Lecture 2.1

METAL CASTINGDisposable Mold Casting

Fundamental of metal casting

Introduction

shrinkage

Typical Cast PartsFigure (a) Typical gray-iron castings used in automobiles, including the transmission valve body (left) and the hub rotor with disk-brake cylinder (front). Source: Courtesy of Central Foundry Division of General Motors Corporation. (b) A cast transmission housing. (c) The Polaroid PDC-2000 digital camera with a AZ191D die-cast high-purity magnesium case. (d) A two-piece Polaroid camera case made by the hot-chamber die-casting process. (a)(b)(c)(d)

Casting of an Aluminum PistonFigure 10.16 Aluminum piston for an internal combustion engine: (a) as-cast and (b) after machining.

Solidification of Pure MetalsFigure 10.1 (a) Temperature as a function of time for the solidification of pure metals. Note that the freezing takes place at a constant temperature. (b) Density as a function of time

Solidification volume shrinkageFig. 2.19 Changes in volume as a metal alloy solidifies.

Cast Structures of Solidified MetalsFigure 10.2 Schematic illustration of three cast structures of metals solidified in a square mold: (a) pure metals; (b) solid-solution alloys; and (c) structure obtained by using nucleating agents. Source: After G. W. Form, J. F. Wallace, J. L. Walker, and A. CibulaFigure 10.3 Development of a preferred texture at a cool mold wall. Note that only favorably oriented grains grow away from the surface of the mold

Alloy SolidificationFigure 10.4 Schematic illustration of alloy solidification and temperature distribution in the solidifying metal. Note the formation of dendrites in the mushy zone.

Solidification of Iron and Carbon SteelsFigure 10.5 (a) Solidification patterns for gray cast iron in a 180-mm (7-in.) square casting. Note that after 11 minutes of cooling, dendrites reach each other, but the casting is still mushy throughout. It takes about two hours for this casting to solidify completely. (b) Solidification of carbon steels in sand and chill (metal) molds. Note the difference in solidification patterns as the carbon content increases. Source: After H. F. Bishop and W. S. Pellini

Basic Types of Cast Structures Figure 10.6 Schematic illustration of three basic types of cast structures: (a) columnar dendritic; (b) equizxed dendritic; and (c) equiaxed nondendritic. Source: Courtesy of D. Apelian

Cast StructuresFigure 10.7 Schematic illustration of cast structures in (a) plane front, single phase, and (b) plane front, two phase. Source: Courtesy of D. Apelian

Chill, columnar and equaixial grainsFig. 2.17 Sketch of solidified grain structure of an alloy: (a) chill crystals; (b) columnar grains; and (c) region of coarse equiaxed grains in centre.

Fluid Flow and Solidification TimeSprue designMass continuityBernoullis theoremReynolds numberChvorinovs Rule

Casting Design and Fluidity TestFigure 10.8 Schematic illustration of a typical riser-gated casting. Risers serve as reservoirs, supplying molten metal to the casting as it shrinks during solidification.Figure 10.9 A test method for fluidity using a spiral mold. The fluidity index is the length of the solidified metal in the spiral passage. The greater the length of the solidified metal, the greater is its fluidity.

Figure 10.10 Temperature distribution at the interface of the mold wall and the liquid metal during the solidification of metals in castingTemperature Distribution during Metal Solidification

Solidified Skin on a Steel CastingFigure 10.11 Solidified skin on a steel casting. The remaining molten metal is poured out at the times indicated in the figure. Hollow ornamental and decorative objects are made by a process called slush casting, which is based on this principle. Source: After H. F. Taylor, J. Wulff, and M. C. Flemings

Solidification Contraction or Expansion

Common Casting DefectsFigure 10.13 Examples of common defects in castings. These defects can be minimized or eliminated by proper design and preparation of molds and control of pouring procedures. Source: After J. Datsko.

Types of Internal and External Chills used in CastingFigure 10.14 Various types of (a) internal and (b) external chills (dark areas at corners) used in castings to eliminate porosity caused by shrinkage. Chills are placed in regions where there is a larger volume of metal, as shown in (c).

Solubility of Hydrogen in AluminumFigure 10.15 Solubility of hydrogen in aluminum. Note the sharp decrease in solubility as the molten metal begins to solidify.

