fine finishing of sic microchannels using abrasive flow...

Indian Journal of Engineering & Materials Sciences Vol. 22, June 2015, pp. 297-306

Fine finishing of SiC microchannels using abrasive flow machining G Venkatesh, Apurbba Kumar Sharma* & Pradeep Kumar

Department of Mechanical and Industrial Engineering, Indian Institute of Technology Roorkee 247 667, India

Received 7 April 2014; accepted 25 November 2014

Finishing of microchannels is a major requirement in many applications including micro fluidics. The finishing of the channel wall/surfaces significantly influence the flow characteristics of the fluid being flown through them. Abrasive flow machining (AFM) is one of the non-traditional finishing processes in which a flexible cutting tool is used to remove the micro asperities from the target surface. It is capable of machining micro-bores, channels, blind holes and intricate geometries, which are difficult to finish using the traditional processes. In the present work, microchannels fabricated using ultrasonic machining (USM) are fine finished by AFM. The input parameters such as extrusion pressure, wt% of processing oil and processing time are varied while finishing the microchannels using a double-acting horizontal type AFM setup. It is observed that surface finish of the microchannel walls got improved by more than 55%. The machining performance is evaluated in terms of the responses-surface finish and material removed. Optimum values of the process parameters are determined for better surface quality.

Keywords: Microchannel, Ultrasonic machining, Abrasive flow machining, Finish machining, Parametric effect

Fabrication of microchannels has gained importance over last few years because of their increasing industrial applications, especially in areas such as cooling of miniature electronic parts/components, biomedical engineering, biochemistry and electophoretic applications. Accordingly, advanced machining techniques have been developed to fabricate microchannels which include laser micromachining1, focused ion beam micro-machining2, micro-electro discharge milling3, micro EDM4 and micro ultrasonic machining5. Micro-ultrasonic machining is one of the best techniques to fabricate the microchannels on hard and brittle materials6. As one of the non-traditional manufacturing processes, micro USM finds its main advantage in machining hard and brittle materials. Machining could be performed in micron and nano level. Sun et al.7 worked on micro USM and produced 20 μm in diameter and 50 μm depth on silicon and quartz plates. USM has been also verified by machining micro features such as blind holes, slots and 3-D cavities for MEMS application by Egashira et al.8 Jain9 worked on ultrasonic machining and fabricated 3D open microchannels on commercially available borosilicate glass and single crystal silicon and investigated material removal rate, tool wear rate, and surface roughness effect on process response. Accordingly micro USM has been used to develop

microchannels on silicon carbide material used in the present study. The major limitations of USM in surface quality and shape precision are greatly affected by machining quality. It was observed that during post fabrication of microchannels through micro USM, the surface gets partially damaged due to pitting action that strongly influences the quality of the microchannels. Thus, a need arises for a secondary finishing process for fine finishing of a microchannel surfaces to achieve high quality surfaces for enhanced performance.

Abrasive flow machining is an innovative finishing technique to finish complex surfaces with close tolerances by removing micro to nano-level material. In this process, material is removed in the form of microchips layer by layer by the flow of pressurized flexible abrasive medium over or through the target surface10. Efficiency of the AFM process depends upon process parameters that are further controlled by three major elements, namely, the machine, work piece fixture (tooling) and flexible medium. The AFM process has been used as a secondary finishing process in many application oriented studies where EDM, USM and TIG etc. were primary processes. The ultimate aim of machining through AFM is to achieve fine finishing of the product. Jung et al.11 worked on finishing of injectors using AFM and recorded that the finishing effect achieved by the

___________ *Corresponding author (E-mail: [email protected])

mailto:[email protected])

INDIAN J. ENG. MATER. SCI., JUNE 2015

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process can enhance engine performance. Kim12 worked on chrome-molybdenum spring collects and conducted deburring test using AFM. Yin et al.13 used 260-500 mm diameter and 25-50 length/diameter ratios of micro-bores of both metal and ceramic materials and studied the polishing effect using AFM process. Lin et al.14 worked on finishing of EDM fabricated micro-holes in stainless steel (SUS 304) and titanium alloy (Ti-6Al-4V) plates by abrasive fluid machining process and reported that AFM process has a control on shape precision and could achieve micro level surface roughness. They also applied Taguchi technique to analyze the response parameters and signal noise ratio14. Tzeng et al.15 carried out finishing of the complex shaped micro slits fabricated by wire electrical discharge machining (Wire-EDM) process and found the significant parameters of the study through Taguchi experimental method. The AFM process capable of polishing wire-EDM damaged surfaces under dry conditions. Pusavec et al.16 analysed the AFM finishing process by developing a movable mandrel based abrasive flow machining process in order to overcome low finishing rate of the AFM16,17.

