- Email: [email protected]
Study of effect of process parameters on titanium sheet metal bending using Nd: YAG laser
Optics & Laser Technology 47 (2013) 242–247
Contents lists available at SciVerse ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Study of effect of process parameters on titanium sheet metal bending using Nd: YAG laser D.P. Shidid a,n, M. Hoseinpour Gollo b, M. Brandt a, M. Mahdavian a a b
School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Bundoora East Campus, PO Box 71, Victoria 3083, Australia Department of Mechanical Engineering, Shahid Rajaee Teacher Training University (SRTTU), Lavizan, Postal Code 16788-15811, Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 May 2011 Received in revised form 26 July 2012 Accepted 30 July 2012 Available online 12 October 2012
Laser Bending is a new non-contact technique of forming sheet-metal components. This method has been extended to bend Titanium components in industry. In Titanium laser bending process serious issues such as oxide film formation and subsequent deleterious changes in material properties are encountered. This paper investigates methods to minimize oxidation with minimal change in bending results for Grade-2 Titanium. Inert gas shielding is used as a means of reducing oxidation. Different gas flow conditions, nozzle positions and inert gas combinations are used to enhance the bend quality and bend angle. These process changes lead to final bending angle increase and decrease in width of Heat Affected Zone (HAZ), section thickness of sheet at HAZ and surface hardness. Also as Grade-2 Titanium is highly reflective, different coatings have been used to improve the absorption of laser beam which resulted in further increase in bending angle. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Laser bending Oxide film Inert gas shield
1. Introduction Titanium is one of the most desirable materials for aerospace, chemical and medical implant fabrication industries because of its properties such as good corrosion resistance, high strength to weight ratio and stability of mechanical and chemical properties at high temperatures. However, initial cost of material and difficult production methods can be major setbacks for extensive utilization of Titanium in industry. Statistics [1] shows that about 90% of Titanium intended to be used in final component is rejected throughout various stages of product development. This includes rejection during fabrication of titanium parts. This may be a result of excess oxidation and distortion caused during welding of work pieces [2] or due to shape imperfections produced with use of impact force in mechanical forming operations in aerospace industry. In order to utilize Titanium in a better way, further research needs to be done to repair and reuse these rejected components. Laser bending is a new non-contact metal forming technique which can be used to avoid or correct these welding distortions. Laser bending is a thermo-mechanical process wherein metal sheets are distorted by a moving, concentrated Laser beam [3]. This process is very easy to control and output can be precisely predicted [4,5]. Along with aerospace n
Corresponding author. Tel.: þ61 423 479 894. E-mail addresses: [email protected] (D.P. Shidid), [email protected] (M.H. Gollo), [email protected] (M. Brandt), [email protected] (M. Mahdavian). 0030-3992/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.optlastec.2012.07.033
industry, laser bending is becoming increasingly popular as a tool for micro scale deformation applications such as micro-forming/ micro adjustment of electronic parts [6] and in-situ distortion repair of turbine blades [7] made of Titanium alloys. Several attempts [7–10] have been made to study behavior of Titanium alloy sheets subjected to laser bending. As thermal conductivity of titanium is significantly low compared to other common high strength materials, its response to Laser treatment is sluggish. Furthermore, Titanium at high temperature (200 1C onwards) reacts with oxygen, nitrogen, carbon and hydrogen present in the surroundings [11] and a surface layer of Titanium Oxide (usually called a-case) is formed. As reported by Li [11], this layer is brittle and hard and also promotes crack propagation. At high temperature levels, oxidation also causes discoloration of surface that leads to change in absorption of laser beam. This makes prediction of process output difficult [9] and it can be a costly and hazardous practice to remove a-case in between laser scans by using mechanical and chemical milling processes. Hence common approach is to avoid a-case formation rather than removing it. To avoid a-case formation, laser bending can be performed in a vacuum chamber, similar to approach taken by Bartkowiak et al. [9] and El Refaey et al. [12] for welding/brazing of titanium components. Major limitation of vacuum chamber processing is that it is not suitable for processing of larger components. Another method used to avoid oxidation as reported by Bartkowiak et al. [9] is inert gas shielding where surface is protected by a jet of inert gas (commonly Argon and/or Helium
D.P. Shidid et al. / Optics & Laser Technology 47 (2013) 242–247
for Titanium). This work shows that shielding with gas nozzles is more effective in avoiding oxidation than processing in a controlled environment. However for that study, only one co-axial nozzle was used to shield top surface with Argon gas. Usually sheets bent using Laser bending process are less than 10 mm thick, hence even with materials having low thermal conductivity, temperature at bottom surface reaches well above 300 1C. This means bottom surface also needs to be shielded from reactive gases. Effect of top and bottom inert gas shields on bending angle has not been studied in any of the previous research. Hence purpose of this research work is to study the effects of shielding top and bottom surfaces on bending angle as well as on bend quality. It was found during this research that apart from shielding the HAZ, inert gas jet also cools the region. Due to simultaneous heating and cooling, temperature gradient was reduced which resulted in reduction in bending angle. This prompted changes in shielding configuration to maximize temperature gradient. Cooling the sheet metal during the bending process also increases surface hardness and reduces heat affected zone [9,13]. Li et al. [11] has suggested that for maintaining an acceptable level of surface hardness, maximum level of oxygen contamination should be less than 2%. Hence, any excess increase in hardness due to shielding needs to be controlled. Aim of this paper is to control increase in hardness and shield the titanium workpiece effectively to improve performance of Laser bending of Titanium sheets and achieve acceptable bend quality. This paper offers a novel coherent picture of the key influencing factors dominating bend quality of Titanium sheets, which has not been presented before.
