纯铜及铜-铝搅拌摩擦焊接头组织与性能研究

IMR OpenIR

	纯铜及铜-铝搅拌摩擦焊接头组织与性能研究
	薛鹏
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	硬态纯铜异种金属微观组织力学性能搅拌摩擦焊接/加工 Workhardened Pure Cu Dissimilar Metals Microstructure Mechanical Properties Friction Stir Welding/processing
摘要	"搅拌摩擦焊接(Friction stir welding, FSW)是针对焊接性差的高强铝合金开发的一种新型固相连接技术。自发明后，FSW技术很快在铝合金领域成功商业应用，并迅速推广到镁合金领域，成为目前两种金属材料焊接的首选工艺。然而，与低熔点的铝合金、镁合金相比，关于高熔点的铜合金以及异种金属之间FSW的研究还较少。本研究对纯铜和铜-铝异种金属的FSW工艺进行了系统研究，并对接头的微观组织和力学性能做了深入、细致的探讨。主要目标为：得到影响纯铜及铜-铝FSW接头质量的关键因素，揭示FSW纯铜接头热影响区及铜-铝接头界面区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：选取硬态纯铜作为研究对象，对获取高质量FSW接头的工艺参数进行了系统研究。研究表明，要获得与母材等强度的FSW接头，需要采用较低的转速，同时FSW过程中需要外加强制冷却，而流动冷却水比静止冷却水更为有效，为避免缺陷产生，程序控制轴肩压下量需要采用较大值。采用外加流动水强制冷却时，FSW硬态纯铜接头的力学性能明显提高，低硬度区宽度变窄，且最低硬度值明显提高。外加流动水强制冷却时，相同焊接参数下焊核区及热影响区的峰值温度和高温区停留时间都明显降低。在400-water参数下，接头热影响区峰值温度仅为130 oC左右，100 oC以上仅停留4 s，几乎不会造成接头热影响区软化，接头热影响区仍然保持母材原始的高位错密度，因此得到了等强度的FSW接头。选择小尺寸的轴肩，采用流动水强制冷却的工艺对纯铜单板进行FSW，研究焊核区，即搅拌摩擦加工(FSP)纯铜加工区超细晶组织的力学行为。FSP纯铜加工区各区域的微观组织基本相同，硬度分布均匀，可认为是均质的材料。FSP超细晶纯铜晶粒均呈现出等轴的再结晶状态，且位错密度较低，大多数晶界比较明锐、平直，在超细的晶粒内部有许多贯穿整个晶粒的退火孪晶界存在。FSP超细晶纯铜晶界取向差分布接近立方结构的随机分布，高角晶界比例较高，在80%-90%之间，且FSP超细晶纯铜的织构也非常弱。FSP超细晶纯铜的硬度和晶粒尺寸之间符合Hall-Petch关系，仅略高于粗晶纯铜的Hall-Petch关系边界线，这与FSP超细晶纯铜弱的织构和低的位错密度有关。相对于粗晶纯铜，FSP超细晶纯铜的强度，尤其是屈服强度有了明显的提高，且具有较高的拉伸塑性，达到了良好的强韧性匹配。通过增加铝含量降低层错能，在超细晶中引入大量的孪晶界、层错及纳米孪晶片层结构，可进一步提高FSP超细晶铜铝合金的强韧性匹配。选取不同厚度的纯铜、纯铝板材，采用对接和搭接两种焊接方式研究了影响FSW异种金属接头成形性及力学性能的关键因素，并对高质量FSW接头的微观组织和力学性能做了深入研究。对于5 mm厚纯铜-纯铝板的FSW对接焊，当纯铜板位于前进侧、搅拌针偏置量不小于2 mm时，可以得到无缺陷的接头。在此条件下，采用较低热输入参数时可获得良好的力学性能。调控FSW参数，避免缺陷产生，同时使Cu-Al界面生成一个均匀连续的金属间化合物(IMC)薄层是获得高质量FSW铜-铝异种金属接头的关键因素。最优参数下FSW接头力学性能优异，拉伸强度系数大于90%，接头可弯曲至180o不发生断裂。在低热输入参数600 rpm-50 mm/min下，采用直径8 mm的大尺寸搅拌针成功实现了FSW纯铜、纯铝的搭接焊。大的搭接面积以及Cu-Al界面上均匀连续的IMC薄层的生成使接头达到了良好的冶金结合，拉伸剪切测试时接头断裂在铝侧热影响区，最大载荷可达2680 N，超过纯铝母材的80%。3 mm厚纯铜-纯铝薄板FSW对焊的研究结果也证实，Cu-Al界面处生成一个均匀连续的IMC薄层是取得良好力学性能的关键。选取优化工艺参数下的FSW铜-铝接头，研究了退火过程中接头界面微观组织演化对力学性能的影响规律。接头初始界面存在一厚度约1 µm的薄层，此薄层可细分为不连续的α-Al固溶体层和两层连续的IMC层。靠近铝侧的IMC层为Al2Cu层，呈现出柱状晶组织；靠近铜侧的IMC层呈现出近似等轴状组织，主要由Al4Cu9和少量的AlCu组成。退火后，FSW铜-铝接头界面层不断增厚，界面上可观察到明显的三层结构，初始的α-Al固溶体层消失，在Al2Cu层与Al4Cu9层之间新形成了一个AlCu层。FSW铜-铝接头力学性能明显受界面IMC层厚度影响，当IMC层厚度大于2.5 µm时，接头开始在界面处断裂。此时，IMC层厚度较小时，接头断口呈现出沿晶断裂模式，裂纹主要在Al2Cu和AlCu层之间扩展；IMC层厚度很大时，接头断口呈现出穿晶断裂模式，裂纹在整个IMC层都可以快速扩展。"
其他摘要	" Friction stir welding (FSW) is a relatively new solid state joining technique which is invented for high strength Al alloys with poor weldabilities. Now, FSW has been successfully applied in commercial joining of Al alloys, and extended into Mg alloys. However, there are few studies on FSW of pure Cu with high melting point and dissimilar metals, compared to those of Al alloys and Mg alloys. In the present study, the effects of FSW parameters on the microstructure and mechanical properties of FSW pue Cu and dissimilar Cu-Al joints were investigated. The aim of this work is to obtain the key parameters for sound FSW pue Cu and dissimilar Cu-Al joints, and investigate the effects of the microstructure of heat affected zone (HAZ) and interface on mechanical properties of FSW pure Cu and dissimilar Cu-Al joints, respectively. Workhardened pure Cu (H state) plates were subjected to FSW, and various parameters were investigated in detail in order to obtaining sound FSW joints. Achieving high quality FSW pure Cu joints needed low rotation rate, and additional flow water cooling. Meanwhile, to avoid defect formation, larger shoulder plunge depth was chosen in FSW program. Under additional water cooling, the mechanical properties of FSW pue Cu joints were enhanced obviously, and the width of low hardness zone was narrowed with enhanced lowest hardness value. The peak temperature and the duration at higher temperatures in the nugget zone and the HAZ were significantly reduced when additional water cooling was applied during FSW. For the 400-water joint, the peak temperature of about 