Magnesium Sheet Research Articles

Springback Suppression of AZ31 Magnesium Alloy Sheet in Draw Bending at Elevated Temperature

In recent years, warm forming of magnesium sheets has been investigated by many researchers since the ductility of the sheets becomes considerably higher due to low CRSS (critical resolved shear stress) at high temperatures (e.g., [1]-[3]). In the present research, the springback of AZ31 magnesium sheet was investigated by performing a draw-bending experiment at several temperatures from 20 °C (room temperature) to 200 °C at drawing speeds ranging from 0.01 to 1.0 mm/s. From the experiment, it was found that the springback was remarkably reduced at 200 °C, especially at a low forming speed, since the flow stress was very low under such a forming condition, and furthermore, the stress relaxation effect was dominant. The effects of temperature and forming speed on springback were discussed.

Analysis of non-equal-channel angular pressing scheme applied for obtaining magnesium sheet in cold state

Deformation patterns of rolling, equal-channel angular pressing and non-equal-channel angular pressing were evaluated. It is noted that when rolling, it is difficult to transfer a circular section preform into a rectangular section with a small thickness. This problem cannot be solved using equal-channel angular pressing. In connection with this, it is proposed to use a non-equal-channel angular pressing pattern to study the cast structure of magnesium. An experimental procedure based on cold extrusion of cylinders with a diameter of 42 mm and a height of 40 mm is described. The strip at the outlet was 40 mm in width and 1 mm in thickness. The percentage reduction of the preform material determined by the ratio of areas was 96 % with a draw ratio of 17. The specific pressures on the punch at the beginning ofthe extrusion process were 1200—1300 MPa, and extrusion force was 1670—1800 kN. The preform was cut into lengths that were rolled at a room temperature into 50 and 10 pm thick foils without intermediate annealing. Rolling was carried out on the Duo mill with a percentage reduction of 12—20 % at an average speed of 0,1 m/s. 20 passes were performed with a total relative reduction of 95 % to make the 50 pm thick foil. The results of computer simulation by the finite element method demonstrated that the constant value of the deformation degree is achieved at a rather considerable distance from the front end estimated as 50 times the thickness of the strip. The deformation rate field was calculated to determine the configuration of the deformation center. Energy costs were estimated. As a result of the completed set of calculated and experimental work, it was possible to establish the following — it is possible to produce a thin sheet preform from a cylindrical cast magnesium blank in one operation at a room temperature. The sheet blank has a level of ductility sufficient for subsequent sheet rolling. The sheet blank obtained in the proposed process has a high level of plastic deformation elaboration created due to the forming pattern with the high level of elongation and shear deformation. Despite the high level of pressure that must be applied to create a comprehensive compression scheme taking into account the lack of the need to heat the preform, energy costs are no higher than in traditional treatment processes.

Investigations of sheet metal forming by vaporizing metal foils

This study aimed to investigate the forming behavior of magnesium alloy by vaporizing metal foils. This process was conducted at room temperature and no heat treatment for the workpiece was required. The free bulging test was applied to identify the formability of the magnesium alloy AZ 31, and the perforated sheet forming test was used to measure the pressure distributions. The effectiveness of the polyurethane plates on generating shock pressure for the metal forming work was firstly examined. The pressure distributions of vaporizing single foils and tailored foils were subsequently discussed. The pressure distributions were mainly determined by the active areas of the foil specimens. AZ 31 magnesium sheets with a thickness of 1 mm and 1.8 mm were formed by the free bulging test under different charging energies. A bulging height of 27.28 mm for the AZ 31 sheet in 1.8 mm was acquired under a charging energy of 4.8 kJ. Vaporizing foils was found to be a feasible technique to conduct magnesium alloy forming without heat treatment for the workpiece.

Stress-state dependent fracture characterisation and modelling of an AZ31 magnesium sheet alloy at elevated temperatures

Abstract Due to a high specific strength, magnesium alloys have a high potential to be considered for lightweight solutions in automotive industry. For the numerical design of forming processes, it is important to describe the yielding as well as the fracture behaviour of a material as precisely as possible. In order to fully characterise the fracture behaviour of an AZ31 magnesium sheet alloy at elevated temperatures, a heated test setup for uniaxial tensile machines was developed. The setup allows an adjustment of the load application angle whereby a stress variation is achieved in the centre of the specimen. In order to determine the fracture strain for different temperatures and for varying stress states, a shear stress specimen (also known as butterfly specimen) was considered to perform mechanical experiments by means of this setup. Using numerical simulations, the specific stress development and strain value in the fracture zone, which is needed to calibrate stress state fracture models, was determined for each loading angle and temperature. For this purpose, an orthotropic yield criterion CPB06, which is suitable for depiction of the particular flow behaviour of magnesium alloys (e. g. compression-tension asymmetry), was used. By this means, sufficient data for the calibration of common stress state based fracture models could be provided and the MMC- (Modified Mohr-Coulomb) fracture model was parameterised.

