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Modelling of the in-flight behaviour of stainless steel powder particles in high velocity oxy-fuel spraying
Journal of Materials Processing Technology 79 (1998) 213 – 216
Modelling of the in-flight behaviour of stainless steel powder particles in high velocity oxy-fuel spraying V.V. Sobolev *, J.M. Guilemany, A.J. Martı´n, J.A. Calero, P. Vilarrubias Centro de Proyeccio´n Te´rmica, Ciencia de Materiales-Ingenierı´a Metalu´rgica, Deparamento de Ingenierı´a Quı´mica y Metalu´rgia, Uni6ersidad de Barcelona, Martı´ i Franque`s, 1, 08028 Barcelona, Spain Received 14 March 1997
Abstract Modelling of the mechanical and thermal behaviour of stainless steel 316L powder particles during high velocity oxy-fuel (HVOF) spraying is presented. This modelling accounts for the combustion process, the gas dynamics inside and outside of the spray gun, gas-particle interactions, acceleration and deceleration of the gas flow, internal heat conduction in the powder particles and particle heating, melting, cooling and solidification. Variations of the times of flight and melting of the particles are studied. Optimal conditions of spraying are predicted. The results agree with experimentally-established HVOF spraying practice. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Modelling; HVOF spraying; Stainless steel powder; In-flight behaviour
1. Introduction
2. Modelling procedure.
In recent decades, high velocity oxy-fuel (HVOF) spraying has been one of the most rapidly developed areas of thermal spraying used in many industrial applications [1,2]. Stainless steel powders are promising in increasing the corrosion resistance of HVOF sprayed coatings. To improve the technology of the HVOF spraying of these powders and to establish its optimum conditions, both the mechanical and the thermal behaviour of the powder particles during their motion must be studied. Mathematical simulation is an effective tool to approach this problem. The purpose of this paper is to provide modelling of the in-flight mechanical and thermal behaviour of the stainless steel powder particles using the models developed in Refs. [3 – 6], to study variations of the particle velocity and temperature with respect to the spraying parameters and to predict the optimum spray conditions.
Mathematical models describing the in-flight behaviour of the powder particles during HVOF spraying are developed in Refs. [3–6]. First, the evolution of the particle velocity Vp along the spraying distance is determined taking into account the combustion process, gas dynamic phenomena inside and outside the spray gun, gas-particle interactions, acceleration and deceleration of the gas flow and specific variations of the drag coefficient with the Reynolds number. Then the heattransfer problem is solved numerically, accounting for the internal heat conduction within a powder particle, heat exchange between the particle and the surrounding gases and particle heating, melting, cooling and solidification at the spraying distance. Variations of the particle flight time tfl and the time of complete fusion of the particle, tfs, are studied. Powder particles made of the stainless steel 316 L are used for modelling, their thermophysical properties being taken from [7]. The parameters of the HVOF spray system correspond to the CDS gun installed at the Centre of Thermal Spraying of the University of Barcelona (Plasma Technik PT100). The powder particle diameter dp = 20–80 mm and particle initial temperature T0 = 1300°C.
* Corresponding author. Tel.: +34 3 4021297; fax: + 34 3 4021638; e-mail: [email protected] 0924-0136/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0924-0136(98)00013-2
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V.V. Sobole6 et al. / Journal of Materials Processing Technology 79 (1998) 213–216
Fig. 1. Variation in particle velocity with spraying distance.
3. Results and discussion
3.1. Particle mechanical beha6iour Variations of the particle velocity at the spraying distance z are presented in Fig. 1, where initial point z = 0 corresponds to the exit from the combustion chamber. Then the particle velocity Vp increases, attains the maximum value Vpm at z =zmv and decreases on moving towards the substrate surface. Acceleration of the particle motion before the attaining of Vpm (zB zmv) becomes more pronounced with a decrease in the particle radius Rp, since the inertia effects decrease with decreasing mass of the particle. The maximum velocity of the particle increases with a decrease in Rp and its axial position zmv is displaced towards the substrate surface when the particle radius increases (Fig. 2). Fig. 3 shows variations of the particle velocities at different spraying distances with respect to the particle radius. With a relatively small spraying distance the value of Vp decreases uniformly with increasing Rp. At greater spraying distances the particle velocity behaves non-uniformly with increase in the particle radius and attains maximum values at particular values of Rp: these values are achieved at Rp =20 mm when z= 0.5 m and at Rp = 25 mm when z =0.6 m.
