Process Development and Numerical Simulation of Mine Equipment Mill Head(End Cover) Castings
Process Development and Numerical Simulation of Mine Equipment Mill Head(End Cover) Castings
Feb 26, 2025
Process Development and Numerical Simulation of Mine Equipment Mill Head(End Cover) Castings
Mill head (end cover) is an important component of the mill machine in mining machinery. The mill is a later-stage processing equipment in the entire mining equipment, which is less impacted by ore and grinds small ores into powder through high-speed rotation and internal grinding balls. The rotation process requires a high resistance to torsion for the castings, thus placing high demands on the internal quality of the entire casting. The mill head (end cover) casting studied in this paper is the main component of the mill product that withstands torsion. The casting has large dimensions and varying wall thicknesses, and defects such as shrinkage, shrinkage cavities, hot cracks, and cold shut are likely to occur during the casting process. The quality of the mill head (end cover) directly determines the overall quality of the mill, hence the casting process design capability of the mill head (end cover) represents the manufacturing level of the mill product. Advanced numerical simulation technology is used to fully simulate the filling, solidification, and cooling processes of the casting process. Based on the residual liquid and solid ratio during solidification, Porosity, Niyama and other criteria, the positions of shrinkage, shrinkage cavities, cracks, and pores can be accurately predicted, which is of great significance for preventing defects such as shrinkage, cracks, and inclusions. Compared with traditional trial-and-error and experimental methods, computational numerical simulation technology has obvious advantages: it is time-saving, cost-effective, accurately predicts defects, and can achieve visualization and quantification of defects. 1 Process Design 1.1 Composition and Testing Requirements of the Casting This casting is a rotating type casting, with a structural diagram as shown in Figure 1, and contour dimensions of φ4,610 mm × 1,556 mm. The material of this casting is carbon steel, with chemical composition as shown in Table 1, and mechanical property requirements as shown in Table 2. The NDT testing requirements are ASTMA609 UT3 level. Figure 1 Mill head(end cover) Casting Schematic Diagram
1.2 Casting Process Analysis The structure of this casting is relatively simple, with two flanges on top and bottom, forming two hot spot areas at the junction of the flanges and the straight cylinder, belonging to a structure with thick middle and thin ends. From the perspective of solidification, this casting itself is not conducive to sequential solidification and must change its own solidification gradient through casting process methods such as risers and chills; at the same time, due to the large wall thickness difference at the two hot spots, there is a tendency for cracks to appear in the later stage of solidification. Two molding schemes are formed through structural analysis as shown in Figure 2. Figure 2a) is the molding schematic of Scheme 1, which sets the hot spot M2 as the upper surface and places a clear riser here. In this scheme, the largest hot spot M1 is located in the lower box part, and it is difficult to set a riser during the feeding process; since the riser can only be a dark riser at this position, the dark riser has low feeding efficiency, large size, and serious segregation, and the later cleaning work is very large; at the same time, the 15° flange is located at the bottom, and the upper part bears 80% of the weight of the sand mold, with a high risk of deformation and cracking during solidification; and this scheme is not conducive to the smooth filling of steel liquid, and severe inclusions occur on the back of the large inclined flange during pouring. Figure 2b) is the molding schematic of Scheme 2, with the large flange facing up, and the steel liquid fills smoothly; a clear riser is directly set at the largest hot spot M1, and a dark riser is set at M2, this scheme has high feeding efficiency and is easy to remove. At the same time, the gating system can be directly set in the dark riser at M2, avoiding the steel liquid directly impacting the sand mold, ensuring the smooth and uniform flow of steel liquid. Through comparison of advantages and disadvantages, Scheme 2 is adopted in the process. Figure 2 Mill head(end cover) Forming Plan 1.3 Process Design
Feeding Process Design
Using the molding scheme of Scheme 2 as shown in Figure 2b), riser and chill design and selection are carried out through the modulus rolling method [3]. Using the formula M=V/S (M—the modulus of the casting, cm; V—the volume of the casting at the modulus, cm3; S—the heat dissipation area of the casting at that place), the modulus of the casting is accurately calculated. M1=V/S=8.9 cm, M riser=1.2M1=10.7 cm, choose the riser size φ600 mm clear riser. The final determined riser size: M2= V/S=4.4 cm, M riser=1.5M2=6.6 cm, choose the riser size φ370 mm dark riser. To increase the feeding efficiency of the riser, a chill is used in the middle of the clear riser to artificially increase the end area. The feeding liquid volume of the riser is calculated to be 42 tons, and the weight of the casting is 39 tons, determining that the riser design is reasonable.
