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Water atomised aluminium alloy powders
Materials Science and Engineering A 383 (2004) 7–13
Water atomised aluminium alloy powders O.D. Neikov∗ , G.I. Vasilieva, A.V. Sameljuk, A.V. Krajnikov Frantzevich Institute for Problems of Materials Science, 3 Krzhyzhanivsky Street, Kiev 03142, Ukraine Received 27 January 2004
Abstract The new rapid solidification (RS) process based on high-pressure water atomisation (WA) of the melt for manufacturing of advanced aluminium alloys was realised in the form of a pilot plant. The problems of safe operation in the course of Al alloy powder production and powder quality were solved by the use of water solutions of inhibitors, by the control of suspension temperature and hydrogen ion exponent (pH), by the hydraulic classification of atomised products, and by the optimisation of dehydration procedure. The rate of powder–water interaction strongly depends on the value of pH. While the rate of room temperature reactions is very slow at pH 3.0–4.0, the increase of pH to 6.0 leads to an intensive powder oxidation. A set of powder metallurgy (PM) alloys for various applications was produced on the base of water atomised powders. The characteristics of tensile strength of such alloys essentially exceed those of cast materials of similar compositions. © 2004 Elsevier B.V. All rights reserved. Keywords: Aluminium powder; Water atomisation; Microstructure; Gas emission; Tensile strength
1. Introduction Many methods are known to realise rapid solidification (RS) of melt. The use of RS technologies, in particular, granular technology, which is based on the centrifugal atomisation of molten metal droplets in water, allows one to develop a set of new alloy compositions [1]. The granular technology provides 0.5–2.5 mm size particles at a cooling rate of ∼104 K/s [2]. The achieved level of mechanical properties is characterised by the following values: ultimate tensile stress, UTS = 520–720 MPa; yield stress, YS = 440–680 MPa and relative elongation EL = 5–10% [3]. However, further improvements of the properties are impeded by the insufficient cooling rate of the centrifugal atomisation process. Therefore, further progress in this area is concerned with the development of new technologies, which would provide higher solidification rates as compared with the granular technology. The well-known water atomisation (WA) process allows one to increase the cooling rate. This technology is widely ∗ Corresponding author. Tel.: +380-44-424-23-80; fax: +380-44-424-21-31. E-mail address: [email protected] (O.D. Neikov).
0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2004.02.030
used for the production of ferrous and some non-ferrous (e.g. copper) metals. However, WA is not applied to aluminium alloys because of two main reasons: (1) explosion risks of the atomisation process due to rapid hydrogen emission during powder–water interaction and (2) high reactivity of aluminium that can cause heavy oxidation of the surface of WA powders. In contrast to the traditional opinion, recent studies showed that WA under optimised conditions can be used for production of high-quality Al alloy powders [4–7]. The main goal of the present study is to estimate quality of ultrahigh-strength Al–Zn–Mg–Cu, weldable high-strength Al–Zn–Mg and elevated-temperature Al–Fe–Cr–Ti powders produced by WA and to determine production regimes, which would ensure explosion-proof operation. Microstructure of the WA powders and mechanical properties of powder metallurgy (PM) alloys consolidated of the powders are studied and tested to estimate quality of the powders. The kinetics of gas emission in the course of interaction of moist powders with the atomising water is measured as a function of water parameters to characterise powder oxidation and to ensure explosion-proof of the WA process. The results obtained are used to optimise technological parameters of powder production.
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O.D. Neikov et al. / Materials Science and Engineering A 383 (2004) 7–13
Fig. 1. Schematic diagram of the pilot plant to produce WA-N Al alloy powders: (1) cooled water reservoir, (2) refrigerating unit, (3) hydrogen index controller, (4) high-pressure water pump, (5) induction furnace, (6) tundish, (7) atomising system, (8) atomising chamber, (9) suspension reservoir, (10) vacuum suspension filter, (11) hydraulic classifier, (12) fine fraction suspension filter, (13) centrifugal pump, (14) water filters, (15) vacuum pump, (16) vacuum dryer.
2. Experimental
gen ion exponent (pH) and the temperature of water and suspension.
2.1. Pilot plant 2.2. Powder composition The present paper focuses on the results of recent developments of a new water atomisation (WA-N) process, realised as a pilot plant. Fig. 1 represents the schematic diagram of the pilot plant for production of aluminium alloy powders. The technological process consists of the following steps. The charge, which is prepared with the use of master alloys, containing alloying elements, is melted in the induction furnace. The ready melt is fed in a tundish, whence it flows out through a calibrated pipe in the bottom. The melt stream is atomised by jets of the high-pressure water through the nozzle in the atomising chamber. Inhibitor from the group of weak electrolytes is introduced into the cooled water reservoir. Water suspension of the atomised powder forms at the bottom of the chamber and then it is poured out into the suspension reservoir (Fig. 1). The suspension is continuously pumped out from the reservoir by the pumps and is supplied to the hydraulic classifier. The suspended powder is classified there into several size fractions. The suspension with coarse fractions is returned into the suspension reservoir and is subjected to the mechanical dehydration by filtering under vacuum. The filtering with a high-performance porous filter dehydrates the suspension with fine fractions. All classified powders are dried in vacuum. The automatic control system of the pilot plant controls the hydro-
Nominal compositions of ultrahigh-strength Al–Zn–Mg– Cu base alloys, elevated-temperature Al–Fe–Cr–Ti base alloys and high-strength weldable Al–Zn–Mg base alloys produced by the WA-N process are shown in Table 1. A set of cast weldable Al–Zn–Mg base alloys and granular weldable 01949 alloy (in Russian classification), which is known as one of the strongest granular Al–Zn–Mg base materials, are also shown in Table 1 and studied for comparison purposes. 2.3. Microstructure study Powder particles were cold pressed into briquettes with porosity about 30% and then electrolytically polished to produce samples for structural investigations. The morphology, microstructure and microchemistry of individual particles and polished samples were studied by scanning electron microscopy (SEM) with energy-dispersion analysis. 2.4. Estimation of cooling rate The cooling rate of melts was estimated with the use of dendrite parameter [1], which is equal to the average size
O.D. Neikov et al. / Materials Science and Engineering A 383 (2004) 7–13
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Table 1 Nominal composition of aluminium alloys, wt.% Alloy No.
