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.     

Stress gradient due to incremental forming of bonded metallic laminates

The residual stress and associated gradient can affect the performance of a material/component during service. Sheet-metal in incremental sheet forming (ISF) due to missing back support may experience high stress gradient across the thickness. The current work is aimed at experimentally analyzing the through-thickness stress gradient in the Cu/steel bonded laminates after ISF deformation. It is found that ISF induces compressive stress gradient, which can be a way greater (about 18 times) than that the rolling process induces in the parent laminates while bonding, specifically when the deformation angle is high. Further, the tool imposes more stress gradient (1–50% depending on the forming conditions) in its motion direction than that in the transverse (or stretching) direction. Moreover, un-strained Cu/steel laminated sheet experiences higher (25%–68%) gradient than that the pre-strained/rolled sheet endures. Regarding the role of technological parameters, high angle, small tool, average step-size and spindle rotation, and low flow-stress induce high stress gradient. The tension tests of the ISFed samples reveal that the post-ISF tensile strength of laminated sheet increases as the stress gradient increases, thus showing a direct relationship between stress gradient and strain hardening in ISF. Finally, models are proposed to predict the stress gradient in the ISFed Cu/steel components.     

Microstructure and texture evolution in low carbon steel deformed by differential speed rolling (DSR) method

The study examined the microstructural and textural evolution of low carbon steel samples fabricated using a differential speed rolling (DSR) process with respect to the number of operations. For this purpose, the samples were deformed by up to 4-pass of DSR at room temperature with a roll speed ratio of 1:4 for the lower and upper rolls, respectively. The DSR technique applied to low carbon steel samples resulted in a microstructure composed of ultrafine ferrite grains, approximately 0.4 µm in size, after 4-pass with a high-angle grain boundary fraction of ~65 %. The microstructural features of the ferrite phase indicated the occurrence of continuous dynamic recrystallization, beginning with the formation of a necklace-like structure of ultrafine equiaxed grains around the elongated grains, which were formed in the early stages of deformation, and ending with ultrafine recrystallized grains surrounded by boundaries with high angles of misorientations. In the pearlite phase, the microstructural changes associated with DSR deformation were presented by the occurrence of bending, kinking, and breaking of the cementite lamellar plates. In addition, the evolution of texture after DSR processing was affected by shear deformation and rolling deformation, leading to the formation of a texture composed of fractions of components with shear texture orientations such as {110} 〈001〉 (Goss) and orientations close to {112} 〈111〉, in addition to rolling texture components consisting mainly of α-fiber and γ-fiber.     

金属板材数控渐进成形工艺的研究进展

为了总结过去十几年国内外学者对板材数控渐进成形工艺技术的研究进展,对渐进成形工艺成形机理方面的研究成果进行了综述,分析了其材料变形的特点;全面概述了近年来国内外学者有关成形工艺参数对成形极限、成形精度、表面质量及能耗和效率的影响方面的研究成果,并介绍了国内外金属板材渐进成形装备的研究进展,最后对新兴的板材渐进成形工艺进行了总结概括。现有研究表明,成形件几何精度、表面质量和成形效率等方面的不足仍然是制约该技术广泛工业化应用的关键问题,同时渐进成形件的形性协同控制机理也亟待研究。     

Microstructure and texture evolution during the drawing of tungsten wires

Tungsten wires develop during their forming process a pronounced fibre texture that causes anisotropic deformation of single grains. The aim of this work is to simulate the crystallographic texture and microstructure evolution that arises during wire drawing using two different texture models. A visco-plastic self-consistent model that allows simulations using a large number of grains is compared with a crystal plasticity finite element model that provides a more detailed insight into the wire’s microstructure. Texture predictions of both models are discussed and quantitatively compared with experimental texture measurements obtained by neutron diffraction. The developed fibre texture causes plane strain deformation of single grains, which induces grain curling. The prediction of grain curling is of importance because it allows studying the residual stresses that trigger splits, at the grain level.     

热轧工艺对Cr12钢表面起皱的影响机制

研究了热轧工艺对Cr12钢表面起皱相关的组织和织构演变的影响,重点关注其对变形组织不均匀和晶粒簇形成的改变。结果表明：随着热轧终轧温度由960降低到850℃,在变形组织中带状晶粒减薄,剪切变形比重增大,含有剪切带的晶粒增多,由表层向中心层逼近,中心层附近光滑带状晶粒减少;在退火组织中由含有剪切带晶粒形成的高r值晶粒簇增...     

