The inclined head pipe is a component of the electric skateboard body, which connects the frame to the steering bar and is connected to the frame through welding technology. In order to achieve lightweight body and improve the endurance of scooters, the material of the inclined head tube is aluminum alloy. Due to the large size, uneven wall thickness, and high design requirements of the product, traditional plastic processing methods cannot meet the forming requirements of the product. Squeeze casting, as a short process and near net forming technology, can produce aluminum alloy castings with complex structures and excellent performance. In practical engineering applications, squeeze casting technology is widely used in the production of structural components such as steering knuckles, engine brackets, and wheel hubs. The characteristic of this process is that the solidification, crystallization, and shrinkage of the metal liquid are completed under high pressure, producing high-performance castings from a denser microstructure.
In order to improve the internal quality of the inclined pipe, the tail gap and the difficult to form positions such as the stepped hole at the pipe mouth are filled in to reduce the difficulty of forming the casting. Subsequently, the gap position is machined through mechanical processing. Setting reinforcing ribs in the middle of the inclined pipe is beneficial for strengthening the strength of the thin walls on both sides. Adding guide ribs at the convex position is beneficial for transmitting pressure, compensating for the shrinkage of the convex position, and preventing subsequent threaded hole processing from being affected by shrinkage or loosening defects.
During the solidification process of molten metal, due to the size and wall thickness of the inclined tube, it is often impossible to achieve sequential solidification, resulting in liquid phase islands at thick and uneven wall thickness positions, leading to shrinkage and porosity inside the casting. AnyCasting can predict potential shrinkage and porosity locations inside castings and quantify the probability of such defects occurring through residual melt modulus. Residual melt modulus at temperature and mold temperature. It can be seen that the distribution of residual bodies is mainly concentrated at the head and tail of the inclined tube. The pipe head is located at the position with the maximum wall thickness of the casting and is far away from the sprue. The occurrence of hot spots between the pipe body and the pipe head leads to the production of residual melt; Due to the complex shape of the tail of the pipe, it is difficult to dissipate heat, resulting in residual molten metal at this location due to the long solidification time; Moderately increasing the pouring temperature and mold temperature can reduce the residual melt modulus. At a pouring temperature of 710 ℃ and a mold temperature of 300 ℃, the corresponding minimum residual melt modulus is 0.418 mm.
As the molten metal begins to solidify, the pressure effect on thin-walled positions such as the pipe body begins to weaken, and the pressure transmission channel is obstructed. At this time, the solid phase rate of the molten metal is 25.88%, as shown in Figure 6b. When the solidification time is 7.4 seconds, the tube body solidifies, and the solid phase rate of the metal liquid is 28.33%. Due to the solidification of the tube body, the local extrusion pressure cannot be transmitted, and the pressure is concentrated at the tube head. When the solidification time is 7.9 seconds, the solid fraction of the metal liquid is 41.84%. As the metal liquid continues to solidify, the extrusion pressure cannot be transmitted to the surface of the casting. By cutting the cross-section, the extrusion pressure is concentrated near the extrusion pin of the pipe head. When the solidification time is 23.5 seconds, the solid phase of the metal liquid is 61.28%. When the solidification time is 31.4 seconds, the position of the pipe head has completely solidified, and pressure cannot be transmitted. The pressure holding process is completed, and the extrusion pin is ready to retract.
Non destructive testing is performed on all surfaces of the inclined tube using X-rays to detect possible defects such as shrinkage porosity, bubbles, and inclusions inside the casting. Place the inclined tube casting on a horizontal moving platform, considering the inclined tube ruler Due to its large size, a single shot inspection cannot obtain a complete view of the internal structure of the casting, and it is necessary to adjust the position of the horizontal moving platform. The results showed that no obvious defects were found in the thick areas of the pipe head, and no inclusions were found inside the casting. The structure of the inclined tube is complex and the wall thickness is relatively thin. After testing, the mechanical properties of the casting blank are: tensile strength of 173 MPa, yield strength of 136 MPa, and elongation of 4.08%, which cannot meet the design requirements (tensile strength ≥ 250 MPa, yield strength ≥ 180 MPa, elongation ≥ 4%). Therefore, T6 heat treatment is needed for the inclined tube casting to improve its strength. Design a 3-level 4-factor orthogonal experiment to explore the optimal T6 heat treatment process parameters for inclined head tubes, as shown in Table 1. The results indicate that, except for sample 1, all other samples meet the requirements Design requirements for inclined head pipes. Among them, sample 9 has the best comprehensive mechanical properties. By analyzing the range of experimental results, the T6 heat treatment process parameters for the inclined tube were finally determined as follows: solid solution temperature of 535 ℃, solid solution time of 8 h, aging temperature of 180 ℃, and aging time of 4 h.
The microstructure of the as cast inclined tube is mainly composed of α – Al and eutectic Si. The α – Al matrix is mostly in the form of coarse dendrites, and the eutectic Si morphology is mostly needle shaped or flake shaped distributed at the grain boundaries, which exacerbates the cutting effect on the α – Al matrix and leads to a decrease in the plasticity of the casting, thereby reducing the mechanical properties of the inclined tube casting. The α – Al dendrites of sample 9 disappeared, and there were no obvious grain boundaries between the matrix. The eutectic Si was uniformly distributed on the α – Al matrix, and the morphology changed from needle like to spherical, greatly reducing its cutting effect on the α – Al matrix.
(1) By optimizing the structure of the inclined head tube extrusion casting, adding reinforcement ribs, flow guide ribs, and sealing the gap between the pipe mouth and the pipe tail, the overall stiffness of the casting is increased, the difficulty of forming the casting is reduced, and internal defects such as shrinkage and porosity are reduced.
(2) Numerical simulation was conducted on the oblique head tube extrusion casting process, and combined with production verification, the process parameters of oblique head tube extrusion casting were determined as follows: pouring temperature of 710 ℃, segmented injection speeds of 200, 90, and 150 mm/s, mold temperature of 250 ℃, local extrusion delay time of 2.5 s, and local extrusion holding time of 25 s.
(3) After T6 heat treatment, the comprehensive mechanical properties of the castings were significantly improved. After solid solution treatment at 535 ℃ for 8 hours and aging treatment at 180 ℃ for 4 hours, the tensile strength of the inclined tube casting is 318.4 MPa, the yield strength is 269.5 MPa, and the elongation is 12.95%. Through metallographic comparison, it was found that after T6 heat treatment, there were no obvious grain boundaries between the matrix, and eutectic Si was uniformly distributed on the α – Al matrix, with a granular morphology that changed from needle like to spherical.