Warm Forming of Mg Sheets: From Incremental to Electromagnetic Forming

Magnesium alloys are generating interest in the automotive and aeronautic industries due to their low density and potential to reduce gross vehicular weight. However, the formability of these alloys is poor and they are very difficult to be formed at room temperature due to their strong basal texture in rolled form. In this paper, the potential of magnesium alloy sheets to be processed at warm conditions is studied for four different forming technologies: incremental forming (IF), deep drawing (DD), hydroforming (HF), and electromagnetic forming (EMF). Forming mechanisms and process window are experimentally characterized by monitoring different process parameters. Special focus is made on the influence of the forming temperature and the strain rate. Thus, experiments at temperatures from room to 523 K (250 °C) and a wide range of strain rates, between 10^?3 up to 10³ s^?1 according to each process nature and scope, are conducted. It is observed that, even the inherent forming rate range of each process vary considerably, increasing forming temperature increases formability for all of these forming processes. In the other hand, an opposing effect of the strain rate is observed between the quasi-static processes (IF, DD, and HF) and the high speed process (EMF). Thus, a detrimental effect on formability is observed when increasing strain rate for quasi-static processes, while a mild increase is observed for EMF.     

Gas‐pressure Forming of an AlMg‐Alloy Sheet at Elevated Temperatures

Forming of automotive leightweight parts using aluminium offers numerous advantages. Compared to other wrought aluminium alloys, in particular AlMg‐alloys generally show a good formability which is favourable for the production of complex parts. However, forming of Mg‐containing alloys at room temperature leads to yielding patterns preventing their implementation for class‐A‐surface applications. Furthermore, the formability of steel still exceeds that of AlMg‐alloys at room temperature. Thus, in the present study, sheet metal forming is applied at a temperature range that is typical for warm forming. It is supposed to profit from the advantages of warm forming like high achievable strains and improved surface quality of the formed part, while not having the disadvantages of long production times and high energy consumption, which is correlated with superplastic forming. Applying fluid‐based sheet metal forming in this paper, nitrogen is used as fluid working medium to satisfy the demand on high temperature resistance. Concerning the blank material used, formability of Mg‐containing aluminium alloys shows strong strain rate sensitivity at elevated temperatures. To figure out the optimal strain rates for this particular process, a control system for forming processes is developed within the scope of this paper. Additionally, FE‐simulations are carried out and adapted to the experiment, based on the generated process data. FE‐investigations include forming of domes (bulging) as well as shape‐defined forming, having the objective to increase formability in critical form elements by applying optimal strain rates. Here, a closed‐loop process control for gas‐pressure forming at elevated temperatures is to be developed in the next stages of the project.     

热轧AZ31镁合金薄板的室温成形性

针对AZ31镁合金板材室温冲压成形较差的特点,采用不同轧制温度获得镁合金板材,使用半球形凸模胀形,绘制镁合金室温成形极限图并分析轧制温度对镁合金板材组织和室温成形能力的影响.发现AZ31镁合金板材的成形性能不仅与晶粒尺寸有关,还与晶粒取向有关.基面织构的减弱可明显提高板材的胀形性能,在基面织构强度相似的情况下,晶粒尺寸对板材的成形性能起决定性影响.      

Study of Forming Limit for Rotational Incremental Sheet Forming of Magnesium Alloy Sheet

As a lightweight material, magnesium is being increasingly used for automotive parts. However, due to a hexagonal-closed-packed (hcp) crystal structure, in which only the basal plane can move, magnesium alloy sheets exhibit a low ductility and formability at room temperature. Press forming of magnesium alloy sheets is conventionally performed at elevated temperatures of 200 °C to 250 °C and thus is known as energy consumed forming. Therefore, in view of an energy saving forming technology, we study magnesium alloy sheet forming by a rotational incremental sheet forming (RISF) at room temperature, where the rotational tool generates local heat of specimen enough to accelerate plastic deformation. The flow curves of the magnesium alloy sheet are obtained and calculated at elevated temperatures, while the yield loci of the magnesium alloy sheet are measured at room temperature. Using RISF, a square cup of 80-mm width, 80-mm length, and 25-mm height is then formed from a magnesium alloy sheet at room temperature. In addition, the strain distribution is obtained and compared with the forming limit curve (FLC) by considering the effect of the tool radius and is found to effectively predict the forming limit of a magnesium alloy sheet in RISF.     

