Abstract
A better understanding of material deformation behaviours with changes in size is crucial to the design and operation of metal microforming processes. In order to facilitate the investigation of size effects, material deformation behaviours needed to be determined directly from material characterizations. This study was aimed at the design and manufacture of a compact universal testing machine (UTM) compatible with a 3D laser-confocal microscope to observe the deformation behaviour of materials in real-time. In this study, uniaxial micro tensile testing was conducted on three different thin (0.05 mm, 0.1 mm, and 0.3 mm) copper specimens with characteristic dimensions at micro scales. Micro tensile experimental runs were carried out on copper specimens with varying grain sizes on the newly developed apparatus under a 3D laser-confocal microscope. Microscale experiments under 3D laser-confocal microscope provided not only a method to observe the microstructure of materials, but also a novel way to observe the early stages of fracture mechanisms. From real-time examination using the newly developed compact testing apparatus, we discovered that fracture behaviour was mostly brought about by the concave surface formed by free surface roughening. Findings with high stability were discovered while moving with the sample grasped along the drive screw in the graphical plot of a crosshead’s displacement against time. Our results also showed very low mechanical noise (detected during the displacement of the crosshead), which indicated that there were no additional effects on the machine, such as vibrations or shifts in speed that could influence performance. The engineering stress-strain plots of the pure copper-tests with various thicknesses or samples depicted a level of stress necessary to initiate plastic flowing inside the material. From these results, we observed that strength and ductility declined with decreasing thickness. The influence of thickness on fracture-strain, observed during tensile testing, made it clear that the elongation-at-break of the pure-copper foils intensely decreased with decreases in thickness. The relative average surface-roughness Ra was evaluated, which showed us that the surface-roughness escalated with the increasing trend of plasticity deformation (plastic strain) ε. For better understanding of the effects of plastic strain on surface roughness prior to material fractures, micro tensile tests were performed on the newly developed machine under a 3D laser-confocal-microscope. We observed that homogeneous surface roughness was caused by plastic strain, which further formed the concave surface that led to the fracture points. Finally, we concluded that surface roughness was one of the crucial factors influencing the fracture behaviour of metallic sheet-strips in metal microforming. We found that this type of testing apparatus could be designed and manufactured within a manageable budget.
Highlights
In this modern world of miniaturization, there is an enormous need for microscale metallic components for industrial applications [1]
Evaluation of local deformation behaviour in a very narrow region on a microscale could be a novel approach to exploring the impacts of size in metal microforming
A novel, small-scale engineering testing instrument for microscale specimens was developed that allowed in situ observations of the surface roughness and fracture mechanisms of materials by 3D laser-confocal microscope
Summary
In this modern world of miniaturization, there is an enormous need for microscale metallic components for industrial applications [1]. The directional/geometrical accuracy and reliability of such microscale metallic parts has been estimated to be in the range of 0.1 μm to 10 μm [1,2]. Various advanced micromanufacturing processes have been invented, and they have been continuously used to manufacture microscale metallic parts such as connectors pins, micro screw and springs [3,4]. Metal microforming is an effective process for large-scale manufacturing of intricate and high-performance microscale metallic segments [5]. Metal microforming is an excellent process because of its notable benefits (for example: high creation rates, close net shapes, limited material waste, and close tolerance) [5]
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