Finite element modelling and experimental investigation of single point incremental forming process of aluminum sheets: influence of process parameters on punch force monitoring and on mechanical and geometrical quality of parts

New trends in sheet metal forming are rapidly developing and several new forming processes have been proposed to accomplish the goals of flexibility and cost reduction. Among them, Incremental CNC sheet forming operations (ISF) are a relatively new sheet metal forming processes for small batch production and prototyping. In single point incremental forming (SPIF), the final shape of the component is obtained by the CNC relative movements of a simple and small punch which deform a clamped blank into the desired shape and which appear quite promising. No other dies are required than the ones used in any conventional sheet metal forming processes. As it is well known, the design of a mechanical component requires some decisions about the mechanical resistance and geometrical quality of the parts and the product has to be manufactured with a careful definition of the process set up. The use of computers in manufacturing has enabled the development of several new sheet metal forming processes, which are based upon older technologies. Although standard sheet metal forming processes are strongly controlled, new processes like single point incremental sheet forming can be improved. The SPIF concept allows to increase flexibility and to reduce set up costs. Such a process has a negative effect on the shape accuracy by initiating undesired rigid movement and sheet thinning. In the paper, the applicability of the numerical technique and the experimental test program to incremental forming of sheet metal is examined. Concerning the numerical simulation, a static implicit finite element code ABAQUS/Standard is used. These two techniques emphasize the necessity to control some process parameters to improve the final product quality. The reported approaches were mainly focused on the influence of four process parameters on the punch force trends generated in this forming process, the thickness and the equivalent plastic deformation distribution within the whole volume of the workpiece: the initial sheet thickness, the wall angle, the workpiece geometry and the nature of tool path contours controlled through CNC programming. The tool forces required to deform plastically the sheet around the contact area are discussed. The effect of the blank thickness and the tool path on the punch load and the deformation behaviour is also examined with respect to several tool paths. Furthermore, the force acting on the traveling tool is also evaluated. Similar to the sheet thickness, the effect of wall angle and part geometry on the load evolution, the distribution of calculated equivalent plastic strain and the variation of sheet thickness strain are also discussed. Experimental and numerical results obtained allow having a better knowledge of mechanical and geometrical responses from different parts manufactured by SPIF with the aim to improve their accuracy. It is also concluded that the numerical simulation might be exploited for optimization of the incremental forming process of sheet metal.     

Strain evolution in the single point incremental forming process: digital image correlation measurement and finite element prediction

Incremental Sheet Forming (ISF) is a relatively new class of sheet forming processes that allow the manufacture of complex geometries based on computer-controlled forming tools in replacement (at least partially) of dedicated tooling. This paper studies the straining behaviour in the Single Point Incremental Forming (SPIF) variant (in which no dedicated tooling at all is required), both on experimental basis using Digital Image Correlation (DIC) and on numerical basis by the Finite Element (FE) method. The aim of the paper is to increase understanding of the deformation mechanisms inherent to SPIF, which is an important issue for the understanding of the high formability observed in this process and also for future strategies to improve the geometrical accuracy. Two distinct large-strain FE formulations, based on shell and first-order reduced integration brick elements, are used to model the sheet during the SPIF processing into the form of a truncated cone. The prediction of the surface strains on the outer surface of the cone is compared to experimentally obtained strains using the DIC technique. It is emphasised that the strain history as calculated from the DIC displacement field depends on the scale of the strain definition. On the modelling side, it is shown that the mesh density in the FE models plays a similar role on the surface strain predictions. A good qualitative agreement has been obtained for the surface strain components. One significant exception has however been found, which concerns the circumferential strain evolution directly under the forming tool. The qualitative discrepancy is explained through a mechanism of through-thickness shear in the experiment, which is not fully captured by the present FE modelling since it shows a bending-dominant accommodation mechanism. The effect of different material constitutive behaviours on strain prediction has also been investigated, the parameters of which were determined by inverse modelling using a specially designed sheet forming test. Isotropic and anisotropic yield criteria are considered, combined with either isotropic or kinematic hardening. The adopted constitutive law has only a limited influence on the surface strains. Finally, the experimental surface strain evolution is compared between two cones with different forming parameters. It is concluded that the way the plastic zone under the forming tool accommodates the moving tool (i.e. by through-thickness shear or rather by bending) depends on the process parameters. The identification of the most determining forming parameter that controls the relative importance of either mechanism is an interesting topic for future research.     

板厚对08Al薄板单点渐进成形中成形极限的影响

目的 针对渐进成形中成形极限测量难的问题,提出一种新的评定成形极限方法.方法 选用08Al为实验材料,通过模拟和实验相结合的方法,研究不同板厚下成形极限角和减薄率的关系,提出利用成形极限角和最大减薄率2个参数组合的方法判断薄板的成形极限,并通过数控实验验证提出方法的准确性,分析板厚对单点渐进成形工艺成形极限的影响.结果...     

