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Casting Aluminum Alloys

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Casting Aluminum Alloys

VADIM S. ZOLOTOREVSKY and NIKOLAI A. BELOVMoscow Institute of Steel and Alloys – State Technical University, 119049,Moscow, 4 Leninsky Pr., Russian Federation

MICHAEL V. GLAZOFFAlcoa Technical Center, Alcoa Center, PA 15069, USA

Amsterdam • Boston • Heidelberg • London • NewYork • OxfordParis • San Diego • San Francisco • Singapore • Sydney • Tokyo

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CONTENTS

Preface ixNotations xiii

1. Alloying Elements and Dopants: Phase Diagrams 1

1.1 The Role of Alloying Elements and Dopants: Basic Alloy Systems 11.2 Phase Diagrams of Ternary Systems 14

1.2.1 The Al–Be–Fe system 141.2.2 The Al–Be–Si system 151.2.3 The Al-Ce-Cu system 161.2.4 The Al–Ce–Fe system 181.2.5 The Al–Ce–Ni system 201.2.6 The Al–Ce–Si system 211.2.7 The Al–Cr–Fe system 221.2.8 The Al–Cr–Mg system 231.2.9 The Al–Cr–Mn system 241.2.10 The Al–Cr–Si system 261.2.11 The Al–Cu–Fe system 261.2.12 The Al–Cu–Mg system 291.2.13 The Al-Cu-Mn system 321.2.14 The Al–Cu–Ni system 341.2.15 The Al–Cu–Si system 361.2.16 The Al–Cu–Zn system 361.2.17 The Al–Fe–Mg system 381.2.18 The Al–Fe–Mn system 391.2.19 The Al–Fe–Ni system 411.2.20 The Al–Fe–Si system 421.2.21 The Al–Mg–Mn system 451.2.22 The Al–Mg–Si system 451.2.23 The Al–Mg–Zn system 471.2.24 The Al–Mn–Ni system 491.2.25 The Al-Mn-Si system 531.2.26 The Al–Ni–Si system 54

1.3 Phase Diagrams of Four-Component Systems 551.3.1 The Al–Be–Fe–Si phase diagram 561.3.2 The Al–Cu–Fe–Mg system 581.3.3 The Al–Cu–Fe–Mn system 581.3.4 The Al–Cu–Fe–Ni system 601.3.5 The Al–Cu–Fe–Si system 621.3.6 The Al–Cu–Mg–Mn system 64

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1.3.7 The Al–Cu–Mg–Si system 641.3.8 The Al–Cu–Mg–Zn system 661.3.9 The Al–Fe–Mg–Mn system 681.3.10 The Al–Fe–Mg–Si system 701.3.11 The Al–Fe–Mn–Si system 741.3.12 The Al–Fe–Ni–Si system 771.3.13 The Al–Mg–Mn–Si system 791.3.14 The Al–Mg–Ni–Si system 79

1.4 Five-Component Phase Diagrams 811.4.1 The Al–Fe–Cu–Mg–Si system 851.4.2 Five-component Systems with Manganese 91

2. Structure and Microstructure of Aluminum Alloys in As-Cast State 95

2.1 Phase Diagrams, Thermodynamics, and Alloy Microstructure 952.2 Equilibrium Thermodynamics and Its Development 97

2.2.1 Classical equilibrium thermodynamics 972.2.2 Equilibrium thermodynamics of concentrationally

non-uniform systems 982.3 Brief Description of Solidification Microstructure Evolution in Casting

Aluminum Alloys via the “Phase-Field’’ Approach 1012.3.1 Phase-field approach applied to solidification 1022.3.2 Dendritic solidification of pure metals 1022.3.3 Phase-field model for solidification of eutectic alloys11 1042.3.4 Solidification microstructure calculations: perspectives and

future work 1062.4 Quantitative Characteristics of Alloy Structure and Methods of its

