Basic Graphite Properties: Influence on Flowability and PM ... - Timcal

<strong>Basic</strong> <strong>Graphite</strong> <strong>Properties</strong>:<strong>Influence</strong> on Flowability and PM Final PartsAuthors: G. Juri and M.Coluccia - Timcal <strong>Graphite</strong> & Carbon, Via Industria, CH-6743 Bodio, SwitzerlandAbstractThe effect of graphite powder with respect to the performance of final PMparts is not fully understood yet. The same is valid for the interaction ofgraphite with other additives, especially lubricants, which plays a major rolefor flowability and agglomeration. The variety of graphite powders used in PMis quite big, from fine natural graphite to primary synthetic grades. For thelatter, the selection of raw materials and the many processing optionsavailable, result in a wide range of property variation.Based on this, the effect of basic graphite properties like crystallinity, PSD,particle shape and density will be studied with respect to the PM application.Standard Fe-based PM mixes will be made with different graphite and carbongrades and their flowability will be determined. The final properties of PM testparts will be analysed with the target of establishing correlations to basicgraphite powder properties.Flowability TestPM Mix: Flowability Test• The role of flowability of powder mixes is often underestimated, but it is crucial for processing andmanufacturing of PM parts.• Many PM mixes do not flow well: key factors are notwell identified, especially with respect to theinteraction of graphite (shape, type, ecc) with metal,additives and lubricants.• Target: identify key properties of graphite powders relatedto flowability (see section “properties”)• Standard flowability test: Hall Flowmeter. Test carried outat ext lab according to MPIF standard->>Results (see Fig. 1) are almost of no use to findcorrelations: differences too small.IdentificationNo <strong>Graphite</strong>F10NG 96/25NG 96/10PG25PG 15 TPE-PG-10Hall Density(g/cm 3 )3.173.043.033.063.013.023.03test @ 50gHall FlowRate (s)3138*N/F3437N/FN/F(”*”= tap assist)Fig 1: Hall Flowmeter: Evaluation of Various <strong>Graphite</strong>sin F-0008 Mix with Base Powder Atomet DB46Comment:• Mix without carbon is definitely better• Tap assist often needed (same with Carney funnel).• Differences very small and repeatibility of singletest runs is low.Conclusion:• Test results give just rough indication• Flow is often much better with productionmix vs lab samplesPT-S Powder TesterPT-S Test: Preliminary Results• Exp. Set up for angle of repose/fall (left)• Camera pictures of angle of repose/fall of F 10 grade (top)• Exp. Set up for aerated/packed density (center)• New powder tester by Hosokawa (PT-S) acquired in 2008.Device measures:- aerated/packed density,- flowability-floodability by angle of repose/of fall/of spatula- calculates flowability index (Carr’s index)• Standard device for coarse powders (>300 mi, friction industry)or fine-but-heavy powders, like graphite for Li-ion batteries• <strong>Basic</strong> correlation-theory: angles of repose/fall decrease for more flowable powders (Carr’s Index increases)• Aerated/packed density data: compressibility factor is calculated as ratio: high values -> less flowable powders• Preliminary test: after screening run, scope of analysis reduced (angle of spatula not analysed)• Accuracy: SD%= 2 - 6 % of average values are realistic for angles and densities• For all graphite grades: data (angles) measured are within a small range corresponding to not good to-bad flowable powders. Compressibility much too highfor good flow• Samples: all samples (Except PG 98/25 and NG 99.5/10) are listed in the section about graphite properties.- PG 98/25 is “equivalent“ to PG 25 in terms of PSD and raw material source, but it is manufactured by a different milling processing technology (and has 98%C).- NG 99.5/10 is a sample with min. carbon 99.5% from same competitor producing also NG 96/10 (sample included in the PM test).

