Fatigue Properties Of Invar Alloy Fabricated By Selective Laser Melting

Invar alloy has attracted extensive attention in the engineering field because of its extremely low coefficient of thermal expansion below Curie temperature(230℃)and excellent mechanical properties at low temperature.Selective laser melting(SLM),as an advanced metal additive manufacturing process,has great advantages in fabricating invar alloy.Based on the actual service environment of invar alloy,the research on the fatigue properties of selected area laser melting invar alloy is of great significance to expand its practical application in aerospace engineering.Firstly,10 groups of different printing process parameters are designed in advance,and the invar alloy formed under the effective forming process parameters is characterized by all-round experiments,including densification and fabricating quality analysis,microstructure,static mechanical properties and so on.Firstly,through the formation quality characterization of 185.3,148.2,123.5,105.8,92.6,82.3 and74.1J/mm~3,it is concluded that when the laser energy density is higher than 74.1J/mm~3,that is,when the scanning speed is greater than 1000mm/s,the relative density of the formed parts is more than 99%.The invar alloy formed under these process parameters has epitaxial columnar crystal growing through the molten pool,with a length of 200-500mm,and its cross section has equiaxed crystal with a diameter of about 100μm.Due to the extremely high cooling rate of SLM process itself,it has subcrystalline structure.Combined with the above characterization results,three parameter groups of laser energy density 74.1,105.8 and 185.3J/mm~3 are finally selected to carry out uniaxial tensile test.The experimental results show that the mechanical properties of selected area laser melting invar alloy are significantly affected by the size and morphology of internal pores or defects,and finally lead to the difference of static mechanical properties.Secondly,the low and high cycle fatigue tests and fatigue failure analysis of invar alloy with three groups of 74.1,105.8 and 185.3J/mm~3 were carried out.Through the low cycle fatigue test,the cyclic stress-strain curve is obtained,which reveals the cyclic hardening and cyclic softening behavior of invar alloy,and it is found that the fatigue behavior of SLM formed invar alloy conforms to the messing effect;Through the high cycle fatigue test,the S-N curve and fatigue ultimate strength are obtained.Combined with the analysis of fatigue failure fracture SEM,the fatigue failure mechanism of invar alloy under each parameter group is inferred,that is,with the gradual increase of laser energy density,the defects in the formed sample are gradually reduced,the premature fatigue failure caused by manufacturing defects is restrained,and the final fatigue ultimate strength is greatly improved.Finally,the threshold value of long crack propagation in the linear elastic rangeΔKth is obtained by the crack propagation experiments with 105.8 and 185.3J/mm~3parameters,and NASGRO model curve parameters also is obtained in the stable crack growth stage.At the same time,combined with the defect size data of failure fracture,it is introduced into EI Tanaka short crack propagation BSB(blocked slip band model)model to successfully predict the influence of internal defect size caused by SLM process on fatigue limit.Further,the numerical solution is carried out by the crack growth analysis software AFGROW to obtain the S-N fatigue life prediction curve under different internal defect sizes.Compared with the experimental data,this method has a good prediction effect on the fatigue life in the stage of high cycle fatigue.