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Study on cross-section clad profile in coaxial single-pass cladding with a low-power laser
ARTICLE IN PRESS
Optics & Laser Technology 37 (2005) 478–482 www.elsevier.com/locate/optlastec
Study on cross-section clad profile in coaxial single-pass cladding with a low-power laser Jichang Liu, Lijun Li Laser Institute, Hunan University, Changsha, Hunan, 410082, PR China Received 23 April 2004; accepted 2 July 2004 Available online 28 October 2004
Abstract In this paper, a model of cross-section clad profile on the substrate in coaxial single-pass cladding with a low-power laser was studied. The static model of powder mass concentration distribution at cold-stream conditions was defined as a Gaussian function. In coaxial single-pass cladding with a low-power laser, since the influence of surface tension, gravity and gas flow on the clad bead could be neglected, the cross-section profile of the clad bead deposited by a low-power laser on the substrate was dominated by the powder concentration at each point on the pool and the time when the material was liquid at this point. The height of each point on the cross-section clad profile was defined as a definite integration of a Gaussian function from the moment at which the melt pool was just arriving at the point to the moment at which the point left the melt pool. In the presented experiment, powder of Steel 63 (at 0.63 wt% C) was deposited on a substrate of Steel 20 (at 0.20 wt% C) at the laser power of 135 W. The experimental results testified the model. r 2004 Elsevier Ltd. All rights reserved. Keywords: Laser cladding; Model; Cross-section profile; Low-power laser
1. Introduction In laser cladding, there existed complex interaction phenomena between laser beam, powder particles and molten region of the substrate. Only those powder particles striking the molten pool adhered to the substrate, whereas particles hitting the solid region ricoheted and were lost [1]. In the present study, valid powder particles were those that adhered to the substrate and formed clad bead. In order to make laser cladding omnidirectional, a coaxial powder feed system was usually used. The performance of the coaxial powder feeder depended upon various gas flow streams, which affected significantly the distribution of the powder stream and the deposition rate in cladding [2]. In coaxial laser-cladding process, metal particles (less than 150 mm) were delivered Corresponding author. Dorm 10, Room 211, Hunan University, Changsha, Hunan Province 410082, China.
0030-3992/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2004.07.010
into a molten pool and clad on the surface of a substrate. In practice, the shielding gas flow rate, carrier gas flow rate, powder feeding rate and geometry and size of the nozzle dominated the powder concentration distribution. Therefore, these parameters could significantly affect the clad profile [3,4]. Jehnming Lin, et al. investigated the powder catchment, distribution and clad profile in coaxial laser cladding [1–5]. The model developed by them showed the powder concentration distribution at cold-stream conditions. They discovered the clad profile was relative to the mode of the powder concentration distribution. However, only the powder that fell on the area within the melt pool was valid for forming clad bead. So, the mode of powder concentration distribution at coldstream conditions could not expresses directly the crosssection clad profile. In this paper, a model of cross-section clad profile on the substrate in coaxial single-pass cladding with a
ARTICLE IN PRESS J.C. Liu, L.J. Li / Optics & Laser Technology 37 (2005) 478–482
low-power laser was developed, and an experiment was used to verify the calculation results.
2. Model of cross-section clad profile 2.1. Model of mass concentration distribution of powder of cold stream In this study, a coaxial laser cladding nozzle, as shown in Fig. 1, was used. In Ref. [2], the Gaussian number concentration distribution mode in the cross section of the powder stream was identified at a coldstream condition. The number concentration was defined as 2 x þ y2 nðx; yÞ ¼ n exp ; (1) 2s2 where n(x, y) was the number of the particles per unit area over the cross-sectional area of the flow, n was the peak particle number concentration in the stream center (peak value of the Gaussian distribution function), which is affected by powder feed rate, x and y were the axis of x- and y-coordinates, respectively, in the cross section of the powder stream on the upper surface of the substrate, s was the stream diameter reaching 60% of the maximum number concentration.
