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.     

Warm forming limits of rare earth-magnesium alloy ZEK100 sheet

Forming limit curves were developed for a rare earth-magnesium alloy, ZEK100-F, at temperatures between 25 and 350 °C in both the rolling (RD) and transverse directions (TD) of the sheet. ZEK100-F contains additions of zirconium (Zr) as a grain refining alloying element and a rare earth addition, neodymium (Nd), that promotes a weakened basal texture allowing enhanced slip activity at lower temperatures. Warm formability measurements were also performed on non-rare earth containing AZ31B-O to examine the relative performance of these two alloys. The ZEK100 material exhibited significantly better room temperature formability over AZ31B-O with a limiting dome height of 29 mm for ZEK100 compared to only 12 mm for the AZ31B-O. At elevated temperatures (250 °C) the difference in formability between the two alloys becomes less pronounced with a LDH of 40 and 36 mm for ZEK100 and AZ31B-O, respectively. What is particularly striking is the pronounced benefit of the rare earth alloyed material at intermediate temperatures, with an LDH of 37 mm at 150 °C which equals the LDH of AZ31B at 250 °C. Similar trends were determined in the measured forming limit curves reported here for the two alloys. The relative performance of the two alloys is largely attributed their initial textures. ZEK100-F also exhibits strongly anisotropic formability (RD versus TD) which can again be attributed to its’ initial texture.     

Role of pre-vertical compression in deformation behavior of Mg alloy AZ31B during super-high reduction hot rolling process

Mg alloy AZ31B plates were processed by hot rolling with different thickness reductions per pass and pre-vertical compression followed by super-high reduction hot rolling (PVCR), respectively. Microstructure evolution, rolling formability variation and mechanical responses were investigated. As reduction per pass increased, the number of shear bands deflecting toward rolling direction increased, resulting in easy crack initiation in and around the bands. With increasing reduction per pass up to 80%, twinning and twinning-induced dynamic recrystallization (DRX) dominated the deformation of the edge material at 350?°C, resulting in local recrystallization with coarse grains and further largest edge-crack degree. Pre-induced {10$\overline{1}$2} tensile twins by pre-vertical compression (PVC) increased number density of nucleation sites for dynamic recrystallization during the subsequent severe rolling, which enhanced the dominant role of continuous dynamic recrystallization. Designed PVCR-b was proved to be a relatively effective method to improve rolling formability of rolled Mg alloy AZ31B plates. With this method, mean grain size of AZ31B plate was significantly refined from ～600?mm to ～14.1?mm and more homogeneous grain size distribution along transverse direction (TD) was achieved. In addition, basal texture intensity was greatly weakened. As a result, tensile anisotropy was distinctly decreased and fracture elongation increased dramatically.     

Modeling high-temperature tensile deformation behavior of AZ31B magnesium alloy considering strain effects

The hot tensile deformation behaviors of AZ31B magnesium alloy are investigated over wide ranges of forming temperature and strain rate. Considering the effects of strain on material constants, a comprehensive constitutive model is applied to describe the relationships of flow stress, strain rate and forming temperature for AZ31B magnesium alloy. The results show that: (1) The effects of forming temperature and strain rate on the flow behaviors of AZ31B magnesium alloy are significant. The true stress–true strain curves exhibit a peak stress at small strains, after which the flow stress decreases until large strain, showing an obvious dynamic softening behavior. A considerable strain hardening stage with a uniform macroscopic deformation appears under the temperatures of 523 and 573 K. The strain hardening exponent (n) increases with the increase of strain rate or the decrease of forming temperature. There are not obvious strain-hardening stages when the forming temperature is relatively high, which indicates that the dynamic recrystallization (DRX) occurs under the high forming temperature, and the balance of strain hardening and DRX softening is easy to obtain. (2) The predicted stress–strain values by the established model well agree with experimental results, which confirm that the established constitutive equation can give an accurate and precise estimate of the flow stress for AZ31B magnesium alloy.     

