Influence of melt superheat on breakup process of close-coupled gas atomization

Availableonlineatwww.sciencedirect.com巴P富c包NcENonferrous MetalsSociety of ChinaTrans. Nonferrous Met. Soc. China 17(2007)967-973Presswww.csu.edu.cn/ysxb/Influence of melt superheat on breakup process ofclose-coupled gas atomizationOUYANG Hong-wu(吹阳鸿武, CHEN Xin(陈欣), HUANG Bai-yun(黄伯云)State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, ChinaReceived 13 October 2006; accepted 2 February 2007Abstract: In close-coupled gas atomization(CCGA), the influences of melt superheat on breakup process are fundamental to obtaindesired or finer powder. Based on a series of Cu atomization experiment under different superheating conditions, the influences ofmelt superheat on breakup process were studied. Experimental results indicate that as the melt superheat is increased to 150, 200, 250and 300 K, the mean particle size(Dso)decreases consequently to 34.9, 32.3, 30.9 and 19.7 um. Theoretical analysis reveals that theprimary breakup and secondary breakup processes are close coupled, and the melt superheat radically influences the melt properties.and plays a crucial role on goveming the filming process of primary breakup and the atomization modes of secondary breakup.Thereexists a strong nonlinear decrease of contact angle of melt to nozzle orifice wall when the superheat is increased from 250 K to 300 K,leading to a marked fall of the film thickness formed in primary breakup, and Dso of copper powders is therefore sharply reduHowever, the log-normal distribution feature of particle size has not been substantially improveKey words: gas atomization; superheat; close-coupled nozzle; powder; particle sizenumerical methods cannot simulate the actual conditions1 Introductionas well. Thusstudies focused on this problemwere all carried out by roughly correlating operationClose-coupled gas atomization(CCGA), with high parameters and particle size while neglecting detailedfine powder productivity, high cooling rate and relatively atomization process, which only leads to individual andlower gas consumption, is increasingly becoming the empirical results[7-12leading technology of manufacturing fine sphericalIn this study, based on the experiment of atomizingmetallic powders[ 1-3]copper melt under different superheats, an"inverseThe main feature of CCGA is that there exists a method"[13] was presented in term of the numericalunique gas recirculation zone below the nozzle, where mode of detailed breakup process to quantitativelyinteractions between melt and high pressure gas become analyze the influences of melt superheat on the particleso complex that it has not been understood well How to size and distribution in CCGAoptimize the operation parameters of CCGA is still an artmore than a science. Despite of a wide range of 2 Experimentalapplication in modern powder processing technologies,the ccga needs more control and enhancement to meetThe experimental procedure was performed with athe demands of specific powder qualities for specific cloose-coupled atomizer. Commercial pure nitrogen wasapplication, especially the particle size(4-6]used as the atomizing gas. The gas resource consists of ahe melt superheat, one of the basic gas atomization band of twelve nitrogen gas cylinders manifold to aparameters, is crucial to the powder particle size and single two-stage high pressure gas regulator, with adistribution. However, it is hard to on- line observe the maximum of 5 MPa. The experimental pressure was 3.5tinuous change ofmelt properties under high temperature and high speed heat中国煤化16o6and1656K,gas flow within complex flow field structure, even the reYHCN MHGcopper is 1 356 K,Foundation item: Projects(10476043; 50574103)supported by National Natural Science Foundation of ChinaCorrespondingauthorOuyaNgHong-wurTel+86-731-8877192;E-mail:oyhw1816@sina.com.cnOUYANG Hong-wu, et al/Trans. Nonferrous Met. Soc. China 17(200namely the superheat is 150, 200, 250 and 300 K), and superheat. Firstly it declines at low pace(34.9, 32.3, andthe accuracy of temperature measurement was +2 K. 30.9 um, respectively), nevertheless, it plummets to 19.7Each atomization process lasted about 90 sum with a sharp fall by 36% at 300 K. So it is necessaryThe morphology of atomized Cu powders was to know why the powder particle sizes allanalyzed by scanning electron microscopy(SEM, Japan same log-normal distribution and what causes the sharpJEOL JSM-5600LV). The