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Gas pore is a common defect in brazed joint.It lowers the brazing rate and affects the properties of joint.Experimental results show that the application of unequal-gap brazing seam effectively decreases the amount and volume of gas pores,and increases brazing rate.This paper establishes a force model of unequal-gap brazing seam,and proposes the constitutive relationship between expulsion force and curvature.The force condition of gas bubble in geometrically different brazing seams were calculated,and the results were verified with ex-periments.The results show that the expulsion force of gas bubble is positively correlated to the curvature of the seam geometry.The gas bubble tends to move towards the direction with large curvature and wider gap.The directional bubble movement is obtained through vary-ing the configuration of gas-liquid interface to meet geometric conditions.Gas bubble accelerates to expulse with arc,hyperbola and cycloid brazing seams,in which the best drainage effects of gas bubble occur for cycloid seams.