Experimental Study on Micromachining of 304 Stainless Steel Under Water Using Pulsed Nd:YAG Laser Beam

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1 5 th International & 26 th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12 th 14 th, 2014, IIT Guwahati, Assam, India Experimental Study on Micromachining of 304 Stainless Steel Under Water Using Pulsed Nd:YAG Laser Beam RasmiRanjan Behera 1, Mamilla Ravi Sankar 1 *, Indrajeet Kumar 2, Ashwini Kumar Sharma 2, Alika Khare 2, J. Swaminathan 3 1* Department of Mechanical Engineering, IIT Guwahati, Guwahati , India 2 Department of Physics,IIT Guwahati, Guwahati , India 3 National Metallurgical Laboratory, Jamshedpur , India 1 rasmi@iitg.ernet.in, 1 *evmrs@iitg.ernet.in, 2 kumar.i@iitg.ernet.in, 2 aksharma@iitg.ernet.in, 2 alika@iitg.ernet.in, 3 jsn@nmlindia.org Abstract Fabrication of miniaturized components is the current subject of research and development, as the applications of these products varies from industry to industry such as aerospace, biomedical, micro-electro-mechanical system (MEMS) etc. These components may include micro-holes, micro-channels, micro-nozzles etc. of size less than 500 µm which are commonly found in various products such as biomedical filters, high-pressure orifices, and fuel injection nozzles etc. It is very difficult to fabricate the micro-features in large number using conventional machining processes due to high tool wear, high heat generation, and change in material properties. Fabrication of such components provides a challenge to the manufacturing industries and to meet the challenges many unconventional machining methods are developed, among which Laser Beam Micromachining (LBµM) is one of them. The main advantages of LBµM are no tool wear, ability to machine complex shapes, high precision, high energy density and less machining time. But defects like recast layer, heat affected zone (HAZ), crack, debris etc. are obtained during laser beam machining in air due to high thermal gradients. Under liquid laser beam machining reduces the temperature gradient, bulges, HAZ and splashing of molten material in order to achieve crack-free machined micro-parts with high ablation rate. In this research work, under water LBµM is carried out using Nd:YAG laser for achieving high quality microchannels on 304 stainless steel. Water layer thicknessandscanningspeed are used as the process parameters where askerf width, kerf depth and surface roughness are used as the microchannel characteristics. Optical microscope and Scanning electron microscope were used to measure and analyze the microchannel characteristics. Keywords:Microchannel, Nd:YAG laser, recast layer, HAZ 1 Introduction Miniaturization of components is the current tendency in the different industries like aerospace, biomedical, chemical, automobile, micro-electromechanical system etc.these components may include micro-holes, micro-channels, micro-mixtures, microreactors etc. of size less than 500 µm.also these microchannels can be used as the permanent marker for product identification in different industries.fabrication of such components provides a challenge to the manufacturing industries and to meet the challenges Laser Beam Micromachining (LBµM) techniques have been developed because of its unique advantages such as high energy density, no tool wear, ability to machine complex shapes, high precision, and high feed rate. During laser micro-machining, when a laser beam is focused on the surface of target material, the material is subjected to different physical phenomena such as heating, softening/melting, vaporization, and explosive removal. However, laser micromachining is a complicated process because laser-material interaction depends on a number of factors such as laser parameters (power density, wavelength, pulse duration, etc.), material properties (thermal conductivity, thermal absorption coefficientetc.) and machining medium (gas, liquids etc.). Recently there have been increasing studies on under liquid laser beam micromachining, in particular, under water. The cooling effect of water can reduce the extent of HAZ, microcracks, recast layer etc. which are more pronounced during machining in air. Both the efficiency and quality of LBµM can be greatly improved. Kruusing(2004, 2004) reviewed the advantages and disadvantages of water assisted laser processing and summarized that it can be applied to cutting, drilling, micromachining, welding, etching, cleaning and laser shock processing. Lu et al. (2004) did experimental investigation on laser drilling of copper, 380-1