2. Material and methods A 550 W multimode Nd:YAG laser system was used for experiments. The wavelength of the laser beam was 1064 nm. The continuous wave (CW) laser beam was delivered to the surface of a workpiece through an optic fiber with a core of 800 mm. The setup was designed in such a way that the laser head was mounted vertically in the Z-direction, while the workpiece could move freely in the horizontal (XY) plane. The distance from the laser head to the workpiece is adjustable and is used to change beam diameter. The table of a CNC milling machine with dimensions 1600 5100 mm2 which was enclosed inside a cabin, as shown in Fig. 1, was used for
243
Table 1 Table of physical and mechanical properties of CP Grade 2 Titanium. Property
Value
Yield strength (MPa) Young’s modulus (GPa) Hardness (as relieved) HRB Density (Kg/m3) Specific heat capacity (J/K) Thermal conductivity (W/mK) Coefficient of expansion (linear)
114 116 72 4507 523 12.1 0.0000085
Table 2 (a) Material parameters Material Length Width Thickness
Commercially pure Grade-2 Titanium 100 mm 70 mm 1.2 mm
(b) Laser parameters Laser type Laser power Beam diameter Scan speed
Nd:YAG 1064 nm 550 W CW Laser 250 W 1.2 mm 3 mm/s
placing the jig and workpiece. The metal plate was mounted in cantilever configuration. The fumes and shield gas were extracted by an exhaust fan that was mounted on the top of the cabin. To facilitate the setting up and monitoring of the operation, a CCD video camera and a light source were installed inside the cabin. These were also used to align the laser beam and monitor the bending process. Real-time images were displayed on the TV screen outside the enclosed cabin. Material used for this research is Commercially Pure Grade 2 Titanium sheet. Physical and mechanical properties according to ASTM B348-99 [14] are summarized in Table 1. For each processing condition experiment, five samples were tested and average values of five samples were plotted against corresponding process parameter for analysis. Bending angle was measured with help of a Laser displacement measuring machine. Rockwell B scale was used to measure surface hardness of sheets. Samples were coated with graphite and thermal paint to improve absorption of laser beam. The properties of material and laser used for experimental study are shown in the following Table 2a and b.
3. Results and discussion 3.1. Coating
Fig. 1. Nd:YAG Laser set-up.
Amount of laser radiation absorbed into a material depends on absorption co-efficient of that material and it can vary due to parameters such as temperature of the workpiece and surface roughness. CP Grade-2 Titanium used in this research is a very reflective material and can only absorb around 20% of incident laser energy (Nd:YAG laser 1064 nm). Low laser energy absorption rates combined with low thermal conductivity make titanium respond sluggishly to laser treatment as compared to other common high strength materials. The comparison of laser bending results for titanium, low carbon steel and stainless steel is shown in Fig. 2. The bending angle increment changes linearly with the scan rate. Titanium demonstrates less increase in bending angle in comparison with other metals. This is due to
244
D.P. Shidid et al. / Optics & Laser Technology 47 (2013) 242–247
high strength, low laser energy absorption coefficient and low thermal conductivity of titanium. Methods used to improve absorption co-efficient of a material usually include processing at higher temperatures, coating workpieces with absorptive coatings and roughening the surface. To avoid any property change in the substrate while improving absorption, use of coatings such as graphite, oxides, phosphates and paints is a common practice in Laser treatment of metals. Depending on substrate, absorption can be improved by 60–80% [15]. Migliore [16] suggested that Graphite is most ideal as it is easy to apply and remove. For this research Graphite 33 seems an ideal choice for coating Titanium sheets. However for laser bending, it is stated in previous work that graphite coating is unreliable and should be avoided as they degrade after few scans and is messy [9,17]. Meanwhile, previous research also suggests that rate of increase of bending angle per pass is significantly higher at the beginning of the process and with the help of absorptive coatings this rate can be further increased. To verify these two hypotheses, Graphite and thermal black paint were applied to titanium sheets as coatings. Graphite powder was dissolved in industrial grade Turpentine and sprayed on sheets until thick, uniform coating was achieved. Aerosol Thermal paint with 500 1C allowable temperature was sprayed uniformly on the sheets. Mean values of five specimens for bending angles are plotted against corresponding pass number. Fig. 3 shows the effect of coatings on bending angle in a multi-pass system. For graphite coated workpieces, a significant increase was observed in bending angle at first scan and angle continued to increase after five passes without any degradation in rate of bending. Possibility of shock ablation is also minimized with
Continuous Wave processing. For thermal paint a high initial increase in absorption was observed, however, as surface temperature of workpiece reached at more than 500 1C, paint burnt off at the first scan and a white layer of burnt chemicals was formed on the surface. This reduced absorption of laser beam. Bending angle per pass, as a result, degraded after first scan. Hence thermal paint does not seem ideal for multi-pass system and would be more suitable for single scan processes such as laser cutting or laser marking operations. 3.2. Inert gas shielding Common gases used in industry for shielding purposes are argon, helium, nitrogen, oxygen and combination of argonþhelium.
