Effect of initial texture on dynamic recrystallization of AZ31 Mg alloy during hot rolling

Wedge-shaped AZ31 plates with two kinds of initial textures were rolled at 573 K to investigate the effect of initial texture on dynamic recrystallization (DRX). The results indicated that the initiation and nucleation of DRX were closely related to the initial texture. The initiation and completion of DRX in the TD-plate were significantly retarded compared with that in the ND-plate. Twin related DRX nucleation was mainly observed in the ND-plate samples; while gain boundary related DRX nucleation was mainly observed in the TD-plate samples. The different DRX behavior between the TD- and ND-plates was attributed to the different deformation mechanism occurring before DRX initiation. For the ND-plate, dislocation glide was considered as the main deformation mechanism accompanied with {1 0 −1 1}-{1 0 −1 2} double twin, which led to the increment of a faster increasing stored energy within the grains. And {1 0 −1 1}-{1 0 −1 2} double twin was mainly found to be DRX nucleation site for the ND-plate. For the TD-plate, {1 0 −1 2} extension twin was the dominant deformation mechanism which resulted in a basal texture with the c-axis nearly parallel to ND. The stored energy caused by dislocation motion was relatively small in the TD-plate before a basal texture was formed, which was considered as the main reason of that DRX was retarded in the TD-plate compared with that in the ND-plate. Based on the difference in deformation mechanism and DRX mechanism caused by the different initial texture, the variation in grain size, micro-texture and misorientation angle distribution in the ND and TD plates were discussed.     

铸态AZ31镁合金板材等温轧制工艺及组织性能研究

为研究铸态AZ31镁合金轧制工艺及轧制后组织性能,通过试验得到不同道次和变形量对铸态AZ31镁合金板材显微组织和力学性能的影响规律,并采用扫描电子显微镜研究了轧制后板材组织.结果表明,铸态AZ31镁合金板材经等温4道次、等变形量轧制后,板材厚度由20mm变化到4.8 mm,抗拉强度和屈服强度分别达到275 MPa和18...     

A numerical and experimental investigation of AZ31 formability at elevated temperatures using a constant strain rate test

The research activity reported in the present paper aims to evaluate the Forming Limit Curves (FLC) of the Mg alloy AZ31 in warm conditions (200 °C) while keeping the equivalent strain rate constant. Specific tools to carry out such a formability test were designed and created: a flat punch (in line with Marciniak’s test) embedding a heating system was adopted in order to heat the central part of the specimen both rapidly and uniformly, where ruptures were forced due to the presence of a driving sheet between the specimen and the punch. A Digital Image Correlation system was also embedded in the formability equipment in order to acquire major and minor strains continuously and evaluate the moment and location of failures. Finite Element simulations were run in order to define punch speed profiles (which differ according to the geometry of the specimen) that were able to keep a constant equivalent strain rate in the region where ruptures were forced. Experimental tests implementing the punch speed profiles were carried out in order to obtain temperature, load and strain data. FLCs at two different strain rate levels (0.02 s⁻¹ and 0.002 s⁻¹) both confirmed and allowed us to quantify the noticeable strain rate effect of such an alloy on the FLC at a temperature of 200 °C. The proposed approach for FLC evaluation is effective for materials whose properties are strongly influenced by the strain rate. Such FLC data can be usefully implemented in numerical simulations of sheet metal forming processes: while tensile tests can be used to determine variations in mechanical behaviour according to the strain rate, the FLCs evaluated in this work allow us to determine the occurrence of strain path-dependent critical conditions according to the strain rate.     

