DEVELOPMENT OF RECYCLABLE Mg-BASED ALLOYS: AZ91D AND AZC1231 PHASE - - PowerPoint PPT Presentation

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ONSULTANTS INC

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University

• f Windsor

GKSS

FORSCHUNGSZE ZENTRUM

DEVELOPMENT OF RECYCLABLE Mg-BASED ALLOYS: AZ91D AND AZC1231 PHASE INFORMATION DERIVED FROM COOLING CURVE ANALYSIS

• A. J. Gesing
• N. D. Reade
• J. H. Sokolowski
• C. Blawert
• D. Fechner
• N. Hort

Mg Technology Symposium, TMS 2010, Seattle

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ONSULTANTS INC

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University

• f Windsor

GKSS

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Motivation

There is a wealth of information to be extracted from a simple cooling rate curve, when it is combined with equally simple bulk alloy composition along with micro-structural and micro-analytical data. Such non-equilibrium thermo-analytical data – acquired from a representative sized UMSA test sample under solidification rates matching those typical of commercial casting processes – are useful for both alloy development and for the development of melt treatment, refining and casting processes. We report on the work performed at the University of Windsor, Ontario and GKSS research center, Germany that significantly expanded the capabilities of the UMSA thermo-analytical system, and applied it to re-interpretation of data for two potentially recyclable Mg alloys, AZ91D and AZC1231.

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Contents

• Determination of alloy phase composition and weight

distribution

• UMSA system calibration by pure Mg solidification

measurement

• Thermal analysis baseline determination
• Thermal peak deconvolution
• Determination of enthalpy of formation of solid phases
• Quantitative evolution of weight fractions of solid

phases during solidification

• Estimate of the evolution of composition of residual

liquid during solidification

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Phases in AZC1231

a phase: HCP-Mg + Al, Zn in solid solution

• Susceptible to corrosion

b phase: Mg17Al12 + Al, Zn, Cu in solid solution

• Forms stable passive layer of Al2O3•MgO
• Partially embeds t phase
• Continuous b phase embedding a and t phases is the most effective

topology for corrosion protection.

t phase: AlMgZnCu solid solution

• Prevents formation of noble Mg-Cu binary intermetallics
• Reduces electrolytic corrosive activity of Cu

Al8Mn5 phase: + Fe and Ni in solid solution

• Prevents formation of noble Al-Fe and Al-Ni binary intermetallics
• Reduces electrolytic corrosive activity of Fe and Ni
• Can be selectively precipitated and settled out of the melt

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Calculation of phase distribution from the average alloy composition and phase micro-composition

Phase Alloy Mg Al Zn Mn Si Cu Fe Ni Calculated Phase Distribution (wt%) Average OES alloy elemental composition (wt%) AZ91D 90.32 8.75 0.67 0.20 0.054 0.006 0.002 0.0006 AZC1231 83.91 11.70 3.04 0.48 0.39 0.47 0.009 0.003 Average EDX elemental phase micro-composition (wt%) a Mg AZ91D 97.22 2.39 0.25 0.07 0.02 0.01 0.01 0.02 86.6 AZC1231 96.16 2.9 0.94 74.6 Al8Mn5 AZ91D 1.33 41.94 1.07 51.86 1.00 0.41 2.00 0.38 0.1 AZC1231 1.44 40.43 0.98 51.68 2.14 0.49 2.83 0.8 b Mg17Al12 AZ91D 53.19 41.68 4.73 0.12 0.07 0.05 0.13 0.04 13.9 AZC1231 51.07 40.02 8.07 0.02 0.02 0.76 0.02 21.8 t MgAlZnCu AZ91D AZC1231 31.22 33.26 11.53 0.68 0.03 22.54 0.05 0.69 1.7 Mg2Si AZ91D 63.38 36.62 0.06 AZC1231 63.38 36.62 1.0

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Comparison of alloy phase distribution

a 74.64% b 21.84% t 1.71% Mg2Si 1.00% Al8Mn5 0.80% AZC1231 a 86.6% b 13.9% Al8Mn5 0.11% Mg2Si 0.06% AZ91D

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Microstructure

• f AZ91D

(1) α Mg, (2) β Mg17Al12, (3) Al8Mn5, (5) MgO oxide skins and pores. SEM-SE 15 kV

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Microstructure

• f AZC1231

(1) α Mg (2) β Mg17Al1 (3) Al8Mn5 (4) τ MgAlZnCu (5) MgO, oxide skins and pores (6) Mg2Si SEM-SE 15 kV

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Commercial purity Mg solidification

The offset between two baseline curves on a cooling rate vs temperature plot at 660 is 0.3 K/s for pure Mg. We subsequently used this observation in extrapolating the quadratic fit to the solid baseline for the alloy cooling rate curves.

