| GH4169 is a precipitation-strengthening type alloy,with a face-centered cubic gamma phase as the matrix,and the body-centered cubic metastable phase ??(Ni3Nb)as the main strengthening phase.The heat treatment method of the GH4169 alloy is mainly based on solid solution and aging.The content of ? phase and grain size are changed by solution,and the reinforced phase with dispersed distribution is obtained through aging treatment to increase the trenghth of the material.The nucleation sites of the ? phase are related to the temperature.When the solution temperature is low,the ? phase nucleates at the ??? phase,but when the temperature i s higher than 900 °C,the nucleation mainly precipitates from the matrix occurring al ong grain boundaries or twin boundaries.In this paper,according to the law of precipitation and growth of ? phase in GH4169 alloy,the samples with different content of ? phase in the intragranular and grain boundary are obtained by solid solution aging treatment,and the tensile properties of the samples are tested at room temperature.The influence of the precipitation position and content of ? phase on the mechanical properties of the alloy under room temperature is studied,and the mechanism of the influence of grain boundary ? on the crack growth stage of GH4169 alloy was explored.Based on the study of the microstructure and mechanical properties of flash welding samples,the influence rule of heat treatment system on the properties of welds and matrix is explored,and the pre-deformation of the welds is enforced to simulate the bulging of the welds.After heat treatment,the mechanical properties of the welds were observed and compared with the matrix under the same heat treatment conditions.The results of the study show that:(1)After the conventional heat treatment process(solid solution + aging),the ? phase mainly precipitates and grows at the grain boundary.The tensile test shows that the presence of the grain boundary ? phase has no obvious effect on the tensile strength,but it will reduce the percentage of area reduction.The instrumented impact test results and fracture observations show that the grain boundary ? phase decreases the toughness by reducing the crack propagation resistance.The microstructure morphology of the intragranular ? phase was obtained by double solution heat treatment with double aging(solid solution + aging + solid solution + aging),and the tensile results showed that the precipitation of a large amount of ? phase in the instrumented would reduce the yield strength of the material.(2)Scanning and EBSD observations revealed that there were no liquefied cracks,dendrites,and ? phases in the heat affected zone of flash welding samples.in the welding seam and nearby zoon,the high-angle grain boundary density increased because of dynamic recrystallization occurs under the high temperature and upsetting force.Under the effect of temperature field,the strengthening phase ?? dissolves in the heat-affected zone.At the same time,carbide(MC)gathers and grows in the welding seam and nearby zoon,which reduces the impact toughness.(3)Through the heat treatment process,the microstructure of the welded joint is changed to meet the forming conditions(bulging or rounding deformation).The tensile results show that the solid solution treatment will significantly reduce the strength of the matrix.And with the increase of solution temperature,percentage of area reduction increases.This is mainly due to the lower ? phase content at higher temperature conditions,which in turn increases crack propagation resistance.However,the mechanical properties of the welded sample after solution treatment have not changed significantly,and similar mechanical properties are maintained at different solution temperatures.After solid solution aging and pre-denaturation treatment,the welded sample and the matrix maintain similar strength indexes.However,due to the presence of coarse grain zone in the welding joint,the percentage of area reduction is lower than that of the base.Therefore,after welding,the welding part undergoes rounding and bulging,and the deformability of the welding joint and the base body tends to be consistent. |
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