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Laser gas nitriding of pure titanium using CW and pulsed Nd:YAG lasers
Surface & Coatings Technology 201 (2006) 1635 – 1642 www.elsevier.com/locate/surfcoat
Laser gas nitriding of pure titanium using CW and pulsed Nd:YAG lasers Edson Costa Santos a , Masanori Morita a , Masanori Shiomi a,⁎, Kozo Osakada a , Masataka Takahashi b a
Department of Mechanical Science and Bioengineering, Graduate School of Engineering Science, Osaka University, Machikaneyama, Toyonaka, Osaka 560-8531, Japan b Inorganic Chemistry Department, Osaka Municipal Technical Research Institute, OMTRI, Morinomya, Joto-ku, Osaka, Japan Received 27 July 2005; accepted in revised form 21 February 2006 Available online 2 May 2006
Abstract The influence of parameters of continuous wave (CW) and pulsed Nd:YAG lasers on the laser gas nitriding (LGN) process of pure titanium models made by selective laser melting is presented. The morphology, microstructure, hardness and chemical composition of the nitrided layers are analysed. The layers formed by the Nd:YAG laser in CW and Q-sw modes show high roughness and porosity, respectively. The morphology of the layers formed by ms-pulsed laser varies from rough to smooth by increasing the peak power from 1 to 3 kW but cracking may occur. The thickness of the layers by using CW and pulsed lasers varies from 10 to 60 μm and the thickness of the layers in Q-sw mode varies from 1 to 2 μm. For pulsed laser experiments, cracking is avoided by using a stepwise pulse shape to decrease the cooling rate. X-ray photoelectron spectroscopy shows that the layers have high nitrogen concentration at the top surface and by grazing incidence X-ray diffraction (GIXD) it is possible to realize that the layer has a gradient TiN/TiN0.3 composition. The smooth and defect free TiN layers formed by ms-pulsed laser do not have any detrimental effect on the fatigue strength of the specimens. © 2006 Elsevier B.V. All rights reserved. Keywords: Laser gas nitriding; Titanium; TiN; Nd:YAG laser; CW; Pulsed
1. Introduction Pure titanium and titanium alloys (e.g. Ti–6Al–4V, Ti– 13Zr–13Nb) are common metals used for the fabrication of medical implants and prostheses. The features of high strength to weight ratio, high biocompatibility and high corrosion resistance are very suitable for the medical area [1]. Recently, selective laser melting (SLM) process, in which physical models can be fabricated directly from CAD data by using laser energy, has been developed and some titanium samples for medical implants have been introduced [2–6]. However, tribological properties such as wear resistance is low compared to other metals used for medical application. In manufacturing of practical models by SLM process for medical application, it is necessary to improve surface quality of the titanium model. Titanium has very high reactivity to interstitial elements (oxygen, nitrogen, carbon and hydrogen) and forms oxides, ⁎ Corresponding author. Tel.: +81 6 6850 6196; fax: +81 6 6850 6199. E-mail address: [email protected] (M. Shiomi). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.02.048
Fig. 1. Illustration of the apparatus used for laser gas nitriding by Q-sw and CW Nd:YAG laser. The average power is 50 W and the laser beam diameter at the focus is of 0.1 mm; in Q-sw mode the pulse length is of 200 ns.
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Fig. 4. The hatching or scanning pitch, the distance between adjacent lines, were varied from 0.1 to 0.4 mm and the nitrogen flow was kept at 10 l/min during experiments. The experiments were done in Ti plates of 0.3, 3 and 5 mm. Fig. 2. Illustration of the apparatus used for laser gas nitriding by pulsed Nd: YAG laser. The average power is 50 W and the peak power, pulse width were varied from 0.2 to 3 kW and 0.5 and 5 ms, respectively.
nitrides, carbides and hydrides during thermal processing if the atmosphere is adequate for the reactions at even low temperatures. At ambient temperature, titanium has an oxide native layer in the order of 3 to 7 nm that is responsible for its high corrosion resistance, but this thin natural oxide layer is easily removed by friction. Nitride and carbide layers increase very much the hardness and tribological properties of titanium. Hydrides are usually avoided because of their detrimental effects on the mechanical properties [7,8]. The nitriding of titanium by plasma, physical vapour deposition, chemical vapour deposition, thermal processing (e.g. salt-bath nitriding) and ion implantation have been performed for the last 25 years. The main disadvantage of these processes is that they require several hours of processing time and/or the specimens are processed at high temperatures altering the bulk mechanical properties of the material [9–12]. Laser gas nitriding (LGN) has several advantages compared to the conventional processes: the process is usually much faster than the conventional nitriding; the thickness of the nitrided layer can be changed from a few micrometers to several tenths of a micrometer; selective area can be coated; and only the surface of the material is affected because of short interaction time [13–21].
Gyorgy et al. studied the nitriding process by using a Nd: YAG laser in Q-sw mode [15]. They indicated that Ti2N was formed and the N/Ti atomic concentration decreased with depth. Geetha et al. studied the microstructure and corrosion resistance of Ti–13Nb–13Zr nitrided by laser [16]. They indicated that TiN, ZrN and TiN0.3 were the main phases in the formed layer and the corrosion resistance of the laser-treated region increased very much. Yilbas et al. studied laser melting of plasma nitrided Ti–6Al–4V alloys [17] and laser treatment followed by PVD TiN coating on Ti–6A–4V [18]. They showed that the rapid solidification in the melting region of the plasma nitrided specimens produced acicular α-phase. They also concluded that the duplex-structure (laser treated in nitrogen followed by PVD) improved the adhesion of the coating to the substrate. Katayama et al. studied the nitriding process of pure titanium and different materials (such as mild steel, nickel, copper and aluminium) by using CW and Nd:YAG lasers [19]. They indicated that the hardness increased after nitriding up to 600 to 850 HV, but cracks were observed. Folkes et al. studied the surface melting and surface nitriding of pure Ti and Ti–15Mo alloy [20]. They indicated that nitrided layers up to 0.3 mm and hardness of around 1000 HV were possible but some cracks were observed. Mordike studied the laser nitrided process by means of a Coherent Everlase CO2 laser [21]. They indicated that corrosion resistance is very much increased by nitriding, but the layers reduce the fatigue limit of the samples.
Fig. 3. Shape of the pulses during laser gas nitriding by pulsed Nd:YAG laser: (a) single pulse shape, frequency = 50 Hz; (b) double pulse shape, frequency = 30 Hz.
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Table 1 Laser gas nitriding by laser in CW mode, P = 50 W, laser beam diameter = 0.1 mm
Fig. 5. Photos of the specimens after torsional fatigue testing: (a) machined sample; (b) laser nitrided layer by CW laser, v = 2 mm/s; (c) laser nitrided layer by CW laser, v = 12 mm/s. The specimens were made by Selective Laser Melting of titanium powder by using the pulsed laser [18–20].
