Powder-Metallurgy for the engineers to undersatnd.ppt

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1. Powder Metallurgy

2. Overview
•History
•Definitions
•Benefits
•Process
•Applications

3. Introduction
•Earliestuseofironpowderdatesbackto3000BC.
Egyptiansuseditformakingtools
•ModerneraofP/MbeganwhenWlampfilamentswere
developedbyEdison
•Componentscanbemadefrompuremetals,alloys,or
mixtureofmetallicandnon-metallicpowders
•Commonlyusedmaterialsareiron,copper,aluminium,
nickel,titanium,brass,bronze,steelsandrefractory
metals
•Usedwidelyformanufacturinggears,cams,bushings,
cuttingtools,pistonrings,connectingrods,impellersetc.

4. Powder Metallurgy
•. . . is a forming technique
Essentially, Powder Metallurgy (PM) is
an art & science of producing metal or
metallic powders, and using them to
make finished or semi-finished products.
Particulate technology is probably the
oldest forming technique known to man
•There are archeological evidences to
prove that the ancient man knew
something about it

5. Powder Metallurgy
•Producing metal or metallic powders
•Using them to make finished or semi-finished
products.
•The Characterization of Engineering Powders
•Production of Metallic Powders
•Conventional Pressing and Sintering
•Alternative Pressing and Sintering Techniques
•Materials and Products for PM
•Design Considerations in Powder Metallurgy

6. History of Powder
Metallurgy
•IRONMetallurgy >
•How did Man make iron in 3000 BC?
•Did he have furnaces to melt iron air blasts, and
•The reduced material, which would then be
spongy, [ DRI ], used to be hammered to a solid or
to a near solid mass.
•Example: The IRON PILLER at Delhi
•Quite unlikely, then how ???

7. Powder Metallurgy
• An important point that comes out :
• The entire material need not be melted to fuse it.
• The working temperature is well below the
melting point of the major constituent, making it
a very suitable method to work with refractory
materials, such as: W, Mo, Ta, Nb, oxides,
carbides, etc.
• It began with Platinum technology about 4
centuries ago … in those days, Platinum, [mp =
1774°C], was "refractory", and could not be
melted.

8. Powder Metallurgy Process
•Powder production
•Blending or mixing
•Powder compaction
•Sintering
•Finishing Operations

9. Powder Metallurgy Process

10. 1. Powder Production
(a) (b) (c)
(a) Water or gas atomization; (b) Centrifugal atomization; (c) Rotating electrode
•Many methods: extraction from compounds, deposition,
atomization, fiber production, mechanical powder
production, etc.
•Atomization is the dominant process

11. Powder Preparation
(a) Roll crusher, (b) Ball mill

12. Powder Preparation

13. 2. Blending or Mixing
•Blending a coarser fraction with a finer fraction
ensures that the interstices between large particles
will be filled out.
•Powders of different metals and other materials may
be mixed in order to impart special physical and
mechanical properties through metallic alloying.
•Lubricants may be mixed to improve the powders’
flow characteristics.
•Binders such as wax or thermoplastic polymers are
added to improve green strength.
•Sintering aids are added to accelerate densification
on heating.

14. Blending
•Tomakeahomogeneousmasswithuniformdistribution
ofparticlesizeandcomposition
–Powdersmadebydifferentprocesseshavedifferent
sizesandshapes
–Mixingpowdersofdifferentmetals/materials
–Addlubricants(<5%),suchasgraphiteandstearic
acid,toimprovetheflowcharacteristicsand
compressibilityofmixtures
•Combiningisgenerallycarriedoutin
–Airorinertgasestoavoidoxidation
–Liquidsforbettermixing,eliminationofdustsandreduced
explosionhazards
•Hazards
–Metalpowders,becauseofhighsurfaceareatovolumeratioare
explosive,particularlyAl,Mg,Ti,Zr,Th

15. Some common equipment geometries used for blending powders
(a) Cylindrical, (b) rotating cube, (c) double cone, (d) twin shell
Blending

16. ME 355 Sp’06 W. Li 16
3. Powder Consolidation
Die pressing
•Cold compaction with 100 –900
MPa to produce a “Green body”.
–Die pressing
–Cold isostatic pressing
–Rolling
–Gravity
•Injection Molding small, complex
parts.

