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Analysis and optimization of surface quality of stainless steel miniature gears manufactured by CO2 laser cutting
Journal Pre-proof Analysis and Optimization of Surface Quality of Stainless Steel Miniature Gears Manufactured by CO2 Laser Cutting Cristina Anghel, Kapil Gupta, T.C. Jen
PII:
S0030-4026(19)31948-5
DOI:
https://doi.org/10.1016/j.ijleo.2019.164049
Reference:
IJLEO 164049
To appear in:
Optik
Received Date:
19 October 2019
Revised Date:
3 December 2019
Accepted Date:
10 December 2019
Please cite this article as: Anghel C, Gupta K, Jen TC, Analysis and Optimization of Surface Quality of Stainless Steel Miniature Gears Manufactured by CO2 Laser Cutting, Optik (2019), doi: https://doi.org/10.1016/j.ijleo.2019.164049
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
Analysis and Optimization of Surface Quality of Stainless Steel Miniature Gears Manufactured by CO2 Laser Cutting
Cristina Anghel1Kapil Gupta1* TC Jen2
1
Mechanical and Industrial Engineering Technology, University of Johannesburg, Johannesburg, South Africa 2 Mechanical Engineering Science, University of Johannesburg, Johannesburg, South Africa Corresponding Author- [email protected]; +27-011-5599081 7225 John Orr Building, University of Johannesburg (DFC) Doornfontein, Johannesburg- 2028 (South Africa)
Abstract
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*
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This paper reports the results of investigation conducted on CO2 laser cutting of miniature gears of
re
stainless steel 304. In this work, analysis of the effects of important laser parameters such as power, cutting speed, focal position, and gas pressure on average surface roughness (Ra) has been
lP
investigated. Stainless steel spur gears having 9.04 mm outside diameter and 4.5 mm face width have been cut using a CO2 laser system with nitrogen as assisted gas. A total of twenty nine experiments
na
have been conducted based on BBD (Box-Behnken Design) technique of response surface methodology where aforementioned laser parameters varied at three levels each. ANOVA study
ur
found focal position as the most significant parameter. Further, the Desirability based optimization of laser parameters obtained best values of Ra- 0.43 µm at laser power- 2407 W, cutting speed- 1.25
Jo
m/min, focal position- (-) 2.4 mm, gas pressure- 12.5 bar. A scanned electron microscopy study also revealed the good surface morphology of the miniature gear machined at optimum parameters.
Keywords: Gear; Laser; Machining, Miniature; Optimization; Surface
Introduction Industrial material processing and manufacturing has shifted towards non-conventional or advanced methods of fabrication due to the accelerate specialized requirements related to geometric accuracy, surface quality, micro-size and shape, and sustainability. One such advanced machining process, extensively used in a wide range of industries, is laser beam cutting (LBC). The process is a controlled thermal, non-contact process whereby a high energy density laser beam is focalized on the workpiece surface and material removal takes place by melting or vaporising the material [1, 2]. A
ro of
jet of gas, delivered coaxially with the laser beam, assist in the ejection of the evaporated and molten material from the cut zone. The advantages of laser beam cutting are manifolds, such as, low wastage, no tool wear, high precision, good dimensional accuracy, and high productivity etc. [1-3].
-p
There are many types of laser cutting systems for a wide range of industrial applications, however the most commonly used industrial lasers are Nd:YAG and CO2 lasers. LBC has been given
re
considerable importance in several manufacturing industries contributing to advancements in high-
lP
tech application areas whereby the miniaturization and microfabrication is an enabling technology. LBC performance and the product quality depend on a large number of process parameters with complex nonlinear relationships that requires difficult and sophisticated modelling and optimization
na
approaches [2-5].These parameters are, type and thickness of the material, laser power, cutting
ur
speed, assist gas pressure, nozzle diameter, stand-off distance, focal position and focal length etc. Miniature gears made from stainless steel 304 are essential components in most of the miniature
Jo
devices requiring motion and/or power transmission such as miniature pumps and motors, scientific instruments, motion reducers, biomedical equipment, and timing devices etc. [6, 7]. Surface quality of these gears play significant role in their functional performance and operating life. There have been some previous attempts to manufacture miniature gears using advanced machining processes to obtain the desired surface quality [6-9]. But, gear fabrication by laser beam cutting or machining hasn’t been explored much. The scope of the work presented in this paper attempts to fulfil this gap.
