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Removal of Surface Spatter and Recast layer by Chemical Cleaning
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 18 (2019) 4763–4772
www.materialstoday.com/proceedings
ICMPC-2019
Removal of Surface Spatter and Recast layer by Chemical Cleaning Ganesh Dongrea, Avadhoot Rajurkara*, Ramesh Gondila, Akhil Mamidwara, Jacob Philipb a
Department of Industrial & Production Engineering, Vishwakarma Institute of Technology, Pune b Dy.General Manager, PPFF, Vikram Sarabhai Space Centre, Thiruvananthapuram, Kerala
Abstract Laser microdrilling has various applications in aerospace and electrical industries in manufacturing components such as nozzle guide vanes, combustion chambers, cooling holes on turbine blades, etc. In this study, cleaning of laser micro-drilled holes has been carried out. These holes were drilled using nanosecond pulsed fiber laser. The holes produced were with defects like surface spatter and recast layer. Therefore defects were cleaned by chemical etching using HNO3+HCl as etchant by keeping workpiece in still solution and passing etchant through the holes using electric pump. Qualitative and quantitative analysis were done using optical microscope. Surface spatter was cleaned in a still etchant solution at room temperature and a good quality etched holes was obtained for 40%HNO3 + 10%HCL when etched for 6 minutes. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019 Keywords: surface spatter, recast layer, laser micro-drilling, chemical etching
1. Introduction In today’s era of miniaturization laser beam drilling plays a major role in the field of automotive, aerospace and electrical industries to produce various applications such as nozzle guide vanes, combustion chambers, cooling holes on turbine blades, etc. The quality that makes it novel is its ability to drill deep holes i.e. with high aspect ratio of up to 1:20. It also provides several advantages such as huge range of materials that can machine, machining at microscale, low-cost, precision machining and less operation time. In laser beam machining laser pulses comes in contact with the material, thus the energy of pulses are captivated by the workpiece which leads to heat accumulation and material is removes by melting and evaporation mechanism.
* Corresponding author.
E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019
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Material removal depends on laser contract time and energy of pulse. Dominance of the material removal mechanism depends on the type of laser used. If milli- or micro-second laser are used then the material removal is by melting of material due to its high contact time which produces holes with low quality. If short or ultrashort lasers (pico-, nano-second lasers) are used then the material removal is by heat conduction, melting and evaporation which produces precise holes with good quality [1]. Hence Nd-YAG nanosecond laser is used to evaluate the parameters. Due to number of influencing parameters, hole with good circularity, minimum taper and minimum heat affected zone is difficult. As during machining, material gets heated which leads to the formation of heat affected zone around the hole, spatter on surface and recast layer on hole wall. Leitz et al [1] studied the metal ablation process by short and ultrashort pulse laser. It was found that pulse duration plays a major role in metal ablation and nanosecond laser gives the highest ablation efficiency per energy as material is less heated. L.S. Jiao et al [2] analyzed the effect of assist liquid on debris formation and taper for femtosecond laser on silicon. A liquid with low boiling point (methanol, ethanol) gives less debris as well as small taper angle. Geng et al [3] tried to reduce diameter and hole taper by using sieve plate and cover plate with varying parameters.Patwa et al [4] drilled holes by IR pulsed laser on Tantalum 2mm plate and found debris and recast layer. They tried to reduce recast layer by using various assist gases and cleaned the recast layer by chemical etching using transene tantalum etchant 111 for 7 minutes.Demir et al [5] investigated the effect of etching time on titanium plate. Microholes were drilled by fiber laser on titanium plate and by using H2SO4 + HF solution was etched for 4min to 28 min. It was found that at 24 minutes complete etching of holes at entrance and exit was achieving. Fishter et al [6] performed experiments to clean recast layer by chemical milling on nickel base superalloy MAR M-200 +Hf. HNO3 + HCl solution was used as etchant for cleaning recast layer at different elevated temperatures for laser and EDM drilled hole separately. It was found that there was change in surface roughness, height and length of workpiece. Nowak et al [7] investigated laser assisted chemical etching on Co, Cr, Ti and Cu in aqueous solution of KOH and phosphoric acid at different concentration and laser power. It was found that with increase in laser power holes of increased diameter and line of large width was producing. Felgueroso et al [8] studied the effect of chemical etching on improvement