Formation Mechanism And Elimination Measures Of Pores In Fusion Welding Of Die Casting Magnesium Alloy

Due to the properties of die casting magnesium alloy and the characteristics offusion welding process, pore issue in fusion welding of die casting magnesium alloy isserious. Therefore, die casting magnesium alloy is usually considered unfavorable tofusion welding in the field of engineering, which limits the development of thetechnology of welded connection and surface defects repair of magnesium alloy diecastings to a certain extent.The plates of die casting magnesium alloy were welded by gas tungsten arcwelding respectively with and without the filler metal. The morphology, distributionfeatures and formation mechanism of pores emerging in different zones of the weldwere investigated by using scanning electron microscopy (SEM), particle size analysissoftware Nano measurer1.2, energy dispersive spectrometer (EDS) and X-raydiffraction (XRD). Furthermore, the decreasing and eliminating measures for poreswere discussed. The main findings are as follows:①Pores in the gas tungsten arc welding of die casting magnesium alloy mainlypresent in the subsurface and near the fusion line in the weld. The area of convexity,melting and porosity all increase with the increasing welding currents. Pores in thewelds mainly inherited from the gas in base metals, and the composition of the gas isgive priority to with N2, H is complementary. The former mainly originate from the gasindrawn during the base metal die casting process, and the latter is derived from theatomic and molecular hydrogen in the base metal. Macro-pores are mainly resulted fromN2，while micro-pores are primarily the hydrogen-induced.②Pores in the weld can be divided into pores in the subsurface, pores in thecenter of weld and pores in the partially melted zone based on the differences of themorphology and distribution features of pores. Pores in the subsurface are induced byhydrogen, which results mainly from the base metal and partially from the H2O in theair and protective gas. And pores in the center of weld, influenced by the fluid force andbuoyancy, are not a serious problem. However, pores in the partially melted zone havefew chances to float out of the molten pool because of the obstacle of the dendrites,solid particles, fluid force and the high viscosity of molten metal, which results in agreat number of large pores in the partially melted zone.③The "dilution effect" of the gas in the molten pool, resulting from adopting low gas hot extrusion AZ61magnesium alloy wire as filler metal, can significantly reducethe porosity, the size and number of coarse pores in the fusion zone, and change thecharacteristics of the distribution of pores, yet it is not a fundamental solution to thepore issue.④Using filler metal of Mg-Zr alloy lead to a small decrease in pore numbercomparing with using AM60filler metal, however, the reduction in the number of poresis not extremely obvious. This is because that Zr can react with H to form irreversiblecompound of ZrH2, which can decrease the pores induced by hydrogen, but it is invalidfor the pores resulted from N2. Comparing with adopting hot extrusion AZ61magnesium alloy wire as filler metal, a further decrease of the porosity is observed inthe welded joints using filler wire AZ61that containing rare earth elements, and theprobable mechanisms are following. On the one hand, the rare earth elements reduceeffectively the hydrogen-induced porosity by increasing the solid solubility of H in themagnesium alloy, reacting with H to form hydrides and accelerating the escape velocityof bubbles. On the other hand, the pore that results from N2is decreased to some extenttoo because the rare earths accelerate the overflow velocity of bubbles.