In this manuscript, weld pool dynamics in laser welding of various series of aluminum alloys were investigated by the in situ X-ray phase contrast imaging system. The experimental results showed that metal irradiated by laser was evaporated immediately, which generated the keyhole. Then metal surrounding the keyhole was melted gradually with the heat from keyhole. The growth rate of keyhole depth had a positive linear correlation with the total content of low boiling temperature elements (TCE),

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... so did the keyhole depth and diameter at the stable stage. Longitudinal view area of the molten pool had a negative linear correlation with the thermal conductivity of aluminum alloy. The measured laser absorption rate had the same variation trend with the ratio of keyhole depth to diameter, and the highest absorption rate of 58% appeared in laser welding of aluminum alloy with TCE equal to 2.1%. Violent fluctuation in keyhole shape was avoided in aluminum alloy with TCE lower than 2.1%, where the surface tension and recoil pressure of metal vapor were balanced. To sum up, the effect of alloy element on weld pool dynamics in laser welding of aluminum alloys was firstly quantified in this manuscript. Aluminum alloy is one of key materials in weight saving of mobile body including electric vehicle (EV). Laser manufacturing has been widely used as a highly efficient method to achieve high quality product in industry 1,2 , where laser is used in welding 3-5 , cutting 6,7 , additive manufacturing 8 , and so on. The interaction between laser and material determines the product quality 9-13 . However, keyhole and molten metal induced by high power density laser are surrounded by solid metal, and these inner phenomena are not observable with the conventional method. Though characteristics of keyhole and molten pool were presented by simulation 14-17 , these calculated results have rarely been validated by experiment directly. Various methods have been adopted to explore the dynamic behavior inside weld pool in laser material processing 18-21 . High speed camera was adopted to observe the movement of molten metal in the weld pool with a specially designed structure 18, 19, 22 . In this method, a glass sheet and a steel sheet were arranged side by side, and their interface was irradiated by laser. By placing a camera at the glass side, it is possible to directly capture the inside behavior of the weld pool, which contributed a better understanding of the keyhole dynamics and flow of the molten pool. However, this specially designed structure changed the heat conduction condition around the weld pool, and the measured keyhole size was clearly larger than the real one in laser processing. Microfocused X-ray transmission in situ observation system was successfully adopted to investigate the keyhole dynamics 20,21 , which contributed an in-depth understanding of the light absorption 23 , stability of the keyhole 24 in laser welding. However, the interface between molten pool and solid metal couldn't been identified with this method due to the marginal difference in absorption performance of X-ray between liquid phase and solid phase. Recently, the X-ray phase contrast imaging system was reported to have the ability to capture the weld pool behavior and porosity formation process clearly 25,26 , which firstly provided an effective method to quantify the real, inner characteristics in welding. Researchers have focused on the weld quality in laser welding of various series of aluminum alloys by both experiments and simulations [27] [28] [29] . These previous researches showed that the aluminum alloy with different