Formability and numerical simulation of AZ31B magnesium alloy sheet in warm stamping process

The potential process for mass production of magnesium alloy components in vehicles—warm stamping process was investigated systematically in the present study. For analyzing the forming process, an accurate numerical model describing the unique characteristics of magnesium alloy sheets under warm forming is very essential. Aiming at this, hardening/softening model for 1.5 mm thickness AZ31B magnesium alloy sheet were firstly constructed based on uniaxial tensile tests. Secondly, semispherical drawing was carried out under the selected temperature to generate experimental forming limit curve (FLC) for AZ31B sheet. Then, friction coefficient was identified using a high-temperature tribo-tester. Finally, numerical simulation was implemented and formability of AZ31B sheet warm forming was verified with experiment. The result shows that the formability, thickness distribution and equivalent strain distribution in simulation agreed well with the actual specimens, which thus provided a good data base for describing the unique characteristics of magnesium alloy sheets under warm forming.     

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.     

Influence of strain and strain rate on microstructural evolution during superplasticity of Mg–Al–Zn sheet

Strain-induced abnormal grain growth was observed along the gage length during high-temperature uniaxial tensile testing of rolled Mg–Al–Zn (AZ31) sheet. Effective strain and strain rates in biaxial forming of AZ31 sheets also affected the nature of grain growth in the formed sheet. For the uniaxial testing done at 400 °C and a strain rate of 10^?1 s^?1, abnormal grain growth was prevalent in the gage sections that experienced true strain values between 0.2 and 1.0. Biaxial forming of AZ31 at 5 × 10^?2 s^?1 and 400 °C also exhibited abnormal grain growth at the cross sections which experienced a true strain of 1.7. Uniaxially tested sample at 400 °C and a strain rate of 10^?3 s^?1, however, showed no abnormal grain growth in the gage sections which experienced true local strain values ranging from 1.0 to 2.3. The normalized flow stress versus temperature and grain size compensated strain rate plot showed that the deformation kinetics of the current AZ31 alloy was similar to that reported in the literature for AZ31 alloys. Orientation image microscopy (OIM) was used to study the texture evolution, grain size, and grain boundary misorientation during uniaxial and biaxial forming. Influence of deformation parameters, namely strain rate, strain, and temperature on grain growth and refinement were discussed with the help of OIM results.     

Analysis of the formability magnesium alloys using a new ductile fracture criterion

An innovative methodology for the determination of forming limits is proposed, based on the strain energy density criterion. In the first section of this paper a modification of the strain energy density criterion, that has mainly been applied for crack propagation in fracture mechanics, is performed, in order to become applicable in metal forming processes. In the second section, experimental methods and Finite Element (FE) analysis for the case of deep drawing forming process are used for the verification of the methodology. Based on the simulation methodology, the forming limits and some process parameters namely, forming temperature, punch radius, punch profile radius and strain rate sensitivity of magnesium alloys AZ31 and WE43 are determined. The optimization results for the studied case show that magnesium alloys have limited formability especially at room temperature; however the formability can be improved by forming at higher temperatures. Finally, formability is improved as the punch and punch profile radii increase up to an optimum value.     

Hydromechanisches Tiefziehen und Hochdruckblechumformung als Verfahren zur Herstellung komplexer Bauteile aus Magnesiumfeinblechen des Typs AZ31B‐0

Hydro Mechanical Deep‐Drawing and High Pressure Sheet Metal Forming as Forming Technologies for the Production of Complex Parts Made of Magnesium Sheet Metal AZ31B‐0 Semi ‐ finished sheet ‐ metal products made of magnesium alloys such as AZ31B are known as better deformable at temperatures in the range of 175 °C ‐ 240 °C. By means of hydroforming technologies, as there are hydro mechanical deep‐drawing and high pressure sheet metal forming, the influence of different forming parameters on the forming results has been investigated. A more complex experimental geometry was deformed applying forming temperatures of 175 °C, 200 °C, 225 °C and 240 °C and accordingly adjusted forces of the blank holder. Concerning the applied forming ‐ methods and experimental parameters the forming results have been evaluated and compared regarding the decrease of sheet thickness and the development of small radii. For some experimental parts, which have been deformed by means of high pressure sheet metal forming at temperatures of 175 °C and 225 °C, supplementary investigations have been carried out in order to determine the evolution of characteristic surface values in dependence on the forming operation. On the basis of these results practical recommendations for the limits of application of aforementioned forming technologies for AZ31B‐0 magnesium sheet metal are given.     

