Accumulative Back Extrusion Process 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.

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.

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.

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.

Microstructure and mechanical properties of commercially pure aluminum processed by accumulative back extrusion

Accumulative back extrusion (ABE), as a new severe plastic deformation (SPD) method, was applied on a commercially pure aluminum, resulting in an ultrafine microstructure. The grain refinement was attributed to the shearing and/or to the mechanical partitioning of the initial coarse grains as well as different dynamic recrystallization mechanisms (i.e., discontinues and continues ones). The effects of imposing several passes along with the strain reversal path method during consecutive ABE passes were discussed. Mechanical characterization revealed that the yield and ultimate strength of pure Al were significantly increased through applying ABE. This was related to grain boundary and dislocation strengthening, as well as to Orowan dislocation bowing mechanisms. The elongation, however, was deteriorated after ABE processing. This was interpreted through the lack of ability of the material to accommodate the ABE-induced strain, inhomogeneous distribution of strain across the ABE-processed workpieces, and the reduced work hardening capacity of the fine-grained aluminum. The formability index and static toughness were also considered to verify the overall mechanical performance of the ABE-processed pure Al.

The semisolid microstructural evolution of a severely deformed A356 aluminum alloy

A cast A356 aluminum alloy was successfully processed through applying a new severe plastic deformation method, accumulative back extrusion (ABE), at the temperature of 300°C. In consequence, a severely deformed microstructure consisting of round and fine Si particles has been obtained. Microstructural evolution of the ABE-processed material soaked for various times at different isothermal temperatures in semisolid state was investigated. It has been shown that the proposed semisolid thermomechanical route would lead to a fine semisolid microstructure, where the spheroidal α-Al globules are surrounded by continuous liquid film. The results indicate that the ABE processing followed by 10min holding at 580°C and/or 6min at 590°C has led to the best thixotropic characteristics, i.e., the high shape factors and low globule sizes. The thixotropic structure characteristics have been also properly discussed in terms of globularization and coarsening mechanisms as well as the kinetics of microstructural evolutions. The almost linear semisolid coarsening behavior of the processed material indicates the outstanding role of Ostwald ripening mechanism in the growth of globules.

Hardness and microstructure homogeneity of pure copper processed by accumulative back extrusion

The present work deals with the microstructure evolution of a pure copper processed by a new severe plastic deformation method. A set of pure copper (99.99%) work-pieces with coarse-grained microstructures was processed by accumulative back extrusion (ABE) method at room temperature. The optical and scanning electron microscopy (SEM) and hardness measurements were utilized to study the microstructural evolution and hardness homogeneity. The results indicated that ABE is a capable process to provide a homogenous grain refined microstructure in pure copper. The observed grain refinement was discussed relying on the occurrence of dynamic restoration processes. The analysis of microstructure and hardness showed outstanding homogeneity improvement throughout the work-pieces as the consecutive ABE passes were applied. The homogeneity improvement was attributed to the propagation of the shear bands and also the heavily deformed regions. A reversing route was also applied in the ABE processing to investigate its effect on the development of microstructural homogeneity. Comparing to the conventional route, the application of the reversing route was found to yield better homogeneity after less passes of the process.

The origin of microstructure inhomogeneity in Mg–3Al–1Zn processed by severe plastic deformation

A Mg–3Al–1Zn alloy has been severely deformed by accumulative back extrusion to investigate the effects of processing parameters (temperature and deformation pass) on the microstructure homogeneity. The results indicate that the activation of different grain refinement mechanisms in magnesium may result in an inhomogeneous microstructure, i.e. a bimodal grain size distribution. However, a rapid rate of homogeneity development has been occurred through applying further subsequent passes. This has been justified considering the imposed strain reversal during accumulative back extrusion processing.

MICROSTRUCTURAL EVOLUTION OF AZ31 MAGNESIUM ALLOY PROCESSED BY A NEW NOBLE SEVERE PLASTIC DEFORMATION METHOD

A new noble severe plastic deformation method, called accumulative back extrusion (ABE), was developed to assist generating ultra fine grain materials. In the present work the ABE process was successfully applied on AZ 31 magnesium alloy up to three passes without any danger of cracks. The results showed that a large shear deformation may introduce through step one, where extensive shear banding and twinning are present in the microstructure. As the second step proceeds via constrained compressive deformation, more deformation inhomogenieties, which may act as preferred nucleation sites for new grains, were introduced in the microstructure. By increasing the number of passes to 3, more homogeneous microstructure with no significant smaller grain size was formed. The strain induced twinning and strain localization, which were led to occurrence of dynamic recrystallization (DRX), were found to be the main reasons of grain refinement during ABE process.

