| The requirements for high quality metal and alloy powders becomes higher and higher with the development of spare parts manufacture. The characteristics of high quality powders are low oxygen content, accurate alloy composition, fine particle size, and high degree of sphericity. Close-coupled gas atomization, with advantages of high atomizing efficiency and making low oxygen content powders, is an ideal choice for making high quality powders. This study is based on close-coupled gas atomization system made by PSI Company. The influence of the tip shape and length of melt delivery tube on tip back pressure value is studied. The results show that the tip shape has a direct impact on tip back pressure. In experiment condition, the column tip can get lower back pressure easier than the taper tip. The tip back pressure becomes lower with the increase of tip diameter and length of melt delivery tube. It also shows periodical variation of the back pressure with the increase of atomizing pressure. Then the mechasim and influence factors of the back pressure formation are discussed. The assembly of the tip shape and length of melt delivery tube are optimized. When the value of atomizing pressure between 2.5MPa～4MPa, the length of taper tip with diameter of 7.0mm between 4.0mm～4.4mm or the length of taper tip with diameter of7.5mm between 3.6mm～4.0mm or the length of colμmn tip between 2.6mm～3.0mm, the tip back pressure can keep on the stable suction.The influence of tip shape and length of melt delivery tube, atomizing pressure and melt superheat on average particle size and productive rate of-400 mesh powder are studied, based on copper alloy pure copper and stainless steel. The results show that:the length of melt delivery tube makes great influence on stability and efficiency of atomization. When the length of melt delivery tube increases from 4.1mm to 4.3mm, the average particle size of CuSnAg alloy powder increases by 5.1% and 6.9%. The tip back pressure increases with the length of melt delivery tube, This trend is beneficial for stability of atomization and bad for atomizing efficiency. Atomizing pressure and melt superheat make great influence on atomizing efficiency. The average particle size of 316 L powder reduces by 17.1% and the value of productive rate of -38μm powder increases by 19.3% respectively, with the atomizing gas pressure increases from 3.0MPa to 3.5MPa. When the melt temperature increases from 1560℃to 1620℃,the first value reduces by 5.4% and the second one increases by 5.2%. So the increase of atomizing gas pressure makes greater influence on atomizing efficiency than the increase of melt temperature. The increase of atomizing gas pressure can result in the increase of gas kinetic energy. The proper increase of atomizing gas pressure can reduces the average particle size and improve fine powder productive rate effectively. However, the increase rate of gas kinetic energy becomes smaller with the increase of atomizing gas pressure, so when the pressure increases to some extent, the increase rate of the atomizing efficiency reduces.The theoretical analysis for the melt break-up and solidification mode based on superheat shows that:the break-up mode turns from bag to film stripping with the increase of superheat, and so the break-up efficiency improves. Calculating on the basis of See-Johnston mode of melt drop solidification time, the result shows that the solidification time turns longer with the increase of superheat, and the solidification time of big melt drop is longer than that of the small drop. Thus, the productive rate of powder clusters and "satellite powder" increase, which can make average particle sizes increase.At last, the average particle sizes of copper, copper alloy and stainless steel powders are calculated based on Miller-Glies model, the results show that the relationship between average particle size of pure copper and copper alloy powder and proportion of gas and metal is in accordance with equation:d50=K(G/m)-1/3,the proportional factor K is respectively 120 and 171. But the stainless steel powder does not agree with the equation, because its complicated compositions result in great difference between physical property of its melt and pure metal melt. |
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