AZ31 Sheet Research Articles

Enhanced Cyclically Stable Plasticity Model for Multiaxial Behaviour of Magnesium Alloy AZ31 under Low-Cycle Fatigue Conditions.

Magnesium alloys, particularly AZ31, are promising materials for the modern automotive industry, offering significant weight savings and environmental benefits. This research focuses on the challenges associated with accurate modelling of multiaxial cyclic plasticity at small strains of AZ31 under low-cycle fatigue conditions. Current modelling approaches, including crystal plasticity and phenomenological plasticity, have been extensively explored. However, the existing models reach their limits when it comes to capturing the complexity of cyclic plasticity in magnesium alloys, especially under multiaxial loading conditions. To address this gap, a cyclically stable elastoplastic model is proposed that integrates elements from existing models with an enhanced algorithm for updating stresses and hardening parameters, using the hyperbolic tangent function to describe hardening and ensure a stabilised response with closed hysteresis loops for both uniaxial and multiaxial loading. The model is based on a von Mises yield surface and includes a kinematic hardening rule that promises a stable simulation of the response of AZ31 sheets under cyclic loading. Using experimental data from previous studies on AZ31 sheets, the proposed model is optimised and validated. The model shows promising capabilities in simulating the response of AZ31 sheet metal under different loading conditions. It has significant potential to improve the accuracy of fatigue simulations, especially in the context of automotive applications.

A flexible yield criterion for strength modeling from biaxial compression to biaxial tension

Accurate strength modeling from equi-biaxial tension (EBT) to equi-biaxial compression (EBC) is critical for the plastic behavior prediction covering the wide-range of stress triaxiality encountered in sheet metal forming. To date, however, few yield criteria are available that can precisely model the initial yield and hardening behavior under six typical stress states between EBC and EBT, simultaneously. Furthermore, there is still a lack of a unified yield criterion for accurate strength modeling across various stress state ranges. To address the issues, a theoretical framework for constructing yield criteria dependent on stress states is provided and a new analytically described isotropic yield criterion is presented in this study. The flexibility in terms of the yield locus and application range is thoroughly explored to make the new yield criterion general. Subsequently, the isotropic yield criterion is extended into an analytically described anisotropic-asymmetric yield criterion. Furthermore, the extended yield criterion is applied to capture the initial yield behavior of DP980, AA5754-O, and AZ31 sheets, and the strain hardening behavior of QP1180 sheets at various stress states ranging from EBC to EBT along different loading directions. The predicted results from the extended criterion agree well with the corresponding experimental findings. The applications demonstrate that the proposed anisotropic-asymmetric yield criterion can effectively model the initial yield and hardening behavior of HCP, BCC, and FCC metal sheets under EBT, EBC, uniaxial tension (UT), plane strain tension (PST), shear (SH), and uniaxial compression (UC) in an analytical way.

GTN poroplastic damage model construction and forming limit prediction of magnesium alloy based on BP-GA neural network

During plastic deformation processes, the ductile solids damage and fracture at the microscopic level originate from the evolution of micro -voids, including nucleation, growth and coalescence of voids. In this study, the fracture caused by damage of the AZ31 magnesium alloy sheet is analyzed using the Gurson-Tvergaard-Needleman (GTN) model. The damage parameters of the GTN model are determined by statistical void volume fractions (VVF) in three damage stages during uniaxial tensile experiments with scanning electron microscopy (SEM). The damage parameters are then optimized by the Back Propagation-Genetic Algorithms (BP-GA) neural network. According to the result, the main GTN damage parameters: the initial void volume, the critical volume fraction, the void volume fraction of the nucleation part, and the void volume fraction when the material finally fails are obtained, respectively. Comparing the simulation with the test, the results of the optimized parameters fit better. The void growth reflects the damage during the deformation process, and the GTN model can accurately predict the ductile damage. Furthermore, the experimental forming limit diagrams (FLDs) of the AZ31 sheet at 200℃ are accurately predicted using the parameters obtained by the GTN model. Good agreement has been observed between the experimental and predicted FLDs.

