AZ31 Sheet Research Articles

Effects of temperature and driver sheet for magnesium alloy sheet in magnetic pulse forming

Thermal effect from warm temperature is always used to improve the formability of AZ31 magnesium alloy sheet. However, it is seldom employed to deform AZ31 sheet in magnetic pulse forming process, due to increasing resistivity and decreasing effect of high strain rate. In this study, Al driver sheets without heating were used to strength effect of high strain rate and drive AZ31 sheet with warm temperature to deform. Method of numerical simulation was used to analyze magnetic pulse forming of AZ31 sheet with driver sheet and temperature. Magnetic flux density and magnetic force with and without Al driver sheet (thickness of 1, 1.5, and 2 mm) and different temperature (25, 100, 150, 200, and 250°C) were investigated. Deformation processes and velocity with Al driver sheet and different temperature were analyzed. The results indicate that it is better for formability of AZ31 sheet to adopt 1-mm Al driver sheet at higher discharge energy and warm temperature.

Fatigue behavior of dissimilar ultrasonic welds in lap-shear specimens of AZ31 and steel sheets

Fatigue behavior of dissimilar ultrasonic welds in lap-shear specimens in magnesium and steel sheets is investigated. The welds were produced using a Sonobond ultrasonic spot welder to join magnesium AZ31B-H24 to two types of steel. The lap-shear specimens have been machined into a dog-bone profile to approximate a linear weld. The lap-shear linear welds were studied based on experimental observations, closed-form stress intensity factor solutions, and a fatigue life estimation model. Optical micrographs of the welds after testing were examined to understand the failure modes of the welds. The micrographs show that the welds tested under cyclic loading failed from kinked fatigue cracks growing through the magnesium sheets. The closed-form stress intensity factor solutions for each side of the weld are used to explain the location of fatigue crack initiation and growth. Two-dimensional finite element analyses of the lap-shear specimens with the welds were carried out to obtain the global and local stress intensity factor solutions for the main cracks and kinked cracks, respectively. A kinked fatigue crack growth model based on the global and local stress intensity factor solutions for finite kinked cracks obtained from the finite element analyses is adopted to estimate the fatigue life of the welds. The fatigue life estimations based on the kinked fatigue crack growth model generally underestimate the experimental results. The kinked crack growth model shows little sensitivity to the welding indentation or the thickness of the steel sheet.

An Inclusive Experimental Investigation on Influences of Different Process Parameters in Warm Incremental Forming of AZ31 Magnesium Sheets

The incremental forming (IF) is a flexible sheet metal forming process, which is extensively used for low-quantity and/or prototype productions. Light alloys, such as magnesium alloys, are also interesting candidates for reducing the weights of different vehicles to reduce their fuel consumption. The present investigation is concerned with the warm incremental forming of AZ31 magnesium sheets. In this regard, the effects of different operational variables, namely the initial temperature, vertical pitch and tool diameter, on various outcome parameters such as actual in-process temperature, drawing depth, major and minor strains (forming limit diagrams), thickness reduction, spring-back, microstructure and hardness of the deformed sample are experimentally studied using the groove test. Among various parameters, it was found that the initial temperature and, consequently, the actual final temperature significantly affected the outputs of the IF process. Regarding this, a transition temperature of about 160 °C was obtained for AZ31 sheets, above which considerable changes were observed in the material behavior and the influences of several operational variables. Based on the experimental findings, a couple of predictive equations are also proposed in this article for estimation of the actual final temperature and the drawing depth of AZ31 samples incrementally deformed via the groove tests.

Deformation mechanism of highly textured AZ31 sheet under high strain rates

AbstractIn this study, in‐plane tension behavior of AZ31 magnesium sheet was measured in rolling direction, transverse direction and 45° direction under high strain rates of 600 s‐1, 1200 s‐1 and 2400 s‐1 at room temperature using a Split Hopkinson Tension Bar (SHTB). The microstructures after high strain rates deformation were observed by optical micrography. The anisotropic behavior was negligible and marginally positive strain rate effects were observed in all deformation directions. Texture analysis of the sheet demonstrated that three possible deformation mechanisms could apply to AZ31 magnesium alloy sheet under high strain rates tension, those are contraction twinning and/or ‐ double twinning, prismatic slip and <c+a> pyramidal slip. The volume fraction of twinning was decreased with strain rate increasing. Dynamic recrystallization took place upon the strain rate of 2400 s‐1 and the dynamic recrystallization mechanism was attributed to the twinning‐induced dynamic recrystallization.

