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Al powder mixture: Coating characteristics and influence of heat treatment on the phase structure
Applied Surface Science 255 (2008) 2538–2544
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Cold spraying of Fe/Al powder mixture: Coating characteristics and influence of heat treatment on the phase structure Hong-Tao Wang, Chang-Jiu Li *, Guan-Jun Yang, Cheng-Xing Li State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 22 May 2008 Received in revised form 21 July 2008 Accepted 22 July 2008 Available online 31 July 2008
In this paper, an iron/aluminum composite coating was deposited by cold spraying using iron and aluminum powder mixtures. The coating was annealed at different temperatures to aim at forming an iron aluminide intermetallic based coating. The results showed that a thick dense Fe/Al composite coating with uniformly distributed Fe and Al particles can be deposited by cold spraying. The compositions of the as-sprayed Fe/Al coating were nearly the same as that of the initial Fe/Al powder mixture. The intensive deformation of particle on impact caused elongation of the grain and disrupted the thin oxide films on powder surface. After annealing at a temperature of 600 8C, an intermediate phase Al5Fe2 coexisted in the deposit with remaining Fe and Al. With increasing annealing temperature to 900 8C, the deposit transformed to mainly FeAl phase with a trace of remaining Fe phase. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Cold spray Microstructure Deposition characteristics Fe/Al Heat treatment
1. Introduction FeAl-based alloys have drawn much attention as materials for high temperature applications because they are low cost intermetallic materials with a relatively low density (5.56 g/cm3) and exhibit good mechanical properties, and excellent corrosion resistance in oxidizing and sulfidizing atmospheres, which is relied on their ability to form a highly protective Al2O3 scale [1–4]. Moreover, Fe and Al, which are raw materials of FeAl phase, are relatively inexpensive and the FeAl intermetallic alloys are lighter than steels or Ni-based alloys. Therefore, FeAl-based alloy is expected as one candidate for substitution of stainless steels or Ni-based superalloys [5]. Although a wide span of research activities on intermetallics has been carried out in the last many years, the commercial applications of iron aluminides as a bulk material was hampered owing to their low ductility and processing problems which result from large difference in the melting points of Al and Fe and exothermic nature of formation of iron aluminides [6]. In order to utilize their excellent properties, the FeAl-based alloys are often used as protective coatings on classical metallic alloys. The coatings are deposited by several coating techniques. Thermal spraying processes such as plasma spraying [7], high-velocity oxy-fuel (HVOF) [8], wire-arc [9] and flame spraying have been widely used to
* Corresponding author. Tel.: +86 29 82660970; fax: +86 29 82660970. E-mail address: [email protected] (C.-J. Li). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.07.127
deposit FeAl-based alloys to protect carbon steels from corrosion. However, the severe oxidation of spray powders during spraying results in high oxide content in the subsequent coating and deteriorates the overall oxidation and corrosion resistances [10]. Recently, the newly emerging cold spraying process provides a promising alternative to fabricate these alloys where no high temperature heat source is used to melt powder particles. Therefore, the deleterious effects inherent to conventional thermal spraying such as oxidation, phase transformation, decomposition, and other problems can be minimized or eliminated [11–14]. Cold sprayed coating is generally deposited through plastic deformation of spray particles. Due to the intrinsic brittleness of FeAl intermetallic compounds at low temperature, it is difficult to deposit FeAl coating directly using a FeAl intermetallic compound powder feedstock. In the previous paper [15], a nanocrystalline FeAl intermetallic compound has been produced by cold spraying of mechanical alloyed Fe(Al) solid solution alloy powder assisted with post-spray heat treatment. It was found that the milled Fe(Al) alloy powder exhibited a refined lamellar microstructure and the as-sprayed coating was dense. The heat treatment at a temperature of 500 8C results in the complete transformation of Fe(Al) solid solution to FeAl intermetallic compound. However, the preparation of a Fe(Al) alloy powder through mechanical alloying process for cold spraying would add additional cost. Therefore, the costeffective approach is to fabricate FeAl intermetallic coating through the use of elemental Fe/Al powder mixture by cold spraying. The low temperature characteristic of cold spraying
H.-T. Wang et al. / Applied Surface Science 255 (2008) 2538–2544
makes it advantageous to deposit composite alloy such as Fe/Al without any significant oxidation during deposition process. In addition, the deformation during cold spraying process would disrupt any thin surface film such as oxides and expose fresh, active material, which was brought into intimate conformal contact under highly localised pressure forming strong atomic bonds. As a result, the cold-sprayed Fe/Al composite alloy may be expected to form an intermetallics by solid-state diffusion in a relatively low temperature. In addition, the near fully dense coldsprayed deposit would not shrink significantly during sintering, unlike the conventional sintering of porous green products [16]. Therefore, the aim of this work was to produce the FeAl intermetallics deposit by cold spraying of a Fe/Al powder mixture followed by heat-treatment. A few papers published on cold-spraying of metal powder mixtures (W/Cu [17], Zn/Al [18], Ti/Al [16] and Ni/Al [19]) show that dense composites can be produced using mixed powder and the heat treatment may lead to the intermetallic phases formation. However, there are no reports focusing on the characterization of cold sprayed Fe/Al composite deposit and its phase transformation during heat treatment. Due to different mechanical properties of individual particles in the mixed powders, the deposition using a Fe/Al powder mixture in cold spraying may differ from that using other powder mixture (such as Ni/Al or Ti/Al) or an alloy powder. Furthermore, the reaction between Fe and Al during heat treatment is also different from that of other elemental powder mixture. Therefore, as the first step of the study on the fabrication of FeAl alloys by cold spraying of Fe/Al powder mixture and subsequent heat treatment, this paper presented the deposition characteristics of cold sprayed Fe/Al composite coating. The phase transformation of the cold sprayed Fe/Al composite coating during heat treatment was primarily examined.
