Hot tearing is a common defect which occurs during solidification of cast alloys with large solidification range. Alloy composition, solidification mode, casting design, melt cleanness and resistance of the mold to free solidification shrinkage are among the main factors affecting the hot tearing tendency of cast components. Determination of the hot tearing tendency and the effective parameters are of great interest to researchers in this field. Structural defects such as oxide bifilms influence the hot tearing occurrence. Poorly controlled molten metal transfer and poor gating system design are the main factors causing the formation of the oxide bifilms. Bifilms are the result of enfolding of the liquid surface into the bulk melt due to melt surface turbulence. Since the melt surface is usually covered by an oxide film, fragmented oxide films may entrain in the melt under turbulent surface conditions. In such case, the dry surfaces of oxide films may come in contact, hence the name ‘‘bifilm’’, and act as cracks for initiation of hot tears during solidification. Bifilms can remain suspended in melts for long periods of time and enter the mold upon pouring. The reduction of bifilm content of castings can be achieved by new gating system designs and possible use of filters. In the present research the effects of three types of gating system design on occurrence of hot tearing in A206 aluminum alloy was studied by computer simulation using PROCAST software and real casting experiments in T-shaped constrained and unconstrained sand moulds. The tensile forces required for tearing and fracturing of the castings in constrained and unconstrained conditions as well as the torn and fractured surfaces of the constrained and unconstrained specimens were analyzed for each gating system design. The results showed that the type of gating system influences the velocity and surface turbulence of the melt and consequently influences the formation of oxide bifilms and hot tearing susceptibility of A206 aluminum alloy. The results of the mould filling and solidification simulations of the castings showed that the naturally pressurized gating system with surge control had resulted in lower surface turbulence during mould filling, decreased hot tearing susceptibility based on the Clyne and Davies criterion and increased cooling rate of the melt. The experimental results of constrained mould castings confirmed the prediction of Clyne and Davies criterion for hot tearing susceptibility, as the most severe hot tearing occurred with the pressurized gating system and the least severe hot tearing occurred with naturally pressurized gating system with surge control. It was also shown that the naturally pressurized gating system with surge control resulted in less oxide bifilm formation and smaller grain size in the hot spots of the castings and, consequently, in improved tensile