热挤压AZ31镁合金的组织结构与蠕变行为

通过对热挤压态AZ31镁合金进行组织形貌观察、内摩擦应力测定及蠕变性能测试,研究了热挤压AZ31合金的组织结构和蠕变行为.结果表明:热挤压AZ31镁合金的组织具有带状结构特征,并沿轧制方向分布,且有β-Mg17Al12相在合金中弥散析出.蠕变期间,位错运动的内摩擦力有较强的温度敏感性,随温度增加,内应力值明显降低,致使合金具有较高的蠕变速率.合金在蠕变期间,大量位错的形成与运动是蠕变初期的变形机制;蠕变稳态阶段,高密度位错逐渐束集形成位错胞,进一步发生蠕变期间的动态再结晶.随裂纹在晶界处萌生使蠕变进入第三阶段,而裂纹沿晶界韧性撕裂扩展是合金的蠕变断裂机制.     

Microstructure evolution and texture development during high-temperature uniaxial compression of magnesium alloy AZ31

Texture development in magnesium alloy AZ31 was studied by uniaxial compression tests at temperatures, strain rates and final strains ranging from 573 to 773 K, 1.0 × 10⁻³ to 5.0 × 10⁻⁵ s⁻¹ and −0.2 to −1.5, respectively. Fiber texture was formed in all of the deformation conditions. The main component of the texture varied depending on deformation conditions; it appeared about 33–38° away from the basal pole after the deformation at higher temperatures and lower strain rates. This can be attributed to the increased activity of the secondary pyramidal slip system. With a decrease in temperatures and an increase in strain rate, the tilting angle of the main component (compression plane) from the basal pole decreased down to about 20°. Construction of a basal fiber texture was detected after deformations at the lowest temperature and high strain rates.     

Quantitative analysis on friction stress of hot-extruded AZ31 magnesium alloy at room temperature

Mg alloy AZ31 with ～79% (volume fraction of scattering less than 30°) basal-fiber texture through hot extrusion exhibits strong grain-size dependent yield strength. Samples with grain sizes varying from 4.5 to 22.3 μm were obtained by altering annealing time durations. The Hall-Petch relations of tension and compression are σ_0.2 = 86+200d^?1/2 and σ_0.2 = 17 + 327d^?1/2, respectively. Considering the correlation between grain orientation and deformation modes, a novel weighted average method of calculating friction stress σ₀ was proposed, and results of calculation agreed with the experimental ones, which can reasonably understand the yielding behavior in tension and compression.     

AZ31B镁合金带材热轧过程组织均匀性及性能研究

本文开展了变形温度为300、350、400 ℃和总压下率分别为15%、30%、45%、60%的AZ31B镁合金带材热轧试验,分析了不同工艺参数对轧后带材的微观组织及力学性能的影响规律。研究表明:随着轧制温度的升高,再结晶百分数增加,晶粒细化显著,组织均匀性增强;当温度达到350 ℃时,由于中间退火保温导致再结晶晶粒长大,使温度进一步升高,对再结晶程度的影响减弱,轧后带材晶粒度和延伸率均有降低;相比温度参数,提升总压下率对晶粒细化效果更为显著,轧制温度为300 ℃,压下率为60%时近表面平均晶粒尺寸由10 μm细化至3.7 μm,中心层晶粒尺寸细化至4.9 μm,组织分布较为均匀;压下率的增加有效改善了组织均匀性,使轧后带材延伸率显著增加,拉伸断口的韧窝增多,且逐渐加深。     

An experimental study of creep‐ratchetting behavior of rolled AZ31B magnesium alloy at room temperature

To study the interaction effect of creep and ratchetting for rolled AZ31B magnesium alloy at room temperature, a series of stress‐controlled tests were designed. In the tests, four loading types with different mean stresses were considered, and dwell loading was applied to explore the creep effect on the ratchetting. The test results indicated that the sequence of ratchetting and creep loading is crucial for the strain evolution. The amount of twinning/detwinning increased as the mean stress decreased, leading to an exhaustion of nonbasal slip during ratchetting, and then suppressed the creep ductility. However, the creep sequence exerted little influence on the strain shift of ratchetting while large amount of twinning/detwinning was involved.     