in mechanical strength on wafers by varying etching time. The mechanical strength increases with increase in etching time. Diepold et al [9] evaluated the effect of different concentration of hydrofluoric acid on smoothing the walls of drilled holes and compared the ultrasonic drilling method with sand blasting and laser machining. The present study is focused on cleaning of the surface spatter and reducing the taper angle by cleaning recast layer on hole wall and bottom. Chemical etching is performed on a drilled hole for achieving the results on SS316 on a developed setup and observed under the optical microscope and SEM. 2. Experimental Details A NUQA 30W Fiber Marking Laser machine manufactured by COHERENT was used for producing diodepumped Nd: YAG laser for experimentation. Machine specifications are as mentioned in Table 1. A Stainless steel 316 workpiece of 0.8mm thickness was used for the study. At first the holes were drilled of around entry diameter 115μmon a workpieceby a diode pumped laser keeping the parameters as shown in Table 2 and then they are further processed. Table 1. Machine Specification NUQA-1064-NA-0030-YZ Output Power
30.0 W
Output Power Adjustment
10 – 100%
Power Stability
± 2.5%
Beam Quality (Nominal)
M2 < 1.5
Mode of Operation
Pulsed
Polarization
Random
Peak Power
10.0 kW
Pulse Energy
1.0 mJ
Pulse Width
100 ± 20 ns
Pulse Repetition Rate (PRR)
30 – 100 kHz
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The two various type of experiments were conducted for cleaning surface spatter and debris, and for removing recast layer from the hole walls and reducing the hole taper separately. Nitric acid (HNO3) and hydrochloric acid (HCl) were used as etchant. Table 2. Parameters used for drilling holes Power
15 W
Frequency
40 kHz
Speed
150 mm/sec
Passes
250,500 & 750
The workpiece was kept in the still etchant solution with the concentration and parameters as mentioned in Table 3 for just cleaning the surface spatter and debris. The experiments were performed by varying the temperature of etchant and keeping other parameters constant for it. For removing the recast layer from the hole walls and reducing the hole taper experiments were conducted on a setup developed during this work as shown in fig 1. In these experiments, etchant solution was pumped through the hole with the help of an electric pump. The setup consisted of two rubber packed mild steel rods with a provision for holding a workpiece and a nozzle of diameter 1mm connected to an electric pump at the top of rods just above the workpiece. The pump was submerged under the etchant solution and thus the solution was passed through the nozzle on the workpiece held between the rods. Experiments were performed by varying one parameter at a time and keeping other parameters constant at room temperature. At first the experiments were conducted by varying time and keeping the concentration of both etchants as shown in Table 4. Later the experiments were performed by varying the concentration of HNO3 first and then th concentration of HCl as shown in Table 5, 6. The etched holes and surface were studied qualitatively and quantitatively with the help of optical microscope.
Fig 1. Chemical etching motorized setup Table 3. Parameters for cleaning surface spatter Etchant Concentration HNO3
40%
HCl
10%
H2O
Remaining
Time
20 min o
Temperature ( C)
27, 40, 60, 70, 80
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G. Dongre et al./ Materials Today: Proceedings 18 (2019) 4763–4772 Table 4. Parameters for determining etching time for removal of recast layer. Etchant Concentration HNO3 HCl
35% 7%
H2O
Remaining
Time (min)
2, 4, 6, 8, 10, 12
Table 5. Parameter for determining the concentration of HNO3 Etchant Concentration HNO3 (%)
25, 30, 35, 40, 45
HCl (%)
10
H2O Time (min)
Remaining 4, 6
Table 6. Parameters for determining the concentration of HCl Etchant Concentration HNO3 (%)
40
HCl (%)
7,10,13
H2O Time (min)
Remaining 6
3. Results and Discussion The amount of surface spatter removal after etching was evaluated at 2 different zones, hole entrance and hole exit. The etching is considered to be complete when the spatter around the hole is completely removed. Fig 2 shows the optical images of holes etched at different temperatures of solution. The optical images were used to measure entry and exit diameters of holes. It was observed that there was no much change in the dimensions of holes after etching. The optical images revealed that the cleaning of surface spatter at room temperature and at elevated temperatures (40,60 and 70) had no much difference. The surface spatter was cleaned without changing the hole dimensions. Since there was no much difference in the cleaning at elevated temperatures, hence it’s better to clean surface spatter at room temperature.
G. Dongre et al./ Materials Today: Proceedings 18 (2019) 4763–4772 Temperature
Room Temperature Before After
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40oC Before
After
Entry
Exit
60oC Before
70oC After
Before
After
Entry
Exit
Fig 2. Entrance and exit optical images of micro drilled holes before and after chemical etching at various temperatures.