The Effect of the Forming Mode on Twinning and Springback in the Bending-Dominated Forming of Magnesium AZ31 Sheet

The Effect of the Forming Mode on Twinning and Springback in the Bending-Dominated Forming of Magnesium AZ31 Sheet

Effect of texture on bending behavior of Mg alloy sheets

The effect of Li elements on the microstructure and texture evolution of AZ31 alloy sheets during room-temperature three-point bending was investigated. The results showed that the conventional c-axis//ND basal texture of the extruded AZ31 sheet could be modified to a TD-elongated fiber texture with a 5 wt% Li addition. As a result, the room-temperature bendability of the Mg–3Al–5Li sheet was improved effectively, and this sheet showed a minimum bending radius of only 2 mm. The superior bendability exhibited surpasses the room-temperature bending capabilities commonly observed in the majority of commercial Mg alloys. The improvement in bendability of the Mg–3Al–5Li sheet was attributed to the TD-elongated texture distribution. This type of texture distribution may offer more effective deformation modes to alleviate thickness strain during bending in comparison to the texture in extruded AZ31 sheets. This study could provide insights into developing Mg alloys with high room-temperature bendability by tailoring texture distribution rather than texture weakening.

Investigating the weld joint characteristics of AZ31 and AZ91 magnesium alloys produced by friction stir welding

ABSTRACT In this work, FSW was used to join AZ31 and AZ91 magnesium (Mg) alloys to investigate the mechanism behind joint formation and its mechanical performance. The cross-sectional microstructure of the weld zone clearly indicated a strong metallurgical bonding between AZ31 and AZ91 sheets in addition to the mechanical interlocking due to the mixing of both base materials. Uniform hardness (62.9 ± 4.5 HV) was recorded in the weld joint and the values were between AZ31 (59.8 ± 2.4 HV) and AZ91 (78.7 ± 19.5 HV). Tensile test results demonstrated decreased strength (145 ± 4.2 MPa) compared with the base alloys (265 ± 7.3 MPa and 190 ± 4.8 MPa for AZ31 and AZ91, respectively. Ductility was noticed as decreased as reflected by the lower % of elongation for the weld joint compared with both AZ31 and AZ91 base materials. The fractography studies of the samples suggest a brittle mode of fracture in the weld joint similar to that of AZ91 base material.

Microstructure and properties of AZ31 alloy by static recrystallisation at lower temperature

The microstructure evolution and mechanical properties of AZ31 Mg sheets that was processed by rolling and subsequent annealing at lower temperature were investigated. The results indicated that few-pass rolling with large deformation is a better strategy to prepare AZ31 Mg sheets with higher deformation restored energy and less rolling defects. After annealing, the static recrystallisation occurred and the recrystallisation degree increases with the annealing temperature and time. Finally, the best comprehensive mechanical properties were obtained by 3-pass rolling with large deformation and annealing at 200 °C for 150 s and the corresponding YS, UTS and TEF of the AZ31 sheet were 225 MPa, 305 MPa and 32%, respectively. The improvement in the microstructure and mechanical properties can be attributed to an optimised static recrystallisation at a relatively low temperatures and in a short time.

High-Speed Laser Beam Welding of Magnesium Alloy AZ31 for Enhancing the Mechanical Properties

