Incremental Equal Channel Angular Pressing Research Articles

Effect of microstructural features on the corrosion behavior of severely deformed Al–Mg–Si alloy

AbstractThe aim of this study was to determine the influence of severe plastic deformation processing and the changes in microstructure resulting therefrom on the corrosion resistance of an Al–Mg–Si alloy. The alloy was processed using incremental equal channel angular pressing, which caused a reduction in grain size from 15 to 0.9 µm. The grain refinement was accompanied by an increase in the number of grain boundaries and dislocations, and by changes in grain orientation. However, there was no change in the size and number of intermetallic particles, which presumably resulted in a constant number of galvanic couplings. Electrochemical experiments revealed only slight differences between the samples before and after processing. Higher potential transients/oscillations upon immersion and increased corrosion currents in the vicinity of corrosion potential point to slightly higher reactivity of the most refined material. This indicates that intermetallic particles are the most crucial microstructural elements in terms of corrosion resistance. Their impact exceeds that of grain boundaries, in particular, at the stage of corrosion initiation. The development of corrosion attack is controlled more by the microstructure of the matrix as the grain refinement resulted in a less pronounced corrosion attack in comparison with the coarse‐grained sample.

Ultrasonic Assisted Incremental Equal Angular Channel Pressing Process of AA 6063

Industrial usage of ultrafine‐grained materials is not common because of their high manufacturing cost of unconventional processing techniques for serial manufacturing. There is a need to increase process efficiency and decrease manufacturing costs for industrial usage of severe plastic deformation methods to manufacture ultrafine‐grained materials. Previously, ultrasonic systems and incremental feeding of materials are used to increase process efficiency. In this experimental study, the ultrasonic system is adapted to an incremental equal angular channel pressing method, called ultrasonic‐assisted incremental equal channel angular pressing (UI‐ECAP). As a reference material of this new system, AA 6063 is selected. Two passes I‐ECAP are applied to the initial material with and without ultrasonic vibrations. The microstructure and mechanical results are investigated for both specimen groups. According to the observations, grain size differences throughout the microstructure decrease with the application of ultrasonic impulses during the deformation. However, UI‐ECAP results in smaller and more equiaxed grain geometry and finer precipitates observed in transmission electron microscope (TEM) analysis. According to the hardness mapping results, UI‐ECAPed sample shows more homogenous hardness distribution than I‐ECAPed sample. The yield strength and ultimate tensile strength of the specimen after two passes of UI‐ECAP are higher in comparison with the I‐ECAP.

Microstructure, tensile properties and formability of ultrafine-grained Al–Mn square plates processed by Incremental ECAP

In the article, plates of commercial alloy 3003 were processed by Incremental Equal Channel Angular Pressing (I-ECAP). A microstructural evaluation revealed that it was possible to obtain a high fraction of high-angle grain boundaries, exceeding 63%, and an average grain size of 0.5 μm. Tensile testing revealed that the material has low ductility, but high mechanical strength, 120% higher than that of the initial material. Plate formability was described using the Lankford parameter and evaluated based on the crystallographic texture. It was concluded that the mean anisotropy was almost identical to that of the initial material, but planar anisotropy was reduced almost to 0. This was explained on the basis of the crystallographic texture. It appears that I-ECAP processed plates have qualities that could make them suitable for further processing by means of metal forming methods.

Similar and dissimilar welds of ultrafine grained aluminium obtained by friction stir welding

Samples from commercially pure aluminium were subjected to various number of passes of Incremental Equal Channel Angular Pressing (I-ECAP) and subsequently welded using Friction Stir Welding (FSW). Similar and dissimilar welds were obtained and investigated in terms of their microstructure and mechanical properties. In the case of similar weld from coarse grained aluminium in the stir zone a decreased average grain size was obtained, which resulted in enhanced microhardness. In the case of samples after I-ECAP, the ultrafine grained regime has not been preserved, which caused a drop in microhardness in the stir zone. Nevertheless, obtained results were still higher than those for coarse grained sample. In all welds the average grain size of 2.1–3.7 μm was obtained. No correlation between the microstructure of base material and stir zone has been found. Tensile tests revealed, that the localization of deformation was obtained in each weld in the area of the biggest average grain size. For dissimilar welds from deformed and undeformed samples a gradient change in microstructure and microhardness was obtained on the cross-section of the welds.

