Titanium alloy melting
Preparation Technology of High Temperature Titanium Alloy
1.Melting method
(1)Vacuum Arc Remelting (VAR)
Vacuum Arc Remelting (VAR) is a melting technology that uses electric arc heating. In a vacuum environment, an electric arc is generated between the raw material and the electrode; the high temperature of the arc rapidly melts the raw material to form a molten pool. The molten metal then flows into the mold below under the action of gravity and solidifies into an ingot after cooling.
Its advantages include mature technology, low equipment investment, and high production efficiency. Although the VAR technology is widely used in titanium alloy melting, it still has problems such as the formation of high/low density inclusions and composition segregation, which affect the mechanical properties of the alloy.
(2)Electron Beam Cold Hearth Melting (EBCHM)
Electron Beam Cold Hearth Melting (EBCHM) is a melting technology that uses high-energy electron beams to heat metals. In a vacuum environment, the kinetic energy of high-speed moving electrons is utilized as the heat source. The high-energy electron beam emitted by the electron gun is focused on the metal raw material, and the generated heat rapidly melts the metal. The molten metal solidifies on the cold hearth and eventually forms an ingot.
Due to the high energy density of the electron beam, EBCHM can heat metals to their melting temperature in a relatively short time, offering the advantages of fast heating speed and precise temperature control. However, the EBCHM technology also has some drawbacks. The manufacturing and maintenance costs of its equipment are relatively high, especially the complexity of the electron gun and vacuum system, which limits its application in small-scale production. Although the electron beam heating speed is fast, there are still certain difficulties in controlling the flow state and temperature of the molten pool, which may lead to challenges in controlling the alloy composition.
(3)Plasma Arc Cold Hearth Melting (PACHM)
Plasma Arc Cold Hearth Melting (PACHM) is a metal melting technology that uses plasma arcs for heating. With inert gas or reducing gas as the medium and the plasma arc emitted by the plasma torch as the heat source, the high temperature generated by the plasma arc melts the metal raw materials. The molten metal solidifies on the cold hearth to form an ingot .
The advantages of PACHM lie in that it operates in an inert atmosphere near atmospheric pressure, which can prevent the volatilization of highly volatile elements and enable precise control of element content. However, it also has disadvantages: the equipment cost is high, the service life of the plasma torch is relatively short, and a large amount of inert gas and refractory materials must be consumed during operation.
2.Powder Metallurgy (PM)
Powder Metallurgy (PM), a preparation method involving powder mixing followed by sintering, is characterized by controllable composition, high material utilization rate, uniform microstructure, reduced segregation, low cost, and a simple process flow.
Some scholars have developed a new near-α high-temperature titanium alloy, Ti-6Al-4Zr-0.5Mo-0.6Si (wt. %), via powder metallurgy. The results show that the PM titanium alloy has a uniform α+β dual-phase microstructure, with uniformly dispersed silicides and no segregation or precipitation of coarse silicides. Other researchers have prepared TA15 high-temperature titanium alloy through powder metallurgy, studied the softening mechanism of the alloy at high temperatures, and further demonstrated the feasibility of powder metallurgy in titanium alloy preparation.
The sintering process is the core part of powder metallurgy material manufacturing, as it directly determines the relative density and mechanical properties of the bulk material after sintering. Prior to sintering, mechanical ball milling (BM) can effectively improve the surface quality and activity of the powder, thereby influencing the final sintering performance.
Among advanced powder metallurgy preparation technologies, Spark Plasma Sintering (SPS) has distinctive features. On one hand, the application of external pressure promotes the contact between powder particles, eliminates porosity, and increases the driving force for sintering, thereby enhancing densification and reducing the required sintering temperature. On the other hand, when pulsed current is applied to the conductive graphite punches and die, plasma is generated to achieve rapid heating. Compared with traditional technologies such as Hot Pressing (HP), Hot Isostatic Pressing (HIP), and Microwave Sintering (MS), SPS offers advantages including fast heating rate, short sintering time, and low energy consumption.
Some scholars have used SPS to rapidly sinter and prepare SiCNWs-reinforced Ti60 high-temperature titanium alloy at a heating rate of 100±5℃/min. This successfully mitigated the chemical reaction between SiC and Ti, further improving the high-temperature strength of the alloy. Other scholars combined BM and SPS technologies, controlling the alloy's microstructure by adjusting the ball-to-powder ratio in BM. This resulted in uniform grain distribution and refined grain size, enabling the preparation of a new Ti-O alloy with excellent yield strength.