Evaluation 1072.5 Non-Equilibrium Solidification of Binary Alloys 114

2.5.1 Microsegregation 1152.5.2 Influence of cooling rate upon solidification and formation of

constituent particles of secondary (excessive) phases 1282.6 Non-Equilibrium Solidification of Multi-Component Alloys 134

2.6.1 Non-equilibrium phase diagrams of multicomponent systems 1342.6.2 Microsegregation in three-component and industrial aluminum

alloys 1452.7 Microstructure of Cast Aluminum Alloys 1542.8 Substructure of Casting Aluminum Alloys 162

2.8.1 Types of dislocation structures in as-cast aluminum alloys ofdifferent systems 162

2.8.2 The influence of solidification conditions upon dislocationmicrostructure 166

2.8.3 The mechanisms of formation of dislocation microstructures incast aluminum alloys 171

2.8.4 Decomposition of aluminum solid solution in the process ofalloy cooling after the completion of solidification 177

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3. Influence of Heat Treatment Upon Microstructure of CastingAluminum Alloys 183

3.1 Homogenizing Heat Treatment 1843.1.1 Dissolution of non-equilibrium constituent particles in the

course of homogenization 1843.1.2 Elimination of microsegregation during homogenization 2003.1.3 Fragmentation and spheroidization of constituent particles 2133.1.4 Changes of grain and dislocation microstructure of aluminum

solid solution in the course of homogenization 2223.1.5 Decomposition of aluminum solid solution in the process of

isothermal heat treatment before quenching 2303.1.6 Development of porosity during homogenization 240

3.2 Aging After Casting and Quenching 240

4. Dependence of Castability and Mechanical Properties onComposition and Microstructure of Aluminum Alloys 247

4.1 Castability 2474.1.1 General characterization of castability 2474.1.2 Concentration dependence of casting properties 258

4.2 Mechanical Properties 2624.2.1 Geometry of elongation diagrams for as-cast and quenched

aluminum alloys, and its connection to the structuraltransformations accompanying deformation 266

4.2.2 Quantitative analysis of relations between tensile mechanicalproperties and structural characteristics of castings 280

4.2.3 Calculations of mechanical properties of castings usingthe totality of microstructural characteristics 295

4.2.4 The influence of casting microstructure upon fracture toughnessand fatigue properties 302

4.2.5 Some regularities in changes of mechanical properties with alloychemical composition 311

5. Industrial Casting Aluminum Alloys 327

5.1 Al–Si Alloys 3275.1.1 General characterization of al–si alloys 3275.1.2 Industrial 4xx and 3xx casting alloys without copper and zinc

(“copper-less’’ alloys) 3365.1.3 Industrial Al–Si alloys with copper and zinc 3515.1.4 Engine piston Al–Si alloys 367

5.2 Alloys on the Basis of the Al–Cu System 3765.3 Al–Mg and Al–Mg–Zn Alloys 386

5.3.1 General characteristic of Al–Mg alloys 3865.3.2 Industrial Al–Mg and Al–Mg–Zn alloys 390

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6. New Alloys 397

6.1 Alloys with Small Amounts of Eutectic 3976.2 General Principles of Alloying for Eutectic Materials 4056.3 High-Strength Alloy AZ6N4 and ATs7Mg3N4 (734) 4206.4 Alloys Doped with Transition Metals for Improved Thermal Stability 4266.5 Alloys with Small Amounts of Silicon (<4%Si) 442

Literature 449

Appendix 1 Compositions of Standard Casting Aluminum Alloys 461

Appendix 2 Principal Characteristics of Binary Phase DiagramsCloser to Aluminum Side 487

Appendix 3 Guaranteed Mechanical Properties of StandardRussian Aluminum Alloys 491

Appendix 4 Recommended Heat Treatments of StandardRussian Casting Aluminum Alloys 499

Appendix 5 Data on Fracture Toughness and Shock Toughness,Fatigue Life, Characteristics of Thermal Stability,Corrosion Resistance, and Castability of Standard Al-Si Alloys 507

Appendix 6 Derivation of Equations Describing Uniaxial Tensile Testingin Finite Deformations 511

A.6.1 The Case of Infinitesimally Small Deformations 513A.6.2 The Case of Finite Deformations 515

Index 523

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PREFACE

By definition, casting alloys are materials used for the production of shape cast-ings, that is aluminum alloy products with complex geometrical shape(s). Castingaluminum alloys are quite widespread and find more and more applications inmodern industry.

