摘要
采用镦制-热处理-磨削-滚压等主要工序制备了Ti-6Al-4V合金螺栓,利用扫描电子显微镜(SEM)和显微硬度计对滚压前后样品进行了显微组织观察和显微硬度测试,通过电子背散射衍射(EBSD)分析了滚压对初生α相晶粒取向的影响,研究了滚压过程中组织演变规律及其与性能之间的关系。结果表明:固溶时效态显微组织由大约30%的等轴初生α相和马氏体构成。经滚压处理后,初生α相晶粒被显著拉长,晶粒之间的排布也更加紧密,并形成晶粒流线。变形过程使得原来具有任意取向的α晶粒位向发生转动,并趋于与自身变形方向保持一致。由于牙底受到的压应力大于牙侧,导致牙底晶粒的畸变程度和变形层厚度更大,其中牙底变形层厚度为112.7μm,牙侧为99μm。固溶时效处理后样品的显微硬度从HV 330~355提升至最高HV 387.7,滚压处理大幅提升了牙底的显微硬度(HV 388.5),但牙侧硬度则有小幅下降(HV 367.3)。
As an α +β alloy,Ti-6Al-4V has become the most widely used titanium alloy in the aerospace fastener industry in recent decades,due to its excellent strength and ductility combination.However,with the rapid development of the aviation industry,high performance bolt fasteners are required.Compared with the method of cutting thread,rolling is more and more widely used due to its high material utilization and fatigue strength.Ti-6Al-4V alloy bolts were prepared by the following main processes:upsetting-heat treatment-grinding-thread rolling in this paper.The microstructure and microhardness of the samples before and after rolling were observed by scanning electron microscope(SEM)and tested by microhardness indenter,respectively.The effect of rolling on crystal orientation of primary α phase was analyzed by electron backscatter diffraction(EBSD).The microstructure evolution during thread rolling and the relationship between microstructure and properties were also studied.The results showed that the cross-section microstructure of the raw material was composed of equiaxed α phase and a small amount of β phase.From the edge to the center,the size of α phase increased from 1.5 to 2.5μm,while the fraction decreased from 91.32%to 88.16%.The microstructure of the longitudinal section was similar to that of the cross section,whereas the grain size and proportion of α phase were about 2μm and 88.66%,respectively.It was worth noting that the distribution ofβphase in the longitudinal section had changed significantly,which was transformed from annular distribution to layered distribution,and the grain orientation between the layers was almost the same.After 960℃/1 h,water quenching(WQ)+482℃/4 h,air cooling(AC)solid solution and aging treatment,the cross-section microstructure of the sample changed into equiaxed primary α phase and transformed β structure.The martensite was the main constituent phase in the transformed β structure,which could be divided into the following two types according to their morphology and initiation site:one was the lath martensite initiated from the high energy grain boundary and grew throughout the whole grain;another was the acicular martensite with a basketweave distribution in the crystal.Due to the different cooling rate in different positions,the size of martensite formed in the center was larger than the edge.Primary α phase of the edge region was uniformly dispersed in the matrix,the grain size was about 5μm,and the volume fraction was 33.75%.From the edge to the center,the volume fraction of primary α phase decreased slightly,while the grain size tended to increase.As for the longitudinal section,the microstructure was similar to the cross section,but the distribution of α phase in the matrix became inhomogeneous.After thread rolling treatment,the volume of the surface metal was redistributed due to the extrusion of the thread rolling die.Based on the law of minimum resistance,the primary α grains would flow and deform along the direction of minimum resistance(the root was deformed to both sides,while the flank was deformed in the direction of the gear profile of thread rolling die).With the increase of deformation,equiaxed α grains were gradually elongated and the arrangement of grains became closer to each other,eventually the grain flow lines were formed.During the deformation process,the α grains with arbitrary orientation started to rotate and gradually tended to be consistent with their own deformation direction.Since the compressive stress at the root was greater than that of flank,the degree of grain distortion and the thickness of deformation layer were also larger for the root structure,among which the thickness of deformed layer of the root and flank were 112.7 and 99μm,respectively.Due to the finer grain size in the edge of original sample,the microhardness of the center(HV 330~340)was smaller than that of the edge(HV 340~355).After solid solution and aging treatment,a bimodal structure composed of equiaxed grains and martensite was obtained,which increased the microhardness of the center and edge of the sample to HV 360~375 and HV 380~390,respectively.Rolling treatment slightly improved the microhardness of the root(HV 388.5),but there was a slight decrease in the flank(HV 367.3).This may be caused by warm deformation at 400℃and a considerable deformation heating because of low thermal conductivity of titanium.During the plastic deformation process,most plastic deformation work was converted into heat energy,which further increased the temperature of the sample and led to the softening of bolt structure.Owing to the high compressive stress in the root,the strain hardening rate of the structure was higher than the softening rate caused by temperature rise,so the microhardness at this position was slightly improved;while the compressive stress in the flank was relatively small,and strain hardening rate of structure was less than the softening rate caused by temperature rise,which led to the decrease of microhardness.
作者
唐伟
余传魁
汪昌顺
吴琳琅
张涵
李成林
Tang Wei;Yu Chuankui;Wang Changshun;Wu Linlang;Zhang Han;Li Chenglin(Aerospace Precision Products Co.Ltd.,Tianjin 300300,China;Tianjin Key Laboratory of Fastening Joint Technology,Tianjin 300300,China;School of Power and Mechanical Engineering,Wuhan University,Wuhan 430072,China)
出处
《稀有金属》
EI
CAS
CSCD
北大核心
2023年第11期1486-1494,共9页
Chinese Journal of Rare Metals
基金
湖北省自然科学基金项目(2022CFB016)资助
关键词
钛合金
高强螺栓
组织演变
螺纹滚压
titanium alloy
high-strength fastener
microstructural evolution
thread rolling