3.1 For determining the etching time From the above observations, the experiments for removing of recast layer from hole walls was performed at room temperature.In order to analyze hole dimensions optical images were taken of microdrilled holes before and after experimentations. Results of experiments performed for determining the etching time are as shown in Table 7.The taper was reduced significantly for etching time 4 min and 6 min. At the time of 4 min the entry diameter was increased to 123μm whereas exit diameter to 73.7μm and also the taper was reduced by around 20% as shown in below Fig 3.
Fig 3. Effect of etching time on hole taper.
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Table 7. Results of microdrilled holes before and after experimentations for determining etching time. Time (Minute)
Entry Dia (μm)
Exit Dia (μm)
Taper, Deg
Entry Ovality
Exit Ovality
Before
After
Before
After
Before
After
Before
After
Before
After
2
112.09
116.88
61.15
66.65
1.808
1.798
0.036
0.021
0.063
0.079
4
119.12
122.9
57.8
73.7
2.163
1.775
0.026
0.032
0.06
0.085
6
118.32
129.06
58.61
77.11
2.117
1.888
0.02
0.033
0.187
0.138
8
116.6
167.04
56.86
110.26
2.112
2.085
0.045
0.053
0.14
0.276
10
116.53
145.43
58.22
78.94
2.054
2.457
0.032
0.018
0.095
0.127
12
120.91
198.82
64.11
92.53
2.004
4.11
0.043
0.104
0.14
0.118
3.2 Effect of etching time on entry diameter With increase in etching time the contact between the pressurized etchant and the hole surface increases due to which material erodes. Thus entry diameter increases with increase in etching time as shown in fig 4, the diameter increases drastically after 8 min of etching. In the range of 2 min to 6 min diameter increases in acceptable range.
Fig. 4 Effect of etching time on Entry diameter
3.3 Effect of etching time on exit diameter As there is increase in etching time the contact between the pressurized etchant and exit surface along with hole periphery increases which results in the removal of recast layer accumulated at hole periphery and exit surface. Thus with increase in etching time exit diameter increases as shown in fig 5.
Fig.5 Effect of etching time on exit diameter
3.4 Effect of etching time on taper: As entry diameter and exit diameter increases with etching time, the exit diameter increases more in comparison with entry diameter thus with increase in etching time hole taper gets reduced till 4 min of etching time as shown in fig 6. The hole taper gets reduced till 6 min and there is much less difference at 8 min between the etched and nonetched hole taper. The taper gets increase drastically after 8 min because of more increase in entry diameter and decrease in material thickness.
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Fig 6. Effect of etching time on hole taper.
3.5 Effect of etching time on hole ovality In the range of 2 min to 8 min there is slightly change in holeovality at entry and till 6 min for exit as shown in fig 7, 8. Above 8 min and 6 min it gets worse respectively, hence etching should be done for less than 8 min for good quality chemical etched holes.
Fig 7. Effect of etching time on entry ovality.
Fig 8. Effect of etching time on exit ovality.
3.6 For determining the concentration of HNO3 The experiments were performed for time 4min and 6 min, and it was observed that there was no much change in the exit diameter and taper for 4 min compare to 6min.The results obtained at 6 min are as shown in Table 8.
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G. Dongre et al./ Materials Today: Proceedings 18 (2019) 4763–4772 Table 8. Results of experiments performed for determining concentration of HNO3. HNO3 Entry Dia (μM) Exit Dia (μM) Taper, Deg Before
After
Before
After
Before
After
-25%
116.498
121.446
59.266
63.567
2.049
2.112
30%
118.835
120.728
51.816
59.517
2.399
2.277
35%
117.75
123.189
45.548
54.293
2.585
2.546
40%
118.232
130.119
51.96
82.674
2.373
1.788
45%
116.149
130.404
67.126
324.474
1.755
-7.279
Figure 9, 10, 11 shows the effect of HNO3 on entry diameter, exit diameter and taper respectively. The taper gets reduce with increase in concentration of HNO3 since the difference between entry diameter and exit diameter gets reduce. At 40% HNO3the minimum taperi.e 1.788o is obtained which is reduced by 25% whereas beyond it i.e for 45% HNO3 the taper angle obtained is negative due to much increase in exit diameter.
