The Forming Mechanism,Microstructure,and Properties Of Zr-based Bulk Metallic Glass Fabricated By Laser Additive Manufacturing

Comparison with the crystalline material,bulk metallic glasses（BMGs）have attracted tremendous attention due to their excellent mechanical,physical and chemical properties.The BMGs have applied in aerospace,automotive industry,optical components,biomedical materials.However,due to the critical cooling rate and size limitation of the BMGs fabricated by conventional fabricating methods,it further restricts their potentially widespread applications as structural and functional materials.Combining the high heating/cooling rate of laser with freedom of design,waste minimization and the ability to manufacture complex structures of additive manufacturing（AM）,laser additive manufacturing technology becomes a potential method to overcome the intrinsic size limitation of BMGs.In this work,based on a point-line-face-body model,the formation mechanism of BMGs fabricated using laser additive manufacturing is studied,and on the basis of which the mechanical property of large-size BMG is probed.Based on a point-line-face-body model,ABAQUS simulations in our thesis have been conducted to investigate the thermal history evolution of different micro-zones（molten pool zone,re-melted zone,and heat-affected zone）during laser additively manufactured BMGs.Results show that the cooling rate and crystallization in micro-zones are different,due to the micro-zones suffered different thermal history during the amorphous form of the different model.The cooling rates in micro-zones of pulse and continuous-wave lasers are far above the critical cooling rate,which ensures that the BMGs fabricated using laser additive manufacturing can overcome the intrinsic size limitation.A progressive BMG forming method is used to study the amorphous structure formation of a single molten pool,temperature interference zone,and BMG,respectively.The successful formation of the amorphous structure in a single deposit is critical for the creation of a bulk amorphous alloy.Crystallization in the remelted zone and heat-affected zone will occur when enough remelting effect and structure relaxation are experienced.A 1.6 mm thickness of Zr-based BMGs are successfully fabricated using laser additive manufacturing.It shows that the amorphization ratio increases with the rise in the number of deposit layers.The deposit layers exhibit excellent corrosion resistance performance and micro-mechanical properties.A Zr-based BMG composite comprising of 82.5 vol.%of the amorphous phase is successfully prepared using laser additive manufacturing.When the pure Zr plate and amorphous powders are used the original materials,combining with the ABAQUS simulations method,results show that the molten pool consists of a fully amorphous structure,while some nanocrystals Al₅Ni₃Zr₂ are found in the remelted zone due to the reheating effect.Composite microstructures in HAZ comprising of amorphous matrix,dendrites Cu Zr and nanocrystals Al₅Ni₃Zr₂ are found owing to multiple effective thermal cycles.The effects of AM processing parameters and scanning strategies on the formation of BMGs are investigated.All AM samples are nearly fully amorphous.However,when the energy density（40 W/60 W）is lower,the samples show lower densification.By optimizing the AM process,a nearly fully amorphous,high relative density sample is fabricated under the parameters laser power of 100W,scanning speed of 2500 mm/s,and the hatch angle of 67°.The optimizing sample exhibits excellent micro-mechanical properties.A nearly fully amorphous Zr₅₀Ti₅Cu₂₇Ni₁₀Al₈ BMG with the dimensions of15 mm×15 mm×10 mm is successfully prepared using laser additive manufacturing.Tensile test results at room temperature in different building directions show that the tensile strength in the horizontal direction is only about467 MPa,which is mainly limited by the deposition defects.The tensile fracture mode is normal stress fracture,almost no plastic deformation.The tensile fractography mainly consists of river-like pattern and cleavage step.Compressive test results at room temperature in different building directions show that the compressive strength in each building direction is about 1465 MPa,and compressive fracture mode is mainly shear fracture.The sample presents a little plasticity before fracture occurs.The compressive fractography mainly consists of vein-like pattern,dimple pattern and cleavage step.