Performance study of a natural polymer based media for abrasive flow machining

Transcription

1 Indian Journal of Engineering & Materials Sciences Vol. 17, December 2010, pp Performance study of a natural polymer based media for abrasive flow machining S Rajesha*, G Venkatesh, A K Sharma & Pradeep Kumar Department of Mechanical and Industrial Engineering, Indian Institute of Technology Roorkee , India Received 14 July 2010; accepted 25 October 2010 Abrasive flow machining (AFM) is a non traditional finishing process used for finishing parts with predominantly irregular geometry. In AFM, material removal and surface finish takes place by flowing viscoelastic abrasive carrier across the surface to be machined. The media (carrier + abrasive) is the key element in the process because of its ability to precisely abrade the selected areas along its flow path. In this study, an attempt is made to develop a new carrier as an alternative to the existing commercially available media. The newly developed carrier is characterized through Fourier transform infrared spectroscopy (FTIR) and Thermogravimetric analysis (TGA). Performance evaluation of the carrier is carried out by considering extrusion pressure, abrasive concentration, viscosity of media, and media flow rate as a process parameters and surface finish improvement and material removal as process responses. The ester based newly developed media is capable of withstanding a temperature to work up to 71 C without changing its characteristics. It is found that the developed carrier is flexible enough to be used in AFM process and performance study reveals that the new polymer based medium yields a good improvement in surface finish as well as material removal. Material removal does not get influenced significantly by the varying media flow rate, but surface finish increases with media flow rate above 796 Pa-s. An operational pressure of 20 bar and abrasive concentration of 50:50 (abrasives: carrier) is observed to be better parameter levels for the conditions attempted in the present study. Keywords: AFM, Abrasive carrier, AFM media, FTIR, TGA Abrasive flow machining is used to deburr, polish or radius surfaces and edges by flowing of semisolid abrasive media over these areas. The process embraces a wide range of feasible applications from critical aerospace and medical components to mass production automobile parts. This process is capable of finishing even the most inaccessible areas, processing multiple holes, slots or edges in one operation. Advances in media formulation and tool design coupled with new capabilities in processing and automation have established the abrasive flow process as a pragmatic solution for satisfying tough manufacturing requirements. The medium acts as a deformable grinding tool. It is the key of AFM process. In AFM process various important process parameters like extrusion pressure, flow volume, flow speed as well as rheology, abrasives, work piece configuration and flow pattern, which affects the process performance were studied 1. The experiments have carried out to study the effect of process parameters like abrasive concentration, abrasive mesh size, numbers of cycles and flow rate of media on *Corresponding author ( rajeshashivanna@gmail.com) material removal and surface roughness of aluminium and brass work pieces, and concluded that the concentration of abrasives in media followed by mesh size, numbers of cycles and flow rate of media are the influencing process parameters on performance 2. Higher abrasive mesh size, both material removal and surface roughness decrease. This is due to the increase in mesh size causes the depth of penetration, as well as width of penetration decreases. Ratio of abrasive particle to the base material of the media (by weight) is also a significant parameter in AFM which could vary from 4:1 to 1:4 with 1:1 as the most appropriate ratio 3,4. Viscosity of the media is one of the significant parameters of the AFM process. So keeping all other parameters constant, an increase in viscosity improves both MR and surface roughness. While selecting the media viscosity, if the passage length is substantially shorter than two times the passage width, use of a higher viscosity media is recommended 5-7. Media flow rate is less influential parameter with respect to material removal. The flow patterns of the media in the work piece passage are strongly affected by the speed of the media. Slower slug flow rates are best for uniform material removal

