AZ31 Sheet Research Articles

Simulation of the bite condition of AZ31 sheet rolling

Sheet rolling is a simple plastic deformation of which, there are many formulas to describe its bite condition, however, there is no rigorous theoretical analysis and result and the applying ranges of these formulas are limited. Although the bite condition is not an important problem in engineering or academy, it is really a term influenced by many factors and it is difficult, maybe impossible to be expressed by an equation. In this paper, by finite element software, the bite condition or the critical friction factor under different thicknesses of AZ31 sheet, rolling reduction, sheet temperature and roller temperature are calculated and with large result data, the change regulation of the bite condition appears which is shown in a colour brick model. This example shows that, avoiding rigorous theoretical analysis, a complex situation can be numerically illustrated well by finite element software.

Effect of Surface Modification on Cumulative Tensile Ductility of AZ31 Magnesium Sheet

Wire brushing and annealing (WBA) process was developed, optimized and utilized to modify the surface layer microstructure of AZ31 automotive magnesium sheet material. The process was carried out using softer brass wire brushes to mitigate the effect of wire brushing on surface quality and damage. The influence of modified surface grain structure and crystallographic texture was studied by continuous uniaxial tension test as well as by a newly proposed multi-step uniaxial stretching and annealing (MUSA) process to assess cumulative uniaxial tensile ductility of AZ31 sheet. A rotational speed of 2800 revolutions per minute for the wire brush with a near-zero depth of cut followed by annealing at 473 K (200 °C) for 60 minutes resulted in acceptable surface quality with a refined grain layer of depth 30 μm, and a modified crystallographic texture on the surface. Material flow behavior, grain microstructure, and texture evolution of WBA-processed material during subsequent MUSA process were analyzed to assess the role of wire brushing in enhancing the MUSA response of AZ31 sheet. Original fully annealed AZ31 sheet (in the non-WBA condition) was also subjected to identical MUSA process for comparison purposes. The results showed improvement in terminal uniaxial tensile ductility of WBA-MUSA-processed material compared to Standard-MUSA material. The ductility improvement is attributed to non-basal texture development and re-distribution of the texture, as well as to grain refinement within the highly deformed surface layer from the combination of WBA and MUSA processes.

Experimental study for rubber pad forming process of AZ31 magnesium alloy sheets at warm temperature

In this paper, forming behavior of AZ31 sheet in rubber pad forming with warm temperature was investigated by experimental method. Rubber pad is always considered as it is temperature resistant. However, some types of rubbers can endure warm temperature, such as polyurethane for 120 °C and silica gel for 250 °C. Thus, forming for coupling with effects of temperature and flexible of rubber pad is proposed in this paper. The forming process of AZ31 sheet experienced bulging first and then gradually contacting with die, which was different from conventional forming. Fracture presented flat at room temperature and fracture became dentation obviously at 100 °C. When the temperature elevated to 200 °C, the fracture does not occur but the polyurethane was melted. The AZ31 sheet could fill the die successfully with silica gel as rubber pad at 200 °C. The effects of different load velocity for forming of AZ31 sheet were investigated. Thickness increased at bottom of deformed part, due to pressure stress acted from surrounding area. The thickness deceased at the bottom corner. Microstructure of deformed AZ31 sheet was also analyzed. It was indicated that dynamic recrystallization and twinning deformation mechanics were activated which had positive influence for improving formability of AZ31 sheet.

Prediction of ductile fracture induced by contraction twinning in AZ31 sheet subjected to uniaxial and biaxial stretching modes

The formability of AZ31 sheets was experimentally measured for uniaxial tension, plane strain stretching, and equi-biaxial stretching modes. The limit strain of the specimen was reasonable under uniaxial tension but was drastically reduced in the biaxial stretching modes. Full-field strain analysis and the observation of the fracture surface revealed that the specimen fractured without the formation of a visible local necking. To predict the fracture behavior using a crystal plasticity-based finite element method, a mean field model was developed to solve the mechanical interplay between the parent and twins. The destabilization of the specimen was simulated by introducing fracture criteria as functions of the accumulated slip inside the contraction twin region. Simulations revealed that strong strain heterogeneity forms in the case of the biaxial stretching modes, and this heterogeneity is apparently reduced under uniaxial tension as a result of the activation of prismatic slip, which produces a uniform strain distribution. Contraction twins nucleate at smaller strain levels in the biaxial stretching modes, and premature failure is predicted in the biaxial stretching modes as a result of the accumulation of slip in the twinned regions. Parametric studies show that even when the volume fraction of twins is small, they have a significant impact on the ductility of AZ31 sheets.

