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Flashcards in Lec 4 Deck (26)
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Powder metallurgy (sintering) basic process

powder metal blended
cold compaction -> sintering
OR hot compaction controlled atmosphere
further processes optional
secondary finishing operations
coining forging infiltration


Adv of powder metallurgy

No waste only semi skilled labour req
final product req little or no finishing


Products produced using powder metallurgy

cutting tools
porous products -> filters
piston rings


How are metal powders produced and how do you decide which process to use

electrolytic deposition
comminution - making particles smaller
economic choice purity physical/chemical properties


Powder production

all particles same size (use sieving as quality control)


What is atomisation (drawing)

liquid metals -> small oriface
stream broken by inert gas/air/water
size of particle based on temp of metal rate of flow, nozzle size and jet characteristics


What is reduction

Hydrogen and CO used as reducing agents
Metal oxides reduced to base metal
powders produced are spongy and porous with uniform spherical or angular shapes


What is electrolyte deposition (drawing)

V Pure method remove pure metal anode, powder remove from cathode
use aqueous solution or fused salts


What is comminution (drawing)

Mechanical process most commonly used crushing and milling
rotating cylinder filled with steel or cast iron balls


What is blending (drawing)

req as powders may have different sizes/shapes
add lubricant allowing particles to flow freely
done in ball mill combine two or more metal powder types -> forming alloy powders to give properties of part
powders fracture and intermingle


What is compaction (drawing)

pressing blended powders together produce a green compact (keeps shape if powder uniform shape and size)
hydraulic or mechanical
obtains req density


Method of compaction - Cold isostatic pressing

Metal powders rubber mould (aids release) room temp


Method of compaction - Hot isostatic pressing

Use high melting point die
inert gas to prevent oxidation (large SA and heat)
100% density


Method of compaction - Powder rolling (drawing)

two rollers produce continuous strip


Method of compaction - Injection moulding (drawing)

fine metal powder blended with polymer or wax binder then pressed greens placed in oven to remove wax then sintered in furance
V complex shapes easy to remove from mould


Method of compaction - Extrusion (drawing)

Powders encased in metal die extruded



DO NOT MELT PARTICLES soften and melt surface - maintain shape
heated in controlled atmosphere below melting point
allow bonding of individual particles
form bridge that then widens


Why do we have a protective atmosphere in sintering

prevent oxidation (powder therefore high SA and high chance of oxidation


What does sintering do

Increases strength density durability and thermal and electrical BUT IT DOES SHRINK therefore allowance should me made


What is the sintering equation

Vs (volume solid) = Vg (volume green) * (1-(change in length/original length)^3
substitute volume solid to density solid and volume green to density green to find out req density of green


Finishing powder metallurgy req or not req

improes properties post sintering
not req if earlier processes done correctly


Method of Finishing - Forging (Diagram)

Work piece shape by compressive force through tool and dies shape sintered compacts to desired shape


Method of Finishing - Coining

Type of forging - slug is coined in completely closed die cavity improves dimensional acc strength and SA


Method of Finishing - Infiltration

Slug lower melting point metal placed against piece
heated to slug melting temp -> infiltrates pores by capillary action
therefore pore free and high density


Method of Finishing - Electroplating

Improved wear and corrosion resistance electrical conductivity and appearence


Powder metallurgy adv and disadv

Eliminates many secondary manufacturing (machining costs) and assembly costs
little waste
wide range of properties - density and porosity
tensile strength of 70MPa to 1250 MPa
BUT initial costs V high for punches and dies
req high production volume 10,000+ to be cost effective
lower tensile strength than wrought or similar composites