IE 21: Casting and Forming Processes

Module 4A: Casting and Powder Metallurgy

Casting

Pouring molten metal into a mold containing a cavity that produces the desired part shape allowing it to solidify, and removing the part from the mold.

Foundry

Factory equipped for making molds, melting and handling molten metal, performing the casting process, and cleaning the finished casting

Foundrymen

What workers who perform casting are called

The Mold

Contains cavity whose geometry determines part shape

What should the size of the cavity be relative to the object?

Slightly oversized to allow for shrinkage of metal during solidification and cooling.

Expendable Molds

Usually made of sand, plaster, ceramics, and similar refractory materials and is broken up to remove the casting; meant to be broken to get the product inside

Binders/Bonding Agents

Used to provide cohesiveness and structural stability

Permanent Molds

Made of metals which maintain their strength at high temperature; can be used repeatedly and provides better heat conductivity to the process

Composite Molds

Made of two or more different materials (sand, graphite, metals); improve mold strength, control cooling rates, and optimize process economics; used when materials are complex

Types of Casting Process

Sand, Permanent Mold, Die, Shell Mode, Investment, Centrifugal

Sand Casting

The traditional method of casting; uses silica sand (SiO2) as the mold material because it resists high temperatures; added with binders like clay, zircon, olivine, etc.`

Two general types of sand in sand casting

Naturally bonded (bank sand) and synthetic (lake sand)

Mulling machines

Used to uniformly and thoroughly mull (mix) sand with additives

Sand Casting Process

Factors to consider in determining the proper sand mold

Surface finish, good permeability, good collapsibility

Open mold (Sand casting)

Simply a container in the shape of the desired part

Closed Mold

Mold geometry is more complex and requires a gating system (passageway) leading into the cavity

Types of Sand Molds

Cold-box, Green-sand, No-bake Molds

Cold-box

Organic or inorganic binders are blended for better dimensional accuracy; more expensive than green-sand

Green-sand

Sand, clay, and water mixed together; least expensive of making molds

No-bake Molds

Synthetic liquid resin mixed with sand; hardens at room temperature

Parts of Sand Mold

Flask

Box containing the mold

Cope

Top half of any part of a 2-part mold

Drag

Bottom half of any part of a 2-part mold (additional parts are called cheeks)

Core

Shape inserted into the mold to form internal cavities or lettering on surface of casting

Core Print

A region used to support the core

Mold Cavity

Hollow mold area in which metal solidifies into the part

Riser

An extra cavity that stores and supplies additional metal to the casting as it shrinks during solidification (blind riser, open riser)

Gating system

Channels used to deliver metal into the mold cavity

Pouring cup

The part where molten metal is poured

Sprue

Vertical channel through which molten metal flows downward

Runners

Horizontal channels

Parting line

Interface that separates the cope and drag of a 2-part mold

Vent

Allows gases and air to escape, preventing defects like porosity and blowholes in the final casting

Well

A cavity at the base of the sprue that helps reduce turbulence and control the flow of molten metal

Hearing furnace

Used to heat the metal to molten temperature sufficient for casting

Turbulence

Inconsistent and irregular variations in the flow

Solidification of Metals

Transformation of molten metal back into solid state; differs if metal is pure metal or alloy

Solidification of Pure Metals

Solidifies at a constant temperature equal to its freezing point (same as melting point)

Solidification of Alloys

Freeze over a temperature range rather than at a single temperature

Solidification Shrinkage

Occurs in nearly all metals because the solid phase has a higher density than the liquid phase (Exception: cast iron with high C content)

Pattern shrinkage allowance

Amount by which mold is made larger relative to final casting size

External Chill

Used to encourage rapid freezing of the molten metal in a thin section of the casting

Riser Design

Must have longer freezing time than the casting; vertical channels that provide a continuous flow of molten metal to eliminate shrinkage as solidification occurs during casting process

Defects in Casting

Segregation, Porosity

Segregation Defects

When the concentration of the solute is not constant throughout the casting; can be reduced by homogenization (achieved through faster colling rate)

Porosity Defects

Caused by shrinkage, trapped gases, or both; detrimental to ductility and fatigue life (must be kept at minimum levels)

Typical shrinkage rage of metal

4% to 7%

Gas porosity

Caused by the gas bubbles

Shell Mold Casting

Box contains fine sand, mixed with thermosetting binder (phenol-formaldehyde) that coats the sand particles

Investment Casting

Also known as LOST-WAX process;

Refractory

Has an unusually high melting point and that maintains its structural properties at very high temperatures

