Study On Microstructures And Properties Of Ti Al Alloys Fabricated By Laser Additive Manufacturing

The density of TiAl alloy is only about half of that of Ni-based superalloy(about4.0g/cm³),and it has excellent high-temperature mechanical properties and oxidation resistance,so it is considered to be the only potential candidate to replace Ni-based alloy in the temperature range of 650-1000?,and has become the frontier hot spot of aerospace materials.However,the inherent brittleness of Ti Al alloy brings difficulties to manufacturing.Due to the high cost of traditional manufacturing methods,poor mechanical properties and difficult to form complex structural components,the wide application of Ti Al alloy is seriously restricted.Based on the classical Ti-48Al-2Cr-2Nb alloy,this paper focuses on improving the room temperature plasticity and high temperature tensile properties of the alloy.The main experimental conclusions are as follows:The deposited layers of Ti-48Al-2Cr-2Nb alloy with different process parameters were fabricated by laser additive manufacturing(LAM)technology.According to the appearance and metallurgical defects of the deposited layer,the optimum forming process of the alloy was determined(laser power 1.4k W,scanning rate 9mm/s,powder feeding 5.67g/min,preheating temperature 350?).Four groups of Ti-48Al-2Cr-2Nb alloy thin-walled samples without metallurgical defects were successfully fabricated using the forming process,and their sizes were about 40mm×6.5mm×60mm.The microstructure and properties of the deposited and annealed alloys were compared and analyzed.The results showed that the content of?₂ phase in the annealed alloys was obviously increased and uniformly distributed on the?matrix compared with the deposited alloy.With the increase of annealing temperature,the microstructure of the annealed alloys changed from fine equiaxed grains?larger equiaxed grains?lamellar structures.Compared with the deposited alloy(469MPa and 1.1%),the room temperature tensile strength and elongation of 1260?/30min/FC annealed alloy(543MPa and 3.7%)were increased by 1.16 times and 3.36 times,respectively.The fracture mechanism was quasi-cleavage fracture.Seven groups of thin-walled samples of Ti-48Al-2Cr-5Nb alloy were fabricated by LAM technology and annealed treatment under 6 different conditions.The results showed that compared with the deposited Ti-48Al-2Cr-2Nb alloy,the size of lamellar clusters in the deposited Ti-48Al-2Cr-5Nb alloy decreased obviously from 53?m to 40?m.The room temperature tensile strength and elongation of deposited Ti-48Al-2Cr-5Nb alloy(510MPa and1.4%)were higher than those of deposited Ti-48Al-2Cr-2Nb alloy(469MPa and 1.1%).With the increase of annealing temperature,the size of lamellar clusters of Ti-48Al-2Cr-5Nb alloy coarsened from 40?m to 370?m,resulting in a gradual decrease in room temperature and high temperature tensile strengths.Compared with the deposited Ti-48Al-2Cr-5Nb alloy(1.4%,2.8%and 4.5%),the elongations of 1260?/1h/FC secondary cycle annealed alloy were the best at room temperature,750?and 850?,and its values were 1.6%,3.8%and 4.8%,respectively.The effects of Ta on the microstructure and properties of Ti-48Al-2Cr-2Nb alloy were studied.The results showed that?/B2 phase appeared in the alloy with the addition of Ta,and the volume fraction of?/B2 phase increased with the increase of Ta content.In addition,the fine equiaxed?phase precipitated at the grain boundaries in Ta=2at.%,the fine equiaxed?phase and?/B2 phase precipitated in the lamellar clusters in Ta=5at.%,and the?lamellar in the lamellar clusters was significantly coarsening in Ta=8at.%,and the?phases were truncated many times in the length direction by the precipitated?₂ and?/B2 phases.The tensile results showed that the comprehensive mechanical properties of 5at.%Ta alloy were the best(the tensile strengths at room temperature,750?and 850?were 677MPa,651MPa and 538MPa,respectively,and the elongations were 1.8%,2.2%and 13.0%,respectively).Compared with Ti-48Al-2Cr-2Nb alloy,the tensile strength at room temperature,750?and 850?increased by 1.44 times,2.08 times,1.90 times,and the elongation increased by 1.64 times,2.0 times,10.0 times,respectively.Combined with the previous research results of Ta on the microstructure and properties of Ti-48Al-2Cr-2Nb alloy,it was determined that the optimum addition of Ta was 5at.%.Seven groups of Ti-48Al-2Cr-2Nb-5Ta alloy thin-walled samples were fabricated by using the optimized process parameters,and their sizes were all about 48mm×6.5mm×53mm.The microstructure and properties of deposited and annealed alloys were compared and analyzed.The results showed that the microstructure of both deposited and annealed alloys was composed of?₂,?and?/B2 phases,and the content of?/B2 phase in lamellar clusters decreased with the increase of annealing temperature.However,the distribution of?/B2 phase around equiaxed?grains increased and the size coarsened gradually.Compared with the deposited alloy(1.8%and 2.2%),the tensile strength of annealed alloys decreased in varying degrees at room temperature,750?and 850?,mainly due to the growth of?/B2 phase size in the alloy.However,the elongation of 1260?/1h/FC secondary cycle annealed alloy at room temperature and 750?(2.2%and 3.4%)was excellent,which was 1.22 times and 1.55 times higher than that at room temperature and 750?,respectively.