| Titanium aluminide(TiAl)-based alloys have the advantages of low density,excellent high temperature strength and creep resistance,and show great application prospect in aerospace industry and automobile industry.Therefore,TiAl-based alloy is regarded as one of the most promising high-temperature structural materials.However,it is very difficult to make TiAl components by the conventional manufacturing processes due to their inherent low room temperature ductility and poor hot deformability.Without the using of knife mold,Metal Additive Manufacturing(MAM)allows for the fabrication of near-fully dense metal components with complex freeform geometries directly from computer-aided design(CAD)models.Selective Laser Melting(SLM)is one of the most promising technologies in the field of MAM.Through melting TiAl alloy powder with the high energy beam laser,it can form any complex structure having high performance and high precision parts theoretically.However,due to the particularity of SLM process,its built parts are quiet different from the parts fabricated by the traditional methods in microstructure and properties.Therefore,it is very important to investigate the microstructure and properties of the TiAl processed by SLM.Therefore,this paper focuses on the forming process,microstructure and mechanical characterization of Ti-45Al-2Cr-5Nb alloy powder processed by SLM.The main conclusions and innovations are as follows:1.The effect of laser power in SLM was studied,and the relationship between microstructure evolution,phases composition and Vickers hardness of SLM-processed Ti-45Al-2Cr-5Nb parts were established.The results suggested that the samples were mainly consists of ?2,? and B2 phases,and the front view and side view of the parts were composed by equiaxed grains and columnar grain,respectively.Across the molten pool,three distinct regions can be clearly distinguished,namely,coarse grain zone,transition zone and fine grain zone.With increasing laser power,the grains size generally grows up,and the crystallographic of the microstructure varies from a dominant strong(0001)orientation to a combination of(0001),(1011)and(1121)orientations.Moreover,the content of high-angle grain boundaries(HAGBs)and a2 phases decrease with the increase of laser power.In addition,due to the reduction of grain size and the decreased content of B2 phase,the Vicker hardness decreases from 580.1+16.4 Hv1/15 to 561.7+16.1 Hv1/15 gradually with the increase of laser power.2.The effect of laser scanning speed in SLM was studied.The results suggested that as increasing the laser scanning speed from 500 mm/s to 800 mm/s,the average grain size generally decreases from 7.11 ?m to 5.43 ?m while their orientations with a combination of(0001),(1011)and(1121)basically remain unchanged.The SLM-produced samples are dominated by HAGBs,and the volume fraction of HAGBs decreases from 91.60%to 86.19%with increasing the laser scanning speed.The results also shows that y and B2 phases are randomly distributed in the ?2 matrix.When increasing the laser scanning speed,the content of ?2 phase decreases,while the contents of ? and B2 phases increase.The orientation relationship between ?2,? and B2 phases can be expressed as:(2021)?2//(200)B2 and(2020)?2//(110)?//(220)B2.Due to the variation of grain size,phase composition and relative density,the nanohardness of the SLM-process parts increases from 7.90±0.32 GPa to 9.49±0.46 GPa when increasing the laser scanning speed from 500 to 800 mm/s.3.The effect of substrate preheating temperature was studied.The results suggested that with increasing the substrate preheating temperature from 298 Kto 623 K,the crystallographic texture of the SLM-processed samples transforms from strong(1011)and(1121)orientations to a strong(0001)orientation firstly,and then to an equal strength of(0001),(1011)and(1121).The samples are dominated by HAGBs.With increasing the substrate preheating temperature,the content of HAGBs increases from 88.2%to 93.9%.The samples mainly consist of ?2,? and B2 phases,and the fine y and B2 grains are randomly distributed in the a2 matrix.The phases transformation mechanism can be expressed as follows:(102)? transforms to(0002)a2 