Accumulative Back Extrusion Research Articles

Grain Refinement, Texture Evolution, and Tensile Performance Improvement of AZ91D Magnesium Alloy Processed by Multi‐Cycle Accumulative Back Extrusion

Accumulative back extrusion (ABE) process is performed on an AZ91D Mg alloy at 310 °C. Microstructure, texture, and mechanical properties affected by the ABE cycles are elucidated in detail. Results reveal that the initial as‐cast grain structure (≈190 μm) is significantly refined (≈2.98 μm) after being imposed by six ABE cycles. The β‐Mg17Al12 phases undergo fragmentation and spheroidization. The dynamic recrystallization (DRX) process is accelerated through the particle stimulated nucleation (PSN) mechanism during initial deformation. Meanwhile, twins are inclined to occur in barren areas of the second phases and play a role of twinning‐induced dynamic recrystallization (TDRX). Grain refinement is governed by the combined function of PSN, TDRX, continuous dynamic recrystallization (CDRX), and discontinuous dynamic recrystallization (DDRX). Basal textures with low intensities are developed due to the fully developed recrystallized grains. Schmid factor analysis indicates that the {0001} <11–20> basal slip and the {11–22} <11–23> pyramidal slip contribute to ABE deformation. Finally, excellent ultimate tensile strength and fracture elongation are obtained after a three‐cycle ABE process, which are estimated to be 296 MPa and 21.43%, respectively. The resulting tensile performance is attributed to the refined grains, fragmented β‐Mg17Al12 particles, and the weakened basal texture.

Improving the dry sliding-wear resistance of as-cast Cu-10Sn-1P alloy through accumulative back extrusion (ABE) process

In this study, the as-cast Cu-10Sn-1P alloy was subjected to multi-pass (1–3 passes) accumulative back extrusion (ABE) process. Also, the effects of the process were investigated on microstructure and dry sliding wear behavior. The results indicated a significant decline in the coefficient of friction (COF) and wear rate of processed samples. The enhanced tribological properties was attributed to the microstructural adjustment, as well as improved hardness and wear active mechanism to change from adhesive/severe abrasive wear to a combination of oxidation and mild abrasive mechanisms.

Evaluating the high temperature superplastic behavior of a thermomechanically processed Al–Cu aluminum alloy through miniaturized testing method

The high temperature superplastic behavior of ultrafine grain A206 aluminum alloy was examined employing miniaturized shear punch jump test method. To achieve a fine-grained microstructure, accumulative back extrusion were applied at 200 °C up to 1, 3 and 5 passes on the as-annealed microstructure. The high temperature mechanical properties of the products were assesses at 100, 200 and 300 °C. The strain rate sensitivity indices (m) of the processed specimens were evaluated under the base strain rate of 0.004 s−1 with the incremental jump to 0.01 s−1, and various temperatures. The temperature of 300 °C was found as the optimum temperature for superplastic deformation of the experimented material, and a considerable increase in the m value was observed after 3 and 5 passes. The highest elongation to fracture value was observed in 5-passes processed specimen at 300 °C where the m value was obtained about 0.35. In this condition, grain boundary sliding was announced to be the dominant deformation mechanism. Lastly, TEM and XRD characterization exposed that the fine Al2Cu precipitates are rarely observed in 1-pass processed microstructure while a high fraction of fine precipitates (7–10 nm) were found in 5-passes processed material. These fine precipitates were thermally stable and coherent with the aluminum matrix, and thus were able to pin the grain boundaries and at the high temperatures and decrease grain growth rate.

Microstructural evolution and mechanical properties of accumulative back extruded duplex (α + β) brass

The microstructural evolution and mechanical performance of α + β brass alloy (Cu-40%Zn) processed through the accumulative back extrusion (ABE) were investigated in this study. The detailed microstructural evolution during the ABE process exhibited a significant difference in deformation mechanisms operating within each phase, consequently altering their grain refinement mechanism. The rotation of the mechanical twinning along the shear bands contributed to the strain accommodation in the α-phase, while the subgrain formation and rotation within the shear bands resulted into a significant α-phase grain refinement at higher ABE passes. On the other hand, the β-phase, which was strain hardened at the early stages of straining, at higher ABE passes, was fragmented into refined subgrains by the highly strained α/β interphase boundaries. The formation of the newly recrystallized grains with ~0.24 μm (α-phase) and ~1.21 μm (β-phases) sizes, mechanical twin boundaries, shear bands and strain incompatibility zones contributed to a significant increase in the ultimate strength from ~350 MPa for the as-received sample to ~800 MPa in the three-pass ABE processed sample.

