5.Metal Alloys

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Metal alloys


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Introduction

Its appropriate to conclude the treatment ofmetallic materials by discussing some of theimportant engineering alloys in term of theircompositions, properties, applications and

fabrication techniques.

Metals alloys by virtue of compositions are oftengrouped to two classes-ferrous and nonferrous.

Ferrous alloys which iron is the principal

constituent include steels and cast irons.

Nonferrous which all the alloys that are not ironbased.


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Fabrication of metals

On occasion the suitability of material for anapplication is dictated by the ease of producing adesired shape and the cost involved.

Metal fabrications techniques are the method by

which metals and alloys are formed andmanufactured into useful products.

They are refining, alloying, and often heattreating process that produce alloys with thedesired characteristics

The method chosen depend on several factorssuch as properties of metal, size and shape offinished product and cost.


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The deformation must be induced by externalforce or stress.

The magnitude must be exceed the yield strengthof the metals.

Most metallic materials are amenable to theprocedure, ductile and capable of somepermanent deformation without cracking orfracturing.

When deformation is achieved at temperatureabove which recrystalline occurs the terms is hotworking.

Cold working is done at room temperature ornear room temperature.


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Classification of metal fabrication

techniques

Metalfabricationtechniques

Formingoperations

Casting Miscellaneous

Powdermetallurgy

WeldingDie Sand InvestmentForging Rolling Extrusion


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Forging Forging is the term for shaping metal by using localized

compressive forces. Forging is manufacturing process where metal is pressed,

pounded or squeezed under great pressure into highstrength parts known as forgings. The process is normally(but not always) performed hot by preheating the metal toa desired temperature before it is worked. It is important tonote that the forging process is entirely different from thecasting (or foundry) process, as metal used to make forgedparts is never melted and poured (as in the castingprocess).

There are basically three methods (or processes) to make a

forged part. Impression Die Forging Cold Forging Open Die Forging Seamless Rolled Ring Forging


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What kind of equipment is used to

make forgings?

Although the styles and drive systems vary widely, aforging can be produced on any of the following pieces ofequipment.

Hammers with a driving force of up to 50,000 pounds,pound the metal into shape with controlled high pressure

impact blows. Presses with a driving force of up to 50,000 tons, squeeze

the metal into shape vertically with controlled highpressure.

Upsetters are basically forging presses used horizontally

for a forging process known as "upsetting".Ring Rollers turn a hollow round piece of metal underextreme pressure against a rotating roll, thereby squeezingout a one-piece ring (with no welding required).


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a

b


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Rolling

Flat rolling or Rolling is defined as thereduction of the cross-sectional area ofthe metal stock, or the general shaping ofthe metal products, through the use of therotating rolls .

It allows a high degree of closed-loopautomation and very high speeds, and isthus capable of providing high-quality,close tolerance starting material forvarious secondary sheet metal workingprocesses at a low cost.


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a

b


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Extrusion The extrusion process is used most frequently to form

plastic and aluminum parts or products. However,ceramics, heavier metals or alloys, and foods such aspasta, are also shaped and manufactured through theextrusion process. Essentially, as long as a material can beeasily liquefied, it can be extruded.

Basically, the extrusion process involves loading smallpieces of raw material or stock into a hopper at one end ofthe extruder. In the case of the plastic extrusion process,this raw material is called pellets, and in the aluminumextrusion process, billets.

The raw stock is fed from the hopper into the extruder,

essentially a heated cylinder, where it is softened. A ram,usually a worm gear, forces the semi-liquid raw materialthrough a smaller chamber, then through a shaped die.

From the die, the extrusion, the string, tube, or rod ofshaped stock, is cooled with water or air, along the run outtable to harden. At the end of the run out table, the

hardened product is cut, wound, shipped, or sent on tofurther refining.


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a

b


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Die casting

Die casting is the process of forcing moltenmetal under high pressure into mold cavities(which are machined into dies). Most die castingsare made from non-ferrous metals, specifically

zinc, copper, aluminium, magnesium, lead, andtin based alloys, although ferrous metal diecastings are possible.

The die casting method is especially suited forapplications where a large quantity of small to

medium sized parts are needed with good detail,a fine surface quality and dimensionalconsistency.


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Die cast parts are found in many placesaround the home. The polished, plated zincdie casting in this kitchen faucet illustratesone of the many finishes possible with diecasting.

These connector housings are examples ofthe durable, highly accurate components thatcan be produced with todays modern diecasting.


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Sand casting

Sand casting is used to make large parts(typically Iron, but also Bronze, Brass,Aluminum). Molten metal is poured into a

mold cavity formed out of sand (natural orsynthetic). The processes of sand castinginclude patterns, sprues and runners,design considerations, and casting

allowance.


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Typical Components of a Two-part Sand Casting Mold


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Investment casting

Investment casting is also known as the lostwax process. This process is one of the oldestmanufacturing processes. The Egyptians used itin the time of the Pharaohs to make gold jewelry(hence the name Investment) some 5,000 years

ago. Intricate shapes can be made with highaccuracy.

In addition, metals that are hard to machine orfabricate are good candidates for this process. Itcan be used to make parts that cannot be

produced by normal manufacturing techniques,such as turbine blades that have complex shapes,or airplane parts that have to withstand hightemperatures.


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Inlet-outlet cover of a valvefor a nuclear power stationproduced using investmentcasting

The investment shellfor casting aturbocharger rotor

A view of the interiorinvestment shows thesmooth surface finishand high level of detail

The completed workpiece

a

b


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Powder metallurgy

Powder metallurgy uses sintering process formaking various parts out of metal powder. Themetal powder is compacted by placing in a closedmetal cavity (the die) under pressure. This

compacted material is placed in an oven andsintered in a controlled atmosphere at hightemperatures and the metal powders coalesceand form a solid.

A second pressing operation, repressing, can bedone prior to sintering to improve the compactionand the material properties.


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Welding process

A weld occurs when pieces of metal are joined by causingthe interface to melt and blend prior to solidifying as auniform metal joint. This process may be caused by heat,pressure or a combination of both. When heat alone isused the process is called fusion welding.

Pressure welding usually involves heating the surfaces to aplastic state and then forcing the metal together. Theheating can be by electric current of by friction resultingfrom moving one surface relative to the other.

The methods and equipment used for welding metal arealso associated with cutting metal. There are a large

number of welding and allied processes including thefollowing.


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5.Metal Alloys

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