At the end of 1940s, vacuum self-consumption electrode arc melting (VAR) technology has basically matured. In terms of melting process, for complex group elements of aerospace grade and rotor grade titanium alloys, three times VAR process is generally adopted in order to improve the uniformity of ingot composition, to reduce the defects such as inclusion and segregation and to improve the consistency of batch quality.
For Ti-6246, Ti-17 and Ti-1023 titanium alloys containing more β-stabilizing elements (Mo, Cr, Fe, etc.), it is necessary to reduce the current density during the last VAR melting to inhibit the generation of severe β-stabilizing element dendritic segregation by controlling the melting and solidification speed.
At present, the development of some applicable to VAR melting, new monitoring means, such as the United States ATI company used to prevent side arc and arc position sensing technology, the operator can observe the arc movement and melted material contour of the three-dimensional image. In addition, some newly developed melting simulation software, capable of numerical simulation of the actual melting process.
In addition to the VAR melting method, cold hearth melting (CHM) has been developed for the preparation of titanium alloy ingots, which is categorized into two types according to the heat source, i.e., electron beam cold hearth melting (EBCHM) and plasma cold hearth melting (PACHM). Practice shows that cold-bed melting is significantly better than VAR melting in eliminating high-density inclusions, low-density inclusions and improving compositional homogeneity, thus realizing "zero-defect" pure manufacturing of titanium alloy materials, and becoming an indispensable melting technology for the production of high-performance, multi-component and high-purity titanium alloys.
At present, the United States has realized the industrialized production of high-quality titanium alloy ingot cold hearth melting, and the cold hearth melting method into the material standards, such as GE standard "high-quality β forging of Ti-17 titanium alloy parts" stipulates that: titanium alloys used for rotating parts give priority to the use of C-grade materials, i.e., cold hearth + vacuum self-consumption melting (HM + VAR); C-grade materials; C-grade + VAR; C-grade materials are preferentially used for titanium alloys. VAR); C grade can replace B grade (three times vacuum autoconsumption melting); other standards "High quality and standard quality α+β forged Ti-6Al-4V titanium alloy parts", "High quality Ti-6Al-4V titanium alloy blades", "High quality α+β forged Ti-6242 titanium parts", etc. The highest grade "HM+VAR" material is also included.
Electron beam cold hearth melting technology is melting titanium alloys inside a closed vacuum furnace chamber, and the technological advances made in recent years include: improving the layout of the hearth, obtaining ingot billets of different shapes (square flat ingots, thick-walled hollow ingots), and improving the production efficiency; and adopting the melting digital simulation technology, which can accurately simulate the three-dimensional morphology of the molten pool, liquid-solid interfacial conditions, the interface conditions of the sidewalls, and the prediction of the temperature of the ingot during the transient solidification stage. U.S. Aerospace Materials Standards Organization developed a single cold hearth production of titanium alloys material standards, that is, AMS6945 (taking into account the volatilization of high vapor pressure Al elements in high vacuum conditions during EBCHM melting, the need to appropriately increase the content of Al), so that a single EBCHM melting of Ti-6Al-4V sheet instead of VAR melting or "HM + VAR " sheet products.
Another important use of electron beam cold hearth melting is the recycling of titanium residues, and TIMET has launched a recycling business called "Toll Melting", which enables closed-loop recycling of customer-supplied titanium scrap residues into ingots or intermediate billets using EBCHM melting, ensuring that customers' raw materials are returned to their product supply chains, further reducing costs. In 2015, VSMPO installed a shell arc melting furnace (SAR: skull arc melting), realizing the recycling of high value-added titanium residues (titanium chips and lumps) and obtaining great economic benefits. According to statistics, China in the last decade or so equipped with nearly 10 sets of electron beam cold hearth melting furnace, respectively, in the Baoti Group, Baosteel Group, Luoyang Shuangrui Wanji, Yunnan Titanium Industry, Qinghai Jouneng Titanium Industry, Panzhihua Cloud Titanium Industry, Shaanxi Tiancheng Aerospace Materials and other enterprises.
China's plasma cold-bed melting furnace there are only three, respectively, for the China Aerospace Development Beijing Aviation Materials Research Institute, the Chinese Academy of Sciences Institute of Metals and Baosteel special steel owned. Plasma cold-bed melting requires inert gas Ar or He as the heat source medium, He is the best plasma medium, the use of He gas to obtain high thermal efficiency, but due to the lack of He gas resources in China, the high price of He gas leads to high melting costs, industrial production is limited. The thermal efficiency obtained by using Ar gas is low, and the melt superheat is small, which affects the melting rate, leading to the slow development of plasma cold-bed melting technology in China. At present, the domestic standards of titanium alloy bars and forgings for aerospace grade and rotor grade (including national military standards and aviation standards) have not yet included the cold-bed melting method. We should make full use of the comprehensive advantageous resources of the domestic aero-engine design institutes, engineering and application research institutes, titanium producers, forgings producers, and main engine factories, etc., and form a joint working group to promote the application of cold-bed melting technology in the manufacture of aerospace grade high-quality titanium alloy materials. Application.
