| Inconel 718 Ni-based superalloy is one of the most used high temperature material. Its high heat cracking sensitivity not only significantly reduces the alloy casting parts usage and yield, and influences the safety and reliability of the parts. Therefore, the microstructure evolution and hot cracking sensitivity of Inconel 718 superalloy welding joint was highly attented by scholars at home and abroad. The welding technology of a new generation of thrust-to-weight ratio power system made of Inconel 718 superalloy thin-wall castings parts is one of the key technology of urgent need in our country.In this paper, the welding joint of the large, complex and thin-wall casting parts of Inconel 718 superalloy was studied. The microstructure of TIG welding joints of different original condition of base metal using different heat input was observed. The width of each subregion of welding joint, grain size, width of dendritic space and the number, size, distribution of precipitates were measured. The equilibrium phase diagram, the equilibrium phase fraction diagram and the solid-liquidus temperature range were calculated. The study of microstructure evolution shows that:(1) When using the same heat input, the sort order of the HAZ(Heat Affect Zone) width among different original condition of base metal is: wrought < as-cast ≈ homogenized. The microstructure in HAZ of all the three original condition of base metal is equiaxed grain, but there is still some dendritic structure trace in the equiaxed grain. The microstructure in PMZ(Partially Melting Zone) of wrought base metal welding joint is equiaxed grain, the as-cast and the homogenized is dendrite. The sort order of the number of M23C6 phase in the HAZ is: wought > homoginized > as-cast. The size of M23C6 phase is little difference. The sort order of the number of MC phase is: as-cast > wrought > homogenized. There is no Laves phase in the HAZ of wrought base metal welding joint, and the number and the size of Laves phase is very nearly the same. The sort order of the total area percentage of the precipitates in the PMZ is: as-cast > homogenized > wrought.(2) As the heat input increased, the width of HAZ and PMZ, the grain size, the dendritic space increased too. The growth rate of grain size in the HAZ of wrought base metal welding joint is approximately 4.6 times of as-cast and homogenized base metal. The total fraction of the MC phase and M23C6 phase in HAZ is decreased as the heat input increase, however, the number and the size of the Laves phase is almost the same. The number of MC phase, M23C6 phase and Laves phase decreased, nevertheless the size increased, as the heat input increase.(3) There is the lamellar segregation, regional segregation of WM(Welding Metal) center and crater and the dendritic microsegregation in WM. The macro- and micro-segregation increased, and the number of Laves phase decrease but size increase, as the heat input increased. The mechanism of phase transformation and the precipitates behavior has been explained based on the solidification theory, the diffusion theory and the phase transition theory.The change rule with temperature of the temperature range of solid-liquid coexistence, the alloy density, the thermal conductivity, the coefficient of linear expansion, the solidification latent heat, the specific heat, the young’s modulus, the poison ratio and the yield strength was calculated by thermodynamic software Jmat Pro. The temperature and stress field, the effect of heat input, closed round hole and multipass welding on plate welding and round hole welding of superalloy sheets were calculated using 3D-FEM method. The results prove that, welding heat stress increased as the heat input increased(1.2 k J/cm), in the meantime, the growth rate of σx(6 MPa), is smaller than σy(8 MPa) in the plate welding, and the growth rate of σt(9 MPa) is smaller than σn(11 MPa) in the round hole welding. Welding heat stress decreased when multi-pass welding.The effect of the original condition of base metal, heat input, casting defect, welding length, multi-layer and multi-pass welding on the Inconel 718 superalloy sheets TIG welding joints hot cracking sensitivity was analyzed by welding experiments, PT and X-ray inspection after welding, and the characterization research of all kinds of hot cracks. The results prove that:(1) The longitudinal and transverse solidification cracks in WM, the liquation cracks in HAZ, the mixed cracks of the WM solidification cracks and the HAZ liquation cracks, crater cracks, the the mixed cracks of the crater cracks and the WM solidification cracks were discovered.(2) The hot cracking sensitive increased as heat input increased.(3) As the heat input not change, the sort order of hot cracking sensitivity among different original condition of base metal welding joint is: as-cast > homogenized > wrought.(4) As the heat input and the original condition of base metal all the same, hot cracking sensitivity of multi-pass welding is larger than the single-pass welding. Mixed cracks are easily formed in the connection region of pre-welding pass crater and the next welding pass WM in the multi-pass welding. Hot cracking sensitivity of parts and components is larger than the plate welding because of restraint intensity of the closure weld is larger. The mechanism of Hot cracking initiation and propagation was revealed by the SEM analysis of the microstructure of welding joint and the fracture surface morphology, alloy phase diagram, the calculation of physical and mechanical properties, thermal elastoplastic 3D-FEM computation.The controlling measure of welding hot cracking during Inconel 718 Ni-based superalloy casting sheets welding was measured, based on the research results of microstructure evolution, the mechanism of welding hot cracking initiation and propagation. It has been verified by small batch production, and proved that it can effectively prevent the hot cracks of Inconel 718 Ni-based superalloy thin-wall casting components for welding. |
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