970428
Thomas Miller
Chicago Powdered Metal Products Company
Francis Hanejko
Hoeganaes
ABSTRACT
Turbine hubs for automatictransmissiontorque
convertersare ideal candidatesfor the powder
metallurgy(P/M) process. The complex shapeof
turbine hubs is costly to producevia conventional
forging and machiningoperations. Increasesin engine
sizeand torquerequirementsby automotivedesigners
requireturbine hubsto possesshigh levelsof
mechanicalproperties. High density P/M
manufacturingtechniques,in combinationwith high
performanceferrousmaterial producescomponents
capableof replacinga forged and machinedturbine hub.
This paperwill review the conversionof a
conventionallyforged and machinedturbine hub usedin
a high torque automatictransmissionto a single pressed
and single sinteredP/M turbine hub. The material used
for the P/M hub was an MPIF FD-O405. Warm
compactionprocessingachievedsignificantly increased
overall sintereddensitiesin the highly stressedinternal
splineregion. Extensivemechanicaland part specific
testingwas conductedto verify the suitability of the
P/M part.
INTRODUCTION
The useof PowderMetallurgy (P/M)
componentshasgrown at a rate of approximately7%
annually since 1990.1During this period, the usageof
PIM componentsin the North American auto sector
increasedfrom 23 poundsper vehicle to in excessof28
poundsper vehicle.' Notable recentapplications
includeconnectingrods powderforged to nearpore-free
Corporation
density,and P/M main bearingend capsthat are pressed
and sinteredto a nominal densityof 6.9 g/cm'2 It was
proposedthat the usageof P/M parts per vehicle could
reach50 poundsby the year 2000.2 Theseadditional
opportunitiesfor P/M can be realized providedthat the
P/M processis capableof producinghigher performance
componentsat lower cost. Traditional methodsto
improve the mechanicalpropertiesof P/M partsinclude
restriking, copperinfiltration, or high temperature
sinteringto achievethe higher density and the
correspondinghigher mechanicalproperties. However,
thesemethodsare costly to implementthus slowing the
growth ofP/M within the automotivesector.
The recentintroduction of the warm compaction
technology3enablesP/M part fabricatorsto single press
and single sinter multi-level complex P/M partsto
densitiesin excessof7.25 g/cm3. Warm compaction
processing,althoughapplicableto all ferrousmaterial
systems,producesthe greatestbenefits when coupled
with high performanceferrousalloy compositions.
Achieving densitiesin excessof 7.25 g/cm3using
diffusion alloyed materialsand molybdenumprealloyed
steelsresult in mechanicalpropertiesthat are
comparableto steelforgings and ductile iron castings.4
This paperwill discussthe conversionof a forged steel
turbine hub (using AISI 1045steel) to a warm
compactedsingle presssingle sinter P/M component.
The complex shapeof the componentprecludedthe
option of double pressingdouble sintering (DP/DS),
and copper infiltration proved unacceptable.
WROUGHT TURBINE HUB PROCESSING
AND THE WARM COMPACTION PROCESS
Turbine hubs in automatictransmissionsare
torquecarrying componentsthat haveboth internal and
externalsplineswith a provision for riveting. Their
function is to transmit torque from the riveted turbine
assemblyto the splined transmissioninput shaft.s
Powdermetallurgy first was usedin the
manufactureof turbine hubs in the early 1960's. The
original P/M turbine hubswere compactedto a
6.2 g/cm3density and subsequentlycopper infiltrated for
higher strength(Figure 1 is a photographof this early
P/M part). As a result of this early pioneeringeffort,
nearlyall current automatictransmissionsnow utilize
P/M turbine hubs. The most commonly specified
material is an MPIF FC-0208,which is a premix of pure
iron powderwith approximately2w/o (weight percent)
copper,0.8w/o graphite plus lubricant.6 Thesecurrent
turbine hubsare processedto a nominal sintereddensity
of 6.8 to 7.0 g/cm3with sectionaldensitiesvarying from
6.6 g/cm3to 7.1 g/cm3.
resultedin the development of new engines with higher
torque for greater towing capabilities. One consequence
of this higher engine output was the performance
demands exceeded the torque carrying capability of
conventionally pressed and sintered turbine hubs. To
guarantee system reliability, the transmission designer
was forced to switch from conventional P/M to
alterl}ative technologies to meet performance demands.