600 610 620 630 640 650 660

• 3
• 2
• 1

1 2 3

• 10

10 30 50 70 90 110 Temperature ( C ) dT/dt (K/s) time (s) dT/dt B mix dT/dt - B mix T

• 100%
• 67%
• 33%

0% 33% 67% 100%

• 3
• 2
• 1

1 2 3 600 620 640 660 680 F solid [J %] dT/dt ( K/s ) Temperature ( C ) B mix dT/dt - B mix B liq B solid F solid

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FORSCHUNGSZE ZENTRUM Pure Mg Thermal Property Data

H0

s = 5.47017E-06 T2 + 2.16869E-02 T - 6.94747

H0

L = 3.429781E-02 T - 5.42735

5 10 15 20 25 30 35 40 45 300 500 700 900 1100 1300 H0 (kJ/mole) Temperature (K)

Mg

H° solid H° liq 25 26 27 28 29 30 31 32 33 34 35 300 500 700 900 1100 1300 Cp (J/mole) Temperature (K)

Mg

Cp solid Cp liquid Cp solid = 1.635961E-08 T3

• 2.7174556E-05 T2

+ 0.0251579 T + 19.350967

Cp liquid = 34.309

Radically different Cp – temperature trend for solid and liquid Mg translates to individual thermal analysis baselines. Step in Cp at melting translates to offset in baselines. Cp is the slope of H0– temperature plot. DH0

solidification decreases on super-cooling

pure Mg.

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UMSA calibration by enthalpy of commercially pure Mg solidification

UMSA setup was calibrated by solidification of a commercially pure Mg sample with known enthalpy of melting. The enthalpy of melting is proportional to the area under the baseline corrected peak on a cooling rate vs time plot. DHsolidifcation at 924K

• 8.49 kJ/mol
• 349.33 J/g

Sample weight 10 g Peak area: ( dT/dt – Bmix ) vs t 224.26 K Proportionality factor 15.58 J/K For our UMSA setup, this proportionality (calibration) factor for cooling rate curve is 15.58 J/K at 924 K. We assume that this value is applicable to solidification at lower temperatures corresponding to the solidification range of AZ91D and AZC1231 alloys.

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Calculation of baseline value for a semi-solid mix

We assume that the baseline for the semi-solid mix (Bmix) is a linear combination of the baseline values for the solid and for the liquid, weighted respectively by the fractions solid (FS) and liquid (FL). An iterative solution is required as calculation of FS and FL depends on the position of the baseline.

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Comparison of Baseline Curves AZ91D and AZC1231

Bliquid= -6.952019E-03 T + 2.166985

• 3
• 2.5
• 2
• 1.5
• 1
• 0.5

0.5 300 350 400 450 500 550 600 650 700 dT/dt (K/s) Temperature (c) AZ91D solidification

dT/dt B liquid B mix B solid

Bsolid = -5.480934E-06 (T-660)2

• 7.599266E-03 (T-660) - 2.70

B liq= -7.799606E-03 T + 2.764391E+00

• 3
• 2.5
• 2
• 1.5
• 1
• 0.5

0.5 300 350 400 450 500 550 600 650 700 dT/dt [K/s] Temperature [C] AZC1231 solidification dT/dt B mix B solid B liq B solid = -5.411848E-06 (T-660)2

• 7.557010E-03 (T-660) - 2.71

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Peak deconvolution for AZ1231 solidification

• 2
• 1.5
• 1
• 0.5

0.5 1 1.5 2 400 450 500 550 dT/dt [K/s] Temperature [C] Deconvolution of Mg2Si and a Mg peaks dT/dt B mix dT/dt - B mix B Mg2Si a Mg

• 1.5
• 1
• 0.5

0.5 1 1.5 300 350 400 450 500 dT/dt [K/s] Temperature [C] Deconvolution of t, b and a peaks dT/dt B mix dT/dt - B mix B thau B beta a Mg b t

On baseline corrected cooling rate vs temperature plot, peak deconvolution involves determining the secondary peak baseline by regression fitting a single polynomial to the primary peak profile both above and below the region of the secondary peak.