Zn
Mg
Cu
Mn
Zr
Sc
Fe
Ti
Cr
Nb
Ni
Co
PM ultrahigh-strength alloys 1Pa 9.5 2P 9.0 3P 9.0 4P 9.0
3.0 3.0 3.0 3.0
1.2 1.2 1.2 1.2
– – – –
0.15 – – –
0.3 – – –
– 0.3 0.6 1.0
– – – –
– – – –
0.2 – – –
– – – –
– – – –
PM elevated-temperature alloys 5P – 6P – 7P –
– – –
– – –
– – –
– – 1.0
– – –
4.0 5.0 5.0
2.0 2.0 2.0
4.0 5.0 5.0
– – –
– – –
– – –
– – – –
– – – 0.5 0.7 – 0.5 0.7 0.3 0.5 0.7 0.3 1.1 (Mn + Zr + Sc)
– – – – –
– – – 0.15 –
– – – 0.15 –
– – –
– – – 0.2 –
– – – 0.5 –
0.1–0.2
0.15
PM and cast weldable high-strength 8P, 8Cb 5.0 9P, 9C 5.0 10P, 10C 5.0 11P, 11C 5.0 12P, 12C 9.0 (Zn + Mg) Granular weldable alloy 01949 4.5–5.5 a b
alloys 3.0 3.0 3.0 3.0 – 2.5–3.45
–
–
0.7–1.0
–
Powder metallurgy alloys. Cast alloys.
of dendrite cells. The cell size was measured with the use of SEM micrographs. The analysis of the dendrite structure allows one to estimate the cooling rate by Eq. (1): a 1/n v= (1) d where v is the cooling rate (in K/s); d, the measured dendrite parameter (in m), and a and n are constants [2]. For high-strength aluminium alloys, a = 100 and a value of n is taken in the interval 0.25–0.5 depending on the grain shape. In particular, n = 1/3 for equiaxed grains. 2.5. Gas emission study The experimental investigation of gas emission kinetics was carried out with the use of a laboratory appliance which consists of a reaction vessel, a measuring dome to collect generated gases, a gas pressure sensor, a unit for pressure compensation and a recording device. The gas collected in the dome was analysed by a chromatograph. The wet powder samples were taken for measurements directly from the chamber of the vacuum filter (Fig. 1), i.e. just after the operation of melt atomisation and before the drying. This is the main methodological distinction of the present experiment from any others. The characteristics of gas emission are usually measured for dry powders when the surface oxide film has been already formed and thus the reactivity of such powders has been already reduced [8]. Evidently, the results obtained in the present paper can differ from the available literature data. The effect of acidity of the atomising water on metal– water interaction was investigated by varying the hydrogen ion exponent pH in a range of 4.0–6.0. A given pH value in
the atomising system was maintained constant by the available automatic control system. The period of metal–water reaction used in the gas emission experiments was up to 2000 h. 2.6. Mechanical properties WA-N powders were consolidated in PM alloys to estimate their mechanical properties. First, the dried powder was pressed to produce pre-forms with porosity about 30% at room temperature in air. The pressed pre-forms were degassed under 0.1 Pa vacuum at elevated-temperature (typically 623–673 K) and then consolidated into dense billets by high-speed impulse loading at 1.0 GPa [9]. The billets were hot extruded in air at extrusion ratios of 17–20 to manufacture semi-products in a form of 6 mm diameter rods. The PM alloys were aged by the T6 regime (738 K × 5.4 ks → water quench → 393 K × 86.4 ks). The ultimate tensile strength, yield strength, elongation and Vickers hardness (HV) were measured with the use of samples cut from the rods.
3. Results and discussion 3.1. Powder microstructure Typical microstructures of the WA-N powders are shown in Fig. 2. The particles usually have irregular shape (see e.g. Fig. 2a). The surface of individual particles is characterised by a highly branched relief with many surface cracks and terraces and pores in the bulk (Fig. 2b). The particles usually exhibit submicron grain structures (Fig. 2c).
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O.D. Neikov et al. / Materials Science and Engineering A 383 (2004) 7–13
Fig. 2. Scanning electron micrographs: (a) powder particles (6P powder), (b) pores and cracks on the powder surface (6P powder), (c) increased magnification micrograph, revealing the grain size (6P powder), (d) surface of the individual particle (6P powder, backscattering electron image), (e) polished metallographic sections of cold pressed powders (4P powder, backscattering electron image) with elemental distributions along the secant line, (f) polished metallographic sections of cold pressed powders (2P powder, backscattering electron image).