Steel forging was one alternative chosen. The complex
shape of the turbine hubs processed by the forging and
machining process proved to be a costly option. What
was needed was a high performance P/M material
process that would survive the higher torque output of
the new engines.
Figure 2 showsschematicallythe manufacturing
sequenceof the wrought turbine hub. AISI 1045bar
stock is hot upsetforged to form the flange and top
cone. After forging, the blank undergoesapproximately
8 distinct machiningoperationsincluding turning and
broachingof the outer (00) and inner diameters(10)
and facing of the flange. The final step is induction
hardeningof the IO spline for strengthand wear
resistance.Figure3 showsthe forged and machined
turbine hub.
Also shown in Figure2 is the manufacturing
sequencefor the warm compactedP/M component. The
powderspecifiedwas an MPIF FD-0405. This material
is a diffusion alloyed powdercontaining4w/o nickel,
1.5w/ocopperand 0.5w/o molybdenum.6This baseiron
was premixedwith graphiteand lubricant; premixing
accomplishedby the ANCORDENSETMpremix
technology. The press-readypowder and compaction
tooling were heatedto the requiredtemperatures
utilizing a Cincinnati IncorporatedEI-TempTMmaterial
delivery system.7 Temperaturecontrol of the die and
heatedpowderwas maintainedwithin +/- 2.5°C of the
setpoint. Compactionwasdone in an 825 ton
Cincinnati Rigid Reflex press. The turbine hubs
weighedapproximately 1100gramseach. Following
compaction,sintering wasdone at conventional
temperaturesusingan endothermicprotective
atmosphere.
Figure 1: Early P/M Turbine Hub
Consumer demand for higher performance
engines in the light truck and van market segments
Prior to production part approval, the warm compacted
turbine hubs were subjected to extensive mechanical
property evaluation. Standard MPIF tensile and fatigue
testing was performed. Preproduction hubs were
evaluated for internal spline durability and internal
spline wear relative to both conventionally compacted
PIM hubs and the forged hubs. This investigation was
done in an MTS test cell designed specifically for torque
testing. The test procedure consisted of fixturing the
Warm Compacted P/M
I
Warm compactpart
Wrought Forging
I
I
Hot upsetforge flange andspacer
I
Bore ID and counter bore spacer
I
Sinter
I
I
Deburr
I
I
I
I
Face top of flange and machine spacer
MachineaD to stepandundercut I
Turn counter bore on ID and burnish
Drill rivet holes
I
Turn aD andundercuton aD
I
Mill slots on bottom of turbinehub
I
I
I
I
BroachIDJ
I
BroachOD
I
I
1,,11 il=~i =1--b=~=I~--=~*~:-bi:~; ~~~:
I.~~~~tl;~He~t T;~~t X~~~;:I-S~'\~c.I
I
Figure 2: Manufacturing steps of the forged steel turbine hub and the warm compacted turbine hub
aD andapplying the designtorqueto the ID spline. For
internal spline durability, the torque was dynamically
appliedfrom 0 to 890 foot poundsthen back to 0 with a
75% spline engagement.The test was terminatedif the
specifiedtorque was not achievedor if the angular
rotation of the ID shaft exceededa specified amount.
Internal spline wear testing was performed in the same
dynamictest cell with test conditions of a constant
torqueof 150foot poundswith 90% spline engagement
and 4 mm of sliding travel per cycle. The hub was
removedat specifiedintervals up to 350,000cyclesfor
measurementof the spline wear.