3.2. Particle thermal beha6iour The different stages of the particle thermal behaviour—heating, melting, heating again, cooling and solidification—were considered. It is clear that solidifi-
cation should be avoided during particle in-flight motion in HVOF spraying, because it is detrimental to the coating structure and properties. However, modelling must also cover this possibility in order to be able to choose the optimum processing conditions. During HVOF spraying, the powder particles are first heated until they reach their solidus temperature (Fig. 4). Then, during the melting period, the mean temperature T of the particle increases slowly due to absorption of the latent heat of fusion. After melting, the particle temperature continues to increase up to the maximum and then starts to decrease as the gas temperature reduces. Finally, the particle temperature for relatively large particles (starting from Rp = 20 mm) achieves the liquidus temperature and enters the thermal interval of solidification (the solid–liquid ‘mushy’ zone) between the temperatures of liquidus and solidus of the stainless steel considered. During the solidification process the particle is cooled very slowly despite the essential decrease in the gas temperature due to the extraction of the latent heat. The periods of melting and solidification increase and the behaviour of the particle temperature becomes more uniform with increasing particle radius. The maximum of the particle temperature Tm decreases with an increase in Rp (Fig. 5). The axial position zmt of Tm varies non-uniformly with increasing particle radius: it increases, attains the maximum value at Rp = 22 mm and then decreases. Variations of the particle temperature at different spraying distances z show that this temperature decrease with an increase in z and finally enters the thermal interval of solidification for relatively large
V.V. Sobole6 et al. / Journal of Materials Processing Technology 79 (1998) 213–216
Fig. 2. Variations in particle maximum velocity and its axial position with particle radius.
Fig. 3. Variation in particle velocity with particle radius for different spraying distances.
value of Rp =30–40 mm (Fig. 6). This means that the powder particles with these values of Rp impinge onto the substrate surface in a partially solidified state.
215
Fig. 5. Variation in particle mean temperature and its axial position with particle radius.
For the development of the coatings on the substrate surface, it is important to know the time of complete fusion of the particle tfs, the time of the particle flight tfl 1 and their ratio z= tfst − fl . Variations of these parameters with respect to the particle radius are given in Fig. 7. The duration of the particle flight tfl increases almost linearly with increasing Rp, while the time of the complete fusion of the particle tfs exhibits more sharp increase with an increase in the particle size until Rp = 30 mm, followed by a slower increase. In the whole range of variation of the particle radius, the powder particles are subjected to complete melting. Later, some of them can start to solidify. The results presented in Figs. 4–6 show that the optimum size distribution for the powder particles considered is Rp = 10–20 mm. In this case, the particles arrive in the liquid
Fig. 4. Variation in particle mean temperature with spraying distance.
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V.V. Sobole6 et al. / Journal of Materials Processing Technology 79 (1998) 213–216
Fig. 6. Variation in particle mean temperature with particle radius for different spraying distances.
Fig. 7. Variations in particle fusion time, flight time and their ratio with particle radius.
state at the substrate surface located at the recommended spraying distance z =0.4 m. From Figs. 1 and 4, it follows that for this size distribution the spraying distance can be extended to z= 0.5 m, i.e. the range of the recommended spraying distances is z= 0.4 – 0.5 m. With such spraying parameters, the velocities and temperatures of the powder particles on the substrate surface enable the building up of quality coatings.
4. Conclusions (1) The particle velocity varies non-uniformly with spraying distance: first it increases, attains the maximum value and then decreases on moving towards the substrate surface. (2) Acceleration of the particle motion before achieving the maximum value and deceleration of its motion afterwards become more pronounced with decreasing particle radius.
.
