2002-01-0668

Effect of Mix Formulation on the Dimensional Stability ofSintered Automotive Components.

Claude Gélinas and Sylvain St-LaurentQuebec Metal Powders Ltd

Copyright © 2002 Society of Automotive Engineers, Inc.

ABSTRACT

The P/M industry is being challenged with increasinglydemanding applications requiring high mechanicalstrength as well as very high dimensional stability.Synchronizer hubs, sprockets and gears are examples ofsuch applications in the automotive industry. Thedimensional stability of P/M sintered parts is influencedby a number of factors including the powder mixformulation, the selection of additives, the blendingprocedures, the handling, compaction, sintering andpost-sintering operations.

A study was conducted to evaluate and quantify theeffect of a few of these factors, namely the mix formula-tion and type of additives on the dimensional stability ofparts made with high performance materials. Statisticaltools were used to identify the significant parameters af-fecting the dimensional stability and quantify their effects.

INTRODUCTION

The development of new materials combined with theimprovement of manufacturing processes during the lastdecades have lead to a significant increase in the level ofstrength that can be reached in P/M sintered parts. Thishas markedly contributed to an increase in the number ofP/M parts used in the automotive industry. However, theachievement of very consistent and stable sintereddimensions within a part and from part to part is of primeimportance to further increase the use of P/M materialsin high precision components such as gears, sprocketsand synchronizer rings. Amongst the numerous meritsof a better dimensional change (DC) control are:

• Reduction of the manufacturing costs by avoidingpost sintering sizing and machining steps.

• Increased usage of high hardness materials thatcannot be machined or calibrated to adjust the finalsize after sintering.

• Reduction of scrapped parts due to failure to meetdimensional specifications.

Several factors affect the dimensional change duringsintering, and thus, the part-to-part dimensionalscattering of P/M components. Table I lists some ofthese important factors [1]. Materials selection is ofcourse of prime importance. In particular, the distributionof the alloying elements including carbon within andbetween parts is crucial for dimensional stability.Alloying elements can be added either to the melt prior toatomization (pre-alloyed), during the annealing treatment(diffusion bonded) or in the powder mixes as elementaladditives. Pre-alloyed powders give a very homogeneousdistribution of the alloying elements within P/M parts buttheir compressibility may be adversely affected. On theother hand, the addition of elemental alloying additives inthe mixes has little effect on compressibility but the riskof segregation is significantly increased. Diffusionbonding reduces segregation [2] by partially bondingalloying elements such as Cu and Ni to the steel particleswith little effect on compressibility. However, the cost ofthese materials is significantly higher than regular mixesof similar compositions.

Table I. Factors affecting the dimensional change ofsintered P/M parts.

Group Factors

MaterialsBase powder (type and chemistry), alloyingtechnology, formulation, type of additives,consistency, flow

Blending Homogeneity, blending technology.Handling Segregation, feeding system

CompactionFilling conditions (shoe), die cavitycomplexity, density variation within andbetween components.

SinteringTemperature, time at temperature, heatingand cooling cycle, atmosphere (carbon andoxygen potential)

Binder treatment, which consists in adding an organicbonding agent during the mixing operation, is anotherroute to reduce the segregation at a lower cost [3]. Therole of the binding agent is to increase the bonding ofelemental alloying, graphite and lubricant additives to thesteel particles, therefore helping to reduce segregationand dusting that may occur during powder handling anddie cavity feeding [4,5]. The binder treatment alsoimproves the flowability of mixes, improving the ease offeeding and the homogeneity of the powder mix withinthe die cavity [6].

The significant improvement in powder flow rate obtainedby binder treatment allows for the use of very fineadditives that would otherwise yield mixes with very poorflow rates or no flow [6]. It was shown in different studiesthat using finer additives is beneficial to the final sinteredproperties. However, their influence on the dimensionalstability of sintered components is complex and not wellknown. For instance, use of fine copper particles inregular mixes is more susceptible to segregation thancoarser particles, while copper with a more irregularshape is less prone to segregation [1].

The object of this paper is to determine the effect ofusing copper and graphite additives of different sizes andshapes on the sintered properties and, more specifically,on the dimensional stability of parts pressed from binder-treated mixes of a Mo pre-alloyed steel powder withelemental Cu, Ni and graphite additives. The effect ofvarying the density on the dimensional change was alsoinvestigated, since a variation of density within andbetween parts is known to contribute to dimensionalscattering.