Fig 9. Effect of HNO3 on entry diameter.
Fig 10. Effect of HNO3 on exit diameter.
Fig 11. Effect of HNO3 on taper angle.
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3.7 For determining the concentration of HCl Since taper was reduced at 40% HNO3, the experiments for determining the concentration of HCl were performed by keeping HNO3 40% and etching time 6 min. The results were obtained as shown in Table 9. Table 9. Results of experiments performed for determining concentration of HCl HCL Entry Dia (μM) Exit Dia (μM) Taper, Deg Before
After
Before
After
Before
After
7%
115.068
124.08
59.955
79.794
1.973
1.648
10%
118.232
130.119
51.96
82.674
2.373
1.72
13%
111.92
128.816
62.389
229.472
1.774
-3.79
The maximum taper was reduced for 10% HCl about 27.5% and for 13% HCl the taper obtained was negative. Effect of concentration of HCl on entry diameter, exit diameter and taper are shown in fig 12, 13,14 respectively.
Fig 12. Effect of HCl on entry diameter.
Fig 13. Effect of HCl on exit diameter.
Fig 14. Effect of HCl on taper angle.
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Finally the parameter obtained for cleaning the recast layer and reducing the hole taper are as shown in Table 10. Figure 15 shows the optical images of microdrilled holes before and after chemical etching for following parameters. Table 10. Parameters for cleaning recast layer. Etchant Concentration HNO3 40% HCl 10% H2O Remaining Time
6 min
Temperature (oC)
Room Temp (27) After
Exit
Entry
Before
Fig 15. Optical images of microdrilled hole before and after etching
4. Conclusion Laser microdrilled holes with fiber laser of nanosecond range is a productive process, but produced holes are with spatter on surface and recast layer along hole periphery and exit surface. Chemical etching with HNO3+HCl solution provides solution for cleaning spatter and recast layer of stainless steel sheet. Surface spatter was removed by keeping the workpiece in a still etchant solution at room temperature with no elevated of temperature. Recast layer from periphery and exit surface was cleaned by passing etchant solution though holes by a electric pump and following results were concluded: 1. Entry diameter and exit diameter increases with etching time which also results in increase of taper above 8 min of etching time. 2. The maximum taper of about 27.5% was reduced for 40% HNO3+ 10% HCl when etched for 6 min. 3. The negative taper was obtained when etched with 45% HNO3 and 13% HCl solutions respectively. Acknowledgement The authors express their sincere gratitude to Indian Space and Research Organization and Pune University for project funding and management of Vishwakarma Institute of Technology for resources. References [1] Karl-Heinz L; Benjamin R; Yvonne R; Andreas O; Michael S: Metal Ablation with Short and Ultrashort Laser Pulses, Physics Procedia 12 (2011), 230-238. [2] Jiao L.S; Ng E.Y.K; Wee L.M; Zheng H.Y: The effect of assist liquid on the hole taper improvement in femtosecond laser percussion drilling, Physics Procedia 19 (2011), 426-430. [3] Geng Y; Wang K; Dong X; Duan W; Mei X; Wang W: Laser drilling of micro-holes with small diameter beyond the limits of focused spot by using a sieve plate or a cover plate, Int J Adv Manuf Technol, DOI 10.1007, 2016. [4] Patwa R; Herfurth H; Flaig R; Christophersen M; Philips B.F: Laser Drilling For High Aspect Ratio Holes and A High Open Area Fraction For Space Applications. [5] Ali Gokhan Demir; Barbara Previtali; Massimiliano Bestetti: Removal of spatter by chemical etching after microdrilling with high productivity fiber laser, Physics Procedia 5 (2010), 317-326. [6] Robert E Fishter; Boca Raton; Henry Lada; Lake Park: Selective chemical milling of recast surfaces, United States Patent, 1983. [7] R Nowak; S Metev; G Sepold: Laser chemical etching of metals in liquids, Materials and Manufacturing Processes (2007), 9:3, 429-446. [8] Gema Cueto- Felgueroso; Josu Barredo; Eneko Cerceda; Lutz Hermanns: Study of the mechanical strength improvement of wafers for EWT solar cells by chemical etching after the drilling process, 26th European Photovoltaic Solar Energy Conference and Exhibition. [9] T Diepold; E Obermeier: Smoothing of ultrasonically drilled holes in borosilicate glass by wet chemical etching, J. Micromech. Microeng. 6 (1996), 29-32.