Ultrafine-Grained Plates of Al-Mg-Si Alloy Obtained by Incremental Equal Channel Angular Pressing: Microstructure and Mechanical Properties

In this study, an Al-Mg-Si alloy was processed using via incremental equal channel angular pressing (I-ECAP) in order to obtain homogenous, ultrafine-grained plates with low anisotropy of the mechanical properties. This was the first attempt to process an Al-Mg-Si alloy using this technique. Samples in the form of 3 mm-thick square plates were subjected to I-ECAP with the 90 deg rotation around the axis normal to the surface of the plate between passes. Samples were investigated first in their initial state, then after a single pass of I-ECAP, and finally after four such passes. Analyses of the microstructure and mechanical properties demonstrated that the I-ECAP method can be successfully applied in Al-Mg-Si alloys. The average grain size decreased from 15 to 19 µm in the initial state to below 1 µm after four I-ECAP passes. The fraction of high-angle grain boundaries in the sample subjected to four I-ECAP passes lay within 53 to 57 pct depending on the examined plane. The mechanism of grain refinement in Al-Mg-Si alloy was found to be distinctly different from that in pure aluminum with the grain rotation being more prominent than the grain subdivision, which was attributed to lower stacking fault energy and the reduced mobility of dislocations in the alloy. The ultimate tensile strength increased more than twice, whereas the yield strength was more than threefold. Additionally, the plates processed by I-ECAP exhibited low anisotropy of mechanical properties (in plane and across the thickness) in comparison to other SPD processing methods, which makes them attractive for further processing and applications.

On the evolution of microstructure and texture in commercial purity titanium during multiple passes of incremental equal channel angular pressing (I-ECAP)

This paper presents an investigation on the evolution of microstructure and deformation characteristics of commercial purity Titanium (CP-Ti) during incremental equal channel angular pressing (I-ECAP). CP-Ti grade 2 was subjected to six passes at 300°C following route BC, using a die with channel angle of 120°. Electron backscatter diffraction (EBSD) technique was used to characterize the microstructure after first, second, fourth and sixth pass, in the flow and transverse plane of the samples. Texture development through subsequent processing was also investigated using pole figures in both planes. Following first pass, the grain boundary maps across both flow and transverse plane showed a high degree of heterogeneity in grain morphology with the presence of elongated and fine grains. Also, misorientation peaks associated with {101̅2} tensile twins and a small fraction of {112̅2} compressive twins were observed in the microstructure. After second pass, microstructure was further refined and the twinning activity was greatly reduced with no noticeable activity after the fourth pass. Remarkable grain refinement was achieved after sixth pass with majority of grains in the ultrafine grain (UFG) range and with a relatively homogenous microstructure. Continuous dynamic recrystallization (CDRX) has been observed during subsequent I-ECAP processing. It was seen that twinning alongside CDRX acted as a dominant grain refinement mechanism during the initial passes of I-ECAP process beyond which slip was dominant deformation behaviour.

Analytical and Experimental Investigation of Process Loads on Incremental Severe Plastic Deformation

From the processing point of view, friction is a major problem in the severe plastic deformation (SPD) using equal channel angular pressing (ECAP) process. Incremental ECAP can be used in order to optimize frictional effects during SPD. A new incremental ECAP has been proposed recently. This new process called as equal channel angular swaging (ECAS) combines the conventional ECAP and the incremental bulk metal forming method rotary swaging. ECAS tool system consists of two dies with an angled channel that contains two shear zones. During ECAS process, two forming tool halves, which are concentrically arranged around the workpiece, perform high frequency radial movements with short strokes, while samples are pushed through these. The oscillation direction nearly coincides with the shearing direction in the workpiece. The most important advantages in comparison to conventional ECAP are a significant reduction in the forces in material feeding direction plus the potential to be extended to continuous processing. In the current study, the mechanics of the ECAS process is investigated using slip line field approach. An analytical model is developed to predict process loads. The proposed model is validated using experiments and FE simulations.

Warm deformation behaviour of UFG CP-Titanium produced by I-ECAP

The objective of the present study is to investigate the deformation behaviour of Ultrafine-grain (UFG) commercial purity Titanium (CP-Ti) at warm temperatures. Firstly, CP-Ti billets were processed through six passes of incremental equal channel angular pressing (I-ECAP) at 300° C using a die channel angle (Φ) of 120°. Uniaxial compression tests were then performed under isothermal conditions on cylindrical samples obtained from the UFG CP-Ti billets. A series of these tests were conducted at different temperatures of 400, 500 and 600 °C and at varying strain rates of 0.01, 0.1 and 1.0 s−1. In each test, the original height of the sample was deformed by ~50% of its original value. The true stress-strain curves obtained, revealed that the flow stress was sensitive to both temperature and strain rate. In general, the flow stress was higher for lower temperatures and higher strain rates. For tests conducted at 400 and 500 °C, the flow stress quickly reaches a peak value, beyond which it exhibits a steady state response where there is no appreciable change in flow stress with increasing strain. The 600 °C tests however shows a strain hardening behaviour. Microstructure of the sample deformed at 600 °C and 0.01 s−1, exhibited significant grain growth.

