Aluminum alloy is a lightweight material frequently used for automotives parts, batteries, airplanes, etc. to reduce greenhouse gas emissions. A highly efficient and stable laser welding method has long been sought for Al alloy because of its poor weldability due to its high heat conductivity and high reflectivity of 1 µm wavelength laser light. Because welding phenomena and defect formation mechanism are still not properly understood due to their complexity, this research aims to investigate the laser welding phenomena of Al alloy. To understand the weld pool and keyhole dynamics during welding, the weld pool was observed with a high-speed camera and X-ray phase-contrast method. The keyhole behavior and solid-liquid interface during laser welding for A1050 can be observed more clearly than in X-ray absorption-contrast method. At a lower welding speed (0.5 m/min), frequent intensive keyhole expansion and high porosity could be observed. Although high porosity could be observed at a higher welding speed (1 m/min), keyhole expansion was barely observed. It is thought that at a low welding speed, intensive evaporation at keyhole bottom leads to unstable keyhole behavior due to high heat input, and as a result, high porosity is formed due to be intake of air into the keyhole. The effect of fan usage was evaluated. In the case without a fan, intensive keyhole expansion was observed frequently. It is supposed that the laser partly interacted with plume, which led to a sudden evaporation due to laser scattering. It was observed that a bubble from the keyhole bottom moved around the keyhole at a high speed, and the bubble was trapped at a solid-liquid interface. Then the bubble grew by absorbing other small bubbles in a melt pool. In all welding conditions, frequent small expansion at the keyhole bottom was observed. Therefore, the porosity formation mechanism is assumed to have a strong relationship with expansion of keyhole bottom, where major porosity was formed.

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