Apatite formability of boron nitride nanotubes

This study investigates the ability of boron nitride nanotubes (BNNTs) to induce apatite formation in a simulated body fluid environment for a period of 7, 14 and 28 days. BNNTs, when soaked in the simulated body fluid, are found to induce hydroxyapatite (HA) precipitation on their surface. The precipitation process has an initial incubation period of ～ 4.6 days. The amount of HA precipitate increases gradually with the soaking time. High resolution TEM results indicated a hexagonal crystal structure of HA needles. No specific crystallographic orientation relationship is observed between BNNT and HA, which is due to the presence of a thin amorphous HA layer on the BNNT surface that disturbs a definite orientation relationship.     

Investigation of glass formability in Al-Co-Y ternary system

The glass formation was investigated in the melt-spun Al-rich Al-Co-Y alloys by the microstructure evolution. The best glass former Al88Co5Y7 with the thickness of 230μm was found, and its glass forming ability （GFA） was higher than that of previously reported Al88Co4Y8 alloy. This result indicates that GFA is strongly sensitive to the alloy composition in the system. The phase selection method is an effective way to locate the optimum glass formation composition rather than Trg and △Tx parameters.     

Numerical formability assessment in single crystals of magnesium

In this paper a rate-sensitive elastic–viscoplastic crystal plasticity constitutive model (CPCM) together with the Marciniak–Kuczynski (M–K) approach have been used to assess the formability of a magnesium single crystal sheet by simulating the forming limit diagrams (FLDs). Sheet necking is initiated from an initial imperfection in terms of a narrow band. A homogeneous deformation field is assumed inside and outside the band, and conditions of compatibility and equilibrium are enforced across the band interfaces. Thus, the CPCM only needs to be applied to two regions, one inside and one outside the band. The FLDs have been simulated under two conditions: (a) the plastic deformation mechanisms are basal, pyramidal 〈c + a〉, and prismatic slip systems, and (b) the plastic deformation mechanisms are basal, pyramidal 〈c + a〉, and prismatic slip systems, as well as extension and contraction twinning systems. The FLDs have been generated for two grain orientations. In the first orientation pyramidal 〈c + a〉 and extension twinning systems, and in the second orientation basal and pyramidal 〈c + a〉 slip systems, as well as contraction twinning systems have favourable orientation for activation. The effects of shear strains outside the necking band, strain rate sensitivity, and c/a ratio on the simulated FLDs in the two grain orientations have been individually explored.     

On the formability of sheet steels

Formability of sheet steel in stamping operation primarily depends on strain hardening exponent (n), average plastic strain ratio ( ) and the maximum strain the material can undergo before the onset of localized necking. The formability parameters (n and ) and the forming limit diagrams have been evaluated for a variety of sheet steel products, extensively used for press forming of components of diverse shapes e.g. extra deep drawing quality auto-body sheets, high strength cold rolled sheets, LPG steel for gas cylinders, austenitic and ferritic stainless steels, etc. The effect of sulphide shape control on formability of hot rolled HSLA steel has also been studied. Additionally, the press performance of auto-body sheets and austenitic stainless steels have been monitored and evaluated at customer’s end for complete information on the formability.     

Stress based prediction of formability and failure in incremental sheet forming

A strain-based forming limit criterion is widely used in sheet-metal forming industry to predict necking. However, this criterion is usually valid when the strain path is linear throughout the deformation process [1]. Strain path in incremental sheet forming is often found to be severely nonlinear throughout the deformation history. Therefore, the practice of using a strain-based forming limit criterion often leads to erroneous assessments of formability and failure prediction. On the other hands, stress-based forming limit is insensitive against any changes in the strain path and hence it is first used to model the necking limit in incremental sheet forming. The stress-based forming limit is also combined with the fracture limit based on maximum shear stress criterion to show necking and fracture together. A derivation for a general mapping method from strain-based FLC to stress-based FLC using a non-quadratic yield function has been made. Simulation model is evaluated for a single point incremental forming using AA 6022-T43, and checked the accuracy against experiments. By using the path-independent necking and fracture limits, it is able to explain the deformation mechanism successfully in incremental sheet forming. The proposed model has given a good scientific basis for the development of ISF under nonlinear strain path and its usability over conventional sheet forming process as well.     

