- Email: [email protected]
Effect of laser parameters on properties of surface-alloyed Al substrate with Ni
ARTICLE IN PRESS Optics and Lasers in Engineering 47 (2009) 971–975
Contents lists available at ScienceDirect
Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
Effect of laser parameters on properties of surface-alloyed Al substrate with Ni S.A. Vaziri a, H.R. Shahverdi a,, M.J. Torkamany b, S.G. Shabestari c a
Department of Materials Engineering, Tarbiat Modares University, PO Box 14115-143, Tehran, Iran National Iranian Center for Laser Science and Technology, PO Box 14665-576, Tehran, Iran c Department of Materials and Metullurgical Engineering, Iran University Science and Technology, Tehran, Iran b
a r t i c l e in fo
abstract
Article history: Received 19 December 2008 Received in revised form 9 April 2009 Accepted 9 April 2009 Available online 17 May 2009
Experimental investigations were carried out to examine the influence of the spot size and peak-power density of a pulsed Nd:YAG laser on the depth of the alloyed layer, the microstructure and the hardness in laser surface alloying of Al with Ni. It was found that the effect of both the peak-power density and the amount of energy absorbed from the laser beam on the depth of the alloyed layer and hardness must be considered simultaneously. In this work, the hardness of the alloyed layer was found to be 10–15 times the value for base Al. Crown Copyright & 2009 Published by Elsevier Ltd. All rights reserved.
Keywords: Aluminum Pulsed laser Surface alloying Surface properties
1. Introduction Laser surface alloying is one of the main methods for improving surface-dependent properties such as wear and corrosion resistance [1–5]. In laser surface alloying, the chemical composition of a material is modified by adding a suitable proportion of alloying elements to the melt pool formed by the laser pulse. The process can be carried out either in a single step or in two steps. In a single step process, alloying elements are fed to the melt pool in wire, powder or gas form. In the two-step process, alloying elements are pre-deposited, such as electroplating, evaporating or sputtering, as the first step. In the second step, the substrate surface, which is to be modified by alloying, is irradiated with a laser pulse [6,7]. Both continuous wave lasers and pulsed lasers can be used for this purpose [8]. The pulse mode is preferable because it affords higher cooling rates and a reduced heat affected zone surrounding the melted region [5]. This stems from the high absorbability of 1064 nm wavelength laser radiation by metals and the short pulse duration of the Nd:YAG laser [9]. The exothermic reaction between aluminum and nickel inside the laser-induced molten pool results in nickel aluminum intermetallic phase precipitates, and hardness and wear resistance is increased [10]. In laser surface alloying, the most important parameters are distribution of the alloying elements and the development of Corresponding author. Tel./fax: +98 2182883307
E-mail address: [email protected] (H.R. Shahverdi).
microstructure in the alloyed layer. Distribution of the alloying element in the alloyed layer remains reasonably uniform if the layer thickness remains small (approximately 400 mm) [11]. The role of the interaction time during laser surface processing of the aluminum, percolated with a thin layer of pure nickel, was the subject of our recent study [12]. The aim of the research reported here was to investigate the alloyed layer properties obtained using various process parameters in the laser surface processing of aluminum electroplated with nickel.
2. Experimental set-up Samples of Al were sliced into square plates of 20 20 mm2 with a thickness of 4 mm. The chemical composition of this Al is presented in Table 1. The hardness of these samples was equivalent to 35 HV. An electroplating unit was used to form a nickel coating on the aluminum pieces. The coating thickness was approximately 30 mm. A model IQL-10 pulsed Nd:YAG laser with a maximum mean laser power of 400 W was used as the laser source. The laser parameter ranges were 1–1000 s–1 for pulse frequency, 0.2–20 ms for pulse duration and 0–40 J for pulse energy. A 5000W-Lp Ophir power meter and LA 300W-LP Joule meter were used to measure average power and pulse energy. The temporal shape of the laser output pulses was squarelike. Full details of the experimental set-up are reported elsewhere [13]. Pure argon gas with a coaxial nozzle at 83–167 10–6 m3/s flow rate was used for shielding.
