Effects of combinatorial water atomization on microstructures and properties of Cu-Sn powder

RARE METALSVol. 23, No. 2, Jun 2004, p. 143Effects of combinatorial water atomization on microstructures andproperties of Cu-Sn powderJIA Chengchang", MA Hongiul:2, JIN Chenghai?, and GE Liqiang2)1) School of Materials Science and Engineing. Univesity of Science and Tchnology Beijng. Bejig 10083. China2)Advanced Technology and MacialsCo. Lud. Bejing 1008100 China(Received 200304-23)Abstract: A couple of additional cooling nozzles were assembled under traditional atomization nozzles in order to improvethe process and produce the powder with fine microstructure and low oxygen. The infuence of the process parameters onthe properties of the powder was investigated. The results show that finer powders with lower oxygen content and more irregular shape can be achieved by combinatorial atomizing process comparing with normal one under the same atomizingpressure.Key words: powder metllurgy; combinatorial nozzle; water atomization; rapid solidification; particle size1 Introductioncludes: contolled silicon power supply with 1000Hz, intermediate frequency induction furmace withAtomization is an important method to producecapacity of 30 kg, atomizing tower and collectionthe metal powder [1-3]. In recent years the develop-chamber. Figure 1 is a sketch map on the main por-ing trend of atomize method system is to producetion of atomizing systerm including combinatorialfine powder with low oxygen. Especially metal in-nozzle.ject molding (MIM) boomingly developed sinceNormal nzzle1980's arouses the large quantity need of the right5and very small metal powder. It requests the particlewie民size of powder be as small as 22 um even finer.Furthermore, the request for the oxygen is also moreAlfixiure colingozlerigorous [4]. This work tries using new water atom-X First ntomizing focusize method to get finer and better quantity powderwith the same parameters such as water pressure. Inthe present method, the affixture cooling nozzle isSccond alomizing focusset under the normal nozzle and the complex nozzlemakes the melt metal be broken under the normalFigure 1 Sketch map on combinatorial atomizingnozzle, then be impacted by the water coming fromsystem.the cooling nozzle. The melt metal drop can be bro-ken two times and be rapidly solidified.2.2 Experimental processCu-Sn bronze alloy (mass fraction of tin is 20%)2 Experimentwas selected as raw material because its powder is2.1 Experiment equipmentswidely used in the powder metallurgy field. NormalThe experiment system used in this work in-and combinatorial water atomization were experi-mented. The flux and pressure of the first atomizingCorresponding author: JIA Chengchang Email: jc@ mater.ustb.edu.cn中国煤化工MYHCNMHG144RARE METALS, Vol. 23, No. 2, Jun 2004water were constant and the flux of second atomiz-d = A/(Vsinc)1)ing water was contolled, in order to research the in-where d is the particle diameter of powder, V is thefluence of the second atomizing on the properties ofvelocity of water flow, A is a constant,a is the spraypowder.angle. The grain size of atomized powder is not onlyCu-Sn bronze aloy was firstly piece melt in. adirectly dependent on the velocity and angle of at-1000 Hz intermediate frequency induction fumace,omizing water flow, but also relative with the designthen pour into a furmace with a nozzle in its bottomand structure of nozzle. The structure of nozzle in-preheated by gas. The powder was washed with al-fluences the constant A. The higher the efficiency ofcohol and dried, then sampled by mesh. The particlenozzle is, the bigger the value of A becomes. Thesize of powder was analyzed by means of theAtomizing kinetic energy of atomizing medium isSA-CP3 optical sedimentation grain size analyze in-transformed to the special surface energy of powderstrument, and the microstructure was observed byparticle in the atomizing process. In this process, theS-250 scanning electron microscope.larger the contacting area between the melt metalAtomizing water and cooling water pressure wereand the atomizing medium is, the higher the trans-12 MPa, Atomizing water current fux was 124 Uforming eficiency is and the finer powder particlesmin; Cooling water curent flux was 0, 40, 60, andbecome. In this research work, melt metal is first80 U min; In addition, pipe diameter is 7 mm.broken by the water flow from atomizing nozzle andbecomes fine melt drop, then more broken by cool-3 Results and discussioning nozzle before its solidification. At the same time3.1 The infuence of Combinatorial atomizing onwater atomizing process includes three stages: (1)powder grain degreethe formation of primal melt drop from the meltFigure 2 gives the infuence of atomizing modemetal, (2) primal melt drop broken to powder parti-on powder grain size. It can be seen that along withcle, (3) aggregation of partial particles. In the Com-the cooling water current flux increasing, Cu-Snbinatorial atomizing process, fine powder particlespowder becomes finer. The mass fraction of -200can be gotten because their aggregation ability be-mesh powder increases from 72% to 79% and pow-came weak due to the action of atomizing nozzle. Itder smaller than 40 um is from 73.9% to 79.6%.has to be seen that there is a possibility of particleThe average particle size of powder decreases frominduration or solidifcation, so overheat temperatureof melt metal is necessary. Furthermore, the distance24.3 um to 21.0 um.between two atomizing focus can not be far.AF F3.2 The influence of Combinatorial atomizing on80 tpowder microstructure50 tThe progress of solidification is properly sepa--8-Normal atomizingrated into two parts, the