Explaining texture weakening and improved formability in magnesium rare earth alloys

Wrought magnesium alloys are rarely used due to their poor formability which is caused by strong textures created during processing. Addition of rare earth (RE) elements including Y, Ce, La, Gd and Nd weakens these strong basal textures and significantly improves formability. Developing a mechanistic understanding of this effect is critical in leading alloy design towards a new class of highly formable magnesium alloys. This fall in texture intensity occurs during recrystallisation and only requires very low solute RE additions, 0·01 at.-% in the magnesium–Ce case. These additions retard dynamic recrystallisation and increase non-basal slip; however, a full understanding of the RE effect has yet to be obtained, with a variety of mechanisms proposed. Recent research in these areas is critically reviewed.     

TRIP 钢无缝管的开发及其成形性分析

本文首次将趋于成熟的相变诱发塑性钢(TRIP钢)生产技术应用到钢管的生产领域,以冷拔钢管为原料,分别利用两阶段热处理和连续热处理两种方式成功开发出具有铁素体、贝氏体、残余奥氏体和少量马氏体组织的薄壁TRIP钢无缝管,并利用环形拉伸以及冷弯等试验手段对其成形性能进行了试验研究。研究结果表明,TRIP钢无缝管具有较好的冷成形性能,可以在内高压成形等领域推广应用。另外,课题组开发的连续热处理设备完全可以用来生产TRIP钢管,为未来工业生产提供了重要参考。     

Local bifurcation and instability theory applied to formability analysis

The design and optimization of both sheet metal formed parts and processes are nowadays carried out virtually making use of numerical tools by finite element analysis. Such virtual try-out approach contributes with significant savings in terms of money, time and effort in the design, production and process set-up of deep drawn parts. The analysis of either forming success operation or surface defects, in each of the development phases, is generally performed by means of the material’s forming limit diagram (FLD), since it allows to define a safe region that reduces the probability of: (i) necking; (ii) wrinkling and (iii) large deformation occurrence. However, the FLD represented in the strain space is known to present some disadvantages. To overcome this problem, Ito and Goya proposed a local bifurcation criterion that defines the critical state for a local bifurcation to set in as a function of the stress level to work-hardening rate ratio, leading to a FLD represented in the stress space. This suggests that the FLD obtained is completely objective in the sense that it is completely independent of the strain or stress history paths (Ito et al. 2000). In this work the Ito and Goya model is used to evaluate formability, as well as fracture mode and direction on the deep drawing of a square cup. Since the analysis is performed based on the stress state, it is also possible to determine an instability factor that “measures” the degree of acceleration by current stress for the local bifurcation mode towards fracture. The selected example highlights the potential use of the criterion which, once combined with the finite element analysis, can undeniably improve the mechanical design of forming processes.     

Post-welding formability of AZ31 magnesium alloy

The plastic flow behaviour and formability of friction stir welded AZ31 magnesium alloys were widely investigated. Flow curves were obtained in extended ranges of temperature (250–350 °C) and strain rate (0.001–0.1 s⁻¹) by means of uniaxial tensile tests; furthermore, forming limit curves were determined using the hemispherical punch method in the same range of temperature but with a constant crosshead speed of 0.1 mm/s. The results were compared with those obtained, under the same experimental conditions, on the base material. The flow stress levels of joint and base material are very similar up the peak of the flow curve although the equivalent strains at the peak and to failure are usually lower than those of base material. However, at the highest temperature and lowest strain rate investigated (350 °C and 0.001 s⁻¹), the flow behaviour of the welded joint tends to be similar to the one of the base alloy. Finally, formability of the friction stir welded material, evaluated in terms of forming limit curves, is usually lower than the one of the base material.     

