spray- state of the art and applicability, International. Thermal Spray Conference, E. Lugscheider and C.C.. Berndt, Ed., March 4-6, 2002 (Essen, Germany), DVS.
Effect of Spraying Parameters on Stainless Steel Particle Velocity and Deposition Efficiency in Cold Spraying B. Sun, R.-Z. Huang, and N. Ohno, H. Fukanuma Plasma Giken Co., Ltd., Toda City, Saitama, Japan
Abstract In the cold spraying process, particle velocity is commonly regarded as the key factor which influences the deposition efficiency and properties of the coating. In the present paper the in-flight particles velocity were measured using a DPV-2000 system. The influences of He and N2 gas pressure and temperature and particle morphology on the particle velocity and deposition efficiency of the coating using stainless steel 316L powders were studied. The microstructure of the coating was examined using optical microscopy. The critical velocity of stainless steel 316L powders was estimated according to the particle velocity distribution and deposition efficiency of the coating. The experiment results suggested that the gas pressure has a more significant influence on the particle velocity and deposition efficiency of the coating than that of the gas temperature. The particle morphology also has significant influence on the particle velocity. The critical velocity of stainless steel 316L powders was in the range of 630 to 680 m/s and it decreased slightly with the gas temperature. Introduction Cold spray is a relatively new high rate material deposition process. In the cold spray process, the powder particles are accelerated to velocities on the order of 300 to 1200 m/s in a supersonic jet of compressed gas generated via a de Laval-type nozzle [1]. As opposed to other thermal spray processes, a significant characteristic of cold spray is the low temperature of the gas, which is usually heated to temperatures up to 650oC before entering the nozzle. The spray particles temperature prior to the impact with the substrate is well below their melting temperature, as a result the spray particles experience little oxidation or decomposition in cold spray [2, 3]. Upon impact with the substrate, the high speed solid-state particles are severely deformed and bond to
the substrate. So far, cold spray was used to spray not only ductile materials such as copper [4, 5], aluminum [6], nickel [7], nickel based alloys [8], zinc [9] but also metal matrix composites [10], cermets [11] and ceramic materials [12]. Previous studies suggested that particle deposition depends on the impact velocity and only the particles with a velocity higher than a critical velocity can be deposited. Below the critical velocity, impacting particles would only cause erosion of the substrate [13, 14]. The experimental and theoretical results showed that the critical velocity is dependent on the properties of powder and substrate materials [15, 16], particle size and geometry [17], particle temperature [18], particle oxygen content [19] and the substrate preparation [20]. This may partially explain that even for the same powder materials the reported critical velocity was somewhat different [19, 21]. Moreover, recent numerical simulation studies on the bonding mechanism of cold spray suggested that the particle velocity corresponding to the onset of shear instability was consistent with the critical velocity [18, 19, 22]. In cold spray process, the spray particles temperature is well below their melt temperature. The energy of deformation is mainly derived from the spray particle kinetic energy. Therefore the particle velocity prior to the impact with the substrate is the key parameters in cold spray. In the present paper, the characteristics of cold spray stainless steel powders were studied. Two types of stainless steel 316L powders in-flight particle velocity were measured using a DPV-2000 system. The influences of the gas species and gas pressure and temperature onto the particle velocity, deposition efficiency and microstructure of the coating were investigated. Furthermore, the critical velocity of stainless steel 316L powders was also estimated according to the particle velocity distribution and the deposition efficiency.
Materials and Experimental Procedures
Table 2: Spray conditions.
The powders used in the experiment were commercially available stainless steel 316L powders with nominal size ranges of -44+16 µm (Micro-melt® 316L) and -45+15 µm (Praxair®, FE-101), respectively. The morphologies of the two types of powders are shown in Fig. 1. The figure shows that the Micro-melt powders present a spherical morphology and the Praxair powders present an angular morphology. Table 1 lists the chemical composition of the two types of powders. Stainless steel 304 with sizes of 50 x 50 x 3 mm was used as a substrate. Prior to spraying, the substrate was sandblasted with #36 brown fused alumina grits. The deposition efficiency was measured by calculating the ratio between the mass of sprayed powder and the deposited mass onto the substrate.
