Aluminum Alloys and Heat Treatments (PDF)

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This document provides an overview of aluminum alloys, their classifications based on alloying elements (e.g., copper, manganese, magnesium), and the effects of heat treatments on their properties. The document also discusses the importance of precipitation hardening in controlling microstructure and mechanical properties.

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Aluminum alloys and specification Aluminium is the world’s most abundant metal and is the third most common element comprising 8% of the earth’s crust. The versatility of aluminium makes it the most widely used metal after steel. Aluminium is derived from the mineral bauxite. Bauxite is converted to...

Aluminum alloys and specification Aluminium is the world’s most abundant metal and is the third most common element comprising 8% of the earth’s crust. The versatility of aluminium makes it the most widely used metal after steel. Aluminium is derived from the mineral bauxite. Bauxite is converted to aluminium oxide (alumina) via the Bayer Process. The alumina is then converted to aluminium metal using electrolytic cells Process. Worldwide demand for aluminium is around 29 million tons per year. Pure aluminium is soft, ductile, corrosion resistant and has a high electrical conductivity. It is widely used for foil and conductor cables, but alloying with other elements is necessary to provide the higher strengths needed for other applications. Aluminium is one of the lightest engineering metals, having a strength to weight ratio superior to steel. However, high purity aluminium has a very low yield strength of about 7 MPa. The tensile strength of pure aluminium is around 90 MPa but this can be increased to over 690 MPa for some heat-treatable alloys. Since there are no allotropic phase transformations in aluminium, much of the control of microstructure and properties relies on precipitation reactions. The solubility of solute in the matrix (α) is therefore of importance. Alloy Designations aluminium alloys classification according to the alloying element 1. Alloying Element None (99%+ Aluminium) - For unalloyed wrought aluminium alloys designated 1XXX, the last two digits represent the purity of the metal. They are the equivalent to the last two digits after the decimal point when aluminium purity is expressed to the nearest 0.01 percent. The second digit indicates modifications in impurity limits. If the second digit is zero, it indicates unalloyed aluminium having natural impurity limits and 1 through 9, indicate individual impurities or alloying elements. 2. Alloying Element Copper - 2XXX In the Al–Cu system, the stable precipitate is CuAl2 but because it is difficult to nucleate, metastable GP1 zones form first. Thus, the free energy curve for GP1 zones is located above that for CuAl2 Figure 1 phase diagram of Al-Cu system where the precipitation of intermetallic compound is the most important. 1 The Al–Cu system is one in which the enthalpy of mixing, ∆HM is positive. It follows that at low temperatures, there will be a tendency for like atoms to cluster, giving rise to a miscibility gap. Copper has the largest solubility, i.e. the smallest enthalpy of solution. Solid solution strengthening is useful but it leads only to an increase of about 40 MPa in the strength of commercial alloys. But the fact that the solubility decreases exponentially with temperature can be used to precipitation harden aluminium alloys. Figure 2 (a) A eutectic phase diagram with a hidden miscibility gap. (b) Free energy of mixing plotted as a function of temperature Precipitation (a) Age hardening involves the rapid cooling of a solid solution from a high temperature to one where it becomes supersaturated so that precipitation begins on ageing. Note that it is not just the solute concentration which is supersaturated but also the vacancy concentration. (b) Age in a way which avoids precipitate–free zones. The latter form either due to vacancy or solute depletion in the vicinity of grain boundaries. (c) Consider the role of metastable precipitates in the development of precipitation hardening. 3. Alloying Element Mn+Mg These are the Al–Mn or Al–Mn–Mg alloys with moderate strength ductility and excellent corrosion resistance. The Al-Mn phase diagram depicts the following intermediate phases: Al12Mn (Al12W-type cubic), Al6Mn (Al6Mn-type orthorhombic), λAl4Mn (hexagonal, space group P63/m), μAl4Mn (hexagonal, P63/mmc), Al11Mn4(HT) (Al3Mn-type orthorhombic), Al11Mn4(LT) (Al11Mn4- type triclinic), Al8Mn5 (Al8Cr5-type rhombohedral), γ (34.5-52 at.% Mn; bcc), and ɛ (55-72 at.% Mn; cph). The Al-Mg phase diagram has the following intermediate phases: Al3Mg2 (Al3Mg2- type cubic, labeled β), R or ɛ (rhombohedral) and Al12Mg17 (A12, αMn-type cubic, denoted γ). There are no intermediate phases in the Mg-Mn system. 