When pouring the interface heat transfer coefficient, the heat exchange process between the casting and the mold begins from the time when the liquid metal contacts the mold. The heat transfer in the mold is the result of the thermodynamic imbalance between the metal, the mold and the surrounding medium.
In the process of mold filling and solidification of liquid metal in lost foam casting, with the gasification and decomposition of the pattern, the front end of the liquid metal will be subjected to a certain degree of chilling at the mold wall. Due to the existence of the coating on the surface of the pattern, the heat transfer will be affected. In addition, the transmission of the decomposition products of the pattern will also affect the heat transfer, These phenomena make the heat transfer characteristics of lost foam casting filling and solidification process very different from that of ordinary cavity casting.
The test principle is to establish a one-dimensional solidification model by using a plate with infinite width and height, measure the temperature curve inside the lost foam casting and the mold through the computer data acquisition system, and calculate the interface temperature drop. The ratio of the heat flow through the interface to the interface temperature drop is H1 = Q/ Δ T is the interface heat transfer coefficient. As shown in Fig. 1.
The test method is to place 4 thermocouples at different distances from the model mold interface in advance. In order to ensure accurate positioning of thermocouples, each thermocouple position is realized by a positioning device. After sand vibration molding, the casting is poured under vacuum and negative pressure, and the computer data acquisition system records the temperature change process of lost foam casting and mold.
The relationship between interface heat transfer coefficient and temperature is shown in Figure 1. The experimental results show that the interfacial heat transfer coefficient is constantly changing during the whole filling and solidification process, and the thickness of coating and the temperature change during pouring have obvious effects on the interfacial heat transfer coefficient. The thicker the coating layer or the higher the temperature at the interface of lost foam casting, the smaller the value of the interfacial heat transfer coefficient H1.
In the lost foam casting method, the surface of the mold is coated with 0.8-1.5mm thick coating, and the casting / Mold Interface belongs to non ideal contact, because when the lost foam casting is cooled and solidified in the mold, its volume will gradually shrink, while the mold will gradually expand when heated. Thus, when the lost foam casting is cooled and solidified in the mold, the interface temperature difference will occur between the lost foam casting and the mold. In the coating layer, heat transfer depends on heat conduction. The solidification process of the casting is affected by the interface heat transfer coefficient H1. The influence of the coating on the heat transfer process in lost foam casting can be simplified as the change of the interface heat transfer coefficient between the casting and the mold. When the pattern is decomposed under the action of liquid metal, the thermal decomposition products of the pattern will enter the nearby dry sand through the coating. The thicker the coating layer, the greater the temperature drop at the metal mold interface, and the smaller the value of the interface heat exchange coefficient H1.