Abrasive Waterjet Micromachining Research Articles

Controlled depth micro-abrasive waterjet milling of aluminum oxide to fabricate micro-molds containing intersecting free-standing structures

Traditionally, photolithography and soft-lithography methods have been used to fabricate micro-molds for casting polymeric microfluidic chips. Because of the time and cost involved, there has been motivation to fabricate microfluidic micro-molds using other techniques. A previous study utilized stainless steel masks cut using abrasive waterjet micro-machining (AWJM) together with abrasive slurry jet micro-machining (ASJM) to mill micro-molds in Al6061 which would allow casting of polymeric microfluidic chips with intersecting micro-channels. However, the deflected slurry jet from the mask edge struck the substrate at oblique incidence and led to two undesirable effects: a trench along the edges of the raised structures representing the channels, and an undercut of the mask. Both effects were linked to the propensity for ductile materials to erode more rapidly at oblique incidence than perpendicular. The present paper demonstrates that these effects can be greatly reduced by using AWJM to create molds in more brittle materials which erode in the opposite fashion, i.e., more rapidly at perpendicular than oblique incidence. To demonstrate this, AWJM was used to machine masks from aluminum oxide and subsequently mill micro-molds with raised intersecting free-standing structures into aluminum oxide substrates. It was found that for molds of identical depth, the undercut and the undesirable erosion in aluminum oxide molds could be reduced by five and four times, respectively, when compared to molds machined in Al6061 using the previous AWJM/ASJM hybrid technique. It was also found that much deeper molds containing intersecting raised free-standing structures of up to 435 μm in height could be made in aluminum oxide, while still maintaining an acceptable surface quality (1.51 μm Ra and 2.47 μm Wa), undercut (25 μm), and undesirable erosion (3%). Finally, since the technique involved only AWJM, it was much more convenient than the previously utilized hybrid AWJM/ASJM.

Integral modeling of abrasive waterjet micro-machining process

Abrasive water jet technology has been widely used to process fine structures on various materials, but there is no complete model that can describe the entire process. In this paper, the integral modeling process of abrasive water jet micro-machining (AWJM) is presented. The three-phase fluid dynamics model inside the nozzle and the surface evolution model are connected by theoretically calculating the jet erosive efficacy distribution before impacting the workpiece. Several improvements over previously presented models, such as taking into account the divergence angle of abrasive particles and the effect of the nonuniform distribution of each phase on the jet erosive efficacy, have been made. The model was used to predict the surface evolution of micro-channel profiles in 316L stainless steel and 6061-T6 aluminium. For both materials, the radial profile of erosive efficacy shows a normal distribution, and the erosive efficacy of 6061-T6 is two orders of magnitude higher than that of 316L. The results show that the model can accurately predict the profiles of the micro-channels resulting from AWJM. In particular, the average prediction error in the width direction is reduced to 4.5%.

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.

Analytical Modeling and Experimental Study of Machining of Smart Materials using Submerged Abrasive Waterjet Micromachining Process

Smart materials are new generation materials which possess great properties to mend themselves with a change in environment. Manufacturing of these materials is a huge challenge, particularly at micron scale due to their superior mechanical properties such as high hardness, high compressive strength and chemical inertness. This research investigates submerged abrasive waterjet micromachining (SAWJMM) process for machining smart ceramic materials. The research also involves experimental study on micromachining of smart materials using an in-house fabricated SAWJMM setup. The study found that SAWJMM process is capable of successfully machining smart materials including shape memory alloys and piezoelectric materials at the micron scale. An analytical predictive model is developed to estimate the MRR during SAWJMM process and the model is found to be capable of accurately predicting the machining results within 10% error. [Submitted 14 January 2018; Accepted 26 May 2018]

Analytical modelling and experimental study of machining of smart materials using submerged abrasive waterjet micromachining process

Analytical modelling and experimental study of machining of smart materials using submerged abrasive waterjet micromachining process

Investigation and modeling of microgrooves generated on diamond grinding wheel by abrasive waterjet based on Box–Behnken experimental design

Precision profile grinding with textured diamond wheels is an alternative for generating microstructures on the ceramic mold for glass molding. In this work, a novel texturing strategy employing abrasive waterjet for superabrasive grinding wheels was proposed to generate controllable microtexture profile on the diamond wheel surface efficiently. The quadratic backward-eliminated regression models were developed using Box–Behnken response surface design in the abrasive waterjet micromachining of diamond grinding wheel sample. The effect of operating parameters and their interaction on the depth and width of groove were investigated. The surface speed and standoff distance were found to be main controlling variables on the depth and width of groove, respectively. A consistently good agreement was confirmed between the predicted values and the experimental values under acceptable errors.

Masked micro-channel machining in aluminum alloy and borosilicate glass using abrasive water jet micro-machining

Abrasive water jets (AWJs) have recently been used to mill unmasked features as narrow as 600 μm. This paper investigated the use of metal masks in order to decrease this minimum possible feature width in Al6061-T6 and borosilicate glass. Although there was under-etching on the channel sidewalls below the mask edges, it was nevertheless found that masked channels could be machined that were between 2–3 times narrower with 11% lower centerline roughness and 44% lower waviness than those created without masks. It was also found that increases in mask thickness led to increases in the channel centerline depth and width, and decreases in the centerline roughness and waviness. Increases in the abrasive mass flow rate increased the channel width, but decreased the depth. Finally, the normalized instantaneous centerline erosion rates in the masked channels decreased faster with depth than in the unmasked cases. Reasons for these trends were discussed in terms of changes in abrasive slurry flow and in the size of the stagnation zone within the channels brought about by the masks. Overall, the study demonstrates that masked AWJ micro-machining of features as narrow as ∼150 μm wide is possible, thus demonstrating the feasibility of the technique for the manufacture of microfluidic and other components.