479
Provided that Rp was the powder flow diameter in which the powder concentration was reduced form the peak value by a factor of e2, a boundary condition was expressed as ! R2p exp 2 ¼ expð2Þ; 2s s ¼ Rp 2:
(2)
Assuming that np was the number of the grains of the powder flow within unit height at x–y plane, Eq. (1) in the infinite x-y plane, O, was integrated as " # ZZ 2 x2 þ y2 n¯ exp dx dy ¼ np ; R2p O
n¼
2np : pR2p
(3)
_ p ; and Given that the powder feed mass rate was m mass concentration was m(x, y), replacing n(x, y) and n _ p ; respectively in Eq. (1), mass with m(x, y) and m concentration was expressed as " # 2 x2 þ y2 _p 2m mðx; yÞ ¼ exp : (4) R2p pR2p
2.2. Assumptions The coaxial jet flow was usually an unsteady-state turbulent flow with variable velocity distribution in the inlet boundary. The unsteady-state powder concentration distribution was too complicated to be solved. For solution of this issue, the following assumptions was given in this study: 1. The coaxial jet flow was a steady-state turbulent flow with constant velocity distribution in the outlet boundary, and the powder concentration distribution in the stream was independent of time. 2. The melt pool was independent of time. 3. Only the powder blown into the melt pool was valid and molten to clad on the substrate. The powder impinging the area out of the melt pool was blown away by the gas flow. 4. The fluid flow and gravity force were neglected.
2.3. Model of cross-section clad profile
Fig. 1. Schematic of coaxial nozzle.
Provided that the origin of the coordinates was at the center of the powder stream on the upper surface of the substrate and the substrate moved relative to the nozzle in the negative x direction, z0 was the stand-off distance, and F and j were the half spraying angles of the powder
ARTICLE IN PRESS J.C. Liu, L.J. Li / Optics & Laser Technology 37 (2005) 478–482
480
flow and laser beam, respectively, the schematic of coaxial laser cladding is shown in Fig. 2. During laser cladding, the melt pool moved. Fig. 3(a) showed a simplified melt pool. Fig. 3(b) showed a 3-D physic mode of the powder concentration distribution on the melt pool. The height of each point on the cross-section clad profile was determined by the momentary concentration (in unit area and unit time) at this point and the time when the temperature at this point is over melt point. It could be expressed by a definite integration of a Gaussian function from the moment at which the melt pool was just arriving at the point to the moment at which the point left out of the melt pool. At a certain moment, assuming that the origin of the coordinates was at the center of the laser beam spot, for any point at which the melt pool passed through the substrate, its coordinates were (x, y), and the coordinates of the pool margin point that first scanned the point (x, y) were (x1, y). The model of height of point (x, y) on the crosssection clad profile could be defined as hðyÞ ¼ W ðyÞ=r; ¼
1 r
Z
t2
_ yÞ dt ¼ mðx; t1
"
_p 2m pR2p
Z
#Z
x1 =v
" exp
x2 =v
# 2ðx2 þ y2 Þ dðx=vÞ R2p
" # x1 _p 2m 2y2 2x2 ¼ exp 2 dx exp 2 prR2p v Rp x2 Rp " # Z " # x1 2 _p 2m 2y 2x2 exp 2 exp 2 dx ¼ prvR2p Rp Rp 0 " # ! Z 0 2x2 þ exp 2 dx Rp x2
Fig. 3. (a) Schematic of melt pool; (b) 3-D physic mode of powder concentration distribution on the melt pool.
pffi " #0 Z 2x 1 Rp _p 2y2 @ 2 m pffiffiffi exp 2 exp a2 da ¼ pffiffiffiffiffiffi p 0 Rp 2prvRp ! Z 0 2 þ pffiffiffi pffi2x exp a2 da p R2 p
¼ W 0 C y ðC 1 þ C 2 Þ=r;
where h(y) and W(y) were the height of point (x, y) and mass of the powder deposited at the corresponding point on the substrate respectively, t1 and t2 were the moment when the point started to melt and the moment when the point started to solidify respectively, v was the scanning speed, r was the density of the clad material and a, W0, Cy, C1 and C2 were defined, respectively, as " # j 2y2 ; C y ¼ exp 2 ; W 0 ¼ pffiffiffiffiffiffi Rp 2pvRp Z 0 2 C 1 ¼ pffiffiffi pffi2x exp a2 da p R1 p
and 2 C 2 ¼ pffiffiffi p
Fig. 2. Schematic of coaxial laser cladding.