particle size distribution was fall at the superheat of 300 K.asured with XRd Micro-Plus LaLls)4 Discussion3 Results4.1 Schematic profile of gas velocity and melt tem-Fig 1the SEM morphologies of copperperature field of CCGAdifferent superheats. All the fourThe unique feature of CCGa is that there is a gasatomized powders show the similar feature, various in recirculation zone like a reverse cone downstream jetdiameter, but all highly spherical. Furthermore, with below the nozzle(orifice), as shown in Fig 4. The top ofgradual rise of melt superheat, the proportion of small the cone is the gas stagnation point, where gas velocityparticles(<20 um)increases whereas that of large goes to zero while pressure rises to maximum. The gasparticles (about 80 um) decreases, as a result, the stream enters into the recirculation zone upstream via thmedium diameter falls. This trend becomes markable stagnation front with a subsonic velocity. Near the meltwhen the superheat is increased to 300 K.orifice,the recirculating gas turns laterally (in radial6. Fig. 2 depicts the particle size distribution. All the direction) towards the circumferential edge. On reachingcurves appear log-normal profiles, which is just the the edge, it encounters the sonic boundary, which forcesbasic feature of close-coupled gas atomized powders. the gas flow to turn downstream; consequently theHowever, the curve of 300 K stands alone, where the recirculating gas is restricted within the cone, andparticle size falls in a wide margin, which shows separated from the outside supersonic flow. Thus, the gasagreement with Fig. l Fig 3 shows the medium diameters velocity distribution within the atomization area isf copper powders under four different conditionsApparently, it declines correspondingly with the rise of temperature fields along the central axis of close-coupledIL中国煤化工CNMHGFig 1 SEM morphologies of Cu powders under different superheat: (a)150 K;(b)200 K; (c)250 K;(d)300KOUYANG Hong-wu, et al/Trans. Nonferrous Met. Soc. China 17(2007)Melt temperature/K300K.010000200Recirculation wakeFlg.2 Particle size distribution under different superheating-400061218243036nationsAtomization distance/mm40Fig-5 Profiles of gas velocity and melt temperature alongcentral axis32.33period (nearly 10s)[14]4.2 Breakup process of melt in CCGAenerally, the whole gas atomization goes throughthree closed coupled stages: primary breakup, secondarybreakup and solidification. The primary breakup processundergoes three orderly steps: film growth, ligament anddroplet formation, which are shown in Fig. 6(15-17Superheat/KFig-3 Mean particle size under different superheating吻AtomizerGas jet exitFig6 Primary breakup of liquid melt: (a) Film growthMelt pour tube(b)Ligament formation;(c) Droplets formationRecirculation eddiesa)In film growth step, firstly the liquid melt columnRecirculation zoneexiting from the delivery tube runs into the recirculatinggas zone; immediately the recirculating gas generateste forces on melt column, where theFig 4 Schematic of gas recirculation zonestagnation pressure is upstream while the ambientsuction is downstream. So those two forces are jatomizer are illustrated in Fig. 5[6]. On top of the umbrella trestles, supporting the liquid column todiagram is the profile of temperature gradient of melt. extruded into film, so called umbrella effectThe melt temperature approximately reduces in a step of25 K per 6 mm accompanying with the liquid melt gas flo中国煤化 iquid film underIt of perturbationstream's breakup, where To is the initial temperature of andCNMHGthat it is tom intothe copper melt. In general, the cooling rate of melt ligaments at half wave-length.droplet in CCGA is so high(about 10-10 K/s) thatc)In droplet formation step, accordinghe whole disintegration process finishes in such a shortayleigh instability, liquid ligament under high970OUYANG Hong-wu, et al/Trans. Nonferrous Met. Soc. China 17(2007)gas flow is dreadfully instable, so it develops instantly asshort waves under the interaction of aerodynamic frictiontension. Furthermore given(a) Flow.Oive wave's oscillation, liquid ligaments areinto fine droplets in the end.E0①D翁During the filming process, the thickness of filmdepends on the contact angle g of the liquid to delivery(ey Flow-O0 Ctube orifice interface. and cannot be thinner than ainimum Hmin(mm)[18]The diameter of ligament obtained from the tearingFig.7 Droplets'secondary breakup modes: (a) Twins, 10.7