A set of interlayers at the sizes 1–5 m is clearly seen on the surface of powder particles of Al–Zn–Mg–Cu and Al–Zn–Mg systems (Fig. 2d). Such interlayers are often enriched with Zn and Mg as well as with Fe and Cu (Fig. 2e). For example, the interlayer Zn and Mg concentrations in 2P–4P alloy powders can reach 22–24 and 24–25% mass, respectively, that correspond to three- and eight-fold enrichments of the bulk concentration. Therefore, relatively high cooling rates do not provide complete homogeneity of the solid solution within an individual particle. The width of the enrichment zone generally correlates with structure refinement, and varies from 0 at some structureless areas of the particle to some hundred nanometers in areas solidified at lower rates. The microstructure of individual particles can vary in a wide range (Fig. 2f). The dendritic parameters are different for different particles and usually vary from 2.5 to 0.2 m that corresponds to cooling rates of 2 × 104 –2 × 106 K/s. In addition to dendritic or subdendritic microstructure, some other areas of individual particles show non-dendritic grains and amorphous structure. Such observations can be explained by a significant intensification of the nucleation during crystallisation caused by a deep super-cooling of the melt.
3.2. Powder oxidation kinetics Oxidation of Al–Zn–Mg–Cu (2P and 4P) and Al–Fe–Cr– Ti (6P) alloys was studied by SEM. Fig. 3 shows the microstructure of polished sections of 2P alloy powder particles after interaction with water during 96 h. Exposure of the powders in water with pH 4.0 at room temperature does not essentially influence their microstructure and chemical composition (Fig. 3a). If pH increases to 6.0, pronounced microstructural changes are seen in 2P (Fig. 3b) and 4P (Fig. 3c) alloy powders. A part of the surface is displayed in backscattered electron images as dark spots, evidencing heavy oxidation of such areas as a result of metal–water interaction. The increase of iron contents from 0.3 wt.% in 2P alloy (Fig. 3b) to 1.0 wt.% in 4P alloy (Fig. 3c) does not change essentially the powder microstructure. The interaction activity of Al–Fe–Cr–Ti alloy powders with water is significantly lower than that of Al–Zn–Mg–Cu and Al–Zn–Mg alloys (cf. Figs. 3 and 4). As seen from Fig. 4, the microstructural changes observed in elevated-temperature Al–Fe–Cr–Ti base powders after a prolonged reaction with water at pH 6.0 come to nothing more than the formation of small islets of the aluminium oxides on the powder surface while no
O.D. Neikov et al. / Materials Science and Engineering A 383 (2004) 7–13
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Fig. 3. Backscattering electron images of polished metallographic sections of cold pressed powders after room temperature interaction with water for 96 h: (a) 2P alloy powder at pH 4.0, (b) 2P alloy powder at pH 6.0, (c) 4P alloy powder at pH 6.0.
Fig. 4. Backscattering electron images of polished metallographic sections of cold pressed 6P alloy powders after room temperature interaction for 2000 h with water at (a) pH 4.0 and (b) pH 6.0.
preferential matrix dissolution is observed in contrast to Fig. 3b and c. 3.3. Macroscopic kinetics of gas emission The kinetic curves of room temperature gas emission measured at pH 4.0 and 6.0 for 2P, 3P and 4P powders are shown in Fig. 5. The rate of powder–water interaction strongly de-
Fig. 5. Gas emission as a function of reaction time for several powders at pH 4.0 and pH 6.0.
pends on the value of pH and it drastically rises with increasing pH. The increase of iron content from 0.3 wt.% in 2P alloy to 1.0 wt.% in 4P alloy does not affect the intensity of gas emission and the experimental curves are identical for all three alloys (Fig. 5). Fig. 6 illustrates the behaviour of 5P, 6P and 7P alloy powders as a function of reaction time at pH 4.0 and pH 6.0 at room temperature. As is seen from Figs. 5 and 6, the reaction activity of Al–Fe–Cr–Ti alloy powders is essentially
Fig. 6. Gas emission as a function of reaction time at room temperature for 5P, 6P and 7P powders at pH 6.0 and pH 4.0.
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O.D. Neikov et al. / Materials Science and Engineering A 383 (2004) 7–13
Fig. 7. Gas emission as a function of reaction time at room temperature for 1P ultrahigh-strength alloy: (1) green powder at pH 3.0, (2) green powder at pH 6.0 and (3) dried and repeatedly moistened powder at pH 6.0.
lower than that of Al–Zn–Mg–Cu alloys that correlates with the results of microstructural studies (Figs. 3 and 4). In particular, the induction period of 2P, 3P and 4P alloys is approximately 30–40 h while that of 5P alloy is 500–600 h. The intensity of gas emission of Al–Fe–Cr–Ti base alloys is approximately 10 times lower in comparison with that of Al–Zn–Mg–Cu base alloys. As seen from Fig. 7, the reaction rate of powders, which were dried and then repeatedly moistened (curve 3), is lower than that of green powders (curve 2). Evidently, the surface oxide film formed during the powder atomisation and drying passivates the powder surface and thus reduces the gas emission. 3.4. Mechanical properties A set of PM alloys was produced with the use of WA-N process for various applications: ultrahigh-strength Al–Zn–Mg–Cu base alloys [10], weldable high-strength Al–Zn–Mg base alloys [11] and elevated-temperature Al–Fe–Cr–Ti base alloys [12]. The level of tensile strength achieved in these works is schematically shown in Fig. 8. The characteristics of room temperature tensile strength are summarised in more detail for PM, cast and granular Al–Zn–Mg base alloys in the T6 condition in Table 2. The results obtained shows that the PM alloys essentially exceed
Fig. 8. Ultimate tensile strength achieved for PM rod specimens produced of WA-N Al alloy powders.
the cast and granular alloys of the same composition in terms of tensile strength. As is known, contamination of cast Al base alloys with iron results in deterioration of plasticity and corrosion cracking resistance. An increase of the iron concentration to 1 wt.% in WA-N alloys does not influence mechanical properties of the PM alloys. Such an effect shows promise for recycling of secondary aluminium, contaminated by iron, with the use of the developed RS technology.