Fracture mechanism of Mg–3Al–1Zn sheet at the biaxial state with respect to forming temperatures

Magnesium (Mg) sheet has been of great interest in automobile industries to make a light-weight design although it has low formability at the room temperature compared with steel sheets. It is required to elevate forming temperature to enhance the formability of Mg sheet, which enables increase of active slip systems in Hexagonal close packed (HCP) crystal structure. This paper demonstrates the effect of forming temperature on the formability of Mg–3Al–1Zn sheet, which is evaluated by the Limit Dome Height (LDH) test at temperature of 423 K, 523 K, and room temperature. The variation of dome heights depending on the forming temperature has been investigated to stand for its formability, and punch stroke and loads have been compared with each other. It has been tried to correlate the fracture mechanism with formability of AZ31 sheet with respect to the forming temperature by investigating the fracture surfaces with optical microscopy (OM) and orientation imaging microscopy (OIM) analyses.     

Research on Thermal Deep-drawing Technology of Magnesium Alloy (AZ31B) Sheets

Forming technology of Mg alloy (AZ31B) sheets can be investigated by thermal deep drawing experiments. In the experiments, the blank holder and die contacting with the blank were heated to the same temperature as the blank by using the heating facility. The circular blank heated in an oven is formed at a temperature range of 100~400℃ to obtain the optimum forming temperature range and the effects of major technical parameters on the workpiece quality.It is found that the blank is brittle at temperatures lower than 200℃.Temperatures higher than 400℃ are not suitable for forming of the sheets because of severe oxidation and wrinkling.AZ31B shows an excellent formability at temperatures from 300 to 350℃ and can be formed into a workpiece with good quality. When the blank holder force is 9 kN, extruded sheets with a thickness of 1 mm can be formed into cups without wrinkling. Workpieces show strong anisotropic deformation behavior on the flanges.     

Influence of thermomechanical processing on superplastic forming of Mg–Al alloys

Abstract

The aim of this paper is to study the influence of the initial microstructure of several Mg–Al alloys on their superplastic formability and on their post-forming microstructure and mechanical properties. Various thermomechanical processing routes, such as annealing, conventional rolling, severe rolling and cross rolling, were used in order to fabricate AZ31 and AZ61 alloys with different grain sizes. These materials were then blow formed into a hat shaped die. It was found that the processing route has only a small effect in the formability of Mg–Al alloys or on the post-forming microstructures and properties due to rapid dynamic grain growth taking place at the forming temperatures. Nevertheless, good formability is achieved as a result of the simultaneous operation of grain boundary sliding and crystallographic slip during forming.     

铸态AZ31镁合金热压缩过程中的再结晶行为

利用Gleeble-1500在温度200～500℃和应变速率0.001～1s-1范围内对铸态AZ31镁合金进行热压缩实验,并对动态再结晶行为进行研究。基于温度-应变速率的变化规律(Zener-Hollomon参数,Z参数),分析了形变温度和应变速率对铸态AZ31镁合金组织结构的影响规律。结果表明:动态再结晶发生后,再结晶晶粒尺寸随着形变温度的降低而减小。随着Z值的增加,动态再结晶作用增强,形变组织细化。为了便于工程应用的参考,给出了相应的热加工三维图。     

Hot working behavior of AZ31 and ME21 magnesium alloys

The plastic deformation and recrystallization behavior of the commercial magnesium alloys AZ31 and ME21 were analyzed in a wide temperature range. Using the conventional hyperbolic sine equation the flow stress dependence on temperature and strain rate was modeled. The activation energy for plastic deformation significantly increased with increasing temperature and delivered values above 180 kJmol^?1 for both alloys in the very high-temperature regime (400–550 °C). At lower temperatures (250–400 °C) the activation energy of the AZ31 alloy was approximately 108 kJmol^?1 considering the peak stress as well as 120 kJmol^?1 considering the flow stress at a strain of 0.5. The stress exponent varied in a range between 4.5 and 6.5. During the high-temperature compression tests a partial recrystallized microstructure was formed, which was distinctly different in AZ31 compared to ME21 due to the different onset of dynamic recrystallization (DRX) mechanisms.     

