Manufacturing Processes Metal Casting I Manufacturing Processes

Outline Introduction Metal Solidification Fluid Flow Fluidity of Molten Metal Heat Transfer Casting Defects

Introduction Casting Pouring molten metal into a mold shaped after the part to be produced, allowing it to harden, and removing it from the mold

Introduction

Introduction Can be used to create complex internal and external part geometries Some casting processes can produce parts to net shape (no further manufacturing operations are required) Can produce very large parts (cast parts weighing over 100 tons have been made) Can be used with any metal that can be heated to its liquid phase Some types of casting are suited to mass production

Examples of Cast Parts Crank handle formed by casting; some areas were machined and assembled after casting

C-clamps formed by casting (left) and machining (right) Examples of Cast Parts C-clamps formed by casting (left) and machining (right)

Examples of Cast Parts Complex part formed by casting Courtesy of Toth Industries

Forms of Casting and Terminology

Introduction Requirements: Mold cavity with desired shape and size Melting process to provide molten metal Pouring process to introduce the metal into the mold Solidification process controlled to prevent defects Ability to remove the casting from the mold Cleaning, finishing and inspection operations

Casting Terminology Flask The box containing the mold Cope The top half of any part of a 2-part mold Drag The bottom half of any part of a 2-part mold Core A shape inserted into the mold to form internal cavities Core Print A region used to support the core

Casting Terminology Mold Cavity The hollow mold area in which metal solidifies into the part Riser An extra cavity to store additional metal to prevent shrinkage Gating System Channels used to deliver metal into the mold cavity Pouring Cup The part of the gating system that receives poured metal Sprue Vertical channel Runners Horizontal channels

Casting Terminology Parting Line / Parting Surface Interface that separates the cope and drag of a 2-part mold Draft Taper on a pattern or casting that allows removal from the mold Core Box Mold or die used to produce cores Casting The process and product of solidifying metal in a mold

Metal Solidification Pure Metals / Alloys Cooling Rate

Pure Metals / Alloys Pure metals solidify at a constant temperature; alloys solidify within a temperature range

Metal Solidification A nucleating agent (inoculant) is a substance that induces grains to nucleate and form at the same time throughout the structure.

Cooling Rate Rapid cooling produces equiaxed (roughly round) grains Slow cooling towards the interior forms long columnar grains that grow towards the center

Metal Solidification Dendrites Tree-like structures that form during the solidification of alloys Slow cooling rates produce dendrites with larger branch spacing; faster cooling rates produce finer spacing; very fast cooling rates produce no dendrites or grains

Metal Solidification

Metal Solidification

Metal Solidification

Fluid Flow Metal is poured through a pouring cup Risers hold and supply metal to prevent shrinking during solidification Gates are designed to prevent contaminants from reaching the mold cavity

Fluidity of Molten Metal The capability of a molten metal to fill mold cavities Viscosity Higher viscosity decreases fluidity Surface tension Decreases fluidity; often caused by oxide film Inclusions Insoluble particles can increase viscosity, reducing fluidity Solidification pattern Fluidity is inversely proportional to the freezing temperature range

Fluidity of Molten Metal Mold design The design and size of the sprue, runners, and risers affect fluidity Mold material and surface Thermal conductivity and roughness decrease fluidity Superheating The temperature increment above the melting point increases fluidity Pouring Lower pouring rates decrease fluidity because of faster cooling Heat transfer Affects the viscosity of the metal

Fluidity of Molten Metal

Heat Transfer The metal that solidifies first is at the wall of the mold; this solid layer thickens as time passes Shrinkage during cooling can change the part dimensions and sometimes cause cracking; it is caused by the metal’s thermal expansion properties and the phase change between liquid and solid.

