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Quantification of Cu Enriched Phases in Synthetic 3XX Aluminum AlloysUsing the Thermal Analysis Technique

M. B. Djurdjevic, W. Kasprzak, C. A. Kierkus, W. T. Kierkus and J. H. SokolowskiNSERC / Ford / University of Windsor Industrial Research Chair in Light Metals Casting Technology

University of Windsor, Windsor, Ontario, Canada

Copyright2001 American Foundry Society

ABSTRACT

The goal of this paper is to demonstrate that it is possible to quantify and to characterize the development of Cu enrichedphases in the 3XX series of aluminum alloys using the Thermal Analysis (TA) system developed at the University ofWindsor. It is shown that several distinct Cu enriched phases are manifested as "peaks" on the first derivative of the coolingcurve. Following a mathematical curve "deconvolution" procedure, it is possible to use the total area under the firstderivative curve to estimate the area fraction of the Cu enriched phases that will be found in the solidified sample. Thisestimation is verified using quantitative metallography (Image Analysis) and chemical analysis (Optical EmissionSpectroscopy). The applicability of the present findings to on-line melt quality control in industrial settings is discussed.

INTRODUCTION

The automotive industry makes frequent use of the 3XX series of aluminum alloys. In order to ensure that cast componentshave good mechanical properties their as-cast microstructures must be closely monitored. Two eutectic microconstituents areprimarily responsible for defining the microstructure in 3XX series alloys: Al-Si and Al-Cu. Both of these eutectics can bedetected on a Thermal Analysis (TA) cooling curve, or more precisely on its first derivative. The solidification of 3XX seriesalloys and the formation of Cu enriched phases can be described as follows (Bäckerud et al., 1986; Caceres et al., 1999;Djurdjevic et al., 1999; Tenekedjiev et al., 1995, Doty et al., 1996):

1. A primary α-aluminum dendritic network forms between 580 - 610oC. The exact temperature depends mainly on theamount of Si and Cu in the alloy. This leads to an increase in the concentration of Si and Cu in the remaining liquid.

2. Between 570 - 555oC (the Al-Si eutectic temperature) the eutectic mixture of Si and α-Al forms, leading to a furtherlocalized increase in the Cu content of the remaining liquid.

3. At approximately 540oC, the Mg2Si and Al8Mg3FeSi6 phases begin to precipitate.4. At approximately 525oC, the “massive” or “blocky” Al2Cu phase (containing approximately 40wt% Cu) forms together

with β-Al5FeSi platelets.5. At approximately 507oC, a fine Al-Al2Cu eutectic phase forms (containing approximately 24wt% Cu). If the melt

contains more than 0.5wt% Mg, an ultra fine Al5Mg8Cu2Si6 eutectic phase also forms at this temperature. This phasegrows from either of the two previously mentioned Al2Cu phases.

Some minor alloying elements present in this type of aluminum alloy are also able to change the morphology of the Cuenriched phases. It has been shown that an increase in the strontium content of the alloy increases the proportion of theblocky Al2Cu and ultra fine Al5Mg8Cu2Si6 Cu phases versus the Al-Al2Cu phase in the 3XX series of alloys. The ratiochanges from 1:3 in low strontium alloys to 9:1 in high strontium alloys (Djurdjevic et al. 1999).

The authors (Djurdjevic et al. 1998) have demonstrated that by using highly sensitive thermocouples at least three Cuenriched phases can be detected on the first derivative of the cooling curve of 3XX series alloys (see Figures 1-5).Metallographic analysis of the TA test samples has confirmed the presence of these phases. Their formation temperaturescan help define the maximum temperature that castings can be exposed to during solution treatment (i.e. by defining thetemperature at which incipient melting will take place). Unfortunately, the total amount of Cu enriched phases present in asample can thus far only be measured using metallographic analysis. This information is critical because these phases play arole in the precipitation phenomena during artificial aging and can have a negative influence on the mechanical properties ofthe alloy.

The present work explores the possibility of quantifying the Cu enriched phases in the 3XX series of aluminum alloys byusing TA. The calculations based on the obtained thermal signatures are compared with Image Analysis (IA) and chemicalanalysis results in order to verify the proposed method.

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EXPERIMENTAL PROCEDURES

MATERIALSSix synthetic 3XX compositions were produced at the Ford Casting Process Development Centre (CPDC) by melting acharge of Al-7wt% Si and Al-9wt% Si base alloys and adding 1, 2 and 4 wt% Cu. The chemical compositions of theresulting alloys, as determined using Optical Emission Spectroscopy (OES) are presented in Table 1.

