Effect of inverted pressurisation profile on deformation characteristics of 5083 aluminium alloy during superplastic forming

Abstract

Decreasing the cycle time for superplastic forming of a commercially available superplastic 5083 aluminium alloy has been studied in the present work by use of an inverted pressurisation profile. A right cylindrical cup with a depth/ diameter ratio of 0·5 could be superplastically gas pressure formed in less than 100 s. The deformation behaviour was similar to that of constant strain forming during the free bulging stage. In this stage, a stress state gradient from the pole to the edge of the formed dome was observed. Plasticity controlled growth of cavities was thought to be the mechanism for the increase of cavity volume fraction during forming. After the centre point of the deformed sheet touched the die surface, the metal flow pattern was found to be different from that of the traditional approach. The minimum thickness was not located at around the bottom corner of the cylindrical cup rather it was located ~ 7.5 mm away from the bottom centre of the cup with radius 20 mm. Significant cavity nucleation and coalescence caused higher cavity growth rates at large strains, owing to the continuous increase in strain rate resulting from the imposed pressurisation profile.     

A Methodology for the Design of Effective Cooling System in Injection Moulding

In this work, the forming behaviour of a commercial sheet of AZ31B magnesium alloy at elevated temperatures is investigated and reported. The experimental activity is performed in two phases. The first phase consists in free bulging test and the second one in analysing the ability of the sheet in filling a closed die. Different pressure and temperature levels are applied. In free bulging tests, the specimen dome height is used as characterizing parameter; in the same test, the strain rate sensitivity index is calculated using an analytical approach. Thus, appropriate forming parameters, such as temperature and pressure, are individuated and used for subsequent forming tests. In the second phase, forming tests in closed die with a prismatic shape cavity are performed. The influence of relevant process parameters concerning forming results in terms of cavity filling, fillet radii on the final specimen profile are analysed. Closed die forming tests put in evidence how the examined commercial magnesium sheet can successfully be formed in complicated geometries if process parameters are adequately chosen.     

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.     

预拉深对气胀钛合金筒形件壁厚均匀性的影响

以钛合金筒形件为研究对象,针对传统气胀成形存在减薄率过大问题,开展了预拉深-气胀复合成形方法研究。采用数值模拟分析了传统气胀成形中筒底减薄率过大的原因,实验研究了不同加载路径对TA15筒形件减薄率的影响。结果表明:气胀成形的加载速度对减薄率有一定的影响,慢速气胀和快速气胀成形件最大减薄率分别为63.2%、54.2%,仅提高加载速度并不能满足壁厚均匀性要求;在有一定预拉深的情况下,通过热拉深-气胀复合成形,最大减薄率可减小到36.7%。热拉深-气胀复合成形工艺可有效改善成形件的壁厚分布均匀性.     

Application of rigid–plastic meso-damage constitutive theory to superplastic bulge forming

Abstract

A rigid–plastic meso-damage constitutive theory is applied to predict the influence of void evolution on the superplastic forming process. Together with this constitutive theory, a simple analytical model is used to simulate the bulge forming process of a spherical dome. The distribution and evolution of damage, deformation, geometrical parameters, and mechanical properties such as strength and ductility throughout the bulging dome are systematically explored. Since a series of numerical results obtained are in good agreement with the actual phenomena and previous experimental results, the applicability of the rigid–plastic meso-damage constitutive theory to superplastic forming processes is considered proven.     

Numerical study of superplastic deformation with superimposed pressure for cavity sensitive materials

Abstract

The non-uniform deformation (necking and thinning) development and fracture of superplastic materials under both uniaxial tension and circular sheet bulging are numerically analysed by considering the effects of strain rate sensitivity and cavity growth with superimposed pressure. It is found that the fracture mode, which is controlled by both strain rate sensitivity and cavity growth rate, can be changed by superimposed pressure from fracture without external necking for cavity sensitive alloys at zero pressure to fracture with necking development or extensive thinning at pressure large enough to completely suppress cavity growth. Fracture mechanism diagrams are presented which enable prediction of the fracture mode to be made as a function of material parameters and pressure conditions for uniaxial tension and bulging.