Laser beam welding is still the focus of research all over the world since new laser sources with more brilliance, higher power, or higher efficiency are being developed. High brilliance leads to thinner fibers when solid-state lasers are used. For welding applications, a thin beam, respective a small focus spot is recommended for low heat input resulting in less deformation. The edge preparation of the welding pieces must be as accurate as possible, and a zero gap is recommended. In earlier research, it was shown, that the gap bridging capacity could be enhanced by the wobbling of small focus spots, as well as refining the grain size in the weld zone by decreasing the focus diameter. Inventions in the optics, like the beam splitting into a core and a ring part, avoid the use of a scanner and can lead to better gap bridging. Nevertheless, the use of a brilliant beam, resulting in a small focus in combination with high power can result in very high welding velocities, just limited by the used machinery. In the present study, a disk laser with 4 kW maximum power and 100 μm focus spot was used to weld 2 mm thick magnesium AZ31 sheets at speeds up to 20 m/min. As expected, the seam width becomes smaller with raising velocity, and some underfill and access material occurred on the surface and the root of the welded sheets. Surprisingly, the texture of the weld seam changed from random at low velocity to a more pronounced texture at high speed with respect to the basal texture of the plate base material. This influences the mechanical behavior, namely the strain to fracture, of the welded joints positively. The high-speed weldments are compared to state-of-the-art weldments of magnesium AZ31, in terms of mechanical strength and elongation to fracture, based on the texture analysis.

Mechanistic investigation of highly bendable magnesium alloy sheet fabricated by short-process manufacturing

In this study, a commercial magnesium alloy AZ31 (Mg-3Al-1Zn-0.3Mn) sheet through a short manufacturing process was found to be ductile and highly formable in bending. Despite possessing a strong basal texture, the short-processed sheet without any annealing can be bent at a small radius, only 0.2 times its thickness in the 90° bending test. Additionally, it could withstand direct deformation by repeated folding-flattening. The in-situ microstructural characterization reveals that extension twin bands with strain localization appear in the bending area. During subsequent flattening, these twin bands underwent detwinning, reducing local strain concentrations and enabling further bending deformation. Such outstanding bend formability originated from the significant 〈a〉-type dislocation loops slipping on the prismatic crystal planes within dynamic-recrystallized grains. These grains underwent a uniform refinement to several microns in the short manufacturing process and exhibited low residual strain. The active prismatic dislocation slip within refined grains was due to its much lower relative activation stress to basal slip (CRSSprism/ CRSSbasal of only ∼1.6) owing to the effective grain boundary hardening. Furthermore, the prismatic dislocation activity was further enhanced when bypassing Al-Mn nano-particles during motion, leaving debris and loops that facilitated easy multiplication.

Joining magnesium and aluminum alloy sheets by a novel hole hemming process

The use of magnesium alloys represents a novel trend in the automotive industry, being used to manufacture high performance vehicle structures. These alloys possess the lowest density among all structural metals. However, the use of joining processes based on plastic deformation of magnesium alloys is restricted by the low formability of these materials at room temperature. In this research work, a novel joining process, called hole hemming, is developed for attaching magnesium AZ31 and aluminum AA6082-T4 alloy sheets in which only one of the sheets should be sufficiently ductile and there is no need for heating or using additional elements, such as rivets. Firstly, the fracture limits of the materials were characterized under different loading states. Then, the process for joining the mentioned material combination was designed with the support of finite element analysis and the critical regions prone to fracture and their loading paths were specified. In addition, the influence of process parameters on the mechanical interlock was investigated to determine process windows. A flexible tool was then developed and, for the first time, novel hole-hemmed joints were manufactured with different process parameters to thoroughly evaluate the performance of the designed process. Finally, single-lap joints were tested under shear loads. The results of this tests show that the novel hole hemming process can suitably join the magnesium and aluminum alloy sheets, with the hole-hemmed joints supporting a maximum load of 2.9 kN with a gradual failure mechanism of hole bearing. Thus, the hole hemming process is shown to be a viable technique for joining materials with very different formability.