Eventually in this study, AFM was chosen for finishing of silicon carbide (SiC) microchannels fabricated by micro USM. The present work mainly focuses on the development of appropriate processing strategy to increase the quality of the finish machined surfaces of microchannel. Details of ultrasonically machined channels, work-piece preparation, varied parameter levels and response optimization results

etc. are discussed in detail elsewhere18. Finish machining of channel surfaces, response surface optimization, effect of the machining parameters and interaction effect on process responses have been discussed with illustrations. Experimental Procedure Fabrication of microchannels

Selection of work-piece material is basically driven by practical requirement. The SiC microchannels are generally used in high temperature heat exchangers, catalytic reactors, electronics cooling, high-power electronics applications and processing of corrosive streams where the thermo mechanical benefits of ceramic materials are desired. Accordingly, SiC was selected as the work-piece material. The chemical composition of the work-piece was confirmed using an Energy Dispersive X-ray Spectroscopy (EDS) and the results are presented in Table 1. Channels of dimension 1 mm width 0.5 mm depth were fabricated on the rectangular flat of 8 mm width 15 mm length through USM using SiC abrasives. The width and depth of the channels were maintained marginally coarse owing to measurement probe requirement. The fabricated channels were then used for fine finishing using the AFM technique. The surface finish of the microchannels fabricated by the USM was recorded prior to the post processing through AFM. The average surface roughness (Ra) was recorded to be 1.4 to 2.0 μm. Figure 1a presents an image of few such microchannels fabricated using

Table 1—Chemical composition of SiC work-piece

Element C O Na Al Si S K Ca total Weight % 37.32 20.79 0.37 0.64 33.61 8.42 0.28 0.53 100

Fig. 1—(a) Typical SiC microchannels machined using USM and (b) Schematic diagram of a SiC microchannel workpiece facilitating abrasive flow action (inset: interaction of the work-piece asperities with the abrasive particles)

VENKATESH et al.: FINE FINISHING OF SiC MICROCHANNELS USING ABRASIVE FLOW MACHINING

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USM. Such microchannels, in general, have a rough profile depending on the processing conditions. A schematic of a general profile is illustrated in Fig. 1b. The asperities as shown in the figure inset are candidates for further smoothening in order to improve fluid flow through such microchannels. A typical SEM micrograph of the microchannel fabricated by USM is shown in Fig. 2. A pit surface was observed on the channel wall. Finishing of microchannel using AFM

The channels fabricated by USM were cleaned using an ultrasonic bath prior to fine finishing through AFM. The developed AFM setup consists of two hydraulic cylinders, two medium cylinders and work-piece tooling at the middle as shown in Fig. 3. The machine is integrated with power supply, hydraulic unit to perform reciprocating action and DC motor to control the media flow rate. In the present work, natural polymer medium19,20 was prepared for the AFM processing by mixing SiC particles (average:

200 mesh) with naphthenic based processing oil which is basically a hydrocarbon oil boiling between 30°C to 200°C (colour: light brown, manufactured by: Standard chemicals). Medium mainly consists of natural polymer with compounds like alkenes, alkanes, sulfoxides, and esters. The carrier can be recovered from the used medium and can be reused. The developed medium was capable of withstanding temperature up to 71°C while disintegrating19

In the AFM setup, the microchannels were placed in such a way that the medium flows axially as shown schematically in Fig. 1b. A special fixture was fabricated to fix the work-piece for the purpose which facilitates the medium flow direction from one cylinder to the other cylinder by constricting the flow through the microchannel. The mechanism ensures physical contact of the abrasive laden medium with the work-piece surface to be finished which consequently improves surface quality. Thus, in AFM process, a fixture plays an important role in enhancing process productivity21. Variables and responses

In the present work, response surface methodology was used to explore the responses over the complete experimental set. Accordingly, a set of twenty experiments were conducted based on central composite rotatable design (CCRD) of response surface methods22. The process parameters and their levels selected for the trials are given in Table 2

