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Keyhole-induced porosity can strongly weaken the joint strength during aluminum alloy laser welding. Two factors are proposed to be responsible for the porosity formation: bubble formation caused by the instability of the keyhole and bubble captured by the solidification interface. Based on numerical and experimental results, the porosity is suppressed with preheating, due to the capillary stability of the keyhole and easy escape of bubble. N2 shielding gas has a minor effect on keyhole stability, but it reduces the porosity number as N2 bubble can dissolve into the liquid aluminum alloy melt pool quickly. The laser-gas metal arc (laser-GMA) welding process can stabilize the metallic plume and enlarge the weld pool. As a result, few spatters and bubbles are formed, and bubbles are easily escaped, both of which contribute to the suppression of porosity. [ABSTRACT FROM AUTHOR]

The instability of the keyhole and bubble being captured by solidification interface are the main factors that affect the formation of porosity in laser welding of both aluminum alloy and steel. In laser welding of aluminum alloy, the more violent fluid flow behind the keyhole, the larger depth to width ratio can cause the keyhole more unstable, and susceptive to collapse that can be described as three necessary steps. The much larger solidification rate and longer escaping distance make the bubble easily captured by the solidification interface, and easily combined. All of these reasons contribute to the formation of larger number and size of porosity in laser welding of aluminum alloy. The numerical and experimental results are in good agreement. [ABSTRACT FROM AUTHOR]

The droplet escape condition is established, a three-dimensional numerical model is built to investigate the behavior of the weld pool in fiber laser welding of 5083 aluminum alloy, the shear stress caused by the metallic vapor flow that has great influence on spatter formation is considered. A high speed photography system is used to capture the transient images of the weld pool, the spatter formation is discussed. The numerical and experimental results show that the spatters generate mainly around the keyhole. Two factors are responsible for the spatter formation: the surface tension of the molten metal around the keyhole is low. The recoil pressure caused by evaporation and the shear stress caused by metallic vapor flow accelerate the upward moving melt around the keyhole. Increasing the welding speed and placing the nozzle at the trailing position can decrease the upward momentum of the molten metal around the keyhole, and stabilize the keyhole, which are help to suppress the formation of spatter. [ABSTRACT FROM AUTHOR]