Development of high-quality titanium alloy ingot smelting technology

               
Update: 04-09-2020
               
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At the end of the 1940s, vacuum consumable electrode ar […]

At the end of the 1940s, vacuum consumable electrode arc smelting (VAR) technology was basically mature. In terms of smelting process, aerospace-grade and rotor-grade titanium alloys with complex components generally use the triple VAR process to improve the composition of the ingot. Uniformity, reduce defects such as inclusions and segregation, and improve batch quality consistency.
For Ti-6246, Ti-17, Ti-1023 titanium alloys containing more β-stabilizing elements (Mo, Cr, Fe, etc.), the current density should be reduced during the last VAR smelting, and the smelting and solidification rate should be controlled to suppress the production Severe β-stabilizing element dendrite segregation.
At present, some new monitoring methods suitable for VAR smelting have been developed. For example, the American ATI company adopts the prevention of side arc and arc position sensing technology, and the operator can observe the arc movement and the three-dimensional image of the contour of the melt. In addition, some newly developed melting simulation software can carry out numerical simulation of the actual melting process.
In addition to the VAR smelting method, a cold hearth smelting technology (CHM) suitable for the preparation of titanium alloy ingots has been developed. According to the different heat sources, it is divided into two types, namely electron beam cold hearth smelting (EBCHM) and plasma cold hearth Smelting (PACHM). Practice has shown that cold hearth smelting is significantly better than VAR smelting in eliminating high-density inclusions, low-density inclusions and improving composition uniformity, so as to achieve "zero defect" pure manufacturing of titanium alloy materials, and become a high-performance, multi-group Yuan and high-purity titanium alloy production indispensable smelting technology.
At present, the United States has realized the industrialized production of high-quality titanium alloy ingot cold hearth smelting, and the cold hearth smelting method has been included in the material standard, such as the GE company standard "High-quality β-forged Ti-17 titanium alloy parts" stipulates: rotating parts Grade C materials are preferentially used for titanium alloys, namely cold hearth + vacuum consumable smelting (HM+VAR); Grade C can replace Grade B (three-time vacuum consumable smelting); other standards "high quality and standard quality α+β forging "Ti-6Al-4V titanium alloy parts", "premium Ti-6Al-4V titanium alloy blades", "premium α+β forged Ti-6242 titanium alloy parts", etc., are also included in the highest level of "HM+VAR" materials.
The electron beam cold hearth smelting technology is to smelt titanium alloys in a closed vacuum furnace chamber. In recent years, the technological advances made include: improving the layout of the hearth, obtaining ingots of different shapes (square slabs, thick-walled hollow ingots), and improving To improve production efficiency; using smelting digital simulation technology, it can accurately simulate the three-dimensional shape of the molten pool, liquid-solid interface conditions, sidewall interface conditions, and temperature prediction during the transient solidification stage of the ingot. The American Aerospace Materials Standards Organization has formulated a material standard for the production of titanium alloys on a single cold hearth, which is AMS6945 (considering the volatilization of Al elements with high vapor pressure under high vacuum conditions during EBCHM smelting, the Al content needs to be appropriately increased), so that one EBCHM The smelted Ti-6Al-4V plate replaces VAR smelting or "HM+VAR" plate products.
Another important use of electron beam cold hearth smelting is the recycling of titanium residues. TIMET has launched a recycling business called "Toll Melting", which can provide customers with closed-loop recycling of titanium scraps and use EBCHM smelting to convert them into ingots or intermediate billets to ensure that customers' raw materials are returned to their products. In the supply chain, closed-loop management is realized to further reduce costs. In 2015, VSMPO installed a skull arc melting furnace (SAR: Skull Arc Melting), which realized the recycling of high value-added titanium residues (titanium scraps, blocks) and obtained huge economic benefits. According to statistics, my country has equipped nearly 10 electron beam cold hearth smelting furnaces in the past ten years. They are located in Baoti Group, Baosteel Group, Luoyang Shuangrui Wanji, Yunnan Titanium Industry, Qinghai Juneng Titanium Industry, and Panzhihua Yun Titanium Industry, Shaanxi Tiancheng Aviation Materials and other enterprises.
There are only 3 plasma-cooled hearth melting furnaces in China, which are owned by China Aviation Development Beijing Institute of Aeronautical Materials, Institute of Metal Research, Chinese Academy of Sciences, and Baosteel Special Steel. Plasma cold hearth smelting requires inert gas Ar or He as the heat source medium. He is the best plasma medium. The use of He gas has high thermal efficiency. However, due to the lack of He gas resources in my country, the high price of He gas leads to excessive smelting costs. High, restricted during industrial production. However, the heat efficiency obtained by using Ar gas is low, and the melt superheat is small, which affects the melting rate, resulting in the slow development of plasma cold hearth melting technology in my country. At present, the domestic aviation-grade and rotor-grade titanium alloy bar and forging standards (including national military standards and navigation aids) have not been included in the cold hearth smelting method. The domestic aeroengine design institute, engineering application research institute, titanium material Manufacturers, forging manufacturers, OEMs and other units have comprehensive superior resources to form a joint working group to promote the application of cold hearth smelting technology in the manufacture of aviation-grade high-quality titanium alloy materials.

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