Improvement of formability for the incremental sheet metal forming process

In order to obtain competitiveness in the field of industrial manufacture, a reduction in the development period for the small batch manufacture of products is required. In order to meet these requirements, an incremental sheet metal forming process has been developed. In this process, a small local region of a sheet blank deforms incrementally by moving a hemispherical head tool over an arbitrary surface. In this work, an incremental sheet metal forming process controlled three dimensionally by a computer has been accomplished. It has been shown by the experiments that a sheet blank is mainly subject to shear-dominant deformation. Therefore, the final thickness strain can be predicted. The uniformity of thickness throughout the deformed region is one of the key factors to improve the formability in the sheet metal forming processes. Using the predicted thickness strain distribution, the intermediate geometry is decided in the manner that a shear deformation is restrained in the highly shear-deformed region and vice versa. This double-pass forming method is found to be very effective so that the thickness strain distribution of a final shape can be made more uniform.     

Modified 2D stylus profilometry and its application to frictional analyses in sheet metal forming operations

Frictional conditions at contact interfaces can have a large influence on the success of some sheet metal forming operations. Topographic surface characterisation is a useful tool for obtaining data necessary for some tribological analyses, and it is proposed that a standard two-dimensional (2D) profilometer can be modified to provide inexpensive three-dimensional (3D) data without any significant loss of performance. A scanning white light interferometer is used to verify this hypothesis, and the results show that while the new profilometry technique is less accurate at low contact areas, it is sufficiently accurate for some purposes. Surface maps are created and the results used to calculate the real contact areas, bearing area curves and hydrodynamic lift pressures for a drawing-quality steel.     

Introduction of laminated supporting tools in water jet incremental sheet metal forming

This contribution presents the introduction of laminated supporting tools in water jet incremental sheet metal forming (WJISMF) process. In WJISMF the main tool is a high-velocity water jet (WJ) instead of a rigid tool. The influence of the main tool trajectory on the forming outcome in WJISMF is observed and optimized in order to reduce the forming time and improve the product quality. Laminated tools are fabricated with abrasive water jet (AWJ) machining from aluminium plates of different thicknesses. The parts formed in this investigation are measured with a coordinate measuring machine (CMM) and results are compared by means of forming time, parts symmetry and achieved geometry, which is defined with the supporting tool.     

Tool path correction algorithm for single-point incremental forming of sheet metal

There exists some error between the manufactured part shape and the designed target shape due to springback of this part after forming. To reduce the error, an iterative algorithm of closed-loop control for correcting tool path of the single-point incremental forming, based on Fast Fourier and wavelet transforms, has been developed. Moreover, the data of the springback shapes, after unloading, of the sheet metal parts formed with the trial and corrected tool paths, used for iterative correction of tool path in the algorithm, are obtained with finite element model (FEM) simulation. Then, a truncated pyramid-shaped workpiece, whose average errors are +0.183/?0.175 mm, was made with the corrected tool path after three iterations solved by the above algorithm and simulation data. The results show that the tool path correction algorithm with Fourier and wavelet transforms is reasonable and the means with FEM simulation are effective. It can be taken as a new approach for single-point incremental forming of sheet metal and tool path design.     

Modeling and simulation for multiple-step incremental air-bending forming of sheet metal

Based on Hill’s yielding criterion and plane strain condition, the explicit expressions of elastoplastic constitutive model are derived in this paper which takes into account the effects of transverse stress, neutral surface shifting, and sheet thickness thinning on the sheet springback of air-bending. Then, this model is embedded into ABAQUS software platform by means of programming. Finally, 3D ABAQUS finite-element models (FEM), used to form the semiellipse-shaped workpiece with super length and large opening of sheet metal, are established, and the multiple-step incremental air-bending forming and springback processes are simulated. The simulation and experiment results show that the data predicted with the new constructed constitutive model under the plane strain condition are in much better agreement with experimental data than those predicted with the constitutive model built-in ABAQUS. It can be taken as a valuable mathematical tool used for multiple-step incremental air-bending forming simulation of large area sheet metal.     