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Enthalpy of formation of solid phases

Enthalpy distribution Phase distribution DHf Solidification temperature range Alloy Phase J% wt% [J/g] [C] AZ91D AZC1231 AZ91D AZC1231 AZ91D AZC1231 AZ91D AZC1231 Alloy average 100.0% 100.0% 100.0% 99.76%

• 298.71 -262.52

650-400 660-345 aMg 90.15% 77.55% 86.56% 74.46%

• 311.09 - 272.74 595-400

578-360 bMg17Al12 8.88% 13.81% 13.93% 21.79%

• 190.52 -165.98

430-400 410-345 tMgAlZnCu 2.05% 1.71%

• 313.76

405-380 Mg2Si 5.07% 0.059% 1.00%

• 1,330

560-500 Al8Mn5 0.97% 1.52% 0.108% 0.80%

• 2,672
• 499.20

650-600 650-600

Enthalpy of formation depends on the composition of the liquid and that of the solid product. Both liquid and solid compositions are different for AZ91D and for AZC1231 resulting in lower absolute DHf values for AZC1231 than for AZ91D. DHf values for tMgAlZnCu, Mg2Si and Al8Mn5 are highly negative, suggesting low electrochemical activity for noble components trapped in these phases.

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Fractions of solid and liquid

Combination of phase weight distribution and thermal analysis information allows quantitative calculation of appearance of each individual phase during solidification. Total fraction solid, FS and remaining fraction liquid, FL are also shown for AZ91D solidification.

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Comparison of phase appearance during solidification of AZ91D and AZC1231

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Evolution of liquid composition during solidification

0.01% 0.10% 1.00% 10.00% 100.00% 350 400 450 500 550 600 650 700 element concentration in the liquid [wt%] Temperature [C] Phases crystallizing out of the solidifying AZ91D liquid a + b Mg17Al12| a Mg | Al8Mn5 Mn Zn Al Mg

Knowing the amount and composition of the phases as they crystallize from the liquid allows estimation of the changes in the elemental composition of the remaining liquid. Composition of each solid phase is assumed to remain constant during the solidification for this estimate.

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University

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FORSCHUNGSZE ZENTRUM Comparison of evolution of liquid composition

during solidification of AZ91D and AZC1231

0.01% 0.10% 1.00% 10.00% 100.00% 350 400 450 500 550 600 650 700 Element concentration in the liquid [wt%] Temperature [C]

Mg Al Zn Cu Mn Si Phases crystallizing out of the solidifying AZC1231 liquid

| a+b | a | Al8Mn5 | t| | Mg2Si |

0.01% 0.10% 1.00% 10.00% 100.00% 350 400 450 500 550 600 650 700 element concentration in the liquid [wt%] Temperature [C] Phases crystallizing out of the solidifying AZ91D liquid a + b Mg17Al12| a Mg | Al8Mn5 Mn Zn Al Mg

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Conclusions

• UMSA thermal analysis is an important tool in minimizing the experimental

effort of new alloy development.

• Uncertainty in the determination of the suitable baseline location is one of

the largest sources of error in thermal analysis. The new baseline calculation procedure accounts explicitly for the semisolid mix during solidification and improves measurement accuracy significantly, especially for the minor phases.

• Quantitative calibration is an extension of UMSA functionality, allowing for

calculation of enthalpies of formation of individual phases.

• Optimization calculation allows reliable quantitative phase weight

distribution determination from the measurements of alloy composition and phase micro-composition data.

• Combination of phase weight, composition and thermal analysis results

allows quantitative details of the solidification process, including the evolution of solid phase content and the residual liquid composition.

• This work significantly expanded the detail of the information derived from

the UMSA thermo-analytical system

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Acknowledgements

The gravity diecasting of the alloys at the TU Clausthal (Dr.-Ing. A. Ditze and Dr.-Ing. C. Scharf) is thankfully acknowledged. This research was partially funded by AUTO21, a member of the Networks of Centers of Excellence of Canada program.

Thank you