Utilisation of fuzzy logic and genetic algorithm to seek optimal corrugated die design for CGP of AZ31 magnesium alloy

ABSTRACT Magnesium alloys are the first choice among other lightweight structural metals like aluminium, titanium and beryllium due to low density and excellent corrosion resistance. However, low slip and cold plastic processing ability limits its applications. Constrained groove pressing (CGP), one among the SPD techniques, is well suited for improving the material properties. Corrugated dies are designed to investigate the deformation behaviour of AZ31 Mg alloy samples. To minimise the number of numerical simulations, Taguchi’s L9 orthogonal array is selected for the grooved die dimensions (viz. groove angle, groove width and coefficient of friction). Elasto-plastic finite element analysis is performed by implementing a multi-criterion-based genetic algorithm optimisation tool for obtaining the optimal die geometry. Reduction in total deformation, increase in the equivalent stress and elastic strain of AZ31 sheet were possible with 50° groove angle, 3 mm groove width and 0.22 friction coefficient. Mamdani-based fuzzy logic soft computing tool is implemented for the model to examine the deformation behaviour of AZ31 Mg alloy. 7.86 % deviation is observed in the fuzzy logic model predictions in comparison with simulation results. The suggested die design is well suitable for multiple CGP passes, to improve the grain refinement in the processed sheet.

High strength-ductility synergy and low anisotropy of Mg-4.5Al-2.6Zn-0.3Mn alloy sheet achieved by annealing after rolling

• The integration of extrusion, rolling and annealing was designed and applied. • Shear bands and twins dominate after rolling with cold rolls. • Shear bands provide nucleation sites for recrystallization during annealing. • The final sheet exhibits high strength-ductility synergy and low anisotropy. • The Mg-4.5Al-2.6Zn-0.3Mn sheet owns potential to replace commercial AZ31 sheets. Mg-4.5Al-2.6Zn-0.3Mn sheets with high strength-ductility synergy and low anisotropy were fabricated by the integration of extrusion, rolling, and annealing, which act as excellent replacements for commercial AZ31 alloys. Finer grains, lower dislocation density, and weaker basal texture endow the as-annealed sheets, instead of the as-rolled ones, with the higher performance.

Effect of Rare Earth and Ca Micro-Alloying on the Microstructure and Anisotropy of AZ31 Magnesium Alloy Sheets

AZ31 magnesium alloy sheets can be applied as 3C electronic product shells. However, AZ31 sheets exhibit obvious anisotropy, which limits their industrial application range. The composition modification of AZ31 is a good choice to improve the anisotropy of the sheets, but the effect of the multi-element micro-alloying on the microstructure and anisotropy of AZ31 alloy sheets is still less investigated. For this aim, the plasticity and anisotropy of AZ31 sheets after adding Ca and rare earth elements such as Y and Gd were investigated in this paper. It is found that, different from the alloying role of single Ca or Ca and Y, the composite addition of 0.2Ca and 0.2Gd can obviously enhance the anisotropy of AZ31 alloy sheets. As a result, the elongation of AZ31 in the rolling direction (RD) direction, 45° direction, and transverse direction (TD) direction is increased from 12.5% to 22%, from 5% to 18.5%, and from 1.5% to 8.5%, respectively. This can be explained as follows: the Al8Mn4Gd phase formed by the addition of Gd consumes a lot of Mn, thus avoiding the formation of brittle phases Al8Mn5 and leading to the improved mechanical properties of the alloy sheets. This study will offer a good solution for enhancing the formability of AZ31 sheets under the condition of not significantly increasing the production costs.

EFFECT OF INITIAL TEMPERATURE ON THE MICROSTRUCTURE AND PROPERTIES OF CRYOGENIC ROLLED AZ31 MAGNESIUM ALLOY

By characterizing the properties of a twin cast-rolled AZ31 sheet after cryogenic rolling, the effects of the initial temperature on the microstructure and mechanical properties of the sheet obtained by cryogenic rolling were studied. The results indicate that the hardness of the alloy was a maximum due to the high-density dislocations in the microstructure when the initial temperature was room temperature. The highest yield strength and elongation appeared at the initial temperature of 200 °C, and the microstructure was mainly fine equiaxed grains. It was observed that fine grains increased, and the yield strength and elongation gradually decreased with the increase of the initial temperature. The overall trend of yield strength and elongation increased first and then decreased. Based on the results of different initial temperature treatments, the optimal microstructure and properties of AZ31 magnesium alloy were acquired when the initial temperature was 200 °C.

