Equal Channel Angular Pressing Material Research Articles

A Novel Method for the Determination of High Temperature FLCs of ECAP-Processed Aluminum AA5083 Sheet Metal

In this study, investigations into the deformation behavior of aluminum AA5083 at elevated temperatures were carried out on a newly developed test rig. The test rig was developed jointly with ZwickRoell GmbH & Co. KG (Germany) and is based on a Nakajima test carried out with heated dies. In this way, statements can be made about the lightweight potential of the alloy. Additionally, equal-channel angular pressing (ECAP) was performed to process the aluminum sheet metal. The conventional ECAP process is mainly used for bulk material in laboratory use and therefore is often not suitable for many industrial applications, especially for large series. The use of sheet metal allows a significant increase in the areas of application. It is documented in conventional ECAP that grain refinement is achieved by the severe plastic deformation. At room temperature this primarily increases the mechanical strength. Formability is improved in fine-grained materials, especially at elevated temperatures, which is related to diffusion-controlled deformation mechanisms and grain boundary sliding. The advantages of ECAP for sheet materials are thus also in lightweight construction and can even optimize the use of the AA5083 alloy. ECAP-route C was used for the process to provide the most homogeneous microstructure possible (180° rotation around the ECAP-axis after the first pass). Nakajima specimens were taken from the processed sheet materials to determine the Forming Limit Curve (FLC) compared to the reference material (four different specimen geometries). FLCs under elevated temperatures (250 °C, 375 °C) were performed on the novel Nakajima test bench. A special feature of the test rig is the rapid heating to avoid microstructural changes. Microscopic examinations were performed after the deformation to study the deformation mechanisms. Differences of the forming and fracture mechanisms between the reference alloy and the ECAP material were found.

The Effect of Process Route on the Mechanical Properties and Microstructure Aluminum Al-Si-Cu-Mg-Mn Alloys

Equal-channel angular pressing (ECAP) is an effective fabrication process to modify ultrafine grain size. It is believed that the smaller grain size could improve the mechanical properties. In the present work, the solution treatment applied to Al-Si-Cu-Mg-Mn alloy. Furthermore, two models of ECAP have been done through one-to-three pass at room temperature. The microstructures were observed by optical microscope (OM) and scanning electron microscope (SEM). The micro-hardness of longitudinal plane was analyzed by vickers hardness test. The strength of ECAP materials at room temperature was obtained by using tensile test. The results show that the ECAP process has improved mechanical properties of alloys, which were the hardness and ultimate tensile strength tend to increase since single pass stage.

Effect of the severe plastic deformation by ECAP on microstructure and phase transformations in Ti-15Mo alloy

Ti-15Mo alloy in a metastable β solution treated condition was processed by equal channel angular pressing (ECAP) at an elevated temperature of 250 °C. The resulting microstructure is highly deformed, contains twins and shear bands, but is not ultra-fine grained. Both the initial solution treated material and the material after ECAP were subjected to ageing at 400 °C and 500 °C in order to study the effect of deformation on phase transformations, namely the α phase precipitation. The phase composition was studied by X-ray diffraction measurement; the microstructure was investigated using conventional EBSD and an advanced method of transmission Kikuchi diffraction (TKD). It was shown that the α phase precipitation is accelerated in areas with higher density of lattice defects, which provide a dense net of preferred sites for nucleation and also fast diffusion paths necessary for accelerated growth. Upon further annealing, discontinuous lamellar coarsening occurs, which had not been previously reported in metastable β-Ti alloys. The microhardness is governed mainly by the formation of ω phase particles. The fraction of ω phase increases during annealing at 400 °C and the specimen aged at 400 °C/16 h shows the highest value of microhardness of 520 HV for both ECAP and undeformed material. Upon annealing at 500 °C, the microhardness is significantly lower.

Impact of Equal Channel Angular Pressing (ECAP) on the machinability of an aluminium alloy (EN AW-6082)

The scope of this paper was to examine the effects of the ECAP procedure on the machinability of aluminium (EN AW-6082) based on turning tests. The behaviour of the ECAP material was investigated and compared to the as-received (AR) material. Since limited information on the machinability of ECAP materials is available, it was necessary to conduct an evaluation on basis of the untreated material. For this purpose, a defined range of cutting parameters, which were raised based on recommendations of the tool manufacturer were used for the first experiments. In order to analyse the effects of ECAP on the machinability the following comparative criteria were pulled up: the quality of the machined surfaces, appearing forces and chips produced.

