Molecular Dynamics Simulation On Selective Laser Melting Of Cu-Zr Amorphous Alloy

Amorphous alloys have long range disorder and short range order with unique mechanical and chemical properties.However,traditional thenology can only fabricate amorphous alloy devices with small size and simple shapes.As a promising additive manufacturing technique,laser selective melting（SLM）is expected to break through the limitation of glass formation ablility and critical size,provides the possibility for manufacturing amorphous alloy devices with large size and complicated shapes.However,some problems such as crystallization of amorphous alloy during SLM greatly affect its microstructure and degrade its mechanical properties.The essence of amorphous crystallization is the transition of atoms from short-medium range order to long range order.In this thesis,we study the evolution of the structure in laser selective melting of Cu-Zr amorphous alloys using molecular dynamics simulations.We have studied the effect of laser processing parameters on the microstructures,crystallization and mechanical properties,and futher clarify the mechanism of amorphous crystallization,to find the way to avoid the occurrence of crystallization.This work mainly includes:（1）The microstructural characteristics,temperatural change and mechanical properties of laser selective melting of Cu₅₀Zr₅₀ amorphous alloy were studied.The lower cooling rate leads to the higher content of perfect icosahedron in powders.However,after laser melting,no obvious correlation exists between the structures of SLM samples and initial powders.The fraction of icosahedron-like clusters in the depositing layer is higher than that in the cladding layer due to the heat affecting of molten pool.However,the fraction of Frank-Kasper polyhedron clusters remains unchanged after laser melting and heat affecting.Moreover,we found the the ultimate tensile strength,the hardness and elastic modulus are similar for SLM and as-cast samples.（2）The effects of laser processing parameters on the microstructure and crystallization behavior of laser selective melting of Cu₁₀Zr₉₀ amorphous alloy were studied.When the laser scan speed is lower,a large number of BCC crystalline phase are formed,accompanied with the nucleation of a few of HCP and FCC crystalline phases.However,with increasing laser scan speed,the fraction of BCC phase significantly decreases,accompanied with more icosahedron-like clusters forming,promoting the formation of amorphous structure.The reason is that higher scan speed greatly reduces the relaxation time of atoms,thus inhibiting the nucleation and growth of crystalline phase.