Nano grained AZ31 alloy achieved by equal channel angular rolling process

Equal channel angular rolling (ECAR) is a severe plastic deformation process which is carried out on large, thin sheets. The grain size could be significantly decreased by this process. The main purpose of this study is to investigate the possibility of grain refinement of AZ31 magnesium alloy sheet by this process to nanometer. The effect of the number of ECAR passes on texture evolution of AZ31 magnesium alloy was investigated. ECAR temperature was controlled to maximize the grain refinement efficiency along with preventing cracking. The initial microstructure of as-received AZ31 sheet showed an average grain size of about 21 μm. The amount of grain refinement increased with increasing the pass number. After 10 passes of the process, significant grain refinement occurred and the field emission scanning electron microscopic (FESEM) micrographs showed that the size of grains were decreased significantly to about 14-70 nm. These grains were formed at the grain boundaries and inside some of the previous larger micrometer grains. Observation of optical microstructures and X-ray diffraction patterns (XRD) showed the formation of twins after ECAR process. Micro-hardness of material was studied at room temperature. There was a continuous enhancement of hardness by increasing the pass number of ECAR process. At the 8th pass, hardness values increased by 53%. At final passes hardness reduced slightly, which was attributed to saturation of strain in high number of passes.     

板料数控渐进成形中变形力的研究

研究了板料数控渐进成形变形力的2种计算方法:按照纯剪切变形的方式计算变形力;按照剪切和弯曲综合变形的方式计算变形力。同时,通过实验测量出渐进成形的实际变形力。通过比较,实验测定的变形力与假定剪切弯曲变形计算的变形力相近,可以认为渐进成形是一种剪切弯曲变形。     

Titanium based cranial reconstruction using incremental sheet forming

In this paper, we report recent work in cranial plate manufacturing using incremental sheet forming (ISF) process. With a typical cranial shape, the ISF process was used to manufacture the titanium cranial shape by using different ISF tooling solutions with and without backing plates. Detailed evaluation of the ISF process including material deformation and thinning, geometric accuracy and surface finish was conducted by using a combination of experimental testing and Finite Element (FE) simulation. The results show that satisfactory cranial shape can be achieved with sufficient accuracy and surface finish by using a feature based tool path generation method and new ISF tooling design. The results also demonstrate that the ISF based cranial reconstruction has the potential to achieve considerable lead time reduction as compared to conventional methods for cranial plate manufacturing. This outcome indicates that there is a potential for the ISF process to achieve technological advances and economic benefits as well as improvement to quality of life.     

Formation of a random texture and ultrafine grains in AA 3003 aluminium alloy during the repeated shear deformation introduced by continuous confined strip shearing

Abstract

To investigate the microstructural development and corresponding texture evolution during repeated shear deformation, specimens of AA 3003 Aluminium alloy were deformed by continuous confined strip shearing based on equal channel angular pressing. Strip specimens were deformed by the shear forming process during up to eight passes, equivalent to effective strains of ~4.8. Texture evolution in the AA 3003 strips during the shear deforming process was studied by comparing the experimentally measured textures with simulated ones. Electron backscattered diffraction was employed to investigate detailed changes in microtextures and microstructures during repeated shear deformation. Softening associated with deformation is believed to be responsible for the formation of ultrafine grains and the random texture resulting from repeated shear deformation.     

Improving geometrical accuracy for flanging by incremental sheet metal forming

Incremental Sheet Forming (ISF) is a manufacturing technology for individualized and small batch production. Among the opportunities this technology provides there is the possibility of a short ramp-up time and to cover the whole production chain of sheet metal parts by using a single reconfigurable machine set-up. Since recent developments proved that manufacturing of industrial parts is feasible, finishing operations such as flanging and trimming gain importance, which are an integral part of manufacturing process chains of many sheet metal parts. This paper analyses the technological capabilities of performing flanging operations by ISF. Due to the localized forming zone and the absence of surrounding clamping devices, ISF exhibits a different material flow than conventional flanging processes. In this paper, the influence of the tool path characteristics, the flange length as well as the flange radius is analysed in order to establish a process window and to compare it to the process limits of conventional flanging operations. Since geometrical deviations occur when flanging operations are performed by ISF, a new adaptive blank holder is developed, which acts in the vicinity of the forming tool and reduces unwanted deformation outside the primary forming zone. The experimental results show the benefits of the adaptive blank holder with respect to geometric accuracy. The established process window and the adaptive blank holder hence contribute to the applicability of incremental flanging operations, such that ISF can be used for all forming and flanging operations along the process chain.     