EXPERIMENTAL PROCEDURE

A water-atomized FL-4400 low alloy steel powdercontaining 0.15% Mn and 0.85% Mo, ATOMET 4401,was selected as the base powder for this study. Thepowder was admixed with 3.70% nickel, 2.65% copperand 0.60% graphite. An EBS-type lubricant was alsoadded and the mixes were binder-treated. The totalamount of lubricant and binder was 0.80% in all themixes. The nickel and different types of copper andgraphite that were used in the study are described inTable II. Two graphite additives were used: a syntheticgrade and a natural grade, the former being purer andfiner. Three copper additives produced by differenttechniques were used: a fine, medium and coarse grade.It is worth noting that these copper powders also havedifferent particle shapes. The same fine nickel powderwas used for all mixes. The four binder-treated mixesprepared with these additives are described in Table IIIwith their apparent density and Hall flow rate. Anapparent density in the 3.10 to 3.15 g/cm³ range with aHall flow rate in the 29 to 31 s/50 g range were obtained.It is worth noting that the binder treatment improved theHall flow rate of the mixes by about 6.5 seconds or 18%.

Table II. Description of the elemental additives used inthe binder-treated mixes.

Additive Size Features

Graphite:

Natural D50 < 9 µm < 3.4% Ash

Synthetic D50 < 6 µm < 0.1% Ash

Copper:

Fine D50 < 9 µm Electrolytic, dendritic

Medium D50 < 20 µm Atomized, irregular

Coarse D50 < 50 µm Atomized, irregular

Nickel D50 ≈ 5 µm Spiky shape with adendritic surface

Table III. Type of graphite and copper used in thebinder-treated mixes with their apparent density and flow

rate (0.85% Mo steel powder with 3.7% Ni, 2.65% Cuand 0.6% Graphite).

Mix A B C D

Graphite Natural Natural Synthetic Synthetic

Copper Coarse Medium Fine Coarse

A.D., g/cm³ 3.08 3.16 3.14 3.16

Hall flow, s/50 g 31.2 29.7 30.9 28.9

Mixes A, B and C were used to evaluate the effect of thetype of additives on the dimensional stability. Mixes Band D were used to study the consistency and stability ofthe sintering process and to validate the methoddeveloped for the measurement of the ring diameters.

Sintered properties of the mixes were determined onstandard TRS bars pressed to densities ranging from 6.7g/cm³ up to 7.1 g/cm³ on a 100 Ton hydraulic press withfloating die. Specimens were sintered for 25 min at1120°C (2050°F) in a 90%N2/10%H2 atmosphere andtempered for 1hr at 205°C (400°F) in air. The proportionand size of pores in specimens pressed to 7.1 g/cm³were evaluated on polished specimens using an imageanalyzer and an optical microscope at a magnification of200X.

For the evaluation of the dimensional stability anddistortion, a thin wall ring was selected since it is moresusceptible to distortion than a TRS bar during sintering.Rings with an outside and inside diameter of 2.06” and1.69” and a thickness of 0.49” (5.2 cm OD by 4.5 cm IDand 1.2 cm thick) were compacted to a density of 7.0g/cm³ on a 100 Ton press with floating die. The positionof rings with respect to the tooling was clearly identifiedin order to assure that the measurements were doneaccording to the same reference point. Rings weresintered in a laboratory belt furnace for 25 minutes at a

temperature of 1120°C in a 90%N2/10%H2 atmosphere.The rings were sintered five per boat with the bottomside facing down. The top and bottom outside diametersof green and sintered rings were measured at threedifferent positions along the circumference: 0°, 120° and240° from the reference point. As illustrated in Figure 1,a positioning fixture was used with a high precision digitalindicator with a resolution of 0.0001” (0.0025 mm) inorder to improve the accuracy and reproducibility of themeasurement. The diameters were measured 0.25 cmbelow the top surface and 0.25 cm above the bottomsurface. The mean top and bottom outside diameterswere calculated from these values. The dimensionalchanges from green size were calculated from thesemean diameters. For the purpose of this work, the partsdistortion or dimensional stability was evaluated bycalculating the difference in dimensional changebetween the top and bottom of the rings.