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The cold spraying system developed in Xi’an Jiaotong University was used to produce Fe/Al composite coating. The detail information of the system was described elsewhere [14]. A spray gun with a converging–diverging de Laval type nozzle of a throat diameter of 2 mm was adopted. The divergent nozzle is 100 mm in length with an exit diameter of 6 mm. Nitrogen gas was employed as both accelerating and powder feeding gases operating at the pressures of 2.0 and 2.5 MPa, respectively, and at a temperature of 350 8C in the pre-chamber of spray gun. Stainless steel plates were used as a substrate. Prior to spraying, the substrate surface was cleaned using acetone and sand-blasted using 24 mesh alumina grits. The standoff distance of the substrate from the gun exit was 20 mm. During deposition, the gun was manipulated by a robot at a traverse speed of 40 mm/s relative to the substrate. The as-sprayed Fe/Al composite coating was annealed at different temperatures in a tubular furnace in argon atmosphere to protect the sample from oxidation. In order to investigate the dynamics of the inter-reaction between Fe and Al, the annealing experiment was interrupted at 500, 600, 700, and 900 8C, respectively. In addition, some samples were annealed at 950 and 1050 8C for 2 h to investigate coating microstructure evolution. The microstructures of the as-sprayed coating and the annealed coating obtained after each interruption were examined by scanning electron microscopy (SEM, Quanta 200, FEI, Czechoslovakia) equipped with energy dispersive X-ray analysis (EDXA) and X-ray diffractometer (XRD, RIKAKU D/MAX-2400) using Cu Ka1 radiation. A differential scanning calorimeter (DSC) was also used to determine the reactions and the onset temperature of the reactions of the deposit during annealing. The microhardness of the initial powder and the coating was tested by the Vickers hardness tester under a load of 20 g and a holding time of 15 s. 3. Results and discussion
2. Experimental materials and procedures 3.1. Deposit microstructure characteristics The commercially available Fe (99.8 wt.%, 54 mm, Youxinglian Nonferrous Metals Ltd., Beijing, China) and Al (99.5 wt.%, 74 mm, Youxinglian Nonferrous Metals Ltd., Beijing, China) powders were used as starting materials to make a powder mixture at an atomic ratio of Fe/Al of 50/50. The mixing process was carried out in a high-energy ball mill with a ball-to-powder weight ratio of 2:1 at a rotation speed of 80 rpm for 30 min. Typical particles of iron and aluminum powders are shown in Fig. 1. Fe powder manufactured through an electrolytic process exhibited an angular morphology as shown in Fig. 1a. Al powder was produced through a gas atomization process and presented a spherical morphology as shown in Fig. 1b.
Fig. 2 shows the cross-sectional microstructure of the assprayed Fe/Al composite deposit in the SEM backscattered mode. It was clear that the coating presented a dense microstructure. Both Fe (light) and Al (dark) can be readily distinguished according to their respective contrast, and were uniformly distributed in the coating. Being softer than iron, aluminum deformed more easily and formed a uniform pore-free matrix, in which Fe appeared as isolated particles with relatively less deformation, as shown in Fig. 2b. The different deformation levels of Fe and Al particles may be attributed to differences in their density (Fe: 7.86 g/cm3, and Al: 2.70 g/cm3, respectively), hardness and particle size and morphol-
Fig. 1. SEM images of the as-received Fe (a) and Al (b) powders.
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H.-T. Wang et al. / Applied Surface Science 255 (2008) 2538–2544
Fig. 2. Cross-sectional images of the as-sprayed iron/aluminum composite deposit in the SEM back-scattered mode: (a) overview; (b) high magnification.