镁合金笔记本电脑外壳的温成形技术研究

采用0.6mm厚的AZ31镁合金材料,以温成形方式获得了复杂外形尺寸的笔记本电脑外壳.通过研究温度对AZ31镁合金板料拉深成形性能的影响发现:180～280℃成形的镁合金电脑外壳零件不会出现断裂和起皱缺陷,但角部高度随成形温度的升高而增加;低于180℃成形,零件圆角出现褶皱;高于280℃成形,圆角被拉裂;并且成形速度和摩擦也会影响零件成形.     

Multi-pass large strain rolling of as-cast AZ31 magnesium alloy

As-cast AZ31 magnesium alloy subjected to multi-pass large strain rolling was investigated. A successive rolling process up to three passes was carried out at 370°C with a pass reduction of 30%. Deformation microstructure characteristics prove that the dynamic recrystallisation (DRX) mode changed with the increase of rolling passes. In the first pass, DRX related to twinning played a dominant role. But in the third pass, DRX grains mainly appeared around the pre-existing grain boundaries. The ultimate strength and elongation of rolled sheets after three passes rolling are enhanced by 37 and 39%, respectively, compared to the as-cast alloy. Meanwhile, the tensile fracture mode was ductile fracture which was different from the ductile–brittle fracture of as-cast.     

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.     

Thermal Conductivity of Magnesium Alloys in the Temperature Range from −125 °C to 400 °C

Magnesium alloys have been widely used in recent years as lightweight structural materials in the manufacturing of automobiles, airplanes, and portable computers. Magnesium alloys have extremely low density (as low as 1738 kg · m^?3) and high rigidity, which makes them suitable for such applications. In this study, the thermal conductivity of two different magnesium alloys made by twin-roll casting was investigated using the laser-flash technique and differential scanning calorimetry for thermal diffusivity and specific heat capacity measurements, respectively. The thermal diffusivity of the magnesium alloys, AZ31 and AZ61, was measured over the temperature range from ?125 °C to 400 °C. The alloys AZ31 and AZ61 are composed of magnesium, aluminum, and zinc. The thermal conductivity gradually increased with temperature. The densities of AZ31 and AZ61 were 1754 kg · m^?3 and 1777 kg · m^?3, respectively. The thermal conductivity of AZ31 was about 25 % higher than that of AZ61, and this is attributed to the amount of precipitation.     

Processing Map of AZ31-1Ca-1.5 vol.% Nano-Alumina Composite for Hot Working

A processing map for extruded AZ31-1Ca-1.5NAl composite has been developed, which exhibited four important domains for hot working. The corresponding temperatures and strain rates associated with these domains are: (1) 250–350°C and 0.0003–0.01 s^?1; (1A) 350–410°C and 0.0003–0.01 s^?1; (2): 410–490°C and 0.002–0.2 s^?1; and (3) 325–410°C and 0.6 s^?1 to 10 s^?1. Dynamic recrystallization (DRX) occurred in all the four domains although different slip mechanisms and recovery processes are involved. Basal slip and prismatic slip dominates deformation in Domains 1 and 1A, respectively, with recovery occurring by climb that is lattice self-diffusion controlled. However, because of the high strain rates in Domain 3, recovery occurs through a climb process, controlled by grain boundary self-diffusion. The recovery mechanism in Domain 2 is cross-slip assisted by pyramidal slip along with basal and prismatic slip. The grain size has a linear relation with Zener–Hollomon parameter in all the domains. At high strain rates, the composite undergoes shear fracture at lower temperatures and intercrystalline fracture at higher temperatures. All of the identified DRX domains are suitable for conducting bulk metal forming processes although the one with the highest strain rates (Domain 3) is preferred for achieving high productivity.     

Galvanostatic Anodic Polarization Curves and Galvanic Corrosion of AZ31B in 0.01 M Na2SO4 Saturated with Mg(OH)2

Measured polarization curves were used in a BEM model of the galvanic corrosion of the Mg alloy AZ31B. The AZ31B galvanostatic anodic polarization curves indicated: (i) the existence of the uni‐positive Mg⁺ ion, (ii) some hydrogen dissolved in the AZ31B metal, and (iii) self corrosion was more important than the applied current density in causing weight loss. Galvanic corrosion of the AZ31B consisted of: (i) corrosion that decreased in depth from the 2024–AZ31B interface, and (ii) heterogeneous corrosion that was denser closer to the 2024–AZ31B interface. There was good agreement between the BEM model and the experimental measurements.