Study On Microstructure And Technology Properties Of Electric Spark Surfacing Welding Of Die Casting Magnesium

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.