Effect Of Shot Peening On Properties Of Additively Manufactured Titanium Alloy

At present,although the static strength of a metallic part that is direct-formed by the additive manufacturing technique can be comparable to its as-forged counterparts,the poor fatigue strength limits its industrial applications.In this study,MSP and LSP are used to modify the mechanical properties of the selective laser melting(SLM)Ti6Al4V titanium alloy.The effect of two shot peening techniques on the property change of SLM-ed titanium alloy are analyzed in terms of its microstructure,microhardness,residual stress,tensile strength and ultra-high cycle fatigue strength.The main research work is as follows:SLM can be used to fabricate complex part with high precision,but typical material defects,such as pores,incomplete fusion holes,and cracks,are easy to introduce during the forming process,which greatly reduces the fatigue properties of a part.This study analyzes the formation mechanisms of the typical defects and find out solutions to reducing defects.Heat treatment is conducted at a temperature of 955°C after surface processing.The results show that the residual stress is greatly decreased after the heat treatment.Meanwhile,the initial acicular ?? martensitic phase of the SLM-ed titanium alloy is decomposed into ?+? in the shape of colony containing parallel ? lamellae.Although the SLM-ed titanium alloy has a low tensile strength,it demonstrates an improved plasticity and extensibility,and decreased anisotropy in the mechanical properties,which may improve its service performance.Confocal laser scanning microscope,metallographic microscope,SEM,micro-hardness tester,tensile testing machine,are used to analyze the effects of laser shot peening(LSP)and mechanical shop peening(MSP)with different processing parameters on property change of the SLM-ed titanium alloy.The results show that,compared with LSP,MSP has significantly increased surface roughness on the processed specimens,and more surface hardening.Higher micro-hardness and residual compressive stress are introduced into the specimens from both processes.With an increased strength due to shot peening,the specimen also has an increase in the maximum residual stress and its influence depth.The LSP processed specimen shows a lower micro-hardness and smaller amplitude of the residual compressive stress,but a larger influence depth.The results of the ultra-high cycle fatigue test show that there is no traditional fatigue limit for the SLMed titanium specimens.The LSP treatment introduces residual compressive stress,high-density dislocations,and grain refinement,which might have deferred the initiation and propagation of the crack,but longer fatigue life has not be achieved for the SLM-ed specimens with too many defects.