AZ31和稀土镁合金微弧氧化膜的对比研究

在相同的工艺参数下制备了AZ31和Mg 10Gd 2Y-0.4Zr稀土镁合金微弧氧化膜层.利用SEM、XRD和EDS对两种陶瓷膜层的组成、微观形貌和元素组成进行了表征;通过电化学测试和盐雾试验评价了两种陶瓷膜层的耐蚀性能;利用显微硬度仪研究了两种陶瓷膜的显微硬度.结果表明:两种陶瓷层的厚度、表面形貌和致密性相似;陶瓷膜由MgO和Mg2SiO4组成,其中MgO为主晶相;与AZ31镁合金陶瓷膜相比,稀土镁合金陶瓷膜中的MgO含量增多,Mg2SiO4含量减少;两种镁合金表面陶瓷化后的耐蚀性和硬度大大提高,AZ31镁合金陶瓷膜耐蚀性优于稀土镁合金陶瓷膜,稀土镁合金陶瓷膜的硬度高于AZ31镁合金陶瓷膜.     

Electro-plastic effect on tensile deformation behaviour and microstructural mechanism of AZ31B alloy

The influences of electropulse on mechanical properties of AZ31B alloy were investigated by the electro-plastic (EP) tensile tests. The results show that the flow stress decreases with the increase of the root mean square (RMS) current density, while the elongation to fracture almost remains unchanged after it reaches a certain value. The higher peak current density can lead to a more potent EP effect when the RMS current density remains approximate. The results of microstructure analysis indicate that, the electropulse can reduce the dynamic recrystallisation temperature and promote the grain boundary sliding. The inverse eutectic reaction (α?+?β?=?L) will take place at the necking zone under high electropulse, which can optimise the deformation mechanism before the liquid phase is too much.     

AZ31镁合金板材轧制过程边裂形成原因分析

针对AZ31镁合金板材轧制过程中出现边部裂纹的问题,采用数值模拟方法研究了AZ31镁合金板材轧制过程中轧制温度对板材边裂的影响,利用实验室热轧试验方式研究了AZ31镁合金板坯宽厚比、轧制道次数以及工作辊直径等工艺参数对镁板边部裂纹的影响.研究表明,边裂的产生多数情况是由于几种因素共同作用的结果,其主要影响因素有轧制温度、道次加工率、轧辊直径以及板坯宽度和厚度等.在其他条件不变的情况下,减少板材宽厚比,可降低边部所受拉应力,有利于减少横向裂纹产生;当b(R·△h)~(1/2)时,随着板材宽度增加,轧制力逐渐升高,边部产生横向裂纹的几率增加;对于相同规格板坯,随着辊径增大,轧制过程中板坯的宽展量和所受摩擦力逐渐增加,有利于发挥板材塑性从而减小边部裂纹产生的趋势.     

Influences of tensile pre-strain and bending pre-deflection on bending and tensile behaviors of an extruded AZ31B magnesium alloy

This paper focused on the influences of tensile pre-strain and bending pre-deflection on the three point bending and uniaxial tensile properties of an extruded AZ31B magnesium alloy. The influences of pre-strain/deflection on bending/tensile curves could be divided into three stages. The results show that: (1) In the elastic stage, considering the variation of specimen’s cross sectional area, the pre-strain/deflection did not affect the measured elastic modulus obtained from both bending and tensile tests. (2) In the transition hardening stage, the specimen presented obvious hardening behaviors on basis of the pre-strain/deflection, the phenomenon was mainly caused by the strain hardening effects produced from previous uniaxial tensile and bending processes. (3) In the large plastic deformation stage/necking stage, as the accumulation of plastic deformations caused by pre-strain/deflection were significant, the specimen’s ability to resist plastic deformation was weakened. Specially, as the tensile pre-strain increased, the bending load decrement rate gradually decreased, and as the bending pre-deflection increased, both the tensile strength and elongation sharply decreased, the accumulated irreversible plastic work promoted the damage process of the magnesium alloy. The influences of tensile pre-strain on the bending behaviors of the magnesium alloy were also analyzed via finite element method.     

