Process repeatability in abrasive jet micro-machining

Abrasive jet micro-machining (AJM) is a useful process for the fabrication of channels and holes in micro-fluidic and microelectronic devices. The small scale of these features and the need to achieve a high degree of accuracy has revealed several problems inherent with traditional abrasive jet equipment. Poor repeatability of the erosion rate in a pressure feed AJM system was traced to uncontrolled variation in the abrasive particle mass flux caused by particle packing and local cavity formation in the reservoir. The introduction of a mixing device within the pressure reservoir ensured that the powder remained loose and able to flow through the orifice to the air stream. This produced a significant improvement in AJM repeatability. Additional factors affecting erosion repeatability such as particle size stratification and the effect of powder level in the reservoir were also examined.     

Characteristics of abrasive slurry jet micro-machining: A comparison with abrasive air jet micro-machining

Abrasive slurry jet micro-machining (ASJM) uses a well-defined jet of abrasive slurry to erode features in a solid target. Compared with abrasive water jet machining (AWJM), the present ASJM system operates at pressures that are roughly two orders of magnitude lower and uses a premixed slurry of relatively low concentration. The objective of the present study was to gain a better understanding of the mechanics of erosion in ASJM by comparing its performance in the micro-machining of holes and channels in borosilicate glass with that of abrasive air jet micro-machining (AJM), a process that is simpler and relatively well understood. A new ASJM system was developed and used to machine blind holes and smooth channels of relatively uniform depth that did not suffer from the significant waviness previously reported in the literature. The effect of particle velocity, particle concentration, jet traverse speed and jet impact angle were examined. A direct comparison of ASJM and AJM results was possible since novel measurements of the crushing strength of the aluminum oxide abrasive particles used in both experiments proved to be unaffected by water. Brittle erosion was shown to be the dominant material removal mechanism in both ASJM and AJM in spite of the significant flow-induced decrease in the local impact angles of many of the particles in ASJM. A new model of the rapid particle deceleration near the target surface helped explain the much smaller erosion rates of ASJM compared with those in AJM. The modeling of the erosion process during the micro-machining of channels showed that the effect of the local impact angle at the leading edge of the advancing jet was much more significant in ASJM than in AJM, primarily due to the narrower focus of the jet impact zone in ASJM. The differences in the water and air flow fields and associated particle trajectories were used to explain the steeper side walls and flatter bottoms of the holes and channels machined with unmasked ASJM compared to those with masked AJM. The respective structures of the water and air jets also explained the much sharper definition of the edges of these features using ASJM compared with maskless AJM. The results of the study show that ASJM can be used to accurately micro-machine channels and holes with a width of 350–500 μm and an aspect ratio of 0.5–1.3 without the use of masks.     

Simulation of erosive smoothing in the abrasive jet micro-machining of glass

Abrasive jet micro-machining (AJM) is a promising technique to machine micro-features in brittle and ductile materials. However, the roughness of micro-channels machined using AJM is generally greater than that from other methods of micro-machining such as wet etching. Previous investigators have suggested that the surface roughness resulting from AJM can be reduced by post-blasting with abrasive particles at a relatively low kinetic energy. This approach was investigated in the present work by measuring the roughness reduction of a reference unmasked channel in borosilicate glass as a function of post-blasting particle size, velocity, dose, and impact angle. Post-blasting the reference channels reduced the roughness by up to 60%. It was observed that post-blasting at shallower angles was more efficient, probably due to the increased amount of edge chipping as opposed to cratering, which contributed to the enhanced removal of profile peaks, leaving a smoother surface. Moreover, post-blasting with smaller particles ultimately resulted in smoother surfaces, but at the penalty of requiring a relatively large particle dose, and consequently a significantly increased channel depth, before reaching the steady-state roughness. Hence, finishing with smaller particles until reaching the steady-state roughness may not be practical when a shallow channel is desired. A previously developed numerical model was modified and used to simulate the post-blasting process leading to the creation of smooth channels as a function of particle size, velocity, dose, impact angle, and target material properties. The model simulated both crater formation (due to growth of lateral cracks) and the chipping of facet edges. Comparisons with centerline roughness measurements for channels in borosilicate glass showed that the model can predict the transient roughness reduction with post-blasting particle dose with a 7% average error.     