ScienceDirect Materials Today: Proceedings 18 (2019) 4763–4772
www.materialstoday.com/proceedings
ICMPC-2019
Removal of Surface Spatter and Recast layer by Chemical Cleaning Ganesh Dongrea, Avadhoot Rajurkara*, Ramesh Gondila, Akhil Mamidwara, Jacob Philipb a
Department of Industrial & Production Engineering, Vishwakarma Institute of Technology, Pune b Dy.General Manager, PPFF, Vikram Sarabhai Space Centre, Thiruvananthapuram, Kerala
Abstract Laser microdrilling has various applications in aerospace and electrical industries in manufacturing components such as nozzle guide vanes, combustion chambers, cooling holes on turbine blades, etc. In this study, cleaning of laser micro-drilled holes has been carried out. These holes were drilled using nanosecond pulsed fiber laser. The holes produced were with defects like surface spatter and recast layer. Therefore defects were cleaned by chemical etching using HNO3+HCl as etchant by keeping workpiece in still solution and passing etchant through the holes using electric pump. Qualitative and quantitative analysis were done using optical microscope. Surface spatter was cleaned in a still etchant solution at room temperature and a good quality etched holes was obtained for 40%HNO3 + 10%HCL when etched for 6 minutes. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019 Keywords: surface spatter, recast layer, laser micro-drilling, chemical etching
1. Introduction In today’s era of miniaturization laser beam drilling plays a major role in the field of automotive, aerospace and electrical industries to produce various applications such as nozzle guide vanes, combustion chambers, cooling holes on turbine blades, etc. The quality that makes it novel is its ability to drill deep holes i.e. with high aspect ratio of up to 1:20. It also provides several advantages such as huge range of materials that can machine, machining at microscale, low-cost, precision machining and less operation time. In laser beam machining laser pulses comes in contact with the material, thus the energy of pulses are captivated by the workpiece which leads to heat accumulation and material is removes by melting and evaporation mechanism.
* Corresponding author.
E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019
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G. Dongre et al./ Materials Today: Proceedings 18 (2019) 4763–4772
Material removal depends on laser contract time and energy of pulse. Dominance of the material removal mechanism depends on the type of laser used. If milli- or micro-second laser are used then the material removal is by melting of material due to its high contact time which produces holes with low quality. If short or ultrashort lasers (pico-, nano-second lasers) are used then the material removal is by heat conduction, melting and evaporation which produces precise holes with good quality [1]. Hence Nd-YAG nanosecond laser is used to evaluate the parameters. Due to number of influencing parameters, hole with good circularity, minimum taper and minimum heat affected zone is difficult. As during machining, material gets heated which leads to the formation of heat affected zone around the hole, spatter on surface and recast layer on hole wall. Leitz et al [1] studied the metal ablation process by short and ultrashort pulse laser. It was found that pulse duration plays a major role in metal ablation and nanosecond laser gives the highest ablation efficiency per energy as material is less heated. L.S. Jiao et al [2] analyzed the effect of assist liquid on debris formation and taper for femtosecond laser on silicon. A liquid with low boiling point (methanol, ethanol) gives less debris as well as small taper angle. Geng et al [3] tried to reduce diameter and hole taper by using sieve plate and cover plate with varying parameters.Patwa et al [4] drilled holes by IR pulsed laser on Tantalum 2mm plate and found debris and recast layer. They tried to reduce recast layer by using various assist gases and cleaned the recast layer by chemical etching using transene tantalum etchant 111 for 7 minutes.Demir et al [5] investigated the effect of etching time on titanium plate. Microholes were drilled by fiber laser on titanium plate and by using H2SO4 + HF solution was etched for 4min to 28 min. It was found that at 24 minutes complete etching of holes at entrance and exit was achieving. Fishter et al [6] performed experiments to clean recast layer by chemical milling on nickel base superalloy MAR M-200 +Hf. HNO3 + HCl solution was used as etchant for cleaning recast layer at different elevated temperatures for laser and EDM drilled hole separately. It was found that there was change in surface roughness, height and length of workpiece. Nowak et al [7] investigated laser assisted chemical etching on Co, Cr, Ti and Cu in aqueous solution of KOH and phosphoric acid at different concentration and laser power. It was found that with increase in laser