0143-8166/$ - see front matter Crown Copyright & 2009 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.optlaseng.2009.04.007
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The pieces were mounted on an X-Y table, the movement of which was controlled by a computer. The experiments were performed in three separate series (A, B, C) to identify the effects of peak-power density (PPD) and absorbed energy. Pulse frequency, pulse duration and scan speed for all tests were fixed, respectively, at 20 s1, 7 ms and 3 mms1. Table 2 shows the parameters used. After laser processing, samples were cut from the middle and the cross-section (perpendicular to the alloyed surface) of the alloyed layer was polished metallographically. The microstructure of the laser-alloyed layer was examined using scanning electron microscopy (SEM) and energy dispersion X-ray analysis (EDX). Small pieces were cut from the samples after laser alloying and X-ray diffraction (XRD) (model Philips XPERT) was carried out on the alloyed surface using Cu Ka radiation. The microhardness profiles were extracted from cross-sections of specimens with a 100 g load at 15 s.
3. Results and discussion 3.1. The effects of pulse PPD and absorption coefficient on the alloyed pool depth The depth and width of the alloyed layer obtained after laser processing experiments are presented in Table 3. Fig. 1 shows the effect of the laser-beam diameter on the depth of the alloyed pool. As shown in Fig. 1, by increasing the laser-
beam diameter for samples A, B and C, the depth of the alloyed pool will decrease. It is well known that the PPD of the laser beam (power per unit area of the beam cross-section) decreases at a rate proportional to the inverse of the square of the beam radius; for example, if the beam radius is increased by a factor of 2, the power density of the beam reduces by a factor of 4 approximately [11]. So the reduction in the power density results in a decrease in the alloyed layer thickness. Fig. 2 shows the effect of the laser PPD on the depth of the alloyed pool. SEM micrographs from cross-sections of samples at high and low PPD are presented in Fig. 3. As can be seen in Fig. 2, by increasing the PPD, the alloyed pool depth was increased. But it was found that at some points (e.g., point 1) even though the PPD was lower than the PPD at other points (e.g., point 2), the alloyed layer was deeper. It is suggested that these results are directly related to absorbed energy from the laser beam. In other words, since the irradiated power at point 1 is greater than that at point 2, the amount of absorbed energy at point 1 is higher than absorbed energy at point 2. Therefore, in the presence of thermal convection and reduced Marangoni convection, which cause the outward radial movement of melts, the depth of the alloyed pool increases faster than for the other conditions. It should be noted that in all experiments, the depth to width ratio is less than one. 3.2. The effects of PPD and absorption coefficient on the amount of alloying elements Fig. 4 shows the effect of beam diameter variations on the amount of Ni in the alloyed layer. Variation of Ni amount versus
Cu
Zn
Si
Fe
Al
0.08
0.14
0.19
0.16
Res.
Table 2 Parameters of laser processing. Peak power (kW)
Pulse energy (J)
Beam diameter (mm)
1.42
10
A1 1
A2 1.5
A3 2
A4 2.5
A5 3
A6 3.5
1.68
11.75
B1 1
B2 1.5
B3 2
B4 2.5
B5 3
B6 3.5
1.85
13
C1 1
C2 1.5
C3 2
C4 2.5
C5 3
C6 3.5
Thickness of alloyed layer (micrometer)
Table 1 The chemical composition (wt%) of the aluminum plate.
300 E=13J (Series C) E=11.75J (Series B)
250
E=10J (Series A)
200 150 100 50 0
0
1
2 Beam diameter (mm)
3
4
Fig. 1. Effect of the beam diameter on the thickness of alloyed layer for samples of A, B and C series.
Table 3 Dimensions of melted pool. Sample name
Width of the alloyed pool (mm)
Depth of the alloyed pool (mm)
Sample name
Width of the alloyed pool (mm)
Depth of the alloyed Sample pool (mm) name
Width of the alloyed Depth of the alloyed pool (mm) pool (mm)
A1 A2 A3 A4 A5 A6
660 812 901 805 810 1070
135 128 110 81 75 60
B1 B2 B3 B4 B5 B6
715 760 956 1085 800 788
141 131 120 117 98 42
830 835 968 1068 1195 1261
C1 C2 C3 C4 C5 C6
260 214 197 185 180 116
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Thickness of alloyed layer (micrometer)
300 250 1
200 150
2
100 50 0
0
2
4
6
8
10
PPD x 105 (W/cm2) Fig. 2. Effect of PPD of the laser on the thickness of the alloyed layer for samples of A, B and C series.
Fig. 3. BSE/SEM Micrographs at 80 magnification from cross-sections of samples: (a) high PPD (peak power: 1.85 kW, beam diameter: 0.5 mm) and (b) low PPD (peak power: 1.42 kW, beam diameter: 1.75 mm). The cross sign shows the sample point from which the EDX analysis is taken.
