Fatigue Crack Growth Behavior Of A Selective Laser Melting Inconel 625

Selective laser melting technology has attracted great attention in aerospace field and it can be used to prepare nickel-based superalloy structure.The research on the properties of laser additive manufactured superalloys were mainly focused on the optimization of microstructure and the comparison with the mechanical properties of traditional process materials.The current research is less about the fatigue properties of materials,especially considering the impact of load characteristics on fatigue performance,which will be beneficial to the engineering structure application of new technology for additive manufacturing.At present,the research on fatigue crack growth behavior of selective laser melting Inconel 625 materials is still lacking.Due to the typical defect sensitivity of additive manufacturing materials,the fatigue crack growth behavior of materials is extremely important based on the knowledge of traditional fatigue and fracture mechanics.In order to study the fatigue crack growth behavior of the selective laser melting Inconel 625 at room temperature,this paper focuses on the stress ratio,overload and building directions of this material.Through the analysis of the testing data,the fatigue crack propagation law of the selective laser melting Inconel625 is obtained.The fatigue crack growth behavior is studied from the perspective of microscopic mechanism using a scanning electron microscope.Then,according to the test data under various stress ratios and building orientations,the crack growth models with good applicability are established from two different ideas.Finally,a numerical simulation method for thermal stress crack propagation of a thin-walled high-temperature combustion chamber structure is given.The main conclusions of this paper are as follows:1.Fatigue crack growth behavior of selective laser melting Inconel 625 compact tensile specimens in two building directions(ST-90° and ST-90°⊥)under different stress ratios(R=0.1,0.5,0.7)are studied.Research indicates:In the threshold region,the selective laser melting Inconel 625 has better fatigue crack growth resistance than the forged materials.The joint effect of microstructure and stress ratio is the main reason for the obvious difference in crack propagation characteristics in the threshold region;In the Paris region,the fatigue crack propagation behavior is no longer sensitive to microstructure.The slight difference in fatigue crack propagation behavior is mainly due to the influence of stress ratio;A higher stress ratio will result in smaller FCG resistance.This is due to the presence of a larger plastic zone and a higher stress level at the crack tip,giving the slip belt sufficient time to fully develop into a crack;The high stress ratio also means a higher stress level,where the crack surface is difficult to contact,and the crack closure effect due to roughness is weakened.2.For the compact tensile specimen of the selective laser melting Inconel 625,consider two building directions(ST-90° and ST-90°⊥)and carry out the test with an overload ratio of 1.5.The results show that the moment of overload leads to the phenomenon of instantaneous acceleration;The size of the plastic zone can be used to describe the severity of the crack propagation rate delay,the larger plastic zone in the ST-90°⊥is,the more obvious crack propagation delay is;Crack growth bifurcation is not the reason of crack propagation delay,the large plastic deformation caused by overload is the main reason of crack closure.3.A Paris unified correction model and an equivalent crack driving force model considering the stress ratio are established.The results show that the model only needs the testing data of fatigue crack growth rate under two stress ratios to determine the model parameters;The prediction results of crack growth rate are basically consistent with the experimental data,especially in the Paris region;At the same time,the crack closure effect of the threshold value is considered,so the predicted value of the threshold value is lower than the test data.For the equivalent crack driving force model,the whole model has strong theoretical support because it is based on the fundamental theory of fracture mechanics.The elastic modulus,Yield stress,stress intensity factor range and crack tip plastic zone size are taken into account in the equivalent driving factor,so that the data of fatigue crack growth rate affected by stress ratios and building orientations in the whole region are compressed into a narrow band,which eliminate the difference of FCG behavior caused by stress ratios and material orientations interference.The model variable acquisition is simple and this model has a wide applicability.4.Based on ABAQUS/FRANC3D software,the crack propagation simulation of the dangerous position of the thin-walled combustion chamber structure has also been carried out.The results show that FRANC3D can well describe crack leading edge shapes,crack propagation paths,and calculate relatively reliable stress intensity factor values.Since the dominant stress direction of thin-walled cylindrical combustor is axial,the crack surface which is not perpendicular to the axial stress will be deflected by the influence of the axial stress;When K_I is not dominant,the crack path and crack shape will become distorted.In this paper,the fatigue crack growth test of selective laser melting Inconel 625 at room temperature was carried out.It provides valuable experimental data for the fatigue performance study of this material.In addition,a modified Paris model,which is not limited to specific materials,and an equivalent crack driving force model with wide applicability and obvious physical mechanism have been developed,which provides a reference for predicting the fatigue crack growth behavior of metal materials under the influence of stress ratios and material orientations.