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Oxide formation in the Sprayform Tool Process
Materials Science and Engineering A 383 (2004) 50–57
Oxide formation in the Sprayform Tool Process S. Hoile a , T. Rayment a , P.S. Grant a,∗ , A.D. Roche b b
a Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK Ford Motor Company, Manufacturing & Process Research and Advanced Engineering, MD3135, 2101 Village Road, Dearborn, MI 48124, USA
Received 27 January 2004
Abstract The Sprayform Tool Process is a novel alternative to CNC machining for the rapid manufacture of hard tooling. The process is based on the robot controlled sprayforming of molten steel droplets onto a ceramic master pattern to form a thick, dimensionally accurate steel shell. This paper describes the Sprayform Tool Process, and investigates the evolution of microstructure in thick steel shells in terms of the oxide fraction as a function of process parameters: (i) spray height; (ii) atomising gas pressure; (iii) wire feed rate; and (iv) manipulating robot velocity. Spray height had the strongest influence on oxide content; by reducing spray height from 160 to 120 mm the oxide fraction in the microstructure was halved. The role of droplet splashing on the subsequent oxidation of splash droplets has been revealed as an important phenomenon controlling sprayed microstructure and oxide fraction. On the basis of the presented experimental results, the implications for the manufacture of large steel shells by the Sprayform Tool Process are discussed. © 2004 Elsevier B.V. All rights reserved. Keywords: Electric arc spraying; Oxidation; Process optimisation; Sprayforming; Steel; Rapid tooling
1. Introduction Electric arc spraying is used traditionally as a cost-effective method for providing wear resistance to small industrial components or corrosion resistance to larger structures such as bridges. In these applications, a single, usually hand held electric arc gun, is used to form incrementally a coating of thickness typically 0.2–10 mm. Coating materials include zinc, aluminium and nickel base alloys as well as many steel grades [1]. More recently, electric arc spraying is being used to manufacture thick (typically 10–30 mm), three-dimensional free standing carbon steel shells for near net shape manufacture of dies or moulds in press tool and polymer injection moulding industries [2]. The process is derived from the thermal spraying of soft zinc tools, in which a molten zinc spray from an electric arc spray gun is deposited onto a sacrificial substrate that is a three-dimensional facsimile of the component to be produced [3]. The deposited shell accurately replicates the substrate topography [4] and is then trimmed, backed and bolstered for use in a range of appli∗ Corresponding author. Tel.: +44-1865-283324; fax: +44-1865-848790. E-mail address: [email protected] (P.S. Grant).
0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2004.02.068
cations. In order to produce steel shells suitable for mass production the thermal history of the steel shell must be controlled carefully to eliminate residual stresses and distortion [5,6]. Furthermore, to obtain sufficient mechanical integrity, corrosion and wear resistance, as-sprayed shells with minimum porosity and maximum strength are required [7]. Fig. 1 shows a sprayformed punch tool manufactured by the Ford Motor Company, which was used to produce over 70,000 pressed steel components in a production application. In comparison with conventionally machined steel or cast iron tooling, sprayformed tools have the potential to reduce tool lead-times by up to 50% and tool costs by up to 30%, depending upon the specific tooling application [8]. For press tooling applications, fracture toughness, machinability and weldability are paramount considerations and mandate that the sprayed steel shells have the lowest oxide fraction. As described subsequently, in the electric arc spraying of steel shells, N2 is used as the atomising and carrier gas for the resulting superheated steel droplets, and also cools both the droplets in-flight to the point of deposition and the surface of the growing sprayformed shell. Sprayforming takes place within an acoustically insulated spray cell with a continuous cross-flow of ambient air to remove over-spray, dust and weld fume to a large multiple filter unit. The use
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Fig. 1. A sprayformed punch tool manufactured by Ford Motor Company.
of N2 atomising gas provides some shrouding of the droplet spray, but nonetheless entrainment of air into the N2 gas stream results in significant in-flight droplet oxidation and shell surface oxidation [9]. This paper provides a description of the Sprayform Tool Process for the production of steel shells from Fe–0.8 wt.% C feedstock wires and investigates the effect of electric arc spraying parameters and spray strategies to minimise the area fraction of oxide.