70
percentage of Ni (wt%)
60 50 40 30 20 200W 10
235W 260W
0
0
1
2 Beam diameter (mm)
3
4
Fig. 4. Effect of laser-spot diameter changes on the amount of Ni in the alloyed layer.
PPD is shown in Fig. 5. It should be noted that the values presented for Ni in Figs. 4 and 5 are the average values obtained from EDAX measurements for five vertical points from the top to
the bottom of the alloyed layer. The amount of nickel variation in melt pool is approximately 10%. Figs. 4 and 5 show that the amount of nickel will be increased by increasing the beam diameter (or decreasing the power density). In other words, by increasing the power density of the laser beam, the alloyed pool depth increases. This leads to the nickel rarefaction and, therefore, the nickel amount in the pool will be decreased relative to what occurs in low-power density cases. Moreover, Fig. 4 shows that the nickel amount corresponding to a peak power of 1.42 kW (series A) is more than the nickel amount at the two other powers. It can be concluded that the absorption coefficient for the cases with low power is lower than the absorption coefficient at higher powers. This phenomenon occurs at higher levels of pulse peak power leading to metal vaporization. As a result, the melt surface will break and a hole (known as a keyhole) appears that acts as a trap for laser energy [14]. In lower power cases, since absorption of energy mostly occurs at the middle of the melt-pool surface, a temperature gradient is produced between the middle and the edge of the melt pool. The convection depends on the derivative surface tension/temperature (ds/dT). When this is smaller than zero, the convection is directed (on the surface) from the middle to the edge of the melt pool. As a result, the melt phase flows and the melt pool has a reduced depth and an increased width. The resulting surface tension and thermal convection will bring Al to the surface and the circumferential region of the pool and this phenomenon leads to an increase in the amount of nickel.
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Amount of Ni (wt%)
60 50 40 30 20 10 0
2
0
4
6
8
10
8000
10000
PPD x 105 (W/cm2) Fig. 5. Variation of Ni amount versus PPD in the alloyed layer.
Microhardness (HV 0.1)
600 500 400 300 200 100 0
0
2000
4000
6000 2)
PPD (W/cm
Fig. 6. Variation of the microhardness values at the centerline of the alloyed pool versus PPD for samples of A, B and C series.
3.3. Hardness of alloyed layer
3.4. XRD The result of XRD that was carried out after the laser surface alloying of aluminum is shown in Fig. 7.
2
1-Al3Ni 2-Al 3-AlNi3
60 [counts]
The effect of the laser-beam PPD on the hardness of the samples is shown in Fig. 6. In this experiment, the laser-spot diameter on the work-piece surface was fixed at 1 mm. Fig. 6 shows that in low PPDs, hardness of the alloyed layer is low. It can be seen in Figs. 2 and 3 that in low PPDs, the depth of the alloyed pool is low and so are alloying and hard phase formation. For PPD between 1800 and 2500 W/cm2, the alloyed pool depth gradually increases. Increasing molten layers of Al and Ni and the absorption of sufficient energy result in interaction between these two elements. Intermetallic nickel aluminide formation will be increased. This means there will be a higher level of hardness in the alloyed layer. The reduction of hardness for higher PPDs (more than 2500 W/cm2) can be related to the increase in the alloyed pool depth (Figs. 1 and 2). This results in the rarefaction and reduction of the nickel amount in the pool (Figs. 4 and 5) and a low level of the hard phase. Meanwhile, the increase in PPD leads to a reduction in the cooling rate and the presence of intermetallic phases and therefore a reduction in the hardness.
80
Al
1
3 1
40 1
20 0
20
1
30
1
40 [°2θ]
50
60
Fig. 7. X-ray diffraction pattern of the laser surface-alloying aluminum with Ni.
The formation of the intermetallic phase in the laseralloying layer was confirmed by the XRD lines. These lines were indexed and compared with the files of nickel aluminide. The result is in good agreement with the experiment. Al3Ni and AlNi3 were the products found in the alloyed layer. The intermetallic phase AlNi3 was seen at 2y ¼ 43.621 and Al3Ni was found at some angles such as 22.8501, 25.8901, 29.6101, 41.8551, 43.6201 and 46.0501.