RESULTS AND DISCUSSION
Figure 3 Forged and Machined Turbine Hub
Table 1 summarizesthe tensile properties,
hardnessvalues,and rotating bending fatigue
characteristicsof both the forged AISI 1045and warm
compactedMPIF FD-0405 materials. Data for the
forged AISI 1045material is presentedin the annealed
and hardenedcondition (the ID is induction hardened).
Propertydata for the FD-0405 is presentedin the
as-sinteredcondition at a density of 7.25 g/cm3(the
overall density of the warm compactedandsintered
turbine hub). Data for the P/M material is presentedin
the as-sinteredcondition only becauseit wasdetermined
that no surfacehardeningof the ID was necessary.
Table 2 presentsthe internal spline durability
test resultsof conventionallycompactedturbine hubs,
forgedturbine hubs,and the warm compactedMPIF
FD-0405hubs. Productionpart validation requiredthat
12 hubsbe testedto 1 million cycles with no failures
with one additional hub testedto 2 million cycles with
no failure. This high torque regime of 0-890-0 foot
poundsexceededthe strengthcapability of the standard
FC-0208material. Failureswere observedbelow the
specifiedminimum value. The forged turbine hub met
the test requirementswith no evidenceof failures.
Warm compactionof the turbine hubsproducedparts
having an overall sintereddensity of 7.25 g/cm3.
Despitethe lack of full density,the warm compaction
processproducedtorque test resultsthat exceededthe
minimum specificationrequirements. That is, twelve
hubswere subjectedto 1 million cycles with no failures
and I hub was testedto two million cycles with no
failure. To evaluatethe durability of the high
performanceP/M hub, one was cycled to failure.
Structuralfailure of the hub occurredat approximately
The as-sinteredyield and tensile strengthsof the
MPIF FD-0405 materialwere equivalentto the forged
AISI 1045steel. Ductility of the steel forging was
higher than the warm compactedP/M part. The fatigue
propertiesdevelopedby rotating bendingfatiguetesting
demonstratedthat the steelforging in the as-forged
condition had a higher fatigue endurancelimit relative
to the as-sinteredwarm compactedP/M part. Specific
part testing was necessaryto determineif the difference
in rotating bendingfatigue performancewasdetrimental
to actual componentperformance.
Table 1: Summary of Standard Mechanical Property Test for Forge and P/M Hubs
For
, ged
AISl1045
".
~m"~cO40:$"
,"""'2"5 """'
~ea~T~t~d
SIccl045 ccc
""i!;;\
cc"'~
CcC~C~ccccc
Yield Strength,psi
TensileStrength,psi
Elongation,%
HRC/B
97 HRB
130,000
16
43 HRC
FatigueLimit, psi
45,000
100,000
62,000
90,000
25
"..t~c",.
96,000
'(
/ """""""'3C"
-,..f!"~""',,,C
62,000
117,000
2.6
17HRC *.
35,000
ApplAve.~t
Table 2: ill Spline Durability Test Results
~yces~~~~r~
lt
F
h
C
Standard P/M hub
1..mi.1l.ioncy~.~
"
at890.ft~lbf.
Failuresobserved
AISI 1045hub
12 parts, no Failures
Warm compact FD-O405
12 parts, no Failures
Specification
12parts,no Failure
~~jJ.~ioncycles
~"
""' at890
,L. 'fi~'lb Ic."
Failuresobserved
1 part, no failure
1 part, no failure
1 part, no failure
O
J
~
cat... 89' 0 .t.
lbc~.~cc
-9,000,000
9 million cycles, significantly in excessof the specified
minimum. Spline durability testing of the high
performancePIM turbine hub demonstratedthat this
processis capableof replacinga steelforging in this
critical high torque application.
The secondcritical performancecriterion of the
turbine hub was internal spline wear. As described
earlier,the analysisof the spline wear was performedin
the sametest cell as the spline durability evaluation.
