Removal of Recast Layer in Laser-Ablated Titanium Alloy Surface by Electrochemical Machining Process

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Congress on Manufacturing and Management (GCMM-2018) Removal14th ofGlobal Recast Layer in Laser-Ablated Titanium Alloy Surface by Electrochemical Machining Process Alloy Surface Removal of Recast Layer in Laser-Ablated Titanium Manufacturing Engineering Society International Conference 2017, MESIC 2017, 28-30 June by Electrochemical Machining Process a a Paiboon Choungthong , Bunchanit Viboon Tangwarodomnukuna* 2017, VigoWilaisahwat (Pontevedra),, Spain a a Department of Production Engineering, Faculty of Engineering, King Mongkut’s University of Technology Thonburi, Bangkok 10140,aThailand

a

Paiboon Choungthong , Bunchanit Wilaisahwat , Viboon Tangwarodomnukun *

Costing models for capacity optimization in Industry 4.0: Trade-off Department of Production Engineering, Faculty of Engineering, King Mongkut’s University of Technology Thonburi, Bangkok 10140, Thailand between used capacity and operational efficiency Abstract a

A. Santanasurface , P. Afonso , A. Zanin , R. Wernke Recast layer formed in the laser-machined is usually fragile and composes of residual stress and micro cracks. The Abstract removal of recast layer is therefore necessary to prevent the contamination of broken off recast particles that could deteriorate the a University of Minho, 4800-058 Guimarães, Portugal functionality service lifelaser-machined of manufactured parts. is Electrochemical machining process can be employed remove the recast Recast layer and formed in the usually fragile and composes of residual stress andto micro cracks. The b surface Unochapecó, 89809-000 Chapecó, SC, Brazil structureof from thelayer workissurface. In necessary this study,tothe recastthe of titanium alloy (Ti-6Al-4V) by laser milling wasthe to removal recast therefore prevent contamination of broken offproduced recast particles that could process deteriorate be removed by electrochemical-machining process. The surface, subsurface and dimensions of laser-milled cavity were functionality andusing service life of manufactured parts. Electrochemical machining process can be employed to remove the recast investigated andsurface. after the electrochemical dissolution. was(Ti-6Al-4V) found that produced most recast can milling be removed structure frombefore the work In this study, the recast of titaniumItalloy by laser process by wasthe to electrochemical machining process and the cavity depth wasThe increased the processing time. The of uselaser-milled of high lasercavity powerwere not be removed by using electrochemical-machining process. surface,with subsurface and dimensions Abstract only increasedbefore the amount of recast, also raised surface imperfections the workpiece suchrecast as micro cracks.byThese investigated and after the but electrochemical dissolution. It wasto found that most canholes be and removed the imperfections can be of reproduced the dissolution, thusbe enlarging the micro holes andhigh cracks well not as electrochemical machining process by and4.0", theelectrochemical cavity depth was increasedwill with the processing The use of laseras power Under the concept "Industry production processes pushed totime. be increasingly interconnected, roughening the work surface. In addition, the electrochemical machining process can facilitate the removal of heat-affected zone only increased the amount of recast, but also raised surface imperfections to the workpiece such as micro holes and cracks. These information based on a real time basis and, necessarily, much more efficient. In this context, capacity optimization by 21% after being dissolved for 5,400 seconds. The implication of thisthus study could provide a guideline for the recast imperfections cantraditional be reproduced by capacity the electrochemical dissolution, enlarging the micro holesprofitability and cracks as removal well as goes beyond the aim of maximization, contributing also for organization’s and value. and the deburring of laser-cut roughening the work surface. surface. In addition, the electrochemical machining process can facilitate the removal of heat-affected zone Indeed, lean management and continuous improvement approaches suggest capacity optimization instead of by 21% after being dissolved for 5,400 seconds. The implication of this study could provide a guideline for the recast removal maximization. The study of capacity optimization and costing models is an important research topic that deserves and the deburring of laser-cut surface. contributions from both the practical andLtd. theoretical perspectives. This paper presents and discusses a mathematical © 2018 The Authors. Published by Elsevier © 2019 The Authors. Published by Elsevier Ltd. model for capacity management based on differentlicense costing models (ABC and TDABC). A generic model has This is an open access article under the CC BY-NC-ND (https://creativecommons.org/licenses/by-nc-nd/4.0/ ) been This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) developed and it was used to analyze idle capacity and to design strategies towards the maximization of organization’s Selection and peer-review under responsibility of the scientific committee of the 14th Global Congress on Manufacturing and © 2018 The Published Elsevier Ltd. of the scientific committee of the 14th Global Congress on Manufacturing and Selection andAuthors. peer-review underbyresponsibility value. The trade-off capacity maximization vs operational efficiency is highlighted and it is shown that capacity Management (GCMM-2018). This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Management (GCMM-2018). optimization hide operational inefficiency. Selection and might peer-review under responsibility of the scientific committee of the 14th Global Congress on Manufacturing and Keywords: Electrochemical; Machining; Laser; Titanium; © 2017 The Authors. Published by Elsevier B.V. Recast Management (GCMM-2018). Peer-review under responsibility of the scientific committee of the Manufacturing Engineering Society International Conference Keywords: Electrochemical; Machining; Laser; Titanium; Recast 2017. a

