Influence Of Processing Parameters Of Slm And Stress Ratio On High-cycle And Very-high-cycle Fatigue Behavior Of Ti-6Al-4V

In this paper,a typical method of additive manufacturing—Selective Laser Melting(SLM)was used to produce Ti-6Al-4V samples.Orthogonal experiment design together with the analysis of variance was used to examine the processing parameters(laser power,scan speed,layer thickness and hatch spacing)of SLM for superior properties of the produced parts,in which nine groups of specimens of Ti-6Al-4V were fabricated.The results clarify that the influence sequence of individual parameter on the porosity is laser power>hatch spacing>layer thickness>scan speed.Ultrasonic fatigue tests(20 k Hz)were conducted to investigate the high-cycle fatigue(HCF)and very-high-cycle fatigue(VHCF)properties of the SLM Ti-6Al-4V.The porosity has significant effects on the HCF and VHCF properties of the SLM Ti-6Al-4V,and the specimen with the lowest porosity has the highest fatigue strength.The validation group of specimens,which were fabricated based on the optimal parameter combination obtained by the orthogonal test,have even lower porosity and higher fatigue strength.This result will provide further analysis and optimization of the properties of SLM Ti-6Al-4V alloy.The fatigue fracture surfaces were observed with SEM,and it was found that fatigue cracks mainly originated from the lack-of-fusion.The inadequate overlap between the inner hatching and the outer contour was the main cause of producing subsurface defects.The process parameters have direct effect on the overlapped quality of adjacent hatching lines and layers,thus affecting the quantity and size of defects.In this paper,the influence of stress ratio(R=–1 and R=0.5)on the HCF and VHCF behavior of Ti-6Al-4V fabricated by SLM was studied.The results showed that the stress ratio affected the crack initiation behavior.Under negative stress ratio,fatigue crack initiated from lack-of-fusion defect,and the feature of rough area(RA)appeared around the defect;while under positive stress ratio,cleavage first occured and then crack propagation happened,and no RA was observed.The method of focused ion beam(FIB)technique was used to prepare the sectional samples in the crack initiation regions of failed specimens.The microstructure morphology near the surface of these sectional samples was observed by transmission electron microscopy(TEM)with selected area electron diffraction method.Experimental results showed that the surface of RA of failed specimens under negative stress ratios is a nanograin layer,but the surface of crack initiation region of failed specimens under positive stress ratios is still original lamellaer grains.The obtained results showed that the“numerous cyclic pressing”mechanism dominates the formation of nanograin layer at the crack initiation region.In this paper,a probabilistic fatigue life prediction model was proposed,and the P-S-N curves corresponding to different failure probabilities were given by combining the S-N data and the defect area on the fatigue fracture.The results showed that most of the experimental data fall near the 50%failure probability curve,which indicates that the failure probability predicted by the model is reasonable.At the same time,the fatigue strength corresponding to 10~9 cycles was predicted by using the proposed model,and the influence degree of different porosity on fatigue strength was analyzed.The predicted S-N curve can be used as a reference for the design of SLM Ti-6Al-4V.