Hot-rolled AZ31 Research Articles

Investigating the corrosion mechanism of AZ31 alloy bimodal microstructure by the quasi-in-situ EBSD characterization and DFT calculation

The grain characteristic and orientation were analyzed using the electron backscatter diffraction (EBSD) for AZ31 hot-rolled alloys. The corrosion initiation of the bimodal microstructure is revealed for the hot-rolled AZ31 alloys obtained by the quasi-in-situ EBSD, showing the preferential corrosion in the fine grain region. Meanwhile, the non-basal crystal plane grain is easier to corrode than the basal crystal plane grain. The adsorption energies of H2O molecule on the crystal planes of magnesium matrix were calculated by the density functional theory method, and a new insight into the corrosion anisotropy of crystal planes was proposed from the surface adsorption energy.

Mechanical anisotropy induced by the competition between twinning and basal slip of AZ31 magnesium alloy under biaxial tension

The mechanical behavior of a hot-rolled Mg AZ31 plate was studied under biaxial tensions along two perpendicular loading axes of x and y in the ND-TD plane, with the x-axis aligned at a 45° counterclockwise angle from the TD (ND and TD refer to the normal direction and transverse direction). A complex strain partitioning in two loading axes is observed with the variation of stress ratio σxx:σyy. For σxx:σyy= 1:1 and 1:1.5, the strains along the x- and y-axes, εxx and εyy, are both positive. However, With the stress ratio of σxx:σyy= 1:3, εyy remains positive, while εxx becomes negative. Interestingly, for σxx:σyy = 1:2, εxx changes from negative during the early stage of loading to positive upon failure. Crystal plasticity simulations and theoretical calculations reveal that the observed strain transition. The reasons for the strain transition with stress ratio is related to the competition between {101¯2} twinning and basal slip. For {101¯2} twinning, the accommodated strains by most of the twins are εxx>0 and εyy>0 for all the stress ratios considered. For basal slip, most of the basal variants accommodate a positive y-axis strain, while for loading cases with low stress ratios, most of the basal variants result in a negative x-axis strain. As the stress ratio decreases, basal slip becomes more active, while the activity of {101¯2} twinning gradually decreases, leading to the observed strain transition. For σxx:σyy = 1:2, the transition from negative to positive is related to the increasing activity of {101¯2} twinning with strain. Theoretical calculations reveal that the competition of basal slip and {101¯2} twinning under biaxial tension is determined by the variation of their Schmid factors with σxx:σyy.

Effect of annealing treatment of hot-rolled AZ31 magnesium alloy on properties and stress corrosion resistance of MAO coatings

PurposeThis study aims to study the formation mechanism of micro-arc oxidation (MAO) coating on AZ31 magnesium alloy and how the annealing process affects its corrosion resistance.Design/methodology/approachThis study involved immersion experiments, electrochemical experiments and slow strain rate tensile experiments, along with scanning electron microscopy, optical microscopy observation and X-ray diffraction analysis.FindingsThe findings suggest that annealing treatment can refine the grain size of AZ31 magnesium alloy to an average of 6.9 µm at 300°C. The change in grain size leads to a change in conductivity, which affects the performance of MAO coatings. The MAO coating obtained by annealing the substrate at 300°C has smaller pores and porosity, resulting in better adhesion and wear resistance.Originality/valueThe coating acts as a barrier to prevent corrosive substances from entering the substrate. However, the smaller pores and porosity reduce the channels for the corrosive solution to pass through the coating. When the coating cracks or falls off, the corrosive medium and substrate come into direct contact. Smaller and uniform grains have better corrosion resistance.

