Force prediction for single point incremental forming deduced from experimental and FEM observations

The aim of the study was to establish practical formulae allowing to predict the forces occurring during the single point incremental forming process. This study has been based on a large set of systematic experiments on the one hand and on results of finite elements modeling simulations on the other. This led to analytical formulae allowing to compute the three main components of the force for five selected materials in function of the working conditions (sheet thickness, wall angle, tool diameter, and step down) with a good precision. Moreover, a general model has been deduced, allowing to compute an approximate value for the force for any material, based on knowledge of the tensile strength only.     

Failure in single point incremental forming

This paper revisits the formability limits of single point incremental forming (SPIF) in the light of fundamental concepts of plasticity and ductile fracture mechanics. The paper has a twofold objective of investigating the limiting strain pairs at fracture in parts showing and not showing signs of necking before cracking and of demonstrating that failure by fracture occurs by tension in crack opening mode I. The overall methodology is based on the combination of circle grid analysis, measurement of the ‘gauge length’ strains at fracture and determination of fracture toughness from experimental tests performed with truncated conical SPIF parts and double edge notched test specimens loaded in tension. The work is performed in aluminium AA1050-H111 and is a step towards comprehension of the circumstances under which failure will occur in SPIF. It is shown that fracture strain pairs of truncated conical parts, fracture forming limit lines (FFLs) determined from conventional sheet formability tests and fracture toughness in crack opening mode I can be merged to create a new understanding of plastic flow and failure by fracture above the onset of necking.     

Effects of three parameters on forming force of the single point incremental forming process

This research has examined the effects of three parameter groups on the forming force of single point incremental forming (SPIF) process. The parameters under study include the material types (sheet aluminum, brass and copper), the forming angles (30°, 40° and 50°), and the tool revolution speeds (200, 400 and 600 rpm). The metal forming was carried out using a spherical edge tool which was pressed onto the metal surface to form work pieces of truncated pyramid shape. In the experiment, the forming forces were measured and analyzed to determine an optimal parameter combination, with regard to the material type, forming angle and revolution speed, for the SPIF process. The experimental results showed that all three parameter groups exerted varying influences over the forming force of the SPIF process. The findings indicated that the sheet brass exhibited the highest force value and that the smaller forming angle contributed to the greater forming force. In addition, the higher tool revolution speed resulted in the lower forming force.

     

On the potential of single point incremental forming of sheet polymer parts

The aim of this paper is to provide an overview into the development and application of single point incremental forming in rapid prototyping and rapid manufacturing of polymer sheet products that will enable the readers to recognize the key influential variables, to identify the process feasibility window, to diagnose possible sources of failure and to understand the routes for selecting the most appropriate materials and operative conditions. The methodology draws from independent determination of mechanical properties and formability limits of polymers to rapid prototyping of truncated conical and pyramidal parts. The investigation is supported by circle grid analysis. Results and observations are explained in the light of theoretical framework based on membrane analysis that is capable of modelling the cold plastic deformation of polymers with pressure-sensitive yield surfaces. The results show that the single point incremental forming of polymer sheets, performed on conventional CNC machining centres, is a cost-effective innovative technology for product development in a manufacturing environment. Applications may span from products with very high depths, taking advantage of the excellent formability of polyethylene terephthalate, to applications in polycarbonate where transparency is kept during cold forming.     

Optimization techniques applied to single point incremental forming process for biomedical application

Single point incremental forming (SPIF) process has the potential to replace conventional sheet forming process in industrial applications. For this, its major defects, especially poor geometrical accuracy, should be overcome. This process is influenced by many factors such as step size, tool diameter, and friction coefficient. The optimum selection of these process parameters plays a significant role to ensure the quality of the product. This paper presents the optimization aspects of SPIF parameters for titanium denture plate. The optimization strategy is determined by numerical simulation based on Box–Behnken design of experiments and response surface methodology. The Multi-Objective Genetic Algorithm and the Global Optimum Determination by Linking and Interchanging Kindred Evaluators algorithm have been proposed for application to find the optimum solutions. Minimizing the sheet thickness, the final achieved depth and the maximum forming force were considered as objectives. For results evaluation, the denture plate was manufactured using SPIF with the optimum process parameters. The comparison of the final geometry with the target geometry was conducted using an optical measurement system. It is shown that the applied method provides a robust way for the selection of optimum parameters in SPIF.     