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4. Conclusions Laser alloying of aluminum with nickel has been studied. The effects of laser parameters on the properties of surface-alloyed Al substrate with Ni were analyzed. The following conclusions can be summarized from the above experimental results: 1. The alloyed layer thickness was strongly dependent on the conditions controlled by the laser parameters. 2. The constituents and their uniformity in the alloyed layer were dependent on the laser absorption regime and the geometry of the alloyed layer obtained. 3. In low PPDs, the alloyed region had an irregular shape, but by increasing the PPD, this shape changed to a uniform one with the expected profile. 4. In medium PPDs (in the range 1800–2500 W/cm2), because of an increase in formation of the hard phase of intermetallic nickel aluminide, the hardness was higher. 5. Laser alloying led to a significant hardening effect. The hardness of the alloyed layer was 10–15 times the value for base Al. References [1] Dube D, Fiset M, Couture A, Nakatsugawa I. Characterization and performance of laser melted AZ91D and AM60B. Mater Sci Eng A 2000;229:38.
975
[2] Ignat S, Sallamand P, Grevey D, Lambertin M. Magnesium alloys laser (Nd:YAG) cladding and alloying with side injection of aluminium powder. Appl Surf Sci 2004;225:124. [3] Dutta majumdar J, Ramesh Chandra B, Galun R, Mordike BL, Manna I. Laser composite surfacing of a magnesium alloy with silicon carbide. Compos Sci Technol 2003;63:771. [4] Liu SY, Hu ID, Yang Y, Guo ZX, Wang HY. Microstructure analysis of magnesium alloy melted by laser irradiation. Appl Surf Sci 2005;252: 1723–31. [5] Almedia A, Anjos M, Vilar R, Li R, Ferreira MGS, Steen WM, et al. Laser alloying of aluminium alloys with chromium. Surf Coat Technol 1995;70:221–9. [6] Vilar R. Laser alloying and laser cladding. Mater Sci Forum 1999;301: 229–52. [7] Joshi SV, Sundararajan G. Lasers for metallic and intermetallic coatings. Surf Eng 1988;1:121–39. [8] Kostrubiec K. Distribution of concentration of gold in the process of laser alloying of the nickel surface layer. J Mater Sci 1991;26:6044. [9] Gordani GR, Razavi RS, Hashemi SH, Nasr Isfahani AR. Laser surface alloying of an electroless Ni–P coating with Al-356 substrate. Opt Lasers Eng 2008;46: 550–7. [10] Senthil Selvan J, Soundararajan G, Subramanian K. Laser alloying of aluminium with electrodeposition nickel: optimization of plating thickness and processing parameters. Surf Coat Technol 2000;124:117–27. [11] Das DK, Prasad KS, Paradkr AG. Evolution of microstructure in laser surface alloying of aluminium with nickel. Mater Sci Eng A 1994;174:75–84. [12] Vaziri A., Shahverdi H.R., Shabestari S.G., Torkamany M.J., Effect of beam interaction time on laser alloying with pulsed Nd:YAG laser, Mater Sci Technol, 2009, (in press). [13] Malek Ghaini F, Hamedi MJ, Torkamany MJ, Sabbaghzadeh J. Weld metal microstructural characteristics in pulsed Nd:YAG laser welding. Scr Mater 2007;56:955–8. [14] Torkamany MJ, Hamdi MJ, Malek F, Sabbaghzadeh J. The effect of process parameters on keyhole welding with a 400 W Nd:YAG pulsed laser. J Phys D: Appl Phys 2006;39:4563–7.
Contents lists available at ScienceDirect
Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
Effect of laser parameters on properties of surface-alloyed Al substrate with Ni S.A. Vaziri a, H.R. Shahverdi a,, M.J. Torkamany b, S.G. Shabestari c a
Department of Materials Engineering, Tarbiat Modares University, PO Box 14115-143, Tehran, Iran National Iranian Center for Laser Science and Technology, PO Box 14665-576, Tehran, Iran c Department of Materials and Metullurgical Engineering, Iran University Science and Technology, Tehran, Iran b
a r t i c l e in fo
abstract
Article history: Received 19 December 2008 Received in revised form 9 April 2009 Accepted 9 April 2009 Available online 17 May 2009
Experimental investigations were carried out to examine the influence of the spot size and peak-power density of a pulsed Nd:YAG laser on the depth of the alloyed layer, the microstructure and the hardness in laser surface alloying of Al with Ni. It was found that the effect of both the peak-power density and the amount of energy absorbed from the laser beam on the depth of the alloyed layer and hardness must be considered simultaneously. In this work, the hardness of the alloyed layer was found to be 10–15 times the value for base Al. Crown Copyright & 2009 Published by Elsevier Ltd. All rights reserved.