<i>In Situ</i> Testing and Heterogeneity of UFG Cu at Elevated Temperatures

Experiments were conducted to investigate deformation-induced processes during in-situ tensile test at elevated temperature. Consequently the microstructure after creep loading was examined by 3D Electron Back Scatter Diffraction (EBSD) technique. The billets of coarse-grained copper were processed by equal-channel angular pressing (ECAP) at room temperature using a die that had an internal angle of 90° between the two parts of the channel and an outer arc of curvature of ~ 20°, where these two parts intersect. The pressing speed was 10 mm/min. To obtain an ultrafine-grained (UFG) material, the billets were subsequently pressed by route Bc by 8 ECAP passes to give the mean grain size ~ 0.7 μm. The constant strain-rate test in tension was performed at 473 K using testing GATAN stage Microtest 2000EW with EH 2000 heated grips which is configured for in-situ electron back scatter diffraction (EBSD) observations. Microstructure was examined by FEG-SEM TESCAN MIRA 3 XM equipped by EBSD detector HKL NordlysMax from OXFORD INSTRUMENT. The tensile test was interrupted by fast stress reductions after different deformation step and observation of microstructure changes was performed. Despite of a considerable interest in ECAP processing method, there are not many works documenting microstructure evolution and changes during creep testing and determining creep mechanisms of ultrafine-grained materials processed by ECAP. It was found that creep resistance of UFG pure Al and Cu is considerably improved after one ECAP pass in comparison with coarse grained material, however, further repetitive pressing leads to a noticeable deterioration in creep properties of ECAP material. Recently it was observed the coarsening of the grains in microstructure of ECAP copper during creep at elevated temperature. It was suggested that creep behaviour is controlled by storage and dynamic recovery of dislocations at high-angle boundaries. In the present work was found that ultrafine-grained microstructure is instable and significant grain growth has already occurred during heating to the testing temperature. Static recrystallization during heating led to the formation of high fraction of special boundaries Σ3 and Σ9. The tensile deformation at 473 K led to the additional grain growth and formation of new grains. Microstructure was investigated also by 3D EBSD.

The Effect of Severe Plastic Deformation Processing on Creep Properties of Metallic Materials

Processing by severe plastic deformation (SPD) may be defined as such metals forming procedure in which a very high strain is imposed on a bulk material. This paper investigates the effect of different equal channel angular pressing (ECAP) routes and number of ECAP passes on resulting microstructure, mechanical properties and creep behaviour of selected materials. The distinction between various ECAP routes (A, B and C) and the difference in number of ECAP passes applied may lead to variations both in the macroscopic distortions of the individual grains and in the capability to develop a reasonably homogeneous and equiaxed ultrafine-grained microstructure. Experimental materials were processed by ECAP at room temperature using a die with an internal angle of 90° between the two parts of the channel. The ECAP pressing was performed by different routes up to 12 ECAP passes. Tensile creep tests were conducted at temperatures 473 - 673 K and at different applied stresses on ECAP materials and, for comparison purposes, on their unpressed states. Microstructure of samples was characterized by scanning electron microscope (SEM) equipped with the electron backscatter diffraction (EBSD) unit. In conclusion, the ECAP processing route and number of applied ECAP passes could play an important role in creep behaviour and their effect may be different for particular materials. The highest differences in processing routes were revealed for materials especially at lower number of ECAP passes. However, a little apparent dependence of the creep properties was observed during subsequent pressing.

Effects of equal-channel angular pressing and accumulative roll-bonding on hydrogen storage properties of a commercial ZK60 magnesium alloy

The hydrogen storage properties of a commercial ZK60 magnesium alloy were investigated after processing by equal-channel angular pressing (ECAP) and accumulative roll-bonding (ARB). Samples were prepared by 6 passes of ECAP and subsequently rolled 25 times. Microstructural characterization was performed using scanning and transmission electron microscopy. The kinetics of hydrogen absorption and desorption were measured in a Sievert-type apparatus. Processing only by ECAP resulted in a very low hydrogen storage capacity of 0.5 wt. %. Additional processing by ARB refined the microstructure and improved the hydrogen absorption. Despite this improvement, the hydrogen capacity was limited to less than 4 wt. %. This limitation was overcome by crushing the materials into small thin plates. The crushed sample from the ECAP material absorbed 4.77 wt. % of hydrogen in 24 h while the capacity of the sample processed by ECAP and ARB was 6.4 wt. %. The results are discussed in terms of morphology and microstructure of the samples.