Microstructure and Texture of MX20 after Conventional Rolling and Rolling from Twin Rolled Cast Strip

Two main impediments are currenty discussed with respect to the industrial application of magnesium sheets. First, the low formability of magnesium sheets requires many rolling passes to roll cast slabs to final gauge, which leads, second, to high costs for the production of magnesium sheets. An alternative cost-saving process chain for magnesium sheets with enhanced properties is the feedstock production by twin roll casting (TRC). In the TRC process, liquid metal proceeds from a furnace over a pipe into a crucible and then flows between a pair of counter rotating, internally cooled rolls. The metal solidifies upon touch with the cooled rolls and gets rolled to a strip. This paper refers to the comparison of the two processing routes on the example of the aluminum-free magnesium alloy MX20 (2 wt% Mn and 0.5 wt% Ca). Both kinds of production processes like casting and twin roll casting have an influence on the microstructure and texture of the feedstock material for the subsequent rolling process. The paper reports on the results of casting and twin roll casting experiments of this alloy. Furthermore, rolling trials are conducted and the deformation behavior of the sheets are presented and discussed with respect to the developed microstructures and textures. The different morphology of precipitates in the cast and twin roll cast feedstock material is used to improve the ductility of the magnesium alloy MX20.

Application of an Evolving Non-Associative Anisotropic-Asymmetric Plasticity Model for a Rare-Earth Magnesium Alloy

Magnesium sheet metal alloys have a hexagonal close packed (hcp) crystal structure that leads to severe evolving anisotropy and tension-compression asymmetry as a result of the activation of different deformation mechanisms (slip and twinning) that are extremely challenging to model numerically. The low density of magnesium alloys and their high specific strength relative to steel and aluminum alloys make them promising candidates for automotive light-weighting but standard phenomenological plasticity models cannot adequately capture the complex plastic response of these materials. In this study, the constitutive plastic behavior of a rare-earth magnesium alloy sheet, ZEK100 (O-temper), was considered at room temperature, under quasi-static conditions. The CPB06 yield criterion for hcp materials was employed along with a non-associative flow rule in which the yield function and plastic potential were calibrated for a range of plastic deformation levels to account for evolving anisotropy under proportional loading. The non-associative flow rule has not previously been applied to magnesium alloys which require the use of flexible constitutive models to capture the severe anisotropy and its evolution with plastic deformation. The non-associative flow rule can provide the required flexibility by decoupling the yield function and plastic potential. For the associative flow rule, such flexibility can only be achieved by multiple linear transformations of the stress tensor resulting in expensive models for calibration and simulations. The constitutive model was implemented as a user material subroutine (UMAT) within the commercial finite element software, LS-DYNA, for general 3-D stress states along with an interpolation technique to consider the evolution of anisotropy based upon the plastic work. To evaluate the accuracy of the implemented model, predictions of a single-element model were compared with the experimental results in terms of flow stresses and plastic flow directions under various proportional loading conditions and along different test directions. Finally, to assess the predictive capabilities of the model, full-scale simulations of coupon-level formability experiments were performed and compared with experimental results in terms of far-field load-displacement and local strain paths. Using these experiments, the constitutive model was evaluated across the full range of representative stress states for sheet metal forming operations. It was shown that the predictions of the model were in very good agreement with experimental data.

