Influence Of Yttrium On Welding Porosity Of Die Casting Magnesium Alloy In Fusion Welding

The die casting Magnesium alloy has high air content due to the serious "volume gas behavior" in die casting process. During fusion welding of die casting magnesium alloy, the porosity problem is very serious due to the high gas content in base materials, which degrades the joint mechanical properties seriously. Therefore, die casting magnesium alloy is seldom welded. With the increasingly extensive application of magnesium alloy, the porosity problem of die casting magnesium alloys in fusion welding need to be solved urgently to promote its further development.Up to now, there are still no consistent conclusions of the formation mechanism of welding porosity of die casting magnesium alloy, and the special remedies are more rarely reported. In this paper, the die casting AM60 magnesium alloy plates were autogenous welded by gas tungsten arc welding, the formation mechanisms of porosity were analyzed. The appropriate element added to the weld seam was determined by the thermodynamic calculation to eliminate nitrogen porosity. The main factors influencing the welding porosity were analyzed and the remedies were discussed via the butt welding of die casting AM60 magnesium alloy plates with different filler wires. In addition, a specific experiment was carried out to verify the rationality of thermodynamic calculation. The main results are as follows:① The nitrogen contained in die casting magnesium alloy exists in the form of molecular, which does not decompose in die casting process. The thermodynamic calculation results indicate the reaction between yttrium and nitrogen is preferable because the nitrogen(molecules form) has a higher chemical affinity with yttrium than magnesium. Therefore, the addition of yttrium can effectively eliminate the nitrogen pores in theory.② Pores with different sizes and shapes distributed in the whole weld seam when autogenous welding of die casting AM60 magnesium alloy. The macro-pores should be nitrogen pores, it can be explicitly observed that the macro-pore inner walls are not truly smooth, which exhibits an irregular array of small striations and some solid particles. The micro-pores should be hydrogen pores, their inner walls are also not smooth, which are similar to the dendrites, in addition, it can be found some much smaller holes located at the dendritic boundaries. It is obvious that the pores in the weld seam mainly belong to nitrogen pores③ The rising velocity of bubbles in the molten pool is very fast when without constraint, and the bigger of the bubbles, the faster of the velocity. In the center of molten pool, the viscosity is very small, so bubbles are easy to escape from the molten pool. However, in the partially melted zone, the bubbles have few chances to float out of the molten pool due to the obstacle of the dendrites and solid particles. Moreover, the rising distance of bubbles is greater in the partially melted zone, because the movement direction of bubbles points to the weld seam central region instead of weld seam surface. In a word, bubbles formed in the partially melted zone have great influences on the welding porosity. If the bubbles can’t get rid of the restriction of partially melted zone, the welding porosity problem will became very serious in partially melted zone. Nevertheless, even the bubbles break through the restriction of partially melted zone, they still always cause serious welding porosity in the weld seam center because that they have not enough time to float out of the molten pool. In addition, the bubbles have more chances to coalesce due to the restriction of partially melted zone, therefore the bubbles originated from partially melted zone always cause bigger welding pores.④ The addition of yttrium resulted in a prominent decrease of porosity rate, especially for the nitrogen pores, which were eliminated completely. The effects of yttrium include the following three aspects: 1) as the prediction of thermodynamic calculation, yttrium reacted with the pre-existing nitrogen, 2) the addition of yttrium increased the fluidity, which resulted in more pores float out of the molten pool, 3) the addition of yttrium restrained the rejection of hydrogen, which leaded to a reduction of hydrogen pores. Obviously, the first effect is most important, though the nitride was not detected directly in the weld seam. Combined the thermodynamic calculation and the fact that magnesium could react with nitrogen during welding process, the reaction between yttrium and nitrogen during welding process was demonstrated, it should be believed that the decrease of welding porosity when welding with the filler wire of Mg-Y alloy was ascribed to the formation of YN...