In-process control strategies for friction stir welding of AZ31 sheets with non-uniform thickness

Two different in-process control strategies were developed and compared with the aim to produce AZ31 magnesium alloy joints by friction stir welding on sheet blanks with a non-uniform thickness. To this purpose, sheets with dip or hump zones were welded by either changing the rotational speed or the tool plunging in order to keep constant the value of the vertical force occurring during the welding stage of the process. The influence of the main process parameters on the tool force, the micro- and macromechanical properties, and the joints microstructures in the dip and hump zones were analyzed. The results showed that using the rotational speed change-based approach, the hump zones are subjected to increased heat input with consequent increase of the heat-affected zone extension and average grain size (up to 11 μm). On the contrary, using the tool plunge change-based approach, the weld seam thickness in the dip zones is reduced, resulting in a decreased joint strength, with respect to tool rotation approach, equal to about 18%.

Effect of effective strain gradient on texture and mechanical properties of Mg–3Al–1Zn alloy sheets produced by asymmetric extrusion

In the present study, Mg–3Al–1Zn (AZ31) alloy sheets were fabricated by conventional extrusion (CE) and asymmetric extrusion (AE) processes. The microstructures, textures and mechanical properties of the AE and CE AZ31 sheets were examined and compared. The results showed that a tilted weak basal texture and microstructure refinement were obtained in the AE AZ31 sheet compared with the CE AZ31 sheet. The reason for the texture weakening and microstructure refinement was ascribed to an introduction of large effective strain gradient throughout the normal direction during the AE process. In addition, the AE AZ31 sheet exhibited lower yield strength and r-value, but larger ductility and n-value. Improving mechanical properties for extruded AZ31 sheet were achieved by the AE process.

Influence of crystallographic texture and grain size on the corrosion behaviour of as-extruded Mg alloy AZ31 sheets

The influences of crystallographic texture and grain size on corrosion behaviour were investigated using the Mg alloy AZ31 sheets produced in the following ways: as-extruded (AR), cross rolling (CR) and unidirectional rolling (UR). The corrosion rate was measured for immersion of the AZ31 sheets in 3.5wt.% NaCl solution using hydrogen evolution, potentiodynamic polarisation curves, and mass loss. The corrosion rate of the AZ31 sheets decreased in the following order: AR, CR, UR. The decrease in corrosion rate was attributed to an increase in basal plane intensity in the crystallographic texture and the reduced grain size.

Tensile behavior and microstructural evolution for AZ31 magnesium alloys sheet at high strain rate

AbstractMechanical behavior and microstructural evolution of AZ31 sheet at high strain rates and elevated temperatures were investigated using a tensile Split-Hopkinson bar. Flow stress and elongation increased with increasing strain rate under high strain rate conditions, but temperature had little effect. Microstructural evolution was analyzed by optical and scanning electron microscopy. Twinning was observed at high strain rates, even at 200°C and 250°C, but not under quasi-static tension. Dynamic recrystallization was observed under quasi-static conditions with increasing temperature, but not at high strain rates. Dimples were observed over the entire temperature range at high strain rates. Increasing ductility was limited compared with that under quasi-static conditions. Dynamic recrystallization occurred at strain rates of 2 000 s−1 and 3 000 s−1 at 250 °C, but the recrystallized grains were very small.

Anisotropy of mechanical and thermal properties of AZ31 sheets prepared using the ARB technique

In the accumulative roll bonding (ARB) technique, repeated stacking of material followed by conventional roll-bonding is carried out. For this process the surfaces are cleaned with ethanol and then joined together by rolling. The rolled material is then cut into two halves, again surface treated and roll-bonded. This process may be repeated several times. For the magnesium alloy AZ31 (Mg-3Al-1Zn) rolling at an elevated temperature of 400 °C is necessary for ARB because of the low plasticity of hexagonal magnesium alloys at lower temperatures. Samples for this study were prepared using 1 to 3 ARB passes through the rolling mill. It was found that the ARB substantially refined the grain size of sheets to the micrometer scale. The microstructure and texture of the deformed samples were studied by light and electron microscopy. The mechanical properties of the ARB samples were explored using tensile test-pieces cut from the sheets with the tensile axis taken either parallel or perpendicular to the rolling direction, where a significant anisotropy in both mechanical properties and Young’s modulus was found. Anisotropy is explained on the basis of the specific microstructure and texture formed during the ARB process.

Mechanical Properties and Friction of AZ31 Magnesium Alloy and Application to the Cylidrical Deep Drawing Process

The mechanical properties such as stress-strain curves and anisotropic parameters at different elevated temperatures are obtained by the computerized screw universal testing machine. The friction testing machine is used to determine the friction coefficient between die and AZ31 sheets at different elevated temperatures. The finite element method is used to investigate the earing of the deep drawing process. In order to verify the prediction of FEM simulation of the earing in the cylindrical cup drawing process, the experimental parameters such as stress-strain curves, anisotropic parameters, fiction coefficient and blank holder force, are as the input data during analysis. The experimental cup height compared with the current simulation result of cylindrical deep drawing process at different elevated temperature.

