Certain alloys containing chromium, nickel, cobalt, molybdenum, etc have a strong affinity for oxygen. If these alloys arc melted in air, the severe oxidation reaction that occurs not only reduces the yield but, more importantly, seriously impairs the final properties and potential performance of the cast product due to the volume of oxide particles (inclusions) left in the final casting. This is the main reason why commercial quantities of these high performance castings are processed in the molten state under vacuum, which ensures that there is minimal oxidation during melting. The necessary vacuum level is easily achieved in modern vacuum melting equipment.
One of the most common uses for such materials is in the production of nickel, iron and cobalt-base superalloy components for use in high-temperature applications in the turbine industry for aerospace and land-based turbine engines. From the primary alloy production through investment casting and to the final heat treatment and brazing into an assembly, all thermal processing is carried out under vacuum (Fig. 1).
Today, vacuum melting and casting furnaces provide high quality materials for use not only in the turbine industry, but also for the automotive, biomedical, chemical and recreational-products industries.
Vacuum precision casting
The product from primary vacuum melting typically is further processed under vacuum as illustrated in Fig. 1. One such application is vacuum precision investment casting (VPIC) for the production of advanced high-quality cast components. The type of furnace used for vacuum precision investment casting (batch and semicontinuous) is dependant on the metallurgical characteristics of the components that are produced; that is, equiaxed or controlled-solidification castings.
Equiaxed casting production
Furnaces for producing equiaxed castings can be either batch or semicontinuous. Conventional processing produces castings having equiaxed, or randomly orixentated, grain structures. In such furnaces, no special consideration is given to controlling the solidification conditions in the mold.
The simplest VPIC furnace design uses a water-cooled steel vacuum vessel containing an induction coil/crucible assembly and facilities to locate molds (Fig. 2). An access door or lid on the furnace allows charging the melt unit with prealloyed bars, as well as inserting a preheated mold into the chamber. The chamber is pumped down to the required vacuum level, and the charge is melted and cast into the mold using a conventional tilt pouring method.
This truly is a batch process because the mold inevitably cools down by the time evacuation and melting are completed (in approximately 20 minutes). Mold heaters can be installed inside the chamber to compensate for mold cool down, but the production rate of such units is not maximized due to the handling time and evacuation time for each charge.
Today, most production VPIC furnaces for equiaxed casting are of the semicontinuous variety. In this case, the furnace consists of two chambers isolated by a large vacuum valve. One chamber contains the melting coil and crucible assembly and the other is used as a mold loading and unloading chamber (Fig. 3).
With the melting chamber under vacuum, the two-chamber design allows loading a preheated ceramic mold into the mold chamber and evacuating the chamber. The interconnecting valve between the two chambers is opened and the mold is transported into the melt chamber by means of either a lift ram or trolley mechanism.
Pouring is carried out immediately, thereby minimizing the time between removal of the mold from the preheat oven and casting in the mold. The filled mold is retracted into the loading chamber and the interconnecting valve is closed. Melting can, therefore, continue uninterrupted in the melting chamber (without ever breaking vacuum) independently of mold handling. This maximizes the production rate of the furnace with the rate-limiting factor being the melting rate of the induction power supply.
Transport times from inserting the mold into the mold chamber to pouring are kept as short as possible (1 to 2 minutes), so there is no appreciable heat loss from the mold with the semicontinuous design.
Other operations in the melting chamber, such as recharging the melt unit with prealloyed bars, taking immersion thermocouple readings, etc., also are carried out using vacuum locks, so there is no need to ever break the vacuum in the melting chamber. This has the advantage that vacuum conditions inside the melting chamber are maintained at the optimum level.
The semicontinuous VIPC furnace can incorporate either a vertical or horizontally arranged mold chamber. Capacity of such furnaces is limited only by the customer's maximum mold size. A 500-kg (1,100 lb) capacity VIPC furnace, supplied by Consarc to Deritend Precision Casting, is the largest VPIC furnace in the UK.
Directionally solidified and single-crystal castings are required for use in turbine applications due to their improved mechanical properties at very high operating temperatures. Furnaces used for controlled solidification have additional design features to exert a high level of control over the solidification process in the mold.
Controlled-solidification furnaces are in general appearance similar to the vertically orientated equiax furnace. However, in addition to the induction melting coil, the upper melting chamber also contains a mold-heating zone to allow heating the ceramic mold to temperatures higher than the alloy liquidus temperature prior to pouring (Fig. 4). The mold-heating zone can be induction or resistance heated and often is split into two or more zones to create a better thermal gradient in the mold.
The molds are placed on a water-cooled chill platen, and a baffled cooling zone also is located directly below the heating zone to create a high thermal gradient for solidification in the cast component. An electromechanical or hydraulic drive system moves the mold into and out from the mold heating zone. The handling system is computer controlled to an exact specification. This level of control is essential during withdrawal of the mold from the heating zone into the cooling zone to create the optimum thermal gradient and, hence, grain orientation in the cast product (Fig. 5).
Furnace technology advancements
Both equiax and controlled-solidification furnaces have grown in size considerably following along with developments in land-based turbine technology. Today, the demand for very large turbine blades has meant that new equiaxed furnaces of 200 to 500 kg (440 to 1,100 lb) capacity and DS/SX furnaces over 100 kg (220 lb) are being requested, compared with earlier furnaces commonly having 50 to 75 kg (110 to 165 lb) capacities.
In past years, all vacuum melting furnaces were manually operated, and it was not uncommon to have significant variances in product quality dependent on the operators involved. Today, PLC and PC based control systems have fully automated the VPIC furnace, and operators merely have to select an appropriate menu for the cast to be carried out. All furnace operations can then be performed automatically, thus maintaining consistency between casts (Fig. 6). Also, data acquisition and SPC facilities are regularly used to allow technical personnel to monitor furnace performance and trend data.
For example, Consarc recently installed a fully automated 100-kg equiax furnace in Japan using a specially developed control system to not only control the casting equipment, but also to interface with the plant network. Production scheduling is automatically downloaded to the machine and interfaced with the casting conditions. The casting process can be carried out automatically with the operator only loading billets and molds.
Induction skull melting
Induction skull melting (ISM) is used for melting and casting metals in a segmented, water-cooled crucible under vacuum or controlled atmosphere by induction. By eliminating all ceramic materials from the melting process, ISM offers a versatile, noncontaminating fast casting process for an ever-increasing variety of applications.
Reactive metals such as titanium and zirconium are today regularly processed in ISM furnaces. In addition, titanium aluminides are recognized as a major growth area in automotive and aerospace applications and show tremendous potential for the future. ISM is the most cost-effective method for producing titanium-aluminide castings directly from raw materials or scrap.
The aerospace industry has for many years sought to eliminate refractory inclusions in superalloy turbine component castings. ISM offers a major step for achieving this goal by providing a clean metal casting process for investment casting of turbine components from barstock materials. This ISM capability has been recognized by the aerospace industry.
The use of ISM for clean metal casting is an area of interest not exclusive to the aerospace and automotive markets. Within the past decade, numerous ISM furnaces have been used for the production of golf club heads, bicycle parts, and other sports related components.