3.2 The families of engineering alloys
3.2.1 Aluminium alloys
Cast aluminium alloys
About 20% of the world production of aluminium is used for cast products.
Aluminium alloys have a relatively low melting temperature, but exhibit a high shrinkage during solidification. Shrinkage of between 3.5 and 8.5%
may occur and allowance has to be made for this in mould design in order to achieve dimensional accuracy in the product.
Aluminium–silicon alloys are the most important of the aluminium casting alloys, and the relevant phase diagram is shown in Fig. 3.9, which is seen to be of simple eutectic form. Slow solidification produces a very coarse eutectic structure consisting of large plates or needles of silicon in an aluminium matrix. The silicon particles are brittle, so castings with this coarse eutectic exhibit low ductility. Refinement of the eutectic improves the mechanical properties of the casting and this can be brought about by rapid cooling, as in permanent mould casting, or by modification, which involves adding sodium salts or metallic sodium to the melt before pouring. Improved tensile strength and improved ductility is the result of this microstructural change, a combination that is rarely encountered in physical metallurgy.
If the silicon content is below 8%, modification is unnecessary, because the primary aluminium phase is present in sufficient quantity to confer adequate
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ductility. The binary Al–Si alloys are generally used where strength is not a primary consideration, e.g. pump casings and automobile engine manifolds.
Copper and magnesium additions are made to enhance the strength of such alloys and more complex compositions (e.g. including nickel additions) lead to improved elevated temperature properties for such applications as piston alloys for internal combustion engines.
Small additions of magnesium allow significant age-hardening of the castings through precipitation of Mg2Si in the aluminium matrix. Doubling of the yield strength may be achieved in this way, and such alloys find use in aircraft and automotive applications.
Aluminium–copper alloys, although less easily cast than Al–Si alloys, respond well to age-hardening heat treatments. Several compositions have been developed with enhanced elevated temperature properties, for example for use as diesel engine pistons.
At% Al
0 10 20 30 40 50 60 70 80 90 100
Liquid
Liquid + Si
Liquid + Al
577
660
Al Si
Si + Al
0 10 20 30 40 50 60 70 80 90 100
Wt% Al
Temperature (°C)
1500 1412 1400 1300 1200 1100
1000 900 800 700 600 500 400 300 200
3.9 The silicon–aluminium phase diagram.
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Aluminium–magnesium alloys, with Mg contents of 4–10%, are characterized by a high resistance to corrosion. Only Al–10% Mg castings respond to heat-treatment. Aluminium–zinc–magnesium alloys have a relatively high eutectic melting point, which make them suitable for castings that are assembled by brazing. The as-cast alloys respond to both natural and artificial ageing.
Cast aluminium and its alloys may be grain refined by the addition of suitable innoculants to the melt. For example, commercial additives based on a master alloy of Al–Ti–B are widely used: these produce intermetallic particles in the melt which act as centres of crystallization for the alloy.
Table 3.1 presents some properties of a selection of cast aluminium alloys.
Wrought aluminium alloys
About 85% of aluminium is used for wrought products, produced from cast ingots by rolling, extrusion, drawing etc. An outline is given in Table 3.2 of the International Alloy Designation System employed for these materials.
We will consider these alloys in two groups, namely those the properties of which are not enhanced by heat-treatment, and those that are.
Non-heat-treatable alloys
There are two important families:
Aluminium–manganese alloys (3xxx series) contain up to 1.25% Mn, which gives rise to solution-hardening. The further addition of magnesium gives a further increase in strength, coupled with high ductility and excellent corrosion resistance. These alloys are widely used for cooking utensils, as well as for beverage cans.