In this work, CO2 laser beam cutting with the assistance of nitrogen gas has been used to cut miniature gears of SS304. The effect of laser machining parameters on surface quality (average surface roughness) of miniature gears has been evaluated and analysed and further process parameter optimization has been done. A scanned electron microscopy study has also been conducted to analyse the surface morphology of the machined gears.
Experimental Procedure
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The experiments were conducted on TRUMATIC L 3020 (Trumph) CO2 laser machine with 3.2 kW maximum output power and nitrogen as assist gas deliver coaxially with the laser beam through a conical shaped nozzle. Figure 1 shows the experimental setup of laser beam machine tool used in the
-p
present work. The involute profile gear samples were cut from 4.5 mm thick sheet of SS 304. The
re
composition of SS304 and specifications of the miniature gear are given in Table 1. Response surface methodology based Box-Behnken Design (BBD) of experiments technique has been used to
lP
design twenty-nine experimental combinations of laser cutting process parameters namely laser power, cutting speed, focal position and gas pressure, each varied at three levels. Table 2 presents the
na
details of process parameters. The levels and values of variable and fixed parameters were decided based on pilot experiments, machine constraints, and literature review. Surface roughness on the cut
ur
edge have been measured using Jenoptik Hommel Etamic Surface profilometer supported by Turbowave V7 software and a TKU 600 probe on three random teeth of the same gear. The
Jo
measurements were repeated thrice to obtain the averaged values. TSCAN make scanned electron microscope VEGA3 has been used to study the surface morphology of miniature gears.
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Figure 1 Experimental setup and sequence of processes for laser beam cutting of SS304 miniature gears.
Manganese 2.00%
Phosphorus 0.045%
Sulfur 0.030%
Silicon 0.75%
Chromium Nickel 18-20% 8-12%
Nitrogen 0.10%
Iron Bala nce
re
Carbon 0.08%
-p
Table 1. Composition of SS 304 and specification of miniature gear Material composition
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Miniature gear specifications Type: external spur, Number of teeth: 10, Outside diameter: 9.04 mm, Thickness: 4.5 mm Pressure angle: 20⁰, Module: 0.750
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Table 2. Details of laser cutting parameters Variable parameters Symbol Parameter Units Laser power Watts A Cutting speed m/min B Focal position mm C Gas pressure bar D Fixed parameters Focal length Nozzle diameter Nozzle stand-off Frequency
-1 1500 1 -1.5 10
0 2000 2 -2.5 13
+1 2500 3 -3.5 16
127 mm 1.7 mm 1 mm 2000 Hz
Results and Discussion Corresponding to all twenty nine experiments, average surface roughness values have been evaluated and further analysis of the effects of process parameters has been done. Successfully conducted
ANOVA study found statistical fitness of the experimental data. Moreover, focal position is identified as the most significant parameter and parameter interactions also played major role. 3D response surface plots for average surface roughness are shown in Figure 2. A low value of surface roughness can be obtained at high laser power and low cutting speed as shown in Figure 2a. For higher power, the temperature of the molten metal on the cut front increases and as combined with a slow cutting speed it keeps the power intensity for longer period of time on the cut front. That resulted in temperature increase beyond the melting point, and consequently partial or total
ro of
vaporization of the material that easily ejected from the cut kerf leaving behind the smoother surface and hence better surface finish. The combination of high laser power and focal position of 2 mm depth from the top surface, and high laser power and gas pressure 16 bar result in the lowest surface
-p
roughness values (Figs 2b-c). For the variation of surface roughness with cutting speed and gas pressure, it is observed that high cutting speed and low gas pressure deteriorate the surface quality
re
that can be explained by the fact that the melt flow velocity is not accelerated by the low gas pressure
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and the faster movement of assisted gas form striation that lead to generate rougher surface (Figure 2e) [10, 11]. Better surface finish can be obtained at some trade-offs between cutting speed and gas
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pressure.
a.
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b.
d.
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c.
f.
