Casting

Casting

A manufacturing process where solid metal is melted, poured into a mold, and solidified into a desired shape.

Foundry

A factory equipped for making molds, melting metal, casting, and finishing parts.

Steps of the casting process

(1) Make mold, (2) Melt metal, (3) Pour into mold, (4) Solidify, (5) Remove from mold.

Advantages of casting

Can create complex geometries and has no size limit.

Disadvantage of sand casting

Poor dimensional accuracy and surface finish.

Example of a large part made by casting

Engine block for automotive vehicles.

Expendable mold processes

Casting processes where the mold is destroyed after the cast part is made.

Material used for permanent molds

Typically metal or ceramic refractory material.

More economical casting process for high production

Permanent mold processes.

Requirements for casting

Mold cavity, melting process, pouring technique, solidification process, mold removal, finishing/inspection.

Cope in a mold

The upper half of the mold.

Oversized mold cavity

Made to account for shrinkage during solidification and cooling.

Riser in casting

A feature to supply molten metal to compensate for shrinkage.

Draft in casting

A taper on patterns or cavities to allow easy removal from the mold.

Heat needed to melt metal for casting

Heat to reach melting point, heat of fusion, and heat to superheat for pouring.

Factors for successful pouring

Pouring temperature, pouring rate, and turbulence control.

Fluidity in casting

The ability of molten metal to flow and fill a mold before freezing.

High surface tension effect on fluidity

Reduces fluidity.

Solidification of pure metal vs alloy

Pure metals solidify at a constant temperature; alloys freeze over a range of temperatures.

Chill zone in a casting

A layer of fine equiaxed grains formed by rapid cooling near the mold wall.

Chvorinov’s Rule

A rule used to calculate total solidification time.

Fastest shape to solidify (same volume)

Cube.

Stages of shrinkage in casting

(1) Liquid contraction, (2) Solidification shrinkage, (3) Solid metal contraction.

Directional solidification

A process where remote areas freeze first to minimize shrinkage voids.

Gas porosity in castings

Caused by entrapped gases rejected during cooling.

Riser’s V/A ratio

Must be higher than the casting’s to ensure proper feeding.

Bernoulli’s Theorem in casting

Describes conservation of energy in fluid flow, used for calculating pouring velocity.

Preventing aspiration in sprue

By tapering the sprue to adjust the cross-sectional area with height.

General design rule for casting gates

Gates should feed into thick sections of the casting.

Avoiding hot spots in casting

Hot spots solidify last, causing shrinkage cavities.

Bulk deformation processes

Metal forming operations causing significant shape change by plastic flow.

The four basic bulk deformation processes

Rolling, forging, extrusion, wire and bar drawing.

Benefit of hot working in bulk deformation

Allows significant shape change due to reduced strength.

Cold working in bulk deformation

Enhances parts strength through strain hardening.

Near net shape in bulk deformation

Parts require little or no subsequent machining.

Typical starting forms for bulk deformation

Cylindrical bars, billets, rectangular billets, slabs.

Rolling in metalworking

A process to reduce sheet or plate thickness by compressive forces.

Percentage of materials undergoing rolling

Over 90%.

Flat rolling usage

Used for reducing the thickness of rectangular cross sections.

Shape rolling

Forming square cross sections into shapes like I-beams.

Backing rolls purpose in rolling mills

Support smaller work rolls and reduce deflection.

Tandem rolling mill operation

Uses two-high mills for continuous rolling reduction.

Defect in flat rolling due to high friction

Wavy edges.

Alligatoring in rolling

Surface splitting caused by excessive deformation.

Role of camber in rolling

Compensates for roll bending to ensure uniform thickness.

Hot rolling vs cold rolling

Hot rolling allows larger deformations due to lower material strength.

Cold-rolled vs hot-rolled improvement

Improved surface finish and dimensional tolerances.

Thread rolling

Cold working process to form threads on parts using dies.

Advantage of thread rolling over machining

Produces stronger threads due to work hardening.

Ring rolling purpose

Produces thin-walled rings of larger diameter from thick-walled rings.

Roll piercing use

Creating seamless tubes by exploiting internal cracks.

Hole size control in roll piercing

A mandrel controls hole diameter.

Roll forging alternative name

Cross-rolling.

Examples of roll-forged products

Tapered leaf springs and knives.

Ball rolling process

Steel balls formed by skew-rolling or upsetting.

Forging definition

Controlled plastic deformation of metal into predefined shapes.

Age of the forging process

Dates back to about 5000 BC.

Advantage of forging over casting

Increased strength due to work hardening.

Types of forging dies

Open-die, impression-die, flashless.

Open-die forging alternative name

Upsetting or upset forging.

Barreling in open-die forging

Caused by friction between work and die surfaces.

Flash in impression-die forging

Constrains metal flow to fill the die cavity.

Limitation of impression-die forging

Not capable of close tolerances; machining is often required.

Flashless forging difference from impression-die forging

No excess flash is created; work is fully constrained.

Coining in forging

A flashless process to mint coins with precise details.

Upsetting use in forging

Forming heads on nails, bolts, and similar hardware.

Swaging definition

Using rotating dies to taper a workpiece radially.

Typical swaged product

Tapered rods or tubes.

Forging equipment applying impact load

Drop hammers (gravity or power).

Hydraulic forging press speed range

0.06-0.30 m/s.

Fastest forging equipment

Counterblow hammer (4.5-9.0 m/s).

Hydraulic presses cost in forging

High initial cost but easier maintenance.

When forging is more economical than casting

For large quantities.

Reducing setup costs in forging

Increasing the number of pieces forged with the same die.

Extrusion definition

Forcing metal through a die to form a uniform cross-section.

Two basic types of extrusion

Direct (forward) and indirect (backward/reverse).

Butt in direct extrusion

The portion of billet that can't be forced through the die.

Limitation of indirect extrusion

Lower rigidity of the hollow ram.

Enhancement of grain structure in cold extrusion

Work hardening.

Disadvantage of cold extrusion

High stresses on machinery and die wear without lubrication.

Defect in high friction extrusion

Pipe defect.

Impact extrusion definition

High-speed cold extrusion, often for collapsible tubes.

Avoidance in extruded cross-section design

Sharp corners and non-uniform thicknesses.

Wire drawing vs extrusion

In wire drawing, work is pulled through the die instead of pushed.

Typical cross-sectional shape in wire drawing

Circular.

Wire drawing vs bar drawing

Wire drawing uses smaller diameter stock (down to 0.03 mm).

Limitation of maximum reduction in drawing

Draw stress exceeding yield strength results in elongation.

Purpose of annealing in wire drawing

Relieves work hardening between dies.

Common product of bar drawing

Metal bars for machining or forging.