogeneity related to ECAP routes.     

Structure and properties of aluminum alloy 1421 after equal-channel angular pressing and isothermal rolling

Sheets from the aluminum alloy 1421 with an ultrafine-grained (UFG) structure and a weak crystallographic texture were prepared by the method of equal-channel angular pressing (ECAP) through a die with channels of a rectangular cross section and by subsequent isothermal rolling. Both operations were carried out at a temperature of 325°C. It is shown that severe plastic deformation (SPD) leads to the formation of a completely recrystallized uniform microstructure with an average grain size of 1.6 µm in the alloy. At room temperature the alloy 1421 demonstrates high static strength (σ_u = 545 MPa, σ_0.2 = 370 MPa) in the absence of a significant anisotropy. At temperatures of hot deformation, the alloy showed ultrahigh elongations under superplasticity (SP) conditions. At a temperature of 450°C and initial deformation rate of 1.4 × 10^?2 s^?1 the maximum elongation at fracture was ～2700%. At static annealing at a temperature of SP deformation, the UFG structure formed in the process of SPD remains stable. The SP deformation is accompanied by an insignificant grain growth and pore formation.     

Using equal-channel angular pressing for refining grain size

Equal-channel angular pressing is an effective tool for attaining ultrafine grain sizes in bulk materials. An important advantage of this technique over conventional metalworking processes, such as extrusion and rolling, is that very high strains may be attained without any concomitant change in the cross-sectional dimensions of the sample. The microstructures introduced by equalchannel angular pressing critically depend on a number of experimental factors, including the nature of the slip systems introduced during the pressing operation and the total strain imposed on the sample. These factors are illustrated by reference to experiments conducted on pure aluminum; results are also included to demonstrate the influence of alloying additions and especially the remarkably small grain sizes that may be achieved in materials having low rates of recovery. For more information, contact T.G. Langdon, University of Southern California, Departments of Materials Science and Mechanical Engineering, Los Angeles, California 90089-1453; (213) 740-0491; fax (213) 740-7797; e-mail langdon@usc.edu.     

Factors influencing microstructural development in equal-channel angular pressing

Processing through the imposition of severe plastic deformation (SPD) provides an opportunity for achieving very significant grain refinement in bulk materials. Although different SPD procedures are available, the process of equal-channel angular pressing (ECAP) is especially attractive because it can be scaled easily to produce relatively large samples. This paper describes the principles of ECAP processing and demonstrates the potential for achieving unusual mechanical properties in the samples subjected to ECAP. Special emphasis is placed on the possibility of attaining a high strain rate superplastic forming capability in the as-pressed materials: examples are presented for an Al−Mg−Sc alloy prepared in the laboratory by casting and for a commercial Al-2024 alloy. This article is based on a presentation made in the 2002 Korea-US symposium on the “Phase Transformations of Nano-Materials,” organized as a special program of the 2002 Annual Meeting of the Korean Institute of Metals and Materials, held at Yonsei University, Seoul, Korea on October 25–26, 2002.     

Characterization of creep properties and creep textures in pure aluminum processed by equal-channel angular pressing

High-purity aluminum was processed by equal-channel angular pressing (ECAP) and then tested under creep conditions at 473 K. The results show conventional power-law creep with a stress exponent of n = 5 which is consistent with an intragranular dislocation process involving the glide and climb of dislocations. It is demonstrated that diffusion creep is not important in these tests because the ultrafine grains produced by ECAP are not stable at this temperature. Texture measurements were undertaken using the high-pressure preferred orientation neutron time-of-flight diffractometer and they reveal significant differences in the evolution of texture during creep in pressed and unpressed specimens. These experimental measurements of texture are in excellent agreement with theoretical textures predicted using a visco-plastic self-consistent model that limits deformation to plastic slip. The calculations provide additional confirmation that creep occurs through an intragranular dislocation process.     

等径角挤压铜合金的分节断裂

本研究提供了等径角挤压不同铜合金的局部剪切、坯锭开裂和分节断裂的实验依据。结果表明,尽管很多参数影响局部剪切,但是合金的硬度和分节断裂与其有着直接的关系,而硬度与合金的成分和相组成有关。在室温下,α-黄铜可以成功进行等径角挤压,而α/β黄铜甚至在350°C下都不能成功进行等径角挤压。利用DEFORMTM 软件模拟了开裂和分节断裂,研究不同参数对分节断裂的影响。结果表明,摩擦力和加工速率对获得完美坯锭影响很小,而利用背压可以很好地减小局部剪切、坯锭裂纹、分节断裂和破坏。利用背压能减小流动局部化,当背压由0提高到600 MPa时,可以提高材料流动均匀性并且使坯锭的均匀性提高36.1%。     

High-strain-rate deformation of titanium using dynamic equal-channel angular pressing

Titanium samples were deformed using equal-channel angular pressing (ECAP). Structural changes upon the uniform and localized high-strain-rate deformation and specific features of the nucleation and propagation of cracks have been studied. A geometrical method of determining the amount of uniform shear strain upon equal-channel angular pressing has been suggested. The method is based on a metallographic examination of the spatial orientation of structural components. The localized deformation leads to the appearance of adiabatic-shear bands. Two band systems are formed: longitudinal and transverse, arranged at an angle to the longitudinal. The occurrence of recrystallization inside the bands indicates local heating of the material to 770–870 K. Specific features of the structure of the adiabatic-shear bands arising in this method of deformation is their large width (to 100 μ m) and a multilayer structure.     