Permanent Mold Cating

Also known as hard-mold casting; two halves of a mold are made from materials such as cast iron, steel, bronze, graphite or refractory metal alloys

Die Casting (Hot Chamber)

Involves the use of a piston, traps a certain volume of molten metal and forces it into the die cavity; low-melting point alloys are usually casted

Centrifugal Casting

Utilizes the inertial forces caused by rotation to distribute the molten metal into mold cavities

Casting Advantages

• Can create complex part geometries

• Can create both external and internal shapes

• Some processes are net shape; others are near net shape

• Can produce very large parts

  • Some methods are suited to mass production

Casting Disadvantages

• Limitations on mechanical properties

• Poor dimensional accuracy and surface finish for some

• Safety hazards to workers due to hot molten materials

Powder Metallurgy

Highly developed method of manufacturing precision metal parts; a “chip-less” process, uses roughly 97% of the starting metal in the finished part; mixing elemental or alloy powders then compacting the mixture in a die

Repacking

Occurs with the elimination of particle bridges; higher compaction pressure means particle deformation is the dominant mode of densification

Basic steps in Powder Metallurgy

1. Powder Production

2. Mixing

3. Forming/Compaction

4. Sintering

5. Optional Operations

6. Finished Product

Powder Production

Step in powder metallurgy where the particle shapes in metal powders, and the processes by which they are produced

Atomization

One method of metal-powder production; the dominant process for producing high-quality metal powders by disintegrating a stream of molten metal into find droplets

Atomization Methods

Gas atomization, water atomization, atomization with a rotating consumable electrode, centrifugal atomization with a spinning disk or cup; directly determines the powder’s shape purity, and particle size distribution

Metal-powder production methods

Reduction, electrolytic deposition, carbonyls, mechanical comminution

Reduction

A metal-powder production method wherein gases such as hydrogen and carbon monoxide are used to reduce metal oxide to its metallic state

Electrolytic deposition

A metal-powder production method that utilizes either aqueous solution or fused salts; the powders produced are among the purest

Carbonyls

A metal-powder production method that lets iron or nickel react with carbon monoxide to form iron and nickel of this type

Mechanical comminution

A metal-powder production method that involves crushing, milling in a ball mill or grinding brittle metals into small particles

Mixing

Second step in powder metallurgy that obtain uniformity in materials, impart special physical and mechanical properties to the product, and can be mixed with lubricants to improve flow characteristics

Forming/Compaction

The step in powder metallurgy wherein the blended powders are pressured into shapes in dies; usually occurs at room temperature at a pressure range of 25-50 tons per square inch

Green compact

Produced when loose powder is compacted; have sufficient strength for in-process handling

Density of the green compact depends on:

Pressure applied, size of particles, friction between the particles and the die walls and punches

Relationship between green compact’s density and elastic modulus

The higher the density, the higher the strength and elastic modulus of the part; since higher metal amount is in the volume

Compaction cycle

1. Cycle start

2. Charge die with powder

3. Compaction begins

4. Compaction complete

5. Ejection of compact

6. Recharging of die

Sintering

Process whereby green compacts are heated in a controlled-atmosphere furnace to a temperature within 70% to 90% of the melting point of metal or alloy

Temperature must be sufficiently high to allow bonding of individual particles but not too high to melt the metals

Optional operations

Coining and sizing, forging, impregnating, infiltration, heat treating, machining, grinding, plating

Coining and sizing

An optional operation that has a purpose to impart dimensional accuracy to the sintered part and to improve its strength and surface finish by further densification

Forging

An optional operation which make products have a good surface finish, good dimensional tolerances, and a uniform and fine grain size

Impregnating

An optional operation that makes components have a continuous supply of lubricant, by capillary action, during their service lives

Infiltration

An optional operation wherein hardness and tensile strength are improved and that the pores are filled

Heat treating

An optional operation for improved hardness and strength; further doing of this after sintering increases material strength

Machining

An optional operation for producing various geometric features by milling, drilling, and tapping

Grinding

An optional operation for improved dimensional accuracy and surface finish`

Plating

An optional operation for improved appearance and resistance to wear and corrosion

Finishing

Examples re grinding with a diamond wheel, lapping and honing, ultrasonic machining, and laser-beam machining

Design considerations

Compact shape must be as simple and as uniform as possible; avoid sharp edges, thin sections, thickness variations, high length-to-diameter rations, and sharp changes in contour

Provision must be made for ejecting the green compact from the die without damaging the compact

Must be made with the widest dimensional tolerances consistent with their applications to increase tool and die life and reduce production costs

Powder Metallurgy vs Casting