Enhanced creep properties of copper and its alloys processed by ECAP

This work describes the effect of equal channel angular pressing (ECAP) on the microstructure and creep properties of pure copper and its two binary alloys with addition of small amounts of Zr or Co. The ECAP pressing was performed at room temperature by route Bc up to 12 passes using a die with an internal angle of 90° between the two parts of the channel. Ultrafine-grained (UFG) microstructure formed through ECAP process has been studied by methods of transmission electron microscopy (TEM) and scanning electron microscopy (SEM) equipped with the electron backscatter diffraction (EBSD) unit. Tensile creep tests were conducted in tension at temperature 673 K and at different applied stresses on ECAP material and, for comparison purposes, on unpressed coarse-grained states of materials under investigation too. It was found that both alloys processed by ECAP exhibited similar character of creep behaviour. Creep resistance was markedly improved after first two ECAP passes in comparison with creep behaviour of unpressed materials. The minimum creep rate of ECAP material may be up to two orders of magnitude lower than that of unpressed material. However, subsequent ECAP passes lead to a decline of creep life and the difference in the minimum creep rate for the ECAP material and unpressed state consistently decreases with increasing number of ECAP passes. Further, ECAP process led to significant improvements in fracture strain. The link between microstructural processes and creep behaviour of pressed copper and its selected alloys is examined in detail.

Microstructural stability after severe plastic deformation of AZ31 Magnesium

Friction stir processing (FSP) and equal channel angular pressing (ECAP) were used to modify the microstructure of twin roll cast (TRC) AZ31 magnesium. The influence of these processes on the microstructural properties of the material was investigated. It was found that both processes produced microstructures with an average grain size of less than 10 pm, suggesting that they have the potential for superplastic deformation. Heat treatments were performed on the TRC, ECAP and FSP materials to assess their microstructural stability. Both the ECAP and TRC material were found to be fairly stable, showing normal grain growth while the FSP material grew substantially at temperatures above 200°C. The activation energy of grain boundary motion of the TRC material was calculated to be 167 kJ/mol.

Assessment of anisotropic tensile strength using a modified shear punch test for Cu–Cr–Zr alloy processed by severe plastic deformation

The effect of severe plastic deformation on the anisotropic strength change was investigated by equal-channel angular pressing (ECAP) experiments with various pass numbers. To overcome the size limitation for the tensile test specimen, this study proposes a shear punch test modified by using a flat-end rectangular punch to evaluate uniaxial tensile strength in multiple directions in the longitudinal and transverse sections of the ECAP-processed Cu–Cr–Zr alloy. The validity of the modified method was verified by the linear relationship between the shear punch test data and tensile test data in the longitudinal direction. Specimens were prepared at three angles (0°, 45° and 90°) with respect to the extrusion direction in the longitudinal sections and the horizontal direction in the transverse sections of the ECAP bar. The results showed that tensile strength deceased when the angle increased from 0° to 90° in both the longitudinal and transverse sections. The longitudinal sections showed a more pronounced anisotropy than the transverse sections. As the amount of plastic deformation by ECAP increased, the degree of the anisotropy decreased in the longitudinal section but increased in the transverse section. Based on the results of this study, the strength of an ECAP material must be assessed in multiple directions in both the longitudinal and transverse sections.

<i>In Situ</i> X-Ray Diffraction Study of Thermal Stability of Cu and Cu-Zr Samples Processed by ECAP

X-ray diffraction (XRD) studies of ECAP (equal-channel angular pressing) materials were performed after annealing and by in-situ measurements in XRD high-temperature chamber for samples prepared by different number of passes and number of revolutions, respectively. Main attention was given to Cu and Cu-Zr samples. Significant dependence on number of passes was found for ECAP samples. In-situ measurements were focused not only on temperature dependence but also on time evolution of the diffraction line profiles. Evaluation in terms of dislocation densities, correlation and crystallite size and its distribution was performed by our own software MSTRUCT developed for total powder diffraction pattern fitting. Abnormal growth of some grains with annealing is well-known for copper and leads to the creation of bimodal microstructure. Therefore a special care must be given to the evaluation and a model of two Cu components (larger and smaller crystallites) was fitted to the data if an indication of some crystallite growth appears either in the XRD line profile shape or in two-dimensional diffraction patterns.

Effect of post-heat-treatment in ECAP processed Cu–40%Zn brass

This study evaluated the grain refinement and mechanical properties of Cu–40%Zn brass processed by equal channel angular pressing (ECAP). The samples were repeatedly pressed to strains as high as 4 at a temperature of 250°C using route C. Recrystallization heat-treatment was carried out at 350°C for 20min. To evaluate the grain boundary character distributions and mechanical properties of ECAP materials, electron backscattered diffraction, Vickers microhardness, and tensile tests were used. Increased passes in ECAP led to notable grain refinement, from 13μm in the initial sample (350°C/180min heat-treated material) to 300nm after 4 passes. The post-heat-treated material after ECAP also sustained refined grain size (2.4μm in the α phase and 4μm in the β′ phase) than that of initial state, which resulted in an increase in tensile strength with no decrease in elongation. Furthermore, coincidence site lattice boundaries derived from annealing twins were more developed than those of the initial material. This paper discusses the enhancement of mechanical properties in terms of grain boundary character distributions development.

