Formability evaluation of a pure titanium sheet in the cold incremental forming process

Owing to its ability to deform a sheet metal locally, the single point incremental forming (SPIF) process produces larger deformations as compared to the conventional forming processes. In the present study, we investigated the effect of some process parameters – pitch, tool diameter, feed rate and friction at the interface between the tool and blank – on the formability of a commercially-pure titanium sheet. Trends between the process parameters and formability are presented in this paper.     

DEFORMATION ANALYSIS OF SHEET METAL SINGLE-POINT INCREMENTAL FORMING BY FINITE ELEMENT METHOD SIMULATION

Single-point incremental forming （SPIF） is an innovational sheet metal forming method without dedicated dies, which belongs to rapid prototyping technology. In generalizing the SPIF of sheet metal, the deformation analysis on forming process becomes an important and useful method for the planning of shell products, the choice of material, the design of the forming process and the planning of the forming tool. Using solid brick elements, the finite element method（FEM） model of truncated pyramid was established. Based on the theory of anisotropy and assumed strain formulation, the SPIF processes with different parameters were simulated. The resulted comparison between the simulations and the experiments shows that the FEM model is feasible and effective. Then, according to the simulated forming process, the deformation pattern of SPIF can be summarized as the combination of plane-stretching deformation and bending deformation. And the study about the process parameters＇ impact on deformation shows that the process parameter of interlayer spacing is a dominant factor on the deformation. Decreasing interlayer spacing, the strain of one step decreases and the formability of blank will be improved. With bigger interlayer spacing, the plastic deformation zone increases and the forming force will be bigger.     

The formability of annealed and pre-aged AA-2024 sheets in single-point incremental forming

The formability of AA-2024 sheets, an aerospace grade material, in the annealed and pre-aged conditions has been investigated in the single-point incremental forming (SPIF) process. The major operating parameters, namely step size, tool radius, and forming speed, of SPIF process were varied over wide ranges, and their effect on the formability was quantified through a response surface method called as central composite rotational design. It was found that the interaction of step size and tool radius is very significant on the formability. Moreover, a variation in the forming speed does not affect the formability of annealed AA-2024 sheet. However, the formability of pre-aged AA-2024 sheet decreases with the increase in the forming speed. Furthermore, the annealed sheet shows higher formability than the pre-aged sheet.     

On the improvement of material formability in SPIF operation through tool stirring action

Single-point incremental forming (SPIF) is a quite new sheet-forming process which offers the possibility to deform complex parts without dedicated dies using a single-point tool and a standard three-axis CNC machine. The process mechanics enables higher strains with respect to traditional sheet-forming processes, but particular attention must be given to the maximum forming angle. In this paper, a new approach is proposed to enhance the material formability through a localized sheet heating as a consequence of the friction work caused by elevated tool rotational speeds. AA1050-O, AA1050-H24, and AA6082-T6 were utilized, and the reached temperatures were recorded by thermocouples, fixed to the sheet using a metal structure. A significant increase in the material formability was observed for both materials, and new formability curves have been built at the varying of the utilized rotational speed.     

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.     

走刀轨迹对双面渐进成形几何精度的影响

在单点渐进成形(single point incremental forming,简称SPIF)加工过程中,成形刀具与夹具间的板料因得不到有效支承而发生翘曲,卸载后板料易回弹,影响制件的成形几何精度;因此,提出了2种新型双面渐进成形(double sided incremental forming,简称DSIF)走刀轨迹,分析了2种轨迹下板料单元的应力和应变状态,并借助有限元分析软件模拟了板料的成形过程,进一步比较了不同走刀轨迹下制件的成形几何精度。仿真数据表明,与单点渐进成形相比,这2种双面渐进成形走刀轨迹能有效改善制件的成形几何精度。     

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.     

Investigating the temperature from friction stir in the SPIF process via an infrared imager

Single point incremental forming (SPIF) is a highly flexible forming process for sheet metal. It has low production costs be-cause the process does not use a die. It is suitable for prototyping and made-to-order production. Currently, the SPIF process employs the concept of heat to increase formability. The idea is to generate heat from friction caused by sliding the tool against the workpiece, called “friction stir”. This research proposed to study the behavior of temperature that occurs when affected by the tool rotation speed and the feed rate, which are both variables affecting friction stir. This research adopted the method of detecting temperature with infrared cameras and online recording data. The camera sensor received data as 8-bit images containing data from 0 to 255. The value of each position represented the temperature level. In this research, the mini-mum-maximum temperature range was set at 80–300 degrees Celsius for forming the hot dipped zinc coat roll steel sheet at a thickness of 0.2 mm using the SPIF process. The variable parameters were the tool rotation speed and feed rate. The tool rotation speed was categorized as high and used no sliding friction with feed rates of 500, 1000, 1500 mm/min. Concerning the results analysis, this study used the relationships between tool rotational speed and feed rate, shown as relative sliding velocity. The results showed with significance that the increase of the maximum temperature in the process corresponded to an increase in relative sliding velocity using a tool rotational speed and feed rate with no relative sliding velocity. The process temperature was close to room temperature. Relative sliding velocity at approximately 6000 and 10000 mm/min caused a maximum temperature of approximately 160–180 and 200–250 degrees Celsius, respectively. Another issue found in the experiment was that not turning the tool reduced the formability of the process.

     

[成形质量的闭环与自适应控制技术]具有开放几何特征钣金件渐进成形的回弹控制与补偿

实现了具有开放几何特征钣金件的渐进成形。提出采用工艺参数优化、多道次渐进成形和轨迹补偿等方法提高钣金件的几何精度。设计了多道次渐进成形方法的中间构型,目的在于增大材料塑性变形并减小几何偏差。采用层切法轮廓轨迹形成两道次成形中第一道次的构型,为了防止第一道次成形时出现破裂,将非固定方向的成形角度设定为75°,第二道次将非固定方向的板料完全成形。采用基于几何补偿的两道次渐进成形方法可以将开放几何特征钣金件的几何偏差控制在0.5 mm左右。     

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.