Standard and Strain Path Change Forming Limit Diagrams of AZ31B Magnesium Sheet at Elevated Temperatures

Effect of strain path change on formability and microstructural characteristics of AZ31B automotive magnesium sheet material is studied at 300°C. The standard forming tests are carried out using Nakazima tests within the environmental chamber. The strain path change tests are performed by in-plane uniaxial, plane strain and balanced biaxial pre-strained specimens in the first stage and Nakazima test in the second stage. The enhancement in the activity of prismatic and pyramidal <c + a> slip as well as a reduction in material anisotropy with increase in temperature is responsible for improvements in formability. Compared to the standard FLD, all uniaxial, plane strain, and balanced biaxial pre-strained materials exhibit higher limit strains. This rise in FLD also incorporates the effect of additional annealing during temperature rise period prior to testing which cannot be avoided with the present experimental set-up and test methodology. The pre-strain effect is reduced by additional annealing effect caused by elevated temperature soaking time. At 300°C, the pre-strained material is almost fully recovered and behaves as the as-received material. Also, significant dynamic recrystallization is observed at 300°C.

Study of Trimming Behavior of Automotive AZ31 and ZEK100 Sheet Materials

The trimming behavior of AZ31 and ZEK100 automotive magnesium sheet materials was investigated using lab-based experiments. The effects of trimming process parameters; trimming speed, clearance and tool setup configuration, on quality of trimmed edge were analyzed. Experimental results indicated strong dependence of trimmed edge quality on trimming process conditions. Clearance between punch and die had the most significant influence on the trimming behavior of AZ31 and ZEK100, both the punch load peak and quality of trimmed edge decreased with increase of clearance. The larger is the clearance, the later the crack initiates. ZEK100 was more sensitive to smaller clearance compared to AZ31. Trimmed edge quality of AZ31 and ZEK100 Mg sheets improved with increase in trimming speeds up to 5 mm/sec. The tool setup configuration with cushion consistently resulted in better trimmed edge quality and especially help with burr height reduction which is a key parameter for assessing the quality of the trimmed edge.

In-situ nanocomposite in friction stir welding of 6061-T6 aluminum alloy to AZ31 magnesium alloy

Prior to friction stir welding (FSW), silicon carbide (SiC) nanopowders were placed inside the groove on the adjacent side of magnesium sheet. In order to make a good distribution of these nanoparticles, first, friction stir processing (FSP) with a pinless tool was conducted on this side. Second, using a frustum pin tool, a combination of two travel speeds (25 and 35 mm/min) and three rotation speeds (550, 600 and 650 rpm) were employed to obtain the optimal conditions for FSW of AZ31 magnesium alloy to 6061 aluminum alloy. The joint fabricated at 35 mm/min and 650 rpm, exhibited improvement of 28% in tensile strength and enhancement about three times in elongation, compared with the joint without nanoparticles. SiC nanoparticles had an effective role on the grain refining of the stirred zone and improving the microstructural characteristics of the banded structure zone. Due to the decreased grain size and the presence of ceramic nanoparticles (SiC) with high value of hardness, the hardness level of the reinforced samples was greater than that of the non-reinforced samples. Fracture of the non-reinforced sample was completely brittle, while in the reinforced samples, fracture features changed to the ductile mode.

Bake-hardenable Mg–Al–Zn–Mn–Ca sheet alloy processed by twin-roll casting

Mg–1.3Al–0.8Zn–0.7Mn–0.5Ca (wt.%) (AZMX1110) alloy sheet produced by low-cost twin-roll casting process exhibits excellent room temperature formability with a large Index Erichsen (I.E.) value of 7.8 mm at room temperature in a solution treated condition (T4). The large I.E. value is associated with a weak basal texture in the T4-condition. This alloy shows rapid age-hardening response at 170 °C, resulting in a significant increase of the flow stress from 198 MPa to 238 MPa within 20 min; such bake hardenability (BH) has never been explored in magnesium sheet alloys before. The microstructure of the bake-hardened sample, characterized by a correlative transmission electron microscopy and atom probe tomography (TEM-APT), reveals that Al, Zn and Ca atoms are segregated to basal <a> dislocations and contribute to the strengthening by pinning dislocation motions, along with the co-clustering of these atoms. This first BH Mg sheet alloy composed of only ubiquitous elements is highly attractive for application to automotive bodies.