MST/724     

空化水射流冲击微成形数值模拟与实验研究

利用空化水射流中空泡溃灭产生的高压冲击波使TA2箔材产生微塑性变形,分析水射流主要工艺参数对微成形质量的影响规律。采用ANSYS/LS-DYNA数值模拟和实验研究对比的方法,通过不同冲击压力和持续时间对成形深度、成形件厚度减薄率影响的数值模拟,并进行成形工件形貌的实验验证。结果显示,对于不同冲击压力峰值和冲击压力持续时间下的板料成形,随冲击压力峰值和持续时间的增加,其成形深度、成形件厚度减薄率呈正比增加;当数值模拟冲击压力峰值Pmax=2.2 GPa、冲击压力持续时间t=40 ns时,与实验入射压力P=20 MPa、冲击时间t=1 min时的试样截面轮廓成形曲线和截面厚度减薄结果比较一致。TA2箔材微成形件圆孔阵列特征在空化冲击区内表面质量良好,具有较大的成形深度及其较好的成形深度一致性。     

An elasto‐plastic analysis of hydrostatic bulging of a circular sheet

Abstract

An incremental finite element method for large elasto‐plastic deformation of metal has been developed. The method takes account of mixed boundary data using the updated Lagrangian formulation variational procedure. The code developed is applicable to the process of hydrostatic bulging of a circular sheet clamped at its periphery. The computation algorithm is based on a convenient explicit form of the hydrostatic pressure load correction stiffness matrix for isoparametric elements. Computer programs are able to determine stress, strain, residual stress, surface profile and spring back. Most predictions agree favorably with experimental results.     

5A06 铝合金板材超塑气胀成形数值模拟

目的改善5A06铝合金板材超塑性气胀成形件壁厚分布。方法采用MARC有限元分析方法,对商业供货态5A06-O铝合金板材(原始厚度为2 mm)的超塑性成形进行数值模拟分析。结果优化后的反吹预减薄变形,使杯形件最终壁厚分布大大改善,最薄处壁厚从单纯正吹胀形时的0.65 mm提高到了0.94 mm,壁厚均匀性指数达到0.079。结论合理的反胀形模具可以增加最小壁厚,达到提高壁厚均匀性的目的。     

An enhanced displacement adjustment method: Springback and thinning compensation

In the paper, we present an enhanced numerical method for the forming tool design optimisation in sheet metal forming. The applied procedures enable a determination of appropriate forming tool geometry so that manufacture of a formed product with specified geometry would be ensured. Apart from springback occurred by the formed part after removal of the forming tools also impact of thinning of the sheet metal during the forming process is considered in the method, and both effects are correspondingly compensated in an iterative procedure of the forming tool geometry determination. The enhanced displacement adjustment method (E-DA) is based on the well-known displacement adjustment (DA) method the application of which is indeed relatively simple, but has proved also, due to an increased number of iterations needed to achieve the required tolerance and possible loss of accuracy, to be less successful when forming of parts with more complex geometry is considered. Computational efficiency in the E-DA method is achieved by applying additional point topology mappings, which establish corresponding interrelations between the discretised point topologies used in the definition of the prescribed product geometry, current tool geometry and on this basis actually computed product geometry, contributing thus significantly in improving the accuracy of communicated data. The advantage of the improved method over the conventional DA method is demonstrated by considering the forming tool design optimisation in channel bending and forming of cylindrically symmetric products.     

Limit Strains Comparison during Tube and Sheet Hydroforming and Sheet Stamping Processes by Numerical Simulation

Hydroforming is a manufacturing process that uses a fluid medium to form a component by using high internal pressure. Tube and sheet hydroforming has gained increasing interest in the automotive and aerospace industries because of its many advantages such as part consolidation, good quality of the formed parts etc. The main advantage is that the uniform pressure can be transferred to every where at the same time. Forming limit is the limit of the component up to that extent it can be formed safely. While analyzing hydroforming process, it is often assumed that the limit strains are identical as that of stamped sheet metal of equivalent material properties. It is not clear if such an assumption is valid. In this paper the forming limit strains during hydroforming is predicted. A series of tube bulge tests for tube hydroforming and limiting dome height test for sheet hydroforming and sheet stamping processes are simulated by a commercial finite element solver to predict the limit strains. Numerical simulation of forming limit strains in tube hydroforming with different internal pressure and different simulation set up with or without axial feeding, while in sheet hydroforming and sheet stamping, by changing the specimen geometry are considered to develop wide range of strain paths in the present work. The effects of process conditions on the forming limit strains are detailed. The comparison of limits strains during hydroforming and stamping processes is presented. Prediction of limits strains is based on a novel thickness based necking criterion.     