2. Experimental Fig. 2 shows a schematic diagram of the sprayforming equipment at Oxford University for the manufacture of sprayformed shells. Four Sulzer Metco SmartArc electric arc spray guns were used to spray Fe–0.8 wt.% C droplets from a wire feedstock (TAFA 38T) using N2 at a pressure of 300 kPa as the atomising gas. The arc spray guns were mounted to a Kuka six-axis programmable industrial robot. The robot moved the gun cluster arrangement over the surface of a ceramic substrate in a pre-determined, repetitive manner termed the ‘path-plan’ [10]. A series of steel shells were sprayformed under different deposition conditions on 305 mm × 305 mm × 40 mm alumina based freeze cast ceramics. Process parameters of spray height, atomising gas pressure, robot velocity, gun current and wire feed rate per gun were varied as shown in Table 1. In all experiments, the arc voltage was maintained at 30 V. Microstructural investigation and measurement of area fraction of oxide in polished cross-sections of the sprayed shell microstructures was carried out by optical microscopy and image analysis using a Zeiss Axioplan2
Fig. 2. Schematic diagram of the experimental arrangement used in the Sprayform Tool Process installed at the Department of Materials, University of Oxford. Table 1 Electric arc spray parameters for experiments to determine their effect on area percent oxide in sprayformed steel shells Run
Spray height (mm)
Robot velocity (mm s−1 )
Gun current (A)
Wire feed rate (g s−1 )
N2 gas pressure (kPa)
1 2 3 4 5 6 7 8 9
160 200 120 160 160 160 160 160 160
400 400 400 400 400 200 100 400 400
150 150 150 150 150 150 150 250 360
1.5 1.5 1.5 1.5 1.5 1.5 1.5 3.0 4.5
300 300 300 200 400 300 300 300 300
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Fig. 3. Typical photomicrograph of the steel shell microstructure.
stereomicroscope with Zeiss image analysis software. Measurements were taken from 163 m × 121 m areas for 50 randomly distributed locations throughout carefully polished (to minimise porosity enlargement) shell cross-sections. Significant care was taken to threshold digital images so that only oxide fraction was measured and the lower fraction of darker porosity was excluded. In all cases, significant contrast between oxide (grey) and porosity (black) was available to discriminate with confidence.
3. Results Fig. 3 shows a typical micrograph of the sprayformed microstructure with relatively low average porosity (black areas) of <2%. Similarly to all the sprayed shells, Fig. 3 shows a heterogeneous microstructure with regions of high and low oxide and porosity fraction. Fig. 3 also shows a number of pre-solidified droplets incorporated into the microstructure, which originated from droplet splashing [8]. In general, the areas of high area
Fig. 4. Photomicrograph showing the formation of ‘columns’ of relatively high oxide fraction (bounded by dashed lines) and increased number of pre-solidified droplets within the shell microstructure.
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Fig. 5. Area percent oxide as a function of wire feed rate per electric arc spray gun, with otherwise standard conditions of 160 mm spray height, 300 kPa nitrogen gas pressure and 400 mm s−1 robot velocity.
Fig. 7. Area percent oxide as a function of atomising gas pressure, with otherwise standard conditions of 1.5 g s−1 wire feed rate, 160 mm spray height, and 400 mm s−1 robot velocity.
fraction oxide were formed in columns that grew from the substrate–surface interface at an angle of ∼45◦ to the vertical as shown in Fig. 4. Regions of relatively high oxide fraction in these columns also contained a higher than average number of the smaller, pre-solidified droplets. Fig. 5 shows that the oxide fraction as a function of wire feed rate per electric arc spray gun increased from 15.5 to
21% as wire feed rate doubled from 1.5 to 3.0 g s−1 and then remained approximately constant as wire feed rate further increased to 4.5 g s−1 . Fig. 6 shows the oxide variation as a function of spray height, indicating spray height had the strongest effect of all parameters investigated on the formation of oxide in the
Fig. 6. Area percent oxide as a function of spray height, with otherwise standard conditions of 1.5 g s−1 wire feed rate, 300 kPa nitrogen gas pressure and 400 mm s−1 robot velocity.
Fig. 8. Area percent oxide as a function of robot velocity, with otherwise standard conditions of 1.5 g s−1 wire feed rate, 160 mm spray height and 300 kPa nitrogen gas pressure.
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Fig. 9. Photomicrographs of shell microstructures manufactured at robot velocities of: (a) 100 mm s−1 and (b) 400 mm s− 1.
sprayed shell microstructure. While spray heights of 160 mm (standard) and 200 mm produced similar oxide fractions of 15.5 and 16%, respectively, a reduction in spray height to 120 mm reduced the oxide fraction by approximately 50%.
Fig. 7 shows there was a surprisingly weak dependence of oxide fraction on atomising N2 gas pressure, with oxide fractions lying in the range 16–18% as atomising gas pressure increased from 200 to 400 kPa.
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Fig. 8 shows the variation in oxide fraction as a function of relative robot velocity over the shell surface. As robot velocity increased from 100 to 400 mm s−1 , the oxide fraction reduced steadily from 18 to 15%. The shell microstructures at robot velocities of 100 and 400 mm s−1 are shown in Fig. 9(a) and (b), respectively. At 100 mm s−1 , Fig. 9(a) shows distinct regions of relatively high oxide fraction and relatively low oxide fraction while Fig. 9(b) shows that the shell manufactured at a robot velocity of 400 mm s−1 again had a heterogeneous microstructure but the scale of all microstructural features was refined significantly.
