Behavior Of Magnesium Alloy Sheet Research Articles

Fracture behavior of AZ31B magnesium alloy under high-temperature ultrasonic-assisted shearing with related ductile-brittle transformation mechanism

A wide width is one of the fundamental requirements for magnesium alloy sheets to fulfill the lightweight needs of the aerospace and rail transportation industries. Nevertheless, ensuring the dimensional integrity of wide-format magnesium alloy sheets is a challenge in most manufacturing facilities. Simultaneously, processing operations are susceptible to encountering production issues, including but not limited to significant loss of raw materials, elevated costs, environmental contamination, and safety risks. Therefore, to fulfill the operational demands of the hot rolling finishing line for wide sheets made of magnesium alloy, the present study employed ultrasonic-assisted shearing (UAS) to carry out a high-temperature shearing test investigation on the AZ31B magnesium alloy sheet. In this research, we employed scanning electron microscopy, optical microscopy, and a micro-hardness tester to investigate the relationship between ultrasonic amplitude and fracture behavior, deformation mechanism, and mechanical properties of magnesium alloy sheets following exposure to ultrasonic vibration. The fracture morphology of the sheet fracture demonstrates that the behavior of the magnesium alloy sheet fractures under ultrasonic vibration changes from ductile fracture under conventional shearing to brittle fracture under ultrasonic-assisted shearing. The primary cause of this transformation between ductile and brittle is the effect of ultrasonic vibration on the material's internal energy. High-energy ultrasonic amplitude action causes magnesium to absorb a significant amount of ultrasonic energy. This results in an increase in the number of dynamic re-crystallization grains in the material organization, a decrease in the number of twins, and significant refinement of the grains. The sheet's hardness also changes along the rolling and normal directions, with a uniform distribution. The magnesium alloy sheet's fracture quality is optimal when the ultrasonic amplitude is 10 μm in the UAS process. This treatment results in notable enhancements in the section's straightness and brightness. Our study aims to advance the mass manufacturing of wide-format magnesium alloy sheets by investigating a cost-effective and high-yield technique for shearing magnesium alloy sheets.

Individual Contribution of Zn and Ca on Age-Hardenability and Formability of Zn-Based Magnesium Alloy Sheet.

This paper reports on the dilemma of the strength and forming behavior of magnesium alloy sheets due to hot rolling and precipitation aging as an obstacle for property adjustment. The effect of the Zn content on the age-hardenability and formability of Mg-Zn-Al-Ca-Mn sheets was investigated. Sheets of two alloys with 2 or 4 wt.% Zn, respectively, were produced by casting and subsequent hot rolling and their microstructure development, precipitation behavior and formability were examined. With higher Zn content the age-hardenability was increased, but at the same time the formability of the sheet decreased, concurrent to the basal-type texture development during rolling. On the other hand, the sheet containing a lower amount of Zn exhibited a weak rolling texture and rather high formability but low age-hardenability. The addition of a larger amount of Zn improved the age-hardenability through the formation of and phases. The basal texture was exhibited due to the consumption of solute Ca due to the formation of the Ca2Mg6Zn3 phase. This study suggests that this contradictory exhibition of the age-hardenability and formability of Ca-containing and Zn-based alloy sheets requires a strategical approach in alloy and process design, which allows tailoring the alloying elements and processing for the respective purpose.

Modeling the effect of pre-straining on mechanical behavior of magnesium alloy sheet

Modeling the effect of pre-straining on mechanical behavior of magnesium alloy sheet

Study on the Time-Dependent Mechanical Behavior and Springback of Magnesium Alloy Sheet (AZ31B) in Warm Conditions.

In this study, the time-dependent mechanical behavior of the magnesium alloy sheet (AZ31B) was investigated through the creep and stress relaxation tests with respect to the temperature and pre-strain. The microstructure changes during creep and stress relaxation were investigated. As the tensile deformation increased in the material, twinning and dynamic recrystallization occurred, especially after the plastic instability. As a result, AZ31B showed lower resistance to creep and stress relaxation due to dynamic recrystallization. Additionally, time-dependent springback characteristics in the V- and L-bending processes concerning the holding time and different forming conditions were investigated. We analyzed changes of microstructure at each forming temperature and process. The uniaxial tensile creep test was conducted to compare the microstructures in various pre-strain conditions with those at the secondary creep stage. For the bending process, the change of the microstructure after the forming was compared to that with punch holding maintained for 1000 s after forming. Due to recrystallization, with the holding time in the die set of 60 s, the springback angle decreased by nearly 70%. Increased holding time in the die set resulted in a reduced springback angle.