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e. Figure 2. 3D surface plots for Ra
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For a focal position of 2.25 mm from the top surface of the workpiece, a 14 bar assist gas pressure is required for a minimum (approx. 0.5 µm) value of surface roughness (Figure 2f). It is clear that the
Jo
variation of the laser cutting parameters in the range opted in the study affects the quality parameter under investigation in various ways, however the analysis reveals that for improved surface roughness, high laser power and low speed with midrange values of focal position and gas pressure are required.
Optimization
Desirability analysis technique developed by Derringer and Suich has been used to minimize the average surface roughness for laser cut miniature gears [12]. Desirability transforms each response Yi into a desirability function di, with values ranging from 0 to 1. If the response value is outside the acceptable region then, di = 0 while, if the response is at its goal target then di = 1. The overall desirability D is then obtained as the weighted geometric average of the individual desirabilities according to the Equation 1: 1
𝐷 = (𝑑1 ∙ 𝑑2 ∙ 𝑑3 ∙ … ∙ 𝑑𝑛 )𝑛
ro of
(Eq.1)
Where,
-p
n is the number of responses
Based upon the objective for response, i.e. to maximize or minimize, different desirability functions
re
can be employed. For minimization, i.e. to obtain as low value of Ra as possible, the desirability
1, 𝑑𝑖 (𝑌𝑖 ) = {(
𝑈𝑖 −𝑌𝑖 𝑈𝑖 −𝐿𝑖
𝑡
) ,
𝐿𝑖 ≤ 𝑌𝑖 ≤ 𝑈𝑖 𝑌𝑖 ≥ 𝑈𝑖
(Eq.2)
na
0,
𝑌𝑖 < 𝐿𝑖
lP
function is as given in Eq 2
where Li, Ui represents the lower and upper acceptable values for the response Yi. The ‘t’ exponent
ur
corresponds to the weight that determine how important it is for the response Yi to be closed to the maximum or minimum. The Desirability prediction results for optimization of Ra are shown in Table
Jo
3 corresponding to a desirability value very close to unity. A confirmation experiment has been conducted to verify the desirability predictions. The optimized value of Ra - 0.43 µm has been obtained at optimum set of parameters as given in Table 3. A very close agreement is found between the results of prediction and confirmation experiment based values therefore, the optimization of surface roughness of laser cut SS304 miniature gear is done successfully.
Table 3. Confirmation experiments results Sl No A B C D Response Desirability Prediction Confirmation Experiment 1 2407 1.25 -2.40 12.5 Ra (µm) 0.428 0.43
Surface Morphology Study A strong influence on the functional performance of miniature parts comes from the nature of the surface quality characteristics where surface morphology and subsurface characteristics play pivotal role [5]. Surface morphology investigation via scanned electron microscope has been done for the
ro of
miniature gears machined at optimum parameters. Figure 3 presents the SEM images of gear tooth surfaces at top, middle and bottom of the cut. For all samples, the tooth flank, at the beginning of the cut, appear to be uniform with no visible cracks, nicks or burrs. There is however a visible waviness on the top part. This may be due to two reasons; the focal point position is way below the surface
-p
therefore, at the top part there is slightly less power intensity, with a slightly higher melts viscosity
re
hence the melt flow displaying the waviness pattern. The other reason may be due to assist gas dynamics at the entry to the cut front where some of the flow may be diverted by the top surface [10,
lP
11]. The middle part shows a very uniform surface. At the bottom, a straight flow pattern is maintained with occasional splatters and the occurrence of small burrs right at the end of the cut. The
Jo
ur
na
burrs are due to re-solidified molten metal that remained attached to the bottom of the cut.
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b b
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aa
Figure 3. SEM images of the different tooth surfaces for the miniature gear machined at optimum parameters
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Further, the laser thermal effects have also been analysed for the gear tooth surfaces obtained before and after optimization. It is known that laser cutting, with its great amount of heat generated and
ur
rapid cooling, may cause changes in the base material at the beam-metal interface as a layered structure on the base material. This layer of affected microstructure is called heat affected zone
Jo
(HAZ) that is undesirable and may compromise the function and lifespan of the machined component or part [10, 11]. The SEM analysis of HAZ for the best gear obtained during twenty seven experiments has been compared with the HAZ for the optimized gear. An appreciable reduction in HAZ is observed from 13 μm to about 9 μm as shown in Figure 4 that is good from the point of view of the improved functional performance and service life of gear.
