Effect Of Mechanical Surface Strengthening On Fatigue Behavior Of Additive Manufactured TC18 Titanium Alloy

TC18 titanium alloy,which has high specific strength and excellent corrosion resistance,can be used in aircraft landing gear to reduce the weight of aircraft.However,its implementations are restricted by the forming difficulties.The forming problems of titanium alloys can be solved by additive manufacture techniques which have the advantages of high flexibility in making arbitrary complex components and high material utilization rate However,the fatigue resistance of additive manufactured materials is insufficient,and it is easy to produce fatigue damage under cyclic alternating loads,which severely shorten the service life of components.In this study,cold hole expansion and shot peening strengthening techniques were used to modify the surface conditions of the additive manufactured TC18 alloy,and the influence regularity and mechanisms of the two strengthening methods on the fatigue life of the material were analyzed as well.The results show that the fatigue life of the specimens can be effectively increased by the two kinds of surface strengthening methods with reasonable parameters.For the cold hole expansion method,specimens with apertures of 12 mm and 16 mm had the best strengthening effect when the magnitude of interference was 4.5%,and their median fatigue life value was about 3.3 times compared with that of the unstrengthened specimens.As the magnitude of interference increased,the fatigue life of the specimens firstly increased and then decreased,but higher than that of the unstrengthened specimens.For the shot peening method,the highest fatigue life was found under the shot peening intensity of 0.15-0.20 A using S230 steel shots,which was 3.43 times comparing with unstrengthened specimens.Besides,the surface roughness had a great influence on the fatigue life of the shot peening specimens if the surface residual compressive stress was similar.By comparing the fatigue fracture surfaces of specimens with and without strengthening,it was found that the fatigue crack source of hole expansion specimens transferred from the surface of hole to the chamfers.Compared with the unstrengthening specimens,the fatigue crack propagation region in the strengthened specimens became larger and flatter,and the crack propagation rate were reduced along the thickness direction of the hole.In addition,the fatigue crack source of shot peening specimens transferred from the surface to the subsurface,and both fatigue crack source and propagation region increased,the roughness of the fatigue region decreased as well.The fatigue cracks always preferred to initiate at the defects wherever they were.From the results of microstructural characterization,microhardness tests and finite element simulation,it was found that the improved fatigue performance of the specimens was caused by the increase of microhardness and residual compressive stress around the hole.The finite element simulation results showed that the thickness of residual compressive stress layer increased with the increasing magnitude of interference.Besides,the residual compressive stress firstly increased and then decreased along the hole radial direction,and finally turned into tensile stress.The lowest compression stress appeared at the entrance surface,followed by the export surface,and the largest one appeared at the middle of the hole wall.In summary,the fatigue resistance of cold hole expansion strengthening specimens was improved by retarding the crack initiation and crack propagation at the same time.The EBSD and nanoindentation results of the strengthening layer indicated that the improvement of the fatigue performance in the shot peened specimens was caused by the grain refinement of the surface layer which was more than ten microns.The reduction of preferential slip(0001)crystal plane along with the increase of the difficult slip(01(?)0)crystal plane in hexagonal crystal structure were the main reasons for improving the fatigue resistance of shot peened specimens.Moreover,the increase of high-angle grain boundaries,microhardness and elastic modulus are also important for the improvement of the fatigue resistance.The TEM results showed that the fatigue crack initiation was effectively inhibited by the amorphous structure and high density dislocations in the surface layer.On the other hand,the early crack propagation was retarded by the dislocation wall,nanocrystalline grains and twin crystals in the subsurface layer.The above reasons together greatly increased the fatigue life of shot peened specimens.In summary,the fatigue resistance of shot peened specimens was improved by retarding the crack initiation and earliest crack propagation.