CASTING

ORIGINS AND CLASSIFICATION

II. CASTING

A metal or an alloy is transformed from a liquid to a crystalline solid by the extraction of thermal energy from the melt during casting in a mold. Casting is usually done in the pri- mary production of metals and in the initial production of alloys, and is the starting point for many discontinuities in engineering components.

Casting of alloys entails the change of a metallic liquid solution into a crystalline solid alloy. Virtually all of the engineering alloys are crystalline solids.³All crystalline solids form inhomogeneously⁴from very small crystals on the relatively cold walls of a casting mold. The crystallization of a solid begins at small and discrete locations called nucleat- ing sites, and alloy crystals grow from these sites into the alloy melt. These separated and individual crystals grow, consuming the melt, until they connect with each other and form a solid. Each of the crystals is called a grain and the place of the meeting of two grains is called a grain boundary. Adjacent grains of one solid phase differ from each other as crys- tals only in the orientations of their pattern of atoms. Their size, shape, and spatial and geometric disposition further characterize the grains in a structure.⁵

The growth of grains in castings is sometimes directional and the most rapid growth tends to proceed in the direction of heat dissipation. An extreme condition of this direc- tionality in castings is called ingotism (Figure 2-2). Directionality of grain structure may create difficulties in ultrasonic examinations, as is experienced in cast stainless steel pip- ing.

When metals and most alloys are solidified, the crystalline structures are denser than the liquids from which they came. Consequently, the casting volume is less than the melt volume after the liquid changes to a crystalline solid. This phenomenon is called solidifi- cation shrinkage. A natural consequence of solidification shrinkage occurs when liquid is entrapped within the crystalline shell of a solidifying casting. Unfilled spaces in the cast solid are created by the shrinkage of the included melt. These voids are called shrinkage voids or shrinkage porosity. A central depression called “pipe” is formed when the shrinkage is concentrated in the top of an ingot (Figure 2-3). Pipe often contains concen- trations of slag and must be cropped from the ingot prior to secondary fabrication.

Solidification shrinkage may induce cracking. This cracking may occur at relatively high temperatures when the cast material is weaker. The cracking due to contraction of the cast solid also may occur at weak locations within the microstructure of an alloy, typ- ically at grain and phase boundaries.

Forms of casting discontinuities include pipe, voids, porosity, microporosity, interden- dritic porosity, slag, and cracking.

3. There is a class of engineering material called metallic glass that currently is used in special applications in relatively small amounts. These are alloys that have some characteristic metallic properties and an amorphous atomic structure.

4. Inhomogeneity of casting nucleation is the local and independent formation of small crystals starting at the liquidus temperature for most alloys and at the solidification temperature for metals.

5. The spatial disposition of grains includes on occasion the nonrandom distribution of orienta- tions of the grains. This material property of a structure is called “texture” and it is a measure of the anisotropy or directionality of material properties in an alloy.

FIGURE 2-3 A cross section through a billet showing pipe from solidification shrinkage.

(Courtesy of C. Hellier.)

FIGURE 2-2 Ingotism in cross section of stainless steel billet. [From Liquid Metals and So- lidification,ASMI (1958).]

Shrinkage voids are simply unfilled macroscopic regions of a casting that have been created by the entrapment of relatively large amounts of melt within the casting. Voids may be found at the last regions of a casting to solidify, and occasionally they appear on the surfaces of castings. Shrinkage voids are potentially deleterious, weakening structures by reducing the continuity of load bearing material. These voids may cause or induce a breach in a boundary, thus allowing subsequent leakage through the wall of a component.

Shrinkage porosity is a distribution of a number of small voids. These small voids are usually localized in clusters and exist in the regions of final solidification. Shrinkage porosity extends from the macroscopic to microscopic scale. Porosity also reduces the load bearing section of a component. If porosity is adequately limited in amount and uni- formly distributed, it may have a negligible effect on the strength of a component. Local- ized porosity in too great a quantity can degrade the mechanical properties of a compo- nent. Porosity also presents a potential problem when it provides a path for the breach of a component fluid boundary. Interconnected pores may readily provide leakage paths. Sur- face porosity may also reduce the fatigue resistance of a component.

Microporosity may be found in the specific form of interdendritic porosity. This oc- curs when the last fluid to solidify is contained between the arms of crystalline growth forms called dendrites. The dendrite arms are always spaced in a regular array and the arm spacing is dependent on the solidification rate. The dendritic forms are usually tran- sient and the final form of the structure is a grain or multiphase structure. However, the interdendritic porosity will remain as a residue of the casting process. This form of dis- continuity is rarely adequate in size, number, or distribution throughout a structure to challenge component integrity.

Gas evolution from melts may also cause porosity in cast structures. Gases are more soluble in liquid alloys than in their crystalline solids. During the crystallization process, the gas is released from the growing crystal into the remaining liquid. If the gas is re- leased in the interior of the casting, it may be entrapped and exist in the form of gas voids or gas porosity (Figure 2-4). Gas porosity has some characteristics different from shrink- age porosity. Unlike shrinkage porosity, gas develops in the solidifying melt with pres- sure. The gases will try to move within the melt to lower-pressure regions. This occasion- ally leads to elongated void structures called wormholes. These elongated pores form as the gas tries to move away from the solid interfaces in the casting. In some cases, this elongated structure will provide a ready leakage path through the boundary of the casting.

Gas porosity, like shrinkage porosity, will decrease the load bearing capacity of a compo- nent.

