三种典型商用2xxx系铝合金搅拌摩擦焊接头的组织与性能研究

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	三种典型商用2xxx系铝合金搅拌摩擦焊接头的组织与性能研究
	张振
学位类型	博士
导师	马宗义 ; 肖伯律
	2012
学位授予单位	中国科学院金属研究所
学位授予地点	北京
学位专业	材料加工工程
关键词	搅拌摩擦焊热处理工艺铝合金材料流变机理组织力学性能 Friction Stir Welding Aluminum Alloy Heat Treatment Material Flow Behavior Microstructure Mechanical Property
摘要	搅拌摩擦焊接 (FSW) 是一种新型的固相连接技术，特别适用于采用传统熔焊方法无法焊接的2xxx和7xxx系铝合金。本研究选择2024-T351 (Al-Cu-Mg)、2219-T6 (Al-Cu)和2014-T6 (Al-Cu-Mg-Mn-Si) 三种典型2xxx系时效强化铝合金，对时效强化铝合金FSW过程中的热点和一些难点问题进行了深入、细致的研究。主要目标是：揭示(a) FSW过程中材料流动特征；(b) FSW工艺参数、微观组织特征与力学性能之间的关系。主要开展了以下几方面的研究工作：首先，分别以2024铝合金的表面包铝层和对接面上的人工氧化物做为标记材料，采用搅拌针“急停”和工具拔出技术，通过调节转速、焊接速度和下压量，研究FSW过程中的材料流动特征。研究表明，接头上、中、下三部分的材料流动特征不同。轴肩主要控制接头顶部的材料流动，形成轴肩区；搅拌针主要控制中部的材料流动，形成搅拌针区；搅拌针底部材料发生了垂直向上的涡旋运动，称之为涡旋区。焊接参数对这三个区域的形状影响显著。200 rpm的低转速下，三个区域区分不明显；随着转速提高到400 rpm时，搅拌针区开始出现不完整的“洋葱环”结构；继续将转速提高到800 rpm时，焊核区组织明显分化，搅拌针区出现完整的“洋葱环”结构。“洋葱环”结构位于搅拌针区氧化物（“S”线）的前进侧。随着焊接速度的提高，轴肩区缩小，搅拌针区扩大。其次，对FSW 2024-T351铝合金接头，考察了焊接工艺参数和自然时效时间对接头组织特别是第二相偏析以及力学性能的影响。研究表明，接头的焊核区中存在很多由无数在晶界聚集的粗大第二相组成的线状偏析带，偏析带主要存在于轴肩区，与周期性的材料流动有关。它们在接头纵截面和水平面上的平均间距等于焊接工具每旋转一周的前进距离。接头的前进和后退侧各存在两个低硬度区：低硬度区I和II，其中，在低硬度区I晶粒发生粗化，GPB区溶解，形成S相并粗化；在焊后长时间（4-12周）自然时效过程中，低硬度区I的硬度保持稳定，低硬度区II的硬度缓慢恢复，随焊接参数和自然时效时间的不同，最低硬度区可能位于低硬度区I或II。拉伸过程中，接头沿最低硬度区或偏析带断裂，偏析带导致高焊接速度FSW接头沿焊核区/热机影响区界面断裂。拉伸强度随焊接速度的增加而升高，不随转速的增加而变化。接头的最高强度系数为94%。再次，对2219-T6铝合金，研究了焊接参数(工具转速、焊接速度、下压量和水冷)和焊后人工时效对FSW接头组织与性能的影响。研究发现，空冷焊接时，可在较宽的转速(400-800 rpm)和焊接速度(100-800 mm/min)范围内，取得致密无缺陷的FSW接头，1200-1600 rpm的较高转速容易导致隧道和空洞缺陷；水冷焊接时，也能在100-800 mm/min的较大焊接速度范围内获得无缺陷的焊接接头；适当增加下压量可减少焊接缺陷的产生。在水冷和空冷接头的前进和后退侧各出现了以一个沉淀相溶解/粗化为特点的最低硬度区，拉伸过程中，铝合金接头可能沿焊核区/热机影响区界面、热机影响区或热影响区断裂，其拉伸强度随焊接速度从100增加到800 mm/min而升高，不随转速的增加而变化。空冷接头的最高强度系数为79%，水冷没有提升接头最低硬度区的硬度和拉伸强度。焊后人工时效并不提升100 mm/min的低焊接速度接头的最低硬度区硬度和拉伸强度，但大幅提高了400和800 mm/min的高焊接速度接头的最低硬度区硬度和拉伸强度。最后，研究了焊接后热处理和水冷对FSW 2014-T6铝合金接头组织与性能的影响。研究发现，接头的前进和后退侧也各出现了一个最低硬度区，焊后人工时效对FSW 2014-T6铝合金接头的力学性能没有影响，而焊后T6热处理可将接头将各区域的硬度恢复至母材水平，但是会导致焊核区发生晶粒异常长大或沿“S”线开裂，损害接头的力学性能。与2219铝合金一样，水冷不能提高FSW 2014-T6铝合金接头的拉伸强度。
其他摘要	" Friction stir welding (FSW) is a relatively new solid state welding technique. It is very suitable for joining aluminium alloys, especially for those usually considered unweldable such as 2xxx and 7xxx series alumium alloys. In the present study, three typical 2xxx series alumium alloys, 2024-T351 (Al-Cu-Mg)、2219-T6 (Al-Cu) and 2014-T6 (Al-Cu-Mg-Mn-Si), were subjected to FSW, and the effects of FSW parameters, water cooling and post-weld heat treatment on the microstructure and mechanical properties of the joints were investigated in detail. The aim of this work is to obtain some insights into (a) material flow during FSW and (b) relationship between the parameters, microstructure characters and mechanical properties of FSW alumium alloys. Firstly, the material flow behavior during FSW was investigated using Alclad layer on the surfaces of 2024Al alloy and the man-made oxidation film on the butting surfaces as mareker material, as well as “stop action weld” and “tool extraction weld” techniques. It was found that the material flow in different regions of the nugget zone was different. The shoulder drove the material to form a shoulder driven zone (SDZ) in the upper region of the workpiece. In the lower region, the pin drove the material to form a pin driven zone (PDZ). A vertically swirl movement occurred beneath the pin, forming a swirl zone (SWZ). The sizes of the three sub-zones were dependent on the welding parameters. The three sub-zones could not be distinguished at a low rotation rate of 200 rpm and were distinguishable at a midlle rotation rate of 400 rpm and were easily distinguishable at a high rotation rate of 800 rpm. With increasing the welding speed, the SDZ shrunk and PDZ enlarged. Secondly, the effects of welding parameters and natural aging time on the microstructure, especially the