【作者】 Ghulam Hussain；

【导师】 高霖；

【作者基本信息】 南京航空航天大学 ， 航空宇航制造工程， 2009， 博士

【摘要】 单点渐进成形技术(SPIF,single point incremental forming)是一种新型钣金加工技术,适用于小批量生产和样件加工。本课题主要研究对象为金属板料在渐进成形中的成形性能,提出了一种测试板料最大成形极限角的新方法。新方法使用一个成形角沿深度方向变化的零件,在工具头的作用下,板料以余弦规律变薄,即加工后的板料厚度等于其原始厚度乘以该处成形角的余弦值。试样在减薄率达到极限值时破裂,破裂处的成形角为0°到90°之间的某一数值,这一数值即可看作是试样材料在单点渐进成形工艺中的最大成形极限角。这种方法可以用单个试件计算出最大成形极限角。而传统的测试方法要使用一系列成形角不同的零件来确定最大成形极限角。这种新测试方法的三个几何参数(即试样零件的口径、零件几何体母线的曲率半径和最初成形角)及试样形状对成形性能的影响已通过实验的方法做了研究。实验结果表明,在特殊的范围内,试样的成形性能随上述三个参数中前两个参数的增大而提高,随第三个参数的增大而降低。在形状方面,实验表明零件的拐角对成形性能是有影响的。以上发现有助于试件的标准化设计。除最大成形角外,成形极限曲线也可以用来描述单点渐进成形的成形性能。本论文中提出了几种测试FLC的新方法,与原有方法的结果做了比较,并指明了原有测试方法的不足,实验发现新方法测出的成形性能要好于原有方法,所以新方法测得的FLC更加精确。在完善了新的测试方法后,研究了工具头半径、加工步长、成形速度和板厚等工艺参数对成形性能的影响。参数的变化范围要远远大于先前的研究。以前的研究中,没有考虑到两个参数的交互作用对成形性能的影响,本课题第一次使用响应面法对交互作用的影响做了研究。在研究纯钛板的成形性能时,提出了一种新的润滑工艺,在钛板表面覆盖一层孔径特定的多孔氧化膜以容纳固体润滑剂,避免工具头与钛板的黏着。研究发现,两个参数的交互作用对成形性能是有影响的,其中工具头半径和加工步长的交互作用、工具头半径和板厚的交互作用对成形性能的影响是非常明显的。为了获得材料的最大成形性能,必须从交互作用的层次去选择参数,例如板厚的增加并非总能提高成形性能,还应当考虑到工具头半径的影响。有限元分析表明,使用小工具头时,接触力和接触应力随板厚的增加而增加,造成了成形性能的降低。成形速度对成形性能的影响是因材料而异的,对于钢板和钛板,成形速度在不同的范围内,对成形性能有积极或消极的作用,但对于铝板,成形性能没有受到成形速度较为明显的影响。一系列经验公式的提出,有助于预测冲压和渐进成形的成形性能。为了确定对成形性能影响最大的材料性能参数,实验对大量的材料进行了实验。实验发现,同之前的观点不同,材料的单拉断面收缩率是与成形性能最相关的参数,并且可以用来评定材料的成形性能。此外,本课题中对比了渐进成形与冲压工艺在成形性能上的区别,渐进成形提高了材料的成形性能,提高的比率随破裂处厚向真应变的增加而增加。最后阐述了成形缺陷可能会造成的影响,首次研究了成形缺陷与成形性能的关系。定义了挤出飞边、向内褶皱和底部凸包三种缺陷,发现三种缺陷的出现均会对成形性能造成影响。对工具头半径、板厚、加工步长、成形角、屈服应力和加工硬化指数等参数对成形缺陷的影响程度做了计算。结果发现挤出飞边和向内褶皱在使用小工具头加工厚板的时候容易出现,其程度随成形角、加工步长和屈服应力的增长而增大。有限元分析结果表明,挤出飞边现象的加剧是由于工具头轴向上的接触力和压应力增加造成的。加工硬化指数是产生底部凸包现象相关性最大的因素,上凸的高度随加工硬化指数、板厚和加工步长的增加而增加,随成形角的增加而减小。有限元分析结果表明,上凸高度随成形角的增加而减小、随板厚的减小而减小是应归因于工具头周围的Von-Mises应力分布。进一步的研究表明,板料的过早破坏是由于成形缺陷和工具头尺寸间存在密切的关系,且在工具头半径很小时易发生。为了避免这种失效,本论文中提出了一个以板厚、加工步长、成形角和屈服应力为变量的经验公式,以计算工具头半径的最小值。更多还原

