Microstructure And Texture Control During Pulsed Laser Additive Manufacturing Of Inconel 718 Alloy

Laser additive manufacturing(LAM),as an advanced manufacturing technology,has the advantages of short manufacturing cycle and high processing flexibility.It can directly forms complex three-dimensional parts,which has been applied to aerospace,biomedical and other fields.The texture is a common phenomenon in LAM parts,resulting in anisotropy of mechanical properties and physical properties.Therefore,it is necessary to tailor the texture of LAM part to improve its performance.The high cooling rate of the pulsed laser can reduce segregation,refine dendrites and improve the mechanical properties.At the same time,the modulated laser could dynamically excite the molten pool and adjusts the molten pool morphology,tailoring the texture of the formed parts.However,the molten pool evolution and the effect mechanism of process parameters on the molten pool morphology and texture are still unclear.In this thesis,the evolution of the melten pool during pulsed laser processing is analyzed to illustrate the pulse laser molten pool control and the dendrite competitive growth.Moreover,the texture formation in the pulsed LAM process are studied.The main purpose of this study is to reveal the formation mechanism of texture,establish a simple relationship between process parameters and texture and realize the control of texture by changing the molten pool morphology.The main research contents and results are as follows:Firstly,the molten pool evolution and the inter-pulse thermal cycles under pulsed laser excitation were investigated.The results show that the molten pool evolution and solidification modes are related to the pulse period.The unconstrained solidification mode occurs in the short-period pulse,because the molten pool cannot reach a quasisteady state.Whereas the constrained solidification mode and unconstrained solidification mode coexist in the long-period pulse,because the molten pool can reach a quasi-steady state.The distribution characteristics of the second phase morphology can be controlled by changing the number of thermal cycles with adjusting the pulse period.This work provides a potential method for the microstructures customization.The influence of pulse parameters on the molten pool temperature and molten pool morphology were studied by modulating the laser in the time domain.The basic mechanism of the texture controlled by the molten pool morphology were also studied.The relationship between process parameters,molten pool morphology and texture was established.The transient temperature of the molten pool exhibits periodical oscillations.The peak temperature decreases with increasing of duration time.However,the minimum temperature and melting time increase with increasing of duration time.At duration time of 20 ms,the bottom of the molten pool morphology is flat,exhibiting uniform grain with a strong fiber texture.At duration time of 60 ms,the molten pool morphology is like fish-scale.Both coarse grains in the molten pool overlapping zones and fine grains in the molten pool center zones are coexist with a weaker cube texture for duration time of 60 ms.Finally,the texture graded component has been fabricated,which indicated that it was feasible to adjust the pulse parameters to realize sitespecific texture.The effect of laser energy distribution on the molten pool morphology,microstructure and texture was studied by modulating the laser in the time-space domain.The effect of heat flux strategy on resulting texture was analyzed.The optimal criterion for dendrite growth under the heat flux strategy is proposed.The molten pool boundaries are approximately parallel and the columnar dendrites obliquely growing with strong fiber texture for the sample of flat top laser.The deep and narrow molten pool morphology is formed for the sample of gaussian laser.Both coarse grains with cube texture in the molten pool overlapping zones and fine grains with a weak fiber texture in the molten pool center zones are coexist for the sample of gaussian laser.The texture type is related to the heat flux strategy.The fiber texture is formed under the heat flux strategy with the single heat flux direction,while the cube texture is formed under the heat flux strategy with the alternate heat flux directions.The influence of the molten pool spatial arrangement on the texture was studied.And the influence of the molten pool boundary on the dendrite growth and competition was analyzed.The z-increment only affects the epitaxial growth of dendrites in the molten pool center.And the growth of dendrite in the building direction is not easy to adjust.The dendrites are directly epitaxially grow under the short pulse period,forming a strong fiber texture with a texture strength as high as 28.6.While the dendrites are not easy to epitaxially grow under the long pulse period.The overlapping position close to the center of the molten pool with decreasing track offset,resulting in increasing the range of the dendrites direct epitaxial growth and reducing the texture of sample.The epitaxial growth of dendrites at the molten pool boundary is related to the angle between the boundary normal angles of adjacent molten pools.The texture of the fabricated part can be tailored by adjusting the molten pools spatial arrangement.For the first time,the sandwich texture features were obtained in powder-fed pulsed LAM of Inconel 718 alloy.Finally,the influence of scanning path and pulse period on texture was studied,considering the asymmetry of molten pool.And the heat treatment response and mechanical properties of the samples were analyzed.The deep molten pool is formed at long period pulse,exhibiting V-shaped grains in the molten pool overlapping zones.The shallow molten pool is formed at short period pulse,exhibiting zigzag grains in the molten pool center zones under the bidirectional path sample.The direct aging heat treatment improves the strength of the sample while maintaining good plasticity,and the elongation reaches 21.18 %,which exceeds the forging standard.