Table 1. Chemical compositions (wt%) of the synthetic alloys.Alloy Si Cu Fe Mg Mn Zn Ti Sr Ni

1A 7.13 0.96 0.12 0.28 0.01 0.01 0.098 0.0033 0.0085A 9.17 1.05 0.12 0.31 0.01 0.01 0.100 0.0042 0.0072A 7.05 1.98 0.13 0.28 0.01 0.01 0.094 0.0027 0.0096A 9.02 2.44 0.12 0.31 0.01 0.01 0.096 0.0063 0.0073A 6.75 4.38 0.12 0.29 0.01 0.01 0.091 0.0029 0.0094A 9.85 4.38 0.14 0.27 0.01 0.01 0.090 0.0035 0.009

MELTING PROCEDUREThe alloys were melted in a 2000 kg capacity reverberatory furnace. During processing, the melt was covered with aprotective nitrogen gas atmosphere to prevent hydrogen and oxygen contamination. No grain refining agents were added tothe melt. The ingots used were pre-modified with Sr.

THERMAL ANALYSIS PROCEDURESamples with masses of approximately 300g ±10g were poured into specially manufactured, ultra light, stainless steel (SS304) cups, (mass = 2.5 ± 0.2 g). Two specially designed, supersensitive K type thermocouples (with extra low thermal timeconstants) were inserted into the melt and temperatures between 750 - 400oC were recorded. The data for TA was collectedusing a high-speed National Instruments data acquisition system linked to a personal computer. Each TA trial was repeatedfour times. Consequently, a total of 24 samples were gathered (the results of three trials were subsequently discarded due toexperimental irregularities).

In order to estimate the precision of the TA measurements, confidence intervals for the total mean liquidus, solidus and twokey eutectic nucleation temperatures were computed using the Student-t procedure. The results of these calculations arepresented in Table 2.

Table 2. Confidence intervals for the four key temperatures determined using thermal analysis system software ( oC).

Confidence Interval 95% 99%Liquidus Temperature 0.381 0.519

Al-Si Eutectic Temperature 0.455 0.621Al-Cu Eutectic Temperature 0.302 0.411

Solidus Temperature 0.391 0.533Total sample size (n) 21

The results suggest that the precision and repeatability of the measuring system was very good. One may be 95% confidentthat the overall mean temperatures presented in this paper would vary by less than +/- 0.5oC if the experiment were replicatedusing the same equipment, under the same conditions (n=21, d.f. = 20, Student-t = 2.086). This variation is similar to therange of error characteristic of the thermocouples used in these trials (which is thought to be approximately +/- 0.5oC).

METALLOGRAPHY AND IMAGE ANALYSISSamples for microstructural analysis were cut from the TA test samples, close to the tips of the thermocouples. The crosssections of the specimens were ground and polished on an automatic polisher using standard metallographic procedures. Thefinal polish was carried out using commercial slurry (Struers OP-U). The samples were observed under a JEOL JSM 5800Scanning Electron Microscope (SEM) using magnifications between X200 and X5000. Qualitative and quantitativeassessments of the chemical compositions of the Cu enriched phases were done using an Energy Dispersive Spectrometer(EDS). For each phase, about 25 separate spot measurements were made and the mean value was calculated. The obtainedchemical compositions were then normalized to 100% and the atomic masses of the constituents in the Cu enriched phaseswere calculated (Table 3).

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Table 3. Chemical compositions of phases observed in the investigated alloys.Type of Cu enriched phases Formulae of Cu enriched Phases

"Blocky" Cu 10.59 Al 4.09 Si 2.82 Mg 2.48"Eutectic" Cu 7.27 Al 12.25 Si 0.47

"Fine Eutectic" Cu 4.14 Al 13.32 Si 1.72 Mg 0.81

The volume fractions of the Cu enriched phases were calculated using the Leica QWin IA software linked to a computerizedLeica DMR microscope, under a magnification of X500. Twenty-five analytical fields were measured for each sample andthe final volume fraction was expressed as a mean value.

Thin foil examinations were carried out on a JEM 2010F Transmission Electron Microscope (TEM). Examinations of thestructure in the bright field and scanning modes were carried out. Additionally, X-ray maps of main alloying elementdistributions (i.e. Al, Si and Cu) were created. The thin foils were made from specimens that were cut by a low speed diamondsaw, ground mechanically and finally thinned by a GATTAN PIPS-Duo Mill ion miller.

