【作者】 闫洪华；

【导师】 张凯锋；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2010， 博士

【摘要】 超塑成形/扩散连接组合技术在钛合金多层构件制造方面取得了发展,但仅就航空航天领域而言,还存在巨大的发展空间,在技术推动和需求牵引的双重作用下,开发现有材料的超塑性和其它连接技术与超塑成形的组合技术研究在宇航结构制造领域具有极其重要的意义。在满足现代飞行器性能要求的前提下开发铝合金的多层结构即可降低制造成本又可减轻重量。但由于铝合金的氧化膜问题使其不容易进行扩散连接,因而通过超塑成形/扩散连接成形铝合金的多层结构就比较困难。5083铝合金是典型的Al-Mg系合金,具有高的物理性能和机械强度,并有优良的可加工性、抗腐蚀性、可焊接性及低廉的价格等优点,因此被广泛应用于航天、航空,船舶等行业。本文以该合金为研究材料,尝试着采用激光连接代替传统的扩散连接并结合超塑成形的组合技术成形铝合金的多层结构。为此,本文详细地对5083铝合金的超塑性及其变形机理、热成形性能、激光焊接性能等方面进行了研究,确定了使该合金获得良好热变形性能和焊接性能的工艺条件,从而为激光连接+超塑成形(LBW+SPF)组合技术成形铝合金的多层结构的发展和应用提供了可靠的依据。初始的商业5083铝合金板晶粒比较粗大,其平均晶粒尺寸远大于10μm,通过热机械处理法获得了平均晶粒尺寸为3μm的细晶板。采用透射电镜对细晶5083在150～300℃温度范围内的退火组织演变进行了研究。确定了对组织超塑性有利的合适的退火温度为250℃。对细晶5083铝合金在温度为380～570℃和应变速率为4.17×10-4～1.0×10-2s-1的范围内进行了单向拉伸实验,结果表明:在同一应变速率下,温度在380～550℃范围时,材料的延伸率随着温度的升高而不断增大;当温度升高到550℃时,材料的延伸率达到最大值530%;但当温度继续升高到570℃时延伸率反而下降;在同一温度下,材料的延伸率随着应变速率的降低而增大。同时,计算了不同温度和应变速率下的应变敏感指数m值。通过金相显微镜和扫描电镜观察并分析了超塑变形过程中的液相行为和空洞行为。为了进一步考察细晶5083铝合金的成形性能,对其进行了非等温拉深实验。研究了不同凹模温度、拉深速度以及压边间隙对该合金非等温拉深成形性能的影响,确定了使该铝合金具有最佳拉深性能的工艺参数。结果表明:在200~300℃的温度范围内,细晶5083铝合金薄板的极限拉深比(LDR)随凹模温度的升高而明显增大,当凹模温度达到275℃时, LDR达到最大值2.9;当在较佳的凹模温度和不同的拉深速度下进行拉深时,细晶5083铝合金非等温拉深工艺在一定的拉深速度范围内对应变速率不敏感,当拉深速度≤2mm/min时均能拉深成功。利用3kW的CO2激光器对细晶5083铝合金薄板进行了激光穿透焊实验研究,接头分别采用对接和搭接两种形式。实验结果得出了两种接头下的最佳焊接工艺参数和接头的显微硬度分布规律。对对接焊试样进行高温拉伸结果表明:焊缝强度高于母材,破坏位置出现在母材一侧。对对接焊板材在500℃进行了自由胀形实验,得到了相对胀形高度为0.59的胀形件。对搭接焊试样进行拉伸结果表明:搭接焊试样在室温下的抗拉强度为186Mpa,约为非细晶母材在室温拉伸时的64%;而在500℃时搭接焊试样的抗拉强度约为细晶母材的91%。根据对细晶5083铝合金搭接激光穿透焊的研究结果,采用获得的最佳焊接工艺参数(激光功率1200kW,焊接速率2mm/min,离焦量0,氩气保护)对不等厚的细晶5083铝合金板按照设计好的连接位置进行激光连接成多层结构,然后采用氩弧焊对多层结构进行封边并留出气道,最后利用超塑成形工艺成形了细晶5083铝合金的三层和四层结构。其中多层结构的面板厚度为1.1mm,芯板厚度为0.9mm。超塑成形工艺参数为:成形温度500℃,成形时间45min,成形压力为1.2MPa。分别分析了成形后的几种多层结构的面板与芯板厚度变化和面板与芯板、芯板之间的连接情况。结果表明:几种多层结构的整体成形均较好,它们的面板与芯板变形都比较均匀,各连接处的结合情况也都较好,几乎没有缺陷发生。更多还原

