Research Progress on Melting Processes of Nickel-Based Superalloys

A comprehensive technical guide on VIM, ESR, VAR, and multi-stage melting technologies

Table of Contents

1. Introduction to Nickel-Based Superalloys
2. Melting Process Requirements
3. Vacuum Induction Melting (VIM)
4. Electroslag Remelting (ESR)
5. Vacuum Arc Remelting (VAR)
6. VIM + PESR Duplex Process
7. VIM + VAR Duplex Process
8. VIM + PESR + VAR Triple Melting Process
9. Process Selection Guidelines
10. Conclusion and Future Outlook
11. Frequently Asked Questions

1. Introduction to Nickel-Based Superalloys

Nickel-based superalloys occupy a uniquely important position in the entire superalloy field, with widespread applications in aerospace, nuclear power equipment, and petrochemical industries. These alloys exhibit excellent strength within the temperature range of 650–1000°C, along with outstanding oxidation and corrosion resistance, driving continuously increasing demand in complex high-temperature environments.

The melting process of nickel-based superalloys represents the primary step in metal material preparation and is a crucial link determining whether the alloy can achieve superior performance. After decades of development, various melting process types have emerged, including single-stage processes like Vacuum Induction Melting (VIM), Electric Arc Furnace Melting (EAFM), and Plasma Arc Furnace Melting (PAFM).

For nickel-based alloys requiring stricter composition control and higher metallurgical quality, additional purification and optimization of alloy ingots beyond single-stage processes becomes necessary. This has led to the development of duplex processes such as VIM+ESR (Electroslag Remelting) and VIM+VAR (Vacuum Arc Remelting), as well as triple melting processes like VIM+ESR+VAR.

2. Melting Process Requirements

Nickel-based superalloys are highly alloyed materials based on nickel as the matrix element, strengthened through the addition of multiple other elements. To meet the complex and demanding service conditions, often more than a dozen strengthening elements are added to nickel-based superalloys.

The addition of multiple strengthening elements results in the formation of various complex phase structures within nickel-based superalloys, including γ matrix phase, γ′ and γ″ strengthening phases, Laves and δ phases, as well as MC and M₂₃C₆ carbides. These multiple elements also cause the alloying degree of nickel-based superalloys to increase rapidly, placing higher demands on melting processes.

For precipitation-strengthened nickel-based alloys, more Al and Ti elements are added during melting to precipitate more γ′ or γ″ strengthening phases. However, Al and Ti are volatile elements that experience significant loss under atmospheric conditions, making them more suitable for VIM processing. Nevertheless, VIM cannot effectively address reactions between high-temperature melts and crucibles, and challenges remain in removing S and P impurity elements, as well as eliminating shrinkage cavities and porosity defects in ingots.

3. Vacuum Induction Melting (VIM)

VIM is the first step in nickel-based superalloy melting, with its primary purpose being to obtain master alloy ingots with chemical compositions meeting requirements, preparing composition and cleanliness for secondary remelting. VIM can effectively control the content of O, N, H gases and harmful elements such as S, P, and Si, achieving precise control over master alloy ingot composition.

VIM operates under vacuum conditions, utilizing electromagnetic induction generated by electrified induction coils to produce eddy current heat in metal charge within the crucible, causing melting. During the melting process, electromagnetic stirring homogenizes the alloy melt composition and enables precise control. The vacuum induction melting process can be broadly divided into four main stages: charging, melting, refining, and pouring.

Melting Stage

The primary purpose of the melting stage is to rapidly melt the metal charge added to the crucible and remove O, N, H gases, non-metallic inclusions, and harmful impurities from the molten metal. During the melting period, two key factors must be properly controlled: melting rate and vacuum degree. This prevents “bridging” phenomena and ensures proper matching between raw material melting rate and harmful gas removal.

Refining Stage

During the refining period after complete melting, refining temperature, refining time, and vacuum degree must be carefully controlled. Nickel-based superalloy melting commonly uses MgO or Al₂O₃ crucibles. Under high-temperature, high-vacuum conditions, MgO and Al₂O₃ decompose, producing metal vapor and continuously supplying oxygen to the metal melt, potentially causing oxygen content to rise rather than fall.