Necking in sheet metal forming. Influence of macroscopic and microscopic properties of materials

The necking of sheet metal is a main problem in forming processes that causes unacceptable thickness decreasing and leads to stop the process. There is a large amount of literature and research papers in this field. But the remaining problems to solve are the applicability and the accuracy of the methods for determining forming limits for various strain paths encountered in industrial processes. In the paper, we demonstrate that the linear stability analysis can provide accurate results considering various constitutive relationships and also considering that now the finite element analyses provide accurate results concerning the main field variables. The method is first presented, and then developments are done in the rigid-plastic case or in the rigid-plastic one accounting for damage by void growth. It is then demonstrated that the proposed method gives accurate results and then can be substituted to classical forming limit analyses.     

Electric assistance hot incremental sheet forming: an integral heating design

Ultrasonic method is promising in oil film thickness measurement, while the commonly used commercial transducers are bulky and then sometimes hinder the application of this technology in field. Using the piezoelectric element is an alternative method and has the advantage of being convenient to be fixed. In this work, the key points of the use of the piezoelectric element are summarized. A calibration rig is set up to test the performance of oil film thickness measurement using piezoelectric element. The results show that using the piezoelectric element can measure the oil film thickness effectively and accurately.     

Sectional multipoint forming technology for large-size sheet metal

Based on the flexibility of multipoint forming, a work piece can be formed in a way section by section, which is named sectional multipoint forming (SMPF) here. With this technique, large-size parts of sheet metal can be manufactured on a small MPF press. In this article, basic principles and deformation characteristics of SMPF are discussed and three typical forming methods are summarized. Partial severe plastic deformation has become the main defect needed to be and wrinkle and dimple have new features in the SMPF process. Numerical simulation is an effective way to predict forming defects. Optimized design of the assortative region can suppress forming defects and improve forming efficiency. Some application examples improved the feasibility of SMPF technology .     

金属板材数控渐进成形支撑CAD模型自动生成

为满足金属板材数控渐进成形中支撑制作的需要,提出一种从板材件的计算机辅助设计模型自动生成支撑计算机辅助设计模型的方法。采用STL数据模型,根据板材件曲面曲率的变化和成形过程中板材的减薄规律,计算出成形过程中板材厚度的变化量,通过对STL模型进行不等距偏置,生成了能够自适应板材厚度变化、并能保证挤压工具头与支撑之间合理间距的支撑计算机辅助设计模型。最后,给出了支撑计算机辅助设计模型生成实例。     

Force-based failure criterion in incremental sheet forming

In incremental sheet forming, a hemispherical tool deforms a sheet moving along a predefined path. The process, useful for making prototypes or small series, is characterized by high flexibility and development times and costs reduction. The part feasibility is strongly influenced by the adopted tool path and, in particular, tool path corrections are requested to obtain the final geometry and to prevent the sheet rupture. To save time and costs, sheet failure criteria are therefore essential. In this paper, a real time force-based sheet rupture criterion is presented and tested on steel, aluminum, and titanium alloys. In particular, it will be shown that the sheet rupture can be related with the tangential forming force, the geometry of the component to be realized, and the sheet material characteristics.     

Spiral tool-path generation with constant scallop height for sheet metal CNC incremental forming

The study of the residual stresses of parts that work under extreme conditions is of great interest, as they will influence the mechanical behavior of the components. This has a huge importance in the automotive and aeronautics sectors, in components such as power shafts or aircraft landing gears. Machining processes produce thermal and mechanical loads causing residual stresses in the surface of the finished component. The influence of the cutting parameters on the surface residual stresses generated during turning of AISI 4340 (40NiCrMo7) treated steel was analyzed. Surface residual stresses were measured using the X-ray diffraction technique and the sin² ?? method. Both magnitude and direction of the residual stresses were assessed in order to understand how the cutting parameters affect the principal stresses directions and the homogeneity of the distribution of stresses. The ANOVA analysis has been used, obtaining the process window to reduce the tensile residual stresses affecting the component behavior against fatigue loads. When feed is increased, more heat is generated during the cutting process, leading to more tensile residual stresses. An increase in cutting speed results in a more adiabatic process (less heating of the part because heat is dissipated through the chip) that also leads to less tensile stresses. It has been observed that the maximum stress is localized around 30° from the cutting direction.     

