Abstract:
The structural characteristics and technological challenges of titanium alloy brush box castings. Based on the analysis of the causes of defects in titanium alloy brush box castings, a high-strength, low-stress titanium alloy brush box casting process is explored and studied. Through production verification, the casting process developed in this study is proven to be successful and effective, providing guidance for titanium alloy casting of similar parts.
1. Introduction
Brush boxes are crucial components in generators, operating in harsh environments. They must not only resist corrosion but also be lightweight, with high dimensional accuracy and excellent mechanical properties. This paper takes the titanium alloy brush box for a new type of generator produced by our company as an example, introduces its structural characteristics and technical requirements, analyzes the technological difficulties in its casting process, and proposes a new casting process design scheme as well as multiple technological measures taken to solve casting defects, thereby ensuring the successful casting of this new type of titanium alloy brush box, improving productivity, reducing production costs, and providing a reference for the casting of similar titanium parts.
2. Product Structural Characteristics and Casting Process Difficulties
2.1 Product Structural Characteristics and Technical Requirements
The titanium alloy brush box has dimensions of 238mm × 33mm × 70mm, a weight of approximately 0.74kg, and a basic wall thickness of 3.5mm, with a minimum wall thickness of 1mm. The dimensional accuracy of the casting requires meeting GB/T 6414 CT6~CT7 grade. Its characteristics include a slender overall shape, small gaps between partitions, and complex structure.
The material of the brush box casting is TC4 alloy, with tensile strength ≥ 890MPa, yield strength ≥ 820MPa, elongation after fracture ≥ 5%, and reduction of area ≥ 10%. The product is delivered after precision machining.
2.2 Analysis of Casting Process Difficulties
The thin wall thickness of the brush box casting, with a minimum wall thickness of 1mm, poses difficulties in gravity pouring and filling, with a risk of incomplete filling. The slender shape of the part may lead to deformation of the castings. The gaps between partitions are 1.5mm, making shell making difficult. The slender cores have low strength and are prone to deformation, cracking, and other issues. The cores, surrounded by molten metal, are susceptible to “fire running” phenomena, making cleanup difficult, which can lead to clogging of partition gaps and scrapping of castings, increasing the difficulty of casting.
3. Casting Process Design Scheme
Due to the complex structure of the brush box, the investment casting process is adopted for production. To successfully cast the brush box, two schemes are designed.
3.1 Scheme One
The brush box is directly connected to the sprue, with the direction pointing vertically upwards. Only a riser and vent holes are set at the upper end of the brush box, and reinforcing ribs are set at weak parts of the casting to prevent deformation. This scheme has a short metal flow path, allowing the molten metal to enter the mold shell smoothly with strong filling ability, good surface quality of the casting, and a high yield rate of 45%.
3.2 Scheme Two
To improve the filling ability, the top-pouring method is adopted for pouring. The advantages of this pouring system are: simple and compact pouring system structure, easy modeling, metal conservation; the metal can easily fill the mold cavity, with the metal temperature being higher on top and lower on the bottom, allowing for sequential solidification from bottom to top, which is conducive to the role of the riser in compensating for shrinkage during casting, preventing defects such as incomplete filling and cold shuts in thin-walled castings.
The brush boxes are placed side by side at the bottom and connected to the sprue through an H-shaped cross gate. The cross gate not only serves as a channel for molten metal to enter the brush box but also acts as a riser to compensate for shrinkage of the brush box. The sprue adopts a conical design to maximize the smooth entry of titanium liquid into the mold shell. Since titanium is reactive at high temperatures and can react with most refractory materials, reaction-induced porosity is common in titanium castings. To further reduce the occurrence of porosity, vent holes are set at parts of the brush box prone to porosity, and tension bars are added to reduce the possibility of deformation of the casting. This scheme has a lower yield rate of only 35%.
4. Simulation Scheme Design
To verify the rationality of the aforementioned pouring system designs, this study will conduct numerical simulation of the brush box casting process based on the ProCAST software. The 3D models of the two pouring schemes are imported into the ProCAST software for preprocessing, with the brush box material set as titanium alloy, the mold shell material as mullite, the contact surface type as COINC, the interfacial heat exchange coefficient as 500W/(m2·°C), the pouring temperature as 1800°C, the pouring speed as 6kg/s, the cooling method as Air_cooling, the initial mold shell temperature as 25°C, gravity casting adopted, and radiation ignored.
5. ProCAST Simulation Results and Analysis
Shrinkage cavities and porosity defects usually occur at positions with slower solidification. This paper uses ProCAST software to analyze and compare the changes in temperature at different locations, quickly locating the positions where shrinkage cavities and porosity defects may occur in the casting.
The changes in the temperature field during the solidification process of the casting in Scheme One at different simulation times. It can be seen that at the beginning of solidification, the casting solidifies rapidly, while the middle runner solidifies slowly. Most parts of the casting solidify almost simultaneously, with a small temperature difference, which is not prone to stress, deformation, and thermal cracking. There is no riser set for compensating shrinkage, saving metal and simplifying the process. The vent holes are set at the top of the casting, which is conducive to gas venting. The disadvantage is that the part where the casting is connected to the sprue solidifies slowly, prone to shrinkage cavities or porosity defects, resulting in less dense structure. The casting simulation results, with defects appearing in areas with slow cooling, and shrinkage porosity or cavity defects also appearing on the brush box body.
The changes in the temperature field during the solidification process of the casting in Scheme Two at different simulation times. It can be seen that at the beginning of solidification, the casting solidifies gradually from bottom to top, with a solidification time of 12.9s. The cross gate can compensate for shrinkage of the casting, with good compensation effects, allowing for the obtainment of dense castings without shrinkage cavities. Liquid metal is mainly concentrated in the sprue and cross gate, with a solidification time of 822s for the cross gate, which is long enough to compensate for shrinkage of the connected casting, reducing the occurrence of shrinkage cavities and porosity, and The casting simulation results, with most defects appearing in the gating system, and only a few defects on the brush box body, indicating a reasonable gating scheme.
Improvements were made to Scheme Two by adding risers at locations prone to defects and adding thermal insulation cotton at the sprue and cross gate positions to further enhance the characteristic of sequential solidification. The gating scheme of Scheme Two was simulated again, with the simulation results.
6. Summary
(1) Based on the structural characteristics of the brush box casting, two process schemes were designed. Numerical simulations were conducted on both schemes to analyze the changes in the temperature field during solidification and the occurrence of defects. Scheme II, which exhibited better overall performance, was selected for further process optimization.
(2) By improving the cooling conditions of the sprue and runner in Scheme II, sequential solidification of the casting was achieved, effectively eliminating shrinkage porosity and shrinkage cavity defects. This met the design requirements of the casting and provided a reference for the process design and optimization of such thin-walled box-shaped castings with high dimensional accuracy requirements.