Effect of Different Microstructure on Corrosion Behavior of Magnesium Alloy Sheets

Effect of Different Microstructure on Corrosion Behavior of Magnesium Alloy Sheets

Warm Forming Behavior of Magnesium Alloy Sheet in Manufacturing of Window Regulator Rail

Recently, as the demands to improve automotive fuel efficiency grow, several attempts are being conducted on the development of lightweight automotive components using magnesium (Mg) alloy sheets having the highest specific strength among metal materials. In a window regulator module which is a switchgear that raises and lowers the automotive side window pane, the weight of the rail accounts for about 40 % of the total weight, while the production cost of the rail is about 10 % of the total cost, that is the weight reduction effect is higher than the cost burden. Therefore, the application of Mg alloy sheets to the window regulator rails is more likely to be commercialized. The aim of this study is to investigate the warm forming behavior of Mg alloy sheets in the manufacturing of window regulator rails at the temperature range from 150 to 250 °C. AZ31B Mg alloy sheets with a thickness of 1.4 mm were used, and their mechanical properties were evaluated at the warm temperature range. The warm forming behavior was predicted by the finite element simulation. Based on the simulation results, the Mg alloy window regulator rail was successfully manufactured without fracture using the warm forming.

Asymmetric Hardening Behavior of AZ31B Magnesium Alloy Sheet with Large Strain at Various Strain Rates

This paper investigates asymmetric hardening behavior of the magnesium alloy sheet of AZ31B at different strain rates when deformation is large. Tensile tests are carried out at the quasi-static state of 0.001 s−1 and the intermediate strain rate of 100 s−1 utilizing the Instron 5583 and the high speed material testing machine (HSMTM), respectively. Compression tests are conducted at the strain rate of 0.001 s−1 and 100 s−1 using a jig fixture mounted on the testing machines to change the loading direction into the opposite direction. A compression testing method is developed to obtain compressive properties with large strain, which includes the attachment of a fork-type clamping device and the design of specimen dimensions to achieve large strain without buckling during the test. Experimental results reveal the asymmetric behavior showing that the tensile yield stress is much larger than the compressive one at both strain rates. The difference between the flow stress level in tension and compression becomes larger when the strain rate is higher. The compressive hardening behavior is very much different from the tensile hardening behavior showing that the compressive flow stress increases remarkably as the deformation proceeds while the tensile flow stress asymptotes to a certain value.

Pulsed Electric Current V-Bending Springback of AZ31B Magnesium Alloy Sheets

The springback of AZ31B magnesium alloy sheets subjected to pulsed electric current-assisted V-bending tests was investigated. To study the effect of pulsed electric current on the bending-dominated deformation behavior of magnesium alloy sheets, the stress–strain responses from the electric current-assisted uniaxial tension and compression tests were experimentally measured. The stress–strain curves under pulsed electric current showed an instantaneous stress drop under the application of the electric pulse. The ductility was enhanced owing to the application of electric current. The electrically assisted V-bending tests helped reduce the springback compared with conventional room-temperature V-bending tests, and the magnitude of the springback reduction increased with the increase in the electric current density. A thermo-mechanical-electrical finite element model for the electrically assisted V-bending test was developed. To consider the Joule heating effect, temperature-dependent hardening and a yield function with strength differential were implemented for simulating the AZ31B sheet. The simulations could reproduce the experimental flow stress curves of the uniaxial tension/compression tests under pulsed electric current, characterized by an instantaneous stress drop and subsequent transient hardening behavior. The simulated springback profiles were in good agreement with the measured V-bending test data, thus validating the proposed finite element modeling procedure.

