Optimization of Hole Taper in Machining of SS316L Alloy Sheet Using Laser Beam Drilling
Satish Namdev1
Automobile Engineering Department, SAMM, Manipal University Jaipur Jaipur -303007, INDIA.e-mail: [email protected]
Anand Pandey2
Mechanical Engineering Department, SAMM, Manipal University Jaipur, Jaipur-303007, INDIA, e-mail:
Arun Kr. Pandey3
Mechanical Engineering Department, Bundelkhand Institute of Engineering &Technology, Jhansi-284128, INDIA, email id: [email protected]
Abstract
SS316L is a most popular super alloy engineering metal having wide applications in medical implants and aerospace fasteners. Mostly, this alloy has been used in aerospace and medical industries. In laser beam machining (LBM) is used thermal energy for machining process. There is very difficult to machining operation like drilling, cutting etc. on this alloy through laser beam. In present research work, laser drilling has been used to create a precise hole of 2mm diameter on SS316L sheet. Aim of this work is to optimize (minimize) taperness of a drilled hole. Experimental models have been prepared in central composite design with 31 experiments. Response surface methodology (RSM) has been used to validate experimental model. Optimization technique genetic algorithm (GA) has been used to minimize hole taper (HT). Significance of input parameters also discussed for hole taper. Hole taper is reduced (improved) by 16.1% from experimental value while using GA.
Keywords-ANOVA, GA, LBPD, RSM, SS316L.
__________________________________________________*****_________________________________________________
I. INTRODUCTION
Stainless steel alloy (SS316L) is highly biocompatible and hard oxide surface. Its unique performance characteristics make it able to use as an advance alloy materials[1]. SS316L are being used to manufacturing of plenty of components for various sector such as medical, aerospace and chemical etc. Laser beam machining (LBM) process is being used for machining operations in macro to micro size dimensions to shape geometrical features viz. cutting, marking, drilling, grooving, of difficult to machine materials [2-3]. Material removal takes place in three sequential steps during LBM process. During the process, a high power laser beam with high density is focused on work piece alloy. Firstly, kinetic energy of laser beam is converted in heat energy and work piece material is being melted. In next step molten material will be evaporate or its chemical state will be change. In third step unwanted material is removed through high gas pressure. There are two types of laser drilling. In trepan drilling, a hole is generated around the circumference by laser beam. Laser beam percussion drilling (LBPD) is like as a punches. LBPD is widely used in industries for create small (micro) sized hole. Number of holes on effusion cooling are fabricated for aircraft components by LBPD [4-6].
Choudhury et al. [7] designed an orthogonal array L9 with four control factors to optimize hole taper for a polymer while drilling with CO2 Laser.
Researchers performed pulsed Nd:YAG laser drilling to get hole (small diameter) on Inconel 718 Sheet. Authors used genetic algorithm (GA) and multiple regression analysis for modelling and optimization geometrical feature for the hole. They prepared quadratic model and got the results more adequate and reliable with predicted value of the responses for geometrical feature of hole as hole circularity & taper [8].
Researchers have used laser trepan drilling on ZTA ceramic 6mm thick plate to generate a hole using. Nd:YAG laser has been used for experiment by researchers[9].
Authors have done experimental work on making a hole on alumina ceramic material by using by laser beam drilling [10]. Researchers had worked on laser cutting of Al2O3 ceramic. They had used laser circular cutting process to
772 Researchers had worked on laser micro drilling of TiAl3 sheet. They prepared an experimental model in CCD. They have taken four process parameters with range to prepare model. Air assist pressure & pulse frequency are significant parameters for hole geometrical feature as hole circularity & hole taper [12].
Based on literature review, prepare experimental run in CCD for decided laser beam parameters and using response surface methodology (RSM) and genetic algorithm (GA) techniques to optimize parameters for best drilled hole to minimize hole taper.
