不同热处理工艺下增材制造TC17钛合金组织与摩擦学性能研究

Microstructure and Tribological Properties of Additively Manufactured TC17 Titanium Alloy Under Different Heat Treatment Processes

研究了不同热处理工艺对定向能量沉积TC17钛合金微观组织与摩擦学性能的影响,分析了热处理过程中的组织演变规律与20 N干滑动磨损下的综合磨损行为。结果表明,在退火与退火后固溶处理阶段,合金主要由初生α相及连续或不连续的晶界α相、β相组成,无相析出区伴随连续晶界α相存在。在退火后固溶处理的基础上进一步进行时效处理,针状次生α相开始析出,且随着时效温度的升高,初生与次生α相开始生长、粗化,无相析出区先消失后出现。硬度与磨损测试结果表明,经580℃时效处理后显微硬度最高(486.93 HV),此时有较强的弥散强化效果,摩擦学性能最优,磨损率仅为0.0451 mg/m。热处理后多种磨损机制混合存在,磨损率、磨损形貌取决于微观结构与氧化层的变化。研究结果为优化TC17合金摩擦学性能与热处理工艺提供了参考。

Objective The TC17 alloy has excellent mechanical strength,fracture toughness,and corrosion resistance and is primarily used in the manufacturing of aero engine disk components.Directed energy deposition(DED)can achieve specific location repairs of damaged parts and 3D printing of complex and large parts.DED of TC17 alloy has been successfully applied to such parts as integral blade disks.However,the low hardness and poor wear resistance of TC17 alloy make it susceptible to fatigue fracture under the harsh use environment of aero engines,which limits its application in the aerospace field.At present,the problems of poor bonding strength,introduction of new defects,and inability to meet various performance requirements in the spraying of hard coatings on titanium alloy surfaces present potential risks and limitations in aerospace applications.Optimized heat treatment has unique advantages in solving these problems.Optimized heat treatment can adjust material properties by adjusting the microstructure.Several studies have been conducted on improving the tensile properties of TC17 by optimizing the heat treatment process,but further research is needed on whether optimized heat treatments can improve the wear resistance of deposited TC17 alloy.Therefore,in this study,the effects of annealing,post-annealing solid solution,and post-annealing solid solution aging treatments on the microstructure,hardness,and tribological properties of deposited TC17 alloy are investigated.The evolution of the structure during heat treatment and the comprehensive wear behavior under 20 N dry sliding wear are analyzed.The results provide a reference for optimizing the tribological properties and heat treatment process of TC17 alloy.Methods The experimental material is TC17 spherical powder,with an average particle size of 66.6μm(Fig.1).The experiment uses the semiconductor laser to generate lasers,and the robot,equipped with coaxial powder feeding,deposits the TC17 powder on the polished TC4 substrate in the argon environment(Fig.2).The process parameters are optimized:laser power of 1600 W,scanning speed of 10 mm/s,powder feeding rate of 11 g/min,overlap rate of 45%,and center protection gas flow rate of 11 L/min.A TC17 alloy sample(size of 75 mm×35 mm×12 mm)is obtained using an N-type scanning path.Samples are taken along the direction of laser deposition,with a sample size of 6 mm×6 mm×6 mm.The TC17 deposition samples are subjected to pre-annealing,annealing solid solution treatment,and annealing solid solution aging treatment(Fig.3),and the microstructure and wear properties of the TC17 deposition and heat-treated samples are characterized by the X-ray diffraction(XRD),energy-dispersive X-ray spectroscope(EDS),scanning electron microscope(SEM),hardness tester,and pin-disk-type friction wear testing machine.Results and Discussions The experimental results indicate that the deposited TC17 alloy consists ofαandβphases,displaying a basketweave structure(Fig.6).After annealing at 840℃,some of the fine α phase dissolves because of the high temperature.After solid solution treatment at 800℃,the primary α phase(α_(P))in the grain interior gradually flattens.During the annealing and postannealing solid solution treatment stages(Fig.7),affected by the diffusion rates of different elements,the grain boundary α phase(α_(GB))is divided into continuousαGB and discontinuousαGB.A phase-free zone(PFZ)appears around the continuous α_(GB) because of the insufficient concentration ofαstabilizing elements at the low-angle grain boundary.After aging at 580℃ based on the post-annealing solution treatment,a large amount of fine needle-like secondary α phase(α_(S))precipitates,and the PFZ disappears.When the temperature rises to 630℃,some of the ultrafineα_(S) redissolves in the β matrix.After aging at 680℃,PFZ reappears with only partial coarsening α_(S) interspersed between α_(P)(Fig.9).Theα_(S) precipitates inside theβgrains,and the randomness of orientation makes the size and quantity ofα_(S) very sensitive to changes in aging temperature.After solution treatment,the average microhardness reaches 425.45 HV,which is higher than the hardness in the as-deposited state(Fig.12).This is attributed to the precipitation and growth ofαP,which increase the volume fraction of the phase,thereby improving the microhardness.After aging treatment,the hardness is further increased,reaching its highest value(~486.93 HV)after aging at 580℃.This is caused by the large amount ofα_(S) precipitation,which achieves the strongest dispersion-strengthening effect.The wear test results show that the wear properties of the heat treatment state are superior to those of the deposition state(Fig.13).Table 3 shows the maximum wear widths and depths in different states.The tribological properties are the best after aging at 580℃,which is attributed to the significant increase in hardness and a large number of secondary phases inhibiting dislocation movement and crack expansion.After heat treatment,a variety of wear mechanisms coexist,and the wear rate and wear morphology depend on the changes in the microstructure and oxide layer.Conclusions A TC17 sample is prepared by DED and then heat-treated.The changes in phase composition,microstructure,microhardness,and tribological properties during heat treatment are studied.The results show that the main microstructure evolution of TC17 alloy during heat treatment includes the growth and coarsening of α_(P);the continuous and discontinuous growth of α_(GB),where the width of PFZ is positively related to the continuous growth of α_(GB);and the precipitation and growth of α_(S),where,as the aging temperature increases,someα_(S) dissolves and some grows to have a clear phase boundary with the β phase.The hardness after heat treatment is higher than that of the deposited state.In the solid solution stage after annealing,the hardness increases with the increase inαphase volume fraction.After further aging,the precipitation of α_(S) achieves dispersion strengthening,and the strengthening effect weakens with the dissolution of α_(S).The hardening effect is higher at 580℃,and the hardness is increased by 20.1%compared with that of deposition state.The tribological performance after heat treatment is better than that in the deposition state.The increase in hardness,secondary phase precipitation,and hard oxide formation are the main reasons for the improvement of the tribological properties.Optimal wear resistance is achieved in the heat treatment systems of 840℃/1 h,air cooling+800℃/4 h,water quenching+580℃/8 h,and air cooling,with a friction coefficient(wear rate)of 0.422(0.0451 mg/m).