4. Discussion In common with many thermally sprayed materials, the microstructure of all the thick steel shells was non-uniform. Areas of relatively high oxide and porosity fractions formed in bands at angles of ±45◦ to the substrate normal. The mechanism by which these columns of deleterious material are formed is [8]: • Steel droplets, typically 0.1 mm in diameter with a velocity of ∼100 m s−1 [12], oxidise in-flight because of entrainment of ambient O2 from the atmosphere in the spray cell. • A proportion of the slightly oxidised droplets that impact the substrate splash and break-up into smaller droplets, which oxidise readily because of their significantly increased surface area to volume ratio. Most of these splash droplets become entrained in the predominantly lateral flow of the atomising gas as it impinges and flows across the surface of the substrate. However, some of the splash droplets re-deposit on perturbations on the previously sprayed surface. • The preferential re-deposition on perturbations causes the perturbations to grow and therefore capture progressively more primary spray or splash droplets. A growing column of relatively high oxide and porosity fraction (associated with the splash droplets) thus propagates through the microstructure. In the experimental arrangement used here, three of the four electric arc spray guns were at 45◦ to the surface normal, hence the columns are similarly propagated preferentially at approximately ±45◦ to the surface normal. Because of arc fluctuations and resulting changes in the local atomising conditions [4,13,14] inherent to the electric arc spray process, perturbations in the surface profile of the deposit during manufacture are inevitable. The amplitude of perturbations in electric arc spraying are relatively large because of the relatively large droplet diameters [14], and similarly account for the heterogeneous nature of electric arc sprayed microstructures. As wire feed rate increased from 1.5 to 4.5 g s−1 the oxide fraction increased by 26% towards a constant fraction at the higher wire feed rates. As the rate at which the consum-
55
able Fe–0.8 wt.% C wire is fed into the arc increases, there is an increase in mean droplet diameter [14] and an average decrease in the specific surface area to volume ratio of the spray. Therefore, it may be expected that there would be a decrease in oxide fraction with increasing wire feed rate. However, this reasoning ignores endemic droplet splashing which occurs in electric arc spraying of Fe–0.8 wt.% C droplets under the conditions investigated [11]. The stability of the droplet at impact can be expressed by the droplet Weber number We: We =
ρν2 d γ
(1)
where ρ is the droplet density, ν the droplet axial velocity, d the droplet diameter and γ is the droplet surface energy. Small values of We indicate a tendency towards droplet stability, whereas high values of We indicate a tendency towards droplet break-up. Previous work has shown that at the relatively extended spray height of 160 mm used here, an increase in the wire feed rate over the range of interest produces small changes in the spray average velocity (∼100 m s−1 ) and temperature (2250 ◦ C) at deposition [15]. Consequently as the wire feed rate increases, the average Weber number of the spray will increase because of the increase in mean droplet diameter as changes to the other parameters in Eq. (1) will be negligibly small. Droplets at higher wire feed rates have an increased tendency for splashing and it is these splash droplets that readily oxidise to produce an increase in oxide fraction. Unfortunately, because shell temperature and wire feed rate are inextricably linked, and the essential requirement to control shell temperatures [3], wire feed rate is not a practical parameter to control oxide fractions in sprayed shells for tooling applications. Increasing the N2 atomising gas pressure (and flow rate) may be expected to increase the partial pressure of N2 in the spray cone and at the shell surface, resulting in the inhibition of both in-flight and shell surface oxidation [16]. Fig. 7 showed a slight increase in oxide fraction with N2 atomising gas pressure. A number of other effects must also be considered: (i) an increase in gas pressure reduces the mean droplet size of the spray, increasing the average surface area to volume and increasing the tendency for oxidation [14]; (ii) smaller droplet diameters cool faster as a function of time and so reduce the rate of oxidation reactions; (iii) smaller droplet diameters have increased axial velocities and the time available for in-flight oxidation decreases; and (iv) N2 atomisation pressures may affect splash droplet oxidation behaviour. Based on the results described here it is proposed that the minimum N2 atomisation pressure investigated of 200 kPa already provides maximum in-flight shrouding of the primary and splash droplets achievable while spraying in an atmospheric ambient environment, and that oxidation slightly increases with increasing N2 gas pressure because of the underlying shift to smaller primary droplet and secondary droplet diameters. However, within the recommended oper-