4. Conclusions 1. The rate of powder–water interaction strongly depends on the value of water hydrogen ion exponent pH. WA-N Al alloy powders show very slow reaction with water at pH 4. The increase of the pH value to 6.0 strongly accelerates the reaction rate and causes pronounced changes in the microstructure and microchemistry of powders. 2. The activity of metal–water reaction is much lower in elevated-temperature Al–Fe–Cr–Ti alloy powders than that in ultrahigh-strength Al–Zn–Mg–Cu alloy powders. 3. The increase of iron content from 0.3 to 1.0 wt.% does not influence the intensity of gas emission. 4. The PM alloy semi-products have significantly higher mechanical properties than the cast and granular alloys of similar compositions.
Table 2 Mechanical properties of 6 mm rods produced by PM and by cast (T6 condition) PM alloy
HV (MPa)
YS (MPa)
UTS (MPa)
EL (%)
Cast and granular alloy
HV (MPa)
YS (MPa)
UTS (MPa)
EL (%)
8P 9P 10P 11P 12P
1640 1770 1900 1900 2010
500 542 596 622 644
575 612 651 655 677
10.4 11.1 7.7 8.8 5.3
8C 9C 10C 11C, 01949
1540 1530 1740 1730
383 452 531 566, 545
433 495 590 608, 610
12.9 13.7 10.3 9.0, 11.4
O.D. Neikov et al. / Materials Science and Engineering A 383 (2004) 7–13
Acknowledgements The research described in this publication was made possible by financial support of the NATO Science for Peace Program (Project SfP 973264) and the US Air Force Office of Scientific Research (STCU Partner Project 061). The authors would like to thank Dr A.I. Sirko of IPMS, Kiev for performing mechanical tests.
References [1] V.I. Dobatkin, V.I. Elagin, V.M. Fedorov, Rapidly Solidified Aluminium Alloys, All-Russian Institute of Light Alloys, Moscow, 1995 (in Russian). [2] G.I. Eskin, Technol. Light Alloys 2 (2000) 17–25 (in Russian). [3] B.I. Bondarev, Y.V. Shmakov, Technology of Manufacturing of Rapidly Solidified Aluminium Alloys, All-Russian Institute of Light Alloys, Moscow, 1997 (in Russian). [4] O.D. Neikov, A.V. Krajnikov, Mater. Sci. Forum 217–222 (1996) 1649–1654. [5] O.D. Neikov, Water atomised powder technology for advanced aluminium alloy production, in: K. Kosuge, H. Nagai (Eds.), Proceedings of the 2000 Powder Metall. World Congress, vol. 1, Kyoto, Japan, 1–16 November 2000, Japan Soc. Powder and Powder Metallurgy, Kyoto, Japan, 2000, pp. 464–466. [6] O.D. Neikov, Rapidly solidified aluminium alloy powder, in: Proceedings of the 1998 Powder Metall. World Congress, vol. 1, Granada, Spain, 18–22 October 1988, European Powder Metallurgy Association, Bellstone Shrewsbury, UK, 1998, pp. 129–134.
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[7] O.D. Neikov, A.V. Krajnikov, A.V. Sameljuk, G.I. Vasilieva, N.G. Chaykina, explosion-security of manufacture processes of advanced water atomized aluminum alloy P/M technology, in: V. Arnold, Chiu-Lung Chu, W.F. Jandeska, H.I. Sanderow (Eds.), Advances in Powder Metallurgy and Particulate Materials, vol. 3, Orlando, Fl, 16–21 June, Metal Powder Industries Federation, Princeton, New Jersey, USA, 2002, pp. 60–69. [8] A.K. Lokenbah, N.A. Zaporina, L.K. Lepiny, Phase composition and structure of oxide films on the fine aluminium powder particles, in: Proceedings of the LatSSR Academy of Sciences, vol. 1, Series Chemic, 1981, pp. 45–54 (in Russian). [9] A.V. Laptev, S.S. Ponomarev, L.F. Ochkas, Solid phase consolidation of fine grained WC-16% wt Co hard metals, J. Adv. Mater. 33 (2001) 42–51. [10] O.D. Neikov, Yu.V. Milman, D.B. Miracle, D.V. Lotsko, A.I. Sirko, N.A. Yefimov, Effect of Sc alloying additions on structure and mechanical properties of PM and cast high-strength Al alloys of same composition, in: Proceedings of the 2001 European Congress and Exhibition on Powder Metallurgy, vol. 2, Nice, France, 22–24 October 2001, European Powder Metallurgy Association, Bellstone Shrewbury, UK, 2001, pp. 219–224. [11] O.D. Neikov, D.V. Lotsko, A.I. Sirko, A.V. Sameljuk, G.E. Thompson, N.P. Zakharova, N.A. Yefimov, Properties of rapidly solidified powder alloys of the Al–Zn–Mg system, Mater. Sci. Forum 396–402 (2002) 1223–1228. [12] O.D. Neikov, Yu.V. Milman, D.B. Miracle, D.V. Lotsko, A.I. Sirko, N.P. Zakharova, Effect of powder size on mechanical properties of elevated temperature aluminium alloys produced by water atomization, in: Proceedings of the 2001 European Congress and Exhibition on Powder Metallurgy, vol. 2, Nice, France, 22–24 October 2001, European Powder Metallurgy Association, Bellstone Shrewbury, UK, 2001, pp. 225–230.