In this paper, laser gas nitriding of pure titanium by using Nd:YAG laser in CW and pulsed modes is presented. The main objective of this paper is to investigate the different properties of the nitrided layers by using the lasers in the different modes. The morphology, microstructure, chemical composition and hardness of the processed layers are analysed. The influence of the coatings on the torsional fatigue strength is also explained. 2. Experimental procedures Figs. 1 and 2 show illustrations of the apparatus used for laser gas nitriding experiments. The laser in Fig. 1 is an ML-7062A Ecomarker from Miyachi. The laser can operate in both Q-sw and CW modes. The pulse width in Q-sw mode is in the order of 200 ns. The maximum average power, P, is 50 W. The laser beam diameter at the focal position (110 mm) is 0.1 mm. In Qsw mode, the layers were made at a pulse energy of around 5 mJ, average power of 30 W, laser beam diameter of 0.6 mm and laser scan speeds (v) of 2 and 4 mm/s. In CW mode, the
Laser scan Nitrided layer speed (mm s− 1) thickness (μm)
Hardness Chemical Lattice (at 20 μm) composition parameter a; c (Å)
2
60
850
4 8 16
49 28.5 28.5
784 566 559
TiN TiN03 – – –
4.224539 2.9667; 4.7831 – – –
layers were made at an average power of 50 W, laser beam diameter of 0.1 mm and laser scan speeds varying from 2 to 16 mm/s. The pulsed laser in Fig. 2 is a LUXSTAR from Lumonics. The maximum peak power is 3 kW and the maximum frequency is 100 Hz. The pulse length (or pulse width) can be varied from 0.5 to 10 ms. The maximum average power is also 50 W. During experiments the peak power was varied from 1 to 2 kW, pulse width from 0.5 to 1 ms and average power from 20 to 50 W. The laser beam diameter at the focal position (149 mm) is 0.75 mm. The laser scan speed was changed from 2 to 20 mm/s. Fig. 3 shows two examples of the shape of the pulses used during the experiments. Since many cracks are found in the experiments with a single-pulse shape, the shape of the pulse is changed to double pulse-shape (stepwise pulse shape) allowing post-heating. The first pulse creates the molten pool and the second pulse decreases the cooling rate. P is the average power, Pp is the main peak power, Ph is the post-heating power, w is the pulse width and Ep is the energy per pulse. Other important processing conditions to be considered during laser gas nitriding are hatching pitch (hp) and nitrogen flow rate. The hatching pitch is the distance between adjacent tracks as showed in Fig. 4. The hatching pitch was varied from 0.1 to 0.4 mm and the flow rate was kept at 10 l/min during all the experiments. Excessive nitrogen gas leaks and pressure in the chamber is almost 1 atm. The hatching pitch and laser scan speed have high influence on the roughness and hardness uniformity of the layer. Smoother and more uniform layers are expected by decreasing the hatching pitch and scan speed [22].
Fig. 6. Top surface of the samples after nitriding: (a) Nd:YAG laser in CW mode; (b) Nd:YAG laser in Q-sw mode.
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Table 2 Laser gas nitriding by laser in Q-sw mode, laser beam diameter = 0.6 mm Laser Average Energy scan power per speed (W) pulse (mm (mJ) s− 1)
Nitrided Roughness Chemical Lattice layer, (μm) composition Parameter a; c thickness (Å) (μm)
2
31.3
5.2
1.95
0.71
2
28.5
4.7
1.26
0.69
4
31.3
5.2
1.11
0.71
4
28.5
4.7
1.04
0.7
TiN TiN03 Ti TiN TiN03 Ti TiN TiN03 Ti TiN TiN03 Ti
4.2299 2.9737;4.8 2.9472;4.6816 4.2241 2.9737;4.7867 2.9494;4.6806 4.2239 2.979;4.7328 2.9488;4.6784 4.2236 2.979;4.7474 2.9492;4.6735
The morphology of the nitrided surfaces and the microstructure of the cross-section of the layers were investigated by optical (Keyence, VHX-100) and scanning electron microscopes (JEOL 6340). The hardness of the cross-section of the layers was evaluated by using an HMV-2000 from Shimadzu and the hardness at the top of the layers was measured by using a Micro Zone Test System (MZT-5 from Akashi). The roughness was measured by using a SJ-400 from Mitsutoyo. Crystalline structures of the films were obtained by a Rigaku X-ray diffractometer RINT2500 Bragg-Brentano diffractometer in both θ–2θ configuration and also by grazing incidence X-ray diffraction (GIXD) with different incident angles (0.5° to 15°). The Cu Kα radiation (λ = 0.154 nm; 40 kV/200 mA) was chosen for excitation. XPS was conducted in order to identify the concentration and chemical bonding states of the elements with a PHI ESCA 5700 Multisystem spectrometer. For XPS investigation the monochromatic Mg Kα radiation (1253.6 eV) was used for excitation. The compositional depth profile was also obtained by XPS by using Ar+ (4 keV) ion beam sputtering. The morphology, depth, microstructure, hardness and chemical composition were analysed on nitrided layers formed on the top of 3 mm thick titanium plates grade 1 supplied by
NILACO. The titanium plates were ground (240 and 400 grit) and ultrasonic cleaned in a mixture of water and 10% ethanol. For analysing the mechanical properties of the layers, torsional fatigue tests were performed on square bars of 7 × 7 mm crosssection and 50 mm of height made by selective laser melting (SLM) of titanium. The details about SLM and the processing parameters for fabrication of the specimens are referred to [2,4– 6]. The stress amplitude during torsional fatigue test was 80 MPa (one million cycles). Fig. 5 shows the specimens after fatigue testing. 3. Laser gas nitriding by using Nd:YAG laser in Q-sw and CW modes Fig. 6 shows the top surface of the layers made by the Nd: YAG laser in CW and Q-sw modes. Tables 1 and 2 show the processing parameters for making the TiN layer, the chemical composition and lattice parameters of the nitrided layers in both modes and the roughness of the layers made by using the laser in Q-sw mode. The roughness of the Ti plates before nitriding was 0.35 μm Ra. By using the CW laser, the layers could be formed up to laser scan speeds of 16 mm/s. The layers made by CW laser have high roughness and could be applied as a roughening process for Ti implants in order to increase their surface area and osseointegration. The thickness of the layers by using the laser in the CW mode was measured by SEM after etching and the microstructure of the specimens after nitriding can be seen in Fig. 7. The layers' thickness varied from 20 to 60 μm by changing the laser scan speed from 16 to 2 mm/s. By increasing the laser scan speed, the energy per line (laser power/scan speed) decreases thus the molten pool is smaller and the TiN layer is thinner. Besides that, increasing the laser scan speed, the interaction time is shorter and the cooling rate is faster, therefore less nitrogen pick-up occurs and consequently the hardness decreases. In Q-sw mode, the layers created at a speed higher than 6 mm/s had very bad adherence to the Ti plate. Decreasing the laser beam diameter, to increase the power density, caused an undesirable burning effect. The thickness of the layers made by the laser in Q-sw mode was measured by comparison of the main Ti X-ray peak intensity at around 40.170° (1 0 1 plane).