(3) With relatively small spraying distances the particle velocity decreases uniformly with an increase in the particle radius. At greater spraying distances, the particle velocity exhibits non-uniform behaviour with increasing particle radius and achieves the maximum values at some particular values of Rp. (4) The powder particles are first heated to their melting point, the particle temperature increases slowly in the melting period. After melting, the particle temperature increases up to its maximum value, after which it starts to decrease and can fall to the solidification temperature. (5) The periods of melting and solidification increase and the behaviour of the particle temperature becomes more uniform with increasing particle radius. Relatively larger particles (Rp = 30–40 mm) seem to impinge onto the substrate surface in a partially solidified state. (6) Within the whole range of the particle radius, the powder particles are subjected to complete melting. Later, some of them start to solidify. For the recommended spraying distance, z= 0.4 m, the optimum size distribution of the powder particles considered is Rp = 10–20 mm. With this size distribution a range of the recommended spraying distances can be extended to z =0.4–0.5. (7) The results obtained are based upon experimentally-validated models and can be used for predictive purposes.
Acknowledgements The authors are grateful to the Generalitat de Catalunya (project 1995 SGR00423) and CICYT (project MAT 96-0426) for financial support. Professor V.V. Sobolev acknowledges sabbatical concession SAB96-0444. J.A. Calero expresses his gratitude to the Generalitat de Catalunya for grant F.I.
References [1] D.W. Parker, G.L. Kutner, Adv. Mater. Process. 7 (1994) 31–35. [2] A.J. Sturgeon, Met. Mater. 8 (1992) 547 – 548. [3] V.V. Sobolev, J.M. Guilemany, Int. Mater. Rev. 41 (1) (1996) 13 – 32. [4] V.V. Sobolev, J.M. Guilemany, J.A. Calero, J. Therm. Spray Technol. 4 (3) (1995) 287 – 296. [5] V.V. Sobolev, J.M. Guilemany, J.A. Calero, J. Mater. Process. Manuf. Sci. 4 (1) (1995) 25 – 39. [6] V.V. Sobolev, J.M. Guilemany, J.C. Garnier, J.A. Calero, Surf. Coat. Technol. 63 (1994) 181 – 187. [7] Metals Handbook, 10th ed., Properties and Selection: Irons, Steels and High-Performance Alloys, vol. 1, ASM Int., Materials Park, OH, 1990.
Modelling of the in-flight behaviour of stainless steel powder particles in high velocity oxy-fuel spraying V.V. Sobolev *, J.M. Guilemany, A.J. Martı´n, J.A. Calero, P. Vilarrubias Centro de Proyeccio´n Te´rmica, Ciencia de Materiales-Ingenierı´a Metalu´rgica, Deparamento de Ingenierı´a Quı´mica y Metalu´rgia, Uni6ersidad de Barcelona, Martı´ i Franque`s, 1, 08028 Barcelona, Spain Received 14 March 1997
Abstract Modelling of the mechanical and thermal behaviour of stainless steel 316L powder particles during high velocity oxy-fuel (HVOF) spraying is presented. This modelling accounts for the combustion process, the gas dynamics inside and outside of the spray gun, gas-particle interactions, acceleration and deceleration of the gas flow, internal heat conduction in the powder particles and particle heating, melting, cooling and solidification. Variations of the times of flight and melting of the particles are studied. Optimal conditions of spraying are predicted. The results agree with experimentally-established HVOF spraying practice. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Modelling; HVOF spraying; Stainless steel powder; In-flight behaviour
1. Introduction
2. Modelling procedure.
In recent decades, high velocity oxy-fuel (HVOF) spraying has been one of the most rapidly developed areas of thermal spraying used in many industrial applications [1,2]. Stainless steel powders are promising in increasing the corrosion resistance of HVOF sprayed coatings. To improve the technology of the HVOF spraying of these powders and to establish its optimum conditions, both the mechanical and the thermal behaviour of the powder particles during their motion must be studied. Mathematical simulation is an effective tool to approach this problem. The purpose of this paper is to provide modelling of the in-flight mechanical and thermal behaviour of the stainless steel powder particles using the models developed in Refs. [3 – 6], to study variations of the particle velocity and temperature with respect to the spraying parameters and to predict the optimum spray conditions.