Thickness distribution and design of a multi-stage process for sheet metal incremental forming

Sheet incremental forming (ISF) is a promising technology. It is inexpensive and does not require particular dies. Sheet thinning, however, has always been one of the forming defects which impede the process’s wide application. Although a multi-stage forming process is supposed to be effective to deal with this problem, it is still uncertain how the process can reduce thickness thinning and there is no applicable rule to determine the favorable number of forming stages. In this work, based on a truncated cone, a finite element method (FEM) model for a double-pass forming was established first. Unlike simplifications in previous studies, with the process of the three-dimensional coordinates in numerical controlled (NC) machining code, the tool trajectory in this simulation model is the same as that in real work. With this approach, it was expected to gain reliability of the simulation result, and then this simulation result and a single-pass forming result were analyzed. The results of the analysis indicate that more uniform thickness distribution in the double-forming process largely benefits from the increase of the total plastic deformation zone. Finally, under the condition of constant volume in deformation, an equation was proposed to work out the right number of necessary forming stages and the rule of this equation was verified with a relatively complex product.     

Tool-path generation for sheet metal incremental forming based on STL model with defects

Tool-path generation is a key issue in sheet metal incremental forming process, and existing approaches either are prone to computationally expensive or cannot be applied to stereolithography (STL) model including defects. Thus, a new tool-path generation method is presented by adopting the thought of generating cutter-location (CL) data directly from corresponding cutter-contact (CC) data. By analyzing the interference characteristics between fillet-end tool and model surface and considering tangential case and intersection case comprehensively, a discrete computational model is proposed to calculate single-layer interference-free CL contour, instead of computing a precise CL point by checking potential interferences from candidate facet, vertices, and edges, respectively. So this method is more efficient and easier for program implementation. Additionally, a fast recursive search algorithm is developed to identify and extract flat area features, and an efficient 2D invalid loops removal algorithm based on decomposition thought is presented to obtain valid CC contours and CL contours with a near linear time-complexity. Implementation tests prove that the new method is effective and robust for STL model with defects, and tool-path achieved is highly precise. It is also applicable to various tool shapes and suitable for planning various types of tool-paths to meet different SMIF process requirements.     

Modeling and experimental validation for truncated cone parts forming based on water jet incremental sheet metal forming

Dieless sheet metal forming technology has many advantages for prototypes and small batch productions, but it is limited by its accuracy. This paper introduces a water jet incremental sheet metal forming (WJISMF) technology for dieless sheet metal forming. It gave a comprehensive study on truncated cone parts forming based on WJISMF, including its forming theoretical model and experimental validation. Firstly, a theoretical model for truncated cone forming based on WJISMF was developed based on plane strain assumption and work-energy theorem. The theoretical model mainly revealed the relationships between the key process parameters (especially water jet pressure) and truncated cone parts forming angle, which was very useful to predict the forming angle in certain water jet pressure or to determine the water jet pressure for different cone angle parts. Then, to validate the theoretical model, truncated cone workpiece was manufactured on a built WJISMF machine. Experimental results show that theoretical model matched the experiment well.     