Fig. 7. Microstructure of the nitrided layers by using the laser in CW mode. Average power = 50 W, hatching pitch = 0.1 mm, laser scan speed = 2 mm/s.
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Fig. 8. SEM micrograph of the top surface of the nitrided layers formed by single-pulse shape: (a) peak power of 1 kW; (b) peak power of 1.5 kW; (c) peak power of 2 kW. Average power = 50 W, pulse length = 1 ms, laser scan speed = 6 mm/s.
Because of the very short pulse length with high energy density with the laser in Q-sw mode, thin layers of 1 to 2 μm could be formed. The roughness of the layers made by the laser in Q-sw mode is low (around 1 μm), but the layer is porous. The hardness of the nitrided layer is related with the lattice parameter [23,24]. Therefore, the TiN layers made by the laser in Q-sw mode are expected to have hardness comparable to the laser in CW mode as the lattice parameters are very close for both conditions shown in Tables 1 and 2. The increase in speed with the Nd:YAG laser in the Q-sw mode caused a slight decrease of the lattice parameters in the TiN and TiN0.3 phases. This is caused by the lower interaction time and consequently less nitrogen pick-up when the laser scan speed is increased. The thickness of the layers made by the laser in CW mode is almost 10 times larger than those made by the laser in Q-sw mode. The movement of the molten pool caused by the difference in surface tension (maragoni movements) and also by the flow of the nitrogen gas may be the reasons for the high roughness of the TiN layers formed in CW mode compared to the ones formed in Q-sw mode. 4. Laser gas nitriding by using pulsed laser By changing the peak power from 1 to 3 kW the morphology of the layers could be changed from rough to smooth surfaces.
Fig. 8 shows the morphology of the titanium nitride layers for single-pulse shape. During single-pulse shape experiments, the peak powers were 1, 1.5 and 2 kW. The TiN layer changes from rough and porous (1 kW) to smooth with high crack density (1.5 and 2 kW); the higher the peak power, the higher the crack density and larger the size of the cracks. The functional properties of the coating are affected by the surface morphology. The cracks or fissures can induce severe corrosion and interface failure between the exposed region of the metallic material and the coating. Avoiding cracks is one of the main concerns in surface coating. The microcracks are formed by high cooling rate in laser processing and increase as the peak power is increased. The formation of large cracks is attributed to the stress developed in a thicker heat affected layer and by successive thermal cycles. In order to decrease the cooling rate and avoid porosity, the double-pulse shape is used. Fig. 9 shows the morphology of the titanium nitride layers for double-pulse shape experiments. The SEM micrograph of the top surface of the coatings shows no cracks. Table 3 shows a comparison between the single-pulse shape and double-pulse shape by using a peak power of 1.5 kW and laser scan speed of 6 mm/s. It is clear that the lattice parameters for both conditions are very close; therefore the nitrided layers are expected to have similar hardness. The layers made by double-pulse shape are much thinner than the ones made by
Fig. 9. SEM micrograph of the top surface of the nitrided layers formed by double-pulse shape (main pulse; post-heating pulse): (a) 1.0 kW–0.5 ms; 0.2 kW–5 ms; (b) 1.5 kW–0.5 ms; 0.2 kW–4.5 ms; (c) 2 kW–0.5 ms; 0.5 kW–1.8 ms.
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Table 3 Laser gas nitriding by laser in ms-pulsed mode, laser beam diameter = 0.75 mm, laser scan speed = 6 mm/s Pulse shape
Main pulse
Post heating
Hardness
Nitrided layer thickness (μm)
Roughness (μm)
Chemical composition
Lattice parameters a; c (Å)
Single
1.5 kW–1 ms 33.3 Hz
–
900
35
0.94
Double (Stepwise)
1.5 kW–0.5 ms 30 H
0.5 kW–1.8 ms 30 Hz
800
10
1.28
TiN TiN03 TiN TiN03
4.225 2.9343;4.7894 4.2222 2.9643;4.7886
single-pulse shape as one can see in Fig. 10. This occurs because by using a stepwise pulse shape, part of the energy is used to decrease the cooling rate and avoid cracking. The hardness of the layer at the top surface was also evaluated by using a MZT-5 from Akashi. The top surface showed hardness varying from 1400 to 1600 HV for most of the conditions. In order to understand these results, depth profiling by XPS was performed and the results can be seen in Fig. 11. For all conditions, a very high concentration of nitrogen at the top surface (up to 40 at.%) was detected. This is the reason for the high hardness measured by the Micro Zone Tester. The layers can be considered as gradient layers formed by rich TiN at the top surface. The nitrogen content decreases with depth and forms TiN0.3 and α-Ti(N) interstitial solution. This may provide very good adhesion strength between the nitrided layers and the metal. In Fig. 12 one can see how the lattice parameter varies for two laser conditions by using grazing incidence angle X-ray (GIXD). The standard lattice parameter for TiN is around 4.225 Å (PDF#87-0632). This shows that the amount of nitrogen (and consequently hardness) decreases with depth and that the higher the peak power, the higher the hardening depth. 5. Influence of the nitrided layers on the torsional fatigue strength
Fig. 13 shows the influence of the coating thickness (scan speed) on the fatigue strength of the samples. For lasers in both CW and pulsed modes, when the laser scan speed increases, which means the coating layer becomes thin, the sample can withstand more cycles without fracturing because the coating layer with porous structure or micro-cracks may shorten the fatigue life. As the hatching pitch is kept the same, thicker the layer, thicker the overlap between the tracks which may cause more defects (micro-cracks) and higher thermal stresses. The results by using the CW laser showed longer fatigue life than those by using the pulsed laser with single-pulse shape. The layer made by pulsed laser with single-pulse shape is characterised by pores and cracks as shown in Fig. 8(a). Comparing the results of Fig. 13, it is possible to conclude that the initiation of the macro-cracks did not change very much for both conditions but the propagation of the cracks was faster for the layers made by the pulsed laser because of the surface defects. Some TiN layers made by pulsed laser in the doublepulse shape (free of surface defects) were also tested. After one million cycles the cracks have propagated on none or maximum two sides of the specimen. It is possible to conclude that the layers made by the stepwise (double-pulse) shape had better adhesion strength and fracture toughness than the layer made by CW laser or pulsed laser in the single-pulse shape. 6. Conclusions
The influence of the TiN layer on the fatigue strength of test pieces made by SLM is investigated. The test pieces are formed by a pulsed Nd:YAG laser at an average power of 50 W, peak power of 1 kW and pulse width of 1 ms. The scan speed and laser spot diameter are 6 mm/s and 0.75 mm, respectively. Some samples are tested without coating and no cracks were observed after one million (106) cycles to the stress level of 80 MPa.