Microstructures and tensile properties of AZ31 magnesium alloy by continuous extrusion forming process

The objective of this study is to investigate the possibility of continuous extrusion forming (Conform process) of AZ31 magnesium alloy. The results indicate that continuous extrusion forming can refine the structure, improve the degree of the structure homogeneity and change the crystal orientation of basal plane and hence enhance the ductility but decrease tensile strength at room temperature. The fracture mechanisms of the material prepared by Conform process change from the mixture of ductile and brittle to the full dimpled rupture compared with the conventional extrudate.     

Deformation characteristics of fine grained magnesium alloy AZ31B thin sheet during rapid gas blow forming

Abstract

Decreasing the forming time in gas blow forming using fine grained Mg alloy AZ31B thin sheet with a thickness of 0·6 mm was studied in this work. Tensile tests and gas blow forming using stepwise pressurisation profiles were performed to explore the deformation behaviour of a fine grained AZ31B Mg alloy sheet. The alloy sheets were successfully deformed into hemispherical domes using two proposed stepwise pressurisation profiles during gas blow forming. As a result, significant reduction in forming time was achieved. Maximum effective deformation rates of 1·02 × 10^–2 and 1·98 × 10^–2 s^–1 were obtained at 300 and 370°C respectively. It was feasible to form a hemispherical dome with a height of 20 mm in less than 80 s at 370°C. The results confirmed that the thickness distribution along the centreline of the formed dome was sensitive to the pressurisation profiles. A higher thinning effect was observed at 370°C due to the higher deformation rate imposed during forming. Grain growth was not a serious problem for forming even at 370°C, and static grain growth should be the major factor resulting in grain growth during forming.     

Effect of D-fructose on the in-vitro corrosion behavior of AZ31 magnesium alloy in simulated body fluid

The effect of addingd-fructose to simulated body fluid(SBF) on the corrosion behavior of AZ31 magnesium(Mg) alloy at 37.C and at a pH of 7.4 was studied by potentiodynamic polarization(PDP), electrochemical impedance spectroscopy(EIS), potentiostatic polarization and hydrogen(H2) collecting techniques,Raman spectroscopy technique, scanning electron microscopy(SEM), energy dispersive spectroscopy(EDS), X-ray diffraction(XRD), X-ray photoelectron spectroscopy analysis(XPS) and Fourier transformed infrared(FTIR). The results demonstrated that the addition of fructose enhanced the deposition of phosphates forming thick and compact corrosion products, which inhibited the transmission of aggressive ions into the Mg substrate. As a result, both the anodic dissolution of Mg and negative difference effect(NDE) were suppressed. Thus, the corrosion resistance of AZ31 Mgalloy in SBF was significantly improved.     

AZ31镁合金室温拉伸微观变形机制EBSD原位跟踪研究

利用电子背散射衍射(EBSD)技术,原位跟踪AZ31镁合金轧制板材室温下沿轧向拉伸时的晶粒取向变化。对变形过程的滑移系和孪晶启动机进行分析。结果表明:变形过程主要由〈a〉基面和柱面滑移系开动而实现,晶粒取向无明显变化,大量〈a〉位错滑移的产生,使得变形后小角度晶界增加明显。晶粒中拉伸孪晶是试样在拉伸变形过程中产生的,而非在试样拉伸后的卸载过程中产生。     

Microstructure and formability of AZ31B magnesium alloy sheet under continuous pulse current

Utilising electropulsing treatment (EPT) to improve the formability of metals is of paramount importance for engineering applications. The effects of EPT on the microstructure and formability of AZ31B magnesium alloy sheet were investigated. The results indicated that the microstructure and mechanical properties were slightly improved with the increase of current density, while the formability was promoted distinctly. Besides, the formability of the specimen after EPT was better than that of the specimen annealed at the same temperature, which indicated that pulse current can effectively increase the formability of the sheet. Further studies confirmed that the athermal effect caused by the pulse current made great contribution to the dislocation mobility and improved the formability of the sheet.     

