3.1 Effect of temperature on U-shaped hole morphology
Fig. 2 shows a U-shaped hole was fabricated by casting with carbon fiber as the core material. It can be seen that a complete through curved hole is achieved and the diameter is very uniform. The overall length of the U-shaped hole is 200 mm, the hole diameter is 2mm and the aspect ratio can reach to 100.
The hole wall morphologies of the U-shaped hole prepared at different temperatures are shown in Fig. 3. It can be seen that the inner wall is very flat, there is no obvious pit defect, and the hole diameter is very uniform.
Fig. 3(a) shows the wall morphology of the U-shaped hole prepared at the temperature of 998K. The roughness test showed that the roughness of the hole wall is 83.264. With the increase of temperature, the roughness of hole wall morphology decreases gradually, and the roughness of the U-shaped hole prepared at 1023K and 1048K are 79.857 and 76.287, respectively, as shown in Fig. 3(b)-(c). King et al reported that the wettability between molten metal and hole core has an important effect on the morphology of hole wall. The decrease in wettability would promote the formation of micro bulges and river ripples on the hole walls, which would increase the roughness of the hole wall. With the increase of temperature, the wettability between carbon fiber and aluminum melt increase gradually, so the roughness of the inner wall of the hole decreased gradually. However, when the experimental temperature was raised to 1073K, the roughness of the inner wall of the hole increased slightly to 89.681. This is because when the melt temperature reaches to 1073K, the carbon fibers will react obviously with the aluminum melt. Pradeep et al reported that the Gibbs Free Energy of the reaction between carbon fiber and the aluminum substrate can reach -163297.4 kJ/mol at 1073K, which is a large negative number for good roughness. As shown in Fig. 4, it can be seen that there are obvious carbon elements at hole wall when the U-shaped hole prepared at 1073K, which indicates that the carbon fiber hole core reacted with the molten metal aluminum, so the roughness of the hole wall is slightly higher.
3.2 Effect of temperature on the behavior of exfoliation corrosion
Exfoliation corrosion is a common type of local corrosion, which has serious harm to the application of aluminum alloy. The exfoliation corrosion is easy to cause the leakage problem for the follow-type cooling channel. Therefore, it is necessary to study the exfoliation corrosion behavior of the U-shaped holes prepared by the casting method. Fig. 5 shows the surface morphologies of the hole wall of the U-shaped hole prepared at 1048K after corrosion treatment at different times.
It can be seen that with the increase of corrosion time, the surface of the hole wall changes obviously. After 12 h, as shown in Fig. 5(a), the surface of the specimen lost luster and appeared slight pitting phenomenon. With the increase of the corrosion time, the pitting corrosion began to increase and the number began to increase. After 48 h, a large number of pitting corrosion and serious discoloration appeared on the surface of the specimen, as shown in Fig. 5(d). With the corrosion time increases, the roughness also changes significantly. The roughness increased from 76.287 to 82.588 after 48 h of corrosion.
Fig. 6 shows the surface morphologies of the specimens prepared at different temperatures after 48 h corrosion treatment. It can be found that the roughness first decreases and then increases with the increase of the preparation temperature. This is due to the large roughness of the hole sample prepared at 998K, the ravines on the rough surface are easy to be the starting point of corrosion during exfoliation corrosion, which will lead to a further increase in hole wall roughness.
With the experimental temperature increase to 1073K, the removal of carbides formed by the reaction between the carbon fibers and the matrix at high temperature from the surface of hole wall during corrosion is an important reason for the increase of roughness. In addition, the microstructure of the sample prepared under the condition 1073K was observed after 48 h exfoliation corrosion, as shown in Fig. 7. It can be seen that there is a phenomenon that silicon will be corroded and peeled from the surface of the U-shaped hole. During the preparation process, the long stay of aluminum melt at high temperature will not only cause the reaction between carbon fibers and aluminum matrix, but also cause the segregation of silicon elements. Under liquid conditions, the values of the mixing enthalpy between carbon and silicon is -39kJ/mol, and between carbon and aluminum is -39kJ/mol, which are much lower than the mixing enthalpy value of -19 kJ/mol between aluminium and silicon. The lower value of the mixing enthalpy between elements in the liquid metal, the easier cause element bonding. Therefore, a high experimental temperature will cause silicon to aggregate on the wall of the hole, which is consistent with the results of Zhang et al. Therefore, the reaction between the carbon fiber and the matrix and the segregation of silicon on the surface of the hole wall reduce the exfoliation corrosion resistance of the U-shaped hole prepared at 1073K.
3.3 Effect of temperature on intergranular corrosion behavior
Fig. 8 shows the microstructure at the cross-section of U-shaped hole samples prepared under different temperatures after 6 h intergranular corrosion. It can be seen that the depth of intergranular corrosion increases with the increase of preparation temperature. The corrosion depth of U-shaped hole prepared at 1023K is only 156μm, as shown in Fig. 8(b). However, when temperature increases to 1073K, the corrosion depth reaches 220μm, as shown in Fig. 8(d). It can be seen that the corrosion goes deep into the matrix along with the eutectic silicon. Because the segregation of silicon elements on the surface of the hole wall at 1073K, it will lead to significant potential difference between the precipitated phase and the aluminum matrix, and promote the intergranular corrosion of aluminum alloy. This is consistent with the research results of Svenningsen et al on the driving force of intergranular corrosion of aluminum alloy. Therefore, reasonable experimental temperature plays an important role for obtaining a follow-type cooling channel with good corrosion resistance.