Study On Rolling Contact Fatigue Performance Of Selective Laser Melted Ti–6Al–4V

Selective Laser Melting(SLM)is most promising addictive manufacturing(3D printing)techniques.Selective laser melting(SLM)can realize the industrial application without or with simple post treatment,such as complex thin-walled parts and lightweight porous parts in aerospace and medical personalized implants,etc.With the rapid development of the technology,SLM holds promise to be applied to all areas of manufacturing.It is quite necessary to study the mechanical properties of SLM parts.Titanium alloy is one of the most widely used materials in aerospace and medical fields.A lot of reserch have been done on the mechanical properties of titanium alloy.However,most study is about the static performance,and the research of its dynamic performance is rare.Lack of dynamic performance data is a major reason which limit the widely application of SLM.Fatigue characteristics reseach is the basis of the dynamic performances tudy.As a common form of fatigue,in this thesis,the rolling contact fatigue(RCF)performance of the SLMed Ti-6Al-4V samples was studied.The study of affecting factors of RCF,the fatigue life test research,failure modes and failure mechanism study was conducted.The main research contents and conclusions are as follows:(1)The density characteristics of SLMed Ti-6Al-4V samples was researched.The density of SLM parts was upto 99.5% of theoretical density,close to the solid density.The density is the basic standards to evaluate SLM forming results.The density has a great effect on the mechanical properties of the SLM parts.The higher the density is,the better the mechanical properties are.Considering the influence of surface quality on the rolling contact fatigue performance,this thesis also studied the influence of the SLM forming process on surface roughness.The results show that the energy density is the main influence factor of surface roughness.Measures to improve the surface roughness was put forward.And results show that the polishing treatment significantly improved the surface quality of SLM.(2)Considering the influence of hardness on the rolling contact fatigue performance,the microhardness of SLMed Ti-6Al-4V samples before and after heat treatment was studied.The microhardness of SLMed Ti-6Al-4V samples was tested and compared with the corresponding brand forging hardness.The microscopic hardness of SLM parts is 373 HV and increased to 403.3 HV after heat treatment,higher than that of forgings(ASTM B381-2010,340 ~ 395 HV).Tensile test of samples was conducted.The tensile strength was 1286 MPa and the elongation rate was 8.3%.Tensile strength is higher than that of traditional castings and forgings,while elongation is lower than that of traditional castings and forgings.The fracture morphology analysis shows that the fracture of SLMed Ti-6Al-4V samples is ductile fracture and the main cause of fracture is unmelted powder,holes and other process defects.(3)The SLMed Ti-6Al-4V samples were processed four different post-treatments and the rolling contact fatigue performance of them was researched.The samples were divided into the as-built group,polishing,heat treatment and heat treatment + polishing group.The rolling contact fatigue life of four groups was tested.It was found that the fatigue life of as-built group was shortest,and the heat treatment and polishing obviously improved the fatigue life of SLM parts,while rolling contact fatigue performance of heat treatment + polishing group was best.(4)The rolling contact fatigue performance of SLM pieces were compared with that of forgings.The results showed that the SLM process performance is superior to the forging process.SLM process improved the wear resistance and micro fatigue performance of titanium alloy.Then the rolling contact fatigue life of titanium alloy was improved.The influence of post-processing on rolling contact fatigue life of SLM parts was studied.It was found that density,microhardness and surface roughness were the main affecting factors of RCF life of the SLM pieces.(5)The rolling contact fatigue failure surface of SLMed Ti-6Al-4V samples was analyzed.The result indicated that the failure modes of different groups were different.The failure of as-built group was cracks mainly caused by surface defects and residual stress.And failure of heat-treated pieces was furrows caused by the roughness and hardness;failure of polishing parts and heat treatment + polishing parts was similar.It respectively was pitting and spalling pitting.And failure of the latter two groups was both caused by the process defects such as holes.The failure modes and mechanism of the two groups were the most similar with that of the traditional parts.