Laser shadowgraphy measurements of abrasive particle spatial, size and velocity distributions through micro-masks used in abrasive jet micro-machining

In abrasive jet micromachining (AJM), a jet of particles is passed through narrow mask openings in order to define the features to be micro-machined. The size and shape of the micro-machined features depends on the distribution of the particle velocity and mass flux through the mask openings. In this work, a high speed laser shadowgraphy technique was used to demonstrate experimentally, for the first time, the significant effect that the mask opening size and powder shape and size have on the resulting distribution of particle mass flux and velocity through the mask opening. In particular, it was found that the velocity through the mask was approximately constant, but different in magnitude than the velocity in the free jet incident to the mask. The measured mass flux distributions were in excellent agreement with a previously developed analytical model, thus directly confirming its validity. Additional measurements also showed that an existing numerical model could be used to predict the velocity distribution in free jets of spherical particles, and, if a modification to the particle drag coefficient is made, in free jets of angular particles. The direct experimental verification of these models allows for their use in surface evolution models that can predict the evolving shape of features micro-machined using AJM.     

Modelling of abrasive particle energy in water jet machining

A phenomenological model of the three-phase flow inside an abrasive water jet machining cutting head has been developed. Several improvements over previously presented models such as taking into account the abrasive particle size distribution, and the effect of breakage of particles on the energy flux have been made. The model has been validated using an extensive set of experimental data with wide variations in cutting-head geometry, operating pressure, and abrasive mass flow rates. The cross-sectional averaged abrasive particle velocity at the exit of the focussing tube has been predicted with good accuracy over the whole range of experiments. In particular, the Pearson correlation between the model and the experimental results is found to be more than 95%, implying the utility of this model in design.     

An analytical model of the effect of particle size distribution on the surface profile evolution in abrasive jet micromachining

An analytical model to estimate the spatial distribution of erosive efficacy across the mask opening in the abrasive jet micromachining (AJM) of substrates is presented. A closed form analytical expression is derived which allows the erosive efficacy in the vicinity of the mask edge to be estimated as a function of the measured abrasive particle size distribution and the width of the mask opening. This analytical expression was used in a previously developed analytical surface evolution model to predict the time dependent eroding surface profiles of micro-holes and micro-channels of various sizes in glass and polymethylmethacrylate (PMMA), using aluminum oxide abrasive powders of different sizes. Use of the measured powder size distributions in the analytical models resulted in excellent agreement between the measured and model predicted channel profiles. The results of the study demonstrate that the particle size distribution and mask opening width can greatly affect the shape and depth of micro-channel profiles. A major improvement over previously developed models is ease-of-application since the erosive efficacy is given by an analytical expression rather than by the use of a computer simulation or a semi-empirical approach.     

A level set methodology for predicting the surface evolution of inclined masked micro-channels resulting from abrasive jet micro-machining at oblique incidence

A novel implementation of narrow band level set methods (LSM) was used to predict the surface evolution of inclined masked micro-channels in glass and poly-methyl-methacrylate (PMMA) made using abrasive jet micro-machining (AJM) at oblique incidence. The resulting inclined PMMA channels had straight walls and rectangular bottoms, while the glass profiles had curved walls and rounded bottoms. The inclined micro-channels rapidly become multi-valued, and the Hamilton–Jacobi type partial differential equation describing their evolution cannot be solved using traditional analytical or numerical techniques. To predict the decrease in particle flux at the mask edge, a previously developed analytical model was generalized from the normal to the oblique incidence case. The local surface velocity function was non-convex, necessitating the development of a modified extension velocity methodology to address the problem of grid ‘visibility’ of the particle flux. The agreement between LSM-predicted and measured surface evolution was fair. Since in its current form the model ignores particle ‘second-strike effects’ and mask wear, it is best suited to predict AJM surface evolution in ductile targets where mask wear is minimal, and not for brittle targets (e.g. glass). Since AJM at oblique incidence can be used to machine three-dimensional suspended micro-structures, the work has important implications for the micro-fabrication of novel MEMS and microfluidic devices.     

Abrasive jet micro-machining of planar areas and transitional slopes in glass using target oscillation

Analytical models are presented which allow the prediction of the shape, sidewall slope, and depth of abrasive jet micro-machined planar areas and transitional slopes in glass using a novel technique in which the target is oscillated transversely to the overall scan direction. A criterion was developed to establish the minimum oscillation velocity to ensure negligible surface profile waviness in the scanning direction. If the oscillation velocity is sufficiently greater than the scanning velocity, the target receives an approximately uniform energy flux, resulting in a high degree of flatness for both masked and unmasked planar areas micro-machined in glass. It was also found that particle scattering from the mask edge caused the sidewalls of a planar area to be very shallow, on the order of a few degrees. Two methods were investigated to machine planar areas with increased sidewall slope using target oscillation: (i) machining micro-channels adjacent to the planned planar area, and (ii) target oscillation with an obliquely oriented nozzle. Among these two methods, target oscillation with an obliquely oriented nozzle created steeper sidewalls and was easier to implement, but it also caused appreciable mask under-etching. A major distinction between the target oscillation approach and a previously published method that was based on the superposition of the erosion profiles of adjacent nozzle scans, is that the latter is capable of machining an arbitrary surface profile over a large area, whereas the present target oscillation technique is intended only for the machining of flat planar areas at a single elevation. For such applications it is the preferred approach.     