Fig. 2—A typical SEM micrograph of the SiC channel surface fabricated by USM

Fig. 3—A two-way horizontal type abrasive flow machine

Table 2—Process parameters and their levels CCRD coded

values Extrusion pressure, EP,

bar. wt% of processing oil

(WPO), % Processing time ( PT),

min. Response parameters

Low (-1) 15 10 5 Middle (0) 20 12.5 7 High (1) 25 15 9

1. Improvement in Surface Finish, ΔRa, μm 2. Material removed, mg

Constant parameters Abrasive mesh size 200 Media Flow Rate, 796 cm3/min Temperature of media, 32 ±2 °C Initial surface roughness, 1.4 -2 µm


300

and the experimental results are presented in Table 3. In the present set of trials, abrasive mesh size and media flow rate were maintained at constant values based on previous investigation20. Three parameters extrusion pressure, wt% of processing oil and processing time were varied with three levels for each parameter based on the setup constraints and processing quality requirements.

The responses such as percentage improvement in surface finish (ΔRa) and material removed (MR) were calculated by using Eqs (1) and (2), respectively. The responses for the trial condition were also predicted using a commercially available software tool Design Expert (version 7.0). The corresponding predicted values of the responses and the errors have been presented in Table 3. The estimated errors for

percentage improvement in surface finish and material removed were found to be ± 5.70 % and ± 7.23 %, respectively.

%100

a

aaa RInitial

RfinalRInitialΔR … (1)

][ weightfinalweightInitialMR … (2)

Regression analysis

The analysis of variance (ANOVA) was carried out at 95% confidence level for surface finish improvement and material removed. The quadratic models of respective response characteristics as a function of both the responses for ΔRa and MR in terms of coded values are presented in Table 4.

Table 3—Experimental trials and corresponding responses with estimated errors

Trial condition

Extrusion pressure (EP), bar

wt% of processing oil (WPO)

Processing time (PT),

min

% improvement in surface

finish,ΔRa,

Material removed, mg

Predicted ΔRa, %

Predicted MR, mg

Error, ΔRa, %

Error, MR, mg

1 20 12.5 7 55.44 14.12 52.28 15.14 5.70 -7.23 2 20 12.5 7 53.23 14.31 52.28 15.14 1.78 -5.80 3 20 12.5 7 52.12 15.56 52.28 15.14 -0.31 2.70 4 20 12.5 7 51.15 14.92 52.28 15.14 -2.21 -1.46 5 20 12.5 7 53.67 14.90 52.28 15.14 2.58 -1.66 6 20 12.5 7 52.19 15.99 52.28 15.14 -0.18 5.31 7 20 10 7 47.12 21.94 47.65 21.80 -1.12 0.60 8 15 12.5 7 39.12 9.91 39.89 9.72 -1.973 1.94 9 20 15 7 57.61 17.05 59.15 16.66 -2.68 2.29 10 25 12.5 7 51.12 14.52 52.42 14.19 -2.53 2.27 11 20 12.5 5 42.12 11.56 43.32 11.59 -2.85 -0.26 12 20 12.5 9 52.12 15.29 52.99 14.74 -1.66 3.62 13 15 15 5 36.54 8.52 35.25 8.42 3.52 1.14 14 15 10 5 23.45 13.12 24.27 13.17 -3.51 -0.36 15 15 15 9 46.14 8.45 46.79 8.71 -1.42 -3.05 16 25 15 9 59.30 15.56 57.97 15.64 2.25 -0.54 17 25 15 5 55.12 13.07 55.56 13.22 -0.79 -1.13 18 25 10 9 45.16 20.95 45.94 21.18 -1.72 -1.09 19 15 10 9 42.14 17.05 41.19 17.04 2.25 0.09 20 25 10 5 39.31 15.30 38.15 15.18 2.96 0.82

Table 4—Regression relations for surface finish improvement and material removed

Responses R-square (%)

Adjusted R-square

(%) Regression model

Surface finish improvement (ΔRa) 97.9 96.1 ΔRa = -162.54 + 11.05 EP -2.85 WPO + 24.79 PT – 0.245 EP2 + 0.1785 WPO2 – 1.032 PT2 + 0.128 EP*WPO – 0.2282 EP*PT – 0.269 WPO*PT ± ε

Material removed (MR) 98.4 96.9 MR = 46.353 + 4.476 EP -17.251 WPO + 8.870 PT - 0.127 EP2 + 0.654 WPO2 – 0.493 PT2 + 0.0557 EP*WPO + 0.0533 EP*PT – 0.179 WPO*PT ± ε

where EP: Extrusion pressure, bar WPO: Processing oil, % PT: Processing time, min and ε = Error