2 Experimental Study on Micromachining of 304 Stainless Steel Under Water Using Pulsed Nd:YAG Laser Beam iron, aluminium and stainless steel in air as well as underwater by using pulsed Nd:YAG laser. It was observed that, both the efficiency and quality of laser processing underwater can be greatly enhanced compared to that of air. The amplitude and duration of laser ablation are 7.9 and 10 times that in air due to water confinement. Mak et al. (2011) delivered a study on nanosecond laser micromachining of GaN/sapphire wafer in water and compared that withair. They illustrated that underwater laser machining improves heat conduction, increases plasma-induced recoil pressure, weakens the plasma shielding effect in water and collapses the cavitation bubbles. Liquid-assisted laser processing (LALP) was applied by Chung and Lin (2010) for achieving through hole with high aspect ratio in glass using CO 2 laser. They found that LALPreduces the defects such as bulges, debris, cracks, scorch and HAZ which produced during drilling in air due to higher temperature gradient.muhammad and Li (2012) reported micromachining of nitinol tube for coronary stent application by using underwater femtosecond laser. They reported that during machining in air, minimum surface roughness is observed at low fluence and high speed. HAZ is obtained at the highest fluence and it reduces with decrease in fluence. But in underwater cutting, high surface quality is obtained at high fluence and low speed. Absence of HAZ is observed in underwater cutting, which is debris-free and recast-free with clean opposite back walls. A comparison between dry and wet laser profile cutting of thin stainless steel tube by using fiber laser was made by Muhammad et al. (2010). Narrower kerf width, lower surface roughness, less dross, absence of back wall damages and smaller HAZ was obtained in case of wet cutting.yan et al. (2011) investigated the underwater CO 2 laser machining of deep cavities in alumina. Due to influence of water dynamics, high recoil pressure is produced in machining region which ejects the molten material from the cavity and prevent re-solidification layer.nath et al. (2010) fabricated through hole in thin stainless steel sheet by using Nd:YAG laser drilling in air as well as under water. It was found that, laser drilling in air can be readily done when the workpiece is placed at the focal point and below the focal point of the lens but not above the focal point. On the other hand, the laser drilling in water can be done when the workpiece is placed above the focal point. In the present experimental study, the micromachining of stainless steel under water using pulsed Nd:YAG laser is investigated. Besides the effect of themedium, the effects of thickness of water filmandscanning speed upon microchannelwidth,depthand surface roughness are investigated. Optical microscope, scanning electron microscope and non-contact type surface profilometerare used to measure and analyze the microchannel quality. 2 Experimental methodologies 2.1 Materials In this work, stainless steel (SS 304) sheet of dimension 50mm X 20mm X 2mmis used as the work material for experimentation.the chemical composition for the material is given in Table Experimental set-up Figure 1 shows the schematic diagram of underwater pulsed Nd:YAG laser machining system. The Q-switched Nd:YAG laser with the wavelength of 532 nm is used for micromachining of stainless steel. This laser machining system consists of various subsystems like laser source and beam delivery unit, power supply unit, Q-switch driver unit and cooling unit. The output from the Q-switched Nd:YAG laser is delivered to the workpiece using a beam delivery system that first bends the laser beam at 90º and focuses it on the workpiece surface through the focusing lens with a focal length of 150 mm. The pulse duration of the laser beam is 10 ns. The pulse frequency of the laser beam is 10 Hz. The laser spot diameter is found to be approximately 10 µm. Beam delivery unit Laser Focal lens Sample Container Water Figure 1 Schematic diagram of Nd:YAG laser machining system Table 1 Chemical composition of 304 stainless steel Elements C Mn S P Si Ni Cr Fe Max. % Balance 380-2

3 5 th International & 26 th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12 th 14 th, 2014, IIT Guwahati, Assam, India 2.3 Experimental procedure The micromachining of 304 stainless steelis performed in liquid medium. Before micromachining, the sample surface is polished by P2000 abrasive grinding paper and cleaned by acetone in order to remove any oil and dust. The sheet is fixed on the container and immersed horizontally in a liquid bath with thickness varies from 6-10 mm. The liquid used in this study is distilled water. The water depth is controlled at a particular thickness above the material surface by the total volume method whose depth is proportional to the volume at a constant area of the container. Thecontaineris designed according to the space available in the setup and is made up of acrylic polymer. The containeris placed on a motorized XY translation stage to enable laser scanning. During experiments, microchannelsare