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Having discussed the birth, extraction, refinement and uses of aluminum, it is now time to see what alloys are available to support the wide range of uses we have detailed previously in Part 2 of this story.

Aluminum alloys can essentially be split into two types as per:

• Those that do not strengthen by heat treatment, strength being provided by alloying the aluminium and by applying mechanical work. These are non-heat treatable

• Those that provide their strength through the process of alloying and by further heat treatment

Non Heat Treatable

The initial strength of these alloys is produced by alloying the aluminum with additions of other elements. These alloys are given numerical prefixes as per:

• pure aluminum alloys (1xxx series),听

• alloyed with manganese (3xxx series),听

• alloyed with silicon (4xxx series), and听

• alloyed with magnesium (5xxx series).

This group relies solely upon two mechanisms to provide strength:

• alloying and,

• cold mechanical work

The 1xxx-series alloys听are of commercial purity (>99% Al) and are used where thermal/electrical conduction or corrosion resistance becomes paramount over strength in design considerations.听

For example, aluminum transmission cables are designed to be used in primary and secondary electrical power distribution as per Figure 1. The conductors are made of听1350-H19听Aluminum series, which makes the cables soft and easy to install in power distribution lines and they can be strengthened by co-cabling with steel. These cables are often designated ACSR standing for Aluminum Conductor Steel Reinforced.

The 鈥淗19鈥� designation refers to the temper or hardness of the material which is detailed below.

Figure 1. Electrical Distribution Towers/Cables where ACSR Aluminum Based Cables may be used

The 3xxx-series alloys听are used in applications where added additional strength and formability are needed on top of excellent corrosion resistance. Typical applications include cooking utensils, pressure vessels, and building products such as siding and gutters.

4xxx Alloys. The 4xxx-series alloys have limited industrial application in their wrought form apart from their use as welding filler material which we will discuss in a later article in our Story of Aluminum.

The 5xxx-series alloys听are used in cases where higher strengths are required. Strength is achieved from large quantities of magnesium as the alloying element. Applications for these alloys include pressure vessels, armour plate, and components for marine and cryogenic service.

A further increase in strength of these alloys is obtained through various degrees of cold working. Cold working is accomplished by rolling, drawing through dies, stretching, or similar operations where a reduction in area is obtained. Regulating the amount of total reduction in the area of the material controls its final properties. The letter 鈥淗鈥� followed by numbers denotes the specific condition obtained.

For example, the first number following the 鈥淗鈥� indicates the basic operations used during or after cold working:听 There are many, many delivery conditions and typical examples are:

• H1 鈥� Cold working only

• H3 鈥� Cold working and stabilized by a heat treatment

The second number following the 鈥淗鈥� indicates the degree of cold work:

• HX2 鈥� Quarter Hard

• HX6 鈥� Three-Quarter Hard

Heat Treatable Aluminum Alloys

These usually are referred to as the 鈥渉eat-treatable鈥� aluminum alloys to distinguish them from the previous alloys in which no significant strengthening can be achieved by heating and cooling.听

Heat treating refers to heating and cooling operations performed for the purpose of changing the mechanical properties, the metallurgical micro structure, or the residual stress state of a metal product.

To increase the strength of these alloys, the heat treatment is a three-step process and is metallurgically based. It鈥檚 not our purpose the delve into complex metallurgy, suffice to say the process consists of:

• A heat treatment

• A quench

• A so-called age hardening step at room temperature or at a designed elevated temperature:听

Again, age hardening is a metallurgical phenomenon which is beyond our scope here. It may also be referred to as precipitation hardening.

The mayor aluminum alloy systems with strengthening by heat treatment are:听

• Those alloying with copper. (2xxx series)

• Those alloying with magnesium and silicon (6xxx series)

• Those alloying with zinc and magnesium (7xxx series)

The 2xxx series, are primarily听aluminum-copper alloys. They find application in screws, fasteners and precision parts for aerospace.

The 6xxx series 颈蝉听an aluminum alloy family which contains magnesium and silicon as predominate alloying components. The 6000 series has excellent properties in the area of extrudability, strength, and high corrosion resistance and may be used in extruded products such as window frames shown in Figure 2.

Figure 2. Extruded Window frames in 6063-T5 alloy

Does anybody looking out of this window ever think that the window frame material started life in the stars many millions of years ago, was deposited on Earth, mined as bauxite, refined, alloyed, and then extruded and made available in your local hardware store?

Moving on, the 7xxx wrought aluminum series features Zinc (Zn) as the primary alloying element. And this series has higher mechanical strength than other aluminum alloys.听 These alloys are used by the aircraft industry, Figure 3, the 7075 Al alloy being the most preferred.听

Figure 3. Typical Military Jet which will include Aluminum Alloys in its make up.

The materials are further identified by the addition of a 鈥�-T鈥� indicating that the alloy has been heat-treated, quenched, or metallurgically hardened. The number following the T indicates which thermal process it has undergone. For example:

听

• T1 鈥� Cooled from an elevated temperature and naturally age hardened

• T2 鈥� Cooled from an elevated temperature, cold-worked, and naturally age hardened

• T6 鈥� heat-treated for metallurgical enhancement and artificially age hardened

• T10 鈥� Cooled from elevated temperature, cold-worked, and artificially aged

So, the above has summarized the several aluminum alloys made available to us via judicious alloying, through cold work and through heat treatment where possible.听

In our next article in this continuing story, we will move toward the weldability of these alloys. We will see which of these materials we have discussed have good, or superior weldability, and which, if any, do not.