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The adjusted and predicted R-square values are well within the range. The detailed ANOVA results for ΔRa and MR were given in Tables 5 and 6 respectively. It is observed from the result that the model is significant and lack of fit is insignificant which are desirable and following the trend. Figure 4 shows the plot between predicted and actual responses. It was observed that the results between the predicted and actual were very close for both surface finish and material removed. This confirms that the model is acceptable; prediction further helps in parameter setting directly in a shop floor and provides enough research scope. Results and Discussion

The data were obtained for finishing of microchannels using AFM. The CCRD of RSM was employed for designing the trial sets. The individual effect and interaction effect of the process parameters on responses are discussed. Further parametric optimization of the factors and their contribution on the responses are plotted. Observations on surface finish improvement

The improvement in surface finish of microchannels increases while applying extrusion pressure up to 20 bar. As the extrusion pressure increases, it leads to increase in axial force resulting in higher ratio of ΔRa

Table 5—ANOVA for fitted RSM model surface finish improvement

Source DF Seq SS Adj SS Adj MS F-value Prob>F Regression 9 1395.89 1390.89 155.090 53.81 < 0.000 (S) Linear 3 956.58 956.58 318.860 110.60 < 0.000 (S) Square 3 362.49 362.49 120.831 41.92 < 0.000 (S) Interaction 3 76.82 76.82 25.608 8.88 < 0.004 (S) Residual Error 10 28.83 28.83 2.883 Lack of fit 5 17.51 17.51 3.502 1.55 0.322* (NS) Total 19 1424.72 Analysis of variance for surface finish (% ΔRa): (Response surface regression: R-square = 97.9%, R-square (adjusted) = 96.89%). where: DF: degree of freedom SS: sum of squares, MS: mean of squares: S-significant; NS: not significant

Table 6—ANOVA for fitted RSM model material removed

Source DF Seq SS Adj SS Adj MS F- value Prob>F (S) Regression 9 218.366 218.366 24.26 68.61 < 0.000 (S) Linear 3 140.747 140.747 46.91 132.66 < 0.000 (S) Square 3 65.041 65.041 21.68 61.31 < 0.000 (S) Interaction 3 12.578 12.578 4.19 11.86 < 0.001(S) Residual Error 10 3.536 3.536 0.35 -- Lack of fit 5 0.982 0.982 0.19 0.38 0.841 (NS) Total 19 221.902 Analysis of variance for material removed (MR): (response surface regression: R-square =98.41%, R-square (adjusted) = 96.97%) where: DF: degree of freedom SS: sum of squares, MS: mean of squares S-significant; NS: not significant

Fig. 4—Predicted and actual responses: (a) surface finish improvement and (b) material removed


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improvements. Further increase in extrusion pressure from 20 to 25 bar, tends to decrease ΔRa improvement. This may be due to the fact that at high pressures, number of active abrasive particles becomes less as the highly viscous medium becomes stiffer (Fig. 5a). Such results had been elsewhere23.

Naphthenic based processing oil used in the present work is primarily for proper mixing and maintaining medium viscosity. Secondly, for proper hold between SiC abrasives and natural polymer is ascertained. It was observed from the Fig. 5b that low wt% of processing oil content, the mixing (hence distribution of the particles) among the ingredients (natural polymer and abrasives) was poor and thus less surface finish is obtained. The improvement of ΔRa increases from 10% to 12% mixing of processing oil in the carrier medium. As the wt% of processing oil increases, the viscosity of the medium approaches the optimal which results in higher rate of ΔRa improvements. Further increase in oil content from 12% to 15%, tends to dilute the medium (reduce the medium viscosity) that triggers decrease in ΔRa improvement. The effects of processing oil on finishing have also been discussed at length by other authors24.