ogies as well. The original powder mixture composition was Fe50Al50 (at.%), namely, 41.7 vol.% Fe and 58.3 vol.% Al. The contents of Al and Fe in the as-sprayed Fe/Al deposit were determined by the image analysis of 10 backscattered SEM micrographs. The result showed that the average contents of Fe and Al in the as-sprayed Fe/Al deposite were approximately 43.6 vol.% and 56.4 vol.%. It can be seen that the relative contents of Fe and Al in the as-sprayed deposit were nearly the same as those of the feedstock powder. This fact indicates that the deposition efficiencies of iron and aluminum during cold spraying were comparable. However, it was reported that during cold spraying of Ti/Al powder mixture [16], the deposition efficiency of titanium was far below that of aluminum. The author argued that the loss of titanium was probably due to the different critical velocities of two powders required to deform and deposit, resulting in a difference in deposition efficiency between Al and Ti. In the present work, the high deposition efficiency of iron during cold spraying of Fe/Al powder mixture was due to its low hardness compared to that of Ti. In addition, Fe powder used in this experiment had an irregular morphology. From an aerodynamics aspect, such morphology was beneficial to the powder acceleration as the drag coefficient of nonspherical particles was larger than that of the spherical particles of the same mass [20]. As a consequence, a larger drag force will be applied to the non-spherical particles, resulting in a higher velocity. The cross-sectional microstructure of the deposit near the top surface is shown in Fig. 3. It can be clearly seen that the Al particles
experienced significant plastic deformation. The shapes of deposited Al particles changed from the initial spherical ball to the irregularity to conform the rough surface of the underlying coating due to their self plastic deformation and the forced deformation by Fe particles on the top, as shown in Fig. 3a. Furthermore, from the SEM image at high magnification as shown in Fig. 3b, it can be found that the microstructure of the impacting zone of the Al particle, evidently exhibiting certain protrusion, was different from that of the inner zone, which indicated that the impacting interface experienced more significant plastic deformation than other zones, as indicated by arrows in Fig. 3b. The typical surface morphologies of the deposit are shown in Fig. 4. The ‘‘splat’’ morphology of powder particles was observed, indicating severe plastic deformation, as shown in Fig. 4a. In addition, it can be clearly seen that small impact craters resulted from particles that did not stick on impact were also evident on the Al particle surface due to its low hardness. The impact craters can be clearly seen in Fig. 4b, as marked by white arrows. Owing to the intensive plastic deformation of deposited particles, the internal strains would be introduced into the deposit during cold spraying process. This aspect would be analyzed in details in following section. Fig. 5 shows the typical morphologies of a fractured cross-section of the Fe/Al composite deposit. The most deposited particles had flattened. However, some small spherical particles were observed in the coating, as indicated by arrow in Fig. 5a, which were nondeformed Al particles. The previous report on cold-sprayed
Fig. 3. Cross-sectional microstructure of the as-sprayed iron/aluminum composite deposit at the top surface zone: (a) overview; (b) high magnification.
H.-T. Wang et al. / Applied Surface Science 255 (2008) 2538–2544
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Fig. 4. SEM images showing surface morphologies of the as-sprayed iron/aluminum deposit: (a) overview and (b) high magnification.
Fig. 5. Morphologies of the fractured iron/aluminum deposit: (a) overview and (b) high magnification.
aluminum coatings [21] showed that a bowl shock wave near substrate surface would be generated during cold spraying process, which would decelerate in-flight particles. While the larger particles have sufficient momentum to maintain their velocity, the smaller lighter particles are decelerated significantly so that they will no longer undergo evident deformation on impact. Fig. 5b exhibits ridges and valleys observed on the surface of an Al particle, which indicates high degree deformation experienced by Al particles. This phenomenon was also observed in cold-sprayed Al coating [22]. Craters and pores in a size of submicrometers imply multiple impacts by spray particles, leading to a higher degree of flattening.
Fig. 6 shows the cross-sectional TEM images of the as-sprayed iron/aluminum composite coating. A particle–particle boundary between Al particles was indicated by black arrows, as shown in Fig. 6a. It can be seen that the particles were in intimate contact at the interface, which would be benefit the elements diffusion across the interface and formation of a metallurgy bonding during postspray heat treatment. In addition, some subgrains also appeared, resulting from intensive deformation, as indicated by white arrows. Fig. 6b shows the aligned and elongated Al grains with a width of about 1 mm and a length of tens of micrometer. These nonequilibrium grain boundaries are characterized by ultrahigh
Fig. 6. Cross-sectional TEM image of the cold sprayed iron/aluminum composite deposit: (a) a particle–particle boundary between Al particles and (b) elongated Al grains.
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H.-T. Wang et al. / Applied Surface Science 255 (2008) 2538–2544 Table 1 Vickers microhardness of Al and Fe phases in the as-sprayed Fe/Al deposit in comparison with those of Al and Fe powders
Fe Al
Fig. 7. Cross-sectional TEM micrograph of the cold sprayed iron/aluminum composite deposit showing the state of oxide layer at the interface.
dislocation densities adjacent to the grain boundaries. This fact provides further evidence that the grains in the deposited particles experienced intensive deformation during cold spraying. With spray powder particles, there is always more or less oxide film on particle surfaces depending on their oxidation state. It was observed that the oxide film has been broken to debris at the contact interfaces. Some of the debris has embedded into the particle, while the debris at the periphery may be cleaned up by the metal jets. Therefore, the oxide film on particle surfaces will be partially deposited into the coating, which may influence the microstructure and performance of the coating. The previous study showed that the oxidation state of Cu particles had significant effect on the critical velocity for particle deposition and thus the coating formation [23]. Fig. 7 shows that the oxide layer with a thickness of about 100 nm existed between a particle–particle boundary. However, it was noticed that oxide film near the contact area has been broken and nearly disappeared due to intensive deformation, as indicated by white arrows. The disrupt of thin surface oxides on the surface of particles would expose fresh, active material, which was brought into intimate conformal contact under highly localised pressure to form strong atomic bonds. As a result, the particle–particle diffusion and formation of metallurgy bonding would be significantly enhanced during post-spray heat treatment.
