Research On The Properties And Crystallization Mechanism Of Fe-based Bulk Amorphous Alloy By Selective Laser Melting

Amorphous alloys have excellent mechanical,chemical and physical properties,such as high strength and hardness,excellent soft magnetic properties,exceptional wear resistance and corrosion resistance,which make them have wide application prospects.However,traditional manufacture methods such as copper mold casting and suction casting are difficult to prepare large-size bulk metallic glasses(BMGs),and the poor plasticity of amorphous alloys at room temperature is therefore difficult to process using traditional methods.This severely restricts the development and application of bulk amorphous alloys.Selective Laser Melting(SLM)is a type of additive manufacturing technology.It prepares parts by layer-by-layer laser melting of metal powders.It has a high cooling rate and is convenient for forming parts with complex structures.Therefore,SLM provides a new path for the fabrication of BMGs with large-size and complex structures.In this paper,Fe₅₅Cr₂₅Mo₁₆B₂C₂ amorphous alloy is chosen as the research object,and SLM is utilized to print Fe₅₅Cr₂₅Mo₁₆B₂C₂ amorphous alloy.The effects of SLM laser power and scanning rate on the forming quality and microstructure of Fe-based bulk amorphous alloys were investigated.The analysis found that the SLM can prepare completely amorphous samples.The statistics of a large number of forming process parameters and forming effects were analyzed.The process window for printing Fe₅₅Cr₂₅Mo₁₆B₂C₂ amorphous alloy by SLM was drawn.The density of the formed sample reached more than 99.0%when the printing parameters are 100 W-300 mm/s.The?45 mm×20 mm fully amorphous Fe-based BMG was successfully fabricated using the best forming process parameters of SLM.The results of X-ray diffraction,differential scanning calorimeter,and transmission electron microscopy were properly confirmed the sample was completely amorphous.The hardness and strength of the BMG were tested.The nano-hardness of the large-sized BMG was measured to be?14 GPa,and the micro-pillar compressive strength was?4500 MPa.In addition,a sample of a 36-tooth gear with tip circle diameter of 41.5 mm was successfully printed by SLM.SLM can indeed form BMG parts with large-sized and complex structures,which will promote the development of BMGs.The finite element simulation(FEM)software was used to model the SLM forming Fe-based bulk amorphous alloy process.FEM results are showing a transient temperature profile for the molten-pool and surrounding areas.Considering that SLM is close to an infinite heat conduction approach,the cooling rate in the molten-pool area is?1.38×10⁶ K/s high.This is several orders larger than the needed critical cooling rate,guaranteeing an amorphous state.Aside from the molten pool,in order to retain a bulk amorphous structure,temperatures and cooling rates of other areas in the as-printed sample are also critically important,such as those in the heat affected zone(HAZ)and the far end.It is noted that,although the temperature in the HAZ can be higher than T_x,the corresponding time window/lifetime of the high temperature period is rather short,i.e.?0.8 ms.During this period of time,in theory the crystallization is possible to occur but,in reality,an incubation period is required which takes?1.87 ms.Nucleation in this case cannot complete and amorphous structure remains due to the kinetic reason.Far end areas experience temperature rises lower than the T_g temperature,leading to no crystallization.The nucleation and growth process were explored,and the crystallization kinetics of Fe-based amorphous alloy was studied using the classic JMA theory.At the same time,the isothermal thermal stability of the amorphous alloy in the supercooled liquid region was studied,and the material was identified as have good thermal stability.The low-power rescanning strategy helps to suppress the generation of defects and improve the quality of the former samples.Hot isostatic pressing is potentially valuable for eliminating sample defects.