Fig. 4. Shielding arrangement.
Fig. 2. Comparison of common materials with Titanium subjected to laser forming.
Fig. 3. Effect of coatings on bending angle.
D.P. Shidid et al. / Optics & Laser Technology 47 (2013) 242–247
245
Fig. 5. Effect of shield gas on bending angle. Gas Velocity at nozzle tip for (a) Air ¼ 15 m/s, (b) Argon ¼ 13.85 m/s, (c) Helium ¼ 28.82 m/s, Outlet temperature for all gases assumed ¼ 201.
Fig. 6. Oxidation in (a) Air, (b) Argon, (c) Argon þ Helium atmospheres.
However, oxygen and nitrogen react with titanium at temperatures higher than 200 1C [9]. Hence, argon and helium are chosen to be shielding gases in this research. Argon and helium gases are inert in nature and do not react with titanium, even at high temperatures. Experiments were performed in compressed air, argon, argonþ helium shield and in argon chamber. The shielding arrangement was as shown in Fig. 4. Different angles ranging from 101 to 451 to were tested for maximum shielding area and for final readings angle for both shields was maintained at 151. Results achieved are discussed in the following sections
3.2.1. Effect of shielding on bending angle Fig. 5 shows the effect of different inert gas configurations on bending angle. Use of Argon as a top surface shield gives more bending angle because of reduction in oxidation and subsequent retention of material ductility. However, it was suspected that impinging top surface with Argon resulted in cooling the top surface and reduction in temperature gradient.
Fig. 7. Side view of the sheet showing increase in section thickness due to oxidation for (a) Air and (b) Argon shield.
Due to thin sheets and to compensate for reduction in temperature gradient, a bottom shield was used to shield the bottom surface. Argon and Helium are preferred choices for processing titanium as nitrogen and carbon dioxide will react with the substrate at high temperatures. Combination of argon as the top surface shield gas and helium as the bottom surface shield (or secondary shield) gas shows maximum bending angle. This is because helium has high thermal conductivity than argon, hence more heat dissipation. Larger the temperature difference between
246
D.P. Shidid et al. / Optics & Laser Technology 47 (2013) 242–247
Fig. 8 Effect of Gas flow rate on surface hardness.
top and bottom surfaces more is the bending angle. Processing time for similar parameters is also lowered due to the cooling effect. Ideally, the bending angle should increase linearly as the number of scans is increased. However, it is apparent from Fig. 5 that the bending angle varies linearly for first few scans and then increases abruptly. This phenomenon is attributed to oxidation of the workpieces. As the workpiece is scanned, multiple times the top surface blackens due to oxidation. The blackening of the surface improves absorption of the beam, hence, increasing the bending angle abruptly. For multi-scan process in this experiment, a pause of 20 s is maintained between each scan to allow the workpiece to cool off. However, as the thermal conductivity of titanium is low, a pause of 20 s is not sufficient to cool the workpiece to the room temperature. Higher initial temperature of the workpiece further increases the absorption coefficient of the workpiece thus increasing the bending angle as the number of scans progress.
3.2.2. Effect on oxidation and HAZ dimensions Fig. 6 shows oxidation on the HAZ. It is observed that reduction in oxidation is significant with two shields to cover both surfaces. This shows the effectiveness of using two shields combination to reduce oxidation on the surface. Inert gas shielding also cools the HAZ effectively resulting in reduced width of the HAZ. It can be seen from Fig. 6 that the width of the HAZ reduces with use of two gases as shields compared to single shields. Section thickness is an important factor influencing angle per pass [17]. As compared to other common high strength materials such as carbon steel and stainless steel, section thickening in titanium is more significant due to the formation of brittle oxide layers. This contributes to reduction in bending angle. As shielding reduces oxidation, it was observed [Fig. 7] that two shield method reduces the section thickening approximately by 40%.