Gating System Design
The pouring weight of this casting is 65 tons. According to the design principle that the gating system of large cast steel parts must ensure rapid filling and smooth flow, the self-developed GS-100 software is used for gating system calculation, determining the outflow velocity of the gating system to be 0.48 m/s, and determining the size of the straight pouring channel to be φ120 mm. According to the open gating system, F straight:F cross:F internal=1:1.2:1.5, the size and quantity of the cross pouring channel and internal pouring channel are calculated. 2 Initial Process Simulation 2.1 Pre-treatment Before Simulation Analysis The three-dimensional solid model of the mill head(end cover) is created using UG software and converted into a file type recognized by the Magma simulation software, imported into the software, and grid division is carried out. The grid division method with equal distance is used, and the grid edge length of the casting, chill, riser, and insulation plate is set to 10 mm. After the division, the total number of grids is 14,000,000, and the number of metal grids is about 1,500,000 [5]. 2.2 Initial Scheme Solidification Simulation Results and Analysis Figure 3 shows the results of various criteria for the casting. Figure 3a) is the shrinkage result diagram. According to the experience value, the shrinkage range is selected as 0%~20%, and this range can accurately indicate the trend of shrinkage and shrinkage cavities. From Figure 3a), it can be seen that shrinkage appears between the two risers, close to the root of the riser neck, showing a deep blue to red display, indicating that this area has the risk of shrinkage defects. According to the solidification theory, in the later stage of solidification of the casting, the part where the isolated liquid phase exists, due to the blocking of the feeding channel, the liquid phase cannot get sufficient feeding, and under the action of volume contraction, shrinkage will occur, and severe cases will produce shrinkage cavities. Figure 3b) is the FSTime display result, and the FSTime result indicates the solidification time of the casting [6]. From Figure 3b), it can be seen that the riser part of the casting is the last solidification area. This result is consistent with the sequential solidification principle of large cast steel parts, avoiding the occurrence of isolated liquid phase areas during solidification of the casting. However, in the middle area between the two risers, the solidification time of the casting is similar, indicating that the casting will have a trend of simultaneous solidification in this area. Simultaneous solidification leads to the occurrence of isolated liquid phase during solidification, thus leading to shrinkage. The simultaneous solidification method can be used when the performance requirements of the cast steel part are not high, but the working conditions of this product are relatively complex and bear high impact, so shrinkage is not allowed. From the comprehensive judgment of the above two criteria, it can be concluded that the casting process has shrinkage defects at the root of the dark riser neck, mainly because the horizontal feeding distance of the casting at this place is not enough. Figure 3 Initial Plan Simulation Results 3 Process Optimization 3.1 Process Improvement There are three methods in casting process design to solve the shrinkage of the casting: 1) Increase the riser to improve the feeding capacity; 2) Set a chill at this place to reduce the modulus of the area that needs feeding, artificially increase the feeding end area, and improve the riser feeding distance; 3) Increase a hot steel adding between the riser and the feeding area to artificially increase the feeding gradient. According to the initial process design process, the riser modulus relationship at this place meets the feeding requirements. If the riser is increased, it will cause waste of steel water and increase production costs. Therefore, considering the cost, the method of setting chills and increasing subsidies is adopted. From the solidification result analysis, the shrinkage part is located at the root of the riser neck, and the solidification time between the two dark risers tends to be simultaneous, without a solidification gradient. Therefore, chills are set in the middle of the two risers to form the end area, and the riser neck large fillet subsidy method is used to increase the feeding area distance, making the entire horizontal feeding comply with sequential solidification. The specific process improvement measures are shown in Figure 4. Figure 4 Blind Casting Head Subsidy Plan 3.2 Process Improvement Solidification Results and Analysis Figure 5 shows the simulation results after the improvement scheme. From the shrinkage simulation results in Figure 5a), it can be found that there are no bright blue and red areas, that is, there is no risk of shrinkage and shrinkage cavities; the FSTime criterion in Figure 3b) shows that the casting has a good solidification time gradient throughout the solidification process, making the casting completely in sequential solidification, and there is no phenomenon of similar solidification time at the root of the riser neck. Figure 6 shows the shrinkage results. Although there is shrinkage in the dark riser, it is because the dark riser provides feeding to the casting, and there are no shrinkage and shrinkage cavities in the casting. Comparing the simulation results of the two process schemes before and after, it can be seen that after the process improvement, the defects at the connection between the riser neck and the casting disappear, and there is no tendency for shrinkage in the casting. After actual production verification, the casting reaches nearly zero welding. Therefore, the process improvement scheme is reasonable and practical. Figure 5 Improved Plan Defect Distribution Chart Figure 6 Improved Plan Porosity Defect Distribution Chart 4 Conclusion Through structural analysis, setting a clear riser at the 15° large plane of the mill head(end cover) and a dark riser at the straight cylinder hot spot can fully change the solidification sequence of the casting and ensure product quality; using Magma software to numerically simulate the solidification process of the mill head(end cover) casting, the results of Porosity and FS Time can effectively analyze the position and cause of shrinkage in the casting; using the method of setting partition chills and large fillet subsidies to increase the riser feeding distance to optimize the initial process of the casting can solve the shrinkage problem at the root of the end riser neck.