According to different estimates, up to 20–30% of all aluminum productsmanufactured worldwide are used for shape castings. Suffice it to say that inaddition to such giants as Alcoa Inc. and Alcan, there are literally hundreds of casthouses in North America. Aluminum castings are also manufactured by differentcompanies that specialize in end materials/products other than aluminum (e.g.,General Motors, Ford, etc.)

Aluminum castings can be and indeed are produced with very substantialamounts of recycled aluminum scrap. For example, in the USA,Western Europe,and Japan up to 75–80% of the overall alloy mass comes from recycled alu-minum/scrap. This is several times higher than the corresponding numbers forwrought aluminum alloys. Earlier it was hypothesized that the general level ofproperties required of cast aluminum products was lower, and it was used mostlyfor the production of non-critical (e.g., not heavily loaded) parts. Indeed, for suchparts the application of recycled aluminum with elevated levels of impurities wasquite acceptable.

However, during the last 10 or 15 years this situation has started to change.Due to considerable improvements in casting technologies, now it is possible toproduce high-quality castings with properties that are comparable to those ofsimilar wrought products. Moreover, this can be done not only for high-qualityalloys, but also for those manufactured with substantial amounts of aluminumscrap. In the latter case the advantage, of course, is in lower production costs.

Significant improvements in the quality of shape castings were achieved due toimproved production processes. Today it is possible to employ modern methodsof molten metal handling, which result in dramatic reduction of harmful non-metallic impurities. Hot isostatic pressing is used to reduce shrinkage porosity. Allthese, and many other, innovations result in significant improvement of aluminumshape casting quality.

There are several important requirements to casting aluminum alloys: goodcorrosion resistance, high level of mechanical properties (such as ultimate ten-sile strength (UTS), yield strength (YS), and elongation (El.)) and, finally, goodcastability. This last property is particularly important; it implies that solidifyingmetal is not prone to hot cracking, possesses excellent fluidity in molten state, andminimal shrinkage porosity. It is because of excellent castability that Al–Si castingalloys (containing more than 4%Si) have retained their leading role among allother casting alloy compositions during the last 60 years, even though the general

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level of all other properties is quite average. Indeed, as far as low temperaturestrength is concerned, Al–Cu and Al–Zn–Mg–Cu alloys are considerably betterthan Al–Si. Creep resistance is the best for Al–Cu–Mn alloys, corrosion resistanceis better for Al–Mg and Al–Zn–Mg alloys. However, mostly due to excellentcastability, more than 90% (!) of all shape castings today are manufactured fromAl–Si alloys.

Obviously, this situation is not normal as it seriously impedes further devel-opment of aluminum alloy shape castings. Evidently, there are two principal waysto approach this important and old problem:

1. Casting technology improvement and development of principally new tech-nological processes that would ensure a high quality of castings made fromalloys with low castability.

2. Development of new casting alloy compositions that would combine excellentlevel of properties with good castability using traditional approaches (e.g., sandcasting, permanent mold casting, etc.).

Today there is no doubt that the automotive industry is the most importantconsumer of aluminum alloy shape castings. Each year the overall volume ofcast aluminum in automotive technologies grows steadily. This is especially trueduring the last 10 years, when the production of “aluminum’’ cars started andthe number of aluminum-intensive vehicles grew rapidly. Such details as cylinderblocks, pistons, other engine parts, frames, and covers of different devices “underthe hood’’ are traditionally cast from aluminum now.

All these complex details and products are manufactured using different castingtechniques and amount to many millions of parts per year.