ð5Þ
Z pffi2x Rp
exp a2 da:
0
It could be seen that if the size of the melt pool was constant, the height of each point on the cross-section clad profile was a function of diameter of powder flow on the upper surface of the substrate, scanning speed and powder feed rate. In case the melt pool profile was defined, h(y) in Eq. (5) could be computed. The real height was difficult to compute due to the irregular boundary of the melt pool. In this study, the melt pool was simplified as a disc. Provided that the radius of the melt pool was 0.8 mm, _ p ¼ 1 g= min; Rp=2.3 mm; the calcuv=200 mm/min, m lation result of the cross-section clad profile was shown in Fig. 4. It was seen that the heights of the points near the edge of the melt pool tended to be zero due to lack of time when they were melted. It was demonstrated that the cross-section clad profile was dependent not only on the
ARTICLE IN PRESS
height (mm)
J.C. Liu, L.J. Li / Optics & Laser Technology 37 (2005) 478–482
0.20 0.18 0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00 -0.5
481
cross-section clad profile
-0.4
-0.3
-0.2
-0.1
0.0 y (mm)
0.1
0.2
0.3
0.4
0.5
Fig. 4. Calculation model of the cross-section clad profile.
concentration distribution of the stream flow, but also on the time for which the laser had traversed.
3. Experiment and discussion The laser-cladding system comprised of four subsystems: a laser unit, a metal powder delivery system, and a CNC machine tool, as shown in Fig. 5. A 500 W CO2 laser was used. The CNC machine tool was a special laser-processing tool. The focus length of the lens in the working head of the laser processing tool was 127 mm (5 in.). The minimum diameter of focused laser beam was about 0.1 mm. The metal powder delivery system consisted of a screw feeder, a powder splitter, soft tubes and a coaxial nozzle. The powder feed rate was decided by the feed screw rate. The powder was carried through the soft tubes by N2 gas flow and then divided into four flows by the powder splitter. The four gas-powder flows were unified to one in the coaxial nozzle. After being focused by the coaxial nozzle, the unified gas-powder flows touched on the substrate or the clad layer. The substrate material utilized in the experiments was Steel 20 (at 0.20 wt% C), and two metal powder materials were Steel 63 (at 0.63 wt% C) and industrial pure iron. The diameter of the metal powder was 45–80 mm. The shield gas was N2 too. The carriage gas flow rate and the shield gas flow rate both were 0.5 m3/h. The temperature at the lab was 20 1C. The laser power was 135 W. The focus of the beam was over the upper surface of the substrate for 1.0 mm. The minimum diameter of the powder flow, Rpmin, was 1.8 mm, and the half spraying angle was 151. Fig. 6 showed the cross-section profiles of the clad beads deposited at the powder feed rate of 1.26 g/min. The sample shown in Fig. 6(a) was deposited at scanning speed of 2 mm/s, and that in Fig. 6(b) was 2.5 mm/s. By comparing of Fig. 6(a) to Fig. 6(b), it was seen that the height of clad bead increased when the scanning speed reduced. Fig. 7 showed the cross-section profiles of the clad beads deposited at scanning speed of 2.5 mm/s. The sample shown in Fig. 7(a) was deposited at powder feed rate of 0.93 g/min, and that in Fig. 7(b) 1.06 g/min. By comparing of Fig. 7(a) to Fig. 7(b), it was seen that the height of clad bead increased with the powder feed rate.
Fig. 5. Experimental setup.
Fig. 6. Cross-section profiles of the clad beads deposited at the powder feed rate of 1.26 g/min.
Fig. 7. Cross-section profiles of the clad beads deposited at laser scanning speed of 2.5 mm/s.