Aluminium–magnesium alloys (5xxx series) contain up to 5% Mg. The alloys owe their strength to work hardening, which occurs at a rate that increases as the Mg content is raised. Over a period of time, the tensile properties may decline due to localized recovery, but special tempers may be
Table 3.1 Typical mechanical properties of some cast aluminium alloys
Alloy Condition Density Young’s Proof UTS Elongation (Mg m–3) modulus stress (MPa) (% in 50 mm)
(GPa) (MPa)
Al–11.5Si (LM6) Sand cast 2.65 71 65 170 8
Al–5Mg–0.5Mn Sand cast 2.65 71 100 160 6
(LM5)
Al–6Si–4Cu– Sand cast ~2.7 71 130 180 1
0.2 Mg–1Zn (LM21)
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Table 3.2 International Alloy Designation System (IADS) for wrought aluminium alloys 4-digit series (xxxx)
Each wrought alloy is assigned a four-digit number of which the first digit is determined by the major alloying element(s) present, thus:
Series Main alloying elements
1xxx Unalloyed aluminium (99% Al minimum)
2xxx Copper
3xxx Manganese
4xxx Silicon
5xxx Magnesium
6xxx Mg and Si
7xxx Zinc
8xxx Lithium
9xxx Unused series
Temper or heat treatment (also applied to Mg alloys)
Suffix letters and digits are added to the alloy number in order to specify the mechanical properties of the alloy and the way in which the properties were achieved, thus:
Suffix letter Basic condition
F As-fabricated
O Annealed wrought products
H Cold worked (strain hardened)
T Heat treated
Suffix digits
First digit Secondary treatment Second digit (H only) Degree of cold work
Recourse to detailed specifications or to manufacturers’ literature is suggested when several digits are included in the temper designation.
applied which stabilize them against this effect. These alloys are widely used in welded applications. Their corrosion resistance makes them suitable for storage tanks and for marine hulls and superstructures. Fine-grained 5xxx alloys have also been used for superplastic forming of panels.
Heat-treatable alloys
There are three important families:
Aluminium–copper alloys (2xxx series). These can contain up to 6.8% copper and this system has been most widely studied as an example of age-hardening.
For example, alloy 2219 (6.3% Cu) is available as sheet, plate and extrusions, as well as forgings, and it can be readily welded. It has relatively high strength in the peak-aged condition, but its peak hardness may be enhanced by about one third by strain hardening the quenched, supersaturated alloy before artificial ageing.
The Al–Cu–Mg alloy known as Duralumin was the earliest age-hardening alloy to be developed, by Wilm in 1906. Alloy 2014 is a development of
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Duralumin and is still widely used for aircraft construction. The addition of copper is deleterious to the corrosion resistance of aluminium, so that sheet material in this group of alloys is often roll clad with pure aluminium or Al–
1% Zn to produce a corrosion-resistant surface layer. This soft surface layer can lead to serious deterioration of the fatigue resistance, however.
Aluminium–magnesium–silicon alloys (6xxx series) are medium-strength alloys, widely used in the form of extruded sections for structural and architectural application, window frames being a typical example. The alloy can be quenched as it emerges from the extrusion press, thus eliminating the need for a separate solution-treatment operation. Optimum properties are again developed by a final ageing treatment.
Aluminium–zinc–magnesium alloys (7xxx series) are the highest-strength family of aluminium alloys. They are readily welded and find wide structural application. An addition of copper is made to reduce the susceptibility to stress-corrosion cracking (SCC), and the Al–Zn–Mg–Cu alloys are widely used in aircraft construction. Alloy 7075 is the most widely known of these and often rather complex heat-treatments have to be applied in order to minimize the propensity to SCC.
A new family of age-hardening aluminium alloys containing lithium (8xxx series) has been developed in recent years. These alloys have the advantage of a lower density and a higher value of Young’s modulus than the 7xxx series. They are designed to substitute for conventional aircraft alloys, with a density reduction of 10% and a stiffness increase of at least 10%. Their purchase price is several times that of existing high-strength aluminium alloys, so their overall economic advantages have to be very carefully weighed when considering a potential application.
Table 3.3 gives the mechanical properties of some wrought aluminium alloys.