Molten-pool Characteristics And Microstructure Control In Quasi-continuous-wave Laser Additive Manufacturing Of Ni-based Alloy

Laser Additive Manufacturing(LAM)has the advantages of high flexibility,short cycle,and freedom from the limitations of part materials and structures.It is widely used for rapid forming of nickel-based complex structure parts.However,due to its inherent lateral confinement solidification mode of moving molten pool,LAM of nickel based superalloys inevitably leads to common problems such as columnar dendritic growth,element segregation,brittle eutectic phase,and structural anisotropy.Therefore,the control of solidification microstructure and mechanical property of LAM fabricated nickel based superalloys is still challenging.In this dissertation,a quasi-continuous-wave laser modulation process was proposed to regulate the molten-pool solidification mode,and microstructure and performance of LAM fabricated parts.It is proposed to conduct research on the dynamic solidification characteristics of molten pool and microstructure formation mechanism during QCW laser modulation by comprehensive research methods of molten-pool online monitoring,molten-pool numerical simulation,microstructure characterization and mechanical property testing.This thesis aims to clarify the dynamic characteristics and heat and mass transfer laws of the molten pool under QCW laser modulation;to reveal the influence of QCW laser modulation parameters on microstructure;to establish the relationship among process,structure and performance,and finally to achieve the control of molten-pool solidification process and mechanical properties of fabricated parts.The main research content and results are as follows:(1)The U-Net deep learning method is proposed to accurately identify and segment the morphology of the molten pool for LAM.The advantages of U-Net method over traditional image processing methods are analyzed.Based on the U-Net extraction method of molten pool boundary,the stability of molten-pool boundary and its impact on microstructure under different laser processing modes were investigated.The results show that,compared to traditional image processing methods,the CNN image segmentation method can effectively remove halo interference caused by plumes and bright spot interference caused by flying particles.It can effectively cope with changes in the shape and contrast of the molten pool caused by sudden changes in speed,thereby achieving efficient,stable,and accurate extraction of molten pool boundary information.Compared with traditional CW laser processing mode,the QCW laser processing mode significantly improves the stability of the molten pool boundary and induces in situ high-frequency remelting of the molten pool,thereby promoting the epitaxial growth of dendrites.(2)The QCW laser modulation process was proposed to control the grain structure and solidification texture during LAM of nickel based superalloys.The influence of QCW laser modulation parameters on dendritic growth and solidification structure was investigated.The formation mechanism of microstructure under typical laser modulation processes was also analyzed.The results show that,the high pulse frequency promotes the epitaxial growth of columnar dendrites and results in a high-strength texture composed of regular zigzag columnar grains.In contrast,the low pulse frequency inhibits the epitaxial growth of dendrites and finally results in random texture composed of equiaxed or stray grains.There are two types of epitaxial growth modes at high pulse frequencies,namely the direct epitaxial growth between adjacent pulses and the cross-layer epitaxial growth along the secondary dendrite direction of the previous layer.The duty ratio mainly affects the dendrite growth direction and dendrite/grain size.The low duty ratio is beneficial for obtaining fine columnar structures with high orientation.The above laser modulation parameters mainly affect the columnar to equiaxed transition(CET)and the epitaxial growth behavior by changing the molten-pool solidification parameters,improving the geometric shape of the molten pool,and inducing the periodically in-situ remelting treatment,thereby determining the final grain structure and solidification texture.(3)The QCW laser modulation process was proposed to comprehensively control the element segregation and Nb-rich phases.The effects of pulse parameters on dendritic growth,element segregation,and Nb-rich phases were studied.The response of microstructure and mechanical property to a standard solution and aging heat treatment were investigated.The results show that,three kinds of Nb-rich phases(Nb-rich Laves,δ-Ni3 Nb and γ″-Ni3Nb)can be comprehensively controlled and the mechanical properties of samples can be improved by optimizing laser modulation parameters.Compared with the coarse columnar dendrites and chain-like Laves phase particles of the CW sample,the refined dendrite structure,the inhibited element segregation and the reduced Laves phases were obtained using the QCW laser processing mode.The high pulse frequency promotes the fine columnar dendrites and linearly distributed Laves phases,while the low pulse frequency tends to produce the fine equiaxed dendrites and discretely distributed Laves phases.The samples fabricated with low pulse frequency exhibit the most ideal heat treatment responses,such as the dissolution most of Laves phase,the dispersion distribution of δ-Ni3 Nb phase and the high density precipitation of Ni3 Nb strengthening phase,which enhance the tensile strength and plasticity of the fabricated samples.(4)A three-dimensional transient mathematical model of molten pool was established to simulate the evolution of molten-pool morphology,temperature distribution and solidification parameters during CW and QCW laser additive manufacturing.The solidification mechanism of molten pool under typical modulation processes was analyzed.The results show that,owing to the periodic input of laser energy,the QCW laser processing mode results in periodic dynamic changes in molten-pool morphology,the temperature field and solidification parameters,and simultaneously induces the periodic in-situ remelting of the molten pool.Compared with the traditional quasi-steady-state translational solidification mode of the molten pool during CW-LAM,the QCW-LAM is featured by the periodic melting and solidification of the molten pool.The high pulse frequency leads to a semi-free solidification mode of molten pool.This solidification mode leads to the inclined molten-pool boundary,the suppressed CET behavior and the high-frequency remelting of molten pool,which promotes the epitaxial growth of columnar dendrites.The low pulse frequency tends to result in a free solidification mode of molten pool.This solidification mode leads to the semicircular molten-pool boundaries,the promoted CET behavior and the limited remelting effect,which thereby inhibits the epitaxial growth of dendrites.Compared to semicircular molten-pool boundary produced under the high duty ratio,the low duty ratio tends to produce the straight molten-pool boundary and the high cooling rate,which promotes the epitaxial growth of dendrites and results in the highly oriented fine columnar structures.