Research On Microstructure And Properties Of TA15 Titanium Alloy By Laser Deposition Manufacturing

To investigate microstructure,high cycle fatigue property and damage tolerance property including crack propagation rate and plane strain fracture toughness of laser deposited TA15titanium alloy,experiments were carried out while optical microscopy?OM?and scanning electron microscopy?SEM?were used.The results indicate that as-deposited microstructure consists of directional column grains with extremely fine basket-weave microstructure.After double-annealing treatment,coarse lamellar-like basket-weave microstructure was obtained.High-cycle fatigue?HCF?test is conducted at room temperature at different stress levels among 720 MPa,760 MPa and800MPa.Fatigue fracture surface is analyzed by scanning electron microscope?SEM?.The results show that laser deposited TA15 titanium alloy has high-cycle fatigue life.The fracture morphology of the fatigue source under 720 MPa shows high tissue sensitivity,and the sensitive size reaches a single slice size.The sensitivity is weak under 800 MPa,and only reaches the alpha lamellar colony size unit.Part of the specimens have pores which are in the range of 40-120 microns.The pore existence makes the high-cycle fatigue life show dispersion which is different from forging and the size and the location of pores have different effects on fatigue life.Pores that are larger and closer to the facture surface reduce high-cycle fatigue life.Although pores exist,laser deposited TA15 titanium alloy still has excellent fatigue properties,which is related to the fine lamellar structure.The fine microstructure increases the crack initiation resistance and improves the high cycle fatigue strength.The crack initiation region was characterized by crystallographic cleavage facets of?lamellar.The crack tended to propagate tortuously,which was related to different orientations of?lamellar.Its propagation direction in certain areas was parallel or approximately perpendicular to?lamellar and the secondary cracks in the crack propagation region helped consuming energy and improved the fatigue life.At last,damage tolerance property including crack propagation rate and plane strain fracture toughness was studied.Of the three conditions,crack in YOZ plane has faster crack propagation rate,with crack in XOY plane relatively slow crack propagation rate.K_IC property presents anisotropism which is subjected to precast crack propagation direction and?grain location.If crack propagation runs the direction perpendicular to?grain boundary,K_IC reaches the maximum86.36MPa?m^1/2.If the?grain locates in the crack propagation plane and is parallel to crack propagation direction,K_IC reaches77.19MPa?m^1/2.If it is perpendicular to crack propagation direction,it has little impact on blocking crack propagation with K_IC the minimum56.16MPa?m^1/2.