Due to their excellent specific strength, corrosion resistance, and relatively lowlabor intensity of production, cast aluminum alloys are also widely used in othertransportation sectors of the economy such as aerospace, marine, and railroadtransportation.

It was mentioned above that in the automotive industry Al–Si alloys find themost widespread application. However, in the aerospace industry a substantialnumber of all castings are made of high-strength Al–Cu alloys of the 2xx series,while in shipbuilding the corrosion-resistant Al–Mg alloys of the 5xx series1 areubiquitous. Alloys of the Al–Mg and Al–Si types are also used in railroad carconstruction (e.g., massive brake gear).

Large amounts of aluminum alloy castings are consumed by the defense indus-try, electronics, nuclear industry, etc. Examples of large cast aluminum partsinclude gaskets of electric motors, wheels of armored vehicles, and tank turrets.

It is obvious that further successes in perfection of already existing and devel-opment of novel casting aluminum alloys will be defined by our understanding oftheir metals science and metals physics, that is our capability to relate alloy prop-erties to their composition and microstructure. In the second half of the 20thcentury this level was significantly raised; however, there are still many questions

1 Here and below the classification of cast alloys adopted by the Aluminum Association will be used.

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and problems that remain unsolved.To a significant degree, such a situation arisesbecause only a limited (and decreasing) number of specialists in several countrieswere involved in aluminum research. For example, in major American univer-sities this area of research is no longer “fashionable’’. Consequently, the actualaluminum research is conducted mostly in technical centers of large industrialcompanies, such as Alcoa Inc.

The present monograph mostly summarizes research conducted at the MoscowInstitute of Steel andAlloys over many decades (Chair of Non-Ferrous Metals), inpart together with Alcoa Inc. (especially during the last 5–7 years). This researchwas initiated by such talented scientists as A.A. Bochvar between 1930 and 1940,I.I. Novikov between 1950 and 1960, and continued by the authors of the presentwork. Many dozens of professors, research scientists, graduate and undergraduatestudents took part in it. The authors would like to express their gratitude toall these numerous researchers. One of the authors (M.V. Glazoff) expresses hissincere gratitude to the Technical Director of the Alcoa Technical Center, Dr.William A. Cassada, III, and to the Division Managers, Dr. Jonell M. Kerkhoffand Dr. Ralph R. Sawtell, for permission to publish this monograph and forcontinuous support of our research efforts.

Finally, it was decided to retain the original nomenclature for most Russiancasting alloys and references used in this monograph. This was done to facilitatedirect discussions between the interested researchers without causing otherwiseinevitable spelling or translation problems. The authors would like to hopethat it will not cause confusion in understanding the corresponding parts ofthis book.

Moscow, Pittsburgh 2007

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NOTATIONS

(Al), (Si) Solid solutions on the basis of aluminum, silicon (and otherelements)

D Grain size of primary dendrites of (Al): Dmin, Dmax– minimaland maximal sizes

d Dendritic parameter of (Al) primary crystalsd′ Subgrain sizeDe Average size of eutectic coloniesde Dendrite arms spacingV c Cooling rate upon solidificationS Specific surface of inclusions (grain boundaries)Cx Concentration of a given component in alloy: C1, C2,

CCu, CFeCx–y Concentration of a component in a phase:C1–2,C2–2,CCu–AlCe, Ca Concentration of component in eutectic and limit

solubility in (Al)�C, Cmin, Cmax Concentration difference, minimal and maximal

concentrationK Distribution coefficient for an elementQV Volume fraction of phases, pores, and eutecticQM Mass fraction of a phase or eutecticm Thickness of the second phase inclusionsI Distance among inclusionsγ Specific weight (density)ρ Density of dislocationsρ1, ρ2 Density of dislocations outside and inside planar subgrain

boundariesb Dislocation Burgers vectora Lattice spacingθ Angle of disorientation between subgrainsDV Coefficient of volume diffusionT Temperatureτ Time

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