Researches On The Structures And Properties Of Bulk Metallic Glass Prepared By Selective Laser Melting

Bulk metallic glass?BMG?owns a series of excellent mechanical,physical and chemical properties,which make them a category of materials for practical applications.However,the limited glass forming ability of metallic glass makes that it is difficult to prepare large-sized and special-shaped BMG by traditional technology,and this restricts the applications of BMG.Selective laser melting?SLM?technology is a fabrication process based on and developed by layer manufacturing,and it provides a new solution for the preparation of large-sized and special-shaped BMG.However,the related researches on the SLM forming of BMG are still underway,and there are many problems need to be solved.Firstly,a Cu-Zr-Al ternary alloy system with lower glass forming ability and lower amorphous phase stability was chosen for the research.The effects of scanning rate,laser power and scanning gap on the structures and properties of the SLM forming Cu-based BMG were studied.The structural mechanism that decides the macroscopic mechanical properties of the SLM forming Cu-based BMG was found.Large-sized and special-shaped Cu-based BMG with whole amorphous structure was prepared by the SLM technology.Based on the work above,the influences and the mechanisms of the heat conduction characteristic during the preparation and the gaseous hydrogenation treatment after the preparation on the structure and performance of the SLM forming BMG were discussed in detail.The following main results are obtained:?1?Based on the research on the SLM forming of Cu₅₀Zr₄₃Al₇ ternary alloy system with lower glass forming ability and lower amorphous phase stability,the energy density criterion for the SLM forming research of BMG was put forward:The scanning speed,laser power and scanning gap parameter can well meet the definition of energy density in the SLM forming process of BMG.The energy density which is too high or too low that is not conducive to the quality improvement of the SLM forming BMG,and there is a suitable energy density range for the specific amorphous alloy system.Through the control of energy density,it is possible to complete the preparation of large-sized and special-shaped BMG with lower glass forming ability by SLM technology.Based on the work above,the concept of critical energy density for the SLM forming research of BMG was also proposed,which provides theoretical guidance for the SLM forming researches of BMG with different alloy systems.?2?The near-net forming of large-sized and special-shaped Cu₅₀Zr₄₃Al₇BMG with whole amorphous structure was accomplished,breaking the limitation of glass forming ability for the Cu-based BMG,and proving that it is viable to prepare BMG with lower glass forming ability by the SLM technology.Meanwhile,the SLM forming Cu-based BMG shown good performance,which included the relative density of 0.970,the excellent corrosion resistance,the compression elasticity modulus of 55.8 GPa and the compression fracture strength of 1044.4 MPa.In addition,the nanoindentation elastic modulus of 129.6 GPa was significantly higher than the compression elasticity modulus,closing to the copper mold cast Cu-based BMG,and it proved that the pore type defects were the main factors that affected the macroscopic mechanical properties of the SLM forming Cu-based BMG.?3?It was found that the mechanical properties of Cu₅₀Zr₄₃Al₇ BMG were anisotropic,which was caused by nearly uni-directional heat flow during the SLM forming process.However,the above differences were not obvious due to the limitation of forming conditions.In addition,through the strict control of the heat flow direction during the solidification process,a typical kind of BMG with significant mechanical properties of anisotropic was prepared,and the influence of uni-directional heat flow on the structures and properties of BMG was researched.Under the heat flow was in the ideal uni-directional condition,the degree of difficulty for the migration and rearrangement of internal atoms was different along different directions during the solidification process of BMG,and this would make a free volume content difference between the sections that vertical and parallel to the uni-directional heat flow.At last,an anisotropy of macroscopic mechanical properties was introduced to the BMG.Based on the work above,the heat flow control scheme was proposed in the SLM forming process of BMG,and it provided a guide for the SLM forming of BMG with mechanical anisotropy.?4?Because of the traditional methods such as heat treatment and thermal isostatic pressure cannot be applied to the post-processing of the SLM forming BMG,a new method of room-temperature gaseous hydrogenation was developed.Firstly,a typical kind of quick quenching forming BMG was selected to validate the feasibility of the new method.It was found that the internal hydrogen atoms of the BMG were weakly adsorbed after the room-temperature gaseous hydrogenation,and mainly concentrated in the free volume structure.The weakly adsorbed hydrogen atoms could effectively promote the proliferation and slippage of shear bands in the compression deformation process of the BMG,and resulted in the compression plasticity of the BMG significantly increased.In addition,the compression plasticity of the SLM forming Cu₅₀Zr₄₃Al₇ BMG was unimproved for the pore type defects after the room-temperature gaseous hydrogenation.However,the increased compression plasticity of the quick quenching forming BMG proved that it is possible to successfully use the method in the SLM forming BMG by eliminating the pore type defects.