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b)
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a)
b)
Figure 4. SEM study of heat affected zone (HAZ) of gear tooth surfaces obtained (a) before optimization (b) after optimization
Conclusions Analysis and optimization of surface quality of SS304 miniature gears machined by laser beam cutting is reported in this paper. The following conclusions can be drawn from this research
It is analysed that high laser power, low cutting speed, and moderate focal position and gas pressure generated lower surface roughness on miniature gear tooth surfaces.
The optimum value of laser process parameters i.e. laser power- 2407 W, cutting speed- 1.25 m/min, focal position- 2.40 mm, and gas pressure- 12.5 bar, produced least average
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roughness 0.43 µm.
The SEM investigation of the cut surface revealed a uniform surface with slight weavings pattern at the top and small dross attached at the bottom. The surface is free of craters, cracks
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and pitting proving that the generation of favourable surface morphology at optimum
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parameters.
The results of this investigation identify the potential of laser beam cutting for manufacturing
Declaration of interests
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of good quality miniature gears.
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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
References
[1] G. Chryssolouris, Laser Machining: Theory and Practice, Springer-Verlag, New York, 1991. [2] R. Schaeffer, Fundamentals of Laser Micromachining, CRC Press, Boca Raton, FL, 2012.
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[3] W.M. Steen, J.Mazumder, Laser Material Processing, 4 Edition, Springer-Verlag, New York, 2010.
[4] U. Klotzbach, A.F. Lasagni, M. Panzner, V. Franke, Laser micromachining, in:
-p
F.A.Lasagni, A.F. Lasagni (Eds.), Fabrication and Characterization in the Micro-Nano Range, Springer-Verlag, Berlin, Heidelberg, 2011, pp. 29-46.
machining
of
nickel-based
superalloy,
Optik
196
(2019)
163199.
lP
laser
re
[5] A. Khan and K. Gupta, Experimental evaluation of surface quality characteristics in
[https://doi.org/10.1016/j.ijleo.2019.163199] [6] K. Gupta, N.K. Jain, Near-Net Shape Manufacturing of Gears by Wire Spark Erosion
na
Machining. Springer Science and Business Media Pvt. Ltd., Singapore, 2016. [7] M.Y. Ali, A.N.M. Karim, E.Y.T. Adesta, A.F. Ismail, A.A. Abdullah, M.N. Idris,
ur
Comparative study of conventional and micro-WEDM based on machining of meso-
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micro sized spur gear, Int. J Precis. Eng. Man. 11 (2010) 779–784. [8] S.K. Chaubey, N.K. Jain, Analysis and multi-response optimization of gear quality and surface finish of meso-sized helical and bevel gears manufactured by WSEM process, Precis. Eng. 55 (2019) 293-309. [9] T. Phokane, K. Gupta, M.K. Gupta, Sustainability Assessment Based Comparative Evaluation of Precision Miniature Gear Manufacturing Processes, In Kapil Gupta Edited,
Innovations in Manufacturing for Sustainability, Springer-Verlag, Berlin, Heidelberg, 2019, pp. 169-182, [10] N. Rajaram, J. Sheikh-Ahmad, S.H. Cheraghi, CO2 laser cut quality of 4130 steel. Int. J Mach. Tool Manu. 43 (2003) 351–358. [11] M. Sharifi, M. Akbari, Experimental investigation of the effect of process parameters on cutting region temperature and cutting-edge quality in laser cutting of AL6061T6 alloy, Optik 184 (2019) 457–463.