2 Figure 3 a) the phase diagram of Al-Mn b) the phase diagram of Al-Mg. Figure 4 ternary phase diagram of Al-Mn-Mg alloy. The strength, at about 110MPa, comes from dispersoids which form in the early stages of solidification. The Mn concentration is restricted to about 1.25 wt% to avoid excessively large primary Al6Mn particles. Magnesium (0.5 wt%) gives solid solution strengthening and the Al– Mn–Mg alloy is used in the H or O conditions. Beverage cans(AA3004) represent the largest single use of aluminium or magnesium. A typical alloy has the chemical composition Al– 0.7Mn–0.5Mg wt%. 3 4. Alloying Element Silicon The binary Al-Si phase diagram was first studied by Fraenkel of Germany in 1908. It is a relatively simple binary diagram where there is very little solubility at room temperature for Si in Al and for Al in Si. Thus, the terminal solid solutions are nearly pure Al and Si under equilibrium conditions. The currently accepted diagram Figure 5 bainary phase diagram of Al-Si alloy. Modifiers like Na shift the eutectic to higher silicon contents, around 14 wt. %, preventing precipitation of hypereutectic Si while refining the structure of the eutectic. This increases both strength and ductility substantially. Figure 6 microstructure of Al-Si alloys 4 5. Alloying Element Magnesium Al-Mg alloys belong to the 5XXX series of aluminum alloys. Compared with other aluminum alloys, the Mg element, as a crucial strengthening element, improves the strength and the corrosion resistance of Al-Mg alloys. Al-Mg alloys can become sensitized when magnesium comes out of solution as a second phase, Al3Mg2, on the grain boundaries, eventually forming a continuous. Figure 6 binary phase diagram of Al-Mg alloy. When exposed to elevated temperature over time (years, in this case) magnesium diffuses from within the grain crystal to form a β-phase precipitate at the grain boundary, in the form of Al3Mg2. The β-phase precipitates can form a continuous, anodic and brittle film on the grain boundaries. Once the grain boundary precipitates becomes continuous, or nearly continuous, the material is considered sensitized. When this sensitized material is exposed to tension in this susceptible state, it is prone to intergranular stress corrosion cracking (IGSCC) Figure 8 Al-Mg alloy intergranular stress corrosion cracking (IGSCC). 5 Figure 9 Effect of temperature on Al-Mg alloy. 6. Alloying Element Zinc 7000 alloys are used above all in automotive industry and architectural applications. These materials exhibit medium strength and ductility at room temperature and can be strengthened by aging treatment. Moreover they are characterized by low quench sensitivity, good corrosion resistance (due to the absence of Cu addition) and good extrudability. Because of their commercial importance, much effort has been spent on investigation of the precipitation process in Al–Zn–Mg alloys. The high strength exhibited in the hardened state is due to a fine distributions of precipitates, notably of the metastable η’ phase MgZn2, produced by artificial aging from a supersaturated solid solution Figure 10 Al-Zn alloy and microstructure. 6 7. Alloying Element Lithium - 8XXX Aluminum alloys containing lithium are under much scrutiny to perform with expectations that they will improve weight and stiffness critical application even against other aluminum alloys and even carbon-fiber composites. Specific benefits of Al-Li alloys are a reduced density and an increased modulus of elasticity with an increase in the specific modulus of 26% compared to alloy 7075-T6. Magnesium and lithium are the only two elemental additions which, when added to aluminum, have the potential ability to decrease its density. Beryllium also decreases the density of aluminum but it is extremely toxic and is a health hazard. Lithium is one of the few elements with substantial solubility in aluminum (4.2 wt % at 600°C/1112°F in a binary aluminum lithium alloy). The potential for aluminum alloy density reduction through lithium additions is evident by comparing its atomic weight (6.94) with that of aluminum (26.98). Lithium additions to aluminum also cause a significant increase in elastic modulus. Each 1% lithium addition of aluminum, up to 4 wt-% lithium, decreases the density by 3%, and increases the elastic modulus by 6%. Military Applications: Certain types of military aircrafts parts like main wing box, center fuselage, control surfaces are made by Al-Li alloys. Al-Li alloys are used as a substitute for conventional Al alloys in helicopters, rockets and satellite systems Figure 11 Al-Li the formation of solid solution and intermetallic compound. 