Microstructure And Mechanical Properties Of TiAl Alloy Processed By Wire Arc Additive Manufacturing

Due to its low density,good resistance against creep and oxidation,high specific strength,TiAl alloy is expected to replace nickel-based superalloy in the temperature range of 800??900?.These properties make TiAl alloys attractive for aerospace field.However,it is difficult to process by conventional thermal processing methods due to its low room temperature plasticity.The process of TiAl alloy by GTA arc additive method not only has a short production cycle and low cost,but also has a high utilization rate of raw materials,which has a huge advantage for the process of TiAl alloy with complex structure.Therefore,in this paper,TC4 wire and ER1100 wire will be sent to molten poor to process TiAl alloy using GTA arc additive technology.The manufacturing process will be optimized to solve the problems of cracks and nonuniform reaction.The microstructure and properties of deposited TiAl alloy in different regions were studied.The effects of Al content and substrate preheating temperature on microstructure and mechanical properties were also investigated.Firstly,the effect of process parameters on the formation of single-layer deposition was investigated.The optimal deposition parameters were selected: TA2 substrate,wire feeding on the opposite side,welding current of 120 A,deposition speed of 100mm/min.With the increase of arc height and the decrease of wire feeding speed,the droplet transition from short circurt transitionto be droplet transition.After the addition of V element,the element and hardness of cross section were more evenly distributed.The content of Al was controlled between 45% and 50%,the preheating temperature reached more than 400?,then there were no obvious cracks existed on the surface and inside of the deposited TiAl alloy.The microstructure and mechanical properties of TiAl alloy in different regions were studied.The top region was fine dendritic,and the dendritic phase was(?2+?)Llamellar,and the interdendrite was L-liquid segregation.There were light lines in the middle region and the layer band region was characterized by full-lamellar structure.The region between two layer bands was characterized by columnar crystal.Large equiaxed ?2 and(?2+?)L grains were found in the bottom regiom.From the top to the bottom,the ? phase was reduced and the ?2 phase was increased.The average grain size increased from 90 um to 450 um,and the grain orientation remained unchanged.The volume fraction of the large angle grain boundary decreased from 84.6% to 56.6%.The compressive strength was increased,while the compressive plasticity was reduced.In terms of hardness,the hardness values of the top region and the bottom region were higher,followed by the middle region,and the mechanical mixing region were the lowest.The effect of Al content on the microstructure and mechanical properties of TiAl alloy in different regions was further studied.In terms of structure,the single ?2 phase changed into full-lamellar with the increase of Al content.The ?m phase started to precipitate at the grain boundary,and finally changed into double structure.With the increase of Al content from 40% to 55%,the content of ? phase and B2 phase increased,while the content of ?2 phase decreased.The average grain size decreased first and then increased.The volume fraction of the large angle grain boundary increased from 72.9% to 82.3%.The TiAl alloy with 50% Al has the highest compressive strength and compression rate,respectively 1762 MPa and 26.1%;while TiAl alloy with 45% Al has the lowest compressive strength and compression rate,respectively 1173 MPa and 10.5%.Finally,the effect of substrate preheating temperature on the microstructure,phase content,grain size,grain boundary angle,microhardness and compression properties of TiAl alloy was studied.The results showed that as the preheating temperature of the substrate increased from 200? to 450?,there was no significant difference in the structure,while the micro-segregation and average content of aluminum decreased.The content of the ? phase increased from 82.3% to 84.6%,and the content of the ?2 phase decreased from 16.0% to 13.0%,and the content of the B2 phase increased from 1.7% to 2.4% with the increase of preheating temperature.The volume fraction of the large angle grain boundary decreases gradually and the average grain size increases gradually.There was little change in microhardness from 458 HV0.2 to 450 HV0.2.In terms of compression performance,the compressive strength increased from 1470 MPa to 1779 MPa,and the compression rate increased from 19.4% to 26.5%.