Lec 4 Flashcards
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
gears cams bushes cutting tools porous products -> filters piston rings
How are metal powders produced and how do you decide which process to use
atomisation reduction 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
Sintering
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
