The growing demand for high melting alloys has led to the need for increasingly more refractory mold materials for the lost wax process. These materials must sustain their chemical, as well as mechanical stability in order to produce high quality castings.

INTRODUCTION
The demand for casting high melting alloys has grown in recent decades due to attractive thermal, mechanical and chemical properties of such alloys. Since its initial development in late 1940's/early 1950's, the lost wax process has enjoyed a healthy growth in casting of high melting alloys in many markets such as aerospace, prosthetic, jewelry, automobile parts, hand tools and leisure equipment.  The casting of high melting alloys presents a special problem due to the high reactivity of these materials in the molten state. Reactivity of high melting alloys requires special shell-making practices to reduce the introduction of casting defects.

Shell materials need to be of sufficiently high refractoriness. As a general practice, a high refractory facecoat slurry is normally applied to the wax pattern to improve refractoriness. Shells need to have sufficient high temperature mechanical stability to ensure dimensional accuracy of cast parts as well. Therefore, shells should neither densify nor creep throughout the process.

The need for use of high refractory shell molds imposes additional challenge to high melting alloy casting. Slurries made of high refractory flours may have a very short lifetime. Lack of sufficient slurry life can make the shell-making process more costly. Moreover, the quality of shells may become unsatisfactory even in batch processes.

THERMODYNAMIC CONSIDERATIONS
At this point the original paper gives technical details on the general chemical reactions that lead to metal - mold reactions.

Reaction 1 describes dissolution of shell material into the liquid metal.

Reactions 2, 3 and 4 characterize the reaction of the molten metal with the mold. 

Reaction 5 characterizes a gaseous reaction product in a cast part.

Reaction 6 represents solid to liquid transformation of ceramic components in the shell mold.

MOLD SURFACE PENETRATION
Beside cast defects caused by chemical reactions between the mold and the molten metal, physical interaction between mold and molten metal can also cause cast defects. Positive rough surface defects may be produced when molten metal penetrates into a mold porosity.

SHELL MAKING CONSIDERATIONS
Proper selection of shell mold material is vital to the successful production of high quality cast parts. However, it should be noted that the actual reaction of molten metal with the mold material is much rarer than supposed. Much of observed defects in a cast part are actually a result of faulty shell mold production. In the following, some of the fundamentals of shell making will be considered.

The lost wax shell making process for high temperature alloys can be summarized in the following generalized steps.

A.   Multi-component slurries are prepared. These slurries are composed of a refractory system and a binder system. The binder system includes at least one condensable inorganic binder.

B.   Facecoat layer(s) are constructed through dip coating and stuccoing of an organic based pattern.

C.   Shell mold is constructed either by applying multiple dip coatings, or by gelling a solid body of slurry around the facecoat layers. If the slurry is an aqueous based slurry, the binder system generally includes a nano-meter size colloidal silica binder. If the slurry is a non-aqueous based slurry, the binder system is generally a silicon alkoxide. In either case, condensation of binders causes permanent siloxane bond formation upon shell drying.

a.   Aging of slurries
Because of the very nature of the shell making process, slurries go through aging. As slurries age, they lose their useful properties, and either become useless, or jeopardize the quality of lost wax shell molds.

There are at least two reasons why lost wax slurries age. First, different components in a slurry may interact with each other, and cause aging over time.

Besides aging due to component interactions, there is yet another reason why investment casting slurries age . Inorganic binders such as colloidal silica are chemically reactive. They permanently bond as the result of siloxane formation upon surface contact. While in the slurry, the possibility of direct surface to surface contact between colloidal silica particles is low, due to the ionic cloud around each particle (diffused double layer). However, once these particles dry, their surfaces will contact and bonding will occur. Drying of particles may occur during many stages of the shell making process. Any stage of the process that promotes formation of a slurry-air interface could potentially cause evaporation of surface water and bond formation between colliding particles. For example, after a part is dipped in a slurry pot, it is removed and the excess slurry allowed to drip off the shell, back into the pot. As the slurry is dripping back to the slurry pot, a large area of slurry-air interface is formed that promotes slurry aging.

The above two factors cause lost wax slurries to age over time and lose their usefulness. Slurry aging due to component interactions may be reduced by properly formulating the slurry. However, solutions to slurry aging due to the formation of slurry - air interface is not sufficiently explored and presently handled only through careful quality control .

b.   Mechanical Properties
Mechanical properties are among the most important properties in a shell mold. Without proper design of mechanical properties, quality and price of a cast part can drastically suffer. This is especially true for casting of high temperature alloys, where molds normally have one or two facecoat layers of high refractory materials. Facecoat layers are thin and susceptible to stress cracks, high temperature erosion and deformation.

The main source of shell layer green strength is siloxane bonds formed by the condensation of silica binder. One way to increase the strength of a shell layer is to add more binder to the slurry.

An increase in the concentration of silica binder results in an increase in green strength of the shell to a point. After a critical binder concentration, shell strength decreases.

It is normally desirable to design slurries at the critical concentration of silica, where shell strength is at maximum and creep is at its minimum value. However, if the reaction between the molten alloy and the relatively low refractory silica becomes a concern, then concentration of silica is reduced. An example of such a case is casting of Ti alloys (reaction 1). Facecoat slurries for Ti shell molds have a very low concentration of silica binder, which increases the possibility of erosion, crack and deformation of the facecoat. One may use an alternative shell making method to substantially increase the green strength at lower silica binder contents. Dry bars infiltrated with colloidal silica show green strengths in excess of 3000 psi.

Summary
Ceramic molds for casting of high temperature alloys have to satisfy certain requirements. Refractory materials in the mold should retain their chemical stability in contact with molten alloy. Mold materials, as a first order approximation, should have DG comparable or preferably more negative than the oxides of molten alloy. In addition, the mold should have high enough liquidus temperature in order to retain its mechanical stability at the cast temperature.
Besides materials requirements, proper processing of molds should also be exercised. Slurries should be prevented from excessive aging.

Electrochemical properties of individual components in the slurry should be reviewed in order to ensure minimal component interactions.  Slurry pots should be monitored regularly to prevent excessive aging due to binder agglomeration at the air-slurry interface. Finally, mechanical properties of the shell need to be designed to ensure defect-free parts. Maximum strength and minimum creep can be achieved at the critical concentration of binder in the slurry. If it becomes desirable to reduce the amount of silica binder, then the infiltration method can be used to optimize strength and creep in a shell system.

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Basic Ceramic Considerations for Lost Wax Processing of High Melting Alloys
H. Frye, Techform
M. Yasrebi, D. H. Sturgis,
PCC Structurals, Inc.

This is an abbreviated version of the original work. For full technical details, please consult the original paper.