Effect of incremental equal channel angular pressing (I-ECAP) on the microstructural characteristics and mechanical behaviour of commercially pure titanium

Incremental equal channel angular pressing (I-ECAP) is one of the continuous severe plastic deformation (SPD) processes. This paper presents the processing of commercial purity titanium (CP-Ti) using a double billet variant of I-ECAP process. Ultrafine-grain (UFG) structure was successfully achieved after six passes of I-ECAP at 300°C. Microstructural evolution and texture development were tracked using EBSD. Analysis revealed continuous dynamic recrystallization (CDRX) as one of the grain refinement mechanism during processing. Room temperature tensile tests carried out before and after six passes, shows significant increase in strength with acceptable levels of ductility. The yield strength was increased from 308 to 558MPa and ultimate tensile strength from 549 to 685MPa. Compression tests conducted at different strain rates shows considerable increase in strength and enhanced strain rate sensitivity after processing. A distinct three-stage strain hardening was observed during compression. However the processed material displayed a loss in strain hardening ability during tensile as well as in compression tests. Detailed microhardness measurements show the evolution of hardness after subsequent passes with a reasonable level of homogeneity after the sixth pass. It is demonstrated that I-ECAP is an effective method for grain refinement in CP-Ti and subsequently improving its mechanical properties.

Influence of incremental ECAP on the microstructure and tensile behaviour of commercial purity titanium

Severe plastic deformation (SPD) is an effective method for producing ultrafine grained (UFG) structures in metals. UFG materials are characterized by an average grain size of <1 µm and mostly high angle grain boundaries. These materials exhibit exceptional improvements in strength, superplastic behaviour and in some cases enhanced biocompatibility. Among various SPD methods available, equal channel angular pressing (ECAP) is the most effective method for obtaining bulk UFG billets. Lately, the interest is towards industrialization of the ECAP technique to enable processing of very long or continuous billets. Incremental ECAP (I-ECAP) developed at University of Strathclyde, offers such possibility. The present work details the processing of commercial purity titanium (CP-Ti), using I-ECAP process, with the objective of improving its strength characteristics. CP-Ti billets were successfully processed for up to four passes at 300 °C using an I-ECAP die with a channel angle of 90°. Electron backscatter diffraction (EBSD) technique was used to characterize the microstructure after first and fourth pass of the process. Analysis of the first pass sample revealed heterogeneous structure with a mixture of elongated and refined equi-axed grains. Moreover, existence of {101 2} tensile twinning in the microstructure was also observed. Remarkable refinement was achieved after fourth pass and ultrafine-grain (UFG) structure was successfully achieved. Room temperature tensile tests carried out on unprocessed and UFG material, display the improvement in strength. The yield strength of the processed material was increased from 308 to 671 MPa and the ultimate tensile strength from 549 to 730 MPa. However, strain-hardening ability of the material was greatly reduced because of processing. Consequently, the material suffers loss in ductility, from 31.9% elongation to failure in the unprocessed form to 21.1% in UFG form. Finally, fracture morphology of the unprocessed and processed CP-Ti displays characteristics of ductile failure. It has been shown that I-ECAP is an effective method for improving strength characteristics of CP-Ti.

New method of producing tailored blanks with constant thickness

The concept of weight-saving in automotive manufacture by using tailored blanks is well established. The methods used to produce extra strength in particular areas of the blank can be based either on increasing material thickness in those areas or keeping the thickness constant but varying the material properties. Typically the first option is used by welding blank patches of different thickness. From the view point of forming blanks into sheet metal products uniform thickness is less problematic and it can be achieved by welding different materials of the same thickness or localised heat treatment. However, these approaches have major limitations: welding introduces discontinuity in material structure and properties while selective heat treatment is difficult to control. A new, original method presented here is based on a local shear deformation of the blank material. The particular process used is incremental equal channel angular pressing. The proposed approach is simulated using finite element modelling and then experimentally verified by producing a constant thickness pure aluminium strip with varying hardness. A discussion of different variants of this approach indicates its potential.

