970428

Thomas MillerChicago Powdered Metal Products Company

Francis HanejkoHoeganaes Corporation

density, and P/M main bearing end caps that are pressedand sintered to a nominal density of 6.9 g/cm'2 It wasproposed that the usage of P/M parts per vehicle couldreach 50 pounds by the year 2000. 2 These additional

opportunities for P/M can be realized provided that theP/M process is capable of producing higher performancecomponents at lower cost. Traditional methods toimprove the mechanical properties of P/M parts includerestriking, copper infiltration, or high temperaturesintering to achieve the higher density and thecorresponding higher mechanical properties. However,these methods are costly to implement thus slowing thegrowth ofP/M within the automotive sector.

ABSTRACT

Turbine hubs for automatic transmission torqueconverters are ideal candidates for the powdermetallurgy (P/M) process. The complex shape ofturbine hubs is costly to produce via conventionalforging and machining operations. Increases in enginesize and torque requirements by automotive designersrequire turbine hubs to possess high levels ofmechanical properties. High density P/Mmanufacturing techniques, in combination with highperformance ferrous material produces componentscapable of replacing a forged and machined turbine hub.

This paper will review the conversion of aconventionally forged and machined turbine hub used ina high torque automatic transmission to a single pressedand single sintered P/M turbine hub. The material usedfor the P/M hub was an MPIF FD-O405. Warmcompaction processing achieved significantly increasedoverall sintered densities in the highly stressed internalspline region. Extensive mechanical and part specifictesting was conducted to verify the suitability of theP/M part.

INTRODUCTION

The recent introduction of the warm compactiontechnology3 enables P/M part fabricators to single pressand single sinter multi-level complex P/M parts todensities in excess of7.25 g/cm3. Warm compactionprocessing, although applicable to all ferrous materialsystems, produces the greatest benefits when coupledwith high performance ferrous alloy compositions.Achieving densities in excess of 7.25 g/cm3 usingdiffusion alloyed materials and molybdenum prealloyedsteels result in mechanical properties that arecomparable to steel forgings and ductile iron castings.4This paper will discuss the conversion of a forged steelturbine hub (using AISI 1045 steel) to a warmcompacted single press single sinter P/M component.The complex shape of the component precluded theoption of double pressing double sintering (DP/DS),and copper infiltration proved unacceptable.

The use of Powder Metallurgy (P/M)components has grown at a rate of approximately 7%annually since 1990.1 During this period, the usage ofPIM components in the North American auto sectorincreased from 23 pounds per vehicle to in excess of28pounds per vehicle.' Notable recent applicationsinclude connecting rods powder forged to near pore-free

WROUGHT TURBINE HUB PROCESSINGAND THE WARM COMPACTION PROCESS

Turbine hubs in automatic transmissions aretorque carrying components that have both internal andexternal splines with a provision for riveting. Theirfunction is to transmit torque from the riveted turbineassembly to the splined transmission input shaft.s

resulted in the development of new engines with highertorque for greater towing capabilities. One consequenceof this higher engine output was the performancedemands exceeded the torque carrying capability ofconventionally pressed and sintered turbine hubs. Toguarantee system reliability, the transmission designerwas forced to switch from conventional P/M toalterl}ative technologies to meet performance demands.Steel forging was one alternative chosen. The complexshape of the turbine hubs processed by the forging andmachining process proved to be a costly option. Whatwas needed was a high performance P/M materialprocess that would survive the higher torque output ofthe new engines.

Powder metallurgy first was used in themanufacture of turbine hubs in the early 1960's. Theoriginal P/M turbine hubs were compacted to a6.2 g/cm3 density and subsequently copper infiltrated forhigher strength (Figure 1 is a photograph of this earlyP/M part). As a result of this early pioneering effort,nearly all current automatic transmissions now utilizeP/M turbine hubs. The most commonly specifiedmaterial is an MPIF FC-0208, which is a premix of pureiron powder with approximately 2w/o (weight percent)copper, 0.8w/o graphite plus lubricant.6 These currentturbine hubs are processed to a nom inal sintered densityof 6.8 to 7.0 g/cm3 with sectional densities varying from6.6 g/cm3 to 7.1 g/cm3.

Figure 2 shows schematically the manufacturingsequence of the wrought turbine hub. AISI 1045 barstock is hot upset forged to form the flange and topcone. After forging, the blank undergoes approximately8 distinct machining operations including turning andbroaching of the outer (00) and inner diameters (10)and facing of the flange. The final step is inductionhardening of the IO spline for strength and wearresistance. Figure 3 shows the forged and machinedturbine hub.