a,*

b

b

Keywords: Cost Models; ABC; TDABC; Capacity Management; Idle Capacity; Operational Efficiency

* Corresponding author. Tel.: +66-2-4709194; fax: +66-2-8729081. address: [email protected] 1.E-mail Introduction * Corresponding author. Tel.: +66-2-4709194; fax: +66-2-8729081. 2351-9789 © 2018 The Authors. Published by Elsevier information Ltd. E-mail address: [email protected] The cost of idle capacity is a fundamental for companies and their management of extreme importance This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) in modern production systems. In general, it is defined as unused capacity or production potential and can be measured Selection peer-review under responsibility of the scientific 2351-9789and © 2018 The Authors. Published by Elsevier Ltd. committee of the 14th Global Congress on Manufacturing and Management in several ways: tons of production, available hours of manufacturing, etc. The management of the idle capacity (GCMM-2018). This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) * Paulo Afonso. Tel.: +351 253 510 761; fax: +351 253 604 741 Selection and peer-review under responsibility of the scientific committee of the 14th Global Congress on Manufacturing and Management E-mail address: [email protected] (GCMM-2018). 2351-9789 Published by Elsevier B.V. Ltd. 2351-9789 ©©2017 2019The TheAuthors. Authors. Published by Elsevier Peer-review underaccess responsibility of the scientific committee oflicense the Manufacturing Engineering Society International Conference 2017. This is an open article under the CC BY-NC-ND (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of the scientific committee of the 14th Global Congress on Manufacturing and Management (GCMM-2018). 10.1016/j.promfg.2019.02.078

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1. Introduction Lasers have been used in macro- to submicro-machining of difficult-to-cut materials, e.g. metal alloys, engineering ceramics and some composite materials, since lasers can provide high energy density with fine resolution to ablate materials precisely and rapidly. Photothermal mechanism basically takes place in the process where the laser-irradiated area is heated, melted and vaporized almost instantly to create a cut. While an amount of laser-irradiated material is vaporized during the ablation process, some material elements are still in the molten status before cooling down and solidifying as a solid structure called recast layer. Titanium alloy (Ti-6Al-4V), which is a famous metal alloy and has high strength-to-weight ratio, has numerously been used in medical, engineering and aerospace applications. Yang et al [1] noted that the recast of titanium alloy obtained after the laser ablation composes of microcrystal and amorphous structures. Since the recast is formed through the rapid solidification, thermal stress is usually developed in the recast where micro-cracks are potentially occurred in the surface and subsurface of the recast layer. This makes the recast fragile and plausibly causes high surface roughness in the lasercut area. These characteristics importantly limit the usability of the laser-cut workpiece in some applications due to the poor surface quality and possible contamination from the broken off recast particles. A post process is therefore needed to remove the recast from the laser-machined surface. A common technique used for the cleaning and polishing of metal surface is electrochemical-machining process. This method is only applicable for the electrically conductive materials as it uses electrical current to pass through the workpiece during the process in order to ionize and dissolve the surface of work material via the electrochemical reactions. The workpiece, which is given as an anode, is submerged in an electrolyte solution together with an electrode assigned as a cathode in the electrochemical cell. Although chloride and nitrate solutions are normally used as the electrolyte [2, 3], other solutions can be employed to induce the electrochemical reactions [4]. Urlea and Brailovski [5] used 1-M HClO4 as the electrolyte solution in the electropolishing of titanium alloy surface, while Xu et al [3] applied 0.5-M sulfuric acid for etching of titanium alloy. Adding 20% ethanol into the mixture of ethylene glycol and NaCl solution can also improve the surface quality of titanium after the electrochemical machining process [6]. The temperature of electrolyte is another factor affecting the electrochemical reactions. Yang et al [7] found that the increase in electrolyte temperature increases the electrical current so as the etching rate of titanium. According to the literatures reviewed above, many types of electrolyte with various degrees of concentration can be employed in the electrochemical machining of titanium alloy. In addition, the influence of electrical parameters and processing time on machining performance can be found in many studies of electrochemical machining/polishing of metals [2, 8, 9]. However, there is very little discussion on the removal of titanium recast caused by the laser ablation. To elaborate and provide a further insight into the recast removal, this paper therefore aims at removing the recast layer in the laser-machined titanium alloy surface by using the electrochemical machining process. The surface, subsurface and dimensions of laser-milled cavity in titanium alloy were investigated before and after the electrochemical machining process. The significance of this work could lead to the improvement of laser-machined surface through the recast removal. In addition, the understanding of recast removal mechanism and the evolution of the laser-ablated surface under the electrochemical dissolution could further advantage the process control and optimization. 2. Materials and methods Titanium alloy (Ti-6Al-4V) was used as a work sample in this study, where laser-milling process was performed to create a cavity in the metal. A nanosecond pulse laser emitting the wavelength of 1064 nm, pulse duration of 120 ns and pulse repetition rate of 30 kHz was employed in the milling process. A focused laser beam having the diameter of 20.3 μm was scanned over the work surface with the area of 1×3 mm and scan speed of 2 mm/s. The setup of laser milling process and milling path used in this study are shown in Fig. 1, noting that the laser scan overlap is kept constant at 50% of the beam diameter. The average laser powers of 25 and 30 W were employed in the milling experiment, and the obtained width, length, depth, taper angle and surface roughness of laser-milled cavity were observed and measured by using a 3D confocal laser microscope (Olympus LEXT OLS4000, Japan) and a scanning electron microscope (SEM). The recast layer and heat-affected zone formed at the surface and subsurface of laser-ablated area were also assessed in the cross-section view of cut samples. The cross section plane was normal