Abrasive slurry jet micro-machining of holes in brittle and ductile materials

This paper investigated the effects of elasticity and viscosity, induced by a dilute high-molecular-weight polymer solution, on the shape, depth, and diameter of micro-holes drilled in borosilicate glass and in plates of 6061-T6 aluminum alloy, 110 copper, and 316 stainless steel using low-pressure abrasive slurry jet micro-machining (ASJM). Holes were machined using aqueous jets with 1 wt% 10 μm Al₂O₃ particles. The 180 μm sapphire orifice produced a 140 μm diameter jet at pressures of 4 and 7 MPa. When the jet contained 50 wppm of dissolved 8 million molecular weight polyethylene oxide (PEO), the blind holes in glass were approximately 20% narrower and 30% shallower than holes drilled without the polymer, using the same abrasive concentration and pressure. The addition of PEO led to hole cross-sectional profiles that had a sharper edge at the glass surface and were more V-shaped compared with the U-shape of the holes produced without PEO. Hole symmetry in glass was maintained over depths ranging from about 80–900 μm by ensuring that the jets were aligned perpendicularly to within 0.2°. The changes in shape and size were brought about by normal stresses generated by the polymer. Jets containing this dissolved polymer were observed to oscillate laterally and non-periodically, with an amplitude reaching a value of 20 μm. For the first time, symmetric ASJM through-holes were drilled in a 3-mm-thick borosilicate glass plate without chipping around the exit edge.The depth of symmetric blind holes in metals was restricted to approximately 150 μm for jets with and without PEO. At greater depths, the holes became highly asymmetric, eroding in a specific direction to create a sub-surface slot. The asymmetry appeared to be caused by the extreme sensitivity of ductile materials to jet alignment. This sensitivity also caused the holes in metals to be less circular when PEO was included, apparently caused by the random jet oscillations induced by the polymer. Under identical conditions, hole depths increased in the order: borosilicate glass > 6061-T6 aluminum > 110 copper > 316 stainless steel. The edges of the holes in glass could be made sharper by machining through a sacrificial layer of glass or epoxy.     

A Taguchi and experimental investigation into the optimal processing conditions for the abrasive jet polishing of SKD61 mold steel

This study introduces an abrasive jet polishing (AJP) technique in which the pneumatic air stream carries not only abrasive particles, but also an additive of either pure water or pure water with a specified quantity of machining oil. Taguchi design experiments are performed to identify the optimal AJP parameters when applied to the polishing of electrical discharge machined SKD61 mold steel specimens. A series of experimental trials are then conducted using the optimal AJP parameters to investigate the respective effects of the additive type and the abrasive particle material and diameter in achieving a mirror-like finish of the polished surface. The Taguchi trials indicate that when polishing is performed using pure water as an additive, the optimal processing parameters are as follows: an abrasive material to additive ratio of 1:2, an impact angle of 30°, a gas pressure of 4 kg/cm², a nozzle-to-workpiece height of 10 mm, a platform rotational velocity of 200 rpm, and a platform travel speed of 150 mm/s. Applying these processing parameters, it is found that the optimal polishing effect is attained using #8000SiC abrasive particles and a 1:1 mixture of water-solvent machining oil and pure water. The experimental results show that under these conditions, the average roughness of the electrical discharge machined SKD61 surface is reduced from an original value of R_a=1.03 μm (R_max: 7.74 μm) to a final value of R_a=0.13 μm (R_max: 0.90 μm), corresponding to a surface roughness improvement of approximately 87%.     

Progress in fluidized bed assisted abrasive jet machining (FB-AJM): Internal polishing of aluminium tubes

This paper deals with the internal finishing of tubular components made from a high strength aluminium alloy (AA 6082 T6) using a fluidized bed assisted abrasive jet machining (FB-AJM) system.Firstly, a Taguchi's experimental plan was used to investigate the influence of abrasive jet speed, machining cycle, and abrasive mesh size on surface roughness and material removal trends. Secondly, the leading finishing mechanisms were studied using combined 3d profilometer-SEM analysis to monitor the evolution of the surface morphology of machined workpieces. Finally, the circumferential uniformity and precision machining of the inner surface of workpieces were tested by evaluating the values of the more significant roughness parameters in different circumferential locations.Consistent trends of surface roughness vs. operational parameters were measured, and significant material removal was found to affect the workpieces during machining. As a result, FB-AJM was found to preferentially machine the asperities and irregularities of the surface, thereby altering the overall surface morphology producing more regular and smoother finishing. Moreover, the good circumferential uniformity and machining accuracy FB-AJM guarantees even on ductile aluminium alloy workpieces ensure that this technology can be applied to a diverse set of industrial components.