Rolled AZ31 Magnesium Alloy Sheets Research Articles

Influence of heat treatment and forming cycle on the precise forming of AZ31 magnesium alloy sheet

Precision forming of 3 mm thick rolled AZ31 magnesium alloy sheet at 90° results in severe cracking and a considerable rebound angle. The effects of different sheet heat treatments and changes in the forming cycle on the value of the forming angle were investigated using 90° precision forming tests. The reasons of sheet cracking were examined using tensile tests and numerical simulations. Microscopic analysis and characterization techniques such as metallographic experiments, SEM observations, and EBSD tests are used to clarify the mechanism of improvement and control of the forming angle value. The results indicate that annealing treatment exhibits high sensitivity to controlling sheet cracking, but low sensitivity to controlling rebound. Among them, after heat treatment at 250 °C+1 h, the sheet no longer cracks during forming. In incomplete annealing, the more small grains produced by recrystallization and the greater the number of dimples in the fracture, the closer the forming angle is to 90°. Full annealing can result in a small reduction in the forming angle value compared to incomplete annealing treatments. After extending the forming cycle for 1 h, the increase in the ratio of dual tensile twin boundaries and low angle grain boundaries resulted in a significant decrease in the value of the forming angle and no cracking of the sheet. The increase of recrystallized tissue on the tensile side and the increase of the KAM value led to the decrease of the forming angle value to the minimum value at 300 °C+1 h heat treatment condition.

Quasi-In-Situ Analysis of As-Rolled Microstructure of Magnesium Alloys during Annealing and Subsequent Plastic Deformation.

In this paper, quasi-in situ experiments were carried out on rolled AZ31 magnesium alloy sheets to track the recrystallization behavior of the rolled microstructure during the heat treatment process and the plastic deformation behavior during the stretching process. The as-rolled microstructures are classified into five characteristics and their plastic deformation behaviors are described. The research shows that annealing recrystallization leads to grain reorganization, resulting in the diversity of grain orientation, and it is easier to activate basal slip. Recrystallization preferentially nucleates in the regions with high stress, while it is difficult for recrystallization to occur in regions with low stress, which leads to the uneven distribution of the as-rolled structure of magnesium alloys. Slip can be better transmitted between small grains, while deformation between large and small grains is difficult to transmit, which can easily lead to the generation of ledges. Incomplete recrystallization is more likely to accumulate dislocations than complete recrystallization, and ledges are formed in the early stage of deformation. Microcracks are more likely to occur between strain-incompatible grains. It is of great significance to promote the application of rolled AZ31 magnesium alloys for the development of heat treatment and subsequent plastic working of rolled magnesium alloys.

Influence of Microstructure and Mechanical Properties on Formability in High Strain Rate Rolled AZ31 Magnesium Alloy Sheets

Influence of Microstructure and Mechanical Properties on Formability in High Strain Rate Rolled AZ31 Magnesium Alloy Sheets

Deformation mechanisms and differential work hardening behavior of AZ31 magnesium alloy during biaxial deformation

Magnesium alloys are frequently subjected to biaxial stress during manufacturing process, however, the work hardening behavior under such circumstance are not well understood. In this study, the deformation mechanisms and differential work hardening behavior of rolled AZ31 magnesium alloy sheets under biaxial loading are investigated. The change of plastic work contours with increasing plastic strain indicates the differential work hardening behavior of AZ31 magnesium alloy under biaxial stress state, resulting in higher macroscopic work hardening rates of biaxial loading than uniaxial loading, with the elastic-plastic transition part of work hardening extended and stage III hardly emerged. Electron backscatter diffraction and Schmid factor analysis confirm the low activation of non-basal 〈a〉 slip during biaxial loading tests. While the thickness strain is primarily accommodated by pyramidal 〈 c + a 〉 slip at the initial stage of biaxial deformation, {10–11} contraction twinning is activated at larger plastic strain. The low activation of non-basal 〈a〉 slip also retards the dynamic recovery and cross-slip of basal and prismatic 〈a〉 slips, leading to the differential work hardening behavior of AZ31 magnesium alloy under biaxial stress state.