渐进成形A3003铝板减薄带分析及数值模拟研究

研究渐进成形过程中板料减薄带的变化,可以提供合理的加工参数,提高板料的成形性能和加工利用率,减少零件破裂失效.基于渐进成形过程中金属板料轮廓的变化与理想情况下轮廓的区别,对渐进成形初始成形阶段A3003铝板减薄带的产生原因和剪切力的变化过程进行了理论分析,并通过有限元模拟分别从未变形区金属板料的长度和强度两个角度对板料渐进成形过程中未变形区下沉的影响,以及成形角度和杨氏模量对变形区回弹的影响两个方面,对减薄带的产生原因进行研究.结果表明:板料未变形区的下沉和变形区的回弹使板料在初始加工阶段形成一段平缓区域,工具头在平缓区域的变形性质发生了变化,平缓区域发生剪切变形导致了板料在初始加工阶段形成了减薄带;渐进成形时减小板料未变形区的长度,增大板料与垂直方向的角度可以一定程度上阻碍减薄带的产生,模拟结果与理论分析相符合.     

Analytical prediction of pressure‐time curve for free inflation of superplastic sheet into hemispherical dome

Abstract

Blow forming is a commonly used production method to deform superplastic sheets. Several papers have appeared to analytically predict the pressurization sequence needed to control the inflation process at a desired strain rate. Some of them, however, are based on an over‐simplified assumption of uniform thickness reduction during the inflation, which often leads to non‐conservative prediction of pressure‐time path. Others have been more rigorous in the formulation, which on the other hand, results in more extensive equations requiring iterative method with a computer program. This paper presents a modified approach. Ignoring the assumption of uniform thinning, a set of physically sound yet easy‐to‐use equations for obtaining pressure‐time curves, heights of the deforming dome, and thickness variations both in space and time, are derived. The analytical results compare favorably with the experimental data and more elaborate finite element solutions.     

Microstructure and texture evolution during incremental sheet forming of AA1050 alloy

In incremental sheet forming higher limiting strain can be achieved compared to the conventional sheet metal forming process, which results in increased formability. The higher level of strain may be accompanied by non-uniform thinning. Thus, the different sections in a component may undergo different levels of deformation. In the present work a truncated cone of AA1050 H14 alloy was formed using the incremental sheetmetal forming (ISF) technique. The deformation mechanism during ISF was studied by investigating the microstructural and texture evolution in the truncated cone along the thickness of the cone wall. High resolution electron backscatter diffraction was performed at different sections of the formed truncated cone. The results show the formation of subgrains in different sections of the cone. At higher strains, grains become thin and elongated which results in grain fragmentation and formation of small grains. These small grains undergo complete recovery process and new grain boundaries (low and high angle) are formed within the thin elongated grains. Further, the evolution of shear texture shows the evidence of shear mode of deformation during incremental sheet forming. Thus, the presence of through thickness shear could be used for understanding the higher forming limit in the ISF process.

     

大拉深比薄壁筒形件充液成形过程数值模拟

目的利用充液成形工艺成形普通拉深工艺难成形的大拉深比筒形件。方法通过理论公式计算了冷冲压工艺成形该制件的道次,利用有限元软件Dynaform对充液成形过程进行了3个步骤模拟,并研究了第1步拉深时初始反胀高度对成形制件减薄率的影响规律。结果利用理论公式计算,传统冲压方法成形拉深比为3.2的筒形件至少需要5个道次,而采用被动式充液成形方法只需要3个道次。每个道次的最大减薄率都在8%以内,最后得到拉深制件的最大减薄率为8.53%,在安全范围以内;第1步充液拉深时,反胀高度分别为1.75,2.75,3.75,4.75,5.75 mm时,得到制件的最大减薄率分别为5.28%,5.08%,4.8%,5.03%,5.03%。结论充液成形工艺较传统冲压工艺可以大大提高板料的成形极限,减少成形道次,成形制件质量好;合适的初始反胀高度,可以减小成形制件壁厚的最大减薄率。     

On the pressure forming of two superplastic alloys

Superplastic forming of the Ti-6Al-4V and Sn-Pb eutectic alloys was attempted using the pressure forming (sheet thermoforming) process. It has been demonstrated that true hemispheres could be formed out of sheets of both the alloys. The thickness strains in both the alloys were less than those predicted theoretically and this could be traced to material flow from the flange and gripped regions. This flow, however, was greater in case of the titanium alloy than the Sn-Pb alloy, on account of the greater strain-rate sensitivity of the former material. Due to the same effect, the thinning factor actually increased with deformation in the titanium alloy, but it decreased on increasing deformation in the Sn-Pb alloy. Within the experimental range, the hold-down pressure (titanium alloy) and initial sheet thickness (Sn-Pb alloy) had very small effects, although the deformation became slightly more uniform on decreasing the hold-down pressure or increasing the initial sheet thickness. The thickness and circumferential strains increased with deformation and in particular when the bulge height (h ₀) to base diameter (D ₀) ratio was greater than 0.35, non-uniformity in deformation along the bulge profile became noticeable. These strains were largest at the pole and its vicinity. On account of its lower strain-rate sensitivity, these effects were more pronounced in the Sn-Pb alloy than in the titanium alloy. Although initially the bulging rate was rapid, later the (h ₀/D ₀) ratio increased linearly with the forming time and at any instant the bulge profile corresponded to an arc of a circle.     