However, if the powder feed rate rose to 1.26 g/min, the height of clad bead deposited at scanning speed of 2.5 mm/s was smaller than that at powder feed rate of 1.06 g/min as shown in Figs. 6(b) and 7(b). This was owing to the decrease in the size of the melt pool. The width of the clad bead shown in Fig. 7(b) was measured as 0.3 mm, and Rp=Rpmin+1.0 tan 151E2.1 mm. The calculated cross-section profile of the clad bead deposited at scanning speed of 2.5 mm/s and powder feed rate of 1.06 g/min was shown in Fig. 8. It was seen that the calculated cross-section clad profile shown in Fig. 8 agreed well with the experimental one shown in Fig. 7(b)). Therefore, the experimental result testified Eq. (5), i.e., the model of cross-section clad profile on the
ARTICLE IN PRESS J.C. Liu, L.J. Li / Optics & Laser Technology 37 (2005) 478–482
482
height (mm)
Cross-section clad profile 0.10 0.09 0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0.00 -0.20
-0.15
-0.10
-0.05
0.00
0.05
0.10
0.15
0.20
y (mm)
Fig. 8. Calculated cross-section profile of the clad bead deposited at a scanning speed of 2.5 mm/s and powder feed rate of 1.06 g/min.
point on the cross-section clad profile was defined as a definite integration of a Gaussian function from the moment at which the melt pool was just arriving at the point to the moment at which the point left out of the melt pool. It was a function of melt pool size, diameter of powder flow on the upper surface of the substrate, laser scanning speed and powder feed rate. In this study, the experimental results agreed well with the calculated ones. The model of cross-section clad profile was demonstrated as a definite integration of a Gaussian function.
substrate in coaxial single-pass cladding with a lowpower laser, was proved. References 4. Conclusions At cold-stream conditions, the static model of the mass concentration distribution of powder was defined as a Gaussian function like that of the number concentration distribution of powder particles. In coaxial single-pass cladding with a low-power laser, the cross-section profile of the clad bead deposited on the substrate was dominated by the powder concentration at each point on the pool and the time when the material was liquid at this point. The height of each
[1] Jehnming Lin. A simple model of powder catchment in coaxial laser cladding. Opt Laser Technol 1999;31:233–8. [2] Jehnming Lin. Concentration mode of the powder stream in coaxial laser cladding. Opt Laser Technol 1999;31:251–7. [3] Jack Beuth, Nathan Klingbeil. The role of process variables in laser-based direct metal solid freeform fabrication. J. Occup. Med. 2001;9:36–9. [4] Jehnming Lin, Bor-Chyang Hwang. Coaxial laser cladding on an inclined substrate. Opt Laser Technol 1999;31:571–8. [5] Jehnming Lin, Bor-Chyang Hwang. Clad profiles in edge welding using a coaxial powder filler nozzle. Opt Laser Technol 2001;33:267–75.
Optics & Laser Technology 37 (2005) 478–482 www.elsevier.com/locate/optlastec
Study on cross-section clad profile in coaxial single-pass cladding with a low-power laser Jichang Liu, Lijun Li Laser Institute, Hunan University, Changsha, Hunan, 410082, PR China Received 23 April 2004; accepted 2 July 2004 Available online 28 October 2004
Abstract In this paper, a model of cross-section clad profile on the substrate in coaxial single-pass cladding with a low-power laser was studied. The static model of powder mass concentration distribution at cold-stream conditions was defined as a Gaussian function. In coaxial single-pass cladding with a low-power laser, since the influence of surface tension, gravity and gas flow on the clad bead could be neglected, the cross-section profile of the clad bead deposited by a low-power laser on the substrate was dominated by the powder concentration at each point on the pool and the time when the material was liquid at this point. The height of each point on the cross-section clad profile was defined as a definite integration of a Gaussian function from the moment at which the melt pool was just arriving at the point to the moment at which the point left the melt pool. In the presented experiment, powder of Steel 63 (at 0.63 wt% C) was deposited on a substrate of Steel 20 (at 0.20 wt% C) at the laser power of 135 W. The experimental results testified the model. r 2004 Elsevier Ltd. All rights reserved. Keywords: Laser cladding; Model; Cross-section profile; Low-power laser
1. Introduction In laser cladding, there existed complex interaction phenomena between laser beam, powder particles and molten region of the substrate. Only those powder particles striking the molten pool adhered to the substrate, whereas particles hitting the solid region ricoheted and were lost [1]. In the present study, valid powder particles were those that adhered to the substrate and formed clad bead. In order to make laser cladding omnidirectional, a coaxial powder feed system was usually used. The performance of the coaxial powder feeder depended upon various gas flow streams, which affected significantly the distribution of the powder stream and the deposition rate in cladding [2]. In coaxial laser-cladding process, metal particles (less than 150 mm) were delivered Corresponding author. Dorm 10, Room 211, Hunan University, Changsha, Hunan Province 410082, China.
