Solid atomization technology and process of molten metal and alloy

Vol.12 No. 3Trans. Nonferrous Met. Soc. ChinaJun.2002[ Article ID] 1003 - 6326( 2002 )3- 0375-04.Solid atomization technology and process of molten metal and alloyCHEN Zhen-hu:(陈振华), YAN Hong-g(严红革),CHEN Gang(陈刚),CHEN Ding(陈鼎)( School of Materials Science and Engineering , Hunan University , Changsha 410082 , China )[ Abstract ] A novel solid atomization technology , which using high velocity gas jet- stream rich in solid salt particles asatomization medium , was developed. The results show that using this novel atomization process can produce various metaland alloy powders with finer particle size , finer microstructures and higher cooling rate than those powders produced bythe conventional gas atomization technology.[ Key words ] powder procssing ; rapid solidification ; powder metallurgy[ CLC number ][ Document code ]A1 INTRODUCTION2 EXPERIMENTALTwo-fluid atomization technology of moltenThe experimental device used to atomize moltenmetal is an effective metal and alloy powdermetal or alloy stream by solid salt particles involved inproduction method in which a high velocity jet-streamgas stream is ilustrated in Fig. 1. The atomizationof gas or high- pressure jet -stream of water is used toexperiments were conducted according to thedisintegrate a molten metal stream into fine dropletsfollowing procedure. NaCl particles were used asto form powders. It has been reported that the yieldatomization medium and stored in a feeding tank.of the powders produced by two-fluid atomizationDuring the atomization procedure , salt particles weretechnology ranges from 30% to 50% of the totaltransmitted to the nozzle by pressurized gas at highoutput of the metal and alloy powders in thvelocity from an air compressor or nitrogen gasworld121 The origin of two fluid atomizationsupply.' The molten metal or alloy was disintegratedtechnology dates back to 1870s when several patentsby the jet-stream of the mixture of NaCl particles andusing air atomization technology to producegas into fine droplets and allowed to fall into water tononferrous metal powders were authorized3l. Duringform powders. After cleaning , filtering and dryingthe Second World War , the Mannesmann Powdertreatment，the powders were collected and ready forProduction Method was developed in German , it wasutilization. The NaCl left in water was recycled.used for making ferrous metal powders to substituteIn this study , the particle size of NaCl powdersmaterials used into guiding hoops of artillery shellsranged from 175 pum to 350 pum. The pressure of thedue to a shortage of copper 41. Over the last eightyyears numerous investigations have been conducted toimprove two-fluid atomization technology ，these了include : changing the structure of atomization nozzle ,8increasing the pressure of atomization medium andimproving the atomization state of molten metal.With the help of these achievements ，two fluidatomization has gradually grown into a perfected?A5technology~ 101，but there has been no any report onhe study of the process with changed atomization↓medium.In the present work , a novel metal and alloypowder production technology has been developed inFig. 1 Schematic diagram of experimental devicewhich a high velocity jet-stream of gas rich in solidusing salt particles as atomization mediumsalt particles is used to disintegrate molten metal and中国煤化工ter lop;3- -Atomizer;lloy stream into droplets to obtain powders. TheIt particles ;technology has been termed as solid atomizationTYHC N M H Galt pricls;technology.The preliminary results of these6- -Feeding tank of salt particles ; 7- Air compressor ;experiments are reported.8- -Storage tank of salt particles①[ Foundation ite1Project ( 50174025 ) spprted by the National Natural Science Foundation of China ; project spprted by the NaturalDoctorate' FUm8Education Minister of China[ Received date ]2001 - 12 - 03 ;[ Accepted date ]2002 - 02- 27376.Trans. Nonferrous Met. Soc. ChinaJun.2002high velocity jet-stream of air or nitrogen gas wasTable 1 Comparison of particle size between0.9士0.1MPa. The volume flow rates of the air orgas atomized powders and powders atomized bynitrogen gaswere 6m/min and 3. 