RESULTS AND DISCUSSION

THERMAL ANALYSIS RESULTSFigures 1 and 2 depict six representative TA cooling curves obtained for alloys 1A, 2A and 3A (Figure 1) as well as 4A, 5Aand 6A (Figure 2). The cooling rate in all six curves was approximately 0.1oC/second and represented the temperaturedifference between the liquidus and solidus temperatures divided by the total solidification time.

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Fig. 1. Cooling curves of alloys with nominal 7 wt% silicon content (Table 1).

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Figures 1 and 2 show that increasing the Si and Cu content of the melt depresses the characteristic temperatures on thecooling curve.

The first derivatives of the cooling curves are presented in Figures 3 to 5. It is apparent that the shapes of the first derivativecurves are strongly dependent on the amount of Si and Cu in the melt.

The number and shape of the peaks visible in the Cu enriched region of the first derivative curves show a strong relationship

with the amount of Cu present in the alloy. Si content does not substantially influence the shape of the Cu enriched eutecticpeaks, although it does influence the duration of α-aluminum dendrite solidification. As can be seen in Figures 3 to 5, adecrease in the Si content by 2 wt% postponed the Al-Si eutectic nucleation by more than 120 seconds. It can also beobserved that an increase in the Cu content decreases the primary solidification time of the α-aluminum, although not to thesame extent as Si.

The precipitation temperature of the Cu enriched phases decreases when Cu increases from 1 to 4 wt%. The Cu enrichedphase represented by the first peak on the cooling curve in Figure 3 (7 wt% Si, 1wt% Cu alloy) began to precipitate at 526oCand the Cu enriched phase represented by the second peak precipitated at 503oC. In Figure 4 (7 wt% Si, 2wt% Cu alloy)three peaks can be observed (although they are clearly "convoluted"). One peak is dominant and the Cu phase representedby this peak began to precipitate 507oC. Increasing the amount of Cu to 4wt% (7wt% Si) further changes the shape of theCu

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Fig. 2. Cooling curves of alloys with nominal 9 wt% silicon content (Table 1).

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Fig. 3. First derivative of the cooling curve with nominal 1 wt% Cu content (Table 1).

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Fig. 4. First derivative of the cooling curve with nominal 2 wt% Cu content (Table 1).

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enriched phase peaks (Figure 5). The precipitation temperatures are also altered. The Cu enriched phase represented by thefirst peak begins to precipitate at 510oC.

Increasing the Cu content from 1 wt% to 4 wt% increased the total solidification time from 1140 seconds (in alloys 1A and5A) to 1180 seconds (in alloys 3A and 4A). However, increasing the Si content (from 7 wt% to 9 wt%) had a negligibleeffect on the total solidification time. In all cases, the masses of the samples were virtually identical.These experiments (Figures 1 to 5) indicate that the Cu enriched phases precipitate at different temperatures depending on theamount of Cu present in the particular 3XX series alloy. The nucleation temperature of the Cu enriched phases can beaccurately read from the first derivatives of the cooling curves and used to define the maximum temperatures that the castingscan be exposed to during the conventional solution treatment process. However, before solution treatment routines can be"tailored" to specific alloys and applications, it is also necessary that the volume fractions of the Cu enriched phases beknown. This will enable researchers to predict the mechanical properties of the castings and to design components accordingto predetermined specifications and requirements. To date, volume fraction assessment has only been possible throughmetallographic analysis.

METALLOGRAPHY AND IMAGE ANALYSIS RESULTSLight Optical Microscopy (LOM) observations combined with Image Analysis (IA) showed that the area fractions of the Siphases and Cu enriched phases increased with additions of Si and Cu. Cu addition from 1 to 4 wt% caused the area fractionof the Cu enriched phases to increase from about 0.6% to about 2.3% (Table 4). It should be noted, however, that it isvirtually impossible to quantify the presence of very fine Cu enriched eutectic phases using IA combined with LOM; in fact,it is difficult to observe them under the SEM.

Fig. 5. First derivative of the cooling curve with nominal 4 wt% Cu content (Table 1).

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Table 4. Comparison of Cu enriched phase area fraction detected by the IA system and determined using thermalanalysis. For the purpose of clarity, the Si and Cu contents of the alloys tested and the volume fractions of the Sieutectic phases are also presented.