At the end of the refining period, alloying adjustments must be completed to control the melt composition within the required range to meet tapping and pouring requirements. Generally, main raw materials such as Ni, Cr, and Fe are placed in the crucible for heating and melting during the melting stage. At the end of refining, active and easily-burned elements like Al and Ti, along with trace elements that need to be added, are introduced with simultaneous electromagnetic stirring to uniformly distribute alloy elements in the melt and reduce compositional segregation.

4. Electroslag Remelting (ESR)

ESR is one of the main processes for superalloy purification melting, with more than half of the superalloy grades in production currently utilizing this melting process. ESR evolved from electroslag welding technology in the 1950s and was widely applied in melting metallurgy fields worldwide during the 1970s and 1980s.

The basic principle of electroslag remelting involves current passing through slag, which generates substantial heat due to high slag resistance. This slag resistance heat gradually melts the electrode requiring remelting. The metal liquid passes through the slag in droplet form for purification, finally completing bottom-to-top solidification in a water-cooled crystallizer.

Slag System Design

The slag system selection, ratio, and amount have decisive effects on the electroslag melting process and ESR ingot quality. During the contact process between metal droplets and liquid slag, a series of slag-metal reactions occur. The slag-metal contact area can exceed 3200 mm²/g, enabling non-metallic inclusions and harmful elements such as S, P, and Sb in metal droplets to be absorbed and removed by the slag. Meanwhile, easily oxidizable elements like Al and Ti fully react with oxides in the slag, achieving good control of alloy cleanliness.

Currently, ESR for nickel-based superalloys mostly uses multi-component slag systems based on CaF₂ with appropriate additions of oxides such as Al₂O₃, CaO, MgO, TiO₂, and SiO₂. Generally, slag selection should satisfy the following requirements:

• Low melting point and viscosity
• Suitable electrical conductivity and high basicity
• Low content of unstable oxides and variable-valence oxides
• Large interfacial tension

Advanced ESR Technologies

Traditional open ESR processes are conducted in atmospheric conditions, inevitably leading to O, N, and H gas absorption and intensifying the burning loss of easily oxidizable elements like Al and Ti. Building on open electroslag remelting, Protected Atmosphere Electroslag Remelting (PESR) and Vacuum Electroslag Remelting (VESR) technologies have been developed.

Additionally, many new electroslag remelting technologies have emerged based on traditional ESR, including Rapid Electroslag Remelting (ESRR), Pressurized Electroslag Remelting (PESR), and Electroslag Continuous Casting (ESCC). These new remelting technologies can effectively improve ESR ingot quality and reduce electroslag remelting costs.

5. Vacuum Arc Remelting (VAR)

VAR is a process technology that uses master alloy ingots from primary melting as remelting electrodes, gradually melting the electrode bars under vacuum atmosphere and low-pressure DC arc using a vacuum consumable furnace, followed by rapid cooling and solidification in a water-cooled copper crystallizer.

With the consumable electrode as cathode, a stable arc zone with temperatures up to 5000 K is generated in vacuum. The electrode bottom gradually melts to form metal droplets, which descend through the arc zone under gravity and fall into the water-cooled crystallizer to form a melt pool, then cool and solidify. During this process, a series of reactions favorable for removing impurities and gases occur. Simultaneously, under forced cooling by the water-cooled crystallizer, it is easy to obtain directionally solidified, compositionally uniform microstructures, resulting in high-quality consumable alloy ingots.

Process Stages

VAR can be broadly divided into four stages according to process steps: consumable electrode welding, arc striking, melting, and hot topping. During the melting stage, appropriate melting parameters including voltage, current, cooling rate, and arc length must be selected. During the hot topping stage, “multi-stage hot topping with low current holding” processes are mostly adopted, gradually reducing current while matching appropriate voltage to improve consumable ingot yield.

Recent Technological Advances

In recent years, based on conventional vacuum consumable remelting, multiple advanced technologies have been developed, including droplet solidification control forming, coaxial power supply, and dynamic real-time weighing control systems.

6. VIM + PESR Duplex Process

VIM+ESR is a commonly used duplex melting process for nickel-based superalloys. However, the ESR process in direct contact with atmosphere inevitably experiences oxygen and hydrogen absorption along with burning loss of easily oxidizable elements. Research indicates that Fe₂O₃ and TiO₂ in liquid slag act as carriers, transferring atmospheric oxygen to the metal pool.

