The Reaearch On The Technology And Properties Of Fe-Cr Alloy Powder Prepared By Close-coupled Vacuum Induction Melting Gas Atomization

With the rapid development and application of laser additive manufacturing technology,higher requirements are put forward for alloy powders.Performance of the powders is determined by the preparation method and technology.The morphology and particle size distribution of alloy powders directly affect the feasibility of laser cladding and other additive manufacturing processes and the quality of products.The metal powders prepared by close-coupled vacuum induction gas atomization have the advantages of small particle size,high sphericity and good fluidity,etc.After long-term development,this technology has become an important method for large-scale preparation of alloy powders.For different powder systems,the specific process is not the same.Fe-Cr alloy has the advantages of high hardness,high cost performance,and has been widely used in surface modification to improve the wear resistance of parts.However,the preparation of Fe-Cr alloy powder for laser additive manufacturing is in urgent need.In this paper,Fe-Cr alloy powder suitable for laser cladding was prepared by the system according to the design idea of the existing powder system,on the basis of assisting to build and debug the industrial production grade close-coupled vacuum induction gas atomization system.The effects of atomization process on the yield,morphology and properties of Fe-Cr alloy powder and its mechanism were systematically studied.A preliminary laser cladding test was carried out on 3Cr13 stainless steel substrate with particle size of 53-105?m.The macroscopic forming,microstructure and hardness of the cladding layer are analyzed to verify the feasibility for laser cladding.The results show that when the atomizing pressure increases from 3.0MPa to 3.8MPa,the yield of fine powder increases gradually and the median particle size of powder decreases.This is mainly due to the increase of atomizing pressure on the metal flow fragmentation ability.When the atomizing pressure continues to increase to4.2MPa,the median particle size of powder increases by 35.56%.The reason is that the high atomizing pressure significantly increases the turbulence of the airflow.The proportion of irregular small particles in the powder increases,which makes it easier to form agglomerated powder.Accordingly,with the increase of atomizing pressure,the fluidity of powder becomes better first and then becomes worse.With the increase of superheat,the yield of fine powder,the median particle size of powder,the fluidity and the bulk density of powder all increased firstly and then decreased.The main reason is that the increase of superheat causes the decrease of the viscosity of molten steel,which promotes the increase of the impact breaking ability of gas and reduces the size of atomized droplets.However,when the superheat is too high,the small particle droplets will collide and agglomerate,leading to the increase of atomized droplets size before cooling.With the increase of pipe diameter,the median particle size of powder increases and the fluidity decreases.This is mainly due to the decrease of the gas-liquid flow ratio due to the increase of the diameter of the liquid guide tube,which leads to the decrease of the breaking ability of the gas to the metal melt in unit time.The median particle size of the powder decreases and the fluidity increases with the increase of the extension length of the liquid guide pipe.The reason is that the extension length of the liquid guide pipe increases,the negative pressure in the reflux area increases and the atomization effect of the liquid is enhanced.Among the above parameters,atomizing pressure has the greatest influence on powder particle size distribution,the extension length of liquid guide pipe mainly affects the stability of atomization process and the superheat affects the crushing and solidification mode of molten steel.In the atomizing process,the reason why the atomizing process stops is that the nonmetallic slag enters the molten steel and blocks the tundish or the liquid guide pipe extending too short.The optimal process of Fe-Cr alloy powder for laser cladding was as follows,atomizing pressure 3.8MPa,superheat 250?,diameter of liquid guide tube 4.5mm,and extension length of liquid guide pipe 2.5mm.The average mass fraction of oxygen content in the whole granular-size section is 0.024% and the alloying element content is controllable.Because the cooling rate of powders with different particle sizes varies greatly,the microstructure of powders changes from microcrystalline to columnar and cellular with the increase of particle sizes.After laser cladding,the cladding layer has good appearance,no pores and no cracks.The Fe-Cr alloy powders prepared by close-coupled vacuum induction gas atomization technology is suitable for laser cladding modification of 3Cr13 martensitic stainless steel and can effectively improve the hardness of the cladding layer.