Study on the Microstructure and Mechanical Property of Ti-47Al-2Cr-2Nb Alloy Fabricated by Selective Electron Beam Melting

Author:Yue Hang Yu

Supervisor:chen yu yong


Degree Year:2019





Titanium aluminide alloys have been considered to be the most attractive candidate materials for aerospace and automobile application due to their low density,high temperature specific strength,excellent oxidation and corrosion resistance.However,it is difficult to fabricate metallic component with complex shape by traditional manufacturing methods for TiAl alloys due to their poor room temperature deformability,which limits their further application.Fortunately,additive manufacturing(AM)technologies,which can fabricate parts from 3D computer-aided design models,possess the capacity of preparing near fully dense components with complex shapes directly.During the fabrication of TiAl alloy by AM,the main advantages of selective electron beam melting(SEBM)for fabricating TiAl alloy are the high preheat temperature,for instance,up to 1100℃.The high preheat temperature of SEBM can release the thermal stress accumulated during AM and avoid the generation of crack,which is significantly important for the intermetallic TiAl alloy.In this study,a Ti-47Al-2Cr-2Nb alloy was fabricated by SEBM.Firstly,metallurgical defects and their forming mechanism were studied systemly during SEBM.Secondly,the influence of fabricating parameters on microstructure,grain size,crystallographic texture and mechanical properties were investigated.Thirdly,the effect of energy density on epitaxial growth and anisotropy of mechanical properties in vertical cross-section were studied,and the phase transformation and microstructure evolution during SEBM process were illustrated finnaly.The effect of fabricating parameters on defects,the characterization of metallurgical defects and their forming mechanism were systematically investigated.The results indicated that the surface of sample using SX scanning strategy exhibited a straight embossment on the edge,while the surface was flat when using SXY scanning strategy.Surface perturbations in central position occurred when manufactured at high energy density fast scanning speed.The porosity decreased gradually with the increasing energy density when the surfaces were flat,and the porosity increased sharply when perturbations occurred in top surface.The content of Al loss gradually increased with the increasing energy density.When the energy density was too low or the surface was uneven,the typical metallurgical defects of TiAl alloy sample produced by SEBM were B2 phase precipitation around partially melted powders.And small pores(diameter<10μm)formed when manufactured at high energy density.The influences of beam current on microstructure,phase composition,grain boundary misorientation,recrystallization and mechanical performances were investigated.The results illustrated that with the enhancing beam current,the microstructure changed from fine duplex toγ/B2 structure.The volume fraction of B2phase and average grain size gradually increased with increasing beam current.The content of grain boundaries misorientation(89±3°)gradually increased and the content of(1-5°)continuously decreased with the enhancing beam current and the recrystallization proceeds sufficiently.The ultimate compression strength(UCS)and the strain(δ)of SEBM-processed TiAl alloy gradually decreased with the increasing beam current.The effects of scanning speed on microstructure,phase composition,texture and tensile property of Ti-47Al-2Cr-2Nb alloy produced by SEBM were investigated systematically.With the scanning speed increasing from 1500 mm/s to 2400 mm/s,the microstructure transformed fromγ/B2 structure with average grain size of 25.6μm into fine duplex microstructure with average grain size of 4.3μm,and the content ofα2 and B2 phase decreased gradually,while the volume fraction ofγphase increased continously.The crystallographic texture showing obviousα2(0001)andγ(111)orientation weakened with the increasing scanning speed.Additionally,the tensile property increased firstly and then decreased with the increasing scanning speed,and the sample produced under 2100mm/s showed the most excellent ultimate tensile strength(UTS)and fracture strain(δ),and they were 684.98 MPa and 0.7%,respectively.The effects of hatch spacing on microstructure,grain boundary characteristic and tensile properties were systematically investigated.The results showed that the microstructure changed from coarse lamellar colony(γ/B2)to inhomogeneously duplex structure in horizontal cross section with the increase of hatch spacing from 85μm to 115μm.The volume fraction of high angle grain boundaries(≥15°)decreased and the volume fraction of low angle grain boundaries(2-15°)increased with the increase of hatch spacing.The SEBM-produced alloy sample H2 exhibited the most excellent room-and high-temperature tensile strength,with UTS being 663 MPa at room temperature and increasing to 724 MPa at 700℃,which indicated that the UTS was improved at 700℃.The influences of energy density on microstructure,aspect ratio,Schmid factor,tensile property and their anisotropy were investigated systematically in the vertical cross-section.The forming mechanism of anisotropy of tensile property and preffered lamellar orientation were discussed detailedly.The results indicated that as the energy density increased from 14.28 to 35 J/mm3,the microstructure transformed from fine-equiaxed grains with a low aspect ratio and a weakly crystallographic texture to elongated grains with a strong texture,which indicated epitaxial growth gradually strengthened.The angles between the lamellar planes and the building direction accumulated 0±5°and 45±5°preferentially with the increase of energy density.The content of misorientation angle45±5°accounted for 56%when energy density was 35 J/mm3.Additionally,the average value of Schmid factor increased gradually from 0.39 to 0.45 and the anisotropy of tensile properties in vertical and horizontal cross-section strengthened with the enhancing energy density.The original microstructures were near lamellar colony(α2/γ)for SEBM-produced TiAl alloy samples under different energy densities.The lamellar structure degraded under in-situ heat treatment during SEBM process.With the increasing energy density,the degradation of lamellar structure became more serious.Lamellar colony(α2/γ)degraded thoroughly and becameγ/B2 lamellar structure with winding phase interface when adopting high energy density,while the microstructure transformed into duplex structure for low energy density,and the microstructure was composed ofγlath and B2phase particle along the primaryα2 lamellae when adopting medium energy density.Three kinds of phase transformations could be observed during SEBM process under different energy densities and they wereα2→γ,α2→B2 andα2+γ→B2.Besides three kinds of phase transformations,the metastable Ti2Al phase was observed during the phase transformationα2→γprocess when adopting low energy density.