Taking the temperature measured by 1 mm and 6 mm thermocouples as the input condition, the interface heat transfer coefficient is calculated by the back calculation program based on the described mathematical model. The working conditions studied in this paper are as follows: the applied pressure is 70MPa, the pouring temperature is 660 ℃, and the initial temperature of the mold is 230 ℃. The maximum difference is 5 ℃. It can be considered that the calculated results are reliable.
As shown in Figure 1, the surface temperature of the casting and the surface temperature of the die are marked as t respectively_ Castingsurf and t_ diesurf。 Where t_ The PQ segment of castingsurf curve represents the change of surface temperature of casting at the moment of melt pouring. At the initial stage of metal solidification, the surface temperature of the casting drops sharply, which indicates that the casting is cooled rapidly at the moment of contact with the die. Corresponding to the sharp drop of casting surface temperature is the sharp rise of die surface temperature, as shown in t_ The VW segment in the diesurf curve is shown. After the sharp decrease of PQ section, the surface temperature of the casting decreases slowly, as shown in QR section in the figure. The slope of Wx curve of die surface temperature decreases gradually.
At 9s, applied pressure is applied to the casting. At the moment when the applied pressure is applied, the surface temperature of the casting suddenly decreases, which is marked as RS segment, as shown in Figure 1. Corresponding to the RS segment, the XY segment represents the sharp rise of the mold surface temperature. These show that the interface heat transfer is greatly enhanced under the action of external pressure. With the decrease of casting surface temperature and the increase of die surface temperature, the temperature difference between them gradually decreases, even less than 30 ℃, as shown in Figure 1. When the surface temperature of the die increases to the maximum, it begins to decrease gradually, and the difference between the surface temperature of the die and the surface temperature of the casting begins to increase gradually.
As shown in Fig. 2, the calculation results of heat flux and heat transfer coefficient of casting die interface corresponding to the temperature calculation results in Fig. 1 are shown. The interface heat flux marked as AI increases sharply and reaches the maximum value of 2.84 MW m-2 at the instant of high temperature melt pouring corresponding to the sharp drop section PQ of casting surface, and the interface heat transfer coefficient marked as AB also increases sharply to a high value 58kwm-2k-1, which indicates that the high temperature melt is rapidly cooled at the moment of contact with the mold, and the rapid heat transfer is caused by the huge temperature difference. After reaching the maximum value, the heat flux at the interface marked as ij section decreases rapidly to 0.7 MW m-2. At the same time, the interfacial heat transfer coefficient also decreased to 6.6kwm-2k-1 and remained for 4S. The change trend of the interfacial heat transfer coefficient and the interfacial heat flux is due to the gradual deterioration of the surface contact between the casting and the die during solidification. With the decrease of heat transfer performance, the solidification process also slows down, which makes the interfacial heat transfer coefficient change little from 5S to 9s.
At the moment when the applied pressure is applied, the interfacial heat transfer coefficient increases to the maximum value of 62.8 kwm-2k-1, which is marked as de segment, as shown in Figure 2. The interface pressure and the interface heat transfer coefficient reach the maximum at almost the same time. This shows that when the interface pressure reaches the maximum, the contact state of the casting die interface also reaches the best. After the applied pressure is applied, it is the pressure holding stage. When the interface pressure between casting and die reaches the maximum value, it begins to decrease. As shown in Figure 2, the heat transfer coefficient of the interface marked as EFG decreases sharply at the beginning, and then keeps a downward trend, but the rate of decline gradually decreases, and finally almost remains unchanged until G point. The EFG segment represents the MT segment corresponding to the interface pressure reduction phase. When the interface pressure reaches the maximum value, the contact state of the casting die interface reaches the best value. At this time, the contact state is formed under the action of the interface pressure. When the interface pressure begins to decline, the contact state of the casting die interface will deteriorate sharply. Later, with the interface pressure gradually decreasing, although the extrusion pressure is decreasing, the tendency of the casting die interface separation is also weakened Therefore, the deterioration of the interface contact state gradually slows down, and the change of the contact state will be very small as long as there is pressure on the interface.
As shown in Figure 2, the GH section represents the change of the interfacial heat transfer coefficient after the interfacial pressure decreases to 0. As the solidification proceeds, the casting is cooled. Under the influence of cooling shrinkage, the interface contact between the casting and the die deteriorates, and the interface heat transfer coefficient decreases gradually.