II. EXPERIMENTAL PROCDURE A. Experimental Setup
All experimental work have been done on 250W Nd:YAG LBM system at RRCAT Indore (India). Compressed gas has been used as assist gas. It is flowing at high pressure through a nozzle of 1 mm diameter. That is co-axial with laser beam. Focal length was 50 mm for focused laser beam. Nozzle diameter was 1 mm with 1064 nm wavelength. 1.8 mm thick SS316L sheet has been used for experimentation. The chemical composition of SS316L sheet is given in Table 1.
SS316L sheet is fixed on CNC table. During laser machining, basic three processes such as immersion, heating and melting of material take place. Melted material is ejaculated by gas pressure. Beam immersion & melting material are controlled by input process parameters.
An appropriate gas pressure is necessary to move out unwanted material fromtargeted surface [13].
Current, pulse frequency, gas pressure & cutting speed have taken as input parameters for experimental work.
Trial experiments have been executed to get possible range of input parameters preferred through drilled hole can be achieved. Input parameters with their levels are given below in Table 2. Due to some limitations with machines (experimental setup), there was not possible to use values of same as theoretical data. Modified theoretical data from experimental data. Experimental data was closer to corresponding theoretical data.
B. Experimental Design
In present research work has been focused to hole taper of SS316L sheet. LTD has been used for testing characteristics of hole on SS316L alloy sheet. Experiments has been performed for drill a 2 mm hole diameter on 1.8mm thick sheet. CCD (central composite design) has been used for design experimental matrix. Experiments have been performed accordingly. Collected data have been investigated to see effect of laser input parameters on hole taper.
Table 1: Chemical Composition of SS316L
Cr Ni Mo
17.06% 10.17 2.1
Fig 1a. Top Diameter
Fig 1b. Bottom Diameter
Four input parameters have been decided with their five levels. Total 31 experiments have been prepared while considering 7 central run. Minitab 16 has been chosen to prepare regression model for validation of design of experiment (DoE).
Fig 2. Hole Taper measurement
C. Evaluation of Hole Taper:
Hole taper (HT) has been selected as a response for hole characterization. Four diameters have taken at interval of 45° for each drilled hole at top and bottom side as shown in fig 1a & fig 1b respectively.
Then, averaged diameter has calculated for top and bottom diameters (Din&Dout) respectively as shown in fig 2. An optical microscope has been used to measure hole diameters with 500X magnification. Hole taper (HT) is calculated by equation 1 [14].
𝐻𝑇 =180𝜋 𝑇𝑎𝑛−1 𝐷𝑖𝑛−𝐷2t𝑜𝑢𝑡 (1)
III. REGRSSION MODEL USING RSM
RSM is a mathematical tool that can be used to make a best model for multi-parameters in experimental data and prepare an optimum experiment design. RSM is trustful arithmetic technique for many applications.
A mathematical relation has been prepared between input parameters and responses. That will help to know behavior of process parameters on response.
Now regression model for higher order may be expressed as given below by equation:
𝑓 𝑥 = 𝑎0+ 𝑓𝑛=1𝑎𝑛𝑝𝑛+ 𝑓𝑛=1𝑎𝑛𝑛𝑝𝑛2+ 𝑓𝑛=1 𝑓𝑚 =𝑓+1𝑎𝑛𝑚𝑝𝑛𝑝𝑚 (2)
774 Where,‘f(x)‘is response, and ‗x‘is responses number, ‗α0‘is constant and ‗αn‘are regression co-efficient, ‗f‘is number of input parameters, while, 𝑝𝑛is linear, 𝑝𝑛2 is square and 𝑝𝑛𝑝𝑚 is interaction of input parameters.
Second order regression model has been prepared. This model is established a mathematical relation among input and output variables. Non important terms may be eliminated from model by using p-value [15, 16].
Regression models for response HT has been given in equation 3. Correlation coefficient has been calculated for models and found to be greater than 0.9 in table 3. It shows values for all experiments are well suited in all models.