Modelling of uniaxially pre-strained and annealed AZ31 magnesium sheet based on microstructural considerations

Material deformation at room temperature and subsequent thermal aspects are analyzed at the microstructural level to understand the uniaxial deformation behavior of AZ31 magnesium sheet from pre-straining and annealing (PSA) process. The sheet is pre-strained in uniaxial tension to various strains by conducting interrupted tests and its uniaxial stress-strain responses are obtained. The pre-strained specimens are then annealed at different temperatures and times and then reloaded in uniaxial tension to obtain their post-anneal stress-strain response up to large strains. Pre and post-annealing mechanical responses are analyzed and correlated with static recovery, grain recrystallization and growth kinetics from annealing. Experimental results are utilized to develop and validate a microstructure-based model to capture the room temperature mechanical and grain-level responses of AZ31 sheet from the PSA process. The model incorporates the effect of recovery on the recrystallization kinetics, as well as non-constant nucleation and growth behavior in order to predict the grain size at the end of recrystallization and within the grain growth stage. In addition, the mechanical properties of PSA materials are predicted by considering the resulting microstructure as a composite of un-recrystallized, recrystallized and coarsened grain regions and by employing the rule of mixture.

Effect of processing route on texture and cold formability of AZ31 Mg alloy sheets processed by ECAP

The present work investigates the influence of different shearing patterns on mechanical and forming behavior at room temperature of equal channel angular pressing (ECAP) processed AZ31 sheets. Hot-rolled AZ31 (Mg-3Al-1Zn, wt%) sheets, with dimensions of 200×200×1.8mm3, were processed at 225°C with channel angle of 110° regarding processing routes A, C and D. Shear deformation induced by ECAP weakens the texture and promotes a broad angular distribution of basal planes in the pressing direction on routes A and C. Especially, processing on route D makes basal planes tilted towards the rolling direction and transverse direction by 90° rotation in the sheet plane. Due to the increased activity of basal <a> slip, ECAP processed sheets exhibit the decrease in yield strength and r-value and increase in uniform strain and n-value in the pressing direction. As a result, route C has the largest uniform elongation in the rolling direction and route D provides a quasi-isotropic hardening behavior at room temperature. The improved formability of the ECAP processed sheets is attributed mainly to low r-value and high n-value. At Nakajima tests, route D provides higher forming limits than the as-rolled sample. The microstructure on route D exhibits a drastic reduction in the fraction of {10−12}-{10−11} double twins. This twinning mode leads to premature failure of the as-rolled sample under biaxial stretching. Consequently, microstructure-texture control induced by ECAP can enhance the cold formability of AZ31 sheets.

Enhanced rollability of Mg[sbnd]3Al[sbnd]1Zn alloy by pulsed electric current: a comparative study

Experiments were designed to compare the rollability of magnesium alloy AZ31 sheet in two rolling schedules at the same rolling temperatures: multi-pass electroplastic rolling (ER) and warm rolling (WR). While cracks occur in WR sheet at an accumulative true strain of −0.24, the sheet can be rolled by ER up to an accumulative true strain of −1.09 without edge-cracking. This rollability enhancement in ER sheet was found to be related to the improved homogeneity of plastic deformation and dynamic recrystallization (DRX), which postpones the crack initiation. A ‘twinning - shear banding - DRX’ microstructure evolution model was proposed to explain the effect of pulsed electric current on the homogeneity of deformation and subsequent DRX. The use of ER sheet could pave the way for low-cost applications of lightweight magnesium components.