Titanium nitride layers by laser gas nitriding were successfully made on the top surface of pure titanium grade 1. The thickness of the layer made by the laser in CW mode varies from 30 to 60 μm and the surface is rough. Nitriding process in CW mode can be considered as a roughening process for titanium. The layers made by Q-sw (pulse length of ns order) laser are
Fig. 10. Microstructure of the nitrided layers by using single-pulse and double-pulse shape: (a) single-pulse shape: 1.5 kW–1 ms, frequency = 33.3 Hz; (b) double-pulse shape: (1.5 kW–0.5 ms)–(0.2 kW–4.5 ms), frequency = 30 Hz.
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Fig. 11. XPS Ar+ depth profiling of specimens after laser gas nitriding. Measurements done for all conditions up to 450 nm (≈ 90 min) showed that the concentration of nitrogen is very high at the top surface. Average power = 50 W, double-pulse shape: (1.5 kW–0.5 ms)–(0.2 kW–4.5 ms), frequency = 30 Hz.
very thin (1 to 2 μm) and have very low roughness but they are porous. Porous surface may be used to increase bone osseointegration. The layer made by the pulsed laser with a pulse length of ms order varies from 10 to 40 μm and can be changed from rough and porous to smooth by varying the peak power from 1 to 3 kW. When the peak power is increased, cracks are observed because of the high cooling rate. It is possible to avoid cracks by using the stepwise pulse shape, with a main pulse to melt the material followed by a pulse to decrease the cooling rate. X-ray showed that the layers are formed by TiN and TiN0.3. XPS analysis helped to understand the high hardness up to 1600 HV of the layers at the top surface: the top layer is formed with very high nitrogen content (40 at.%). XPS, X-ray and hardness measurements suggest that the layers can be considered as gradient layers formed by TiN, TiN + TiN0.3, TiN0.3 + α-Ti(N) and Ti in depth. The layers have some detrimental effect on the fatigue strength of the specimens: the thicker the layer, the higher the decrease on the fatigue strength. On the other hand, the smooth layers free of defects made by the stepwise pulse shape
Fig. 12. Variation of the ‘c’ lattice parameter in grazing incidence angle X-ray diffraction (GIXD) of the nitrided layers. Average power = 50 W, frequency = 30 Hz.
showed very good behaviour and almost no detrimental influence on the fatigue strength. References [1] D.M. Brunette, P. Tengvall, M. Textor, P. Thomsen, Titanium in Medicine: Material Science, Surface Science, Engineering, Biological Response and Medical Applications, Springer, 2001. [2] E.C. Santos, M. Shiomi, K. Osakada, K.F. Abe, J. Mech. Eng. Sci., Part C, Proc. Instn. Mech. Engrs., IMechE 218 (2004) 711. [3] F. Abe, E.C. Santos, K. Osakada, M. Shiomi, J. Mech. Eng. Sci., Part C, Proc. Instn. Mech. Engrs., IMechE 217 (2003) 1. [4] E.C. Santos, M. Shiomi, K. Osakada, F. Abe, Proc. of the 13th Solid Freeform Symposium, August 2001, Austin, Texas, August 2001, p. 180. [5] T. Laoui, E.C. Santos, K. Osakada, M. Shiomi, M. Morita, S.K. Shaik, N.K. Tolochko, F. Abe, Proc. of the Laser Assisted Net Engineering (LANE2004), Erlanger, Germany, September 2004, p. 475. [6] E.C. Santos, M. Shiomi, M. Morita, K. Osakada, F. Abe, Proc. of the 5th International Symposium on Laser Precision Microfabrication, Nara, Japan, May 2004, p. 268. [7] M.L. Wasz, F.R. Brotzen, R.B. McLellan, A.J. Griffin, Int. Mater. Rev. 41 (1) (1996) 1. [8] M.J. Donachie, Titanium: A Technical Guide, ASM, Metals Park, OH, 1988. [9] ASM international, Metals Handbook, vol. 5, Surface engineering, USA, December 1994. [10] R. Wei, T. Booker, C. Rincon, J. Arps, Surf. Coat. Technol. 186 (2004) 305. [11] P.K. Ajikumar, M. Kamruddin, R. Nithya, P. Shankar, S. Dash, A.K. Tyagi, B. Raj, Surf. Coat. Technol. 51 (2004) 361. [12] V. Fouquet, L. Pichon, A. Straboni, M. Drouet, Surf. Coat. Technol. 186 (2004) 34. [13] E.C. Santos, M. Shiomi, M. Morita, K. Osakada, F. Abe, Proc. of the 2004 Japanese Spring Conference for the Technology of Plasticity, Tokyo, Japan, May 2004, p. 107. [14] E.C. Santos, M. Morita, M. Shiomi, K. Osakada, M. Takahashi, Proc. of the 2004 Japanese Autumn Conference for the Technology of Plasticity, Tokyo, Japan, November 2004, p. 503. [15] E. Gyorgy, A.P. del Pino, P. Serra, J.L. Morenza, Surf. Coat. Technol. 173 (2003) 265. [16] M. Geetha, U.K. Mudali, N.D. Pandey, R. Asokamani, B. Raj, Surf. Eng. 20 (1) (2004) 68. [17] B.S. Yilbas, J. Nickel, A. Coban, M. Sami, S.Z. Shuja, A. Aleem, Wear 212 (1997) 140. [18] B.S. Yilbas, S.Z. Shuja, Surf. Coat. Technol. 130 (2000) 152.
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(a)
(b) 106
Number of cycles in torsional fatigue test
Number of cycles in torsional fatigue test
106
105
Crack initiation Crack propagated at all sides Fracture 104
0
2
4
6
8
10
12
14
16
18
105
Crack initiation Crack propagated at all sides Fracture 104 0
2
Laser scan speed [mm/s]
4
6
8
10
12
14
16
18
Laser scan speed [mm/s]
(c) Number of cycles in torsional fatigue test
106
105
Crack initiation Crack propagated at all sides Fracture 104
CW
Pulsed Single-pulse
Pulsed Double-pulse
Fig. 13. Influence of the laser gas nitriding on the torsional fatigue strength of the titanium models: (a) CW laser mode, average power = 50 W. (b) pulsed laser mode, 1 kW–1 ms–50 Hz. (c) comparison between the fatigue life of layers made by laser in CW (P = 50 W), single pulse shape (Pp = 1 kW) and double pulse shape (Pp = 1 kW, Ph = 0.2 kW), laser scan speed = 6 mm/s. [19] S. Katayama, A. Matsunawa, A. Morimoto, S. Ishimoto, Y. Arata, ICALEO, LIA, Los Angeles, November 1983. [20] J. Folkes, D.R.F. West, W.M. Steen, Laser Surface melting and alloying of titanium, Laser Surface Treatment of Metals (1986) Series E: Applied Sciences, no. 115 NATO ASI Series 451. [21] B.L. Mordike, Laser gas alloying, Laser Surface Treatment of Metals (1986) Series E: Applied Sciences, no. 115 NATO ASI Series 389.
[22] S. Das, Proc. of the 12th Solid Freeform Fabrication Symposium, Austin, Texas, August 2001, p. 102. [23] J.P. Bars, E. Etchessahar, J. Debuigne, J. Less-Common Met. 52 (1977) 51. [24] H.A. Wriedt, J.L. Murray, Bull. Alloy Phase Diagr. 8 (4) (1987) 378.
