Study On The Microstructure And Properties Of Laser Near Net Forming High Strength And Toughness Titanium Alloy

High-strength and toughness titanium alloy has the characteristics of high fracture toughness and high strength,and has wide applications in aviation and aerospace.The traditional process for preparing large titanium alloy structural parts has the disadvantages of long procedures,complicated processes,high costs and high requirements for manufacturing equipment.Additive manufacturing,or 3D printing technology,is a digital manufacturing technology that uses a high-energy beam as a heat source to form the component through layer-by-layer material stacking.It has the advantages of short forming cycle,low manufacturing cost and moldless manufacturing.There are a total of four technologies widely used in metal 3D printing:laser near net shape forming(LENS),laser selective melting(SLM),electron beam fuse deposition(EBF),and electron beam selective melting(SEBM).The laser near-net forming technology has obvious advantages such as rapid melting characteristics,dense as-deposited structure,and no need for post-processing.It is especially suitable for the processing of large and complex structural parts for aerospace.In this paper,the Ti-1300 titanium alloy spherical powder was fabricated by plasma rotation electrode process.The microstructure and properties of LENS-formed Ti-1300 titanium alloy were studied.On the one hand,the microstructure and properties of deposited samples of LENS-formed Ti-1300 titanium alloy were studied.On the other hand,the effects of annealing,solid solution aging(STA),annealing slow cooling and aging(BASCA)heat treatment and cyclic cooling and aging heat treatment on the microstructure and mechanical properties of laser near net forming Ti-1300 titanium alloy were explored.Mainly got the following conclusions:Laser near-net-shape Ti-1300 titanium alloy macroscopic grains are composed of a mixture of columnar and equiaxed grains,and the grain size runs through multiple deposited layers.The columnar crystal region were formed by a fine needle-like αphase,while the equiaxed crystal region were formed a rod-shaped α phase.The tensile properties of the deposited alloy are σb=1160MPa,σ0.2=1106MPa,δ=8.7%,ψ=23%.Good plasticity but low strength.When annealing the laser near net shape forming Ti-1300 titanium alloy,it was found that when the annealing temperature was changed from 680℃ to 860℃,the number of a phase decreased continuously until disappeared.in the β phase regionIt was found that the alloy’s microstructure was composed of typical sheet βtransformation structure after solid solution and aging(STA)in the β phase region.When the solid solution temperature is constant,as the aging temperature increases,the thickness of the as increases continuously,and the a phase content hardly changes.The strength of the alloy gradually decreases and the plasticity gradually increases.It was found that the alloy’s microstructure was composed of rod-like α phase,necklace-like grain boundary α-phase and needle-shaped secondary α-phase after solid solution and aging(STA)in the α+β phase region.When the aging temperature is constant,the primary α phase content decreases with the increase of the solution temperature,while the primary α phase morphology is not greatly affected.The strength of the alloy increases with the increase of the solution temperature,and the plasticity shows an opposite trend.In the microstructure of BASCA heat-treated alloy,the a phase is rod-shaped and intermittent,and the aged α phase is obviously needle-shaped.When the furnace cooling rate is 5℃/min,its tensile strength and yield strength reach 1248MPa and 1206 MPa,respectively,and the elongation and sectional shrinkage reach 6.0%and 25%,respectively.The best match between plasticity and strengthAfter cyclic heat treatment and aging heat treatment,the alloy microstructure consists of slab primary α phase,equiaxed primary α phase,and needle-shaped secondary α phase.When the lower cycle temperature is 600℃,its mechanical properties reach the best,where σb=980MPa,σ0.2=907MPa,δ=16.0%,ψ=33%.In summary,the conventional annealing and solution aging heat treatment processes have very limited adjustment of laser near-net-shape forming Ti-1300 titanium alloy microstructure.The BASCA heat treatment process can fully adjust the a phase morphology and distribution in the alloy microstructure,and its strength and plasticity can achieve the best match.The cyclic cooling and aging heat treatment process can adjust the morphology and distribution of the a phase to the maximum extent,the plasticity of the alloy is best.