Keywords: Aluminum Pulsed laser Surface alloying Surface properties
1. Introduction Laser surface alloying is one of the main methods for improving surface-dependent properties such as wear and corrosion resistance [1–5]. In laser surface alloying, the chemical composition of a material is modified by adding a suitable proportion of alloying elements to the melt pool formed by the laser pulse. The process can be carried out either in a single step or in two steps. In a single step process, alloying elements are fed to the melt pool in wire, powder or gas form. In the two-step process, alloying elements are pre-deposited, such as electroplating, evaporating or sputtering, as the first step. In the second step, the substrate surface, which is to be modified by alloying, is irradiated with a laser pulse [6,7]. Both continuous wave lasers and pulsed lasers can be used for this purpose [8]. The pulse mode is preferable because it affords higher cooling rates and a reduced heat affected zone surrounding the melted region [5]. This stems from the high absorbability of 1064 nm wavelength laser radiation by metals and the short pulse duration of the Nd:YAG laser [9]. The exothermic reaction between aluminum and nickel inside the laser-induced molten pool results in nickel aluminum intermetallic phase precipitates, and hardness and wear resistance is increased [10]. In laser surface alloying, the most important parameters are distribution of the alloying elements and the development of Corresponding author. Tel./fax: +98 2182883307
E-mail address: [email protected] (H.R. Shahverdi).
microstructure in the alloyed layer. Distribution of the alloying element in the alloyed layer remains reasonably uniform if the layer thickness remains small (approximately 400 mm) [11]. The role of the interaction time during laser surface processing of the aluminum, percolated with a thin layer of pure nickel, was the subject of our recent study [12]. The aim of the research reported here was to investigate the alloyed layer properties obtained using various process parameters in the laser surface processing of aluminum electroplated with nickel.
2. Experimental set-up Samples of Al were sliced into square plates of 20 20 mm2 with a thickness of 4 mm. The chemical composition of this Al is presented in Table 1. The hardness of these samples was equivalent to 35 HV. An electroplating unit was used to form a nickel coating on the aluminum pieces. The coating thickness was approximately 30 mm. A model IQL-10 pulsed Nd:YAG laser with a maximum mean laser power of 400 W was used as the laser source. The laser parameter ranges were 1–1000 s–1 for pulse frequency, 0.2–20 ms for pulse duration and 0–40 J for pulse energy. A 5000W-Lp Ophir power meter and LA 300W-LP Joule meter were used to measure average power and pulse energy. The temporal shape of the laser output pulses was squarelike. Full details of the experimental set-up are reported elsewhere [13]. Pure argon gas with a coaxial nozzle at 83–167 10–6 m3/s flow rate was used for shielding.
0143-8166/$ - see front matter Crown Copyright & 2009 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.optlaseng.2009.04.007
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The pieces were mounted on an X-Y table, the movement of which was controlled by a computer. The experiments were performed in three separate series (A, B, C) to identify the effects of peak-power density (PPD) and absorbed energy. Pulse frequency, pulse duration and scan speed for all tests were fixed, respectively, at 20 s1, 7 ms and 3 mms1. Table 2 shows the parameters used. After laser processing, samples were cut from the middle and the cross-section (perpendicular to the alloyed surface) of the alloyed layer was polished metallographically. The microstructure of the laser-alloyed layer was examined using scanning electron microscopy (SEM) and energy dispersion X-ray analysis (EDX). Small pieces were cut from the samples after laser alloying and X-ray diffraction (XRD) (model Philips XPERT) was carried out on the alloyed surface using Cu Ka radiation. The microhardness profiles were extracted from cross-sections of specimens with a 100 g load at 15 s.