The increased oxygen content in the metal pool intensifies the burning loss of elements like Al and Ti. Although adding deoxidizers (such as Al, CaSi, Mg) to the slag pool can achieve effective deoxidation, this simultaneously changes slag composition and causes changes in certain easily oxidizable element contents in the ESR ingot.

Slag System Optimization Research

Regardless of the electroslag remelting technology used, slag system design and ratio remain core factors for achieving purified remelting. Researchers worldwide have conducted extensive studies on slag system design and remelting alloy quality.

Studies have shown that developing a CaF₂-CaO-Al₂O₃-SiO₂-MgO five-component slag system with lower surface tension and viscosity values can produce certain alloys with uniform composition and low O and N contents. Research on increasing CaO content in CaF₂-CaO-Al₂O₃-MgO-TiO₂ five-component slag systems from 5% to 36% demonstrated that O content in alloys decreased from 33.3×10⁻⁶ to 10×10⁻⁶, while S content dropped from 20×10⁻⁶ to 6.5×10⁻⁶, indicating CaO content significantly affects desulfurization.

Designing appropriate slag systems matched to alloy characteristics and developing new electroslag remelting technologies have become important foundations for nickel-based superalloy purification melting.

7. VIM + VAR Duplex Process

Compared to ESR, VAR is a technology that does not require slag, so the remelting process is not hindered by slag skin on the ingot surface affecting heat transfer. Simultaneously, cooling media (such as He gas) can be introduced between the solidifying ingot and crystallizer to enhance cooling, resulting in shallower melt pools with faster cooling. This facilitates obtaining consumable ingots with finer, more dense microstructures and smaller segregation.

Melting Rate Effects

Melting rate significantly affects the microstructure evolution and metallurgical quality of VAR consumable ingots. Research on three melting rates (low, medium, high) for VIM+VAR consumable processes has shown that as melting rate increases, the central region of consumable ingots transforms from columnar to equiaxed crystals, and Laves phase size and content at the same location also increase, indicating element segregation intensifies with increasing melting rate.

White Spot and Black Spot Defects

Researchers have also studied white spot and black spot defects in VIM+VAR double vacuum consumable processes. Studies have found that arc deflection and excessively long arc lengths cause melt pool temperature and flow field disturbances, leading to floaters and fallen pieces that cannot completely melt, forming white spots. Stable diffuse arcs are key to avoiding white spot formation.

Research on black spot formation in VAR alloy ingots and their relationship with process parameters has found that black spots formed from ingot center to half-radius positions are formed by interdendritic Nb and Mo element segregation, closely related to cooling rate.

Cooling Enhancement with Helium Gas

With rapid development in aerospace and gas turbine industries, consumable ingot diameter requirements have gradually increased. This places higher demands on cooling intensity during VAR. Therefore, introducing cooling media (such as He gas, Ar gas) between consumable ingots and crystallizers has become an effective means of enhancing cooling conditions during remelting.

Using short arc control and He gas to improve cooling solidification rate are effective measures for weakening element segregation and preventing white/black spots in alloys.

8. VIM + PESR + VAR Triple Melting Process

Since VIM+PESR and VIM+VAR duplex processes each have certain limitations, they cannot meet the melting requirements for alloys with higher microstructure and performance requirements. The VIM+PESR+VAR triple process can combine PESR’s advantages of sulfur and oxygen removal and impurity reduction with VAR’s advantages of reduced segregation, achieving ingot performance optimization and meeting more stringent melting requirements.

Comparative Research Results

Currently, numerous scholars and institutions worldwide have conducted research on triple melting processes for nickel-based superalloys. Comparative studies between VIM+PESR and VIM+PESR+VAR melting processes on large-scale Φ508 mm superalloys demonstrate significant improvements.

Compared to VIM+PESR, after VIM+PESR+VAR triple melting, the content of harmful elements S and O in ingots decreased significantly, while inclusion quantity and size also reduced. Room temperature and high-temperature tensile properties improved considerably.

