As one of the important components of the balance suspension system for heavy-duty commercial vehicles, the balance axle bracket connects the leaf spring and the frame, and transmits them The force between transfers. Heavy commercial vehicles operate under complex working conditions, requiring high structural strength for the balance shaft bracket, which directly affects the driving safety of commercial vehicles. Therefore, when developing balance shaft bracket products, not only should the rationality of structural design be considered from a functional perspective, but good manufacturing processability is also a factor that must be considered in product design to ensure the quality requirements of the balance shaft bracket. The iron mold sand coating casting process is a process that combines the advantages of both metal mold casting and shell mold casting. This process maintains a gap of 6-10mm between the inner cavity and the mold of the iron mold, and injects and fills the gap with coated sand to solidify at a certain temperature. At this time, the coated sand tightly covers the inner cavity of the iron mold, thereby obtaining the required mold cavity for casting. The casting is made using this mold cavity. Due to the fact that the external mold is a thick iron mold, the presence of the iron mold greatly improves the heat dissipation rate during the casting process, thereby refining the grain structure of the casting and enhancing its mechanical properties. At the same time, the iron casting process with sand coating can control the thickness of the sand coating layer according to the structure of the casting, in order to achieve sequential solidification and reduce the probability of casting defects. However, the structure of the balance shaft bracket for heavy-duty commercial vehicles is complex and the casting height is high, making it difficult to design the iron sand covered casting process, and there are few relevant research reports.
The key to the design of the casting process is to adopt the iron type sand covered casting technology, based on the structural characteristics and mechanical performance requirements of the balance shaft bracket, and combined with the actual production process, to ensure that there are no shrinkage or loosening defects inside the balance shaft bracket. The preliminary design casting process plan is as follows: place the balance shaft vertically and keep the maximum projection surface of the casting in a horizontal state, which can ensure the external quality of the balance shaft root; Adopting side injection casting, two pieces per box; The base of the balance shaft is relatively thick and prone to hot spots, so a cold iron is placed at the base of the balance shaft; The initial temperature of the outer iron type is 180-220 ℃, the initial temperature of the sand layer is 70-90 ℃, the pouring temperature is 1410-1420 ℃, the pouring speed is 30 cm/s, and the pouring radius is 30 mm.
The casting process plan was used for sample production, and the surface quality of the sample was good. The casting was initially processed and then assembled onto a bench testing machine for bench testing. However, during the bench testing process, the root of the balance shaft broke first, and the fatigue life of the balance shaft bracket did not meet the minimum standard. Dissecting the balance shaft bracket casting, it was found that there were serious shrinkage and loosening defects in the thick part at the root of the balance shaft.
The solidification process of the casting shows that there is a large isolated liquid phase zone in the thick area at the root of the balance shaft. Due to the lack of iron liquid to supplement shrinkage, shrinkage and porosity defects may occur. Using the residual melt modulus method to predict possible defects in the balance shaft support casting, the typical cross-section at the root of the balance shaft support casting where defects appear. From the defect prediction results, it can be seen that the thick area at the root of the balance shaft will produce obvious shrinkage and porosity defects during the solidification process, which is consistent with the actual location of the defects in the trial castings. In order to improve the casting quality of the balance shaft bracket, it is necessary to improve and optimize its initial casting process plan.
The overall wall thickness of the balance shaft bracket varies greatly, and the thicker areas are connected to the surrounding thin-walled structures. Therefore, during the solidification process, the shrinkage channel is easily cut off, and the probability of shrinkage porosity in the casting is high. This is the reason for the shrinkage porosity defect at the root of the balance shaft bracket. Based on the defect situation of castings in the trial production and simulation of samples, the original process plan was optimized by adding a riser at the thick position at the root of the balance shaft, and using the riser to compensate for shrinkage. At the same time, due to the long vertical distance between the lower part of the casting and the sprue, the metal liquid has a certain impact force on the lower cavity of the casting during the filling process. To address this issue, an sprue is added to the lower part of the casting to buffer the impact force generated by the vertical fall of the metal liquid and reduce the impact on the sand layer. At the same time, the added sprue can also provide contraction channels for the bolt holes on both sides of the balance shaft bracket.
The simulation results of the casting filling process after optimizing the process plan show that due to the addition of an internal gate at the lower part of the casting, the metal liquid can directly enter the lower part of the mold cavity through the newly added internal gate. Afterwards, the metal liquid from the upper part of the casting gradually flows into the lower part of the mold cavity. At this time, there is already some metal liquid in the lower part of the casting, reducing the filling drop and preventing the impact of vertically falling metal liquid on the sand layer. As the filling time increases, the metal liquid fills the lower part of the casting cavity from bottom to top, and there is no splashing of the metal liquid, filling the maximum cross-sectional position of the casting. At this point, the liquid level of the metal liquid has become stable. When the filling rate reaches 85%, most of the mold cavity of the casting is already filled with molten metal, and the liquid level continues to fill to the riser and balance shaft, ultimately completing the entire filling process. The entire filling process has a good effect, with the metal liquid filling the entire cavity from bottom to top. Compared with the pre optimized process plan, the metal liquid filling process in the optimized process plan is smooth and has less impact on the sand layer.
The solidification of castings starts from the surface layer, followed by the solidification of the thin-walled positions of the castings first; As the temperature decreases, the casting begins to solidify towards the center around the balance axis and riser area. Due to the presence of the riser, the metal liquid at the root of the balance shaft is always connected to the metal liquid in the riser before it completely solidifies. The riser provides a shrinkage channel for the thick area at the root of the balance shaft, and the riser area finally solidifies, thus reducing the probability of shrinkage and porosity defects at the root of the balance shaft. The comparison between the prediction of casting defects under the optimized casting process scheme and the actual cutting results of the trial production samples shows that both simulation and trial production results show that the shrinkage and porosity defects at the root of the balance shaft have been completely eliminated. That is, during the solidification process of the casting, the root of the balance shaft solidifies before the riser, introducing the shrinkage and porosity defects into the riser, thereby ensuring the internal quality of the balance shaft support casting body. In addition, the surface quality of the entire casting is good and meets the production process requirements. Through bench testing of the optimized process plan and trial production of the balance shaft bracket, it was found that there was no damage to the balance shaft bracket before reaching the fatigue life requirements, which meets the usage requirements.
Conclusion
(1) The sample produced by the initial casting process plan of the balance shaft bracket showed significant shrinkage and porosity defects at the root of the balance shaft, which was due to the slow solidification of the thick and large area at the root of the balance shaft, resulting in isolated liquid phase The area is not affected by the shrinkage caused by iron liquid supplementation.
(2) On the basis of the initial casting process plan for the balance shaft support, improvements were made, including the addition of a riser at the base of the balance shaft to compensate for shrinkage, so that the base of the balance shaft solidifies before the riser, and then in the casting process Add an internal gate at the bottom of the component to reduce the impact of molten metal on the sand coating.
(3) The simulation and trial production results under the optimized casting process scheme for the balance shaft bracket show that the shrinkage and porosity defects at the root of the balance shaft have been completely eliminated, and the surface and internal quality of the casting are good, which is in line with the requirements of the balance shaft bracket casting Production requirements.