Regression Equation in uncoded units for hole taper as mentioned below:
HT = -38.9 + 4.604𝑝1 + 0.0376𝑝2 + 0.0111𝑝3 + 5.161𝑝4 – 0.2136 𝑝12+ 0.000080 𝑝22 + 0.000564𝑝32 - 0.1742𝑝42 - 0.002773𝑝1𝑝2+ 0.002958𝑝1𝑝3- 0.0694𝑝1𝑝4 - 0.000245𝑝2𝑝3- 0.004779𝑝2𝑝4 - 0.003809𝑝3𝑝4 (3)
IV. VALIDATION OF REGRESSION MODELS A. Theoretical Validation
Table 3: Regression coefficients for responses HT
S R-sq R-sq (adj) 0.506 87.18% 75.96%
Table 4: ANOVA result for response HT
Source DF F-Value p-Value
Regression 14 7.77 0
Linear 4 7.47 0.001
Square 4 1.6 0.223
2-Way Interaction 6 8.73 0
Error 16
Lack-of-Fit 9 5.31 0.019
Pure Error 7
Total 30
Table 5: p-values of input parameters for response HT
Parameters p-value Gas Pressure 0.001 Current 0.116 Cutting Speed 0 Pulse
Frequency 0.005
Model validation has been done based on ANOVA results. R-Sq, p-value and F-ratio are taken to validate the model. R-sq for response is shown in table 3. F-ratio and p-value are calculated and shown in table 4 for response.
Confidence level should not be less than 95% for calculated f-ratio for responses. P-values should not be more than 0.05 for considered responses. Correlation coefficient (R) should not be less than 0.9 for measured response [15]. In present work, Confidence level is above 95% for F-ratio and p-value is less than 0.05 for response HT. Correlation coefficient (R) is 0.93 for HT. Therefore, regression model is acceptable for all characteristics for predicted response value.
B. Experimental Validation
Predicted value of theoretical models have been compared with actual values of experimental models. This comparison shows minor difference between predicted and experimental values. Because, some realistic conditions like temperature, pressure and vibration of CNC worktable during perform experiment. To validate experimental model, calculated mean percentage error (MPE) [17] for response HT is 1.23% for 2mmhole diameter. Above mentioned MPE values are in acceptable limit for all responses for drilled hole diameter [18].
V. RESULT ANDDISCUSSION A. Significance of Process Parameters
P-values have been found out for input parameters by ANOVA technique using MINITAB 2017 software.
Confidence level has been considered above 95%. P-value should not be more than 0.05 for input parameters. It is clear from table 5 that GP, CS and PF are playing significant role to minimize hole taper shown in table 5.
B. Parametric Analysis - Improving hole taper (HT):
Response surface plots are showing significantly effect of gas pressure, cutting speed and pulse frequency in fig 3a and fig 3b.
Fig 3a. Response surface plot HT vs GP and CS
Response surface diagram fig 3a is showing, hole taper is maximum at initial value of gas press. While gas pressure is increased up to its maximum value, hole taper is going to down accordingly. Hole taper is reducing with cutting speed. Hole taper is minimum at minimum cutting speed.
CURRENT 200
PULSE FREQUENCY 1 0 Hold Values
7.5 9.0
4 6
66.0 7.5
9.0 0
10.5
50 25 0
775 8
T H
D E E P S G N I T T U C E
R U S S E R P S A G
urface Plot of HT vs CU
S T ING SPEED, GAS PRESSURE T
CURRENT 200 CUTTING SPEED 40
Hold Values
7.5 9.0
2 4 6
66.0 7.5 8
10.5
10 8
12 8
T H
Y C N E U Q E R F E S L U P E
R U S S E R P S A G
urface Plot of HT vs PUL
S S FREQUENCY, GAS PRESSUREE
776 Response surface diagram figure 3b is showing hole taper is decreasing with decresing pulse frequency. While performing drilling operation at low pulse frequency with minimum cutting speed, during operation high assist gas pressure will remove unwanted molten material from the surface. Hole diameter will be improve at top side as well as bottom side and hole taper will be reduced.