3 m2/minsolid salt particlesrespectively. Induction heating was adopted. TheAlloy or Sample Mean particle ParticleAtomizationsuperheat of the molten metal or alloy was betweenmetalnumbersize/ pumsize/ (umtechnology150K and 200K. The diameter of the melt streamAl-12%Si 1-188.2229. 19 Gas atomizationwas 3.5~ 3.8mm. The flow rate of NaCl particlesAl-12%Si1-242. 7313.27 Solid atomizationwas 1.5~4. 0 kg/ min.A laser diffraction particle size measuring meterb2-179. 448.65 Gas atomizationmade by Malvern Instruments Cooperation in Britain2b2-245. 414. 06 Solid atomizationand a Fisher Particle Size Measuring Meter were usedSn3-178. 1815. 39 Gas atomizationto test particle size of the powders. Standard .6n3-236.697.26 Solid atomizationmetallography techniques were used to examine themicrostructure of the particles. Chemical analyzingZn4-176. 428.64 Gas atomizationwas used to measure the residual NaCl content in the4-244.303.89 Solid atomizationas- prepared powders.)u5-184.2228. 67 Gas atomization3 RESULTS AND DISCUSSION5-240.2814.26 Solid atomization3. 1Mean particle size and particle sizedistribution of solid atomized powders3.2Effect of atomization process parameters onA series of comparative experiments wereparticle size of metal powdersconducted to examine the atomization effect of moltenIn this technology , the state of salt particles is ametal using solid salt particles. The mean particle sizekey parameter. Its effect on particle size of thof the produced powders andtheir particle sizeatomized powders was systematically investigated.distribution were measured. The results are given inThe result shows that the higher the mass fluid rateTable 1 and Table 2.and velocity of salt particles and the finer the saltThe mean particle size of NaCl atomized powdersparticles，the finer the atomized powders are.is about 50% smaller than that of the powdersGenerally speaking , the mass fluid rate and velocityproduced with gas atomized process. The particle sizeof salt particles depend on the volume fluid rate anddistribution of the former is much narrower than thatvelocity of the carrier gas. In the presentof the later. In contrast to the conventional highexperiments , the pressure of air and nitrogen are bothvelocity gas jet-stream atomization process，the finercontrolled at 0. 8 MPa , the volume fluid rate of thempowders with narrower particle size distribution canare kept at 6m2/min and 3.3 m'/min respectively ,be obtained by this new developed technology underthe variables are the feeding rate of salt particles withotherwise equal process parameter conditions , such assame particle size. The mean particle sizes of the as-super-heat temperature , diameter of molten metal orprepared Al-12%Si alloy powders are 42. 73 pum andalloy stream and its fluid rate and structure of the jet60.42 pum respectively when the feeding rate of salnozzle.particles are 4 kg/min and 2. 1 kg/ min respectively.Table 2 Comparison of particle size distribution between gas atomized powdersand powders atomized by solid salt particlesParticle size distribution/pmAlloy orSample0~1010~ 3030~5050~ 7070~ 100100~ 150 > 150AI- 12%S(g)121.788.7316. 2(14.5017.5618.04.23.19Al-12%S(s)8. 8425.1625. 6016.64.8.102. 662.8012.8616.6513.9115.7017. 8020.. 282210. 5123. 7920. 1912.138. 8712. 003212.9212.0716. 80中国煤化工13.61.24.0029.3722.036.657. 60MHCNMHG422.199.899.7615.8319.0715. 8617. 40Z(39. 6823.8924.8213. 4513. 188.146.84C(g)1.508.9126.7014 6515. 219.0523.209.056. 100.93* gzation ,S一-Solid atomization :mass flow of salt particles is 4 kg/ minVol.12 No .3Solid atomization technology and process of molten metal and alloy377 .It is obvious that the particle size of the as preparedpowders were greatly affected by the feeding rate of[a)salt particles. According to theory analysis ,it can beconcluded that if the fluid rate of carrier gas is given ,then the higher the pressure of the carrier gas and thehigher the fluid rate of salt particles and the higherthe dynamic energy of the salt particles ，the betterthe atomization effect of the molten metal or alloy.Because of the limitation of the experimental device ,it is hard to quantitatively examine the effect of20Amatomization parameters on particle size of the as-(6)prepared powder that requires further research.3.3Cooling rate of atomized metal and alloypowder particlesThe thermal conductivity of air and nitrogen gasis range in 0.015~0.06W m-“K-1 , for water it isranges in 0.65~0.70W m-l K-1 , for NaCl and_20pam_KClthey are 1.15W m-l K-1and6.53W m-' K-1 at 272K. The specific heat ofNaCl and KCl are 854} kg h K-1 andFig.2 Microstructures of atomized Al-Si690] kg-h K- 1 respectively ，their density aralloy particles prepared by two processes2.17g/cm’and1. 