(b)
Chemical composition of the powders (weight %).
Component Micro-Melt Praxair
C
Si Mn
P
S
Ni
Cr
Mo
.025 .53 1.11 .025 .006 16.3 10.1 2.08 .024
1
11
16
He
N2
1.0 - 3.0
1.5 - 3.0
Praxair
1.0 - 2. 5
200 – 400
Micro-melt
150 - 300
1.5 - 3.0
Praxair
150 - 300
200 - 400
Micro-melt
Driving gas pressure (MPa) Driving gas temperature (oC) Powder feed gas pressure (MPa)
N2
Driving gas pressure + 0.3 MPa
Spray distance (mm)
20
Traverse speed (mm/s)
50
Powder feed rate (g/min)
10
Experimental Results Particle Velocity Particle velocity is seen as the key factor in the cold spray process. The influence of the spray parameters on the microstructure and properties of the deposited coatings will be determined through the change of the particle conditions. In the present paper, the influences of gas species and gas pressure and gas temperature on two types of stainless steel particles velocity were investigated.
Figure 1: Powders morphology of a) Micro-melt 316L and b) Praxair FE-101. Table 1:
He
Powder feed gas
2
The cold spray system developed by Plasma Giken Co., Ltd. having a de Laval type converging/diverging nozzle, was used to deposit the coatings in the experiment. Table 2 lists the experiment conditions. The in-flight particle velocity was measured at the spray distance of 20 mm from the exit of the nozzle at the centre line of the particles flow, using a DPV-2000 system ( Tecnar Automation Ltd., Canada ). The substrate was removed during the particle velocity measurement process. For the radiation intensity of the in-flight particles is too low to be de detected by the sensor of the DPV-2000 system in cold spray, an additional CPS-2000 system with a laser source ( HPLD - 785 - 3W ) was equipped to illuminate the spray particles. By collecting monochromatic light scattered by the particles, the velocity and diameter of the particles can be measured by DPV-2000 system. The microstructure of the stainless steel coatings was examined using optical microscope (Olympus, BX51M).
Figure 2 shows the influences of He gas pressure and gas temperature on the Micro-melt particles velocity distribution measured with the DPV-2000 system. The average particle velocities increase from a value of 400 to 610 m/s when the He gas pressure changed from 1.0 to 3.0MPa at a fixed temperature of 200 oC. The gas temperature has no distinct influence on the particle velocity distribution and the average particle velocity keeps approximately constant at a value of 550 m/s at a fixed gas pressure of 2.0 MPa. 40
40
Normalized volume percent (%)
(a)
Driving gas
30
1.0MPa
150 C
2.0MPa
200 C
o o o
30
3.0MPa
250 C o
300 C
Micro-melt 20
He T = 200 oC
10
0
Micro-Melt He P = 2.0 MPa
20
10
0
200
400
600
800
Particle Velocity (m/s)
(a)
1000
1200
0
0
200
400
600
800
1000
1200
Particle Velocity (m/s)
(b)
Figure 2: Influence of the He gas a) pressure and b) temperature on the Micro-melt particles velocity distribution. Figure 3 shows the influence of the He gas pressure and gas temperature on the Praxair particles velocity distribution. It can be seen that the average particle velocity increased significantly from 480 to 830 m/s with the increase of gas
pressure from 1.0 to 2.5 MPa at a fixed gas temperature of 200 o C. While the gas temperature has almost no influence on the particle velocity distribution. The average particles velocity is approximately constant at about 700 m/s. 40
30
1.0MPa
Praxair
1.5MPa
He T = 200 oC
Praxair He P = 2.0 MPa
o
150 C o
200 C
30
2.0MPa
o
250 C
2.5MPa
o
300 C
20
20
10
10
0
0
200
400
600
800
1000
0
1200
0
200
Particle Velocity (m/s)
400
600
800
1000
1200
Particle Velocity (m/s)
(a)
(b)
Figure 3: Influence of the He gas a) pressure and b) temperature on the Praxair particles velocity distribution.