Study on anisotropic mechanical behavior of AZ31 magnesium alloys under free-end torsion

In this study, the mechanical behavior and microstructure of a hot-rolled AZ31 magnesium alloy sheet under free-end torsion with torsional axis along rolling direction (RD), normal direction (ND) and 45° in ND-RD plane, have been investigated both experimentally and numerically. The mechanical behavior under torsion, including stress-strain response, Swift effect and strain hardening rate, present obvious anisotropy. The Swift effect is sensitive to the direction of torsion axis and exhibits remarkable anisotropy with axial elongation in ND sample and axial shortening in RD and 45° samples. The axial elongation or shortening is determined by both slips and extension twin, with extension twin exerting a greater influence on the extent of the axial deformation. During shear straining, the dominant deformation mode transforms basal slip to prismatic slip with the axial direction deviating from ND to RD, while the second-order pyramidal ⟨c+a⟩ slip is nearly not activated during whole deformation attributed to its high critical resolved shear stress. Based on the effective Schmid factor (ESF) of deformation modes, the grains with favored orientation would get activation of {101‾2} extension twin due to the high ESF, resulting in the different twin volume fractions. Additionally, the activation of twin relative to basal slip has a connection with the increasing trend of normalized strain hardening rate.

A Study of {10-12} Twinning Activity Associated with Stress State in Mg-3Al-1Zn Alloy during Compression

An eight-sided prism sample, obtained from a hot-rolled AZ31 magnesium alloy sheet, was compressed at room temperature along the transverse direction to investigate the influence of local strain on twinning behavior using electron backscatter diffraction (EBSD) measurements, hardness distribution, and metallographic observations. The octagonal surface of the sample was divided into distinct regions based on hardness distribution and metallographic observations. Combined analysis of the Schmid factor (SF) and the strain compatibility factor (m’) was employed to study twin variant selection. Basal on SF ratio distribution, the Schmid factor criterion, can predict over 75% of observed twin variants in regions A and D (normal stress samples). In contrast, 64% of twin variant selection behavior in region C (shear stress sample) can be effectively explained using a pure shear model. Twin variants with high strain compatibility factors may prefer activation to reduce stress concentration. The strain compatibility factor is more appropriate than the Schmid factor for analyzing the effect of local strain on the selection behavior of twin variants.

Influence of Gd and Li additions on the microstructural evolution and mechanical properties of hot-rolled AZ31 alloy

Thermomechanical processing and alloying are two common approaches to enhance the mechanical properties of Mg. Thus, the effects of Gd and Li elements, as two elements with various properties, on the microstructure and mechanical properties of a hot-rolled and consequently annealed AZ31 alloy were analyzed. The microstructure and texture was analyzed by scanning electron microscopy and electron back scattered diffraction. Accordingly, the mean grain size of 46 μm in the AZ31 alloy was reduced to 41 and 26 μm in the AZ31–2.9Gd and AZ31–2.3Li alloys, respectively. Grain refinement occurred due to continuous dynamic recrystallization and twinning-induced recrystallization in all of the alloys. Textural evolution showed that neither Gd nor Li could inhibit the occurrence of basal texture. Room temperature mechanical properties were assessed by using the shear punch test (SPT) method. The results exhibited that the strength of the AZ31 alloy was improved from 116 MPa to 139 MPa and 141 MPa by respective additions of Gd and Li. Strength enhancement was mainly attributed to the grain boundary strengthening and second-phase particles hardening. One promising observation was the simultaneous enhancement of shear strength and ductility by the addition of Gd and Li. This observation was ascribed to the more uniform distribution of second-phase particles and lower volume fraction of twins as potential sites for crack initiation. As long as mechanical strength is concerned, results favor the use of Li as alloying element in Mg, especially due to the more noticeable lightweighting potential compared to Gd.

Dynamic recrystallization and twinning behavior of magnesium alloy during hot tension

