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.     

Multi-response optimization on single-point incremental forming of hyperbolic shape Al-1050/Cu bimetal using response surface methodology

In present work, experimental and numerical investigations were carried out on single-point incremental forming of explosive bonded clad sheets. The sheets were produced by explosion welding from 1050 aluminum alloy and C10100 copper alloy sheets. A generatrix of hyperbolic curve was utilized as profile of final shapes formed by SPIF process. During some investigations, the interaction and main effect of the process parameters viz. tool diameter, step down, rotational speed, and sheet arrangement were evaluated on the fracture depth and wall thickness at fracture using ANOVA method. For experimentation, a customized design table was built with three quantify and one qualify factors in two levels. The design table totally provides four input factors and two responses in 12 runs. The responses are fracture depth and wall thickness. A multi-response optimization was conducted to find optimum values for input parameters using response surface methodology (RSM) and the confirmatory experiment revealed the reliability of RSM in this regard. Moreover, predictive models were presented in confidence interval of 95% to formulate the relationship between the responses and the input factors using RSM approach. Additionally, a finite element analysis was carried out on the SPIF based on optimal input parameters to depict reaction force changing, thickness variation, and stress distribution.     

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.     

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.

     

Thickness improvement in single point incremental forming deduced by sequential limit analysis

Multistage forming is one of the most practical solutions to avoid severe thinning in single point incremental forming (SPIF). A successful implementation of multistage SPIF is strongly dependent on an appropriate deformation path. In this paper, firstly, a simplified modeling technique is proposed using sequential limit analysis. It is shown that sequential limit analysis can predict the thickness distribution faster than an equivalent model in a commercial finite element modeling code like Abaqus can. The reliability of the model is assessed by comparing experimental and simulated results for single-stage and multistage SPIF cones. This model is utilized to study the effect of various deformation paths on the thickness distribution. As a result, a new multistage strategy is designed and implemented to form a 70° wall angle cone in three stages. The thickness distribution of the cone is improved significantly compared to cones formed by a single-stage and a conventional three-stage strategy. Besides this improvement, the new multistage SPIF can be carried out in much less time.     

数控渐进成形中的鼓包问题分析

针对渐进成形过程中的鼓包问题,分析了该现象的形成原因,利用简单试验法分析了零件成形角度、成形深度、成形零件直径、零件形状对鼓包问题的影响规律;并采用三因素两水平的正交试验方法分析了工具头直径、板料厚度、每层进给量及其交互作用对成形结果的影响。结果表明:板料成形角度越小鼓包高度越大,通过优化成形工艺参数可以减小鼓包高度,采用渐进成形正成形方法可以从根本上解决鼓包问题。     

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.     

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.

     

基于响应面法的单点增量成形过程变形能优化

单点增量成形过程中的变形能对加工成本控制及工具头与材料之间的热效应和摩擦效应有直接影响。以典型圆锥形制件为研究对象,采用BBD实验方法,设计四因素三水平实验方案,利用响应面法研究工具头直径d、层间距Z、板厚t和成形角α对变形能的影响,并得到变形能的多元二次预测模型,最后以变形能最小为目标对该模型进行优化。实验结果表明：板厚对变形能的线性影响最显著,随着板厚的增大变形能增大,工具头直径越大所需变形能越大,成形角增大时所需的变形能增大;变形能最小的工艺参数组合是工具头直径4.0mm、层间距0.95mm、板厚0.57mm、成形角45°。     

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.     

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.     

Deformation mechanics in single-point and accumulative double-sided incremental forming

Single-point incremental forming (SPIF) uses one small hemispherically ended tool moving along a predefined toolpath to locally deform a completely peripherally clamped sheet of metal such that the sum total of the local deformations yields the final desired shape of the sheet. While SPIF is characterized by greater formability than conventional forming processes, it suffers from significant geometric inaccuracy. Accumulative double-sided incremental forming (ADSIF) is a substantial improvement over SPIF in which one hemispherically ended tool is used on each side of the sheet metal. The supporting tool moves synchronously with the forming tool, therefore acting as a local but mobile die. ADSIF results in considerably enhanced geometric accuracy and increased formability of the formed part as compared to SPIF. In light of the aforementioned advantages of ADSIF as compared with SPIF, an investigation of the mechanics associated with the ADSIF process, which has yet to be presented in the literature, is warranted. The present study sheds light on the differences in deformation mechanisms between SPIF and ADSIF. Finite element analyses are performed to simulate deformation in the two processes, and a detailed analysis of the deformation history is presented. It is shown that the presence of the supporting tool in ADSIF elicits substantial differences in the plastic strain, hydrostatic pressure, and shear strains as compared to SPIF. The implications of these trends on the prevalent modes of deformation in ADSIF along with possible explanations for increased formability observed in the process arediscussed.     

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.     

Optimized bending angle distribution function of contour plate roll forming

Regarding the roll forming of contour plates, traditional roll forming techniques often result in peak longitudinal strain during the forming process, which causes defects such as longitudinal bowing and springback. The bending angle, sheet thickness, and the number of forming passes are all important process parameters that cause the aforementioned problems. This study proposes quantifying the projection track regarding the edge of the profile section in the horizontal plane that follows a cubic curve and uses the curve function to reasonably distribute the bending angle to study the maximum forming strain and the average strain for all passes of the sheet between the forming passes. Simultaneously, the influence law of the sheet thickness and number of forming passes was studied. Based on theoretical simulation and experimental verification, the optimal bending angle distribution function is A1: y?=?x³?+?x²?+?x. When the thickness of the sheet is 1.5 mm, both the maximum forming strain between the passes during roll forming and the average strain of all passes are the smallest, which are 3.91 and 0.609%, respectively. Moreover, with the increase in the number of passes, the longitudinal bowing and springback decrease to varying degrees.     