3.2.3. Effect on surface hardness As mentioned above, cooling effect due to shielding of irradiated zone also increases surface hardness of the material. To study this effect Rockwell hardness (scale B) tests were performed for single laser pass with different gas flow rates. Results in Fig. 8 show that hardness increases linearly with increase in gas flow rate. Increased hardness may contribute to decrease in final bending angle obtained over multiple passes, hence, is compared to bending angle values for corresponding gas flow rates.
The bending angle decreases rapidly for gas flow rate of 15– 20 lpm and later stabilizes. For optimal shielding efficiency and hardness increase, gas flow rate of 25 lpm is suggested for above mentioned process parameters.
4. Conclusions Effects of inert gas shielding on laser bending of titanium sheets were studied in this research from different practical point of views and the following conclusions are drawn 1. Coating has significant effect on bending angle for first pass which is important in multi-scan system as bending angle per pass degrades as number of passes is increased. Therefore use of graphite coatings is recommended for improved absorption of the laser beam. 2. Argon and helium gas shielding for top and bottom surfaces respectively has significant effect on bending angle as temperature gradient is increased. 3. Rapid heating due to concentrated laser irradiation and cooling due to inert gas shielding increases the hardness of the titanium plate. This hardening coupled with excessive oxidation makes the plate more brittle by reducing the ductility and this contributes to reduction in final bending angle. However, this effect is overshadowed by reduction in oxidation, HAZ width, section thickness and overall better quality of the bend. This study provides an insight on the possible method of controlling distortion during welding process.
References [1] Barnes J, Brandt M, Sun S, Scherrer M. Laser assisted machining of titanium – LMCut benefits – An OEM perspective. In Fourth Australian JSF advanced technology and innovation conference, Melbourne, 2010. [2] Sheng Liu, Mahdavian M, Aswin D, Ding S. Experimental study of temperature and clamping force during Nd:YAG laser butt welding. Journal of Optics and Laser Technology 2009;41:665–852. [3] Geiger M, Vollertsen F. The mechanisms of laser forming. CIRP Annals – Manufacturing Technology 1993;42:301–4. [4] Hosseinpour Gollo M, Moslemi Naeini H, Liaghat GH, Jelvani S, Torkamany MJ. A numerical and experimental study of sheet metal bending by pulsed Nd:YAG laser with DOE method. Advanced Materials Research 2010; 83–86:1076–83. [5] Hosseinpour Gollo M, Moslemi Naeini H, Liaghat GH, Torkamany MJ, Jelvani S, Panahizade V. An experimental study of sheet metal bending by pulsed Nd:YAG laser with DOE method. International Journal of Material Forming 2008;1:137–40. [6] Tam AC, Laser processes for precise microfabrication of magnetic disk-drive components, RIKEN Review No. 32 (January, 2001): Focused on Laser Precision Microfabrication (LPM2000), 2001.
D.P. Shidid et al. / Optics & Laser Technology 47 (2013) 242–247
[7] Marya M, Edwards GR. A study on the laser forming of near-alpha and metastable beta titanium alloy sheets. Journal of Materials Processing Technology 2000;108:376–83 2001. [8] Walczyk DF, Vittal S. Bending of titanium sheet using laser forming. Journal of Manufacturing Processes 2000;2:258. [9] Bartkowiak K, Edwardson SP, Dearden G, Watkins KG. 2-D laser forming comparative study on Nd:YAG of titanium alloy Ti-6Al-4V In 23rd international congress on applications of lasers and electro-optics (ICALEO 2004) San Franscisco, vol. 97, October 4–7 2004. [10] Marya M, Edwards GR. An analytical model for the optimization of the laser bending of titanium Ti-6Al-2Sn-4Zr-2Mo. Journal of Materials Processing Technology 2002;124:337–44. [11] LI X, Xie J, Zhou Y. Effects of oxygen contamination in the argon shielding gas in laser welding of commercially pure titanium thin sheet. Journal of Materials Science 2005;40:3437–43.
247
[12] El Refaey A, Tillmann W. Characterization of titanium/steel joints brazed in vacuum. Supplement to the Welding Journal (AWS and Welding Research Council) May 2008:113s–8s. [13] Cheng J, Yao Lawrence Y. Cooling effects in Multiscan Laser Forming. Journal of Manufacturing Processes 2001;3:60–72. [14] ASTM International, Standard Specification for Titanium and Titanium Alloy Bars and Billets, in B348-11, ed. West Conshohocken, PA, 2011. [15] Edwardson SP, Abed E, Carey C, Edwards KR, Bartkowiak K, Dearden G, et al,. Key factors influencing the bend per pass in Laser forming. In: laser materials processing conference, 2007, p. 506. [16] Migliore L. Heat treating with Lasers. Advanced Materials & Processes 1998 August 1998. [17] Edwardson SP, Griffiths J, Edwards KR, Dearden G, Watkins KG. Laser forming: overview of the controlling factors in the temperature gradient mechanism. In: Proceedings of the Institution of Mechanical Engineers, 224, 1031.