A mechanism-driven plasticity model for deformation by glide and twinning and its application to magnesium alloys

The plastic deformation of two different rolled magnesium sheets (AZ31 and ZE10) under quasi-static tensile and compressive loading conditions at room temperature is studied. Beside glide by dislocation motion, deformation twinning leads to evolving flow stress asymmetry and evolving plastic anisotropy in these alloys. These mechanisms cause a significant change in the shape of the yield surface. A phenomenological plasticity model in which the primary deformation mechanisms, slip and (extension) twinning, are treated separately is developed here and incorporated in the finite element framework. Deformations caused by these mechanisms are modelled by a symmetric and an asymmetric plastic potential, respectively. The hardening functions are coupled to account for the latent hardening. The necessary input for model parameter calibration is provided by mechanical tests along different orientations of the rolled sheets. Tensile tests of flat notched samples and shear tests of sheets are furthermore incorporated in the parameter optimisation strategy, which is based on minimising the difference between experimental behaviour and FE result. The model accounts for the characteristic tension-compression asymmetry and the evolution of strain anisotropy. Both, the convex-up and the concave-down shaped stress-strain response are predicted. The method is further probed on the simulation of forming operations.

Effect of deformation twinning on forming limit analysis of polycrystalline magnesium

A formability assessment using the crystal plasticity model is conducted to investigate the effect of deformation twinning on the formability of polycrystalline magnesium sheet. For presenting the polycrystalline behaviors of magnesium, the homogenization-based finite element method is introduced, and the Marciniak-Kuczyńsk type sheet necking analysis is demonstrated. Two kinds of deformation twinning model, i.e., the asymmetric slip type and van Houtte’s models are adopted, and the effect of those models on the forming limit analysis is numerically discussed. It is revealed that the lattice reorientation due to deformation twinning plays an important role in plastic flow localization of magnesium, while shear deformation caused by deformation twinning has an insignificant effect.

Modeling and experimental validation of friction self-piercing riveted aluminum alloy to magnesium alloy

Friction self-piercing riveting (F-SPR) process has been proposed to achieve crack-free joining of low-ductility materials by combining SPR process with the concept of friction stir processing. The inhibition of cracking in an F-SPR joint is related to the in-process temperature as well as plastic deformation of materials, which are controlled by the process parameters, i.e., spindle speed and feed rate. However, the relationship between F-SPR process parameters and the temperature characteristics within the joint has not been established. In the current study, a coupled thermal-mechanical model based on solid mechanics was setup to study the F-SPR process of aluminum alloy and magnesium alloy. Temperature and strain rate-dependent material models and preset crack surface method were integrated in the model and geometry comparisons were conducted for model validation. Based on this model, the evolutions of temperature and plastic deformation in the rivet and the sheets of an F-SPR joint were obtained to reveal the formation mechanism of the joint. The temperature distribution and evolution of the sheet materials were correlated with F-SPR process parameters, and a critical spinning speed of 2000 rpm at a feed rate of 1.35 mm/s was determined capable of inhibiting cracking in the magnesium sheet.

Microstructure and mechanical properties of Al/Cu/Mg laminated composite sheets produced by the ARB proces

In the present study, an Al/Cu/Mg multi-layered composite was produced by accumulative roll bonding (ARB) through seven passes, and its microstructure and mechanical properties were evaluated. The microstructure investigations show that plastic instability occurred in both the copper and magnesium reinforcements in the primary sandwich. In addition, a composite with a perfectly uniform distribution of copper and magnesium reinforcing layers was produced during the last pass. By increasing the number of ARB cycles, the microhardness of the layers including aluminum, copper, and magnesium was significantly increased. The ultimate tensile strength of the sandwich was enhanced continually and reached a maximum value of 355.5 MPa. This strength value was about 3.2, 2, and 2.1 times higher than the initial strength values for the aluminum, copper, and magnesium sheets, respectively. Investigation of tensile fracture surfaces during the ARB process indicated that the fracture mechanism changed to shear ductile at the seventh pass.

Texture and Anisotropy Parameters of Pressed MA14 Alloy Pipes Produced by Granule Metallurgy Methods

The effect of the granular technology on the formation of the crystallographic texture during the extrusion of MA14 magnesium alloy pipes with a diameter of 105 mm and a wall thickness of 5 mm was studied. The pipes were manufactured from pressed workpieces fabricated by hot molding compaction of magnesium granules, which were produced by centrifugal spraying of melt in a helium gas medium at the cooling rate of 104–105 K/s. The texture of the pipes was characterized by the arrangement of the prism poles along the axial direction; the reduction of the wall thickness of a pipe significantly exceeded that of its diameter and, as a result, a basal texture similar to that of magnesium sheets was formed in its walls. A procedure was proposed for calculating the yield locus making it possible to estimate the yield stresses for any kind of stress on the basis of Hill’s yield criterion for anisotropic materials.

