On the effect of gating system type on occurrence of hot tears in Al–4 wt% Cu castings

In this study, effects of gating system type on hot tearing occurrence of Al–4 wt% Cu castings were investigated using the instrumented constrained T-shaped casting method. Three different gating systems, i.e. a conventional pressurized gating system, a conventional unpressurized gating system and a naturally pressurized gating system with surge control arrangement, were used for this purpose. The presence of oxide bifilms in torn surfaces were studied using scanning electron microscopy and energy-dispersive x-ray spectroscopy. It was shown that hot tearing severity could be predicted from the final slope of the load–time curves. Hot tearing susceptibility was highest for the conventional pressurized gating system. This was associated with the high surface turbulence of the melt during mould filling which would result in the formation of such defects as oxide bifilms. For the naturally pressurized gating system, the maximum load tolerable by the solidifying casting was much larger than those for conventional gating systems and severity of hot tears formed were much less significant. Hot tearing occurrence in all the castings was closely associated with the presence of thick oxide layers. In other words, greater amounts of oxide bifilms were observed on the hot tearing surfaces of the more severely torn castings.