Normalized volume percent (%)
40
40 1.5 MPa 2.0 MPa 2.5 MPa 3.0 MPa
30
200 oC 300 oC 30
Micro-Melt
20
400 oC
Micro-Melt N P = 3.0 MPa
20
N T = 300 oC
2
2
10
10
0
0 0
200
400
600
800
1000
1200
0
200
400
600
800
1000
1200
Particle Velocity (m/s)
Particle Velocity (m/s)
(a)
(b)
Figure 4: Influence of the N2 gas a) pressure and b) temperature on the Micro-melt particles velocity distribution.
Normalized volume percent (%)
40
40 1.5 MPa 2.0 MPa 2.5 MPa 3.0 MPa
30
200 oC 300 oC
30
20
N T = 300 C 2
10
0
200
400
600
800
Particle Velocity (m/s)
(a)
1000
1200
100
100
0
200
400
600
800
1000
1200
Particle Velocity (m/s)
(b)
Figure 5: Influence of the N2 gas a) pressure and b) temperature on the Praxair particles velocity distribution. The influence of the N2 gas pressure and temperature on the Micro-melt particle velocity is shown in Fig. 4. The average
Praxair N gas
200 oC
2
10
0
Deposition Efficiency The deposition efficiency of the Praxair powders sprayed using N2 gas and He gas are shown in Fig. 6. For N2, the spraying deposition efficiency increased significantly with gas pressure and gas temperature as shown in Fig. 6a. The highest deposition efficiency was 53.2% for a temperature of 400 oC and a pressure of 3.0 MPa. For He, the influence of the gas pressure on the deposition efficiency is more significant than the gas temperature. The deposition efficiency is higher than 80% for gas pressures increased to 2.0 MPa even when the gas temperature is as low as 200 oC. When the gas pressure changed from 1.5 to 2.5 MPa the deposition efficiency sprayed
400 oC
20
0
In comparison with the Fig. 2 to Fig. 5, it is evident that for the experiment parameters range used, the He gas pressure has a bigger effect on the particle velocity than the gas temperature. For N2, both the gas pressure and gas temperature have influence on the particle velocity. Moreover, it is obvious that the Praxair particles has a higher velocity than the Micro-melt particles at the same spray conditions.
Praxair N P = 3.0 MPa
Praxair o
The Praxair particle velocity distribution sprayed by the N2 gas at different gas pressure and temperature is shown in Fig. 5. The average particle velocity increased from 530 to 600 m/s with the increase of gas pressure from 1.5 to 3.0 MPa at a gas temperature of 300 oC. It can be seen that the gas temperature also has significant influence on the particle velocity distribution. The average particle velocity increased from 570 to 640 m/s when the gas temperature changed from 200 to 400 o C at a gas pressure of 3.0 MPa.
Deposition efficiency (%)
Normalized volume percent (%)
40
particle velocity increased from 430 to 530 m/s with the increase of gas pressure from 1.5 to 3.0 MPa at a gas temperature of 300 oC. Different to the behavior observed previously for the He gas, the N2 gas temperature has a relatively significant influence on the particle velocity distribution as shown in Fig. 4b. The average particle velocity increased from 500 to 550 m/s when the gas temperature increased from 200 to 400 oC at a fixed gas pressure of 3.0 MPa.