Alloy Vol. % of Cu-richphases, (TA)

Area Fraction % ofCu-rich phases, (IAS) Cu, wt.% Si, wt.% Area Fraction % of

Si phases, (IAS)

1A 1.55 0.63 0.96 7.13 6.96

2A 2.91 1.22 1.98 7.05 5.86

3A 6.06 2.35 4.38 6.75 6.20

4A 5.96 2.02 4.38 8.95 10.70

5A 1.70 0.60 1.05 9.17 9.47

6A 3.41 1.32 2.44 9.02 9.02

For this reason, preliminary TEM investigations were done on thin foils of pure Al-Si-Cu alloy (6.6wt% Si and 3.5wt% Cu)(Kasprzak, et al. 2000). Under a magnification of 22,000X this study revealed the presence of ultra fine Al-Cu eutectics.Additional x-ray mapping showed the presence of single Si crystals inside the Al-Cu eutectic (Figure 6). This confirms thepostulate that the solidification of the Al-Si eutectic continues until the solidus temperature is reached.

Additional SEM observation, combined with X-ray spot microanalysis for the 3XX series alloy was performed to identify themorphology and stoichiometry of the observed Cu enriched phases. This analysis confirmed the earlier assertion that Cuenriched phases appear in three main morphologies: blocky, eutectic type and fine eutectic type (Figure 7). Quantitative x-ray microanalysis for revealed the stoichiometries of the phases (Table 3) presented in Figures 6 and 7.

It should be noted that a complete evaluation of the morphology and corresponding stoichiometry of the Cu enriched phasesis beyond the scope of the present paper. To establish the crystallization sequence of the Cu enriched phases and thecorresponding stoichiometry with relation to the TA results quenching experiments will be necessary.

The variance in the Si content of the investigated alloys between 7 and 9wt% is associated with a change in the area fractionof the Si phases from about 6% to about 10% (Table 4). The imperfect agreement between these two measurements can beexplained by two factors: First, the IA measurements do not take into account small Si crystals that can not be resolved bythe LOM (Figure 6) or the Si that is dissolved in the aluminum matrix. Second, because the cast samples are heterogeneousand because only a finite number of regions were evaluated using the IA, these measurements may not be preciselyrepresentative of the entire sample.

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Fig. 6. Micrographs obtained from TEM of the Al-Cu eutectic found in the thin foil made from pure Al-Si-Cu alloy(6.58wt% Si and 3.52wt% Cu):

SEI - Secondary Electron Image (SEI - scanning mode) of the Al-Cu eutectic,Al - X-ray distribution map of Al,Si - X-ray distribution map of Si,Cu - X-ray distribution map of Cu.

Al

Si Cu

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x 22K

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PROCEDURE FOR THE DETERMINATION OF THE "ENERGY SIGNATURE" OF THE COPPER ENRICHEDPHASES FROM THERMAL ANALYSISDetermination of the total Cu enriched phase area fraction by metallography is a time consuming and laborious procedure;therefore, it can not be used as an on-line measurement tool, or as a method of controlling casting quality in a foundryenvironment. This paper proposes a new technique for estimating the total volume of Cu enriched phases based on the resultsobtained using TA.

A comparison of the total area fraction of the Cu enriched phases determined using IA with a detailed analysis of the "energysignature" (ES) of the Cu enriched phases of each alloy tested shows that the two measurements are almost perfectly correlated(see Figure 10).

In this paper, the ES of the Cu enriched phases is defined as the ratio of the area between the first derivative of the cooling curveand the hypothetical solidification path of the Al-Si eutectic to the total area between the first derivative of the cooling curve andthe Base Line (BL) (Kierkus and Sokolowski, 1999). The rationale of this assumption is based on:

1. The IA results, which permit one to postulate that the solidification of the Al-Si eutectic continues until the solidustemperature is reached.

2. The total latent energy evolved during alloy solidification is the sum of the energy released by all of the phases involved inthe process.