Sand castingSAND CASTING BENEFITS

Least Expensive Casting Process Castings can be up to Several Tons Less Expensive than Machining Shapes from Bar Stock Can Cast Intricate Shapes Can be Used with Most Pourable Metals and Alloys

HOW IT WORKS

The Sand Casting ( Green Sand ) molding process utilizes a cope ( top half ) and drag ( bottom half ) flask set-up. The mold consists of sand, ( usually silica ), clay and water. When the water is added it develops the bonding characteristics of the clay, which binds the sand grains together.

When applying pressure to the mold material it can be compacted around a pattern, which is either made of metal or wood, to produce a mold having sufficient rigidity to enable metal to be poured into it to produce a casting. The process also uses coring to create cavities inside the casting. After the casting is poured and has cooled the core is removed. The material costs for the process are low and the sand casting process is exceptionally flexible. A number of metals can be used for castings in sizes from ounces to many thousand pounds. The mold material is reclaimable, with between 90 and 95% of the sand being recycled, although new sand and additions are required to make up for the discarded loss. These features, combined with the relative ease of mold production, have ensured that the green sand molding process has remained as the principal method by which castings are produced.

The sand casting processes

Mold makingFIGURE 2-9 Steps in making a green sand mold using a semi-automatic molding process. (D. L. Zalensas, ed., Aluminum Casting Technology, 2nd ed., 1997, p. 187). Reprinted withpermission from American Foundry

Design for Ease of Removal from MoldFigure Taper on patterns for ease of removal from the sand mold

THE SAND The sand used for green sand molding is critical and determines the favorable or unfavorable outcome of the casting. It controls the tolerances, surface finish and the repeatability while in production. Remembering that the tolerances on sand castings are usually wider than the other casting methods.

The sand must exhibit the following characteristics:

FLOWABILITY: The ability to pack tightly around the pattern. PLASTIC DEFORMATION: Have the ability to deform slightly without cracking so that the pattern can be withdrawn. GREEN STRENGTH: Have the ability to support its own weight when stripped from the pattern, and also withstand pressure of molten metal when the mold is cast. PERMEABILITY: This allows the gases and steam to escape from the mold during casting.

All of these requirements are dependent on the amount of active clay present and on the water content of the mixture.

DEFINITIONS 1. POURING CUP: This is where the metal is poured into the mold. 2. SPRUE: The vertical channel from the top of the mold to the gating and riser system. Also, a generic term used to cover all gates, runners and risers. 3. RUNNER: The portion of the gate assembly that connects the sprue to the casting in gate or riser. 4. GATE: The end of the runner in a mold where molten metal enters the mold cavity. 5. RISER: A reservoir of molten metal provided to compensate for the contraction of the metal as it solidifies. 6. MOLD CAVITY: The impression in a mold produced by the removal of the pattern. When filled with molten metal it forms a casting. 7. COPE: Upper or top most section of a flask, mold or pattern. 8. PARTING LINE: A line on a pattern or casting corresponding to the separation between the parts of a mold. 9. DRAG: Lower or bottom section of a flask, mold or pattern.

Production Steps in Sand-CastingFigure Outline of production steps in a typical sand-casting operation.

Design for Ease of Removal from MoldFigure Taper on patterns for ease of removal from the sand mold

Sequence of Operations for Sand-CastingFigure Schematic illustration of the sequence of operations for sand casting. (a) A mechanical drawing of the part is used to generate a design for the pattern. Considerations such as part shrinkage and draft must be built into the drawing. (b-c) Patterns have been mounted on plates equipped with pins for alignment. Note the presence of core prints designed to hold the core in place. (d-e) Core boxes produce core halves, which are pasted together. The cores will be used to produce the hollow area of the part shown in (a). (f) The cope half of the mold is assembled by securing the cope pattern plate to the flask with aligning pins and attaching inserts to form the sprue and risers. Continued on next slide.

Sequence of Operations for Sand-Casting, Cont.(g) The flask is rammed with sand and rthe plate and inserts are removed. (h) The drag half is produced in a similar manner with the pattern inserted. A bottom board is placed below the drag and aligned with pins. (i) The pattern , flask, and bottom board are inverted; and the pattern is withdrawn, leaving the appropriate imprint. (j) The core is set in place within the drag cavity. (k) The mold is closed by placing the cope on top of the drag and securing the assembly with pins. The flasks the are subjected to pressure to counteract buoyant forces in the liquid, which might lift the cope. (l) After the metal solidifies, the casting is removed from the mold. (m) The sprue and risers are cut off and recycled, and the casting is cleaned, inspected, and heat treated (when necessary).