Research On Microstructural Modification And Strengthening Mechanism Of Laser Additive Manufacturing Cu-15Ni-8Sn Alloys

With its high strength and elasticity,the Cu-15Ni-8Sn alloy has recently become increasingly popular as a potential substitute for Be-Cu alloys.However,its susceptibility to tin segregation from tin supersaturation has greatly limited the preparation of this alloy.Although powder metallurgy,melt-spinning,and spray forming have achieved certain effects in controlling the element segregation of this alloy,these methods are still hindered by their long process flow,limited product shape and size,and low material utilization.Laser additive manufacturing(LAM)based on powder freeform fabrication can effectively inhibit the segregation of alloying elements,refine the grain size,and create a large number of dislocations with its technical advantages of rapid solidification.Recently,with the concepts of "additive manufacturing+" and "extreme manufacturing",hybrid manufacturing technologies that integrate additive manufacturing technology and various processing technologies have begun to emerge.As an extreme manufacturing technology for material preparation,it can obtain material microstructures and properties that are difficult to obtain by conventional methods,and has outstanding technical advantages in material modification.Previous studies have shown that pre-deformation aging can significantly improve the strength of Cu-15Ni-8Sn alloys.However,few reports further explore the potential of the Cu-15Ni-8Sn alloy using "additive manufacturing+" extreme manufacturing technology combined with laser additive manufacturing technology and pre-deformation aging strengthening.Herein,dense and fine-grained Cu-15Ni-8Sn alloy bulk samples were prepared by two additive manufacturing processes:laser melting deposition(LMD)and selective laser melting(SLM),which solved the problem of macro segregation of Sn element,and their mechanical properties were better than those of casting counterpart.In order to further improve the strength of LAM Cu-15Ni8Sn alloy,three post-treatment strengthening processes,i.e.,"direct aging","solid solution→aging"|,"solid solution→pre-deformation→aging",were studied according to its characteristics of rapid solidification non-equilibrium microstructure.The effects of rapidly solidified non-equilibrium microstructure,solid solution microstructure,and deformation microstructure on the aging behavior of Cu-15Ni-8Sn alloy during isothermal aging at 400℃ were systematically analyzed,as well as the strengthening mechanism and its influencing factors.In the study of laser additive manufacturing of Cu-15Ni-8Sn alloy and its post-treatment strengthening,the main findings are as follows.(1)The microsegregation of Sn in the LAM Cu-15Ni-8Sn alloy and the pre-deformation before aging both weakened the effect of aging strengthening and lead to an uneven distribution of the spinodal structures,ordered phases(D022 and L12),and lamellar structures.(2)Dislocation substructures change the morphologies of spinodal structures and ordered phases,affecting the variant types of the D022 ordered phases.The honeycomb network structure of "original segregation phase+ordered phase" formed around the cellular crystal after the direct aging of the SLM sample improves its strength and electrical conductivity.(3)The spinodal structure,ordered phase,discontinuous precipitation γ phase,and lamellar structure all can promote the strength of the Cu-15Ni-8Sn alloy.The ordered phase L12 which is coherent with the matrix has the greatest strengthening effect,and the slow transition from D022 to L12 is the main reason why the alloy is not easily overaged.(4)The formation of the network structures of the discontinuous precipitation y phases at the grain boundaries mainly accounted for the reduction in the plasticity of the Cu-15Ni-8Sn alloy.(5)The pre-deformation aging strengthening of the LAM samples significantly shortened the preparation process of the high-strength Cu-15Ni-8Sn alloy,and the yield strength reached 1329～1389 MPa.The large deformation promotes the preferential nucleation and growth of the discontinuous precipitation y phase along the elongated grain boundaries and shear bands.The smaller the grain size before deformation,the more uniform the distribution of the discontinuous precipitation γ phase.In addition,the stacking faults and L-C dislocations on the dislocation cell wall coexist with the ordered phase for a long time,which inhibits the nucleation of the γ phase and has a good strengthening effect.