Also shown in Figure 2 is the manufacturingsequence for the warm compacted P/M component. Thepowder specified was an MPIF FD-0405. This materialis a diffusion alloyed powder containing 4w/o nickel,1.5w/o copper and 0.5w/o molybdenum.6 This base ironwas premixed with graphite and lubricant; premixingaccomplished by the ANCORDENSETM premixtechnology. The press-ready powder and compactiontooling were heated to the required temperaturesutilizing a Cincinnati Incorporated EI- TempTM materialdelivery system.7 Temperature control of the die andheated powder was maintained within +/- 2.5°C of theset point. Compaction was done in an 825 tonCincinnati Rigid Reflex press. The turbine hubsweighed approximately 1100 grams each. Followingcompaction, sintering was done at conventionaltemperatures using an endothermic protectiveatmosphere.

Figure 1: Early P/M Turbine Hub

Prior to production part approval, the warm compactedturbine hubs were subjected to extensive mechanicalproperty evaluation. Standard MPIF tensile and fatiguetesting was performed. Preproduction hubs wereevaluated for internal spline durability and internalspline wear relative to both conventionally compactedPIM hubs and the forged hubs. This investigation wasdone in an MTS test cell designed specifically for torquetesting. The test procedure consisted of fixturing the

Consumer demand for higher performanceengines in the light truck and van market segments

Warm Compacted P/M Wrought Forging

I Warm compact part I

I Sinter I

I Deburr I

I Machine aD to step and undercut I

I Hot upset forge flange and spacer I

I Bore ID and counter bore spacer I

Face top of flange and machine spacer

I Drill rivet holes I

I Turn aD and undercut on aD I

I Mill slots on bottom of turbine hub I

I Broach IDJ

Turn counter bore on ID and burnish

Broach OD

I 1,,11 il=~i =1-- b =~=I~-- =~*~:-bi:~; ~~~: I

I I.~~~~tl;~He~t T;~~t X~~~;:I-S~'\~c. I

Figure 2: Manufacturing steps of the forged steel turbine hub and the warm compacted turbine hub

aD and applying the design torque to the ID spline. Forinternal spline durability, the torque was dynamicallyapplied from 0 to 890 foot pounds then back to 0 with a75% spline engagement. The test was terminated if thespecified torque was not achieved or if the angularrotation of the ID shaft exceeded a specified amount.Internal spline wear testing was performed in the samedynamic test cell with test conditions of a constanttorque of 150 foot pounds with 90% spline engagementand 4 mm of sliding travel per cycle. The hub wasremoved at specified intervals up to 350,000 cycles formeasurement of the spline wear.

RESULTS AND DISCUSSION

Table 1 summarizes the tensile properties,hardness values, and rotating bending fatiguecharacteristics of both the forged AISI 1045 and warmcompacted MPIF FD-0405 materials. Data for the

Figure 3 Forged and Machined Turbine Hub

forged AISI 1045 material is presented in the annealedand hardened condition (the ID is induction hardened).Property data for the FD-0405 is presented in theas-sintered condition at a density of 7.25 g/cm3 (theoverall density of the warm compacted and sinteredturbine hub). Data for the P/M material is presented inthe as-sintered condition only because it was determinedthat no surface hardening of the ID was necessary .

The as-sintered yield and tensile strengths of theMPIF FD-0405 material were equivalent to the forgedAISI 1045 steel. Ductility of the steel forging washigher than the warm compacted P/M part. The fatigueproperties developed by rotating bending fatigue testingdemonstrated that the steel forging in the as-forgedcondition had a higher fatigue endurance limit relativeto the as-sintered warm compacted P/M part. Specificpart testing was necessary to determine if the differencein rotating bending fatigue performance was detrimentalto actual component performance.

Table 2 presents the internal spline durabilitytest results of conventionally compacted turbine hubs,forged turbine hubs, and the warm compacted MPIFFD-0405 hubs. Production part validation required that12 hubs be tested to 1 million cycles with no failureswith one additional hub tested to 2 million cycles withno failure. This high torque regime of 0-890-0 footpounds exceeded the strength capability of the standardFC-0208 material. Failures were observed below thespecified minimum value. The forged turbine hub metthe test requirements with no evidence of failures.Warm compaction of the turbine hubs produced partshaving an overall sintered density of 7.25 g/cm3.Despite the lack of full density, the warm compactionprocess produced torque test results that exceeded theminimum specification requirements. That is, twelvehubs were subjected to 1 million cycles with no failuresand I hub was tested to two million cycles with nofailure. To evaluate the durability of the highperformance P/M hub, one was cycled to failure.Structural failure of the hub occurred at approximately

Table 1: Summary of Standard Mechanical Property Test for Forge and P/M Hubs

~m"~cO40:$"""i!;;\,"""'2"5 """'

/ """""""'3C"

"..t~c",. -,..f!"~""',,,C

Yield Strength, psiTensile Strength, psi

Elongation, %HRC/B

Fatigue Limit, psi

For ged,AISl1045".

