Research On The Solidification Mechanism And Performance Control Of Cu-Fe Monotectic Alloy Produced By Laser Additive Manufacturing

Cu-Fe immiscible alloy with good electrical conductivity,thermal conductivity,ductility,toughness of the alloy component Cu element and the high strength,high hardness,excellent wear resistance and magnetic properties of the alloy component Fe element,is widely used in the preparation of electrical engineering switches,large-scale integrated circuits and electrified railway overhead wires,etc.It also has a good application prospect in the field of electronic industry,automobile and aviation.At present,the research on Cu-Fe immiscible alloys focuses on revealing the formation of microstructure and the mechanism of liquid phase separation(LPS),and improving segregation and delamination of Cu-Fe immiscible alloys during conventional solidification.Due to the constraints of sample size and complex preparation process,few studies on the properties of Cu-Fe immiscible alloys have been made,which greatly limits the industrial application.In this project,laser additive manufacturing technologies(laser melting deposition(LMD)and selective laser melting(SLM))with high cooling rate were used to successfully prepare bulk Cu-Fe immiscible alloys.Research results were obtained as follow:To solve the problem that Cu-Fe immiscible alloy is prone to segregation and delamination,LMD combined with mechanical alloying(MA)was used to produce particle dispersion reinforced Cu-Fe immiscible alloys by adjusting Fe content.Based on theoretical calculation,the physical model of microstructure evolution was established to reveal the LPS mechanism of Cu-Fe immiscible alloys.The Fe-rich liquid phase is separated by nucleation and growth mechanism,and the nucleus of Fe-rich liquid phase grow and coarsen by diffusion,Ostwald ripening,and collision and coagulation.The collision and coagulation are driven by Brownian motion,Stokes motion,and Marangoni migration.Since Marangoni migration is the dominant mechanism,the size of Fe rich particles increases from the bottom to the top of the molten pool.Furthermore,the properties of particle dispersion reinforced Cu-Fe immiscible alloys were studied.The hardness of Cu-Fe immiscible alloy(?153 HV_0.2)is uniformly distributed and slightly higher than that of brass(?137.7 HV_0.2).During the electrochemical corrosion,Fe-rich particles are preferred to corrosion,which can realize the cathodic protection of Cu-rich matrix,making the corrosion resistance of Cu-Fe immiscible alloy better than that of brass.And it shows good soft magnetic properties:Saturated magnetization of 9.19 emu/g and the residual magnetization and coercivity are 0.13 emu/g and 9.25 Oe,respectively.During rapid solidification,“single hole”and“multi hole”Fe-rich particles reinforced Cu-Fe immiscible alloys were formed in situ due to Kirkendall effect.The corrosion resistance of Cu-Fe immiscible alloy reinforced by“multi hole”Fe-rich particles is better than that of“single hole”immiscible alloy and brass after 5 days immersion in 3.5 wt.%Na Cl solution.In addition,the cooling rate was controlled by adjusting the laser spot size to shorten the period of nucleation,diffusion,coarsening,and collision and coagulation of the Fe-rich droplets in the LPS process.So that the size and distribution of the dispersed particle as reinforcement phase were refined and optimized,respectively.The hardness,wear resistance,and corrosion resistance of Cu-Fe immiscible alloy with finer Fe-rich particles and more uniformly embedded in Cu-rich matrix are more excellent.In view of the segregation and delamination of LMD-produced Cu-Fe immiscible alloy with high Fe content and the limitation of the sample size,the bulk Cu-Fe immiscible alloy with good macroscopic quality was successfully prepared by SLM.The heterogeneous“fiber/particle-twin”Fe-rich phase is embedded in the Cu-rich phase matrix.The microstructure evolution of Cu-Fe immiscible alloy was analyzed,and the mechanism of plastic deformation and crack propagation was discussed.During fracture failure,twin as a toughening mechanism plays a role of crack bridging,which can effectively shield the stress concentration at the crack tip,thus inhibiting the crack propagation.Based on the mechanism of LPS,Ti B₂ particles as nucleating agent were introduced to control the nucleation of LPS,optimize the“fiber/particle-twin”Fe-rich phase structure in Cu-Fe immiscible alloy,and improve the hardness and wear resistance of Cu-Fe immiscible alloy.The mechanisms of strengthening and wear of Cu-Fe immiscible alloy were analyzed and discussed.The fibrous Fe₂P with high hardness strengthens the Cu-rich matrix,which makes it difficult for Ti B₂ particles to move or remove.It can enhance the support effect of Ti B₂ particles during wear to improve the bearing capacity of Cu-rich matrix and protect Cu-rich matrix from wear.The wear resistance of Cu-Fe immiscible alloy is significantly improved.In view of the“strength-ductility”inversion relationship of Cu-Fe immiscible alloy,316L stainless steel powder was selected to reduce the stacking fault energy of mixed powder and overcome the fracture failure of Fe-rich phase during deformation.The“fiber/particle-twin/stacking fault”structure with good strength and ductility was formed in situ.Cu-Fe immiscible alloy shows good mechanical properties.The strength of Cu-Fe immiscible alloy is about 590±10 MPa and the elongation is about 8.9±1%.It is attributed to fine-grain strengthening,back stress strengthening and solid solution strengthening caused by Cu-rich matrix with fine equiaxed and columnar grains and supersaturated solid solution Fe,and the interface strengthening and twin boundary/fault strengthening caused by“fiber/particles-twin/fault”Fe-rich phase.