Vacuum die casting (Vacuum-Die Casting, VDC) is a technology that can be used by titanium casting manufacturers to improve casting quality and reduce costs.
As the application of titanium castings in the aerospace industry continues to grow, various manufacturers are committed to finding production methods that can reduce production costs to replace high-cost titanium components, especially in today’s fierce world economic competition. Therefore, titanium castings with lower cost and similar mechanical properties to forgings can not only replace existing titanium parts, but also replace parts of other materials. VDC technology was developed to produce high-quality, low-cost titanium castings. Typical applications for its castings include aircraft fuselages (such as bulkheads, brakes, gears, hinges, etc.) and other aerospace and industrial parts.
Die casting technology is used to produce aluminum, magnesium, zinc, copper-based alloy castings, and has been used in the machinery manufacturing industry for many years. This type of die casting is produced by melting the metal in the atmosphere and then injecting the molten metal into the mold under high pressure. The casting is a net shape part or near net shape part, and the final shape part can be obtained by slight treatment or processing after casting. This process has a short processing cycle, and the time from the metal melt to the net shape part is usually less than 15s.
The requirements for titanium die-casting technology are completely different from traditional alloys. The most important thing is that the metal melting chamber and the mold cavity must maintain a high vacuum, otherwise, the oxygen content of the casting will increase, which cannot meet the technical conditions of aviation alloys. The method of vacuum die-casting titanium castings is the same as the standard die-casting process, except that the vacuum time of the melting chamber / mold cavity and the melting time of the titanium alloy are extended. The vacuum die-casting process uses a separate charge of titanium alloy and induction shell melting (ISR). Compared with the continuous casting method of die-casting aluminum, the melting time is 5min more.
Compared with other methods such as investment casting, vacuum die-casting titanium casting process is simple, so the cost is generally lower. The vacuum die casting process does not require dewaxing, shelling and chemical cleaning, and the process is reduced by nearly half. Compared with the investment casting or forging process, it can save about 30% of the cost. Because the vacuum die-casting mold is directly in contact with the molten titanium liquid, its service life is shortened, and the cost of the mold part of the mold casting method is greater than that of the investment casting method.
At present, the vacuum die casting method can only cast some integral, single-sided, simple-shaped titanium castings, while investment casting can cast very complex shapes and hollow castings. In addition, vacuum die casting can only cast up to 12 pieces at a time, the maximum size of castings is 61cmx46cmx25cm, and the maximum mass is 18kg. The titanium alloys that can be cast are Ti-6Al-4V, Ti-6Al-2Sn-4Zr-2Mo, Ti-15V-3Al-3Cr-3Sn and Alloy C.
The comparative study on the structure and mechanical properties of Ti-6Al-4V alloy casting rods in vacuum die casting and investment casting shows that the two casting rods are treated by hot isostatic pressing at 899 ℃ and 103MPa for 2h, and then at 834 ℃ Next, after 2h vacuum annealing, the grain size of the vacuum die-casting rod is about 100μm, and the grain has an arbitrary α-β structure, while the grain size of the investment casting rod is 750μm; the yield strength of the vacuum die-casting material (930. 79MPa) is about 12% higher than the yield strength of investment casting materials (827.37MPa), close to the yield strength (896.32MPa) of forgings, rolling-annealed parts; the fracture toughness Kq of vacuum die castings is 142.8MPa; √m is close to investment castings, but higher than the value of annealed forgings—87.9MPa√m.
From the surface microstructure of investment casting Ti-6Al-4V and vacuum die casting material, the alpha layer of the investment casting is close to 375μm, which is due to the reaction between the titanium melt and the oxygen stored in the ceramic mold. The alpha layer is very brittle and can be eliminated by chemical etching in an acid bath before loading, but the size of the casting is reduced. Vacuum die casting uses induction shell smelting with a water-cooled copper crucible. The casting system and the mold use metal materials.