Obtaining a nanostructure in titanium by equal-channel angular pressing

Conclusions 1. We have developed a technological setup for isothermal equal-channel angular pressing (ECAP) for hard-to-deform materials that serve at up to 500°C and specific loads on the punch of up to 2000 MPa. 2. We established the process parameters that provide fabrication of defectless preforms from titanium of grade VT1-0 with a nanostructure and obtained specimens 100 mm long and 20 mm in diameter. 3. We used the method of grids to study the strain state of preforms obtained by the ECAP method and established the great influence of the geometry of the channels, in particular, of the back angle of intersection of the pressing channels, on the uniformity of the distribution of shear strain in the plane of flow of the material. 4. Rotation of a preform of titanium VT1-0 after each cycle of ECAP by 90° about the longitudinal axis provides a homogeneous structure with grains 0.2 – 0.5 μm in size after eight treatment cycles. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 9, pp. 27 – 31, September, 2000.     

Evolution of nanoscale porosity during equal-channel angular pressing of titanium

Small-angle neutron scattering (SANS) analysis and transmission electron microscopy evidence suggest the occurrence of nanoscale porosity in commercial-purity titanium processed by equal-channel angular pressing (ECAP). SANS data were produced at two different facilities (GKSS, Germany; and Los Alamos, USA) and were analysed using three different methods. The results are consistent and yield a conclusive picture of the distribution of the scattering centres, which are believed to be associated with nanoporosity. Back pressure applied during ECAP tends to reduce the average pore size, which also depends on the processing route used. The results of the study strongly suggest that ECAP leaves a footprint in titanium in the form of a population of polydispersed nanovoids, which may play an important role in subsequent processing of the material.     

圆形等通道转角挤压过程三维数值模拟

基于网格重划分技术,采用有限元软件Marc对等通道转角挤压(Equal Channel Angular Pressing,ECAP)进行数值模拟,得到了模具转角φ=90°、ψ=0°以及不同摩擦条件下圆形试样ECAP变形过程的网格、载荷变化以及等效应变分布规律。结果表明:由于剪切变形和外摩擦作用,底部网格产生畸变,出口端部形成半球形;圆形ECAP过程经历开始变形、稳定变形和终了三个阶段,在开始变形阶段载荷急剧增加,进入稳定阶段后变化平稳,但由于摩擦和加工硬化作用载荷继续增加;与无摩擦情况相比,摩擦的存在增加了试样与模具接触的接触面积、载荷值以及变形的不均匀性。此外,试样中心较表面变形更均匀,而中间截面相比头尾变形较为均匀。     

A convergent-beam electron diffraction study of strain homogeneity in severely strained aluminum processed by equal-channel angular pressing

Aluminum of commercial purity was processed by equal-channel angular pressing (ECAP) through two, four and eight passes at room temperature. A series of [1 1 4] convergent-beam electron diffraction (CBED) zone axis patterns were obtained using an electron probe with a diameter of 20 nm. Observations were recorded both immediately adjacent to the grain boundaries and in the grain interiors. Symmetry breaking of the higher-order Laue zone (HOLZ) lines was observed adjacent to the boundaries after two and four passes but not in the grain interiors. Pattern simulation of the CBED patterns taken from the two- and four-pass samples adjacent to the boundaries revealed a homogeneous strain with compressive and shear components. The presence of these homogeneous strains demonstrates that the internal stresses associated with the deformation of aluminum at room temperature are localized in the close vicinity, to within ∼20 nm, of the grain boundaries.     

Effect of the velocity of equal-channel angular pressing on the formation of the structure of pure aluminum

Transmission electron microscopy, X-ray diffraction, and electron back-scattering pattern analysis have been used to investigate the effect of the speed of equal-channel angular pressing (ECAP) at room temperature on the formation of ultrafine-grained structure in pure aluminum. It has been established that eight ECAP passages with a pressing speed of 3.3 × 10^?2 mm/s results in the formation in aluminum of a substantially inhomogeneous grain structure with a grain size in the range of 1–27 μm (average size 3.0 μm). An increase in the speed of pressing by an order of magnitude leads to an increase in the level of internal stresses and dislocation density, an increase in the upper boundary of the interval of the grain-size distribution and in the average grain size (to 3.4 μm), and a decrease in the number of boundaries with high-angle misorientations. It is assumed that these changes are connected with the fact that processes of dislocation-structure relaxation have no time to occur during the ECAP at high pressing speeds.     

Corrosion of ultra-fine grained copper fabricated by equal-channel angular pressing

Corrosion of ultra-fine grain (UFG) copper fabricated by equal-channel angular pressing (ECAP) has been investigated in comparison with that in recrystallized coarse grain (CG) copper. Corrosion current was estimated by a Tafel extrapolation method to examine the kinetics of corrosion in a modified Livingstone etchant, which is sensitive to dislocations and grain boundaries. UFG copper exhibited a lower corrosion current in comparison with that in its recrystallized coarse grain (CG) counterpart despite the fact that the dislocation density and total fraction of grain boundaries are much greater in UFG copper than in CG copper. Corrosion damage on the surface of UFG copper is macroscopically rather uniform whereas obvious attack at grain boundaries and selective corrosion of some grain interiors were observed in CG copper.