power holes of increased diameter and line of large width was producing. Felgueroso et al [8] studied the effect of chemical etching on improvement in mechanical strength on wafers by varying etching time. The mechanical strength increases with increase in etching time. Diepold et al [9] evaluated the effect of different concentration of hydrofluoric acid on smoothing the walls of drilled holes and compared the ultrasonic drilling method with sand blasting and laser machining. The present study is focused on cleaning of the surface spatter and reducing the taper angle by cleaning recast layer on hole wall and bottom. Chemical etching is performed on a drilled hole for achieving the results on SS316 on a developed setup and observed under the optical microscope and SEM. 2. Experimental Details A NUQA 30W Fiber Marking Laser machine manufactured by COHERENT was used for producing diodepumped Nd: YAG laser for experimentation. Machine specifications are as mentioned in Table 1. A Stainless steel 316 workpiece of 0.8mm thickness was used for the study. At first the holes were drilled of around entry diameter 115μmon a workpieceby a diode pumped laser keeping the parameters as shown in Table 2 and then they are further processed. Table 1. Machine Specification NUQA-1064-NA-0030-YZ Output Power
30.0 W
Output Power Adjustment
10 – 100%
Power Stability
± 2.5%
Beam Quality (Nominal)
M2 < 1.5
Mode of Operation
Pulsed
Polarization
Random
Peak Power
10.0 kW
Pulse Energy
1.0 mJ
Pulse Width
100 ± 20 ns
Pulse Repetition Rate (PRR)
30 – 100 kHz
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The two various type of experiments were conducted for cleaning surface spatter and debris, and for removing recast layer from the hole walls and reducing the hole taper separately. Nitric acid (HNO3) and hydrochloric acid (HCl) were used as etchant. Table 2. Parameters used for drilling holes Power
15 W
Frequency
40 kHz
Speed
150 mm/sec
Passes
250,500 & 750
The workpiece was kept in the still etchant solution with the concentration and parameters as mentioned in Table 3 for just cleaning the surface spatter and debris. The experiments were performed by varying the temperature of etchant and keeping other parameters constant for it. For removing the recast layer from the hole walls and reducing the hole taper experiments were conducted on a setup developed during this work as shown in fig 1. In these experiments, etchant solution was pumped through the hole with the help of an electric pump. The setup consisted of two rubber packed mild steel rods with a provision for holding a workpiece and a nozzle of diameter 1mm connected to an electric pump at the top of rods just above the workpiece. The pump was submerged under the etchant solution and thus the solution was passed through the nozzle on the workpiece held between the rods. Experiments were performed by varying one parameter at a time and keeping other parameters constant at room temperature. At first the experiments were conducted by varying time and keeping the concentration of both etchants as shown in Table 4. Later the experiments were performed by varying the concentration of HNO3 first and then th concentration of HCl as shown in Table 5, 6. The etched holes and surface were studied qualitatively and quantitatively with the help of optical microscope.
Fig 1. Chemical etching motorized setup Table 3. Parameters for cleaning surface spatter Etchant Concentration HNO3
40%
HCl
10%
H2O
Remaining
Time
20 min o
Temperature ( C)
27, 40, 60, 70, 80
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G. Dongre et al./ Materials Today: Proceedings 18 (2019) 4763–4772 Table 4. Parameters for determining etching time for removal of recast layer. Etchant Concentration HNO3 HCl
35% 7%
H2O
Remaining
Time (min)
2, 4, 6, 8, 10, 12
Table 5. Parameter for determining the concentration of HNO3 Etchant Concentration HNO3 (%)
25, 30, 35, 40, 45
HCl (%)
10
H2O Time (min)
Remaining 4, 6
Table 6. Parameters for determining the concentration of HCl Etchant Concentration HNO3 (%)
40
HCl (%)
7,10,13
H2O Time (min)
Remaining 6
3. Results and Discussion The amount of surface spatter removal after etching was evaluated at 2 different zones, hole entrance and hole exit. The etching is considered to be complete when the spatter around the hole is completely removed. Fig 2 shows the optical images of holes etched at different temperatures of solution. The optical images were used to measure entry and exit diameters of holes. It was observed that there was no much change in the dimensions of holes after etching. The optical images revealed that the cleaning of surface spatter at room temperature and at elevated temperatures (40,60 and 70) had no much difference. The surface spatter was cleaned without changing the hole dimensions. Since there was no much difference in the cleaning at elevated temperatures, hence it’s better to clean surface spatter at room temperature.
