High-Tech Melting Processes Improve Aerospace Materials
High-performance metals are those metals that are exposed to some combination of high stresses, high and/or fluctuating temperatures, and corrosive conditions where high reliability is essential.
Characterized by complex and carefully controlled alloying, uniform composition, low levels of contaminants and a fine or controlled grain structure, the application base for high-performance metals has been steadily expanding through advancement of materials technology and recognition of how the proper materials can “make or break” an end product. The main driver for improvements in high-performance materials, however, started with and remains in the aerospace industry.
Overview of Production Paths
Steel and specialty-materials processing follows two general production paths: flat products and long products. Both start with primary melting, where the initial raw materials (virgin raw materials and/or scrap or revert) are melted and refined to a specific chemistry. The molten metal can then be further processed by hot-charging methods if originally air melted (e.g., ladle treatment or hot charge into a vacuum induction melting furnace) or cast into ingots for remelting and/or downstream processing.
High-performance alloys will generally all be processed through some kind of remelting, but depending on the final use, some will start with air melt (plus ladle/degassing treatment) while the highest performing materials are melted under vacuum.
Arc Furnace/Air Induction/Ladle Treatment
Early materials were all produced via open-air furnaces, primarily (since World War II) in basic oxygen furnaces (BOF) or electric arc furnaces (EAF). As technology improves, induction furnaces have increasingly been used. BOF and EAF processing excel at large-scale production of basic steels at minimum cost. However, the exposure to air, use of fluxes, graphite electrodes and other potential contaminants mean that their suitability for the production of the most critical materials is limited.
As remelting developed, ladle treatment for the removal of some of these impurities was begun. The result was that some materials originally designed to use vacuum induction melting as the primary melt process are now melted as “air melt plus ladle treatment” in the production process, followed by electroslag remelting, vacuum arc remelting or both. Many structural components of aircraft that require high reliability without being subjected to high temperatures – such as the flap tracks in the wings, engine mounting brackets and landing-gear structural components – are now produced via these combined processes.
Vacuum Induction Melting
Vacuum induction melting (VIM) on something approaching a commercial scale was first introduced in Germany in 1917. However, the true commercialization of the process took the creation of more powerful vacuum pumpsets during and after World War II and the higher demands being placed upon materials by the development of jet engines for aircraft and rocket motors.
While conceptually simple – place a coreless induction furnace inside a vacuum chamber – the mechanics of VIM furnaces have continually improved for several decades as demands for higher productivity and reduced maintenance drive furnace design advances. What started as a single vacuum chamber around a furnace in the early days has progressed to multi-chamber designs featuring separate melt, mold, tundish and material-charging chambers (Fig. 1).
The reduced pressure (vacuum) in the melt chamber is used to both refine the molten alloy and protect it from attack by atmospheric gases:
1. Reactive ingredients (e.g., Ti) can be protected from atmospheric attack. In general, lower pressures improve the protective effect.
2. Most chemical reactions that will eliminate impurities from metals and alloys are enhanced at reduced pressure.
3. Dissolved gases and contaminants with elevated vapor pressure can be removed from liquid molten metals at low pressures.
These features combine to provide unique advantages over the air-melt and ladle-treated materials covered earlier, including excellent control over the complete alloy chemistry – not just the alloy composition but also beneficial trace elements and harmful impurities. The reproducibility of alloy chemistries within narrow target ranges results in highly consistent material properties, even on a heat-to-heat basis. Since two of the driving factors behind quality control for the aerospace market are repeatability and reproducibility, both in process and properties, VIM is a key component in aerospace high-performance alloys.
While the global capacity of VIM is a very small percentage of global steel production, it is nevertheless extremely important for high-performance alloys in general and the aerospace industry in particular.
Early developments in melting high-performance alloys (e.g., those for the space program) focused on cold-hearth refining methods (plasma and electron-beam melting) due to their ability to focus heat in specific areas of the melt region. This allows for improved refining characteristics because impurities can be selectively heated or cooled, which evaporates or solidifies them on the edge of the water-cooled copper hearth, removing them from the final ingot.
Cold-hearth refining equipment is technically complex and has high processing costs due to the high energy input needed to overcome the rapid heat removal through the copper hearth. Improvements in VIM and remelting processes have largely reduced the use of electron-beam and plasma melting for superalloys. Cold-hearth technologies continue to be utilized in melting reactive and refractory materials such as titanium, tantalum, niobium and zirconium.
Even while primary melt processes were undergoing improvement, various organizations were looking at ways to introduce cleaner material through remelting methods. The two methods (Fig. 2) discussed here have gained the greatest commercial acceptance and application, covering a wide variety of materials.
Both electroslag remelting (ESR) and vacuum arc remelting (VAR) take a VIM or air-melt (and typically degassed) ingot as an electrode, melting it via the namesake method and solidifying the new ingot in a water-cooled copper mold, also known as a crucible. The fundamental differences in the two processes lead to one or the other being favored based on the material being melted and the ultimate end use, with the highest-quality materials utilizing VIM, ESR and VAR.
ESR was first proposed by Hopkins in the 1930s in the U.S., but development was largely carried out in the Soviet Union before the process gained wider acceptance. In the ESR process, the electrode tip is immersed in a molten slag, typically of the CaF2-CaO-Al2O3 group. The slag acts as a resistance heating element in conjunction with the alternating current that flows through the electrode, driving up the temperature of the slag and melting the electrode drop by drop (Fig. 3).