Feedstock powder
Cold sprayed deposit
91 5 Hv0.02 34 2 Hv0.02
171 15 Hv0.02 63 7 Hv0.02
XRD peaks of the as-sprayed coating were the same as those of feedstock powder, consisting only of pure Fe and Al phases. No aluminum or iron oxides were observed in the as-sprayed deposit due to low temperature characteristic of the cold spray process and low oxygen content, being well below detectable levels. Furthermore, no peaks of iron aluminide intermetallic phases appeared in the as-sprayed deposit. Since the deposited particles experienced intensive plastic deformation during cold spraying, an internal strain would be induced in the as-sprayed deposit. It is well known that the internal strain would cause the peak broadening in X-ray diffraction analysis. The full-widths at the half maximum (FWHM) of Fe and Al diffraction peaks (calculated by the computer software ‘‘Origin 7.0’’ [24]) were listed in Fig. 8b. It can be seen that the FWHM of all diffraction peaks in the as-sprayed deposit were larger than those of feedstock powder. However, the increment of the FWHM of the as-sprayed deposit was nearly same for all diffraction peaks and about 30%. 3.3. Deposit microhardness The mean Vickers microhardness values of Fe and Al splats in the as-sprayed deposit and those of the feedstock powders are listed in Table 1. It can be seen that the microhardness values of Fe and Al splats in the iron/aluminum composite deposit were higher than those of feedstock powder. The higher microhardenss of iron/ aluminum deposit was attributed to work hardening by the splatted particles with intensive deformation during cold spraying process. In addition, the hardness increment of Fe and Al splats in the as-sprayed deposit was nearly same, being about 80% compared with the starting powders. This fact also indicated that the hardening degree of Fe and Al particles during cold spraying process was nearly the same, which was coincident with the increase of the FWHM of the diffraction peaks of the as-sprayed deposit in comparison of the feedstock powder.
3.2. Deposit phase compositions
3.4. DSC analysis of the deposit
The XRD patterns of the feedstock powder and the as-sprayed iron/aluminum composite deposit are shown in Fig. 8. The main
The thermal response of the composite coating was studied using a differential scanning calorimeter (DSC) in a temperature
Fig. 8. XRD patterns of the feedstock powder and the as-sprayed deposit (a) and the FWHM of the diffraction peaks of the feedstock powder and the as-sprayed deposit (b).
H.-T. Wang et al. / Applied Surface Science 255 (2008) 2538–2544
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addition, there was no evidence for any remaining free Al, therefore, it may be noted that at 700 8C Fe2Al5 coexisted with free iron in the deposit. X-ray analysis of the sample obtained at 900 8C showed that the diffraction peaks of Fe2Al5 phase disappeared completely and mainly diffraction peaks of FeAl phase were present in the XRD pattern with a fraction of remaining Fe (as shown in Fig. 10d). The trace of Fe in the annealed deposit was attributed to the limited diffusion time in the present test. This fact means that the Fe2Al5 phase completely transformed to FeAl phase under this condition. As a result, FeAl intermetallic compound can be formed through heat treatment of cold sprayed Fe/Al composite alloy. It was well known that heat-treatment temperature would have significant influence on the microstructure of the deposit. Further work is now being undertaken to investigate the microstructure evolution of the iron/aluminum composite deposit during heat treatment. Fig. 9. DSC curve of the cold-sprayed Fe-50Al composite deposit.
4. Conclusions range from room temperature up to 800 8C at a heating rate of 10 8C/min. As shown in Fig. 9, a characteristic peak appeared on the DSC curve, indicating a strong exothermic reaction between Fe and Al with an onset temperature of around 625 8C and peak maximum at a temperature of around 632 8C. This exothermic event was due to the formation of Fe2Al5 phase according to XRD analysis in the next section. In addition, no phase transformations were detected during cooling from the maximal temperature achieved in the DSC run. This result indicates that the above transformation was not reversible. 3.5. Phase evolution during the heat treatment of the Fe/Al deposit Fig. 10 shows XRD patterns of the iron/aluminum composite deposit obtained after quenched from the temperatures of 600 8C, 700 8C, and 900 8C. The result obtained after the interruption at a temperature of 600 8C indicates that phase transformation took place and the formation of intermediate phase Fe2Al5 occurred (as shown in Fig. 10b). However, Fe and Al were still the main phases in this case. This fact indicates that only a small quantity of Fe and Al reacted through solid diffusion under this condition. With quenching temperature rising to 700 8C, the intensive reaction between Fe an Al took place because the liquid Al phase quickly covered the free iron surface through capillary action. As a result, the content of Fe2Al5 phase in the deposit was obviously increased and on the contrary, the content of Fe and Al phases decreased, which is evident from the relative intensity of the diffraction peaks of different phases in the XRD patterns (as shown in Fig. 10c). In
It was found that a dense iron/aluminum composite coating was produced by cold spraying using blend mixture of iron and aluminum powders. The composition of the as-sprayed Fe/Al composite coating was nearly the same as that of the feedstock powder. Analysis of the microstructure of iron/aluminum composite coating suggested that mechanical interlocking of splats was predominant bonding mechanism. The intensive deformation of particle tended to elongate the grain and disrupt thin surface oxides on the powder particle surfaces. The DSC analysis confirmed that Fe2Al5 was the first intermetallic phase formed upon heating through either solid state interdiffusion or temporary liquid state sintering. The Vickers microhardness values for the iron and aluminum splats in the as-sprayed deposit were higher than those of the feedstock powder due to working harden effect during particles deposition process. The heat treatment of the iron/ aluminum deposit at 900 8C leads to the formation of FeAl intermetallic phase. The present results clearly showed that cold spraying of Fe/Al powder mixture with subsequent annealing treatment is a promising process to fabricate FeAl intermetallic coating. Acknowledgements The present work is supported by the Key Project of the Ministry of Education of China (No. 106145), National Basic Research Program of China (grant No. 2007CB707702), and Science Fund for Distinguished Young Scholars (grant No. 50725101). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
Fig. 10. XRD patterns of the interrupted samples at various temperatures: (a) RT; (b) 600 8C; (c) 700 8C; (d) 900 8C.