Microstructure inhomogeneity in ultra-fine grained bulk AZ91 produced by accumulative back extrusion (ABE)

This paper focuses on the effects of the accumulative back extrusion (ABE) process on microstructure inhomogeneity and room temperature mechanical properties of AZ91 magnesium alloys. The results show that ABE processing leads to significant grain refinement with an average grain size of ∼2 μm in the shear zone from initial grain size of ∼150 μm at the central region of a specimen subjected to high shear through 1 pass at 300 °C. In the outer region of the sample which is in contact with die surface and central region which is in contact with inner punch land, there is no significant change in the microstructure, but some Mg17Al12 precipitation occurs around the massive Mg17Al12 phase after ABE. In the other regions subjected to deformation channel of the specimen, there is more grain refinement along with increased Mg17Al12 dissolution. It is also shown that the ABE process affects the morphology of the Mg17Al12 phase. Microhardness measurements verified the microstructure inhomogeneity results.

Shear banding phenomenon during severe plastic deformation of an AZ31 magnesium alloy

Severe plastic deformation was applied on a wrought AZ31 magnesium alloy by a new method called accumulative back extrusion (ABE). Instabilities of plastic flow in the form of localized shear bands were experimentally observed during ABE processing of the AZ31 alloy. The obtained microstructures show the appearance of shear bands in ABE processed specimens, the extent of which was observed to be decreased by increasing the temperature. The restricted flow (due to the deformation geometry) was discussed as the main cause of the latter behavior. A noticeable grain refinement was observed inside the shear bands which was attributed to the occurrence of continuous dynamic recrystallization inside the bands. To analyze the homogeneity of mechanical properties, the microhardness variations from the deformed bulk to the shear bands were measured and interpreted. The role of shear banding in grain refinement with no harmful effect on material soundness was explained.

Shear deformation and grain refinement during accumulative back extrusion of AZ31 magnesium alloy

The accumulative back extrusion (ABE), as a new-noble severe plastic deformation (SPD) technique, has been employed to clarify the microstructural evolutions of AZ31 magnesium alloy during severe deformation. The latter has been explored using a 3D finite element analysis along with the microstructural investigations. The distribution of ABE generated shear strain (SS) and its corresponding microstructures have been thoroughly studied. The results indicated that the restrictions of material flow during ABE processing had been led to the mechanical shear bands generation in the microstructure. In addition, the occurrence of continuous dynamic recrystallization (CDRX) within the bands has resulted in a local grain refinement in those areas. Consequently a bimodal structure including the fine recrystallized grains along with the elongated ones has been developed. The effect of deformation mode on the microstructural refinement has been also discussed through considering the developed SS history and the related microstructural refinement.

Processing of AZ31 magnesium alloy by a new noble severe plastic deformation method

In the present investigation a wrought magnesium alloy (AZ31) has been processed applying the accumulative back extrusion (ABE) method. This was performed through different thermomechanical processing routes (different ABE deformation passes at temperatures of 80–380°C). The results indicate that AZ31 alloy may successfully be deformed through ABE processing even at temperatures as low as 80°C. Following the ABE processing a sophisticated microhardness testing was conducted and thorough microstructural observations were undertaken using optical microscopy. The results show that the equiaxed submicron size grains have been achieved. As the number of passes was increased, a more homogeneous microstructure with finer mean grain size was obtained. It was also found that increasing the temperature resulted in larger mean grain size and also higher microstructural homogeneity.

Deformation homogeneity in accumulative back extrusion processing of AZ31 magnesium alloy

The ultra-fine grained materials produced by severe plastic deformation (SPD) methods have been widely investigated in the last decades due to the enhanced material properties such as improved tensile strength, hardness, toughness, fatigue life, and the optical and electrical related properties. The microstructure and mechanical properties of the deformed materials are strongly dependent on the amount and the homogeneity of strain achieved during the SPD processes. Accumulative back extrusion (ABE) method is a novel severe plastic deformation (SPD) technique which has been recently developed for fabrication of bulk ultra-fine structured materials. The knowledge of strain distribution and deformation behavior during ABE processing may assist the better understanding of the whole process. Therefore the present work has been conducted to examine the strain distribution and deformation behavior during ABE process through applying 3D finite element simulations with Abaqus/Explicit for different stages of deformation. In order to validate the simulation results, a grid method assessment was employed. The results demonstrated that a homogenous equivalent plastic strain of 4–5 has been gained after applying one pass ABE. In addition the results indicate that the distribution of normal strain is inhomogeneous. The microstructural observations have revealed a proper structural homogeneity after completion of an ABE cycle.

Plastic deformation analysis of accumulative back extrusion

AbstractFinite element analysis for the recently developed plastic deformation process, accumulative back extrusion, is carried out to understand the plastic deformation behavior during the processing in terms of geometry, equivalent strain, principal stress, and load history. The simulated results show that the deformation is highly nonuniform, and that it is almost impossible to achieve strain homogeneity by repeating the accumulative back extrusion process. In addition, the folding defects generated in the workpiece with cycles of accumulative back extrusion can cause surface fracture.