A study on optimal design of process parameters in single point incremental forming of sheet metal by combining Box–Behnken design of experiments,response surface methods and genetic algorithms

Incremental forming is a sheet metal forming process characterized by high flexibility; for this reason, it is suggested for rapid prototyping and customized products. On the other hand, this process is slower than traditional ones and requires in-depth studies to know the influence and the optimization of certain process parameters. In this paper, a complete optimization procedure starting from modeling and leading to the search of robust optimal process parameters is proposed. A numerical model of single point incremental forming of aluminum truncated cone geometries is developed by means of Finite Element simulation code ABAQUS and validated experimentally. One of the problems to be solved in the metal forming processes of thin sheets is the taking into account of the effects of technological process parameters so that the part takes the desired mechanical and geometrical characteristics. The control parameters for the study included wall inclination angle (α), tool size (D), material thickness (Th_ini), and vertical step size (In). A total of 27 numerical tests were conducted based on a 4-factor, 3-level Box–Behnken Design of Experiments approach along with FEA. An analysis of variance (ANOVA) test was carried out to obtain the relative importance of each single factor in terms of their main effects on the response variable. The main and interaction effects of the process parameters on sheet thinning rate and the punch forces were studied in more detail and presented in graphical form that helps in selecting quickly the process parameters to achieve the desired results. The main objective of this work is to examine and minimize the sheet thinning rate and the punch loads generated in this forming process. A first optimization procedure is based on the use of graphical response surfaces methodology (RSM). Quadratic mathematical models of the process were formulated correlating for the important controllable process parameters with the considered responses. The adequacies of the models were checked using analysis of variance technique. These analytical formulations allow the identification of the influential parameters of an optimization problem and the reduction of the number of evaluations of the objective functions. Taking the models as objective functions further optimization has been carried out using a genetic algorithm (GA) developed in order to compute the optimum solutions defined by the minimum values of sheet thinning and the punch loads and their corresponding combinations of optimum process parameters. For validation of its accuracy and generalization, the genetic algorithm was tested by using two analytical test functions as benchmarks of which global and local minima are known. It was demonstrated that the developed method can solve high nonlinear problems successfully. Finally, it is observed that the numerical results showed the suitability of the proposed approaches, and some comparative studies of the optimum solutions obtained by these algorithms developed in this work are shown here.     

铜铝复合板渐进成形回弹缺陷研究

为了讨论不同工艺参数对复合板渐进成形回弹的影响,建立了包含工具头、复合板材和复合界面的三维有限元模型,通过对比T形剥离结果得到复合界面内聚力单元参数,通过实验验证了有限元模型的可靠性.实验结果表明,成形角对双金属复合板渐进成形回弹最敏感,工具头直径对回弹敏感度最低,另外,内聚力单元能有效模拟双金属复合板结合界面,模拟误...     

弹体温成形工艺改进的研究

针对弹体温成形实际生产过程中模具寿命低及壁厚差大的问题,通过对温成形工艺参数的分析,提出弹体温成形过程的改进方案,即在温挤压工步之前增加预成形工步,在温挤压工步之后增加变薄拉深工步.经对改进工艺的计算机模拟及实验验证表明,此改进的工艺能够减小壁厚差,降低凸模温升,延长凸模寿命.     

Experimental research on micro-deep drawing processes of pure gold thin sheet using DLC-coated female die

Micro-forming has been considered as an alternative for micro-fabrication method of micro-parts for its massive production characters. With pure gold thin sheet, the manufacture of micro-parts, e.g., micro-cup is still a delicate and challenging task and has received less attention. Since there are size effects of friction when a liquid lubricant is applied, the forming process becomes sensitive to the friction for the small strength of pure gold shin sheet, and the traditional deep drawing process cannot easily adapt to these kinds of micro-parts. This study presents the results of research conducted on micro-cup deep drawing process with pure gold thin sheet using diamond-like carbon film and polyethylene film to reduce the friction. In particular, the effect of lubrication condition on punch load, surface profile, reduction of thickness, and accuracy of inner diameter are investigated and discussed. Experimental results show that the diamond-like carbon-coated female die leads to lower punch load, high surface quality, uniform thickness of sheet, and high diameter accuracy. The diamond-like carbon film presents excellent wear-resistance properties under high contact pressure and large plastic deformation of thin sheet. The results indicate that micro-forming using diamond-like carbon-coated female die shows potentiality to be feasible alternative to micro-parts of pure gold thin sheet.     

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.

     

Analysis of superplastic blow-forming in a conical closed die

A mathematical model using the finite-difference method has been proposed in this work to examine the plastic deformation behavior of the sheet during blow-forming in a conical closed die. In the formulation of this mathematical model, nonuniform thinning in the free bulged region and the contact condition including the sticking and sliding friction modes between the sheet and die are considered. Effects of various forming parameters such as the die entry radius, friction coefficient, inclined angle of the die, etc., upon the optimized pressurization profile, forming time and the thickness distribution of products were discussed systematically. Furthermore, experiments on superplastic blow-forming in a conical closed die were carried out using 8090 Al-Li sheets. It is found that the theoretical predictions agree with experimental results. The thickness distributions of the SPF-ed product obtained with different friction coefficients along the die entry, sidewall and bottom are closer to the experimental data than those obtained with a constant friction coefficient.