Formation Mechanism Of Defects And Control In Selective Laser Melting (SLM) 3D Printing Of Amorphous Alloys

Amorphous alloys have a series of excellent properties such as high strength,high elasticity,high hardness,and excellent corrosion resistance and catalytic properties benefiting from their disordered atomic structure.However,it exhibits poor processability due to extremely brittleness at ambient temperature.At present,the two major approaches of manufacturing amorphous alloy components have limitations.Copper mold casting method is difficult to achieve complex components owing to the requirement of high cooling rate accompanies with the poor fluidity of the melt.Moreover,thermoplastic forming?TPF?is only suitable for the fabricating of fine parts,because the amorphous alloys have a narrow temperature range of thermoforming,it is difficult to form compnents with large-size and complex structure.Selective Laser Melting?SLM?3D printing?also called as Additive Manufacturing?technique seems to be a promising method to solve the problems.Since only small volumes of materials are molten and cooled,as the laser beam with high energy density across the powder bed during SLM,the cooling rate reaches up to 10⁴–10⁶K/s that is higher than the critical cooling rates required for most amorphous alloy systems?10²–10⁴K/s?,can effectively inhibit the occurrence of crystallization during the forming process.However,the steep temperature gradient caused by high heating and cooling rates around molten pools,induces huge thermal stress during SLM process,which,in turn,causes generation of micro-cracks in the 3D printed amorphous alloy components.The fabrication of crack-free Fe-based amorphous alloy components have not been achieved so far.Although micro-cracking can be completely suppressed in Zr-based amorphous alloy systems due to its high toughness,the defects such as incomplete surface,high porosity during solidification,seriously degrade the quality of the 3D printed components.In addition,heterogeneous thermal expansions and contractions induced by inhomogeneous thermal distribution during SLM,inevitably result in residual stresses.Especially for the amorphous alloys with intrinsic brittleness and low fracture toughness,the effects of thermal stress,defects and residual stress are more serious.However,the origin of micro-cracks,the formation mechanism of defects,the distribution and evolution of residual stress in 3D printed amorphous al oy components are still unclear.Based on the above background,in this work,we carried out a systematic study on the origin of micro-cracks and influences of micro-pores on crack generation in a Fe-based amorphous alloy with the composition of Fe_43.7Co_7.3Cr_14.7Mo_12.6C_15.5B_4.3Y_1.9from experimental and simulation aspects.The results revealed that both location and size of micro-pores played important roles in the formation of micro-cracks in the 3D printed Fe-based amorphous alloy.Micro-pores at the edges between molten pool and heat affect zone led to high thermal stress concentration,which eventually caused the formation of micro-cracks.In addition,it was found that thermal stress increased with the decrease of pore size.Based on experiment and finite element method?FEM?simulation,a critical pore-size of 30?m was induced for initiation of micro-cracks in the Fe-based amorphous alloy studied.According to fracture mechanisms of materials,the fracture toughness plays a crucial role in hindering micro-crack propagation.Compared to the Fe-based amorphous alloy systems,Zr-based amorphous alloy systems with high fracture toughness could dissipate much fracture energy due to the formation of large plastic zone,and moderates the stress concentration in front of crack tip,accordingly,shield the crack initiation or crack propagation.A systematic study was conducted on the influence of energy densities on defects evolution of the final layer?which could be considered as the traces of melt flow during SLM?by both experimental characterization and computational fluid dynamics?CFD?simulation during SLM Zr-based amorphous alloy.The following conclusions can be drawn,at a relatively low energy density,insufficient laser energy input led to incomplete melting of particles and?balling?,discontinuous melting track gradually formed as a consequence.With the increasing energy density,the dominant irregular-shaped open porosities were caused by the unstable melt flow,and could not be filled with melt materials of the next layer and then remained in the as-fabricated samples.The metallurgical pores with small size on the surfaces of sample fabricated under an appropriate energy density,could be filled with melt materials and then be closed.Almost fully dense Zr-based amorphous alloy component can be produced by process optimization.The FEM combined with experiments and theoretical analysis were introduced to estimate the residual stress in selective laser melting of a Zr-based amorphous alloy.The results revealed that the XY cross scanning strategy exhibits relatively low residual stress by comparison with X and Y strategies,and the residual stress becomes serious with increasing bar thickness.The residual stress,on the other hand,can be tuning by annealing or preheating the substrate.These results provide a new route to improve the forming quality of 3D printed amorphous alloy components with a large scale and complex structure.