Dissolved gas emanating near the surface of a component may cause splitting and de- forming of material near the surface. The appearance and name of this form of surface and near-surface discontinuity is “blistering.”

There are occasions when a casting mold may contain regions that are not completely filled. These discontinuities in structure are called casting cavities. This condition is caused by blockage of the melt from the mold cavity by early solidification within a mold passage.

Shrinkage and the stresses arising from shrinkage after casting may be adequate to rupture a casting, as shown in Figure 2-5a. This is called solidification shrinkage crack- ing and it may occur through the crystal grains of an alloy or around the boundaries of grains and solid phases. Shrinkage cracks may later propagate in an alloy and cause failure during subsequent heating and mechanically forming. This damage is a form of hot cracking and hot tearing; see Figure 2-5b. Hot cracking usually is considered to be internal cracking and hot tearing involves cracks open to the surface of a casting. These solidification and hot cracking discontinuities are deleterious to service and are unac- ceptable.

Inclusions of foreign objects in casting typically occur when pieces of refractory are broken off into the melt. The refractory may come from the primary production process or from refractories used in casting molds and pouring crucibles.

There is another form of discontinuity characteristic of casting called a cold shut. This is a discontinuity on the surface of a casting caused by a stream of liquid metal solidifying on and not fusing with a previously solidified part of the component. A cold shut may also refer separately to a plugging of a channel in a mold by early solidification, which then prevents the entire mold cavity from filling, resulting in casting cavities.

A scab is a surface discontinuity that has a rough and porous texture, usually with a cavity underneath caused by refractory inclusion near the surface. Scabs are more com- monly found in thin sections of castings.

A mold parting line may give rise to a geometric discontinuity on the casting called a casting seam. A mold parting line seam is one indication of a casting process.⁶

An inherent cause of microscopic inhomogeneity in materials comes from phase sepa- ration in alloy solids. A phase is a region of matter that is homogeneous and physically distinct from its surroundings, independent of the size, form, or disposition of the phase.

Most engineering alloys are composed of structures that are mixtures of numerous micro- scopic phases that are inhomogeneous in chemistry and/or crystal structure. This is a con- sequence of the natural separation of phases that occurs over discrete regions of tempera- ture and chemistry, as shown in equilibrium phase diagrams.

FIGURE 2-4 Gas porosity in alumiunum alloy die casting. (Courtesy of R. B. Pond, Jr.)

6. There are casting processes, such as investment casting, which leave no mold parting lines.

Parting lines may also be dressed out of a component after casting.

(b)

FIGURE 2-5 (a) Radiograph of shrinkage in a casting. (b) Radiograph of hot tear in a cast- ing. (Courtesy of C. Hellier.)

(a)

In metallurgical systems, an equilibrium phase diagram is an empirical map of the phases in alloys that exist as a function of temperature and composition. Although most engineering alloys generally are composed of many different phases and characteristic geometric combinations of phases called constituents, the principle of chemical phase separation may be seen by example. A common type of binary phase diagram called a bi- nary eutectic is found in the lead–tin system (Figure 2-6).

There are three points illustrated in the lead–tin diagram in Figure 2-6. Point one is in the single-phase liquid region, and the structure is a liquid solution with a uniform chem- istry of lead and tin. At point two, the phase region is composed of two phases, a liquid and a solid. In this two-phase liquid and solid region, the liquid and the solid are not com- posed of the initial alloy chemistry and the points on the bounding lines at the temperature selected define these two chemistries. In the example, the solid is a lead-rich crystal struc- ture with tin atoms replacing some of the lead atoms. This alloy solid is called a solid so- lution. In the mixture of the crystalline solid and the liquid alloy at point two, the liquid is richer in tin than is the solid. Neither of the phases is of the original alloy composition, but in combination they must yield the original composition.⁷

Point three of Figure 2-6 is in the two-phase solid region named alpha and beta (␣+

␤). Here is found a mixture of two different crystalline solids. The alloy is composed of a lead-rich crystalline solid solution, with tin atoms replacing some of the lead atoms, and a tin-rich crystal, with lead atoms replacing some of the tin atoms. A micrograph of this structure is seen in Figure 2-7. Like most engineering alloys, this solid is inhomogeneous on the microscopic level.

FIGURE 2-6 The equilibrium binary phase diagram for lead-tin. (Courtesy of R. B. Pond, Jr.)

7The average of the liquid and the solid composition is the alloy composition.

Microscopic mixtures of different solid phases are the rule for engineering alloys.

The physical properties of the different phases are different. The separation of the dif- ferent solids on a microscopic scale does not necessarily represent a structural disconti- nuity of concern. However, the sizes, shapes, and distributions of alloy solid phases characterize the microstructure, which is critical in determining the physical properties of the alloy.

Macroscopic segregation in the casting operation can give rise to inhomogeneities that are deleterious in an alloy. Macroscopic chemical segregation is a condition that often oc- curs in very large alloy castings, such as ingots. The segregated glassy material formed during casting is called slag. Relatively large local aggregates of slag challenge the me- chanical strength of an alloy.

There are also changes in alloy microstructure that occur at relatively high tempera- tures where a nonequilibrium structure changes to an equilibrium structure. Graphitiza- tion of carbon and molybdenum steels is a good example of this phenomenon. The car- bides in this alloy steel will decompose to iron and graphite at high temperatures and the location of the decomposition will favor highly stressed regions. Therefore, highly stressed regions of these steels will change their microstructure over time and generate weak particles of graphite. These graphitic regions may eventually initiate cracks.