In our previous article in Industrial Heating (“Basics of Specialty Melting,” November 2012), we provided an overview of high-tech melting processes and how they improved aerospace materials. Using electroslag remelting (ESR) and/or vacuum arc remelting (VAR), ingots produced via primary melting of the raw materials are refined. In both cases, remelting is typically on the scale of multiple tons, with the final ingot then being forged and machined to its final purpose.
Some ESR furnaces can produce ingots in excess of 100 tons. On the other end of the scale, there is an equally interesting suite of applications in the form of various small to medium casting furnaces. These utilize similar high-performance materials that are typically prepared as “master alloy” in a vacuum induction melting furnace (VIM) to produce castings for the aerospace, power-generation and, to a lesser but growing degree, automotive industries.
The casting of metals has been with humanity for as long as metals have been melted. Archaeological finds date back approximately 5,700 years for copper and bronze materials, and evidence of copper making (with or without tin to form bronze) dates back 7,500 years. Many improvements and variations in casting processes were developed over time as materials improved, but casting truly took off with the debut of the assembly line and the increasing demands that cars and World War II military equipment placed on cast components.
These increasing demands, in quality and quantity, led to the further development of casting processes. Sand casting in particular made tremendous strides in applying automation to regulate the mixing of the sand used for molds, greatly improving batch-to-batch repeatability of the sand mixing and the castings made in those molds. However, the demand for precision near-net-shape components during World War II could not be met by the relatively slow process of sand casting plus machining. Instead, investment casting was pursued.
Investment casting, also known as “lost-wax casting,” dates back thousands of years. It was primarily used for the casting of ornaments and jewelry due to the level of detail that could be cast into the pieces. This as-cast detail and ability to hold precise dimensions brought it to the attention of dentists in the late 19th and early 20th centuries, culminating in the invention of a new lost-wax casting machine by William H. Taggart of Chicago, who described his invention in a 1907 paper.
It took the demands of the nascent jet-engine aerospace industry to further develop the dental investment-casting process. Initially, jet engines used turbine blades made by sand casting. However, these blades then had to be machined on every surface because the sand castings were not precise enough for the requirements of the rapidly rotating turbines. The resulting demand in machining swamped the machine-tool capacity of the time and brought the near-net-shape precision of investment casting to the attention of the aerospace industry. Post-WWII, investment casting expanded into many other industries and applications, though the driving force behind advances in the technology of investment casting remains the gas turbine industry (aerospace and power generation).
Why high-performance materials?
In order to increase the efficiency of a gas turbine (higher thrust for a jet engine, more energy per unit of fuel consumed for a power station), the combustion temperature must increase. Long ago, this required blades, vanes and other components in the hottest parts of aircraft engines to be made from vacuum-cast superalloys. Jet engines utilize lightweight but high-temperature components, frequently made of titanium, in certain locations. Titanium reacts with typical refractory crucible materials when molten, but it can be melted in a “cold copper” (water-cooled) crucible, heated by an induction power supply and coil.
On the ground, most parts for automotive applications are still air cast. Specialized casting applications, however, are increasing. Automotive turbocharger turbine wheels are perhaps the most obvious application, but many engine components are being examined in light of increasing fuel-efficiency requirements from national governments. Some parts, like engine valves (Fig. 1), are already produced in titanium or a titanium alloy but are not in widespread use. However, some engines for high-end cars further this trend and utilize even more high-performance materials to reduce weight while improving performance.
Refractory Crucible Casting Furnaces
Today’s typical vacuum precision investment casting (VPIC) furnace can come in a wide variety of sizes and furnace orientations (horizontal or vertical) to suit both the product being cast and the space available to install and operate the furnace and related equipment.
The simplest furnaces are often batch units, so called because the melting chamber will be opened to atmosphere after every pour – a batch process. Both the induction melting system and the mold table will be held in the same chamber, and there may or may not be additional chambers for taking immersion thermocouple readings, molten-metal samples or making additions to the melt. Original vacuum furnaces were batch units, since it took time to figure out how to make effective isolation valves. Today, they meet simple process needs at a lower capital equipment cost, if also typically with a reduced yield due to a variety of factors.
The alternative to a batch furnace is a semi-continuous furnace, so called because the melt chamber will remain under vacuum for multiple heats while the mold chamber, separated from the melt chamber by a vacuum isolation valve, will be opened during and after each heat to load and unload the mold. There will also be a materials charging chamber to load the master alloy barstock (or other charge material) into the melt chamber while the latter is under vacuum and often an immersion thermocouple device with its own vacuum lock as well. Although complex, because they can be more specifically designed to the desired process, these furnaces generally have improved yields or are capable of making product that cannot be made in a batch furnace.
Apart from batch versus semi-continuous furnaces, the main differentiation between furnaces that utilize refractory crucibles for melting is the microstructure of the castings they produce – equiaxed grains or directionally solidified (DS).
Equiax Casting Furnaces
The majority of castings made have an equiaxed solidification microstructure consisting primarily of small equiax grains. The grains will vary in size and number depending on the structure of the part and how the mold is prepared for the casting. Many equiax casting furnaces are built in a vertical configuration (Fig. 2), which saves shop floor space. Some are built horizontally. These are typically larger furnaces and/or units that utilize a centrifugal casting process that spins the mold to help fill small cross-sectional thicknesses in the castings. Sometimes horizontal furnaces are used when headroom is restricted.
Equiax castings are the workhorse of the casting industry. Technically, pretty much everything air cast would be considered equiaxed, given how the parts solidify. Usually, the term is specifically used in the vacuum casting industry to differentiate from DS castings. The parts themselves are often made as castings to take advantage of weight savings, cost reductions due to eliminating forging or machining operations, and similar reasons familiar to anyone who has looked at shifting parts from a fabricated component to a casting when particular alloys are not required.