Finite element analysis of die offsetin two-turn incremental ECAP

Severe plastic deformation (SPD) is the process used to produce ultra fine grain(UFG), metals are characterised by improved mechanical properties which make them suitable for advanced structural applications. Apart from various SPD techniques, two turn incremental equal channel angular pressing (ECAP), S-shape channel discussed in this paper to double the effective strain in a single press. Unfortunately, processed UFG metals are rarely used in industrial applications due to lack of advancement in ECAP technique. Analysis of ECAP technique by changing die geometry and process parameter helps to produce defect free UFG metals. ECAP channel continuously pressed a very long billet in which repetitions accumulate large plastic strain. Two turn ECAP is useful to increase the productivity. However, any changes in channel angle, corner angle as well as offset length are cause for variation in mechanical properties of metals. In order to check the die suitability of two-turn ECAP with variation of effective strain, punch load, a FEA simulation is carried out and the suitable tool geometry and process kinematics are established. Die channel sat two different offset length are used to study the flow behaviour of metal through 900 channel angel. The mode of material flow is the same as in the well established classical L-shape ECAP process following route C pass, while continuous character and improved productivity suggest that new S-shape ECAP process might be suitable for grain refinement of metals on an industrial scale.

Finite element analysis of die angle at constant offset length in two-turn ECAP

Severe plastic deformation (SPD) is the process used to produce smaller grain in the range of 1-pm. These materials have been classified as equiaxed ultrafine grain (UFG) metals. These metals are characterised by high defect density and improved mechanical properties which make them suitable for advanced structural applications. From all available SPD techniques, two turn incremental equal channel angular pressing (ECAP), S-shape channel discussed in this paper to double the effective strain in single press. Analysis of ECAP technique by changing die geometry and process parameter helps to produce defect free UFG metals. ECAP channel continuously pressed a very long billet in which repetitions accumulate large plastic strain. Two turn ECAP is useful to increase the productivity. However, any changes in channel angle, corner angle as well as offset length are cause for variation in mechanical properties of metals. In order to check the die suitability of two-turn ECAP with variation of die angle a finite element analysis (FEA) simulation is carried out and the suitable tool geometry with process kinematics are established. ECAP channels at two different channel angles are used to study the flow behaviour of metal through 15mm offset length. The mode of material flow is the same as in the well-established classical L-shape ECAP process following route C pass, while continuous character and improved productivity suggest that new S-shape ECAP process might be suitable for grain refinement of metals as well as minimization of deformation load on an industrial scale.

Producing High‐Strength Metals by I‐ECAP

Incremental equal channel angular pressing (I‐ECAP) is used in this work to produce ultrafine‐grained (UFG) pure iron, aluminum alloy 5083, commercial purity titanium (grade 4), and magnesium alloy AZ31B. Pure iron is processed at room temperature, aluminum alloy at 200 °C, titanium at 320 °C, and magnesium alloy at 150 °C. Strength improvement, attributed to the grain refinement below 1 μm, is reported for all processed materials. The yield strength increase is the most apparent in pure iron, reaching almost 500 MPa after one pass of I‐ECAP, comparing to 180 MPa in the as‐forged conditions. UFG titanium, aluminum, and magnesium alloys obtained in this study reached yield stress of 800, 350, and 300 MPa, respectively, in each case exhibiting the yield strength increase by at least 30%, comparing to the alloys processed by conventional metal forming operations such as forging and rolling.

Microstructure and mechanical properties of friction stir welded joints made from ultrafine grained aluminium 1050

In order to obtain ultrafine grained structure, commercially pure aluminium (Al 1050) plates were subjected up to 8 passes of Incremental Equal Channel Angular Pressing (IECAP) following route C. Plates in different stages of IECAP were joined using Friction Stir Welding (FSW). All welded samples were investigated to determine their mechanical properties and structure evolution in the joint zone. The joining process reduced mechanical strength of material in the nugget zone, which was explained by the grain growth resulting from temperature rise during FSW. Nevertheless, the obtained results are promising in comparison to other methods of joining aluminium.

