Titanium alloy is widely used in aerospace, military and other fields, with the rapid development of titanium alloy casting process, especially investment precision casting, the quality of titanium alloy castings is increasingly concerned. The use of ProCAST simulation technology to carry out mold filling and solidification analysis of castings is conducive to reducing the time of continuous trial and error in the product design process, and overcoming the difficulties in process design and saving product development time through the dynamic filling and shrinkage process of the riser in the design process of the casting riser.
The titanium alloy joints of a special automobile door are one of the typical parts that are prone to fracture during the current service, and the structure is relatively simple, but the casting has isolated hot joints, which is easy to produce shrinkage holes. ZTC4 titanium alloy for commonly used articulation parts is produced by investment precision casting process, and the processing process is: wax mold – coating – melting – pouring – mechanical processing thermal isostatic pressing – scanning – thermal treatment – mechanical processing – finished products. In the production process of the hinged parts, it was found that there were large pressure pits at the connection between the casting body and the riser of multiple batches, and the subsequent repair welding affected the performance of the casting, resulting in fracture during actual use. The cause of defects such as shrinkage holes of titanium alloy castings is that the titanium alloy liquid has low superheat, pouring and cooling under vacuum, and its shrinkage capacity is not as good as that of cast steel, and the shrinkage distance is short. The shape and size of the titanium alloy riser have a great influence on the shrinkage effect.
The problem of shrinkage holes at the root of the hinged part is largely due to changes in the shape and size of the large opening. The riser is a cavity used to store molten metal in the mold during the casting process, which replenishes the metal when the casting is formed, and has the effect of preventing shrinkage, shrinkage, exhaust and slag collection. In response to this problem,
ProCAST software was used to simulate risers of different sizes and shapes, analyze the distribution of hot joints and craters, and obtain riser schemes that can eliminate shrinkage holes, providing reference for actual production.
1 Model building
1.1 Hinges and their chemical composition
The material of the articulation is ZTC4, which belongs to the commonly used cast titanium alloy in the aerospace and military fields, the material belongs to the medium temperature alloy, is one of the most widely used and used titanium alloys, used in this study .The articulation components are shown in Table 1.
Tab.1 Chemical composition of the hinge
| C | O | N | H | Fe | Al | V | Other impurities | Ti |
| 0.02 | 0.11 | 0.02 | 0.003 4 | 0.19 | 6.5 | 3.9 | 0.4 | Allowance |
Fig.1 3D structure of the hinge
1.2 Finite element model
According to the different shapes of castings, the shapes of commonly used risers are cone-shaped, trapezoidal ellipsoidal, and trapezoidal with arc. The size or diameter of the riser of the investment precision titanium alloy casting should not be less than 3 times the thickness of the shrinkage hot node, and the height of the riser is usually taken as 1.5~2.0 times of its thickness or diameter. In the process of riser design, it is necessary to consider the smooth and tidy connection between the riser and the casting, so as to avoid inclusions and stains or even local chemical reactions here [10]. Before designing the gating system of the hinged parts, the distribution of shrinkage holes in the casting without risers is first analyzed, and it can be seen from Figure 2 that there are isolated hot joints in the square structure of the casting “H” shaped protrusions, which are prone to shrinkage holes. In this study, according to the shape of the hinged part and the distribution of shrinkage holes when the casting body solidifies, five different schemes of risers were set for simulation analysis, as shown in Figure 3, scheme 1 set 2 trapezoidal gates with slope at the bottom of the casting, and 2 at the upper end perpendicular to the position of the root shrinkage hole round riser; Scheme 2 On the basis of scheme 1, change the round riser at the top to a trapezoidal riser with a slope to increase the shrinkage of molten metal; Scheme 3 rotates the casting counterclockwise 90° on the basis of schemes 1 and 2, and sets a trapezoidal with a slope at the upper end perpendicular to the position of the root shrinkage hole Mouth; Scheme 4 On the basis of scheme 3, a trapezoidal riser with a slope is provided at the top of the base plate; Scheme 5 On the basis of scheme 4, change the riser perpendicular to the upper end of the root crater hole to a conformal riser.
