Characteristic analysis of lost foam casting based on ProCAST simulation

The material of a flat casting is aluminum alloy alsi17cu3. The liquidus temperature is 636 ℃, the solidus temperature is 492 ℃ and the pouring temperature is 745 ℃; The data of foam and dry sand are in ProCAST’s own database. The whole model consists of four parts: hollow sprue, two foam plates and dry sand mold.

1) Difference from traditional sand casting

In terms of filling, the difference in filling sequence between the two can be seen from the filling time cloud diagram. lost foam casting starts from the inner gate in a fan shape and advances to the far end, and the final filling position is the farthest end of the local distance from the inner gate; The traditional sand mold casting filling tends to fill the bottom of the casting at first, and then slowly surge up from the bottom until the whole cavity is filled. Finally, the filling position is the farthest end of the casting in the direction of gravity.

From the filling pattern of the two molten metal in the transverse runner, it can be found that due to the existence of foam and negative pressure, the process of lost foam casting’s filling force originates from the process of foam combustion giving up the cavity space, and is not sensitive to gravity. The front of the filling mold has a larger temperature swing. The gas back pressure of this layer will hinder the further filling of the mold cavity by the liquid metal. Therefore, the liquid metal at the filling front of lost foam casting generally presents a concave arc shape and tends to move forward attached to the mold wall with low air gap pressure; The dynamic force of sand mold filling is gravity acceleration. The liquid metal at the front of filling always tends to flow vertically downward, and the liquid metal flows downward in a full convex arc. Different filling order and filling shape will lead to different filling results, so the defect types and the location of the same type of defects in the filling process of the two processes will be very different.

In the solidification process, it can be seen from the cloud diagram of solidification time that there is little difference between the two processes. In fact, due to the certain thermal insulation characteristics of refractory coating and dry sand compared with traditional sand mold, the solidification time of lost foam casting should be longer. When lost foam casting is carried out, this characteristic needs to be simulated by appropriately reducing the heat transfer coefficient between metal and mold.

In addition, the solidification sequence of lost foam casting and sand mold castings is generally the same. Generally speaking, the mold filling time of castings is relatively short (small and medium-sized parts are generally a few seconds to tens of seconds). In a relatively short time, for high-temperature molten metal such as cast steel and cast iron, the difference in the initial temperature field between them will not be too large, and the final solidification results will not be very different, The change of temperature field during solidification only shows that lost foam casting lags behind the sand mold. Therefore, generally, when only the solidification process of lost foam casting needs to be simulated, the calculation of lost foam casting filling process can also be ignored (generally time-consuming), and the pouring temperature can be directly used as the initial temperature of the casting to conduct simple temperature field simulation or the coupling of temperature field with stress and micro model to estimate the solidification process results of the casting.

2) Influence of gas back pressure factors

For software simulation of lost foam casting after version 10, it is recommended that gas = 2 in flow parameters, considering the role of gas in mold filling and the role of gas back pressure, and gas = 1 considering the existence of gas but not the influence of back pressure. In terms of filling time, the consideration of gas back pressure (Filltime=1.704s) is longer than that of Filltime=1.347s without considering the gas filling time. It reflects that there is a certain choke effect in the filling process of gas backpressure. Improving sand mold and coating permeability and decreasing foam density can reduce the air gap pressure at the filling front and facilitate smooth filling. When gas = 1 is set, the position and range of air entrapment can be estimated. As shown in the figure, air entrapment is easy to occur near the final filling position of the casting, while air entrapment generally occurs in the turbulent area in sand casting.