Water atomised aluminium alloy powders O.D. Neikov∗ , G.I. Vasilieva, A.V. Sameljuk, A.V. Krajnikov Frantzevich Institute for Problems of Materials Science, 3 Krzhyzhanivsky Street, Kiev 03142, Ukraine Received 27 January 2004
Abstract The new rapid solidification (RS) process based on high-pressure water atomisation (WA) of the melt for manufacturing of advanced aluminium alloys was realised in the form of a pilot plant. The problems of safe operation in the course of Al alloy powder production and powder quality were solved by the use of water solutions of inhibitors, by the control of suspension temperature and hydrogen ion exponent (pH), by the hydraulic classification of atomised products, and by the optimisation of dehydration procedure. The rate of powder–water interaction strongly depends on the value of pH. While the rate of room temperature reactions is very slow at pH 3.0–4.0, the increase of pH to 6.0 leads to an intensive powder oxidation. A set of powder metallurgy (PM) alloys for various applications was produced on the base of water atomised powders. The characteristics of tensile strength of such alloys essentially exceed those of cast materials of similar compositions. © 2004 Elsevier B.V. All rights reserved. Keywords: Aluminium powder; Water atomisation; Microstructure; Gas emission; Tensile strength
1. Introduction Many methods are known to realise rapid solidification (RS) of melt. The use of RS technologies, in particular, granular technology, which is based on the centrifugal atomisation of molten metal droplets in water, allows one to develop a set of new alloy compositions [1]. The granular technology provides 0.5–2.5 mm size particles at a cooling rate of ∼104 K/s [2]. The achieved level of mechanical properties is characterised by the following values: ultimate tensile stress, UTS = 520–720 MPa; yield stress, YS = 440–680 MPa and relative elongation EL = 5–10% [3]. However, further improvements of the properties are impeded by the insufficient cooling rate of the centrifugal atomisation process. Therefore, further progress in this area is concerned with the development of new technologies, which would provide higher solidification rates as compared with the granular technology. The well-known water atomisation (WA) process allows one to increase the cooling rate. This technology is widely ∗ Corresponding author. Tel.: +380-44-424-23-80; fax: +380-44-424-21-31. E-mail address: [email protected] (O.D. Neikov).
0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2004.02.030
used for the production of ferrous and some non-ferrous (e.g. copper) metals. However, WA is not applied to aluminium alloys because of two main reasons: (1) explosion risks of the atomisation process due to rapid hydrogen emission during powder–water interaction and (2) high reactivity of aluminium that can cause heavy oxidation of the surface of WA powders. In contrast to the traditional opinion, recent studies showed that WA under optimised conditions can be used for production of high-quality Al alloy powders [4–7]. The main goal of the present study is to estimate quality of ultrahigh-strength Al–Zn–Mg–Cu, weldable high-strength Al–Zn–Mg and elevated-temperature Al–Fe–Cr–Ti powders produced by WA and to determine production regimes, which would ensure explosion-proof operation. Microstructure of the WA powders and mechanical properties of powder metallurgy (PM) alloys consolidated of the powders are studied and tested to estimate quality of the powders. The kinetics of gas emission in the course of interaction of moist powders with the atomising water is measured as a function of water parameters to characterise powder oxidation and to ensure explosion-proof of the WA process. The results obtained are used to optimise technological parameters of powder production.
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O.D. Neikov et al. / Materials Science and Engineering A 383 (2004) 7–13
Fig. 1. Schematic diagram of the pilot plant to produce WA-N Al alloy powders: (1) cooled water reservoir, (2) refrigerating unit, (3) hydrogen index controller, (4) high-pressure water pump, (5) induction furnace, (6) tundish, (7) atomising system, (8) atomising chamber, (9) suspension reservoir, (10) vacuum suspension filter, (11) hydraulic classifier, (12) fine fraction suspension filter, (13) centrifugal pump, (14) water filters, (15) vacuum pump, (16) vacuum dryer.
2. Experimental
gen ion exponent (pH) and the temperature of water and suspension.
2.1. Pilot plant 2.2. Powder composition The present paper focuses on the results of recent developments of a new water atomisation (WA-N) process, realised as a pilot plant. Fig. 1 represents the schematic diagram of the pilot plant for production of aluminium alloy powders. The technological process consists of the following steps. The charge, which is prepared with the use of master alloys, containing alloying elements, is melted in the induction furnace. The ready melt is fed in a tundish, whence it flows out through a calibrated pipe in the bottom. The melt stream is atomised by jets of the high-pressure water through the nozzle in the atomising chamber. Inhibitor from the group of weak electrolytes is introduced into the cooled water reservoir. Water suspension of the atomised powder forms at the bottom of the chamber and then it is poured out into the suspension reservoir (Fig. 1). The suspension is continuously pumped out from the reservoir by the pumps and is supplied to the hydraulic classifier. The suspended powder is classified there into several size fractions. The suspension with coarse fractions is returned into the suspension reservoir and is subjected to the mechanical dehydration by filtering under vacuum. The filtering with a high-performance porous filter dehydrates the suspension with fine fractions. All classified powders are dried in vacuum. The automatic control system of the pilot plant controls the hydro-
Nominal compositions of ultrahigh-strength Al–Zn–Mg– Cu base alloys, elevated-temperature Al–Fe–Cr–Ti base alloys and high-strength weldable Al–Zn–Mg base alloys produced by the WA-N process are shown in Table 1. A set of cast weldable Al–Zn–Mg base alloys and granular weldable 01949 alloy (in Russian classification), which is known as one of the strongest granular Al–Zn–Mg base materials, are also shown in Table 1 and studied for comparison purposes. 2.3. Microstructure study Powder particles were cold pressed into briquettes with porosity about 30% and then electrolytically polished to produce samples for structural investigations. The morphology, microstructure and microchemistry of individual particles and polished samples were studied by scanning electron microscopy (SEM) with energy-dispersion analysis. 2.4. Estimation of cooling rate The cooling rate of melts was estimated with the use of dendrite parameter [1], which is equal to the average size
O.D. Neikov et al. / Materials Science and Engineering A 383 (2004) 7–13
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Table 1 Nominal composition of aluminium alloys, wt.% Alloy No.