Prediction and control of pillow defect in single point incremental forming using numerical simulations

Pillows formed at the center of sheets in Single point incremental forming (SPIF) are fabrication defects which adversely affect the geometrical accuracy and formability of manufactured parts. This study is focused on using FEA as a tool to predict and control pillowing in SPIF by varying tool size and shape. 3D Finite element analysis (FEA) and experiments are carried out using annealed Aluminum 1050. From FEA, it is found out that the stress/strain state in the immediate vicinity of the forming tool in the transverse direction plays a determinant role on sheet pillowing. Furthermore, pillow height increases as compression in the sheet-plane increases. The nature of in-plane stresses in the transverse direction varies from compressive to tensile as the tool-end geometry is changed from spherical to flat. Additionally, the magnitude of corresponding in-plane stresses decreases as the tool radius increases. According to measurements from the FEA model, flat end tools and large radii both retard pillow formation. However, the influence of changing tool end shape from hemispherical to flat is observed to be more important than the effect of varying tool radius, because the deformation zone remains in tension in the transverse direction while forming with flat end tools. These findings are verified by conducting a set of experiments. A fair agreement between the FEM and empirical results show that FEM can be employed as a tool to predict and control the pillow defect in SPIF.

     

A new approach for deformation history of material elements in hole-flanging produced by single point incremental forming

This paper combines plasticity and circle grid analysis to investigate the deformation mechanics and failure in hole-flanging produced by single point incremental forming (SPIF). The approach is based on circle grid analysis and allows tracing strains and stresses along the deformation history of material to compare their maximum achievable values against necking and fracture limits in the principal strain and stress spaces. The overall methodology draws from the independent characterization of necking and fracture limits by means of sheet metal formability tests to the appraisal of strain loading paths in hole-flanging with blanks having different pre-cut hole diameters. The work is supported by experimentation in aluminium AA1050-H111 and the overall investigation widens previous research in the field by presenting the first set of experimental data covering the history of material strains, stresses and their corresponding formability limits.     

TA1板单点渐进成形壁厚变化规律的数值模拟研究

钛及其合金应用越来越广泛,目前单点渐进成形技术已经开始运用于钛及其合金的成形加工,但仍存在成形件壁厚过度减薄的问题,严重影响了成形件的成形质量。文章以TA1钛合金板材成形方锥形件为研究对象,运用Abaqus有限元仿真软件,依据单一变量原则,分别探究工具头直径、底面边长、板材原始厚度和螺距等工艺参数对成形件平均壁厚减薄率与最大壁厚减薄率的影响规律。结果显示:增大工具头直径,平均壁厚减薄率随之增大,最大壁厚减薄率随之减小;平均壁厚减薄率和最大壁厚减薄率都随着底面边长的增加而增加;板材原始厚度增加,平均壁厚减薄率与最大壁厚减薄率都随之减小;随着螺距的增加,平均壁厚减薄率逐渐减小,最大壁厚减薄率逐渐增大。     

Comparison of plastic strains on AA5052 by single point incremental forming process using digital image processing

In this study, a surface based strain measurement was used to determine the formability of the sheet metal. A strain measurement may employ manual calculation of plastic strains based on the reference circle and the deformed circle. The manual calculation method has a greater margin of error in the practical applications. In this paper, an attempt has been made to compare the formability by implementing three different theoretical approaches: Namely conventional method, least square method and digital based strain measurements. As the sheet metal was formed by a single point incremental process the etched circles get deformed into elliptical shapes approximately, image acquisition has been done before and after forming. The plastic strains of the deformed circle grids are calculated based on the nondeformed reference. The coordinates of the deformed circles are measured by various image processing steps. Finally the strains obtained from the deformed circle are used to plot the forming limit diagram. To evaluate the accuracy of the system, the conventional, least square and digital based method of prediction of the forming limit diagram was compared. Conventional method and least square method have marginal error when compared with digital based processing method. Measurement of strain based on image processing agrees well and can be used to improve the accuracy and to reduce the measurement error in prediction of forming limit diagram.

     

On the forming uniformity during a single layer forming of electromagnetic incremental forming

Aiming to the precise control of forming quality in electromagnetic incremental forming (EMIF), deformation behaviors of a spherical crown-shaped thin-wall     

Plane strain extrusion by sequential limit analysis

In this paper, the analysis of plane strain forward extrusion through wedge shaped dies is implemented by using a finite element procedure based on a dual (upper bound) formulation. Since the von Mises yield criterion is employed and expressed in an inequality form, stresses do not appear in this upper bound formulation. As a result, neither complicated stress updating nor rigid zone treatment is needed in this approach. The effect of material strain hardening can be incorporated in the analysis by updating local yield stresses stepwise with deformation history. The computed results are first compared with existing analytical solutions in good agreement. Effects of friction, die semiangle, height reduction ratio and material strain hardening on the extrusion load of aluminum are then investigated.     