Heat Transfer

Heat Transfer

Casting Defects Metallic Projections Cavities Discontinuities Defective surface Incomplete Casting Incorrect dimensions or shape Inclusions

Casting Defects

Casting Defects Porosity may be caused by shrinkage and/or gases Thin sections solidify faster than thick sections; therefore the molten metal cannot be supplied to thick regions that are solidifying Gases become less soluble in a metal as it cools and solidifies, causing it to be expelled and sometimes form or expand porosity

Casting Defects

Casting Defects Chills Pieces of material placed in the mold to speed up heat transfer in thicker areas of the part to prevent shrinkage porosity Internal chills are left within the cast part; external chills are removed

Chills

Summary Casting involves melting metal and allowing it to solidify in the desired shape Casting allows the creation of parts that would be difficult or uneconomical to make by machining

Manufacturing Processes Metal Casting II Manufacturing Processes

Outline Sand Casting Shell Mold Casting Composite Molds Expendable Pattern Casting Plaster Mold Casting Ceramic Mold Casting Investment Casting Pressure Casting Vacuum Casting Die Casting Centrifugal Casting Squeeze Casting and Semisolid Metal Forming Casting Single Crystals Rapid Solidification Melting Design Considerations

Examples of Cast Parts

Examples of Cast Parts

Typical Casting Metals Aluminum Aluminum-silicon alloy Aluminum-copper Brass Gray cast iron Copper Lead Steel

Casting Processes

Sand Casting Uses a mold made of compressed sand; after the metal solidifies, the sand is broken away

Sand Casting Pattern Full sized model of the part Core Full sized model of the interior surfaces of the part Sand Silica (SiO2) 90% sand 3% water 7% clay

Sand Casting

Sand Casting

Example of a Sand Casting Mold

Sand Casting Advantages: Almost no limit on size, shape, weight or complexity; low cost; almost any metal Limitations: Relatively poor tolerances and surface finish; machining often required; low production rate Common metals: Cast irons, steel, stainless steel, casting alloys of aluminum and copper, magnesium and nickel

Sand Casting Size limits: 1 oz – 6000 lb Thickness limits: As thin as 3/32 in, no maximum Tolerances: 1/32 in for the first 6 in, .003 in for each additional inch; additional increment across the parting line Draft allowance: 1 - 3° Surface finish: 100 -1000 µin

Shell Casting Casting process in which the mold is a thin shell (typically 3/8 inch) made of sand held together by a thermosetting binder

Shell Casting

Shell Casting Advantages: Higher production rate than sand casting; high dimensional accuracy and smooth finish Limitations: Requires expensive metal patterns; resin adds to cost; part size is limited Common metals: Cast irons, casting alloys of aluminum and copper

Shell Casting Size limits: 1 oz minimum; usually less than 25 lb; mold area usually less than 500 in2 Thickness limits: 1/16 – ¼ in depending on material Tolerances: .005 in/in Draft allowance: ¼ - ½° Surface finish: 50 – 150 µin

Composite Molds Made from 2 or more different materials Good for complex shapes such as turbine blades

Expendable Pattern Casting Polystyrene pattern vaporizes on contact with molten metal

Foam Pattern of an Engine Block

Plaster Mold Casting Uses a mold made of plaster (gypsum) with talc and silica, which is broken away after the metal solidifies The mold has a relatively low thermal conductivity; a somewhat uniform grain structure can be produced

Plaster Mold Casting Advantages: High dimensional accuracy and smooth finish; can make net- or near-net-shaped parts Limitations: Lower temperature nonferrous metals only; long molding time; mold material is not reusable; maximum size limited Common metals: Primarily aluminum and copper

Plaster Mold Casting Size limits: 1 oz – 15 lb Thickness limits: As thin as .025 in Tolerances: .005 in on the first 2 in; .002 in per additional inch Draft allowance: ½ - 1° Surface finish: 50-125 µin

Ceramic Mold Casting Uses a mold made of refractory ceramic materials which can be used for high-temperature applications

Ceramic Mold Casting

Ceramic Mold Casting Advantages: Intricate detail, close tolerances, smooth finish Limitations: Mold material is expensive and not reusable Common metals: Ferrous and high-temperature nonferrous metals are most common; can be used with alloys of aluminum, copper, magnesium, titanium and zinc

