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Abstract. The present work studies the effect of eutectic silicon particle morphology on the fluidity of 4045 aluminum alloy filler through dynamic spreading experiment, filling test and welding seam experiment. The experimental results indicate that the morphology of eutectic silicon particle has an excellent influence on the melting speed at the initial melting stage of brazing process: the alloy loaded with smaller eutectic silicon particles exhibits a faster melting speed and better fluidity. When the alloy completely melts along with those internal eutectic silicon particles in a variant morphology, particles sizes have limited impact on the fluidity. In addition, it is discovered that alloy introduced with smaller particles possesses a much lower surface activation energy; furthermore, the proportional relationship the ratio of length to diameter (S) and surface activation energy (Q) is verified in this article, and is consistent with the formula: Q=2.35S+94.47.