3. Results and discussion 3.1. The effects of pulse PPD and absorption coefficient on the alloyed pool depth The depth and width of the alloyed layer obtained after laser processing experiments are presented in Table 3. Fig. 1 shows the effect of the laser-beam diameter on the depth of the alloyed pool. As shown in Fig. 1, by increasing the laser-
beam diameter for samples A, B and C, the depth of the alloyed pool will decrease. It is well known that the PPD of the laser beam (power per unit area of the beam cross-section) decreases at a rate proportional to the inverse of the square of the beam radius; for example, if the beam radius is increased by a factor of 2, the power density of the beam reduces by a factor of 4 approximately [11]. So the reduction in the power density results in a decrease in the alloyed layer thickness. Fig. 2 shows the effect of the laser PPD on the depth of the alloyed pool. SEM micrographs from cross-sections of samples at high and low PPD are presented in Fig. 3. As can be seen in Fig. 2, by increasing the PPD, the alloyed pool depth was increased. But it was found that at some points (e.g., point 1) even though the PPD was lower than the PPD at other points (e.g., point 2), the alloyed layer was deeper. It is suggested that these results are directly related to absorbed energy from the laser beam. In other words, since the irradiated power at point 1 is greater than that at point 2, the amount of absorbed energy at point 1 is higher than absorbed energy at point 2. Therefore, in the presence of thermal convection and reduced Marangoni convection, which cause the outward radial movement of melts, the depth of the alloyed pool increases faster than for the other conditions. It should be noted that in all experiments, the depth to width ratio is less than one. 3.2. The effects of PPD and absorption coefficient on the amount of alloying elements Fig. 4 shows the effect of beam diameter variations on the amount of Ni in the alloyed layer. Variation of Ni amount versus
Cu
Zn
Si
Fe
Al
0.08
0.14
0.19
0.16
Res.
Table 2 Parameters of laser processing. Peak power (kW)
Pulse energy (J)
Beam diameter (mm)
1.42
10
A1 1
A2 1.5
A3 2
A4 2.5
A5 3
A6 3.5
1.68
11.75
B1 1
B2 1.5
B3 2
B4 2.5
B5 3
B6 3.5
1.85
13
C1 1
C2 1.5
C3 2
C4 2.5
C5 3
C6 3.5
Thickness of alloyed layer (micrometer)
Table 1 The chemical composition (wt%) of the aluminum plate.
300 E=13J (Series C) E=11.75J (Series B)
250
E=10J (Series A)
200 150 100 50 0
0
1
2 Beam diameter (mm)
3
4
Fig. 1. Effect of the beam diameter on the thickness of alloyed layer for samples of A, B and C series.
Table 3 Dimensions of melted pool. Sample name
Width of the alloyed pool (mm)
Depth of the alloyed pool (mm)
Sample name
Width of the alloyed pool (mm)
Depth of the alloyed Sample pool (mm) name
Width of the alloyed Depth of the alloyed pool (mm) pool (mm)
A1 A2 A3 A4 A5 A6
660 812 901 805 810 1070
135 128 110 81 75 60
B1 B2 B3 B4 B5 B6
715 760 956 1085 800 788
141 131 120 117 98 42
830 835 968 1068 1195 1261
C1 C2 C3 C4 C5 C6
260 214 197 185 180 116
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Thickness of alloyed layer (micrometer)
300 250 1
200 150
2
100 50 0
0
2
4
6
8
10
PPD x 105 (W/cm2) Fig. 2. Effect of PPD of the laser on the thickness of the alloyed layer for samples of A, B and C series.
Fig. 3. BSE/SEM Micrographs at 80 magnification from cross-sections of samples: (a) high PPD (peak power: 1.85 kW, beam diameter: 0.5 mm) and (b) low PPD (peak power: 1.42 kW, beam diameter: 1.75 mm). The cross sign shows the sample point from which the EDX analysis is taken.
70
percentage of Ni (wt%)
60 50 40 30 20 200W 10
235W 260W
0
0
1
2 Beam diameter (mm)
3
4
Fig. 4. Effect of laser-spot diameter changes on the amount of Ni in the alloyed layer.