In this study, the method of static pouring was adopted, and the gating system with different shapes and sizes of risers was used, and the combined gating system designed by the 5 schemes was shown in Figure 4, in order to simulate the speed and accuracy of the calculation, right The same parts are meshed in different sizes, with the grid size of the straight runner set to 8 mm, the grid size of the cross runner set to 6 mm, the size of the riser and casting set to 3 mm, and the finite element model. And because the actual production of medium-sized shells requires 11 layers of slurry, it has been simulated In the process, the thickness of each layer of slurry is approximated to 1.3 mm, that is, the thickness of the set shell is 14 mm, a total of 1 554 465 individual meshes are divided, and the final finite element model is shown in Figure 5.
In the actual production process, the temperature of the mold before pouring also has an effect on the fluidity and filling of the titanium alloy, and the initial temperature of the poured mold shell is set to 300 °C, the pouring temperature of the molten metal is set to 1 720 °C, the pouring time is set to 5 s, and the other relevant simulation parameters are shown in Table 2~4.
Fig.2 Shrinkage distribution of the hinge body without the gating system
Fig.3 Five riser schemes with different shapes and sizes
Fig.4 Molding scheme of the castings
Fig.5 Meshing of the castings
Tab.2 Related parameters in the simulation
| environment/℃ | Mold shell initial | Pouring /℃ | Pouring time | Pouring furnace spokes Emissivity | Pouring speed |
| 30 | 300 | 1720 | 5 | 0.91 | 5.76 |
Tab.3 Heat transfer coefficient between molten metal and mold shell
| temperature/℃ | 0 | 25 | 1600 | 1650 | 2000 |
| 换热系数 /(W·m-2·k-1) | 30 | 30 | 100 | 600 | 600 |
Tab.4 Emissivity of the mold shell
| temperature/℃ | 30 | 600 | 800 | 1000 | 1200 | 1400 | 1600 | 1800 | 2000 |
| Emissivity | 0.9 | 0.71 | 0.62 | 0.56 | 0.51 | 0.48 | 0.47 | 0.46 | 0.45 |
2 Simulation results and analysis
2.1 Analysis of hot joints under different risers
Due to the relatively complex structure and pouring process of multi-scheme simulation analysis of castings, in order to ensure smooth and rapid flow and complete and smooth filling, it is necessary to prevent the occurrence of cold isolation during the filling process. With the flow of molten metal, the temperature of high-temperature titanium liquid decreases, and the molten metal flows into the casting cavity and gradually flows into the riser; As the distance traveled by the molten metal increases, the temperature of the bottom runner gradually decreases. The temperature near the gate is high, and vice versa. And according to the structural analysis of the casting, the temperature of the casting in the same position is lower than that of the wall thickness. Filling solidification affects the quality of subsequent castings, and titanium alloy castings are prone to defects such as shrinkage holes, shrinkage looseness, porosity, and inclusions during the solidification process. Therefore, the scheme design of the gating system needs to meet the requirements that liquid titanium can quickly and smoothly fill the mold cavity from bottom to top from the same direction, without eddies, splashes and cut-offs, and the gas in the cavity can be smoothly discharged outside the mold.