The formability of aluminum foam sandwich panels

The present paper aims to study the formability of Aluminum Foam Sandwich (AFS) panels. At now, the final shape of foamed devices is directly obtained through the foaming process itself and no further shaping steps are expected. In any case, further manufacturing processes may be exploited to produce more complex parts. Among forming operations, bending can be regarded as one of the simplest processes for both study and application. Besides, bending tests may yield interesting information about material properties. With regard to the metal foams characterization, several bending tests on AFS panels fabricated by Alulight® were carried out by varying the process conditions. A universal testing machine was employed for this purpose, collecting data about the deformed geometry and about load vs. displacement. Even though the samples deformation was related to the occurrence of foam cells failure or collapse, once the process was terminated, they still retained a significant bending strength. The metal foams properties were also investigated using both non-destructive and mechanical tests. In particular, thickness measurements (using an ultrasonic feeler), X-ray analysis and foam density measurements were carried out on AFS specimens before the execution of both upsetting and bending tests. Finite Element simulations of the foam bending process were performed to investigate stress and strain distributions on the specimens. In particular, an isothermal plane strain model of the bending process was setup using the FEM commercial code Deform 2D. The results of this study were used to produce closed structure components (square shapes) by combining three 90° bends. A further improvement consisted in joining the open ends, to enhance shear and torsion resistance. Among joining techniques conventional welding processes (Tungsten Inert Gas—TIG and laser) and a non-conventional method (Friction Stir Welding—FSW) were investigated. Finally, the mechanical properties of the joints were characterized using both three and four point bending tests.     

Ti及TiAl合金的电致增塑性

TiAl合金和Ti合金由于缺少滑移系,是典型的难变形材料.本文探索应用高密度脉冲电流改善其成形性.所选择的材料是TiAl合金 (Ti₂AlNb)、TC4合金(Ti-6Al-4V)、TA15合金和TA1-A.研究了脉冲电流处理对变形损伤的影响,研究结果表明,在一定条件下因塑性变形而产生的微裂纹可以被治愈,并形成局部的再结晶组织,预塑性变形试样的塑性可以被完全恢复,极限变形量得到提高.对退火态的板材进行了高密度电流脉冲处理试验分析,试验结果表明,电流脉冲处理显著地改变了板材的力学性能:均匀延伸率提高、屈服应力和屈强比下降.这对改善钛合金板材成形性具有重要的意义.选用冷轧态板材进行了脉冲电流处理实验,研究结果表明,脉冲电流处理可以大幅度地提高冷轧钛合金板材的塑性,同时材料强度不降低,由于晶粒的细化作用,强韧性得到改善.本文的研究结果表明高密度脉冲电流是改善难变形合金成形性的一种有效方法.     

Microstructure and press formability of a cross-rolled magnesium alloy sheet

Cross-rolling, in which the roll axis is tilted by 7.5° towards the TD-direction, was carried out on a commercial magnesium alloy. The (0002) texture intensity of the cross-rolled specimen was lower than that of the unidirectionally rolled specimen, and the (0002) texture of the cross-rolled specimen was inclined about 10° towards the TD-direction. Also, the grain size of the cross-rolled specimen was smaller than that of the unidirectionally rolled specimen. As a result of the Erichsen tests at 433-493 K, the press formability of the cross-rolled specimen was higher than that of the unidirectionally rolled specimen. The high formability of the cross-rolled specimen is attributed to both the modification of (0002) texture and the enhancement of grain refinement by the cross-rolling.     

三种强度级别的双相钢成形性能研究

对三种不同强度级别、不同厚度的双相钢进行了系统的成形性试验,综合分析了双向钢的力学性能、杯突性能、冷弯性能、成形极限。研究表明:双向钢的延伸率A及加工硬化指数n值随强度级别上升而下降;杯突试验结果表明,杯突的高度随双相钢强度级别上升而下降,随材料厚度升高而升高;双相钢冷弯性能随材料强度级别上升而下降;双轴应变下的胀形试验表明双向钢具有良好的成形性;基于软钢的FLD0(是指成形极限曲线FLC与纵坐标e1的交点的纵坐标值,它是FLD(成形极限图)上的重要的特征点)经验公式并不能直接套用在双向钢上。     

Improvement in formability of aluminum alloy sheet by enhancing geometrical hardening

Individual grains in a polycrystal rotate during plastic deformation. This leads to a change in the crystallographic texture, and results in an increase or decrease of the macroscopic flow stress of the material. Such a change of strength as a result of grain rotations is called geometrical or texture hardening/softening. In the present study, for textured aluminum alloy sheets, the geometrical hardening/softening effect in the in-plane plane-strain stretching mode is numerically investigated using a generalized Taylor-type polycrystalline model. It is found that the cube texture () exhibits significant geometrical hardening when the major stretching direction is inclined at 45° relative to the orthotropic axes, and that a cube texture rotated about the normal direction (ND) shows a notable degree of geometrical hardening for any in-plane orientation of the sheet. Using the Marciniak-Kuczyński-type approach, forming limits for these textured sheets are analyzed. It is found that geometrical hardening definitely enhances the formability. It is, therefore, strongly suggested that texture control guided by the present results may be highly effective in producing aluminum alloy sheets with higher formability.     