An elasto-plastic constitutive model of magnesium alloy sheet at warm forming temperature under strain path changes

In the current work, a continuum-based approach is discussed in order to describe the plastic hardening behavior of magnesium alloy sheets under strain path changes at warm forming temperature. The constitutive model is represented to use an anisotropic distortional yield function combining a stable component and a fluctuating component. The stable component includes the yield function that represents the material’s anisotropy or the strength effect between tension and compression. The evolution of the fluctuating component is reformulated based on recently developed HCP metal’s distortional hardening model at room temperature. The modified models were implemented into finite element (FE) software via user-material subroutine. Predicted stress-strain curves of tension-compression-tension (TCT) and compression-tension-compression (CTC) loadings for two pre-strains were compared with experimental results. Overall, the proposed model could give reliable agreement with experimental data.

Numerical study of inelastic behavior of magnesium alloy sheets during cyclic loading-unloading

Magnesium alloys exhibit significant inelastic behavior during unloading, especially when twinning and detwinning are involved. It is commonly accepted that noteworthy inelastic behavior will be observed during unloading if twinning occurs during previous loading. However, this phenomenon is not always observed for Mg sheets with strong rolled texture. Therefore, the inelasticity of AZ31B rolled sheets with different rolled textures during cyclic loading-unloading are numerically investigated by elastic viscoplastic self-consistent polycrystal plasticity model. The incorporation of the twinning and detwinning scheme enables the treatment of detwinning, which plays an important role for inelastic behavior during unloading. The effects of texture, deformation history, and especially twinning and detwinning on the inelastic behaviors are carefully investigated and found to be remarkable. The simulated results are in good agreement with the available experimental observations, which reveals that the inelastic behavior for strongly rolled sheets is very different than that for extruded bars.

Distortional hardening concept for modeling anisotropic/asymmetric plastic behavior of AZ31B magnesium alloy sheets

A continuum-based approach for describing the plastic hardening behavior of magnesium alloy sheets subjected to non-proportional strain path changes at room temperature is presented. The constitutive model is based on an anisotropic distortional yield function that combines a stable component and a fluctuating component. The stable component initiates the yield criterion that characterizes the typical strength differential (SD) effect between tension and compression in magnesium alloys at room temperature. The evolution of the fluctuating component is reformulated based on its cubic metal counterpart to represent the deformation behavior of magnesium alloys, which consists of slip- and twinning-dominant deformation modes. To validate the proposed model, a predictor–corrector numerical algorithm was implemented in a finite element (FE) program by using a user material subroutine, and its numerical accuracy was evaluated by iso-error map analysis. Comparison of the experimental and simulated results obtained for various loading path changes showed that, although the model was not formulated using the kinematic hardening concept, the model could reproduce complex features of the stress–strain responses of an AZ31B magnesium alloy sheet under non-proportional loading paths, i.e., asymmetric hardening behavior under tension and compression, the sigmoidal nature of the hardening curve during monotonic compression and compression followed by tension.

Constitutive modelling of magnesium alloy sheets under strain path changes

In this study, a continuum-based approach for the description of the plastic hardening behavior of magnesium alloy sheets subjected to non-proportional strain path changes is discussed. The constitutive model is based on an anisotropic distortional yield function combining a stable component and a fluctuating component. The stable component initiates the yield criterion that characterizes the typical strength differential between tension and compression in magnesium alloys at room temperature. The evolution of the fluctuating component is reformulated based on its cubic metal counterpart to represent the deformation nature of magnesium alloys that consist of slip and twin dominant modes. The model is not formulated with a kinematic hardening rule, but it reasonably reproduces complex features of the stress-strain responses under the load reversal in magnesium alloy sheet: i.e., asymmetric hardening behavior under tension and compression, sigmoidal nature of hardening curve during monotonic compression and compression followed by tension, strong anisotropy etc.

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.