Application of Taguchi Method on Biaxial Stretch Forming of Friction Stir Processed Mg AZ31B Alloy

The present study describes the effect of friction stir processing parameters on formability of Mg AZ31B sheet under biaxial stretching. The formability of friction stir processed sheet was studied by limiting dome height test in biaxial strain deformation mode. The experiments were carried out as per the Taguchi parametric design concepts and an L9 orthogonal array was used to study the influence of various combinations of process parameters. Statistical optimization technique, ANOVA was used to determine the optimum levels and to find the significance of each process parameter. The results indicate that the traverse speed is the most significant factor followed by the rotational speed and the tilt angle in deciding the formability of friction stir processed magnesium alloy. In addition, mathematical model was developed to establish relationship between the different process variables with formability by regression analysis.     

高温轧制对AZ31B镁合金组织和性能的影响

研究了高温轧制工艺对AZ31B镁合金微观组织、织构以及性能的影响规律.在轧制状态下,随着轧制温度从450℃升高至525℃,合金组织内部动态再结晶逐渐增多,孪晶数量不断减少,同时组织的均匀性也得到了改善,基面织构强度也呈下降的趋势.经350℃保温60min退火之后,合金板材内部发生了完全再结晶,孪晶组织消失,显微组织更均...     

镁合金的超塑性与损伤定量分析

研究了轧制AZ31B镁合金板材的超塑性与空洞损伤，对拉伸试样在超塑性变形各阶段轴剖面的空洞进行了观察，通过对空洞演化的分析建立了空洞体积分数与变形程度的定量关系。研究结果表明：AZ31B镁合金板材在一定的变形条件下具有良好的超塑性；变形伴随着空洞的形核、长大，继而发生空洞的连接，导致材料断裂；空洞体积分数随着变形程度的增加呈指数规律变化。     

AZ31镁合金的研究现状

综述了国内外对AZ31镁舍金的研究现状。讨论了主要合金元素对AZ31镁合金的组织和性能的影响,介绍了AZ31镁合金的晶粒细化、塑性成形技术的研究现状。对AZ31镁合金的发展前景进行分析。     

非线性组合硬化条件下镁合金板料变形回弹预测

变形回弹作为金属板料成形的主要缺陷之一,如何提高变应变路径条件下的回弹预测精度一直是研究者们面临的难题.本文针对镁合金变形特点,提出了同时考虑同向硬化、动态硬化和屈服圆畸变的本构模型.以0.8 mm厚AZ31B镁合金板料为研究对象,施加不同预拉伸后进行弯曲变形试验,观察了不同预变形对回弹规律的影响.同时结合有限元分析ABAQUS-Explicit (Vumat)和ABAQUS-Implicit (Umat)对板料的变形及回弹过程进行模拟仿真,对比试验与模拟结果,验证动态硬化对于镁合金板料变形回弹的重要影响.      

基于Gurson模型的镁合金板材温热冲压成形研究

在Gurson损伤模型的基础上,采用有限元数值模拟与温热冲压实验相结合的方法,对镁合金板材温热冲压成形过程中的材料损伤过程进行了预测.考虑了板材的塑性各向异性行为,通过用户自定义材料子程序VUMAT将损伤模型嵌入到有限元软件ABAQUS/Explicit中.采用单轴拉伸试验数据与有限元数值模拟结果进行迭代,确定了Gurson模型所需要的材料参数.使用ABAQUS模拟得到了镁合金板材温热冲压过程中微孔洞的演变及分布规律.通过扫描电子显微镜,对不同温度下的AZ31镁合金板材由孔洞增长和聚合引起的内部损伤演化进行了观察分析.研究结果表明,板材中微孔洞的分布与实验数据相吻合,说明本文所提出的方法可以应用于金属板材温热冲压成形性能预测.      