High-Temperature Wear Mechanisms of a Severely Plastic Deformed Al/Mg2Si Composite

The present work was primarily conducted to study the wear behavior of as-received and severely deformed Al-15%Mg2Si in situ composites. The severe plastic deformation was applied using accumulative back extrusion (ABE) technique (one and three passes). The continuous dynamic recrystallization (CDRX) was recognized as the main strain accommodation and grain refinement mechanism within aluminum matrix during ABE cycles. To investigate the wear properties of the processed material, the dry sliding wear tests were carried out on both the as-received and processed samples under normal load of 10 and 20 N at room temperature, 100 °C, and 200 °C. The results indicated a better wear resistance of processed specimens in comparison to the as-received ones at room temperature. In addition, the wear performance was improved as the ABE pass numbers increased. These were related to the presence of oxide tribolayer. At 100 °C, the as-received material exhibited a better wear performance compared to the processed material; this was attributed to the formation of a work-hardened layer on the worn surface. At 200 °C, both the as-received and processed composites experienced a severe wear condition. In general, elevating the temperature changed the dominant wear mechanism from oxidation and delamination at room temperature to severe adhesion and plastic deformation at 200 °C.

Alleviation of Mechanical Anisotropy in Ultrafine/Nano-grained AZ31 Magnesium Alloy

An ultrafine/nano-grained AZ31 magnesium alloy was processed through multipass accumulative back extrusion at 280 and 380 °C. Texture analysis was conducted to determine preferred crystallographic orientations developed during multipass deformation. The anisotropy in mechanical properties of the processed materials was studied by compressive and tensile testing. The results implied that the accumulative back extrusion effectively reduces the anisotropy of yield strength, ultimate strength and strain-to-fracture during compressive and tensile loading. The operations of different twinning/slip systems during compressive and tensile deformation were discussed using Schmid factors measurements. A decrease in twinning-related anisotropy, as well as the contribution of different slip systems in strain accommodation, could reduce the mechanical anisotropy of ultrafine/nano-grained AZ31 alloy. The compressive strength of the two-pass processed material was lower along extrusion direction than transverse direction; however, the result was reverse after four passes. The tensile behavior implied that the contribution of basal slip might accommodate strain along length as well as the thickness of the specimen, while prismatic slip may assist in narrowing the width of the specimen and elongating the length.

Grain Refinement through Shear Banding in Severely Plastic Deformed A206 Aluminum Alloy

The present work is conducted to study the microstructure and texture evolutions in an as‐cast A206 aluminum alloy after applying severe plastic deformation. Toward this end, the material is severely deformed through accumulative back extrusion (ABE) technique at 200 °C and followed by assessing the room temperature mechanical properties of the products. The macro shear‐bands formation in the highly strained regions can result in grain refinement through the geometric dynamic recrystallization mechanism. A significant refinement is also characterized within the micro shear‐bands; this is attributed to the intensified substructure development and the occurrence of continuous dynamic recrystallization. The corresponding inverse pole figure maps show similar orientation for these newly refined grains with the parent ones. A random texture is produced through sub‐grain rotation to dissimilar orientation at the intersection of micro‐bands. The assessment of mechanical properties of the processed materials reveal significant increase in both yield and ultimate tensile strength values. The hardness profiles also demonstrate a relatively homogenous microstructure after three and five ABE passes holding a mean hardness value of 183 Vickers.

Enhancing the strength and ductility in accumulative back extruded WE43 magnesium alloy through achieving bimodal grain size distribution and texture weakening

The microstructure of a rare earth containing magnesium alloy, Mg-4.35Y-3RE-0.36Zr wt%, was engineered through applying accumulative back extrusion (ABE) process. Toward this end, the predetermined ABE cycles were applied at 400°C up to five passes under a punch speed of 5mm/min to study the ultrafine grained microstructure formation and its corresponding texture modification in the experimental material. A variety of bimodal grain size distributions were developed at all deformation conditions. In addition, the dissolution of eutectic phase stimulated the probability of dynamic precipitation of β phase during deformation. The latter caused a pinning effect on the grain boundary and gave rise to an inhomogeneous grain growth thereby intensified a bimodal grain size distribution (bimodality). In addition, the capability of experimental material to shear band formation during straining, even after one ABE pass, induced the level of bimodality. A remarkable grain refinement was achieved inside the shear bands due to the higher magnitude of shearing strain. Furthermore, the shear bands intersections provided suitable conditions for well defined ultrafine grain formation in between primary bands. The formation of noticeable number of these ultrafine grains within the shear bands could decrease the basal intensity thereby inducing a significant texture weakening effect. The obtained results indicated a significant improvement in both the strength (yield and ultimate) and elongation to fracture of the processed material. This was justified considering the effects of grain size, the level of bimodality and the texture weakening.