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ating range of the electric arc spray guns of gas pressure 200–400 kPa, varying the N2 gas pressure does not provide sufficiently marked changes in shell oxide fraction to be useful. Fig. 8 showed that increasing the robot velocity over the shell surface from 100 to 400 mm s−1 had little effect on oxide fraction since droplet dynamic and thermal history are unaffected by robot velocity. However, Fig. 9 showed that the scale of microstructural features is reduced at higher robot velocities. As the leading edge of the advancing spray cone moves over the sprayed surface, it presents a ‘step’ of material, which is deposited progressively over the surface [17]. At the robot relative velocity of 100 mm s−1 and under the conditions used in this study, this step height is ∼200 m which corresponds to the larger scale bands shown in Fig. 9(a). At 400 mm s−1 , the step height reduces to ∼50 m, again consistent with the refinement of microstructural features in Fig. 9(b). Therefore, while robot velocity is an important variable for controlling the scale of microstructural features it has no significant effect on the shell oxide fraction. Fig. 6 showed that spray height had the strongest effect on oxide fraction, primarily because of the reduction in the time-of-flight of droplets to deposition and the resulting reduction in in-flight oxidation. Reductions in oxide fraction of ∼50% over previous processing conditions were achieved. Spray height is also one of the easiest processing variables to adjust without affecting other aspects of the process. The typical spray height for the Sulzer Metco SmartArc electric arc guns is between ∼100 and 150 mm, whereas the spray height of 160 mm used as standard here is derived from the restricted geometry of the electric arc spray gun cluster configuration [18]. Further experiments have shown that if the spray height is reduced below 100 mm, then excessive build-up of splash droplets on the end of the electric arc spray guns may occur.
5. Conclusions The Sprayform Tool Process utilises multiple electric arc guns mounted on a six-axis robot and has been used to manufacture thick 305 mm × 305 mm steel shells by deposition onto a sacrificial freeze cast ceramic facsimile under carefully controlled thermal conditions. Because of the need to reduce oxide fractions in the steel shells for use in the most demanding press tool applications, process parameters of wire feed rate, atomising gas pressure, robot velocity and spray height have been investigated in terms of their influence on the formation and fraction of oxide within the sprayformed shell microstructure. • Within the recommended range of operating pressures, N2 gas pressure had the weakest effect on shell oxide fraction. • Although decreasing wire feed rate had a significant effect on reducing shell oxide fraction, wire feed rate
was inextricably linked to shell temperature and therefore could not be used to independently control oxide fraction. • Robot relative velocity over the shell surface was strongly related to the scale of microstructural features but had little effect on oxide fraction, and in general, robot velocities should be as high as possible. • Reductions in spray height decreased oxide fraction and offered a practical means to minimise the formation of deleterious oxide. However, a minimum spray height was established, beyond which further reduced spray heights caused re-deposition of splash droplets on the electric arc spray guns. • In order to rationalise the observed trends in oxide formation and to explain as-sprayed steel shell microstructures, droplet splashing and subsequent oxidation behaviour have been shown to be important.
Acknowledgements The authors would like to thank the United Kingdom Engineering and Physical Sciences Research Council and the Ford Motor Company, USA for financial support, and Nuffield Bursary student Christof Bachmann for metallographic assistance.
References [1] L. Pawlowski, The Science and Engineering of Thermal Spray Coatings, Wiley, 1995. [2] A.P. Newbery, P.S. Grant, R.M. Jordan, A.D. Roche, T.C. Carr, in: Proceedings of the Fifteenth International Thermal Spray Conference, Nice, France, 25–28 May 1998, ASM International, OH, pp. 1223. [3] C.K. Chua, K.H. Long, S.L. Ho, Int. J. Adv. Manuf. Technol. 15 (1999) 609. [4] B. Zu, X. Liu, S. Ma, Z. Chen, in: Proceedings of the Fifteenth International Thermal Spray Conference, Nice, France, 25–28 May 1988, ASM International, OH, pp. 1039. [5] R.M. Jordan, A.D. Roche, United States Patent 5952056 (1999). [6] M.T. Lusk, R. Jaramillo, A.D. Roche, C. Mgbokwere, S. Deshpande, in: R. Srinivasan, S.L. Semiatin, A. Beaudoin, S. Fox, Z. Jin (Eds.), Microstructural Modeling and Prediction During Thermomechanical Processing, The Minerals, Metals and Materials Society, Pennsylvania, 2001, pp. 75–84. [7] X. Wang, J. Heberlein, E. Pfender, W. Gerberich, J. Therm. Spray Technol. 8 (1999) 565. [8] P.S. Grant, T. Rayment, Z. Djuric, S. Hoile, P. Jones, S. Duncan, A.D. Roche, D. Field, in: Proceedings of the Seventh Time Compression Technologies Conference, Manchester, UK, 16–17 October 2002, Rapid News Publications PLC and Time-Compression Technologies, Cheshire, 2002. [9] R.A. Neiser, M.F. Smith, R.C. Dykhuizen, J. Therm. Spray Technol. 7 (1998) 537. [10] P.D.A. Jones, S.R. Duncan, T. Rayment, P.S. Grant, UK Patent Application GB/0115731.2 (2001). [11] A.P. Newbery, P.S. Grant, J. Therm. Spray Technol. 9 (2000) 250. [12] Z. Djuri´c, P.S. Grant, Modell. Simul. Mater. Sci. Eng. 9 (2001) 111.