8 Alloying Element Tin Al–Sn alloys are simple eutectic binary alloy systems with solid solutions of a wide range of compositions and are well known as lubricating compounds, with the Al–Sn used as a surface coating for sliding bearings. The alloys also excel in high temperature stability. The surface finishing of the bearings is provided by dip coating or sputtering in industrial applications. Al- Sn is an immiscible alloy (unmixable) and it is called as soft tribological alloy. However, the system of Al-Sn has a wide miscibility gap in the liquid phase and does not form solid solution, as well as has big density difference between the two components, these is a very sedimentary tendency in conventional casting processes leading to insufficient alloying. Al in Sn melt with different temperature. According to the phase diagram of Al-Sn, it can be seen that no intermetallic compound is formed between Al-Sn, which means that there is no reaction diffusion at the Al/Sn solid-liquid interface during the dissolution process, but only interdiffusion behavior 7 Figure 11 Al-Sn system and the diffusion of Sn in Al at 500C. Heat treatments of Al alloys Aluminum alloys are classified as either heat treatable or not heat treatable, depending on whether the alloy responds to precipitation hardening, the key characteristic being that the alloying elements show greater solubility at elevated temperatures than at room temperature. Solution heat treatment involves heating the aluminum and alloys to a temperature slightly below the eutectic melting temperature. The objective of solution heat treatment is to maximize the amount of solute in solid solution. This requires heating the material close to the eutectic temperature and holding the material at temperature long enough to allow the alloy to become a homogenous solid solution. After solution heat treatment, the material is quenched to maintain the solute in supersaturated solid solution. Then the aging process for aluminum starting which involves either natural aging or artificial aging. Figure 12 a) solution treatment of AA 2017 and aging process b) strengthen mechanism for AA2024. The aging process is the key to form the required mechanical properties. Figure 13 the effect of the aging process on the mechanical properties. 8 Finally, it is important to compare the mechanical properties of aluminum alloys with steel specially for yeild point which it is the main for the designers to estimat the allowable stress. From the figure below it can be noteced that the yield strength for aluminum alloys is equal to defferent types of steel. Series 6 and 5 have similar to bake hardened steel and series 7xxx is higher than micro alloyed steel and austentic St.St. FIGURE 14 the comparison of Al alloys mechanical properties with steel alloys. However; the heat treatments of the alloys may improve the strength to higher. The table below shows the type of heat treatment and aging condition and classification. 9 Corrosion The natural alumina film (2–10 nm thick) on aluminium protects in neutral environments but not in alkaline or in strong acids (with the exception of concentrated nitric acid which is a strong oxidising agent). The oxide film can be thickened by immersion in hot acid to some 1–2 μ m. Even thicker films (10–20 μ m) can be obtained by anodising aluminium or zirconium. This involves making the component an anode in dilute H2SO4 solution. The film contains a cellular structure of open pores; they can be sealed by boiling in water which makes the cells expand by hydration. On drying the cells remain closed. The cells can be filled with dye before sealing to produce coloured aluminium. An increase in the current density and voltage during anodising causes microscopic arcing which locally induces the oxide to fuse and solidify rapidly. With sufficient arcing, a tenacious, hard and fully dense alumina coating is formed. This plasma electrolytic oxidation process can be exploited in making components such as rollers, which require wear resistance. Zinc in solid solution raises the Al–Zn electrode potential; such alloys are therefore used for cladding and as galvanic anodes for sacrificial protection. The presence of intermetallic compounds in an aluminium alloy reduces corrosion resistance. For example, iron and silicon compounds are regions where the alumina film is weakened. As a result, pure aluminium corrodes at a much lower rate than alloys. Indeed, pure aluminium is often used to clad aluminium alloys to protect against corrosion. Fatigue There are two major difficulties. Coherent precipitates are cut by dislocations; each passage of a dislocation shears the particle, producing steps at the entry and exit sites, thereby reducing the particle cross–section on the slip–plane (Fig. 6). This makes it easier for a subsequent dislocation to cut the particle. Slip then tends to focus on particular planes, leading to stress concentrations which promote fatigue. It is better therefore to have a mixture of fine, coherent and bigger semi– coherent precipitates so that the danger of inhomogeneous slip is reduced. Fig. : The effect of a dislocation passing through a coherent precipitate. Fatigue is also initiated at pores in thick aluminium components. This can only be controlled by careful processing, and by rolling deformation where this is permitted. 10

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