Laser gas nitriding of pure titanium using CW and pulsed Nd:YAG lasers Edson Costa Santos a , Masanori Morita a , Masanori Shiomi a,⁎, Kozo Osakada a , Masataka Takahashi b a
Department of Mechanical Science and Bioengineering, Graduate School of Engineering Science, Osaka University, Machikaneyama, Toyonaka, Osaka 560-8531, Japan b Inorganic Chemistry Department, Osaka Municipal Technical Research Institute, OMTRI, Morinomya, Joto-ku, Osaka, Japan Received 27 July 2005; accepted in revised form 21 February 2006 Available online 2 May 2006
Abstract The influence of parameters of continuous wave (CW) and pulsed Nd:YAG lasers on the laser gas nitriding (LGN) process of pure titanium models made by selective laser melting is presented. The morphology, microstructure, hardness and chemical composition of the nitrided layers are analysed. The layers formed by the Nd:YAG laser in CW and Q-sw modes show high roughness and porosity, respectively. The morphology of the layers formed by ms-pulsed laser varies from rough to smooth by increasing the peak power from 1 to 3 kW but cracking may occur. The thickness of the layers by using CW and pulsed lasers varies from 10 to 60 μm and the thickness of the layers in Q-sw mode varies from 1 to 2 μm. For pulsed laser experiments, cracking is avoided by using a stepwise pulse shape to decrease the cooling rate. X-ray photoelectron spectroscopy shows that the layers have high nitrogen concentration at the top surface and by grazing incidence X-ray diffraction (GIXD) it is possible to realize that the layer has a gradient TiN/TiN0.3 composition. The smooth and defect free TiN layers formed by ms-pulsed laser do not have any detrimental effect on the fatigue strength of the specimens. © 2006 Elsevier B.V. All rights reserved. Keywords: Laser gas nitriding; Titanium; TiN; Nd:YAG laser; CW; Pulsed
1. Introduction Pure titanium and titanium alloys (e.g. Ti–6Al–4V, Ti– 13Zr–13Nb) are common metals used for the fabrication of medical implants and prostheses. The features of high strength to weight ratio, high biocompatibility and high corrosion resistance are very suitable for the medical area [1]. Recently, selective laser melting (SLM) process, in which physical models can be fabricated directly from CAD data by using laser energy, has been developed and some titanium samples for medical implants have been introduced [2–6]. However, tribological properties such as wear resistance is low compared to other metals used for medical application. In manufacturing of practical models by SLM process for medical application, it is necessary to improve surface quality of the titanium model. Titanium has very high reactivity to interstitial elements (oxygen, nitrogen, carbon and hydrogen) and forms oxides, ⁎ Corresponding author. Tel.: +81 6 6850 6196; fax: +81 6 6850 6199. E-mail address: [email protected] (M. Shiomi). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.02.048
Fig. 1. Illustration of the apparatus used for laser gas nitriding by Q-sw and CW Nd:YAG laser. The average power is 50 W and the laser beam diameter at the focus is of 0.1 mm; in Q-sw mode the pulse length is of 200 ns.
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Fig. 4. The hatching or scanning pitch, the distance between adjacent lines, were varied from 0.1 to 0.4 mm and the nitrogen flow was kept at 10 l/min during experiments. The experiments were done in Ti plates of 0.3, 3 and 5 mm. Fig. 2. Illustration of the apparatus used for laser gas nitriding by pulsed Nd: YAG laser. The average power is 50 W and the peak power, pulse width were varied from 0.2 to 3 kW and 0.5 and 5 ms, respectively.
nitrides, carbides and hydrides during thermal processing if the atmosphere is adequate for the reactions at even low temperatures. At ambient temperature, titanium has an oxide native layer in the order of 3 to 7 nm that is responsible for its high corrosion resistance, but this thin natural oxide layer is easily removed by friction. Nitride and carbide layers increase very much the hardness and tribological properties of titanium. Hydrides are usually avoided because of their detrimental effects on the mechanical properties [7,8]. The nitriding of titanium by plasma, physical vapour deposition, chemical vapour deposition, thermal processing (e.g. salt-bath nitriding) and ion implantation have been performed for the last 25 years. The main disadvantage of these processes is that they require several hours of processing time and/or the specimens are processed at high temperatures altering the bulk mechanical properties of the material [9–12]. Laser gas nitriding (LGN) has several advantages compared to the conventional processes: the process is usually much faster than the conventional nitriding; the thickness of the nitrided layer can be changed from a few micrometers to several tenths of a micrometer; selective area can be coated; and only the surface of the material is affected because of short interaction time [13–21].
Gyorgy et al. studied the nitriding process by using a Nd: YAG laser in Q-sw mode [15]. They indicated that Ti2N was formed and the N/Ti atomic concentration decreased with depth. Geetha et al. studied the microstructure and corrosion resistance of Ti–13Nb–13Zr nitrided by laser [16]. They indicated that TiN, ZrN and TiN0.3 were the main phases in the formed layer and the corrosion resistance of the laser-treated region increased very much. Yilbas et al. studied laser melting of plasma nitrided Ti–6Al–4V alloys [17] and laser treatment followed by PVD TiN coating on Ti–6A–4V [18]. They showed that the rapid solidification in the melting region of the plasma nitrided specimens produced acicular α-phase. They also concluded that the duplex-structure (laser treated in nitrogen followed by PVD) improved the adhesion of the coating to the substrate. Katayama et al. studied the nitriding process of pure titanium and different materials (such as mild steel, nickel, copper and aluminium) by using CW and Nd:YAG lasers [19]. They indicated that the hardness increased after nitriding up to 600 to 850 HV, but cracks were observed. Folkes et al. studied the surface melting and surface nitriding of pure Ti and Ti–15Mo alloy [20]. They indicated that nitrided layers up to 0.3 mm and hardness of around 1000 HV were possible but some cracks were observed. Mordike studied the laser nitrided process by means of a Coherent Everlase CO2 laser [21]. They indicated that corrosion resistance is very much increased by nitriding, but the layers reduce the fatigue limit of the samples.
Fig. 3. Shape of the pulses during laser gas nitriding by pulsed Nd:YAG laser: (a) single pulse shape, frequency = 50 Hz; (b) double pulse shape, frequency = 30 Hz.
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Table 1 Laser gas nitriding by laser in CW mode, P = 50 W, laser beam diameter = 0.1 mm
Fig. 5. Photos of the specimens after torsional fatigue testing: (a) machined sample; (b) laser nitrided layer by CW laser, v = 2 mm/s; (c) laser nitrided layer by CW laser, v = 12 mm/s. The specimens were made by Selective Laser Melting of titanium powder by using the pulsed laser [18–20].
