摘要
基于侧吹保护条件下的光纤激光非穿透深熔焊接工艺,模拟了构件的服役环境,对GH3128搭接接头进行了不同循环次数的900℃真空热处理,并对接头组织和显微硬度、室温与高温拉伸及蠕变拉伸性能进行了分析测试,通过分析组织与断口,对GH3128搭接接头的高温力学性能进行了评估。研究结果表明,与无热处理接头相比,热处理后的接头的室温与高温拉伸性能分别提升了35%和20%,蠕变性能大幅提高。接头组织分析结果表明:热处理后的接头枝状组织消熔,晶粒呈粗大等轴晶状态;各断口均呈现出“抛物线”状韧窝,开口方向与拉伸力方向一致,呈韧性断裂特征,且无显微裂纹产生;热处理次数对接头显微硬度、室温及高温拉伸性能的影响不大;随着热处理次数的增加,接头蠕变性能呈减小趋势。
Objective GH3128 has the benefits of strong heat, pressure, and corrosion resistance, and is extensively employed in high-temperature components of active thermal protection structures in the aerospace field. Laser welding is the primary manufacturing process of active thermal protection components, and the lap joint is the primary joint form. The thermal load condition of the active thermal protection components is harsh, which puts forward higher demands on the hightemperature mechanical properties of lap joints. In this study, in view of the joint forms and high-temperature short-time work characteristics of active thermal protection components, the microstructure and mechanical properties of GH3128 lap joints under different vacuum heat treatment conditions are investigated on the basis of fiber laser non-penetrating deep fusion welding process under the condition of side blowing protection.Methods The test materials are GH3128 plates with the solution state, the upper plates’ size is 200 mm×150 mm×1 mm, and the lower plates’ size is 200 mm×150 mm×2 mm. The light source is a fiber laser with a wavelength of 1060--1070 nm. The beam focusing parameter is Kf=8 mm. mrad, the transmission fiber core diameter is 200 μm, the output coupling collimator’s focal length is 200 mm, and the focusing lens’ focal length is 300 mm. High purity Ar gas is employed as the protection gas, and the flow rate is 8 L/min. The nozzle’s inner diameter is 8 mm, the phosgene spacing is 2 mm, the nozzle’s output length is 6 mm, and the protective gas output angle is 50°. The welding process parameters are as follows: the laser output power is 1500 W, the welding speed is 2 m/min, and the defocusing is 0. After welding, the mechanical properties of GH3128 lap joints are tested, and the GH3128 lap joints are subjected to 900 ℃ vacuum heat treatment with different cycles. The weld is corroded with aqua regia(6 m LHCl+2 m LHNO 3), cleaned with alcohol, and used as a metallographic sample. An optical microscope is employed to observe the weld morphology. The fracture and microstructure are observed using a scanning electron microscope. The weld hardness is tested using a microhardness tester with a load of 100 g and loading time of 15 s. The tensile testing machine is employed to test the tensile properties of each batch of samples at room temperature. The tensile testing machine is employed to test the weld’s tensile properties at 900 ℃. At 900 ℃, the equipment is employed to test the weld’s creep property. The tensile force is 800 N and the time is 6 h.Results and Discussions The joints are typical "nail head" weld shapes in deep fusion welding, and there are circular sporadic pores near the weld’s root at the joint surface’s lower part. The joint microstructures are primarily columnar crystals, symmetrically dispersed along the weld center line, and the growth direction is perpendicular to the fusion line(Fig. 4). The columnar crystals near the "nail head" at the upper part of the weld are longer compared with the columnar crystals at the joint surface and the lower weld. After vacuum heat treatments with different cycles, the weld joint’s grain size becomes larger and there are no microcracks around the porosity. The microhardness of lap joints under different heat treatment cycles is higher than that of the base metal, and the microhardnesses of the weld joints and base metal have no visible change(Fig. 6). The tensile properties demonstrate that the GH3128 lap joints’ tensile strength at high temperatures decreases by about 50% compared to that at the room temperature. After different heat treatments, the joint’s tensile strength at the room temperature increases by about 35%. The joint’s tensile strength increases by about 20% at 900 ℃(Fig. 7). Vacuum heat treatment can enhance the joints’ tensile strength, but the number of heat treatment cycles has little impact on the joints’ tensile strength at room temperature and high temperature. The creep property test findings demonstrate that the creep curve’s slope of joints without heat treatment is the largest. The slope of the joints’ curves after heat treatment increases slightly with the increase in numbers of heat treatment cycles, but it is far less than the slope of the specimen’s creep curves without heat treatment(Fig. 10). The microstructure of lap joint faces after heat treatments with different cycles is observed and examined. The findings demonstrate that the microstructure at the joints’ central position without heat treatment is primarily made of small columnar dendrites, while the columnar dendrite structure at both sides of the central line of the joints is slightly larger in size. After the first heat treatment, the grain boundaries of coarse equiaxed grains appears at each position of the joint interface, and the dendrite structure demonstrates the sign of melting. After heat treatments with five cycles, the columnar dendrite structure at each position of the interface melts within the equiaxed grain. After each heat treatment, there is no microcracks on the bonding surface(Fig. 12). After heat treatment, the joints’ coarse equiaxed grain may be the reason for enhancing of tensile and creep properties of joints at high temperatures.Conclusions Compared with the joints without heat treatment, the tensile properties of GH3128 lap joints at room temperature and high temperature increase by 35% and 20%. Simultaneously, the joints’ creep property is significantly enhanced, and the joints’ maximum creep strain decreases from 1.08%(without heat treatment) to 0.12%( after heat treatments with five cycles). The analysis of the joints’ microstructure demonstrates that the joints’ grain size becomes larger and the dendrite structure in the grain is gradually melted after heat treatments with five cycles. After heat treatments with five cycles, the joints’ microstructure is coarse equiaxed grain, and the dendrite structure is entirely melted. The joints’ fracture morphology demonstrates that there are "parabolic" dimples on each fracture, and the opening direction is consistent with the direction of the tensile force, demonstrating ductile fracture characteristics without microcracks. The number of heat treatment cycles has little impact on the microhardness, and tensile properties at room temperature and high temperature. With the increase in the number of heat treatment cycles, the joints’ creep properties decrease, and the joints’ maximum creep strain increases from 0.05%(after heat treatment with one cycle)to 0.12%(after heat treatments with five cycles).
作者
袁振飞
蒋劲
程智伟
杜欣
祁百鑫
武强
杨顺华
肖荣诗
Yuan Zhenfei;Jiang Jin;Cheng Zhiwei;Du Xin;Qi Baixin;Wu Qiang;Yang Shunhua;Xiao Rongshi(High-Power and Ultrafast Laser Manufacturing Laboratory,Faculty of Materials and Manufacturing,Beijing University of Technology,Beijing 100124,China;Science and Technology on Scramjet Laboratory,China Aerodynamics Research and Development Center,Mianyang 621000,Sichuan,China)
出处
《中国激光》
EI
CAS
CSCD
北大核心
2022年第21期164-172,共9页
Chinese Journal of Lasers
基金
国家自然科学基金面上项目(51775009)
高超声速冲压发动机技术重点实验室开放课题(STSMY-KFKT-2020003)。
