Fabrication of micro-channel arrays on thin metallic sheet using internal fluid pressure: Investigations on size effects and development of design guidelines

Micro-feature (channel, protrusion, cavity, etc.) arrays on large area-thin metallic sheet alloys are increasingly needed for compact and integrated heat/mass transfer applications (such as fuel cells and fuel processors) that require high temperature resistance, corrosion resistance, good electrical/thermal conductivity, etc. The performance of these micro-feature arrays mainly affects the volume flow velocity of the reactants inside the arrays which directly controls the rate of convection mass/heat transport. The key factors that affect the flow velocity include channel size and shape, flow field pattern, flow path length, fluid pressure, etc. In this study, we investigated these micro-feature arrays from the manufacturability perspective since it is also an important factor to be considered in the design process. Internal fluid pressure (hydroforming) technique is investigated in this study with the specific goals to, first, understand if the so-called "size effects" (grain vs. feature size) are effective on the manufacturability of thin metallic sheet into micro-channels, and second, to establish design guidelines for the micro-channel hydroforming technique for robust mass production conditions. Thin stainless steel 304 blanks of 0.051 mm thick with three different grain sizes of 9.3, 10.6, and 17.0 μm were used in hydroforming experiments to form micro-channels with the dimensions between 0.46-1.33 and 0.15-0.98 mm in width and height, respectively. Based on the experimental results, the effect of the grain size on the channel formability was found to be insignificant for the grain size range used in this study. On the other hand, the effect of the channel (feature) size was shown to dominate the overall formability. In addition, FE models of the process were developed and validated with the experimental results, then used to conduct a parametric study to establish micro-channel design guidelines. The results from the parametric study suggested that in order to obtain the maximum aspect ratio (height-to-width ratio) a small channel width should be used. Even though a large channel width would result in a higher channel height, the height-to-width ratio was found to be lower in this case. In addition, higher aspect ratio could be obtained by using larger corner radius (R_d), wider distance between adjacent channels (W_int), or less number of channels. On the other hand, the variation in draft angle (α) between 5° and 20° in combinations with the other channel geometries was found to be insignificant on the channel formability/height. All in all, these channel parameters (W, R_d, W_int, α, channel number, etc.) should be taken into account simultaneously in the design process in order to obtain such a design of the micro-feature arrays that would meet both performance and manufacturing requirements and constraints.