Casting alloys are defined by their chemical composition and according to the major alloying elements present in the alloy. The alloying elements and impurities present in an alloy together with the cooling rate, determine which constituents will precipitate. These major alloying elements are added to increase the strength and other properties of aluminum casting alloys.
Role of Strontium Modification
The improvement in mechanical properties has generally been attributed to variations in the morphology and size of the eutectic silicon particles. Silicon content and its morphology in the structure have a significant influence on the mechanical properties of the alloys. Chemical modifiers are elements which are used mainly for transforming the morphology of the coarse acicular silicon in the eutectic structure of Al-Si alloys into a finer or more fibrous form in order to improve the mechanical properties. The most common modifying elements used are sodium and strontium, [31] although nowadays Sr is mostly employed for modification purposes, in the form of Al-Sr master alloys. The modification mechanism of the Si particles and changes in their morphology has been described by Mondolfo. [29] Although the presence of strontium can change the morphology of the silicon particles, when the Al-Si alloy contains more than 0.05 wt% Sr, the formation of undesirable Sr-compounds such as Al2SrSi2 contributes to a decrease in mechanical properties. [32] The addition of Sr is associated with an increase in the amount of porosity and inclusions.
It has been reported that the iron intermetallic volume fraction is lower in the Srmodified alloys compared to non-modified alloys. The lower volume fraction of the iron intermetallics in the Sr-modified alloys may be attributed to the effects of Sr modification on the fragmentation of iron intermetallics in the as-cast Al-Si alloys. [33] Peyman et al. [34] studied the influence of Sr addition on intermetallic compound morphologies in an Al-SiCu-Fe cast alloy; they observed that the length and volume fraction of the β-phase decreases and that the number density of particles increases upon adding 0.015 % Sr, indicating that Sr proves to be an effective element in the modification and shortening of the β-intermetallic phase particles.
It has been observed that the addition of strontium acts as an obstacle for the nucleation of the β-needles, by reducing the number of sites ultimately available for nucleation. As a result, the β-iron phase precipitates at a smaller number of sites, leading to the precipitation of needles which are larger compared to those in the non-modified alloy.[35] These studies [34, 35] contradict each other. It has been found that for 319 alloy containing 0.46% Fe and which solidified at a slow cooling rate, the optimum Sr levels lie closer to the limit of 400 ppm. As the iron level increases, the optimum Sr level will be observed to shift towards the higher limit. [36]
Silicon content and its morphology in the structure have a significant influence on the mechanical properties of the alloys . As the silicon content increases, the tensile strength of the alloy is enhanced; however, at the same time, the brittle nature and flake morphology of the unmodified silicon phase will affect the alloy ductility adversely. After modification, both tensile strength and elongation are improved, with the greatest improvement being observed in the elongation. [37]
Role of Iron
Any element which is not classified as an alloying component is termed an impurity, and as such it is deemed to have a negative effect on the castability, mechanical properties and heat treatment of aluminum alloys. Among such elements, iron is a common impurity in aluminum alloys arising from a number of sources. It is usually considered detrimental for Al-Si based casting alloys.[33,48] Iron can enter the melt during further downstream melt activity through two basic mechanisms: (i) liquid aluminum is capable of dissolving iron from unprotected steel tools and furnace equipment, or (ii) iron may also enter an aluminum melt via the addition of low-purity alloying materials such as Si, or via the addition of scrap that contains higher background iron than the primary metal.
Manganese is a common alloying element used as an addition to neutralize the effect of iron and to modify the morphology and type of the intermetallic phases formed. Manganese is added to many of the alloys for two main purposes: (i) to increase hightemperature strength and creep resistance through the formation of the high melting point compounds; Cu2Mn3Al20, MnCrAl12, Al15(Fe,Mn)3Si2 and more complex compounds containing chromium and nickel; and (ii) to correct the embrittling effect of Fe.
The detrimental effect of iron begins at a somewhat low primary Fe-level but becomes far more serious once a critical Fe-level, dependent on the alloy composition, is exceeded. The critical iron level is directly related to the concentration of both Si and Mg in the alloy; this concentration controls the formation of Fe phases, mainly β-Al5FeSi, αAl15(Fe,Mn)3Si2 and π-Al8FeMg3Si6. Increasing Fe leads to a gradual reduction in the elongation, impact strength, and tensile strength of Al-Si alloys, while the Brinell hardness and yield strength are reported to increase gradually. Iron contents of up to 0.2% improve the tensile strength, while higher levels reduce the tensile strength and elongation, and increase hardness. [49] It has been reported that the percentage of the β-phase rapidly increases with the iron concentration for a given concentration of Mn .
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Table des matières
CHAPTER 1 DEFINING THE PROBLEM
1.1 INTRODUCTION
1.2 OBJECTIVES
CHAPTER 2 REVIEW OF THE LITERATURE
2.1 ALUMINUM ALLOYS
2.1.1 Classification of Aluminum Alloys
2.1.1.1 Aluminum-Silicon Casting Alloys
2.2 Al-Si-Mg ALLOY SYSTEM
2.3 INTERMETALLIC PHASES IN Al-Si-Mg ALLOYS
2.4 ROLE OF ALLOYING ELEMENTS AND IMPURITIES
2.4.1 Role of Strontium Modification
2.4.2 Role of Magnesium
2.4.3 Role of Iron
2.4.4 Role of Beryllium
2.4.5 Role of Titanium
2.4.6 Combination Effect of Beryllium with the Other Alloying Elements
2.5 HEAT TREATMENT OF Al-Si-Mg ALLOYS
2.6 MECHANICAL PROPERTIES OF Al-Si-Mg ALLOYS
2.6.1 Tensile Testing
2.7 QUALITY INDEX CHARTS FOR ALUMINUM FOUNDRY
ALLOYS
2.8 Al-Cu-Mg-Zn WROUGHT ALLOYS
CHAPTER 3 EXPERIMENTAL PROCEDURES
3.1 INTRODUCTION
3.2 ALLOY PREPARATION AND MELTING PROCEDURES
3.3 MELTING AND CASTING PROCEDURES
3.3.1 Thermal Analysis
3.3.2 Preparation of Tensile Bars
3.4 HEAT TREATMENT PROCEDURES
3.5 TENSILE TESTING
3.6 METALLOGRAPHY
CONCLUSION
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