Influence of Temperature and Loading Rate on the Forming Limit Behaviour of Twin-Roll Cast, Rolled and Heat-Treated AZ31 as a Function of the Stress State

Magnesium and its alloys have high potential for lightweight applications in the automotive and aerospace industries. In order to design parts for new applications with optimized mechanical properties and higher, more economic production rates, the forming limit behaviour of thin sheets (t &lt; 1.0 mm) has to be known for different temperatures and loading rates. In this study, forming limit curves of 0.8 mm thick AZ31 sheet were measured for deformation at 200 °C and 250 °C and at loading rates of 1 mm/s and 10 mm/s with the Nakajima test. The investigations showed that an increase in temperature from 200 °C to 250 °C tends toward higher forming limit values for all stress states. In contrast, an increase in the loading rate from 1 mm/s to 10 mm/s induces a reduction in formability. It can be seen that the temperature, loading rate, and stress state influence the force-distance curves, the distribution of the local major strains, and the sheet thickness reduction.

Strain-induced selective grain growth in AZ31 Mg alloy sheet deformed by equal channel angular pressing

In this work the microstructure development during annealing of AZ31 sheets processed by equal channel angular pressing (ECAP) is presented. Samples with dimensions of 200×200×1.8mm3 were processed in a single ECAP pass at 225°C using a channel angle of 130°. Subsequent isothermal and isochronal heat treatments were used to reveal the microstructure stability of the ECAPed sheets at temperatures between 180 and 300°C. A systematic analysis of the microstructure development was conducted using optical microscopy and electron backscatter diffraction (EBSD). It was found that a texture modification produced by the profuse activation of 101-2 extension twinning during ECAP is lost during annealing as a result of thermal detwinning. Moreover, a bimodal microstructure composed by small and coarse grains was observed at 250°C and 300°C as a result of the fast growth of a few grains. The reason for the selective grain growth is related with the activation of strain induced grain boundary migration which was stimulated by the large stored strain contained in the as-ECAP processed sheets. The dissolution of Mg17Al12 particles during annealing is also seen as a possible factor for the onset of rapid grain growth.

An effective approach called the composite extrusion to improve the mechanical properties of AZ31 magnesium alloy sheets

In the present study, a new effective composite extrusion to produce the AZ31 magnesium alloy sheets was carried out and compared with the conventional direct extrusion. The microstructures, textures and mechanical properties of AZ31 magnesium alloy sheets obtained by two different extrusion processes at room temperature were investigated. The results demonstrated that the ductility of the extruded AZ31 sheet was increased from 18.5% to 22.5% and the ultimate tensile stress (UTS) was also higher by the composite extrusion, compared with those by the conventional direct extrusion. The improved mechanical properties of the composite extruded samples were mainly attributed to the more homogeneous and finer microstructure near the center as well as the weakened basal texture which were depended on the introduction of shear deformation caused by the fiction between billets during the composite extrusion. Therefore, the composite extrusion could be a simple but effective extrusion process to improve the mechanical properties of AZ31 magnesium alloy sheets.

Experiment and crystal plasticity analysis on plastic deformation of AZ31B Mg alloy sheet under intermediate temperatures: How deformation mechanisms evolve

In order to investigate the evolution of deformation mechanisms of AZ31B Mg alloy sheet and their correlation with material property development under intermediate temperatures, systematic experimental examination and in-depth crystal plasticity analysis on the material are performed. The mechanical responses of AZ31B Mg alloy sheet are measured under uniaxial tension and compression along RD (rolling direction), TD (transverse direction) and ND (normal direction) directions and over the temperature range 100 ∼ 300°C. The evolution of anisotropy (r-value), texture, and microstructure with respect to temperature and strain are examined. Two uncommon deformation behaviors in AZ31 sheet deformation are observed in the experiments for the first time. Firstly, the slope of r-value versus strain curve in RD compression at lower temperature changes from a positive to a negative one when temperature rises to 200°C and 300°C. Secondly, micro-bands are formed during RD compression at 200°C, in contrast to uniform equiaxed dynamic recrystallization (DRX) grains in RD tension and ND compression. The evolution of deformation mechanisms in the uniaxial tension/compression deformation is analyzed with the visco-plastic self-consistent (VPSC) model that takes the variation of r-value into account. The correlation between the experimental observations and deformation modes evolution is probed. It is found that the negative slope of activity ratio of prismatic to basal slip is highly consistent with the change in r-value with strain during uniaxial compression at 200°C. The different rotation ‘room’ caused by basal slip in uniaxial compression at 200°C, together with the restriction of grain boundary, leads to higher misorientation evolution rate in loading direction than in the other directions, and then results in the formation of micro-bands.