In this paper, laser gas nitriding of pure titanium by using Nd:YAG laser in CW and pulsed modes is presented. The main objective of this paper is to investigate the different properties of the nitrided layers by using the lasers in the different modes. The morphology, microstructure, chemical composition and hardness of the processed layers are analysed. The influence of the coatings on the torsional fatigue strength is also explained. 2. Experimental procedures Figs. 1 and 2 show illustrations of the apparatus used for laser gas nitriding experiments. The laser in Fig. 1 is an ML-7062A Ecomarker from Miyachi. The laser can operate in both Q-sw and CW modes. The pulse width in Q-sw mode is in the order of 200 ns. The maximum average power, P, is 50 W. The laser beam diameter at the focal position (110 mm) is 0.1 mm. In Qsw mode, the layers were made at a pulse energy of around 5 mJ, average power of 30 W, laser beam diameter of 0.6 mm and laser scan speeds (v) of 2 and 4 mm/s. In CW mode, the
Laser scan Nitrided layer speed (mm s− 1) thickness (μm)
Hardness Chemical Lattice (at 20 μm) composition parameter a; c (Å)
2
60
850
4 8 16
49 28.5 28.5
784 566 559
TiN TiN03 – – –
4.224539 2.9667; 4.7831 – – –
layers were made at an average power of 50 W, laser beam diameter of 0.1 mm and laser scan speeds varying from 2 to 16 mm/s. The pulsed laser in Fig. 2 is a LUXSTAR from Lumonics. The maximum peak power is 3 kW and the maximum frequency is 100 Hz. The pulse length (or pulse width) can be varied from 0.5 to 10 ms. The maximum average power is also 50 W. During experiments the peak power was varied from 1 to 2 kW, pulse width from 0.5 to 1 ms and average power from 20 to 50 W. The laser beam diameter at the focal position (149 mm) is 0.75 mm. The laser scan speed was changed from 2 to 20 mm/s. Fig. 3 shows two examples of the shape of the pulses used during the experiments. Since many cracks are found in the experiments with a single-pulse shape, the shape of the pulse is changed to double pulse-shape (stepwise pulse shape) allowing post-heating. The first pulse creates the molten pool and the second pulse decreases the cooling rate. P is the average power, Pp is the main peak power, Ph is the post-heating power, w is the pulse width and Ep is the energy per pulse. Other important processing conditions to be considered during laser gas nitriding are hatching pitch (hp) and nitrogen flow rate. The hatching pitch is the distance between adjacent tracks as showed in Fig. 4. The hatching pitch was varied from 0.1 to 0.4 mm and the flow rate was kept at 10 l/min during all the experiments. Excessive nitrogen gas leaks and pressure in the chamber is almost 1 atm. The hatching pitch and laser scan speed have high influence on the roughness and hardness uniformity of the layer. Smoother and more uniform layers are expected by decreasing the hatching pitch and scan speed [22].
Fig. 6. Top surface of the samples after nitriding: (a) Nd:YAG laser in CW mode; (b) Nd:YAG laser in Q-sw mode.
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Table 2 Laser gas nitriding by laser in Q-sw mode, laser beam diameter = 0.6 mm Laser Average Energy scan power per speed (W) pulse (mm (mJ) s− 1)
Nitrided Roughness Chemical Lattice layer, (μm) composition Parameter a; c thickness (Å) (μm)
2
31.3
5.2
1.95
0.71
2
28.5
4.7
1.26
0.69
4
31.3
5.2
1.11
0.71
4
28.5
4.7
1.04
0.7
TiN TiN03 Ti TiN TiN03 Ti TiN TiN03 Ti TiN TiN03 Ti
4.2299 2.9737;4.8 2.9472;4.6816 4.2241 2.9737;4.7867 2.9494;4.6806 4.2239 2.979;4.7328 2.9488;4.6784 4.2236 2.979;4.7474 2.9492;4.6735
The morphology of the nitrided surfaces and the microstructure of the cross-section of the layers were investigated by optical (Keyence, VHX-100) and scanning electron microscopes (JEOL 6340). The hardness of the cross-section of the layers was evaluated by using an HMV-2000 from Shimadzu and the hardness at the top of the layers was measured by using a Micro Zone Test System (MZT-5 from Akashi). The roughness was measured by using a SJ-400 from Mitsutoyo. Crystalline structures of the films were obtained by a Rigaku X-ray diffractometer RINT2500 Bragg-Brentano diffractometer in both θ–2θ configuration and also by grazing incidence X-ray diffraction (GIXD) with different incident angles (0.5° to 15°). The Cu Kα radiation (λ = 0.154 nm; 40 kV/200 mA) was chosen for excitation. XPS was conducted in order to identify the concentration and chemical bonding states of the elements with a PHI ESCA 5700 Multisystem spectrometer. For XPS investigation the monochromatic Mg Kα radiation (1253.6 eV) was used for excitation. The compositional depth profile was also obtained by XPS by using Ar+ (4 keV) ion beam sputtering. The morphology, depth, microstructure, hardness and chemical composition were analysed on nitrided layers formed on the top of 3 mm thick titanium plates grade 1 supplied by
NILACO. The titanium plates were ground (240 and 400 grit) and ultrasonic cleaned in a mixture of water and 10% ethanol. For analysing the mechanical properties of the layers, torsional fatigue tests were performed on square bars of 7 × 7 mm crosssection and 50 mm of height made by selective laser melting (SLM) of titanium. The details about SLM and the processing parameters for fabrication of the specimens are referred to [2,4– 6]. The stress amplitude during torsional fatigue test was 80 MPa (one million cycles). Fig. 5 shows the specimens after fatigue testing. 3. Laser gas nitriding by using Nd:YAG laser in Q-sw and CW modes Fig. 6 shows the top surface of the layers made by the Nd: YAG laser in CW and Q-sw modes. Tables 1 and 2 show the processing parameters for making the TiN layer, the chemical composition and lattice parameters of the nitrided layers in both modes and the roughness of the layers made by using the laser in Q-sw mode. The roughness of the Ti plates before nitriding was 0.35 μm Ra. By using the CW laser, the layers could be formed up to laser scan speeds of 16 mm/s. The layers made by CW laser have high roughness and could be applied as a roughening process for Ti implants in order to increase their surface area and osseointegration. The thickness of the layers by using the laser in the CW mode was measured by SEM after etching and the microstructure of the specimens after nitriding can be seen in Fig. 7. The layers' thickness varied from 20 to 60 μm by changing the laser scan speed from 16 to 2 mm/s. By increasing the laser scan speed, the energy per line (laser power/scan speed) decreases thus the molten pool is smaller and the TiN layer is thinner. Besides that, increasing the laser scan speed, the interaction time is shorter and the cooling rate is faster, therefore less nitrogen pick-up occurs and consequently the hardness decreases. In Q-sw mode, the layers created at a speed higher than 6 mm/s had very bad adherence to the Ti plate. Decreasing the laser beam diameter, to increase the power density, caused an undesirable burning effect. The thickness of the layers made by the laser in Q-sw mode was measured by comparison of the main Ti X-ray peak intensity at around 40.170° (1 0 1 plane).