2 408 INDIAN J. ENG. MATER. SCI., DECEMBER 2010 and high slug flow rates produces large edge radii 3,6. According to the US Patent 7 the silly putty substance used extensively for media is made by mixing dimethyl silicon oil with a boron compound and it is named as polyborosiloxane. Polyborosiloxane mixed with abrasives and used as a media for AFM process and inferred that the temperature is an important variable in AFM process. Further, commercial grade putty with abrasives and varnish oil in the media to maintain viscosity and found that the media viscosity decreases with increasing shear rate. Also observed that the material removal rate and surface finish increase with increasing viscosity of media 8,9. Extrusion pressure, abrasive concentration and grain size affect the axial cutting forces, force ratio (radial force to axial force), active grain density and their influence on reduction of the surface roughness 10. Silicon polymer as a media for AFM process and finished internal primitives of SG cast iron (600 grade) and concluded in terms of improvement of surface finish 11. Silicon polymer used to remove the recast layers of complex hole produced by WEDM and to enhance the roughness of the WEDM surface 12. An attempt in the direction of developing new media based on viscoelastic carrier for AFM process, and characteristic like, shear rate, creeping time and frequency and the percentage ingredients of developed natural rubber media was reported 13,14. In the present study, a natural polymer based media has been developed using standard laboratory procedure. The performance study of newly developed polymer was carried out by considering silicon carbide (SiC) abrasive particle. The experiment was carried out by considering extrusion pressure, abrasive concentration, viscosity of media, and media flow rate as a process parameter and surface finish improvement and material removal as a process response with three levels. Experimental Procedure Development of abrasive carrier Development of a carrier was carried out keeping in view its commercial available carrier ability and service performance. New media mainly consists of natural polymer mainly containing easter group. The new carrier was fabricated using standard laboratory procedure. Newly developed polymer media was mixed with Napthanic based processing oil to maintain the desired viscosity which is one of the major process parameters of abrasive flow machining process. Figure 1 shows the highly viscous and deformable new polymer abrasive carrier. The commercially available media consist of di-methyl silicon oil and boric trioxide 90:10 by weight ratio respectively 15. Characterization of media Fourier transform infrared spectroscopy analysis was used to identify the type of compounds present in the new media. In addition, Thermogravimetric analysis were conducted under atmosphere air of 200 ml/min flow rate, with a heating rate of 10 C/min over a range of temperature of C with reference of alumina powder material. The sample weights were kept at 12.5±0.2 mg for all TGA experiments. It is important to extract sufficient information about the structure of the compound which was selected for developing the abrasive carrier. FTIR analysis is a vibrational spectroscopy used to identify the type of compounds present in the carrier. The current FTIR analysis was carried out using Thermo Nicolet Nexus model. Figure 2 shows the FTIR analysis of the new media. The peaks show different compounds present in the material. It is observed that the presence of Fig. 1 Natural polymer based deformable abrasive carrier Fig. 2 FTIR analysis of new polymer based carrier

3 RAJESHA et al.: ABRASIVE FLOW MACHINING 409 alkenes and esters are more dominating which provide the elastic nature to the carrier. Thus, it is flexible enough to be used in AFM process. Thermogravimetric analysis was used to know the thermal stability of the carrier to measure the weight loss against the time and temperature. Figure 3 shows the effect of temperature on derivative weight loss of the carrier. The characteristic indicates that the presently used alternate media is capable of withstanding a temperature to work up to 71 C without changing its characteristics. Maximum processing temperature rise was reported up to 10 C above the room 8. Thus, the present media can be safely used in AFM application in which processing temperature hardly comes near to its critical temperature (71 C). Figure 4 illustrates morphology and the purity of silicon carbide abrasive particle through field Fig. 3 TGA analysis of the new polymer based carrier emission scanning electron microscope (FE-SEM) (model: LEO, VP-435) were analysed. These SiC abrasive particles were used to evaluate the performance of the new polymer abrasive carrier. The particles do not have a defined geometry, however, carries sharp cutting edges. The average size of the abrasives is 75 micron. Performance study of new media The performance evaluation of the newly developed media was conducted in a laboratory developed double acting, horizontal type AFM set-up shown in Fig. 5. The Teflon tooling developed to facilitate the media flow and holding a cylindrical work piece is given in Fig. 6. The cylindrical work pieces for finishing the inner surfaces were made of brass. Its main applications include in the areas where low friction is required such as locks, gears, bearings, doorknobs, ammunition, and valves for plumbing and electrical applications. Work piece were prepared by drilling followed by reaming to maintain an initial surface roughness in the range of µm and dimension of 16 mm OD 8 mm ID 20 mm length. Few such work pieces ready for machining (AFM) are shown in Fig. 7. Work pieces were cleaned by acetone and subsequently measurements of the initial surface roughness and weight were carried out. The surface roughness of the work piece was measured in three different locations using perthometer (Mahr, model M2) and average surface roughness (R a ) values were calculated. The media cylinder was filled with abrasive media in different proportions (carrier: abrasive) by weight Fig. 4 FE SEM analysis of SiC abrasive practical