Effect of Severe Plastic Deformation and Subsequent Silicon Spheroidizing Treatment on the Microstructure and Mechanical Properties of an Al–Si–Mg Alloy

This study investigated the synergetic effects of severe plastic deformation and subsequent heat treatment on the characteristics of an A356 alloy. The severe deformation by accumulative back extrusion (ABE) at 300 °C substantially refined the α–Al primary phase and Si particles, but did not homogeneously redistribute the Si particles. ABE also improved the strength but did not enhance the ductility. To make a compromise between strength and ductility, a subsequent heat treatment at 540 °C was carried out. It was shown that severe plastic deformation substantially accelerated the silicon spheroidization. Heat treatment increased the ductility of the material from ∼8% (in the deformed condition) to ∼15%. This was discussed with emphasis on restoration of the matrix, spheroidization of the Si particles, and redistribution of the Si particles within the α–Al matrix.

Microstructure, Texture, and Tensile Properties of Ultrafine/Nano-Grained Magnesium Alloy Processed by Accumulative Back Extrusion

An AZ31 wrought magnesium alloy was processed by employing multipass accumulative back extrusion process. The obtained microstructure, texture, and room temperature tensile properties were characterized and discussed. Ultrafine-grained microstructure including nano-grains were developed, where the obtained mean grain size was decreased from 8 to 0.5 μm by applying consecutive passes. The frequency of both low angle and high angle boundaries increased after processing. Strength of the experimental alloy was decreased after processing, which was attributed to the obtained texture involving the major component lying inclined to the deformation axis. Both the uniform and post-uniform elongations of the processed materials were increased after processing, where a total elongation of 68 pct was obtained after six-pass deformation. The contribution of different twinning and slip mechanisms was described by calculating corresponding Schmid factors. The operation of prismatic slip was considered as the major deformation contributor. The significant increase in post-uniform deformation of the processed material was discussed relying on the occurrence of grain boundary sliding associated with the operation of prismatic slip.

Strain-induced nano recrystallization in AZ31 magnesium: TEM characterization

Shear bands were developed during accumulative back extrusion of Mg-3Al-1Zn alloy at 130 °C. Transmission electron microscopy was employed to study the substructural evolution prior to and during the development of macro shear bands. The results illustrated that nanograins with mean size of 100 nm were formed within the bands. The observations revealed that flow localization in magnesium is preceded by interactions of deformation twins, dislocations and micro shear bands. The substructure within the bands is evolved through development of rectangular subgrains as well as subdivision of elongated grains. These substructures transformed subsequently into equiaxed nano-sized grains through continuous recrystallization. EBSD analysis confirmed the contribution of extension twinning during shear band development.

Compressive Deformation Behavior of an Ultrafine/Nano Grained AZ31 Magnesium Processed by Accumulative Back Extrusion

Abstract An AZ31magneium alloy was processed through accumulative back extrusion (ABE) process at 280 °C up to six passes. Compressive deformation behavior of the processed materials was studied by employing uniaxial compression tests at room temperature. The results indicated that grains of 80 nm to 1 µm size were formed during accumulative back extrusion, where the mean grain size of the experimental material was reduced by applying successive ABE passes. A deformation texture characterizing with the basal plane mainly lie inclined to the deformation axis was developed. Compressive yield and maximum compressive strengths were measured to increase by applying successive extrusion passes, while the strain-to-fracture dropped. The evolution of mechanical properties was explained relying on the grain refinement effect as well as texture change. It was described that the share of different deformation mechanisms and developing of shearing regions near the grain boundaries may influence the deformation behavior of the ultrafine/nano grained AZ31 alloy.