Fig 2: <strong>Graphite</strong> PSD• TIMREX: no relevant difference at same d90 (also with diff. process),except PG 98/25. Competitor grades: broader PSD curve (e.g. bigger uniformity value).PG 98/25 is very similar.Fig 3: Flowability-Moisture Samples analysed as such and after drying:• small variation of angles at different moisture levels• aerated/packed density increase with decreasing moisturePT-S test: ResultsFig 4: Angle of repose/fall for PM graphite gradesComment to Fig. 4:• all 25 µm grades are close, 10 µ as well, BUT with smaller angles!!!->> flow should be better (according to theory)• All competitors grades show very small angles, indicating slightlybetter flow propertiesFig 5: Aerated/packed density and compressibility for PM graphite gradesComment to Fig. 5:• At same PSD: competitor grades NG 96/25 and NG 99.5/10 havemuch bigger density, both aerated and packed. PGs have lowestdensity, F grades are intermediate (effect of raw material plusprocessing).• PG 98/25 shows highest values of all grades, due to different millingprocess than PG 25 (and slightly higher d90).Flowability-PT-S Test: ConclusionAnalysis of angles: not accurate enough (for different graphite grades with similar PSD) to find correlationsAerated/packed density: show some marked differences. Correlation with processing/graphite properties available based on existing know-how.- PG grades: lowest scott density, BET and max. oil absorption ->> most flaky shape (milling process).- PSD curve is narrower than competitor grades.- Compaction properties are the same.Grade PG 98/25: “new“ process version gives properties similar to competitor grades, i.e. broad PSD and very high density.- Performance in PM mix and final parts to be checkedMoisture: not crucial for graphite (being very low), but more for lubricants or additives. To be kept under controlFlowability test on pure graphite powder: cannot highlight small differences. Interaction with additives is more relevant for flow of mix ->> Test tobe made on PM mixes with selected metal powder and additives (next step). PT-S tester should give more accurate results on PM mixes, in view ofhigher density and better flow properties (vs pure graphite)

Powder Metallurgy Data - Sintered <strong>Properties</strong>Fig 6: Density of sintered samples based on 3 typical PM mixes.Effect of graphite typeFig 7: Dimensional change DC from die of sinteredsamples based on 3 typical PM mixes. Effect of graphite typeFig 8: Transverse rupture Strength TRS of sinteredsamples based on 3 typical PM mixes. Effect of graphite typeFig 9: Apparent hardness HRBw of sintered samplesbased on 3 typical PM mixes. Effect of graphite typeTest Parameters• Test according to MPIF procedures withdifferent graphite grades.• Powder Mixes made with 0.80% <strong>Graphite</strong>and 0.80% Acrawax C (F-0008, FC-0208and FN-0208)• Metal powders: Fe Ancorsteel 1000B, 165Copper and 123 Nickel• Evaluation of results:- Standard TRS bars molded at 6.60, 6.90and 7.08 g/cm3 nominal density(short LD,MD and HD).- Sintering in a production continuous beltfour-zone furnace. Temperature 1200F,1400F, 2050F and 2050F with 97%N2-3%H2. Sintering time approx. 20-25 minutes.- <strong>Graphite</strong> sample listed in Section on<strong>Graphite</strong> <strong>Properties</strong>Comment:• Density - data at medium level (Fig 6) show typical trend:- FN mixes show highest density values- F mixes are intermediate- FC mixes give lowest values. It is known that control of Cu-melting is crucial to achieve high density.• Density - Effect of graphite- NGs give higher density, excepted for FC mix. It is well known that the lower springback of NGs is animportant factor to achieve higher density (green and sintered).• DC- Data in Fig 7 (at HD) show typical trend, which is reverse compared to sintered density, i.e.- FN-mixes give lowest DC values, F-mixes are intermediate and FC-mixes yield higher values.- Note that due to “simplified“ nature of test, in some cases DC values are higher than for industrial partsprocessed with optimized process parameters.• DC- Effect of carbon- it is well known that increasing %C (alloyed) improves strength and hardness, but also DC.- In view of the above synthetic grades F 25 or F10 should results in bigger DC than NGs, but in fact theeffect is much smaller than expected, especially with Cu-mixes. This is probably due to faster C-diffusionof F grades see “Conclusion“).Conclusion• Test results confirm general trends and typical features of the 3 types of mixed chosen, as known from technical liteature, although test (sintering) parameternot optimized (fixed).• In terms of density NG grades give some advantages in F and FN mixes. In FC mix SG graphites F10 and F25 are equivalent or better.• DC - PG grades show equivalent performance to competitor grades, whereas NG 96/10 shows highest values of DC of all grades.• DC - F-grades - Due to the higher carbon % one would expect that synthetic grades like F 25 or F10 give bigger DC than NGs. In fact, the effect is much smallerthan expected, especially with Cu-mix.- Recent publications (by QMP in MPR, March 2008, pp 24-27) have shown that, at same level of sintered carbon, synthetic graphite grades yield a much betterdimensional control of parts than NGs. The data show that faster diffusion of carbon inhibits Cu-diffusion in the matrix after melting, thus resulting in smallerdimensional change of parts. Moreover, the effect is almost independent of PSD of graphite, when SG is used, instead of NG. Due to slower C diffusion thelatter requires use of smaller PSD to compensate this effect.- The data by QMP shows that with FC mixes DC values in the range of 0.020-0.025% with small sigma deviations can be achieved in industrial conditions,thanks to higher reactivity of the TIMREX F grades.• Summarizing, the data in our study are in line with previous data showing the advantages of TIMREX grades F10 and F25 in applications, where high density,strength, hardness are a priority.The dimensional change values are not smaller than for NGs, but they are more consistent (small sigma), allowing betterdimensional control of parts. This advantage can be exploited to a maximum extent in FC formulations, that require control of „dilation“effects related to Cumelting,in order to achieve optimum performance of final parts.

