邓彩艳 ,李建宏 ,龚宝明 ,王东坡,许全军
AuthorsHTML:邓彩艳 ^{1, 2}，李建宏 ¹ ，龚宝明 ^{1, 2}，王东坡 ^{1, 2}，许全军 ¹
AuthorsListE:Deng Caiyan,Li Jianhong,Gong Baoming,Wang Dongpo,Xu Quanjun
AuthorsHTMLE:Deng Caiyan^{1, 2}，Li Jianhong¹，Gong Baoming^{1, 2}，Wang Dongpo^{1, 2}，Xu Quanjun¹
Unit:1. 天津大学材料科学与工程学院，天津 300350；
2. 天津市现代连接技术重点实验室，天津 300350

Unit_EngLish:1. School of Materials Science and Engineering，Tianjin University，Tianjin 300350，China；
2. Tianjin Key Laboratory of Advanced Joining Technology，Tianjin 300350，China

Abstract_Chinese:采用 USF-300 超声疲劳试验机对焊态和热处理态的 7050-T7451 铝合金搅拌摩擦焊(FSW)接头进行超高周 疲劳试验，通过扫描电镜(SEM)、电子背散射衍射(EBSD)、透射(TEM)等手段对接头微观组织和疲劳断口进行观 察，研究焊后热处理(PWHT)对 7050-T7451 铝合金搅拌摩擦焊接头超高周疲劳行为的影响．结果表明：当循环次数 为 1×109 时，热处理态的疲劳强度比焊态提升了 25%，并且随着应力水平的降低提升的幅度越大．焊态接头疲劳试 件的失效位置大多位于热影响区，而热处理态试件大部分断裂于搅拌区，试件的断裂位置与疲劳裂纹萌生机制有 关．对于焊态接头试件，疲劳裂纹起源于表面粗大第二相处；而对于热处理态接头，搅拌区内部异常长大的晶粒是 主要的疲劳裂纹源．热处理后热影响区内部粗化沉淀相含量减少、晶界无析出带的宽度减小，使得热影响区的薄弱 属性得到改善，而搅拌区内细小等轴晶的异常长大降低了搅拌区的疲劳强度，这也是焊态和热处理态接头超高周疲 劳行为出现差异的主要原因．
Abstract_English:welding(FSW)joints were performed using USF-300 ultrasonic fatigue testing machine. The influence of post weld heat treatment(PWHT)on VHCF behaviors of 7050-T7451 aluminium alloy friction stir welded joints was investigated by scanning electron microscopy(SEM)，electron backscattered diffraction(EBSD)and transmission electron microscopy(TEM). Results reveal a 25% increase of the fatigue strength of the heat-treated joints compared with that of the welded joints at 1×109 cycles. Moreover，the rate of increase becomes higher as the stress level decreases. The fatigue crack mostly initiates at the heat affected zone(HAZ)of the welded joints，while most of the heat-treated joints are broken at the stir zone. Fracture locations of the specimens are mainly related to the fatigue crack initiation mechanism. Fatigue cracks originate from the coarse secondary phases on the surface of the welded joints while fracture mainly initiates from abnormally grown grains at the stir zone for the heat-treated joints. After heat treatment， reduced number of coarse precipitated phases and decreased width of precipitate-free zone in the grain boundary are observed. These signify improved properties of the HAZ. The weakening of the fatigue strength of the stir zone is attributed to the abnormal growth of fine equiaxed crystals. This is also found to be the main reason for the difference in VHCF behaviors between the welded and heat-treated joints.
Keyword_Chinese:7050-T7451 铝合金；搅拌摩擦焊；超高周疲劳；焊后热处理
Keywords_English:7050-T7451 aluminium alloy；friction stir welding；very high cycle fatigue；post weld heat treatment

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