S. Hoile et al. / Materials Science and Engineering A 383 (2004) 50–57 [13] T. Watanabe, X. Wang, E. Pfender, J. Heberlein, Thin Solid Films 316 (1998) 169. [14] A.P. Newbery, P.S. Grant, Powder Metall. 46 (2003) 229. [15] P.S. Grant, A.P. Newbery, Surf. Coat. Technol. (August 2003), submitted for publication. [16] G. Jandin, H. Liao, Z.Q. Feng, C. Coddet, Mater. Sci. Eng. A349 (2003) 298.
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[17] M.K. Hedges, A.P. Newbery, P.S. Grant, Mater. Sci. Eng. A326 (2002) 79. [18] T. Rayment, P.S. Grant, in: Proceedings of the Thirteenth International Federation for Heat Treatment and Surface Engineering Congress/International Surface Engineering Congress, Columbia, OH, 7–10 October 2002, ASM International, OH.
Oxide formation in the Sprayform Tool Process S. Hoile a , T. Rayment a , P.S. Grant a,∗ , A.D. Roche b b
a Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK Ford Motor Company, Manufacturing & Process Research and Advanced Engineering, MD3135, 2101 Village Road, Dearborn, MI 48124, USA
Received 27 January 2004
Abstract The Sprayform Tool Process is a novel alternative to CNC machining for the rapid manufacture of hard tooling. The process is based on the robot controlled sprayforming of molten steel droplets onto a ceramic master pattern to form a thick, dimensionally accurate steel shell. This paper describes the Sprayform Tool Process, and investigates the evolution of microstructure in thick steel shells in terms of the oxide fraction as a function of process parameters: (i) spray height; (ii) atomising gas pressure; (iii) wire feed rate; and (iv) manipulating robot velocity. Spray height had the strongest influence on oxide content; by reducing spray height from 160 to 120 mm the oxide fraction in the microstructure was halved. The role of droplet splashing on the subsequent oxidation of splash droplets has been revealed as an important phenomenon controlling sprayed microstructure and oxide fraction. On the basis of the presented experimental results, the implications for the manufacture of large steel shells by the Sprayform Tool Process are discussed. © 2004 Elsevier B.V. All rights reserved. Keywords: Electric arc spraying; Oxidation; Process optimisation; Sprayforming; Steel; Rapid tooling
1. Introduction Electric arc spraying is used traditionally as a cost-effective method for providing wear resistance to small industrial components or corrosion resistance to larger structures such as bridges. In these applications, a single, usually hand held electric arc gun, is used to form incrementally a coating of thickness typically 0.2–10 mm. Coating materials include zinc, aluminium and nickel base alloys as well as many steel grades [1]. More recently, electric arc spraying is being used to manufacture thick (typically 10–30 mm), three-dimensional free standing carbon steel shells for near net shape manufacture of dies or moulds in press tool and polymer injection moulding industries [2]. The process is derived from the thermal spraying of soft zinc tools, in which a molten zinc spray from an electric arc spray gun is deposited onto a sacrificial substrate that is a three-dimensional facsimile of the component to be produced [3]. The deposited shell accurately replicates the substrate topography [4] and is then trimmed, backed and bolstered for use in a range of appli∗ Corresponding author. Tel.: +44-1865-283324; fax: +44-1865-848790. E-mail address: [email protected] (P.S. Grant).
0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2004.02.068
cations. In order to produce steel shells suitable for mass production the thermal history of the steel shell must be controlled carefully to eliminate residual stresses and distortion [5,6]. Furthermore, to obtain sufficient mechanical integrity, corrosion and wear resistance, as-sprayed shells with minimum porosity and maximum strength are required [7]. Fig. 1 shows a sprayformed punch tool manufactured by the Ford Motor Company, which was used to produce over 70,000 pressed steel components in a production application. In comparison with conventionally machined steel or cast iron tooling, sprayformed tools have the potential to reduce tool lead-times by up to 50% and tool costs by up to 30%, depending upon the specific tooling application [8]. For press tooling applications, fracture toughness, machinability and weldability are paramount considerations and mandate that the sprayed steel shells have the lowest oxide fraction. As described subsequently, in the electric arc spraying of steel shells, N2 is used as the atomising and carrier gas for the resulting superheated steel droplets, and also cools both the droplets in-flight to the point of deposition and the surface of the growing sprayformed shell. Sprayforming takes place within an acoustically insulated spray cell with a continuous cross-flow of ambient air to remove over-spray, dust and weld fume to a large multiple filter unit. The use
S. Hoile et al. / Materials Science and Engineering A 383 (2004) 50–57
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Fig. 1. A sprayformed punch tool manufactured by Ford Motor Company.