Experiments and constitutive modeling of deformation behavior of a magnesium sheet during two-step loading

Two-step tension tests, namely pre-tension in transverse direction followed by re-tension along different directions, were performed on an AZ31 magnesium alloy sheet. Compared with monotonic tension, early yielding phenomenon (strain path changes induced softening) was observed in the second loading of two-step tension. Moreover, the degree of softening increases with increasing loading angle from transverse direction to rolling direction. To model the deformation behavior, especially the early yielding phenomenon of AZ31 magnesium alloy sheet in two-step tension, more fluctuating components were introduced in the HAH model. The early yielding phenomenon of AZ31 magnesium alloy sheet in two-step tension was qualitatively well described by the extended HAH model.

Twinning-induced shear banding and its control in rolling of magnesium

Rolling of magnesium sheets is challenging at temperatures below 200 °C due to the strain localization and shear banding associated with the twinning activity. In this study, magnesium sheets with basal, off-basal (90° tilted), and mixed (50% basal + 50% off-basal) textures are rolled between room temperature and 165 °C to understand and control the twinning-induced localizations. While the fraction of strain-localized regions increases from 0.1 to 0.6 with strain and temperature, the intensity of them are controlled by the starting textures. The sheet with basal texture develops the most intense localizations at room temperature, and fails by shear banding at 0.16 strain. Off-basal sheet, on the other hand, has similar fraction of twins and localizations but deforms to the strain of 0.36 without shear banding. Maximum uniform strains increase with temperature and reach to 0.60, 0.50, and 0.33 at 165 °C for off-basal, mixed, and basal textures, respectively. When the fraction and intensity of localizations are incorporated to a model treating the continuum as a composite, it is possible to capture the shear banding and failure during rolling. The model correctly predicts the maximum strains for a given starting texture and temperature.

Effect of upper-die temperature on the formability of AZ31B magnesium alloy sheet in stamping

To ensure the temperature and formability of magnesium sheet metal transferring from a continuous oven, stamping of magnesium alloy sheet normally requires a setting of stationary heating oven in which lower-punch and binder are heated simultaneously. This paper studied the effect of unheated or extra heated upper-die with different temperatures on AZ31B magnesium sheet metalʼs formability in stamping. Forming limit curves (FLC) under different temperature upper-die were generated to analyze the temperature effect, which shows that the formability decrease significantly as upper-die temperature drops. Moreover, temperature fields simulated through thermo-mechanical coupling FEM simulation and microstructure of different specimen region indicate inhomogeneous mechanical properties due to temperature gradient, which results in different modes of crack initiation and propagation. Finally, FLC result was verified through a tryout stamping of automobile hinge cover.

Determination of the Hot Cracking Tendency of a Twin-Roll Cast AZ31 Magnesium Alloy by Means of Gleeble Tests

The knowledge about the formation of hot cracking in magnesium alloys, such as in twin-roll cast magnesium sheets and strips, is fundamental for a good quality of the strips during the further processing by rolling or welding and minimize the reject. Hot cracking often occurs in the so-called mushy zone, when solid phases and melt coexist, at temperatures where the material no longer exhibits ductility. For the evaluation of the hot cracking tendency of an alloy, the width of the HTBR (High-temperature brittleness range) can be used. On the basis of a test on a Gleeble HDS-V40, the HTBR was determined for a twin-roll cast AZ31 magnesium alloy. The transition between ductile forming behaviour and complete brittle reaction of the AZ31 alloy is confirmed by the observation of the fracture surfaces (determination of the fracture type) in the scanning electron microscope (SEM) and is located at 555 °C. The HTBR shows a range 35 K.

Processing Effects on the Formability of Magnesium Alloy Sheets

As a generalized semi-finished product, the use of magnesium sheets requires addressing two major aspects of their processing: their microstructure and texture control, which are both essential for the forming behavior of such sheets during their forming to parts. Further, the processing of such sheets is complex, and therefore expensive, and requires simplification. In this work, magnesium alloys AZ31, ZE10, and ME21 are investigated in the form of conventionally rolled sheets, as well as in the form of extruded sheets. Their microstructural and textural development are correlated to their mechanical and forming properties. During extrusion, strong textures develop that hinder stretch-forming operations, even in rare earth-containing alloys. Chemical composition and process parameters have a significant impact on the texture development, and enable the design of sheet materials with weak textures and potentially enhanced formability.