Investigation on a Deep Drawing and In‐Process Electromagnetic Calibration

Extending forming limits is one of the most important aims of research work in production engineering. One possibility to improve material formability is the application of high strain rates, which can be realized e.g. by means of electromagnetic forming (EMF). A further extension of the forming limits can be achieved by a beneficial combination of EMF and quasistatic forming operations, which allow exploiting the complementary advantages of the different technologies involved. This approach will be described on the basis of a deep drawing and inprocess electromagnetic sheet metal forming calibration in this paper. Thereby, the design as well as the subsequent analysis of the components as well as the combined process plays a distinctive role. Furthermore, the stages of development regarding the integrated tool coil will be presented and the resulting examples discussed. Finally, the setup of the integrated process as well as the feasibility will be shown on an exemplary semi‐industrial workpiece.     

Extended Forming Limits using Laser‐assisted Hydroforming

The motivation of the presented research work was to provide an approach to reduce the high fluid pressures and clamping forces needed in hydroforming presses to cup small form elements. Using local heating, small form elements like domes and creases can be formed at very low pressures of 2‐3 MPa, whereas cold forming requires pressures which are 20‐50 times higher. Apart from the proportion of forming temperature and work pressure, temperature distribution is very important, and can be adjusted by a special laser beam shaping optic or scanning processing head. The second method is more flexible in regard to element sizes and outlines, but has a lower thermal efficiency. Line network analyses were carried out showing great improvements in the resulting strain distribution. In order to characterise the general improvement of the material's formability, forming limit curves (FLC) were generated, using the bulge‐test. The results prove the extended forming limit of the laser‐assisted warm cupping process. For the investigations different materials were used: the deep drawing steel DC05, the aluminium alloy 5182 and the magnesium alloy AZ31.     

Relationships between process parameters and temperature field of roll casting for wide sheet made of AZ61 magnesium alloy: Finite element method

In recent years, wide sheet made of AZ61 wrought magnesium alloys has been widely studied and applied in industry. Thin roll-casting technology for the new wrought magnesium alloy can provide acceptable quality wide and thin sheet made of AZ61 magnesium alloy. To study the influences of roll-casting process parameters on temperature field for wide and thin sheet made of AZ61 magnesium alloy plates, some simplification and assumptions have been done by characteristics of magnesium alloy. Two-dimensional FEM model for roll-casting has been established along casting direction. Simulations of temperature fields of the plates have been done by using finite element analysis ANSYS software. A series of researches on the temperature distributions under different process parameters (pouring temperature, heat-transfer coefficients and casting speeds) have been done. The simulation results and the literature about the casting process of the relevant theory are the same. The simulation results show that the process parameters of rapid-casting process for AZ61 magnesium alloy are mutual influenced on the temperature fields of wide sheet made of AZ61 magnesium alloy.     

Effects of ultrasonic vibration on plastic deformation of AZ31 during the tensile process

An investigation on the plastic behavior of AZ31 magnesium alloy under ultrasonic vibration(with a frequency of 15 kHz and a maximum output of 2 kW)during the process of tension at room temperature was conducted to reveal the volume effect of the vibrated plastic deformation of AZ31.The characteristics of mechanical properties and microstructures of AZ31 under routine and vibrated tensile processes with different amplitudes were compared.It is found that ultrasonic vibration has a remarkable influence on the plastic behavior of AZ31 which can be summarized into two opposite aspects: the softening effect which reduces the flow resistance and improves the plasticity,and the hardening effect which decreases the formability.When a lower amplitude or vibration energy is applied to the tensile sample,the softening effect dominates,leading to a decrease of AZ31 deformation resistance with an increase of formability.Under the application of a high-vibrating amplitude,the hardening effect dominates,resulting in the decline of plasticity and brittle fracture of the samples.     