segregetation of second-phase particle, and mechanical properties of 5 mm thick FSW 2024Al-T351 joints were investigated. In SDZ, the periodic material flow resulted in segregation bands that consisted of countless segregation phases at the grain boundaries. The average spacing of the segregation bands on the longitudinal and horizontal cross sections was equal to tool advancement per revolution, that is, the ratio of welding speed to rotation rate (V/R). In HAZ, FSW thermal cycle resulted in two low hardness zones (LHZs): LHZ I with the grain coarsening and dissolution of GPB (Guinier-Preston-Bagaryatsky) zones, as well as the formation and coarsening of S (Al2CuMg) phases; and LHZ II with only the complete dissolution of GPB zones. After 4-12 months of natural aging, the hardness was essentially unchanged at LHZ I but recovered at LHZ II. The lowest hardness distribution zone was located in either LHZ I or LHZ II. The FSW joints fractured along LHZs or the segregation bands. The segregation bands resulted in unusual fracture behavior along the segregation bands in the FSW joints obtained under high welding speed. The tensile strength of the FSW joints increased with increasing the welding speed and could also be slightly enhanced by the long-term post-weld natural aging, but was independent of the rotation rate. The maximum strength coefficient of FSW joint was 94%. Thirdly, the effect of FSW parameters (rotation rate, welding speed, plunge depth, and water cooling) and post-weld artifical aging heat treatment on the microstructure and mechanical properties of 5.6 mm thick 2219Al-T6 joints was investigated in detail. Under air cooling condition, the sound FSW joints could be obtained under lower rotation rates of 400-800 rpm and welding speeds of 100-800 mm/min; higher rotation rates of 1200-1600 rpm easily led to the tunnel and void defects. Under water cooling condition, the sound FSW joints could be obtained under welding speeds of 100-800 mm/min and the rotation rate of 800 rpm. For both air and water cooled FSW joints, the FSW thermal cycle resulted ina low hardness zone (LHZ) on both retreating side (RS) and advancing side (AS). The LHZs may be located at the interface between the nugget zone (NZ) and the thermo-mechanically affected zone (TMAZ), at the TMAZ, or at the heat affected zone (HAZ) under the varied welding parameters. The tensile strength of FSW 2219Al-T6 joints increased when increasing the welding speed from 100 to 800 mm/min, and was weakly dependent on the rotation rates. The FSW 2219Al-T6 joints fractured along the LHZs on the RS. The maximum strength coefficient of FSW joint was 79%. Water cooling did not enhance the tensile strength of FSW 2219Al-T6 joints. The post artificial aging did not enhance the the tensile strength of FSW 2219Al-T6 joints obtained under the low welding speed but largely increased the the tensile strength of FSW 2219Al-T6 joints obtained under the high welding speed. Finally, the influence of post weld artificial aging, T6 treatments and water cooling on the microstructure and mechanical properties of 5-6 mm thick FSW 2014Al-T6 joints were investigated at the welding speeds of 100-800 mm/min and the rotation rates of 400-800 rpm. It was found that artificial aging did not alter the grain structure but T6 heat treatment caused the abnormal grain growth at the nugget zone. The tensile strength of the joints could not be enhanced by the artificial aging treatment but were improved by the T6 treatment. The effectiveness of T6 treatment is related with the distribution of “S” line, Water cooling did not enhance the tensile strength of FSW 2014Al-T6 joints."
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64479
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	张振. 三种典型商用2xxx系铝合金搅拌摩擦焊接头的组织与性能研究[D]. 北京. 中国科学院金属研究所,2012.