In this study, {10–12} tensile twins (TTs) were induced in hot-rolled AZ31 Mg sheets via hot tension along the normal direction. The results demonstrate that the volume fraction of TTs at 2 mm/min is greatly lower than their counterparts at 0.5 mm/min at 100 °C and 200 °C. Dynamic recrystallization (DRX) can decrease twin nucleation rate during hot tensile when the strain rate reaches 2 mm/min. Decrease in critical resolved shear stress of non-basal slips at 200 °C enhances the deformation capability among grains, which dramatically reduces the volume fraction of TTs. Hence, TTs are hardly observed in 200°C-2mm/min sample. Tensile temperature significantly affects the morphology of TT lamellae. Grain boundaries prevents the longitudinal growth of the TTs, and then the thickness of TTs begins to grow to coordinate the deformation, resulting in fine TT lamellae in 100°C-0.5 mm/min. Compared with AR, the basal texture intensity of 100°C-2mm/min and 200°C-2mm/min increases and the occurrence of DRX is the main reason, while basal texture weakening of 100°C-0.5 mm/min and 200°C-0.5 mm/min depends on the formation of TTs. Compared with samples at 200 °C, the samples at 100 °C have higher tensile stresses. The maximum stress of 100°C-0.5 mm/min is 64.97 MPa when tensile strain reaches 0.15 owing to its highest dislocation density. 200°C-2mm/min possesses the highest hardness among the four tension samples, which is associated with the largest fraction of DRXed grains (68.8%) and its refinement strengthening effect. Dislocation strengthening exerts more influence on the hardness of 100°C-0.5 mm/min.

Effect of Ca Precipitation on Texture Component Development in AZ Magnesium Alloy.

To enhance the formability of magnesium alloys, inhibition of basal texture development by the particle-stimulated nucleation (PSN) effect has attracted significant interest. However, its contribution to texture development is not easily observed due to the separation of texture from the conventional deformation behavior. This study aims to separate the Ca texture from the deformation behavior of AZX611 alloy and quantify it using scanning electron microscopy with electron backscatter diffraction (SEM-EBSD). Since Ca in the AZ61 magnesium alloy precipitated as Al2Ca, the hot-rolled magnesium alloys AZ31, AZ61, and AZX611 were used. High temperature compression was conducted at 723 K, the strain rate 0.05/s and 0.005/s and the true strain up to −1.0. Dynamic recrystallization was observed in each specimen and the Ca-free alloys showed dislocation glide at high strain rates and solute drag at low strain rates. When the dislocation glide dominated, basal texture was strengthened. In contrast, solute drag caused non-basal texture development. Precipitation hardening caused AZ61 to have higher flow stress than those of the Ca-free alloys by the PSN effect; its texture was observed separately because the PSN grain growth around the precipitation and orientation was specific, similar to the one developed at the solute atom drag.

Investigation on microstructure and mechanical properties of hot-rolled AZ31 Mg alloy with various cryogenic treatments

AZ31 Mg alloy sheet, prepared by a novel process combining hot rolling and various cryogenic treatments, has been investigated on its microstructure and mechanical properties. The results show that twins and the Al8Mn5 phase were introduced in the sample with rapid cooling and rapid heating and slow heating and slow cooling, which can significantly impede the movement of dislocations, while twins also can effectively coordinate deformation. Compared to the conventional hot rolling sample, the average grain size and texture strength of the cryogenic treatment samples were refined and increased by ∼24% and∼50%, respectively. The dislocation density of the coarse-grained region in the sample with slow heating and slow cooling is low, and the prismatic <a> and pyramid <c+a> slips were activated, which can improve the ductility of the alloy. In comparison to conventional hot rolling, the ultimate tensile strength and elongation of AZ31 Mg alloy after rapid cooling and rapid heating were increased by ∼15% and ∼37%, respectively, while the elongation after slow heating and slow cooling increased by ∼71%. The improvement of the mechanical properties of Mg alloy with cryogenic treatment is attributed to the synergistic effect of the refined grain, twins, residual dislocations, precipitates, texture and non-basal slip.

Evaluation of static recrystallization and grain growth kinetics of hot-rolled AZ31 alloy

The static recrystallization/grain growth kinetics of the AZ31 (Mg-3Al-1Zn, wt%) alloy were investigated employing Vickers microhardness and electron backscatter diffraction (EBSD) measurements. The AZ31 alloy was subject to a hot-rolling for 70% of thickness reduction and then annealed at various temperatures (150°C, 250°C, and 350°C) from 5 min to 24 h. First, the static recrystallization kinetics were analysed by means of the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model. The results showed that the recrystallization occurred under two regimes both involving their own Avrami exponent/ activation energy. In regime I, the Avrami exponent was found in the range of 1.5-0.35 depending on the annealing temperature and activation energy of 74.1±5.7 kJ×mol-1. In regime II, an identical value of Avrami exponent was found (0.1-0.2) and a very low activation energy of 14.8±0.7 kJ×mol-1 was found for all annealing conditions. Non-random nucleation sites such as shear bands were considered as the main factor responsible for the deviation from the JMAK model. Moreover, the grain growth kinetics was well fitted by the general equation where . Accordingly, Qg = 109± 0.2 kJ×mol-1 which is median between grain boundary diffusion and bulk diffusion values for Mg and its alloys. The derived activation energies were discussed in terms of influencing factors such as solute drag, formation of basal texture, and microstructural heterogeneities like shear bands and twinning.