VALIDATION OF THE MEASUREMENT METHOD – Inorder to validate the precision and the accuracy of themethod, the bottom and top diameters of green andsintered rings were also measured using an industrialcoordinate measuring machine (CMM). This type ofmachine is used in the industry to precisely determinethe size of parts of different shapes. For the purpose ofthis work, nine contact points equally distributed alongthe circumference were read. From these nine readings,the CMM calculates an ideal diameter that minimizes thesum of the squares of distances between thesemeasured points and the calculated ideal diameterpoints. Measurements were done on two series of 15rings pressed from materials B and D. Figure 2illustrates the relation between the mean outsidediameters measured using the two methods. Anexcellent correlation factor (R2) of 0.9796 was obtainedbetween the two sets of data, clearly indicating that themethod developed in the lab and used in this study wasvery precise even though the in-house method wasfound to give slightly lower values than the CMM method:a difference of about 0.0015” for both green and sintered

parts. The difference may be related to the fact thatmeasurements were done at different positions relativeto the bottom and top surfaces (0.16 cm below the topand above the bottom surfaces for the CMM method).

RESULTS AND DISCUSSION

EFFECT OF DENSITY AND ADDITIVES FINENESS ONSINTERED PROPERTIES (BARS) – The sinteredproperties before and after tempering were evaluated onbars pressed to different densities. Figure 3 shows theeffect of green density on the dimensional changemeasured after sintering and tempering and thetransverse rupture strength after tempering for materialsA, B and C. The dimensional change after temperingwas about 0.02% to 0.04% more negative than aftersintering for all the mixes. Also, the use of fineradditives, mainly fine copper, resulted in a more negativedimensional change, the difference between mixes Aand C being around 0.28% at a given density.

Increasing the density resulted in a linear increase of thedimensional change, the variation being similar for all themixes, i.e., an increase in dimensional change of about0.05% for each increment of 0.1 g/cm³ in density. Thisshows that the size and shape of additives have nosignificant effect on how the dimensional change varieswith density. Therefore, it appears that the use of fineradditives may not improve the dimensional stability ofparts in which the density is not homogeneouslydistributed.

The use of very fine copper and graphite was alsobeneficial to the rupture strength. Indeed, the TRS was20 to 40 kpsi higher for mix C than for mixes A and B.However, no significant difference in TRS was obtainedbetween mixes A and B at 6.7 g/cm³ and 6.9 g/cm³ evenif there was a significant difference in the size of copper.It is interesting to note that the rupture strength of mix Astarted to level off between 6.9 g/cm³ and 7.1 g/cm³

Figure 2. Relation between outside diametersmeasured using two different methods.

Figure 1. Fixture used to measure the outsidediameters of green and sintered rings.

R2 = 0.9796

2.063

2.065

2.067

2.069

2.071

2.073

2.075

2.063 2.065 2.067 2.069 2.071 2.073 2.075

Diameter measured with CMM, in

Dia

met

er m

easu

red

wit

h d

igit

al

ind

icat

or,

in

Green Parts

Sintered Parts

while it continued to increase linearly for mixes B and C.This difference in behavior is likely related to the porositythat was significantly different in these materials. Indeed,image analysis of pores in parts pressed at 7.1 g/cm³revealed that the number of large pores was significantlylarger for mix A than for mixes B and C, as shown inFigure 4. For instance, there was about 10 pores permm² having a length greater than 50 µm compared to2.3 in mixes B and C. In addition, no pore larger than

100 µm was detected in mixes B and C while the longestpore analyzed in mix A was close to 250 µm. Thepresence of such large pores in parts made with mix Aclearly reduced the beneficial effect of increasing densityon strength. These large pores were likely formed by the

large Cu particles that melted during sintering andpenetrated the surrounding steel grain boundaries.

CONSISTENCY AND STABILITY OF THE SINTERINGPROCESS – Two series of 15 rings pressed from mixesB and D were used for this preliminary test intended forassessment of the stability of the sintering treatment.Rings were loaded in groups of 5 of a given mix in a boatand fed into the sintering furnace 30 minutes apart byalternating mixes B and D. The 3 boats per mix and 5positions per boat were numbered. The meandimensional changes from green size were measured forthe top and bottom diameters of all the rings made frommixes B and D. A statistical analysis of the DC resultswas made in order to find out any effect of the boat orposition of the rings in the boat during sintering. AThree-Way or three factor analysis of variance (Three-Way ANOVA) was carried out with the mix, the boat andthe position within the boat as the variables. Thedetailed results are reported in Table IV. For thestatistical analysis, DF refers to the degrees of freedomor number of levels in each factor, F is the F distributionstatistical test to compare the variation between andwithin the factors (analysis of variance) and P value isthe probability of being wrong in concluding that there isa true difference between the groups.