Change of anisotropic tensile strength due to amount of severe plastic deformation in aluminum 2024 alloy

Abstract Strength change in anisotropic principal axis directions of an over-aged aluminum 2024 (Al 2024) alloy subjected to equal channel angular pressing (ECAP) was studied. This alloy was subjected to several stages of severe plastic deformation by ECAP. Various strength values were measured using the tension test, the small punch (SP) test and Rockwell and Vickers hardness tests. The SP test was employed due to the size limitation for the tension test specimen in the transverse direction of the ECAP sample. In a previous paper [8] , we proposed equations that can assess the tensile strengths in the anisotropic principal axis directions of an anisotropic material using SP test data and showed that the proposed strength assessment equations were very accurate. These equations were used in this study. As the amount of plastic deformation by ECAP increased, the tensile strength in the longitudinal direction of the ECAP-processed Al 2024 alloy was increased. However, considerable change of strength, which is generally predicted, was not observed in this study. In the result of the tensile strength in the transverse direction, the strength even decreased after an equivalent plastic strain of approximately 3.0. It was argued that this decrease was due to the dynamic recovery of the dislocation density during or after ECAP processes at 150 °C. This argument was supported by comparison with the results of the tension test and the hardness test. Based on the results of this study, the strength of an ECAP material must be assessed for both the longitudinal and transverse directions.

Macro‐ and Nanomechanical Properties and Strain Rate Sensitivity of Accumulative Roll Bonded and Equal Channel Angular Pressed Ultrafine‐Grained Materials

AbstractSeveral processes of severe plastic deformation are suitable for the production of materials with ultrafine‐grained microstructures which are known to exhibit high strength and often good ductility as well as strain rate sensitive behavior. The most promising ones are equal channel angular pressing (ECAP) for bulk material and accumulative roll bonding (ARB) for the production of sheet material. In order to evaluate the influence of the process on these mechanical properties and the strain rate sensitivity, tensile tests, and nanoindentation tests were performed on material produced up to similar effective plastic strains of εARB = 6.4 and εECAP = 6.3. It could be shown that the macroscopic strength is slightly higher for ARB than for ECAP material and vice versa in nanoindentation. Independent of the testing method, the strain rate sensitivities and activation volumes are similar for both materials. Thus, both processes performed up to similar effective plastic strains lead to comparable improvements in the mechanical properties. Additionally it could be shown, that this comparison allows the identification of the dominant deformation mechanism which is responsible for the observed strain rate sensitivity.

The influence of ECAP on the small punch creep of Al–4 vol.% Al4C3 composite

The creep behaviour of a composite based on an aluminium matrix reinforced by 4 vol.% Al4C3 was studied at temperatures of 623 and 723 K by small punch testing with a constant force. The composite was tested in two different states: (i) as received by mechanical alloying with hot extrusion (HE) as the final operation and (ii) with equal channel angular pressing (ECAP) superimposed on the hot-extruded material. The ECAP does not improve the observed creep resistance. The reduction of force leading to the same deflection rate is not very significant. This points out that the ECAP process of the present composite, which produces substantial strengthening at lower temperatures, is not accompanied by pronounced weakening of creep resistance at elevated temperatures. The threshold force in the ECAP material is about 5 N weaker than in the HE material.

Assessment of tensile strength using small punch test for transversely isotropic aluminum 2024 alloy produced by equal channel angular pressing

The microstructures of metallic materials become extremely refined and their mechanical strengths exhibit considerable anisotropy when subjected to severe plastic deformation by equal channel angular pressing (ECAP). The small punch (SP) testing method was employed as a substitute for the standard tension test to assess the anisotropic tensile strength of materials produced by ECAP. SP testing is a viable alternative to standard tension tests because ECAP samples are usually limited to approximately 10 mm in length in the transverse direction perpendicular to the extrusion (longitudinal) direction of the ECAP process and the dimensions of the SP test specimen are 10 mm × 10 mm × 0.5 mm. The SP tests were performed using specimens of over-aged aluminum 2024 alloy subjected to three ECAP passes at 150 °C and extracted by wire cutting along the longitudinal and transverse axes. The failure surfaces of SP specimens after testing showed that failure was caused by shear deformation and that ECAP-processed materials displayed anisotropic strength. Based on these results, a method is proposed to assess tensile strength along both the transverse and the longitudinal axes of ECAP materials exhibiting anisotropic strength. The validity of the proposed method was verified through a comparison of its performance evaluating plastic flow strength with measurements obtained by tension tests along the longitudinal axis.