     

Pyramid as test geometry to evaluate formability in incremental forming: Recent results

The present work has been undertaken with an objective to fill the gaps of previous studies and to explore guidelines to standardize the test specimen for evaluating formability with a single specimen in single point incremental forming (SPIF). Two candidate geometries for formability testing (i.e. varying wall angle pyramidal frustum and varying wall angle conical frustum) have been compared by varying geometrical parameters and materials. The critical size in horizontal plane (i.e. half-side length/curvature radius) and critical initial forming angle have been identified and compared for the two geometries. The critical size in horizontal plane has been found to be different for the two geometries. The critical initial forming angle has been found to be same for the two geometries. For various sheet materials, the difference in the formability of VWACF and VWAPF shows a dependence upon the percent reduction in area at tensile fracture.     

Consideration of geometric nonlinearity in rigid-plastic finite element formulation of continuum elements for large deformation

A rigid—plastic finite element formulation for the continuum elements employing the geometric nonlinearity during an incremental time step is presented. In sheet metal deformation, the displacement for each step is considerably large even though the effective strain increment is very small. For such large displacement problems, geometric nonlinearity must be considered. In the elastic—plastic finite element using continuum elements, general incremental formulations to include the geometric nonlinearity are available. However, in the conventional rigid—plastic finite element analysis using continuum, elements, the geometric nonlinearity has not been considered properly during an incremental time step. In this paper, in order to incorporate geometric nonlinearity to rigid—plastic continuum elements during a step, the convected coordinate system is introduced. To show the stability of strain distributions by the effect of geometric nonlinearity according to incremental step size, two sheet metal forming processes, stretching and deep drawing process, are analysed with various step sizes. Then the computed results using the derived equation are compared with those obtained without considering geometric nonlinearity.     

Incremental forming of Mg alloy sheet at elevated temperatures

In the present study, incremental forming of Mg alloy sheet at elevated temperatures was attempted with local heating apparatus. The device is composed of several halogen lamps and designed to move with forming tool for local heating in deformation zone. In order to investigate the influences of process parameters to incremental formability of AZ31 alloy sheet, a series of incremental forming tests of AZ31 for cone and pyramid type of simple models were carried out under various process conditions. Experiments were performed under various temperatures, feeding depth per cycle and inclination angles and the results were analysed.     

Failure mechanisms in single-point incremental forming of metals

The last years saw the development of two different views on how failure develops in single-point incremental forming (SPIF). Today, researchers are split between those claiming that fracture is always preceded by necking and those considering that fracture occurs with suppression of necking. Each of these views is supported by convincing experimental and theoretical arguments that are available in the literature. This paper revisits failure in SPIF and presents a new level of understanding on the influence of process variables such as the tool radius that assists the authors to propose a new unified view on formability limits and development of fracture. The unified view conciliates the aforementioned different explanations on the role of necking in fracture and is consistent with the experimental observations that have been reported in the past years. The work is performed on aluminium AA1050-H111 sheets and involves independent determination of formability limits by necking and fracture using tensile and hydraulic bulge tests in conjunction with SPIF of benchmark shapes under laboratory conditions.     

Studying Formability Limits By Combining Conventional and Incremental Sheet Forming Process

In this work it is assessed the potential of combining conventional and incremental sheet forming processes in a same sheet of metal.This so-called hybrid forming approach is performed through the manufacture of a pre-forming by conventional forming,followed by incremental sheet forming.The main objective is analyzing strain evolution.The pre-forming induced in the conventional forming stage will determine the strain paths,directly influencing the strains produced by the incremental process.To conduct the study,in the conventional processes,strains were imposed in three different ways with distinct true strains.At the incremental stage,the pyramid strategy was adopted with differ-ent wall slopes.From the experiments,the true strains and the final geometries were analyzed.Numerical simulation was also employed for the sake of comparison and correlation with the measured data.It could be observed that single-stretch pre-strain was directly proportional to the maximum incremental strains achieved,whereas samples subjected to biaxial pre-strain influenced the formability according to the degree of pre-strain applied.Pre-strain driven by the prior deep-drawing operation did not result,in this particular geometry,in increased formability.     

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.     

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.     

Analysis of drawbead process by static-explicit finite element method

The problem analyzed here is a sheet metal forming process which requires a drawbead. The drawbead provides the sheet metal enough tension to be deformed plastically along the punch face and consequently, ensures a proper shape of final products by fixing the sheet to the die. Therefore, the optimum design of drawbead is indispensable in obtaining the desired formability. A static-explicit Finite element analysis is carried out to provide a perspective tool for designing the drawbead. The finite element formulation is constructed from static equilibrium equation and takes into account the boundary condition that involves a proper contact condition. The deformation behavior of sheet material is formulated by the elastic-plastic constitutive equation. The finite element formulation has been solved based on an existing method that is called the static-explicit method. The main features of the static-explicit method are first that there is no convergence problem. Second, the problem of contact and friction is easily solved by application of very small time interval. During the analysis of drawbead processes, the strain distribution and the drawing force on drawbead can be analyzed. And the effects of bead shape and number of beads on sheet forming processes were investigated. The results of the static explicit analysis of drawbead processes show no convergence problem and comparatively accurate results even though severe high geometric and contact-friction nonlinearity. Moreover, the computational results of a static-explicit finite element analysis can supply very valuable information for designing the drawbead process in which the defects of final sheet product can be removed.