Laser Additive Manufacturing Of CoCrFeNi High-entropy Alloy And Its High-cycle Fatigue And High-Speed Impact Behavior

High-entropy alloys exhibit excellent strength-toughness performance under harsh conditions such as ultra-low/high temperatures,strong radiation,corrosive environments,and high strain rates,mainly due to their high lattice distortion,strong metallic chemical bonds,stable phase,achieving "cocktail effect" properties.Traditional manufacturing methods such as casting and arc melting easily produce defects such as segregation,hot cracks,inclusions,and large differences of structure in surface and center.Additive manufacturing,as a process with a faster cooling rate during forming,can effectively reduce segregation and hot cracks,and can adjust the micro structure of specimens through the design of process parameters to customize performance.Laser powder bed fusion(L-PBF)technology is currently a high-precision metal additive technology with excellent mechanical properties.High-entropy alloys manufactured by L-PBF demonstrate the advantages of high-entropy alloys and additive manufacturing technology.CoCrFeNi high-entropy alloy manufactured by L-PBF has become a material that provides excellent performance in extreme environments and complex conditions.This paper focuses on the microstructure of CoCrFeNi high-entropy alloys in different forming directions based on the L-PBF forming process,and their evolution laws and mechanical behavior under high-cycle tension-tension fatigue and Hopkinson bar high-speed impact.The deformation mechanism and twinning mechanism under different wor-king conditions are the main concern,and the anti-fatigue and anti-impact performance of CoCrFeNi high-entropy alloys manufactured by L-PBF are analyzed.This can provide a scientific basis for the strength design and service evaluation of additively manufactured parts of this type of alloy.The main research work and important conclusions of this paper include:(1)The influence of L-PBF process elements on microstructure and static mechanical properties:experimental research and establishment of the L-PBF process window of CoCrFeNi high-entropy alloy,with a relative density of 99.956%.The optimized parameters are laser power(P)of 300W,scanning speed(v)of 700mm/s,hatching space(h)of 0.09mm,and layer thickness(t)of 0.04mm.The influence of laser scanning path on microstructure,including grain orientation,grain morphology,and substructure,and the mechanism of microstructure strengthening and toughening of additively manufactured specimens are revealed.Under the same grain size,the microstructure of additively manufactured specimens endows them with better strength than cast specimens.(2)High-cycle fatigue performance and microstructure evolution of CoCrFeNi manufactured by L-PBF:Using material characterization methods such as EBSD and TEM,the microstructure of TD direction(the loading direction of the TD direction specimen is perpendicular to the construction direction)specimens in different fatigue loads and different life stages are analyzed to determine the conditions for inducing nano-twinning and improving their fatigue performance through the effect of twin boundaries on dislocation motion.The fatigue performance of TD and BD direction(the loading direction of the BD direction specimen is parallel to the construction direction)specimens manufactured by L-PBF is analyzed by studying the effect of construction direction on fatigue performance under fatigue loads of σmax=200,250,300,350,and 450 MPa.Under high-cycle fatigue loads of σmax above 250 MPa,the fatigue life of BD direction specimens under the same fatigue load is higher than that of TD direction specimens.The fatigue fracture mechanism of specimens is analyzed by combining fracture morphology and microstructure characterization.The impact resistance of CoCrFeNi high-entropy alloys manufactured by L-PBF is analyzed by the Hopkinson bar impact test,and the deformation mechanism and failure mode of specimens are discussed under different impact velocities.