PPD is shown in Fig. 5. It should be noted that the values presented for Ni in Figs. 4 and 5 are the average values obtained from EDAX measurements for five vertical points from the top to
the bottom of the alloyed layer. The amount of nickel variation in melt pool is approximately 10%. Figs. 4 and 5 show that the amount of nickel will be increased by increasing the beam diameter (or decreasing the power density). In other words, by increasing the power density of the laser beam, the alloyed pool depth increases. This leads to the nickel rarefaction and, therefore, the nickel amount in the pool will be decreased relative to what occurs in low-power density cases. Moreover, Fig. 4 shows that the nickel amount corresponding to a peak power of 1.42 kW (series A) is more than the nickel amount at the two other powers. It can be concluded that the absorption coefficient for the cases with low power is lower than the absorption coefficient at higher powers. This phenomenon occurs at higher levels of pulse peak power leading to metal vaporization. As a result, the melt surface will break and a hole (known as a keyhole) appears that acts as a trap for laser energy [14]. In lower power cases, since absorption of energy mostly occurs at the middle of the melt-pool surface, a temperature gradient is produced between the middle and the edge of the melt pool. The convection depends on the derivative surface tension/temperature (ds/dT). When this is smaller than zero, the convection is directed (on the surface) from the middle to the edge of the melt pool. As a result, the melt phase flows and the melt pool has a reduced depth and an increased width. The resulting surface tension and thermal convection will bring Al to the surface and the circumferential region of the pool and this phenomenon leads to an increase in the amount of nickel.
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Amount of Ni (wt%)
60 50 40 30 20 10 0
2
0
4
6
8
10
8000
10000
PPD x 105 (W/cm2) Fig. 5. Variation of Ni amount versus PPD in the alloyed layer.
Microhardness (HV 0.1)
600 500 400 300 200 100 0
0
2000
4000
6000 2)
PPD (W/cm
Fig. 6. Variation of the microhardness values at the centerline of the alloyed pool versus PPD for samples of A, B and C series.
3.3. Hardness of alloyed layer
3.4. XRD The result of XRD that was carried out after the laser surface alloying of aluminum is shown in Fig. 7.
2
1-Al3Ni 2-Al 3-AlNi3
60 [counts]
The effect of the laser-beam PPD on the hardness of the samples is shown in Fig. 6. In this experiment, the laser-spot diameter on the work-piece surface was fixed at 1 mm. Fig. 6 shows that in low PPDs, hardness of the alloyed layer is low. It can be seen in Figs. 2 and 3 that in low PPDs, the depth of the alloyed pool is low and so are alloying and hard phase formation. For PPD between 1800 and 2500 W/cm2, the alloyed pool depth gradually increases. Increasing molten layers of Al and Ni and the absorption of sufficient energy result in interaction between these two elements. Intermetallic nickel aluminide formation will be increased. This means there will be a higher level of hardness in the alloyed layer. The reduction of hardness for higher PPDs (more than 2500 W/cm2) can be related to the increase in the alloyed pool depth (Figs. 1 and 2). This results in the rarefaction and reduction of the nickel amount in the pool (Figs. 4 and 5) and a low level of the hard phase. Meanwhile, the increase in PPD leads to a reduction in the cooling rate and the presence of intermetallic phases and therefore a reduction in the hardness.
80
Al
1
3 1
40 1
20 0
20
1
30
1
40 [°2θ]
50
60
Fig. 7. X-ray diffraction pattern of the laser surface-alloying aluminum with Ni.
The formation of the intermetallic phase in the laseralloying layer was confirmed by the XRD lines. These lines were indexed and compared with the files of nickel aluminide. The result is in good agreement with the experiment. Al3Ni and AlNi3 were the products found in the alloyed layer. The intermetallic phase AlNi3 was seen at 2y ¼ 43.621 and Al3Ni was found at some angles such as 22.8501, 25.8901, 29.6101, 41.8551, 43.6201 and 46.0501.
ARTICLE IN PRESS S.A. Vaziri et al. / Optics and Lasers in Engineering 47 (2009) 971–975
4. Conclusions Laser alloying of aluminum with nickel has been studied. The effects of laser parameters on the properties of surface-alloyed Al substrate with Ni were analyzed. The following conclusions can be summarized from the above experimental results: 1. The alloyed layer thickness was strongly dependent on the conditions controlled by the laser parameters. 2. The constituents and their uniformity in the alloyed layer were dependent on the laser absorption regime and the geometry of the alloyed layer obtained. 3. In low PPDs, the alloyed region had an irregular shape, but by increasing the PPD, this shape changed to a uniform one with the expected profile. 4. In medium PPDs (in the range 1800–2500 W/cm2), because of an increase in formation of the hard phase of intermetallic nickel aluminide, the hardness was higher. 5. Laser alloying led to a significant hardening effect. The hardness of the alloyed layer was 10–15 times the value for base Al. References [1] Dube D, Fiset M, Couture A, Nakatsugawa I. Characterization and performance of laser melted AZ91D and AM60B. Mater Sci Eng A 2000;229:38.
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