The Origin of Fracture in the I-ECAP of AZ31B Magnesium Alloy

Magnesium alloys are very promising materials for weight-saving structural applications due to their low density, comparing to other metals and alloys currently used. However, they usually suffer from a limited formability at room temperature and low strength. In order to overcome those issues, processes of severe plastic deformation (SPD) can be utilized to improve mechanical properties, but processing parameters need to be selected with care to avoid fracture, very often observed for those alloys during forming. In the current work, the AZ31B magnesium alloy was subjected to SPD by incremental equal-channel angular pressing (I-ECAP) at temperatures varying from 398 K to 525 K (125 °C to 250 °C) to determine the window of allowable processing parameters. The effects of initial grain size and billet rotation scheme on the occurrence of fracture during I-ECAP were investigated. The initial grain size ranged from 1.5 to 40 µm and the I-ECAP routes tested were A, BC, and C. Microstructures of the processed billets were characterized before and after I-ECAP. It was found that a fine-grained and homogenous microstructure was required to avoid fracture at low temperatures. Strain localization arising from a stress relaxation within recrystallized regions, namely twins and fine-grained zones, was shown to be responsible for the generation of microcracks. Based on the I-ECAP experiments and available literature data for ECAP, a power law between the initial grain size and processing conditions, described by a Zener–Hollomon parameter, has been proposed. Finally, processing by various routes at 473 K (200 °C) revealed that route A was less prone to fracture than routes BC and C.

Tailored Sheared Blanks Produced by Incremental ECAP

Incremental equal channel angular pressing (I-ECAP) is a process used for production of continuous ultrafine grained bars, plates and sheets. Normally the thickness of the processed billet is kept unchanged in consecutive passes to enable repetitive insertion into the same die. This is achieved by controlling the bottom dead centre of the reciprocating punch. However, if a final product requires being thinner and therefore longer, the bottom position of the punch can be lowered before the last pass. Going further, the bottom position of the punch can be changed during the process, which opens up a possibility to vary billet thickness along its length. Such a product, especially sheet, can serve as a preform for further metal forming operations and is known as tailored blank. This paper will show examples of varying thickness sheets produced by different configurations of I-ECAP. Experimental and finite element results will be presented.

The role of microstructure and texture in controlling mechanical properties of AZ31B magnesium alloy processed by I-ECAP

Mechanical properties of AZ31B magnesium alloy were modified in this work by various processing routes of incremental equal channel angular pressing (I-ECAP) followed by heat treatment. Possible strategies for improving ductility and strength of the alloy were investigated. Processing by routes A and BC showed that texture plays predominant role in controlling mechanical properties at room temperature. Four passes of I-ECAP by route C followed by annealing enhanced ductility up to 0.35 of true strain. It was found that tensile twinning was important in accommodating strain during tensile testing, which resulted in a very good hardening behaviour. The yield strength was improved to 300MPa by refining grain size to 0.8µm in I-ECAP at 150°C. The obtained structure and properties were shown to be stable up to 150°C. True strain at fracture was increased to 0.2 after annealing at 150°C without lowering strength.

Grain refinement in technically pure aluminium plates using incremental ECAP processing

Ultrafine grained materials are capable of superplastic elongation at strain rates approximately two orders of magnitude faster than those currently employed for commercial superplastic forming operations. However, such operations require the material in the form of thin sheets. Therefore, in this work, a new approach to produce ultrafine grained plate samples using a modified equal channel angular pressing (ECAP) method, namely incremental ECAP, was proposed. Unlike conventional ECAP, incremental ECAP works in small steps in which deformation and feeding are associated with two different tools acting asynchronously. Eight passes were applied to technically pure aluminium, with the sample rotation by 90° around the Z axis, which resulted in two full rotations and accumulated strain equal to 9.2. It was demonstrated that grain refinement under these conditions occurs very efficiently. Eight passes resulted in grain size reduction to below 500nm and very high fraction of high angle grain boundaries of about 80%. This was attributed to the activation of different slip systems in consecutive passes (thanks to sample rotation) and the lack of redundant strain, which results in early establishment of equiaxial grain structure. These two features confirm incremental ECAP to be one of the most effective severe plastic deformation methods in terms of grain size refinement and high angle grain boundaries formation.

The Effect of Initial Grain Size on Formability of AZ31B Magnesium Alloy during I-ECAP

The goal of this work was to investigate formability of AZ31B magnesium alloy during incremental equal channel angular pressing (I-ECAP). Square billets were processed using different routes of I-ECAP at temperatures varying from 125 °C to 250 °C. The billets were obtained from commercially available coarse-grained, hot-extruded rod and fine-grained, hot-rolled plate. A strong influence of the initial microstructure on processing temperature was reported. Fine-grained samples were successfully processed at 200 °C, while coarse-grained ones must have been heated up to 250 °C to avoid fracture. A gradual temperature decrease with subsequent passes allowed successful pressing at 150 °C. Processing using various routes of I-ECAP showed that a billet rotation before the last pass had strong influence on mechanical properties. The results of experiments were plotted on the diagram of allowable processing temperature for AZ31B. It was found that the relation between the minimum temperature in I-ECAP and the initial grain size could be described by a logarithmic equation.