Effects of runner system design on the mechanical strength of Al-7Si-Mg alloy castings

Effects of runner systems on the mechanical strength of Al-7Si-Mg alloy castings have been investigated by employing different cross-sectional shapes of runners. Computational fluid dynamics (CFD) modelling was used to identify the flow behaviour and the oxide film distribution in the castings. The X-ray radiography was then used to examine the general distribution of defects in the castings from different running systems. Four-point bending method was applied. The scatter of bend strength results was quantified by Weibull statistics. The oxide films and casting defects existing on the fracture surface of the casting specimens were observed by scanning electron microscopy (SEM). Both of numerical and experimental results showed that the vortex-flow runner system (VR) could effectively control the ingate velocity and keep its value lower than 0.5 m s^-1, avoiding the generation of surface turbulence and the consequential entrainment of oxide films. The results from both the experiment and simulation support the conclusion that the use of VR can produce castings with fewer oxide film inclusions and more reliable mechanical properties compared with the castings using conventional runner systems.