Study On The Numerical Simulation Of Multi-point Forming Process For Retiary Sheet Metal

With the scientific progress and rapid development of industrial standards, the variety of new industrial products is developing at an incredible speed, and so is the renewal of the old products, the requirement of forming mold increase urgently. One solution to catching up with time is to explore new flexible technological process. Multi-point forming (MPF) is a flexible forming process for 3D sheet metal. Its basic idea is to make traditional mold discrete, using a series of height adjustable elements (little punch) to combine forming surface. MPF can rapidly achieve required surface, save a lot of modeling time and cost compared with traditional mold, one MPF system can be used for different purposes. MPF has been applied to vehicles covering parts, medical engineering (titanium alloy retiary sheet forming), architectural decoration and other fields. And in ship hull plate, pressure vessels, aircraft skin, urban sculpture and many other fields have broad application prospects. MPF equipment has been successfully used in the torsion plates forming of Beijing Stadium (nest project) for 2008 Olympic Games. So it is great importance to carry the MPF technique into effect.Computer aidding digital sheet metal forming, it is the development trend of the forming field. MPF technology adapt to the demond of digital sheet metal forming, it can be able to realize flexible, rapid and accurate forming part. In this paper, MPF technology is regarded as the core, CAD/CAM/CAT is regarded as a means to explore the surface modeling and detection method of MPF process; Studying digital MPF based on closed-loop forming by finite element numerical simulation, discussing deformation characteristics of sheet in forming process, putting forward some means and methods to control the forming defects and improve the accuracy of part.The main contents and conclusions are as follows:1. Study of MPF surface modeling methodConstructing NURBS surface based on the data points has been analysed in detail. To consider surface reconstruction for the absenting cranium as the representative for MPF surface modeling has been explored in depth. In the measurement for CT photograph, taking the method based on wavelet theory to detect the edge of cranium surface, adopting the function of Modified Quadratic Shepard to construct the rectangular mesh from scattered points.The denoise function for measured data is realized based on Wavelet Transform. After that, the NURBS surface was interpolated.2. Study of numerical simulation method for MPF and wrinkling of sheet metal in forming processWrinkling is main defect in sheet metal forming process. Based on basic numerical simulation theory of sheet metal forming, wrinkling criterion is given and the formula for calculating wrinkling critical stress is derived. The finite element model for wrinkling numerical analysis is established. The MPF processes of spherical surface parts of titanium alloy sheet metal, retiary sheet metal with hole sparse distribution and sheet metal with square hole density distribution were simulated. The results show that thickness and deformation are important factors affect the wrinkling. The spherical surface part of retiary sheet metal is not prone to wrinkle under the same condition. With sheet metal thickness increasing and deformation diminishing, wrinkles are weakened gradually. The reasons for the spherical surface part of retiary sheet metal is not prone to wrinkle are also analyzed, and the non-wrinkling critical graphs for spherical surface part with different sheet were obtained. According to existing conditions, the forming test was done, the testing result complies with the simulation result suitably, and they conform to the non-wrinkling critical graph for spherical surface part. The method how to eliminate wrinkling defects has been discussed, the conclusion is that varying deforming path MPF and sectional MPF techniques are effective to restrain wrinkling.3. Research on fracture of sheet metal in MPF process based on finite element analysisFracture is main expressional form of tensile instability in sheet metal forming process. The tensile stability numerical description of isotropic sheet and orthotropic sheet can be realized by calculating tensile stability factor. The forming limit diagram(FLD)of sheet metal is obtained by computer simulation technology, and judging the fractures whether appear during the process based on FLD. The MPF in drawing process of titanium alloy sheet metal, retiary sheet metal with hole sparse distribution and sheet metal with square hole density distribution were simulated. The simulation results indicate that the drawing capability of sheet metal with square hole is bad, the drawing capability of retiary sheet metal with hole is better a little than sheet metal with square hole, and the drawing capability of sheet metal is best because of its good continuity. At the same time, with sheet metal thickness increasing, tensile property of sheet is strengthened gradually. The relationship graph between critical drawing height and blank-holder force with different sheet metal were fitted by studying drawing test. A fracture critical curve for forming part of different sheet metal was obtained. In order to restrain fracturing defect, the forming mode which sheet metal is sandwiched by steel paddings is proposed. The simulation and testing results demonstrate this forming mode is suitable for titanium alloy retiary sheet metal part with large deformation.4. Research on numerical simulation of springback in MPF process and springback control methodThe MPF processes of spherical surface parts of different sheet metal with different thickness and curvature radius were simulated by explicit-implicit algorithm, springback trend and curvature distribution have been concluded. The simulation results indicate, the influence of thickness and deformation to springback is same to traditional sheet metal forming essentially, that is reducing thickness and deformation can deteriorate the springback influence. Under the same condition, the springback ratios of retiary sheet metal are less than that of sheet metal. The relationship curve of springback ratios and thickness, curvature radius in spherical surface has been concluded. Based on the springback analysis, a method which can restrain springback-alternating forming is given. The analysis results show that the products with high precision and low residual stress are available by the multi-point alternating forming method, and the precision and uniformity of the parts are also improved with more alternating times. At the same time, the stretch forming mode with flexible clamp equipment which can aiso suppress springback is proposed, a simple flexible clamp equipment and interrelated finite element mode have been made. The simulation result and test show that the part with good quality, well-proportioned and small springback ratios can be obtained when the flexible clamp equipment is used.5. Technical study of the MPF digital process for titanium alloy cranial prosthesis based on finite element analysisMPF technology has many advantages in small batch production of sheet metal part, for example low cost and high accuracy, it is very suitable for customized products.Based on MPF technology, a novel digital manufacturing method of titanium alloy cranial prosthesis was developed in this paper. The MPF processes of titanium alloy cranial prosthesis of retiary sheet metal with different elastic cushion thickness, different steel padding thickness and different friction condition were simulated. The simulation results indicate that the dimple defect can be suppressed effectively when the thickness of elastic cushion is exceed 6mm, and the wrinkles can be restrained when 08Al steel paddings with 0.7mm thickness are used. The course which from 3D surface reconstruction based on CT data to forming titanium alloy cranial prosthesis with good quality is introduced detailedly. The testing result shows that the efficiency of cranial prosthesis can be improved when MPF digital technology is adopted, and the technical feasibility is validated primely by actual cases.6. Research on key techniques to multi-point closed-loop forming and part accuracy controlThe usual course of multi-point closed-loop forming and PID algorithm have been put forward, closed-loop forming simulation with PI controller has been conducted. The result proves the target surface can be obtained by closed-loop forming with PI controller. On the basis of non-contact detection for 3D surface, the two step registration method combining ICP algorithm and Genetic Algorithm (GA) optimization is proposed. At first step, the self adaptive GA is employed to aligning the data in different coordinate system, and then ICP algorithm is employed to register the data precisely by using the result of GA as the initial value. The actual cases show the registration accuracy is improved by this method. The error analysis of customized titanium alloy cranial prosthesis has been done, the result shows that the accurate MPF can be realized by MPF system and 3-D laser measurement system.