0030-3992/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2004.07.010
into a molten pool and clad on the surface of a substrate. In practice, the shielding gas flow rate, carrier gas flow rate, powder feeding rate and geometry and size of the nozzle dominated the powder concentration distribution. Therefore, these parameters could significantly affect the clad profile [3,4]. Jehnming Lin, et al. investigated the powder catchment, distribution and clad profile in coaxial laser cladding [1–5]. The model developed by them showed the powder concentration distribution at cold-stream conditions. They discovered the clad profile was relative to the mode of the powder concentration distribution. However, only the powder that fell on the area within the melt pool was valid for forming clad bead. So, the mode of powder concentration distribution at coldstream conditions could not expresses directly the crosssection clad profile. In this paper, a model of cross-section clad profile on the substrate in coaxial single-pass cladding with a
ARTICLE IN PRESS J.C. Liu, L.J. Li / Optics & Laser Technology 37 (2005) 478–482
low-power laser was developed, and an experiment was used to verify the calculation results.
2. Model of cross-section clad profile 2.1. Model of mass concentration distribution of powder of cold stream In this study, a coaxial laser cladding nozzle, as shown in Fig. 1, was used. In Ref. [2], the Gaussian number concentration distribution mode in the cross section of the powder stream was identified at a coldstream condition. The number concentration was defined as 2 x þ y2 nðx; yÞ ¼ n exp ; (1) 2s2 where n(x, y) was the number of the particles per unit area over the cross-sectional area of the flow, n was the peak particle number concentration in the stream center (peak value of the Gaussian distribution function), which is affected by powder feed rate, x and y were the axis of x- and y-coordinates, respectively, in the cross section of the powder stream on the upper surface of the substrate, s was the stream diameter reaching 60% of the maximum number concentration.
479
Provided that Rp was the powder flow diameter in which the powder concentration was reduced form the peak value by a factor of e2, a boundary condition was expressed as ! R2p exp 2 ¼ expð2Þ; 2s s ¼ Rp 2:
(2)
Assuming that np was the number of the grains of the powder flow within unit height at x–y plane, Eq. (1) in the infinite x-y plane, O, was integrated as " # ZZ 2 x2 þ y2 n¯ exp dx dy ¼ np ; R2p O
n¼
2np : pR2p
(3)
_ p ; and Given that the powder feed mass rate was m mass concentration was m(x, y), replacing n(x, y) and n _ p ; respectively in Eq. (1), mass with m(x, y) and m concentration was expressed as " # 2 x2 þ y2 _p 2m mðx; yÞ ¼ exp : (4) R2p pR2p
2.2. Assumptions The coaxial jet flow was usually an unsteady-state turbulent flow with variable velocity distribution in the inlet boundary. The unsteady-state powder concentration distribution was too complicated to be solved. For solution of this issue, the following assumptions was given in this study: 1. The coaxial jet flow was a steady-state turbulent flow with constant velocity distribution in the outlet boundary, and the powder concentration distribution in the stream was independent of time. 2. The melt pool was independent of time. 3. Only the powder blown into the melt pool was valid and molten to clad on the substrate. The powder impinging the area out of the melt pool was blown away by the gas flow. 4. The fluid flow and gravity force were neglected.
2.3. Model of cross-section clad profile
Fig. 1. Schematic of coaxial nozzle.