Ceramic Mold Casting Size limits: Several ounces to several tons Thickness limits: As thin as .05 in, no maximum Tolerances: .005 in on the first inch; .003 in per additional inch Draft allowance: 1° Surface finish: 75-150 µin

Investment Casting Uses a wax pattern which is coated with refractory materials to form a mold; the wax is then melted out and the mold cavity is filled with metal Can be used for high precision complex shapes from high melting point metals that are not readily machinable

Investment Casting

Example of a Wax Injection Mold

Example of a Wax Pattern

Example of a Coated Pattern

Example of Finished Castings

Investment Casting Advantages: Excellent surface finish; high dimensional accuracy; nearly unlimited intricacy; almost any metal; no flash or parting line Limitations: Expensive patterns and molds; high labor costs; limited size Common metals: Mainly aluminum, copper and steel; also used with stainless steel, nickel, magnesium and precious metals

Investment Casting Size limits: As small as 1/10 oz; usually less than 10 lb Thickness limits: As thin as .025 in, less than 3 in Tolerances: .005 in on the first inch; .002 in per additional inch Draft allowance: none required Surface finish: 50-125 µin

Pressure Casting Pressure casting forces the metal up into the mold chamber by applying a small amount of pressure

Vacuum Casting

Permanent Mold Casting (Pressure/Vacuum) Advantages: Good surface finish and dimensional accuracy; metal mold causes rapid cooling and fine grain structure; molds can be used up to 25 000 times Limitations: High initial mold cost; shape, size and complexity are limited; mold life is very limited with metals with high melting points Common metals: Alloys of aluminum, magnesium and copper most common; iron and steel can be used in graphite molds; alloys of lead, tin and zinc also used

Permanent Mold Casting (Pressure/Vacuum) Size limits: Several ounces to about 150 lb Thickness limits: Minimum depends on material but generally thicker than 1/8 in; maximum about 2 in Tolerances: .015 in for the first inch and .002 in for each additional inch; .01 in added across the parting line Draft allowance: 2 - 3° Surface finish: 100 - 250 µin

Die Casting Another form of permanent mold casting; molten metal is forced into the mold cavity at pressures ranging from .7 MPa - 700 MPa

Die Casting

Die Casting

Example of a Die Casting Mold

Centrifugal Casting Uses a rotating mold to form hollow cylindrical parts such as pipes, gun barrels and lamp posts

Vertical Centrifugal Casting

Centrifugal Casting Advantages: Can produce a wide range of cylindrical parts; good dimensional accuracy and cleanliness Limitations: Limited shape; spinning equipment may be expensive Common metals: Iron, steel, stainless steel, alloys of aluminum, copper and nickel

Centrifugal Casting Size limits: Up to 10 ft in diameter and 50 ft in length Thickness limits: Wall thickness .1 – 5 in Tolerances: Outer diameter within .1 in; inner diameter within about .15 in Draft allowance: 1/8 in / ft Surface finish: 40 - 100 µin

Semicentrifugal Casting Uses a rotating mold to form parts with radial symmetry, such as wheels with spokes

Squeeze Casting A combination of casting and forging; a die applies pressure as the metal solidifies

Casting Single Crystals Uses a slow crystal-growth solidification procedure to produce parts made of a single crystal with no grain boundaries A helical constriction only allows one crystal of favorable orientation to grow into and fill the mold chamber

Casting Single Crystals

Rapid Solidification Cools metal rapidly at rates as high as 106 K/s so that it cannot crystallize and instead forms an amorphous glasslike structure

Melting Furnaces Cupola Crucible Furnace Induction Furnace

Melting Furnaces Cupola A vertical cylindrical furnace used for melting cast iron

Melting Furnaces Crucible furnace Melts metal without direct contact with a burning fuel mixture

Melting Furnaces Induction furnace Uses an alternating magnetic field to heat the metal

Design Considerations

Design Considerations

Design Considerations

Design Considerations

Casting Alloys

Summary A variety of casting processes are available for different applications Design considerations must be taken to prevent casting defects