17. Compaction
•Press powder into the desired shape and size in dies
using a hydraulic or mechanical press
•Pressed powder is known as “green compact”
•Stages of metal powder compaction:

18. •Increasedcompactionpressure
–Providesbetterpackingofparticlesandleads
to↓porosity
–↑localizeddeformationallowingnewcontacts
tobeformedbetweenparticles
Compaction

19. •Athigherpressures,thegreendensityapproaches
densityofthebulkmetal
•Presseddensitygreaterthan90%ofthebulkdensityis
difficulttoobtain
•Compactionpressureuseddependsondesireddensity
Compaction

20. W. Li
Friction problem in cold
compaction
•The effectiveness of pressing with a single-acting punch is
limited. Wall friction opposes compaction.
•The pressure tapers off rapidly and density diminishes away
from the punch.
•Floating container and two counteracting punches help
alleviate the problem.

21. •Smaller particles provide greater strength mainly due to
reduction in porosity
•Size distribution of particles is very important. For same
size particles minimum porosity of 24% will always be
there
–Box filled with tennis balls will always have open space between
balls
–Introduction of finer particles will fill voids and result in↑ density

22. •Because of friction between (i) the metal particles and (ii)
between the punches and the die, the density within the
compact may vary considerably
•Density variation can be minimized by proper punch and
die design
(a)and (c) Single action press; (b) and (d) Double action press
(e) Pressure contours in compacted copper powder in single action press

23. A 825 ton mechanical press for compacting metal powder

24. Cold IsostaticPressing
•Metal powder placed
in a flexible rubber
mold
•Assembly pressurized
hydrostatically by
water (400 –1000
MPa)
•Typical: Automotive
cylinder liners →

25. 4. Sintering
•Parts are heated to 0.7~0.9 T
m.
•Transforms compacted mechanical
bonds to much stronger metallic
bonds.
•Shrinkage always occurs:sintered
green
green
sintered
V
V
shrinkageVol


_ 3/1
_









sintered
green
shrinkageLinear



26. Sintering –Compact Stage
•Greencompactobtainedaftercompactionisbrittleand
lowinstrength
•Greencompactsareheatedinacontrolled-atmosphere
furnacetoallowpackedmetalpowderstobondtogether

27. Carried out in three stages:
•First stage: Temperature is slowly increased so that all
volatile materials in the green compact that would
interfere with good bonding is removed
–Rapid heating in this stage may entrap gases and
produce high internal pressure which may fracture
the compact
Sintering –Three Stages

28. •Promotes vapor-phase
transport
•Because material
heated very close to MP,
metal atoms will be
released in the vapor
phase from the particles
•Vapor phase resolidifies
at the interface
Sintering: High temperature stage

29. •Third stage: Sintered product is cooled in a
controlled atmosphere
–Prevents oxidation and thermal shock
Gases commonly used for sintering:
•H
2, N
2, inert gases or vacuum
Sintering: High temperature stage

30. Liquid Phase Sintering
•During sintering a liquid phase, from the lower MP
component, may exist
•Alloying may take place at the particle-particle interface
•Molten component may surround the particle that has
not melted
•High compact density can be quickly attained
•Important variables:
–Nature of alloy, molten component/particle wetting,
capillary action of the liquid

31. Hot Isostatic Pressing (HIP)
Steps in HIP

32. Combined Stages
•Simultaneouscompaction+sintering
•Container:HighMPsheetmetal
•Containersubjectedtoelevated
temperatureandaveryhighvacuumto
removeairandmoisturefromthepowder
•Pressurizingmedium:Inertgas
•Operatingconditions
–100MPaat1100C

33. •Producescompactswithalmost100%
density
•Goodmetallurgicalbondingbetween
particlesandgoodmechanicalstrength
•Uses
–Superalloycomponentsforaerospace
industries
–FinaldensificationstepforWCcuttingtools
andP/Mtoolsteels
Combined Stages

34. (i)Slip is first poured into an absorbent mould
(ii)a layer of clay forms as the mould surface absorbs water
(iii)when the shell is of suitable thickness excess slip is poured away
(iv)the resultant casting
Slip-Casting