Table3 summarizesthe wear test resultspresentedas
relative measurements.That is, the maximum wear
allowed by the specificationis equatedto 1. Any wear
numberlessthan 1 implies the wear was lessthan the
specifiedmaximum, thus meetingthe specification.
Any numbergreaterthan I implies the wear exceeded
the specification,thus not meetingthe specification.
Table 3: Summary ofm Spline Wear Tests
Conventionallycompactedand sintered
FC-O208turbine hubswere evaluated. Thesehubs
exhibited excessivewear in this high torque application
with a relative wear value of 1.4. Wear measurements
of the forged hub showedno wear at the specified
number of cycles. However,it is noted that the ID
spline of the AISI 1045hub was induction hardenedto a
hardnessvalue 40 HRC. Without this hardening
operation,the forged steelturbine hubs would not meet
the specifiedminimum wear perfonnance. Weartesting
of the warm compactedMPIF FD-O405showedno
measurablewear at the specifiednumber of test cycles.
The absenceof wear in the warm compactedturbine
hubs is significant becausethesecomponentswere
evaluatedin the as-sinteredcondition. The apparent
hardnessof the P/M componentswas approximately
15 HRC. The warm compactionP/M processcoupled
with the MPIF FD-0405high perfonnancematerial
systemeliminatedthe needfor heattreating of the
internal spline thus significantly reducingmanufacturing
cost and simplifying the manufacturingflow.
Figure 4 is a photomicrographof the as-sintered
microstructureof the warm compactedturbine hub. It
consistsof lamellar pearlite,bainite, nickel rich
martensiteareas,and ferrite. This unique microstructure
was responsiblefor the excellentwear performance
demonstratedin the as-sinteredcondition.
Figure 3: Photograph of the Warm Compacted
Tnrhinp Hnh
Figure 4: Microstructure of the As-Sintered
Turbine Hub
14
13r
-~ 12rt
11
r
-<>- MeasuredPorosity
,
~:
~~~=~I~i~:===~=
,
~
j
~--
,
'-'
c-
"j 7.40
-0- CalculatedDensity
10 t
~"r
,oJ"
'
"""
~
"-
~ ~~
,
7.30 E
~
'-'
7.20 .~:Go
-
.in
0
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0
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Q)
...
~
rI)
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:2-:
9
c=
v
0
7.10 "0
v
~
"3
7.00 C.)
~
U
~
8
-v-
7
6
5
--J--
0
5
10
15
20
25
30
Distancealong the ill Spline from the Bottom (mm)
Figure5: PorosityDistribution Along the m Spline
As mentionedearlier,the overall sintered
densityof the turbine hub was 7.25 g/cm3. To achieve
this high sintereddensity, the greencompactwas
compactedto approximately97% of the pore free
densityof the premix. Onebenefit of compactionto this
high greendensity is the greateruniformity of density
throughoutthe finished part. To documentthis, a
productionprocessedhub was sectionedand examined
for porosity distribution alongthe length of the spline
usingquantitativeimage analysis. Figure 5 is a plot
showingthis density profile for a production part. The
drop off in density in the small neutral zone is
approximately0.15 g/cm3comparedwith the average
sectionaldensity of about 7.3 g/cm3. This reduced
densitydifferential is a significant improvementrelative
to the resultsachievedwith conventionalcompactionin
which this density differential can be as great as 0.4
g/cm3.
In addition to the mechanicalproperty testing,
extensivedimensionaland weight capability studies
wereperformedon the process. The resultsindicated
that the variationsmeasuredwere consistentwith the
v~ri~tinn~nhtainedin conventionalP/M Drocessin2:.
SUMMARY
Warm compactionof an 1100gram multi-level
P/M turbine hub producedan overall as-sintereddensity
of approximately7.25 g/cm3. High densitieswere
achievedin the critically stressedID regionsof the part.
