Mesoscopic Thermodynamic Behavior And Processing Control Of Selective Laser Melting Ni-based Alloy Powder

Inconel 718 superalloy having excellent mechanical properties is highly required for the fabrication of the complex-geometry hot-end components that have been used in the applicaitons of high thrust-to-weight ratio engine and high-temperature nuclear reactors as the industrial applications rapidly developed.Nevertheless,the conventional process having the shortcomings of high cost in production of models,long fabrication cycle and complex process restricted the extensive industrial application of Ni-based superalloys.This thesis mainly focused on the influence of processing parameters on the densification behavior,heat and mass transfer occurred in neighbouring tracks and layers,evolution of metallugical defects,microstructure development,and mechanical properties of SLM-processed Inconel 718 alloy with the combination of experimental and numerical analysis.Based on the aboved results,the relations between processing parameters(such as material composition,laser process)and packed status of powder-bed,microstructure evolution,development of in-situ reinforcements,mechanical properties and high-temperature oxidation property were discussed,favoring the production of high-performance Ni-based superalloys components.The mesoscopic physical model concerning the reaction of high-energy laser beam and Inconel 718 alloy was established,and the influence of processing parameters on migration of bubble within the molten pool and resultant densification were investigated.It was found that the thermal convection within molten pool was enhanced as increasing the laser power,which would drive the bubbles to escape from the pool and the resultant high densification.The effect of hatch spacing on the heat and mass transfer within laser-induced track/track during SLM was discussed.The laminar feature would sufficiently wet the neighbouring areas of pool and drive the molten liquid to fill the porosity among powder particles,thereby resulting in a high densification.Moreover,the influence of processing parameters on the depth,heat and mass transfer within laser-induced layer/layer was discussed.It could be found that a high linear laser energy density could significantly reduce the viscosity and surface tension of molten liquid,which was benefit to decrease the number of metallugical defects and produce a well bonding between the current layer and previously fabricated layer.As for the mechanical properties of SLM-processed Inconel 718 alloy,the influence of scan strategies(i.e.,layered alternating scan strategy with an orientation of each over-building layer,island/ layered alternating scan strategy)on the densification,microstructure and mechanical properties was investigated.An increase in the angel of layered alternating orientation could reduce the cooling rate of the liquid/ solid interface,leading to the transformation of microstructure from column to cellular morphology.As compared with the layered alternating scan strategy,the island/ layered alternating scan strategy could further reduce the thermal accumulation and residual stress by means of island scan strategy.Meanwhile,the SLM-processed Inconel 718 alloy was dominanted with a cellular microstructure and an excellent tensile strength over 1155.6 MPa.The mesoscopic model of discrete WC/Inconel 718 composite powder was proposed,and the influence of average diameter of poweder particles on the the flow-ability,stacking status,thermodynamics within molten pool and processing quality of components of components was investigated.Meanwhile,the corresponding underlying relationship of powder particle,layering process and processing quality was concluded.The results revealed that when the powder particle with an undersized size(d50<25 ?m),it could aggregate each other induced by dominated Van der Waals force,directly leading to an inhomogeneous powder-bed and the pore within the initial powder-bed could be dragged into pool easily.In this case,the porosity formed due to the instability of liquid.By contrast,the powder particle with a large sized tended to stack discretely within powder-bed despite of alleviating the aggregation of powder particles with a dominated gravity,causing a reduction of operated temperature within pool and insufficient spread of liquid.As a result,the metallugical defects formed in the ultimate component.The relation of processing parameters and the interficial thermodynamics and fragmentation of reinforcement was studied to reveal its underlying fragmentation behavior during SLM.A relative low linear laser energy density could result in a formation mechanism of dissolution-diffusion-fragmentation,while the formation mechanism of fragmentation-dissolution-diffusion was revealed during SLM using a large linear laser energy density.The influence of the processing parameter on the interficial heat and mass transfer of WC/Inconel 718 and microstructure evolution was investigated,and disclosed that the Inconel 718 alloy was reinforced by the multi-phases(i.e.Ni2W4 C,(Nb,M)C and WC).Meanwhile,the growth of in-situ Ni2W4 C phase was primarily relied on the thermodynamics within molten pool.On the other hand,the development of granular(Nb,M)C carbide was strongly depended on the nucleation and growth during SLM.Hence,the underlying mechanism of microstructure evolution of SLM-processed WC/Inconel 718 composite was revealed by tailoring the thermodynamics within molten pool during SLM.The effect of weight percentage of WC particle on the mechanical properties of SLM-processed WC/Inconel 718 composite was studied.The experimental results showed that the increase of WC weight percentage could enhance the tensile strength of composite,but it could also resulting in the nonuniform of microstructure and a reduction in densification.The tensile strength achieved to 1399.3MPa with a desired elongation of 22.2% of the SLM-processed WC/Inconel 718 composite with the WC weight percentage of 15 wt.%.Moreover,the influence of weight percentage of WC particle on the high-temperature oxidation behavior of composite was investigated,and the attendant high-temperature anti-oxidation mechanism was revealed based on the thermodynamics and dynamics of composite during oxidation.It was found that the oxidation film of WC/Inconel 718 composites formed under 850 °C with an oxidation time of 100 h was consisted of two layers,i.e.the outer oxidation layer and the inner oxidation layer.The former one was composed of R-Ti O2,Cr2O3,Ni Fe2O4 and Cr2 Ni O4,and the latter one was formed with a dense Cr WO4 layer due to the enrichment of W element having a high outer diffusion rate at the interface between outer oxidation and composite.The high-temperature oxidation behavior of SLM-processed WC/Inconel 718 composites mainly followed the oxidation kinetics of tkwp=?2.Moreover,the composite with a WC weight percentage of 15 wt.% possessed a relatively lower increase in oxidation weight of 0.44 mg/cm2 after experiencing a oxidation time of 100 h at 850 °C,revealing an excellent resident of high-temperature oxidation of the SLM-processed WC/Inconel 718 composite.