| As a Ni-based superalloy,Inconel 718 has received much technological interest and has been widely used in aerospace and energy industries,due to unique properties,such as high strength,strong creep resistance and good corrosion resistance.Nevertheless,it is difficult to manufacture high-quality parts with complex shape by conventional casting methods and forging processes,because of the high hardness and low thermal conductivity of Inconel 718 alloy.However,the emerging disruptive processing technology“additive manufacturing”has unique advantages in the preparation of complex-shaped nickel-based superalloys.Selective laser melting(SLM)is a typical“additive manufacturing”processing technology.It uses laser as a heat source to selectively melt Inconel 718 alloy powder,and adjusts the process parameters to control the melting and solidifica tion process,so as to effectively control the microstructure and performance,and finally achieves high-quality components.Therefore,as the research object,Inconel 718alloy was prepared by SLM in this work.We study the characteristics of micro-molten pool of single track,and the structure and mechanical properties of block specimens.Thus,the establishment of the correlation of“thermal behavior in the molten pool-microstructure-mechanical properties”provides theoretical and technical support for the high-quality preparation of Inconel 718 alloy components formed by SLM.The finite element method was used to simulate the temperature field of the Inconel 718 alloy by SLM,and the thermal behavior of micro-molten pool of single track and the multi-layer block was obtained.The thermal behavior results of micro-molten pool of single track show that in the same micro-melt pool,when the scanning speed is 1450 mm/s,the largest temperature gradient and solidification velocity difference between the bottom and the upper part of the molten pool can reach 0.99×10~7 K/m and 0.35 m/s,respectively;When the laser power is 200 W,the largest temperature gradient and solidification velocity difference between the bottom and the upper part of the molten pool can reach 0.79×10~7 K/m and 0.49 m/s,respectively.The thermal behavior of the multi-layer bulk micro-melt pool shows that when the hatch distances increase from 50μm to 80μm,the temperature gradient decreases from 1.99×10~7 K/m to 1.14×10~7 K/m,and the maximum solidification rate increases from 0.71 m/s to 1.92 m/s;as the scanning speed increases from 1000 mm/s to 1450 mm/s,the maximum temperature gradient decreases from 1.4×10~7K/m to 1.21×10~7 K/m,and the maximum solidification rate increases from 1.12 m/s to 1.52 m/s;with the laser power increases from 280 W to340 W,the maximum temperature gradient increases from 1.13×10~7 K/m to1.49×10~7 K/m,and the maximum solidification rate decreased from 1.66 m/s to 0.69m/s.The Molten pool morphology of single track under different process parameters was studied,and three typical melting modes of t he molten pool were obtained,which are conduction mode(N<0.5),transition mode(0.5<N<1),and keyhole mode(N>1).The microstructure changes from cellular grain at the bottom to cellular dendrite at the top of molten pool under three melting modes.The as-deposited microstructure of single track is dominated byγphase,and there are few Laves phases on the dendrites,and the solidification path is L→L+γ→γ+Laves.Comparing preferential growth direction of grains in the three modes,the conduction mode is closer to the<001>direction,with larger low-angle grain boundaries and stronger(001)texture;whereas the grain orientation of keyhole mode is the mixing direction of<001>,<101>,and<111>;and the transition mode is between the two directions.In the three modes,the keyhole mode has the smallest grain size,and the nano-hardness is higher;the average nano-hardness of the deposition zone and the remelting zone are 11.38±0.45 GPa and 9.73±0.29 GPa,respectively.The solidified microstructure in the multi-layer bulk micro-molten pool is also transformed from cellular crystals to cellular dendrites;the interlayer structure is composed of columnar crystals that grow epitaxially and can pass through several layers of powder;in the block samples,there are not onlyγphase and Laves phase,but also more nano twins.Furthermore,we investigated effects of hatch distances,scanning speed and laser power on the morphology,microstructure and tensile properties of Inconel 718alloy by SLM.The results indicated that with the hatch distance increasing from 50μm to 80μm,the depth and width of the molten pool decrease;the solidified microstructure is composed of columnar crystals,and the dendrite spacing decreases;the average size of grains decrease,the preferential growth direction of grains is closer to the<001>direction,and the crystallographic texture is weakened;When the hatch distance is 70μm,the ultimate tensile strength and elongation reach the maximum,which are respectively 1001±20 MPa and 17.54±1.15%.As the scanning speed increases from 1000 mm/s to 1450 mm/s,the depth and width of the molten pool decrease;The solidified microstructure changes from cellular dendrite to columnar dendrites,and the dendrite spacing decreases;the average s ize of grains decreases,and the preferential growth direction of the grains is mainly in the<001>direction,there are<101>and<111>orientations,and the texture strength is correspondingly weakened;When the scanning speed increases to 1300 mm/s,the ultimate tensile strength and elongation reach the maximum,which are respectively1014±19 MPa,19.04±1.12%.With the laser power increases from 280 W to 340 W,the depth and width of the molten pool show an increasing trend;the solidified microstructure tends to transform from cellular dendrite to cellular,and the dendrite spacing increases;the average size of grains increases,and the preferential growth direction of the grains is mainly in the<001>direction.With the increase of the laser power,the preferential growth direction of<101>appears,and the texture is weakened.When the laser power is 320 W,the tensile performance is the best.The tensile strength and elongation are 1018±15 MPa and 20.11±1.89%,respectively.When hatch distance,scanning speed,laser power and powder layer thickness are same,different scanning strategies directly affect the compactness,crystallographic texture and mechanical properties of the component.Compared with the scanning strategy that rotates 90°and 105°layer by layer,when the scanning strategy is 67°,the component is relatively dense and the average size of grains is the smallest.the preferential growth direction of grains is the mixing direction of<001>,<101>,and<111>,and the texture is the weakest.When the scanning strategy is 67°,the tensile performance is the best,and the tensile strength and elongation are 1035±19 MPa and 22.82±0.78%,respectively. |
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