Dynamic recrystallization behavior and microstructural evolution of Mg alloy AZ31 through high-speed rolling

High-speed rolling (HSR) is known to improve the workability of Mg alloys significantly, which makes it possible to impose a large reduction in a single pass without fracture. In the present study, dynamic recrystallization (DRX) behavior and microstructural and textural variations of Mg alloy AZ31 during a HSR process were investigated by conducting rolling with different imposed reductions in the range of 20%–80% at a high rolling speed of 470 m/min and 400 °C. High-strain-rate deformation during HSR suppresses dislocation slips but promotes twinning, which results in the formation of numerous twins of several types, i.e., {10–12} extension twins, {10–11} and {10–13} contraction twins, and {10–11}–{10–12} double twins. After twinning, high strain energy is accumulated in twin bands because their crystallographic orientations are favorable for basal slips, leading to subsequent DRX at the twin bands. Accordingly, twinning activation and twinning-induced DRX behavior play crucial roles in accommodating plastic deformation during HSR and in varying microstructure and texture of the high-speed-rolled (HSRed) sheets. Area fraction of fine DRXed grains formed at the twin bands increases with increasing rolling reduction, which is attributed to the combined effects of increased strain, strain rate, and deformation temperature and a decreased critical strain for DRX. Size, internal strain, and texture intensity of the DRXed grains are smaller than those of unDRXed grains. Therefore, as rolling reduction increases, average grain size, stored internal energy, microstructural inhomogeneity, and basal texture intensity of the HSRed sheets gradually decrease owing to an increase in the area fraction of the DRXed grains.     

Effect of glycine addition on the in-vitro corrosion behavior of AZ31 magnesium alloy in Hank's solution

This study reports on the effect of the addition of Glycine to Hank's solution on the in-vitro corrosion behavior of AZ31 magnesium (Mg) alloy at 37 ℃ and a pH of 7.4 studied by using potentiodynamic polarization (PDP),hydrogen collecting techniques and electrochemical impedance spectroscopy (EIS) in combination with surface characterization techniques such as optical microscopy (OM),scanning electron microscopy(SEM),energy dispersive spectroscopy (EDS),X-ray diffraction (XRD) and X-ray photoelectron spectroscopy analysis (XPS).The results reveal that adsorption of glycine initially subdues the dissolution ofAZ31 Mg alloy while in long run it enhances the dissolution of the alloy due to the commencement of the chelation effect ofglycine with Ca2+ released from hydroxyapatite.The chelation ofglycine with Ca2+induces the formation of cracks in the surface film which further promotes the dissolution ofAZ31 Mg alloy thereby forming a porous corrosion products layer on the surface of the alloy.As a result,both the continuous dissolution of AZ31 magnesium alloy and the hydrogen evolution rate (HER) are enhanced with increasing the immersion time in Hank's solution.     

加工方式对AZ31变形镁合金热拉伸流变应力的影响

针对不同方法制备的AZ31镁合金薄板,利用热拉伸试验机和金相显微镜对其在不同温度和变形速率下的流变应力进行了实验研究.结果表明:挤压、交叉、热轧和冷轧等方法制备的AZ31镁合金薄板的应力-应变曲线基本特征是相同的.峰值流变应力随变形温度的升高和应变速率的降低而降低,在低温时具有明显的厚度效应;当温度大于350℃时峰值流变应力几乎不随板材厚度变化而变化;应变速率小于1.0×10-2s-1,变形温度大于150℃下所有AZ31薄板的延伸率均δ≥45%;单向轧制薄板的各向异性随温度提高减小.