machined on the steel sheet by focusing the laser beam on workpiece surface and moving it with respect to workpiecein a reciprocating motion for a distance of 10 mm along the 20 mm width of the sample. In presence of water the focal length of lens increases with the height of water level. The increase is nearly 0.33 h, where h is the water level height above the focal point in air [Nath et al. (2010)]. Under water laser beam micromachining is carried out by changing the position of workpiece with respect to the focal point of the lens. Preliminary experiments are carried out to determine the appropriate processing parameters to be used for the study. Scanning speed (SS) and water layer thickness or water height (WH)are varied in the experiments and their variation ranges are shown in Table 2. Laser pulse energy (LPE) and laser scanning pass number (SP No.) are fixed at 60 mj and 8 respectively, in order to obtain a certain depth of microchannel. The whole experiment is carried out at room temperature. The width, depth and surface roughness of microchannelsare measured by using noncontact type surface profilometer. The microchannelsare subsequently characterized by using scanning electron microscopy (SEM) and optical microscopy (OM). Table 2 Experimental input parameters and their ranges Input parameters Ranges Laser pulse energy(mj) 60 Water layer thickness(mm) 6-10 Scanning speed(µm/s) Scanning pass number 8 3 Results and Discussion 3.1 Effect of water layer thickness upon kerf width, kerf depth and surface roughness Kerf width (µm) Kerf depth (µm) Surface roughness (µm) SS = 150 µm/s Water layer thickness (mm) SS = 150 µm/s Water layer thickness (mm) SS = 150 µm/s Water layer thickness (mm) Figure 2Effect of water layer thickness on kerf width, kerf depth and (c) surface roughness. Figure 2 shows the effect of water layer thickness on kerf width, kerf depth and surface roughness of microchannel at fixed 60 mj laser pulse energy, 150 µm/s scanning speed and 8 scanning pass number. It can be seen that, the kerf width gradually decreases with increase in water layer thickness as shown in Figure (c) 380-3

4 Experimental Study on Micromachining of 304 Stainless Steel Under Water Using Pulsed Nd:YAG Laser Beam 2. This is mainly due to increase in water layer leads to more absorption of laser energy. So, heat transfer to material is reduced. Thus less melting and evaporation occurs. So, kerf width decreases. The same trend is found for kerf depth with increase in water layer thickness as shown in Figure 2. This is also due to more energy absorption with increase in water layer thickness. So, less penetration of heat to the material which results decrease in kerf depth.figure 2(c) shows the effect of the water layer thickness on surface roughness, R a. It can be observed that the surface roughness decreases with increase in water layer thickness. At high level of water layer, the energy transfer to the material is less, so less amount of debris and recast layer is formed and fewer amounts of recast layer and debris are deposited on the material. Thus high quality surface is obtained Conversion at higher layer water layer thickness. Heat effect Microchannel Figure 4Surface profilometryphotographs of microchannels at different settings of water layer thickness 7 mm and 9 mm keeping constant scanning speed = 150µm/s, laser pulse energy = 60 mj and scanning pass number = 8. µm µm Figure 3 Optical microscopy photographs of microchannels at different settings of water layer thickness 7 mm, 10 mm keeping constant scanning speed = 150µm/s, laser pulse energy = 60 mj and scanning pass number = 8. Figure 3 (a-b) shows the optical microscopy images of microchannel machined at different water layer thickness of 7 mm and 10 mm. And it is clearly seen that for the higher water layer thickness, the thermal effect and surface oxidation along the cut are smaller. This is mainly because of less thermal distortion at higher water layer thickness due to more energy absorption. Figure 4 (a-c) shows the surface profilometry images of microchannel as water layer thickness varying from 7 mm and 9 mm. The kerf depths are decreasing with increase in water layer thickness due to more energy absorption by water. And it is clearly observed that bulges and debris are absent due to hydrodynamic flow of water. In under water laser beam micromachining, when a laser beam is delivered onto the water surface, a portion of the laser energy is absorbed by water. Due to the high absorption of water, a rapid heating and vaporization process is induced. So, many bubbles are formed and a keyhole in water is generated as a channel for the other portion of energy penetrating. The energy arrived onto the workpiece surface is absorbed for melting and/or vaporization of workpiece surface. Then, the molten particles are removed by the water flow by thermal convection or bubbles movement

