Study On Formation Mechanisms Of Porosity And The Cracks In Laser Welding Of Magnesium Alloy Thin Plates

The formation mechanisms of porosity and hot cracks in laser welding of AZ31B and AZ61 magnesium alloy thin plates are investigated. The influence of different welding conditions on the sensitivity to porosity and cracks are studied by changing the welding speed, laser power and gas protection methods, etc.? ? Morphology of microstructure and weld defects is observed by using microscopes and an scanning electronic microscope.Experimental results show that the narrow weld joints produced by the laser welding of both magnesium alloys have good weld bead appearances and that the welding deformation is quite small. No visible cracks or porosity on the weld surface can be found. However, porosity and cracks are found inside the sectioned weld under microscopic observation. Pores in AZ31B and AZ61 welds are found to be scattered in the weld. Pores at the bottom part of the weld are found to be in small sizes, whilst large pores are found to be mainly distributing along the fusion line. Most of the pores are of a circular or sphere shape, with a few of exceptions of an oval shape, but whirlpool pores that are usually formed because of keyhole instability in laser welding are not found. To a certain extent, the amount of porosity is sensitive to welding speeds, but barely sensitive to laser power. Preheating of workpiece, surface cleaning before welding and choice of other proper welding parameters can reduce the porosity tendency. It is also found that AZ61 has a higher porosity tendency than AZ31B. This can be explained by the assumption that the higher aluminum content in the former has an effect in expanding the solidification temperature range, thus more hydrogen would precipitate from the welding pool.It is also shown that both magnesium alloys are sensitive to hot cracking. Tiny crystallization cracks in AZ31B and AZ61 welds are found in the welds produced under different welding conditions. These crystallization cracks develop along the grain boundary. In addition, in some AZ61 welds, the bifurcated crystallization cracks and the liquefied cracks are found, whilst such cracks are not found in AZ31B weld joints under the same welding condition. It is hence concluded that AZ61 is more sensitive to hot cracking than AZ31B.The formation mechanisms of porosity and the cracks are analyzed and discussed comprehensively in this paper. The pores formed in laser welding are primarily hydrogen porosity, which is mainly due to the sudden drop of the solubility of hydrogen in magnesium during the cooling process, and as a result, hydrogen is precipitated and bubbles are produced. When the bubbles fail to escape from the welding pool, they stay as gaseous pores in the weld. There are three phases in the formation of bubbles, including the initiation of bubbles, the growth of the bubbles, and the escape of the bubbles from the molten pool. Both atomic and molecular hydrogen make a contribution to the formation of pores. Because of the high gaseous pressure and the high surface tension, the keyhole instability has little influence on pore formation during the process of laser welding of magnesium alloys.The formation of crystallization cracks is explained by the postulate that aluminum, magnesium and other alloying elements react together to form low melting-point eutectic films during the cooling process. The low melting-point eutectic films crack under the welding tensile stress caused by thermal expansion and contraction of magnesium alloy. The formation mechanism of liquefied cracks is similar to crystallization cracks. But for liquefied cracks, the low melting-point eutectic films are formed during the remelting of grain boundaries in the heat affected zone. Since AZ61 has a higher content of aluminum, which has an effect of expanding the crystallization temperature range, it explains well that AZ61 is more sensitive to hot cracking than AZ31B.