Cyclic bending under tension of alloy AZ31 sheets: Influence on elongation-to-fracture and strength

This paper describes the main results from an experimental investigation into the consequence of cyclic bending under tension on elongation-to-fracture (ETF) and strength of AZ31 sheets. The deformation is imparted on test specimens of the alloy using a recently built continuous-bending-under-tension (CBT) apparatus conceived to increase ETF relative to simple tension (ST). The apparatus pulls the specimen in tension with a certain velocity while the specimen reciprocates through a set of three rollers, which impart a given amount of bending. The parameter space consisting of bending depth, crosshead velocity, and sheet thickness is explored to achieve the greatest ETF for the alloy. However, only moderate improvements in ETF are obtained. The small improvements are attributed to the relatively uniform elongation of the alloy during both CBT and ST with a small amount of the remaining ductility to deplete throughout the sheet by CBT after exhausting the uniform elongation. Measurements of the grain structure and texture evolution using electron-backscattered diffraction and neutron diffraction reveal slip dominated deformation with some twinning followed by de-twinning. The behavior of the alloy upon CBT processing to a certain number of CBT cycles followed by heat treatments (HT) is also investigated to achieve tradeoffs in strength and ductility. Results and insights from these investigations are presented and discussed. Significantly, it is shown that the strength of the alloy can be increased for over 30% while preserving at least 5% of its ductility.

Investigation into the microstructure, tensile properties and bendability of Mg–Al–Zn/Mg-xGd laminated composite sheets extruded by porthole die

In this work, laminated composite sheets of Mg alloys AZ31 and Mg-xGd are fabricated through co-extrusion using a pothole die. The asymmetric deformation of the dissimilar Mg alloys during the extrusion is analyzed by combining the results of experiments and numerical simulations. The microstructure, tensile properties and bendability of the laminated composite sheets are comparatively investigated. The results show that the sheet AZ31/Mg-1.2Gd obtains a flat profile, uniform composite layers and high bonding strength of 131 MPa because of the relatively symmetric deformation of the Mg alloys AZ31 and Mg-1.2Gd during the co-extrusion. Compared with the sheet AZ31, the bendability of the laminated composite sheet AZ31/Mg-xGd is significantly improved. In addition, when the Mg-xGd alloys, instead of the AZ31 alloy, are placed in the outer layer of the specimens during bending experiments, the laminated composite sheets get higher bendability. With the actions of the weakened texture and activation of non-basal slip in the composite layer containing the Gd element, both the ductility and bendability of sheet AZ31/Mg-1.6Gd reach the highest levels among those of the laminated composite sheet, corresponding to the elongation of 29% and the ultimate bending angle of 101 °, respectively. The analysis of the asymmetric deformation of dissimilar Mg alloys during the extrusion and the discussion of the effects of Gd content on the microstructure, tensile properties, and bendability in this study provides a comprehensive insight into the fabrication of high-formability laminated composite sheets of Mg alloys.

Study on Microstructure and Properties of Single‐Pass Online Heating Rolling of Mn‐Containing Magnesium Alloy Sheets

Mg–2.0Zn–1.5Mn (ZM21) alloy and Mg–1.0Mn–0.5Al (MA10) alloy extruded bars have excellent mechanical properties and low processing cost. Herein, these two alloys sheets are rolled at different temperatures and reductions by online heating rolling. In addition, their mechanical properties and microstructure characteristics are also studied through electron backscattered diffraction. The results show that the yield strength of rolled ZM21 sheets is gradually improved with increasing rolling strain and temperature and further exhibits good comprehensive mechanical properties. The mechanical properties of ZM21 rolled sheets are slightly lower than that of AZ31 rolled sheet and have the potential to substitute the AZ31 sheet. However, MA10 alloy rolled sheet herein exhibits poor mechanical properties, indicating that the mechanical properties of MA10 alloy highly depend on the microstructures resulting from the processing parameters. The mechanism of poor mechanical properties and different mechanical behaviors of extruded bars and extruded sheets are discussed.