The temperature field of the hinged part filling process is shown in Figure 5, when the casting is filled for 3 s, the molten metal begins to flow into the mold from the cross runner, and the flow rate of the molten metal is 0.5 m/s. When the filling time is 3.5 s, that is, the filling rate is 70%, the flow rate of molten metal in the mold is 0.33 m/s. When the filling time is 4 s, the filling rate is 80%, the flow rate of the molten metal is stable at 0.3 m/s, until the 5 s filling is completed, the flow rate of the molten metal in the mold is stable at about 0.3 m/s, and there is no obvious turbulence, splashing and pouring dissatisfaction, that is, the metal liquid filling is stable and the gating system scheme is reasonably designed. From the perspective of the various time periods of filling, liquid metal titanium is poured from the direct pour
The channel starts to flow into the transverse runner, and the molten metal flows from the inner runner into the cavity. With the filling time, the molten metal is smoothly gravitationally cast, the flow is smooth and rapid, and there is almost no dark red area in the casting body area, avoiding scattered and isolated hot joints. The casting filling and forming effect is good, and the gating system design and the number of gates meet the requirements. The results of the hot joint distribution of the solidification field at the end of the filling can be seen
Out, the hot joints in the castings under different risers are basically isolated at the root of the casting. In the early stage of molding, molten metal gravity casting; With the filling process, the entire cavity of the articulation is basically liquid metal titanium; At the end of molding, a small amount of solidification occurs in some areas of the casting, and as the solidification progresses, the molten metal in the casting cavity first solidifies, followed by the riser on the casting, and finally the inner runner and the outer runner. However, as the solidification progresses, the thick wall in the casting cavity solidifies after the riser, and the solidification rate of this part is higher than that of other positions in the casting, and the shrinkage channel is blocked by the dendrite formed. As shown in Figure 7, in the first 4 riser design schemes, the riser is solidified before the hot joint, the shrinkage channel is closed, and in the subsequent solidification process, the riser has no effect on the casting, and the casting can only rely on self-contraction. Select a typical node at the last solidification position of the casting, and track its temperature over time. As shown in Figure 8, the metal liquid enters the sprue at the beginning of the filling process at a node temperature of 1 720 °C. As the filling progresses, the molten metal flows through straight runners, cross runners and shunts into nodes and nodes
The point itself cools to equilibrium, the temperature is maintained at about 1 650 °C, at 158.4 s, the node temperature drops to 1 600 °C, has been completely solidified, after 158.4 s, the node is completely solidified, the metal flow channel is closed, no metal flows into the node, the node temperature continues to decrease, and the furnace cools to room temperature. At 158.4 s, the temperature of each part of the casting drops below the solidus temperature, indicating that the casting has been completely solidified, and the molten metal at the position of the isolated hot joint shrinks freely to form a cavity, that is, shrinkage holes.
Therefore, try to design the fifth riser according to the shape of the casting structure, that is, the maximum wall thickness, according to the solidification heat node, the riser neck is solidified after the maximum wall thickness, and the casting can be completely supplemented by the riser
Shrinkage, indicating that this riser design is more reasonable.
Fig.6 Distribution of filling field in different time periods: (a) 3 s, 60% filling rate, (b) 3.5 s, 70% filling rate, (c) 4 s, 80% filling rate,(d) 4.5 s, 90% filling rate, (e) 5 s, 98% filling rate
Fig.7 Distribution of thermal nodes under different schemes
Fig.8 Curve of typical node temperature of casting changing with time
2.2 Analysis of craters under different risers
Figure 9 shows the simulated distribution of shrinkage holes with porosity set to 30 at different risers. As can be seen from the figure, the shrinkage holes of the risers of the first 2 schemes are slender and concentrated in thick area reinforcements perpendicular to the riser retractor
, the large shrinkage hole of scheme 1 is 2.86 cm3, and the slender type of scheme two is large
The size of the craters is 2.95 cm3
, this large shrinkage hole on the strength of the casting,
Plastic toughness has a great impact and must be avoided. Scheme 3 ~ 5 In order to avoid the slender shrinkage as shown in schemes 1 and 2, rotate the casting counterclockwise by 90°, this mold combination method can effectively avoid the slender shrinkage holes, but the root of the casting riser of solutions 3 and 4 has 2.62 cm3
The size of the shrinkage hole, the shrinkage hole in the rest of the part is basically effectively eliminated. Scheme 5: According to the shape of the casting, the top riser is designed, according to the post-treatment results, it can be seen that the shrinkage holes in the casting are almost all eliminated, and the overall defect of the casting is almost none, because the casting should be subjected to hot isostatic pressing treatment.
The shrinkage holes present in scheme 5 can be completely removed according to experience to meet the production requirements of hinged parts.
3 Experimental verification
In order to ensure that the experimental results and the simulated results are comparable, the casting is directly pressed with metal molding. In order to prevent the shrinkage of the wax mold, the wax mold is placed with integral molding cold wax, and the shrinkage rate is designed according to 1.5%,
The pickling amount is 0.6 mm on one side, and the mold is assembled according to the conformal riser designed in scheme 5, and the mold situation is shown in Figure 10, and the investment mold is still poured at 100 kg vacuum self-consumption shell furnace at 1 720 °C. The mechanical properties of the castings were obtained by a tensile test at room temperature, and the experimental equipment was equipped with the Shimadzu AGS-X3000KNX universal material test machine, and two were taken along with the casting for hot isostatic pressing treatment (i.e. at 120 MPa).