[16]
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[21] R. Morgan, P. Fox, J. Pattison, C. Sutcliffe, W. O’Neill, Mater. lett. 58 (2004) 1317– 1320. [22] K. Balani, A. Agarwal, S. Seal, J. Karthikeyan, Scripta Mater. 53 (2005) 845–850. [23] C.-J. Li, W.-Y. Li, H.L. Liao, J. Therm. Spray Technol. 15 (2006) 212–222. [24] Software Origin v7.0, OriginLab Corporation, Northampton, MA, USA, 2002.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Cold spraying of Fe/Al powder mixture: Coating characteristics and influence of heat treatment on the phase structure Hong-Tao Wang, Chang-Jiu Li *, Guan-Jun Yang, Cheng-Xing Li State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 22 May 2008 Received in revised form 21 July 2008 Accepted 22 July 2008 Available online 31 July 2008
In this paper, an iron/aluminum composite coating was deposited by cold spraying using iron and aluminum powder mixtures. The coating was annealed at different temperatures to aim at forming an iron aluminide intermetallic based coating. The results showed that a thick dense Fe/Al composite coating with uniformly distributed Fe and Al particles can be deposited by cold spraying. The compositions of the as-sprayed Fe/Al coating were nearly the same as that of the initial Fe/Al powder mixture. The intensive deformation of particle on impact caused elongation of the grain and disrupted the thin oxide films on powder surface. After annealing at a temperature of 600 8C, an intermediate phase Al5Fe2 coexisted in the deposit with remaining Fe and Al. With increasing annealing temperature to 900 8C, the deposit transformed to mainly FeAl phase with a trace of remaining Fe phase. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Cold spray Microstructure Deposition characteristics Fe/Al Heat treatment
1. Introduction FeAl-based alloys have drawn much attention as materials for high temperature applications because they are low cost intermetallic materials with a relatively low density (5.56 g/cm3) and exhibit good mechanical properties, and excellent corrosion resistance in oxidizing and sulfidizing atmospheres, which is relied on their ability to form a highly protective Al2O3 scale [1–4]. Moreover, Fe and Al, which are raw materials of FeAl phase, are relatively inexpensive and the FeAl intermetallic alloys are lighter than steels or Ni-based alloys. Therefore, FeAl-based alloy is expected as one candidate for substitution of stainless steels or Ni-based superalloys [5]. Although a wide span of research activities on intermetallics has been carried out in the last many years, the commercial applications of iron aluminides as a bulk material was hampered owing to their low ductility and processing problems which result from large difference in the melting points of Al and Fe and exothermic nature of formation of iron aluminides [6]. In order to utilize their excellent properties, the FeAl-based alloys are often used as protective coatings on classical metallic alloys. The coatings are deposited by several coating techniques. Thermal spraying processes such as plasma spraying [7], high-velocity oxy-fuel (HVOF) [8], wire-arc [9] and flame spraying have been widely used to
* Corresponding author. Tel.: +86 29 82660970; fax: +86 29 82660970. E-mail address: [email protected] (C.-J. Li). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.07.127
deposit FeAl-based alloys to protect carbon steels from corrosion. However, the severe oxidation of spray powders during spraying results in high oxide content in the subsequent coating and deteriorates the overall oxidation and corrosion resistances [10]. Recently, the newly emerging cold spraying process provides a promising alternative to fabricate these alloys where no high temperature heat source is used to melt powder particles. Therefore, the deleterious effects inherent to conventional thermal spraying such as oxidation, phase transformation, decomposition, and other problems can be minimized or eliminated [11–14]. Cold sprayed coating is generally deposited through plastic deformation of spray particles. Due to the intrinsic brittleness of FeAl intermetallic compounds at low temperature, it is difficult to deposit FeAl coating directly using a FeAl intermetallic compound powder feedstock. In the previous paper [15], a nanocrystalline FeAl intermetallic compound has been produced by cold spraying of mechanical alloyed Fe(Al) solid solution alloy powder assisted with post-spray heat treatment. It was found that the milled Fe(Al) alloy powder exhibited a refined lamellar microstructure and the as-sprayed coating was dense. The heat treatment at a temperature of 500 8C results in the complete transformation of Fe(Al) solid solution to FeAl intermetallic compound. However, the preparation of a Fe(Al) alloy powder through mechanical alloying process for cold spraying would add additional cost. Therefore, the costeffective approach is to fabricate FeAl intermetallic coating through the use of elemental Fe/Al powder mixture by cold spraying. The low temperature characteristic of cold spraying
H.-T. Wang et al. / Applied Surface Science 255 (2008) 2538–2544
makes it advantageous to deposit composite alloy such as Fe/Al without any significant oxidation during deposition process. In addition, the deformation during cold spraying process would disrupt any thin surface film such as oxides and expose fresh, active material, which was brought into intimate conformal contact under highly localised pressure forming strong atomic bonds. As a result, the cold-sprayed Fe/Al composite alloy may be expected to form an intermetallics by solid-state diffusion in a relatively low temperature. In addition, the near fully dense coldsprayed deposit would not shrink significantly during sintering, unlike the conventional sintering of porous green products [16]. Therefore, the aim of this work was to produce the FeAl intermetallics deposit by cold spraying of a Fe/Al powder mixture followed by heat-treatment. A few papers published on cold-spraying of metal powder mixtures (W/Cu [17], Zn/Al [18], Ti/Al [16] and Ni/Al [19]) show that dense composites can be produced using mixed powder and the heat treatment may lead to the intermetallic phases formation. However, there are no reports focusing on the characterization of cold sprayed Fe/Al composite deposit and its phase transformation during heat treatment. Due to different mechanical properties of individual particles in the mixed powders, the deposition using a Fe/Al powder mixture in cold spraying may differ from that using other powder mixture (such as Ni/Al or Ti/Al) or an alloy powder. Furthermore, the reaction between Fe and Al during heat treatment is also different from that of other elemental powder mixture. Therefore, as the first step of the study on the fabrication of FeAl alloys by cold spraying of Fe/Al powder mixture and subsequent heat treatment, this paper presented the deposition characteristics of cold sprayed Fe/Al composite coating. The phase transformation of the cold sprayed Fe/Al composite coating during heat treatment was primarily examined.