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na
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re
-p
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[12] G Derringer, R. Suich, Simultaneous optimization of several response variables, J Qual. Technol. 12 (1980) 214-219
PII:
S0030-4026(19)31948-5
DOI:
https://doi.org/10.1016/j.ijleo.2019.164049
Reference:
IJLEO 164049
To appear in:
Optik
Received Date:
19 October 2019
Revised Date:
3 December 2019
Accepted Date:
10 December 2019
Please cite this article as: Anghel C, Gupta K, Jen TC, Analysis and Optimization of Surface Quality of Stainless Steel Miniature Gears Manufactured by CO2 Laser Cutting, Optik (2019), doi: https://doi.org/10.1016/j.ijleo.2019.164049
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
Analysis and Optimization of Surface Quality of Stainless Steel Miniature Gears Manufactured by CO2 Laser Cutting
Cristina Anghel1Kapil Gupta1* TC Jen2
1
Mechanical and Industrial Engineering Technology, University of Johannesburg, Johannesburg, South Africa 2 Mechanical Engineering Science, University of Johannesburg, Johannesburg, South Africa Corresponding Author- [email protected]; +27-011-5599081 7225 John Orr Building, University of Johannesburg (DFC) Doornfontein, Johannesburg- 2028 (South Africa)
Abstract
ro of
*
-p
This paper reports the results of investigation conducted on CO2 laser cutting of miniature gears of
re
stainless steel 304. In this work, analysis of the effects of important laser parameters such as power, cutting speed, focal position, and gas pressure on average surface roughness (Ra) has been
lP
investigated. Stainless steel spur gears having 9.04 mm outside diameter and 4.5 mm face width have been cut using a CO2 laser system with nitrogen as assisted gas. A total of twenty nine experiments
na
have been conducted based on BBD (Box-Behnken Design) technique of response surface methodology where aforementioned laser parameters varied at three levels each. ANOVA study
ur
found focal position as the most significant parameter. Further, the Desirability based optimization of laser parameters obtained best values of Ra- 0.43 µm at laser power- 2407 W, cutting speed- 1.25
Jo
m/min, focal position- (-) 2.4 mm, gas pressure- 12.5 bar. A scanned electron microscopy study also revealed the good surface morphology of the miniature gear machined at optimum parameters.
Keywords: Gear; Laser; Machining, Miniature; Optimization; Surface
Introduction Industrial material processing and manufacturing has shifted towards non-conventional or advanced methods of fabrication due to the accelerate specialized requirements related to geometric accuracy, surface quality, micro-size and shape, and sustainability. One such advanced machining process, extensively used in a wide range of industries, is laser beam cutting (LBC). The process is a controlled thermal, non-contact process whereby a high energy density laser beam is focalized on the workpiece surface and material removal takes place by melting or vaporising the material [1, 2]. A
ro of
jet of gas, delivered coaxially with the laser beam, assist in the ejection of the evaporated and molten material from the cut zone. The advantages of laser beam cutting are manifolds, such as, low wastage, no tool wear, high precision, good dimensional accuracy, and high productivity etc. [1-3].
-p
There are many types of laser cutting systems for a wide range of industrial applications, however the most commonly used industrial lasers are Nd:YAG and CO2 lasers. LBC has been given
re
considerable importance in several manufacturing industries contributing to advancements in high-
lP
tech application areas whereby the miniaturization and microfabrication is an enabling technology. LBC performance and the product quality depend on a large number of process parameters with complex nonlinear relationships that requires difficult and sophisticated modelling and optimization
na
approaches [2-5].These parameters are, type and thickness of the material, laser power, cutting
ur
speed, assist gas pressure, nozzle diameter, stand-off distance, focal position and focal length etc. Miniature gears made from stainless steel 304 are essential components in most of the miniature
Jo
devices requiring motion and/or power transmission such as miniature pumps and motors, scientific instruments, motion reducers, biomedical equipment, and timing devices etc. [6, 7]. Surface quality of these gears play significant role in their functional performance and operating life. There have been some previous attempts to manufacture miniature gears using advanced machining processes to obtain the desired surface quality [6-9]. But, gear fabrication by laser beam cutting or machining hasn’t been explored much. The scope of the work presented in this paper attempts to fulfil this gap.
In this work, CO2 laser beam cutting with the assistance of nitrogen gas has been used to cut miniature gears of SS304. The effect of laser machining parameters on surface quality (average surface roughness) of miniature gears has been evaluated and analysed and further process parameter optimization has been done. A scanned electron microscopy study has also been conducted to analyse the surface morphology of the machined gears.