Contents lists available at SciVerse ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Study of effect of process parameters on titanium sheet metal bending using Nd: YAG laser D.P. Shidid a,n, M. Hoseinpour Gollo b, M. Brandt a, M. Mahdavian a a b
School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, Bundoora East Campus, PO Box 71, Victoria 3083, Australia Department of Mechanical Engineering, Shahid Rajaee Teacher Training University (SRTTU), Lavizan, Postal Code 16788-15811, Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 May 2011 Received in revised form 26 July 2012 Accepted 30 July 2012 Available online 12 October 2012
Laser Bending is a new non-contact technique of forming sheet-metal components. This method has been extended to bend Titanium components in industry. In Titanium laser bending process serious issues such as oxide film formation and subsequent deleterious changes in material properties are encountered. This paper investigates methods to minimize oxidation with minimal change in bending results for Grade-2 Titanium. Inert gas shielding is used as a means of reducing oxidation. Different gas flow conditions, nozzle positions and inert gas combinations are used to enhance the bend quality and bend angle. These process changes lead to final bending angle increase and decrease in width of Heat Affected Zone (HAZ), section thickness of sheet at HAZ and surface hardness. Also as Grade-2 Titanium is highly reflective, different coatings have been used to improve the absorption of laser beam which resulted in further increase in bending angle. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Laser bending Oxide film Inert gas shield
1. Introduction Titanium is one of the most desirable materials for aerospace, chemical and medical implant fabrication industries because of its properties such as good corrosion resistance, high strength to weight ratio and stability of mechanical and chemical properties at high temperatures. However, initial cost of material and difficult production methods can be major setbacks for extensive utilization of Titanium in industry. Statistics [1] shows that about 90% of Titanium intended to be used in final component is rejected throughout various stages of product development. This includes rejection during fabrication of titanium parts. This may be a result of excess oxidation and distortion caused during welding of work pieces [2] or due to shape imperfections produced with use of impact force in mechanical forming operations in aerospace industry. In order to utilize Titanium in a better way, further research needs to be done to repair and reuse these rejected components. Laser bending is a new non-contact metal forming technique which can be used to avoid or correct these welding distortions. Laser bending is a thermo-mechanical process wherein metal sheets are distorted by a moving, concentrated Laser beam [3]. This process is very easy to control and output can be precisely predicted [4,5]. Along with aerospace n
Corresponding author. Tel.: þ61 423 479 894. E-mail addresses: [email protected] (D.P. Shidid), [email protected] (M.H. Gollo), [email protected] (M. Brandt), [email protected] (M. Mahdavian). 0030-3992/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.optlastec.2012.07.033
industry, laser bending is becoming increasingly popular as a tool for micro scale deformation applications such as micro-forming/ micro adjustment of electronic parts [6] and in-situ distortion repair of turbine blades [7] made of Titanium alloys. Several attempts [7–10] have been made to study behavior of Titanium alloy sheets subjected to laser bending. As thermal conductivity of titanium is significantly low compared to other common high strength materials, its response to Laser treatment is sluggish. Furthermore, Titanium at high temperature (200 1C onwards) reacts with oxygen, nitrogen, carbon and hydrogen present in the surroundings [11] and a surface layer of Titanium Oxide (usually called a-case) is formed. As reported by Li [11], this layer is brittle and hard and also promotes crack propagation. At high temperature levels, oxidation also causes discoloration of surface that leads to change in absorption of laser beam. This makes prediction of process output difficult [9] and it can be a costly and hazardous practice to remove a-case in between laser scans by using mechanical and chemical milling processes. Hence common approach is to avoid a-case formation rather than removing it. To avoid a-case formation, laser bending can be performed in a vacuum chamber, similar to approach taken by Bartkowiak et al. [9] and El Refaey et al. [12] for welding/brazing of titanium components. Major limitation of vacuum chamber processing is that it is not suitable for processing of larger components. Another method used to avoid oxidation as reported by Bartkowiak et al. [9] is inert gas shielding where surface is protected by a jet of inert gas (commonly Argon and/or Helium
D.P. Shidid et al. / Optics & Laser Technology 47 (2013) 242–247
for Titanium). This work shows that shielding with gas nozzles is more effective in avoiding oxidation than processing in a controlled environment. However for that study, only one co-axial nozzle was used to shield top surface with Argon gas. Usually sheets bent using Laser bending process are less than 10 mm thick, hence even with materials having low thermal conductivity, temperature at bottom surface reaches well above 300 1C. This means bottom surface also needs to be shielded from reactive gases. Effect of top and bottom inert gas shields on bending angle has not been studied in any of the previous research. Hence purpose of this research work is to study the effects of shielding top and bottom surfaces on bending angle as well as on bend quality. It was found during this research that apart from shielding the HAZ, inert gas jet also cools the region. Due to simultaneous heating and cooling, temperature gradient was reduced which resulted in reduction in bending angle. This prompted changes in shielding configuration to maximize temperature gradient. Cooling the sheet metal during the bending process also increases surface hardness and reduces heat affected zone [9,13]. Li et al. [11] has suggested that for maintaining an acceptable level of surface hardness, maximum level of oxygen contamination should be less than 2%. Hence, any excess increase in hardness due to shielding needs to be controlled. Aim of this paper is to control increase in hardness and shield the titanium workpiece effectively to improve performance of Laser bending of Titanium sheets and achieve acceptable bend quality. This paper offers a novel coherent picture of the key influencing factors dominating bend quality of Titanium sheets, which has not been presented before.