o
80
250 C
80
2
o
o
300 C
200 C
o
60
60
350 C
o
250 C
o
o
400 C
300 C
40
40
20
20
He gas Praxair
0
1
1.5
2
2.5
Gas pressure (MPa)
(a)
3
3.5
0 0.5
1
1.5
2
2.5
3
Gas pressure (MPa)
(b)
Figure 6: Deposition efficiency of the Praxair powder sprayed using a) N2 and b) He gas.
at a temperature of 300 oC was lower than that sprayed at 200 and 250 oC as shown in Fig. 6b. It is because when the He gas temperature was at 300 oC, with the increase of gas pressure higher than 1.5 MPa, the adhesion of spray powders on the nozzle wall occurs during spraying which result in a decrease in the deposition efficiency. For Micro-melt powders, the deposition efficiency sprayed using N2 gas is too low and therefore only the He gas sprayed deposition efficiency was measured as shown in Fig. 7. It can be seen that when the gas temperature is higher than 100 oC, the gas pressure has more significant effect on the deposition efficiency than the gas temperature. The highest deposition efficiency was 48% for a temperature of 200oC and a pressure of 3.0 MPa. However, no particle deposition occurs when the gas pressure is equal to 1.0 MPa.
Figure 8 shows typical microstructures of the Praxair coatings. The examination of the microstructures of the coatings revealed that dense coatings can be deposited both using N2 and He gas. It can also be observed that the Praxair coating mainly consists of highly deformed particles, eliminating the presence of pores at the particles interfaces, which can be attributed to the accumulative peening effect resulting from the successive impacts of particles [23]. In comparison with the Praxair coatings, pores are present at the particle interfaces in the Micro-melt coatings as shown in Fig. 9, which implies that the deformation of the impacting particle was insufficient.
100 o
Deposition efficiency (%)
100 C 80
He gas Micro-melt
o
150 C o
200 C 60
o
250 C
(a) He gas, 2.0 MPa, 250 oC
40
20
0 0.5
(b) He gas, 2.0 MPa, 300 oC
Figure 9: Typical microstructures of the Micro-malt coatings. 1
1.5
2
2.5
3
3.5
Gas pressure (MPa)
Figure 7: Deposition efficiency of the Micro-melt powder sprayed using He gas. Microstructure
(a) N2 gas, 2.0 MPa, 250 oC
(b) N2 gas, 3.0 MPa, 400 oC
(c) He gas, 1.0 MPa, 200 oC
(d) He gas, 2.0 MPa, 250 oC
Figure 8: Typical microstructures of the Praxair coatings.
Critical Velocity In cold spray, it is well known that for a given powder material a critical velocity exists, and that only the particles with a velocity higher than this critical velocity can be deposited. For particles velocities below the critical velocity, the impacting particles would only cause substrate erosion. Recent study on the bonding mechanism of the cold spray process suggested that the critical velocity is mainly dependent on the thermo mechanical properties of the powder and substrate materials [24], particle temperature [18] and the oxygen content in the spray particles [19]. In the present work, the critical velocity of the Praxair and Micro-melt particles was estimated according to the particle velocity distribution and the deposition efficiency obtained from experiments. The particles cumulative volume fraction as a function of the particle velocity can be calculated according to the particle velocity distribution as shown in Fig. 2 to Fig. 5. Assuming that only the particles with a velocity higher than the critical velocity can be deposited, therefore a point, on which the cumulative volume fraction is equal to the value of one minus deposition efficiency, can be determined on the cumulative volume fraction curve. The corresponding particle velocity is regarded as the critical velocity. Due to the erosion effect of the un-deposited low speed particles on the previously deposited coating, the real critical velocity may be slightly lower than the calculated value. Figure 10a shows that the critical velocity of Praxair and Micro-melt powders were in the range of 630 to 680 m/s and it increased slightly with the increase of gas pressure. However, the critical velocity decreased slightly for both the Praxair and Micro-melt
Critical Velocity (m/s)
powders when the gas temperature increased from 200oC to 400oC, as shown in Fig. 10b. 750
750
700
700
650
650 600
600 o
550
Praxair, He, 200 C
550
o
Praxair, N , 300 C
Praxair, He, 2.0 MPa Praxair, N , 3.0 MPa
2
500
o
2
500
Micro-melt, He, 200 C
Micro-melt, He, 2.0 MPa Micro-melt, N , 3.0 MPa
o
Micro-melt, N , 300 C
2
2
450 0.5
steel powders, therefore it can be deduced that the difference in particle velocity sprayed at the same parameters can be attributed to the difference in the particle morphology, which influence the coefficient of drag force between the spray particle and gas and consequently influence the particle velocity.
