Aiming at ZM2 magnesium alloy export castings with closed oil circuit, the magnesium alloy special-shaped tube castings with closed oil circuit were successfully developed through the preparation of non-ferrous alloy low-emission coated sand core with flame retardant effect, and the use of simulation software to optimize the casting process and flux-free gas-protected melting process. The magnesium alloy special-shaped tube casting with closed oil circuit has been successfully developed.
Key words: ZM2 magnesium alloy; closed oil circuit; casting technology
Magnesium is the lightest metal structural material (density 1.8 g / cm3), and has the characteristics of high specific strength, high specific stiffness, and good damping performance. ZM2 belongs to Mg-Zn-RE-Zr series magnesium alloy. Due to the addition of rare earth elements of rhenium and cerium, its casting performance is better, the tendency to shrink is reduced, and its high temperature mechanical properties and fatigue strength are reduced And certain heat-resistant mechanical parts, play a role in energy saving and emission reduction, vibration and noise reduction. Figure 1 is a magnesium alloy special-shaped pipe casting exported to the United Kingdom.The material is ZM2 (rare earth element is greater than 1.0%), and the surface hardness is greater than 60 HB. 0317 mmx57 mm, casting dimension tolerance CT9, closed oil circuit is cast, the inner surface of the oil circuit is round and smooth.
1 Casting Process Design
Due to the reactivity of magnesium alloys, it is easy to react with air, moisture, and silica sand during the casting process.Therefore, there is a certain degree of casting difficulty. We use furfur resin sand molding and add flame retardants to the molding sand.In order to avoid flux inclusion, we use Flux-free gas protection for smelting and pouring; the closed annular oil path must be cast, that is, the slender annular core must have a certain strength, and it is easy to collapse and clean up after pouring is completed.Because it is a magnesium alloy casting, it is necessary to avoid the reaction between the molding sand and the magnesium liquid. Therefore, the prepared sand-coated core with flame-retardant ability is adopted to ensure the formation of the oil circuit and the surface quality.
1.1 Determination of Chemical Composition
Within the composition range that meets the requirements of the ZM2 standard, Zn has a better solid solution strengthening and aging strengthening effect in magnesium alloys, but within the standard range, the higher the Zn, the larger the crystallization temperature range, the poorer the casting properties of the alloy, and the larger the tendency to shrink. Therefore, the lower limit of Zn is 3.5% -4.0%; Zr has a small solid solubility in magnesium alloys and has a strong grain refining effect, but excessive Zr is liable to precipitate and segregate, and Zr is selected to be 0.6% -0.8%; rare earth elements It can improve the casting performance of magnesium alloy, improve the high temperature performance and fatigue strength, and the RE composition is controlled at 1.2% ~ 1.4% 2
1.2 Modeling and Core Casting
Furan resin sand molding is adopted.To prevent the reaction of magnesium liquid with the gas in the mold and the material in the molding sand, sulfur and boric acid are mixed into the molding sand in a certain proportion for the molding operation. The sulfur can form sulfur dioxide gas during the pouring process and prevent the magnesium liquid In contact with air, boric acid can form a layer of enamel protective film on the surface of magnesium solution to prevent oxidation and combustion of magnesium alloy. The sealed oil circuit of this casting uses ordinary coated sand, and 0.8% -1.5% boric acid is added to mix.The addition of boric acid is to flame retardant the sand core, prevent the magnesium liquid from reacting with the sand core, causing sand removal difficulties, and the oil surface Leveling and other issues. The core is made on the hot core machine.Due to the slender annular oil path diameter, it is easy to break during the core making.The core is added in the middle and the auxiliary connecting and connecting bridges have good core strength. An air hole is drilled at the core head for exhaust, as shown in FIG.
1.3Magnesium alloy is susceptible to slag inclusion defects due to its pouring process.
It is necessary to ensure smooth filling and no spatter, otherwise secondary oxidation slag inclusion is easy to form. Therefore, the pouring system adopts an open bottom pouring ring pouring system with a cross-section ratio of FEFaFa = 1: 1.5: 2. The bottom diameter of the riser is designed by 1.2 times the number of molds at the hot junction.
1.4 Numerical Simulation of Casting
UG is used to perform 3D modeling, draw the gating system, use simulation software to simulate the filling and solidification process of the casting process, check the rationality of the gating system, and predict the shrinkage and porosity to improve the success rate of production.
1.5 Smelting and Pouring
ZM2 smelting uses resistance furnace smelting, using raw materials as pure as possible. The smelting and pouring processes are performed under gas protection, and the ratio of mixed protective gas is SF, CO0-1: 100, and the protection effect is good. Smelting is completed at 780 ℃ for 10-15 minutes, and the temperature is lowered to 730t5 ℃ for pouring.
1.6 Heat Treatment and Performance Test
The fracture of the casting is fine and free of inclusions, and the aging treatment of the ZM2 special-shaped tube: the casting is placed flat in the furnace, heated to 330 ± 5 ℃, maintained for 2 hours, and then maintained at 175 s ℃ for 14 hours, and then air-cooled. The single-mold sand casting test rod was heat-treated together with the furnace, the tensile strength reached 235 MPa, the elongation reached 4.6%, and the surface hardness of the casting reached 65 HB, which fully met the standards and customer requirements.
2 Product Trial Production and Process Adjustment
After the process was determined, after trial production and radiographic inspection, the first batch of cast products had atmospheric pores and sponge shrinkage defects. The coated sand is ordinary cast steel coated sand.The resin is added in a large amount, and the hot core generates a large amount of gas when heated.The appearance of the atmospheric holes is relatively regular, and most of them are located between the two risers, which may be the part. It cools quickly and is not easy to vent; the sponge-like shrinkage is in the area below the riser, which has a large thermal node, and the pouring height of some risers is low. Switch to non-ferrous alloy special low-gas-emission coated sand. Under the condition of ensuring the core strength, expand the core head drilling and depth. Dry the core at 200 ℃ for 1 hour before using it. Sheet to strengthen the exhaust of this part; place cold iron at the bottom of the riser corresponding to the bottom box to increase the top-to-bottom temperature gradient and enhance the riser shrinkage. Sand, use after drying. After the process improvement, the second batch of castings eliminated the defects of atmospheric pores. After radiographic inspection, it reached the acceptance requirements of 11 types of castings. After the product was sent to the UK, the customer passed the re-inspection, and the chemical composition, mechanical properties and internal quality were qualified. .
By using non-ferrous alloy low-gas-emission coated sand core mixed with boric acid flame retardant material, it can be used to produce magnesium alloy castings with complex cavities, combined with flux-free gas protection melting process and reasonable pouring feeder process. The alloy casting has a low impurity content, and its mechanical properties and corrosion resistance are higher than those of conventional smelted magnesium alloy castings.