Fig 4. Fitness value curve & optimum parameters for HT
C. Optimization Using GA
GA is robust optimization technique. GA gives best result depend upon on accepted progression because of its intrinsic benefits. GA starts at random with a set of solutions of coded factors. Fitness value is determined for evaluated individual string. Subsequently, GA operator‘s such as reproduction, mutation and crossover make an updated population. This population has been calculated & tested to meet standards. Maximum generations in solution are used to end the algorithm [13], [19]-[21].
There is 2mm hole diameter for optimization. Objective function has been defined separately in MATLAB R2018 as eq. 3 i.e. HT.
As mentioned, objective function (HT) have to be minimize within range of parameter as shown in table 2. Minimum value of objective function will minimize HT.
Objective function has been defined. Now, for this analysis, population type is double vector, population size = 50, crossover probability = 0.8, no of generations =400, mutation probability = constraint dependent wereconsidered.
Program was run & ended afterward 400 generations in MATLAB. Optimized values of fitness functions for responses has been shown in figure 4 for response HT is optimized after 70th generations respectively for hole diameter 2mm. Mean curve values are touching with best fitness curve without any extra improvements‘ diameters for different responses.
Optimized value of response HT from GA and best value of response from experiments for 2mm hole diameter shown in table 6. Comparative result (Table 6) shows 16.1% improvement for response HT. Confirmation experiments have done at optimal input parameters (table 6) to examine above mentionedimprovements.
Table 6: Comparison between optimal value and best value for response
Responses Best value from experiments
Optimal value from GA
%
Improvement Optimal parameters
Hole taper 2.05 1.72 16.1 6 150 80 12
VI. RECAST LAYER (RC)THICKNESS
In the present investigation, formation of recast layer thickness can be seen as shown in Fig 5. This may be due to the fraction of this molten liquid re-solidification, adhering to the sidewalls of the hole produced surfaces [22]. The measured recast layer thickness at settings as shown in fig 5 is 85.25micronm for hole diameter 2 mm.
Fig 5. Recast layer thickness
VII. CONCLUSIONS
Laser trepan drilling has been done on SS316L plate of 1.8mm thickness. Pulsed Nd:YAG laser was used for 2mm hole diameter. The conclusions of presented work as follows:
a. Current and pulse frequency play significant role in REDUCING HOLE TAPER.
b. Mathematical model is well fitted in arrangement with experimental values for hole taper. Because, mean prediction error was 1.25%.
c. Optimal outcomes show that hole taper has been reduced by 16.01% at gas pressure= 6 kg/cm2, cutting speed = 80 mm/min, current = 150 amp, and pulse frequency = 12 Hz.
REFERENCES
[1] B.T. Rao, R. Kaul, P. Tiwari, and A.K. Nath, ―Inert gas cutting of titanium sheet with pulsed mode CO2 laser‖, Optics and Lasers in Engineering, vol. 43(12), 2005, pp. 1330–1348.
[2] L.V. Shanjin, and W. Yang, An investigation of pulsed laser cutting of titanium alloy sheet. Optics and Lasers in Engineering, vol. 44(10), 2006, pp. 1067–1077.
[3] D.A. Dornfeld, J.S. Kim, H. Dechow, J. Hewson, and L.J. Chen, ―Drilling burr formation in titanium alloy SS316L‖. CIRP Annals - Manufacturing Technology, vol. 48(1), 2006, pp. 73–76.
[4] A.K. Dubey, and V. Yadava, ―Experimental study of Nd:YAG Laser beam machining—An overview‖. Journal of Materials Processing Technology, vol. 195(1–3), 2006, pp. 15–26.
[5] G.D. Gautam, and A.K.Pandey, ―Pulsed Nd:YAG laser beam drilling: A review‖, Optics and Laser Technology, vol. 100, 2018, pp. 183–215.
[6] G.K.L. zNg, and L. Li, ―Repeatability characteristics of laser percussion drilling of stainless-steel sheets‖, Optics and Lasers in Engineering, 2003, vol. 39(1), pp. 25-33.
