Effect of closed-couple gas atomization pressure on the performances of Al-20Sn-1 Cu powders

Available online at www.sciencedirect.com。ScienceDirectRARE METALSELSEVIERR4RE METALS, VoL 27, No. 4, Aug 2008,p. 439Effect of closed-couple gas atomization pressure on the performances ofAl-20Sn-1Cu powdersZHAO Xinming, XU Jun, ZHU Xuexin, and ZHANG ShaomingNational Engineering and Technological Research Center for Non-ferrous Metals Composites, General Research Instiute for Non-ferrous Metals, Beijing 100088, ChinaReceived 29 August 2007; received in revised form 29 November 2007; accepted 10 December 2007AbstractA-20Sn-1Cu powders were prepared by gas atomization in an argon atmosphere with atomizing pressures of 1.1 and 1.6 MPa. The charac-teristics of the powders are determined by means of dry sieving, scanning electron microscopy (SEM), optical microscopy (OM), and X-raydiffractometry (XRD). The results show that the powders exhibit a bimodal size distribution and a higher gas pressure results in a broad sizedistribution. All particles in both cases are spherical or nearly spherical and stllites form on the surface of coarse particles. Dendnitic andcellular structures coexist in the particle. With decreasing particle diameter, the secondary dendrite arm spacing (SDAS) decreases and thecooling rate increases. The particles processed under high gas atomization pressure (1.6 MPa) exhibit a lower SDAS value and a highercooling rate than those of the same size under low gas atomization pressure (1.1 MPa), The XRD resuts show that the Sn content increaseswith decreasing particle size.Keywords: powder production; Al-Sn-Cu alloy; gas atomization; rapid solidification; particle size distribution1. Introductionence of atomization gas pressure on the powders was inves-tigated in terms of the powder particle size distribution,The gas atomization technique is widely used to producemorphology, and microstructure.a wide range of ulrafine spherical metal alloy powders,which have very attractive material properties because of the2. Experimentalhigh solidification rate [1-4]. The gas atomization process isdescribed as the nozzle generates a high velocity gas streamThe atomization runs were performed in an indus-that disintegrates the liquid metal stream into droplets,trial-scale closed-couple discrete jets gas atomizer. Argonwhich subsequently spheroidize, cool, and freeze into metalwas used as the atomizing agent. A mixture of pure elementpowder particles ranging from 1 μm to 1 mm in diameterAl (99.80 wt.%), Sn (99.99 wt.%), and electrolytic copper[2-3]. Two types of gas atomization nozzles are used in low(99.99 wt%) with the nominal composition of Al-20Sn-1Cuand moderate pressure powder metallurgy production. Onewas prepared in a magnesia crucible by means of a me-is closed, confined, or close-coupled nozzle; another is opendium-frequency induction fumace under an argon atmos-or free fall nozzle [5]. The closed-coupled nozzle is far morephere. The alloy was heated to 100 K above the equilibriumefficient for atomization than the free fall nozzle because theliquids to ensure sufficient mixing of all elerments. A ho-high velocity atomization gas flow is very close to the metalmogenization time of about 10 min was adopted. To reducestream and the pressure loss is small before impact with thethe impurity, the melting and atomization chambers weremelt.evacuated and argon was backfilled to a pressure of 1.05 XThe particle size and surface morphology are important105 Pa prior to melting and atomization. The primary at-characteristics [6], which can be affected by the operationalomization variable used in this study is gas pressure. Theparameters of gas type, gas pressure, melt flow rate, andvalues of pressure. are 1.1 MPa and 1.6 MPa, respectively.melt superheat [3, 5]. Since the pressure of atomization gasThe中国煤化工wed to cool down tois one of the important parameters, in this article, the influ- ambieCiphere, and then col-MHCNMHGCorresponding author: XU Jun E-mail: xujun@grinm.com440RARE METALS, VoL. 27, No. 4, Aug 2008lected.describe the spread of powder size distibution. Fig. 1(b)The experiment of powder particle size distribution wasshows that when P = 1.6 MPa, dg4 and dso are 47.7 μm andcarried out on a standard sieving machine. After vibrating129.6 um, respectively, and dga and dso are 67.4 μm andfor 15 min, the weight of the powders retained on each sieve151.6 μum when P= 1.1 MPa. By cormparing the standardwas measured and converted into a percentage of the totaldeviation σ= 2.72 (P= 1.6 MPa) and σ= 2.24(P= 1.6powder sample. Samples of powders were mounted in bake-MPa), it can be seen that a higher gas pressure results in alite and polished by following the standard metallographicbroader size ditribution.procedures. A dilute Keller's reagent solution (95vol.