Studies On Microstructure And Properties Of Selective Laser Melted Ti6Al4V Alloy Treated By Ultrasonic Surface Rolling Process

Titanium alloys are known as the suitable lightweight structural materials due to their excellent properties,such as high-specific strength and excellent corrosion resistance.These performance advantages explain their increasing and preferential use in the aeronautic industry and biomedical engineering in recent years.As a novel addictive manufacturing technique,selective laser melting?SLM?use a high-energy laser beam as the heat source to melt the powder materials to fabricate the complex three-dimensional components directly from a digital model.This way not only breaks through the geometry restriction of the traditional subtractive manufacturing but also has a significant advantage in the preparation of the refractory materials.However,the complex thermal interactions,splashes,pores and residual stress affect the mechanical properties of the SLMed parts during the SLM process.Hence,it is necessary to optimize the SLM parameters and the post-treatment of the SLMed parts.Generally,ultrasonic surface rolling process?USRP?can be used to improve the surface finish and hardness,introduce the compressive residual stress and modify the microstructures of the surface by surface plastic deformation.Three post-treatment strategies including heat treatment,USRP,and direct current assisted ultrasonic surface rolling process?DC-USRP?are used to strengthen the SLMed Ti6Al4V parts with the optimum performance.Correspondingly,the effects of USRP and DC-USRP on the microstructure,wear,and fatigue properties of the SLMed Ti6Al4V parts were analyzed.Subsequently,the related strengthening mechanisms were focused in this thesis.?1?Laser powers and scanning speeds are the crucial processing parameters affecting the roughness,porosity and microstructures of the SLMed Ti6Al4V alloy.It is found that the laser powers of 200-250 W and the scanning speeds of 850-1150 mm/s cause the minimum surface roughness and porosity.During this parameter interval,the increase of laser scanning speed has a negligible influence on the tensile strength and yet elevates the elongation of the SLMed parts.The optimum comprehensive mechanical properties?ultimate tensile strength 1350 MPa,fracture elongation 7.8%?of the SLMed samples are fabricated at a laser power of 250 W and a scanning speed of 1150 mm/s.It is found that the heat treatment has a significant effect on the mechanical properties of the SLMed parts.Correspondingly,the micro-hardness and ultimate tensile strength of the SLMed parts decrease as the temperature of heat treatment increases,while its fracture elongation increases.The maximum fracture elongation and tensile strength reaches 14.5%and 1025 MPa at a temperature of 1000°C respectively.?2?USRP and DC-USRP can effectively change the surface roughness,residual stress distributions,microstructures and hardness of samples.The appropriate increase of the vibration amplitude,load force and rolling ball diameter can reduce surface roughness and improve residual compressive stress amplitude and depth.Correspondingly,the optimum USRP parameters are vibration amplitude of 10?m,loading force of 1000 N and tip diameter of 10mm.In this condition,the sample can obtain the surface roughness of 0.25?m,the residual stress amplitude of 1315 MPa,and a residual stress depth of more than 400?m.In addition,the deformation microstructure occurs in the surface of the samples when the vibration amplitude exceeds 9?m.In the deformed zone,the microstructure exhibits a gradient change from the coarse lamellar?structure to the ultrafine lamellar grain,ultrafine equiaxed grain and nanograin corresponding to from the interior matrix to the exterior surface.The dislocation density increases first and then decreases with the decreasing of depth from the surface,especially reaching the highest value of 3.2×10¹⁴ m^-2 at the depth of 200?m.Meanwhile,the intensity of texture decreases,and the preferred orientations are gradually changed to the rolling direction from the matrix to the surface.This is attributed to the dislocation motion.Due to the large strain inside the samples resulted from the USRP treatment,the high-density dislocations are initiated inside the grains due to the crystal slips and entangled along the deformation direction to develop into the subgrain boundaries,which results in refining the initial lamellar structures.The deformation layer exhibits a strong work hardening.The highest hardness reaches about 5.57 GPa near the surface,which is about 33%higher than that of the matrix.The strengthening mechanism can be attributed to the grain boundary strengthening and the dislocation strengthening,in which the maximum contribution to the hardness by grain boundary strengthening is about 1.51 GPa at the depth of 10?m and the maximum contribution to the hardness by dislocation strengthening is about 0.51 GPa at the depth of 200?m.?3?Regardless of dry or lubricant wear conditions,the wear resistance of the SLMed Ti6Al4V alloy is improved by the heat treatment,USRP and DC-USRP.The DC-USRP treated samples can obtain the best performance of the wear resistance.When the sliding time is 2 min under the condition of the dry wear,the wear rate is decreased from 7.0×10^-4 mm³/?N·m?of the SLMed sample to 1.8×10^-4 mm³/?N·m?of the DC-USRP one,and thus the wear resistance of the DC-USRP treated sample is nearly 5.5 times than that of the SLMed one,respectively.When the sliding time is 15 min under the condition of the lubrication wear,the wear rate of DC-USRP treated sample is only 1×10^-6 mm³/?N·m?and the wear resistance is 40 times higher than that of the SLMed one.However,the wear resistance decreases with the increase of sliding time after the treatment of the USRP and DC-USRP.The wear mechanism of the SLMed Ti6Al4V and heat-treated Ti6Al4V displayed the abrasive wear,adhesive wear and delamination,while that of USRP and DC-USRP shows only the abrasive wear.The reasons for the improved wear resistance of the USRP and DC-USRP treated samples are?i?the improved surface shear deformation resistance due to the increased hardness,?ii?delamination inhibited by the surface residual compressive stress,and?iii?the changed wear mechanism caused by the reduced surface roughness at a condition of lubrication wear.?4?Heat treatment improved dramatically the ultra-high-cycle fatigue performance of the SLMed Ti6Al4V alloy.Correspondingly,the fatigue limit of heat treatment is 100 MPa higher than that of the SLMed one at the cycle times of 10⁸ to 10⁹.However,the ultra-high-cycle fatigue properties is decreased significantly after the treatment of USRP.The reduction of the ultra-high-cycle push-pull fatigue performance can be attributed to the residual tensile stress inside the specimen,which leads to the rapid initiation and growth of the cracks during the fatigue test.The multiple cracks initiations and its propagations perpendicular to the axial direction of the samples could be responsible for the decreased very-high-cycle torsional fatigue life,which result from the combination of the residual compressive stress generated by USRP and torsional shear stress.