Additive manufacturing of high performance AZ31 magnesium alloy with full equiaxed grains: Microstructure, mechanical property, and electromechanical corrosion performance

• AZ31 alloy with full of equiaxed grains was obtained by GTAW-based additive manufacturing. • Relationship between the microstructure and properties of AMed AZ31 alloy were studied. • Comparison between the corrosion resistance of hot-rolled and AMed AZ31 alloy was conducted. Additive manufacturing (AM) technology has drawn tremendous attention in producing lightweight and complex metallic components. In this work, a thin wall of AZ31 Mg alloy with fine equiaxed grains was fabricated via gas tungsten arc-based wire arc additive manufacturing (WAAM-GTA) technology. The grain structure, dislocation, and texture of the hot-rolled and WAAM-GTA AZ31 samples were analyzed by EBSD, while the matrix and dispersed phases were characterized by EDS and X-ray diffraction. The 3D porosity of samples was examined quantitatively by an X-ray computed tomography (XCT). The results show that dominated equiaxed α-Mg grain structure was obtained together with negligible precipitated phase. The mechanical properties of WAAM-GTA AZ31 were superior to their cast counterparts and close to those of wrought AZ31, and their correlation to microstructure and defects were explored. Moreover, the WAAM-GTA AZ31 showed excellent electromechanical corrosion performance in 3.5 % NaCl solution as compared to the hot-rolled AZ31 plate. The WAAM-GTA technology therefore offers new routes to fabricate AZ31 Mg components with equiaxed grain structure, as well as favorable mechanical properties and electromechanical corrosion performance.

Effects of annealing treatment on the microstructure and corrosion behavior of hot rolled AZ31 Mg alloy

The annealing treatments were conducted at various temperatures and time using hot rolled AZ31 Mg sheets. The effects of annealing temperature and time on the microstructure and corrosion rate were investigated. The rolled AZ31 Mg consisted of coarse deformed grains with high stored energy, deformation twins, and recrystallized grains with free strain, showing the highest corrosion rate of 10.3 mm/year. The annealing effectively slowed down the corrosion rate of AZ31 Mg by increasing the grain size and decreasing the numbers of dislocations, twins and second phase particles. A high annealing temperature of 450 °C and a long holding time of 5 h further reduced the corrosion rate of AZ31 Mg to quite low level of 3.5 mm/year.

Effect of hot rolling on microstructure, crystallographic texture, and hardness of AZ31 alloy

In this study, relationships between microstructure, texture, and hardness of hot-rolled AZ31 alloy were investigated. It was found that the grain size decreased into the ultra-fine domain after 60% hot deformation, reaching a grain size of 1 μm. Several dynamic recrystallization (DRX) mechanisms activated after 40% and 60% deformations including discontinuous dynamic recrystallization (DDRX), twin dynamic recrystallization (TDRX), continuous dynamic recrystallization (CDRX), and particle stimulated nucleation (PSN). With increasing the thickness reduction, the intensity of basal texture first increased, then slightly decreased, and finally significantly increased to 15.7 multiple of random density. The average misorientation angle decreased when the thickness reduction increased owing to the formation of a very strong {0001} basal texture. With increasing the strains, the average microhardness increased because of decreasing grain size and increasing twin fraction and shear band density. There were no differences between the hardness values of RD–TD and RD–ND planes for AZ31 samples after 10% and 40% hot rolling and these samples were isotropic. After 60% deformation, the hardness of the RD–TD plane (73.5 HB) was higher than that of the RD–ND one (68.4 HB) that resulted in hardness anisotropy due to the generation of very strong basal texture.

