Study On Low Cycle Fatigue And Rolling Contact Fatigue Performance Of Ti-6Al-4V Parts Processed By SLM

As one of the most important titanium alloys in industrial applications,Ti-6Al-4V alloy is widely used in aerospace,marine engineering,automotive,energy,chemical and biomedical fields due to its characteristics of low density,high strength,strong corrosionresistance and good biocompatibility.Selective laser melting(SLM)additive manufacturing(AM)technology can directly form near-net-shaped metal parts by stacking materials layer by layer,without considering the limitation of cutting tools,moulds and fixture tools,so it shows great advantages in fabricating complex geometry Ti-6Al-4V parts that cannot be processed by traditional manufacturing methods.However,compared to traditional manufacturing technologies,the distinct manufacturing process of laser rapid melting and cooling makes SLM form different microstructures on Ti-6Al-4V parts,and hence different part performance.Fatigue performance has an important effect on the service life and reliability of mechanical parts.At present,the research on Ti-6Al-4V processed by SLM mainly focuses on the material properties under static load and high cycle fatigue(HCF)load,while the research on low cycle fatigue(LCF)and rolling contact fatigue(RCF)properties are less.The material occurs LCF or RCF failure under actions of cyclic stress close to or exceeding its yield strength or cyclic contact stress.Therefore,the thesis carries out the study of LCF and RCF performance of Ti-6Al-4V parts processed by SLM.The main research contents are as follows:(1)The LCF properties and failure mechanism of as-built and annealed SLM Ti-6Al-4V materials at different strain amplitude levels are studied.The cyclic softening properties of the SLM Ti-6Al-4V are analyzed by stress–strain hysteresis loops and the progression curves of the stress amplitude with cycles.The cyclic Ramberg–Osgood and the Basquin–Coffin–Manson models are adopted to analyze the cyclic stress-strain and the strain–life relationship for the SLM Ti-6A1-4V,based on which the LCF performance parameters are determined.The fatigue fracture surfaces are observed by using scanning electron microscopy(SEM),and the results indicate that fatigue cracks originate from the surface or subsurface defects.The comparison of fatigue life test results shows that the as-built SLM demonstrated a comparable LCF performance with the wrought material.The fatigue life of SLM Ti-6A1-4V is reduced after annealing treatment.(2)Study on LCF damage model of SLM Ti-6A1-4V.Based on the elastic-plastic constitutive theory,continuum damage mechanics,and LCF test data of SLM Ti-6Al-4V,the material parameters of combined kinematic and isotropic hardening constitutive model and continuous damage model for SLM Ti-6Al-4V are calibrated.A comprehensive finite element model of combined kinematic and isotropic hardening and LCF damage for SLM Ti-6Al-4V is established.The analysis of elastic-plastic behavior,damage evolution and failure for SLM Ti-6Al-4V under LCF load is carried out based on the finite element model.The simulation results of the stress-strain hysteresis loop and the LCF life are in good agreement with the test results under different strain amplitude levels,which proves that the finite element model can not only reasonably describe the stress-strain behavior of SLM Ti-6Al-4V parts under different strain levels,but also predict the LCF life.(3)Study on LCF properties of Ti-6Al-4V porous structure processed by SLM.Triply Periodic Minimal Surface(TPMS)method is adopted to design Gyroid and Diamond porous structures.Based on the combined kinematic and isotropic hardening model and constitutive damage model of SLM Ti-6Al-4V,the finite element model for LCF analysis of TPMS porous structure is established.The stress-strain behavior and damage failure mechanism of the porous structure under the LCF load are studied.The influence of different cell types on the LCF performance of the porous structure under the same volume fraction is analyzed.It is found that the stress distribution of the Diamond porous structure is more uniform under fatigue load,and Diamond porous structure has higher LCF resistance.(4)The method of LCF constrained topology optimization design for SLM parts is proposed.Based on local stress-strain method,the SIMP interpolation model fatigue constrained topology optimization theory and the LCF performance parameters of SLM materials,LCF constrained topology optimization design process for SLM parts is established,and the design steps are in detail.The structure optimization design of SLM parts under LCF load is realized.Take hinge bracket parts as an example,the structural optimization design is carried out to verify the effectiveness of the LCF constrained topology optimization design method for SLM parts.(5)The RCF properties of SLM Ti-6Al-4V under different post-treatment conditions are studied.Based on the friction and wear theory and contact mechanics,the influence of different surface characteristics on the RCF performance of SLM parts is analyzed,and the results are compared with the experimental results of wrought Ti-6Al-4V parts.It is found that the RCF performance of the SLM Ti-6Al-4V is superior to those of the wrought parts and strengthened after polishing and annealing treatment.The worn surfaces are observed by using an optical microscope,and the results indicate that surface and subsurface defects are the main causes of fatigue crack initiation and the residual stresses enhance fatigue crack growth.