Noteworthy advantages of using aluminum alloys Light weight Good machinability Good formability High electrical / thermal conductivity High corrosion resistance

Important alloying elements in aluminum alloy systems •Copper (2xxx) Manganese (3xxx) Silicon (4xxx) Magnesium (5xxx) Zinc (7xxx) • Cr, Ti and Zr are used in some alloys for grain refinement

Wrought alluminum alloys There exists an internationally agreed classification system for wrought alloys. Classification for wrought aluminium alloys. 8XXX Miscellaneous alloys, e.g. aluminium-lithium alloys 7XXX Al - Zn - Mg alloys 6XXX Al - Mg - Si alloys 5XXX Al - Mg alloys 4XXX Al - Si alloys 3XXX Al - Mn alloys 2XXX Al - Cu alloys 1XXX Al of 99% minimum purity Each alloy is described by a four digit number

Designations of wrought aluminum alloys Non-heat-treatable alloys 1xxx series (super purity & commercial purity aluminum) 3xxx series (Al-Mn & Al-Mn-Mg alloys) 5xxx series (Al-Mg alloys) 8xxx series (Miscellaneous alloys) Heat-treatable alloys 2xxx series (Al-Cu & Al-Cu-Mg alloys) 6xxx series (Al-Mg-Si alloys) 7xxx series (Al-Zn-Mg and Al-Zn-Mg-Cu alloys) Designation of each alloy includes a further letter and number indicating the temper, or condition of the alloy.

Denoting the thermal/mechanical treatment of wrought alloys O: annealed H: work hardened H12: quarter hard; H14: half hard: H18: full hard T1: naturally aged after hot working T4: solution treated, quenched and naturally aged T6: solution treated, quenched and artificially aged T8: same as T6, except cold worked before aging

Principle of age-hardening Age hardening requires a decrease in solid solubility of the alloying elements with decreasing temperature.

The Al-Zn phase diagram shows the rapid decrease of the solubility of zinc in aluminum with decreasing temperature.

5.65 wt% solubility of Cu in Al at 548C The solubility drops down to 0.02 wt% at room temperature Precipitation hardening is thus possible. Al-Cu phase diagram (Al-rich side)

Heat treatment usually involves the three following stages: 1. Solution treatment at a relatively high temperature to dissolve the alloying elements. 2. Rapid cooling or quenching usually to room temperature to obtain supersaturated solid solution (SSSS) of these elements in aluminum. 3. Controlled decomposition of the SSSS to form a finely dispersed precipitates, normally accompanied with ageing at appropriate temperature(s).

Heat treatment for precipitation hardening • Solution heat treatment: at T0, all the solute atoms A are dissolved to form a single-phase (α) solution. • Rapid cooling across the solvus line to exceed the solubility limit. This leads to a metastable supersaturated solid solution at T1. Equilibrium structure is α+β, but limited diffusion does not allow β to form. • Precipitation heat treatment: the supersaturated solution is heated to T2 where diffusion is appreciable – finely dispersed particles rich in element B start forming : aging.

Precipitation hardening in Al-Cu System

Heat treatment for precipitation hardening Discs of Cu atoms 1 or 2 monolayers thick

Surface treatments Many coatings are commercially used – organic coatings, chemical conversion coatings and platings Most important are anodizing and hard coating – coating is aluminum oxide, electrochemically induced. The coating provides a good corrosion barrier and wear resistance.s

Corrosion Aluminum is susceptible to pitting and accelerated corrosion seawater and halide environments. These environments should be avoided. Pure aluminum has better corrosion resistance than any of the aluminum alloys. This is because alloy micro-constituents impair the protective oxide film. To improve the corrosion resistance, Al-Cu and Al-Zn-Mg alloys are sandwiched between two pure aluminum sheets and rolled to produce the composite Alclad.

Designations of cast aluminum alloys Alloy designation Major alloying elements 1xx.x 99.5 min. aluminum 2xx.x Copper 3xx.x Silicon + copper or magnesium 4xx.x Silicon 5xx.x Magnesium 6xx.x Unused series 7xx.x Zinc 8xx.x Tin 9xx.x Other element Last digit after decimal point indicates the product form (0: casting, 1: ingot)

Casting techniques The most common aluminium casting techniques are; 1) Sand casting 2) Die casting - gravity casting - high pressure die casting - low pressure die casting - vacuum die casting - squeeze casting Selection of casting process depends upon alloy composition which is related to controlled characteristics such as solidification range,fluidity, susceptibility to hot-cracking. Castability 3xx.x > 4xx.x > 5xx.x > 2xx.x > 7xx.x Si,Cu,Mg Si Mg Cu Zn

Modification of microstructure Apart from fast cooling to refine the microstructure, modification can be carried out by adding certain alkali fluorides to the melt prior to pouring. Additions of Sr or Na change eutectic microstructure from needle-like or lamellar to fibrous. A concentration of 0.02% Sr is enough to cause 100% modification to fibrous structure.

Eutectic silicon crystals with acicular morphology in an unmodified sample of alloy A365. Etchant: Keller's reagent Eutectic silicon crystals with fibrous morphology in a modified sample of alloy A365. Etchant:Keller's reagent

Why add modifiers Controlling silicon morphology Improving mechanical properties Improving machinability Reducing hot tearing Reducing heat treatment times Controlling porosity distribution Improving die filling Suppressing primary silicon formation Reducing die sticking

Cast aluminium alloys are widely used for transport applications, e.g., Cast engine block