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MEC130 2 4 8 10 15 > Lec 4 > Flashcards

Lec 4 Flashcards

1
Q

Powder metallurgy (sintering) basic process

A 
powder metal blended 
cold compaction -> sintering
OR hot compaction controlled atmosphere 
further processes optional 
secondary finishing operations
coining forging infiltration
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2
Q

Adv of powder metallurgy

A

No waste only semi skilled labour req

final product req little or no finishing

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3
Q

Products produced using powder metallurgy

A 
gears
cams
bushes 
cutting tools 
porous products -> filters
piston rings
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4
Q

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

A 
atomisation
reduction
electrolytic deposition
comminution - making particles smaller
economic choice purity physical/chemical properties
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5
Q

Powder production

A

all particles same size (use sieving as quality control)

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6
Q

What is atomisation (drawing)

A

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

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7
Q

What is reduction

A

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

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8
Q

What is electrolyte deposition (drawing)

A

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

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9
Q

What is comminution (drawing)

A

Mechanical process most commonly used crushing and milling

rotating cylinder filled with steel or cast iron balls

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10
Q

What is blending (drawing)

A

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

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11
Q

What is compaction (drawing)

A

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

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12
Q

Method of compaction - Cold isostatic pressing

A

Metal powders rubber mould (aids release) room temp

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13
Q

Method of compaction - Hot isostatic pressing

A

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

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14
Q

Method of compaction - Powder rolling (drawing)

A

two rollers produce continuous strip

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15
Q

Method of compaction - Injection moulding (drawing)

A

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

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16
Q

Method of compaction - Extrusion (drawing)

A

Powders encased in metal die extruded

17
Q

Sintering

A

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

18
Q

Why do we have a protective atmosphere in sintering

A

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

19
Q

What does sintering do

A

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

20
Q

What is the sintering equation

A

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

21
Q

Finishing powder metallurgy req or not req

A

improes properties post sintering

not req if earlier processes done correctly

22
Q

Method of Finishing - Forging (Diagram)

A

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

23
Q

Method of Finishing - Coining

A

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

24
Q

Method of Finishing - Infiltration

A

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

25
Q

Method of Finishing - Electroplating

A

Improved wear and corrosion resistance electrical conductivity and appearence

26
Q

Powder metallurgy adv and disadv

A

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

MEC130 2 4 8 10 15 (6 decks)

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