The main benefits of ESR can be considered as:
1. Chemical reactions between the slag and the molten metal
3. Attack on and dispersal of nonmetallic inclusions by the slag
3. Improved ingot structure due to the progressive solidification within the water-cooled copper crucible
The presence of the metallurgically active slag means that chemical changes can and do occur in the transition of the metal from electrode to ingot. Indeed, this was by and large one of the main reasons for the development of the ESR process. Desulfurization was not possible in early steelmaking processes, and even after the initial desulfurization techniques were developed, ESR remained the preferred method for this purpose if higher ingot quality was desired.
Today, the ease of desulfurization in the primary melting process (or ladle treatment) means that remelting focuses more on the final structure of the ingot and its inclusion content. In fact, with the advent of inert-gas melting (melting inside a hood containing an argon or argon-nitrogen mixture), the aim is to maintain a constant slag and alloy composition from the bottom of the ingot to the top.
The removal of nonmetallic inclusions that are in the electrode is a large benefit of ESR. While the ESR process, unlike the VAR process, will not degas the metal or remove any oxides, any inclusions will be reduced in size via interactions with the slag and will be finely dispersed through the ingot structure. As larger inclusions are primary crack propagation sites, the reduction in size and dispersal of inclusions in an ESR ingot can greatly extend the lifetime of any component made from this material.
Finally, the solidification structure of an ESR ingot is driven by the close proximity to the water-cooled copper crucible, producing a superior structure for later forging after any heat treatments required by the ingot. However, a slag skin also solidifies on the crucible, surrounding the ingot and restricting thermal transfer, which limits the diameter of highly segregation-prone materials in ESR.
That said, ESR furnaces to produce ingots of more than 2 meters in diameter with weights over 250 tons have been built. These are mainly seeing use in power generation and general forging-materials production, greatly reducing the revert produced from the oversize ingots otherwise used for forging. These ESR furnaces have used single electrodes of up to 150 tons or multiple electrodes that are “exchanged” into the crucible in turn. Depending on the crucible shape, ESR furnaces can also produce slabs and round-corner-square ingots, giving rise to further applications for the process.
Vacuum Arc Remelting
VAR was developed primarily in the 1950s, largely in the U.S. Initial work was done with titanium, but the benefits for high-performance materials such as the newly developed nickel-based superalloys meant that VAR was quickly applied to the production of those materials.
Unlike ESR, there is no slag or other material between the electrode and ingot, and as such there is no chemical change to the material during the remelting process. Instead, direct current is used to establish a metal vapor arc after the atmosphere is evacuated from the furnace head and crucible (Fig. 4). This arc supplies the heat that melts the electrode tip.
The lack of the ESR slag, and the external skin that it leaves on the ingot, gives improved thermal conductivity to the water-cooled copper crucible. This reduces solidification time for the ingot center for VAR ingots compared to the same diameter ESR ingots with a concurrent improvement in material properties and reduction in segregation defects.
Other advantages of the VAR process come from the exposure of the molten metal to direct vacuum and the flotation effect of the open-metal pool on low-density inclusions. The vacuum exposure allows for some degassing to occur, particularly hydrogen but potentially some nitrogen as well. (Oxygen is usually present in oxide inclusions, which are less susceptible to evaporation.) There are some undesirable losses in low-vapor-pressure elements such as magnesium. Typically, electrodes meant for VAR processing will be poured with an excess of these elements since the loss (or recovery) percentage is predictable.
The flotation of inclusions in the open-metal pool is key to VAR-process cleanliness and popularity. Since low-density inclusions will float on the surface, and there is no slag pool to restrict movement, convective and gas-flow forces push these inclusions toward the crucible wall, where they freeze into the surface of the forming ingot. It is then quite simple to remove these inclusions from downstream processing by grinding or turning the surface of the VAR ingot, resulting in excellent cleanliness and material properties in the remaining material.
One of the key features in the manufacture of the highest-performance alloys today is the extensive processing done to get a super clean, and thus highly reliable, product. To this end, the most critical materials utilize not just VIM, ESR or VAR but all three.
VIM is used to provide a top-quality electrode with precisely targeted chemistry, which is then taken to the ESR furnace. The ESR melt ensures that there are no gross inclusions in the material while also providing a solid, shrinkage-free electrode to the VAR furnace.
The ESR ingot surface is cleaned and then used as an electrode in the VAR furnace. This final step removes all of the fine inclusions from the ESR material to the surface, giving the best ingot structure and cleanest possible material for downstream processing.
From the late 1960s to the present, aircraft turbine engines have improved remarkably:
• Maximum available thrust increased ~6x
• Compression ratio increased 3-4x
• Thrust-to-weight ratio improved by ~7x
• Specific fuel consumption is approximately halved
• Reliability (measured by in-flight shutdowns) improved more than 10x
As a result, most current-design medium and medium-large aircraft have two engines instead of three or four, with consequent reduced drag and improved fuel efficiency. These improvements would not have been possible without the high-tech melting processes described in this paper, particularly VIM, ESR and VAR furnaces.
Just as early aeronautics and the U.S. space program drove improvements in computers and numerous other technologies, improvements in aerospace materials and their processing methods have gradually made their way into more mainstream products, enabling higher temperatures (and thus efficiency) in power-generation systems, harsher conditions for oil pipelines and lighter-weight components for high-performance automotive applications, to name just a few. The broadening of applications for these melting methods will only continue.
The author would like to thank R. Roberts of Consarc for information provided through many years of conversations regarding VIM, ESR, VAR and other melting methods; and also R. Schlatter for all of his special insights into vacuum and controlled-atmosphere melting. IH
For more information: Contact Aaron Teske, technical & sales manager, Asia; Consarc Corporation, 100 Indel Ave., P.O. Box 156, Rancocas, NJ 08073-0156; tel: 609-267-8000 x174; fax: 609-267-1366; e-mail: email@example.com; web: www.consarc.com