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In aluminum gravity castings, a confluence weld defect could be found on the top flat surface of a thin-wall casting, such as a car engine cylinder head housing or a box shape casting. Such defect became a weakest point of mechanical property of the casting and it would shorten the service life of the casting.
In present study, three types of runner systems were designed in order to understand the cause of confluence weld defect. In the result, the formation of a confluence weld is determined by the velocity difference between two metal filling-fronts. As the difference is greater, the position of confluence weld where two fronts met is distance further away from center line of the casting geometry. During casting experiments, the velocities for two metal filling-fronts were measured by means of voltage data recorder and high speed videoing with the help of LED lights. They were also predicted by computational modeling. Two types of confluence weld above the top surface were discovered. In single-gating system, the difference is greater and the defect is more notable. In double-gating system, the different become less.
In all of runner systems, the position of a confluence weld was observed from the surface of top plate region of the casting experiment and it also was predicted by computational modeling. Furthermore, the strength of the confluence weld was evaluated by four-point bending method.