Fig. 7. Microstructure of the nitrided layers by using the laser in CW mode. Average power = 50 W, hatching pitch = 0.1 mm, laser scan speed = 2 mm/s.
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Fig. 8. SEM micrograph of the top surface of the nitrided layers formed by single-pulse shape: (a) peak power of 1 kW; (b) peak power of 1.5 kW; (c) peak power of 2 kW. Average power = 50 W, pulse length = 1 ms, laser scan speed = 6 mm/s.
Because of the very short pulse length with high energy density with the laser in Q-sw mode, thin layers of 1 to 2 μm could be formed. The roughness of the layers made by the laser in Q-sw mode is low (around 1 μm), but the layer is porous. The hardness of the nitrided layer is related with the lattice parameter [23,24]. Therefore, the TiN layers made by the laser in Q-sw mode are expected to have hardness comparable to the laser in CW mode as the lattice parameters are very close for both conditions shown in Tables 1 and 2. The increase in speed with the Nd:YAG laser in the Q-sw mode caused a slight decrease of the lattice parameters in the TiN and TiN0.3 phases. This is caused by the lower interaction time and consequently less nitrogen pick-up when the laser scan speed is increased. The thickness of the layers made by the laser in CW mode is almost 10 times larger than those made by the laser in Q-sw mode. The movement of the molten pool caused by the difference in surface tension (maragoni movements) and also by the flow of the nitrogen gas may be the reasons for the high roughness of the TiN layers formed in CW mode compared to the ones formed in Q-sw mode. 4. Laser gas nitriding by using pulsed laser By changing the peak power from 1 to 3 kW the morphology of the layers could be changed from rough to smooth surfaces.
Fig. 8 shows the morphology of the titanium nitride layers for single-pulse shape. During single-pulse shape experiments, the peak powers were 1, 1.5 and 2 kW. The TiN layer changes from rough and porous (1 kW) to smooth with high crack density (1.5 and 2 kW); the higher the peak power, the higher the crack density and larger the size of the cracks. The functional properties of the coating are affected by the surface morphology. The cracks or fissures can induce severe corrosion and interface failure between the exposed region of the metallic material and the coating. Avoiding cracks is one of the main concerns in surface coating. The microcracks are formed by high cooling rate in laser processing and increase as the peak power is increased. The formation of large cracks is attributed to the stress developed in a thicker heat affected layer and by successive thermal cycles. In order to decrease the cooling rate and avoid porosity, the double-pulse shape is used. Fig. 9 shows the morphology of the titanium nitride layers for double-pulse shape experiments. The SEM micrograph of the top surface of the coatings shows no cracks. Table 3 shows a comparison between the single-pulse shape and double-pulse shape by using a peak power of 1.5 kW and laser scan speed of 6 mm/s. It is clear that the lattice parameters for both conditions are very close; therefore the nitrided layers are expected to have similar hardness. The layers made by double-pulse shape are much thinner than the ones made by
Fig. 9. SEM micrograph of the top surface of the nitrided layers formed by double-pulse shape (main pulse; post-heating pulse): (a) 1.0 kW–0.5 ms; 0.2 kW–5 ms; (b) 1.5 kW–0.5 ms; 0.2 kW–4.5 ms; (c) 2 kW–0.5 ms; 0.5 kW–1.8 ms.
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Table 3 Laser gas nitriding by laser in ms-pulsed mode, laser beam diameter = 0.75 mm, laser scan speed = 6 mm/s Pulse shape
Main pulse
Post heating
Hardness
Nitrided layer thickness (μm)
Roughness (μm)
Chemical composition
Lattice parameters a; c (Å)
Single
1.5 kW–1 ms 33.3 Hz
–
900
35
0.94
Double (Stepwise)
1.5 kW–0.5 ms 30 H
0.5 kW–1.8 ms 30 Hz
800
10
1.28
TiN TiN03 TiN TiN03
4.225 2.9343;4.7894 4.2222 2.9643;4.7886
single-pulse shape as one can see in Fig. 10. This occurs because by using a stepwise pulse shape, part of the energy is used to decrease the cooling rate and avoid cracking. The hardness of the layer at the top surface was also evaluated by using a MZT-5 from Akashi. The top surface showed hardness varying from 1400 to 1600 HV for most of the conditions. In order to understand these results, depth profiling by XPS was performed and the results can be seen in Fig. 11. For all conditions, a very high concentration of nitrogen at the top surface (up to 40 at.%) was detected. This is the reason for the high hardness measured by the Micro Zone Tester. The layers can be considered as gradient layers formed by rich TiN at the top surface. The nitrogen content decreases with depth and forms TiN0.3 and α-Ti(N) interstitial solution. This may provide very good adhesion strength between the nitrided layers and the metal. In Fig. 12 one can see how the lattice parameter varies for two laser conditions by using grazing incidence angle X-ray (GIXD). The standard lattice parameter for TiN is around 4.225 Å (PDF#87-0632). This shows that the amount of nitrogen (and consequently hardness) decreases with depth and that the higher the peak power, the higher the hardening depth. 5. Influence of the nitrided layers on the torsional fatigue strength
Fig. 13 shows the influence of the coating thickness (scan speed) on the fatigue strength of the samples. For lasers in both CW and pulsed modes, when the laser scan speed increases, which means the coating layer becomes thin, the sample can withstand more cycles without fracturing because the coating layer with porous structure or micro-cracks may shorten the fatigue life. As the hatching pitch is kept the same, thicker the layer, thicker the overlap between the tracks which may cause more defects (micro-cracks) and higher thermal stresses. The results by using the CW laser showed longer fatigue life than those by using the pulsed laser with single-pulse shape. The layer made by pulsed laser with single-pulse shape is characterised by pores and cracks as shown in Fig. 8(a). Comparing the results of Fig. 13, it is possible to conclude that the initiation of the macro-cracks did not change very much for both conditions but the propagation of the cracks was faster for the layers made by the pulsed laser because of the surface defects. Some TiN layers made by pulsed laser in the doublepulse shape (free of surface defects) were also tested. After one million cycles the cracks have propagated on none or maximum two sides of the specimen. It is possible to conclude that the layers made by the stepwise (double-pulse) shape had better adhesion strength and fracture toughness than the layer made by CW laser or pulsed laser in the single-pulse shape. 6. Conclusions
The influence of the TiN layer on the fatigue strength of test pieces made by SLM is investigated. The test pieces are formed by a pulsed Nd:YAG laser at an average power of 50 W, peak power of 1 kW and pulse width of 1 ms. The scan speed and laser spot diameter are 6 mm/s and 0.75 mm, respectively. Some samples are tested without coating and no cracks were observed after one million (106) cycles to the stress level of 80 MPa.
Titanium nitride layers by laser gas nitriding were successfully made on the top surface of pure titanium grade 1. The thickness of the layer made by the laser in CW mode varies from 30 to 60 μm and the surface is rough. Nitriding process in CW mode can be considered as a roughening process for titanium. The layers made by Q-sw (pulse length of ns order) laser are
Fig. 10. Microstructure of the nitrided layers by using single-pulse and double-pulse shape: (a) single-pulse shape: 1.5 kW–1 ms, frequency = 33.3 Hz; (b) double-pulse shape: (1.5 kW–0.5 ms)–(0.2 kW–4.5 ms), frequency = 30 Hz.