4 410 INDIAN J. ENG. MATER. SCI., DECEMBER 2010 Fig. 5 The double acting AFM set-up percentage. The tooling with the work piece was placed between the two media cylinders shown in Fig. 5. The media was pushed from one cylinder to the other using hydraulic cylinder, which is operated by hydraulic power unit. The experimentation was performed with the set of three levels of process parameters as shown in Table 1. After machining, the tooling was removed from the setup and work piece were cleaned by acetone. The final roughness and weight were measured to calculate the percentage improvement in surface finish and material removal by using the Eqs (1) and (2) respectively. Fig. 6 Tooling for AFM Fig. 7 Brass work pieces used in the trials. Table1 Different process parameters selected for the experimentation S. No Process parameters Levels Independent parameters 1. Extrusion pressure (bar) Abrasive carrier concentration (% by weight) 60:40 50:50 40:60 3. Viscosity of media (Pa-s) Media flow rate (cm 3 /min) Constant parameters 5. Abrasive particle size (Mesh Size) Processing time (min) 5min 7. Temperature of media ( 0 C) 35 ± 2 8. Initial surface roughness (µm) 0.9 to1.2 Process response 9. Surface finish (improvement) (%) 10. Material removal (mg)

5 RAJESHA et al.: ABRASIVE FLOW MACHINING 411 Surface finish improvement= (initial Ra final Ra ) 100 (%) initial R a (1) Material removal (MR)=[initial wt-final wt)] 1000 (mg)... (2) Results and Discussions Experiments were carried out for evaluating performance of the newly developed polymer based carrier and SiC abrasive mixed media using the laboratory developed AFM set-up. Performances were evaluated based on the influence of the process parameters like, extrusion pressure, abrasive concentration, viscosity of media, and media flow rate on surface finish improvement and material removal. Influence of extrusion pressure Figure 8 shows trend curves of surface finish (SF) improvement and material removal. The surface finish improvement increases with the increasing extrusion pressure. At lower pressure surface finish and material removal are low due to shearing energy produced through abrasive particle at 10 bar extrusion pressure to shear the peeks is not enough. Therefore, only the top crests of the asperities will get sheared off and remaining part of the peeks observed in the surface roughness. The SF and MR sharply increase from 10 to 20 bar pressure while compared with 20 to 30 bar pressure. As the extrusion pressure increases from 10 to 20 bar, it leads to increase in axial force resulting in higher rates of both the SF and MR improvement. Further increase in extrusion pressure from 20 to 30 bar, however, does not cause any further improvements in the rate of SF improvement and MR when compared to as illustrated by reduced slope 20 bar pressure ( m 2 < m 1 ), This may be due to the fact that at higher pressure, number of active abrasive particles becomes less as the media becomes less flexible. The higher the pressure the abrasive in the media spins itself and not actively participating in the machining action 16. Effect of media concentration Typical effects on SF and MR due to variation in the abrasive concentration is given in Fig. 9. Density of the media increases with increasing abrasive concentration leading to an increase in SF improvement and MR. In a lean formulation, less amount of abrasive particles (hence fewer cutting edges) are present in the media leading to observed lower surface roughness improvement and MR. This is due to less amount of active abrasive particles participating the abrasive process. A significant improvement in SF and MR was observed at 50:50 concentration. This is due to more active abrasive particle present in the media and taking part during the abrasion that leads to significant improvement in both the responses. Further increase in abrasive concentration (40:60) causes a decline in surface finish improvement curve; however, MR gets is appreciably improved. This is due to more number of abrasive particles taking part in machining process and continues to remove fresh materials from the work surface contributing to the observed increase in MR. On the other hand, large number of fresh/sharp cutting edges available with the larger amount of abrasives in a given volume of carrier additional scratching of the already improved surface. It is interesting to note from the illustrating that the effect of abrasive concentration on rate of enhancement in surface quality and MR is similar up to an optimum concentration of 50:50, beyond which surface quality Fig. 8 Effect of extrusion pressure on SF and MR Fig. 9 Effect of abrasive concentration on SF and MR