Plastic deformation mechanism of ultra-fine-grained AA6063 processed by equal-channel angular pressing

Equal-channel angular pressing (ECAP) is an established method to produce ultra-fine-grained (UFG) materials with extraordinary mechanical properties. However, different methods to characterize the complex microstructure give contradictory results. Therefore an ECAP-processed UFG aluminum alloy AA6063 was characterized by various electron-microscopy-based methods such as backscattered electron contrast, electron backscatter diffraction, transmission electron microscopy and low-voltage scanning transmission electron microscopy. Only a skilful combination of all methods reveals the complete information about the microstructure which is needed to understand the results of the mechanical testing by nanoindentation, tensile tests and strain-rate jump test. The main difference is the amount of low-angle grain boundaries and high-angle grain boundaries which determine hardness, tensile and yield strength and strain-rate sensitivity of ECAP materials produced by different numbers of passes and routes. This is explained by the interaction of dislocations with the different grain boundaries.

Thermo-mechanical response of Al 6061 with and without equal channel angular pressing (ECAP)

The thermo-mechanical responses of Al 6061 before and after equal channel angular pressing (ECAP) at different strain rates and temperatures were measured. Al 6061 was solution heat treated before ECAP pressing at room temperature and subjected to up to three passes. After pressing, the billets were aged at 100 °C for 2 days. An as-received Al 6061-T651 was studied similarly to investigate the differences between processed and non-processed specimens. The responses of ECAP material were determined at −30, 22, 125 and 250 °C, and at strain rates from 10 −5 to 2530 s −1; the 6061-T651 specimens were subjected to uniaxial compressive loading at −31, 22, 85, 150, 230 and 315 °C, and strain rates ranging from 10 −5 to 2200 s −1. It was found that, the ECAP process increases the strength versus the T651 condition. Additionally, the Al 6061 ECAP is not sensitive to strain rate at room and lower temperatures, but the sensitivity increases as the number of passes and/or temperature are increased and this is the same for the non-processed material. Increasing the number of passes increases the flow stress at room and lower temperatures, has almost no effect at 125 °C and decreases at 250 °C. For both materials, the dynamic flow stress is higher than the stress at quasi-static strain rates even when the quasi-static strain rate regime is insensitive to strain rate. The Al 6061 has strong texture after one pass but steadily increases as the number of passes are increased. This is the first study that reports on the thermo-mechanical responses of ECAP and non-ECAP Al 6061 at such a wide range of strain rates, including dynamic, and temperatures.

Corrosion Characteristics of an Ultrafine-Grained Al-Mg-Si Alloy (AA6082)

The corrosion behaviour of the aluminium alloy, AA6082, processed by equal-channel angular pressing (ECAP) after different passes (route E, room temperature) was studied in comparison to the coarse-grained counterpart. The results of the electrochemical investigations (cyclovoltammetry; electrochemical impedance spectroscopy, EIS) are presented in correlation with the microstructure before and after the corrosion examinations. Both, chemical (precipitations, phases) and physical (dislocations, high-angle grain boundaries, grain size, low-angle grain boundaries) inhomogeneities characterize the microstructure of this commercially used Al-Mg-Si alloy. Results indicate an improved resistance against pitting of the ECAP material expressed by a reduced pitting density of up to 50 % and lower pit depths. EIS measurements and microstructural examinations (scanning electron microscopy, transmission electron microscopy, 3D topography measurement) confirm that ECAP modifies the number, size and distribution of these inhomogeneities, which leads to a more favourable corrosion behaviour.

Effect of Equal-Channel Angular Pressing (ECAP) on Creep in Aluminium Alloys

This paper examines the effect of equal-channel angular pressing (ECAP) on creep behaviour of pure aluminium, binary Al-0.2wt.%Sc alloy and ternary Al-3wt.%Mg-0.2wt.%Sc alloy. The ECAP was conducted at room temperature with a die that had a 90° angle between the channels and 8 repetitive ECAP passes followed route BC. Constant stress compression creep tests were performed at 473 K and stresses ranging between 16 to 80 MPa on ECAP materials and, for comparison purposes, on the initial coarse-grained materials. The results showed that the creep resistance of the ECAP processed Al-Sc and Al-Mg-Sc alloys was markedly deteriorated with respect to unpressed coarse-grained materials.