| With the advantages of custom shape design,personalized customization,high shape accuracy,resource-saving,short processing cycle,and intelligent optimization of part performance,selective laser melting(SLM)has huge application prospects in some crucial fields of the national economy and people's livelihood,such as aviation,medical,education,energy.However,before the SLM fabricated parts are put into practical application,there are still many critical scientific problems to be solved urgently in the mechanical properties,especially the long-term service performance.In this study,a series of studies on the SLM fabricated Inconel 718 were conducted.which include the establishment of a high-temperature creep experiment system suitable for miniature specimens,the characterization of the three-dimensional grain structure of the SLM fabricated alloy,and the effect of heat treatment conditions and the characteristics of the SLM-fabricated specimens(such as the sampling position,loading direction,and surface condition)on the high temperature creep properties of the material.This research will not only serve as important theoretical guidance for clarifying the structural characteristics and exploring the creep performance optimization methods of the SLM fabricated Inconel 718,but also provide an application reference to establish the high-temperature mechanical property certification method for the SLM fabricated components.The main results obtained in the study are as follows:Two sets of high temperature creep test systems(a small punch creep test system and a uniaxial creep test system with the digital image correlation characterization function)for miniature specimens were designed and assembled.The small punch creep test system can be used to evaluate the creep performance of the disc specimens with argon protection at the temperature ranging from room temperature to 800? and the load ranging from 50 to 1000 N.The loading accuracy and deformation resolution of the small punch creep test system are ± 0.1 N and 10 ?m,respectively.The uniaxial creep test system can be used to evaluate the creep performance of the uniaxial specimens at the temperature ranging from room temperature to 800?,and the load ranging from 0 to 2000 N.The loading accuracy and deformation resolution of the uniaxial creep test system are±0.1 N and 0.09 ?m,respectively.Moreover,for the uniaxial creep test system,the digital image correlation technique was used to measure the whole field strain of the specimen during the creep testing.The unique advantages of the two creep testing systems in characterizing the high temperature creep performance of miniature specimens were proved by the experiments,showing the results of the devices are correct and credible.2.The characterization of the 3D grain structure of the SLM fabricated Inconel 718 shows that the 90° cross-printing specimen comprises columnar grains with a length of up to 1 mm and coarse V-shaped grains with an average height of 81.3 ?m.The sample presents a periodic distribution of grain size and texture strength.The energy density,texture strength,and grain size reach the maximum values at the center of the melting pool,resulting in the formation of epitaxially grown columnar grains.The energy density,texture strength,and grain size all reach the minimum value at the edge of the melting pool,leading to layered V-shaped grains.It is recommended that the periodic structure can be simplified into structural units which are more reasonable to analyze and simulate for mechanical properties.The investigation of the small punch creep(SPC)life indicates that the SLM fabricated Inconel 718 shows different SPC properties and fracture behaviors from traditional forging and casting Inconel 718.At 650?/600 N,the short-term SPC life of the SLM fabricated Inconel 71 8 specimen loaded along the building direction(BD)is equivalent to that of the as-forged specimens,but is much lower than that of the as-cast specimen.The Laves phase distributed on the grain boundaries is an important reason for the fracture of the SLM fabricated specimen.When loading in the BD,the columnar grain boundary is perpendicular to the tensile stress at the local position,which further reduces the creep life of the specimen.In the SLM fabricated specimen,the different directional grains and dendrites near the "channel-channel" melting pool boundary can lead to the crack deflected.The effect of heat treatment on the SPC life of SLM fabricated Inconel 718 shows that under the same loading direction,the HA1-treated specimens have the longest SPC life,while the SA-treated specimens have the shortest SPC life.This is because there are fewer harmful ? phases at the grain boundaries and more ?' and ?" strengthening phases in the grains of the HA1-treated specimens,while the distribution of precipitates in the SA-treated specimens is in the opposite trend.The complete recrystallization in HA2 treatment increases the SPC life of the Z specimens loaded along the BD but reduces the creep life of the X specimens loaded along the scanning direction(SD),making the two types of specimens with the same life.The specimens with columnar grains show better small punch creep resistance than the specimens with larger equiaxed grains.Under the same heat treatment conditions(HA1.HSA,and SA),the SPC life of the X specimen is much longer than that of the Z specimens,which is mainly caused by the spatial multi-level grain structure.Theoretical calculations show that in the SLM process,the higher energy input or the larger overlap rate of the melting pool helps to reduce the area percentage of V-shaped grains,thereby further improve the creep life of the specimens.The influence of different sampling positions,loading directions,and surface conditions on the creep were examined.The result shows that the bottom specimen has the longest creep life due to the higher degree of recrystallization,while the top specimen has the lowest creep life due to a large number of V-shaped grains.Cracks in the 0°-loaded specimen did not easily propagate along the transverse direction due to the restriction by the columnar grains,thus has a longer creep life.In comparison,the 90°-loaded specimen with cracks easily propagated along the columnar grain boundary and thus has a shorter creep life.In the 45°-loaded specimen,the crack network structure was formed due to the complicated stress state,and inclined cracks in the 67°direction are generated.Thus,the creep life of the 45°-loaded specimen is the shortest.For the specimens with a rough surface,the creep cracks originated at the location of surface defects,resulting in a slightly lower creep life than that of specimens with a smooth surface. |
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