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The cold spraying system developed in Xi’an Jiaotong University was used to produce Fe/Al composite coating. The detail information of the system was described elsewhere [14]. A spray gun with a converging–diverging de Laval type nozzle of a throat diameter of 2 mm was adopted. The divergent nozzle is 100 mm in length with an exit diameter of 6 mm. Nitrogen gas was employed as both accelerating and powder feeding gases operating at the pressures of 2.0 and 2.5 MPa, respectively, and at a temperature of 350 8C in the pre-chamber of spray gun. Stainless steel plates were used as a substrate. Prior to spraying, the substrate surface was cleaned using acetone and sand-blasted using 24 mesh alumina grits. The standoff distance of the substrate from the gun exit was 20 mm. During deposition, the gun was manipulated by a robot at a traverse speed of 40 mm/s relative to the substrate. The as-sprayed Fe/Al composite coating was annealed at different temperatures in a tubular furnace in argon atmosphere to protect the sample from oxidation. In order to investigate the dynamics of the inter-reaction between Fe and Al, the annealing experiment was interrupted at 500, 600, 700, and 900 8C, respectively. In addition, some samples were annealed at 950 and 1050 8C for 2 h to investigate coating microstructure evolution. The microstructures of the as-sprayed coating and the annealed coating obtained after each interruption were examined by scanning electron microscopy (SEM, Quanta 200, FEI, Czechoslovakia) equipped with energy dispersive X-ray analysis (EDXA) and X-ray diffractometer (XRD, RIKAKU D/MAX-2400) using Cu Ka1 radiation. A differential scanning calorimeter (DSC) was also used to determine the reactions and the onset temperature of the reactions of the deposit during annealing. The microhardness of the initial powder and the coating was tested by the Vickers hardness tester under a load of 20 g and a holding time of 15 s. 3. Results and discussion
2. Experimental materials and procedures 3.1. Deposit microstructure characteristics The commercially available Fe (99.8 wt.%, 54 mm, Youxinglian Nonferrous Metals Ltd., Beijing, China) and Al (99.5 wt.%, 74 mm, Youxinglian Nonferrous Metals Ltd., Beijing, China) powders were used as starting materials to make a powder mixture at an atomic ratio of Fe/Al of 50/50. The mixing process was carried out in a high-energy ball mill with a ball-to-powder weight ratio of 2:1 at a rotation speed of 80 rpm for 30 min. Typical particles of iron and aluminum powders are shown in Fig. 1. Fe powder manufactured through an electrolytic process exhibited an angular morphology as shown in Fig. 1a. Al powder was produced through a gas atomization process and presented a spherical morphology as shown in Fig. 1b.
Fig. 2 shows the cross-sectional microstructure of the assprayed Fe/Al composite deposit in the SEM backscattered mode. It was clear that the coating presented a dense microstructure. Both Fe (light) and Al (dark) can be readily distinguished according to their respective contrast, and were uniformly distributed in the coating. Being softer than iron, aluminum deformed more easily and formed a uniform pore-free matrix, in which Fe appeared as isolated particles with relatively less deformation, as shown in Fig. 2b. The different deformation levels of Fe and Al particles may be attributed to differences in their density (Fe: 7.86 g/cm3, and Al: 2.70 g/cm3, respectively), hardness and particle size and morphol-
Fig. 1. SEM images of the as-received Fe (a) and Al (b) powders.
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Fig. 2. Cross-sectional images of the as-sprayed iron/aluminum composite deposit in the SEM back-scattered mode: (a) overview; (b) high magnification.