of N2 atomising gas provides some shrouding of the droplet spray, but nonetheless entrainment of air into the N2 gas stream results in significant in-flight droplet oxidation and shell surface oxidation [9]. This paper provides a description of the Sprayform Tool Process for the production of steel shells from Fe–0.8 wt.% C feedstock wires and investigates the effect of electric arc spraying parameters and spray strategies to minimise the area fraction of oxide.
2. Experimental Fig. 2 shows a schematic diagram of the sprayforming equipment at Oxford University for the manufacture of sprayformed shells. Four Sulzer Metco SmartArc electric arc spray guns were used to spray Fe–0.8 wt.% C droplets from a wire feedstock (TAFA 38T) using N2 at a pressure of 300 kPa as the atomising gas. The arc spray guns were mounted to a Kuka six-axis programmable industrial robot. The robot moved the gun cluster arrangement over the surface of a ceramic substrate in a pre-determined, repetitive manner termed the ‘path-plan’ [10]. A series of steel shells were sprayformed under different deposition conditions on 305 mm × 305 mm × 40 mm alumina based freeze cast ceramics. Process parameters of spray height, atomising gas pressure, robot velocity, gun current and wire feed rate per gun were varied as shown in Table 1. In all experiments, the arc voltage was maintained at 30 V. Microstructural investigation and measurement of area fraction of oxide in polished cross-sections of the sprayed shell microstructures was carried out by optical microscopy and image analysis using a Zeiss Axioplan2
Fig. 2. Schematic diagram of the experimental arrangement used in the Sprayform Tool Process installed at the Department of Materials, University of Oxford. Table 1 Electric arc spray parameters for experiments to determine their effect on area percent oxide in sprayformed steel shells Run
Spray height (mm)
Robot velocity (mm s−1 )
Gun current (A)
Wire feed rate (g s−1 )
N2 gas pressure (kPa)
1 2 3 4 5 6 7 8 9
160 200 120 160 160 160 160 160 160
400 400 400 400 400 200 100 400 400
150 150 150 150 150 150 150 250 360
1.5 1.5 1.5 1.5 1.5 1.5 1.5 3.0 4.5
300 300 300 200 400 300 300 300 300
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Fig. 3. Typical photomicrograph of the steel shell microstructure.
stereomicroscope with Zeiss image analysis software. Measurements were taken from 163 m × 121 m areas for 50 randomly distributed locations throughout carefully polished (to minimise porosity enlargement) shell cross-sections. Significant care was taken to threshold digital images so that only oxide fraction was measured and the lower fraction of darker porosity was excluded. In all cases, significant contrast between oxide (grey) and porosity (black) was available to discriminate with confidence.
3. Results Fig. 3 shows a typical micrograph of the sprayformed microstructure with relatively low average porosity (black areas) of <2%. Similarly to all the sprayed shells, Fig. 3 shows a heterogeneous microstructure with regions of high and low oxide and porosity fraction. Fig. 3 also shows a number of pre-solidified droplets incorporated into the microstructure, which originated from droplet splashing [8]. In general, the areas of high area
Fig. 4. Photomicrograph showing the formation of ‘columns’ of relatively high oxide fraction (bounded by dashed lines) and increased number of pre-solidified droplets within the shell microstructure.
S. Hoile et al. / Materials Science and Engineering A 383 (2004) 50–57
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Fig. 5. Area percent oxide as a function of wire feed rate per electric arc spray gun, with otherwise standard conditions of 160 mm spray height, 300 kPa nitrogen gas pressure and 400 mm s−1 robot velocity.
Fig. 7. Area percent oxide as a function of atomising gas pressure, with otherwise standard conditions of 1.5 g s−1 wire feed rate, 160 mm spray height, and 400 mm s−1 robot velocity.
fraction oxide were formed in columns that grew from the substrate–surface interface at an angle of ∼45◦ to the vertical as shown in Fig. 4. Regions of relatively high oxide fraction in these columns also contained a higher than average number of the smaller, pre-solidified droplets. Fig. 5 shows that the oxide fraction as a function of wire feed rate per electric arc spray gun increased from 15.5 to
21% as wire feed rate doubled from 1.5 to 3.0 g s−1 and then remained approximately constant as wire feed rate further increased to 4.5 g s−1 . Fig. 6 shows the oxide variation as a function of spray height, indicating spray height had the strongest effect of all parameters investigated on the formation of oxide in the
Fig. 6. Area percent oxide as a function of spray height, with otherwise standard conditions of 1.5 g s−1 wire feed rate, 300 kPa nitrogen gas pressure and 400 mm s−1 robot velocity.
