Impact properties of al-10%si alloys and experimental design

ALUMINUM CASTING ALLOYS

Aluminum is relatively malleable compared to other materials, and it is well-suited to metal-forming applications. In its pure form, however, aluminum is possessed of low strength; for which reason it is typically alloyed with other elements for strengthening purposes. A number of metals may be alloyed with aluminum, but only a few are used as major alloying elements in commercial aluminum-based alloys; others are used as supplements to alloying additions for the improvement of alloy properties and characteristics. The effects of these alloying additives on the properties of aluminum depend on the individual elements and the specific amounts added, as well as on their interaction with aluminum and with each other. Major additions are primarily used for strengthening, while other elements are used to obtain specific microstructural characteristics which may include a finer grain size, higher critical recrystallization temperatures, or else to block the harmful effects of certain impurities.

Aluminum and its alloys are used in many aspects of modern life, from soda cans and household foil to the automobiles and aircraft in which people travel. The alloying elements in aluminum alloys may be present in the form of solid solution, dispersoids, precipitates within the grain, or intermetallic compounds at the grain boundaries. Due to the multiplicity of the alloying elements, many different phases precipitate during solidification and subsequent cooling .

Depending on their method of fabrication, aluminum alloys may be divided into two major groups: cast and wrought alloys. Cast aluminum alloys are classified on the basis of their chemical composition; there is, however, no internationally accepted system of nomenclature which has been adopted for identifying them. These alloys incorporate many advantages compared to other materials and processing methods, although continuous improvements will be necessary in the future for them to maintain their competitive advantage.

Aluminum casting alloys are the most versatile of all common foundry alloys. For large productions, the three main casting processes are sand casting, permanent mold casting and high pressure die casting. Aluminum is also castable by means of the lost foam process, as well as the plaster, centrifugal and shell mold processes. Wrought alloys differ from cast alloys in that they can be shaped by deformation. Both aluminum cast and wrought alloys may be separated into heat-treatable and non-heat-treatable alloys, where the alloys are strengthened using heat treatments in the first case, and work hardening in the second.

ALUMINUM-SILICON ALLOYS

Aluminum alloys containing silicon as the major alloying element are highly satisfactory from the point of view of castability, weldability, thermal conductivity, and excellent corrosion resistance; in particular, they also display good retention of physical and mechanical properties at elevated températures. It is for this reason that Al-Si casting alloys have usually constituted 85 to 90% of the total of aluminum cast parts produced.

Al-Si-Cu-Mg Alloy System

The 319 (Al-Si-Cu-Mg) casting alloys represent the workhorse of aluminum foundry alloys. In this group, silicon provides good casting characteristics while copper provides high strength and machinability at the expense of somewhat reduced ductility and lower corrosion resistance. Based on the Al-Si system, the alloy contains copper and magnesium as the main alloying elements. Its silicon content ranges from 5.5 to 6.5 wt%, and copper content varies from 3.0 to 4.0 wt%.

Al-Si-Mg Alloy System

Another important group of alloys in the Al-Si system are Al-Si-Mg alloys, which are hardened by Mg2Si, such as the 356 alloy. Magnesium is the basis for strength and hardness development in heat-treated Al-Si alloys. In the heat-treated condition, the hardening phase Mg2Si has a solubility limit corresponding to approximately 0.7% Mg. Beyond this limit, no further strengthening occurs nor does matrix softening take place. At room temperature, quantities of magnesium exceeding 0.3% Mg will be present as Mg2Si. An increase of magnesium, within the alloy range, results in increased strength at the expense of ductility. Magnesium also has a beneficial effect on corrosion resistance. By including additional elements, it is possible to improve the mechanical properties of Al-SiMg alloys.

With regard to these alloys, iron is considered an impurity originating in the process of mining aluminum from the ore. It often appears in the form of AlFeSi intermetallics at the grain boundaries, causing a severe loss of ductility in the alloy; strength may also be noticeably affected. As a result, the iron-content is kept significantly low in premium quality alloys which are used for aircraft and aerospace castings requiring high-grade quality properties. Copper is present primarily as an impurity in Al-Si-Mg alloys and decreases the sensitivity of the alloy to quench rates. It also increases the stress-hardening effect as well as the strength in the T6 temper. Higher copper content decreases ductility and resistance to corrosion, while additions of manganese, chromium, and zirconium inhibit recrystallization during solution treatment. Manganese additions increase creep and fatigue resistance and, to some extent, counteract copper in neutralizing the corrosion susceptibility of the alloy. Manganese converts the crystallization of needle-like intermetallic phases to cubic or globular forms, such as Chinese script morphology, which have less harmful characteristics. This type of morphology improves tensile strength, elongation and ductility. Furthermore, it should be noticed that small amounts of manganese (usually Mn:Fe = 1:2) play a positive role by breaking up the iron needles When added in a higher ratio or in the presence of chromium, depending on the melt temperature, manganese produces a hard multi-component intermetallic compound, commonly referred to as sludge, which affects the mechanical properties of the casting. Lead and bismuth may be deemed useful additions for improving the machining characteristics of the alloy .