Tailoring microstructure and texture of Mg–3Al–1Zn alloy sheets through curve extrusion process for achieving low planar anisotropy

• Symmetric curve extrusion (SCE) and asymmetric curve extrusion (ACE) processes were firstly proposed to fabricate AZ31 Mg alloy sheets. • The SCE and ACE sheets exhibited unique textures which were ascribed to the introduction of additional flow velocity difference along transverse direction during extrusion. • The ACE sheet exhibited the lowest basal pole intensity because of the introduction of extra asymmetric shear deformation during ACE process. Curve extrusion processes, including symmetric curve extrusion (SCE) and asymmetric curve extrusion (ACE) processes, were proposed to fabricate Mg–3Al–1Zn (AZ31) alloy sheets with the aim of improving planar anisotropy. For comparison, traditional extrusion (TE) was conducted on producing AZ31 sheets. The dynamic recrystallization and flow behavior of AZ31 Mg alloy during extrusion were examined and analyzed, and the final microstructures and mechanical properties of AZ31 sheets were investigated. In comparison with the TE sheet, finer grain size, more uniform microstructure and weaker tilted basal texture were achieved in the SCE and ACE sheets. Both the SCE and ACE sheets exhibited unique textures which basal poles tilted to extrusion direction (ED) and new transverse direction (TD)-tilted texture component was developed. The reasons were mainly ascribed to the introduction of additional flow velocity difference along TD during extrusion. Furthermore, the ACE sheet exhibited the lowest basal pole intensity, which was attributed to the introduction of extra asymmetric shear deformation during extrusion process. As a result, the ACE sheet showed the lowest yield strength and r -value, but highest elongation and n -value. Improving the planar anisotropy of AZ31 sheet was achieved by the ACE process.

Improving the fatigue performance of AZ31 sheet with hole via electromagnetic cold expansion process

Improving the fatigue performance of AZ31 sheet with hole via electromagnetic cold expansion process

Through-Thickness Microstructures and Yield Strength Enhancement for AZ31 Mg Sheets Treated by Ultrasonic Nanocrystal Surface Modification

An ultrasonic nanocrystal surface modification (UNSM) technique was applied to a 1-mm thick AZ31 magnesium sheet. UNSM is a relatively new surface modification technique in which a hard, hemispherical tip (2.38 mm in diameter) strikes the surface at an ultrasonic frequency to induce plastically deformed gradient microstructures and deep compressive residual stresses through the thickness. After the UNSM treatment, the through-thickness microstructures were thoroughly investigated using electron microscopy and electron backscatter diffraction analysis. The through-thickness microstructures revealed zones that were severely deformed (down to 200 µm from the surface) and twin-dominated (200~300 µm deep from the surface). The severely deformed zone consisted of shear banding, grain subdivision and reorientation, due to the strong plastic deformation, accompanied by the formation of {10<math xmlns="http://www.w3.org/1998/Math/MathML"><mover accent='true'><mn>1</mn><mo>−</mo></mover></math>2} tensile twins (despite compressive strikes by the hemispherical tip), {10<math xmlns="http://www.w3.org/1998/Math/MathML"><mover accent='true'><mn>1</mn><mo>−</mo></mover></math>1}-{10<math xmlns="http://www.w3.org/1998/Math/MathML"><mover accent='true'><mn>1</mn><mo>−</mo></mover></math>2} double twins and {10<math xmlns="http://www.w3.org/1998/Math/MathML"><mover accent='true'><mn>1</mn><mo>−</mo></mover></math>1} compression twins. The cause for tensile twinning was examined through a literature survey. In the twin-dominated zone, the twining activity prevailed as the slip activity gradually decayed through the thickness. The UNSM-induced hardness and microstructure enhancement was found to be effective down to about 300~400 μm deep from the surface. Finally, the source of the increase in yield strength after the UNSM treatment of the AZ31 sheet was analyzed, and focused on individual cases of microstructural enhancement in the severely deformed zone and the twin zone, and the compressive residual stress.