2. Material and methods A 550 W multimode Nd:YAG laser system was used for experiments. The wavelength of the laser beam was 1064 nm. The continuous wave (CW) laser beam was delivered to the surface of a workpiece through an optic fiber with a core of 800 mm. The setup was designed in such a way that the laser head was mounted vertically in the Z-direction, while the workpiece could move freely in the horizontal (XY) plane. The distance from the laser head to the workpiece is adjustable and is used to change beam diameter. The table of a CNC milling machine with dimensions 1600 5100 mm2 which was enclosed inside a cabin, as shown in Fig. 1, was used for
243
Table 1 Table of physical and mechanical properties of CP Grade 2 Titanium. Property
Value
Yield strength (MPa) Young’s modulus (GPa) Hardness (as relieved) HRB Density (Kg/m3) Specific heat capacity (J/K) Thermal conductivity (W/mK) Coefficient of expansion (linear)
114 116 72 4507 523 12.1 0.0000085
Table 2 (a) Material parameters Material Length Width Thickness
Commercially pure Grade-2 Titanium 100 mm 70 mm 1.2 mm
(b) Laser parameters Laser type Laser power Beam diameter Scan speed
Nd:YAG 1064 nm 550 W CW Laser 250 W 1.2 mm 3 mm/s
placing the jig and workpiece. The metal plate was mounted in cantilever configuration. The fumes and shield gas were extracted by an exhaust fan that was mounted on the top of the cabin. To facilitate the setting up and monitoring of the operation, a CCD video camera and a light source were installed inside the cabin. These were also used to align the laser beam and monitor the bending process. Real-time images were displayed on the TV screen outside the enclosed cabin. Material used for this research is Commercially Pure Grade 2 Titanium sheet. Physical and mechanical properties according to ASTM B348-99 [14] are summarized in Table 1. For each processing condition experiment, five samples were tested and average values of five samples were plotted against corresponding process parameter for analysis. Bending angle was measured with help of a Laser displacement measuring machine. Rockwell B scale was used to measure surface hardness of sheets. Samples were coated with graphite and thermal paint to improve absorption of laser beam. The properties of material and laser used for experimental study are shown in the following Table 2a and b.
3. Results and discussion 3.1. Coating
Fig. 1. Nd:YAG Laser set-up.
Amount of laser radiation absorbed into a material depends on absorption co-efficient of that material and it can vary due to parameters such as temperature of the workpiece and surface roughness. CP Grade-2 Titanium used in this research is a very reflective material and can only absorb around 20% of incident laser energy (Nd:YAG laser 1064 nm). Low laser energy absorption rates combined with low thermal conductivity make titanium respond sluggishly to laser treatment as compared to other common high strength materials. The comparison of laser bending results for titanium, low carbon steel and stainless steel is shown in Fig. 2. The bending angle increment changes linearly with the scan rate. Titanium demonstrates less increase in bending angle in comparison with other metals. This is due to
244
D.P. Shidid et al. / Optics & Laser Technology 47 (2013) 242–247
high strength, low laser energy absorption coefficient and low thermal conductivity of titanium. Methods used to improve absorption co-efficient of a material usually include processing at higher temperatures, coating workpieces with absorptive coatings and roughening the surface. To avoid any property change in the substrate while improving absorption, use of coatings such as graphite, oxides, phosphates and paints is a common practice in Laser treatment of metals. Depending on substrate, absorption can be improved by 60–80% [15]. Migliore [16] suggested that Graphite is most ideal as it is easy to apply and remove. For this research Graphite 33 seems an ideal choice for coating Titanium sheets. However for laser bending, it is stated in previous work that graphite coating is unreliable and should be avoided as they degrade after few scans and is messy [9,17]. Meanwhile, previous research also suggests that rate of increase of bending angle per pass is significantly higher at the beginning of the process and with the help of absorptive coatings this rate can be further increased. To verify these two hypotheses, Graphite and thermal black paint were applied to titanium sheets as coatings. Graphite powder was dissolved in industrial grade Turpentine and sprayed on sheets until thick, uniform coating was achieved. Aerosol Thermal paint with 500 1C allowable temperature was sprayed uniformly on the sheets. Mean values of five specimens for bending angles are plotted against corresponding pass number. Fig. 3 shows the effect of coatings on bending angle in a multi-pass system. For graphite coated workpieces, a significant increase was observed in bending angle at first scan and angle continued to increase after five passes without any degradation in rate of bending. Possibility of shock ablation is also minimized with
Continuous Wave processing. For thermal paint a high initial increase in absorption was observed, however, as surface temperature of workpiece reached at more than 500 1C, paint burnt off at the first scan and a white layer of burnt chemicals was formed on the surface. This reduced absorption of laser beam. Bending angle per pass, as a result, degraded after first scan. Hence thermal paint does not seem ideal for multi-pass system and would be more suitable for single scan processes such as laser cutting or laser marking operations. 3.2. Inert gas shielding Common gases used in industry for shielding purposes are argon, helium, nitrogen, oxygen and combination of argonþhelium.
