热处理对激光沉积制造TA15/Ti_(2)AlNb组织和性能的影响

Microstructure and Properties of Laser Deposition TA15/Ti_(2)AlNb with Heat Treatment

应用激光沉积制造(LDM)技术制备TA15/Ti_(2)AlNb复合结构,对试样进行多种热处理,并观察其显微组织,测试其拉伸性能,分析其断口形貌和显微硬度。结果表明,TA15侧组织进行去应力退火后,α相逐渐增大并彼此截断,产生短小棒状α相;当退火温度达到900℃时,组织为较均匀的网篮组织;固溶时效处理后,出现部分粗化的α相与细长的α相。Ti_(2)AlNb侧随着退火温度的升高,O相逐步转变为B2相和α_(2)相;固溶时效处理后,O相较为充分地析出。过渡区界面在固溶时效处理后基本消失。TA15/Ti_(2)AlNb抗拉强度在热处理后有所降低,但塑性较好;沉积态拉伸试样断裂机制为解理断裂,退火态、固溶时效态拉伸试样断裂机制为半解理半韧性断裂;断裂位置均在过渡界面附近。TA15侧显微硬度高于Ti_(2)AlNb侧,950℃×1 h空冷+800℃×4 h空冷固溶时效处理后两侧显微硬度差异最小。

Laser deposition manufacturing(LDM)technology is a digital advanced manufacturing technology emerged in recent years.Titanium alloy materials prepared by this technology are widely used in various fields.In the process of laser deposition manufacturing,problems such as deformation and cracking of the deposited sample may occur.Heat treatment is an effective means to control the microstructure and has a significant effect on improving the mechanical properties of the sample.This paper focused on the effects of nine heat treatment processes on the microstructure and mechanical properties of TA15/Ti_(2)AlNb direct transition titanium alloy samples,and the experimental results were compared and analyzed.The laser deposition manufacturing system used in this article consisted of semiconductor laser device,synchronous powder feeding system,four-way coaxial powder feeding nozzle,oxygen analyzer and focusing system and so on.The whole depositing process was carried out under an argon atmosphere.The vacuum-dried TA15 and Ti_(2)AlNb titanium alloy powders were used and TA15 plates were used as the substrates.First,TA15 powders were deposited on the substrate along the z-axis direction,and then Ti_(2)AlNb powders were deposited.After the depositions were completed,heat treatments were performed and corresponding specimens were prepared.Optical microscope(OM)was used to observe the microstructure of the samples after different heat treatments,the electronic universal testing machine was used to stretch the tensile specimens at room temperature,scanning electron microscope(SEM)was used to observe the fracture morphology and the microhardness tester was used to test the microhardness of samples after different heat treatments.After the stress relief annealing,αphase gradually increased and cut off each other on the TA15 side,producing short rod-shapedαphases.As the annealing temperature increased,the length-width ratio ofαphase increased,andαphases in different directions crossed and cut off each other,showing basket-like microstructure.When the annealing temperature reached 900℃,the microstructure of TA15 side was relatively uniform.After solution aging treatment,a part of coarseαphases and slenderαphases appeared in the microstructure of TA15 side.After the stress relief annealing,the light gray B2 phases were heated and theα_(2)phases were precipitated along the grain boundary on Ti_(2)AlNb side.with the increasing of the annealing temperature,O phases gradually transformed into B2 phases andα_(2)phases.After the solution aging treatment,O phases precipitated again and on Ti_(2)AlNb side O phases andα_(2)phases were uniformly distributed in the matrix B2 phases.The transition interface was very obvious in the as-deposited state.As the annealing temperature increased,the originally clear transition interface became more and more insignificant,and the transition interface gradually disappeared after solution aging treatment.The tensile fractures were analyzed by energy spectrometer,and the fracture position of the TA15/Ti_(2)AlNb samples in the as-deposited,annealed,and solution-aged states were all near the transition interface.Compared with the as-deposited state,the tensile strength of TA15/Ti_(2)Al-Nb tended to decrease after annealing or solution aging heat treatment,but the plasticity tended to increase.The as-deposited fractures had river-like patterns and cleavage steps,so they were cleavage fractures.The annealed and solution-aged tensile fractures had both dimples and cleavage steps,so the fracture mechanisms belonged to semi-cleavage and semi-ductile fractures.The microhardness of TA15 side was higher than that of Ti_(2)AlNb side.After 950℃×1 h air cooling and 800℃×4 h air cooling treatment,the difference of microhardness between the two sides was the smallest and the microstructure of the transition zone was also relatively more uniform.The heat treatment could effectively improve the microstructure of the transition interface of the TA15/Ti_(2)AlNb deposited samples,making the structures of both sides more uniform and increasing the plasticity without significantly reducing the strength of the samples.So,by proper heat treatment the TA15/Ti_(2)AlNb deposited samples could get a better comprehensive performance.The fracture position of each tensile sample was near the transition interface,resulted in the great property difference of these two alloys of TA15 and Ti_(2)AlNb.In the future,much effort should be done on the optimization of laser manufacturing process parameters and the design of gradient transition layers of TA15/Ti_(2)AlNb composite structure,in order to obtain better properties.