However, some components are more particular in alloy selection. The aerospace and power-generation industries demand this, although more industrial applications have emerged in corrosion-resistant applications and the high temperatures involved in some oil and gas work. For example, some high-performance alloys specifically contain additional elements to act as grain-boundary strengtheners, to increase the temperature a part can withstand, or to impart additional oxidation or corrosion resistance.
Parts requiring complex internal cooling can indicate the use of ceramic cores inside the initial wax part, which survives the removal of the wax after the mold shelling process and leaves behind the core. After casting, the core can be removed by a variety of processes, typically including an acid leach. However, development continues on cores that can maintain dimensional stability at pouring temperatures while being easier to remove from the casting.
The casting of equiax components in a vacuum furnace is relatively similar to air casting with the allowance for the use of a mold lock in the vacuum chamber to bring the mold to the induction melting furnace. Ceramic shell molds will be preheated in an oven prior to pouring to minimize the temperature shock on the shell and reduce the possibility of ceramic inclusions in the final part. They are then transferred to a mold table or mold plate in the mold chamber on the VPIC furnace.
After closing the outer door, evacuating the air from the mold chamber and opening the vacuum isolation valve to the melt chamber, the mold is then moved to just under the pour lip of the induction furnace and filled. The procedure is then reversed as the mold is removed from the furnace. Some equiax molds then have an exothermic material added to promote part filling, while others just rely on thicker shell or refractory cloth padding to retain heat in key areas. The molds are allowed to cool in air, and the shell is removed from the part. Further downstream, processing will depend on the exact demands of the specific part.
Directional-Solidification Casting Furnaces
The other type of microstructure is considered directionally solidified (DS), with a subset of single crystal (SC or sometimes SX) for a DS casting with no grain boundaries in the part. The vast majority of these parts are used in the hottest regions of aerospace and power-generation turbines. One of the principle reasons for the development of the DS process was to extend the creep life of these parts at their extreme operating-temperature conditions.
Parts that are made as DS castings typically require a certain number of columnar grains across the part to utilize the grain-boundary strengtheners in the alloy. In contrast, single-crystal alloys dispense entirely with the grain-boundary strengtheners since the point is to not have any grain boundaries at all. Both types of parts are routinely cored, as previously described. This allows cooling air to flow through the part, which is typically an airfoil surface of a rotating blade or static vane in a turbine (though other static parts are also made by these methods).
In the casting process, DS casting molds are typically preheated outside the furnace to dry them and reduce the time required for preheating inside the chamber. This is still performed at a lower temperature than equiax casting molds to reduce thermal cycling. Inside the melt chamber of the DS VPIC is a resistance or induction heater. Induction heaters (Fig. 3),
while more expensive initially, are generally preferred over resistance heaters due to their efficiency and reduced maintenance and replacement-parts requirements. This heater will be used to slowly heat the ceramic shell mold to a point above the melting point of the alloy being poured but below the melting point of the ceramic.
Once the mold heating cycle is nearly complete, the metal to be poured will be melted in the furnace above the mold heater, and the alloy will be poured at the specified time. At this point, the slow withdrawal process will begin. Unlike equiax furnaces, which typically utilize a plain-steel mold table to support the molds, DS/SC furnaces utilize a water-cooled copper plate under the mold. This plate acts as a heat sink to draw heat out of the alloy, initially freezing a plethora of individual equiaxed grains against the copper. As the mold withdraws from the mold heater, however, grains with a more favorable orientation relative to the heat sink (i.e. those that will grow directly away from the water-cooled copper plate) will grow more quickly than less favored grains, eliminating many of the initial equiaxed grains.
DS parts desire multiple grains. These grains then grow through the withdrawal process, which is typically kept at a fairly steady withdrawal rate. In SC parts, however, an additional grain selector is used to eliminate all but one of the grains formed by the initial pour, which then grows throughout the part. As both types of molds are withdrawn from the mold heater, additional cooling methods can be used to try to increase the temperature differential between the mold heater and the region below. IH
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With the development of superchargers and turbochargers for aircraft, mainly to force more air into the combustion stage to bring up the pressure when flying at higher altitudes, engine designers realized that similar functionality could also be applied to other types of engines for efficiency improvements. Due to the need for advanced high-temperature metals in the turbocharger, this was initially done on larger engines only, such as ship and locomotive diesel engines. Today, they are common on every diesel engine and even many gasoline engines.
One reason why turbochargers are more common is the easy availability of the high-performance alloy turbocharger wheels for the automotive engines. The parts require a moderately complex wax injection die to produce a pattern for the lost-wax process. The part is nowhere near as complicated as aerospace castings because it has reasonably thick wall sections and no coring. Because of this, they can be produced in specifically developed “turbocharger” furnaces.
This is a small-scale batch vacuum casting furnace that locates the induction coil in air on the outside of a small vacuum bell jar. This vacuum chamber is made of a nonconductive material to allow the magnetic field to couple with the metal barstock inside. The barstock is held in a cup on top of the mold and inserted such that the metal is mostly within the induction field.
The bottom end of the bar is tightly held in the mold’s refractory and acts as a plug, remaining solid while the bulk of the bar is melted. After the bar is melted in, conductive heat from the molten metal will melt the “plug,” and the metal will self-pour into the mold.
While the overall yield from this type of furnace is generally lower than can be attained from a dedicated semi-continuous equiax furnace, the small size of the equipment and small mold size mean that the overall capital investment cost – both for the furnace and for ancillary equipment (e.g., mold setup, handling, shell systems, etc.) – is reduced. As a result, this type of equipment is a decent entry point into the specialty casting industry.