Effect of entrained air in abrasive water jet micro-machining: Reduction of channel width and waviness using slurry entrainment

The erosion produced by abrasive water jets is a result of the complex interaction of the three-phase jet (water-air-abrasive) and the target. The objective of the present work was to isolate the effect of the entrained air in abrasive water jet micro-machining (AWJM) by comparing milled surfaces made by AWJM with those produced by high-pressure slurry jet micro-machining (HASJM) while maintaining a constant particle velocity. An existing model developed for abrasive particle velocities in AWJM was modified and used to predict the particle velocity in HASJM, and then verified using a double disc apparatus (DDA). The model allowed prediction of the conditions required to achieve average particle velocities of ~255m/s using the two systems with the same 38µm garnet particles. Under this condition of equal particle velocity, there was a very large reduction in the centerline waviness, Wa, of micro-channels made in SS316L and Al60661-T6 using HASJM; typically 3.4 times smaller waviness than found in channels made with AWJM using the same dose of particles. This was found to be mainly due to the much larger variation in abrasive flow rate in AWJM brought about by the air/particle entrainment system, rather than any fundamental change in erosion mechanism due to the impact of the nonhomogeneous three phase air/particle/water in AWJM. The centerline roughness, Ra, was approximately the same in both processes at a traverse velocity of Vt=4572mm/min and a nozzle angle of 90°. For both processes, Wa and Ra increased with an increase in pressure and abrasive particle dose, and decreased with an increase in nozzle angle. For micro-channels of a given depth, the widths of those made with HASJM were 26% narrower than those produced with AWJM, mainly due to the wider jet that resulted from the entrained air in AWJM, again at the same particle velocity. The erosion rate in HASJM was found to be about 27% lower mainly because of a smaller width of micro-channels in HASJM.

Abrasive waterjet micro-machining of channels in metals: Model to predict high aspect-ratio channel profiles for submerged and unsubmerged machining

Recent advances in the development of miniaturized nozzles have made possible the use of abrasive waterjets to perform controlled-depth micro-milling. Haghbin et al. (2015) discussed the effects of the surrounding fluid in the micro-machining of shallow channels in 316L stainless steel and 6061-T6 aluminum using a prototype nozzle having an orifice diameter of 127μm and a 254μm mixing tube diameter. This paper uses those results to develop a new surface evolution model that predicts the size and shape of relatively deep micro-channels resulting from unsubmerged and submerged abrasive water jet micro-machining (AWJM). For both unsubmerged and submerged AWJM, and for both materials, the erosive efficacy distribution changed suddenly after the initial formation of the channel. The initially wide distribution was due to backflow of the abrasive slurry along the channel walls, which did not occur once the channel was formed and most of the flow was directed along the channel length. A novel approach in which two different erosive efficacy expressions are sequentially used in a surface evolution equation is presented and shown to accurately predict the evolving surface topography for micro-channels up to aspect ratios of 3.

Abrasive waterjet micro-machining of channels in metals: Comparison between machining in air and submerged in water

Abrasive water jet technology can be used for micro-milling using recently developed miniaturized nozzles. Abrasive water jet (AWJ) machining is often used with both the nozzle tip and workpiece submerged in water to reduce noise and contain debris. This paper compares the performance of submerged and unsubmerged abrasive water jet micro-milling of channels in 316L stainless steel and 6061-T6 aluminum at various nozzle angles and standoff distances. The effect of submergence on the diameter and effective footprint of AWJ erosion footprints was measured and compared. It was found that the centerline erosion rate decreased with channel depth due to the spreading of the jet as the effective standoff distance increased, and because of the growing effect of stagnation as the channel became deeper. The erosive jet spread over a larger effective footprint in air than in water, since particles on the jet periphery were slowed much more quickly in water due to increased drag. As a result, the width of a channel machined in air was wider than that in water. Moreover, it was observed that the instantaneous erosion rate decreased with channel depth, and that this decrease was a function only of the channel cross-sectional geometry, being independent of the type of metal, the jet angle, the standoff distance, and regardless of whether the jet was submerged or in air, in either the forward or backward directions. It is shown that submerged AWJM results in narrower features than those produced while machining in air, without a decrease in centerline etch rate.

Impact Erosion of Quartz Crystals by Micro-Particles in Abrasive Waterjet Micro-Machining

Abrasive waterjet (AWJ) micro-machining is a precision processing technology with some distinct advantages. To understand the machining process, the erosion mechanism is presented and discussed when micro-particle impacting on a quartz crystal specimen. It is found that three types of impressions are formed which are craters, micro-dents and scratches. Small-scale craters including crashed zones and radial cracks are associated with plastic flow and subsurface micro-cracks that decrease the material strength, but cause little material removal, while large-scale craters including conchoidal fractures caused by the propagation of lateral cracks dominate the volume change of the specimen. Micro-dents are produced by the impact of particles possessing small kinetic energies, and scratches are generated by particle sliding or rolling over the target surface and make a negligible contribution to material removal. The crater volume generated by the impact of individual particle is then discussed with respect to particle impacting velocity and impact angle. It shows that an increase in particle impact angle or particle velocity increases the crater volume due to the increased conchoidal fractures during the impact process.