99 g/cmt11]respectively.(a)- -Atomized with air ;According to Jones' S viewpoint, the cooling(b )-Atomized with solid salt particlesmethod of molten droplets during atomization is byheat convection on the interface of the droplet and3.4 Explosion problem of salt particles duringgas , the heat conductivity h can be calculated by theatomization processfollowing equation :The expansion coefficients of NaCl and KClh=2k+0.6(kpfC (”归(1)crystal are 44X 10-°/K and 36X 10-°/K at 300Kwhich are much higher than that of common metalswhere k ，ρ，C and μ are the heat conductivity，and alloy , so it is very easy for them to explodedensity , specific heat and viscosity of the atomizationduring quick heating. Obvious explosion phenomenonmedium respectively ; V stands for relative velocity ofof the salt particles occurred during the presentthe droplet and the cooling medium ，and d theatomization procedure which is beneficent fordiameter size of the particle. The higher theatomization of molten metal stream and make thecalculated h，the higher the cooling rate of thephenomenon of melt wrapping up salt particles harddroplet.to take place.Referring to the data of every parameter fordifferent medium ，it is obvious that the cooling3.5 Corrosion effect of salt on metal powderefficiency of NaCl and KCl powders is much higherThe residual salt in the as- prepared powdersthan that of water and gas , the cooling rate of theatomized by solid salt particles is very easy to bepowder atomized by solid salt particles is also higher.cleaned out by water washing treatment. T he residualFig.2 shows the microstructures of eutectic Al-SiNa+ion concentration canbe reduced to;alloy powder ，the primary silicon phase in the saltconsiderable low level after water washing , filteringatomized particles is much finer than that obtained byand drying treatment. Table 3 shows that waterair atomization method. The cooling rate of a certainparticle can be calculated with the following equation :Table 3 Na ion concentration in powders atomized个= cxk(2)by solid salt particles after water washing treatmentwhere T is cooling rate ,c and k are constants ,入is平l ions concentration afterthe space between the two secondary dendrite crystal中国煤化工ater washing treatmentarms. In present study ，λ is about only 1 μmA1- d.MYHCNM HG0.016% .according to Fig.2 , so the cooling rate is in the rangeAl-12%S( s)1-20.012%of 10*~ 10'K/s according to Eqn.(2). It is higherby one to two orders of magnitude than that of theSr(g)3-10. 005%conventionalatomization process ，10-10*K/57.Sr(s)3-20. 006%378 .Trans. Nonferrous Met. Soc. ChinaJun.2002washing treatment can lower the concentration of Naobtained by the newly developed technology underions in the powders atomized by solid salt particles tootherwise equal process parameters conditions , thethat of air atomized powders. This kind of powdercooling rate of the particles obtained by thishas not been corroded by salt after several months'technology is higher.storage.2) Particle size of the as prepared powders isobviously affected by the state of salt particles in the3.6 Application prospects of this new atomizationcarrier gas stream. The higher the relative amount oftechnologythe salt particles in the gas jet-stream , the higher theTheessencepresentatomizationvelocity of the salt particles , the finer the size of thetechnology is to use high velocity jet -stream of airsalt particle and the finer the size of the as- atomizedcarrying high concentration of solid powder as a mainpowder particles.medium of atomization to atomize molten stream ofmetal or alloy. It is a simple and applicable[ REFERENCES ]production technology for fine metal powdercomparingwithotherconventional powder[1 ] Fritz V Lenel. Powder Metallurgy Principles anctechnology. To fabricate high quality of metalApplications [ M ] New Jersey : MPIF Princeton ,1992 ,13.powders proper solid salt particles as atomizationmedium must be chosen carefully.' The salt particles[2] Alan Lawley. 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