Modelling dynamic recrystallisation in magnesium alloy AZ31

In this paper, a dynamic recrystallisation (DRX) viscoplastic self-consistent (VPSC) model is created to predict the continuous DRX (CDRX) and twin-assisted DRX (TDRX) behaviour within an AZ31 magnesium (Mg) alloy over temperatures ranging from 25 °C to 200 °C. At this temperature range, the active deformation mechanisms and physical phenomena participating in the strain accommodation and texture evolution of Mg alloys are: a basal slip, a prismatic slip, c+a pyramidal slip, {101‾2} tensile twinning, and DRX. In order to predict which deformation modes are activated for this particular alloy and their influence on the DRX mechanism, a temperature sensitive VPSC model combined with a dislocation density hardening law and a composite twin grain model is used. The model is validated against a comprehensive set of experimental data including flow curves and texture evolution, realised on a hot-rolled AZ31 Mg alloy deformed at different temperatures between 25 °C and 200 °C under uniaxial tensile loads parallel to, perpendicular to, and at 45° offset from the rolling direction of the processed material. Furthermore, a parametric study into parameters such as grain size, dislocation density, and crystallographic orientation of the nucleating DRX grains influencing the DRX kinetics and, subsequently, the plastic material behaviour of the alloy is carried out and discussed.

On the evolution of microstructure, texture and corrosion behavior of a hot-rolled and annealed AZ31 alloy

The microstructure and texture evolution of an AZ31 alloy were investigated after hot rolling and subsequent annealing using electron backscatter diffraction (EBSD). First, the alloy was hot-rolled at 350 °C up to low, medium and high strain (20, 50 and 85% of thickness reduction, respectively). The alloy samples where then annealed at 350 °C for 2, 10 and 60 min. The effect of strain level and annealing on corrosion behavior in seawater was also evaluated using electrochemical tests. At low strain, the microstructure was characterised by the absence of twinning, mainly due to the prior thermo-mechanical history of the as-received alloy. However, various modes of twinning were observed at medium strain. At high strain, the dynamic recrystallization process resulted in a microstructure with a typical basal texture. The results demonstrate that twins are responsible for the deviation of {0002} basal poles from normal towards the transversal direction. Annealing at 350 °C for up to 60 min led to normal grain growth in all the samples. In medium and highly strained samples, the deformation texture was retained, while the low strain sample underwent noticeable changes due to the absence of dynamic recrystallization. A synergetic effect of grain refinement and texture weakening was responsible for the alloy's enhanced corrosion resistance.

Fracture micro-mechanisms in hot-rolled AZ31 and AZ31-Sr magnesium alloys

Micro specimens of hot-rolled AZ31 and AZ31 + 0.8 wt% Sr magnesium alloys were tensioned in the rolling direction, and fracture micro-mechanisms were studied by in-situ real-time optical observations, electron microscopy, and DIC analysis. In-situ observations indicated that strain localization on the microstructural shear bands developed during tension was responsible for the fracture of the AZ31 specimens. DIC analysis demonstrated a substantial strain heterogeneity in the AZ31 sample, with the most intense concentration on the tension-induced shear bands. Neither rolling nor tension shear bands were observed in AZ31-Sr alloy. Instead, many cracks were formed on the specimen surface, whose propagation led to the final fracture. Compared to the AZ31 alloy, the Sr-containing AZ31 alloy experienced a more uniform strain distribution during deformation. Most of the twins inherited from the rolling process were perpendicular to the rolling direction, which were identified as the source of crack initiation, especially in the Sr-added alloy. Microstructural evidence showed that the contribution of precipitates in damage of Sr-added AZ31 alloy was higher than that in AZ31 alloy. However, cracks were mostly initiated from the twin boundaries, not the precipitate stringers. The variations of plastic anisotropy coefficient (R-value) were described as a function of applied strain using DIC analysis. It was found that incorporation of Sr into AZ31 alloy reduced its pronounced anisotropy.

