Microstructure And Fatigue Crack Growth Behavior Of Ti-6Al-4V Alloy Fabricated Via Laser Melting Deposition

As one of the important metal additive manufacturing technologies,laser melting deposition technology is based on the principle of layer-by-layer stacking manufacturing,which can realize the forming of parts with complex structures while shortening the production cycle time,and has received increasing attention from more and more industries in recent years.Many metal materials have been developed for laser fusion deposition,among which TC4 titanium alloy is widely used in the aerospace industry,ships and automobiles due to its excellent overall mechanical properties.However,the complex interaction between the high-energy laser beam and the material during laser melting deposition forms a fine and chaotic of microstructures,resulting in high strength but low plasticity of TC4 titanium alloys,especially the resistance to fatigue crack growth,which is directly related to the damage tolerance properties,needs to be improved.In this paper,TC4 titanium alloy samples were prepared by laser melting deposition technique,the samples were heat treated at different temperatures,and the relationship between microstructure and microhardness,static tensile properties were investigated by OM,SEM,XRD and other material analysis test methods.The anisotropy of fatigue crack growth behavior was studied,and the fatigue crack growth rates of three sampling directions,XY,ZY and YZ,were tested.The ?KT values were determined by segmental fitting of the Paris equation,the crack growth paths and fracture surface morphology were observed,and the effects of microstructure and residual stress on the fatigue crack extension behavior were discussed.The main conclusions are as follows:(1)During the process of complete melting to solidification,TC4 titanium alloy undergoes ?+??????liquid phase????+?/?' phase transformation process.The needle-like a' martensite is formed under high cooling rate,and the high temperature gradient and solidification rate lead to the columnar growth of prior-? grains along the deposition direction.After heat treatment at 850-900?,?' decomposes into ?+? and ?phase is formed between the coarsened p phase interface.When the heat treatment temperature is 1050?,the columnar ? grains transform into equiaxed grains with a fine Widmanstatten structure inside.(2)The needle-like ?'martensite leads to high strength and low ductility in the deposited specimens.?' martensite is decomposed after heat treatment at 900?,which increases the elongation by about 80%.During static stretching,the columnar ? grain boundaries hinder the dislocation and slip of ?-phase,resulting in lower ductility along the horizontal stretching direction than the vertical direction.(3)The ?KT was 13.6 MPa(?)and 15 MPa(?)for the as-built and heat-treated specimens,respectively.When ?K was less than ?Kt,the crack tip was affected by the columnar ? grain and uniaxial residual stresses,and in the ZY direction,the edge tensile residual stresses reduced the fatigue resistance of the deposited specimens.For the XY direction,the effect of columnar ? grain on the fatigue crack extension rate is comprehensive,while in the YZ and ZY directions the effect is localized.When ?K is greater than ?KT,crack growth is influenced by the inherent fatigue crack growth resistance of the material.