The interaction of abrasive medium with the target surface was calculated in terms of time of interaction and termed as processing time or machining time. It was the total time for which the work-piece surface was exposed to abrasive action during the AFM process. Figure 5c illustrates that during the initial stages of processing time, surface finish improves sharply, further drop in percentage surface finish improvement can be attributed to the fact that the sharp abrasives edges get blunt as machining progresses that results in only marginal improvements in finish on continued machining. Such parametric effects on finishing of the micro-surfaces were also explored by other researchers.15

Figure 6(a-c) shows the interaction effect of the response parameters on improvement in surface finish. It was observed from the plots that surface finish improvement with respect to extrusion pressure follows a similar trend as explained above. Surface finish improves with increase in wt% of processing oil, since it leads to decrease in viscosity of the medium. This results in higher rate of ΔRa improvements. The surface finish increases with increase in processing time up to a moderate level even in combination with extrusion pressure and wt% of processing oil (Fig. 6a and c). However, the

Fig. 5—Response curves showing the individual effect of process parameters on ΔRa

Fig. 6—Combined effect of (a) processing time and extrusion pressure, (b) extrusion pressure and wt% of processing oil, and (c) processing time and wt% of processing oil on surface finish improvement


303

finishing rate declines further on increase of the values of the parameters. The moderate level of all the three parameters considered here is found to be favourable for providing best improvement in surface finish. This is further substantiated by the minimum error (-0.18%) obtained between the experimentally obtained ΔRa values and the predicted values of the ΔRa corresponding to the trial #6 as presented in Table 3. However, the highest experimentally obtained ΔRa was found corresponding to the higher levels of the three parameters as shown in the trial #16 (Table 3). This can be explained due to the fact of dominating influence of processing time which is highest at 9 min. Similarly, decreasing trend in the ΔRa can be clearly seen with the increase in extrusion pressure, although, the combination of higher extrusion pressure and higher wt% of oil maintaining a better ΔRa value (Fig. 6a and b).

Observations on material removed Effect of process parameters on material removed

in AFM has been explored by many authors25-27. Removal of material layer by layer in the order of micrometers is a general phenomenon in the AFM process. Figure 7 illustrates the effect of the parameters such as, extrusion pressure, wt% of processing oil and processing time on material removed of microchannel work-piece using AFM process. It was observed from the trend curves that there is an increase in MR with increase in extrusion pressure. In each trend curve, the process parameter of interest was varied from its low to high level, whereas the values of the rest of the parameters were maintained at the middle level (Table 1). The MR increases significantly from 15 to 20 bar. As the extrusion pressure increases, it leads to increase in axial force resulting in higher rate of MR. Further increase in extrusion pressure from 20 to 25 bar, however does not cause significant improvements in

MR. (Fig. 7a). It was observed from Fig. 7b that when wt% of processing oil increases, MR decreases. This is due to the fact that at higher wt% of processing oil mixed with the medium leads to decrease in the medium viscosity. Thus at lower viscosity of the medium, the abrasives get pushed back upon hitting the work-piece surface and hence become less effective and resulting low MR.

Figure 7c shows that during the initial stages of processing time, a significant improvement is observed in MR. The observed drop in the rate of increase in MR can be attributed to the fact that the sharp abrasive edges get blunt due to prolonged machining that results in only marginal improvements in MR. Results on effect of process parameters on material removed were explored in length for different machining conditions26.

Typical response surfaces illustrating the effect of combinations of process parameters on the process responses are presented in Fig. 8a and c. It was observed from the surface plots that the material removal increases with increase in extrusion pressure and processing time follows a similar trend. In case of processing oil, the material removal decreases with increase of processing oil (Fig. 8c). Figure 8a and 8b reveal that extrusion pressure has domination effect on the MR even in combination with processing time and wt% of processing oil. In fact, processing time has marginal effect on all the combinations have been attempted. In fact the bandwidth of variation of processing time is also considerably narrow due to which significant variation could not be observed. Optimization

The objective of optimization was to maximize surface finish and to maximize material removed from the machined surface subject to the constraints of the working limits of three variables (Table 2). The parametric optimization was carried out using the software tool MINITAB (version 15). Figure 9 shows the percentage contribution of processing parameters on the process performance as observed from the ANOVA results. It was found that the extrusion pressure is the most influencing factor in case of surface finish improvement; whereas processing oil is the most influencing parameter for MR. Extrusion pressure has the highest percentage contribution (43.78%), and wt% of processing oil was contributing maximum 44.49% on ΔRa and MR, respectively. However, in both the response factors, the marginal contribution was observed by processing time.