A temperature-dependent elasto-plastic constitutive model for magnesium alloy AZ31 sheets

For warm forming simulations of magnesium alloy sheets, a constitutive model is proposed for describing the temperature-dependent asymmetric cyclic behavior of magnesium alloy sheets. The asymmetric hardening behavior is classified into three modes − twinning (T), untwinning (U), and slip (S) − depending on the corresponding dominant deformation mode. The yield criterion uses two separate yield functions that correspond to the twinning/untwinning and slip dominant deformation modes. Though this model is phenomenological, it adopts the concept of the deformation mechanism as magnesium alloy sheets exhibits significantly different behavior by the active deformation mechanisms for a wide range of temperatures. To obtain the model parameters, the constitutive model requires cyclic behavior at room temperature and tensile behavior at high temperatures. A numerical algorithm for efficiently integrating the constitutive equations is presented for switching deformation paths and active deformation modes. For verification, the predictions by the proposed constitutive model are compared with measurements for the simple shear behavior and the high temperature cyclic behavior.

Constitutive modeling for anisotropic/asymmetric hardening behavior of magnesium alloy sheets: Application to sheet springback

The constitutive model for the unusual asymmetric hardening behavior of magnesium alloy sheet presented in a companion paper (Lee, M.G., Wagoner, R.H., Lee, J.K., Chung, K., Kim, H.Y., 2008. Constitutive modeling for anisotropic/asymmetric hardening behavior of magnesium alloy sheet, Int. J. Plasticity 24(4), 545–582) was applied to the springback prediction in sheet metal forming. The implicit finite element program ABAQUS was utilized to implement the developed constitutive equations via user material subroutine. For the verification purpose, the springback of AZ31B magnesium alloy sheet was measured using the unconstrained cylindrical bending test of Numisheet (Numisheet ’2002 Benchmark Problem, 2002. In: Yang, D.Y., Oh, S.I., Huh, H., Kim, Y.H. (Eds.), Proceedings of 5th International Conference and Workshop on Numerical Simulation of 3D Sheet Forming Processes, Jeju, Korea) and 2D draw bend test. With the specially designed draw bend test the direct restraining force and long drawn distance were attainable, thus the measurement of the springback could be made with improved accuracy comparable with conventional U channel draw bend test. Besides the developed constitutive models, other models based on isotropic constitutive equations and the Chaboche type kinematic hardening model were also considered. Comparisons were made between simulated results by the finite element analysis and corresponding experiments and the newly proposed model showed enhanced prediction capability, which was also supported by the simple bending analysis adopting asymmetric stress–strain response.

Temperature and strain rate effect incorporated failure criteria for sheet forming of magnesium alloys

As formability of magnesium alloys is highly dependent upon temperature and strain rate at elevated temperatures, a new approach of incorporating strain rate and temperature effects into a failure criterion is necessary. In the present study, two failure criteria were proposed to predict the fracture behavior of magnesium alloy sheets deforming at elevated temperatures. The two failure criteria were developed based on the effective strain or damage parameters and both could be formulated as a function of Zener–Holloman parameter. The experimental data obtained from the series of tensile tests conducted in wide range of temperature and strain rate were used to find the correlation between the damage value at fracture (or fracture strain) and Zener–Holloman parameter. Comparison between the experimental results and the prediction by FEM incorporated with the failure criteria indicated that the proposed criteria were useful in predicting the fracture behavior of magnesium alloy sheets deforming during deep drawing process.

Constitutive modeling for anisotropic/asymmetric hardening behavior of magnesium alloy sheets

Magnesium alloy sheets have been extending their field of applications to automotive and electronic industries taking advantage of their excellent light weight property. In addition to well-known lower formability, magnesium alloys have unique mechanical properties which have not been thoroughly studied: high in-plane anisotropy/asymmetry of yield stress and hardening response. The reason of the unusual mechanical behavior of magnesium alloys has been understood by the limited symmetry crystal structure of HCP metals and thus by deformation twinning. In this paper, the phenomenological continuum plasticity models considering the unusual plastic behavior of magnesium alloy sheet were developed for a finite element analysis. A hardening law based on two-surface model was further extended to consider the general stress–strain response of metal sheets including Bauschinger effect, transient behavior and the unusual asymmetry. Three deformation modes observed during the continuous in-plane tension/compression tests were mathematically formulated with simplified relations between the state of deformation and their histories. In terms of the anisotropy and asymmetry of the initial yield stress, the Drucker–Prager’s pressure dependent yield surface was modified to include the anisotropy of magnesium alloy. The numerical formulations and characterization procedures were also presented and finally the correlation of simulation with measurements was performed to validate the proposed theory.