【作者】 陈勇；

【导师】 游国强；

【作者基本信息】 重庆大学 ， 材料加工工程， 2012， 硕士

【摘要】 镁合金铸件常因局部出现的铸造缺陷、机械加工和运输过程中产生的表面损伤而使铸件成为不合格产品。因此，需要修补以降低生产成本，提高经济效益。传统的修复方法存在一些缺点和不足，难以获得满意的修复质量。与传统的修复技术相比，电火花堆焊技术克服了一些缺点并兼有一些优点。本文旨在通过电火花堆焊工艺对压铸AZ81镁合金表面缺陷进行修复，探寻一条能够获得较好修复效果的镁合金铸件表面缺陷修复工艺，以期对镁合金铸件的修复工艺改进提供一定的理论基础和指导。实验表明：压铸AZ81镁合金电火花堆焊接头主要由母材(BM)和焊缝区(WZ)和极薄的热影响区(HAZ)组成。没有出现过热组织和粗大晶区，焊缝组织表现为非常规的亚快速凝固组织，成分和组织分布均匀。在一定范围内，随着脉冲占空比和放电频率的增加，堆焊接头横截面宏观形貌有所改善，实现冶金结合的界面更多，但仍会有少量界面熔合不良。冶金结合界面主要有两种类型：一种界面是由母材微区重熔后与堆焊熔滴非均匀混合互熔后结晶而成，另一种界面是母材发生超薄层熔化与堆焊熔滴快速结晶而成。通过测试两组试验典型接头的显微硬度，得出堆焊区显微硬度略低于母材硬度。当脉冲占空比和放电频率在一定范围内上升时，堆焊区显微硬度略有升高，且显微硬度值变化幅度减小。接头界面结合处合金元素发生了相互扩散，过渡平稳。参数优化后焊缝区Mg元素的烧损和蒸发量减少。焊缝中几乎未出现传统气孔缺陷，但是存在明显的堆焊层间未熔合和夹渣现象。随着脉冲占空比和放电频率的增加，层间未熔合缺陷减少，堆焊区组织疏松缺陷基本消失。在对压铸AZ81镁合金表面缺陷修复的工艺研究中，得出了一条能够获得较好修复效果的工艺参数：占空比50%-60%，放电频率300HZ-350HZ，电压50V，氩气流量6L/Min。进一步研究发现，修复尺寸越小的表面缺陷或损伤，该工艺效果越好。更多还原

【Abstract】 Due to local casting defects, surface damages produced during machining andtransportation process, the substandard products of casting magnesium alloy shall bewelded up to reduce production costs and improve economic efficiency.The satisfactory repair quality is somewhat difficult to be obtained by thetraditional repair techniques, due to some of their shortcomings and deficiencies.However, compared with the traditional repair techniques, electric spark surfacingwelding has some advantages to overcome some shortcomings. Through the repairexperiment of the surface defects of the die-cast AZ81magnesium alloy by electricspark surfacing welding, this article aims to explore a processing method with betterrepair effects of the surface defect of casting magnesium alloy, and to provide furthertheoretical basis and guidance of the improvement of the repair process of magnesiumalloy castings.The theoretical analysis and experimental results showed that the welding joints ofthe die casting AZ81magnesium alloy by electric spark surfacing welding consist ofbase metal (BM), weld zone (WZ) and a very thin layer of heat-affected zone (HAZ),with no overheated structure and coarse grain zone. The characteristic of themicrostructure in weld zone is non-conventional sub-rapid solidification, with uniformcomposition and microstructure and extremely small grains.The macroscopic morphology of the surfacing welding joint in the cross-sectionimproved with the increase of the pulse duty cycle and the discharge frequency, withmore interface metallurgical bonding and less interface bad fusion. There are two maintypes of metallurgical bonding interface: One is formed after the remelting micro zoneof base metal and the melt drip of surfacing welding heterogeneously mixed, mutuallymelted and then crystallized; another interface is formed after the ultrathin layer of thebase metal and the melt drip of surfacing welding rapidly crystallized. Themicrohardness test of two sets of typical welding joints showed that the microhardnessof surfacing weld zone was slightly lower than that of the base metal. Themicrohardness and its uniformity of surfacing weld zone slightly increased when thepulse duty cycle and the discharge frequency increased within a certain range,.The interdiffusion of alloy elements and smooth transition in interfacial juncture ofthe interface occured. After the parameter optimization, the burning loss and evaporation of Mg element in the weld zone reduced. Almost no traditional welding airhole defect appeared in the weld zone, but there was obvious lack of fusion in surfacinglayer and slag inclusion. The defects of lack of fusion in surfacing layer reduced withthe increase of the pulse duty cycle and the discharge frequency, and surfacing loosedefects disappeared. In the process of surface defects repair of the die-casting AZ81magnesium alloy, a processing method with better repair effects of the surface defect isobtained, and its process parameters are as follows: duty cycle of50%-60%, thedischarge frequency of300HZ-350HZ, the voltage of50V, the argon flow of6L/Min.Further studies showed that the smaller the size of surface defects or damage, the betterthe repair effects.更多还原