Provided that the origin of the coordinates was at the center of the powder stream on the upper surface of the substrate and the substrate moved relative to the nozzle in the negative x direction, z0 was the stand-off distance, and F and j were the half spraying angles of the powder
ARTICLE IN PRESS J.C. Liu, L.J. Li / Optics & Laser Technology 37 (2005) 478–482
480
flow and laser beam, respectively, the schematic of coaxial laser cladding is shown in Fig. 2. During laser cladding, the melt pool moved. Fig. 3(a) showed a simplified melt pool. Fig. 3(b) showed a 3-D physic mode of the powder concentration distribution on the melt pool. The height of each point on the cross-section clad profile was determined by the momentary concentration (in unit area and unit time) at this point and the time when the temperature at this point is over melt point. It could be expressed by a definite integration of a Gaussian function from the moment at which the melt pool was just arriving at the point to the moment at which the point left out of the melt pool. At a certain moment, assuming that the origin of the coordinates was at the center of the laser beam spot, for any point at which the melt pool passed through the substrate, its coordinates were (x, y), and the coordinates of the pool margin point that first scanned the point (x, y) were (x1, y). The model of height of point (x, y) on the crosssection clad profile could be defined as hðyÞ ¼ W ðyÞ=r; ¼
1 r
Z
t2
_ yÞ dt ¼ mðx; t1
"
_p 2m pR2p
Z
#Z
x1 =v
" exp
x2 =v
# 2ðx2 þ y2 Þ dðx=vÞ R2p
" # x1 _p 2m 2y2 2x2 ¼ exp 2 dx exp 2 prR2p v Rp x2 Rp " # Z " # x1 2 _p 2m 2y 2x2 exp 2 exp 2 dx ¼ prvR2p Rp Rp 0 " # ! Z 0 2x2 þ exp 2 dx Rp x2
Fig. 3. (a) Schematic of melt pool; (b) 3-D physic mode of powder concentration distribution on the melt pool.
pffi " #0 Z 2x 1 Rp _p 2y2 @ 2 m pffiffiffi exp 2 exp a2 da ¼ pffiffiffiffiffiffi p 0 Rp 2prvRp ! Z 0 2 þ pffiffiffi pffi2x exp a2 da p R2 p
¼ W 0 C y ðC 1 þ C 2 Þ=r;
where h(y) and W(y) were the height of point (x, y) and mass of the powder deposited at the corresponding point on the substrate respectively, t1 and t2 were the moment when the point started to melt and the moment when the point started to solidify respectively, v was the scanning speed, r was the density of the clad material and a, W0, Cy, C1 and C2 were defined, respectively, as " # j 2y2 ; C y ¼ exp 2 ; W 0 ¼ pffiffiffiffiffiffi Rp 2pvRp Z 0 2 C 1 ¼ pffiffiffi pffi2x exp a2 da p R1 p
and 2 C 2 ¼ pffiffiffi p
Fig. 2. Schematic of coaxial laser cladding.
ð5Þ
Z pffi2x Rp
exp a2 da:
0
It could be seen that if the size of the melt pool was constant, the height of each point on the cross-section clad profile was a function of diameter of powder flow on the upper surface of the substrate, scanning speed and powder feed rate. In case the melt pool profile was defined, h(y) in Eq. (5) could be computed. The real height was difficult to compute due to the irregular boundary of the melt pool. In this study, the melt pool was simplified as a disc. Provided that the radius of the melt pool was 0.8 mm, _ p ¼ 1 g= min; Rp=2.3 mm; the calcuv=200 mm/min, m lation result of the cross-section clad profile was shown in Fig. 4. It was seen that the heights of the points near the edge of the melt pool tended to be zero due to lack of time when they were melted. It was demonstrated that the cross-section clad profile was dependent not only on the
ARTICLE IN PRESS
height (mm)
J.C. Liu, L.J. Li / Optics & Laser Technology 37 (2005) 478–482
0.20 0.18 0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00 -0.5
481
cross-section clad profile
-0.4
-0.3
-0.2
-0.1
0.0 y (mm)
0.1
0.2
0.3
0.4
0.5
Fig. 4. Calculation model of the cross-section clad profile.
concentration distribution of the stream flow, but also on the time for which the laser had traversed.