[7] Choudhury, I.A.; Chong, W.C.; Vahid, G. Hole qualities in laser trepanning of polymeric materials. Optics and Lasers in Engineering 2012, 50 (9), pp. 1297–1305.
[8] R. Goyal, and A.K. Dubey, ―Quality Improvement by Parameter Optimization in Laser Trepan Drilling of Superalloy Sheet‖, Materials and Manufacturing Processes, vol. 29(11-12), 2014, pp. 1410-1416.
778 [9] S.K. Saini, A.K. Dubey, B.N. Upadhyay , and A. Choubey ―Study of hole characteristics in Laser Trepan Drilling
of ZTA‖, Optics and Laser Technology, vol. 103, 2018, pp. 330–339.
[10] E. Kacar, M. Mutlu, E. Akman, A. Demir, L. Candan, T. Canel, V. Gunay, and T.Sınmazcelik, ―Characterization of the drilling alumina ceramic using Nd:YAG pulsed laser‖, Journal of Material Process Technology, vol. 209 (4), 2009, pp. 2008–2014.
[11] Y.C. Lee, and M.H. Cheng, ―CO2 laser processing on ceramic substrates of light emitting diode assisted by compressed gas‖, Machining Science and Technology, vol.19, 2015, pp. 400–415.
[12] R. Biswas, A.S. Kuar, S. Sarkar, and S. Mitra, ―A parametric study of pulsed Nd:YAG laser micro-drilling of gamma-titanium aluminide‖, Optics and Laser Technology, 42, 2010, pp. 23–31.
[13] A. K. Pandey, and A. K. Dubey, ―Modeling and optimization of kerf taper and surface roughness in laser cutting of titanium alloy sheet‖, Journal of Mechanical Science and Technology, vol. 27, 2013, pp. 1-10.
[14] R. Goyal, A.K. Dubey, Modelling and optimization of geometrical characteristics in laser trepan drilling of titanium alloy, Journal of Mechanical Science and Technology, vol. 30, 2016, pp. 1281–1293.
[15] B. S. Yilbas, ―Study of affecting parameters in laser hole drilling of sheet metals‖, Journal of Engineering Materials and Technology, vol.109, 1987, pp. 282-287.
[16] A.K. Pandey, A.K. Dubey, Simultaneous optimization of multiple quality characteristics in laser cutting of titanium alloy sheet, Opt. Laser Technol. 44 (2012) 1858–1865.
[17] T. Olmez, ―The optimization of Cr (VI) reduction and removal by electrocoagulation using response surface methodology,‖ Journal of Hazardous Materials, vol. 162(2-3), 2009, pp. 1371–1378.
[18] S. Baroutian, M. K. Aroua, A. A. A. Raman, and N. M. N. Sulaiman, ―A packed bed membrane reactor for production of biodiesel using activated carbon supported catalyst,‖ Bioresource Technology, vol. 102, no. 2, pp.
1095–1102, 2011.
[19] D. Montgomery, Design and Analysis of Experiments, John Wiley & Sons, New York, NY, USA, 2001.
[20] S. Marimuthu, M. Antar, J. Dunleavey, D. Chantzis, W. Darlington, and P. Hayward, ―An experimental study on quasi-CW fibre laser drilling of nickel superalloy‖ Optics and Laser Technology, vol. 94, 2017, pp. 119–127.
[21] Y. Liu, R. Zhang, W. Li, J. Wang, X. Yang, L. Cheng, and L. Zhang, ―Effect of machining parameter on femtosecond laser drilling processing on SiC/SiC composites‖, International Journal of Advanced Manufacturing Technology, vol. 96(5–8), 2018, pp. 1795–1811.
[22] N.I. Morar, R. Roy, J. Mehnen, S. Marithumu, S. Gray, T. Roberts, and J. Nicholls, ―Investigation of recast and crack formation in laser trepanning drilling of CMSX-4 angled holes‖, International Journal of Advanced Manufacturing Technology, vol. 95(9-12), 2018, pp. 4059–4070.
