Study on Corrosion Cause of the Aluminum Wheel Mold During Low Pressure Die-casting Process

       Study on Corrosion Cause of the Aluminum Wheel Mold During Low Pressure Die-casting Process

CHEN Zhenming1 , LIN Jiahua2 , FU Jiapan2 , CHEN Qingxun1 , ZHAO Haidong2

(1. Foshan Nanhai Superband Mould Co., Ltd., Foshan 528234, China; 2. Nation Engineering Research Center for Metallic Net Shape Forming, South China University of Technology, Guangzhou 510640, China)

Abstract： The corrosion phenomenon in wheel mold during low pressure die casting process was investigated with optical microscope and scanning electron microscope. The results show that the surface of mold cavity reacts with high-temperature aluminum liquid after coating falling off, which results in soldering in the mold surface. Then, the transition layer is formed and it composes of Fe-Al-Si intermetallic compound. Finally, after the cyclic action of high-temperature aluminum liquid, the transition layer continuously diffuses and falls off from the mold surface, and it eventually causes corrosion of the mold.

Key words： #aluminum alloy wheel; #low pressure die-casting; #mold; #soldering; #corrosion

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As a precision casting technology, low-pressure casting is currently the main method for manufacturing aluminum alloy wheels for automobiles [1-3]. In actual production, low-pressure casting molds, especially the bottom mold, are subjected to repeated impacts of high-temperature melts. The working environment is harsh and erosion, aluminum sticking, and corrosion are very likely to occur [4,5], which affects the service life of the mold and increases the investment cost of the mold. This research takes low-pressure casting wheel molds as the object and studies the causes of its corrosion, aiming to provide a basis for mold maintenance and mold failure prevention in actual production.

1 Analysis of samples

Figure 1 shows a corroded low-pressure casting wheel bottom mold. The bottom mold is used to produce 18-inch aluminum alloy wheel castings and its material is H13 steel. The sample was cut from the corroded gate of the bottom mold by wire cutting, and the microstructure characteristics of the corroded area and the non-corroded area of the sample were compared and analyzed by optical microscope (Leica DMI 5000M) and scanning electron microscope (FEI Quanta 200).

2 Results and Analysis

2.1 Microstructure Analysis

Figures 2(a) and 2(b) show the front and side schematic diagrams of the analyzed sample, respectively. Under an optical microscope, 50x metallographic photographs were continuously obtained along the surface of the mold cavity, and then the panoramic images of the corroded area and the uncorroded area were synthesized (Figures 2c and 2d). For further analysis, the corroded area was divided into area 1, area 2, and area 3, as shown in Figure 2(c). It can be seen that in both the corroded area 3# and the uncorroded area, two layers of abnormal tissue were observed near the mold cavity surface, with a clear boundary with the mold substrate.

Figures 3 to 5 show the microstructures of the corroded area 1, area 2, and area 3, respectively.

(1) In the microstructure of region 1, two different organizational morphologies were observed (Fig. 3a), which were separated by an obvious “interface” (as shown by the dotted line in Fig. 3a). Near the “interface”, on the side close to the mold matrix, an obvious overheated structure appeared (Fig. 3d). This phenomenon was mainly caused by the excessively high local temperature of the mold, which led to the recrystallization of the grains。

(2) In region 2, aluminum adhesion is clearly observed, and its microstructure has two types (Figures 4b and 4c). Along the direction from the mold substrate to the surface, the microstructure characteristics of Figure 4(b) are H13 structure, abnormal structure (similar to region 1), transition structure, and aluminum adhesion layer, while Figure 4(c) is H13 structure, transition structure, and aluminum adhesion layer.

(3) In the microstructure of region 3, abnormal structure is also observed (Figure 5a), forming a clear boundary with the mold substrate (dashed line in Figure 5a). Figure 5b is a partial enlarged view of Figure 5a. It can be observed that the structures on both sides of the "interface" are obviously different, and an obvious overheated H13 steel structure appears on the side close to the mold substrate (similar to region 1).

2.2 Energy spectrum analysis

Based on the observation results of the microstructure, energy spectrum analysis was performed on the characteristic parts to determine the composition of each organization. Figure 6 shows the energy spectrum analysis results of the organizations on both sides of the "interface" in corrosion area 1. The side close to the mold substrate mainly contains Fe, Cr, Si, V and other elements, which are normal H13 mold steel components (4Cr5MoSiV); while a large number of C, O and Mn elements appear on the side close to the cavity surface. Therefore, the material organization and composition on both sides of the "interface" are different. Combined with the analysis of the processing method of the bottom mold gate, it can be determined that the abnormal organization observed in the corrosion area and the uncorroded part is the welding material of multi-pass welding, in which the C element is caused by the welding core material, and the O element is caused by the inhalation during the welding process.

Figure 7 shows the line scan result of the aluminum sticking area from the mold substrate to the cavity surface in corrosion area 2 (Figure 4c). It can be seen that the transition layer contains elements such as Fe and Al. The reason is that the Al element in the aluminum melt and the Fe element in the mold steel form FeAl intermetallic compounds on the mold surface[6], and then form FeAlSi intermetallic compound transition layer with aluminum melt. Its thermal conductivity is low, resulting in serious aluminum sticking.

During the low-pressure casting process, the coating on the surface of the bottom mold cavity is subjected to the combined effects of the impact of aluminum liquid during filling and the friction between the casting and the mold during demolding[7,8]. When the bonding force between the coating and the mold surface is insufficient, the coating will be detached, resulting in the exposed part of the mold surface being directly corroded by the high-temperature aluminum liquid in the next mold. In addition, due to the high temperature of the bottom mold, especially the gate part near the riser, the coating sprayed has a weak bonding force with the mold and is easy to fall off. Finally, as the casting production cycle continues, mold erosion continues to worsen, and local peeling occurs. Therefore, in corrosion areas 1 and 2, due to the repeated erosion of high-temperature aluminum liquid, the repair welding materials peeled off. Only a small amount of multi-pass welding structure similar to that in area 3 was observed microscopically, while corrosion and shedding of the gate of the bottom mold was observed macroscopically...

3 Conclusion

During low-pressure casting production, due to the lack of coating protection, the mold steel is in direct contact with the high-temperature aluminum liquid. The Fe element in the mold steel and the Al element in the aluminum melt form FeAl intermetallic compounds on the mold surface, and then form a FeAlSi intermetallic compound layer with the aluminum melt. Its thermal conductivity is low, resulting in severe aluminum sticking, and ultimately causing mold corrosion and even partial peeling. In order to ensure the life of the mold and the quality of the casting, aluminum sticking must be removed in time during production. The coating layer is the best measure to prevent the diffusion of aluminum sticking and the formation of the transition layer.

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