3. Experiment and discussion The laser-cladding system comprised of four subsystems: a laser unit, a metal powder delivery system, and a CNC machine tool, as shown in Fig. 5. A 500 W CO2 laser was used. The CNC machine tool was a special laser-processing tool. The focus length of the lens in the working head of the laser processing tool was 127 mm (5 in.). The minimum diameter of focused laser beam was about 0.1 mm. The metal powder delivery system consisted of a screw feeder, a powder splitter, soft tubes and a coaxial nozzle. The powder feed rate was decided by the feed screw rate. The powder was carried through the soft tubes by N2 gas flow and then divided into four flows by the powder splitter. The four gas-powder flows were unified to one in the coaxial nozzle. After being focused by the coaxial nozzle, the unified gas-powder flows touched on the substrate or the clad layer. The substrate material utilized in the experiments was Steel 20 (at 0.20 wt% C), and two metal powder materials were Steel 63 (at 0.63 wt% C) and industrial pure iron. The diameter of the metal powder was 45–80 mm. The shield gas was N2 too. The carriage gas flow rate and the shield gas flow rate both were 0.5 m3/h. The temperature at the lab was 20 1C. The laser power was 135 W. The focus of the beam was over the upper surface of the substrate for 1.0 mm. The minimum diameter of the powder flow, Rpmin, was 1.8 mm, and the half spraying angle was 151. Fig. 6 showed the cross-section profiles of the clad beads deposited at the powder feed rate of 1.26 g/min. The sample shown in Fig. 6(a) was deposited at scanning speed of 2 mm/s, and that in Fig. 6(b) was 2.5 mm/s. By comparing of Fig. 6(a) to Fig. 6(b), it was seen that the height of clad bead increased when the scanning speed reduced. Fig. 7 showed the cross-section profiles of the clad beads deposited at scanning speed of 2.5 mm/s. The sample shown in Fig. 7(a) was deposited at powder feed rate of 0.93 g/min, and that in Fig. 7(b) 1.06 g/min. By comparing of Fig. 7(a) to Fig. 7(b), it was seen that the height of clad bead increased with the powder feed rate.
Fig. 5. Experimental setup.
Fig. 6. Cross-section profiles of the clad beads deposited at the powder feed rate of 1.26 g/min.
Fig. 7. Cross-section profiles of the clad beads deposited at laser scanning speed of 2.5 mm/s.
However, if the powder feed rate rose to 1.26 g/min, the height of clad bead deposited at scanning speed of 2.5 mm/s was smaller than that at powder feed rate of 1.06 g/min as shown in Figs. 6(b) and 7(b). This was owing to the decrease in the size of the melt pool. The width of the clad bead shown in Fig. 7(b) was measured as 0.3 mm, and Rp=Rpmin+1.0 tan 151E2.1 mm. The calculated cross-section profile of the clad bead deposited at scanning speed of 2.5 mm/s and powder feed rate of 1.06 g/min was shown in Fig. 8. It was seen that the calculated cross-section clad profile shown in Fig. 8 agreed well with the experimental one shown in Fig. 7(b)). Therefore, the experimental result testified Eq. (5), i.e., the model of cross-section clad profile on the
ARTICLE IN PRESS J.C. Liu, L.J. Li / Optics & Laser Technology 37 (2005) 478–482
482
height (mm)
Cross-section clad profile 0.10 0.09 0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0.00 -0.20
-0.15
-0.10
-0.05
0.00
0.05
0.10
0.15
0.20
y (mm)
Fig. 8. Calculated cross-section profile of the clad bead deposited at a scanning speed of 2.5 mm/s and powder feed rate of 1.06 g/min.
point on the cross-section clad profile was defined as a definite integration of a Gaussian function from the moment at which the melt pool was just arriving at the point to the moment at which the point left out of the melt pool. It was a function of melt pool size, diameter of powder flow on the upper surface of the substrate, laser scanning speed and powder feed rate. In this study, the experimental results agreed well with the calculated ones. The model of cross-section clad profile was demonstrated as a definite integration of a Gaussian function.
substrate in coaxial single-pass cladding with a lowpower laser, was proved. References 4. Conclusions At cold-stream conditions, the static model of the mass concentration distribution of powder was defined as a Gaussian function like that of the number concentration distribution of powder particles. In coaxial single-pass cladding with a low-power laser, the cross-section profile of the clad bead deposited on the substrate was dominated by the powder concentration at each point on the pool and the time when the material was liquid at this point. The height of each
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