Optimization of a progressive microforming process and in-process material behavior

• Main Author: Ehsan Ghassemali
• Other Authors: Chua Beng-Wah
• Format: Theses and Dissertations
• Language: English
• Published: 2014
• Subjects: DRNTU::Engineering::Manufacturing::Product engineering; DRNTU::Engineering::Materials::Material testing and characterization; DRNTU::Engineering::Mathematics and analysis::Simulations; DRNTU::Engineering::Mechanical engineering::Mechanics and dynamics
• Online Access: https://hdl.handle.net/10356/55374
• Institution: Nanyang Technological University

In this context, a progressive open-die micro-forging/extrusion process was introduced and developed for production of axi-symmetric micro-metallic components. Such a system has unique advantages over existing micro-forming processes. First, it circumvented the issues of handling small billets needed for extruding pins of very small diameter. Second, such a process of using a strip metal can be more productive, since a progressive process can be implemented. Third, the progressive manner provides the benefit of using a blanking process to remove the formed pin from the strip metal, eliminating the possibility of buckling damage if an ejector pin was used in the process. Investigating the microstructural evolution during the process interestingly revealed the existence of a dead-metal-zone (DMZ) at the micro-pins’ surface in some circumstances. Finite element (FE) simulation showed that due to the openness nature of the process, the material flows in two different directions, i.e. towards the die orifice and away, leads to develop a neutral plane in the deforming material. Knowing the location of the neutral plane using FE simulation, together with the shape of the DMZ obtained from the experiments, helped us in understanding the cause for this behavior. Having that found, selection of the optimum tooling dimensions (i.e. punch diameter) was suggested as a way to control the behavior of the DMZ in the pins’ microstructure. Using FE simulation, it was shown that the friction inside the die orifice plays a more significant role on material behavior, rather than friction on the die top surface. Statistical analysis using ANOVA method provided an analytical solution for determination of the friction factor based on the relative punch and die diameters. Experimental investigations revealed the prevention of the formation of the DMZ at the pin surface by increasing the initial grain size. This behavior was attributed to the role of the subgrain size on the deformation. Moreover, micro-compression tests on the micro-pins having different grain sizes revealed no significant size effect with respect to the mechanical behavior, even if the number of grains over the diameter of the micro-pins falls below its critical value. To justify the reason laying under this fact, a recovery annealing cycle was applied on the micro-pins to change the substructure properties without altering the mean grain size. A surprising drop in the flow stress of the recovery-annealed micro-pins implied the importance of considering subgrain size rather than grain size over the diameter of component for the size effect investigation. Besides, phenomenological studies showed that there are two main phenomena competing during the open-die forming process; (i) Upsetting, (ii) Extrusion. Since the main “pin forming” occurs during the Extrusion phenomenon (the last stage of the process), it can be used for process optimization. To do so, the Upper Bound Theory was successfully modified for finding the critical strip thickness, after which the Extrusion is the dominant phenomenon. This provided an analytical solution for optimization of the process by minimizing the material wastage. Lastly, the effect of different lubricants on the material flow was investigated. It was shown that due to the process scale, it is difficult to put and keep the lubricant inside the die orifice, and hence no significant lubrication effect on the material behavior could be observed experimentally. Theoretical studies using the developed Upper Bound model, however, showed that successful lubrication inside the die orifice could significantly affect the final part dimension.