Advantages for Large-Scale Ingots

Research on element segregation behavior in triple-melted Φ508 mm large-scale superalloy ingots and bars has shown that although Nb, Ti, Mo and other elements gradually intensify in segregation from ingot edge to center, after high-temperature homogenization and forging, alloy bars show no “black spots” or “white spots” macroscopic segregation, with high distribution uniformity of internal Nb, Ti, Mo elements, and microscopic segregation basically eliminated. This demonstrates the unique advantages of triple processing for large-scale ingot melting.

Beyond wrought superalloys, the triple process has also been applied to powder superalloy melting. Through this method, O, N, S and other impurity element contents in powder alloys can be reduced to below 1×10⁻⁶, achieving ultra-high purity master alloys for powder metallurgy applications.

9. Process Selection Guidelines

The selection of nickel-based superalloy melting processes primarily depends on alloy composition and quality requirements. The following table compares the advantages, disadvantages, and applicable melting conditions of various processes.

VIM+ESR Process Considerations

VIM+ESR can further control S, P, and other impurity elements along with non-metallic inclusions to lower levels, obtaining dense ingots with good surface quality and improving alloy hot workability. However, due to slag skin impeding heat dissipation, severe element segregation exists at the ingot core. Additionally, active elements like Al and Ti still experience some burning loss even under protective atmosphere, and as ESR ingot length increases, composition deviation between head and tail further enlarges.

Considering ESR advantages and disadvantages, VIM+ESR is more suitable for nickel-based superalloys requiring higher cleanliness, smaller ingot specifications, and lower control requirements for volatile elements.

VIM+VAR Process Considerations

VIM+VAR eliminates contamination from atmosphere, molds, and refractory materials, has no slag shell, and benefits from good heat dissipation conditions between ingot and crystallizer, reducing core segregation and achieving more uniform composition distribution. However, this method cannot effectively remove S, P, and other impurity elements, produces larger-sized inclusions in greater quantities, and results in poorer ingot surface quality.

Considering VAR technology advantages and disadvantages, VIM+VAR is more suitable for nickel-based superalloys requiring larger ingot specifications, more uniform microstructure, and higher control requirements for volatile element content.

Current Ingot Size Capabilities

Due to ESR and VAR technology limitations, duplex process VIM+ESR or VIM+VAR melted nickel-based superalloy ingot diameters are currently controlled within 660 mm domestically, while international producers have begun exploring larger ingot sizes for melting and application. Using VIM+ESR/PESR+VAR triple processes to prepare larger specifications with higher cleanliness nickel-based superalloy ingots is a trend and urgent requirement for modern industrial development.

10. Conclusion and Future Outlook

The rapid development and continuously improving application requirements of nickel-based superalloys place higher demands on master alloy ingot melting processes. Selecting appropriate melting processes according to alloy grade and quality requirements is the foundation for ensuring ingot melting efficiency and quality.

Key Findings

• Given the significant influence of melting process route selection and parameter control at each melting stage on alloy ingot microstructure, composition, and inclusions—which have decisive effects on formed alloy mechanical properties—establishing the constitutive relationship of “melting route – process parameter control – microstructure evolution – mechanical properties” is an important foundation for achieving high-quality nickel-based superalloy smelting.
• The VIM+ESR+VAR triple process can combine advantages of various melting technologies to achieve melting of ultra-large specification ingots with higher cleanliness, representing the development trend for high-quality nickel-based superalloy melting processes.
• Further research and application of triple processes is needed, including selecting high-quality raw materials, optimizing precise control of process parameters at each melting stage, improving technical and management requirements during melting, and ensuring continuity and coordination between melting stages.

Future Development Directions

The triple process has been widely applied to high-quality wrought nickel-based superalloy purification melting, while in casting, powder metallurgy, and even metal additive manufacturing powder master alloy ingot areas, many scholars and institutions are conducting further research on triple melting processes.

Continued advancement in precise and automated control of VAR process parameters, including droplet solidification control forming, coaxial power supply, and dynamic real-time weighing control technologies, will be essential for meeting increasingly stringent quality requirements while expanding ingot size capabilities.

11. Frequently Asked Questions

This article provides technical guidance on nickel-based superalloy melting processes. For specific applications, please consult with FUSHUN METAL’s technical team for tailored recommendations based on your requirements.