Zn
Mg
Cu
Mn
Zr
Sc
Fe
Ti
Cr
Nb
Ni
Co
PM ultrahigh-strength alloys 1Pa 9.5 2P 9.0 3P 9.0 4P 9.0
3.0 3.0 3.0 3.0
1.2 1.2 1.2 1.2
– – – –
0.15 – – –
0.3 – – –
– 0.3 0.6 1.0
– – – –
– – – –
0.2 – – –
– – – –
– – – –
PM elevated-temperature alloys 5P – 6P – 7P –
– – –
– – –
– – –
– – 1.0
– – –
4.0 5.0 5.0
2.0 2.0 2.0
4.0 5.0 5.0
– – –
– – –
– – –
– – – –
– – – 0.5 0.7 – 0.5 0.7 0.3 0.5 0.7 0.3 1.1 (Mn + Zr + Sc)
– – – – –
– – – 0.15 –
– – – 0.15 –
– – –
– – – 0.2 –
– – – 0.5 –
0.1–0.2
0.15
PM and cast weldable high-strength 8P, 8Cb 5.0 9P, 9C 5.0 10P, 10C 5.0 11P, 11C 5.0 12P, 12C 9.0 (Zn + Mg) Granular weldable alloy 01949 4.5–5.5 a b
alloys 3.0 3.0 3.0 3.0 – 2.5–3.45
–
–
0.7–1.0
–
Powder metallurgy alloys. Cast alloys.
of dendrite cells. The cell size was measured with the use of SEM micrographs. The analysis of the dendrite structure allows one to estimate the cooling rate by Eq. (1): a 1/n v= (1) d where v is the cooling rate (in K/s); d, the measured dendrite parameter (in m), and a and n are constants [2]. For high-strength aluminium alloys, a = 100 and a value of n is taken in the interval 0.25–0.5 depending on the grain shape. In particular, n = 1/3 for equiaxed grains. 2.5. Gas emission study The experimental investigation of gas emission kinetics was carried out with the use of a laboratory appliance which consists of a reaction vessel, a measuring dome to collect generated gases, a gas pressure sensor, a unit for pressure compensation and a recording device. The gas collected in the dome was analysed by a chromatograph. The wet powder samples were taken for measurements directly from the chamber of the vacuum filter (Fig. 1), i.e. just after the operation of melt atomisation and before the drying. This is the main methodological distinction of the present experiment from any others. The characteristics of gas emission are usually measured for dry powders when the surface oxide film has been already formed and thus the reactivity of such powders has been already reduced [8]. Evidently, the results obtained in the present paper can differ from the available literature data. The effect of acidity of the atomising water on metal– water interaction was investigated by varying the hydrogen ion exponent pH in a range of 4.0–6.0. A given pH value in
the atomising system was maintained constant by the available automatic control system. The period of metal–water reaction used in the gas emission experiments was up to 2000 h. 2.6. Mechanical properties WA-N powders were consolidated in PM alloys to estimate their mechanical properties. First, the dried powder was pressed to produce pre-forms with porosity about 30% at room temperature in air. The pressed pre-forms were degassed under 0.1 Pa vacuum at elevated-temperature (typically 623–673 K) and then consolidated into dense billets by high-speed impulse loading at 1.0 GPa [9]. The billets were hot extruded in air at extrusion ratios of 17–20 to manufacture semi-products in a form of 6 mm diameter rods. The PM alloys were aged by the T6 regime (738 K × 5.4 ks → water quench → 393 K × 86.4 ks). The ultimate tensile strength, yield strength, elongation and Vickers hardness (HV) were measured with the use of samples cut from the rods.
3. Results and discussion 3.1. Powder microstructure Typical microstructures of the WA-N powders are shown in Fig. 2. The particles usually have irregular shape (see e.g. Fig. 2a). The surface of individual particles is characterised by a highly branched relief with many surface cracks and terraces and pores in the bulk (Fig. 2b). The particles usually exhibit submicron grain structures (Fig. 2c).
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Fig. 2. Scanning electron micrographs: (a) powder particles (6P powder), (b) pores and cracks on the powder surface (6P powder), (c) increased magnification micrograph, revealing the grain size (6P powder), (d) surface of the individual particle (6P powder, backscattering electron image), (e) polished metallographic sections of cold pressed powders (4P powder, backscattering electron image) with elemental distributions along the secant line, (f) polished metallographic sections of cold pressed powders (2P powder, backscattering electron image).