Microstructure, Texture, and Mechanical Properties of Continuously Extruded and Rolled AZ31 Magnesium Alloy Sheets

It is difficult to fabricate magnesium alloy sheets at room temperature due to their hexagonal close-packed structure. In this paper, we proposed a new process that can be used to fabricate AZ31 magnesium alloy sheets with fine-grained microstructure and good mechanical properties. AZ31 magnesium alloy sheets with dimensions of 160 mm × 8 mm were obtained successfully by a continuous extrusion technique. The extruded sheets were then rolled by two different rolling routes, Route 1: the extruded sheet was heated at 350 °C and then rolled without heating of rolls and Route 2: the sheet was rolled without any previous heating by heating rolls at 280 °C. The microstructure and texture evolution as well as mechanical properties of the continuously extruded and rolled sheets were examined experimentally. The magnesium alloy sheets with relatively fine grains of 4.5-8.3 µm were obtained by continuous extrusion followed by rolling processes. The tensile strength of continuously extruded AZ31 Mg alloy sheets can be effectively increased by rolling using heating rolls (Route 2), while the ductility was enhanced by rolling heated sheets (Route 1). The mixed microstructure, consisting of refined DRX grains and deformed grains, contributes to enhancement in tensile strength of the sheet samples rolled by Route 2 due to grain-boundary strengthening and work hardening. Lower temperature and larger rolling reduction are two main factors for improving the tensile strength of AZ31magnesium alloy sheets.

Role of twinning on the stress and strain behaviors during reverse loading in rolled magnesium alloy sheets

Reverse loading from compression to tension was performed on a rolled AZ31 magnesium alloy sheet to systematically study the effects of twinning and detwinning on the stress and strain behaviors after stress reversal. A crystal plasticity finite-element simulation was also conducted to study the underlying deformation mechanisms. This paper consists of the following three findings. (1) A sigmoidal curve occurred irrespective of compressive strain, while the sigmoidal curve became less pronounced as the compressive strain increased. The simulation results showed that the shift from detwinning-dominated to slip-dominated deformation became more gradual as the compressive strain increased; thus, the sigmoidal curve became less pronounced. (2) The yield stress after stress reversal increased when the sheet was annealed prior to reverse loading. The simulation results suggested that one of the mechanisms was that the residual stresses generated during compression acted as back stresses to detwinning activity; thus, the yield stress was increased by removing the residual stresses by annealing. (3) The Lankford value after stress reversal was relatively small and further decreased during the first work-hardening stage, whereas it increased during the second stage. The simulation results indicated that the thickness strain was composed of both slip and detwinning activities in the first stage, while only slip activity contributed toward the thickness strain in the second stage, which resulted in the difference in the evolution of the Lankford value between the first and second stages.

Properties of Rolled AZ31 Magnesium Alloy Sheet Fabricated by Continuous Variable Cross-Section Direct Extrusion

Rolling is currently a widely used method for manufacturing and processing high-performance magnesium alloy sheets and has received widespread attention in recent years. Here, we combined continuous variable cross-section direct extrusion (CVCDE) and rolling processes. The microstructure and mechanical properties of the resulting sheets rolled at different temperatures from CVCDE extrudate were investigated by optical microscopy, scanning electron microscope, transmission electron microscopy and electron backscatter diffraction. The results showed that a fine-grained microstructure was present with an average grain size of 3.62 μm in sheets rolled from CVCDE extrudate at 623 K. Dynamic recrystallization and a large strain were induced by the multi-pass rolling, which resulted in grain refinement. In the 573-673 K range, the yield strength, tensile strength and elongation initially increased and then declined as the CVCDE temperature increased. The above results provide an important scientific basis of processing, manufacturing and the active control on microstructure and property for high-performance magnesium alloy sheet.

Microstructure and Properties of Rolled Magnesium Alloy Thick Plate at Different Positions

The microstructure and properties of rolled AZ31 magnesium alloy sheet with the change of normal position were investigated by electronic universal testing machine, ZEISS optical microscopy and scanning electron microscopy. The tensile test results showed that the mechanical properties of sample selected at the depth of 8mm, which is 2/5 of the whole thickness counting from rolling plane are closest to the properties for whole sheet. Microstructure characterization indicated that a large number of tiny grains are distributed around the large grains. The majority of these grains are equiaxed grains, and the few twins could be observeded. The fracture mechanism was ascribed to mixed ductile and brittle fractures.

Analysis of the microstructure and deformation mechanisms by compression along normal direction in a rolled AZ31 magnesium alloy

The twinning and slipping modes of a rolled AZ31 magnesium alloy sheet were investigated through in situ compressive tests. A method based on scanning electron microscopy and electron backscattered diffraction measurements was used to identify activated twinning and slip systems. {10-12} tensile twins were present, but only occasionally. Contraction twins were not observed. Post deformation examination revealed the dominance of pyramidal <c+a> slip.