Experimental and numerical study on the ECARed magnesium AZ31 alloy

One of the best methods for applying severe plastic deformation (SPD) to the sheets is equal channel angular rolling (ECAR) process. In this paper, the effect of ECAR process on the AZ31 alloy has been evaluated. Firstly, the process has been accomplished on the magnesium specimens. Then, the effect of ECAR process on mechanical properties of AZ31 alloy such as: microhardness, strength and microstructure have been assessed. For evaluation of the ECAR parameters on the samples, numerical method also has been utilized. Finally, the results indicate that the microhardness after 8th pass ECAR has been increased about 45%. Also, the average grain size has been decreased about 94% after 8th pass ECAR. The yielding stress (Y.S) and ultimate tensile strength (UTS) of AZ31 sheet were decreased by increasing the pass number while, elongation has been improved.

Enhanced mechanical behavior and reduced mechanical anisotropy of AZ31 Mg alloy sheet processed by ECAP

AZ31 Mg alloy sheets were processed by equal channel angular pressing (ECAP) at 225°C with a channel angle of 110° regarding a strain path change. In consideration of mechanical anisotropy of Mg sheets, the sheet was processed by ECAP along the rolling direction in the first pass. Subsequently, it was processed along the transverse direction by 90° rotation in the sheet plane. This shear deformation led to grain refinement and developed basal texture with tilted basal planes towards the pressing direction. The presented processing route, which is denoted as route D, reduced mechanical anisotropy and enhanced hardening behavior. Consequently, AZ31 sheet, which was processed by ECAP on route D, exhibited an almost isotropic hardening at room temperature with improved ductility.

Enhancement of mechanical properties and corrosion resistance of magnesium alloy sheet by pre-straining and annealing

Pre-straining and annealing processes were carried out to enhance the mechanical properties and corrosion resistance of AZ31 sheet by modifying the microstructure and texture. The results showed that by a small tensile deformation along the weak basal textured orientation, the pre-strained AZ31 samples showed higher strength and improved anisotropy after annealing. Furthermore, because the texture reinforcement effect obtained by the pre-deformation was retained during annealing, the corrosion resistance of pre-strained and annealed specimens was enhanced compared with the as-extruded sheet, which was characterised by the evolved hydrogen and the immersed surfaces. This study suggested that appropriate pre-straining accompanied annealing was a potential technique to improve the comprehensive properties of the magnesium alloy sheets.

On the Development of Mg Sheets with Improved Formability by Applying Additional Shear Strain during Processing

Reducing vehicle weight and emissions by lightweight design is a major goal of the automotive industry. Magnesium as the lightest structural metal offers a significant weight saving potential compared to steel and aluminum. However, the poor formability of magnesium semi-finished products (e.g. sheets) has hindered the massive application of this metal. This poor formability arises due to the formation of strong textures. Typically the basal planes of the HCP-structure align parallel to the sheet plane. Such preferred orientation of the basal planes limits the ability of basal &lt;a&gt; slip to accommodate plastic strain in the sheet plane and is thus unfavourable for ductility and formability of the sheets, especially at room temperature. A combination of new technologies could help to alleviate the strong textures formed in Mg sheets. In this regard the utilization of twin-roll casting (TRC) in combination with processing techniques which apply additional shear strain such as differential speed rolling (DSR) and/or equal channel angular pressing (ECAP) could refine the microstructure and modify the resulting deformation texture. In this study an AZ31 strip (3Al-1Zn-Mg Bal. wt.%) produced by TRC was hot rolled at 400 °C using a rolling speed of 10 m/min. A speed ratio of 10% between rolls was used. Rolled samples with dimensions of 200 x 200 x 1.8 mm receive a single pass of ECAP at 200 °C. After processing the microstructure of the samples were analyzed by means of optical microscopy, electron backscatter diffraction and X-ray diffraction. The results showed that the microstructure can be refined and the texture altered in comparison to conventionally rolled AZ31 sheets. The samples processed by ECAP show an increase in formability up to 50% higher than conventionally rolled sheets. This demonstrates the potential of using shear deformation for processing Mg sheets.