Table des matières

CHAPTER 1 DEFINITION OF THE PROBLEM
1.1 INTRODUCTION
1.2 OBJECTIVES
1.3 THESIS LAYOUT
CHAPTER 2 REVIEW OF THE LITERATURE
2.1 INTRODUCTION
2.2 ALUMINUM CASTING ALLOYS
2.3 ALUMINUM-SILICON ALLOYS
2.3.1 Al-Si-Cu-Mg Alloy System
2.3.2 Al-Si-Mg Alloy System
2.4 MICROSTRUCTURE AND FEATURES OF Al-Si ALLOYS
2.5 MECHANICAL PROPERTIES OF Al-Si ALLOYS
2.5.1 Hardness Test
2.5.2 Tensile Testing
2.5.3 Instrumented Impact Testing
2.6 EFFECTS OF MELT TREATMENT
2.6.1 Modification of Al-Si Alloys
2.6.1.1 Types of Chemical Modifier
2.6.1.2 Effect of Modification on Mechanical Properties
2.6.1.3 Effect of Modification on Melt Quality
2.6.2 Grain Refinement
2.6.2.1 Grain Refinement by Adding Titanium
2.6.2.2 Grain Refinement by Adding Boron
2.6.2.3 Grain Refinement by Adding Titanium with Boron
2.6.2.4 Effect of Grain Refinement on Properties
2.6.2.5 Effect of Grain Refinement on Melt Quality
2.6.3 Mutual Poisoning Effect of Modification and Grain Refinement in Al-Si Casting Alloys
2.7 ALLOYING ELEMENTS
2.7.1 Role of Copper and Magnesium in Al-Si Alloys
2.7.2 Role of Iron and Manganese in Al-Si Alloys
2.7.2.1 Formation of Iron-Intermetallics during Solidification
2.7.2.2 Effect of |3(AlFeSi) on the Properties of Al-Si Alloys
2.7.2.3 Effect ofa-Fe on the Properties of Al-Si Alloys
2.8 HEAT TREATMENT
2.8.1 Solution Heat Treatment
2.8.2 Quenching
2.8.1 Aging
2.9 EFFECTS OF TRACE ELEMENTS ON MATERIALS
2.9.1 Tin in Al-Si Alloys ,
CHAPTERS EXPERIMENTAL PROCEDURES
3.1 INTRODUCTION
3.2 EXPERIMENTAL PROCEDURES
3.2.1 Preparation of Alloys and Melting Procedures
3.2.2 Casting Procedures
3.2.3 Metallography-Microstructural Examination
3.2.4 Heat Treatment
3.2.5 Mechanical Testing
3.2.5.1 Tensile Testing
3.2.5.2 Hardness Testing
3.2.5.3 Impact Testing
CHAPTER 4 EFFECT OF ALLOYING ELEMENT-ADDITIONS ON THE MICROSTRUCTURE, HARDNESS, AND TENSILE PROPERTIES OF Al-10.8%Si NEAR-EUTECTIC ALLOY
4.1 INTRODUCTION Il l
4.2 CHARACTERIZATION OF MICROSTRUCTURE
4.2.1 Characteristics of Silicon Particles
4.2.2 liter-metallic Phases
4.2.2.1 Iron-Rich Intermetallics
4.2.2.2 Copper-Rich Intermetallics
4.2.2.3 Al-Cu-Mg-Si Phases
4.3 POROSITY MEASUREMENTS
4.4 HARDNESS
4.4.1 Effects of Melt Treatment
4.4.2 Effects of the Addition of Fe and Mn
4.4.3 Effects of the Addition of Cu and Mg
4.5 TENSILE PROPERTIES
4.5.1 Effects of Melt Treatment
4.5.2 Effects of the Addition of Fe and Mn
4.5.3 Effects of the Addition of Cu and Mg
CHAPTER 5 IMPACT PROPERTIES OF Al-10%Si ALLOYS AND EXPERIMENTAL DESIGN
5.1 IMPACT PROPERTIES
5.1.1 Introduction
5.1.2 Effects of Melt Treatment
5.1.3 Effects of the Addition of Fe and Mn
5.1.4 Effects ofthe Addition of Cu and Mg
5.1.5 Relation Between Impact Energy and Ductility
5.2 STATISTICAL ANALYSIS
5.2.1 Introduction
5.2.2 Factorial Design of Experiment
5.2.3 Results and Discussion
CHAPTER 6 ADDITION OF TRACE ELEMENTS
6.1 INTRODUCTION
6.2 EFFECTS OF LEAD, BISMUTH, AND TIN ON MICROSTRUCTURE
AND MECHANICAL PROPERTIES OF AN EXPERIMENTAL Al10.8%Si ALLOY
6.2.1 Characterization of Microstructure
6.2.1.1 Effects on Microstracture of Adding Pb, Bi, and Sn Individually
6.2.1.2 Effects on Microstracture of Adding Pb, Bi, and Sn in
Combination
6.2.2 Mechanical Properties
6.2.2.1 As-Cast Condition
6.2.2.2 T6 and T7 Heat-Treated Condition
6.3 INFLUENCE OF TIN ADDITION ON THE MICROSTRUCTURE AND
MECHANICAL PROPERTIES OF B319.2 AND A356.2
ALLOYS
6.3.1 Microstructure
6.3.1.1 Alloy B319.2
6.3.1.2 Alloy A356.2
6.3.2 Porosity
6.3.3 Mechanical Properties
6.3.3.1 Hardness
6.3.3.2 Tensile Properties
6.3.3.3 Impact Properties
CHAPTER 7 CONCLUSIONS AND RECOMMENDATIONS
7.1 CONCLUSIONS
7.2 SUGGESTIONS FOR FUTURE WORK
REFERENCES

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