Fig. 4. Shielding arrangement.
Fig. 2. Comparison of common materials with Titanium subjected to laser forming.
Fig. 3. Effect of coatings on bending angle.
D.P. Shidid et al. / Optics & Laser Technology 47 (2013) 242–247
245
Fig. 5. Effect of shield gas on bending angle. Gas Velocity at nozzle tip for (a) Air ¼ 15 m/s, (b) Argon ¼ 13.85 m/s, (c) Helium ¼ 28.82 m/s, Outlet temperature for all gases assumed ¼ 201.
Fig. 6. Oxidation in (a) Air, (b) Argon, (c) Argon þ Helium atmospheres.
However, oxygen and nitrogen react with titanium at temperatures higher than 200 1C [9]. Hence, argon and helium are chosen to be shielding gases in this research. Argon and helium gases are inert in nature and do not react with titanium, even at high temperatures. Experiments were performed in compressed air, argon, argonþ helium shield and in argon chamber. The shielding arrangement was as shown in Fig. 4. Different angles ranging from 101 to 451 to were tested for maximum shielding area and for final readings angle for both shields was maintained at 151. Results achieved are discussed in the following sections
3.2.1. Effect of shielding on bending angle Fig. 5 shows the effect of different inert gas configurations on bending angle. Use of Argon as a top surface shield gives more bending angle because of reduction in oxidation and subsequent retention of material ductility. However, it was suspected that impinging top surface with Argon resulted in cooling the top surface and reduction in temperature gradient.
Fig. 7. Side view of the sheet showing increase in section thickness due to oxidation for (a) Air and (b) Argon shield.
Due to thin sheets and to compensate for reduction in temperature gradient, a bottom shield was used to shield the bottom surface. Argon and Helium are preferred choices for processing titanium as nitrogen and carbon dioxide will react with the substrate at high temperatures. Combination of argon as the top surface shield gas and helium as the bottom surface shield (or secondary shield) gas shows maximum bending angle. This is because helium has high thermal conductivity than argon, hence more heat dissipation. Larger the temperature difference between
246
D.P. Shidid et al. / Optics & Laser Technology 47 (2013) 242–247
Fig. 8 Effect of Gas flow rate on surface hardness.
top and bottom surfaces more is the bending angle. Processing time for similar parameters is also lowered due to the cooling effect. Ideally, the bending angle should increase linearly as the number of scans is increased. However, it is apparent from Fig. 5 that the bending angle varies linearly for first few scans and then increases abruptly. This phenomenon is attributed to oxidation of the workpieces. As the workpiece is scanned, multiple times the top surface blackens due to oxidation. The blackening of the surface improves absorption of the beam, hence, increasing the bending angle abruptly. For multi-scan process in this experiment, a pause of 20 s is maintained between each scan to allow the workpiece to cool off. However, as the thermal conductivity of titanium is low, a pause of 20 s is not sufficient to cool the workpiece to the room temperature. Higher initial temperature of the workpiece further increases the absorption coefficient of the workpiece thus increasing the bending angle as the number of scans progress.
3.2.2. Effect on oxidation and HAZ dimensions Fig. 6 shows oxidation on the HAZ. It is observed that reduction in oxidation is significant with two shields to cover both surfaces. This shows the effectiveness of using two shields combination to reduce oxidation on the surface. Inert gas shielding also cools the HAZ effectively resulting in reduced width of the HAZ. It can be seen from Fig. 6 that the width of the HAZ reduces with use of two gases as shields compared to single shields. Section thickness is an important factor influencing angle per pass [17]. As compared to other common high strength materials such as carbon steel and stainless steel, section thickening in titanium is more significant due to the formation of brittle oxide layers. This contributes to reduction in bending angle. As shielding reduces oxidation, it was observed [Fig. 7] that two shield method reduces the section thickening approximately by 40%.