Experimental and numerical research on process formability in magnetic pulse forming of AZ31 magnesium alloy sheets

The plain strain of AZ31 magnesium alloy sheet in magnetic pulse forming was investigated by numerical simulation and experimental method. Combination of uniform pressure coil and Holmberg’s specimen was employed to evaluate the plain strain of AZ31 sheet. The numerical simulation for magnetic pulse plain strain of AZ31 sheet is performed by means of ANSYS FEA software. The magnetic flux density of uniform pressure coil was distributed uniformly, especially at the center of gauged area of AZ31 sheet directly in relation to the deformation behavior of AZ31 sheet. The velocity of typical point increases as increasing energy, and the more position closes to the center of sheet the higher velocity achieves. The forming height is increased with increasing discharge voltage. Compared with C=768 µF and C=1536 µF, the capacitance of 1152 µF is more effective for forming, which is confirmed by experiments. The peak velocity at the center of sheet is about 105 m/s. The major strains of magnetic pulse plane strain lie approximately in the strain ranges of 5.83-6.45%. However, the 3.22-3.82% (major strain) are the limit strains in quasi-static condition. The experimental results indicate that the major strain of AZ31 sheet improves about 75% compared with the quasi-static case.

Effect of texture on the electromagnetic shielding property of magnesium alloy

Wrought magnesium alloys commonly exhibit strong texture, which significantly affects the properties of alloys. The effect of texture on the electromagnetic shielding property of these alloys as promising shielding materials is rarely reported. In the present study, the electromagnetic interference shielding effectiveness (SE) of AZ31 sheets with different texture intensities was investigated by coaxial cable method within the testing frequency range of 30–1500MHz. Results indicated that the SE of the sheets gradually increased with the strengthening of the basal texture intensity. The SE increment was mainly attributed to the improvement in the reflection attenuation of the incident electromagnetic wave, which was induced by the expansion of the impedance mismatch between the air and sheet surface. Furthermore, the relationship among texture, conductivity, and impedance was also discussed.

Thermal effects in magnetic pulse forming of magnesium alloy sheet

Thermal effects in magnetic pulse forming of magnesium alloy sheet were investigated by finite element software ANSYS. The adiabatic temperature produced in magnetic pulse forming was due to the high forming velocity. Distribution of adiabatic temperature is affected by discharge parameters, such as voltage and capacitance. Distribution of adiabatic temperature is predicted by C-P model and J-C model which are always used to describe the forming behaviors of material in high velocity forming. Predicted distribution of adiabatic temperature by C-P model and J-C model increases with increasing of discharge voltage, but decreases clearly with capacitance increasing, particularly at the center position of the deformed part. The highest temperature is 77.7 and 94 °C at C = 1152 μF/U = 2.75 kV (limit forming parameters) predicted by C-P model and by J-C model, respectively. The relationship between velocity and adiabatic temperature is also analyzed. Preliminary study for the effect of adiabatic temperature on the micro-structure of AZ31 sheet in magnetic pulse forming is explored by the analysis for fracture morphology of the deformed sheet.

Constitutive Modeling of Magnesium Alloys with Distortional Hardening

Anisotropic mechanical behavior is one of the key factors restricting the processing procedure of magnesium alloys. This pronounced anisotropy, however, cannot be characterized by classical isotropic or kinematic hardening due to the constant shape of yield surfaces during plastic deformation. Therefore, the shape evolution of yield surfaces, also known as distortional hardening is the main way to capture the anisotropic behavior. Based on elasto-plasticity theory at finite strain, constitutive model with distortional hardening for Mg alloys is proposed. The thermodynamical consistency is proved. The anisotropic mechanical behavior of AZ31 sheet is demonstrated after material parameters calibration.

Effect of Additional Shear Stress on Microstructure Evolution of AZ31 Sheet by Differential Speed Rolling

Asymmetric rolling of deformed magnesium alloys sheet can accelerate refine the grain size, reduce the basal texture, and improve the subsequent formability. This happens because the additional shear stress and strain are introduced during the differential speed rolling. This paper investigates the relationship of shear stress and microstructure evolution by the experiment in conjunction with the FEM (Finite Element Method) simulation. The differential speed ratio (1, 1.2 and 1.5) and rolling reduction (8% and 15%) were executed at room temperature. The additional shear strain field is simulated for corresponding asymmetric rolling conditions. The results showed that the additional shear stress and strain were the essential influence factors of microstructure evolution and formability of AZ31 sheet for the differential speed rolling. It is helpful in enhancing and improving the formability of rolled magnesium alloys.