Thermodynamically consistent constitutive modeling of distortional hardening for wrought Mg alloys under non-proportional loading

Strong anisotropic mechanical behaviour due to texture is one of the key factors restricting the processing procedure of wrought magnesium alloys. This pronounced anisotropy, however, cannot be characterized by only classical isotropic or kinematic hardening due to the constant shape of yield surface during plastic deformation. Therefore, the shape evolution of yield surface, also known as distortional hardening is the main way to capture the anisotropic behaviour. 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 behaviour of AZ31 sheet under non-proportional loading is demonstrated after model parameters identification.

Improvement of planar isotropy, mechanical properties and corrosion resistance of extruded Mg-3Al-1Zn alloy sheet by special grain re-orientation

The extruded Mg-3Al-1Zn (AZ31) wide sheet with 500 mm in width was possessed of a sub-strong c-axis∥transverse direction (TD) texture component in addition to owning primary typical strong c-axis∥normal direction (ND) basal texture. The asymmetric distribution of grain orientations around sheet normal gave rise to strong planar anisotropy in mechanical properties owing to different initial texture on slip and twinning during deformation along different directions, which further restricted the application of the wide sheet. Even more regrettably, the c-axis∥TD texture component arranged prismatic planes with high surface energy exposing to the sheet surface, which decreased the corrosion resistance of the sheet markedly. A special grain re-orientation process was carried out on the AZ31 sheet by consuming the c-axis∥TD textured matrix via special pre-twinning, and finally a symmetric weak basal texture was introduced. As a result, the anisotropic degree between tensile yield strengths along different directions of the sheet was minimized from ∼125 MPa to only ∼10 MPa, and the average ultimate strength of the optimized sample increased ∼30 MPa compared with that of the as-extruded sheet. Meanwhile, texture modification based on re-orientating the initial c-axis∥TD textured matrix decreased the number fraction of prismatic planes exposing to the sheet surface, which contributed to enhance the corrosion resistance of the sheet effectively. It seemed that special grain re-orientation through appropriate pre-deformation was an efficient way to deal with the unsatisfied comprehensive performances of magnesium alloy sheets.

The dislocation-twin interaction and evolution of twin boundary in AZ31 Mg alloy

The reaction of lattice dislocation with twin boundary plays a crucial role in the plastic deformation of magnesium alloys. In this study, we visit the basal dislocation-twin interaction in a hot-rolled AZ31 sheet through pre-compression along rolling direction (RD) and subsequent compression along 45° of RD and normal direction (ND), with focus on the twin boundary (TB) structure evolution and nucleation structure characterization. It is found that basal dislocation slip dominates the compression deformation when the strain along 45° of RD and ND is less than 14%; when the strain reaches 14%, new deformation modes are initiated. When the strain is in the range of 5%–14%, basal-prismatic (BP/PB) boundaries are created by dislocation-twin interaction. Meantime, the number of the BP/PB boundaries increase linearly with strain, leading to an extremely incoherent TB. When the strain reaches 14%, {101¯2} twin nucleates from parent of previous {101¯2} twin and {303¯4} twin, a twinning mode not reported before, nucleates from previous {101¯2} TB. Based on the direct experimental observations, the nucleation mechanisms of the new nucleuses are proposed. Moreover, TBs of these new nucleuses present faceted structures and previous {101¯2} twin-new {303¯4} twin interaction results in a low-angle asymmetrical tilt boundary. These correlations will significantly benefit the development of crystal plasticity modeling and enhance meso-scale understanding of structural evolution in hexagonal close-packed materials.

Evolution of Microstructure and Texture in AZ31 Alloy Subjected to Gas Forming

In this study, AZ31 sheets were subjected to gas forming using a stepped geometry die and employing two different temperatures (400°C and 450°C) and exposure times (60 s and 120 s). Microstructural examinations revealed that the density of the mechanical twins may be taken as indicative of the progress of recrystallization together with the relative grain size. Although gas forming was successfully applied, the severe residual texture inherited from the initial material was found to persist and was little affected by the operating conditions and the multiaxial stress state of the process.

Stress intensity factor solutions for welds in lap-shear specimens of dissimilar sheet materials with and without kinked cracks

Global stress intensity factor solutions for welds with various widths in lap-shear specimens of magnesium alloy AZ31 and high-strength low-alloy steel sheets are developed using finite element analyses. Results are compared to and validated with the analytical solutions based on the beam bending theory and semi-infinite planes with connection. Analyses are also conducted using modified elastic constants with a bimaterial constant of zero for specimens with small kinked cracks. Results indicate that the stress intensity factor solutions for a vanishing kink length can be approximated by the available analytical solutions for the zero bimaterial constant for fatigue cracks growth models. The results of the global stress intensity factor solutions and the local stress intensity factor solutions for kinked cracks with the experimentally observed kink angle as functions of the kink length can be adopted for fatigue life estimations of ultrasonic welds in lap-shear specimens of magnesium and steel sheets.