Effect of layered microstructure and its evolution on superplastic behaviour of AA 8090 Al–Li alloy

Abstract

Superplastic forming grade sheets of AA 8090 Al–Li alloy were observed to contain layers of different microstructure and microtexture across the thickness cross-section. Superplastic behaviour and its relationship to the concurrent microstructural and microtextural evolution of this sheet were studied at 803 K by tensile testing of specimens taken from the full thickness and the near surface and midthickness layers. Initially, the surface layers contained nearly equiaxed and relatively coarse grains with a strong S {123}〈634〉 type texture, whereas the midthickness section had elongated fine grains and a dominant Bs {011}〈211〉 texture. The stress–strain rate (σ–ε) curves exhibited minimum flow stress for the full thickness material. Varying levels of grain growth and texture weakening occurred in the above two layers, the extent of which depended on whether the layers were in separated form or as coexistents in the full thickness material. The maximum values of strain rate sensitivity index for the full thickness, surface, and centre materials were 0.82, 0.64, and 0.56, respectively. The corresponding ductility values were 475, 420, and 286% at ε=1×10^-3 s^-1.     

An investigation into thickness distribution in single point incremental forming using sequential limit analysis

Single point incremental forming (SPIF), needing no dedicated tools, is the simplest variant of incremental sheet metal forming processes. In the present work, a simplified model of SPIF of a truncated cone, capable of predicting the thickness distribution, has been developed using sequential limit analysis (SLA). The obtained results were validated experimentally and compared with thickness predictions obtained from an explicit shell FE model implemented in Abaqus. It is shown that SLA is capable to solve the thickness prediction problem more accurately and efficiently than the equivalent FEA approach. As an application of the proposed model, the effect of the diameter of the hemispherical tool tip and the step down on the thickness distribution and the minimum thickness in a 50° cone is studied using SLA. By introducing bending and stretching zones in the wall of the cone, variations of the minimum thickness by changing the tool diameter and the step down are discussed.     

A Polycrystal Model to Evaluate Mechanical Properties of Asymmetrically Rolled AL Sheets

The asymmetric rolling process (ASR) differs from conventional rolling (CR) through the use of different roll circumferential velocities. Using proper parameters, asymmetric rolling imposes intense shear deformations across the sheet thickness, leading not only to the occurrence of shear texture, but also to grain refinement [1]. Some shear texture components are known to improve plastic strain ratio values, and consequently formability. In Simões et al. [4], a AA1050-O sheet was asymmetrically rolled and annealed. Shear texture was obtained, as opposed to typical gamma-fiber texture obtained on sheets rolled through the conventional process. Shear tests were used to evaluate strength and formability. A polycrystal plasticity model, as formulated by Gambin [2] and implemented by Alves de Sousa [3], was employed to evaluate texture evolution and to give a sounding theoretical basis for the improved mechanical properties on sheets after the process. For FCC materials, this approach avoids the uniqueness issue related to the choice of the set of active slip systems by applying a regularized Schmid Law. Consequently, it generates yield surfaces with smooth corners where the normal vector is always uniquely defined. In the following sections, implementation guidelines are given. The accuracy of simulation results and the advantages of the asymmetric rolling process, when compared to conventional rolling, are the main topics of discussion.     

Rigid plastic model of incremental sheet deformation using second‐order cone programming

This paper describes a method for numerically modelling the incremental plastic deformation of shells and applies the method to incremental sheet forming (ISF). An upper bound finite element shell model is developed based on sequential limit analysis under the rigid plastic assumption, which is solved by manipulating the problem into the form of a second‐order cone program (SOCP). Initially, the static upper bound plate problem is investigated and the results are compared with the existing literature. The approach is then extended to a shell formulation using a linearized form of the Ilyushin yield condition and two methods for treating the Ilyushin condition are presented. The model is solved efficiently using SOCP software. The resulting model shows good geometric agreement when validated against an elasto‐plastic model produced using existing commercial software and with measurements from a real product produced using ISF. Copyright © 2008 John Wiley & Sons, Ltd.     

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.     

Effect of asymmetrical rolling and annealing the mechanical response of an 1050-o sheet

The asymmetrical rolling process has been studied as a way to promote intense shear deformations across the sheet thickness. These shear deformations may lead, given the proper conditions, to the development of shear texture components ({001}<110>, {111}<110> and {111}<112>) and also grain refinement. In this work, a 1050-O sheet is asymmetrically rolled and annealed. Conventional rolling is also performed, for comparison purposes. Shear texture components are obtained for the asymmetrically rolled specimens, and seem to be retained after annealing. Differences in mechanical response between asymmetrical and conventionally rolled specimens, as well as texture evolution after heat treatment processing are inferred based on experimental tensile and shear tests. Numerical simulations are used to help explain the differences found on experimental tests. It is proven that it is difficult to spread shear texture through the entire sheet thickness from a general asymmetric rolling process. Based on the fact, future research is discussed at closure.