6 412 INDIAN J. ENG. MATER. SCI., DECEMBER 2010 Fig. 10 Effect of media viscosity on SF and MR Fig. 12 Profile of improvements in surface finish with respect to extrusion pressure and abrasive concentration. Fig. 11 Effect of media flow rate on SF and MR does not show any further significant improvement. This is indicative of suitable abrasive carrier concentration at 50:50. Effect of carrier viscosity The effects of media viscosity on surface finish and MR are shown in Fig. 10. Surface finish improvement is not much significantly dependent on the viscosity of the media. Material removal on the other hand increases with increase in media viscosity. At higher viscosity, media gets stiffer and the abrasive particle holding capacity increases against the machining surface. At low viscosity of the media, abrasive particles gets pushed towards the centre of the flow when come in contact with the material surface, leading to lower MR. Effect of media flow rate Effects of media flow rate on surface finish and MR are graphically shown in the Fig. 11. Surface finish improvement is seen lean from level 1 to 2. Further, it improved significantly from level 2 to 3. This is due to machining surface being exposed to more abrasives with fresh cutting edges taking active Fig. 13 Profile showing extrusion pressure dependence on MR. part in abrading the work piece surface. Further, velocity of the abrasive particles increases with the media flow rate leading to removal of asperity peeks in sub micron level. This results in fine surface finish and low MR. Material removal remains constant with increasing media flow volume indicates its role as insignificant. Selection of extrusion pressure and abrasive concentration Selection of appropriate ranges/vales of process parameters is critical for optimal performance of a process. Effect of the most significant process parameters extrusion pressure and abrasive concentration on surface finish and material removal rate are as shown in the Figs 12 and 13 respectively. It is observed from the Fig.12 that surface finish improves significantly with pressure and abrasive concentration at initial levels (10-20 bars). However, further increase in pressure results in improvement in surface finish only with 50:50 abrasive concentration; the rate of improvements with other concentrations (60:40 and 40: 60) decreases significantly with higher

7 RAJESHA et al.: ABRASIVE FLOW MACHINING 413 operating pressure. Similar observations can be made for the material removal profile also barring the concentration of 50:50 (Fig. 13). The MR clearly diminishes with higher operating pressure setting at this concentration. Thus, a media with a concentration of 50:50 can be operated at an extrusion pressure of 20 bar without compromising much on both the process responses. Conclusions The present study was carried out characterise a newly developed natural polymer based AFM media to study its performance in terms of the improvement in finish surface quality and material removal. Experimentation was carried out to understand behaviour of abrasive carrier with different levels of important AFM process parameters. Following conclusions were drown: (i) (ii) (iii) (iv) (v) The major components of the new carrier are esters alkenes and alkanes. Thus, it is an ester group polymer. The new polymer media flexible enough to be used in AFM process. Surface finish improvement and material removal improve with higher extrusion pressure and abrasive concentration. But rate of improvement is declining above 20 bar extrusion pressure indicating that higher extrusion pressure is not recommended. Surface finish is constant throughout the experimental levels of media viscosity, but with increases in media viscosity above 640 Pa-s the material removal increases. Material removal remains consistent with the varying media flow rate, but surface finish increases with media flow rate above 796 Pa-s. Acknowledgments The authors would like to thank the Department of Science and Technology, Government of India, for their financial support to this research (DST Grant No: SR/S3/MERC-106/2007). References 1 Rhoades L J, J Mater Process Technol, 28 (1991) Jain V K & Adsul S G, Int J Mach Tools Manuf, 40 (2000) Williams R E & Rajurkar K P, ASME, PED, 38 (1989) Mc Carty, US Pat , Rhoades L J, Non-Tradit machining Conf Proc, December 1985, Williams R E, Rajurkar K P & Rhoades L J, Trans ASME, J Eng Ind, 114 (1982) Rob R, et al., US Pat , Davies & Fletcher, Proc Inst Mech Eng Pt C: J Mech Eng Sci, 209 (1995) Jain V K, Ranganatha C & Muralidhar K, J Mach Sci Technol, 5 (2001) Gorana V K, Jain V K & Lal G K, Int J Mach Tools Manuf, 44 (2004) Raju H P, K Narayanasamy, Y G Srinivasa & Krishnamurthy R, J Mater Process Technol, 166 (2005) Wang A C & Weng S H, J Mater Process Technol, 192 (2007) Kar K Kamal, Ravikumar N L, Tailor Piyushkumar B, Ramkumar J & Sathiyamoorthy D, J Mater Process Technol, 209 (2008) Kar K Kamal, Ravikumar N L, Tailor Piyushkumar B, Ramkumar J & Sathiyamoorthy D, J Manuf Sci Eng, 31 (2009) McGregor R R, Verona & Earl Leathen, US Pat , Reddy K M, Sharma A K & Kumar P, J Eng Manuf: Proc I Mech E Pt B, 222 (2008)