130 oC and the duration of 4 s above 100 oC indicated no or significantly reduced annealing soften effect was achieved in the HAZ. In this case, initial high dislocation density in the base metal (BM) was reserved in the HAZ, resulting into the equal-strength FSW pure Cu joints. FSW was conducted on single pue Cu plate with small shoulder under additional water cooling, and the ultrafine grained (UFG) structure in the nugget zone, i.e., the processed zone in friction stir processing (FSP) pure Cu, was investigated in detail. The microstructure of the FSP UFG pure Cu was characterized by equiaxed grains with low dislocation density and predominant high-angle grain boundaries (HAGBs) as high as ~90%. Furthermore, many annealing twin boundaries (TBs) were frequently observed in the fine grains. The hardness value and the grain size of FSP UFG pure Cu obey the Hall-Petch relationship, and was silight higher than the borderline of the coarse grained (CG) Cu Hall-Petch line, which was related to the low dislocation density and weak texture in FSP UFG Cu. Compared to the CG Cu, significantly enhanced strength was achieved in FSP UFG Cu with good tensile ductility. Moreover, introducing aboundant TBs, stacking faults and TB layers into the UFG structure by reducing the stacking fault energy can further ehance the strength and ductility simultaneously. Pue Cu and pue Al plates with different thickness were FSWed by butt and lap manners under various parameters in order to obtain sound FSW dissimilar Cu-Al joints, and the microstructure and mechanical properties of the joints were investigated in detail. For the 5 mm thickness Cu, Al plates, defect-free FSW butt joints were achieved when the Cu plate was in advanced side and the pin offset was not samller than 2 mm, and sound mechanical properties can be obtained at low rotation rates. Under optimum parameter, the welding coefficient was higher than 90%, and the joints were bended to 180o without frature. The sound mechanical properties were related to the uniform and continues thin intermetallic compound (IMC) layer of ~1 µm thick at the Cu-Al interface, and this was also approved in 3 mm thickness FSW Cu-Al butt joints. Under low heat input of 600 rpm-50 mm/min, 3 mm thickness Cu plate and Al plate were successfully lap FSWed using a larger pin of 8 mm in diameter. The joints fractured at HAZ in Al side during tensile shear tests, and the maximum load was as high as 2680 N, which was higher than 80% of BM. Lager lap area and the uniform and continues thin IMC layer at the Cu-Al interface resulted in the sound mechanical properties of the FSW Cu-Al lap joints. The evolution of the interface microstructure of the 5 mm FSW Cu-Al butt joint under optimum parameter during annealing process was observed, and the effects on the mechanical properties were also investigated in detail. Initial Cu-Al interface was a thin layer of ~1 µm thick, with one discontinues α-Al solution layer and two continues IMC layers. The IMC layer near the Al side was Al2Cu which exhibited columnar structure, and the IMC in the other layer were equiaxed Al4Cu9 and few AlCu grains. After annealing, the Cu-Al interface was thickened, and exhibited three layers. The α-Al solution layer disappeared and a new AlCu layer formed between Al2Cu and Al4Cu9 layers. When the IMC thickness was larger than 2.5 µm, FSW Cu-Al joints fractured at the interface. In this case, intergranular fracture was observed between Al2Cu and AlCu layers when the thickness of theIMC layer was relatively small; however, transgranular fracture occurred at the whole IMC layers when the thickness was very large."
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64472
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	薛鹏. 纯铜及铜-铝搅拌摩擦焊接头组织与性能研究[D]. 北京. 中国科学院金属研究所,2012.