35. •Slip:Suspensionofcolloidal(smallparticlesthat
donotsettle)inanimmiscibleliquid(generally
water)
•Slipispouredinaporousmoldmadeofplaster
ofparis.Airentrapmentcanbeamajorproblem
•Aftermoldhasabsorbedsomewater,itis
invertedandtheremainingsuspensionpoured
out.
•Thetopofthepartisthentrimmed,themold
opened,andthepartremoved
•Application:Largeandcomplexpartssuchas
plumbingware,artobjectsanddinnerware

36. 5. Finishing
•The porosity of a fully sintered part is still significant (4-15%).
•Density is often kept intentionally low to preserve
interconnected porosity for bearings, filters, acoustic barriers,
and battery electrodes.
•However, to improve properties, finishing processes are
needed:
–Cold restriking, resintering, and heat treatment.
–Impregnation of heated oil.
–Infiltration with metal (e.g., Cu for ferrous parts).
–Machining to tighter tolerance.

37. Special Process: Hot compaction
•Advantages can be gained by combining consolidation and
sintering,
•High pressure is applied at the sintering temperature to bring
the particles together and thus accelerate sintering.
•Methods include
–Hot pressing
–Spark sintering
–Hot isostatic pressing (HIP)
–Hot rolling and extrusion
–Hot forging of powder preform
–Spray deposition

38. Atomization
•Producealiquid-metal
streambyinjecting
moltenmetalthrougha
smallorifice
•Streamisbrokenbyjets
ofinertgas,air,orwater
•Thesizeoftheparticle
formeddependsonthe
temperatureofthemetal,
metalflowratethrough
theorifice,nozzlesize
andjetcharacteristics

39. Electrode Centrifugation
Variation:
A consumable electrode
is rotated rapidly in a
helium-filled chamber.
The centrifugal force
breaks up the molten tip
of the electrode into
metal particles.

40. Finished Powders
Fe powders made by atomization Ni-based superalloy made by
the rotating electrode process

41. Reduction
•ReducemetaloxideswithH
2/CO
•Powdersarespongyandporousandtheyhaveuniformly
sizedsphericalorangularshapes
Electrolyticdeposition
•Metalpowderdepositsatthecathodefromaqueous
solution
•Powdersareamongthepurestavailable
Carbonyls
•ReacthighpurityFeorNiwithCOtoformgaseous
carbonyls
•CarbonyldecomposestoFeandNi
•Small,dense,uniformlysphericalpowdersofhighpurity
P/M Process Approaches

42. P/M Applications
►Electrical Contact materials
►Heavy-duty Friction materials
►Self-Lubricating Porous bearings
►P/M filters
►Carbide, Alumina, Diamond cutting tools
►Structural parts
►P/M magnets
►Cermets
►and more, such as high tech applications

43. Advantages / Disadvantages P/M
•Virtually unlimited choice of alloys, composites, and associated
properties.
–Refractory materials are popular by this process.
•Controlled porosity for self lubrication or filtration uses.
•Can be very economical at large run sizes (100,000 parts).
•Long term reliability through close control of dimensions and
physical properties.
•Very good material utilization.
•Limited part size and complexity
•High cost of powder material.
•High cost of tooling.
•Less strong parts than wrought ones.
•Less well known process.

44. Powder Metallurgy Disadvantages
oPorous !! Not always desired.
oLarge components cannot be produced on
a large scale [Why?]
oSome shapes [such as?] are difficult to be
produced by the conventional p/m route.
•WHATEVER, THE MERITS ARE SO
MANYTHAT P/M,
•AS A FORMING TECHNIQUE, IS GAINING
POPULARITY

45. References
•Wikipedia Powder Metallurgy
(http://en.wikipedia.org/wiki/Powder_metallurgy)
•Wikipedia Sintering
(http://en.wikipedia.org/wiki/Sintering)
•All about powder metallurgy http://www.mpif.org/
•Powder Metallurgy -
http://www.efunda.com/processes/metal_proces
sing/powder_metallurgy.cfm
•John Wiley and Sons –Fundamentals of Modern
Manufacturing Chapter 16 (book and handouts)