Fabrication of high aspect ratio free-standing structures using abrasive water jet micro-machining

The abrasive water jet micro-machining of high aspect ratio free-standing structures such as fins used in heat sinks was investigated. The aim was to fabricate the maximum number of fins per unit length with the highest aspect ratio into Al6061-T6, in order to maximize heat transfer potential. Adjacent straight channels at various offsets were machined using a micro-nozzle, thus leaving the free-standing structures in between. The effect of varying nozzle traverse speed, number of passes, water jet pressure, channel offset, and abrasive mass flow rate on the quality of the resulting free-standing structures was studied. Under some conditions, an undesirable 'leveling' phenomenon was found to occur, i.e. the tops of the free-standing structures eroded due to particle impacts at the periphery of the jet, and secondary slurry flow from within the channels between the structures. In general, conditions that led to a higher erosion rate (higher water jet pressures, lower nozzle traverse speeds, and higher abrasive mass flow rates) were found to minimize the leveling at a given offset, because the channels between the structures rapidly became deep, and secondary slurry flow was directed along the channel, rather than up the sidewalls. A threshold minimum offset to avoid this phenomenon was also determined. Contrary to previously reported results, it was found that the jet had ample energy to erode the target material even at effective stand-off distances (distance between nozzle and channel bottom) greater than 10 mm, provided that sufficiently large abrasive particles are used. With these factors in mind, it was demonstrated that high-quality free-standing structures with aspect ratios of more than 40 could be fabricated.