Hot-rolled AZ31 Magnesium Alloy Research Articles

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.

Texture modification and mechanical properties of AZ31 magnesium alloy sheet subjected to equal channel angular bending

Analysis of the recently proposed equal channel angular bending (ECAB) process is provided on thin hot-rolled AZ31 magnesium alloy sheets. In particular, effects of deformation temperature and strain path on the texture evolution and mechanical properties are systematically investigated under single pass ECAB at various temperatures and multi-pass ECAB process that involves changes in strain paths. It is found that simultaneous activation of multiple twinning types is successfully introduced during ECAB, which results in obvious tilted component of basal texture. Attributed to the domination of extension twins, weaker basal textures are detected after both single pass ECAB at 150 °C and three cross passes ECAB at 200 °C. After annealing, the basal texture is further weakened via twin-related recrystallization and the annealed microstructure is featured with mixture of basal and non-basal orientated grains. Additionally, the effect of grain orientation on the mode of plastic deformation and the roles of grain orientation and grain boundary on the local strain accommodation are coherently studied. This study reveals that over 60 % increase of uniform elongation with marginal reduction of tensile strength less than 5 % can be achieved for single pass ECAB at 150 °C and three cross passes ECAB at 200 °C, which is the result of larger fraction of grains favored with extension twinning and better local strain accommodation.

Preparation and Effect Analysis of {10–12} Extension Twins at the Edge of AZ31 Magnesium Alloy Plate

The edge samples of AZ31 magnesium alloy plate after hot rolling and annealing were compressed along the rolling direction (RD) under different deformation conditions. Then, the optimal conditions for the generation of {10–12} extension twins and the microtexture evolution of edge samples were studied by using EBSD. The results show that the texture strength of the hot-rolled AZ31 magnesium alloy plate was significantly greater than that of the middle part. When the compressive strain along the RD was small at the edge of the rolled piece at room temperature, a large number of extension twin layers were generated. Grain matrix and twins coexisted. As the strain increased to 8.19%, the grains completely transformed into extension twins. Along with the generation of extension twins, hard-oriented grains were consumed in large quantities and the weakening effect of the basal texture was proportional to the volume fraction of the extension twins, while at high temperature (250 °C), the slip dominated the plastic deformation process.

Texture–Microstructure Correlation in Hot-Rolled AZ31

The paper presents a correlation between microstructure and crystallographic texture in hot-rolled AZ31 magnesium alloy. The microstructure and texture evolution are investigated using optical microscopy, field emission scanning electron microscopy, X-ray diffraction, respectively. The results indicated discontinuous dynamic recrystallization, twinning dynamic recrystallization, and particle stimulated nucleation as the main mechanisms during hot deformation. The average size of grains decreased from 19 to 1 μm as the rolling reduction increased from 0 to 60%. A large area fraction of new grains was observed at 40% thickness reduction. At the beginning of plastic deformation, basal slip and extension twinning increased the intensity of {0001} basal fiber texture. Dynamic recrystallization had a considerable effect on weakening the basal texture. Interestingly, some unusual highly stable components including $$ (01\bar{1}3)[7\bar{8}13],(01\bar{1}3)[\bar{2}110] , $$ and $$ (01\bar{1}3)[14\bar{5}\bar{3}] $$ existed at all deformation conditions and their intensity remained nearly unchanged.

The Effect of Crystal Orientation on Damping Capacity in Rolled AZ31 Magnesium Alloy

The effect of crystal orientation on damping capacity is studied on hot rolled AZ31 magnesium alloy. AZ31 magnesium alloys which was machined out parallel to rolled direction and perpendicular to rolled direction were investigated about the relationship between the initial texture and damping capacity. The specimens are annealed at 623 K and time ranging from 30 to 180 minutes. After heat treatment recrystallization is occurred. Then, average grain size and damping capacity are increased with increasing annealing time. Two types of specimens show different main component of texture and damping capacity respectively. It is found that initial texture was affected on damping capacity.