【Abstract】 The technology of superplastic forming and diffusion bonding (SPF/DB) has made the development for multi-sheet structure manufacturing of titanium alloy. However, there is a huge space for development in aviation and aerospace fields. So, the researches on superplasticity of existing materials and technologies of other bonding methods combined with superplastic forming have extremely important significance. To develope the multi-layer structure of aluminum alloys can reduce manufacturing cost and weight under meeting the performance requirements of modern aircraft. However, the obstinate oxide film of aluminum alloy leads to that diffusion bonding of the alloy is not easy. This problem exists so that the multi-layer structure of aluminum alloy by SPF/DB becomes more difficult.Al-Mg based 5083 Al alloy has shown broad applications in aerospace, aviation, shipping and other departments for its special characteristics, such as low price, moderate strength, good corrosion resistance and high formability with moderate superplasticity. In this paper, the superplasticity and its deformation mechanism, foming properties, laser beam welding performance of 5083 Al alloy were sdudied in detail. The good forming and welding process parameters were determined by experiments. These provide a reliable basis for the development and application of multi-layer structure of alumimum alloys using the technique of laser beam welding + superplastic forming (LBW+SPF). To this end, this paper has been focused on the sdudies as following:The microstructure with uniform equiaxed fine gain (average size of 3μm) was obtained using the thermo-mechanical processing (TMP) for the 5083 Al alloy plate. The microstructure evolution of the fine-grained 5083 Al alloy has been firstly investigated at annealing temperatures of 150～300℃. The results indicate that the suitable annealing temperature is about 200℃in terms of Organization superplasticity. Uniaxial tensile test was carried out at a temperature range of 500～570℃and a strain rate range of 4.17×10-4～1.0×10-2 s-1. Maximum tensile elongation 530% was obtained at 550℃and strain rate 4.17×10-4s-1, at which maximum strain rate sensitivity index m of 0.68 was attained. The cavities and fracture were observed during uniaxial tensile deformation of the alloy by scanning electronic microscopy (SEM). It indicated that linkage of cavities in large region would induce failure of the material. Moreover, presence of liquid phase at grain boundary also influenced superplastic deformation and behavior of cavities.The effect of temperature of female die, deep drawing speeds, blankholder gap and lubricating condition on non-isothermal deep technology of fine-grained 5083 Al alloy was studied under temperature range of 200～300℃. Experimental results show that, fine-grained 5083 Al alloy sheets have good deep drawing formability when temperature of female die is higher than 250℃. When the temperature of female die was 275℃, the limiting drawing ratio (LDR) of the material reached maximum 2.9. Under the optimum female-temperature and different punch speed, the influences of strain rate on the non-isothermal deep drawing process of fine-grained 5083 Al alloy is a little. When the punch speed is equal or lesser than 2mm/min, the deep drawing will be well.The laser full penetration welding without filler of fine-grained 5083 Al alloy sheets is investigated with 3kW CO2 laser, butt and lap joints were used in two forms. Butt weld penetration test results show that, the welding surface with high quality is obtained under appropriate laser power and welding speed. The profile of microhardness traverse across the weld exhibits like W shape and that the heat affected zone (HAZ) has serious softening. The results of high temperature tension show that the tensile strength of welded joint is greater than the base metal, which is the side of fractures. Optimal relative height of 0.59 of the free bulging specimen is obtained at 500℃. This result shows that laser butt-welding sheets of fine-grained 5083 Al alloy possess good high temperature formability. The thickness of 1.1mm and 0.9mm for the fine-grained 5083 Al alloy sheets in lap welding penetration test results show that, the profile of microhardness traverse across the weld exhibits like W shape and that the heat affected zone has serious softening. The results of tensile test at room temperature show that the tensile strength of welded joint is 186Mpa, which is about 64% of a general 5083 base metal. And the tensile strength of the welded specimen is about 91% that of the base metal at 500℃. Multi-sheet structures of an aluminum alloy were fabricated through laser beam welding combined with superplastic forming technique. The novel welding design and bonding have been developed to improve the formation quality of the structure for the alumimum alloy. Forming process parameters are: forming temperature is 500℃, forming time is 45min, and forming pressure is 1.2MPa.The distribution in thickness within the formed structure and bonding conditions between face- and core-sheets were investigated. It reveals homogeneous deformation and well-bonding property of the structure. This verifies the feasibility of the processing procedures for the multi-sheet structures of an aluminum alloy.更多还原