Titanium has unique properties that allow its use in certain applications where other metals cannot be used. Particularly, titanium alloys are effective in aerospace and high-performance structures due to their high strength-to-weight ratios. The chemical process industry also takes advantage of the benefits of titanium; titanium alloys often are used in certain corrosive environments where no other metal system will perform adequately. Titanium rapidly forms a stable oxide film in oxidizing and neutral aqueous solutions, and it is nearly immune to corrosive attack in nitric acid, bleaches, and oxidizing halide salts.
Despite these attributes, the application of titanium often is limited by its relatively high cost. For example, in cast form, titanium costs several times more than stainless steels. Several factors influence the cost of titanium:
- It is very reactive with oxygen and nitrogen and must be melted, cast, and cooled under vacuum or inert atmosphere
- It must be cast into special nonreactive molds, which inherently are expensive due to the high cost of the oxide mold material (typically zirconia)
- Material use often is quite low due to the difficulty of recycling revert (sprues, gates, risers and defective castings that normally are remelted) and scrap
- Casting of titanium is extremely difficult, often necessitating substantial rework and upgrade of the castings
A melting and casting system for titanium, known as induction skull melting (ISM), was developed to minimize these drawbacks. The process offers several advantages for titanium investment casting, including fast cycle times compared with those of vacuum induction melting. The process uses a water-cooled copper crucible, thus eliminating contamination associated with typical vacuum induction melting ceramic crucibles (figure 1). Also, the process is unrivaled in its ability to use revert and scrap metal. These advantages make ISM an attractive process for the production of a variety of low-cost, high-quality titanium castings.
Titanium Investment Casting
The production of investment molds used for titanium casting is similar to the production of investment molds for ferrous casting except for some very important differences. The major difference is in the investment slurry formulation. For ferrous casting, the investment molding media usually consists of zircon, silica, and alumina/silica. However, due to titaniumns strong affinity for oxygen, these same oxide refractories cannot be used to construct molds for titanium casting. Molds made of such refractories result in titanium castings having unacceptable surface finish and gross porosity. Also, a very deep brittle reaction layer (alpha case) is formed, which leads to difficulty in machining due to the high hardness of the surface layer. The alpha case also can lend itself to crack initiation and propagation in higher strength titanium alloys such as Ti-6Al-4V.
To avoid these problems, investment molds used for casting titanium must be made out of special high-stability refractories such as zirconia, thoria, and yttria. Another factor that must be carefully controlled by the titanium caster is ash content of the pattern wax. While high ash content of pattern wax can lead to excessive defects for the ferrous caster, but it can lead to scrapped castings for the titanium caster. "Troublesome" problems for the ferrous caster often are devastating to a titanium caster. Also, the fluidity of molten titanium is very poor, which leads to shrinkage and gas defects, as well as no-fill, or misrun, defects. Because of this, titanium alloys often are centrifugally cast to aid in the filling of thin sections (figure 2).
Titanium Melting and Casting Practice
For many metals, mechanical properties of castings can be lower than those of wrought alloys. However, titanium castings often have comparable, and often superior, mechanical properties to those of wrought products. Also, properties associated with crack propagation and creep resistance can be superior in titanium castings. Because of this, titanium castings often are substituted for forged and machined parts. Table 1 shows some typical chemical compositions and mechanical properties for pure titanium and the often-used Ti-6Al-4V alloy.
While titanium castings have very good mechanical properties, actual casting of titanium using the traditional vacuum consumable electrode method has several drawbacks. This method uses a vacuum arc furnace to melt a portion of a titanium electrode into a water-cooled copper crucible. When the desired amount of metal is molten, the remaining electrode is quickly withdrawn and the crucible is tilted to pour the metal into the mold. The consumable electrodes are titanium billets or forgings, which are very expensive compared with titanium scrap and revert. Even titanium electrodes made of revert are comparatively expensive due to the labor intensive operation of welding the revert into a suitable electrode. Because the material is melted into a water-cooled crucible, there is very little superheat available to aid in the castability of titanium. Even worse, the only location of the melt that contains a noticeable amount of superheat is near the arc itself at the top of the molten pool. Therefore, the first metal entering the mold cavity is hotter than the last metal, which results in poor solidification profiles in the castings. In addition, the necessary use of prealloyed electrodes for vacuum arc remelting (VAR) of a melt for producing castings makes it extremely difficult and expensive to produce nonstandard grades of titanium.