G. Dongre et al./ Materials Today: Proceedings 18 (2019) 4763–4772 Temperature
Room Temperature Before After
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40oC Before
After
Entry
Exit
60oC Before
70oC After
Before
After
Entry
Exit
Fig 2. Entrance and exit optical images of micro drilled holes before and after chemical etching at various temperatures.
3.1 For determining the etching time From the above observations, the experiments for removing of recast layer from hole walls was performed at room temperature.In order to analyze hole dimensions optical images were taken of microdrilled holes before and after experimentations. Results of experiments performed for determining the etching time are as shown in Table 7.The taper was reduced significantly for etching time 4 min and 6 min. At the time of 4 min the entry diameter was increased to 123μm whereas exit diameter to 73.7μm and also the taper was reduced by around 20% as shown in below Fig 3.
Fig 3. Effect of etching time on hole taper.
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Table 7. Results of microdrilled holes before and after experimentations for determining etching time. Time (Minute)
Entry Dia (μm)
Exit Dia (μm)
Taper, Deg
Entry Ovality
Exit Ovality
Before
After
Before
After
Before
After
Before
After
Before
After
2
112.09
116.88
61.15
66.65
1.808
1.798
0.036
0.021
0.063
0.079
4
119.12
122.9
57.8
73.7
2.163
1.775
0.026
0.032
0.06
0.085
6
118.32
129.06
58.61
77.11
2.117
1.888
0.02
0.033
0.187
0.138
8
116.6
167.04
56.86
110.26
2.112
2.085
0.045
0.053
0.14
0.276
10
116.53
145.43
58.22
78.94
2.054
2.457
0.032
0.018
0.095
0.127
12
120.91
198.82
64.11
92.53
2.004
4.11
0.043
0.104
0.14
0.118
3.2 Effect of etching time on entry diameter With increase in etching time the contact between the pressurized etchant and the hole surface increases due to which material erodes. Thus entry diameter increases with increase in etching time as shown in fig 4, the diameter increases drastically after 8 min of etching. In the range of 2 min to 6 min diameter increases in acceptable range.
Fig. 4 Effect of etching time on Entry diameter
3.3 Effect of etching time on exit diameter As there is increase in etching time the contact between the pressurized etchant and exit surface along with hole periphery increases which results in the removal of recast layer accumulated at hole periphery and exit surface. Thus with increase in etching time exit diameter increases as shown in fig 5.
Fig.5 Effect of etching time on exit diameter
3.4 Effect of etching time on taper: As entry diameter and exit diameter increases with etching time, the exit diameter increases more in comparison with entry diameter thus with increase in etching time hole taper gets reduced till 4 min of etching time as shown in fig 6. The hole taper gets reduced till 6 min and there is much less difference at 8 min between the etched and nonetched hole taper. The taper gets increase drastically after 8 min because of more increase in entry diameter and decrease in material thickness.
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Fig 6. Effect of etching time on hole taper.
3.5 Effect of etching time on hole ovality In the range of 2 min to 8 min there is slightly change in holeovality at entry and till 6 min for exit as shown in fig 7, 8. Above 8 min and 6 min it gets worse respectively, hence etching should be done for less than 8 min for good quality chemical etched holes.
Fig 7. Effect of etching time on entry ovality.
Fig 8. Effect of etching time on exit ovality.
3.6 For determining the concentration of HNO3 The experiments were performed for time 4min and 6 min, and it was observed that there was no much change in the exit diameter and taper for 4 min compare to 6min.The results obtained at 6 min are as shown in Table 8.
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G. Dongre et al./ Materials Today: Proceedings 18 (2019) 4763–4772 Table 8. Results of experiments performed for determining concentration of HNO3. HNO3 Entry Dia (μM) Exit Dia (μM) Taper, Deg Before
After
Before
After
Before
After
-25%
116.498
121.446
59.266
63.567
2.049
2.112
30%
118.835
120.728
51.816
59.517
2.399
2.277
35%
117.75
123.189
45.548
54.293
2.585
2.546
40%
118.232
130.119
51.96
82.674
2.373
1.788
45%
116.149
130.404
67.126
324.474
1.755
-7.279
Figure 9, 10, 11 shows the effect of HNO3 on entry diameter, exit diameter and taper respectively. The taper gets reduce with increase in concentration of HNO3 since the difference between entry diameter and exit diameter gets reduce. At 40% HNO3the minimum taperi.e 1.788o is obtained which is reduced by 25% whereas beyond it i.e for 45% HNO3 the taper angle obtained is negative due to much increase in exit diameter.