ogies as well. The original powder mixture composition was Fe50Al50 (at.%), namely, 41.7 vol.% Fe and 58.3 vol.% Al. The contents of Al and Fe in the as-sprayed Fe/Al deposit were determined by the image analysis of 10 backscattered SEM micrographs. The result showed that the average contents of Fe and Al in the as-sprayed Fe/Al deposite were approximately 43.6 vol.% and 56.4 vol.%. It can be seen that the relative contents of Fe and Al in the as-sprayed deposit were nearly the same as those of the feedstock powder. This fact indicates that the deposition efficiencies of iron and aluminum during cold spraying were comparable. However, it was reported that during cold spraying of Ti/Al powder mixture [16], the deposition efficiency of titanium was far below that of aluminum. The author argued that the loss of titanium was probably due to the different critical velocities of two powders required to deform and deposit, resulting in a difference in deposition efficiency between Al and Ti. In the present work, the high deposition efficiency of iron during cold spraying of Fe/Al powder mixture was due to its low hardness compared to that of Ti. In addition, Fe powder used in this experiment had an irregular morphology. From an aerodynamics aspect, such morphology was beneficial to the powder acceleration as the drag coefficient of nonspherical particles was larger than that of the spherical particles of the same mass [20]. As a consequence, a larger drag force will be applied to the non-spherical particles, resulting in a higher velocity. The cross-sectional microstructure of the deposit near the top surface is shown in Fig. 3. It can be clearly seen that the Al particles
experienced significant plastic deformation. The shapes of deposited Al particles changed from the initial spherical ball to the irregularity to conform the rough surface of the underlying coating due to their self plastic deformation and the forced deformation by Fe particles on the top, as shown in Fig. 3a. Furthermore, from the SEM image at high magnification as shown in Fig. 3b, it can be found that the microstructure of the impacting zone of the Al particle, evidently exhibiting certain protrusion, was different from that of the inner zone, which indicated that the impacting interface experienced more significant plastic deformation than other zones, as indicated by arrows in Fig. 3b. The typical surface morphologies of the deposit are shown in Fig. 4. The ‘‘splat’’ morphology of powder particles was observed, indicating severe plastic deformation, as shown in Fig. 4a. In addition, it can be clearly seen that small impact craters resulted from particles that did not stick on impact were also evident on the Al particle surface due to its low hardness. The impact craters can be clearly seen in Fig. 4b, as marked by white arrows. Owing to the intensive plastic deformation of deposited particles, the internal strains would be introduced into the deposit during cold spraying process. This aspect would be analyzed in details in following section. Fig. 5 shows the typical morphologies of a fractured cross-section of the Fe/Al composite deposit. The most deposited particles had flattened. However, some small spherical particles were observed in the coating, as indicated by arrow in Fig. 5a, which were nondeformed Al particles. The previous report on cold-sprayed
Fig. 3. Cross-sectional microstructure of the as-sprayed iron/aluminum composite deposit at the top surface zone: (a) overview; (b) high magnification.
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Fig. 4. SEM images showing surface morphologies of the as-sprayed iron/aluminum deposit: (a) overview and (b) high magnification.
Fig. 5. Morphologies of the fractured iron/aluminum deposit: (a) overview and (b) high magnification.
aluminum coatings [21] showed that a bowl shock wave near substrate surface would be generated during cold spraying process, which would decelerate in-flight particles. While the larger particles have sufficient momentum to maintain their velocity, the smaller lighter particles are decelerated significantly so that they will no longer undergo evident deformation on impact. Fig. 5b exhibits ridges and valleys observed on the surface of an Al particle, which indicates high degree deformation experienced by Al particles. This phenomenon was also observed in cold-sprayed Al coating [22]. Craters and pores in a size of submicrometers imply multiple impacts by spray particles, leading to a higher degree of flattening.
Fig. 6 shows the cross-sectional TEM images of the as-sprayed iron/aluminum composite coating. A particle–particle boundary between Al particles was indicated by black arrows, as shown in Fig. 6a. It can be seen that the particles were in intimate contact at the interface, which would be benefit the elements diffusion across the interface and formation of a metallurgy bonding during postspray heat treatment. In addition, some subgrains also appeared, resulting from intensive deformation, as indicated by white arrows. Fig. 6b shows the aligned and elongated Al grains with a width of about 1 mm and a length of tens of micrometer. These nonequilibrium grain boundaries are characterized by ultrahigh
Fig. 6. Cross-sectional TEM image of the cold sprayed iron/aluminum composite deposit: (a) a particle–particle boundary between Al particles and (b) elongated Al grains.
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Fe Al
Fig. 7. Cross-sectional TEM micrograph of the cold sprayed iron/aluminum composite deposit showing the state of oxide layer at the interface.
dislocation densities adjacent to the grain boundaries. This fact provides further evidence that the grains in the deposited particles experienced intensive deformation during cold spraying. With spray powder particles, there is always more or less oxide film on particle surfaces depending on their oxidation state. It was observed that the oxide film has been broken to debris at the contact interfaces. Some of the debris has embedded into the particle, while the debris at the periphery may be cleaned up by the metal jets. Therefore, the oxide film on particle surfaces will be partially deposited into the coating, which may influence the microstructure and performance of the coating. The previous study showed that the oxidation state of Cu particles had significant effect on the critical velocity for particle deposition and thus the coating formation [23]. Fig. 7 shows that the oxide layer with a thickness of about 100 nm existed between a particle–particle boundary. However, it was noticed that oxide film near the contact area has been broken and nearly disappeared due to intensive deformation, as indicated by white arrows. The disrupt of thin surface oxides on the surface of particles would expose fresh, active material, which was brought into intimate conformal contact under highly localised pressure to form strong atomic bonds. As a result, the particle–particle diffusion and formation of metallurgy bonding would be significantly enhanced during post-spray heat treatment.