Mechanical experiments on the superplastic material ALNOVI-1, including leak information

In subatomic particle physics, unstable particles can be detected with a so-called vertex detector, placed inside a particle accelerator. A detecting unit close to the accelerator bunch of charged particles must be separated from the accelerator vacuum. A thin sheet with a complex 3D shape prevents the detector vacuum from polluting the accelerator vacuum. Therefore, this sheet has to be completely leak tight. However, this can conflict with restrictions concerning maximum sheet thickness of the product. To produce such a complex thin sheet, superplastic forming can be very attractive in cases where a small number of products is needed. In order to predict gas permeability of these formed sheets, many mechanical experiments are necessary, where the gas leak has to be measured. To obtain insight in the mechanical behaviour of the used material, ALNOVI-1, tensile experiments were performed to describe the uniaxial stress-strain behaviour. From these experiments, a high strain rate sensitivity was measured. The flow stress of this material under superplastic conditions was low and the material behaved in an isotropic manner upon large plastic strains. The results of these experiments were used to predict the forming pressure as a function of time in a free bulge experiment, such that a predefined target strain rate will not be exceeded in the material. An extra parameter within these bulging experiments is the application of a hydrostatic pressure during the forming process. Such a pressure postpones the nucleation and growth of internal cavities, which means that higher plastic strains can be reached before failure. Results from these experiments showed that at higher hydrostatic pressures, higher bulges were made. All these bulges were leak tested, showing also that higher hydrostatic pressures lead to a lower void volume fraction at higher hydrostatic pressures, since these bulges were more leak tight at the same bulge height than bulges made without the application of this pressure. This article describes the setup and results of the uniaxial (tensile) and biaxial (bulging) experiments on the superplastic aluminium ALNOVI-1.     

Experimental investigation and finite element modeling on profile forming of conical component using Al 3003(O) alloy

Profile forming of sheet metal is a special technique that offers flexibility and cost-effectiveness in the metal forming process, requiring no high capacity presses or set of dies, thus meeting the ever increasing demand for small batch production and rapid prototyping. This paper demonstrates the formation of sheet metal part using the CNC controlled hemispherical tool and analyses the various parameters underlying this mechanism – like maximum wall angle, surface roughness (Ra) and thinning of sheet. Analysis of the microstructure is carried out using Scanning electron microscopy and Energy-dispersive X-ray spectroscopy microanalysis test. In this study, Aluminum sheet of grade Al 3003(O) with 1 mm and 1.25 mm thickness is used as a work piece. The paper also presents an explicit numerical simulation using the standard finite element code ABAQUS and the experimental and numerical results are validated.     

Effect of bulge shape on wrinkling formation and strength of stainless steel thin sheet

Embossing and restoration is a simple method to increase the strength of thin sheet metals by creating concentric pattern of wrinkles onto thin sheet-metal plates. SUS304 stainless steel sheets of 0.4 mm thickness were experimentally bulged using hemispherical punches, and then compressed between two flat platens of a testing machine to produce the wrinkles. The process simulation was carried out using finite element method to investigate the effect of bulge shape and height on the wrinkling formation and sheet strength. Thickening of the plate was observed during the compression step and after the thinning that is occurred during the bulging step. Different restoration patterns were observed as bulge height increases. Also, the number of wrinkles within a pattern and its distribution throughout the plate were depending upon the bulged shape. The pressure bearing capacity of the compressed part rapidly accelerates with the increase of wrinkles inclusion. Virtual deflection tests were carried also out using the finite element method (FEM) to evaluate the effect of the wrinkles pattern on the thin plate rigidity or strength. The results showed that the technique is effective in enhancing the strength, the flexural deflection and reducing the failure risk of thin sheet metals parts. This can contribute in the weight reduction of sheet metal parts. The observed patterns can be utilized in sheet decoration.     

反向压力对板材粘性介质压力胀形的影响研究

为研究带有反向压力粘性介质压力胀形过程中接触条件对板材成形性的影响规律,利用DEFORMTM-2D结合韧性断裂准则对覆层板粘性介质压力胀形过程进行有限元分析.结果表明:接触表面无摩擦单纯依靠反向压力能够提高板材成形极限,随着接触表面摩擦系数增大,板材壁厚分布愈均匀,板材的破裂位置由试件顶端转移到凹模圆角处,板材成形极限显著提高.因此,在三维应力状态下有效控制板材所受法向压力和界面摩擦力可以提高板材成形性.