%H2O-2.5vol.% HNO-1.5vol.% HCI-1vol.% HF) was se-(a■P=I.1 MPalected to etch the polished cross-sections of the samples. Thephase morphology and microstructure of the particles were20- I口P-1.6 MPaobserved by optical microscopy (OM). The OM was alsoused to measure the secondary dendrite arm spacing, and to15calculate the cooling rate of the experienced powders usingan equation. The examined method of arm spacing was thesame as Ref. [7]. The surface morphology of the powderswas examined in a HITACHI S-4500 scanning electron mi-croscope (SEM). The phase in the powders was identifiedby X-ray diffractometry (XRD) technique using Cu Kau ra-25 38-45 53-62 75-106 150-180diation with a wavelength h= 0.105406 nm.25-38 45-53 62-75 106-150 >180Paticle size 1 um3. Results and discussion1003.1. Powder size distribution90|(he powder size ditribution of the atomizedAl-20Sn-1Cu alloy and the cumulative weight distributiondependence on the particle size at different gas pressures areshown in Fig. 1. It is seen that the size distribution is quitesimilar in both cases and there is a bimodal with the maxima50occurring at about 25 μm and 150 μm as shown in Fig. 1(a).40From Fig. 1(b), the mass mean powder diameter (dso) was30obtained as 47.7 um forP= 1.6 MPa and 67.4 μum for P=20 I. P=I.1 MPa▼P=1.6 MP'a1.1 MPa, respectively. The results show that the mass me-dian powder diameter (dso) decreases as the atomization gas204060 80100120140160 180200pressure increases, which is in agreement with other studiesPaticle size/ um[8-11].The relationship between the powder size distribution andFig. 1. Powder size distribution of the atomized Al-20Sn-1Cuthe atomization process is mostly empirical. A typical andalloy (a) and cumulative weight distribution (b) dependence onthe particle size at diferent gas pressures.available empirical equation for the atomization of metalscomes from the analysis of a free-fall atomization process3.2. Surface morphology of the particlesby Lubanska [1, 12] In the present study, the only variableis the atomization gas pressure (P). Therefore, the variety ofFig. 2 shows the scanning electron micrograph (SEM) ofP value leads to a change of the aspiration pressure at thethe atomized Al-20Sn-1Cu powders. It can be observed thatmelt orifice, which will cause the melt flow rate alter (high all particles in both cases are spherical or nearly spherical. Inor low). During the atomization process of liquids, the pow-addition, the amount of large particles atP= 1.1 MPa isder size distribution will be influenced by the primary andconsiderably more than that at P = 1.6 MPa. The surface ofsecondary breakup [13]. Most of the fine powders appear tothe particle exhibits cellular-dendritic with short secondarybe created by the secondary breakup of the large globulesdendrites growth morphology, which is a clear characteristicthat are produced by a relatively coarse primary stage and中国煤化工3ere are a few smallthis will result in a bimodal characteristic.particidefined as sallitesThe standard deviation, defined as σ= dgy/dso, can also in botYHCNMHGZhao XM. et al, Effecet of closed couple gas atomization pressure on the performances of AI-20Sn-1Cu powders44120 um50umFig. 2. Scanning electron micrograph of the atomized Al-20Sn-1Cu powders: (网)at P= 1.1 MPa; (b) at p= 1.6 MPa; (c) surfacemorphology of the particle, P= 1.1 MPa; (d) stellites P= 1.6 MPa.Nichiporenko and coworker [14-15] found that whatever trum of solidification structures, depending on the alloythe surface tension of the metal, atomized particles will becomposition and solidification conditions. Fig. 3 shows thespherical if there are no factors (such as protective films oroptical micrograph of the cross section of theatomizedhigh melting point oxides) which impede the transformationAl-20Sn-1Cu powders. It can be seen that two microstruc-to the spherical shape. In the present study, oxygen in thetures coexist in the particle. One is dendritic structure; theatmosphere of the atomization chamber is kept constant at position of nucleation is indicated with a white arTow, where0.02 vol.