Fabrication,Structure And Properties Of 3D Printed Amorphous Alloys By Selective Laser Melting

Bulk metallic glasses(BMGs)are new kinds of materials developed from the last midcentury.Due to their disordering atomic structure,BMGs possess a series of excellent properties,including high strength and large elastic limit,excellent corrosion resistance and soft magnetic property.However,poor manufactuability at room temperature restrains its engineering applications as structural materials.Thus,the development of novel processing techniques is essentially required for extending the scope of applications of BMGs.Selective laser melting(SLM)known as one of the powderful 3D printing techniques is a promising route to solve the manufacturing bottleneck of BMGs.In this thesis,we designed and constructed a new setup of SLM device,and then chose Fe-based and Zr-based amorphous alloy systems as examples to study the maunfractuability of BMGs.The microstructures,thermal behaviors and mechanical properties have been systematically investigated by means of X-ray diffraction(XRD),optical microscopy(OM),scanning electron microscopy(SEM),transmission electron microscopy(TEM),nano-indentation,thermal analysis,static compressive test,finite element method simulation(FEM)and so on.The specific contents of the thesis are briefly decribed as follows:(1)Based on the the specific requirements for 3D printing of BMGs,a novel SLM device with two powder feeding modes was constructed.This setup contains five parts: optical system,powder feeding system,atmosphere protection system,software control system and cooling system.It possesses two working modes: One is the hopper feeding mode,in which powders are transported by the hopper to laser scanning area,which is specially appropriate for the selection of amorphous alloy systems and processing parameter optimization,as a small amount of feedstock powders are used,The another is cylinder feeding mode,in which powders are pushed from the powder cylinder to laser scanning area,which is suitable for the fabrication of large components.In addition,the device can be protected by inert atmosphere after vacuumizing,which saves the usage of inert gas to the greatest extent.After several debugging,the device can be stably operated and reach the expectation,which is propitious to the research of SLM 3D prting of BMG.(2)According to the microstructural evolution of SLMed Fe-based BMG under different processing parameters,it was found that high energy density caused serious crystallization in heat affected zones(HAZs),thus resulting in the reduction of amorphous content of the printed sample.Based on the temperature field by finite element simulation,it was revealed that high laser energy density make the nano-crystallines in HAZ have more time to nucleate and grow.In order to clarify the relationship between the processing parameters and amorphous content in the 3D printed parts,a mathematical model was established to predict the amorphous content under different energy densities.(3)Bulk Zr-based BMGs with high amorphous content(83%)and low porosity(0.12%)were sucessfully fabricated after parameters optimization of SLM process.The as-printed BMGs were composed of molten pools and HAZs,the molten pool exhibits fully amorphous,while the HAZ exhibits mixing structure of amorphous phase and nano-crystallines.Due to the high fracture toughness of amorphous alloy system,the generation of micro-cracks were restrained around the holes,thus high compression strength(1.5GPa)was obtained.However,the intrinsic annealing effect due to repeated scanning in SLM process caused degraded plasticity and fracture toughness compared with the casting BMG.(4)The structural heterogeneity of SLMed Zr-based BMG was studied in terms of crystallization kinetic analyses.Two types of heterogeneity were revealed:(i)The transformation of SLMed Zr-based BMG is lack of incubation time during isothermal crystallization process because of the existance of nano-crystallines in HAZ.(ii)The differences of amorphous phase in composition and microstructure between molten pool and HAZ lead to various crystallization mechanisms.The amorphous phase in molten pool follows a mode of three-dimensional diffusion controlled growth with increasing nucleation rate,while that in HAZ follows a mode of three-dimensional diffusion controlled growth on the preexisting nucleation sites.The heterogeneity cause two softening points of SLM Zr-based BMG and higher micro-hardness in HAZ.(5)Finally,the effect of heat treatment in super-cooled liquid state on SLMed Zr-based BMG was also investigated.The results indicate that heat treatment can incresae the content of "free volume" in SLMed Zr-based BMG,and decrease the modulus and hardness in both molten pools and HAZs.However,the heat treatment ultimately results in poor performance.Finite element simulation reveals that the heterogeneity of molten pool and HAZ will cause micro-cracks around the boundary of them during heat treatment process,which is the main reason for the deterioration of performance.