Combiningthe benefitsof the warm compaction
technologywith a high performancematerial system,
the needfor induction hardeningof the ID splinewas
eliminated. This resultedin a simplified manufacturing
processand an overall part cost reduction. Extensive
preproductiontrials demonstratedthat the warm
compactionprocessis a viable manufacturing
alternativefor the productionof complex multi-level
shapes,in particular automatictransmissionturbine
hubsfor high torque engines. This paperdescribedthe
conversionof a forged hub to one producedvia this high
performanceP/M material processingsystem. Torque
testing of preproductionhubs demonstratedequivalent
performanceto the wrought componentin termsof both
ID spline durability andill spline wear.
CONCLUSIONS
REFERENCES
1.) Warm compactionprocessingof a complex
multi-level turbine hub producedan overall sintered
densityof 7.25 g/cm3with high sintereddensities
achievedin the criticallD spline region.
I.) Capus,JosephM., "Metal Powder,A Global Survey
of Production,Application and Markets 1992- 200I",
SecondEdition, ElsevierAdvancedTechnology,June
1996.
2.) Warm compactionprocessingof a high
torqueautomatictransmissionturbine hub successfully
replaceda forged, machined,and heattreatedAISI 1045
steelpart. The P/M processsignificantly reducedthe
processingsteps. The reducednumberof processing
stepsresultedin overall lower manufacturingcost.
2.) White, Donald G., "Powder Metallurgy in 1995",
AdvancedMaterials & Processes,August 1995,pp 49 51.
3.) Rutz, H.G., Hanejko F.G., "High Density Processing
of High PerformanceFerrousMaterials", Advancesin
Powder Metallurgy and Particulate Materials
3.) Utilizing an MPIF FD-O405material
composition,the P/M turbine hub was usedin the
as-sinteredcondition eliminating the requirementfor
induction hardeningof the 10 spline as requiredin the
forged componentit replaced.
4.) Standardtensile and fatigue testing of the
P/M material demonstratedthat the P/M componentwas
equivalentin yield and tensile propertiesto the wrought
steel. The P/M part had a lower rotating bendingfatigue
strength.
5.) 10 spline durability and 10 spline wear
testing of the warm compacted P/M component met and
surpassedthe requirements specified by the end user.
6.) Manufacturingvariability of the warm
compactionprocesswas equivalentto the variability of
the conventionalcompactionprocesses.
ACKNOWLEDGMENTS
Vol, 5, Compiled by C. Lall and A.J. Neupaver,pp 117133,Metal PowderIndustriesFederation,Princeton.NJ.
4.) Luk, S.H., Rutz, H.G., Hanejko,F.G., "A New
Method For ManufacturingHigh PerformancePowder
Metallurgy Components'"Advancesin Powder
Metallurgy and Particulate Material-1994, Vol. 5,
Compiled by C. Lall and A.J. Neupaver,pp 135-]63,
Metal PowderIndustriesFederation,Princeton.NJ
5.) Ford EngineeringSpecification,Part Number
ESF4DP-7926-AA,Rev Frame2, May 1989.
6.) "Material Standardsfor PIM Structural Parts- MPIF
Standard35", Metal PowdersIndustry, Federation,
Princeton,NJ, 1994.
7.) Unkel, R., "Introducing EI- TempTM,A New PIM
Part Pressing System", Advances in Powder Metallurgy
and Particulate Material-I 994, Vol. 5, Compiled by C.
Lall and A.J. Neupaver, pp 135-163, Metal Powder
IndustriesFederation,Princeton.NJ
The authorswish to thank Mr. Frederick
Geromettaand Mr. J. S. Khanujaof the Ford Motor
Companyfor their supportof this programand their
suggestionsin the preparationof this paper.
Additionally, we want to thank the managementof
ChicagoPowderedMetal ProductsCompanyand
HoeganaesCorporationfor their permissionto publish
this paper. In particular,we acknowledgethe assistance
of all Chicago PowderedMetals laboratoryand
productionpersonneland Jerry Golin, SteveKolwicz
and Tom Murphy of the HoeganaesCorporation
Metallographic laboratoryfor their assistancein sample
preparationand analysis.
- 1994,