Research On The Surface Integrity And Mechanical Behavior Of Additive-manufactured Ti6Al4V Alloy Assisted By Temperature Field During Ultrasonic Roller Burnishing

Low density,high specific strength,good thermal stability,strong corrosion resistance,and excellent biocompatibility make titanium alloys widely used in highperformance engineering applications such as aerospace,petrochemicals,marine engineering,and medical devices.However,the chemical activity and low thermal conductivity and elasticity of titanium alloys make them difficult to process.Laser direct energy deposition(L-DED)is an additive manufacturing technology that uses lasers as a heat source to melt and deposit high-performance materials.It has the advantages of high design freedom,fast construction speed,large size of formable parts,and vacuum environment manufacturing,making it one of the solutions for forming difficult-toprocess material parts.However,the L-DED forming process involves multiple energy field interactions,which leads to unstable melt pools and easy cracking and porosity formation,and low forming surface quality reduces the comprehensive performance of formed parts.As a typical high-strength alloy,the main weakness of titanium alloys is their sensitivity to stress concentration in fatigue strength,and over 80% of cracks in highstrength component fatigue failures start from surface scratches,scratches,or inclusions.Based on the above research background,Ti6Al4 V was used as the research object material to prepare additive samples by L-DED forming method,and three postprocessing techniques,turning,ultrasonic burnishing(UB),and ultrasonic burnishing coupled with heat treatment(UB/HT),were used to process the additive samples.The three prepared samples were compared and analyzed in terms of forming surface integrity,micro-mechanical properties of surface modified layers,room temperature tensile properties,and axial tensile-compression fatigue properties,and relevant research conclusions were drawn.The main research content and conclusions include:(1)Both UB and UB/HT processes can significantly improve the forming surface integrity of Ti6Al4 V additive samples,and the forming surface integrity of UB/HTtreated samples is more excellent.Turning is a material removal process that can eliminate some surface cracks,impurities,deposition tracks,and unmelted particles of the sample,but it does not improve the surface porosity and defects of the sample,and it will form turning marks on the processed surface.UB and UB/HT processes are non-material removal processes.The process mechanism is to force the surface material to undergo plastic deformation,flow from the surface peak to the valley,and achieve "cutting peaks and filling valleys." Therefore,it can significantly reduce surface roughness and porosity,while forming a certain thickness of surface modified layer and introducing residual stress fields.Compared with UB-treated samples,due to the assistance of temperature fields,the material is more easily deformed,and the surface modified layer of UB/HT-treated samples is thicker,the average grain size is smaller,and the surface microhardness is higher.However,due to the relaxation effect of residual stress in the plastic deformation zone of the sample during the insulation process,the surface residual stress value of the UB/HT-treated sample is smaller than that of the UB sample.Comprehensive comparison and evaluation of the forming surface integrity of the samples show that the forming surface integrity of UB/HT-treated samples is more excellent.(2)Both UB and UB/HT processes can improve the micro-mechanical properties of Ti6Al4 V additively manufactured specimens,while UB/HT-treated specimens exhibit superior micro-mechanical properties.Nanoindentation test results show that,compared to the UB-treated specimens,the UB/HT-treated specimens have higher nanohardness,shallower indentation depth,and lower creep deformation,indicating better deformation and creep resistance.Three different processed specimens were calculated for yield strength,and the results demonstrated that the UB/HT-treated specimens have the highest yield strength.(3)Both UB and UB/HT processes improve the strength of Ti6Al4 V additively manufactured specimens while reducing their tensile ductility at room temperature.According to the results of room temperature tensile tests,the machined specimens exhibit a ductile fracture,while the UB and UB/HT-treated specimens exhibit a ductilebrittle mixed fracture.Combined with the stress-strain curve,microstructure,and mathematical model analysis of the room temperature tensile test,the ultrasonic rolling process enhances the specimen’s strength through dislocation and grain boundary strengthening.However,it also leads to strain hardening,which reduces the material’s plasticity.(4)Both UB and UB/HT processes can increase the fatigue strength of Ti6Al4 V additively manufactured specimens,and the fatigue cycle number of UB/HT-treated specimens is 2.5 times that of UB-treated specimens.The analysis of the fracture morphology of the specimens indicates that both UB and UB/HT processes can suppress the surface initiation of cracks and transfer the fatigue source to the material’s interior.The significant improvement in fatigue life due to the UB/HT process is the result of the combined effects of surface quality improvement,residual stress introduction,microstructure refinement,and work hardening.