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to the laser scanning direction, where the examined surface was ground and polished by emery papers and polishing pad with alumina slurry before being etched by a chemical agent of 10% nitric acid, 5% hydro fluoric acid and 85% distilled water for 20 seconds. All laser-ablated samples were machined by the electrochemical process with an attempt to remove the recast layer as well as spatters deposited on and nearby the cavity surface. The workpiece was connected to an anode in the electrochemical cell and submerged in an electrolyte solution composed of 97% concentrated sulfuric acid and distilled water with a mixing ratio of 1 to 2 by volume. A 4×4 mm stainless steel wire mesh having the mesh size of 16 was used as an electrode and placed parallel to the workpiece surface with a gap distance of 20 mm. A constant DC voltage of 27 V was applied to the workpiece and electrode as shown in Fig. 2. The polished surface, subsurface and dimensions of cavities were later observed by the microscope at the processing time of 360 and 5,400 seconds, and the results were also compared to those obtained before the electrochemical machining process. Laser

Length

Workpiece

Width

50% scan overlap

Y axis

Laser beam diameter X axis (a)

(b) Fig. 1. Schematics of (a) laser milling process and (b) milling path used in this study.

Workpiece (Anode)

Stainless steel wire mesh (Cathode)

Laser-machined cavity Electrolyte Fig. 2. Setup of electrochemical machining process.

3. Results and discussion Surface morphologies of titanium alloy ablated by the average laser powers of 25 and 30 W are shown in Fig. 3 and 4, respectively. The dimensions and average surface roughness of cavities obtained before and after electrochemical machining process is listed in Table 1. By comparing the width and length of cavities formed before and after machining process, the results were found to be insignificantly different. However, the cavity depth remarkably increased after the etchining process and further deepened with the increased etchining time. Two possible reasons could lead to this feature. Firstly, the peak of uneven recast surface has a higher electrical potential than the other regions, and this typically causes more vigorous electrochemical reaction at the recast and then results in a more material removed. Secondly, the structure of recast layer has less density than the base metal since it composes of cracks and holes in the layer. These characteristics allow electrolyte to flow into the recast for dissolving the metal and also flushing the debris of recast elements away from the cavity, thus promoting the cut

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depth. Since the recast was greatly removed and the cavity was deeper after the electrochemical machining process, the taper angle then became smaller and a steeper cut profile was attained accordingly. Regarding Fig. 3 and 4, the recast was significantly removed from the cavity after the electrochemical machining process and a more uniform cut profile was also achievable. It can be noticed that the milling process induced by 30W laser power (Fig. 4) resulted in a great amount of recast and large deviation of cavity surface after the ablation. Regarding Fig. 4(a), the laser-milling path was started on the right side of the cavity and moved towards the left side with the scan overlap of 50% of the laser beam diameter. The longer the workpiece is irradiated by a laser beam, the more amount of heat energy is accumulated in the work material. This importantly enlarges the heat-affected zone, increases the amount of molten material and deepens the cavity depth. When the laser beam approaches the left side of the cavity or moves along the last milling track, a significant amount of molten material are formed and some of them are ejected from the molten pool with the aid of recoil pressure. According to the experiments, these molten elements were gradually piled up on the vicinity of the milling track and subsequently solidified to become a bulge of recast on the right side of the cavity as shown in Fig. 4(a). First milling track