A set of interlayers at the sizes 1–5 m is clearly seen on the surface of powder particles of Al–Zn–Mg–Cu and Al–Zn–Mg systems (Fig. 2d). Such interlayers are often enriched with Zn and Mg as well as with Fe and Cu (Fig. 2e). For example, the interlayer Zn and Mg concentrations in 2P–4P alloy powders can reach 22–24 and 24–25% mass, respectively, that correspond to three- and eight-fold enrichments of the bulk concentration. Therefore, relatively high cooling rates do not provide complete homogeneity of the solid solution within an individual particle. The width of the enrichment zone generally correlates with structure refinement, and varies from 0 at some structureless areas of the particle to some hundred nanometers in areas solidified at lower rates. The microstructure of individual particles can vary in a wide range (Fig. 2f). The dendritic parameters are different for different particles and usually vary from 2.5 to 0.2 m that corresponds to cooling rates of 2 × 104 –2 × 106 K/s. In addition to dendritic or subdendritic microstructure, some other areas of individual particles show non-dendritic grains and amorphous structure. Such observations can be explained by a significant intensification of the nucleation during crystallisation caused by a deep super-cooling of the melt.
3.2. Powder oxidation kinetics Oxidation of Al–Zn–Mg–Cu (2P and 4P) and Al–Fe–Cr– Ti (6P) alloys was studied by SEM. Fig. 3 shows the microstructure of polished sections of 2P alloy powder particles after interaction with water during 96 h. Exposure of the powders in water with pH 4.0 at room temperature does not essentially influence their microstructure and chemical composition (Fig. 3a). If pH increases to 6.0, pronounced microstructural changes are seen in 2P (Fig. 3b) and 4P (Fig. 3c) alloy powders. A part of the surface is displayed in backscattered electron images as dark spots, evidencing heavy oxidation of such areas as a result of metal–water interaction. The increase of iron contents from 0.3 wt.% in 2P alloy (Fig. 3b) to 1.0 wt.% in 4P alloy (Fig. 3c) does not change essentially the powder microstructure. The interaction activity of Al–Fe–Cr–Ti alloy powders with water is significantly lower than that of Al–Zn–Mg–Cu and Al–Zn–Mg alloys (cf. Figs. 3 and 4). As seen from Fig. 4, the microstructural changes observed in elevated-temperature Al–Fe–Cr–Ti base powders after a prolonged reaction with water at pH 6.0 come to nothing more than the formation of small islets of the aluminium oxides on the powder surface while no
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Fig. 3. Backscattering electron images of polished metallographic sections of cold pressed powders after room temperature interaction with water for 96 h: (a) 2P alloy powder at pH 4.0, (b) 2P alloy powder at pH 6.0, (c) 4P alloy powder at pH 6.0.
Fig. 4. Backscattering electron images of polished metallographic sections of cold pressed 6P alloy powders after room temperature interaction for 2000 h with water at (a) pH 4.0 and (b) pH 6.0.
preferential matrix dissolution is observed in contrast to Fig. 3b and c. 3.3. Macroscopic kinetics of gas emission The kinetic curves of room temperature gas emission measured at pH 4.0 and 6.0 for 2P, 3P and 4P powders are shown in Fig. 5. The rate of powder–water interaction strongly de-
Fig. 5. Gas emission as a function of reaction time for several powders at pH 4.0 and pH 6.0.
pends on the value of pH and it drastically rises with increasing pH. The increase of iron content from 0.3 wt.% in 2P alloy to 1.0 wt.% in 4P alloy does not affect the intensity of gas emission and the experimental curves are identical for all three alloys (Fig. 5). Fig. 6 illustrates the behaviour of 5P, 6P and 7P alloy powders as a function of reaction time at pH 4.0 and pH 6.0 at room temperature. As is seen from Figs. 5 and 6, the reaction activity of Al–Fe–Cr–Ti alloy powders is essentially
Fig. 6. Gas emission as a function of reaction time at room temperature for 5P, 6P and 7P powders at pH 6.0 and pH 4.0.
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Fig. 7. Gas emission as a function of reaction time at room temperature for 1P ultrahigh-strength alloy: (1) green powder at pH 3.0, (2) green powder at pH 6.0 and (3) dried and repeatedly moistened powder at pH 6.0.
lower than that of Al–Zn–Mg–Cu alloys that correlates with the results of microstructural studies (Figs. 3 and 4). In particular, the induction period of 2P, 3P and 4P alloys is approximately 30–40 h while that of 5P alloy is 500–600 h. The intensity of gas emission of Al–Fe–Cr–Ti base alloys is approximately 10 times lower in comparison with that of Al–Zn–Mg–Cu base alloys. As seen from Fig. 7, the reaction rate of powders, which were dried and then repeatedly moistened (curve 3), is lower than that of green powders (curve 2). Evidently, the surface oxide film formed during the powder atomisation and drying passivates the powder surface and thus reduces the gas emission. 3.4. Mechanical properties A set of PM alloys was produced with the use of WA-N process for various applications: ultrahigh-strength Al–Zn–Mg–Cu base alloys [10], weldable high-strength Al–Zn–Mg base alloys [11] and elevated-temperature Al–Fe–Cr–Ti base alloys [12]. The level of tensile strength achieved in these works is schematically shown in Fig. 8. The characteristics of room temperature tensile strength are summarised in more detail for PM, cast and granular Al–Zn–Mg base alloys in the T6 condition in Table 2. The results obtained shows that the PM alloys essentially exceed
Fig. 8. Ultimate tensile strength achieved for PM rod specimens produced of WA-N Al alloy powders.
the cast and granular alloys of the same composition in terms of tensile strength. As is known, contamination of cast Al base alloys with iron results in deterioration of plasticity and corrosion cracking resistance. An increase of the iron concentration to 1 wt.% in WA-N alloys does not influence mechanical properties of the PM alloys. Such an effect shows promise for recycling of secondary aluminium, contaminated by iron, with the use of the developed RS technology.
