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In microforming processes it is preferred to not to use lubricants, due to their complex behavior in micro-scale. Nevertheless, using lubricants could increase the life time and decrease the required forming load. Thus, it is necessary to study and develop an analytical solution for different lubrication conditions in microforming processes. A previously studied upper bound model was modified in this study for various lubrication conditions in an open-die micro-forging/extrusion process. Two approaches were chosen for identifying the friction factor in the model: (i) global friction factor, (ii) localized friction factor. Comparison of the modeling results with the experimental showed the reliability of the second approach, providing a better fit. Published version

In metal deformation, size effect is generally attributed to the interactive effect of grain size and specimen dimension. This work shows, however, that relative substructure dimensions should also be considered. 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.

Effects of substructural dimensions on the mechanical properties of micro-pins produced by an open-die micro-extrusion/forging process were studied. Micro-pins of diameter 0.3 mm were manufactured from copper strips, having different initial grain sizes. Micro-compression tests on the micro-pins revealed no significant size effect, even if the number of grains over the diameter of the micro-pins falls below its critical value. However, relaxation of the as-formed substructure using recovery annealing led to a surprising drop in the flow stress of the micro-pins. This was explained and attributed to the number of subgrains over the diameter of the micro-pins, showing the important role of subgrains rather than grains in determining the mechanical properties. ASTAR (Agency for Sci., Tech. and Research, S’pore) Accepted version

Friction effects during a progressive microforming process for production of micropins of various diameters were experimentally investigated and were analytically modeled, using a hybrid friction model. The response surface method and ANOVA analysis were used to generalize the findings for various pin diameters. Besides, it was shown that to get an accurate result in simulation, the friction model must be considered locally instead of a global friction model for the whole process. The effect of friction factor on the final micropart dimensions (the effect on the instantaneous location of the neutral plane) and the forming pressure were investigated. The results showed a reduction in the friction factor as die diameter increased. Following that, the optimum frictional condition to obtain the highest micropart aspect ratio was defined as the maximum friction on the interface between the die upper surface and the punch surface, together with a minimum friction inside the die orifice.

Surface modifications are often employed to improve the corrosion properties of magnesium alloys. In this study, the eco-friendly magnesium hydroxide (Mg(OH)2) conversion coatings were successfully applied on AZ31 substrates through hydrothermal treatment with the assistance of tungstate ions. Phase composition, surface and interface morphologies, surface composition, and adhesion property were characterised using X-ray diffraction, scanning electron microscope, energy dispersive spectroscopy and scanning scratch tester respectively. The coating obtained was fairly dense and uniform. Coating thickness increased with the increase of hydrothermal treatment time, but longer times ( > 6 h) has a detrimental effect on the coating quality. The combined effect of adsorption and complexing of tungstate ions can effectively change the growth of Mg(OH)2 crystal from anisotropic to isotropic. This effect was crucial to produce the dense and uniform conversion coating. Scratch test indicated that the conversion coating adhered to the substrate strongly. Electrochemical testing revealed that the conversion coating caused AZ31 to be of higher corrosion potential with a much lower corrosion current density. This current study provides a promising surface treatment method for magnesium alloys towards their further applications.

The interactive effect of grain size and specimen dimensions on the material flow and microstructural evolution was studied in a progressive open-die microforming process. Particular interest was paid on the effect of the number of grains over the initial specimen thickness, on the evolution of the dead metal zone (DMZ) in the final micro-component's microstructure. Such a DMZ is deemed unfavorable for mechanical properties of the pin. Interestingly, experimental results revealed that the DMZ can be removed at the pin surface by increasing the initial grain size. This behavior was attributed to the role of the strain gradient on the deformation. In the aspects of the forming load and dimensional measurements of the final parts, there were no significant size-effects observed in this process. This is because the neutral plane, which demarcates the two directions of material flow in the open-die forging/extrusion process, determines the amount of material flow towards the die orifice, regardless of the initial grain size. ASTAR (Agency for Sci., Tech. and Research, S’pore) Accepted version

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. DOCTOR OF PHILOSOPHY (MAE)