Hot Gas Forming And Microstructure-Performance Control Of Ti-22Al-24Nb-0.5Mo Alloy Square-Section Tubular Part

With the rapid development of aerospace industry,the aircrafts with higher speed and the engine with larger thrust-to-weight ratio are requested for research.Thus a higher requirement for structural materials on heat-resistant and lightweight becomes increasingly urgent.In this case,Ti₂AlNb-based alloys combined with the excellent performance of high specific strength,good oxidation resistance and sufficient creep resistance at elevated temperatures,have great potential in applying in the field of aerospace.Ti₂AlNb-based alloys have multiple phase transformations and compositions,resulting in abundant microstructure morphologies.Moreover,mechanical properties of the Ti₂AlNb-based alloys were very sensitive to the microstructures,which were determined by the thermal processing and heat treatment history.To achieve the relationship between microstructure and mechanical properties,the effects of microstructural type and O-morphologies on mechanical properties were studied in this paper.Besides,the hot gas forming and followed ageing treatment was systematicly investigated to fabricate Ti-22Al-24Nb-0.5Mo alloy quadrate-section tubular parts with superior mechanical properties.At last,the hot gas forming-in situ heat treatment process was designed to achieve the integrated shape-forming and performance-controlling for the Ti-22Al-24Nb-0.5Mo alloy tubular parts.Ti-22Al-24Nb-0.5Mo hot-rolled sheet with a thickness of 2.0 mm was adopted in this paper.The microstructure and mechanical property of the initial sheet were dectected by SEM,EBSD,TEM and tensile test at high temperature.Three typical microstructure types for the Ti-22Al-24Nb-0.5Mo alloy,i.e.equixed microstructure,duplex microstructure,lamellar microstructure,were obtained through different heat treatment.The corresponding effect on tensile property and creep property at 750°C was investigated.The duplex microstructure consists of the equixed?₂ which can prevent the grougth of?/B2 grains and the lamellar O phase which can resist the dislocation motion,resulting in good tensile property and creep resistance.To furtherly investigate the effect of O-morphologies?phase content,average width and average aspect-ratio?on mechanical property,the duplex microstructure with different O-morphologies was achieved by different ageing treatment on 970°C-solutioned Ti-22Al-24Nb-0.5Mo alloy.The O-morphologies were measured and counted.Then,the relationships of O-morphologies versus ageing temperature and ageing time were established.The phase volume fraction was dependent on the ageing temperature;The average width was increased with the increasing of ageing temperature and ageing time;While the average length-width ratio was decreased with the increasing of ageing temperature and ageing time.The room temperature hardness and 750°C tensile test were performed for the alloy with different O-morphologies and the relationships of O-morphologies versus mechanical property were also established.The higher volume fraction,finer width and higher length-width ratio is facilitated for the improve of room temperature hardness;While the lower volume fraction,finer width and higher length-width ratio is facilitated for the improve of yield strength at 750°C?In order to obtain the microstructure with excellent formability,the effect of heat treatment on formability of Ti-22Al-24Nb-0.5Mo alloy was studied.Additionally,the tensile tests at 950°C-990°C were conducted on the Ti-22Al-24Nb-0.5Mo alloy with excellent formability.The high-temperature defoamation behavior and the evolution of microstructure and mechanical property were investigated.Moreover,the tube billets were fabricated by U-O forming and laser-beam welding.In order to study the hot forming characteristics of compoents with irregular-section,the dies applied in hot gas forming of quadrate tube were designed.Besides,the effects of gas forming paramters on wall-thickness,corner size,microstructure evolution and mechancial property were investigatted.After forming at 970°C-990°C,the Ti-22Al-24Nb-0.5Mo alloy quadrate tubes with relative uniform wall-thickness and equixed microstructure were obtained.The post ageing treatment was performed to gain the duplex microstructure.After aging at 800°C,the elongation at fracture at 750°C was158.1%higher than that in the quenched state,which exhibited superior comprehensive mechanical properties.Based on the conventional hot gas forming and post ageing treatment,the gas forming-in situ heat treatment process with short process and low energy consumption was designed and the effect of airflow pressure on microstructure evolution and mechanical property was investigated.During the airflow cooling,the O phase with high aspect-ratio was precipated and the O-morphologies were dependent on the cooling speed.As the airflow pressure decreased,the cooling speed of the quadrate tube gradually reduced,the phase vollume fraction and average width increased,but the aspect-ratio decresed.Acorrdingly,the microhardness at room temperature decreased but the elongation at 750°C increased.The O-morphologies can be controlled by airflow pressure,thereby the integrated hot gas forming-in situ heat treatment process can realize the goal of controlling the shape and performance simultaneously for the Ti-22Al-24Nb-0.5Mo alloy components.