Characterization of the microstructure, texture and mechanical properties of 7075 aluminum alloy in early stage of severe plastic deformation

The present investigation was conducted to study the microstructural, mechanical and texture evolutions of a 7075 aluminum alloy at the early stage of severe plastic deformation. In this regard, three different techniques were elaborated (for just one pass) to ensure maintaining different deformation modes. These were done applying Multi-Axial Forging (MAF), Accumulative Back Extrusion (ABE) and Friction Stir Processing (FSP) under predetermined thermomechanical condition based on the Zener-Hollomon value. The macrostructures of the deformed materials demonstrate a unique flow pattern and distribution of grain refinement zones, especially in ABE and MAF methods. The EBSD characterization results reveal the presence of accumulated micro-shear bands, which have been ended to an outstanding grain refinement. The results show occurrence of both discontinues and continues dynamic recrystallization (mainly in the stir zone) in the FSPed materials. The mechanical examinations display a significant improvement of both yield and ultimate tensile strength values in all methods. Moreover, the tensile tests were applied in different directions to evaluate the mechanical anisotropy of the deformed materials. The results exhibit higher capability of FSP in delivering more isotropic properties in comparison to the other techniques. In addition, the macro-texture results approve the stronger effect of FSP on fading the initial rolling anisotropy out.

Evolution of microstructure and mechanical properties in a hypoeutectic Al–Si–Mg alloy processed by accumulative back extrusion

This study demonstrates the evolution of microstructure and mechanical properties of a hypoeutectic Al–7Si–0.4Mg (A356) alloy processed by accumulative back extrusion (ABE) at temperatures ranging from 200 to 500°C. ABE processing is one of the new severe plastic deformation techniques enabling one to produce relatively large ultrafine-grained materials in a cylindrical shape. One complete pass of ABE was estimated to introduce a reasonably homogeneous effective strain of ~3 as calculated by finite element analysis. Microstructural observation showed that globular α-Al primary phase was subdivided into fine substructures and Si particles having a fibrous shape were fragmented and spheroidized within the eutectic constituent through ABE processing. There was no evidence of homogeneous distribution of the fine Si particles in the α-Al phase after ABE. Mechanical testing at room temperature showed that both yield strength and ultimate tensile strength of the A356 alloy dramatically increased through ABE, especially at lower processing temperatures, as compared with the as-cast condition whereas there was no significant reduction in ductility at all processing temperatures. The experimental results were discussed with emphasis on the microstructure evolution involving dynamic recrystallization and deformation behavior including strengthening mechanisms and strain hardening in the Al–Si alloy.

The evolution of γ-Mg17Al12 intermetallic compound during accumulative back extrusion and subsequent ageing treatment

Accumulative back extrusion (ABE) processing, as a novel severe plastic deformation (SPD) method, has been recently justified to be capable of modifying the microstructural characteristics of alloys. In line to its ongoing researches, the present work has been planned to study the evolution of γ-Mg17Al12 intermetallic phase during ABE and subsequent ageing treatment in a high Al-bearing Mg–Al–Zn alloy. The behaviour of γ intermetallic has been systematically examined as following points of view: (i) strain–temperature-dependent morphology changes, (ii) strain-induced dissolution, and (iii) re-ageing behaviour as a function of time and temperature. Aiming to analyse the morphology of eutectic γ compound with respect to the strain and temperature, 2D projections of effective diameter, shape factor and globularity have been made in strain/temperature graphs. The processing conditions (strain and temperature) corresponding to the desired and undesired morphologies are introduced and microstructurally explained through underlying plasticity mechanisms, i.e., ‘necking-thinning-particle separation’ and ‘brittle fragmentation.’ The former mechanism is suggested to be in relation with partial strain-induced dissolution of eutectic γ phase, leading to generation of a supersaturated solid solution. This has resulted to the observation of ‘off-stoichiometry’ phenomena in Mg17Al12 phase and has been justified through dislocation-assisted deformation mechanism at elevated temperature. Surprisingly, a unique re-ageing behaviour has been found for the obtained solid solutions, where a modified kinetics and morphology of γ phase precipitation were characterized. The altered precipitation behaviour is attributed to the specific defect structure achieved by SPD acting as fast diffusion channel for Al solutes.

EBSD characterization of repetitive grain refinement in AZ31 magnesium alloy

A polycrystalline Mg–3Al–1Zn was subjected to high plastic strains by applying accumulative back extrusion (ABE) process. Electron back scatter diffraction analyses were used to explore the grain refinement mechanisms. A novel grain refinement trend was found at large strains, where new recrystallized grains were repetitively refined through dynamic recrystallization. The grain size was diminishing continuously up to the fourth ABE pass, after which no further grain refinement was noticed upon successive passes.