Twinning and recrystallization behavior of AZ31 Mg-rolled sheets during the corrugated wide limit alignment process

In this study, tensile twins (TTs) were induced in hot-rolled AZ31 Mg sheets via the corrugated wide limit alignment (CWLA) process. The microstructure evolution demonstrated that, compared to the as-received AZ31 Mg alloy sample, the twinned sample exhibited a rolling direction-tilted texture with {101‾2} TT boundaries in the 15–23% range. Moreover, distinct twin variants appeared in the matrix grains. Dynamic recrystallization (DRX) occurred during CWLA treatment when the temperature reached 300 °C, and a refined microstructure was formed. Discontinuous DRX was the primary mechanism observed during the CWLA process, and twin-assisted DRX was also initiated in the deformation stage. After CWLA deformation, the formability was improved. The 0.2% yield strength decreased by 46 MPa, the ultimate tensile strength increased by 30 MPa, and the fracture elongation remained the same as that of the annealed as-received samples, all of which resulted in enhanced formability of the alloy sheets.

Significant disparity of non-basal dislocation activities in hot-rolled highly-textured Mg and Mg-3Al-1Zn alloy under tension

It is well-established that Mg-3Al-1Zn (AZ31) alloy exhibits much better tensile ductility than pure Mg. However, the underlying mechanisms for such difference still remain relatively unexplored at the dislocation level. In this work, we deformed hot-rolled highly-textured pure Mg and AZ31 samples with similar initial microstructures (i.e., grain size and texture) under tension along the rolling direction. Apparent shear banding was only observed in the pure Mg samples from the early stage of the deformation. Shear bands could act as easy paths for crack propagation, leading to low ductility in pure Mg. Apparent shear banding was not noted in deformed AZ31 samples. We then investigated the deformation mechanisms at the dislocation level using transmission electron microscopy. Systematic tilting experiments and statistical analyses of multiple grains at different strain levels revealed a significant disparity of non-basal dislocation activities between pure Mg and AZ31. For pure Mg, <c + a> dislocations were activated since the early stage of plastic deformation. For AZ31, <c + a> dislocations were mostly absent at all strain levels, even in the strain-to-failure samples. Non-basal <a> dislocations, including prismatic and pyramidal <a> dislocations, were observed. The promotion of the non-basal <a> dislocation activities and the suppression of <c + a> dislocations in AZ31 are expected to offer more sustainable hardening, which could elucidate the absence of apparent shear banding and much-improved ductility in AZ31 compared to pure Mg.

Microstructure and Mechanical Properties of Hot-Rolled AZ31 Mg Alloy Subjected to Cryogenic Treatment After Air Cooling

Microstructure and Mechanical Properties of Hot-Rolled AZ31 Mg Alloy Subjected to Cryogenic Treatment After Air Cooling

4D X-ray tomography characterization of void nucleation and growth during deformation of strontium-added AZ31 alloys

We report the influence of strontium on the evolution of microstructural damage during tensile deformation of hot-rolled AZ31 alloys tested parallel to the rolling, transverse, and diagonal directions using a 4D (3D X-ray tomography plus time) technique. X-ray computed tomography images were collected at several strains during each tensile test, and again after the fracture had occurred, to quantify the nucleation, growth, and coalescence of voids. Adding strontium leads to the formation of Al4Sr particles in the microstructure. Cracking of these particles provides the primary micromechanism of damage nucleation. Increasing strontium content increases the size and number of Al4Sr particles, thereby increasing the number of potential sites for void nucleation. Despite this, the ductility of AZ31, as well as its strength, when tested in all three directions, is enhanced by adding Sr. Nanoindentation tests confirm that this is accomplished by reducing the matrix strength through a decreasing role of aluminum solid solution hardening, which is more than compensated by an increase in precipitation strengthening. Fracture occurs through the interactions of shear bands with second-phase particles. Shear fracture is dominant on the fracture surfaces of the strontium-free alloys, while void growth induced dimples are found in much greater number on the fracture surfaces of the strontium-added alloys. Void growth is accurately described by optimization of the Rice-Tracey model using regression analysis. This study explains why strontium additions to AZ31 can be used to improve both strength and ductility by controlling the evolution of damage during deformation.