AZ31 마그네슘 합금의 어닐링 이후 내부마찰 거동

Specimens were machined out from hot-rolled AZ31 magnesium alloy, and deformed at 623K with rolling reduction of 30%. After hot rolling, specimens were annealed at various range of temperature and time. In this study, static recrystallization was occurred during heat treatment, however, variation of main component and intensity of texture was not revealed. The results of microstructure observation, damping capacity test and dislocation mechanism indicated that increasing of damping capacity was caused by grain growth. It means that grain size is effective factor to damping capacity.

Effect of Sn Addition on Corrosion Properties of As-Cast and Hot-Rolled AZ31 Magnesium Alloys

In this study, the effect of Sn addition on corrosion resistance of as-cast and hot rolled AZ31 magnesium alloy was investigated. Sn additions were made by 0.2 wt%, 0.5 wt% and 1 wt%. An electric resistance furnace was used to produce alloys. Hot rolling process was performed at 350 °C by 40% thickness reduction at one rolling pass. Microstructure characterizations were performed by optical (OM) and scanning electron microscope (SEM). Immersion tests and electrochemical analyses were performed to investigate the corrosion resistance of the alloys. A 3.5% NaCl working solution at room temperature was used in both corrosion tests. The results showed that Sn addition decreased the primary dentrite size and restricted the growth of secondary dentritic arm. The as-cast structures transformed to dynamically recrystallized grain structures after hot-rolling process in all the alloys. Corrosion resistance of AZ31 magnesium alloy tended to decrease with Sn addition. This decrease was more clear in homogenized and hot-rolled states while there were some flactuations in as-cast states.

Pyramidal slips in high cycle fatigue deformation of a rolled Mg-3Al-1Zn magnesium alloy

The high cycle fatigue behavior of a hot-rolled AZ31 magnesium alloy were investigated at high frequencies (97.3Hz) and different stress amplitudes (50MPa, 60MPa, 70MPa, 90MPa, 110MPa) by using a tension-compression fatigue test at room temperature. The results show that the stress amplitude significantly influences the activation of fatigue mechanisms. When the stress amplitudes were 50 and 60MPa, which is close to the fatigue strength, only basal dislocations were observed, no obvious twins and <c + a> dislocations can be observed. Grains were refined under high cyclic numbers led by continuous dynamic recrystallization (CDRX). When the stress amplitudes increased to 70, 90 and 110MPa, which were higher than the fatigue strength, few {10–12} tensile twins, {10–11}-{10–12} double twins and <c + a> pyramidal dislocations were observed in the fatigued samples, and no obvious grain refinement was observed, only a small amount of twins in the material. Pyramidal slip is one of the deformation mechanisms in magnesium alloy during high cycle fatigue deformation when the stress amplitude is higher than the fatigue strength.

Study on the paired twinning behavior in a hot rolled AZ31 magnesium alloy via interrupted in situ compression

Continuous {101̅2} twin bands across neighboring grains was observed in a hot rolled AZ31 during the interrupted compression along the rolling direction. The variant selection in paired twins was studied by Schmid factor (SF) and geometric misorientation relationship. The results show that most mated twins possess high rank Schmid factors (SFs), however, they does not share the same twin variant. Furthermore, EBSD analysis indicates that the formation of paired twins is correlated with the orientation relationship between adjacent grains. Based on these observation and analysis, the mechanism of “sequential twinning” nucleation and synergetic growth during deformation are proposed. Above all, this twinning behavior is capable of transmitting the localized shear and releasing stress at the former twin-termination interfaces, which plays an important role in accommodating plastic strain and mediating multiple grains' deformation

A comparative study between uniaxial compression and plane strain compression of Mg–3Al–1Zn alloy using experiments and simulations