4. Conclusions 1. The rate of powder–water interaction strongly depends on the value of water hydrogen ion exponent pH. WA-N Al alloy powders show very slow reaction with water at pH 4. The increase of the pH value to 6.0 strongly accelerates the reaction rate and causes pronounced changes in the microstructure and microchemistry of powders. 2. The activity of metal–water reaction is much lower in elevated-temperature Al–Fe–Cr–Ti alloy powders than that in ultrahigh-strength Al–Zn–Mg–Cu alloy powders. 3. The increase of iron content from 0.3 to 1.0 wt.% does not influence the intensity of gas emission. 4. The PM alloy semi-products have significantly higher mechanical properties than the cast and granular alloys of similar compositions.
Table 2 Mechanical properties of 6 mm rods produced by PM and by cast (T6 condition) PM alloy
HV (MPa)
YS (MPa)
UTS (MPa)
EL (%)
Cast and granular alloy
HV (MPa)
YS (MPa)
UTS (MPa)
EL (%)
8P 9P 10P 11P 12P
1640 1770 1900 1900 2010
500 542 596 622 644
575 612 651 655 677
10.4 11.1 7.7 8.8 5.3
8C 9C 10C 11C, 01949
1540 1530 1740 1730
383 452 531 566, 545
433 495 590 608, 610
12.9 13.7 10.3 9.0, 11.4
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Acknowledgements The research described in this publication was made possible by financial support of the NATO Science for Peace Program (Project SfP 973264) and the US Air Force Office of Scientific Research (STCU Partner Project 061). The authors would like to thank Dr A.I. Sirko of IPMS, Kiev for performing mechanical tests.
References [1] V.I. Dobatkin, V.I. Elagin, V.M. Fedorov, Rapidly Solidified Aluminium Alloys, All-Russian Institute of Light Alloys, Moscow, 1995 (in Russian). [2] G.I. Eskin, Technol. Light Alloys 2 (2000) 17–25 (in Russian). [3] B.I. Bondarev, Y.V. Shmakov, Technology of Manufacturing of Rapidly Solidified Aluminium Alloys, All-Russian Institute of Light Alloys, Moscow, 1997 (in Russian). [4] O.D. Neikov, A.V. Krajnikov, Mater. Sci. Forum 217–222 (1996) 1649–1654. [5] O.D. Neikov, Water atomised powder technology for advanced aluminium alloy production, in: K. Kosuge, H. Nagai (Eds.), Proceedings of the 2000 Powder Metall. World Congress, vol. 1, Kyoto, Japan, 1–16 November 2000, Japan Soc. Powder and Powder Metallurgy, Kyoto, Japan, 2000, pp. 464–466. [6] O.D. Neikov, Rapidly solidified aluminium alloy powder, in: Proceedings of the 1998 Powder Metall. World Congress, vol. 1, Granada, Spain, 18–22 October 1988, European Powder Metallurgy Association, Bellstone Shrewsbury, UK, 1998, pp. 129–134.
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[7] O.D. Neikov, A.V. Krajnikov, A.V. Sameljuk, G.I. Vasilieva, N.G. Chaykina, explosion-security of manufacture processes of advanced water atomized aluminum alloy P/M technology, in: V. Arnold, Chiu-Lung Chu, W.F. Jandeska, H.I. Sanderow (Eds.), Advances in Powder Metallurgy and Particulate Materials, vol. 3, Orlando, Fl, 16–21 June, Metal Powder Industries Federation, Princeton, New Jersey, USA, 2002, pp. 60–69. [8] A.K. Lokenbah, N.A. Zaporina, L.K. Lepiny, Phase composition and structure of oxide films on the fine aluminium powder particles, in: Proceedings of the LatSSR Academy of Sciences, vol. 1, Series Chemic, 1981, pp. 45–54 (in Russian). [9] A.V. Laptev, S.S. Ponomarev, L.F. Ochkas, Solid phase consolidation of fine grained WC-16% wt Co hard metals, J. Adv. Mater. 33 (2001) 42–51. [10] O.D. Neikov, Yu.V. Milman, D.B. Miracle, D.V. Lotsko, A.I. Sirko, N.A. Yefimov, Effect of Sc alloying additions on structure and mechanical properties of PM and cast high-strength Al alloys of same composition, in: Proceedings of the 2001 European Congress and Exhibition on Powder Metallurgy, vol. 2, Nice, France, 22–24 October 2001, European Powder Metallurgy Association, Bellstone Shrewbury, UK, 2001, pp. 219–224. [11] O.D. Neikov, D.V. Lotsko, A.I. Sirko, A.V. Sameljuk, G.E. Thompson, N.P. Zakharova, N.A. Yefimov, Properties of rapidly solidified powder alloys of the Al–Zn–Mg system, Mater. Sci. Forum 396–402 (2002) 1223–1228. [12] O.D. Neikov, Yu.V. Milman, D.B. Miracle, D.V. Lotsko, A.I. Sirko, N.P. Zakharova, Effect of powder size on mechanical properties of elevated temperature aluminium alloys produced by water atomization, in: Proceedings of the 2001 European Congress and Exhibition on Powder Metallurgy, vol. 2, Nice, France, 22–24 October 2001, European Powder Metallurgy Association, Bellstone Shrewbury, UK, 2001, pp. 225–230.