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Fig. 11. XPS Ar+ depth profiling of specimens after laser gas nitriding. Measurements done for all conditions up to 450 nm (≈ 90 min) showed that the concentration of nitrogen is very high at the top surface. Average power = 50 W, double-pulse shape: (1.5 kW–0.5 ms)–(0.2 kW–4.5 ms), frequency = 30 Hz.
very thin (1 to 2 μm) and have very low roughness but they are porous. Porous surface may be used to increase bone osseointegration. The layer made by the pulsed laser with a pulse length of ms order varies from 10 to 40 μm and can be changed from rough and porous to smooth by varying the peak power from 1 to 3 kW. When the peak power is increased, cracks are observed because of the high cooling rate. It is possible to avoid cracks by using the stepwise pulse shape, with a main pulse to melt the material followed by a pulse to decrease the cooling rate. X-ray showed that the layers are formed by TiN and TiN0.3. XPS analysis helped to understand the high hardness up to 1600 HV of the layers at the top surface: the top layer is formed with very high nitrogen content (40 at.%). XPS, X-ray and hardness measurements suggest that the layers can be considered as gradient layers formed by TiN, TiN + TiN0.3, TiN0.3 + α-Ti(N) and Ti in depth. The layers have some detrimental effect on the fatigue strength of the specimens: the thicker the layer, the higher the decrease on the fatigue strength. On the other hand, the smooth layers free of defects made by the stepwise pulse shape
Fig. 12. Variation of the ‘c’ lattice parameter in grazing incidence angle X-ray diffraction (GIXD) of the nitrided layers. Average power = 50 W, frequency = 30 Hz.
showed very good behaviour and almost no detrimental influence on the fatigue strength. References [1] D.M. Brunette, P. Tengvall, M. Textor, P. Thomsen, Titanium in Medicine: Material Science, Surface Science, Engineering, Biological Response and Medical Applications, Springer, 2001. [2] E.C. Santos, M. Shiomi, K. Osakada, K.F. Abe, J. Mech. Eng. Sci., Part C, Proc. Instn. Mech. Engrs., IMechE 218 (2004) 711. [3] F. Abe, E.C. Santos, K. Osakada, M. Shiomi, J. Mech. Eng. Sci., Part C, Proc. Instn. Mech. Engrs., IMechE 217 (2003) 1. [4] E.C. Santos, M. Shiomi, K. Osakada, F. Abe, Proc. of the 13th Solid Freeform Symposium, August 2001, Austin, Texas, August 2001, p. 180. [5] T. Laoui, E.C. Santos, K. Osakada, M. Shiomi, M. Morita, S.K. Shaik, N.K. Tolochko, F. Abe, Proc. of the Laser Assisted Net Engineering (LANE2004), Erlanger, Germany, September 2004, p. 475. [6] E.C. Santos, M. Shiomi, M. Morita, K. Osakada, F. Abe, Proc. of the 5th International Symposium on Laser Precision Microfabrication, Nara, Japan, May 2004, p. 268. [7] M.L. Wasz, F.R. Brotzen, R.B. McLellan, A.J. Griffin, Int. Mater. Rev. 41 (1) (1996) 1. [8] M.J. Donachie, Titanium: A Technical Guide, ASM, Metals Park, OH, 1988. [9] ASM international, Metals Handbook, vol. 5, Surface engineering, USA, December 1994. [10] R. Wei, T. Booker, C. Rincon, J. Arps, Surf. Coat. Technol. 186 (2004) 305. [11] P.K. Ajikumar, M. Kamruddin, R. Nithya, P. Shankar, S. Dash, A.K. Tyagi, B. Raj, Surf. Coat. Technol. 51 (2004) 361. [12] V. Fouquet, L. Pichon, A. Straboni, M. Drouet, Surf. Coat. Technol. 186 (2004) 34. [13] E.C. Santos, M. Shiomi, M. Morita, K. Osakada, F. Abe, Proc. of the 2004 Japanese Spring Conference for the Technology of Plasticity, Tokyo, Japan, May 2004, p. 107. [14] E.C. Santos, M. Morita, M. Shiomi, K. Osakada, M. Takahashi, Proc. of the 2004 Japanese Autumn Conference for the Technology of Plasticity, Tokyo, Japan, November 2004, p. 503. [15] E. Gyorgy, A.P. del Pino, P. Serra, J.L. Morenza, Surf. Coat. Technol. 173 (2003) 265. [16] M. Geetha, U.K. Mudali, N.D. Pandey, R. Asokamani, B. Raj, Surf. Eng. 20 (1) (2004) 68. [17] B.S. Yilbas, J. Nickel, A. Coban, M. Sami, S.Z. Shuja, A. Aleem, Wear 212 (1997) 140. [18] B.S. Yilbas, S.Z. Shuja, Surf. Coat. Technol. 130 (2000) 152.
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(a)
(b) 106
Number of cycles in torsional fatigue test
Number of cycles in torsional fatigue test
106
105
Crack initiation Crack propagated at all sides Fracture 104
0
2
4
6
8
10
12
14
16
18
105
Crack initiation Crack propagated at all sides Fracture 104 0
2
Laser scan speed [mm/s]
4
6
8
10
12
14
16
18
Laser scan speed [mm/s]
(c) Number of cycles in torsional fatigue test
106
105
Crack initiation Crack propagated at all sides Fracture 104
CW
Pulsed Single-pulse
Pulsed Double-pulse
Fig. 13. Influence of the laser gas nitriding on the torsional fatigue strength of the titanium models: (a) CW laser mode, average power = 50 W. (b) pulsed laser mode, 1 kW–1 ms–50 Hz. (c) comparison between the fatigue life of layers made by laser in CW (P = 50 W), single pulse shape (Pp = 1 kW) and double pulse shape (Pp = 1 kW, Ph = 0.2 kW), laser scan speed = 6 mm/s. [19] S. Katayama, A. Matsunawa, A. Morimoto, S. Ishimoto, Y. Arata, ICALEO, LIA, Los Angeles, November 1983. [20] J. Folkes, D.R.F. West, W.M. Steen, Laser Surface melting and alloying of titanium, Laser Surface Treatment of Metals (1986) Series E: Applied Sciences, no. 115 NATO ASI Series 451. [21] B.L. Mordike, Laser gas alloying, Laser Surface Treatment of Metals (1986) Series E: Applied Sciences, no. 115 NATO ASI Series 389.
[22] S. Das, Proc. of the 12th Solid Freeform Fabrication Symposium, Austin, Texas, August 2001, p. 102. [23] J.P. Bars, E. Etchessahar, J. Debuigne, J. Less-Common Met. 52 (1977) 51. [24] H.A. Wriedt, J.L. Murray, Bull. Alloy Phase Diagr. 8 (4) (1987) 378.