Magnesium anisotropic behavior is enhanced by different deformations paths and modes. The mechanical anisotropy during plastic deformation of a Mg–3Al–1Zn magnesium alloy was examined by uniaxial compression (UC) and plane strain compression (or channel-die CD) at room temperature. The mechanical behavior and the observed microtextures were compared with the predictions obtained from different crystal plasticity models. A hot rolled AZ31 magnesium alloy plate with a strong basal texture was stressed along four different directions to initiate different deformation mechanisms. In both UC and CD, the compression along the transverse direction generated mainly {10−12} extension twins, while compression along the normal direction (Z=ND) was initially accommodated by pyramidal 〈c+a〉 slip followed by contraction twinning. During compression at 45° of the normal direction to the transverse direction, both slip and twinning took part in the deformation. The sample compressed in CD along the rolling direction and with the normal direction constrained was deformed by prismatic slip and tension twinning. It was found that basal slip was an important deformation mechanism in all the samples. Simulations were made with different models, and the simulated flow stresses were higher in CD than that in UC, but slightly underestimated in case of uniaxial compression. Modeling also corroborated higher {10−11} compression twinning activity in CD than that in UC, and that compression twinning resulted in a decrease in flow stress in CD. To fit with the experimental flow stresses, it was necessary to take into account an additional hardening caused by twin boundaries. In our model, the {10−12} twin boundaries were estimated to increase the critical resolved shear stress (CRSS) of prismatic slip by about 30%. However, twin induced hardening on pyramidal 〈c+a〉 and basal slip of about 10% is also necessary if contraction twinning do not soften the material.

Role of tension twins on warm deformation behavior of AZ31 Mg alloy

Twinning has been known to play an important role on plastic deformation of wrought magnesium alloys. Role of tension twins on warm deformation has thus been investigated in this study under two different loading conditions, viz. tensile and compressive loading at 250 °C using hot-rolled AZ31 magnesium alloy. Tension twins were observed to form under a compressive loading perpendicular to the c-axis and also under a tensile loading along the c-axis at 250 °C. Various morphologies and volume fractions of tension twins were generated under different loading conditions, but they exhibit different flow curves and work hardening behavior during the warm deformation. The ‘Hall-Petch type hardening effect’ appeared to contribute significantly in an early stage of deformation below the strain of ∼0.1, while texture hardening effect became dominant above the ∼10% strain in RD-C specimen. ND-T specimen was, on the other hand, found to generate tension twins continuously up to fracture, but dislocation slip became dominant with increasing strain above ∼10%. Twins were also found to affect dynamic re-crystallization behavior during warm deformation.

Hot tensile deformation and fracture behaviors of AZ31 magnesium alloy

The hot tensile deformation and fracture behaviors of the hot-rolled AZ31 magnesium alloy were studied by uniaxial tensile tests with the temperature range of 523–723K and strain rate range of 0.05–0.0005s−1. Effects of deformation parameters on the strain hardening rate, strain rate sensitivity, microstructural evolution and fracture morphology were discussed. The results show that: (1) The flow curves show a considerable strain hardening stage and no obvious diffuse necking stage under the relatively low temperatures (523 and 573K). (2) The elongation to fracture increases with the increase of the deformation temperature. But, the sharp drop of the elongation to fracture under 723K and 0.0005s−1 results from the synthetical effects of the grain growth, inverse eutectic melting reaction (α+β=L) and the incipient melting of α matrix. (3) For the case with the deformation temperature of 623K and relatively low strain rates, the fracture mechanism is the combination of the void coalescence and intergranular fracture. (4) Under the deformation temperature of 723K, the fine recrystallized grains experienced a rapid growth and the deformation mechanism is the dislocation creep with the help of inverse eutectic liquid phase. (5) The presence of proper amount of liquid phase on the grain boundary changes the deformation mechanisms, and makes great contribution to the high ductility. However, it will deteriorate the material ductility if the amount of liquid phase is too much.

Strengthening and toughening of magnesium alloy by {1 0 −1 2} extension twins

The mechanisms of strengthening and toughening of hot-rolled AZ31 magnesium alloy by {1 0 −1 2} extension twins during strain path changed recompression are studied in the current paper. Under the favorable orientation, {1 0 −1 2} extension twinning can occur in {1 0 −1 2} twins generated by precompression along the rolling direction. Grain refinement by extension twins dramatically enhances both yield stress and peak stress of reloading along the transverse direction without any degradation in elongation, while the effect is closely related with the level of prestrain.