The same batch of attached cast R5 specimens held at 910 °C for 2 h at argon pressure and cooled with the furnace to below 300 °C was carried out in accordance with GB/T 228.1. The method of forming cold wax + metal mold + static pouring is adopted to effectively ensure the contour size of the casting, and the size inspection of the casting is carried out
The inspection includes the use of joint arm scanning and casting digital model comparison, the detection of castings, according to the best fitting result deviation of ± 0.5 mm required size preliminary inspection and casting 100% dimensional inspection by professional inspectors according to the casting drawing and PCP size control table for casting size final inspection, so that the structural contour size of 150 mm can be controlled within ±0.3 mm. There are basically no visible flow marks, cold insulation and microcracks on the surface of the casting, and there is no obvious pressure on the root of the casting after hot isostatic pressing
The pit is shown in Figure 11, the internal quality of the casting is inspected by X-ray, in accordance with MFS0705 regulations, and evaluated according to the ASTM E 192 standard radiographic reference negative, as shown in Table 5, internal defects are evaluated according to the wall thickness of the casting, the articulation casting is evaluated using Kodak 125 X-ray negative, the fluorescence inspection method is using Class I A method, and the fluorescent liquid is grade 3
Sensitivity. The X-ray detection results and the simulated results are consistent, as shown in Figure 12, the casting X-ray detection results show that the shrinkage hole is on the riser, the overall casting has almost no shrinkage hole, after testing, the internal quality of the hinged casting and the fluorescent surface quality are better, and the casting fluorescence once pass rate is more than 70%, X
The pass rate of light inspection is more than 75%, the number of defects is small, and after a small amount of repair, the final inspection of castings meets the requirements of technical conditions. And the room temperature tensile mechanical properties of castings after hot isostatic pressure annealing
The test data is shown in Table 6, which meets the requirements of the technical protocol and meets the requirements of putting into production.
Fig.9 Shrinkage cavity distribution under different risers
Fig.10 Actual modelling scheme
Fig.11 Actual casting produced
| 铸件壁厚 /mm | 标准板厚 /mm | 单个气孔 | 串状气孔 | 分气孔 | 缩孔 /mm | 分散缩孔 | 集中疏松 | 低密度夹杂 | 高密度夹杂 |
| ≤9.53 | 6.35 | 3 | 5 | 4 | 1 | 5 | 3 | 5 | 4 |
| >9.53~15.88 | 12.7 | 4 | 5 | 5 | 1.2 | 6 | 7 | 4 | 4 |
| >15.88~25.4 | 19.05 | 6 | 4 | 5 | 1 | 6 | 7 | 5 | 4 |
Fig.12 X-ray scan results
Tab.6 Mechanical properties of the casting
| 抗拉强度 /MPa | 屈服强度 /MPa | 伸长率 /% | 断面收缩率 /% |
| 910 | 840 | 8.0 | 17 |
4 conclusion
(1) The use of ProCAST technology to carry out filling solidification analysis of castings is conducive to reducing the time of trial and error in the product design process, and dynamically compensating through risers in the design process of casting risers
process, overcome the difficulties of process design and save product development time.
(2) Through multi-scheme simulation analysis, different spruer schemes were adopted to solve the problem of concentrated shrinkage holes at thick hot joints of casting roots.
(3) The shrinkage hole designed by conformal riser structure was reduced by 94.8% compared with the previous one, the shrinkage hole at the root of the casting was reduced from 2.11 cm3 to 0.11 cm3, the shrinkage hole was approximately all proposed, X-ray flaw detection was carried out, and the detected shrinkage hole defect was consistent with the simulation results, and the first-time qualification rate of casting fluorescence was 70%
Above, the X-ray inspection pass rate is more than 75%, the number of defects is small, after a small amount of repair, the final inspection of the casting meets the technical requirements.