Part accuracy improvement in two point incremental forming with a partial die using a model predictive control algorithm

As a flexible forming technology, Incremental Sheet Forming (ISF) is a promising alternative to traditional sheet forming processes in small-batch or customised production but suffers from low part accuracy in terms of its application in the industry. The ISF toolpath has direct influences on the geometric accuracy of the formed part since the part is formed by a simple tool following the toolpath. Based on the basic structure of a simple Model Predictive Control (MPC) algorithm designed for Single Point Incremental Forming (SPIF) in our previous work Lu et al. (2015) [1] that only dealt with the toolpath correction in the vertical direction, an enhanced MPC algorithm has been developed specially for Two Point Incremental Forming (TPIF) with a partial die in this work. The enhanced control algorithm is able to correct the toolpath in both the vertical and horizontal directions. In the newly-added horizontal control module, intensive profile points in the evenly distributed radial directions of the horizontal section were used to estimate the horizontal error distribution along the horizontal sectional profile during the forming process. The toolpath correction was performed through properly adjusting the toolpath in two directions based on the optimised toolpath parameters at each step. A case study for forming a non-axisymmetric shape was conducted to experimentally validate the developed toolpath correction strategy. Experiment results indicate that the two-directional toolpath correction approach contributes to part accuracy improvement in TPIF compared with the typical TPIF process that is without toolpath correction.     

Experimental and theoretical analysis of the thickness distribution in multistage two point incremental sheet forming

The International Journal of Advanced Manufacturing Technology - Incremental sheet forming (ISF) has received increasing attention with its advantages of economy and flexibility for small batch...     

基于正交试验设计的单点渐进成型极限的研究

单点渐进成型技术是近些年发展起来的一种板料成型方法,确定板料的成型极限是该技术应用于工业生产的前提条件之一.利用UG软件建立了成型零件三维模型并编写了数控程序,采用正交试验法对板料2A12在单点渐进成型中的成型极限进行了研究.实验结果表明,成型过程中的润滑状况对板料成型极限的影响最大;以机油为润滑剂、背吃刀量为O.1mm、进给速度为2000mm/min时,板料2A 12的最大成型极限角为72.5°.     

A new parameter and its effect on the formability in single point incremental forming: A fundamental investigation

A new parameter, blank stiffness, with a potential effect on the formability in single point incremental forming (SPIF) has been introduced and investigated. Various plates with a square hole at the center and half-side length of the square ranging from 12–56mm were used as backing plates for blanks. It is shown that with a decrease in the size of hole/work-piece, there is an increase in the blank stiffness. This increase in the stiffness in turn adversely affects the formability in SPIF process.     

1060铝材双点渐进成形几何精度的研究

针对单点渐进成形几何精度差的问题,提出了一种双点渐进成形加工技术。以1060铝为研究对象,确定目标零件,从成形轨迹和进给量两个方面编制双点渐进成形工艺和单点渐进成形工艺。然后根据编制的工艺在渐进成形设备上编制数控程序,制作了单点与双点渐进成形样品。再使用FARO便携式三坐标测量仪,测量了双点渐进成形与单点渐进成形样品的几何精度。最后,提出了一种对比分析数据的方法,对单点渐进成形和双点渐进成形的几何精度进行了定量的对比分析。实验结果表明,在工艺参数相同的前提下,双点渐进成形的几何精度远高于单点渐进成形的几何精度。     

多道次双点渐进成形数值模拟与实验研究

针对多道次双点渐进成形中材料变形规律问题,建立了典型圆锥台件双点渐进成形有限元模型,采用螺旋轨迹对其多道次成形规律进行了数值模拟,对模拟结果中节点坐标和位移、单元应变和壁厚数据进行了提取,分析了成形过程中节点流动、单元应变及壁厚分布规律,并通过在实验板料表面印制圆形网格和制作标记点进行了多道次双点渐进成形实验,对成形零件网格变化、标记点径向移动和零件壁厚进行了测量分析,验证了模拟结果的正确性。研究结果表明,多道次双点渐进成形中材料产生明显的径向流动,且随着成形道次增多而增大,同时,成形道次越多零件壁厚均匀性越好;另外,采用变间隔角度设计方法,可改善多道次双点渐进成形零件壁厚分布均匀性,且随着成形道次增多作用越明显。     

单点渐进成形设备数控系统总体设计

本文采用深圳固高科技有限公司生产的GT400系列控制器作为数控系统的运动控制器件,基于Visual C++设计平台,使用面向对象技术设计开发了渐进成形开放式数控系统软件,并进行了软件的测试。