Main failure mode of aluminum alloy die-casting molds is thermal fatigue. Melting temperature of aluminum alloy is 600~760℃, surface temperature of die-casting mold cavity is as high as over 600℃, and thermal fatigue failure accounts for about 70%. Thermal fatigue is mainly due to thermal stress being greater than thermal fatigue strength limit during die casting cycle. Thermal stress is caused by temperature fluctuations in die-casting mold. Therefore, it is becoming increasingly important to understand evolution of die-casting mold temperature and formation of thermal balance during aluminum alloy die-casting cycle. Zhao Xinyi, Hsieh, Li Zhaoxia, Zhang Guangming and others respectively studied effects of cooling process, cooling water temperature and cooling pipe diameter, mold preheating temperature, pouring temperature and other factors on temperature field of magnesium alloy and aluminum alloy castings and molds. However, there are no reports on influence of mold materials on thermal balance. Mold materials have a very important impact on temperature, thermal balance and service life of aluminum alloy die-casting molds.
This article uses PRO/E and PROCAST software, taking front cover aluminum alloy die-casting as an example, through simulation of temperature field during die-casting cycle, study formation of thermal balance of die-casting mold, influence of mold material on temperature curve at different positions from surface of mold cavity, analyze influence of mold material on formation of thermal balance to provide guidance for selecting reasonable mold materials.

 

Figure 1 is a diagram of aluminum alloy parts of front cover, which is made of A390 aluminum alloy. Figure 2 is a simplified model of fixed and movable die parts.

Preheating temperature of movable and fixed molds is 200℃, pouring temperature is 700℃, heat transfer coefficient between casting and mold is 1500W/(m2·C), heat transfer coefficient between molds is 1000W/(m2·C), heat transfer coefficient between mold and air is 5W/(m2·C), heat exchange coefficient between mold and release agent is 500W/(m2·C). Since molten metal fills mold cavity instantly, and at the same time, this simulation focuses on mold, so filling process is not considered. Cycle period is 30s. Filling starts at 0s, mold is opened at 15s, die casting is rolled out at 20s, release agent is sprayed at 23s, spraying ends at 25s, and mold is closed at 29s.

Mold materials use H13, ceramics and pure copper with widely different properties. Main factors affecting mold temperature are thermal conductivity and specific heat capacity of mold material. Figure 3 shows thermal conductivity of mold material.

 

 

Five nodes on middle section of movable mold are selected as analysis objects. Figure 4 shows cross-sectional position and node position diagram. Nodes 1 to 5 respectively represent the five points from top to bottom in Figure 4(b). Node 1 is on cavity surface, and node 5 is the farthest away from cavity surface.

 

Figure 5 is a schematic diagram of temperature rise of die-casting mold from preheating temperature To to steady state. Die-casting mold increases from average preheating temperature To before the first die-casting cycle to steady-state temperature Tmin. It can be seen from Figure 5 that temperature (Ti) of a certain point on mold at the beginning of any die-casting cycle before reaching a steady state can be expressed as sum of mold preheating temperature and temperature increment (δT) of each die-casting cycle experienced before die-casting cycle:
T i =δT i-1 +δT i-2 +δT i-3 + …… +T 0 =T 0 +ΔT (1)
Temperature changes at each point in die-casting mold are realized through continuous die-casting simulation. Each working cycle consists of several stages: cooling, mold opening, spraying release agent, and mold closing. Continuing simulation process of above temperature field, T max and T min remain constant when thermal equilibrium is reached, and temperature increment (δT) of each die-casting cycle is zero.

 

Figure 6 shows 5-node temperature change curve during continuous die casting of H13, copper and ceramic mold materials. As can be seen from Figure 6, after about 50 die-casting cycles from preheating temperature of 200℃, temperature change of die-casting mold tends to be stable, and die-casting mold enters a thermal equilibrium state. During this thermal balance formation process, as cycle proceeds, temperatures of five nodes on mold surface increase δT in the overall temperature each cycle, which is not exactly same. δT value is the largest at the beginning of die-casting cycle. As number of die-casting times increases, δT becomes smaller and smaller, and reaches zero when thermal equilibrium is reached. Node 1 closest to cavity surface has the largest temperature fluctuation (T max -T min ) in each cycle. Temperature fluctuations of nodes farther away from cavity surface are smaller. Temperature fluctuations of the five nodes in Figure 6(a) are not very different. The most important reason is that thermal conductivity of copper is very large, which causes temperature to transfer quickly from cavity surface to substrate or from substrate to surface, so temperature fluctuation thickness is relatively large. Among the five points of ceramic mold material in Figure 6(b), only node 1 has temperature fluctuations, and the other four nodes have basically no temperature fluctuations in each cycle. This is mainly due to fact that thermal conductivity of ceramics is very low, and depth of temperature change in each die-casting cycle is very thin. In Figure 6(c), temperature of nodes 1 to 4 of H13 mold material fluctuates, but only node 5 does not fluctuate. This is due to thermal conductivity of H13 being between that of copper and ceramic.

 

Figure 7 (a) shows temperature change curves of node 1 of three mold materials. T max of node 1 after thermal equilibrium is 582℃ for ceramic material, 529℃ for H13, and 506℃ for copper. This shows that the smaller thermal conductivity, the higher maximum temperature of cavity surface point. Fluctuation amplitude (T max -T min ) of node 1 is 142℃ for ceramic material, 64℃ for H13, and 43℃ for copper. Temperature fluctuation amplitude of cavity surface point is the largest for ceramic materials, followed by H13, and copper is the smallest. This shows that when cavity surface absorbs and dissipates heat, the smaller thermal conductivity, the more difficult it is for heat to diffuse from cavity surface to mold base or from base to surface, and the greater surface point temperature fluctuation range. Figure 7(b) shows temperature change curves of node 2 of the three mold materials. Node 2 is inside cavity and at a certain distance from surface of cavity. Temperature fluctuation amplitude of cavity subsurface of the three mold materials is smaller than temperature fluctuation amplitude of cavity surface. This is because subsurface is not in direct contact with high-temperature aluminum and low-temperature release agents. Heat absorption and heat dissipation are completed through heat conduction of mold material between this point and cavity surface. Therefore, its sensitivity to temperature is weaker than that of cavity surface. At subsurface point 2, copper has the largest fluctuation amplitude, followed by H13, and ceramic material has the smallest amplitude. This is determined by thermal conductivity of various materials.

(1) A three-dimensional finite element model of aluminum alloy die-casting of front cover was established using PRO/E and PROCAST.
(2) Temperature change curves of five nodes at different distances from cavity surface were analyzed. It is pointed out that mold enters thermal equilibrium after 50 die-casting cycles. Temperature fluctuations at points closer to the cavity surface are greater.
(3) Effects of H13, copper and ceramic mold materials on temperature curves of cavity surface points and subsurface layers were analyzed. The greater thermal conductivity, the smaller temperature fluctuation amplitude of cavity surface points, and the greater cavity subsurface temperature fluctuation amplitude.