Enhancement of dimple formability in sheet metals by 2-step forming

In this study, a 2-step stamping model with an additional 1st stamping tool is proposed to reduce stamping flaws in the curved parts of dimples in nuclear fuel spacer grids. First, the strains in the curved part of the dimple are analyzed and compared with strain solutions for pure bending. A reference 2D FE (finite element) model of the 1-step stamping is established, and the corresponding maximum strain is obtained. FE solutions are obtained for various process variable values for the 1st stamping tool used in the 2-step stamping model. Based on these solutions and applying the RSM (response surface method), strains are expressed as a function of process variables. This function then serves to evaluate optimum process variable values. Finally, by transferring these optimum values to a 3D FE model, we confirm the enhanced formability of the proposed 2-step stamping model.     

Design of texture for improved formability of high-strength steel

The effect of crystallographic textures on the formability of BCC steel sheets has been studied by using crystalline plasticity finite element analysis (FEA) and experiments. It was confirmed that one of the important reasons why the conventional high-strength steel sheet has poor formability was due to lack of {111} fiber texture components —γ-fiber texture—. In this paper, a texture adjusted design method is proposed to improve the formability of conventional high-strength steel sheets. First, an artificial γ-fiber texture is defined in terms of a rotationally symmetric Gaussian distribution of deviation angles, which has a certain scatter width along the given γ-fiber skeleton line in Euler space. The analytic textures are designed by introducing the artificial γ-fiber texture into the conventional high-strength steel model. The blending coefficient corresponding to the {111}/{001} volume fraction ratio is selected as the design parameter. Then, an optimum crystallographic texture of steel sheet is found through the limit dome height (LDH) formability tests by employing as objective function, which is evaluated by a maximum thinning ratio of the deformed sheet. Further, it is demonstrated that the sheet with the optimum texture shows the best straining in VDI benchmark stamping tests.     

Correlation between anisotropy in the normal-state mass renormalization and anisotropy in the superconducting energy gap for zinc

A nonlocal pseudopotential model for the Fermi surface and band structure of zinc together with the cyclotron resonance data is used to calculate the orbital average of the normal-state mass renormalization _CR ⁱ for numerous cyclotron orbits on the Fermi surface of zinc. We find that the mass renormalization _k is constant for each sheet of the Fermi surface but that it varies from sheet to sheet. Using the Eliashberg equation, this anisotropy in _k is correlated to anisotropy in the superconducting energy gap _k . For zinc we predict three distinct energy gaps, in the ratio of 2.30:1.76:1.00, corresponding respectively to the lens, the monster, and the cap sheets of the Fermi surface. Experimental evidence for this anisotropy in _k is provided by various measurements of the electronic properties in the superconducting state such as the low-temperature specific heat, microwave absorption, ultrasonic attenuation, effect of alloying on the critical temperature, and the temperature dependence of the critical field. We show that the results of these experiments are consistent with our predictions.Research supported by the National Science Foundation and the Advanced Research Project Agency. Submitted to the Department of Physics, The University of Chicago, in partial fulfillment of the requirements for the Ph.D. degree.     

Effect of annealing on formability of aluminium grade 19000

Forming limit diagrams are used by the stampers to solve sheet metal forming problems. In practice, sheet metals have been subjected to various combinations of strain. Necking during sheet metal forming, sets the limit to which the sheet metal can be formed. Forming limit diagram is an effective tool to evaluate the formability of sheet metal in various strain conditions. The information upon the formability of the sheet metal is important for both sheet metal manufacturers and users. In this work, a study has been made on the formability of aluminium 19000 grades annealed at three different temperatures namely 160 °C, 200 °C and 300 °C for sheet thickness of 2.00 mm. The tensile properties and formability parameters were experimentally evaluated and they are related to forming limit diagram. Strain distribution profiles obtained from the forming experiment have been analyzed. The fractured surface of the formed samples were viewed using scanning electron microscope (SEM) and the SEM images were correlated with fracture behaviour and formability of sheet metal. The sheet which is annealed at 300 °C has been found to possess good drawability and stretchability compared to other two annealed sheets.