Plastic anisotropy and fracture behavior of AZ31 magnesium alloy

The effects of texture and abnormal large grains on the plastic anisotropy and fracture behavior of hot-rolled AZ31 magnesium alloy were investigated. Uniaxial tensile deformation behaviors of samples with tensile axis tilting 0°, 15°, 30°, 45°, 60°, 75° and 90° to normal direction (ND) respectively were addressed. Tensile deformation anisotropy was observed for samples with different angles to ND. The results show that the specimens with the angle from 0° to 30° exhibit relatively lower yielding strength due to the extension twinning. However, basal slip and prismatic slip are the dominant deformation modes for the specimens with angles larger than 45°. Macro-fractures are parallel to the length direction of abnormal large grains in the specimens with angles less than 60°, while those are serrated fracture edge for specimens with angles 75° and 90°.

Grain growth kinetics of a fine-grained AZ31 magnesium alloy produced by hot rolling

Abstract A study on the kinetics of grain growth of a fine-grained, hot-rolled AZ31 magnesium alloy was carried out by isothermal and isochronal annealing treatments. The grain size of the as-rolled and subsequently annealed sheet samples was determined by metallographic observation using the linear intercept method. It was found that for annealing at temperature in the range of 523–723 K, the effect of annealing temperature, T, and time, t, on the grain growth could be well interpreted by the kinetic equation, D n − D 0 n = k t , where k = k 0 e − E g / R T . Based on the experimental data on grain growth obtained by annealing treatments, the grain growth exponent, n, and the activation energy, Eg, were figured out, and these grain growth kinetic parameters were compared with those for other magnesium alloys reported in recent literatures.

Effect of the reduction ratio per pass on the microstructure of a hot-rolled AZ31 magnesium alloy sheet

The effect of the reduction ratio per pass on the microstructural evolution of an AZ31 gravity cast plate during hot rolling was evaluated systematically. {10\( \bar 1 \)2} extension twinning was observed in large grains after an earlier rolling pass, but it disappeared upon a subsequent rolling pass due to the reorientation of the grains in an unfavorable direction caused by repetitive rolling. The finer grains were newly formed at the boundaries of the original large grains and twin boundaries by dynamic recrystallization. The grain size distribution was changed from a bimodal distribution upon a small total reduction to a homogeneous distribution upon a large total reduction due to the formation of finer grains that continued at the grain boundaries of large grains by repetitive hot rolling. The size and distribution of the grains were affected not only by the reduction ratio per pass but also by the total reduction (number of rolling passes).

Study of the Effect of Microstructure on the Anisotropy of AZ31 Magnesium Alloy Thin Sheet

Stronger (0001) basal plane textures mainly include two types: and in AZ31 magnesium alloy thin sheet were formed after hot-rolling. The texture types of hot-rolled AZ31 magnesium alloy thin sheet after annealing at 523K and 673K respectively were as same as that of hot-rolled thin sheet without annealing, but texture intensities became weaker, especially after annealing at 673K. The strong texture of hot-rolled thin sheet caused anisotropy of mechanical properties (tensile strength b, yield strength s and elongation ) significantly , and the anisotropy reduced with the decrease of texture intensity after annealing. Besides texture, the grain shape also effects anisotropy of mechanical properties. The anisotropy becames more significantly with the increase of relative difference of grain diameter between transverse and longitudinal directions.

The room temperature mechanical properties of hot-rolled AZ31 magnesium alloy

A small specimen test technique was employed to evaluate the mechanical properties of a set of hot-rolled AZ31 magnesium alloy at room temperature. The effect of equivalent rolling strain and temperature on the room temperature mechanical behavior of experimental alloy was studied using the shear punch testing (SPT) method. To verify the latter the standard uniaxial tension tests were also carried out. The results indicated that the room temperature strength and ductility of rolled AZ31 alloy is increased by decreasing the rolling temperature and increasing the equivalent rolling strain. In addition, the current work explores the existence of a linear relationship between shear punch test data and the universal tensile strength and elongation.