Effect Of Heat Treatment On Fracture Toughness Of 3D Printed TC4 Titanium Alloy

In this paper,microstructural the characteristics of the deposited and heat-treated TC4 titanium alloy formed by Selective Laser Melting（SLM）and its influence on fracture toughness and fatigue crack growth rate were studied.the experiental result are listed as following:（1）The microstructure of the deposited cross-section showed a large number of acicular martensiteα’andβphases,and the longitudinal section showed coarse columnar crystals growing along the forming direction.The intragranular acicular martensiteα’phase and the columnar crystals form 45°angled.After the heat treatment,the needle-likeα’phase transformed into the lathαphase,producing anα+βlath structure.（2）With the increase of heat treatment temperature,theαlayer gradually coarsens,and the cracks generally expanded along theα/βtwo-phase interface.The whole direction of crack propagation is controlled by a cluster composed of long slats,while the microscopic of crack propagation is controlled by thick slats.Direction,resulting in an increase in the degree of tortuous path,the fracture toughness gradually increased from43.1 MPa·m^0.5 in the deposited state to 109 MPa·m^0.5 after heat treatment at 950℃/2h/FC.（3）The heat-treated specimens were prefabricated with fatigue cracks parallel to the forming direction and perpendicular to the forming direction,and the fatigue crack growth rate test was carried out.It was found that with the increase of heat treatment temperature,the fatigue crack growth rate of the lathαphase coarsened and the fatigue crack growth rate gradually reduced.The sample is mainly cleavage fracture,and the secondary cracks gradually increase.A turning point in the Pairs zone was found in the sample after 950℃/2h/FC heat treatment.Comparing the two sets of specimens parallel to the forming direction and perpendicular to the forming direction,the fatigue crack growth rate of the former is faster.The former samples mostly extend perpendicular to theαphase,that is,cut through theαphase and expand forward.The latter mostly expanded parallel to theαphase,which is along the two phase interface ofα/β.