The second phase particles and mechanical properties of 2124 aluminum alloy processed by accumulative back extrusion

An age-hardenable 2124 aluminum alloy was severely deformed by accumulative back extrusion (ABE) method up to three passes at 100 and 200°C. The characteristics of the second phase particles were studied using scanning electron microscopy. The results indicated that the size of primary particles had been reduced after the first ABE pass where even much finer particles was obtained as the successive passes were applied. In addition, the secondary particles were fragmented into finer pieces after ABE at 100°C, whereas a particle coarsening was realized as the deformation temperature rose to 200°C. The latter was attributed to the Ostwald ripening mechanism. However, the volume fraction of secondary particles was significantly decreased after three ABE passes at 200°C due to the occurrence of deformation induced dissolution. Additionally, the tensile properties of the processed materials were measured utilizing a miniaturized tensile testing method. The results were justified considering the evolution of the second phase particles.

Modification of the grain structure, γ phase morphology and texture in AZ81 Mg alloy through accumulative back extrusion

The capability of accumulative back extrusion (ABE), as a recently developed severe plastic deformation technique, has been considered to modify the microstructural characteristics of a Mg–Al–Zn alloy composing higher Al content. The results indicate that applying the ABE process up to five passes led to simultaneous modification of the γ phase morphology, grain structure and deformation texture of the experimental alloy. The morphology of the eutectic γ phase has been sequentially altered from initial coarse network to elongated and finally spherical morphologies with an average globularity and diameter of 0.85 and 7.5μm, respectively. This has been justified considering the mechanical fragmentation and thermal disintegration of the eutectic particles through necking phenomena. In addition the initial coarse grain structure of the cast alloy (~330μm) has also been significantly refined (d~1μm). This substantial grain refinement is attributed to the dynamic recrystallization enhancement in connection with pinning effect of crushed γ particles during successive passes. The typical basal texture has been modified to a weak random texture having grains with their normal distribution in a desirably wide range of deviation angles from normal direction. The occurrence of particle stimulated nucleation, particle pinning, and shear banding phenomena are suggested as the main reasons causing the weak scattered deformation texture.

An investigation into the homogeneity of microstructure, strain pattern and hardness of pure aluminum processed by accumulative back extrusion

The present work focuses on the evolution of microstructure and mechanical characteristics of commercially pure aluminum during accumulative back extrusion (ABE) processing. The electron backscatter diffraction (EBSD) analysis and microhardness measurements were utilized to study the effect of pass number and deformation route on the microstructure and hardness across the ABE-processed specimens. The distributions of effective strain and shear stress during ABE were studied using finite element analysis with DEFORMTM 3D software. The results demonstrated that an effective plastic strain of about 2 was introduced after each separate pass of ABE. This process produced a high-strength ultrafine-grained material where the grain refinement was attributed to the shearing and/or mechanical subdivision of the initial coarse grains along with dynamic restoration processes. The distributions of both the simulated strain and the measured microhardness were inhomogeneous after the first ABE pass, but thereafter outstanding hardness homogeneity was achieved when the consecutive ABE passes were applied. Moreover, the application of the reversing route was found to introduce a high level of hardness homogeneity after each applied ABE pass.

Characterization of ultrafine and nano grained magnesium alloy processed by severe plastic deformation

A wrought AZ31 magnesium alloy was severely deformed through a novel continuous severe plastic deformation technique, called accumulative back extrusion. The microstructure and texture obtained after applying multiple deformation passes at a temperature range of 80–380°C were characterized. After a single pass at 80°C an ultrafine/nano grained microstructure was obtained, while the initial texture was completely replaced by a new fiber basal texture, 40° inclined to transverse direction. As the processing temperature increased, the obtained texture intensities were strengthened, though the c-axis of crystals gradually rotated towards transverse direction and a <10–11> fiber texture parallel to normal direction was developed. Moreover, repetitive accumulative back extrusion was associated with tendency of basal plane to lie parallel to transverse direction, while the orientation of prismatic planes showed a random distribution around normal direction. After eight passes the most noticeable texture included fiber basal texture oriented almost parallel to the transverse direction, and <10–10> perpendicular to the extrusion direction and <10–11> parallel to the normal direction. The maximum texture intensity was decreased as the number of passes increased, which is attributed to strain path change involved during each consecutive accumulative back extrusion pass as well as promoted contribution of non-basal slip systems.