Mathematical models describing the in-flight behaviour of the powder particles during HVOF spraying are developed in Refs. [3–6]. First, the evolution of the particle velocity Vp along the spraying distance is determined taking into account the combustion process, gas dynamic phenomena inside and outside the spray gun, gas-particle interactions, acceleration and deceleration of the gas flow and specific variations of the drag coefficient with the Reynolds number. Then the heattransfer problem is solved numerically, accounting for the internal heat conduction within a powder particle, heat exchange between the particle and the surrounding gases and particle heating, melting, cooling and solidification at the spraying distance. Variations of the particle flight time tfl and the time of complete fusion of the particle, tfs, are studied. Powder particles made of the stainless steel 316 L are used for modelling, their thermophysical properties being taken from [7]. The parameters of the HVOF spray system correspond to the CDS gun installed at the Centre of Thermal Spraying of the University of Barcelona (Plasma Technik PT100). The powder particle diameter dp = 20–80 mm and particle initial temperature T0 = 1300°C.
* Corresponding author. Tel.: +34 3 4021297; fax: + 34 3 4021638; e-mail: [email protected] 0924-0136/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0924-0136(98)00013-2
214
V.V. Sobole6 et al. / Journal of Materials Processing Technology 79 (1998) 213–216
Fig. 1. Variation in particle velocity with spraying distance.
3. Results and discussion
3.1. Particle mechanical beha6iour Variations of the particle velocity at the spraying distance z are presented in Fig. 1, where initial point z = 0 corresponds to the exit from the combustion chamber. Then the particle velocity Vp increases, attains the maximum value Vpm at z =zmv and decreases on moving towards the substrate surface. Acceleration of the particle motion before the attaining of Vpm (zB zmv) becomes more pronounced with a decrease in the particle radius Rp, since the inertia effects decrease with decreasing mass of the particle. The maximum velocity of the particle increases with a decrease in Rp and its axial position zmv is displaced towards the substrate surface when the particle radius increases (Fig. 2). Fig. 3 shows variations of the particle velocities at different spraying distances with respect to the particle radius. With a relatively small spraying distance the value of Vp decreases uniformly with increasing Rp. At greater spraying distances the particle velocity behaves non-uniformly with increase in the particle radius and attains maximum values at particular values of Rp: these values are achieved at Rp =20 mm when z= 0.5 m and at Rp = 25 mm when z =0.6 m.
3.2. Particle thermal beha6iour The different stages of the particle thermal behaviour—heating, melting, heating again, cooling and solidification—were considered. It is clear that solidifi-
cation should be avoided during particle in-flight motion in HVOF spraying, because it is detrimental to the coating structure and properties. However, modelling must also cover this possibility in order to be able to choose the optimum processing conditions. During HVOF spraying, the powder particles are first heated until they reach their solidus temperature (Fig. 4). Then, during the melting period, the mean temperature T of the particle increases slowly due to absorption of the latent heat of fusion. After melting, the particle temperature continues to increase up to the maximum and then starts to decrease as the gas temperature reduces. Finally, the particle temperature for relatively large particles (starting from Rp = 20 mm) achieves the liquidus temperature and enters the thermal interval of solidification (the solid–liquid ‘mushy’ zone) between the temperatures of liquidus and solidus of the stainless steel considered. During the solidification process the particle is cooled very slowly despite the essential decrease in the gas temperature due to the extraction of the latent heat. The periods of melting and solidification increase and the behaviour of the particle temperature becomes more uniform with increasing particle radius. The maximum of the particle temperature Tm decreases with an increase in Rp (Fig. 5). The axial position zmt of Tm varies non-uniformly with increasing particle radius: it increases, attains the maximum value at Rp = 22 mm and then decreases. Variations of the particle temperature at different spraying distances z show that this temperature decrease with an increase in z and finally enters the thermal interval of solidification for relatively large
V.V. Sobole6 et al. / Journal of Materials Processing Technology 79 (1998) 213–216
Fig. 2. Variations in particle maximum velocity and its axial position with particle radius.
Fig. 3. Variation in particle velocity with particle radius for different spraying distances.
value of Rp =30–40 mm (Fig. 6). This means that the powder particles with these values of Rp impinge onto the substrate surface in a partially solidified state.