Casting Using Induction Skull Melting
The ISM process uses a water-cooled copper crucible to avoid contamination of reactive alloys. However, unlike copper VAR crucibles, the ISM crucible is segmented. The segments in the crucible allow the use of an induction power source to apply a magnetic field to the metal charged inside the crucible. Without the segments, the induction coil would serve only to melt the copper crucible. In essence, the unsegmented copper crucible is not a crucible at all, but merely a charge. With the segments, the magnetic field supplied by the induction coil passes through the crucible segments and couples with the titanium (or other metal) charged inside the crucible. The charge is melted, and a thin shell freezes along the crucible base and walls of the crucible. This shell, or skull, contains the molten metal. The metal essentially is melted within its own shell (figure 3).
There are several unique advantages of induction skull melting for investment casting of titanium. Melting stock for ISM basically can be anything that physically fits into the crucible, including ingot, plate, tubing, turnings, sponge, compacts, cobble, powder, and revert. Ideal melting stock is chopped up thick plate scrap, which ensures high-quality melting stock at "scrap" prices. Because the charge is melted by means of an applied magnetic field, there is no need to fabricate an electrode. Even loosely charged material will quickly melt using ISM. Typically, an ISM heat consists of about 70% virgin material and 30% revert.
The strength of castings is influenced by the oxygen content of titanium; that is, higher oxygen produces higher strength titanium. Therefore, the oxygen content of the raw material is carefully controlled. High-oxygen revert is mixed with low oxygen virgin material and vice-versa. Also, titanium dioxide can easily be added to the melt to raise the oxygen level if necessary. This results in castings that contain repeatable oxygen levels from heat to heat.
The ISM process also allows much greater freedom in charging and alloying. For instance, material can be added directly to the melt, which allows for maximum charging weights, as well as control of high vapor-pressure alloying additions such as manganese. Also, the molten metal can be held for extended periods of time to allow complete dissolution of refractory metal alloying additions such as tantalum and tungsten.
After the molten metal is poured (figure 4), a thin shell, or skull, remains in the crucible. The skull can be quickly removed, which allows fast preparation to melt a different alloy. For instance, a 40-kg heat of commercially pure titanium can be followed by a 40-kg heat of Ti-6Al-4V, which can be followed by a 40-kg heat of zirconium with little delay and no chance of cross-contamination. ISM has been used to produce 120 heats in 18 hours; cycle time can be as short as 6 minutes from pour to pour. The process also allows for unmatched flexibility in alloying.
Selected alloy systems produced via ISM are shown in Table 2.
The benefits of the induction skull melting process for titanium investment casting translate into the capability of producing high-quality castings at a lower price. The cost benefits associated with ISM revolve around the elimination of electrode fabrication (labor intensive) and the ability to use lower cost, yet fully certified (chemical composition), melting stock. Production data for an ISM casting unit and a VAR casting unit are compared in Table 3; these are actual data accumulated during two years of production of Ti-6Al-4V golf-club heads in a casting facility in Taiwan. Data show that the ISM process is superior to the standard VAR process for high-volume casting work such as golf-club production. The ISM process also is attractive for producing critical aerospace components, as well as prototype castings.
Some typical titanium castings produced by ISM are shown in Figure 5.
Induction skull melting is a proven method for the production of titanium and reactive-alloy castings. ISM capabilities include:
- Capable of melting nearly any alloy. ISM has been used for casting more than 3,000 different reactive alloys.
- Current melting and casting capability of ISM is 100 kg of molten titanium; scale up from this capacity is promising.
- ISM can be used with pour-to-pour cycle times of only 6 minutes.
- ISM is ideal for small alloy-research melting units for casting weights as low as 1 kg.
- ISM basically can use any form of melting stock allowing for low-cost casting.
- ISM is a proven workhorse method for the production of titanium castings. The water-cooled copper crucible often lasts well over 10,000 heats. A crucible life of more than 100,000 heats has been reported by one high-production ISM user.
- ISM offers unmatched homogeneity of alloys due to its inherent molten metal stirring caused by the magnetic field of the induction coil.
As titanium casting becomes more common, the investment caster needs a melting and casting process having as much flexibility as possible. Induction skull melting offers such flexibility allowing the investment caster to be competitive with other production methods in an ever-changing market.