<strong>Graphite</strong> Powders for PM - Key <strong>Properties</strong>The property summary includes data on natural graphite grades, like E-PG 10 , PG 25, and on primary synthetic grades, like F 10 and E-F 25. Forcomparison, data on two typical natural graphite grades used in PM are also shown. They are denominated with NG 96/25 and NG 96/10, based ontheir carbon content and their PSD (d90). All analysis is made on powder samples in loose or compact form, but without addition of binders oradditives.<strong>Graphite</strong> Powders for PM: Summary of <strong>Properties</strong>Grades E-PG10 NG 96/10 F10 PG 25 NG 96/25E-F 25Ash (810°C)(%wt)3.4453.3490.0283.5244.3130.032Xylene density(g/cm3)2.2802.2682.2532.2702.2792.255Lc(nm)16211791227234167C/2(nm)0.33570.33570.33590.33560.33560.3357d10(µm)2.82.02.14.33.44.2d50(µm)6.35.55.29.89.111.8d90(µm)12.211.810.619.321.026.4BET(m2/g)9.613.915.06.67.08.6Oil adsorption(%wt)165.8123.8141.9163.2136.0136.0Scott density(g/cm3)0.0620.1250.0900.0770.1320.126Springback(%)9.37.413.78.58.814.2PD@0.47 t/cm2(g/cm3)1.7041.6841.6211.7761.8971.686PD@2.5 t/cm2El. Resist. XYBend. Strgth(g/cm3)(m.cm)(N/mm2)1.9681.549.21.9691.8012.31.8651.988.22.0141.238.52.0011.739.11.9071.525.3Fig 10: Analysis of ash, surface area (BET), particle size distribution(d90 by laser diffraction) and apparent density (scott ).Scott density is plotted on the right axis.Table 1 – Analysis results on different graphite powders for PMConclusionTypical ash content of synthetic grades is very low in comparison to natural graphite grades (higher carbon).BET increases with finer PSD, but is also related to the surface structure of grains (see F-grades vs PG-grades) and it is very likely proportional to reactivity.Scott density increases with d90: differences at same PSD indicate different shape and/or surface structure of particles, mainly due to processing or source(see NG 96/10).Grain shape-surface texture: the oil adsorption data contribute to highlight such features in combination with BET, Scott density, PSD (and SEM imaging,not shown) ->> PG family is the most flaky material and has narrowest PSD curve. Effect on flowability not clear.Crystallinity: Lc values are reduced by fine milling, as can be seen from data on 10 and 25 micron grades.Lc of TIMREX F-grades is slightly lower than for NG grades, but nevertheless it results in higher reactivity, i.e. higher carbon diffusion at lower temperature.At same PSD, springback increases for decreasing Lc, i.e. synthetics have slightly higher springback. Thus pressed density of compacts is higher for NGs thanfor SGs and this usually leads to higher density of PM parts.El. Resistivity of powder compacts is correlated to crystallinity, particle size and shape, but also to final density of samples (100% graphite). The same isvalid for the bending strength. For these reasons, NG samples show lower resistivity and higher strength.Final SummaryFlowability - <strong>Graphite</strong> propertiesFlowability data by Hall Flowmeter are suitable to characterize PM mixes in production, but not to investigate correlations with graphite properties (low accuracy).PT-S powder tester gives more accurate data, although graphite powders for PM fall in the range of not well-flowable powders. Data of angles of repose/fall are notprecise enough, but aerated/packed density shows bigger differencies.Density data show some features of TIMREX grades that can be directly related to graphite properties and milling process. <strong>Properties</strong> like ash, springback, scottdensity and crystallinity are relevant parameters for the selection of a suitable graphite grade for one specific application.Competitor grades NG 96/25 and NG 99.5/10 show slight advantage in terms of flow properties.PG 98/25: the different processing option applied to this PG grade shows interesting results, but the final performance in PM parts has still to be evaluated.Test will be repeated on PM mixes, that, thanks to higher density, should give better results. Of course additive selection is crucial for test result evaluation.PM studyIn PM parts made from mixes of F-0008, FC-0208 and FN-0208 type, TIMREX PG and F grades show performance comparable to competitor’s NG grades.It is noteworthy, that commercial grade NG 96/10 shows the highest values of DC for NGs, indicating that, beside ash content, some other factor is relevant for theperformance of NG 96/10 in comparison to E-PG 10. The analysis results on the graphite powder (BET, Scott, etc) indicate a difference in shape and/or surfacetexture, that is most probably due to a different processing technique.The results on the high-purity grades F 10 and F 25 show that they are especially suitable for high-density applications, with high strength and hardness required.Recent publications on industrial test confirm that the high C% and the high reactivity of F-grades lead to PM parts characterized by small values of DC (especiallyin FC-mix) with a very high reproducibility, not achievable with NG grades.Contact address:Dr. Giovanni Juri, Timcal Ltd, Via Industria, CH-6743 Bodio, Switzerland,Ph.: +41 91 873 2010, Fax: +41 91 873 2009, e-mail:g.juri@timcal.com