62,000

90,00025

97 HRB

45,000

~ea~T~t~dcc"'~ SIccl045 cccCcC~C~ccccc

96,000

130,00016

43 HRC

100,000

62,000117,000

2.617 HRC *.

35,000

'( ApplAve.~t

Table 2: ill Spline Durability Test Results

1..mi.1l.ioncy~.~"at890.ft~lbf. ~~jJ.~ioncycles"' at890 'fi~'lb ~"" ,L. Ic."

h lt C F O J~yces~~~~r~

89' 0 ~ lbc~ cat... .t. .~cc

Standard P/M hub Failures observed Failures observedAISI 1045 hub 12 parts, no Failures 1 part, no failure

Warm compact FD-O405 12 parts, no Failures 1 part, no failure -9,000,0001 part, no failure12 parts, no FailureSpecification

9 million cycles, significantly in excess of the specifiedminimum. Spline durability testing of the highperformance PIM turbine hub demonstrated that thisprocess is capable of replacing a steel forging in thiscritical high torque application.

The second critical performance criterion of theturbine hub was internal spline wear. As describedearlier, the analysis of the spline wear was performed inthe same test cell as the spline durability evaluation.Table 3 summarizes the wear test results presented asrelative measurements. That is, the maximum wearallowed by the specification is equated to 1. Any wearnumber less than 1 implies the wear was less than thespecified maximum, thus meeting the specification.Any number greater than I implies the wear exceededthe specification, thus not meeting the specification.

Table 3: Summary ofm Spline Wear Tests

Conventionally compacted and sinteredFC-O208 turbine hubs were evaluated. These hubsexhibited excessive wear in this high torque applicationwith a relative wear value of 1.4. Wear measurementsof the forged hub showed no wear at the specifiednumber of cycles. However, it is noted that the IDspline of the AISI 1045 hub was induction hardened to ahardness value 40 HRC. Without this hardeningoperation, the forged steel turbine hubs would not meetthe specified minimum wear perfonnance. Wear testingof the warm compacted MPIF FD-O405 showed nomeasurable wear at the specified number of test cycles.The absence of wear in the warm compacted turbinehubs is significant because these components wereevaluated in the as-sintered condition. The apparenthardness of the P/M components was approximately15 HRC. The warm compaction P/M process coupledwith the MPIF FD-0405 high perfonnance materialsystem eliminated the need for heat treating of theinternal spline thus significantly reducing manufacturingcost and simplifying the manufacturing flow.

Figure 4 is a photomicrograph of the as-sinteredmicrostructure of the warm compacted turbine hub. Itconsists of lamellar pearlite, bainite, nickel richmartensite areas, and ferrite. This unique microstructurewas responsible for the excellent wear performancedemonstrated in the as-sintered condition.

Figure 3: Photograph of the Warm CompactedTnrhinp Hnh

Figure 4: Microstructure of the As-SinteredTurbine Hub

14

13 r12 rt

-<>- Measured Porosity

-0- Calculated Density "j 7.40

7.30

7.20

7.10

7.00

-~'-'c-.in0...0

~"0Q)...~rI)~Q)

:2-:

,,

~ ~~~=~I~i~:===~= 11 j ,oJ" ' """ ~ "- , , r ~: ~-- ~"r

10 t ~ ~~ -

~9

8 -v-

7

6

5 --J--

3015 20 250 5 10

Distance along the ill Spline from the Bottom (mm)

Figure 5: Porosity Distribution Along the m Spline

SUMMARYAs mentioned earlier, the overall sintereddensity of the turbine hub was 7.25 g/cm3. To achievethis high sintered density, the green compact wascompacted to approximately 97% of the pore freedensity of the premix. One benefit of compaction to thishigh green density is the greater uniformity of densitythroughout the finished part. To document this, aproduction processed hub was sectioned and examinedfor porosity distribution along the length of the splineusing quantitative image analysis. Figure 5 is a plotshowing this density profile for a production part. Thedrop off in density in the small neutral zone isapproximately 0.15 g/cm3 compared with the averagesectional density of about 7.3 g/cm3. This reduceddensity differential is a significant improvement relativeto the results achieved with conventional compaction inwhich this density differential can be as great as 0.4

g/cm3.