【Abstract】 Single point incremental forming (SPIF) is a new sheet metal forming process. It has potential to replace conventional sheet forming processes in order to produce small batches at low cost and in short delivery time.This study is focused on the formability in SPIF. A new test to evaluate maximum wall angle, a formability measure, in SPIF was devised. This test makes use of geometry whose wall angle continuously increases from 0o to 90o along the depth. Therefore, the test specimen, depending upon the thinning limit of sheet, fractures somewhere between 0o and 90o. This enables the new test to provide maximum wall angle using single specimen and renders it promising over the existing test in which a series of specimen is required to be formed. Detailed investigations on the effect of variation in the geometrical parameters, namely horizontal curvature in a plane perpendicular to tool axis, generatrix radius and initial wall angle, and change in the shape of test specimen upon the results of newly proposed test were also performed. It was found that the maximum wall angle, in specific range, increases with increase in the former two geometrical parameters and decreases with increase in the last parameter. Also, a shape with corners has an adverse effect upon the maximum wall angle. These findings will helpful to standardize the test specimen. In addition to the test for maximum wall angle, new methods to determine forming limit curve, another formability representative in SPIF, were also devised. Forming limit curves from new methods were compared with the one obtained from the existing method. The new methods showed much higher formability than the existing one, thus enabling determination of accurate FLC.The effect of process parameters, namely tool radius, step size, forming speed and sheet thickness, upon the formability (i.e., maximum wall angle) was investigated by varying parameters over wider ranges, as compared to the previous studies on this subject. Several materials including aluminum, steel and pure titanium were employed. To study the effect of interactions of parameters, which has not been studied by former researchers, statistical designs were prepared using response surface methodology. However, before conducting investigations for titanium, a lubrication method was developed according to which a titanium oxide film with specific pore size and thickness was found to be an essential perquisite to avoid sticking of titanium to tool tip. The studies regarding the effect of process parameters upon formability revealed that the interaction of tool radius and step size and the interaction of tool radius and sheet thickness are very influential. It was found that in order to maximize the formability, one parameter should be chosen keeping in view its reciprocal parameter involved in interaction. Also, an increase in the sheet thickness, contrary to previous researchers, does not always cause an increase in the sheet formability; rather the outcome depends upon the tool radius chosen. According to FEA, excessive increase in contact pressure and stresses with increase in sheet thickness is responsible for decreasing formability when small radii tools are employed to process thick sheets. The influence of forming speed upon formability was found to be material dependent. For steels and pure titanium, it appeared as a very significant parameter: it, depending upon the speed range, can negatively or positively affect the formability. However, it did not prove substantially influential for aluminum sheets.To identify the most relevant material property influencing the formability in SPIF, the correlations of formability with material properties were also examined by employing wide range of materials. It was found that the reduction in area at tensile fracture, in contrast to the previous finding on this subject, is the most influential material property. Moreover, a comparison between the sheet formability in SPIF and stamping was drawn out. It was found that SPIF enhances sheet formability and improvement in formability caused by SPIF increases with increase in true thickness strain at tensile fracture. A set of empirical models was also developed using which one can foresee the forming ability of stamping and SPIF processes.Lastly, study regarding the forming defects and their effect upon the formability in SPIF was carried out. This kind of work was first time undertaken in literature and has been reported in the current thesis. Three forming defects, namely metallic wall, fold-in and bulge, were identified. It was found that the appearance of any of these defects can adversely influence the sheet formability. The effect of relevant process parameters, namely tool radius, sheet thickness, step size, wall angle, yield stress and hardening exponent, upon their intensity was statistically analyzed. It was found that metallic wall and fold-in appear only when thick sheet is processed with small radius tool. And after their appearance, their intensity increases as the wall angle, step size and yield stress increases. FEA explained that the increase in wall formation with increase in wall angle is because of increase in contact pressure and compressive stresses along tool axis. The hardening exponent appeared as the most influential parameter for the development of bulge. The bulge height showed increase with increase in hardening exponent, sheet thickness and step size and decrease with increase in wall angle. FEA revealed that the decrease in bulge height with increase in wall angle and decrease in sheet thickness is due to localization of von-mises stresses around the tool. In-depth experimental analysis showed that premature sheet failure due to appearance of a forming defect is closely linked with the radius of tool employed. Early fracture occurs when the tool radius is very small. In order to avoid such a failure, an empirical model in terms of four process parameters, namely sheet thickness, step size, wall angle and yield stress, was developed to compute the minimum safe limit of tool radius.更多还原