Multilayer Mg-Stainless Steel Sheets, Microstructure, and Mechanical Properties

Different multilayer Mg AZ31 and SS304L steel sheet combinations were prepared with different volume fractions of Mg. Isolated stress–strain curves of the Mg layers showed significant improvements in the strength and elongation of multilayer samples. Results indicated that in the most extreme situation with the lowest Mg volume fraction (Vf = 0.39), the ultimate strength was increased by 25 pct to 370 MPa and the elongation was improved by 70 pct to 0.34. Investigation of the fracture surface showed that failure occurs by the coalescence of cracks close to the interface region. The improved strength of the multilayer samples was due to the combined effect of surface crack prevention by the steel layer and the higher work-hardening rate caused by the possible increased activity of non-basal systems. It is suggested that the stronger work-hardening behavior and the enhanced activity of non-basal systems in the multilayer samples were due to the formation of new stress components in the transverse direction. The larger the volume fraction of steel in the multilayer, the longer the distance remaining unstrained before the UTS.

Refinement Strengthening of AZ31 Magnesium Alloy by Warm Constrained Groove Pressing

The grain refinement and its effect on the mechanical properties of AZ31 magnesium alloys were investigated. The grain refinement was conducted using one of the severe plastic deformation methods, warm constrained groove pressing (CGP) technology. The effective strain was analyzed using the DEFORM-3D software. It was found that the effective strain distribution in AZ31 sheet was homogeneous after a complete four-stage of CGP at 473 K. The average grain size was reduced from 20 μm to 9 μm and the hardness was improved from 55.84 HBW to 69.16 HBW after a complete CGP process at 473 K. The yield stress and ultimate tensile strength increase with grain refinement, from 132 MPa and 240 MPa to 155 MPa and 285 MPa, respectively, following the Hall-Petch relationship. Subjected to annealing at 530 K for 15 minutes, the average grain size of CGP sample decreases slightly and the elongation increased up to 16.2%, close to 17.72% for the pre-CGP samples. DOI: http://dx.doi.org/10.5755/j01.ms.23.1.14392

Improved stretch formability of AZ31 sheet via grain size control

It is reported that stretch formability of Mg alloys enhances with increasing grain size. In order to understand the effects of grain size, the evolution of microstructure during tensile test was investigated comparing with mechanical properties such as work hardening coefficient n-value) and Lankford value (r-value). An increase in grain size promotes the activation of {10 1̅2} tensile twins and most of the tensile twins formed in the grains whose basal poles tilted to the rolling direction (RD) by around 20 degrees during tensile loading toward RD. Promoting tensile twinning contributes to an increase of n-value induced by the interaction between tensile twins and basal dislocations, and a decrease of r-value associated with releasing stress concentration on grain boundaries continuously activating basal dislocation. In the present study, the high formability is associated with tensile twinning up to a certain grain size and then in coarser grain size, it deteriorates by changing the matrix grains into hard orientations.

Method for Outlet Temperature Control during Warm Rolling of AZ31 Sheets with Heated Rolls

Method for Outlet Temperature Control during Warm Rolling of AZ31 Sheets with Heated Rolls

Effect of dimple patterning conditions of Periodical Straining Rolling on microstructures and mechanical properties of AZ31 sheets

Magnesium alloys are well-known for the attractive physical properties such as the lowest density among the structural metallic materials, high specific strength, high vibration absorption and so on. However, the strong (0001) basal texture of wrought magnesium alloys causes the poor plastic formability at room temperature in spite of high Lankford value. Texture and microstructure control of magnesium alloys are quite necessary to improve the cold sheet formability in order to prevail the wider application of wrought magnesium alloy products. The authors have already proposed the novel rolling process called “Periodical Straining Rolling (PSR)” using the small grooved roll to introduce periodically localized plastic strain in rolled sheet and confirmed that its effectiveness of the proposed rolling can realize the micro texture and microstructure control. This paper is aimed to discuss the effects of control of spatial strain distribution on microstructure, micro texture and mechanical properties by using various dimple patterning conditions of PSR process. The microstructure and micro texture observations of AZ31 magnesium alloy sheets PSR processed under the several dimple patterning conditions (two-sided PSR processing conditions) evince that the weak crystallographic orientations are largely prominent by changing dimple patterning conditions. As a result, the higher Erichsen values can be effectively achieved by changes in dimple patterning as compared to the conventional flat rolling process.