【作者】 李超；

【导师】 张凯锋；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2012， 博士

【摘要】 超塑成形是制造复杂薄壁轻合金结构件的有效方法之一，已经在许多领域得到应用。但由于低应变速率和高温的要求，常规的超塑成形工艺能耗高、效率低，导致成本较高。本文试图对超塑成形的加热方法进行探索，实现脉冲电流快速加热。还将利用脉冲电流的电致超塑性效应提高轻合金板材的超塑成形性能。关于电流对材料超塑性能的影响，已有相关的研究报道，但工作主要集中在电致超塑性理论方面，而实际的工程应用还鲜有报道。为此，本文将脉冲电流引入到轻合金材料的超塑成形工艺中，将脉冲电流的焦耳热效应与电致超塑性效应结合，提出一种脉冲电流辅助超塑成形新方法，并设计了相应的工艺装备、制定了工艺流程、确定了工艺参数。工艺试验结果表明，该方法对AZ31镁合金及TC4钛合金等轻合金板材的超塑成形十分有效，成形试件质量良好，工艺过程所需时间减少50%，能耗降低65%左右。根据电热学理论，详细分析了轻合金板材脉冲电流加热过程中的加热速率、加热温度等与材料热物理性能及电流参数间的关系，为合理地制订加热规范奠定了理论基础。利用有限元方法分析了平板及异形试件加热时的温度场分布，并通过在电极与试件之间放置TC4金属片（具有低导热系数、高电阻率的物理特性）的方法，提高了加热坯料温度场的均匀性。通过加热试验对上述理论分析进行了试验验证，结果表明，该加热方法高效、节能、可靠。为揭示脉冲电流作用下轻合金板材的超塑变形机理，利用光学显微镜（OM）、透射电子显微镜（TEM）等方法分析了不同原始显微组织的AZ31镁合金及TC4钛合金板材在脉冲电流辅助超塑成形工艺条件下变形时的微观组织演变规律，包括晶粒长大、动态再结晶（DRX）和晶界滑移（GBS）等，讨论了脉冲电流对位错形貌及位错运动的影响。研究发现脉冲电流可在一定程度上抑制成形过程中晶粒长大、促进再结晶形核，细化成形后的微观组织。此外脉冲电流还可以促进晶界滑移，提高位错运动能力。这些都有助于材料超塑变形能力的提高。针对超塑成形中的空洞问题，应用电热学及热弹性理论分析了在脉冲电流作用下，V形空洞附近的温度场及应力场。并分析了脉冲电流对空洞形核及长大的影响，利用扫描电子显微镜（SEM）观察了AZ31镁合金超塑变形前后的的空洞形貌。研究发现，脉冲电流可在V形空洞尖角附近形成局部高温区及局部压应力区，在一定程度上抑制V形空洞在垂直电流方向的发展；此外脉冲电流还可提高空洞形核率，促进小尺寸空洞的长大，从而使得空洞大小及分布更加均匀，避免了材料的过早断裂。更多还原

【Abstract】 As one of the effective method to manufacture complex and thin wall lightalloy structures, superplastic forming has been applied in many fields. However, thistechnology has the defect of high cost owing to high energy consumption and lowefficiency caused by the deformation condition of high forming temperature and lowstrain rate. Therefore, the pulse current heating method of superplastic foming hasbeen studied in this paper to achieve fast and high efficiency heating. Moreover, theelectro-superplastic effect of pulse current has been utilized to improve the fomingproperty of the light alloy sheet.Most present studies about the electro-superplastic focused on the theory, andthere has little literatures concerning the craft application of electro-superplstic.Therefore, the pulse current has been applied into the process of light alloy sheetsuperplastic forming in this paper, and the Joule heating effect andelectro-superplstic effect of pulse current have been associated and applied. A newprocessing method named pulse current auxiliary superplastic forming (PCASPF)has been proposed, and the processing device has been design, and the technologicalprocess and the processing parameter have been determined. From the results of theforming test, it is known that this method is effective for superplastic foming of lightalloy sheet such as AZ31magnesium alloy and Ti-6Al-4V titanium alloy. The timeof the whole processing reduced to50%, and the energy consumption of the wholeprocessing reduced to35%.Based on the electrothermics theory, the heating rate and heating temperatureof light alloy sheets with different performances during pulse current heating hasbeen studied systematically, which is necessary to determine the heating processingparameters. The temperature fields of the flat plate and semisphere plate duringpulse current heating have been analysed by the method of finite element simulation.The uniformity of the temperature field has been improvement by placing theTi-6Al-4V titanium alloy sheet between the specimen and the electrode. The heatingtest has been executed to check the theory analysis. Form the results of heating test,it is known that this heating method is high efficiency and energy conservation.To disclosure the deformation mechanism of light alloy superplstic forming bythe action of pulse current, the analysis methods of OM and TEM have been used toanalyse the microstructure evolution, involving grain growth dynamicrecrystallization (DRX) and grain boundary slipping (GBS), of AZ31magnesium alloy and Ti-6Al-4V titanium alloy with different original microstructure duringPCASPF, and the effect of pulse current on the dislocation morphology andmovement has also been studied. Form the research, it is known that the graingrowth speed has been reduced, the dynamic recrystallization has been accelerated,and the microstructure has been refined by the pulse current. Furthermore, the grainboundary slipping and the dislocation movement have also been accelerated by thepulse current, which are benefit to improve the superplastic forming ability of thelight alloy sheet.Based on the electrothermics and thermoelasticity theory, the temperature andstress fields by the action of pulse current near the cavities have been studied. Theeffect of pulse current on the cavity nucleate and cavity growth has been analysed,and the cavitied morphology of AZ31magnesium alloy during PCASPF has beenobserved by the method of SEM. Form the results, it is concluded that there exist thelocal high temperature and compression stress zone near the cavity tips, whichrestrained the cavity continues growth along the tip direction. Moreover, the cavitynucleate rate and small cavity growth have been accelerated by the pulse current,which made the cavities size and cavities distribution more uniform, and avoided thepremature fracture during superplastic forming.更多还原