Fig 9. Effect of HNO3 on entry diameter.
Fig 10. Effect of HNO3 on exit diameter.
Fig 11. Effect of HNO3 on taper angle.
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3.7 For determining the concentration of HCl Since taper was reduced at 40% HNO3, the experiments for determining the concentration of HCl were performed by keeping HNO3 40% and etching time 6 min. The results were obtained as shown in Table 9. Table 9. Results of experiments performed for determining concentration of HCl HCL Entry Dia (μM) Exit Dia (μM) Taper, Deg Before
After
Before
After
Before
After
7%
115.068
124.08
59.955
79.794
1.973
1.648
10%
118.232
130.119
51.96
82.674
2.373
1.72
13%
111.92
128.816
62.389
229.472
1.774
-3.79
The maximum taper was reduced for 10% HCl about 27.5% and for 13% HCl the taper obtained was negative. Effect of concentration of HCl on entry diameter, exit diameter and taper are shown in fig 12, 13,14 respectively.
Fig 12. Effect of HCl on entry diameter.
Fig 13. Effect of HCl on exit diameter.
Fig 14. Effect of HCl on taper angle.
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Finally the parameter obtained for cleaning the recast layer and reducing the hole taper are as shown in Table 10. Figure 15 shows the optical images of microdrilled holes before and after chemical etching for following parameters. Table 10. Parameters for cleaning recast layer. Etchant Concentration HNO3 40% HCl 10% H2O Remaining Time
6 min
Temperature (oC)
Room Temp (27) After
Exit
Entry
Before
Fig 15. Optical images of microdrilled hole before and after etching
4. Conclusion Laser microdrilled holes with fiber laser of nanosecond range is a productive process, but produced holes are with spatter on surface and recast layer along hole periphery and exit surface. Chemical etching with HNO3+HCl solution provides solution for cleaning spatter and recast layer of stainless steel sheet. Surface spatter was removed by keeping the workpiece in a still etchant solution at room temperature with no elevated of temperature. Recast layer from periphery and exit surface was cleaned by passing etchant solution though holes by a electric pump and following results were concluded: 1. Entry diameter and exit diameter increases with etching time which also results in increase of taper above 8 min of etching time. 2. The maximum taper of about 27.5% was reduced for 40% HNO3+ 10% HCl when etched for 6 min. 3. The negative taper was obtained when etched with 45% HNO3 and 13% HCl solutions respectively. Acknowledgement The authors express their sincere gratitude to Indian Space and Research Organization and Pune University for project funding and management of Vishwakarma Institute of Technology for resources. References [1] Karl-Heinz L; Benjamin R; Yvonne R; Andreas O; Michael S: Metal Ablation with Short and Ultrashort Laser Pulses, Physics Procedia 12 (2011), 230-238. [2] Jiao L.S; Ng E.Y.K; Wee L.M; Zheng H.Y: The effect of assist liquid on the hole taper improvement in femtosecond laser percussion drilling, Physics Procedia 19 (2011), 426-430. [3] Geng Y; Wang K; Dong X; Duan W; Mei X; Wang W: Laser drilling of micro-holes with small diameter beyond the limits of focused spot by using a sieve plate or a cover plate, Int J Adv Manuf Technol, DOI 10.1007, 2016. [4] Patwa R; Herfurth H; Flaig R; Christophersen M; Philips B.F: Laser Drilling For High Aspect Ratio Holes and A High Open Area Fraction For Space Applications. [5] Ali Gokhan Demir; Barbara Previtali; Massimiliano Bestetti: Removal of spatter by chemical etching after microdrilling with high productivity fiber laser, Physics Procedia 5 (2010), 317-326. [6] Robert E Fishter; Boca Raton; Henry Lada; Lake Park: Selective chemical milling of recast surfaces, United States Patent, 1983. [7] R Nowak; S Metev; G Sepold: Laser chemical etching of metals in liquids, Materials and Manufacturing Processes (2007), 9:3, 429-446. [8] Gema Cueto- Felgueroso; Josu Barredo; Eneko Cerceda; Lutz Hermanns: Study of the mechanical strength improvement of wafers for EWT solar cells by chemical etching after the drilling process, 26th European Photovoltaic Solar Energy Conference and Exhibition. [9] T Diepold; E Obermeier: Smoothing of ultrasonically drilled holes in borosilicate glass by wet chemical etching, J. Micromech. Microeng. 6 (1996), 29-32.