Feedstock powder
Cold sprayed deposit
91 5 Hv0.02 34 2 Hv0.02
171 15 Hv0.02 63 7 Hv0.02
XRD peaks of the as-sprayed coating were the same as those of feedstock powder, consisting only of pure Fe and Al phases. No aluminum or iron oxides were observed in the as-sprayed deposit due to low temperature characteristic of the cold spray process and low oxygen content, being well below detectable levels. Furthermore, no peaks of iron aluminide intermetallic phases appeared in the as-sprayed deposit. Since the deposited particles experienced intensive plastic deformation during cold spraying, an internal strain would be induced in the as-sprayed deposit. It is well known that the internal strain would cause the peak broadening in X-ray diffraction analysis. The full-widths at the half maximum (FWHM) of Fe and Al diffraction peaks (calculated by the computer software ‘‘Origin 7.0’’ [24]) were listed in Fig. 8b. It can be seen that the FWHM of all diffraction peaks in the as-sprayed deposit were larger than those of feedstock powder. However, the increment of the FWHM of the as-sprayed deposit was nearly same for all diffraction peaks and about 30%. 3.3. Deposit microhardness The mean Vickers microhardness values of Fe and Al splats in the as-sprayed deposit and those of the feedstock powders are listed in Table 1. It can be seen that the microhardness values of Fe and Al splats in the iron/aluminum composite deposit were higher than those of feedstock powder. The higher microhardenss of iron/ aluminum deposit was attributed to work hardening by the splatted particles with intensive deformation during cold spraying process. In addition, the hardness increment of Fe and Al splats in the as-sprayed deposit was nearly same, being about 80% compared with the starting powders. This fact also indicated that the hardening degree of Fe and Al particles during cold spraying process was nearly the same, which was coincident with the increase of the FWHM of the diffraction peaks of the as-sprayed deposit in comparison of the feedstock powder.
3.2. Deposit phase compositions
3.4. DSC analysis of the deposit
The XRD patterns of the feedstock powder and the as-sprayed iron/aluminum composite deposit are shown in Fig. 8. The main
The thermal response of the composite coating was studied using a differential scanning calorimeter (DSC) in a temperature
Fig. 8. XRD patterns of the feedstock powder and the as-sprayed deposit (a) and the FWHM of the diffraction peaks of the feedstock powder and the as-sprayed deposit (b).
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addition, there was no evidence for any remaining free Al, therefore, it may be noted that at 700 8C Fe2Al5 coexisted with free iron in the deposit. X-ray analysis of the sample obtained at 900 8C showed that the diffraction peaks of Fe2Al5 phase disappeared completely and mainly diffraction peaks of FeAl phase were present in the XRD pattern with a fraction of remaining Fe (as shown in Fig. 10d). The trace of Fe in the annealed deposit was attributed to the limited diffusion time in the present test. This fact means that the Fe2Al5 phase completely transformed to FeAl phase under this condition. As a result, FeAl intermetallic compound can be formed through heat treatment of cold sprayed Fe/Al composite alloy. It was well known that heat-treatment temperature would have significant influence on the microstructure of the deposit. Further work is now being undertaken to investigate the microstructure evolution of the iron/aluminum composite deposit during heat treatment. Fig. 9. DSC curve of the cold-sprayed Fe-50Al composite deposit.
4. Conclusions range from room temperature up to 800 8C at a heating rate of 10 8C/min. As shown in Fig. 9, a characteristic peak appeared on the DSC curve, indicating a strong exothermic reaction between Fe and Al with an onset temperature of around 625 8C and peak maximum at a temperature of around 632 8C. This exothermic event was due to the formation of Fe2Al5 phase according to XRD analysis in the next section. In addition, no phase transformations were detected during cooling from the maximal temperature achieved in the DSC run. This result indicates that the above transformation was not reversible. 3.5. Phase evolution during the heat treatment of the Fe/Al deposit Fig. 10 shows XRD patterns of the iron/aluminum composite deposit obtained after quenched from the temperatures of 600 8C, 700 8C, and 900 8C. The result obtained after the interruption at a temperature of 600 8C indicates that phase transformation took place and the formation of intermediate phase Fe2Al5 occurred (as shown in Fig. 10b). However, Fe and Al were still the main phases in this case. This fact indicates that only a small quantity of Fe and Al reacted through solid diffusion under this condition. With quenching temperature rising to 700 8C, the intensive reaction between Fe an Al took place because the liquid Al phase quickly covered the free iron surface through capillary action. As a result, the content of Fe2Al5 phase in the deposit was obviously increased and on the contrary, the content of Fe and Al phases decreased, which is evident from the relative intensity of the diffraction peaks of different phases in the XRD patterns (as shown in Fig. 10c). In
It was found that a dense iron/aluminum composite coating was produced by cold spraying using blend mixture of iron and aluminum powders. The composition of the as-sprayed Fe/Al composite coating was nearly the same as that of the feedstock powder. Analysis of the microstructure of iron/aluminum composite coating suggested that mechanical interlocking of splats was predominant bonding mechanism. The intensive deformation of particle tended to elongate the grain and disrupt thin surface oxides on the powder particle surfaces. The DSC analysis confirmed that Fe2Al5 was the first intermetallic phase formed upon heating through either solid state interdiffusion or temporary liquid state sintering. The Vickers microhardness values for the iron and aluminum splats in the as-sprayed deposit were higher than those of the feedstock powder due to working harden effect during particles deposition process. The heat treatment of the iron/ aluminum deposit at 900 8C leads to the formation of FeAl intermetallic phase. The present results clearly showed that cold spraying of Fe/Al powder mixture with subsequent annealing treatment is a promising process to fabricate FeAl intermetallic coating. Acknowledgements The present work is supported by the Key Project of the Ministry of Education of China (No. 106145), National Basic Research Program of China (grant No. 2007CB707702), and Science Fund for Distinguished Young Scholars (grant No. 50725101). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
Fig. 10. XRD patterns of the interrupted samples at various temperatures: (a) RT; (b) 600 8C; (c) 700 8C; (d) 900 8C.
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