1
1.5
450
2
2.5
3
3.5
100
150
200
250
300
350
400
450
o
Gas pressure (MPa)
Gas temperature ( C)
(a)
(b)
Figure 10: The critical velocity of stainless steel as a function of the gas a) pressure and b) temperature. Discussions In the cold spray process, a supersonic jet of compressed gas is generated via a converging/diverging de Laval nozzle. For a supersonic gas flows, the gas velocity in the nozzle is determined by the gas species, gas temperature and nozzle geometry. Thus, the gas pressure does not affect the gas velocity. However, the gas pressure influence the gas density due to the change of gas mass flow rate. Therefore, the gas velocity can be increased by using a lower molecular weight gas, by increasing inlet gas temperature and using large expansion ration nozzles. For spray particles, its velocity is dependent on gas velocity and density, particle size and density and the coefficient of drag force between the spray particle and the gas [1]. In comparison Fig. 2 to Fig. 5, it can be seen that particle velocity sprayed using He gas is higher than that sprayed using N2 gas at the same spray parameters for both the Praxair and Micro-melt particles. It is because the molecular weight of He gas is much lower, which leads to a higher He gas velocity in the nozzle, and this effect is beneficial for the particle velocity in the He gas flow. It can also be seen that the gas pressure strongly influences the particle velocity for both He gas and N2 gas, due to the fact that increasing the gas pressure leads to an increase in the gas mass flow rate and consequently the gas density is increased. Therefore, the drag force on the spray particles is increased and results in an increased particle velocity. Increasing the gas temperature will improve the gas velocity due to the decrease of the gas density during the gas expansion. The increase in gas velocity and decrease in gas density results in a contradictory effect on particle velocity for the gas temperature. For N2, gas temperature strongly influences the particle velocity while for He gas the influence is less significant. It can be attributed to the fact that N2 gas has higher molecular weight and the gas density in the nozzle is higher than the He gas. Considering there is no significant difference in the size and density for the two types of stainless
Increasing the particle velocity leads to a higher deposition efficiency owing to the high impacting kinetic energy which beneficial for the deformation of the impacting particles and the bonding with the substrate. In the experiment, the He gas sprayed Praxair coatings achieved deposition efficiencies over 80% and the deposition efficiency of He sprayed Praxair powders is relatively low. This results were consistent with the particle velocity distribution. The experiment results show that both the gas pressure and temperature has slight influence on the critical velocity. For cold spray, different from the other thermal spray process, the heating of the spray particle is mainly realized in the converging region of short distance of the nozzle. The particle temperature is strongly influenced by the gas temperature. Therefore, the decrease of the critical velocity can be attributed to the decrease of the yield strength of the powder materials with an increase of the gas temperature. For gas pressure, the influence mechanism on the critical velocity was not well understood yet. Conclusions Two types of stainless steel 316L powders were sprayed using either He or N2 as process gas. The experiment results show that, compared with the gas temperature, the gas pressure has a significant influence on the particle velocity and deposition efficiency. The investigation of deposition efficiency and microstructure of the coating shows consistency with the particle velocity distribution. The cold spray of Praxair stainless steel coatings consisted of highly deformed particles sprayed using He and N2 gas. The critical velocity of stainless steel was in the range of 630 - 680 m/s and decreased slightly with the gas temperature. References 1. 2. 3.
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