Table IV. Effect of the mix, the boat and position in theboat on the DC from green size measured in top and

bottom of the rings: results of the Three-Way ANOVA.

Statistical Analysis Results (95% confidence level):

Bottom Top

Variable DF F P F P

Mix

Boat

Position

1

2

4

27.4

2.4

0.2

0.0008

0.15

0.92

17.7

2.8

0.3

0.003

0.12

0.84

Least Square Means of the DC for “Mix”:

Mix Bottom Top

B

D

0.059%

0.108%

0.037%

0.067%

At a 95% confidence level, only the material appeared asbeing a statistically significant source of variation (F>5.32and P 50 µm L > 75 µm L > 100 µm

-0.30

-0.20

-0.10

0.00

0.10

0.20

6.6 6.7 6.8 6.9 7 7.1 7.2

Green Density, g/cm³

D.C

. vs

gre

en s

ize,

%as sintered

as tempered

160

180

200

220

240

260

280

6.6 6.7 6.8 6.9 7 7.1 7.2

Green Density, g/cm³

T.R

.S.,

kp

si

Mix A

Mix BMix C

Mix A

Mix B

Mix C

DC in top and bottom of rings. The results are plotted inFigure 5 for the 15 rings of mixes B and D. Again, theboat and position in the boat did not have a significanteffect on the DC variation at a 95% confidence level. Onthe other hand, the material had a significant effect. Theaverage in DC variation (top versus bottom) with itsstandard deviation as well as its standard error arereported for the two mixes in Figure 6. Mix B containingthe medium size copper and natural graphite exhibited abetter stability in DC variation with an average of 0.022%versus 0.044% for mix D containing the coarse copperand synthetic graphite. Moreover, the standard deviationwas also much smaller for mix B: 0.012% versus 0.023%for mix D. Thus, both the average and the standarddeviation in DC variation were reduced by half in mix B.

EFFECT OF MIX FORMULATION ON DIMENSIONALSTABILITY – For this study, 5 rings were pressed fromeach mix A, B and C and measured (3 diameters in topand bottom). The 5 rings of a given mix were loaded in aboat and the three boats fed into the sintering furnace 30minutes apart each other. After sintering, the diameterswere measured again. The average and standard

deviation of the green and sintered diameters are givenin Table V. In general, the standard deviations are verysmall indicating a good reproducibility from ring to ringand position to position for a given mix. Green diametersare very similar for the three mixes and indicate that thetop of the rings is slightly larger than the bottom. On theother hand, after sintering, the diameters vary muchmore from top to bottom and from mix to mix. Thedifference in mean diameters between top and bottom ofrings increases from mix C to B to A (from finer tocoarser additives in the binder-treated mix). Thedimensional changes from green size measured in topand bottom of rings are reported in Figure 7.

Table V. Green and sintered mean diameters (inches)with their standard deviation in parentheses.

Mix Top Bottom Difference

Green diameters

A2.0672

(0.0001)2.0668

(0.0001)0.0004

B2.0672

(0.0001)2.0669

(0.0001)0.0003

C2.0674

(0.0001)2.0671

(0.0001)0.0003

Sintered diameters

A2.0709

(0.0002)2.0689

(0.0003)0.0020

B2.0682

(0.0002)2.0666

(0.0002)0.0016

C 2.0646(0.0003)

2.0643(0.0002)

0.0003

In general, and as seen on the TRS bars, the DCdecreases with the fineness of the copper additive:0.14%, 0.02% and -0.14% for the mixes containingcoarse (A), medium (B) or fine (C) copper additivesrespectively. Contrary to the previous series of results,the DC is now slightly larger in the top of the ringsindicating that the sintering was not identical. A two-wayanalysis of variance was carried out with the mix andposition of the rings in the boat as input variables. At a

Figure 6. Mean in DC variation from top to bottomdiameters with its standard deviation and standard error.

Figure 7. Dimensional change measured on top andbottom diameters of the 5 rings pressed and sintered

from mixes A, B and C.

Figure 5. Difference between DC from green size in thetop and in the bottom of rings made from mixes B and D.