This concept is demonstrated in Figures 8 and 9. Figure 8 presents the first derivative of the cooling curve (FD) and the Base Linecurve (BL). The area between the two curves, from the liquidus state (l) to the solidus state (s) is proportional to the latent heat ofsolidification of the alloy (in this example, Alloy 2A from Table 1). If the two aforementioned assumptions are correct, then theregression line between the arbitrarily selected state (a) and the solidus state (s) (as shown in Figure 9) is a part of the solidificationpath of the Al-Si eutectic (AlSi,e). Therefore, it is evident that the area between path (a)-(s) and the first derivative of the coolingcurve (FD) should be proportional to the latent heat of solidification of the Cu enriched phases. The proportionality constant inboth cases; the total latent heat of alloy solidification and the latent heat of the solidification associated with the Cu enrichedphases is the "apparent specific heat" of the alloy. The determination of the "apparent specific heat" of an alloy is the subject ofongoing investigation by the authors. It should also be noted that preliminary "deconvolution"of the area between the (FD) and(AlSi,e) curves into three separate peaks has been performed. This should permit one to quantify the three compounds presentedin Table 4. The results are promising and will be presented in subsequent publications.The results of the Cu enriched phase determinations are presented in Table 3 and in Figure 10. The high correlation observedon the regression plots (Figure 10) shows that it is possible to estimate the volume fraction of Cu enriched phases from theTA analysis experiments without resorting to IA.

Fig. 7. SEM micrographs (BSE images) with characteristic morphology of Cu enriched phases found in theinvestigated alloys;1. the blocky (#1) and eutectic type (#2),2. fine eutectic type (#3).

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Fig. 8. First derivative of the cooling curve (FD) with its Base Line (BL) and the hypothetical solidification path of theAl-Si eutectic (AlSi,e).

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CONCLUSIONS

A comprehensive understanding of melt quality is of paramount importance for the control and prediction of actual castingcharacteristics. This paper has shown that TA can be extended to quantify the total volume fraction of the Cu enriched phases inthe 3XX series of aluminum alloys. Future work should confirm that on-line quantitative control of the Cu enriched phases ispossible using TA.

ACKNOWLEDGEMENTS

The authors would like to express their appreciation to the Natural Sciences and Engineering Research Council of Canada(NSERC) and to the Ford Motor Company for their sponsorship and Ms. E. Moosberger for her assistance with thepreparation of this manuscript. Special thanks to P. Gallo for sample preparation and Q. Ren for Cu enriched phasemeasurement using the IA technique as well as to Dr. M. Niewczas (McMaster University) for his assistance with the TEManalysis.

REFERENCES

Bäckerud, L., Chai, G. and Tamminen, J., “Solidification Characteristics of Aluminum Alloys” Volume 2,AFS/SKANALUMINIUM, Oslo (1986).

Caceres, C. H., Djurdjevic, M. B., Stockwell, T. J. and Sokolowski, J. H., "The Effect of Cu Content on the Level ofMicroporosity in Al-Si-Cu-Mg Casting Alloys", Scripta Materialia, Vol. 40, pp. 631-637 (1999).

Djurdjevic, M., Sokolowski, J., and Stockwell, T., “A Metallographic and Thermal Analysis of the Effect of Sr on the AlCuEutectics in the 319 Alloy ” NSERC/Ford/University of Windsor Industrial Research Chair in Light Metals CastingTechnology Confidential Report, June 11, pp. 1-15 (1998).

Fig. 10. Relationship between IA and TA measurements and the chemical compositions of the investigated alloys.

[IA, vol%] = 0.3721xR2 = 0.971

[Cu, wt%] = 0.7173xR2 = 0.994

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2.5

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Vol

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ses,

(IA

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3.5

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IA, vol. %Cu , wt. %

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Djurdjevic, M., Stockwell, T. and Sokolowski, J., “The Effect of Strontium on the Microstructure of the Aluminum-Si andAluminum-Cu Eutectics in the 319 Aluminum Alloy”, International Journal of Cast Metals Research , No. 12, pp.67-73 (1999).

Doty, H. W., Samuel, A. M. and Samuel, F. H., “Factors Controlling the Type and Morphology of Cu-Containing Phases inthe 319 Aluminum Alloy”, 100th AFS Casting Congress, Philadelphia, Pennsylvania, USA, April 20-23, pp. 1-30(1996).

Kasprzak, W., Niewczas, M. and Sokolowski, J. H., NSERC/Ford/University of Windsor Industrial Research Chair in LightMetals Casting Technology Confidential Report, in progress (2000).

Kierkus, W. T. and Sokolowski, J. H., "Recent Advances in Cooling Curve Analysis: A New Method of Determining the'Base Line' Equation", AFS Transactions (1999).

Tenekedjiev, N., Mulazimoglu, H., Closset, B. and Gruzleski, J., “Microstructures and Thermal Analysis of Strontium-Treated Aluminum-Si Alloys”, American Foundrymen’s Society Inc., Des Plaines, Illinois, USA (1995).