Experimental Procedure
ro of
The experiments were conducted on TRUMATIC L 3020 (Trumph) CO2 laser machine with 3.2 kW maximum output power and nitrogen as assist gas deliver coaxially with the laser beam through a conical shaped nozzle. Figure 1 shows the experimental setup of laser beam machine tool used in the
-p
present work. The involute profile gear samples were cut from 4.5 mm thick sheet of SS 304. The
re
composition of SS304 and specifications of the miniature gear are given in Table 1. Response surface methodology based Box-Behnken Design (BBD) of experiments technique has been used to
lP
design twenty-nine experimental combinations of laser cutting process parameters namely laser power, cutting speed, focal position and gas pressure, each varied at three levels. Table 2 presents the
na
details of process parameters. The levels and values of variable and fixed parameters were decided based on pilot experiments, machine constraints, and literature review. Surface roughness on the cut
ur
edge have been measured using Jenoptik Hommel Etamic Surface profilometer supported by Turbowave V7 software and a TKU 600 probe on three random teeth of the same gear. The
Jo
measurements were repeated thrice to obtain the averaged values. TSCAN make scanned electron microscope VEGA3 has been used to study the surface morphology of miniature gears.
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Figure 1 Experimental setup and sequence of processes for laser beam cutting of SS304 miniature gears.
Manganese 2.00%
Phosphorus 0.045%
Sulfur 0.030%
Silicon 0.75%
Chromium Nickel 18-20% 8-12%
Nitrogen 0.10%
Iron Bala nce
re
Carbon 0.08%
-p
Table 1. Composition of SS 304 and specification of miniature gear Material composition
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Miniature gear specifications Type: external spur, Number of teeth: 10, Outside diameter: 9.04 mm, Thickness: 4.5 mm Pressure angle: 20⁰, Module: 0.750
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Table 2. Details of laser cutting parameters Variable parameters Symbol Parameter Units Laser power Watts A Cutting speed m/min B Focal position mm C Gas pressure bar D Fixed parameters Focal length Nozzle diameter Nozzle stand-off Frequency
-1 1500 1 -1.5 10
0 2000 2 -2.5 13
+1 2500 3 -3.5 16
127 mm 1.7 mm 1 mm 2000 Hz
Results and Discussion Corresponding to all twenty nine experiments, average surface roughness values have been evaluated and further analysis of the effects of process parameters has been done. Successfully conducted
ANOVA study found statistical fitness of the experimental data. Moreover, focal position is identified as the most significant parameter and parameter interactions also played major role. 3D response surface plots for average surface roughness are shown in Figure 2. A low value of surface roughness can be obtained at high laser power and low cutting speed as shown in Figure 2a. For higher power, the temperature of the molten metal on the cut front increases and as combined with a slow cutting speed it keeps the power intensity for longer period of time on the cut front. That resulted in temperature increase beyond the melting point, and consequently partial or total
ro of
vaporization of the material that easily ejected from the cut kerf leaving behind the smoother surface and hence better surface finish. The combination of high laser power and focal position of 2 mm depth from the top surface, and high laser power and gas pressure 16 bar result in the lowest surface
-p
roughness values (Figs 2b-c). For the variation of surface roughness with cutting speed and gas pressure, it is observed that high cutting speed and low gas pressure deteriorate the surface quality
re
that can be explained by the fact that the melt flow velocity is not accelerated by the low gas pressure
lP
and the faster movement of assisted gas form striation that lead to generate rougher surface (Figure 2e) [10, 11]. Better surface finish can be obtained at some trade-offs between cutting speed and gas
Jo
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na
pressure.
a.
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b.
d.
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re
-p
c.
f.
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e. Figure 2. 3D surface plots for Ra
ur
For a focal position of 2.25 mm from the top surface of the workpiece, a 14 bar assist gas pressure is required for a minimum (approx. 0.5 µm) value of surface roughness (Figure 2f). It is clear that the
Jo
variation of the laser cutting parameters in the range opted in the study affects the quality parameter under investigation in various ways, however the analysis reveals that for improved surface roughness, high laser power and low speed with midrange values of focal position and gas pressure are required.
