| Nickel-based superalloy is an important part of superalloy.It has high temperature strength,good corrosion resistance and excellent high temperature fatigue resistance in the range of 650?1000?.Incone718 alloy has excellent performance at high temperature.SLM(Selective Laser Melting)technology is especially suitable for forming complex shapes of aviation superalloy parts.Therefore,it is of great significance to study the fracture mechanism in ultra-high cycle fatigue rang of Inconel718 superalloy formed by SLM for promoting the engineering application of Inconel718 superalloy.In this paper,Inconel718 superalloy formed by SLM is taken as the research object.Based on the factors that affect the fatigue properties of the material,such as microstructure,microhardness and tensile strength,advanced material analysis methods such as OM,SEM and EBSD are adopted.The microstructure,grain size,distribution and morphology of precipitated phase and high Angle grain boundary of SLM forming and forging superalloy were observed.In this paper,ultrasonic fatigue tests of SLM forming and forging IN718 specimens at room temperature and high temperature were carried out.The fatigue fracture characteristics were analyzed,and the effects of internal defects and microstructure discontinuity on the fatigue behavior of specimens at high temperature were discussed.The following are the research results of this paper.(1)The grain size of the SLM specimen is larger and more uneven than that of the forgings.There are a large number of Laves and?phases at the grain boundary of the SLM alloy,and the microstructure consists of a large number of tiny cellular crystal structures.The SLM alloy have less high angle grain boundaries than the forging alloy.After heat treatment,the hardness of SLM forming specimen is significantly increased,and the hardness of longitudinal section is higher than that of transverse section.The tensile strength of the SLM forming specimen is higher than that of the forging specimen at both room temperature and high temperature,but the elongation is lower.(2)At room temperature,the S-N curves of both SLM and forged alloy show a"step shape",and the crack initiation position of the SLM forming specimen is significantly related to its life.When the life is between 10~5-10~7cycles,the crack initiation occured on the surface,and when the life is over 10~7cycles,most of the crack initiation occured in the interior.The fracture failure mode of the specimens with surface initiation is surface slip,while the crack source of the specimens with internal initiation is circular keyhole.The crack initiation modes of forging specimens are surface slip,surface cleavage and internal cleavage.(3)At high temperature,the S-N curve of SLM specimen is in the shape of"double linear",and the S-N curve of forgings is in the shape of"step".The position of crack initiation of both SLM forming and forging specimens is related to their life.When the life of SLM specimen is less than 10~8cycles,the crack initiation is mainly on the surface,and when the life is more than 10~8cycles,the crack initiation is mainly in the interior.When the life of the forgings is between 10~5-10~7cycles,cracks are all initiated on the surface,and when the life is over 10~7cycles,cracks are all initiated on the inside.When the life is less than 10~8cycles,the fatigue properties of SLM and forging specimens are similar.When the life is above 10~8cycles,the fatigue performance of SLM specimen is slightly worse,and the crack source is internal defect and discontinuous area of microstructure.During the high temperature fatigue process,more Laves and?phases precipitated at the grain boundaries of the forgings as the potential sources of fatigue cracks,and more large Angle grain boundaries and twin boundaries,which slowed down the growth of micro-cracks.Therefore,the fatigue properties of the forging specimens in the ultra-high cycle fatigue stage at high temperature were slightly better than those of the SLM specimens.(4)The SLM alloy has similar fatigue properties at room temperature and high temperature when its life is less than 10~8cycles.Above 10~8cycles,the fatigue performance at room temperature is better than that at high temperature,and the difference becomes more obvious with the increase of life.The influence of high temperature on the fatigue properties of SLM alloy is mainly through the following ways.The?phase and Laves phase formed on the grain boundary lead to the embrittlement of grain boundary,which leads to the intergranular fracture more easily.Carbide and oxygen elements can promote crack source formation,accelerate crack propagation,and even induce secondary crack formation.The microstructure discontinuity dominates the fatigue behavior at the ultrahigh cycle fatigue stage at high temperature. |
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