Fig. 7—Response curves showing the individual effect of process parameters on MR


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The response optimization was performed to investigate optimized inputs, resultant local and global solutions on finishing of microchannels. Table 7 shows the predicted inputs, outputs, local and

global solutions. Composite desirability (D) evaluates how the parametric settings optimize a set of responses overall. Desirability has a range of zero to one. One represents the ideal case; zero indicates that one or more responses are outside their acceptable limits. The achieved composite desirability (D) of the global solution (0.9289) is fairly close to 1, which indicates that the optimized input values satisfactorily align with the targeted output values. A test-case was conducted to confirm the optimized input values and the corresponding results are shown in Table 8. It was observed that the error percentage of surface finish improvement and material removed are 3.31% and 0.79% (Table 8). The obtained results appear well within a practically achievable range. Therefore, the obtained result can be useful to predict the optimum values of the desired outputs in the same experimental condition. SEM analysis of finish machined microchannels

Scanning electron microscopy (SEM) was used to observe the topography of the machined surfaces. Figure 10 shows a typical surface of a SiC microchannel work-piece before and after AFM. The pit surface was clearly observed on the un-finished microchannel surface. After finishing with AFM process, pits get levelled up and a glazed look on the

Fig. 8—Combined effect of (a) processing time and extrusion pressure, (b) extrusion pressure and wt% of processing oi, and (c) processing time and wt% of processing oil on material removal

Fig. 9—Contribution of process parmeters on ΔRa and MR

Table 7—Optimized results for predicted optimized inputs and responses

Predicted optimized inputs Predicted optimized output Local and global solutions

Extrusion pressure

wt% of processing oil

Processing time

%ΔRa MR, mg

Composite desirability (D)

Local solution 22.43 15 5 55.01 14.41 0.7300 Local solution 24.96 10.28 9 46.45 19.99 0.8561 Local solution 24.78 10 5.88 42.98 18.08 0.7245 Local solution 22.12 10.17 6.29 46.52 20 0.8573 Global solution 21.29 10.62 9 50.61 20 0.9289


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surface was observed (Fig. 10b). No cracks were visible at this magnification, although embedding of abrasive particles was observed occasionally. It was clear from the SEM images that during AFM, the asperity peaks of the ultrasonically machined surface get trimmed and the topography becomes significantly finer. Figure 10c shows some embedded abrasives on the finished surface. The surface roughness of the finish machined (AFMed) surface got improved by approximately 59% as compared to the un-finished microchannel surface (Fig. 10d). Thus such microchannels should be more effective in applications like micro-fluidics and bio applications. The improvement in surface

topography can be further enhanced with the use of even finer abrasives. Conclusions

The present study was carried out to evaluate the performance of the AFM process in terms of the improvement in surface quality of a microchannel of SiC which was fabricated by micro-USM. Trials were carried out using RSM for finishing the microchannels having average initial surface roughness (Ra) of extrusion pressure, wt% of processing oil and processing time at different levels of parameters. A natural polymer based medium was used in the process that works in the principle of

Fig. 10—SEM micrograph of SiC micro channel specimen (a) micro channel machined by USM, (b) micro channel finished by AFM (at 200#, 15%, 25 bar, 5 min) [insets: schematic of the channel surface of the SEM image], (c) at 200#, 12.5 %, 20 bar, 7 min and (d) at 200#, 15%, 25 bar, 9 min

Table 8—Error percentage of the predicted and measured responses

Predicted inputs Predicted outputs Measured outputs Error % EP WPO PT %ΔRa MR %ΔRa MR %ΔRa MR

21.29 10.62 9 50.61 20.03 52.34 19.87 3.31 -0.79


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extrusion. The material is removed in the form of tiny chips owing to the abrasive-asperity interactions during the extrusion under applied pressure. The results have been derived using various expressions and commercial analysis software tools. Most of the results are presented with help of 3-D plots (in terms of response surfaces). The major conclusions from the present work are as follows: (i) The abrasive flow machining approach is

effective in finishing hard microchannels of 1 mm width.

(ii) A significant improvement (up to 59.3%) in surface finish could be obtained using the AFM technique.

(iii) The extrusion pressure and wt% of processing oil have significant influence on the process while the effect of processing time was the least.

(iv) Extrusion pressure has the highest contribution in finishing of microchannels, whereas wt% of processing oil had contributed the most in material removal in finishing of a microchannel.

(v) The process is capable of removing the pits and other surface irregularities of the candidate surface; the AFMed surface gets glazed with the increasing machining time.

Acknowledgments

The authors are thankful to the Department of Science and Technology, Government of India, New Delhi for providing financial assistance to this work through DST Project Grant No. DST–384–MID References 1 Cheng, K S & Midorikawa K, Opt Lett, 28 (2003) 55-56. 2 Reyntjens S & Puers R J, Micromech Microeng, 11 (2001)

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