Stress relaxation in rolled AZ31 magnesium alloy sheets

Stress relaxation in rolled AZ31 magnesium alloy sheets

Microstructure and texture evolution in multi-pass warm rolled AZ31 magnesium alloy

Electron Backscatter Diffraction (EBSD) is employed to characterize the microstructure and texture established during the process of warm rolled AZ31 magnesium alloy sheets. The grain size was refined from 17.4 μm to 3.8 μm after 4 pass rolling. Texture of as-rolled sheets was expressed by (0002) basal texture, and the texture intensity was increased with the rolling pass increasing. The mechanical properties of as-rolled sheets were greatly improved by warm rolling.

Work-Hardening Behavior upon Reverse Loading in a Rolled AZ31 Magnesium Alloy Sheet

The objective of this study was to understand the work-hardening behavior of a rolled AZ31 magnesium alloy sheet upon reverse loading. Firstly we carried out an in-plane compression-tension test of a rolled AZ31 magnesium alloy sheet with various compressive strains. The sigmoidal curve was exhibited during tension regardless of the amount of compressive strain, but a shape of the curve was clearly different depending on the compressive strain. To understand the mechanism of this difference, a crystal plasticity finite-element simulation was carried out. The simulation result showed that the above difference in the shape was owing to the difference in the activities of slip and twinning systems during tension depending on the compressive strain.

Effect of pre-strain on work-hardening behavior of magnesium alloy sheets upon cyclic loading

This paper reports the effect of pre-strain on the work-hardening behavior of rolled AZ31 magnesium alloy sheets during in-plane cyclic loading. The work-hardening behavior of the alloy remained almost unchanged when tensile strain was applied before cyclic loading. However, the work-hardening behavior was significantly affected when a compressive strain was applied. First, the resulting stress–strain curve was not sigmoidal upon tension in some cases, depending on the magnitudes of the applied compressive pre-strain owing to the inversion of the loading direction from tension to compression before the second increase in the work-hardening rate. In other words, a sigmoidal stress–strain curve certainly arose upon tension when compressive pre-strain was applied and the loading direction was not inverted from tension to compression. It was found that the strain at the beginning of the second increase had a high correlation with the volume fraction of twins. Second, the change in the rate of work-hardening at the beginning of tension became sharp as compressive pre-strain increased, probably owing to the effect of activation of detwinning on the stress–strain curve, which became increasingly significant as the compressive pre-strain increased the volume fraction of twins.

Grain size and texture effects on deformation behavior of AZ31 magnesium alloy

Cold rolled AZ31 magnesium alloy sheet was subjected to friction stir processing to generate four average grain sizes ranging from 0.8 to 9.6μm. The processed material exhibited a strong basal fiber texture with the c-axis tilted about 35–55° towards the processing direction. The grain size and texture dependence of mechanical behavior were evaluated by using tensile testing along two orthogonal directions. Remarkably high ductility of ∼65% was achieved in relatively coarse grained material that fractured without developing necking when tested in the processing direction. The ductility decreased significantly to ∼10% for ultrafine grained material as the tensile yield strength increased from ∼53MPa to ∼180MPa. Grain size had limited influence on ductility of processed material tested in transverse direction, but reduced the uniform elongation to ∼2% for ultrafine grained material which exhibited ∼320MPa yield strength. Accompanying the significant anisotropy in tensile strength in two directions, the deformation of processed AZ31 in the processing direction was mainly accommodated through basal slip and extension twinning (except for ultrafine grained material); however, the deformation of material in transverse direction was dominated by non-basal slip. Influences of grain size and texture on mechanical behavior were studied in terms of work-hardening and deformation mechanisms.

OS0318 マグネシウム合金板の除荷時変形挙動に及ぼす双晶活動の影響

This paper presents a crystal-plasticity finite-element simulation of deformation behavior during unloading in a magnesium alloy sheet considering twinning and detwinning. A detwinning model proposed by the authors was employed. The detwinning model could be employed in the framework of a twinning model proposed by Van Houtte (1978), with which shear strain and lattice rotation due to detwinning could be taken into account in the crystal-plasticity finite-element simulation. The material parameters were determined so as to obtain reasonable fits with stress-strain curves obtained from experiments of a rolled AZ31 magnesium alloy sheet. A loading-unloading process under in-plane compression was simulated, focusing on the activities of twinning and detwinning. A pronounced inelastic deformation arose during unloading in the simulation, and this tendency was in good agreement with the experimental result. We found that the pronounced inelastic deformation arose because of not only the activation of detwinning during unloading but also the dispersion of crystal orientations owing to the activity of twinning during loading.