Warm compaction of an 1100 gram multi-levelP/M turbine hub produced an overall as-sintered densityof approximately 7.25 g/cm3. High densities wereachieved in the critically stressed ID regions of the part.Combining the benefits of the warm compactiontechnology with a high performance material system,the need for induction hardening of the ID spline waseliminated. This resulted in a simplified manufacturingprocess and an overall part cost reduction. Extensivepreproduction trials demonstrated that the warmcompaction process is a viable manufacturingalternative for the production of complex multi-levelshapes, in particular automatic transmission turbinehubs for high torque engines. This paper described theconversion of a forged hub to one produced via this highperformance P/M material processing system. Torquetesting of preproduction hubs demonstrated equivalentperformance to the wrought component in terms of bothID spline durability and ill spline wear.

In addition to the mechanical property testing,extensive dimensional and weight capability studieswere performed on the process. The results indicatedthat the variations measured were consistent with thev~ri~tinn~ nhtained in conventional P/M Drocessin2:.

E~'-':Go.~c=v

0"0v~"3C.)

~U

CONCLUSIONS REFERENCES

1.) Warm compaction processing of a complexmulti-level turbine hub produced an overall sintereddensity of 7.25 g/cm3 with high sintered densitiesachieved in the criticallD spline region.

I.) Capus, Joseph M., "Metal Powder, A Global Surveyof Production, Application and Markets 1992 - 200 I",Second Edition, Elsevier Advanced Technology, June1996.

2.) White, Donald G., "Powder Metallurgy in 1995",Advanced Materials & Processes, August 1995, pp 49 -51.

2.) Warm compaction processing of a hightorque automatic transmission turbine hub successfullyreplaced a forged, machined, and heat treated AISI 1045steel part. The P/M process significantly reduced theprocessing steps. The reduced number of processingsteps resulted in overall lower manufacturing cost.

3.) Rutz, H.G., Hanejko F.G., "High Density Processingof High Performance Ferrous Materials", Advances inPowder Metallurgy and Particulate Materials - 1994,

Vol, 5, Compiled by C. Lall and A.J. Neupaver, pp 117-133, Metal Powder Industries Federation, Princeton. NJ.

3.) Utilizing an MPIF FD-O405 materialcomposition, the P/M turbine hub was used in theas-sintered condition eliminating the requirement forinduction hardening of the 10 spline as required in theforged component it replaced.

4.) Luk, S.H., Rutz, H.G., Hanejko, F.G., "A NewMethod For Manufacturing High Performance PowderMetallurgy Components'" Advances in PowderMetallurgy and Particulate Material-1994, Vol. 5,Compiled by C. Lall and A.J. Neupaver, pp 135-]63,Metal Powder Industries Federation, Princeton. NJ

4.) Standard tensile and fatigue testing of theP/M material demonstrated that the P/M component wasequivalent in yield and tensile properties to the wroughtsteel. The P/M part had a lower rotating bending fatiguestrength. 5.) Ford Engineering Specification, Part Number

ESF4DP-7926-AA, Rev Frame 2, May 1989.

6.) "Material Standards for PIM Structural Parts - MPIFStandard 35", Metal Powders Industry, Federation,Princeton, NJ, 1994.

5.) 10 spline durability and 10 spline weartesting of the warm compacted P/M component met andsurpassed the requirements specified by the end user.

7.) Unkel, R., "Introducing EI- TempTM, A New PIMPart Pressing System", Advances in Powder Metallurgyand Particulate Material-I 994, Vol. 5, Compiled by C.Lall and A.J. Neupaver, pp 135-163, Metal PowderIndustries Federation, Princeton. NJ

6.) Manufacturing variability of the warmcompaction process was equivalent to the variability ofthe conventional compaction processes.

ACKNOWLEDGMENTS

The authors wish to thank Mr. FrederickGerometta and Mr. J. S. Khanuja of the Ford MotorCompany for their support of this program and theirsuggestions in the preparation of this paper.Additionally, we want to thank the management ofChicago Powdered Metal Products Company andHoeganaes Corporation for their permission to publishthis paper. In particular, we acknowledge the assistanceof all Chicago Powdered Metals laboratory andproduction personnel and Jerry Golin, Steve Kolwiczand Tom Murphy of the Hoeganaes CorporationMetallographic laboratory for their assistance in samplepreparation and analysis.