Study On Microstructure And Properties Of Fe-based Bulk Metallic Glass And Its Composites Fabricated By Selective Laser Melting

Fe-based bulk metallic glasses（Fe-based BMGs）have attracted great attentions because of their ultra-high yield strength,hardness,excellent wear and corrosion resistance and the low cost,which show great application value and development potential in many fields such as aerospace,precision instruments,medical devices and new energy,etc.However,the limited amorphous forming ability and intrinsic brittleness make it difficult for traditional methods such as copper mold casting and powder metallurgy to fabricate Fe-based BMG components with large size and complex geometry,which seriously limits their application as engineering structural materials.Selective laser melting（SLM）technique is characterized by high forming accuracy and cooling speed,which is expected to break out the limitation of formation size and geometry of BMG and shed light on the fabrication of BMGs.Even though,there still exist some problems such as micro-cracks,pores,crystallization and difficult to quickly optimize process parameters during SLM process.In addition,the intrinsic brittleness as the common problem of BMGs results into the poor deformation ability.It is urgent to realize the toughening of BMGs.These above problems are the hotspots and difficulties of current research.In this thesis,FeCrMoCB amorphous alloy powders were selected for SLM process,and the researches mainly focused on the parameter of BMG based on temperature filed simulation and intelligent algorithm optimization,adjustment and control of the microstructure and mechanical properties of BMG by scanning strategy,the improved brittleness and the toughening mechanism of BMG by adding Cu,and the investigation of the micro-mechanical and tribological properties of BMG.The main conclusions are summarized as follows:（1）Study on the parameter of SLM-fabricated FeCrMoCB BMG based on temperature field simulation and intelligent algorithm optimization.The temperature field and the evolution of molten pool of SLM-fabricated FeCrMoCB BMG under different parameters were systematically investigated by finite element analysis method.Combined with the single factor experimental method,the high amorphous content formation window with the laser power of90 W,powder layer thickness of 30μm and scanning speed ranging from 300 to 900 mm/s was acquired based on the XRD and DSC results.And then a back propagation neural network model optimized by genetic algorithm was established for the prediction and optimization of SLM processing parameters.The optimized parameters were:laser power of 90 W,powder layer thickness of 30μm,scanning spacing of 90μm and scanning speed of 300 mm/s.FeCrMoCB BMG components with large size and complex geometry were successfully fabricated by using the optimized parameters.（2）Effect of scanning strategy on the microstructure and mechanical properties of SLM-fabricated FeCrMoCB BMG.It is found that the residual stress caused by temperature gradient and material expansion and contraction are the main reseasons of crack formation during the SLM process of FeCrMoCB BMG.To solve this,a scanning strategy combined with checkboard scanning and laser melting was proposed to adjust the heat flux and relief the residual stress though plastic rheology when the FeCrMoCB BMG was heated to supercooled liquid region.The dense Fe-based BMG free of cracks was fabricated by using checkboard scanning and laser melting,in which the residual stress was decreased from 572 MPa to 168MPa.The effect of scanning strategy on the formation quality,microstructure,thermal stability,mechanical and tribological properties of SLM-fabricated FeCrMoCB BMG was also systematically investigated.The results showed that the scanning strategy had no significant effect on the microstructure and thermal stability of SLM-fabricated FeCrMoCB BMG,but the formation quality and mechanical and tribological properties were greatly improved.The hardness,elastic modulus,friction coefficient and wear rate were 15±0.1 GPa,275±11|GPa,0.21,1.29×10^-5 mm³·N^-1·m^-1,respectively.We in-depth analyzed the wear mechanism and found that oxidation wear and fatigue wear were the dominated wear mechanism for the SLM-fabricated FeCrMoCB BMG.（3）Study on the toughening mechanism of SLM-fabricated FeCrMoCB/Cu BMGC by Cu addition.To solve the intrinsic brittleness of BMG,FeCrMoCB/Cu BMGCs with well toughness were successfully fabricated via SLM by using FeCrMoCB/Cu composite powders.The effect of Cu content on the densification behavior,microstructure and thermal stability of SLM-fabricated FeCrMoCB/Cu BMGCs was systematically investigated.The results showed that the agglomeration of brittle element in the molten pool can be effectively improved and the stress concentration can be avoided when the Cu content is beyond 40 vol%.FeCrMoCB/Cu components with large formation size and dense microstructure can be fabricated.In addition,the toughening mechanism was also in depth investigated.The results showed that the added Cu could effectively inhibit the expansion of the single shear bands and lead to the mismatch of composite interface deformation and promoted the formation of multiple shear bands,and finally effectively improved the plastic deformation ability of amorphous alloys.When the volume fraction of Cu is 40 vol%,the compressive plastic strain and fracture strength of FeCrMoCB/Cu BMGC reached～8%and～900 MPa,respectively.（4）The investigation of the micro-mechanical and tribological properties of SLM-fabricated FeCrMoCB/Cu BMGC.The micro-and mac-mechanical properties of SLM-fabricated FeCrMoCB/Cu BMGC under different work conditions were systematically investigated to verify its reliability as engineering structural materials.Firstly,the microhardness and modulus of SLM-fabricated FeCrMoCB/Cu BMGC were investigated under different peak load,and the creep properties in different regions were also deeply studied.It is found that the micro-mechanical properties showed obvious peak load dependent effect.The microhardness and modulus of Cu increased while that for FeCrMoCB decreased when the load increased.The microhardness of interface between Cu and FeCrMoCB decreased and the modulus increased when the load increased.In addition,the tribological properties and wear mechanism under different normal loads and sliding speeds were also systematically investigated.The results showed that the tribological properties and wear mechanism of SLM-fabricated FeCrMoCB/Cu BMGC were depended on the normal load and sliding speed.It is found that the tribological properties are related to the stability of the friction layer formed on the wear surface.The crystal-amorphous composite structure provides it with excellent wear resistance.The friction coefficient and wear rate were only 0.33 and 4.52±1.54×10^-6 mm³·N^-1·m^-1,respectively when the normal load was 30 N and sliding speed was 250 rpm.