%, and liquid melts do not react with argon gas toeliminate the infuence on the shape of the particle. Thus, theparticle is spherical or nearly spherical, as shown in Figs.2(a) and (b).The formation of sacllites in Fig. 2(d) is related to thedifferent solidification rates with different particle diameters[16]. There exists a lower solidification rate for large parti-cles owing to a low heat transfer rate, and a higher solidif-cation rate for small ones. As a result of various solidifica-tion rates, during the atomization process, the larger dropletsmay be still in liquid or semi-solid state while the small ones10 umhave already solidifed entirely. The collision of large andsmall particles in the gas turbulence let small particles attachFig. 3. Optical micrograph of cross section of powders at P =to large ones firmly.1.1 MF中国煤化工3.3. Metallographic examinationstheopposite side. TheRapidly solidified alloy powders exhibit a broad spec-YH;CN MH GeoposteThe micro-442RARE METALS, VoL. 27, No. 4, Aug 2008structure in atomized powders depends on the velocity of thediameter and the particles processed under high gas atomi-solid-liquid interface [9, 17]. Initally, the velocity is large inzation pressure exhibit lower SDAS values than those of thethe undercooled droplet and generates cellular structure nearsame size under low gas atomization pressure. The SDASthe nucleation position. The velocity of the solid-liquid in-was related to the cooling rate and the fllowing relationterface decreases as the solid-liquid interface crosses thwas found [18]:particle, because the recalescence of melt occurs with an= 49.37-0.3(1)consequent increase of the interface temperature, and the so-where,名is SDAS (um), and T is the cooling rate (K:s).lidification front develops into dendritic structure.Fig. 4(b) depicts the results of these calculations. It is seen3.4. Secondary dendrite arm spacing (SDAS) and coolingthat the cooling rate increases with decreasing particle di-rateameter and the particles at high gas atomization pressureIt is generally accepted that the cooling rate of particles(1.6 MPa) exhibit a higher cooling rate. The difference ofdepends strongly on the particle size. Fig. 4 shows thethe cooling rate relates to the effective heat transfer coff-measured secondary dendritic arm spacing and calculatedcient of particles, which should be higher for high gas at-cooling rates as a function of the particle diameter. Fig. 4(a)omization pressure than for low atomization pressure [19].indicates that the SDAS decreases with decreasing particle(a)8.0*10*.0*10*。Pel!MPa6.0x10* t之5.0x103-4.0x10°9”3.0*10*. -0-P=1.6MPa2.0x10*-8 1.0x100.(40 80 120 1602001080 120 160 200d1 μmd/μmFig. 4. Measured secondary dendritic arm spacing (a) and calculated cooling rates (b) as a function of particle diameter forAl-20Sn-1Cu atomized powders.3.5. XRD analysis of the powdersabout 180 um and 25 um. The XRD patterms provide evi-The phases in the Al-20Sn-1Cu atomized powders weredence that a multiphase composition consisting of Al and Snidentified by X-ray difractometry (XRD). The XRD patternexists in all samples. The relative amount of Sn in the fineof each powder size range was compared with the standard powders (~25 um) is higher than that in the coarse onesdiffraction files from each specific phase. Fig. 5 shows two(~180 um), providing evidence that the Sn content increasestypical XRD patterns of atomized powders with the sizes ofwith the decrease of the particle size.■Al4. Conclusions口Sn(1) The Ar gas atomized AI-20Sn-1Cu powder exhibits a于|bimodal sizce distribution with the maxima occuring atabout 25 um and 150 μm in both cases (P= 1.1 MPa and P会|= 1.6 MPa). The mass mean powder diameter (dso) is 47.7。。.μm for P= 1.6 and 67.4 um for P=1.1 MPa, respectively.The comparison of the standard deviation indicates that ahigher gas pressure results in a broader size distribution.(2) The dendritic and cellular structures coexist in one20406080particle SDAS decreases with decreasing particle diameter20/0)and th中国煤化工atomization pesureFig. 5. XRD patterns of dfferent size Al-20Sn-1Cu atomizedexhibiY片CNMHGB rates increase withpowders, P= 1.1 MPa: (田) ~180 pm; () ~25 pm.the deZhao XM. et al, Effct of closed .couple gas atomization pressure on the performances of AI-20Sn-1Cu powders443atomization pressure exhibit a higher cooling rate.Vol 9), ASM Intermational, OH, 2004: 71.(3) XRD results show that the Sn content increases with[8] Zambon A, Nitrogen versus helium: effects of the choice ofthe decrease in particle size.the atomizing gas on the structures of FespNigSioBo andFez2Niz6TaySgB17 powders, Mater Sei. Eng. 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