Confluence weld in an aluminum gravity casting

In aluminum engine housing castings, made by gravity casting process, a confluence weld defect can occasionally be found on their top flat surface. To understand the formation of the defect, three types of runner systems were designed in this study. At the top flat surface of the aluminum casting, the location of the confluence weld, where two filling fronts of the aluminum melt merge, could be determined by the velocity difference of the two fronts. The greater the difference, the further the weld location was away from the center of the top surface. The movement of the front was detected by a time-logger and indirectly filmed by a camera in the casting experiments. The filling profile was simulated by computational modeling. Two forms of the weld could be characterized by the velocity difference. In single-gating experiment, the difference was greater than that in double-gating experiment and the defect was more prominent. In all runner systems, the mechanical properties of the castings containing the confluence welds were examined by four-point bend test. Weibull probability plots were used to assess the fracture mechanisms for the three runner systems.

► Confluence welds are formed and designed by three types of runner systems. ► Two distinctive welding mechanisms and appearances are found. ► One is that oxides are pushed away from the welding and a strong weld is formed. ► In the other, stable oxides are created on the stopped front and its weld is weak. ► Weibull probability plots are used to assess these two welding mechanisms.