5 5 th International & 26 th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12 th 14 th, 2014, IIT Guwahati, Assam, India 3.2 Effect of scanning speed upon kerf width, kerf depth and surface roughness Kerf width (µm) Kerf depth (µm) WH = 10 mm Scanning speed (µm/s) WH = 10 mm Scanning speed (µm/s) surface roughness also decreases withincreasing scanning speed. When scanning speed increases, less melting occurs with formation of less recast layer and debris due to less interaction between laser and material. Thus high quality surface is obtained at higher scanning speed, where the surface roughness is found to be minimal with R a = 0.29 µm. 3.3 Analysis of microchannels based on SEM micrographs Recast layer Debris Microchannel HAZ Surface roughness (µm) (c) WH = 10 mm (c) Microchannel Scanning speed (µm/s) Figure 5Effect of scanning speed on kerf width, kerf depth and (c) surface roughness. It can be observed that, both the kerf dimensions i.e. kerf width and kerf depth decreases with increase in scanning speed as shown in Figure 5(a and b). With increase in scanning speed, the interaction time between laser and material is decreased. So, the heat transfer to the material decreases which reduces the kerf dimensions. Figure 5(c) shows the effect of the scanning speed on surface roughness, R a. It can be seen that the Figure 6SEM photographs of microchannel, different layers of microchannel and (c) enlarged view of machining zone.water layer thickness = 10 mm, scanning speed = 312µm/s, laser pulse energy = 60 mj and scanning pass number =

6 Experimental Study on Micromachining of 304 Stainless Steel Under Water Using Pulsed Nd:YAG Laser Beam A sample is analyzed under scanning electron microscope (SEM) as shown in Figure6. The parameters used for machining of this microchannelare laser pulse energy = 60 mj, scanning speed = 312µm/s, water layer thickness = 10 mm and scanning pass number = 100. The surface topography shows the different layers of microchannel such as recast layer, HAZ and conversion layeras observed from figure 6. The microstructures are different for different layers as shown in figure 6. Very less debris are present in the microchannel due to ejection of debris by the water flow. Figure 6 (c) shows the enlarged view of machined surface. 4 Conclusions Laser micromachining tests are carried out on 304 stainless steel substrate, using under water pulsed Nd:YAG laser, in order to determine the best working parameters to obtain a given output. The SEM results show that how under water Nd:YAG laser ablation affects the machined microchannel morphology. The microchannel quality, roughness, depth, width are dependent on the ranges of input parameters.various output parameters such as kerf width, kerf depth and surface roughness decrease with the increase in water layer thickness and scanning speed. The optimum input parameters for machining of minimum microchannel size with high surface quality in the current experimental study are found to be 10 mm (water layer thickness) and 350 µm/s (scanning speed). Good quality channel i.e. channel with minimum recast layer and debris can be generated during under water laser beam micromachining process. References Chung, C.K. and Lin, S. L. (2010), CO 2 laser micro machined crack less through holes of Pyrex 7740 glass, International Journal of Machine Tools & Manufacture, Vol. 50, pp Kruusing, A. (2004), Underwater and water-assisted laser processing: Part 1 general features, steam cleaning and shock processing, Optics and Lasers in Engineering, Vol. 41, pp Kruusing, A. (2004), Underwater and water-assisted laser processing: Part 2 Etching, cutting and rarely used methods, Optics and Lasers in Engineering, Vol. 41, pp Lu, J., Xu, R. Q., Chen, X., Shen, Z. H., Ni, X. W., Zhang, S. Y., &Gao, C. M. (2004), Mechanisms of laser drilling of metal plates underwater, Journal of Applied Physics, Vol. 95(8), pp Mak, G. Y., Lam, E. Y. and Choi, H.W.(2011), Liquidimmersion laser micromachining of GaN grown on sapphire, Applied Physics A, Vol. 102, pp Muhammad, N. and Li, L. (2012), Underwater femtosecond laser micromachining of thin nitinol tubes for medical coronary stent manufacture, Applied Physics A, Vol. 107, pp Muhammad, N., Whitehead, D., Boor, A. and Li, L. (2010), Comparison of dry and wet fibre laser profile cutting of thin 316L stainless steel tubes for medical device applications, Journal of Materials Processing Technology, Vol. 210, pp Nath, A. K., Hansdah, D., Roy, S., &Choudhury, A. R. (2010), A study on laser drilling of thin steel sheet in air and underwater, Journal of Applied Physics, Vol. 107(12), pp :1-9. Yan, Y., Li, L., Sezer, K., Wang, W., Whitehead, D., Ji, L., Bao, Y. and Jiang, Y. (2011), CO 2 laser underwater machining of deep cavities in alumina, Journal of the European Ceramic Society, Vol. 31, pp