| Nickel-based superalloy parts fabricated by laser additive manufacturing have been widely applied in aerospace,energy and other fields.Traditional laser additive manufacturing usually adopts a Gaussian distributed laser source,which has high residual stress,large temperature gradient,and is prone to the formation of coarse columnar crystals and obvious Laves phase.In this investigation,the circular oscillating laser is proposed to carry out laser additive manufacturing.The laser-powder coupling characteristics,molten pool stirring,dynamic and static mechanical behavior and particle reinforcement mechanical properties of circular oscillating laser are studied.The purpose of this thesis is to promote the transformation from columnar crystal to equiaxed crystal of nickel-based superalloy for additive manufacturing,inhibit the precipitation of Laves phase,improve the mechanical properties of the alloy,and provide theoretical and technical reference for the application of circular oscillating laser in additive manufacturing.The main scientific research works are as follows:An oscillating laser deposition manufacturing platform was established,and the spatial oscillating of laser beam was realized by galvanometer scanning.The design concept of annular powder feeding and circular oscillating laser beam coupling with annular powder stream was proposed,and an annular coaxial nozzle with powder-mixing and water-cooling chamber was developed.The influence of laser-powder coupling of circular oscillating laser on the geometric morphology and microstructure evolution of GH3536 alloy deposited layer were studied.The annular coaxial powder feeding head realized circumferential uniform water cooling,and changed the four-channel powder flow into a circular uniform distribution.Compared with Gauss laser,the dilution ratio and the heat affected zone of deposited layer fabricated by circular oscillating laser are decreased by 80.91%and by 40.18%,respectively.Multi-physical field numerical models of circular oscillating laser and Gaussian laser deposition were established.The differences in heat transfer,melt flow and solidification behavior between circular oscillating laser and Gaussian laser deposition were studied by combining with multi-channel deposition experiments.The change rule of GH3536 alloy microstructure morphology and grain size was predicted by calculating the temperature gradient G and solidification front velocity R.The circular oscillating laser stirring the melt pool makes the melt temperature and flow velocity periodically change.Compared with Gaussian laser,the convection effect is stronger,the temperature gradient on the surface and inside of the molten pool is high and the difference is small,and the solidification rate is higher,which promotes grain refinement.The high temperature and stirring effect of oscillating laser enhanced convection is conducive to the diffusion of refractory elements such as Mo and Cr,avoiding the formation of Laves phase by enrichment in the liquid phase.The experimental results show that the volume fraction of Laves phase formed by circular oscillating laser is 30.03%lower than that of Gaussian laser.In order to study the influence of microstructure on mechanical behavior of GH3536 alloy deposited by circular oscillating laser,the tensile properties and fracture mechanism of as-deposited GH3536 alloy were analyzed by quasi-static tensile test.Compared with Gaussian laser,the toughness of GH3536 alloy deposited by circular oscillating laser is stronger due to more equiaxed crystals and less Laves phase,and the maximum elongation is increased by 91.36%.However,the strength of the alloy is slightly reduced due to less columnar dendrites and a large amount of remelted boundaries,with a maximum reduction of 9.96%.Three heat-treatment schemes of solution treatment（1177℃/1 h）,aging（720℃/8 h）and solution treatment+aging（1177℃/1 h+720℃/8 h）were formulated for GH3536 alloy deposited by circular oscillating laser.The results show that after solution treatment,the microstructure is completely recrystallized,and the long-stripped Laves phases are dissolved into granular shape;after aging treatment,a large number of chain-like carbides are precipitated on the grain boundaries.The effect of solution+aging treatment on the mechanical properties of as-deposited GH3536 alloy is the best,and the tensile strength and elongation are increased by 22.99%and 9.70%respectively,exceeding the standard of aircraft high-temperature alloy billet specification.A dynamic compression finite element model based on J-C constitutive and damage is established by using the Split Hopkinson Pressure Bar（SHPB）experiment to study the dynamic mechanical damage and plastic deformation behavior of GH3536alloy deposited by circular oscillating laser under high strain rate.The as-deposited GH3536 alloy had obvious strain rate strengthening phenomenon.Increasing the strain rate increases the dislocation density inside the SHPB specimens,and the dynamic yield strength of the sample increases from 792.86 MPa to 1175.16 MPa when the strain rate increases from 1000 s-1 to 5000 s-1.After the SHPB experiment,the specimens undergo large plastic deformation,the grains rotate dynamically and recrystallize to form an insulated shear band under the action of conjugate shear force,and the width of the shear band and the refinement extent of the internal grains increase with the increase of the strain rate,and the slip mode changes from linear to wave-shaped.In view of the shortcomings of traditional GH3536 alloy,such as low strength,poor wear and corrosion resistance,a design concept of high proportion WC ceramic particle reinforcement was proposed,and the 16 wt.%WC particles reinforced GH3536alloy was successfully fabricated by the circular oscillating laser.Under the stirring effect of circular oscillating laser,the WC particles are uniformly distributed in the deposited layer,and the tensile and compressive yield strength are increased by 19.15%and 28.56%respectively;The wear loss rate is reduced by 89.17%,and the corrosion resistance in Na Cl solution is improved by 44.9%.WC particles and carbide hard phase formed after melting and decomposition of WC hinder the dislocation movement which forms fine grain strengthening and dispersion strengthening,and enhances the strength of GH3536 alloy.During the dry sliding friction,WC particles and hard carbides prevent the GH3536 deposited layer from contacting with the grinding ring,improving the wear resistance;During the corrosion process,WC particles act as physical barriers to prevent metal from undergoing oxidation reaction,and its shielding effect is higher than the micro-cell effect caused by carbides,which significantly improves the surface corrosion resistance of the particle reinforced alloy. |
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