Fig. 8. Area percent oxide as a function of robot velocity, with otherwise standard conditions of 1.5 g s−1 wire feed rate, 160 mm spray height and 300 kPa nitrogen gas pressure.
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Fig. 9. Photomicrographs of shell microstructures manufactured at robot velocities of: (a) 100 mm s−1 and (b) 400 mm s− 1.
sprayed shell microstructure. While spray heights of 160 mm (standard) and 200 mm produced similar oxide fractions of 15.5 and 16%, respectively, a reduction in spray height to 120 mm reduced the oxide fraction by approximately 50%.
Fig. 7 shows there was a surprisingly weak dependence of oxide fraction on atomising N2 gas pressure, with oxide fractions lying in the range 16–18% as atomising gas pressure increased from 200 to 400 kPa.
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Fig. 8 shows the variation in oxide fraction as a function of relative robot velocity over the shell surface. As robot velocity increased from 100 to 400 mm s−1 , the oxide fraction reduced steadily from 18 to 15%. The shell microstructures at robot velocities of 100 and 400 mm s−1 are shown in Fig. 9(a) and (b), respectively. At 100 mm s−1 , Fig. 9(a) shows distinct regions of relatively high oxide fraction and relatively low oxide fraction while Fig. 9(b) shows that the shell manufactured at a robot velocity of 400 mm s−1 again had a heterogeneous microstructure but the scale of all microstructural features was refined significantly.
4. Discussion In common with many thermally sprayed materials, the microstructure of all the thick steel shells was non-uniform. Areas of relatively high oxide and porosity fractions formed in bands at angles of ±45◦ to the substrate normal. The mechanism by which these columns of deleterious material are formed is [8]: • Steel droplets, typically 0.1 mm in diameter with a velocity of ∼100 m s−1 [12], oxidise in-flight because of entrainment of ambient O2 from the atmosphere in the spray cell. • A proportion of the slightly oxidised droplets that impact the substrate splash and break-up into smaller droplets, which oxidise readily because of their significantly increased surface area to volume ratio. Most of these splash droplets become entrained in the predominantly lateral flow of the atomising gas as it impinges and flows across the surface of the substrate. However, some of the splash droplets re-deposit on perturbations on the previously sprayed surface. • The preferential re-deposition on perturbations causes the perturbations to grow and therefore capture progressively more primary spray or splash droplets. A growing column of relatively high oxide and porosity fraction (associated with the splash droplets) thus propagates through the microstructure. In the experimental arrangement used here, three of the four electric arc spray guns were at 45◦ to the surface normal, hence the columns are similarly propagated preferentially at approximately ±45◦ to the surface normal. Because of arc fluctuations and resulting changes in the local atomising conditions [4,13,14] inherent to the electric arc spray process, perturbations in the surface profile of the deposit during manufacture are inevitable. The amplitude of perturbations in electric arc spraying are relatively large because of the relatively large droplet diameters [14], and similarly account for the heterogeneous nature of electric arc sprayed microstructures. As wire feed rate increased from 1.5 to 4.5 g s−1 the oxide fraction increased by 26% towards a constant fraction at the higher wire feed rates. As the rate at which the consum-
55
able Fe–0.8 wt.% C wire is fed into the arc increases, there is an increase in mean droplet diameter [14] and an average decrease in the specific surface area to volume ratio of the spray. Therefore, it may be expected that there would be a decrease in oxide fraction with increasing wire feed rate. However, this reasoning ignores endemic droplet splashing which occurs in electric arc spraying of Fe–0.8 wt.% C droplets under the conditions investigated [11]. The stability of the droplet at impact can be expressed by the droplet Weber number We: We =
ρν2 d γ
(1)
where ρ is the droplet density, ν the droplet axial velocity, d the droplet diameter and γ is the droplet surface energy. Small values of We indicate a tendency towards droplet stability, whereas high values of We indicate a tendency towards droplet break-up. Previous work has shown that at the relatively extended spray height of 160 mm used here, an increase in the wire feed rate over the range of interest produces small changes in the spray average velocity (∼100 m s−1 ) and temperature (2250 ◦ C) at deposition [15]. Consequently as the wire feed rate increases, the average Weber number of the spray will increase because of the increase in mean droplet diameter as changes to the other parameters in Eq. (1) will be negligibly small. Droplets at higher wire feed rates have an increased tendency for splashing and it is these splash droplets that readily oxidise to produce an increase in oxide fraction. Unfortunately, because shell temperature and wire feed rate are inextricably linked, and the essential requirement to control shell temperatures [3], wire feed rate is not a practical parameter to control oxide fractions in sprayed shells for tooling applications. Increasing