大型U形板材工件渐进滚弯成形数值模拟

利用ABAQUS有限元分析平台,对大型U形板材工件渐进滚弯成形及其回弹过程进行数值模拟,其中工件材料类型选择普通低碳钢。针对半椭圆形工件形状,提出用半径不同的五段圆弧逼近的几何规划。在此基础上,根据加工过程可能出现的缺陷,合理设计滚弯道次,调整ABAQUS参数设置。用模拟优化的工艺参数成形加工半椭圆形板材工件,经origin8拟合配准,其各段曲率半径最大误差小于5%。模拟结果表明,用渐进滚弯方法成形加工半椭圆形工件是可行的,且模拟过程满足准静态响应的要求。     

Effects of forming parameters on temperature in frictional stir incremental sheet forming

Frictional stir Incremental sheet forming (ISF) is a new technology used to fabricate parts of hard-to-form materials without using heating equipment. Thus far, limited information is known about the effects of main forming parameters, except spindle speed of the tool, on the temperature of formed sheet in friction-stir ISF. The effects of six forming parameters, namely, sheet thickness, tool vertical step, tool diameter, spindle speed, feed rate, and wall angle of the formed part, were identified using the design of experiment of orthogonal array, analysis of response tables and graphs, and analysis of variance. Results show that spindle speed, feed rate, sheet thickness, and tool vertical step significantly affect the temperature of the sheet. In addition, the temperature of the sheet is significantly increased by increasing sheet thickness, tool vertical step, and spindle speed but significantly decreased with increasing tool feed rate.

     

Modelling drawbeads in sheet metal forming

In this paper, we describe the numerical modelling of drawbead forces required to draw a sheet metal through a bead with a constant cross section. The model is formulated using an elasto-plastic large strains finite element method combined with an improved numerical technique taking into account the contact and friction conditions, based on linear programming techniques and fixed point conditions. The numerical simulations were carried out with various drawbead geometries and the results are expressed in terms of drawing restraining force versus drawing movement, as a function of the drawbead geometry, gap conditions and friction conditions. With this procedure it is also possible to determine the main deformation paths in the drawbead region where the bending strain predominates over the membrane strain. The results obtained are compared with experimental data and the agreement proves to be good. The calculated results can be used as a basis to obtain better approximations of the drawbead constitutive equations to be used in general 3D FE simulation codes in cases where it is impossible to exactly determine the drawbead geometry.     

Study on multiple-step incremental air-bending forming of sheet metal with springback model and FEM simulation

A mathematical model of springback radius was developed with dimensional analysis and orthogonal test. With this model, the punch radius could be solved for forming high-precision semiellipse-shaped workpieces. With the punch radius and other geometrical parameters of a tool, a 2D ABAQUS finite-element model (FEM) was established. Then, the forming process of sheet metal multiple-step incremental air bending was simulated with the FEM. The result showed that average errors of the simulated workpiece were +0.68/?0.65 mm, and provided the process data consisting of sheet feed rate, punch displacement and springback angle in each step. A semiellipse-shaped workpiece, whose average errors are +0.68/?0.69 mm, was made with the simulation data. These results indicate that the punch design method is feasible with the mathematical model, and the means of FEM simulation is effective. It can be taken as a new approach for sheet metal multiple-step incremental air-bending forming and tool design.     

A new sheet metal forming system based on the incremental punching, part 1: modeling and simulation

In heavy forging, a manipulator is indispensable to assist and help the precision of the forming process. This paper presents a multi-system simulation methodology combining the forging finite element method (FEM) simulation and the kinematics analysis to evaluate the mutual reaction loads between the forging process and the assisting manipulator. The forging is realized by the thermal–mechanical FEM simulation and the kinematics movements are analyzed based on the statics and dynamics modeling of the manipulator. The reaction load generating from the forging process to the manipulator clamps is treated as an input parameter for the kinematics analysis system, which will then calculate the movement of the manipulator. And this movement is regarded as the passive compliant movement constraint and applied on the forging process through the manipulator clamps. Using this coupled system, the study compares the reaction loads with and without the active vertical compliant movement and/or the passive horizontal compliant movement and reveals the effect of these compliant movements on the reaction loads.