Last milling track

(a)

(b)

(c) Fig. 3. Cavity profiles formed by 25-W laser milling process: (a) before and after electrochemical machining process for (b) 360 and (c) 5,400 seconds. Table 1. Dimensions and surface roughness of cavities obtained before and after electrochemical machining process. Average laser power = 25 W Width Length Depth (mm) (mm) (mm) Before machining After machining for 360 s After machining for 5400 s

Ra (μm)

Average laser power = 30 W Width Length Depth (mm) (mm) (mm)

6.39

1.01

2.99

1.11

3.00

0.273

Taper angle (deg) 33.96

Ra (μm)

0.149

Taper angle (deg) 85.6

1.12

3.00

0.325

31.72

8.55

0.99

2.98

0.384

28.89

11.64

1.12

3.02

0.359

28.47

8.95

0.99

2.99

0.582

9.34

11.43

11.94

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Although the recast layer was remarkably removed from the cavities, the average surface roughness (Ra) of cavities was insignificantly improved after the machining process as noted in Table 1. It is anticipated that the chemical reactions of sulfuric acid play a crucial role in etching the metal surface underneath the recast layer [10]. Micro-cracks and other surface imperfections caused by the laser ablation process were reproduced and became more apparent with the increased etching time as shown in Table 2 and 3. According to the micrographs shown in Table 2, some recasts are still deposited in the cavity after the etching time of 360 seconds. By considering the morphology of recast, it could be noted that the recast layer can conceal the uneven surface of laser-ablated area to some extent. This might be a reason of the low roughness value of the non-etching condition for the 25-W case (Table 1). When the recast was removed through the electrochemical dissolution, the actual cavity surface was then exposed. According to the observation, it is not unreasonable to note that the cavity surface induced by laser was not substantially smoother than the recast, so that the surface roughness values obtained before and after the electrochemical machining process were not significantly different. Although the increase in etching time can clean off the recast layer from the cavity, the chemical reaction of acidic electrolyte further etched the micro cracks making them larger and more discernable than the short etching time. Last milling track

First milling track

(a)

(b)

(c) Fig. 4. Cavity profiles formed by 30-W laser milling process: (a) before and after electrochemical machining process for (b) 360 and (c) 5,400 seconds.

By considering the morphology of cavity induced by 30-W laser power (Table 3), the cavity surface was full of micro holes after the electrochemical machining process. This feature could be relevant to the high heat input conducting toward the metal. Some alloying elements as well as impurities having low boiling temperature likely vaporized from the laser-irradiated surface, thus forming micro-holes in work surface. Nevertheless, these microholes were filled or covered by the molten elements during the laser ablation that was later solidified as the recast layer. Regarding the results, the micro-holes can be evidenced after the electrochemical machining process and the holes became larger with the increased etching time. This finding is similar to the study of Baek et al [11] in which the micro-holes are expanded with the etching time and this feature also affects the tensile strength of work material. The chemical reaction induced by sulfuric acid in the electrolyte solution was expected to enlarge the micro holes in the cavity surface and the micro cracks along the cavity edges through the pitting corrosion.

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Table 2. Surface morphology of titanium alloy obtained before and after the electrochemical machining process when the laser power of 25 W was used. Before electrochemical machining Overall

After electrochemical machining for 360 s

After electrochemical machining for 5,400 s

Edge region

Recast

Recast Cracks

Center region

Recast

Table 3. Surface morphology of titanium alloy obtained before and after the electrochemical machining process when the laser power of 30 W was used. Before electrochemical machining Overall

After electrochemical machining for 360 s

After electrochemical machining for 5,400 s

Edge region

Recast Cracks Center region

Micro-holes

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The laser-milled cavities obtained before and after the electrochemical machining process were cross-sectionally cut to observe their subsurface characteristics as shown in Fig. 5. With regard to the results, the average thickness of recast layers formed by the milling process under the laser power of 25 and 30 W was 56 and 109 μm, respectively. Even though most recast was removed from the cavity via the electrochemical dissolution after 5,400 seconds, the heat-affected zone (HAZ) was still apparent in the subsurface of cavity. Fine acicular martensite structure and fine grain size were found in the HAZ. After the electrochemical machining process, the width of HAZ was reduced from 60 and 64 μm to 52 and 33 μm for 25 and 30 W laser milling conditions.