and(110)?,then the residual ? phase orderly transforms to B2,and finally the residual Bz phase and incompletely transformed y phase homogeneously distribute in the ?2 matrix.With the increase of substrate preheating temperature,the nanohardness of SLM-processed parts increases from 7.57±0.38 GPa to 8.47±0.42 GPa.4.The effect of heat treatment was studied.The results suggested that as increasing the annealing temperature from 298 K to 1473 K,the average grain size of the samples from the top view generally grows up from 5.81?m to 9.98 ?m,while their orientations with a combination of(0001),(1011)and(1121)basically remain unchanged.With increasing the annealing temperature,the content of B2 phase decreases,while the content of a2 and y phases increase.The y and B2 phases in a range of several nanometers are randomly distributed in the a2 matrix.The phases evolution mechanism and orientation relationship are:(200)??(1120)? +(111)? and(2021)?3//(110)?//(110)B2.During the heat treatment process,the nanohardness H ranges from 8.36±0.42 GPa to 9.75±0.49 GPa and Young's Moludus G ranges from 179.60±8.98 GPa to 199.26±9.96 GPa.During the heat treatment process,the evolution of y phase texture can be summarized as:fiber texture intensity decreased first and then increased,while the fiber texture maintain {101}<121>orientation does not change.The recrystallization texture intensity first increases and then decreases,and also the recrystallization texture maintaining {124}<211>orientation remains unchanged.The intensities of the cube texture and R texture also show the first increased and then decreased trend.The ?2 phase of the SLM-processed TiAl alloy possesses a D019 structure(hcp)with the presence of a hexagon diagonal of A1 atoms occupying the hexahedral interstitial position of the hcp Ti lattice.The B2 phase has a bcc structure,in which the Ti and Al atoms are located on the upper and lower layers of the bcc structure,while the Cr and Nb atoms are situated at the center of the bcc unit.The y phase shows a L10(bcc)structure,where Ti atoms occupy the eight angles as well as center of the upper and lower surfaces of L10 structure,while Al atoms are located in the center of the four side faces of L10 structure.The structure change from ?(B2)phase to ?(?2)is likely achieved by the dispelling of Nb and Cr atoms and through transforming from the“...Ti-Nb/Cr-Al-Ti-Nb/Cr-Al...”stacking sequence in the(110)plane of the bcc structure to the“...Ti-Al-Ti-Ti-Ti-Al-Ti-Ti...”stacking sequence in the(0001)plane of the DO19(hcp)structure.The structural change from a(a2)to ? is likely achieved by the shifting of Ti and A1 atoms and through transforming from the"...Ti-Al-Ti-Ti-Ti-Al-Ti-Ti...”stacking sequence in the(0001)plane of the D019(hcp)structure to the“...Ti-Al-Ti-Al...”stacking sequence in the(110)plane of the L10(bcc)structure.5.The effect of TiB2 content on microstructure evolution and mechanical properties of SLM-processed Ti-45Al-2Cr-5Nb/TiB2 metal matrix composite were investigated.The results suggested that with increasing the content of TiB2,the grains size generally decreases while their crystallographic orientations range from a strong(0001)direction to a mixture of(1011)and(1121)directions.The content of ?2 phase decreases from 76.2%to 51.2%,while the contents of ? and B2 phases increases from 22.3%to 43.7%and 1.5%to 2.5%respectively with increasing the content of TiB2.The orientation relationships of ?2,?,B2,TiB2 and TiB can be concluded as:(1120)?2//(110)B2//(111)?//(1010)TiB2,(2020)?2//(111)TiB//(110)? and(0002)?2//(0001)TiB2.With the increase content of TiB2,the nanohardness of SLM-processed TiAl/TiB2 MMC firstly increases and then decreases.The prismatic fiber intensity is larger than the basal and pyramidal fibers intensity.Meanwhile,the intensities of the three fibers increase with the increase of TiB2 content.There are two forms of TiB2 reinforcements distributed in the TiAl-based matrix,which can be concluded as the needlelike micro-TiB2 and irregular nano-TiB2.The nano-TiB2 with the size measurements of 10 nm in length and 3-5 nm in width uniformly distributes in the TiAl-based matrix. |
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