THE EFFECT OF {10¯11}–{10¯12} DOUBLE TWINNING ON THE MICROSTRUCTURE, TEXTURE AND MECHANICAL PROPERTIES OF AZ31 MAGNESIUM ALLOY SHEET DURING ROLLING DEFORMATION

{1011}—{1012} double twin is the most common twin type being observed in the rolled AZ31 magnesium alloy,especially at low or moderate rolling temperature.Four types of rolled AZ31 magnesium alloy sheets were produced by rolling strong basal textured AZ31 plates at 150℃and 300℃respectively through different rolling paths by two passes till total reduction 17%. The rolling paths are normal and cross rolling respectively:the rolling directions of the two passes for normal rolling are mentally paralleled while perpendicular to each other for the cross rolling.Sheets are characterized in terms of microstructure and texture using optical microscope and scanning-electron microscope equipped with EBSD detector,and their mechanical performances are measured by uniaxial tensile tests at room temperature and a strain rate of 0.001 s~(-1).Tensile samples are cut parallel or perpendicular to the final rolling direction to obtain the mechanical anisotropics.The effects of the {1011} {1012} double twins occurred during rolling on the microstructure.texture and mechanical properties of the rolled sheets are discussed and related to each other.The results show that the fraction of {1011}-{1012} double twins varies widely in different rolled sheets,which are determined by the rolling temperature.The twins in different rolled sheets are arranged in different ways.In the normal rolled sheets,the traces of twins are approximately perpendicular to the final rolling direction,while in the cross rolled sheets there are both traces of twins parallel and to the final directions.This phenomenon is related to the variant selection of twinning during rolling deformation. And the variant selection law is related to the rolling direction.For the same reason,the textures of twins in the rolled sheets produced through different paths are different.The textures of twins will add onto the bulks,and increases the textural difference between the bulks.The differences between mechanical properties of the rolled sheets are also related to the different textures and microstructures of the sheets caused by twins.

THE DEVELOPMENT OF THE MICROSTRUCTURE AND TEXTURE IN COLD ROLLED AZ31 MG ALLOY SHEETS

Formability is very important parameter of magnesium alloy sheets and it would be related to the texture of sheet metals. In this study, magnesium alloy sheets with strong {0002} texture were cut along the angles of 0, 12.5, 25 and 37.5 degrees to rolling direction (RD). Prepared samples were rolled at room temperature condition. Cold rolled AZ31 magnesium alloy sheets along the angles of 0, 12.5, 25, 37.5 and 45 degrees to rolling direction were investigated microstructure and texture with optical microscopy and x-ray diffractometer, respectively.

Numerical-Experimental Characterization of a Superplastic AZ31 Magnesium Alloy

In this work the superplastic behaviour of a hot rolled AZ31 magnesium alloy sheet under a biaxial tension test with the blow forming technique is presented and reported. The specimen dome height and its thickness distribution, during and after the test, have been used as characterizing parameters. A numerical FE model of the test has been developed in order to easily characterize the material and to directly analyze experimental results. The influence of the rolling cycle on the microstructure and consequently on the material behaviour has been also analyzed. A synergic use of experimental results and of the numerical model has been done for finding material constants in different situations. The material flow parameters have been found and results are presented.

Texture of AZ31 Magnesium Alloy Sheet Heavily Rolled by High Speed Warm Rolling

Magnesium alloy sheets had to be rolled at elevated temperature to avoid cracking. The poor workability of magnesium alloy is ascribed to its hcp crystallography and insufficient activation of independent slip systems. Present authors have succeeded in 1-pass heavy rolling of AZ31 magnesium alloy sheet below 473K by raising rolling speed above 1000m/min. Heavy reduction larger than 60% can be applied by 1-pass high speed rolling even at room temperature. The improvement of workability at lower rolling temperature is due to temperature rise by plastic working. The texture of heavily rolled AZ31 magnesium alloy sheet is investigated in the present study. The texture of sheets rolled 60% at room temperature was &lt;0001&gt;//ND basal texture. At the rolling temperature above 373K, the peak of (0001) pole tilted ±10-15 deg toward RD direction around TD axisto form a double peak texture. The texture varied through thickness. At the surface, the (0001) peak tilted ±10-15 deg toward TD direction around RD axis to form a TD-split double peak texture. The direction of (0001) peak splitting rotated 90 deg from the surface to the center of thickness. Heavily rolled magnesium alloy sheets have non-basal texture. The sheets having non-basal texture are expected to show better ductility than sheets with basal texture.

Texture and microstructure changes in asymmetrically hot rolled AZ31 magnesium alloy sheets

Asymmetrically hot rolled AZ31 magnesium alloy sheets exhibited a texture gradient, where the intensity of {0002} basal textures decreased from the upper surface through the center to the lower surface. After subsequent annealing, the intensity of {0002} components was reduced significantly throughout the thickness and the grains were refined possibly by discontinuous recrystallization.