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金属学报  2017, Vol. 53 Issue (3): 358-368    DOI: 10.11900/0412.1961.2016.00311
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2219铝合金搅拌摩擦焊中的局部液化现象及对接头力学性能的影响
康举1,2,梁苏莹3,吴爱萍1(),李权1,4,王国庆5
1 清华大学机械工程系摩擦学国家重点实验室 北京 100084
2 华北电力科学研究院有限责任公司 国网冀北电力有限公司电力科学研究院 北京 100045
3 首钢工学院 北京 100144
4 首都航天机械公司 北京 100076
5 中国运载火箭技术研究院 北京 100076
Local Liquation Phenomenon and Its Effect on Mechanical Properties of Joint in Friction Stir Welded 2219 Al Alloy
Ju KANG1,2,Suying LIANG3,Aiping WU1(),Quan LI1,4,Guoqing WANG5
1 State Key Laboratory of Tribology, Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China
2 State Grid Jibei Electric Power Co. Ltd. Research Institute,North China Electric Power Research Institute Co. Ltd., Beijing 100045, China
3 Shougang Institute of Technology, Beijing 100144, China
4 Capital Aerospace Machinery Company, Beijing 100076, China
5 China Academy of Launch Vehicle Technology, Beijing 100076, China
引用本文:

康举,梁苏莹,吴爱萍,李权,王国庆. 2219铝合金搅拌摩擦焊中的局部液化现象及对接头力学性能的影响[J]. 金属学报, 2017, 53(3): 358-368.
Ju KANG, Suying LIANG, Aiping WU, Quan LI, Guoqing WANG. Local Liquation Phenomenon and Its Effect on Mechanical Properties of Joint in Friction Stir Welded 2219 Al Alloy[J]. Acta Metall Sin, 2017, 53(3): 358-368.

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摘要: 

在2219-T8铝合金搅拌摩擦焊(FSW)接头焊核区(NZ)发现了局部液化现象。采用热处理和热模拟2种方法对比分析了局部液化区组织的特点,采用显微硬度计和原位拉伸实验研究了局部液化对接头力学性能的影响。结果表明,局部液化是由NZ中的Al2Cu相与周围基体的共晶反应,即发生了组分液化所致。液化区为液相+α(Al)相的半固态组织,冷却时高熔点组元α(Al)相先析出,同时在搅拌挤压和材料流动的作用下发生固液分离,形成离异共晶。液化区的硬度低于NZ正常区域,是NZ微区试样断裂的起源,降低了NZ的抗拉强度和延伸率。不过,由于液化区域是局部的、极少量的存在于NZ中,其对接头拉伸性能的影响远小于整体均发生弱化的热机械影响(TMAZ)的影响。

关键词 2219-T8铝合金搅拌摩擦焊局部液化原位拉伸力学性能    
Abstract

Al alloy 2219 (AA2219) exhibits excellent mechanical properties in a wide temperature range from -250 ℃ to 250 °C, indicating great potential for application in aerospace structures. Compared to fusion welding, friction stir welding (FSW) could significantly improve mechanical properties of the AA2219 joints. Since invented by the welding institute (TWI) of UK in 1991, FSW has been treated as a solid-state joining technique by the commercial companies, which has been in an agreement in most scientific researchers. However, recently a controversy that has been raised over the viewpoint that FSW is a strict solid-state process, and some observations of liquation have been reported, especially in the stir zone of friction stir spot welding (FSSW) joint. However, the phenomenon of liquation in FSW AA2219 joints has not been reported previously. Therefore, the aim of this work is to reveal the evidence of local liquation during FSW AA2219-T8 and its effect on mechanical properties of the joints. In this work, AA2219-T8 plates (8 mm thick) were friction stir welded at a welding speed of 180 mm/min and a rotation speed of 800 r/min using a welding tool with threaded pin. Heat treatment and thermal simulation experiments were carried out to contrast the characteristics of the local liquation regions. A Vickers microhardness testing machine and an in situ SEM imaging tensile test facility were employed to study the effect of local liquation on mechanical properties of the joints. The results showed that the microstructures in the local liquation regions were divorced eutectic, and its formation was related to the coupled thermal-mechanical interaction during the FSW process. In the FSW process, the local high temperature led to constitutional liquation. During the cooling period, the semisolid mixtures decomposed into α(Al) matrices and θ (Al2Cu) particles under stir and material flow actions. The liquation regions had a lower value of hardness than the normal regions in the nugget zone (NZ), making the liquidation region susceptible to cracks initiation and decreasing the ultimate tensile strength and elongation for a local liquation region contained NZ sample. However, the negative effect of local liquation regions on the mechanical properties of the FSW AA2219-T8 joint was less than that of the thermo-mechanically affected zone (TMAZ), since the local liquation regions were only localized and tiny fractions being in the NZ, whereas the TMAZ was whole softened.

Key words2219-T8 Al alloy    friction stir welding (FSW)    local liquation    in situ tensile testing    mechanical property
收稿日期: 2016-07-18     
图1  接头中取样位置示意图
图2  SEM原位观察拉伸试样No.1尺寸(No.2试样平行段长度为10 mm)
图3  2219-T8铝合金FSW接头BM和NZ中的第二相粒子形貌
图4  2219-T8铝合金FSW接头NZ中的局部液化现象
图5  局部液化区域在NZ次表层中的OM暗场像
图6  2219-T8铝合金经不同热处理条件后的凝固组织形貌
图7  NZ正常试样(NZ-1)和含局部液化组织试样(NZ-2)的工程应力-应变曲线
图8  NZ-1和NZ-2试样的起裂位置
图9  NZ疲劳试样裂纹的萌生和扩展行为
图10  FSW-Joint试样的工程应力-应变曲线和拉伸过程中的裂纹演变
图11  NZ-1和NZ-2的断口形貌
图12  疲劳试样断口形貌
图13  NZ-1、NZ-2和FSW-Joint试样的抗拉强度和延伸率
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