Optimization
Desirability analysis technique developed by Derringer and Suich has been used to minimize the average surface roughness for laser cut miniature gears [12]. Desirability transforms each response Yi into a desirability function di, with values ranging from 0 to 1. If the response value is outside the acceptable region then, di = 0 while, if the response is at its goal target then di = 1. The overall desirability D is then obtained as the weighted geometric average of the individual desirabilities according to the Equation 1: 1
𝐷 = (𝑑1 ∙ 𝑑2 ∙ 𝑑3 ∙ … ∙ 𝑑𝑛 )𝑛
ro of
(Eq.1)
Where,
-p
n is the number of responses
Based upon the objective for response, i.e. to maximize or minimize, different desirability functions
re
can be employed. For minimization, i.e. to obtain as low value of Ra as possible, the desirability
1, 𝑑𝑖 (𝑌𝑖 ) = {(
𝑈𝑖 −𝑌𝑖 𝑈𝑖 −𝐿𝑖
𝑡
) ,
𝐿𝑖 ≤ 𝑌𝑖 ≤ 𝑈𝑖 𝑌𝑖 ≥ 𝑈𝑖
(Eq.2)
na
0,
𝑌𝑖 < 𝐿𝑖
lP
function is as given in Eq 2
where Li, Ui represents the lower and upper acceptable values for the response Yi. The ‘t’ exponent
ur
corresponds to the weight that determine how important it is for the response Yi to be closed to the maximum or minimum. The Desirability prediction results for optimization of Ra are shown in Table
Jo
3 corresponding to a desirability value very close to unity. A confirmation experiment has been conducted to verify the desirability predictions. The optimized value of Ra - 0.43 µm has been obtained at optimum set of parameters as given in Table 3. A very close agreement is found between the results of prediction and confirmation experiment based values therefore, the optimization of surface roughness of laser cut SS304 miniature gear is done successfully.
Table 3. Confirmation experiments results Sl No A B C D Response Desirability Prediction Confirmation Experiment 1 2407 1.25 -2.40 12.5 Ra (µm) 0.428 0.43
Surface Morphology Study A strong influence on the functional performance of miniature parts comes from the nature of the surface quality characteristics where surface morphology and subsurface characteristics play pivotal role [5]. Surface morphology investigation via scanned electron microscope has been done for the
ro of
miniature gears machined at optimum parameters. Figure 3 presents the SEM images of gear tooth surfaces at top, middle and bottom of the cut. For all samples, the tooth flank, at the beginning of the cut, appear to be uniform with no visible cracks, nicks or burrs. There is however a visible waviness on the top part. This may be due to two reasons; the focal point position is way below the surface
-p
therefore, at the top part there is slightly less power intensity, with a slightly higher melts viscosity
re
hence the melt flow displaying the waviness pattern. The other reason may be due to assist gas dynamics at the entry to the cut front where some of the flow may be diverted by the top surface [10,
lP
11]. The middle part shows a very uniform surface. At the bottom, a straight flow pattern is maintained with occasional splatters and the occurrence of small burrs right at the end of the cut. The
Jo
ur
na
burrs are due to re-solidified molten metal that remained attached to the bottom of the cut.
ro of -p re
b b
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aa
Figure 3. SEM images of the different tooth surfaces for the miniature gear machined at optimum parameters
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Further, the laser thermal effects have also been analysed for the gear tooth surfaces obtained before and after optimization. It is known that laser cutting, with its great amount of heat generated and
ur
rapid cooling, may cause changes in the base material at the beam-metal interface as a layered structure on the base material. This layer of affected microstructure is called heat affected zone
Jo
(HAZ) that is undesirable and may compromise the function and lifespan of the machined component or part [10, 11]. The SEM analysis of HAZ for the best gear obtained during twenty seven experiments has been compared with the HAZ for the optimized gear. An appreciable reduction in HAZ is observed from 13 μm to about 9 μm as shown in Figure 4 that is good from the point of view of the improved functional performance and service life of gear.
c
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b)
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a)
b)
Figure 4. SEM study of heat affected zone (HAZ) of gear tooth surfaces obtained (a) before optimization (b) after optimization
Conclusions Analysis and optimization of surface quality of SS304 miniature gears machined by laser beam cutting is reported in this paper. The following conclusions can be drawn from this research
It is analysed that high laser power, low cutting speed, and moderate focal position and gas pressure generated lower surface roughness on miniature gear tooth surfaces.
The optimum value of laser process parameters i.e. laser power- 2407 W, cutting speed- 1.25 m/min, focal position- 2.40 mm, and gas pressure- 12.5 bar, produced least average
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roughness 0.43 µm.
The SEM investigation of the cut surface revealed a uniform surface with slight weavings pattern at the top and small dross attached at the bottom. The surface is free of craters, cracks
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and pitting proving that the generation of favourable surface morphology at optimum
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parameters.
The results of this investigation identify the potential of laser beam cutting for manufacturing
Declaration of interests
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of good quality miniature gears.
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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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