initial nucleation of crystals40Combinatorialand the subsequent growth of these nuclei by theatomizing20|accretion of molecules from the melt.In the solidification process of alloys, the micro-structure is dependent on the growth ratio of solid2010608(phase from melt. If the growth ratio is lower thanParticle size 1 umdiffusion ratio in the melt, dendrite structure may beFigure 2 Influence of atomizing mode on the grainformed.size of Cu-Sn powders.The second dendrite distance can be used to esti-Nozzle is a key part in the atomizing process.mate the cooling velocity and homogeneous (There is an atomizing model [4]powder [5]. When the composition of alloy is cer-中国煤化工MYHCNMHGJia c.C. et al, Effects of combinatorial water atomization on microstructures and properties of ..145erally at 1/2-1/3.2 = B(d77dr)-"2)Figure 3 gives the microstructure of powderswhere d77dt is cooling velocity,心is second dendritemade from normal and Combinatorial atomizingdistance, B and n are experience constant, n is gen-process.4M 20Figure 3 Dendrite structure of powder: (a) Normal atomizing; (b) Combinatorial atomizing.The measured result shows that the second den-In the water atomizing process, as the melt dropdrite distance of normal atomized powder is aboutis broken, its special surface area rapidly increases1.8 um and that of combinatorial atomized powderand oxidation reaction happens between melt dropis about 1.0 pum. From this result it can be calculated(or particle) and surrounding water vapor, formingthat the cooling velocity increases from 10 K/s tooxidation film on the particle surface [6].10* K/s. In other words, Combinatorial atomizingxMe(s) + yH2O(g) = Me2O, + yH(g)(3process can largely increase the cooling velocity.During the metal particle was cooled by water, asteam film is formed around the particle. Because of3.3 Infuence on oxygen content in powderits low thermal conduction coefficient, the steamFigure 4 shows the infuence of water currentfilm can break down the heat transform from particleflux from cooling nozzle (second atomizing) on theto environment and accelerate the oxidation reaction.oxygen content in powder. When the water currentIn this process, as soon as the melt drop forms, it isflux from cooling nozzle is zero, the process is nor-cooled by water flow, then the oxygenation condi-mal atomizing.tion is destroyed. At the same time turbulence flowformed around the metal particles can break thesteam film and accelerate the thermal transfer frommetal particle to cooling medium. So the oxygena-tion reaction can be slowed and the oxygen contentin metal particles can be reduced.0.2|3.4 Influence of combinatorial water atomizingon the powder particle shapeFigure 5 gives the photographs of water atom-0.1 Lized powder. It can be seen that the powder of the02040608(combinatorial atomizing has more iregular shapeWater current fIlux 1 L'min-'than the normal atomizing process. This is becausethe second atomizing water flow can reduce the sur-Figure 4 Influence of water current flux on the oxy-face energy of melt metal drop and increase thegen mass fraction in powder.中国煤化工MYHCNMHG146RARE METALS, VoL. 23, No. 2, Jun 2004cooling rate, and the ability for particle to becomeDuring atomizing process, the solidification timeball debases.14s and blling time tb can be expressed as fllows [7].4s= (Dpm)(6h) { CpIn[(Tm - T)(T,- T)] + HI(Ts- T)}(4where D is melt drop diameter, Pm is density of melt,forming sphere, so the powder particles become ir-Tis the temperature of atomizing medium, Tm is theregular shape. This demonstrates that lower viscositytemperature of original melt, Ts is the solidificationand higher surface tension of melt are propitious totermperature, H is meling latent heat of metal, and hthe sphere of powder. In the combinatorial atomizingis thermal conduction cofficient.process, the scond atomizing water flow can de-h= [(3ntfu)(4Vo)(0n*-r)crease the surface tension and increase the viscositywhere n1 is the radius of powder particle after beingof melt, which makes against the sphere of powder.spheroid, r2 is the radius of powder particle beforeTherefore, the shape of powder particle is more ir-being spheroid, μ is viscosity of melt, σ is surfaceregular.tension of melt, and V is volume of powder particle.4 ConclusionIn the combinatorial atomizing process, the cool-ing rate of melt can be increased from 10'K/s to 10*K/s by the effect of second atomizing water. Cu-Snpowder with finer particle size and lower oxygencan be produced under the same atomizing pressure.References1I] German R.M., Powder Melallury Science [M],Metal Powder Industry Federation, New Jersey, 1984:2] Miller SA, Close-coupled gas atomization of metallloys [1], Meral Powder Repor, 1987, 42 (0): 702.3] Hopkins W.O.. High performance RSP gas atoniza-tion-critical areas of equipment design D, MetalPowder Report, 1987, 42 (10): 706.4] German R.M., Modern Developments in P/M [M],MPIF, N, 1981: 325.5] Wang PX, Powder Meallungy (In Chinese) [M],Mtallurgical Industry Press, Beijing, 1984: 24.6] Tunbrg T. and Nyborg L, Surface reaction duringFigure 5 Shape of atomized powder particles: (a)water atomization and sintering austenitic stainlessNormal atomizing; (b) Combinatorial atomizing.steel powder [J], Powder Metalurgy, 1995, 38 (2):The shape of atomized powder particle is primar-120.ily determined by o and 1. If b < ts, the melt drop[7] Nichiporenko S and Naida YI, Heat Exchange be-tween Metal Particles and Gias in Atomization Proc-has enough time for balling before its soldification,ess [J], Powder Metall. Mel.Ceram, 1968, 67 (7): 509so the powder particles become sphere. Contrarily, if1> 1, the melt drop will be solidified before its中国煤化工MYHCNMHG