| 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.
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