the N2 atomising gas pressure (and flow rate) may be expected to increase the partial pressure of N2 in the spray cone and at the shell surface, resulting in the inhibition of both in-flight and shell surface oxidation [16]. Fig. 7 showed a slight increase in oxide fraction with N2 atomising gas pressure. A number of other effects must also be considered: (i) an increase in gas pressure reduces the mean droplet size of the spray, increasing the average surface area to volume and increasing the tendency for oxidation [14]; (ii) smaller droplet diameters cool faster as a function of time and so reduce the rate of oxidation reactions; (iii) smaller droplet diameters have increased axial velocities and the time available for in-flight oxidation decreases; and (iv) N2 atomisation pressures may affect splash droplet oxidation behaviour. Based on the results described here it is proposed that the minimum N2 atomisation pressure investigated of 200 kPa already provides maximum in-flight shrouding of the primary and splash droplets achievable while spraying in an atmospheric ambient environment, and that oxidation slightly increases with increasing N2 gas pressure because of the underlying shift to smaller primary droplet and secondary droplet diameters. However, within the recommended oper-
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ating range of the electric arc spray guns of gas pressure 200–400 kPa, varying the N2 gas pressure does not provide sufficiently marked changes in shell oxide fraction to be useful. Fig. 8 showed that increasing the robot velocity over the shell surface from 100 to 400 mm s−1 had little effect on oxide fraction since droplet dynamic and thermal history are unaffected by robot velocity. However, Fig. 9 showed that the scale of microstructural features is reduced at higher robot velocities. As the leading edge of the advancing spray cone moves over the sprayed surface, it presents a ‘step’ of material, which is deposited progressively over the surface [17]. At the robot relative velocity of 100 mm s−1 and under the conditions used in this study, this step height is ∼200 m which corresponds to the larger scale bands shown in Fig. 9(a). At 400 mm s−1 , the step height reduces to ∼50 m, again consistent with the refinement of microstructural features in Fig. 9(b). Therefore, while robot velocity is an important variable for controlling the scale of microstructural features it has no significant effect on the shell oxide fraction. Fig. 6 showed that spray height had the strongest effect on oxide fraction, primarily because of the reduction in the time-of-flight of droplets to deposition and the resulting reduction in in-flight oxidation. Reductions in oxide fraction of ∼50% over previous processing conditions were achieved. Spray height is also one of the easiest processing variables to adjust without affecting other aspects of the process. The typical spray height for the Sulzer Metco SmartArc electric arc guns is between ∼100 and 150 mm, whereas the spray height of 160 mm used as standard here is derived from the restricted geometry of the electric arc spray gun cluster configuration [18]. Further experiments have shown that if the spray height is reduced below 100 mm, then excessive build-up of splash droplets on the end of the electric arc spray guns may occur.
5. Conclusions The Sprayform Tool Process utilises multiple electric arc guns mounted on a six-axis robot and has been used to manufacture thick 305 mm × 305 mm steel shells by deposition onto a sacrificial freeze cast ceramic facsimile under carefully controlled thermal conditions. Because of the need to reduce oxide fractions in the steel shells for use in the most demanding press tool applications, process parameters of wire feed rate, atomising gas pressure, robot velocity and spray height have been investigated in terms of their influence on the formation and fraction of oxide within the sprayformed shell microstructure. • Within the recommended range of operating pressures, N2 gas pressure had the weakest effect on shell oxide fraction. • Although decreasing wire feed rate had a significant effect on reducing shell oxide fraction, wire feed rate
was inextricably linked to shell temperature and therefore could not be used to independently control oxide fraction. • Robot relative velocity over the shell surface was strongly related to the scale of microstructural features but had little effect on oxide fraction, and in general, robot velocities should be as high as possible. • Reductions in spray height decreased oxide fraction and offered a practical means to minimise the formation of deleterious oxide. However, a minimum spray height was established, beyond which further reduced spray heights caused re-deposition of splash droplets on the electric arc spray guns. • In order to rationalise the observed trends in oxide formation and to explain as-sprayed steel shell microstructures, droplet splashing and subsequent oxidation behaviour have been shown to be important.
Acknowledgements The authors would like to thank the United Kingdom Engineering and Physical Sciences Research Council and the Ford Motor Company, USA for financial support, and Nuffield Bursary student Christof Bachmann for metallographic assistance.
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