Boat 1 Boat 2 Boat 3

Boat 1

Boat 2 Boat 3

Mix

B D

DC

Var

iati

on

(T

op

-Bo

tto

m),

%

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

Standarddeviation

Standarderror of

the mean}

{Ring

A1 A2 A3 A4 A5 B1 B2 B3 B4 B5 C1 C2 C3 C4 C5

Dim

ensi

on

al c

han

ge,

%

-0.20

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

0.20

0.25TopBottom

Ring

B2 B4 B6 B8 B10B12

B14D2 D4 D6 D8 D10

D12D14

DC

Var

iati

on

(T

op

-Bo

tto

m),

%

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10

0.11

Boat 1 Boat 2 Boat 3

Boat 1 Boat 2 Boat 3

95% confidence level, only the material appeared asbeing a statistically significant source of variation.Another approach to evaluate the dimensional stabilityand part distortion is to consider the differences betweendimensional changes in the top and bottom of rings. Theaverage in absolute DC variation (top versus bottom)with its standard deviation and standard error arereported in Figure 8 for the three mixes.

It readily appears that the DC variation within parts is notstrictly related to the extent of DC itself. Indeed, mixes Aand C exhibit a DC of 0.14% during sintering (a growth inthe former mix and a shrink in the latter mix) but the DCvariation is much lower in the latter. For these rings, it isclear that the best dimensional stability is achieved withmix C containing the fine copper and synthetic graphiteadditives: lowest average and standard deviation in DCvariation (0.013% ± 0.006%). The worst results wereobtained with mix A containing the coarse copper and

natural graphite (0.077% ± 0.010%). As discussedabove, the melting of such coarse copper particlesduring the sintering creates more disorder in thestructure and consequently more distortion (morelocalized liquid, larger pores). By substituting such acoarse copper with a medium size copper, thisphenomenon likely decreased and an improvement inDC variation was achieved (0.061% ± 0.011% for mix B).

CONCLUSION

The influence of using copper and graphite of differentsize and morphology on the sintered properties,especially the dimensional stability of rings made withbinder-treated FLNC-4405 mixes was studied. Usingfine additives resulted in much smaller dimensionalvariations and distortion within and between parts. Italso promoted shrinkage during sintering and animprovement of the transverse rupture strength andapparent hardness. The number of large pores insintered parts was also found to be significantly reducedby the use of fine copper. On the other hand, the sizeand morphology of additives had no effect on how the

DC varied with density. It is therefore highly beneficial touse fine additives when tight dimensional control isrequired. However, the use of very fine additives isknown to adversely affect the flow rate of regular mixesand binder-treatment appears to be necessary in order toget adequate flow rate and die cavity feeding. The nextstep in this program would be to validate the benefit ofusing binder-treated mixes with fine additives on thedimensional change stability of parts on a productionscale.

ACKNOWLEDGMENTS

The authors thank L. Thibodeau for his collaboration andsupport with the experimental portion of this work.

REFERENCES

1 M. Larsson and J. Rassmus, “Higher tolerances byimproved materials and processes”, Proc. 1998 P/MWorld Congress, Granada, EPMA, Vol. 4, pp 379-384.

2 Th. Esser and al., “Dimensional stability of iron-copper steel”, Advances in P/M, Proc. Con.,Pittsburgh, MPIF, 1990, Vol. 1, pp 245-260.

3 R. Guo, “Applications of the binder treated Mo-Prealloyed premixes as alternatives to diffusionbonded powders”, Proc. 2000 P/M World Congress,JPMA, Kyoto, pp 499-502.

4 F. Gosselin, M. Gagné and Y. Trudel, “Segregation-free blends: Processing parameters and productproperties”, Proc. World Con. on P/M, The instituteof Metals, London, 1990, Vol 1, p. 297.

5 C. Gélinas. F. Chagnon and Y. Trudel, “Optimizingproperties of binder-treated ferrous powderpremixes”, Advances in P/M & Particulate Materials,Proc. Int. Conf., Seattle, MPIF, 1995, Vol. 1, pp 3-45/3-56.

6 M. Carvalho, R. Pegorer and D. Rodrigues, “Use oforganic binders in powder mixes to improvedimensional stability of sintered steels”, Proc. 1998P/M World Congress, Granada, EPMA, Vol. 4, pp404-408.

Boat 1 Boat 2 Boat 3

Boat 1

Boat 2 Boat 3

Mix

A B C

DC

Var

iati

on

(T

op

-Bo

tto

m),

%

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10

Standarddeviation

Standarderror of

the mean}

{

Figure 8. Difference in DC between the top and bottomdiameters for rings pressed from mixes A, B and C.