Intracrystalline Substructures Control and Strength-Toughness Optimization of Laser Solid Formed TC4-DT Alloy

Author:Zhao Zhuang

Supervisor:chen jing


Degree Year:2018





With the changes of the design concept in aviation structural industry,advanced titanium alloy parts have gradually developed into the damage tolerance titanium alloy with excellent comprehensive mechanical properties.TC4-DT is a kind of damage tolerance titanium alloy with medium strength and high toughness,which has been widely used in advanced aerospace field.Laser Solid Forming(LSF)is a kind of metal additive manufacturing technology used to directly fabricate three-dimensional metallic components with complex structures.Recently,LSF has been recognized as an attractive technique for fabricating expensive aerospace parts rapidly and cost-effectively.However,the typical microstructures of the LSFed TC4-DT titanium alloy are usually comprised of fine and disordered basket-weave or lamellar intracrystalline structures.It is why the LSFed TC4-DT titanium alloy is stronger than its wrought alloy.But for industrial applications its tensile ductility and crack propagation resistance(directly related to the damage tolerance properties)ought to be substantially improved.Therefore,in the present study,the intracrystalline sub-structures formation mechanism,tuning method and the resulting strength-toughness behavior will be investigated.Therefore an important scientific basis can be established for the application of LSF technology in the field of advanced aerospace.Main research contents and corresponding results are as follows:(1)The intracrystalline sub-structures characteristics deposited with two kinds of laser have been investigated.The α/β interface phase with a fcc crystal structure is observed in the diode laser solid formed TC4-DT titanium alloy specimens.The formation of the α/βinterface phase is mainly attributed to the rapid cooling rate and significant interfacial segregation of vanadium element.The oc/p interface phase hinders dislocation motion and forms deformed twin substructure during the tensile test.This is an important factor,which significantly decreases the mean free path for dislocation slip,to improve the strength of the LSFed TC4-DT titanium alloy.Moreover,the twinning-induced plasticity effects in the oc/p interface phase further increase the plastic deformation capacity.This results in higher elongation for the LSFed TC4-DT titanium alloy.(2)The intracrystalline sub-structures characteristics of LSFed TC4-DT titanium alloy with different build dimension(section geometries)have been investigated.The samples with large build dimension exhibit fine basket-weave microstructure,due to the fast heat loss.While under the small size deposition process,the samples show coarsening colony micro structure with strong transformation texture as a result of variant selection under the relative slow cooling rate.The formation process of the colony microstructure conforms to the interfacial instability mechanism,and the orientation of αWGB is the same as that of theαGB they originate from.(3)A microstmcture consisting of primary equiaxed-α,ultrafine αs lamellar and retainedβ phase has been obtained in the LSFed TC4-DT titanium alloy through an innovative sub critical triple heat treatment(STHT).The elongation of the STHTed samples is significantly improved comparing with that of as-deposited samples,while,the strength is also exceeding ASTM standards,with 905.SMPa in σb.The globularization mechanism of theα laths is mainly attributed to the polygonization of the inherent dislocation during subcritical annealing.The successive occurrences of dislocation slip and shear deformation in the αe are found to significant improvement of the tensile ductility.(4)The sub-structure selection map of the LSFed TC4/TC4-DT titanium alloy is established,and the predicted microstmcture features are basically consistent with the experimental microstmcture.In the lower laser energy density and larger build dimension,a needle α’ martensite is formed inside the LSFed TC4-DT titanium alloy specimens.With the increase of laser energy density or decrease of build dimension,the intracrystalline sub-structure is gradually evolved to the basket-weave microstructure-colony microstmcture.(5)By selecting the appropriate laser process strategy,the LSFed TC4-DT titanium alloy composed of full colony microstmcture can be obtained,which exhibits excellent crack propagation resistance.Compared with basket-weave microstmcture,da/dN of the colony microstmcture exists obvious a inflection point in the Pairs region.Before the inflection point,da/dNⅠ=1.78E-14.(△K)7.01253.After the inflection point,da/dNⅡ=1.54E-7.(△K)3 240767.(6)The relationship between the microstructure parameters and mechanical properties of LSFed TC4-DT titanium alloy has been revealed.Further,based on the dislocation slip,a quantitative prediction model of the microstructure-mechanical properties has been established.For the basket-weave microstmcture,the main microstmcture parameters that governed the tensile propertie and crack propagation resistance are the intracrystalline a laths.For the Widmanstatten microstmcture,the colony size significant effects the tensile properties.The crack propagation curve of the Widmanstatten microstmcture exhibits two typical regions.In Pairs-1 regions,both the a laths thickness and colony size have significant influence on the crack propagation path.While,in Pairs-Ⅱ regions,crack propagation path is not sensitive to the α laths,and the main microstmcture parameters controlling the crack propagation path is α colony.