215
Fig. 5. Variation in particle mean temperature and its axial position with particle radius.
For the development of the coatings on the substrate surface, it is important to know the time of complete fusion of the particle tfs, the time of the particle flight tfl 1 and their ratio z= tfst − fl . Variations of these parameters with respect to the particle radius are given in Fig. 7. The duration of the particle flight tfl increases almost linearly with increasing Rp, while the time of the complete fusion of the particle tfs exhibits more sharp increase with an increase in the particle size until Rp = 30 mm, followed by a slower increase. In the whole range of variation of the particle radius, the powder particles are subjected to complete melting. Later, some of them can start to solidify. The results presented in Figs. 4–6 show that the optimum size distribution for the powder particles considered is Rp = 10–20 mm. In this case, the particles arrive in the liquid
Fig. 4. Variation in particle mean temperature with spraying distance.
216
V.V. Sobole6 et al. / Journal of Materials Processing Technology 79 (1998) 213–216
Fig. 6. Variation in particle mean temperature with particle radius for different spraying distances.
Fig. 7. Variations in particle fusion time, flight time and their ratio with particle radius.
state at the substrate surface located at the recommended spraying distance z =0.4 m. From Figs. 1 and 4, it follows that for this size distribution the spraying distance can be extended to z= 0.5 m, i.e. the range of the recommended spraying distances is z= 0.4 – 0.5 m. With such spraying parameters, the velocities and temperatures of the powder particles on the substrate surface enable the building up of quality coatings.
4. Conclusions (1) The particle velocity varies non-uniformly with spraying distance: first it increases, attains the maximum value and then decreases on moving towards the substrate surface. (2) Acceleration of the particle motion before achieving the maximum value and deceleration of its motion afterwards become more pronounced with decreasing particle radius.
.
(3) With relatively small spraying distances the particle velocity decreases uniformly with an increase in the particle radius. At greater spraying distances, the particle velocity exhibits non-uniform behaviour with increasing particle radius and achieves the maximum values at some particular values of Rp. (4) The powder particles are first heated to their melting point, the particle temperature increases slowly in the melting period. After melting, the particle temperature increases up to its maximum value, after which it starts to decrease and can fall to the solidification temperature. (5) The periods of melting and solidification increase and the behaviour of the particle temperature becomes more uniform with increasing particle radius. Relatively larger particles (Rp = 30–40 mm) seem to impinge onto the substrate surface in a partially solidified state. (6) Within the whole range of the particle radius, the powder particles are subjected to complete melting. Later, some of them start to solidify. For the recommended spraying distance, z= 0.4 m, the optimum size distribution of the powder particles considered is Rp = 10–20 mm. With this size distribution a range of the recommended spraying distances can be extended to z =0.4–0.5. (7) The results obtained are based upon experimentally-validated models and can be used for predictive purposes.
Acknowledgements The authors are grateful to the Generalitat de Catalunya (project 1995 SGR00423) and CICYT (project MAT 96-0426) for financial support. Professor V.V. Sobolev acknowledges sabbatical concession SAB96-0444. J.A. Calero expresses his gratitude to the Generalitat de Catalunya for grant F.I.
References [1] D.W. Parker, G.L. Kutner, Adv. Mater. Process. 7 (1994) 31–35. [2] A.J. Sturgeon, Met. Mater. 8 (1992) 547 – 548. [3] V.V. Sobolev, J.M. Guilemany, Int. Mater. Rev. 41 (1) (1996) 13 – 32. [4] V.V. Sobolev, J.M. Guilemany, J.A. Calero, J. Therm. Spray Technol. 4 (3) (1995) 287 – 296. [5] V.V. Sobolev, J.M. Guilemany, J.A. Calero, J. Mater. Process. Manuf. Sci. 4 (1) (1995) 25 – 39. [6] V.V. Sobolev, J.M. Guilemany, J.C. Garnier, J.A. Calero, Surf. Coat. Technol. 63 (1994) 181 – 187. [7] Metals Handbook, 10th ed., Properties and Selection: Irons, Steels and High-Performance Alloys, vol. 1, ASM Int., Materials Park, OH, 1990.