Recast

HAZ

Recast

HAZ

50 μm

50 μm

(a)

(b)

Recast HAZ HAZ 50 μm (c)

50 μm (d)

Fig. 5. Subsurface of laser-milled cavities obtained (a,c) before and (b,d) after electrochemical machining process for 5,400 seconds when the laser power of (a,b) 25 and (c,d) 30 W was used.

4. Conclusions Electrochemical machining process was employed to remove the recast layer of titanium alloy (Ti-6Al-4V) induced by the laser milling process. The effects of laser power and electrochemical processing time on the surface and subsurface features of the laser-milled cavity in titanium alloy were examined in this study. The findings and implication of this work can be concluded as follows:  The recast layer can be removed by the electrochemical machining process. The cavity depth was found to significantly increase with the processing time, while the width and length of cavity were marginally changed. In addition, a more uniform cavity profile was achievable after the recast removal.  Using higher laser power resulted in a greater amount of recast as well as larger deviation of cavity profile. Although most recast can be dissolved and washed away from the cavity surface after 5,400 seconds, the average surface roughness of cavities was not remarkably improved. The micro-holes and cracks were more

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apparent in the laser-milled cavity after the electrochemical dissolution particularly when higher laser power was used. These negative features could plausibly be minimized by optimizing the electrolyte concentration and processing time in order to prevent the over etching of work surface. Besides the reduction of recast layer, the width of heat-affected zone was found to decrease by about 21% after the electrochemical machining process. The implication of this study could provide an essential guideline for the recast removal induced by the electrochemical machining process. The manufacturing of metal parts that require deburring and surface finishing after the laser machining process will be benefited from this work. Further investigations are, however, needed to enable the surface polishing all together with the recast removal for improving the performance of this surface preparation process.

Acknowledgements This research was financially supported by the Thailand Research Fund and the Office of the Higher Education Commission, Ministry of Education, Thailand (Grant No. MRG6080010 and MRG5180330). References [1] C. Yang, X. Mei, W. Wang, K. Wang, G. Jiang, Recast layer removal using ultrafast laser in titanium alloy, The International Journal of Advanced Manufacturing Technology, 68 (2013) 2321-2327. [2] G.Q. Wang, D. Zhu, H.S. Li, Fabrication of semi-circular micro-groove on titanium alloy surface by through-mask electrochemical micromachining, Journal of Materials Processing Technology, 258 (2018) 22-28. [3] Z. Xu, X. Chen, Z. Zhou, P. Qin, D. Zhu, Electrochemical Machining of High-temperature Titanium Alloy Ti60, Procedia CIRP, 42 (2016) 125-130. [4] S.S. Anasane, B. Bhattacharyya, Experimental investigation on suitability of electrolytes for electrochemical micromachining of titanium, The International Journal of Advanced Manufacturing Technology, 86 (2016) 2147-2160. [5] V. Urlea, V. Brailovski, Electropolishing and electropolishing-related allowances for powder bed selectively laser-melted Ti-6Al-4V alloy components, Journal of Materials Processing Technology, 242 (2017) 1-11. [6] D. Kim, K. Son, D. Sung, Y. Kim, W. Chung, Effect of added ethanol in ethylene glycol–NaCl electrolyte on titanium electropolishing, Corrosion Science, 98 (2015) 494-499. [7] L. Yang, A. Lassell, G.P.V. Paiva, Further study of the electropolishing of Ti6Al4V parts made via electron beam melting, in: Solid Freeform Fabrication Symposium, USA, 2015, pp. 1730-1737. [8] Z. Xu, J. Liu, D. Zhu, N. Qu, X. Wu, X. Chen, Electrochemical machining of burn-resistant Ti40 alloy, Chinese Journal of Aeronautics, 28 (2015) 1263-1272. [9] Z. Baicheng, L. Xiaohua, B. Jiaming, G. Junfeng, W. Pan, S. Chen-nan, N. Muiling, Q. Guojun, W. Jun, Study of selective laser melting (SLM) Inconel 718 part surface improvement by electrochemical polishing, Materials & Design, 116 (2017) 531-537. [10] B.D. Craig, D.S. Anderson, A. International, Handbook of Corrosion Data, ASM International, 1994. [11] S.M. Baek, A.V. Polyakov, J.H. Moon, I.P. Semenova, R.Z. Valiev, H.S. Kim, Effect of surface etching on the tensile behavior of coarseand ultrafine-grained pure titanium, Materials Science and Engineering: A, 707 (2017) 337-343.