The Research Of Close-Coupled Gas Atomization Technology For Additive Manufacturing 18Ni300 Alloy Powders

The close-coupled gas atomization technology can prepare metal powders with small particle size,high sphericity and good flowability.After a long period of development,this technology has become the main method to prepare powders,especially metal and alloy powders for additive manufacturing.However,due to the lack of revealing the mechanism of close-coupled gas atomization flow field and studying on the influence of atomization process parameters on powder characteristics,the current close-coupled gas atomization technology has some common problems,that included low yield of small particle size powder and poor stability of atomization process.Based on the above reasons,in order to reveal the basic characteristics of close-coupled gas atomization flow field and guide the design of atomizer,this paper used Ansys Fluent software to simulate the atomization flow field.The 18Ni300 mold steel with great application prospects in the field of metal additive manufacturing was selected as the research object,and the influence of different atomizer structures on the stability of atomization process,as well as the influence of atomizing process parameters and atomizing medium on the characteristics of 18Ni300 alloy powder was studied in combination with experiments.The research results are as follows:There was a reverse cone reflux zone in the typical flow field of close-coupled gas atomization.The gas velocity of the gas retention point which is at the top of the reflux zone was zero.The bucket-like shock wave was on the outside of the recirculation zone.And the intersectant bucket-like shock waves formed arcuate normal shock,which was Mach disk.The structure parameter of annular gap and the main process parameter of atomization pressure had significant influence on the atomization flow field.When the annular gap was 1.2 mm and the atomization pressure was 4.5 MPa,the reflux zone was the largest,the maximum velocity was 671 m/s,the minimum temperature was 74 K and the maximum Mach number was 4.66.Based on the simulation results,the annular gap of the atomizer was 1.2 mm.The suction pressure of the atomizer was tested experimentally.The results showed that with the atomization pressure increased,the suction pressure of the nozzle decreased first and then increased.The change trend of suction pressure with pressure of inner tangential inlet atomizer was gentler than vertical inlet atomizer,and atomized flow field of inner tangential inlet atomizer was more stable.On the other hand,the new design catheter reduced the occurrence of "metal tumors" in the process of gas atomization,which improved the stability of the close coupled gas atomization process.The improved atomizer can perform stable atomization.With the atomization pressure and metal melt superheat temperature increased,the width of size distribution and median diameter of volume(x50)of 18Ni300 metal powder decreased first and then increased,while the yield of 15-53 um powder,degree of sphericity and apparent density increased first and then decreased.When the atomization pressure was 4.5 MPa and the metal melt superheat temperature was 255 K,the x50 and width of size distribution reached the minimum value,the yield of powder was the highest,the degree of sphericity and apparent density were the best.The result of atomization pressure was consistent with the numerical simulation prediction results.The metal powder atomized by nitrogen had smaller x50 and width of size distribution,higher yield of 15-53 um powder,better degree of sphericity and apparent density than that atomized by argon atomization.The content of oxygen element in powder prepared by two kinds of atomizing media was little different,but the content of nitrogen element in powder prepared by argon atomization was much lower than that prepared by nitrogen atomization.