Microstructure Control And Mechanical Property Optimization Of Laser Solid Formed TC21 Titanium Alloy

With the design philosophy of aviation industry evolving from static strength design to damage tolerance design,the materials selection criteria has been changed accordingly and promoted the applications of damage tolerance titanium alloys.Laser solid forming（LSF）is an additive manufacture technique for fabricating high-performance complex metallic parts,which has been widely researched in the past decades.Especially,the LSF process has been considered as an appropriate and cost-effective technology to fabricate titanium alloy products.However,the macrostructure of LSFedα+βtitanium alloys usually consists of coarse columnar priorβgrains.The coarse columnarβgrains will deteriorate the mechanical properties and lead to anisotropy.Given the deficiency of strongly textured columnar grains and the desire of randomly textured equiaxed grains,a method to fabricate equiaxed priorβgrains is urgently needed to be developed.In the present study,microstructure evolution,microstructure controlling method and anisotropy of mechanical properties of LSFed TC21 titanium alloys will be investigated.A brief introduction to the present work and the main achievements obtained are as follows:1.The inhomogeneous microstructure and mechanical properties of LSFed TC21 titanium alloy based on the empirical processing parameters（1）The microstructure and texture characterization of LSFed TC21 titanium alloy based on the empirical processing parameters were investigated.The results showed that the as-deposited TC21 titanium alloy exhibits a macrostructure comprising coarse columnar priorβgrains and exhibits strong<100>fiber texture.TC21 titanium alloy which contains medium alloying elements shows quite inhomogeneousαlaths along the build direction as the precipitation ofαlaths is sensitive to the thermal history they experienced.Due to the Burger orientation relationship and variants selection,theαphase texture is directly affected by the original solidifiedβgrains texture.（2）The room temperature tensile properties and fracture mechanism of the LSFed TC21titanium alloy containing columnar priorβgrains were investigated.The tensile tests showed significant anisotropic mechanical properties under two different heat treatment conditions.The horizontal samples exhibit stronger strength than the vertical samples.And the ductility of the horizontal sample is much inferior to that of the vertical samples.The inferior ductility of the horizontal samples is caused by larger amounts ofα_GB layers andβgrain boundaries which perpendicular to the tensile direction.Coarsening ofα_GB layers can further reduce the ductility.2.Priorβgrain morphology evolution and control of LSFed TC21 titanium alloy（1）When the LSFed TC21 titanium alloy were deposited by deliberately increasing the powder feed rate,the microstructure of each cladding layer is composed of two regions:（i）randomly orientated cellular structure region caused by partially melted powders at the top of each cladding layer;and（ii）epitaxial cellular structure region adjacent to the fusion line.A novel design to fabricate wholly equiaxed priorβgrains structure in LSFed titanium alloy was proposed based on the experimental results.The lower laser energy density is critical for the survival of the captured powders.The underlying cladding layer temperature is also an important control parameter for the preservation of the equiaxed dendrites when a new cladding layer was deposited.In general,the equiaxed priorβgrains were obtained with lower laser energy density and a lower underlying cladding layer temperature.（2）The formation mechanism and texture characterization of the columnar priorβgrains were studied.The results showed that both vertical and inclined columnar priorβgrains were obtained,and in the samples with a thicker cladding layer the columnarβgrains show a larger inclined angle.The tilted molten pool boundary resulted in the deviation of heat flow from the build direction and will affect the finalβgrains morphologies and orientations.3.Mechanical property and fracture mechanism of LSFed TC21 titanium alloy containing equiaxed priorβgrains（1）The texture intensity of the priorβgrains andαphase was reduced in the LSFed TC21titanium alloy containing equiaxed priorβgrains.Besides,the repetitive pauses during the LSF process lead to a more homogeneous microstructure.The texture of priorβgrains andαphase almost unaffected when the heat treatment temperature is belowβtransition temperature.However,the microstructure characterization（αlaths width and volume fraction）is strongly depended on the heat treatments.（2）The room temperature tensile properties and anisotropy of the LSFed TC21 titanium alloy containing equiaxed priorβgrains were investigated.The results showed the tensile properties of the equiaxed priorβgrains are superior than the columnar priorβgrains.The aging treated sample are anisotropy while the sotion+aging treated sample are isotropy.The yield strength of the samples increased with a decreaingαlaths width,and follows the Hall-Petch relationship.The anisotropy of the LSFed TC21 titanium alloy containing equiaxed priorβgrains can be contributed to the inhomogeneous distribution of theαlaths.（3）The fracture mechanism of the LSFed TC21 titanium alloy containing equiaxed priorβgrains were investigated.The result showed that voids could nucleate at theα/βinterfaces,βgrain boundaries and theαlaths.In the samples containg equiaxed priorβgrains,the cracks can be confined in a single priorβgrain and delayed the occurrence of fracture.4.Mechanical property optimization of LSFed TC21 titanium alloyTC21 titanium parts containing equiaxed priorβgrains and homogeneous microstructure can be obtained by adjusting the LSF processing parameters and heat treatment,respectively.The mechanical properties comparable to the wrought could be achieved by appropriate microstructure control.