3.2.3. Effect on surface hardness As mentioned above, cooling effect due to shielding of irradiated zone also increases surface hardness of the material. To study this effect Rockwell hardness (scale B) tests were performed for single laser pass with different gas flow rates. Results in Fig. 8 show that hardness increases linearly with increase in gas flow rate. Increased hardness may contribute to decrease in final bending angle obtained over multiple passes, hence, is compared to bending angle values for corresponding gas flow rates.
The bending angle decreases rapidly for gas flow rate of 15– 20 lpm and later stabilizes. For optimal shielding efficiency and hardness increase, gas flow rate of 25 lpm is suggested for above mentioned process parameters.
4. Conclusions Effects of inert gas shielding on laser bending of titanium sheets were studied in this research from different practical point of views and the following conclusions are drawn 1. Coating has significant effect on bending angle for first pass which is important in multi-scan system as bending angle per pass degrades as number of passes is increased. Therefore use of graphite coatings is recommended for improved absorption of the laser beam. 2. Argon and helium gas shielding for top and bottom surfaces respectively has significant effect on bending angle as temperature gradient is increased. 3. Rapid heating due to concentrated laser irradiation and cooling due to inert gas shielding increases the hardness of the titanium plate. This hardening coupled with excessive oxidation makes the plate more brittle by reducing the ductility and this contributes to reduction in final bending angle. However, this effect is overshadowed by reduction in oxidation, HAZ width, section thickness and overall better quality of the bend. This study provides an insight on the possible method of controlling distortion during welding process.
References [1] Barnes J, Brandt M, Sun S, Scherrer M. Laser assisted machining of titanium – LMCut benefits – An OEM perspective. In Fourth Australian JSF advanced technology and innovation conference, Melbourne, 2010. [2] Sheng Liu, Mahdavian M, Aswin D, Ding S. Experimental study of temperature and clamping force during Nd:YAG laser butt welding. Journal of Optics and Laser Technology 2009;41:665–852. [3] Geiger M, Vollertsen F. The mechanisms of laser forming. CIRP Annals – Manufacturing Technology 1993;42:301–4. [4] Hosseinpour Gollo M, Moslemi Naeini H, Liaghat GH, Jelvani S, Torkamany MJ. A numerical and experimental study of sheet metal bending by pulsed Nd:YAG laser with DOE method. Advanced Materials Research 2010; 83–86:1076–83. [5] Hosseinpour Gollo M, Moslemi Naeini H, Liaghat GH, Torkamany MJ, Jelvani S, Panahizade V. An experimental study of sheet metal bending by pulsed Nd:YAG laser with DOE method. International Journal of Material Forming 2008;1:137–40. [6] Tam AC, Laser processes for precise microfabrication of magnetic disk-drive components, RIKEN Review No. 32 (January, 2001): Focused on Laser Precision Microfabrication (LPM2000), 2001.
D.P. Shidid et al. / Optics & Laser Technology 47 (2013) 242–247
[7] Marya M, Edwards GR. A study on the laser forming of near-alpha and metastable beta titanium alloy sheets. Journal of Materials Processing Technology 2000;108:376–83 2001. [8] Walczyk DF, Vittal S. Bending of titanium sheet using laser forming. Journal of Manufacturing Processes 2000;2:258. [9] Bartkowiak K, Edwardson SP, Dearden G, Watkins KG. 2-D laser forming comparative study on Nd:YAG of titanium alloy Ti-6Al-4V In 23rd international congress on applications of lasers and electro-optics (ICALEO 2004) San Franscisco, vol. 97, October 4–7 2004. [10] Marya M, Edwards GR. An analytical model for the optimization of the laser bending of titanium Ti-6Al-2Sn-4Zr-2Mo. Journal of Materials Processing Technology 2002;124:337–44. [11] LI X, Xie J, Zhou Y. Effects of oxygen contamination in the argon shielding gas in laser welding of commercially pure titanium thin sheet. Journal of Materials Science 2005;40:3437–43.
247
[12] El Refaey A, Tillmann W. Characterization of titanium/steel joints brazed in vacuum. Supplement to the Welding Journal (AWS and Welding Research Council) May 2008:113s–8s. [13] Cheng J, Yao Lawrence Y. Cooling effects in Multiscan Laser Forming. Journal of Manufacturing Processes 2001;3:60–72. [14] ASTM International, Standard Specification for Titanium and Titanium Alloy Bars and Billets, in B348-11, ed. West Conshohocken, PA, 2011. [15] Edwardson SP, Abed E, Carey C, Edwards KR, Bartkowiak K, Dearden G, et al,. Key factors influencing the bend per pass in Laser forming. In: laser materials processing conference, 2007, p. 506. [16] Migliore L. Heat treating with Lasers. Advanced Materials & Processes 1998 August 1998. [17] Edwardson SP, Griffiths J, Edwards KR, Dearden G, Watkins KG. Laser forming: overview of the controlling factors in the temperature gradient mechanism. In: Proceedings of the Institution of Mechanical Engineers, 224, 1031.