Room Temperature Shear Band Development in Highly Twinned Wrought Magnesium AZ31B Sheet

Failure mechanisms were studied in wrought AZ31B magnesium alloy after forming under different strain paths. Optical micrographs were used to observe the shear band formation and regions of high twin density in samples strained under uniaxial, biaxial, and plane strain conditions. Interrupted testing at 4?pct effective strain increments, until failure, was used to observe the evolution of the microstructure. The results showed that shear bands, with a high percentage of twinned grains, appeared early in the samples strained under biaxial or plane strain tension. These bands are similar to those seen in uniaxial tension specimens just prior to failure where the uniaxial tensile ductility was much greater than that observed for plane strain or biaxial tension conditions. A forming limit diagram for AZ31B, which was developed from the strain data, showed that plane strain and biaxial tension had very similar limit strains; this contrasts with materials like steel or aluminum alloys, which typically have greater ductility in biaxial tension compared to plane strain tension.     

Developing an Empirical Relationship to Predict Corrosion Rate of AZ31B Magnesium Alloy under Sodium Chloride Environment

Magnesium alloys have gained considerable interest as a structural material for automotive and aerospace applications due to their low-density, high specific strength and good castability. As a consequence, these light alloys have a promising future. The limitation of low corrosion resistance restricts their practical applications. Corrosion behaviour of the AZ31B magnesium alloy was evaluated by conducting immersion corrosion test in NaCl solution at different chloride ion concentrations, pH value and immersion time. An attempt was also made to develop an empirical relationship to predict the corrosion rate of AZ31B magnesium alloy. Three factors, five level, central composite rotatable design matrix was used to minimize the number of experimental conditions. Response surface methodology was used to develop the relationship. The developed relationship can be effectively used to predict the corrosion rate of AZ31B magnesium alloy at 95 % confidence level.     

Investigation of Elongation at Fracture in a High Speed Sheet Metal Forming Process

This paper investigates the dynamic elongation at fracture of conventional steels, advanced high strength steels and nonferrous metals, such as aluminium and magnesium alloys. Dynamic tensile tests were carried out using a high speed material testing machine at various strain rates ranging from 0.001/s to 200/s. The results show that the elongation at fracture of sheet metals does not simply decrease with the increase of the strain rate. The elongation of SPCC, SPRC450R, TRIP600 and AZ31 decreases when the tests are carried out under the quasi‐static state at the strain rate of 0.1/s, but increases again when the tests are carried out at the strain rate of 0.1/s up to the strain rate of 200/s. Furthermore, DP600 and AA7003‐T7 show the tendency that the tensile elongation increases as the strain rate increases. This tendency is related to the microstructure and forming history of the sheet metal. It is concluded that localized strain rate hardening in the necking region induces the enlargement of the necking region and thus the increased elongation. This phenomenon is worth being considered to predict the fracture of sheet metal products in high speed sheet metal forming.     

Nitride Nanoparticle Addition to Beneficially Reinforce Hybrid Magnesium Alloys

This study is aimed at understanding the function of two nitride nanoparticles regarding altering the mechanical properties of hybrid magnesium alloys in relation to nanoparticle-matrix reactivity. Nitride nanoparticles were selected for reinforcement purposes due to the affinity between magnesium and nitrogen (in parallel with the well-known magnesium-oxygen affinity). AZ91/ZK60A and AZ31/AZ91 hybrid magnesium alloys were reinforced with AlN and Si₃N₄ nanoparticles (respectively) using solidification processing followed by hot extrusion. Each nitride nanocomposite exhibited higher tensile strength than the corresponding monolithic hybrid alloy. However, AZ91/ZK60A/AlN exhibited slightly lower tensile ductility than AZ91/ZK60A, while AZ31/AZ91/Si₃N₄ exhibited higher tensile ductility than AZ31/AZ91. The formation of high strain zones (HSZs) (from particle surfaces inclusive) during tensile deformation as a significant mechanism supporting ductility enhancement was addressed. AZ91/ZK60A/AlN exhibited lower and higher compressive strength and ductility (respectively) compared to AZ91/ZK60A, while AZ31/AZ91/Si₃N₄ exhibited higher and unchanged compressive strength and ductility (respectively) compared to AZ31/AZ91. Nanograin formation (recrystallization) during room temperature compressive deformation (as a toughening mechanism) in relation to nanoparticle-stimulated nucleation (NSN) ability was also discussed. The beneficial (as well as comparative) effects of the respective nitride nanoparticle on each hybrid alloy are studied in this article.